Note: Descriptions are shown in the official language in which they were submitted.
WO 2023/084121 PCT/EP2022/082024
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BARRIER-COATED CELLULOSE-BASED SUBSTRATE FOR LAMINATED
PACKAGING MATERIAL
Technical field
The present invention relates to a barrier-coated paper or cellulose-
based substrates, for use as a barrier sheet in a laminated packaging material
for oxygen-sensitive products, and to a method of manufacturing the barrier-
5 coated paper or cellulose-based substrate, by applying at least one gas
barrier coating of at least one gas barrier material to a total thickness from
2
to 5000 nm.
Background of the invention
10 Packaging containers of the single use disposable type for liquid
foods
are often produced from a packaging laminate based on paperboard or
carton. One such commonly occurring packaging container is marketed under
the trademark Tetra Brik Aseptic and is principally employed for aseptic
packaging of liquid foods such as milk, fruit juices etc, sold for long term
15 ambient storage. The packaging material in this known packaging container
is
typically a laminate comprising a bulk or core layer, of paper, paperboard or
other cellulose-based material, and outer, liquid-tight layers of
thermoplastics.
In order to render the packaging container gas-tight, in particular oxygen gas-
tight, for example for the purpose of aseptic packaging and packaging of milk
20 or fruit juice, the laminate in these packaging containers normally
comprises
at least one additional layer, most commonly an aluminium foil.
On the inside of the laminate, i.e. the side intended to face the filled
food contents of a container produced from the laminate, there is an
innermost layer, applied onto the aluminium foil, which innermost, inside
layer
25 may be composed of one or several part layers, comprising heat sealable
thermoplastic polymers, such as adhesive polymers and/or polyolefins. Also
on the outside of the bulk layer, there is an outermost heat sealable polymer
layer.
The packaging containers are generally produced by means of modern,
30 high-speed packaging machines of the type that form, fill and seal
packages
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from a web or from prefabricated blanks of packaging material. Packaging
containers may thus be produced by reforming a web of the laminated
packaging material into a tube by both of the longitudinal edges of the web
being united to each other in an overlap joint by welding together the inner-
and outermost heat sealable thermoplastic polymer layers. The tube is filled
with the intended liquid food product and is thereafter divided into
individual
packages by repeated transversal seals of the tube at a predetermined
distance from each other below the level of the contents in the tube. The
packages are separated from the tube by incisions along the transversal
seals and are given the desired geometric configuration, normally
parallelepipedal, by fold formation along prepared crease lines in the
packaging material.
The main advantage of this continuous tube-forming, filling and sealing
packaging method concept is that the web may be sterilised continuously just
before tube-forming, thus providing for the possibility of an aseptic
packaging
method, i.e. a method wherein the liquid content to be filled as well as the
packaging material itself are reduced from bacteria and the filled packaging
container is produced under clean conditions such that the filled package may
be stored for a long time even at ambient temperature, without the risk of
growth of micro-organisms in the filled product. Another important advantage
of the Tetra Brik -type packaging method is, as stated above, the possibility
of continuous high-speed packaging, which has considerable impact on cost
efficiency.
Packaging containers for sensitive liquid food, for example milk or juice,
can also be produced from sheet-like blanks or prefabricated blanks of the
laminated packaging material of the invention. From a tubular blank of the
packaging laminate that is folded flat, packages are produced by first of all
building the blank up to form an open tubular container capsule, of which one
open end is closed off by means of folding and heat-sealing of integral end
panels. The thus closed container capsule is filled with the food product in
question, e.g. juice, through its open end, which is thereafter closed off by
means of further folding and heat-sealing of corresponding integral end
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panels. An example of a packaging container produced from sheet-like and
tubular blanks is the conventional so-called gable-top package. There are
also packages of this type which have a moulded top and/or screw cap made
of plastic.
5 A layer of an aluminium foil in the packaging laminate provides gas
barrier properties quite superior to most other gas barrier materials. The
conventional aluminium-foil based packaging laminate for liquid food aseptic
packaging is still the most cost-efficient packaging material, at its level of
performance, available on the market today.
10 Any other material to compete with the foil-based materials must be
cost-efficient regarding raw materials, have comparable food preserving
properties and have a comparably low complexity in the converting of
materials into a finished packaging laminate.
Among the efforts of developing non-aluminium-foil materials for liquid
15 food carton packaging, there is also a general incentive towards developing
pre-manufactured films or sheets having high and multiple barrier
functionalities, which may replace the aluminium-foil barrier material in the
conventional laminated packaging material, or which may combine several
separate barrier layers in the laminated material and adapt it to conventional
20 processes for lamination and manufacturing.
Preferred types of such alternative, more environmentally sustainable
barrier material are barrier-coated paper substrates made by aqueous
dispersion coating or vapour deposition coating onto thin paper carrier
substrates. There are various aqueous dispersion coating processes and
25 vapour deposition coating processes and material recipes for such coatings,
and there is a need for cost-efficient barrier materials of this "non-foil"
type,
i.e. non-aluminium-foil, having improved properties for use in packaging
laminates for liquid food packaging, regarding barrier properties, in
particular
towards gases, such as oxygen gas.
30 An earlier patent publication W0201 1/003565A1 discloses a non-
alum inium-foil packaging material comprising a pre-coated and metallised
Kraft paper substrate for the purpose of induction heat sealing.
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The earlier patent publication W02017/089508A1 discloses how
improved barrier properties may be obtained from a metallised paper in a
similar packaging laminate, by selecting a paper substrate providing optimal
properties. Such a metallised paper substrate provided not only improved
5 barrier properties, but also indicated better stability of the metallised
layer for
induction heat sealing purposes.
There remains, however, a need for further improved oxygen gas
barrier properties over prior art gas-barrier coated paper substrates. There
is
also an increased need for improved properties regarding recyclability and
10 environmental sustainability of the materials used for gas-barrier
coated
paper substrates and laminated packaging materials containing them.
Disclosure of the invention
15 It is, accordingly, an object of the present invention to provide
improved
barrier-coated cellulose-based substrates, for laminating into packaging
materials.
It is also an object of the invention to provide barrier-coated papers or
cellulose-based substrates providing good gas barrier properties as well as
20 improved recyclability and sustainability, which fulfil the needs of
future
sustainable laminated packaging materials.
It is a further general object of the invention to provide barrier-coated
papers or cellulose-based substrates for improving laminated packaging
materials for oxygen-sensitive products, such as non-foil laminated packaging
25 materials for liquid, semi-liquid or wet food products, which do not
contain
aluminium foil but still have good gas and other barrier properties suitable
for
long-term, aseptic packaging at reasonable cost.
A particular object, is to provide barrier-coated papers or cellulose-
based substrates enabling, relative to aluminium foil barrier materials, cost-
30 efficient, non-foil, paper- or paperboard-based, laminated packaging
materials, and packaging with good gas and water vapour barrier properties,
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as well as recyclability and a sustainable environmental profile, for the
purpose of manufacturing packages for long-term, aseptic food storage.
These objects are thus attainable according to the present invention by
the barrier-coated cellulose-based substrate and the method of manufacturing
5 the barrier-coated cellulose-based substrate, as defined in the appended
claims.
Summary of the invention
According to a first aspect of the invention, there is thus provided a
barrier-coated cellulose-based substrate, for use as a barrier sheet in a
laminated packaging material for oxygen-sensitive products, comprising a
cellulose-based substrate having a density of at least 900 kg/m3, and a
grammage from 30 to 80 g/m2, and applied on a first side of the cellulose-
based substrate, at least one gas barrier coating of at least one gas barrier
material to a total thickness from 2 to 7000 nm, such as from 2 to 5000 nm
such as from 2 to 4000 nm, the gas barrier material excluding nanocrystalline
cellulose, NCC, wherein the barrier-coated cellulose-based substrate further
comprises a ductile base layer pre-coating, which is applied by means of
dispersion coating and subsequent drying, onto the surface of the first side
of
the cellulose-based substrate and positioned beneath the at least one gas
barrier coating, the barrier-coated cellulose-based substrate thus being
suitable for providing gas barrier properties in a laminated packaging
material
and in packages made thereof.
The gas barrier coating may be a barrier dispersion coating, applied by
means of dispersion or solution coating, and/ or a barrier deposition coating,
applied by means of a vapour deposition method.
In an embodiment, the barrier dispersion coating comprises a polymer
selected from the group consisting of vinyl alcohol polymers and copolymers,
such as from the group consisting of polyvinyl alcohol, PVOH, and ethylene
vinyl alcohol, EVOH, starch and starch derivatives, xylan, xylan derivatives,
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nanofibrillar cellulose/ microfibrillar cellulose, NFC/ MFC, and blends of two
or
more thereof.
The barrier deposition coating may instead, or additionally, comprise a
vapour deposition coating of a material selected from metals, metal oxides,
inorganic oxides and carbon coatings. In an embodiment, the barrier
deposition coating is a vapour deposition coating selected from the group
consisting of an aluminium metallisation coating and aluminium oxide, AlOx,
and preferably it is an aluminium metallisation coating.
The at least one gas barrier coating may comprise a barrier dispersion
coating, first applied by means of dispersion or solution coating onto the
ductile base layer pre-coating, and a barrier deposition coating, subsequently
applied by means of a vapour deposition method, onto the barrier dispersion
coating.
The ductile base layer pre-coating may be made from an aqueous
composition comprising a polymer binder material having inherent ductility
properties selected from the group consisting of styrene-butadiene copolymer
(SB) latex, styrene acrylate copolymer (SA) latex, other latexes of acrylate
polymers and copolymers, such as vinyl acrylic copolymer latex and vinyl
acetate acrylate copolymer latex, and of bio-based polymer materials.
The ductile base layer pre-coating may advantageously further
comprise a filler material, to further smoothen the surface of the cellulose-
based substrate. The filler may be of inorganic material and in a preferred
embodiment comprise laminar particles of an inorganic compound. Such
laminar fillers may contribute further to barrier properties in the material,
by
the creation of overlapping mineral flakes or lamellae, thus preventing
migration of small molecules through the material. Such laminar inorganic
particles may be clays, such as kaolin clay or bentonite clays, silicates and
talcum particles.
In an embodiment, the ductile base layer pre-coating may be applied
by means of aqueous dispersion coating at an amount from 2 to 15 g/m2,
such as from 5 to 15 g/m2, such as from 8 to 15 g/m2, such as from 10 to 15
g/m2, dry weight.
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The cellulose-based substrate may further have a second ductile
coating on its opposite side, which may be of the same type as the ductile
base layer pre-coating on the first side of the substrate.
The barrier-coated cellulose-based substrate of the invention may be
5 used in a laminated packaging material, which may further comprise a
first
outermost liquid tight material layer and a second innermost liquid tight
material layer. The second innermost, liquid tight material layer forms the
contact layer towards the product to be packed in a packaging container
formed from the laminated packaging material, and may also be heat-
10 sealable to itself or other thermoplastic materials.
The laminated packaging material may further comprise an additional
layer of paper or paperboard or other cellulose-based material, constituting a
bulk layer.
Thus, for the purpose of carton packaging of oxygen-sensitive food
15 products, such as liquid, semi-liquid or wet food products, such a
laminated
packaging material may comprise a bulk layer of paper or paperboard or
other cellulose-based material, a first outermost liquid tight, material
layer, a
second innermost liquid tight and optionally heat sealable material layer and,
arranged on the inner side of the bulk layer, between the bulk layer and the
20 second innermost layer, the barrier-coated cellulose-based substrate of
the
first aspect.
Such a laminated packaging material may be used in a packaging
container, which may be intended for packaging of liquid, semi-liquid or wet
food. Such a packaging container is manufactured at least partly from the
25 laminated packaging material, or is in its entirety made of the
laminated
packaging material.
In a second aspect of the invention, a method of manufacturing the
barrier-coated cellulose-based substrate of the first aspect is provided. The
method comprises a first step of providing a cellulose-based substrate, having
30 a first side and a second side, as a moving web in a roll-to-roll system, a
second step of applying a first aqueous dispersion of a ductile base layer pre-
coating composition, onto the first side of the moving cellulose-based
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substrate, optionally applying a second aqueous dispersion of a ductile
coating composition onto the other side of the moving substrate, and
subsequently drying the applied ductile base layer pre-coating, and the
optional second ductile coating composition, by forced evaporation, a third
5 step of calendering the pre-coated and dried cellulose-based substrate to
obtain a density of at least 900 kg/m3, such as at least 1000 kg/m3, and a
fourth step of applying a gas barrier coating by means of dispersion coating a
second aqueous dispersion or solution of a barrier composition, onto the first
side of the moving cellulose-based substrate and the ductile base layer pre-
coating, the gas barrier material excluding nanocrystalline cellulose, NCC,
and subsequently drying the applied barrier dispersion coating by forced
evaporation, and/ or by means of vapour depositing a barrier deposition
coating onto the first side of the moving cellulose-based substrate and the
ductile base layer pre-coating, to a total gas barrier coating thickness from
2
to 7000 nm, such as from 2 to 5000 nm.
In an embodiment, the fourth step of the method comprises a first
operation of applying a gas barrier coating by dispersion coating a second
dispersion or solution of a barrier composition, onto the first side of the
moving cellulose-based substrate with the ductile base layer pre-coating, and
20 drying the applied gas barrier coating by forced evaporation, and a
subsequent, second operation of vapour depositing a barrier deposition
coating onto the first side of the moving cellulose-based substrate with the
ductile base layer pre-coating and the gas barrier coating of the first
operation.
25 It has hitherto been assumed that improved gas barrier properties from
barrier-coated papers may be achieved by sourcing better cellulose-based
substrates, which inherently provide gas barrier properties when further
laminated to any polymer layers, and/or by coating thicker layers of the
barrier
coating materials having the inherent gas barrier properties. It has recently
30 become better understood, however, that the interface portion between
the
barrier coatings and the cellulose-based substrate may play a key role for the
optimal performance of subsequently applied coatings, which contribute most
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with the gas barrier properties. It has been found that optimal performance
may be achieved by adding a ductile base layer pre-coating onto the surface
of the high-density cellulose-based substrate. By doing so, it has been seen
that the barrier qualities of the cellulose-based substrate itself do not need
to
5 be so
high, and the amount of coated gas barrier polymer as a barrier pre-
coating may even be reduced. The resulting gas barrier properties in a
packaging laminate will thus be improved, if a ductile base layer pre-coating
is
used, although it does not contribute with significant gas barrier properties
itself, i.e. the gas barrier properties are not inherent to the material(s) of
the
ductile base layer pre-coating. The ductile base layer pre-coating composition
should be selected to provide a ductile foundation as well as to provide an
even, dense, compatible pre-coating surface to receive any further gas barrier
coating. The material selected for the ductile base layer pre-coating does not
need to contribute with inherent gas barrier properties, however.
15 The ductile base layer pre-coating material is advantageously applied
in the form of an aqueous composition comprising a polymer binder material
having inherent ductility properties selected from the group consisting of
styrene-butadiene copolymer (SB) latex, styrene acrylate copolymer (SA)
latex, other latexes of acrylate polymers and copolymers, such as vinyl
acrylic
copolymer latex and vinyl acetate acrylate copolymer latex, and of bio-based
polymer materials. The first aqueous dispersion of the ductile base layer pre-
coating composition may further contain a filler material, such as clay or
other
inorganic particles or pigments, and/or cellulose fibrils.
The barrier-coated cellulose-based substrate obtained by the above
25 described
method and coating layer configuration, provides improved gas
barrier properties to a laminated packaging material, as well as to packaging
containers made therefrom, and may also improve the recyclability and
sustainability profile of such packaging materials and packaging containers.
30 Detailed description
With the term "long-term storage", used in connection with the present
invention, is meant that the packaging container should be able to preserve
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the qualities of the packed food product, i.e. nutritional value, hygienic
safety
and taste, at ambient conditions for at least 1 or 2 months, such as at least
3
months, preferably longer, such as 6 months, such as 12 months, or more.
With the term "package integrity", is generally meant the package
5 tightness, i.e. the resistance to leakage or breakage of a packaging
container.
The term encompasses the resistance of the package to intrusion of
microbes, such as bacteria, dirt, and other substances, that may deteriorate
the filled food product and shorten the expected shelf-life of the package.
One main contribution to the integrity of a package from a laminated
10 packaging material is provided by good internal adhesion between adjacent
layers of the laminated material. Another contribution comes from the material
resistance to defects, such as pinholes, ruptures and the like within each
material layer itself, and yet another contribution comes from the strength of
the sealing joints, by which the material is sealed together at the formation
of
a packaging container. Regarding the laminated packaging material itself, the
integrity property is thus mainly focused on the adhesion of the respective
laminate layers to its adjacent layers, as well as the ability of the
individual
material layers to withstand thermal and mechanical loads, e.g. during folding
and sealing to form packaging containers. Regarding the sealing of the
20 packages, the integrity is mainly focussed on the quality of the sealing
joints,
which is ensured by well-functioning and robust sealing operations in the
filling machines, which in turn is ensured by adequately adapted heat-sealing
properties of the laminated packaging material.
The term "liquid or semi-liquid food" generally refers to food products
25 having a flowing content that optionally may contain pieces of food.
Dairy and
milk, soy, rice, grains and seed drinks, juice, nectar, still drinks, water,
flavoured water, energy drinks, sport drinks, coffee or tea drinks, coconut
water, wine, soups, jalapenos, tomatoes, sauce (such as pasta sauce), beans
and olive oil are some non-limiting example of food products contemplated.
30 Further examples of other oxygen-sensitive food products, possible to
package and protect with the laminated packaging materials of the present
disclosure are e.g. dry and/or fatty foods, such as milk powders and other
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powdered food. Examples of fatty foods are cheese, butter and spreads.
Such packaging may be flow-wrap packaging or form, fill, seal (FFS)
packaging, e.g. in bags. It may also be packaging in a jar, tray, lidded
spread
container, collapsible tube, clam-shell package, sleeve, envelope or wrapper.
5 In these applications, the packaging material typically undergoes folding
or a
similar type of stress (e.g. creasing, stretching), which make the packaging
material based on the barrier-coated cellulose-based substrate of the present
disclosure particularly suitable.
The term "aseptic" in connection with a packaging material and
10 packaging container refers to conditions where microorganisms are
eliminated, in-activated or killed. Examples of microorganisms are bacteria
and spores. Generally an aseptic process is used when a product is
aseptically packed in a packaging container. For the continued asepticity
during the shelf-life of the package, the package integrity properties are of
15 course very important. For long-term shelf-life of a filled food
product, it may
furthermore be important that the package has barrier properties towards
gases and vapours, such as towards oxygen gas, in order to keep its original
taste and nutritional value, such as for example its vitamin C content.
With the term "bulk layer" is normally meant the thickest layer or the
20 layer containing the most material in a multilayer laminate, i.e. the
layer which
is contributing most to the mechanical properties and dimensional stability of
the laminate and the structural stability of packaging containers folded from
the laminate, based on thick paper, paperboard or carton. It may also mean a
layer providing a greater thickness distance in a sandwich structure, which
25 further interacts with stabilising facing layers, which have a higher
Young's
modulus, on each side of the bulk layer, in order to achieve sufficient
mechanical properties, such as bending stiffness, for achieveing structure
stability of formed packaging containers.
The term "dispersion coating" herein relates to a coating technique in
30 which an aqueous or substantially aqueous dispersion, suspension, emulsion
or solution of a polymer is applied to the surface of a substrate layer,
usually
in the form of a continuous web, to form a solid, substantially non-porous
film
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after drying. The term "dispersion" covers thus also any suspension, emulsion
or solution or mixes thereof, that would be capable of providing such a
coating after drying. Polyvinyl alcohols (PVOH, PVAL) are typical polymers
suitable for dispersion coating, but may for example at high saponification
5 degrees in practice rather be polymer solutions, or mixes of dispersed
and
dissolved PVOH. A dispersion-coated barrier layer or coating is formed by a
dispersion coating, also called "liquid-film coating", techniques. The aqueous
dispersions may comprise fine polymer particles, and thus be a latex.
The term "latex" as used herein refers to a composition comprising an
10 aqueous suspension or dispersion or emulsion of polymer particles, which
can be natural polymers, synthetic polymers, synthetic polymers derived from
biomasses or combinations thereof.
Grammages of papers were determined according to the official test
method of ISO 536:2019 by the unit g/m2, while thickness and density were
15 determined according to ISO 534:2011, by the units pm (m) and kg/m3,
respectively.
Thickness measurements of coated polymer layers on paper may be
measured and estimated by taking sliced section samples of the structure and
studying them in a SEM microscope. The slicing may be done using e.g. a
20 cryo microtome.
OTR was measured with Oxtran 2/21 (Mocon) equipment based on
coulometric sensors, and evaluated according to ASTM F1927-14 and ASTM
F1307-14. See further description of OTR test methods in connection with the
Examples.
25 The method for determining OTR on flat material identified the amount
of oxygen per surface and time unit at passing through a material at a defined
temperature, a given atmospheric pressure, i.e at an atmosphere of 21 %
oxygen (unless otherwise stated), during 24 hours. The method for
determining OTR on packages identified the amount of oxygen per time unit
30 entering the package at a defined temperature, a given atmospheric
pressure,
i.e at an atmosphere of 21 % oxygen (unless otherwise stated), during 24
hours.
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Water vapour transmission rate (VVVTR) measurements were carried
out by a Permatran 3/33 (Mocon) instrument (norm: ASTM F1249-13 using a
modulated infrared sensor for relative humidity detection and VVVTR
measurement) at 38 C and 90% driving relative humidity gradient.
5 Surface roughness was measured according to TAPP! 555 om-15,
being the same as ISO 8791-4. The cellulose-based substrate suitable for
carrying barrier-coatings for the purpose of the invention is not limited to a
certain type of paper, but includes also other cellulose-based substrates,
based on any type of native cellulose, fibrous or micro-/nano- fibrillar
cellulose
10 with various degree of crystallinity. The invention is not applicable,
however,
to substrates from plastics or polymers, such as films made from regenerated,
i.e. dissolved and subsequently precipitated, chemically modified cellulose
polymers.
It has been seen that the combination of a base-layer pre-coating and
15 gas barrier coating(s), as of the present invention, may improve the gas
barrier properties of a paper substrate beyond what was hitherto believed
possible.
To be suitable for a final barrier-coating step by means of a vapour
deposition coating process, the fibrous part of a substrate needs to be thin,
20 such as 60 g/m2 or below, such as 50 g/m2 or below, preferably 45 g/m2
or
below, for reasons of efficiency and production economy and to avoid
blistering of the coating, due to air being entrapped in a fibrous cellulose-
based part of a substrate. On the other hand, cellulose-based substrates
thinner or with a lower grammage than 30 g/m2 may be mechanically too
25 weak and/or less dimension stable, when they are coated with wet
dispersions and subsequently dried, thus exhibiting shrinkage or curling
problems or even web breaks. It is thus more preferred to use cellulose-
based substrates having a grammage of from 30 to 70 g/m2, such as from 30
to 65 g/m2, such as from 35 to 60 g/m2, such as from 35 to 55 g/m2, such as
30 from 35 to 50 g/m2.
Furthermore, the pre-coated cellulose-based substrate has a high
density of at least 900 kg/m3 and should also have a dense and smooth
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surface, for best possible interface to a further gas barrier coating applied
to
it. Accordingly, the barrier-coated cellulose-based substrate should be a pre-
coated and calendered paper, such as a pre-coated, super-calendered paper
. The calendering or super-calendering operation, in addition to a higher
5 density of the substrate, also accomplishes an improved bonding between,
or
"integration of", the pre-coating layer and the surface of the paper
substrate.
The cellulose-based substrate for use according to the invention, may
be formed from cellulose fibres comprising at least 50% by dry weight of
chemical pulp, such as sulphate pulp. Chemical pulp is used to obtain
10 toughness in a paper for high-speed coating and converting processes and
for use in final packages.
Sulphate or "Kraft" pulp may be advantageous for improved repulping
in recycling, and general dewatering of the fibres.
For the purpose of recycling and good dewatering ability, the fibres of
15 the cellulose-based substrate should have a Canadian Standard Freeness
(CSF) higher than 300 ml, such as higher than 350 ml, such as higher than
400 ml, as measured by ISO 5267-2:2001. Correspondingly, the fibres of the
cellulose-based substrate should have a Schopper-Riegler value lower than
40 degrees SR, such as lower than 36 degrees SR, such as lower than 32
20 degrees SR, as measured according to ISO 5267-1:1999.
Softwood pulp may provide strength/toughness properties in the
resulting paper, and may be comprised in the pulp by at least 50 wt%.
Preferably thus, the cellulose-based substrate comprises at least 50 wt%,
such as from 60 to 100 wt%, such as from 70 to 100 wt% Kraft softwood
25 cellulose, such as bleached Kraft softwood cellulose.
It has been seen that for some uses, such as for liquid-tight packaging
of wet or liquid or viscous flowing products, it may be advantageous to use an
as thin as possible cellulose-based substrate, because then less polymer
may be needed in adjacent liquid-tight layers, or heat-sealable material
30 layers.
For optimal barrier properties by means of minimal amount of barrier
material, it has thus hitherto been seen that a gas barrier coating, to be
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coated to a few micrometers or nanometers of thickness only, onto a paper or
cellulose-based substrate, needs a further foundation of a ductile base layer
pre-coating to first be applied to the cellulose-based substrate.
The ductile base layer pre-coating may be obtained from an aqueous
5 composition comprising a polymer binder material having inherent ductility
properties selected from the group consisting of styrene-butadiene copolymer
(SB) latex, styrene acrylate copolymer (SA) latex, other latexes of acrylate
polymers and copolymers, such as vinyl acrylic copolymer latex and vinyl
acetate acrylate copolymer latex, and of bio-based polymer materials.
10 The ductile base layer pre-coating may be obtained from obtained
from
an aqueous composition comprising a bio-based polymer binder material
having inherent ductility properties, selected from the group comprising
starch
derivatives, polyisoprene, lignin-based polymers, alginates, gums, and soy-
based proteins and latexes of one or more such bio-based polymer binder
15 material.
The ductile base layer pre-coating may further comprise a filler
material.
The ductile base layer pre-coating may be applied by means of suitable
dispersion coating techniques, such as blade coating, rod coating, bar
coating, smooth roll coating, reverse roll coating, lip coating, air knife
coating,
curtain flow coating, dip coating and slot die coating methods, and
subsequent drying to evaporate the dispersion medium, normally water, by
forced convection drying. Preferably, the ductile base layer pre-coating is
applied by blade or rod coating technology and subsequent drying. The term
aqueous dispersion coating includes coating of aqueous compositions of
polymer emulsions, dispersions, suspensions, solutions and latex
formulations, and also when such compositions further comprise pigments,
inorganic particles or other filler material.
The ductile base layer pre-coating may be obtained from an aqueous
latex composition, such as a latex selected from the group comprising
styrene-butadiene latex (SB-latex), methylstyrene-butadiene latex, styrene
acrylate latex (SA-latex), acrylate latex, such as vinyl acrylic copolymers
and
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vinyl acetate acrylate latex, styrene-butadiene-acrylonitrile latex, styrene-
acrylate-acrylonitrile latex, styrene-butadiene-acrylate-acrylonitrile latex,
styrene-maleic anhydride latex, styrene-acrylate-maleic anhydride latex,
mixtures thereof, or bio-based latex made with plant-based polymer
materials. Also e.g. styrene-acrylate latex or styrene-butadiene latex may be
at least partly derived from biomass to provide similar performance with an
improved carbon footprint.
The ductile base layer pre-coating may in an embodiment be made
from substantially plant-based material sources, by producing an aqueous
latex composition comprising an emulsion of a bio-based polymer binder
material, such as selected from the group comprising starch derivatives,
including modified starches and crosslinked starches, polyisoprene, lignin-
based polymers, alginates and gums, such as guar gum, and soy-based
proteins, including latex compositions of the type "Ecosphere" from
Ecosynthetix, "Vytex" from Vystar, "NeoLigno" from Stora Enso, "OC-Binder"
from Organoclick, "Polygal" surface coatings from Polygal. The bio-latex
composition Ecosphere from Ecosynthetix is for example an aqueous latex
of crosslinked starch particles. A latex may be manufactured by aqueous
emulsion polymerization. Alternatively, as in the case of manufacturing a
latex
from a biopolymer, the biopolymer material such as starch may be plasticized
under shear force to suitable particle size, and subsequently be crosslinked
by the addition of a crosslinking agent. Thereafter, the biopolymer particles
may be added to a water dispersion to form an aqueous latex or suspension
of the particles.
The ductile base layer pre-coating composition may thus in an
embodiment be made from a plant-based or bio-based polymer and be
applied in the form of an aqueous latex of such polymer.
The ductile base layer pre-coating may thus be made from an aqueous
latex composition comprising a polymer material having inherent ductility
properties selected from the group consisting of styrene-butadiene
copolymers (SB), styrene acrylate copolymers (SA), other acrylate polymers
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and acrylate copolymers, such as vinyl acrylic copolymers and vinyl acetate
acrylate copolymers, and of aqueous latexes of bio-based polymer materials.
In a further embodiment, the ductile base layer pre-coating may be
made from an aqueous latex composition comprising a bio-based polymer
material having inherent ductility properties selected from the group
comprising starch derivatives, including modified starches and crosslinked
starches , polyisoprene, lignin-based polymers, alginates, gums, and soy-
based proteins.
In yet a further embodiment, the ductile base layer pre-coating may be
made from an aqueous latex composition comprising crosslinked starch
particles.
The latex composition may further comprise inorganic filler particles,
such as kaolin clay or other laminar clay compounds, silica particles, talcum
particles and/or calcium carbonate, at from 1 to 80 weight-% of the dry
content, such as from 1 to 70 wt-%, such as from 1 to 50 wt-%, such as from
1 to 40 wt-%, such as from 30 wt-%, such as from 1 to 20 wt-% of the dry
content. The filler content may further support ductility, while also
providing
sufficient flexibility and reducing tensions in the pre-coating, such that the
pre-
coating can follow the cellulose-based part of the substrate as it is folded,
without obtaining cracks in the pre-coating itself.
The latex composition may alternatively, or also, comprise organic filler
material, such as microfibrillated cellulose.
In an embodiment, the ductile base layer pre-coating may comprise
from 4 to 45 wt%, such as from 4 to 35 wt%, such as from 4 to 25 wt%, such
as from 4 to 20 wt%, such as from 4 to 16 wt% of the polymer binder material
having inherent ductility properties, and from 55 to 96 wt%, such as from 65
to 96 wt%, such as from 75 to 96 wt%, such as from 80 to 96 wt% of the filler
material, dry weight, and optionally further compounds, such as thickening
agents and crosslinking compounds, at additive amounts. Such additive
amounts would be included only at up to 10 wt% of the ductile base layer pre-
coating, based on dry weight.
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In another embodiment, the ductile base layer pre-coating may
comprise from 10 to 20 wt% of the polymer binder material having inherent
ductility properties, from 75 to 85 wt-% of an inorganic filler, from 3 to 5
wt%
of a crosslinking compound, such as starch, and from 1 to 2 wt% of a
5 thickening agent, based on dry weight.
The filler material may be an inorganic filler selected from the group
comprising clays, such as nano-clays including bentonite clays, kaolin clay,
talcum, CaCO3, and silica particles.
Preferably, the filler material may be an inorganic laminar compound,
such as bentonite clay or kaolin clay. Specifically suitable such laminar clay
minerals may be laponite, kaolinite, dickite, nacrite, halloysite, antigorite,
chrysotile, pyrophyllite, montmorillonite, hectorite, saponite, sauconite,
sodium tetrasilicic mica, sodium taeniolite, commonmica, margarite,
vermiculite, phlogopite, xanthophyllite and the like. A specific type of such
nano-clay laminar particles are those of montmorillonite, e.g. sodium-
exchanged montmorillonite (Na-MMT).
The ash content of the thus pre-coated cellulose-based substrate may
accordingly be from 15 to 25 wt%, such as from 15 to 23 wt%, as determined
by IS01762:2019. The same range of ash content applies to a gas-barrier
20 coated and pre-coated cellulose-based substrate, i.e. 15-25 wt%.
The ductile base layer pre-coating composition may be applied at a
grammage from 2 to 15 g/m2, such as from 5 to 15 g/m2, such as from 8 to 15
g/m2, such as from 10 to 15 g/m2, dry weight.
The polymer of the ductile base layer pre-coating may be selected to
25 exhibit a glass transition temperature from -30 to +30 degrees Celsius,
such
as from -30 to +20 degrees Celsius, to provide inherent ductility to a paper
substrate coated with a base layer pre-coating thereof.
The cellulose-based substrate may further have a second ductile
coating on its opposite side, which may be of the same type as the ductile
30 base layer pre-coating on the first side of the substrate.
The second ductile coating composition may be applied by means of
aqueous dispersion coating to an amount from 1 to 10 g/m2, such as from 1 to
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7 g/m2, such as from 1 to 6 g/m2, such as from 1 to 5 g/m2, such as from 2 to
g/m2, dry weight. Particularly suitable latex compositions may be described
as having a latex polymer chemical composition a styrene-butadiene
copolymer or a styrene-acrylic copolymer. Such latex polymers are miscible in
5 water at any proportion. Other particularly suitable latex polymers may
be
anionic aqueous dispersions of copolymers of n-butyl acrylate and styrene.
The ductile base layer pre-coating should be coated directly onto, and
adjacent, the surface of the paper or cellulose-based substrate. The paper
may allow moisture to migrate outwards through the laminated packaging
material, and the ductile base layer pre-coating material may also allow such
water vapour migration, to prevent unfavourable entrapment of moisture near
a moisture sensitive barrier coating of e.g. PVOH or EVOH. Any moisture
migrating through the material from the inside liquid food product in the
package will slowly be further transported via the paper layer and the
15 paperboard bulk layer of the laminated packaging material towards the
outside of the packaging container. The cellulose-based substrate and the
paperboard bulk layer may then "breathe away" the humidity from the barrier
pre-coating and thus keep the moisture content within the gas barrier
coating(s) substantially constant over time.
20 The
ductile base layer pre-coating may be calendered after coating
onto the cellulose-based substrate and forced convection drying. A
calendering operation further integrates the ductile base layer pre-coating
with the cellulose surface of the substrate, by improving the surface bonding
while at the same time creating a smooth surface of the ductile base layer
25 pre-coating. The calendering operation may be a super-calendering operation
involving multiple high-pressure roller nips and at least one thermo-roller
nip,
such as more than 4 high-pressure roller nips, such that the density of the
pre-coated cellulose-based substrate may be increased to at least 900 kg/m3,
such as above 1000 kg/m3, such as above 1100 kg/m3. The temperature of
30 the thermo-roller may provide a surface temperature of from 100 to 300
C,
such as from 100 to 240 C.
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The solids content in the aqueous latex coating applied may be in the
range of from 45 to 70 wt-%, such as from 45 to 60 wt-%, such as from 47 to
55 wt-%, such as from 48 to 51 wt-%.
The most suitable type of system for dispersion coating of the ductile
5 pre-coating or coating compositions are blade coaters or rod coaters, where
a
large amount of coating composition is applied to the paper and the surplus is
scraped off again. The coating composition is applied onto the substrate by
an applicator. Common applicators may be jet applicators, roll applicators and
short dwell time applicators (SDTA). An advantage of roll applicators is that
10 bad formation of a substrate paper is less critical for the
roll applicator and it
is therefore well suited when thick coatings are desired. Blades that are used
to scrape off excessive amount of coating are made of steel and may be
equipped with a ceramic tip that makes them last longer. After the coating is
applied it passes trough a drier, usually an IR-drier, hot air drier or a
cylinder
15 drier.
The grammage of the ductile, pre-coated cellulose-based substrate
may in an embodiment be from 40 to 80 g/m2, such as from 40 to 75 g/m2,
such as from 40 to 70 g/m2, such as from 40 to 65 g/m2.
The thickness of the ductile, pre-coated cellulose-based substrate may
20 be from 35 to 70 pm, such as from 35 to 65 pm, such as from
40 to 60 pm,
such as from 45 to 60 pm.
The first, top side of the ductile, pre-coated cellulose-based substrate
may exhibit a very low porosity.
The ductile base layer pre-coating may be applied as an aqueous
latex composition having a solids content from 48 to 51 wt%, and a Brookfield
viscosity from 100 to 1000 mPa.s, a pH from 5.5 to 8.
The ductile base layer pre-coating may further be applied as an
aqueous composition by means of blade or rod coating, at an amount from 5
to 15 g/m2, dry weight.
The thus obtained pre-coated and dried cellulose-based substrate may
subsequently be super-calendered, such as at a calender nip pressure of at
least 100 kN, such as at least 200 kN, such as 300kN or above, and at a
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thermo-roller surface temperature from 100 to 300 C, such as from 100 to
240 C, such as from 150 to 240 C. The cellulose-based substrate including
the ductile base layer pre-coating, may be calendered to obtain an air
permeance value lower than 100 nm/(Pa.$), which is the lower limit of the
range within which the test method ISO 5636-5:2013 is applicable, and further
to an air permeance value lower than 1 nm, such as from 40 to 900
pm/(Pa.$), such as from 40 to 800 pm/(Pa.$), such as from 100 to 700
pm/(Pa.$), such as from 200 to 500 pm/(Pa.$), as determined by the SCAN-P
26:78 test method. It is believed that the remarkably dense surface obtained
by the ductile pre-coating base layer of the high-density cellulose-based
substrate further promotes the effect of robustness of gas barrier properties
beyond what has hitherto been seen, when subsequently coated with thin gas
barrier coatings.
The ductile, pre-coated cellulose-based substrate may have a surface
roughness of the first, top side lower than 100 ml/min Bendtsen, such as
lower than 80 ml/min Bendtsen, such as lower than 50, such as lower than
30, such as lower than 20 ml/min Bendtsen, as measured according to SS-
IS08791-2:2013.
A different measurement of the surface roughness is the Parker Print
Surface (PPS) roughness, measured according to TAPP! 555 om-15, being
the same as ISO 8791-4.
The PPS roughness of the first, top side ductile, pre-coated cellulose-
based substrate surface is preferably lower than 3.0 pm, such as lower than
2.8 pm, such as lower than 2.5 pm, such as lower than 2.2 pm, such as
lower than 2.0 pm, such as 1.8 pm or below, for a further improved gas
barrier coating performance, as determined by the above test method.
The cellulose-based substrate including the ductile base layer pre-
coating as well as the at least one gas barrier coating, may have a PPS
surface roughness lower than 3.0 pm, such as lower than 2.8 pm, such as
lower than 2.5 pm, such as lower than 2.2 pm, such as lower than 2.0 pm,
such as 1.8 pm or below, as measured according to TAPP! 555 om-15, being
the same as ISO 8791-44.
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The density of the ductile, pre-coated cellulose-based substrate may be
higher than 900 kg/m3, such as higher than 1000 kg/m3, such as higher than
1100 kg/m3, such as higher than 1200 kg/m3. The pre-coated cellulose-based
substrate may be calendered, such as preferably supercalendered, to obtain
the high density and surface smoothness.
A lower surface roughness provides for a perfect interface to
subsequently applied adjacent layers and coatings, with a reduced number of
imperfections such as pinholes, and unevenness in a coating layer.
Consequently, the coating or further layer may be applied at a higher quality,
or at a lower thickness, or both. For a same coating thickness of a gas
barrier
coating, better oxygen barrier properties are thus obtained in the coating
itself.
The pre-coated cellulose-based substrate, including the ductile base
layer pre-coating, may have a density of at least 950 kg/m3, such as at least
1000 kg/m3, such as at least 1100 kg/m3, as measured according to ISO
534:2011, and a grammage from 35 to 75 g/m2, such as from 40 to 75 g/m2,
such as from 45 to 70 g/m2, such as from 45 to 65 g/m2.
The application of the ductile base-layer pre-coating, in conjunction
with the calendering process of the thus coated cellulose-based substrate,
provides a gain in ductility of the pre-coated cellulose-based substrate and
consequently also in the final gas-barrier coated material, which will result
in
improved gas barrier properties also in laminated packaging materials, such
that the properties are more resistant to breakage, cracking and damage in
the process of converting into a package, as well as in handling and
distribution of the package.
The increased ductility reduces the tendency of the material to form
cracks in the paper and thereby also reduces the tendency to formation of
cracks in the gas barrier coatings, thanks to a re-distribution of stress and
strain over a larger surface area, e.g. during folding operations.
The at least one gas barrier coating, which provides the barrier-coated
cellulose-based substrate of the invention with its basic gas barrier
properties,
may be a gas barrier dispersion coating, applied by means of dispersion or
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solution coating, and/ or a barrier deposition coating, applied by means of a
vapour deposition method.
Gas barrier coatings applied by means of coating of aqueous
dispersion or solution of gas barrier compositions may comprise polymers
which have inherent gas barrier properties, and which are food safe and
environmentally sustainable both regarding recyclability and in industrial
coating and lamination processes. Such polymers are thus water dispersible
and/or dissolvable in water and may be applied by means of an aqueous
"dispersion coating" process, or a so called "liquid film coating" process.
Non-
aqueous or only partly aqueous coating compositions, such as those based
on alcohols or mixtures of alcohol and water, could also be suitable for
achieving the good results from this invention. They would, however, likely be
less suitable from environmental sustainability point of view, than purely
water-based coating compositions.
In an embodiment, the barrier dispersion coating comprises a polymer
selected from the group consisting of vinyl alcohol polymers and copolymers,
such as from the group consisting of polyvinyl alcohol (PV0H) and ethylene
vinyl alcohol (EVOH), starch, starch derivatives, xylan, xylan derivatives,
nanofibrillar cellulose/ microfibrillar cellulose (NEC! MFC), and of blends of
two or more thereof.
In a further embodiment, the barrier dispersion coating is applied by
means of dispersion or solution coating at a total amount of from 0.2 to 6
g/m2, such as from 0.5 to 5 g/m2, such as from 0.5 to 4 g/m2, such as from 0.5
to 3.5 g/m2, such as from 1 to 3.5 g/m2, such as from 1 to 3 g/m2, dry weight.
Processes suitable for coating of low dry-content gas-barrier polymer
dispersion/ solution compositions are broadly any suitable wet coating
methods, such as gravure roll coating, smooth roll coating, reverse roll
coating, wire bar coating, blade coating, lip coating, air knife coating, and
curtain flow coating methods. The experiments for the present invention were
performed by means of smooth roller coating, but it is believed that any of
the
above or other liquid film coating methods that would contribute to generate a
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homogeneous layer with smooth and even coated surface would be suitable
for providing gas barrier coatings out the invention.
In a more specific embodiment, the barrier dispersion coating
compositions are based on the two most common types of polymers and
5 coplymers suitable for dispersion coating, based on on vinyl alcohol
monomers, i.e. polyvinyl alcohol (PVOH) and ethylene vinyl alchol (EVOH).
The gas barrier polymer may preferably be PVOH, because it provides
good film formation properties, gas barrier properties, cost efficiency, food
compatibility and odour barrier properties.
10 A PVOH-based gas barrier composition performs best when the PVOH
has a degree of saponification of at least 98 %, preferably at least 99 %,
although also PVOH with lower degrees of saponification will provide oxygen
barrier properties.
On the other hand, EVOH may be advantageous by providing some
15 moisture resistance to the barrier material, since the copolymer
comprises
ethylene monomer units. The amount of ethylene monomer units depends on
the choice of EVOH grade, but its presence will be at the expense of some
oxygen barrier property, in comparison to pure PVOH. Conventional EVOH
polymers, are normally intended for extrusion and are not possible to disperse
20 or dissolve in an aqueous medium in order to produce a thin liquid film
coated
barrier film of 3,5 g/m2 or below. It is believed that EVOH should comprise a
rather high amount of vinyl alcohol monomer units to be water-dispersible and
that the properties should be as close to those of liquid film coating grades
of
PVOH as possible. An extruded EVOH layer is thus not an alternative to a
25 liquid film coated EVOH, because it inherently has less similar
properties to
PVOH than EVOH grades for extrusion coating, and because it cannot be
applied at a cost-efficient amount below 5 g/m2 as a single layer by extrusion
coating or extrusion lamination.
Nano-crystalline cellulose, NCC, is a form of nano-cellulose but is not
30 the same as "microfibrillar cellulose", "MFC" (CMF) or "nanofibrillar
cellulose",
NFC (CNF)".
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MFC/ NEC may thus contain longer particles, so-called "fibrils" having a
width of 10-1000 nm, and a length of at least 1 pm, such as up to 10 pm,
such as up to 100 pm.
Both MFC and NEC have an aspect ratio of 50 or above, while NCC/
5 CNC may be defined to have an aspect ratio below 50, e.g. in accordance
with ISO/TS 20477:2017 and the draft TAPP! norm WI3021.
The term "NCC", is used for shorter particles and "rod-like" particles,
having a width of 3-100 nm, and a length from 100 too above 1000 nm, such
as from 100 to 3000 nm, such as from 100 to 1000 nm, such as from 100 to
10 500 nm. The majority of the NCC particles in the composition
should have this
dimension, may be from 100 to 500 nm length, such as from 100 to 200 nm
and with a small width of from 3 to 100 nm.
The barrier dispersion coating composition may further comprise from
about 1 to about 20 weight %, of an inorganic laminar compound based on
15 dry coating weight, such as exfoliated nanoclay particles, such as
bentonite.
Thus, the barrier layer may include from about 99 to about 80 weight % of the
polymer based on the dry coating weight. An additive, such as a dispersion
stabiliser, defoamer or the like, may also be included in the gas barrier
composition, preferably in an amount of not more than about 1 weight %
20 based on the dry coating. The total dry content of the
composition is
preferably from 5 to 20 weight-%, such as from 7 to 15 weight-%.
A further possible additive in the barrier pre-coating composition may
be a polymer or compound with functional carboxylic acid groups, in order to
improve the water vapour and oxygen barrier properties of a PVOH coating.
25 Suitably, such polymer with functional carboxylic acid groups
is selected from
among ethylene acrylic acid copolymer (EAA) and ethylene methacrylic acid
copolymers (EMAA) or mixtures thereof. In one embodoment, such a barrier
layer mixture may essentially consist of PVOH, EAA and an inorganic laminar
compound. The EAA copolymer may be included in the barrier layer in an
amount of about 1-20 weight %, based on dry coating weight.
It is believed that some further improved oxygen and water barrier
properties may result from an esterification reaction between the PVOH and
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the EAA at an increased drying temperature, whereby the PVOH is
crosslinked by hydrophobic EAA polymer chains, which thereby are built into
the structure of the PVOH. Crosslinking can alternatively be induced by the
presence of polyvalent compounds, e.g. metal compounds such as metal-
5 oxides. Such mixtures are, however, more expensive because of the cost of
the additives and may be less preferred from a recyclability point of view.
Thus, while it is more preferable to use a barrier dispersion coating
from a pure PVOH or EVOH composition, advantageous gas barrier results
may be obtainable also with barrier dispersion coatings comprising further
additives as described above.
The barrier dispersion coating may thus be applied at a total amount of
from 0.2 to 5 g/m2, such as from 0.2 to 4 g/m2, more preferably from 0.5 to 4
g/m2, such as from 0.5 to 3.5 g/m2, such as from 1 to 3 g/m2, dry weight.
Below 0.2 g/m2, there will be no gas barrier properties achieved at all, while
above 3.5 g/m2, the coating may bring less cost-efficiency to the packaging
laminate, due to high cost of barrier polymers in general and due to high
energy cost for evaporating off the liquid. A recognisable level of oxygen
barrier is achieved by PVOH at 0.5 g/m2, and above, and a good balance
between barrier properties and costs is normally achieved between 0.5 and
3.5 g/m2.
In an embodiment, the barrier dispersion coating may be applied in two
or even three consecutive steps with intermediate drying, as part-layers.
When applied as two part-layers or "part-coatings", each layer may suitably
be applied in amounts from 0.2 to 2.5 g/m2, preferably from 0.5 to 1.5 g/m2,
25 and allows a higher quality total layer from a lower amount of liquid
gas
barrier composition. More preferably, the two part-layers may be applied at
an amount of from 0.5 to 1.5 g/m2 each.
For the unexpected improvement of the invention, the barrier
dispersion coating shall thus not be coated directly onto the paper or
cellulose-based substrate but shall be preceded by a first ductile base layer
pre-coating of a different polymer and material composition than the gas
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barrier material composition, to prepare the substrate surface for the
application of the gas barrier coating. It is believed that the particular
properties of the aqueous ductile base layer pre-coating composition promote
a dense and even base layer top surface for further gas barrier coating and a
5 compatible adhesion chemistry and wettability for the subsequent
application
of for example a polyvinyl alcohol-based gas barrier coating. The main
improvement effect is however that the first ductile base layer pre-coating
has
the capability to absorb stress and strain on the barrier-coated cellulose-
based substrate as it is folded and abused, when used in a laminated
packaging material.
According to another embodiment, the base-layer pre-coated cellulose-
based substrate has on the surface of its first, pre-coated side instead, or
in
addition, a vapour deposition coating of a gas barrier material selected from
metals, metal oxides, inorganic oxides and amorphous diamond-like carbon
15 coatings. The vapour deposition coating may be applied by means of
physical
vapour deposition (PVD) or chemical vapour deposition (CVD), for example
by plasma enhanced chamical vapour deposition (PECVD). It may more
specifically be selected from the group consisting of an aluminium
metallisation coating and aluminium oxide, AlOx. Preferably it is an aluminium
metallisation coating.
In a further embodiment, the barrier-coated cellulose-based substrate
has on its first, top-side surface a first coating of a gas barrier material
formed
by coating and subsequent drying of a dispersion or solution of an aqueous
gas barrier composition, and further a vapour deposition coating of a gas
25 barrier material, such as selected from metals, metal oxides, inorganic
oxides
and amorphous diamond-like carbon, applied onto the first barrier dispersion
coating.
The barrier-coated cellulose-based substrate may thus be coated with
a gas barrier material by means of vapour deposition coating onto its top-side
30 surface to a thickness of from 2 to 80 nm, such as from 2 to 50 nm, such
as
from 2 to 45 nm.
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The vapour deposited barrier coating to finally be coated onto the top-
side surface of the cellulose-based substrate, is applied by means of physical
vapour deposition (PVD) or chemical vapour deposition (CVD), for example
by plasma enhanced chamical vapour deposition (PECVD).
5 Generally, below 5 nm the barrier properties may be too low to be
useful and above 200 nm, such as above 100 nm, such as above 50 nm,
depending on the type of vapour deposition coating, the barrier coating may
be less flexible and, thus, more prone to cracking when applied onto a
flexible
substrate and would also cost more.
10 Other
examples of vapour deposition coatings are aluminium oxide
(A10x, A1203) and silicon oxide (SiOx) coatings. Generally, PVD-coatings of
such oxides are more brittle and less suitable for incorporation into
packaging
materials by lamination, while metallised layers as an exception do have
suitable mechanical properties for lamination material despite being made by
15 PVD.
Normally, an aluminium metallised layer inherently has a thin surface
portion consisting of an aluminium oxide due to the nature of the
metallisation
coating process used.
In an embodiment, such an aluminium metallised layer has been
20 applied to
an optical density (OD) of from 1.8 to 2.5, preferably from 1.9 to
2.2. At an optical density lower than 1.8, the barrier properties of the
metallised film may be too low. At above 2.5, on the other hand, the
metallisation layer may become brittle, and the thermostability during the
metallisation process will be low due to higher heat load when metallising the
25 substrate
film during a longer time. The coating quality and adhesion may
then be negatively affected.
Other coatings may be applied by means of a plasma enhanced
chemical vapour deposition method (PECVD), wherein a vapour of a
compound is deposited onto the substrate under more or less oxidising
30 circumstances. Silicon oxide coatings (SiOx) may, for example, also be
applied by a PECVD process, and may then obtain very good barrier
properties under certain coating conditions and gas recipes.
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DLC defines a class of amorphous carbon material (diamond-like
carbon) that displays some of the typical properties of diamond. Preferably, a
hydrocarbon gas, such as e.g. acetylene or methane, is used as process gas
in a plasma for producing a coating of amorphous hydrogenated carbon
barrier layer applied by a PECVD vacuum process, i.e. a DLC. DLC coatings
applied by PECVD under vacuum provide good adhesion to subsequently
laminated, adjacent polymer or adhesive layers in a laminated packaging
material. Particularly good adhesion to adjacent polymer layers, are obtained
with polyolefins and in particular polyethylene and polyethylene-based co-
polymers.
The at least one gas barrier coating may comprise a barrier dispersion
coating, first applied by means of dispersion or solution coating onto the
ductile base layer pre-coating, and a barrier deposition coating, subsequently
applied by means of a vapour deposition method, onto the barrier dispersion
coating.
A barrier-coated cellulose-based substrate may further be provided,
which further comprises a ductile base layer coating applied also onto the
backside of the substrate. Such a further ductile coating on the other side of
the cellulose-based substrate ensures optimal performance upon further
abuse and fold-forming of a laminated packaging material comprising the
barrier-coated cellulose-based substrate, such that better oxygen barrier
properties of finally shaped and filled packaging containers are obtained.
The backside of the substrate may thus optionally also be further
coated with at least one gas barrier coating of at least one gas barrier
material as defined in any of the above embodiments.
The gas barrier coated cellulose-based substrate obtained by the
above method, thus provides excellent low OTR and low VVVTR also after
lamination into a laminated packaging material and further after fold-forming
and sealing operations of such a laminated material into packages.
A carton-based laminated packaging material for packaging of oxygen-
sensitive products may comprise a bulk layer of paper or paperboard, a first
outermost, liquid tight material layer, a second innermost liquid tight,
material
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layer and, arranged on the inner side of the bulk layer of paper or
paperboard,
towards the inside of a packaging container made from the packaging
material, between the bulk layer and the second innermost layer, the barrier-
coated cellulose-based substrate of the invention.
5 A paper
or paperboard bulk layer may have a thickness of from about
100 pm up to about 600 pm, and a surface weight of approximately 100-500
g/m2, preferably about 200-300 g/m2, and may be a conventional paper or
paperboard of suitable packaging quality.
For low-cost aseptic, long-term packaging of liquid food, a thinner
10 packaging laminate may be used, having a thinner paper core layer. The
packaging containers made from such packaging laminates are not fold-
formed and more similar to pillow-shaped flexible pouches. A suitable paper
for such pouch-packages usually has a surface weight of from about 50 to
about 140 g/m2, preferably from about 70 to about 120 g/m2, more preferably
15 from 70 to about 110 g/m2. As the barrier-coated substrate in this
invention in
itself may contribute with some stability to the laminated material, the paper
layer corresponding to a "bulk" layer may be even thinner, and interact with
the barrier cellulose-based substrate in a sandwich interaction to still
produce
a laminated packaging material having the desired mechanical properties
20 altogether.
The barrier-coated paper or cellulose-based substrate may be bonded
to the bulk layer by an intermediate adhesive, or thermoplastic polymer
bonding layer, thus binding the un-coated surface of the barrier-coated paper
to the bulk layer. The bonding layer may be a polyolefin layer, such as in
25
particular a layer of a polyolefin copolymer or blend, such as including in
the
majority ethylene monomer units. The bonding layer may bond the bulk layer
to the barrier-coated cellulose-based substrate by melt extrusion laminating
the molten bonding polymer as a layer between the webs and simultaneously
pressing the three layers together, while being forwarded through a
30
lamination roller nip under simultaneous cooling, thus providing a laminated
structure by extrusion lamination. Melt extrusion lamination requires a
sufficient amount of molten polymer, in this case typically a polyolefin, such
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as low density polyethylene, to bind the two colder surfaces together A
sufficient amount is typically from 12 to 20 g/m2, possibly from 12 to 15
g/m2.
Other suitable bonding or tie layers in the interior of the laminated
material, such as for example between the bulk or core layer and the barrier-
coated cellulose-based substrate, or between the innermost, liquid tight and
heat sealable layer and the barrier-coated paper substrate, may also be so-
called adhesive thermoplastic polymers, such as modified polyolefins, which
are mostly based on LDPE or LLDPE co-polymers or, graft co-polymers with
functional-group containing monomer units, such as carboxylic or glycidyl
functional groups, e.g. (meth)acrylic acid monomers or maleic anhydride
(MAH) monomers, (i.e. ethylene acrylic acid copolymer (FAA) or ethylene
methacrylic acid copolymer (EMAA)), ethylene-glycidyl(meth)acrylate
copolymer (EG(M)A) or MAH-grafted polyethylene (MAH-g-PE). Another
example of such modified polymers or adhesive polymers are so called
ionomers or ionomer polymers. Preferably, the modified polyolefin is an
ethylene acrylic acid copolymer (FAA) or an ethylene methacrylic acid
copolymer (EMAA).
The barrier-coated cellulose-based substrate may be bonded to the
bulk layer by wet application of an aqueous dispersion of an adhesive
composition comprising an adhesive polymer binder onto one of the web
surfaces to be laminated and pressing the two paper webs together while
they are forwarded through a lamination roller nip, thus providing a laminated
structure by wet lamination. The moisture of the aqueous adhesive
composition is absorbed into the fibrous cellulose network of the two paper
layers, and partly evaporating with time, during the subsequent lamination
processes. There would thus be no need for a forced drying step. The barrier-
coated cellulose-based substrate may thus be laminated to the bulk layer by
from 0.5 to 6 g/m2, such as from 1 to 5 g/m2, such as from 1 to 5 g/m2,dry
weight, of an interjacent bonding composition comprising a binder selected
from the group consisting of acrylic polymers and copolymers, starch, starch
derivatives, cellulose derivatives, polymers and copolymers of vinyl acetate,
polymers and copolymers of vinyl alcohol, copolymers of styrene-acrylic latex
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or styrene-butadiene latex or adhesive bio-latexes. For best possible
environmental and sustainability profile, adhesive binders originating from
plants or non-fossil sources are preferred.
Such a low amount of an interjacent bonding composition is only
5 possible to apply by aqueous dispersion or solution coating of a polymer
binder, and is not possible to apply by extrusion coating or extrusion
lamination of a single-layer polymer melt due to the nature of the molten-
layer
extrusion process. Since the surfaces of the layers to be bonded together are
both made of cellulose, such wet lamination is made by the absorption of the
aqueous medium into the respective cellulose layers, and thus a thin and dry
bonding layer at the interface between the two layers may be formed.
Suitable materials for the outermost and innermost liquid-tight layers
may be thermoplastic polymers, such as polyolefins such as polyethylene and
polypropylene horno- or co-polymers, preferably polyethylenes and more
preferably polyethylenes selected from the group consisting of low density
polyethylene (LOPE), linear LOPE (LLDPE), single site catalyst metallocene
polyethylenes (m-LLDPE) and blends or copolymers thereof. Such
thermoplastic polymers also have the advantage of being readily weldable,
i.e. heat sealable, to the same or similar polymers and to other materials
with
thermoplastic behaviour. According to an embodiment, the outermost heat
sealable and liquid-tight layer may be an LDPE, while the innermost heat
sealable, liquid-tight layer may be a blend composition of m-LLDPE and
LOPE for optimal lamination and heat sealing properties.
The outermost layer may be merely protective towards liquid and dirt,
25 such that any sealing of the outside surface to another surface or item,
such
as an opening device or the like, will be carried out by an additional glue or
hot melt. For packaging of products having lower requirements regarding
strength of seals and tightness, this is also valid regarding the second
innermost layer. For packaging of liquid, semi-liquid, viscous flowing
products
30 and wet food, the packaging container qualities are more dependent on
that
the second innermost layer is also heat-sealable to produce strong and tight
packages that can carry the filled product under all circumstance in handling
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and distribution, and thus itmay be required that the second innermost layer
is
liquid-tight as well as heat sealable.
The same thermoplastic materials, as listed regarding the outermost
and innermost layers, such as polyolefins and in particular polyethylene-
5 based materials, may also be suitable in bonding layers in the interior
of the
laminated material, i.e. between a bulk or core layer, such as paper or
paperboard, and the barrier-coated cellulose substrate. The thermoplastic
bonding layer may thus be a polyethylene layer, such as a low density
polyethylene (LDPE) layer.
10 The second innermost liquid tight, heat sealable polyolefin layer may
be a pre-manufactured film comprising the same or similar polyolefins, as
described above, for improved robustness of the mechanical properties of the
packaging material. Due to the manufacturing process in film blowing and film
casting operations, and optional subsequent film orientation operation steps,
15 the polymers of such films acquire different properties from what is
possible
from merely (co-) extrusion coated polyolefin layers. Such a pre-
manufactured polymer film may thus contribute to the mechanical robustness
of a laminated packaging material and to mechanical strength, package
integrity and further reduced loss of barrier properties of formed and filled
20 packaging containers from the laminate packaging material.
A laminated packaging material may have a pre-manufactured polymer
film laminated between the barrier-coated cellulose-based substrate and the
second innermost liquid tight material layer, for improved robustness of the
mechanical properties of the laminated packaging material. A pre-
25 manufactured film has a higher degree of orientation of the polymer of
which
it is made, and thus has different mechanical properties to a merely extrusion
coated or extrusion laminated layer of the same or corresponding polymer.
Thus, by incorporating such a film in the structure, the laminated material
alttogether may be made stronger and better resistant to downstream tough
30 treatment of the material. Such pre-manufactured films may be avoided in
the
material, as they add costs both from sourcing of materials point of view and
from the lamination operation point of view. Pre-manufactured films may have
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different mechanical properties and may be ranging from biaxally oriented,
tough films obtained by mere extrusion cast films to films manufactured by
film blowing and inherent polymer orientation occuring in that process, or
with
additional subsequent orientation. Alternatively, polymer materials that are
5 merely extrusion coated or extrusion laminated may be used.
The second innermost layer of a liquid tight, heat sealable material may
be a polyolefin, preferably a blend of low density polyethylene, LDPE, and
metallocene-catalysed (using a single-site or constrained-geometry catalyst),
linear low density polyethylene, m-LLDPE. This is the type of polymer most
10 used today for the innermost layer, for best balanced liquid tightness
and heat
sealability properties, and which generates best possible package integrity of
heat sealed packaging containers. By choosing the composition of this layer
carefully, the amount of polymer in this layer may be optimised to be as low
as possible while still producing strong and reliable packages filled with
15 product.
The second innermost liquid tight, heat sealable material layer may be,
or comprise, a pre-manufactured polymer film, the film comprising a heat-
sealable thermoplastic polymer material and, optionally, a further layer of a
material for providing improved robustness of the mechanical properties of
20 the laminated packaging material.
The purpose of any one of the previously listed specific examples of
laminates, is to add complementary properties to the laminated packaging
material, when using merely the barrier-coated cellulose-based substrate as
such, as a gas barrier material in the laminate structure, or wherein the
25 applied coating provides only some gas barrier properties, or when the
coating has only moisture-sensitive gas barrier materials. By laminating the
barrier-coated celulose-based substrate to a further polymer film, which may
add further moisture-resistance or water vapour barrier properties, the at
least
two different barrier materials may interact to provide further enhanced total
30 barrier properties to the total laminate structure. Typical examples of
such a
pre-manufactured films, adding at least barrier properties towards water
vapour, may be metallised films and polymer films including filler materials,
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such as flake-shaped mineral fillers or other small particles, which help to
delay diffusion of water vapour through the laminated stucture. The
necessary bonding layer between the barrier-coated cellulose-based
substrate and the further barrier film may ensure such enhanced barrier
5 properties, since the interjacent bonding layer acts as a "cushion" and
further
"gas or vapour migration interface" in the laminated structure.
The outer- and innermost, liquid-tight layers and the lamination layers
in the interior of the laminate structure, do not normally and inherently add
high barrier properties to migrating gas molecules or small molecules. Their
10 purpose is to provide a direct barrier to water in liquid form from
penetrating
through to the cellulose-based bulk material and other paper layers. The
liquid barrier layers also prevent water vapour from migrating to the
cellulose
to the extent that it gets wet, but are not capable of keeping the moisture
content of the laminated structure at zero or at the low level of "dry" paper
15 (which is at about 7-8 % in an environment at ambient temperature, i.e.
at 23
deg C, and 50 % relative humidity, RH). The moisture content in the
laminated carton material of a packaging container filled with liquid is
usually
rather high and migration through the material occurs, unless there is a
further water vapour barrier included, such as an aluminium foil, a vapour
20 deposited metallisation layer, other vapour deposition coating,
inorganic
material layer or other polymer material layer.
A laminated packaging material as described above may provide good
integrity when transformed into filled packaging containers, by good adhesion
between the adjacent layers within the laminated construction and by
25 providing good quality of the barrier coatings and the ductile base
layer pre-
coating, each and in combination. Especially, for the packaging of liquids,
and
wet food, it is important that the inter-layer adhesion within the laminated
packaging material, as well as the oxygen gas barrier properties, is
maintained also under wet packaging conditions.
30 A packaging container formed from a described laminated packaging
material may be partly sealed, filled with liquid or semi-liquid food and
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subsequently sealed, by sealing of the packaging material to itself,
optionally
in combination with a plastic opening or top part of the package.
To conclude, robust and reliable packages for liquid food packaging for
long term shelf-life and storage may be obtained by the barrier-coated
cellulose-based substrate and the laminated packaging material comprising it,
as defined by the invention, thanks to the over-all improved barrier
properties
provided that is even maintained during folding of the packaging material. The
laminated packaging material structure works better for the forming into fold-
formed packages, both regarding the good adhesion between the barrier-
coated substrate and the other layers of the laminated material and regarding
the improved contribution to gas barrier properties from the barrier-coated
substrate itself. The latter is likely due to an improved flexibility and
resistance to stress and deformation, thanks to the ductile base layer pre-
coating in the barrier-coated cellulose-based substrate. It has been seen that
the ductile pre-coating is able to prevent the barrier coating and innermost
liquid-tight layers to be strained to a degree resulting in crack openings.
Most
likely the ductile pre-coating re-distributes the high local stress and strain
in
the cellulose based substrate, barrier coating and innermost liquid-tight
layers
during folding. The strain level therfore becomes smaller and distributed over
a larger area.
Examples and description of preferred embodiments
In the following, preferred embodiments of the invention will be
described with reference to the drawings, of which:
Fig. la and lb schematically show in cross-section embodiments of
barrier-coated cellulose-based substrates according to the invention,
Fig. 2a shows a schematic, cross-sectional view of an example of a
laminated packaging material, comprising the barrier-coated cellulose-based
substrate of Fig. la,
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Fig. 2b is showing a schematic, cross-sectional view of a further
example of a laminated packaging material comprising a barrier-coated
cellulose-based substrate as of Fig. lb,
Fig. 3a shows schematically a method, for dispersion coating a base
5 layer or barrier pre-coating composition onto a cellulose-based
substrate,
Fig. 3b shows schematically a method, for melt (co-) extrusion coating
layer(s) of a thermoplastic heat sealable and liquid-tigth polymer onto a web
sustrate, to form innermost and outermost layers of a packaging laminate of
the invention,
10 Fig. 4a is showing a diagrammatic view of a plant for physical vapour
deposition (PVD) coating, by using a solid metal evaporation piece, onto a
substrate film,
Fig. 4b is showing a diagrammatic view of a plant for plasma enhanced
chemical vapour deposition (PECVD) coating, by means of a magnetron
15 plasma, onto a substrate film,
Fig. 5a, 5b, 5c and 5d are showing typical examples of packaging
containers produced from a laminated packaging material comprising a
barrier-coated cellulose-based substrate according to the invention,
Fig. 6 is showing the principle of how such packaging containers are
20 manufactured from the packaging laminate in a continuous, roll-fed, form,
fill
and seal process,
Fig. 7 is a diagram showing the effect of surface roughness on the
oxygen transmission rate of different paper substrates, and
Fig. 8 shows schematically in cross-section an embodiment of a
25 cellulose-based substrate pre-coated with a ductile base layer.
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Examples
Example 1
Paper property characteristics were evaluated in laboratory scale to
provide the most suitable cellulose-based substrate for this invention.
Table 1 below demonstrates the main properties used for selecting the
most suitable paper substrate. Several paper structures were tested
regarding their porosity (represented by Gurley air resistance), their
mechanical resistance (represented by elongation and TEA-tensile energy
absorption) and their surface smoothness (represented by Bendtsen and PPS
roughness values) characteristics. The porosity of a paper can be measured
either as an air resistance with the unit s/(100 ml) as determined by Tappi
T460 om-02, or as an air permeance with the unit pm/(Pa s), as determined
by IS05036-5:2013. For papers with very low porosity the air permeance,
with the unit pm/(Pa s), nm/(Pa s), or pm/(Pa s), is determined by SCAN-P
26:78.
Among the papers tested, the structures that presented the lowest
porosity, with the highest Gurley air resistance value, at times not even
detectable by the equipment due to a too high value, were papers 1, 2 and
11.
30
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Table 1 - Evaluated paper substrates with different porosity, mechanical
resistance and surface roughness properties
Paper Roughness
Samples
Paper gram (Bendtsen) PPS (u Elongation m)
Gurley TEA (J/mz)
(%)
type mage (mum) (s/dl)
(g/m2) Wire Felt Wire Felt
MD CD MD CD
Paper
A GPP 32 241.8 222.4 5.552 5.572 *
1.69 3.892 33.08 42.42
1
Paper
GPP 391.6 336 * *
2.456 3.932 86.42 80.34
2 58
Paper
123.5
CP 156.3 184.2 4.774 4.475 41.29 2.232 7.638 78.55
45 2
Paper
CP 60 8.4
9.4 1.358 1.442 9637 1.875 8.369 74.77 216.4
6
Paper
CP 40
22.5 18.7 1.34 1.058 530.5 2.068 4.371 52.88 61.83
7
Paper
NC
354.8 39.5 5.533 2.505 97.58 4.174 3.471 92.23 60.53
8 40
Paper
NC 40
475.9 51.9 5.922 2.821 114.1 3.342 3.456 68.47 49.44
9
Paper
CP 9.5
9.7 1.325 1.187 231.6 2.309 6.467 51.18 79.06
40
Paper
CP 21 18 2.076 1.772 *
1.575 2.162 46.89 20.92
11 55
5 (*) Properties that exceeded the equipment's detection limit due to their
high
value. GPP means grease proof paper, CP means pre-coated paper and NC
means non-coated paper.
The most suitable combination of the evaluated paper properties was
10 found to be the one described under K in Table 1 above. In order to
confirm
that the structure was the most suitable for gas barrier application, the base
layer pre-coating and the gas barrier coating formulations were applied to
validate the oxygen transmission levels achieved with the selected paper
base. The ductile base layer pre-coating and the barrier coatings were
applied on the felt side, i.e. the top side, of the papers.
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A summary of the analyses performed for coated papers is shown in
Table 2. The surface roughness of the pre-coated papers was evaluated in
PPS. The effect of the base layer pre-coating on the paper roughness
represents the change (in %) of roughness, measured for coated papers, in
5 comparison to the roughness measured for uncoated papers.
The paper substrates (A, B, F, G and K) were further coated with one
or two layers of a nanocrystalline cellulose, NCC, gas barrier solution (total
wet thickness of - 25 pm). The coatings were evaluated for coating weight,
roughness, kit oil, grease resistance and oxygen barrier. Evaluation of these
10 barrier coating properties indicates the effects of the type
of base paper sheet
and of its initial roughness on further gas barrier coating performance.
Kit oils were evaluated according to the Tappi T559 pm96 standard
(KIT test). In this test, the organic solvent mixture is dropped onto the
surface
of the coated papers, followed by inspecting whether the organic solvents
15 have penetrated the barrier layer and are absorbed by the
paper. For this
evaluation, grade 7-12 KIT test mixtures were used.
Oxygen barrier properties for Table 2 were evaluated according to
ASTM D3985 and F1927-50 standards, using a MOCON sensor instrument,
Ox-Tran model 2/22. Measurements were taken at 23 C and 70% relative
20 humidity 1 atm, i.e. 100 % oxygen. For papers with higher initial surface
roughness, OTR was measured after applying 2 coating layers (due to the
failure of 1 layer to form a good barrier). For papers with low surface
roughness, samples with 1 coating layer were evaluated (due to good barrier
formation).
30
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Table 2 - Summary of analyses performed for coated papers selected for
barrier application
P Rough Numb Rough Dry Effect of KIT Grease OTR
g
a -ness er of ness coating coating rati
resistance 70% RH,
p PPS, gas PPS, weight, on ng after
ml/m2/day
e pm, barrier pm g/m2 rough- creasing
r un- coatin (coate ness, `)/0
coated g d)
layers
A 5.572 2 4.11 9.17 -38.7 12 Yes 9.72
B * 2 6.99 9.8 -13.8 12 Yes 35.06
F 1.442 1 1.50 4.18 +5.6 12 No 30.5
G 1.058 1 1.78 5.08 +14.8 12 Yes 3.06
K 1.772 1 2.66 5.5 +10.4 12 Yes 3.42
The effect of roughness on the oxygen transmission rate of barrier-
coated papers is shown in Figure 7.
As shown in Table 2 and as observed, surface roughness of the
substrate paper has a substantial effect on the final barrier performance
after
coating. First, the higher the surface roughness of the base paper, the
greater
weight of the applied gas barrier coating is needed, although even when wet,
greater thicknesses were applied to the different papers. This can be seen for
paper A and B, which have relatively high surface roughness and
consequently required high gas barrier coating weights. Also, on papers with
higher roughness measurements, usually >3 pm (PPS) as observed for A and
B, the NCC coating barrier solution reduced the roughness of the paper. For
papers with low surface roughness, there is a slight increase in roughness,
but in terms of pm, the roughness remains very low.
Secondly, as can be seen in Table 2 and Figure 7, the lower the
roughness of the base paper (usually <3 pm), the better the barrier
performance will be after barrier application. For paper structures such as G
and K, with low roughness, it can be seen that only one coating layer was
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sufficient to achieve excellent oxygen and grease barriers, while for papers A
and B, an insufficient oxygen barrier was achieved, although a high coating
weight was applied. This implies that not only is barrier performance
improved, but the number of layers needed to form the barrier is also
5 decreased. After testing the grease resistance of the papers, all papers
demonstrated an excellent grease barrier (KIT 12). Moreover, all papers
except paper F demonstrated grease resistance after creasing of the paper. It
seems that the roughness of paper has lower effect on the grease barrier
compared to the oxygen barrier.
10 The performance of paper F was different from the other papers tested.
At low roughness, paper F showed insufficient barrier performance in terms of
oxygen transmission and grease resistance. Based on the properties of Paper
K, combining a low surface roughness with low porosity, and providing good
oxygen barrier properties, it may be concluded that not only the roughness
15 affects the performance of the barrier, but also the composition of the
paper,
i.e. its porosity, and the composition of its surface.
Paper surface roughness is a very important parameter that will affect
the effectiveness of the coated barrier layer, in terms of number of layers
applied, weight of coating applied and barrier performance. A PPS roughness
20 value lower than 3 pm is required to be able to coat with just one layer
and
achieve good barrier performance. Surface roughness is not the only
important parameter to consider, as each paper's composition and resistance
properties can also affect performance.
Therefore, after studying and evaluating all the paper substrate
25 structures described above and considering the combination of paper air
resistance, i.e. porosity, surface roughness properties and the paper's
performance in barrier tests after application on paper, the paper structure
named as K was chosen as the most suitable structure to continue the scale-
up studies and to design a proof-of-concept for validation of barrier
30 performance.
Paper K is pre-coated and supercalendered, having a density above
1100 kg/m3 and is produced from 100 % softwood Kraft pulp.
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Example 2
5 The pre-coated high-density paper K is pre-coated on both sides, has
a grammage of 55 g/m2 and all of its content is repulpable. Figure 8
represents the structure of the pre-coated thin paper K, determined to be the
best combination to ensure performance of further gas barrier coatings
coated onto its top surface. The structure shown in figure 8 has ductile
10 coatings applied to the surfaces of the top side as well as the bottom
side of
the paper, which will be described below. These ductile base layer coatings
allow for optimal combination of low porosity and roughness to achieve the
expected oxygen barrier performance. Theductile base layer pre-coating is
applied directly onto the paper fibers. Directly on top of the heaviest layer
on
15 the top side (or felt side) of the paper (B in Figure 8), the gas
barrier coating is
to be applied, in a later step.
The fiber composition represented as A in Figure 8, is formed only from
cellulose pulp fibers. These fibers are from bleached kraft softwood fibers
and
may have a grammage of 30 to 50 g/m2. The grammage of the coated paper
20 will then be from 40 to 65 g/m2. The base paper for paper K has an
uncoated
grammage of 35 g/m2 and a ductile, pre-coated grammage of 55 g/m2.
The top and bottom surfaces of the fibrous composition A, were thus
coated with aqueous pre-coatings of ductile dispersion compositions. These
coatings had a pH of 5.5 to 8, solid content of 48 to 51%, Brookfield
viscosity
25 of 100 to 1000 mPa.s, while the coated material exhibits a Tg ranging from -
30 to 0 and 0 to 30 ( C), and may described in terms of chemical composition
as an aqueous dispersion of a styrene-butadiene binder copolymer, a so-
called SB-latex, and a kaolin clay filler. The pre-coating composition used
comprised about 15 wt% of SB-latex, about 80 wt% of kaolin filler material,
30 about 4 wt% crosslinking starch compound and further additives, such as
less
than 2 wt% of a thickening agent.
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As illustrated in figure 8, the coating layer B applied onto the paper top
side may have a weight of 5 to 15 g/m2, such as from 10 to 15 g/m2, applied
as a single layer. The coating composition illustrated in C, applied onto the
bottom/ back of the paper, may have a grammage from 1 to 10 g/m2 applied
5 in a single layer and is also obtained from an aqueous dispersion
comprising
a styrene-butadiene copolymer and kaolin. The coated paper as illustrated by
the set of A, B and C may thus have a final grammage between 40-65 g/m2.
The thus pre-coated paper K as illustrated in Figure 8, with layers A, B
and C had a grammage of 55 g/m2 and final porosity lower than 100 nm/(Pa
10 s) as determined by IS05636-5:2013, which is the lower limit of the
applicable
range of the test method, and further lower than 1 nm, as determined by
SCAN-P 26:78. The pre-coated paper had a PPS roughness lower than 2 pm,
i.e. about 1.8 pm, on its top side. The final, NCC gas-barrier-coated paper K
as of Table 2 exhibited a felt side surface roughness of about 2.8. Thus, the
15 pre-coated and barrier-coated paper of the invention may have a PPS
surface
roughness of lower than 3 pm, such as from 0.5 to 3 pm.
Example 3
Mechanical resistance properties of the pre-coated cellulose-based
20 substrates used were explored to demonstrate the importance and
contribution of each layer of the pre-coated paper composition to the final
application of the barrier-coated substrates.
Material failure by nucleation and propagation of a crack in the
substrate will create openings and discontinuities in the barrier coating
layer,
25 thus destroying the final package barrier. The more elongation the
substrate
can undergo before failure the more resilient is the substrate to various
loads,
such as bending during package forming. By becoming more flexible, the
structure after pre-coating with a ductile material and subsequent drying and
calendering, allows the pre-coated paper structure to more effectively resist
30 bending and the initiation of breaks in its surface.
This behavior of reduced stiffness of the structure containing cellulose
fibers and the top and backside coatings after calendering can be illustrated
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with the reduction of Young's Modulus as shown in tables 3 and 4, in both MD
and CD directions of the paper. The reduction in Young's Modulus means that
the stiffness of the total material structure decreases, which allows the
material to gain more flexibility and thus become more resilient and resistant
5 to creasing and folding processes.
The application of the flexible or ductile base layer pre-coating on the
fibrous layer allows for a gain in ductility of the final material, which will
result
in a material more resistant to breakage, cracking and damage in the process
of converting a package.
Table 3 ¨ Average of the values obtained for the stress x strain curves of
each
pre-coated paper layer material described in Figure 7 (i.e. ABFGK ), as well
as the influence of the individual coating layers on final ductility of each
of the
materials in the MD direction of the paper
Layer Description Maximum Elongation Energy Break- Modul
or of the layer Load [N] at Max. at ing
us
struct- and surface load [mm] Maxim Point
[MPa]
ure treatment urn Load
Load [N]
[J]
A 100% fiber 20.58 0.30 0.005 7.55
2418.7
layer
BAC A pre- 18.05 0.32 0.004 6.19
1648.3
coated on
both sides
BAC BAC
14.60 0.42 0.005 7.85 1269.0
structure
after
calendering
Table 4 ¨ Average of the values obtained for the stress x strain curves of
each
pre-coated paper layer material described in Figure 7 (Le. ABFGK ), as well
as the influence of the individual coating layers on final ductility of the
materials
in the CD direction of the paper
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Layer Description Maxi Elongation Energy Breaking Modul
or of the layer mum at Max.
at Point us
compo and surface Load load [mm] Maxim Load [N] [MPa]
sition treatment [N] Urn
Load
[J]
A 100% fiber 15.00 0.62 0.007
2.99 1108.4
layer
BAC A pre- 14.94 0.85 0.010 4.35
862.5
coated on
both sides
BAC BAC 14.20 0.98 0.010 4.61
634.7
structure
after
calendering
The mechanical resistance properties of the pre-coated and uncoated
papers evaluated here, when in conjunction with the laboratory barrier
application exploration results, provide an understanding of the excellent
5 performance achieved. For example, the elongation at maximal load
increases as the cellulose-based substrate or paper is pre-coated
(measuremrents "B") and further as the pre-coated paper is calendered
(measurements "C"). This confirms the beneficial effect of providing a more
ductile/flexible material as substrate for the gas barrier, so that the gas
barrier
coated substrate can withstand higher elongation without cracking during
conversion of packaging material into packages.
Example 4
The invention described herein, in addition to bringing unique features
15 that allow for reaching high oxygen barrier levels, is therefore a
solution
capable of replacing non-renewable components in packaging, has high
content of renewable materials, and is 100% repulpable or recyclable
according to the PTS RH 021/97 standard.
For recycling the thin, ductile, pre-coated paper substrate selected for
the application developed here, in addition to the performance of
repulpability
tests according to PTS RH 021/97, a repulpability study was also performed
in the laboratory.
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To evaluate repulpability in the laboratory, a sample of known weight of
paper was dispersed at 23 C using a device with capacity of 3000
revolutions/min, for 20 minutes of agitation. After the 3rd minute of
stirring, it
was already possible to see that the paper is repulpable. No lumps were
5 detected in the water suspension, indicating that the paper was already
dispersed in the water.
Example 5
A gas barrier coating formulation based on NCC was used for coating
the flexible pre-coated paper substrate, which can be characterized as
containing 5 to 22% solids, and having a viscosity of 600 to 2500 2 m Pa-s.
The chemical components of the formulation were based on NCC (CNC),
PVOH and starch-based materials and contained at least 50% of the
renewable materials (NCC and starch), such as at least 50% of NCC, and is
100% recyclable and biodegradable.
Nanocelluloses have challenging rheologies, so when added to blends
with other polymers and components, they cause a change in rheology by
altering the final viscosity of the suspension. NCCs (Nanocrystalline
Cellulose
or Cellulose Nanocrystals) are extracted from hardwood or softwood
20 cellulosic pulp, and all of their dimensions are on the nanometric
scale. The
NCCs used have all their dimensions on the nanometric scale, with widths
ranging from 5 to 20 nm and lengths from 150 to 400 nm.
A double layer gas barrier layer of nanocrystalline cellulose or PVOH
or other gas barrier material may be formed on the top side surface of the
ductile pre-coated paper. A first gas barrier coating creates a gas barrier
layer onto the ductile base layer pre-coating of the top side of the paper,
and
a second gas barrier coating provides a second gas barrier layer onto the
first
gas barrier coating layer.
The sequence of barrier layer application is essential in this case, onto
the ductile pre-coated paper. The first gas barrier disperson layer is applied
onto the ductile base layer pre-coating on the top side surface of the paper,
by means of bar coating, and this layer is dried using infrared and hot air to
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form a homogeneous gas barrier layer on the surface. The application speed
used in the tests was 300 to 600 m/m in to ensure good quality of gas barrier
coating layers formed on the ductile pre-coated paper surface. The first
formed gas barrier dispersion layer prepares the surface to receive the
5 second barrier dispersion layer, which is also formed by means of bar
coating, onto the dried first barrier dispersion layer. The second gas barrier
dispersion layer applied also goes through a process of film forming and
drying using infrared and hot air for drying. It should be noted that the
optimal
range for drying using hot air tested for the formulation and the ductile pre-
coated paper is from 90 to 150 C.
Drying at evaluated temperatures does not cause loss of barrier levels,
and provides good formation of a coating film on the pre-coated paper
surface, even at higher temperatures. Finally, to ensure proper winding and
cooling of the barrier-coated paper substrate, after drying, the coated paper
15 should be rolled at temperatures below 40 C. This secures the formation
of a
homogenous film, ensuring high oxygen- and grease-barrier performance.
Example 6
Different conditions were tested for larger scale application of a gas
20 barrier coating on the surface of paper K, described above. The gas
barrier
coating was applied on a pilot coating machine that allowed for using
different
technologies, however, bar or blade coating methods were the application
technologies used to form the gas barrier coating layers applied onto the
ductile pre-coating B, as illustrated in Figure 8, with a total dry coating
amount
25 of from 0.5 to 5 g/m2. Application bars are smooth, and their diameters
can
vary, here bar diameters from 18 to 30 mm were used. The tests were made
by applying the gas barrier directly onto the ductile pre-coated base paper as
illustrated in Figure 8.
In a final prototype for proof-of-concept and upscaling,the
30 characteristics of the ductile pre-coated papers used in the development
were
as described in Table 5 below.
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Table 5 ¨ Characteristics of the paper used in large-scale test
PPS PPS SCAN-P
Paper Paper Layer B Layer C
top bottom 26:78
Tests Paper th.ickness density coating coating
(g/m2) layer layer (pm/Pa.
(gm) (kg/m3) (g/m9 (g/m9
(11m) (11m)
s)
Large-
scale 1222-
55-60 45-50 12-15 5 2.07 2.1 206
Proto- 1333
type
In this upscaling work, it was thus seen that pre-coated paper
substrates having a lower surface roughness and a lower porosity generally
provided better results and provided improved oxygen barrier levels in formed
and filled packages.
The barrier formulation containing NCC was thus applied onto the
ductile pre-coating B on the top side of the paper substrate A (Fig. 8), i.e.
onto
the top surface with the highest pre-coating weight, by forming a 2-layer gas
barrier coating of 2 x 2.5 g/m2 of NCC, resulting in package OTR 21%02
(cc/pkg/24h) of 0.016 at 0.2 atm and 50% RH, according to ASTM F1307-14.
The aqueous dispersion containing cellulose nanocrystals was thus
applied to surface B in a first step, thereby consolidating a final gas
barrier
layer measuring about 2.5 g/m2, which prepared the surface to receive the
second gas barrier dispersion coating layer. After applying the first layer,
it is
dried by infrared heating and hot air convection. A second layer measuring
about 2.5 g/m2 was applied onto the first gas barrier coating, to form a total
2-
layer gas barrier coating of about 5 g/m2. After applying the second layer, it
is
dried by infrared heating and hot air, and finally, the solidified resin is
cooled
to a temperature below 40 C, to prevent bonding between sheets of paper
with a barrier already applied on the surface (bonding the sheets with the
still
active, uncooled resin).
The amount applied was limited to 2.5 g/m2 for each part-layer, to
prevent excess water on the cellulose-based substrate. Using the pre-coated
paper as described in Figure 7 allows for greater dimensional stability
control
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as the flexible pre-coating B acts as a surface preparer as well as impacting
the surface roughness and porosity of the substrate.
The application technology used for forming this gas barrier coating
was a bar coating method. A bar diameter of 24 mm was used. The bar used
5 for applying the barrier dispersion was rotating in the same direction as
the
paper winding with speed from 20 to 80 rpm, or in the opposite direction to
the paper winding direction with speed from 80 to 160 rpm .
Example 7
10 The paper
stubstrate K from Table 1, as well as the previously best
tested substrate paper type for carrying oxygen barrier coatings, i.e. a
"Superperga WS 32 gsm Parchment FL109" greaseproof paper from Nordic
Paper, were for comparison (Comparative Example 1), laminated in the same
way with LDPE on their respective top sides at 20 g/m2.
15 The comparative paper was measured to have a surface roughness on
the top side, i.e. the side to be barrier-coated, of about 36 ml/min Bendtsen.
It
comprised cellulose refined to a higher degree, i.e. cellulose of smaller
fibrous/fibrillar molecules, to provide its dense surface and medium paper
density of merely 865 kg/m3. The comparative paper is not 100 % recyclable,
20 but leaves a reject of water-swollen, low-molecular cellulose.
The paper substrate K used in the invention had a top side Bendtsen
roughness of about 20 ml/mm.
The oxygen transmission rates through the flat laminated papers of
Example 1 and Comparative Example 1 were measured using a coulometric
25 detector and the evaluation was done according to ASTM F1927-14 by the
unit cm3/m2/24h at 0.2 atm oxygen pressure and a moisture level of 50%
relative humidity. The results from the measurements are presented in Table
6.
30 Table 6. OTR for flat papers laminated with PE, cm3/m2/24h at 0.2 atm,
50%RH
Comparative Example 1:
Example 1: Pre-coated
Superperga WS 32 g/m2 paper
Parchment FL109 GPP
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Paper Highly refined grease- High-density
(super-
proof paper, with a calendered) and
ductile
density of 865 kg/m3 base layer pre-
coated
Sulphate SW paper
Flat paper OTR 0.2 42 71
atm, 23 C, 50%
RH
From the results in Table 6, it is seen that the best previously studied
paper substrate as a laminated sample provides better initial, inherent OTR
performance than the base layer pre-coated paper substrate of the invention.
The pre-coated paper substrates from Example 7 but without the
laminated layers of LDPE on both sides, were thus instead coated with a first
gas barrier coating of polyvinyl alcohol, PVOH, or NCC, respectively, onto the
ductile base layer pre-coating (top side of paper substrate). The PVOH used
was obtained from Kuraray and had a degree of hydrolysis of at least 98 %,
i.e. Poval 6-98. The dispersion of PVOH was applied by means of a smooth
roller coating method in pilot-scale equipment, and the wet amount applied of
the aqueous dispersion of the PVOH was about 15 weight-%. For the purpose
of anti-foaming, 0.05 volume- /0 of 1-octanol was added to the PVOH. The
Brookfield viscosity at 23 C of the PVOH dispersion barrier composition was
500-800 mPa.s.
The rotation speed of the coating roller was 160 rpm, with rotation in
opposite direction compared to the web running direction. The coating was
applied in two steps, with a dry gram mage of the first and second layer of
1.6
g/m2 and 1.6 g/m2, respectively.
A combination of infrared irradiation IR and hot air was used for drying
the coated layers and the surface temperature was kept at below 100
degrees Celcius while the web speed was about 300 m/m in during coating.
In a different coating operation in the same pilot-scale equipment, a
dispersion of NCC was instead applied, by means of a smooth roller coating
method. The amount applied of the aqueous dispersion of the NCC was
about 19-20 weight-%. The Brookfield viscosity at 23 C of the aqueous NCC
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dispersion barrier composition was <2000 m Pas and the viscosity at 50 C
was 1000-1200 mPa-s. Both IR and hot air was used for drying the coated
layers and the web speed was about 300 m/min during coating. The rotation
speed of the coating roller was 80 rpm, with rotation in opposite direction
5 compared to the web running direction. The coating was applied in two
steps,
with a dry grammage of the first and second layer of 1.9 g/m2 and 2.7 g/m2,
respectively.
The pre-coated and gas-barrier coated papers were then laminated
with 20 g/m2 LDPE on its top sides and the oxygen transmission rates were
evaluated as for Example 7 and for Comparative example 1, but also
including 80% moisture level. The oxygen transmission rates are shown in
Table 7.
Table 7. OTR for pre-coated and barrier-coated papers laminated with PE.
PVOH NCC 4.6gsm
3.2gsm
Flat paper OTR 0.2 atm, 23 0.31 0.13
C, 50% RH
Flat paper OTR 0.2 atm, 23 7.93 17.2
C, 80% RH
From the results in Table 7, it is seen that the OTR performance of the
pre-coated and barrier-coated paper is significantly improved compared to the
20 pre-coated paper without a gas barrier coating in Table 6. Both the PVOH
coating and NCC coating improves the OTR performance. It is however noted
that both types of coating are sensitive to moisture, with higher OTR for 80%
RH compared to OTR at 50% RH.
Example 8
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The pre-coated and gas-barrier dispersion-coated papers from
Example 7 were metalized to an optical density of about 1.8. The metalized
barrier papers were then extrusion coating laminated with paperboard and
polymers to provide a packaging material according to the following structure:
//Outside 12 g/m2 LDPE/ Duplex CLC 80 mN, 200 g/m2, paperboard
bulk layer/ LDPE 20 g/m2 bonding layer/ barrier-coated paper substrate
(with 2x barrier coating -4 g/m2/ Al metal OD -1.8/ Adhesive layer EAA
copolymer 6 g/m2/ blend LDPE+m-LLDPE 29 g/m2//
The liquid paperboard was prior to lamination creased to facilitate sub-
sequent folding of packages. Lamination of the packaging material was
carried out in a flexible pilot laminator with three extrusion coating
stations.
The lamination speed was about 400 m/m in.
The Duplex CLC paperboard was a clay-coated paperboard of the
conventional type, and the m-LLDPE is a metallocene-catalysed linear low
density polyethylene. The barrier-coated side of the paper substrate was
directed in the laminated structure towards the inside (corresponding to the
inside of a packaging container manufactured from the laminated material).
The adhesive polymer EAA and the innermost heat-sealable layer were
coextrus ion coated together onto the barrier-coated paper and the outermost
layer of LDPE was extrusion coated onto the outside of the paperboard.
The oxygen transmission rates through the flat laminated packaging
materials of Example 8 were measured using a coulometric detector and the
evaluation was done according to ASTM F1927-14 by the unit cm3/m2/24h at
0.2 atm oxygen pressure and at moisture levels of 50% and 80% relative
humidity.
Comparative Example 2
The previously tested paper substrate also used in Comparative
Example 1, was gas-barrier coated and laminated into a packaging material
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structure in a substantially corresponding way, except for the ductile base
layer pre-coating not being included.
The paper was first coated in two consecutive dispersion coating
operations with 1.5 g/m2 each of PVOH (Poval 15-99, fully saponified
5 PVOH), with drying after each step. Then it was metallised to an optical
density of about 2.3.
The barrier-coated comparative paper substrate was further laminated
into a packaging material structure as follows:
10 //Outside 12 g/m2 LDPE/ Duplex CLC 80 mN, 200 g/m2, paperboard
bulk layer/ LDPE 20 g/m2 bonding layer/ barrier-coated paper substrate
(with 2x barrier coating -1.5 g/m2/ Al metal OD -2.3/ Adhesive layer
EAA copolymer 6 g/m2/ blend LDPE+m-LLDPE 19 g/m2//
15 The results of OTR measurements on the laminated packaging
materials of Example 8 and Comparative Example 2 are presented in Table 8.
Table 8. OTR for flat laminated packaging materials
Comparative Example 8-1: Example
8-2:
Example: Pre-coated Pre-
coated
Superperga WS paper with NCC paper with
32 g/m2 barrier coating PVOH
barrier
Parchment coating
FL109 GPP
Laminate structure: 80 m N 80 m N 80 mN
/LDPE/ paperboard paperboard 1.5 paperboard 4.6
paperboard 3.2
/LDPE/ paper + g/m2 PVOH; Met g/m2 NCC; Met g/m2 PVOH;
pre-coating + to OD -2.3 to OD -1.8 Met to
OD -1.8
barrier coating +
met/ inside PE
polymers/
Flat laminate OTR 1.3*
1 atm, 23 C, 50%
RH
Flat laminate OTR 1.3*
1 atm, 23 C, 80%
RH
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Flat laminate OTR 0.26** 0.39* 0.38*
0.2 atm, 23 C,
50% RH
Flat laminate OTR 0.26¨ 0.38' 0.36'
0.2 atm, 23 C,
80% RH
*Measured at oxygen pressure 1 atm/ 0.2 atm
Converted to oxygen pressure 0.2 atm
It can be seen in Table 8 that the OTR values for packaging material
5 using a
pre-coated and barrier coated paper are higher than for packaging
material using a highly refined grease-proof paper, even though the barrier
coating grammages were higher on the pre-coated paper of the invention.
This can be attributed to that the OTR contribution from the paper itself is
lower for the pre-coated paper compared to the highly refined grease-proof
10 paper, see Table 7.
It is also noted that the packaging material using pre-coated and
barrier coated paper are not sensitive to an increased moisture level.
Example 9
15 Packages
were produced in a Tetra Pak E3/CompactFlex filling
machine. This type of filling machine has the capacity to fill portion
packages
at a speed of 9000 packages/hour and a flexibility that allows for quick
change between different package formats. Packages were in the format of
Tetra Brike with a volume of 200 ml.
20 The Oxygen transmission rate of packages (filled, emptied and dried)
was measured with packages mounted on a special holder; inside the
package nitrogen is purged; the outside of the package is exposed to the
environment surrounding the instrument. When oxygen permeates through
the package into the nitrogen carrier gas, it is transported to the
coulometric
25 sensor. The sensor reads how much oxygen that leaks into the nitrogen gas
inside the package. OTR is then evaluated according to ASTM F1307-14, at
0.2 atm (surrounding air containing 21% oxygen). The measurement unit is
cm3/package/24h.
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Table 9. OTR for packages, including loss factors.
Packaging material Comparative Example 8-1: Example
8-2:
used Example 2: Pre-coated Pre-
coated
Superperga WS paper with paper
with
32 gsm NCC barrier PVOH
barrier
Parchment coating coating
FL109 GPP
Package format Tetra Brik Tetra Brik Tetra
Brik
Aseptic 200S Aseptic 200S Aseptic
200S
Package volume 200 ml 200 ml 200 ml
Laminate area per 0.030 m2 0.030 m2 0.030
m2
package
Calculated, 0.0078 0.0117 0.0114
theoretical* OTR per
package, 0.2 atm, 23
C, 50% RH
Measured OTR per 0.075 0.016 0.013
package 0.2 atm, 23
C, 50% RH
Loss factor 9.6 1.4 1.1
measured/theoretical,
23 C, 50% RH
*Using the package laminate area and the corresponding OTR value for flat
packaging material in Table 2.
5 It is
interestingly noted from Table 9, and surprisingly too, that even
though the OTR values for flat laminated packaging materials using pre-
coated and barrier coated paper are higher than for flat laminated packaging
materials using a highly refined grease-proof paper, as seen from Table 7, the
package OTR values are improved by using the pre-coated and barrier
coated paper. The measured package OTR is 0.013-0.016
cm3/package/24h/0.2 atm instead of 0.075. When dividing the measured OTR
values with the corresponding theoretically calculated ones it is also seen
that
the loss factors are close to 1 when using pre-coated and barrier coated
paper, compared to a loss factor of 9.6 when using a highly refined grease-
15 proof paper. It is evident that the gas barrier performance on the flat
laminated packaging material is preserved even after folding of the packages,
when using pre-coated and barrier coated paper. This is attributed to the
ductile base layer pre-coating, which is able to reduce the effect of stress
concentrations in the paper. Stress concentrations are otherwise an origin for
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cracks to initiate and propagate through the paper. If the paper cracks, a
very
thin barrier coating is not able to withstand the high stress and will
consequently also crack, thus imparting the gas barrier. Ductile materials in
particular, are able to withstand high strain and being able to re-distribute
the
5 stress concentrations in the paper during folding.
Further, relating to the attached figures:
In Fig. la, there is shown, in cross-section, an embodiment of a barrier-
coated cellulose-based substrate 10a, of the invention. The substrate lla is a
10 paper made from a major proportion of cellulose fibers from sulphate
softwood pulp, having a grammage of 35 g/m2, first provided with a ductile
base layer pre-coating 12a on its top side, by applying a latex or biopolymer
binder composition, such as specifically in this example an SB-latex binder
composition further also comprising an inorganic laminar filler material, by
15 means of aqueous dispersion coating and subsequent drying to evaporate
the
water. The dry weight of the applied ductile base layer pre-coating is about
12
g/m2. Optionally, a further, second, ductile coating 15a, of the same
composition as the ductile base layer pre-coating 12a, may be applied in the
same manner onto the opposite, uncoated side of the paper substrate 11a.
20 The dry weight of the, second, ductile coating is about 5 g/m2. The thus
pre-
coated paper substrate is subsequently super-calendered by passing several
high pressure roller nips and at least one thermoroll applying a surface
temperature of from 100 to 240 C.
Further, the paper substrate has a gas barrier coating 13a made from a
25 barrier dispersion or solution coating of PVOH, Povale 6-98 from
Kuraray,
applied onto the surface of the ductile base layer pre-coating 12a. The gas
barrier coating 13a is thus applied by means of aqueous dispersion coating
and subsequently dried to evaporate the water, preferably as two consecutive
part-coating steps with drying in between and after. The total dry weight of
the
30 PVOH barrier dispersion coating is about 3.5 g/m2. Further and
optionally, the
barrier dispersion-coated paper substrate may have an aluminium barrier
deposition coating 14a, i.e. an alum inium-metallised layer, applied onto the
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dried surface of the barrier dispersion coating 13a, and by physical vapour
deposition to an OD of about 1.8.
Fig. lb shows, in cross-section, a further embodiment of a barrier-
coated cellulose-based substrate 10b, of the invention. The same paper is
used as the cellulose-based substrate as in Fig. la, and is coated with a
first
ductile, base layer pre-coating 12b of the same composition as used in
Fig.la, at a dry weight amount of about 12 g/m2. A further, second, ductile
coating 15b, of the same composition as the ductile base layer pre-coating
12b, is applied in the same manner onto the opposite, uncoated side of the
paper substrate 11b. The dry weight of the second, ductile coating is about 5
g/m2.
In this embodiment, there is no gas barrier dispersion coating applied,
but the first ductile base-layer pre-coating 12b is directly coated with a gas
barrier deposition coating 14b of an alum inium-metallised layer, to an OD of
about 2. The second, ductile coating 15b is, on the other hand, coated with a
gas barrier coating 13b made from a barrier dispersion or solution coating of
PVOH, Poval 6-98 from Kuraray, as described in Fig. 1a. The total dry
weight of the PVOH barrier dispersion coating is about 3.5 g/m2.The resulting
barrier-coated paper substrate thus has one gas barrier coating applied on
each side of the paper, each with a ductile coating beneath it, as a bridging
layer between the surface of the paper substrate and the respective gas
barrier coating, 14b and 13b.
In Fig. 2a, a laminated packaging material 20a for liquid carton
packaging is shown, in which the laminated material comprises a paperboard
bulk layer 21a of paperboard, having a bending force of 80 mN and a
grammage weight of about 200 g/m2, and further comprising an outer liquid
tight and heat sealable layer 22a of low density polyethylene applied on the
outside of the bulk layer 21a, which side is to be directed towards the
outside
of a packaging container produced from the packaging laminate. The layer
222 is transparent to show the printed decor pattern 272, applied onto the
bulk layer of paper or paperboard, to the outside, thus informing about the
contents of the package, the packaging brand and other information targeting
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consumers in retail facilities and food shops. The polyethylene of the outer
layer 22a is a conventional low density polyethylene (LDPE) of a heat
sealable quality, but could also include further similar polymers, including
LLDPEs. It is applied at an amount of about 12 g/m2. An innermost liquid tight
5 and heat sealable layer 23a is arranged on the opposite side of the bulk
layer
21a, which is to be directed towards the inside of a packaging container
produced from the packaging laminate, i.e. the layer 23a will be in direct
contact with the packaged product. The thus innermost heat sealable layer
23a, which is to form strong transversal heat seals of a liquid packaging
container made from the laminated packaging material, comprises one or
more in combination of polyethylenes selected from the groups consisting of
LDPE, linear low density polyethylene (LLDPE), and LLDPE produced by
polymerising an ethylene monomer with a C4-C8, more preferably a C6-C8,
alpha-olefin alkylene monomer in the presence of a metallocene catalyst, i.e.
15 a so called metallocene ¨ LLDPE (m-LLDPE). This innermost layer of
polyethylenes is applied at an amount of about 29 g/m2.
The bulk layer 21a is laminated to the uncoated side (i.e. having no gas
barrier coating applied) of the barrier-coated paper substrate 10a, from Fig.
1a, i.e. 25a, having also an aluminium barrier deposition coating 14a, i.e. an
20 alum inium-metallised layer, applied onto the dried surface of the
barrier
dispersion coating 13a, by physical vapour deposition to an OD of about 1.8,
by an intermediate bonding layer 26a of a low density polyethylene (LDPE).
The intermediate bonding layer 26a is formed by means of melt extruding it
as a thin polymer melt curtain between the two paper webs and thus
25 laminating the bulk layer and the barrier-coated paper substrate to each
other, as all three layers pass through a cooled press roller nip. The amount
applied of the intermediate bonding layer 26a is about 20 g/m2.
The innermost heat sealable layer 23a may consist of one layer or
alternatively of two or more part-layers of the same or different kinds of
LDPE
30 or LLDPE or blends thereof, and is well adhered to the metallised barrier
deposition coating surface 14a of the barrier-coated paper substrate 10a, by
an intermediate coextruded tie layer 24a at an amount of about 6 g/m2, e.g. of
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ethylene acrylic acid copolymer (EAA), which thus bonds the innermost heat
sealable layer(s) to the barrier coated paper substrate 10a, by applying the
layers together in one single melt coextrusion coating step of layers 24a and
23a.
5 In order to reduce the amount of the thermoplastic polymer fraction in
recycling processes, such as extrusion laminated polyethylene polymers, and
for improved repulpability of the packaging material in recycling processes,
the lamination layer 26a, which is bonding the barrier-coated cellulose-based
substrate 25a to the bulk layer 21a, may be a thin layer of a wet laminated
10 polymer binder instead, obtained from drying of a dispersion-coated
aqueous
adhesive composition. Such a lamination step is performed in an efficient cold
or ambient lamination step at industrial speed without any energy-consuming
drying operation needed to accelerate the evaporation of the water. The dry
weight of such a bonding layer would in such an embodiment only need to be
15 about 6 g/m2, or preferably lower, and be made from a polymer which is
readily re-dispersible in water such that it is repulpable into the fraction
of
cellulose fibres in a carton fibre recycling process.
In a further embodiment, the back-side of the paper substrate lla may
first be coated with a second, ductile coating 15a, of the same or a similar
20 composition as the ductile base layer pre-coating 12a, at a dry weight
of
about 5 g/m2, as described in connection to Fig. 1 a, and then the bonding
layer 26a may comprise a similar aqueous adhesive composition as the
ductile base layer pre-coating composition 15a. In a different embodiment, the
paper substrate lla may remain uncoated on the backside, while the amount
25 of such bonding layer 26a may be higher, such as from 10 to about 12
g/m2,
to simultaneously produce one single layer 26a, behaving both as a ductile
base layer 15a and a lamination bonding layer 26a.
Alternatively, a bulk layer 21a may be laminated to the uncoated side
(i.e. having no gas barrier coating applied) of a barrier-coated paper
substrate
30 10a", from Fig. la, however not having the optional aluminium barrier
deposition coating 14a, by the same methods as described above. On the
inside of the barrier-coated barrier substrate there is instead laminated a
pre-
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manufactured polymer film substrate having a barrier deposition coating,
applied by means of a vapour deposition method, such as a metallisation
coating laminated to the barrier-coated cellulose based substrate. The
polymer film substrate may be laminated to the barrier-coated cellulose-based
5 substrate by an interjacent bonding layer of a polymer, either by means
of
extrusion lamination of a thermoplastic bonding layer between the two barrier-
coated webs, or by wet lamination of an aqueous adhesive. The metallised
polymer film substrate may comprise a heat sealale material layer on the side
of the polymer film substrate facing away from the metallisation coating, to
form the second, innermost liquid-tight and heat sealable material layer.
Alternatively, the metallised polymer film substrate may be further extrusion
coated with the second, innermost liquid-tight and heat sealable material
layer
23a. The layers and the materials and polymers are otherwise be the same
as in the laminated packaging material of fig. 2a, and described above.
15 In Fig. 2b, a different laminated packaging material 20b, for liquid
carton packaging, is shown, in which the laminated material comprises a
paperboard core layer 21b, having a bending force of 80 mN and a
grammage weight of about 200 g/m2, and further comprises an outer liquid
tight and heat sealable layer 22b of LDPE applied on the outside of the bulk
20 layer 21b, as described in Fig. 2a. Furthermore, a similar innermost
liquid
tight and heat sealable layer 23b is arranged on the opposite side of the bulk
layer 21b, as described above in Fig. 2a.
The bulk layer 21b is laminated to the barrier-coated paper substrate
described in Fig. 1 b, by means of wet lamination with an intermediate bonding
25 layer 26b of a thin layer of adhesive polymer, obtained by applying an
aqueous dispersion of a polyvinyl acetate adhesive, or a starch adhesive,
onto one of the surfaces to be adhered to each other and subsequently
pressing together in a roller nip. This lamination step is thus performed in
an
efficient cold or ambient lamination step at industrial speed without any
30 energy-consuming drying operation needed to accelerate the evaporation of
the water. The dry amount applied of the intermediate bonding layer 26b is
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from 3 to 5 g/m2 only, which entails that there is no need for drying and
evaporation of the bonding layer.
Thus, the amount of thermoplastic polymer can be significantly reduced
in this lamination layer, in comparison to the conventional melt extrusion
5 laminated bonding layer of LDPE, described in Fig. 2a.
This resulting laminated packaging material 20b thus has a barrier-
coated cellulose based substrate as described in Fig. lb, having a gas barrier
coating applied on each side, with the barrier deposition layer 14b, in this
case the metallised layer, being directed towards the inside and the innermost
10 layer 23b, and the barrier dispersion coating 13b directed towards the
bulk
layer 21b. Both gas barrier coatings 14b and 13b have each a ductile coating
beneath it, acting as a flexible, cushioning layer 12b and 15b, respectively.
In yet further embodiments of either the laminated structure of Figure
2a (not shown), or the laminated structure of Figure 2b (not shown), the
15 innermost liquid-tight layer 23a" or 23b" may consist of a pre-
manufactured,
blown film, comprising LDPE or LLDPE polymers in any blends thereof, and it
may be laminated to the barrier-coated paper substrate, to the surface of its
barrier deposition coating, i.e. the aluminium metallisation, by means of an
intermediate, melt extrusion laminated bonding layer 24a or 24b, comprising a
20 thicker tie layer of EAA than used in Fig. 2a or 2b, or a more simple
bonding
layer of LDPE, which is from 12 to 20 g/m2, such as from 12 to 18 g/m2, thick.
Alternatively, the pre-manufactured blown film 23a or 23b may be
laminated to the metallised coating by means of another wet lamination
bonding layer, of an aqueous adhesive of an acrylic (co)polymer adhesive
25 layer 24a" or 24b", at ambient (cold) temperature, at an amount from 3
to 5
g/m2. As stated above, if the barrier deposition coating 14a is not applied to
the barrier-coated paper substrate, a barrier deposition coating may be
applied to the pre-manufactured film instead, by vapour deposition coating.
In Fig. 3a, an embodiment of a principal process of aqueous dispersion
30 coating 302 is shown, which may be used for applying a gas barrier
coating
12 from an aqueous gas barrier composition onto a substrate, or for applying
a ductile base layer pre-coating from an aqueous latex composition.
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Alternatively, it may be used for applying an aqueous adhesive composition
for wet laminating two webs together, of which at least one web has a fibrous
cellulose surface. The web of cellulose-based substrate 31a (e.g. the paper
11a, llb from Fig. la, 1b) is forwarded to the dispersion coating station 32a,
where the aqueous dispersion composition is applied by means of rollers onto
the top surface of the substrate. The aqueous dispersion composition may
have an aqueous content of from 80 to 99 weight-%, -`)/0, in the case of
barrier
compositions, thus there may be be a lot of water on the wet coated substrate
that needs to be dried by heat, and evaporated off, to form a continuous
coating, which is homogenous and has an even quality with respect to barrier
properties and surface properties, i.e. evenness and wettability. The drying
is
carried out by a hot air dryer 33a, which also allows the moisture to
evaporate
and be removed from the surface of the substrate. The substrate temperature
as it travels through the dryer, may be kept constant at a temperature of
below 100 C, such as below 90 C, such as from 70 to 90 C, in order to
avoid defects in the coating. Drying may be partly assisted by irradiation
heat
from infrared IR-lamps, in combination with hot air convection drying. For the
coating of the ductile base layer pre-coating, however, the aqueous content is
much lower and then also less drying will be needed.
A resulting web of a ductile base layer pre-coated paper substrate 34a
may optionally be calendered by passing through at least one high pressure
roller nip, and is then forwarded to cool off and further wound onto a reel
for
intermediate storage and later further subjected to gas barrier coating
operations. The further coating operations may be vapour deposition coating
of a barrier deposition coating 14, or a further dispersion coating operation
of
a gas barrier composition, as described above, to provide a barrier-coated
cellulose-based substrate.
Fig. 3b shows a process 30b for the final lamination steps in the
manufacturing of the packaging laminate 20a or 20b, of Fig. 2a and 2b,
respectively, after that the bulk layer 21a, 21b first has been laminated to
the
barrier-coated cellulose-based substrate 10a or 10b of Fig. la or Fig. 1 b,
(i.e.
25a or 25b of Fig. 2a and 2b respectively).
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As explained in connection to Fig. 2a and 2b, the bulk layer paperboard
21a;21b may be laminated to the barrier-coated paper substrate 10a;10b;
25a; 25b by means of wet, cold dispersion adhesive lamination, or by means
of melt extrusion lamination. The adhesive may be applied by means of a
same or similar method as described in connection to Fig. 3a, however not
requiring drying, or very little heating.
The resulting paper pre-laminate web 31b is forwarded from an
intermediate storage reel, or directly from the lamination station for
laminating
the paper pre-laminate. The non-laminated side of the bulk layer 21a; 21b,
i.e.
its print side, is joined at a cooled roller nip 33 to a molten polymer
curtain 32
of the LDPE, which is to form the outermost layer 22a; 22b of the laminated
material, the LDPE being extruded from an extruder feedblock and die 32b.
Subsequently, the paper pre-laminated web, now having the outermost layer
22a;22b coated on its printed side, the outside, passes a second extruder
feedblock and die 34b and a lamination nip 35, where a molten polymer
curtain 34 is joined and coated onto the other side of the pre-laminate, i.e.
on
the barrier-coated side of the paper substrate 10; 25a;25b. Thus, the
innermost heat sealable layer(s) 23a are coextrusion coated onto the inner
side of the paper pre-laminate web, to form the finished laminated packaging
material 36, which is finally wound onto a storage reel, not shown.
These two coextrusion steps at lamination roller nips 33 and 35, may
alternatively be performed as two consecutive steps in the opposite order.
According to another embodiment, one or both of the outermost layers
may instead be applied in a pre-lamination station, where the coextrusion
coated layer is first applied to the outside of the (printed) bulk paperboard
layer or onto the metallisation coating of the barrier-coated paper substrate,
and thereafter the two pre-laminated paper webs may be joined to each other,
as described above.
According to a further embodiment, the innermost layers of the heat
sealable and liquid-tight thermoplastic layers are applied in the form of a
pre-
manufactured film, which is laminated to the coated side of the barrier-coated
paper substrate 10.
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As explained in connection to Fig. 2a and 2b, such an innermost layer
23a"; 23b" may be laminated to the barrier-coated paper substrate 10 by
means of wet, cold dispersion adhesive lamination, or by means of melt
extrusion lamination.
5 Fig. 4a is a diagrammatic view of an example of a plant 40a for
physical vapour deposition, PVD, of e.g. an aluminium metal coating, onto a
web substrate of the invention. The dispersion-coated paper substrate 41 is
subjected, on its coated side, to continuous evaporation deposition 40, of
evaporised aluminium, to form a metallised layer of aluminium or,
alternatively
10 to a mixture of oxygen with aluminium vapour, to form a deposited
coating of
aluminium oxide. The coating is provided at a thickness from 5 to 100 nm,
preferably from 10 to 50 nm, to form the barrier-coated paper 43 of the
invention. The aluminium vapour is formed from ion bombardment of an
evaporation source of a solid piece of aluminium 42. For the coating of
15 Aluminium oxide, also some oxygen gas may be injected into the plasma
chamber via inlet ports.
Fig. 4b is a diagrammatic view of an example of a plant 40b for plasma
enhanced chemical vapour deposition coating, PECVD, of e.g. hydrogenated
amorphous diamond-like carbon coatings onto a web substrate of the
20 invention. The web substrate 44a is subjected, on one of its surfaces,
to
continuous PECVD, of a plasma, in a plasma reaction zone 45 created in the
space between magnetron electrodes 46, and a chilled web-transporting
drum 47, which is also acting as an electrode, while the substrate is
forwarded by the rotating drum, through the plasma reaction zone along the
25 circumferential surface of the drum, and subsequently wound onto a
roller as
a barrier-coated web substrate 44b. The plasma for deposition coating of an
amorphous DLC coating layer may for example be created from injecting a
gas precursor composition comprising an organic hydrocarbon gas, such as
acetylene or methane, into the plasma reaction chamber. Other gas barrier
30 coatings may be applied by the same principal PECVD method, such as
silicon oxide coatings, SiOx, then starting from a precursor gas of an
organosilicon compound. The PECVD plasma chamber is kept at vaccum
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conditions by continuously evacuating the chamber at outlet ports 48a and
48b.
Fig. 5a shows an example of a packaging container 50a produced from
a packaging laminate. The packaging container is particularly suitable for
5 beverages, sauces, soups or the like. Typically, such a package has a
volume
of about 100 to 1000 ml. It may be of any configuration, but is preferably
brick-shaped, having longitudinal and transversal seals 51a and 52a,
respectively, and optionally an opening device 53. In another embodiment,
not shown, the packaging container may be shaped as a wedge. In order to
10 obtain such a "wedge-shape", only the bottom part of the package is fold
formed such that the transversal heat seal of the bottom is hidden under the
triangular corner flaps, which are folded and sealed against the bottom of the
package. The top section transversal seal is left unfolded. In this way the
only
partly folded packaging container is still easy to handle and dimensionally
15 stable enough to put on a shelf in the food store or on any flat
surface.
Fig. 5b shows an alternative example of a packaging container 50b
produced from an alternative packaging laminate. The alternative packaging
laminate is thinner by having a thinner paper bulk layer, and thus it is not
dimensionally stable enough to form a parallellepipedic or wedge-shaped
20 packaging container, and is not fold formed after transversal sealing
52b. The
packaging container will remain a pillow-shaped pouch-like container and be
distributed and sold in this form.
Fig. 5c shows a gable top package 50c, which is fold-formed from a
pre-cut sheet or blank, from the laminated packaging material comprising a
25 bulk layer of paperboard and the barrier-coated paper substrate of the
invention. Also flat top packages may be formed from similar blanks of
material.
Fig. 5d shows a bottle-like package 50d, which is a combination of a
sleeve 54 formed from a pre-cut blank of the laminated packaging material,
30 and a top 55, which is formed by injection moulding plastics in combination
with an opening device such as a screw cork or the like. This type of
packages are for example marketed under the trade names of Tetra Tope
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and Tetra Everoe. Those particular packages are formed by attaching the
moulded top 55 with an opening device attached in a closed position, to a
tubular sleeve 54 of the laminated packaging material, sterilizing the thus
formed bottle-top capsule, filling it with the food product and finally fold-
5 forming the bottom of the package and sealing it.
Fig. 6 shows the principle as described in the introduction of the
present application, i.e. a web of packaging material is formed into a tube 61
by overlapping the longitudinal edges 62, 62' of the web and heat sealing
them to one another, to thus form an overlap joint 63. The tube is
continuously filled 64 with the liquid food product to be filled and is
divided
into individual, filled packages by repeated, double transversal seals 65 of
the
tube at a pre-determined distance from one another below the level of the
filled contents in the tube. The packages 66 are separated by cutting between
the double transversal seals (top seal and bottom seal) and are finally shaped
15 into the desired geometric configuration by fold formation along
prepared
crease lines in the material.
Fig. 7 shows the effect of surface roughness on the oxygen
transmission rate of the coated papers listed in Table 2, at 70% RH
(ml/m2.day).
20 Fig. 8 shows the principal structure of a the ductile cellulose-based
substrate A, as pre-coated with a ductile base layer pre-coating B on its top
side, and further, optionally coated with a similar ductile coating C on its
backside.
As a final remark, the invention is not limited by the embodiments
25 shown and described above, but may be varied within the scope of the
claims.
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