Note: Descriptions are shown in the official language in which they were submitted.
i CA 02923974 2016-03-10
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Packaging Material Having a Barrier Coating Based on Starch, and Coating
Mass, Method, and Device for Producing such a Barrier Coating
TECHNICAL FIELD
The invention relates to a packaging material comprising a starch-based
barrier
coating, to a method for producing a packaging material comprising a starch-
based
barrier coating, to a starch-based coating compound, to the use thereof for
surface-
sealing of packaging materials, and to a device for producing the multi-layer
packaging
material according to the invention.
PRIOR ART
In 2009, the Cantonal Laboratory of Zürich pointed out the problem of the
migration of
mineral oils from cardboard packaging to so-called "dry" food in scientific
publications.
This problem arises in particular in the case of packaging made of recycling
cardboard.
The inks from the newspaper production, but also the inks for printing the
cardboard
packaging, were identified as main sources for the mineral oil contamination.
However,
due to the fact that the migration occurs mainly via the gas phase, it cannot
be ruled
out that mineral oil residues from secondary packaging or from adjacent
packaging
reaches into the food when using virgin fiber-based cardboard as well.
119 samples of dry food from various packaging types were examined with regard
to
their mineral oil content (mineral oil saturated hydrocarbons (MOSH)) in a
publication
by A. Vollmer et al. from the year 2011 (Eur Food Res Technol (2011) 232:175-
182).
In addition to these saturated aliphatic and cyclic hydrocarbons (MOSH), the
aromatic
hydrocarbons (mineral oil aromatic hydrocarbons (MOAH)) in the form of
mixtures of
predominantly alkylated aromatic hydrocarbons play an important role as
relevant
contaminants. Food without additional repackaging absorb up to 70% of the MOSH
and a large portion of the MOAH from the packaging via the gas phase
(migration).
The mineral oil contaminants thus far exceed the maximum permitted levels.
According
to this study, additional inner packaging made of paper or polyethylene (PE)
was not
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able to limit the mineral oil migration. However, the authors came to the
conclusion that
inner packaging made of polypropylene (PE), acrylate-coated PE, polyethylene
terephthalate (PET) or films comprising an aluminum coating effectively
prevented the
MOSH migration into the dry food at least for a period of 3 months.
In its Opinion No. 008/2010 of December 9, 2009, the German Federal Institute
for
Risk Assessment (Deutsches Bundesinstitut far Risikobewertung (BfR)) came to
similar conclusions. The elimination of direct contact of recycling paper and
cardboard
with dry food comprising a large specific surface by using inner bags (e.g.
PET films)
comprising a barrier effect are proposed as short-term courses of action in
the
publication, which can be obtained under
http://www.bfr.bund.de/defiragen und antworten zu mineraloel ueberoaenoen aus
verpackungsmaterialien auf lebensmittel-50470.html.
Such additional inner packaging, however, are not desired for economic reasons
on
the one hand, because it increases the price of the packaging, and, on the
other hand,
because it is non-environmental, because it increases the volume of waste.
The paper industry thus attempts to use further approaches to deal with the
problem of
the mineral oil contamination in the packaging sector. On the one hand,
attempts are
made to eliminate the sources for the contamination, in that fresh cardboard
material
and mineral oil-free inks are used. The contaminations can be prevented in
this way.
Foregoing the use of recycling paper, however, is extremely disadvantageous
from an
environmental and economic aspect.
Cleaning the old paper prior to producing the cardboard would also be a
suitable way
to prevent the mineral oil contamination of the food. To adhere to the
migration limits, it
will most likely be necessary to remove up to 99% of the mineral oil from the
old paper.
However, the suitable large-scale methods for this still do not exist and it
is furthermore
already foreseeable that this is an extremely cost- and resource-intensive
approach.
Further approaches lie in the use of multi-layer laminates and in the
application of
barrier layers to the inside of cardboard packaging, so as to eliminate the
migration of
the mineral oil contamination via the gas phase. It gives the manufacturers of
paper
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and cardboard packaging, in particular of cartonboard and corrugated cardboard
liners
the opportunity to quickly react to the existing demand for new packaging
solutions,
without having to forego the use of recycling paper, which is extremely
worthwhile
ecologically.
It is proposed in W013076241A2 to use aqueous polyvinyl acetate-based
dispersions
to produce a coating on film-shaped substrates to reduce the diffusion of oil-
containing
substances from packaging in food and medical products.
However, the polyvinyl acetate-based barrier layer is not soluble in water and
thus
interferes with the recycling of a cardboard, which is treated in this manner,
in recycling
processes, as they are currently common. In addition, the use of a synthetic
coating,
which has a poor biodegradability, tarnishes the look of the cardboard
packaging as a
packaging solution, which is extremely worthwhile ecologically. Due to the
fact that
such coatings are also comparatively expensive, there is still a need for cost-
efficient
ecologically worthwhile coating means comprising a good barrier effect.
Starch-based coatings are also known in the paper industry. In the coating
industry, for
example, they are used in coating colors to provide an increased stability to
cardboard,
or to improve the paper characteristics, such as, e.g., to reduce dust and to
smooth
surfaces for a better printability.
When being cooked in solution, starch already develops a high viscosity in the
case of
small portions of solids. For example, a 10% starch solution can have a
viscosity of
>5,000 mPas at 40 C and is thus unusable in common coating processes. On
principle, partially degraded starches comprising average molecular weights of
Mw<<1,000,000 g/mol are used so as to keep the viscosity of the starch
solution and
thus of the coating compound within manageable limits (for example a 50%
starch
solution comprising a viscosity of <3,000 mPas at 40 C), in the case of higher
portions
of solids.
Starch coatings from such degraded or partially degraded starches are
relatively brittle
and break easily in response to the production of the cardboard packaging.
Such
coatings do not withstand in particular the mechanical stresses, in response
to folding
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and creasing of the cardboard packaging, so that undesired surface defects are
created in the coating, through which, in turn, mineral oil residues can
migrate into the
packaged food. Such compositions are thus unusable as barrier coating, because
the
coating must be flexible, so as to be capable of being creased and folded, for
example
when processing the coated packaging material to form folding boxes.
Methods for coating paper and cardboard are generally known to the person of
skill in
the art. A generally common device for coating, in particular paper and
cardboard, as it
corresponds to the prior art, is illustrated schematically in Fig. 1.
Significant
components of such a device for coating a web 2 are the coating aggregate 4
and the
adjacent drying device, on principle consisting of an IR radiant heater 8 and
a hot air
drying system 9. The web is thereby guided by so-called guide rollers 3. The
web can
be unrolled from a roll 1 and can be rolled up on such a roll 1' again after
the drying.
However, this is not absolutely necessary: the coating can also represent an
intermediate step in a multi-step continuous process, for example.
The coating compound can be applied to the substrate surface, e.g., by means
of
applying by doctor, blade coating, casting, rolling, spraying, printing or
other methods,
which are suitable to apply liquid compounds. A common embodiment of a coating
aggregate 4 is illustrated in an exemplary manner in Figure 1. The coating
compound
is applied to the paper via an application roller 5, which runs through a so-
called
coating sump 6. The excess coating compound is wiped off by means of a doctor
or a
blade 7. The coated web 2' is guided across the dryer 8, 9 and is thereby
dried in such
a manner that it can be formed into a paper roll 1' again in a roll-up device,
without
sticking together.
The demands on the mechanical characteristics and on the barrier effect of the
coating
thus conflict with the demands by the coating process. At present, this
problem has not
yet been solved satisfactorily.
It is thus an object of the invention to provide improved compositions for the
coating of
packaging materials, which can be used in an economically worthwhile manner,
which
can be applied to planar/extensive/flat/sheet substrates in an efficient and
cost-
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effective manner, and which thereby simultaneously have an excellent barrier
characteristic.
It is furthermore an object of the present invention to provide for an
improved method
for coating paper and a device, which is suitable for doing so.
5 DESCRIPTION OF THE INVENTION
These objects are solved by means of the features of the independent claims.
Preferred embodiments are reflected in the dependent claims.
According to a first aspect, the present invention relates to a multi-layer
packaging
material, comprising a planar/extensive/flat/sheet substrate and at least one
starch-
based barrier layer applied thereto.
The invention furthermore relates to a coating compound, which is suitable to
produce
the packaging material according to the invention, and which is preferably
used for the
production of the packaging material.
A further aspect of the invention relates to a method for producing this
packaging
material as well as to a device, which is suitable to carry out this method.
The invention further relates to the use of a starch-based coating compound
for
surface-sealing packaging materials, to prevent or to reduce the migration of
lipophilic
contaminants, which are contained in the packaging material, for example, into
the
packaged good. The coating compounds according to the invention are preferably
used for this.
Lipophilic contaminants in terms of the present invention comprise mineral oil
contaminants, which originate from residues of printing inks in recycled
paper, for
example. The contaminants comprise linear, cycled and aromatic hydrocarbons,
in
particular MOSH and/or MOAH.
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The method for producing a packaging material comprising a starch-based
barrier
coating or barrier layer, respectively, differs significantly from known
methods in that a
coating compound comprising a highly-molecular starch in the form of a
suspension of
starch particles is applied to a substrate, which is to be coated, and that
the
suspension on the substrate is gelatinized "in situ" on the substrate with the
help of hot
steam, preferably of water vapor, which can include further integral parts, if
applicable,
and is thus solidified to form a barrier coating. In a preferred embodiment,
this has the
result that, in the finished state, the coating, which is located on the
substrate, has
particles of more or less highly destructured starch granules, which are
connected/bonded to one another, and thus clearly differs from starch
coatings, which
are produced from dissolved starch.
As natural polymer from re-growing raw material sources, starch has the
crucial
advantage that the price is low and that the environmental balance is positive
as
compared to the synthetic products. A further advantage is that starch has
already
established itself in the paper industry and does not represent an
interference factor in
the existing recycling processes.
According to the present invention, a coating compound in the form of a low-
viscous
suspension of granular starch comprising long-chained or highly molecular
starch
molecules, respectively, is used, which can be applied efficiently with a high
solids
content to a suitable substrate or to a carrier layer, respectively, of a
packaging
material, preferably to paper or cardboard, and which is subsequently
gelatinized and
formed into a film by means of heat, preferably by means of steam, more
preferably
water vapor. The application of the starch compound in the form of a
suspension
thereby makes it possible to combine a high solids content in the application
compound with a relatively low viscosity, as it is required for coating, while
simultaneously using starch types, which have preferably not been degraded,
thus
long-chained starch types. Surprisingly, it turned out that long-chained
starch types, in
particular in the form of a suspension, can be used in a worthwhile manner for
coating
and that the use of these long-chained starch types in the barrier coating
furthermore
leads to significantly improved mechanical characteristics of the coating. The
average
molecular weight K, of the starches used according to the invention should be
at least
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500,000g/mol, preferably at least 1,000,000g/mol. The abbreviation Mw in the
present
patent application thereby always refers to the weight average of the
molecular weight.
The coating compound according to the invention represents a suspension, the
solids
portion of which includes starch as main component. It is in accordance with
the
invention that these suspended starch particles are agglutinated, that is,
gelatinized, to
form a cohesive barrier coating, which significantly limits or completely
prevents the
migration of lipophilic contaminants, in particular of MOSH (Mineral Oil
Saturated
Hydrocarbons) and MOAH (Mineral Oil Aromatic Hydrocarbons), into the packaged
product by means of the external impact of heat and moisture immediately after
applying to the substrate ("in situ"). The barrier effect is crucially
dependent on the fact
that a barrier coating, which is virtually free from surface defects (e.g.
pinholes), is
obtained across the entire surface, and that the barrier coating is flexible
and not
sticky, so that the paper can be processed by means of the creasing and
folding
techniques, which are common in the packaging industry, without the barrier
coating
being damaged or impacted in a noteworthy manner thereby. This is the only way
to
ensure that the packaged food is protected sufficiently against the mineral
components
passing through, even in the case of longer storage periods.
Starch is the main component of the coating compound according to the
invention. In
terms of the invention, starch, the average molecular weight Mw of which is at
least
500,000g/mol, preferably at least 1,000,000g/mol and more preferably at least
2,000,000g/mol (determined via GPC-MALLS), is used in the case of at least one
coating process, thus in the case of at least one layer of the barrier coating
or barrier
layer, respectively. The average molecular weights are determined by means of
GPC-
MALLS (gel permeations chromatography by means of multi-angle laser light
scattering) after pressure cooking the starch at 150 C in a closed autoclave.
The "in
situ" gelatinization according to the invention in combination with the use of
aqueous
coating compounds comprising a high content of highly-molecular starches
allows the
use of coating compounds, which can be processed well, which have a low
viscosity,
and which have a relatively low water content, so that the water quantity
applied to
reach the desired layer thicknesses/starch quantities can be dried off
subsequently by
means of the systems, which are available and which are common in the paper
industry, without having to make large investments for more powerful drying
sections.
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Further advantages, which are associated with the reduction of the water
quantity,
which is applied to the paper substrate to be coated, will be explained below.
It is an
enormous advantage not only with regard to the additional system costs, when
the
quantity of water, which must be dried off, can be kept as low as possible.
The lower the molecular weight, the more brittle the starch becomes, even in
the case
of higher plasticizer contents. On the other hand, the viscosity of starch in
solution
increases massively with the molecular weight of the starch. The viscosity in
solution is
dominated primarily by the largest starch macromolecules. As a result, a 10%
solution
of desired large starch macromolecules can already result in a viscosity of
>5,000mPas. However, a lower viscosity is desired for applying the starch
compound
to the paper. Solutions comprising a water content of at least 90% would be
necessary
to apply a sufficient quantity of suitable high-molecular starch
macromolecules to the
paper. To obtain the same layer thickness with a 10% solution as with a 50%
solution,
roughly 10-times the amount of water is required. Water, which must dried off
at high
costs. To obtain a 50% pourable starch solution, the starch must be degraded
very
strongly and the mechanical characteristics thereof are then far different
than the
required characteristics.
The approach according to the invention of not applying the starch to a
packaging
material, which is to be coated, for example a paper, as a solution, but to
use at least a
part of the starch in the aqueous coating compound in the form of non-
gelatinized,
suspended starch granules, provides a way out of this situation. When using
such
granular starch, however, it must be ensured that the starch is at least
partially
released from the granules on the substrate, that is, that the starch is
gelatinized, so
that the large starch macromolecules contained therein can take effect. To be
able to
gelatinize the starch granules, sufficient water must be present, and the
granules must
be heated up to the gelatinizing temperature. Water thereby diffuses into the
granules
and a phase conversion in the starch granule then occurs under the influence
of the
water, whereby the partially crystalline structure converts into an amorphous
structure.
Due to the fact that the starch granules absorb a lot of water hereby, even a
highly
fluid compound solidifies thereby. In response to a planar application, a film
comprising
a certain stability is obtained. The suitable barrier film results from
subsequent drying.
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Native starches are present in the form of starch granules. These granules are
birefringent under the polarization microscope. The person of skill in the art
is well
aware that starch granules can be gelatinized, for example in aqueous
solutions. In
response to gelatinizing, the starch granules absorb water and thereby swell
considerably. Gelatinized starch granules also burst and disintegrate easily,
for
example under shear, into fragments, which can then also dissolve completely,
until a
genuine molecular solution is created. The transition from non-swollen starch
via
gelatinized starch granules to dissolved starch can be subdivided into the
following
stages:
Stage 1: the crystallinity of the starch is at most partially destroyed, under
the
polarization microscope
stage 1.1: maximally 5% of the granules are no longer birefringent
stage 1.2: 5 - 10% of the granules are no longer birefringent
stage 1.3: 10 - 20% of the granules are no longer birefringent
stage 1.4: 20 - 30% of the granules are no longer birefringent
stage 1.5: 30 - 40% of the granules are no longer birefringent
Stage 2: the crystallinity of the starch is substantially destroyed, under the
polarization
microscope
stage 2.1: 40 ¨ 50% of the granules are no longer birefringent
stage 2.2: 50 - 60% of the granules are no longer birefringent
stage 2.3: 60 - 80% of the granules are no longer birefringent
stage 2.4: 80 - 100% of the granules are no longer birefringent
Stage 3: maximally 5% of the granules are birefringent
stage 3.1: and 1 ¨ 10% of the granules have burst
stage 3.2: and 10 ¨ 20% of the granules have burst
stage 3.3: and 20 ¨ 30% of the granules have burst
stage 3.4: and 30 ¨ 50% of the granules have burst
stage 3.5: and 50 ¨ 70% of the granules have burst
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stage 3.6: and 70 ¨ 100% of the granules have burst
Burst starch granules are characterized in that the starch granules have tears
on the
surface and/or that the previously relatively smooth surface has been deformed
5 significantly (e.g. wrinkled surface). In addition to starch particles,
which are still
present as whole granules, starch particles, which have disintegrated into
fragments,
can also be present. However, the starch granules as well as the fragments can
still be
recognized as entities.
Stage 4: no birefringence whatsoever is observed, the starch granules are
10 substantially destroyed.
Stage 4.1: fragments of starch granules are still present, the majority of the
starch is
present in dissolved form
Stage 4.2: the starch is present in completely dissolved form
In the context with the present patent application, a granular starch is
identified as a
starch, which is destructured to stage 1.5 at the most. Preferably, the starch
is
destructured up to stage 1.4 at most, more preferably up to stage 1.3, more
preferably
up to stage 1.2, and most preferably up to stage 1.1 at most. Starches
comprising one
of these destructuring stages can also be identified as non-gelatinized
starches.
In terms of the present invention, starches of destructuring stages 2 and 3
are
considered to be gelatinized starch or gelatinized starch particles.
Gelatinized starches
thus have a destructuring degree in the range of at least stage 2.1 to stage
3.6 at the
most. In ascending order, stages 2.2, 2.3 and 2.4 in each case also represent
preferred lower limits. In descending order, stages 3.5, 3.4, 3.3, 3.2 and 3.1
in each
case also represent preferred upper limits.
A dissolved starch is destructured to at least stage 4.1. According to a
particularly
preferred embodiment of the present invention, the dissolved starch is present
as
genuine molecular solution.
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The destructuring degree of the starch particles can be determined easily
under the
polarization microscope, for example with a 200-fold magnification, by means
of the
above graduations.
Unless specified otherwise, it applies for the present patent application that
all of the
plasticizer portions are based on the sum of starch plus plasticizer. The
portion of the
thickening agent(s) is based on the sum of starch plus thickening agent. The
portion of
the additive(s) is based on the dry mixture or composition, respectively.
Unless
specified otherwise, the information % by weight always refers to proportion
by weight
per proportion by weight.
Coating compound
After subtracting an optionally comprised filler component, the aqueous
coating
compound according to the invention comprises the following portions of
components:
a) 10 ¨ 75% by weight of granular starch,
b) optionally < 50% by weight of dissolved starch,
C) 0 ¨ 70% by weight of plasticizer, wherein the plasticizer portion is
based on
starch plus plasticizer,
d) 25 ¨ 90% by weight of water,
wherein this coating compound ¨ as specified in more detail below ¨ has a
viscosity in
the range of 50 ¨ 5,000mPas at 40 C, measured by means of a Brookfield
viscometer
at a rotational speed of 10Orpm. The starch follows as sum from granular and
dissolved starch. In the context of the present invention, the proportions by
weight in
each case refer to the dry substances and not to the substances in their
commercially
available form, in which they include a certain water content as moisture.
Water
content in each case refers to all of the water, that is, the supplied water
plus the
water, which is present in the substances as moisture. The portions of the
components
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of the disclosed compositions, with the exception of the filler(s) are always
chosen in
such a manner that the sum results in 100% by weight.
In accordance with an embodiment according to the invention, the coating
compound
consists of the components a) to d) comprising the above-mentioned portions
and one
or a plurality of the thickening agents defined below comprising the specified
portions.
In accordance with a further embodiment according to the invention, the
coating
compound consists of the components a) to d).
The coating compound can be used directly to coat the substrate.
Viscosity and pH-value of the coating compounds
The lower limit of the viscosity of the coating compounds according to the
invention in
mPas, measured at 40 C by means of a Brookfield viscometer at a rotational
speed of
100 rpm, is 50, preferably 70, more preferably 100, most preferably 150.
The upper limit of the viscosity in mPas is 5,000, preferably 3,000, more
preferably
2,500, more preferably 2,200, more preferably 2,000, more preferably 1,800,
more
preferably 1,600, most preferably 1,500.
If the viscosity is too low, an application weight, which is too low, and thus
a barrier
coating, which is too thin, is obtained when using a starch-based coating
compound. If
the viscosity is too high, an application weight, which is too high, is
obtained, or a
controlled, even application is impacted disproportionately when using a
starch-based
coating compound.
The pH-value of the coating compound according to the invention is preferably
> 4,
preferably > 5, preferably > 6, more preferably > 6.5, most preferably > 6,7
and, on the
other hand, preferably < 10, preferably < 9, preferably < 8.5.
According to a further aspect, the present invention relates to the use of the
above-
explained coating compound for the surface sealing of packaging materials, to
prevent
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or to reduce the migration of lipophilic contaminants, such as mineral oil
residues, in
particular MOSH and/or MOAH contained in the packaging material.
In particular, the coating compound according to the invention is used for
surface
sealing the inner sides of packaging.
Barrier layer
After subtracting an optionally comprised filler component (which will be
defined in
more detail below), the barrier coating or barrier layer, respectively,
according to the
invention comprises the following components:
a) 30 ¨ 100% by weight of starch,
b) 0 ¨ 70% by weight of plasticizer, wherein the plasticizer portion is
based on
starch plus plasticizer,
wherein
c) > 20% by weight of the starch has a molar mass NI, of >
1,000,000g/mol,
and
d) > 20% by weight of the starch is present in the barrier coating in the
form of
gelatinized starch particles, and
e) the barrier coating has a surface weight of 5 ¨ 80 g/m2.
Gelatinized starch particles
In the barrier coating, which is applied to a substrate, >20% by weight,
preferably
>30% by weight, more preferably >50% by weight, more preferably > 70% by
weight,
more preferably > 90% by weight, more preferably > 95% by weight, most
preferably >
99% by weight of the starch is present in the form of gelatinized starch
particles. The
gelatinized starch particles contribute to the advantageous mechanical
characteristics,
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such as the flexibility of the coating, in particular if macromolecules, which
move easily
and which are chosen from the thickening agents, are located between the
particles.
The gelatinized starch particles of the barrier layer follow from the granular
starch,
which is used in the coating compound. In other words, the gelatinized starch
or the
gelatinized starch particles, respectively, of the barrier layer have a higher
destructuring degree than the granular starch, which is used in the coating
compound.
A starch, in the case of which at least 40%, preferably at least 50%, more
preferably at
least 60%, more preferably at least 80% of the starch granules are no longer
birefringent, are/is identified as gelatinized starch particles or gelatinized
starch.
Gelatinized starch particles or gelatinized starch also comprises starch in a
state, in
which maximally 5% of the starch granules are still birefringent, and wherein
at least
1%, preferably at least 10%, more preferably at least 20%, more preferably at
least
30%, more preferably at least 50%, more preferably at least 70% of the starch
granules have burst.
Molecular weight of the starch
To provide for the required mechanical characteristics of the barrier coating,
it is first
and foremost the fraction of the largest macromolecules, which is relevant. >
20% by
weight, preferably > 30% by weight, more preferably > 40% by weight, more
preferably
> 50% by weight, most preferably > 60% by weight of the starch of the barrier
coating
are to thus have a molar mass IM, of > 1,000,000g/mol.
In a preferred embodiment > 20% by weight, preferably > 30% by weight, more
preferably > 40% by weight, more preferably > 50% by weight, most preferably >
60%
by weight of the starch of the barrier coating are to have a molar mass KA, of
>
2,000,000g/mol.
In a further preferred embodiment, > 20% by weight, preferably > 30% by
weight, more
preferably > 40% by weight, more preferably > 50% by weight, most preferably >
60%
by weight of the starch of the barrier coating are to have a molar mass M,,,,
of >
3, 000, 000g/mol.
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In a further preferred embodiment, > 20% by weight, preferably > 30% by
weight, more
preferably > 40% by weight, more preferably > 50% by weight, most preferably >
60%
by weight of the starch of the barrier coating are to have a molar mass Mw of
>
5,000,000g/mol.
5 In a further preferred embodiment, > 20% by weight, preferably > 30% by
weight, more
preferably > 40% by weight, more preferably > 50% by weight, most preferably >
60%
by weight of the starch of the barrier coating are to have a molar mass Mw of
>
10,000,000g/mol.
As already mentioned above, the average molecular weight Mw of the used
starches is
10 at least 500,000g/mol, preferably at least 1,000,000g/mol. It turned out
to be
advantageous, when this weight average of the molecular weight distribution Mw
of the
starch of the barrier coating in g/mol is > 500,000, preferably > 1,000,000,
more
preferably > 2,000,000, more preferably > 2,500,000, more preferably >
3,000,000,
more preferably > 4,000,000, more preferably > 5,000,000, more preferably >
15 7,000,000, particularly preferably > 10,000,000.
On principle, the upper limit of the average molecular weight Mw of the used
starches,
in particular of the granular starch, is not limited, except for by the
natural conditions.
Preferably, the upper limit is 50 million g/mol.
Due to the fact that in the case of the production methods according to the
present
invention, the used starch molecules can virtually not be degraded with regard
to the
molecular weight thereof, the above-mentioned information relating to the
molecular
weights of the starch in the barrier coatings can also be transferred to the
coating
compounds. Here, the information relates in particular to the used granular
starch,
which can be found as gelatinized starch particles in the coating.
Surface weight
The barrier effect of the barrier coating increases as the surface weight
increases. The
surface of papers, pasteboard and cardboard is not really smooth, but has a
more or
less pronounced roughness ¨ a type of mountain and valley landscape. To obtain
a
CA 02923974 2016-03-10
16
good barrier effect, the valleys must initially be filled and the layer should
then also still
cover the highest mountain peaks. Due to the fact that rougher papers are also
used in
the packaging industry for price reasons, and due to the fact that a good
barrier layer
is to also be obtained for these papers, it is necessary that thicker barrier
coatings can
be applied. A barrier effect, which is as good as possible, and rougher papers
thus
require a larger application quantity or a larger surface weight,
respectively. However,
the material and process costs are also increased with this, in particular
because thick
layers can only be obtained by repeatedly applying coating compounds, wherein
the
applied layer must be dried in-between in each case. In addition, the
flexibility of the
barrier coating is reduced as the thickness increases.
The upper limit of the surface weight (dry compound) of the barrier in g/m2 is
thus
preferably 80, preferably 70, more preferably 60, more preferably 55, more
preferably
50, more preferably 45, more preferably 40, more preferably 35, more
preferably 30,
most preferably 25.
On the other hand, the lower limit for the surface weight of the barrier in
g/m2 is 3,
preferably 5, more preferably 7, more preferably 9, most preferably 10.
Granular starch
A starch, in the case of which at least 60%, preferably at least 70%, more
preferably at
least 80%, more preferably at least 90%, more preferably at least 95%, most
preferably approximately 100% of the starch granules are birefringent, is
identified as
granular starch. In response to the gelatinization, the starch granules lose
their
birefringent character, which is due to the partially crystalline structure of
the starch
granules. Birefringent is thus synonymous with non-gelatinized. Suspended in
water,
birefringent starch granules only absorb water to a limited extent, hardly
swell, and act
like solid particles. They create only little viscosity. The starch granules
absorb water
only after being heated to the gelatinizing temperature and swell strongly,
and then
create a highly distinct viscosity.
The upper limit for the portion of the granular starch in the coating compound
in % by
weight, after deducting an optionally comprised filler component, is 75 at the
most,
CA 02923974 2016-03-10
. ,
17
preferably 70 at the most, preferably 65 at the most, more preferably 60 at
the most,
more preferably 56 at the most, more preferably 53 at the most, more
preferably 49 at
the most, most preferably 47 at the most.
After deducting an optionally comprised filler component, the lower limit for
the portion
of the granular starch in the coating compound in % by weight is at least 10,
preferably
at least 15, preferably at least 20, more preferably at least 25, more
preferably at least
28, more preferably at least 31, most preferably at least 34.
The preferred weight average of the molecular weight distribution Mw of the
granular
starch in g/mol is > 500,000, preferably > 1,000,000, preferably > 2,000,000,
preferably
> 2,500,000, preferably > 3,000,000, preferably > 4,000,000, preferably >
5,000,000,
preferably > 7,000,000, particularly preferably > 10,000,000.
With regard to the origin and the preparation, any granular starches or
mixtures
thereof can on principle be used as granular starch. For example, they can be
used in
the native state, as well as in the physical and/or chemically/enzymatically
modified
state.
With regard to the origin, root starches, such as, for example, potato
starches or
tapioca starches are preferred, because, as compared to starches of a
different origin,
they have low gelatinizing temperatures, and because the solidification or
gelification,
respectively, of the coating compound to a barrier coating is thus already
possible at
low temperatures. Tapioca starch is particularly preferred. Tapioca starch is
colorless
and flavorless and genetically modified alternatives of tapioca starches are
not yet
known. Pea starch is further preferred, because it turned out that it has
particularly
good film-forming characteristics.
In a preferred embodiment, the granular starch is used in the native, that is,
in the non-
modified state. Usable characteristics can be obtained herewith at low costs.
In a further preferred embodiment, substituted granular starches, such as
starch esters
and starch ethers, are used, such as, for example, hydroxpropylated or
acetylated
starches. These modifications lead to a particularly high expansibility of the
barrier
CA 02923974 2016-03-10
18
coating, which is an important advantage in response to the creasing and
folding of the
barrier coating. In the alternative, oxidized starches are used.
Hydroxpropylated
starches are particularly preferred.
In a further preferred embodiment, cross-linked granular starches are used, in
particular cross-linked starch esters or cross-linked starch ethers,
respectively, for
example starch phosphates and starch adipates. Preferably, the cross-linking
is
pronounced lightly. Such starches are commercially available. By increasing
the
molecular weight, which is associated with the cross-linking, improved
mechanical
characteristics are obtained. Cross-linked hydroxypropylated starches, in
particular
lightly cross-linked hydroxypropylated starches are particularly preferred. In
terms of a
simpler processing, non-cross-linked hydroxypropylated starches are preferred.
According to a further embodiment of the present invention, the granular
starch is a
non-cross-linked starch or a mixture of non-cross-linked starches.
In a preferred embodiment, substituted granular tapioca starch is used, in
particular
hydroxypropylated tapioca starch. Preferably, cross-linked substituted tapioca
starch,
such as, for example, hydroxypropylated starch phosphate.
In a further preferred embodiment, substituted granular pea starch is used, in
particular
hydroxypropylated pea starch. Preferably, cross-linked substituted pea starch,
such as,
for example, hydroxypropylated starch phosphate
The amylose content of the granular starch(es) in % by weight is preferably <
60, more
preferably < 50, more preferably < 40, more preferably < 37, more preferably <
35. It
turned out that high amylose contents can lead to a reduced expansibility of
the barrier
coating.
The amylose content of the granular starch(es) in % by weight is preferably >=
0, more
preferably > 0.5, more preferably > 0.7, more preferably > 1, more preferably
> 2.5,
most preferably > 5. Amylose contents, which are too low, can lead to a
reduced
expansibility of the barrier coating.
CA 02923974 2016-03-10
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According to a preferred embodiment, so-called "waxy" starches are not used in
the
coatings and the coating compounds according to the invention.
Granular starches comprising a dextrose equivalent (DE) of < 3, more
preferably < 1,
most preferably < 0.7, more preferably < 0.5, more preferably < 0.2, more
preferably <
0.1, particularly preferably < 0.05 are further preferred. The dextrose
equivalent of a
polysaccharide mixture identifies the percentage of reducing sugar in the dry
substance. It corresponds to the quantity of glucose (= dextrose), which had
the same
reducing power per 100 g of dry substance. The DE value is a measure for how
far the
polymer degradation has occurred. In the case of high DE values, inferior
mechanical
characteristics are obtained. The dextrose equivalent is determined according
to the
ISO standard 5377.
According to preferred embodiments of the invention, granular starches are
used,
which are approved for applications for contact with food.
Dissolved starch
The coating compound can optionally also comprise dissolved starch. In the
case of
aqueous systems, the dissolved form of starch is typically obtained by means
of the
following measures, which are well-known to the person of skill in the art:
cooking the
starch, in particular by means of a jet cooker, heating to a temperature above
the
gelatinizing temperature, dissolving extruded, amorphous starch, using pre-
gelatinized
starch. As defined above, dissolved starch can be present in the form of a
genuine
molecular solution, but fragments of destructured starch granules can also be
present.
Gelatinized starch granules disintegrate easily into fragments under shear and
can
then dissolve completely. The gelatinized starch granules also disintegrate
into
fragments by means of cooking them for a longer period of time, and a genuine
molecular solution is finally created. A molecular solution is obtained very
quickly by
means of the jet cooker.
Dissolved starch can be used just as the thickening agents mentioned below, to
increase the viscosity of the coating compound and to modify the
characteristics of the
barrier coating. According to the present invention, the use thereof is
optional.
CA 02923974 2016-03-10
With regard to suitable starch and preferred types, substantially the same
statements
as were made with regard to the granular starch, apply for the dissolved
starches.
Limitations relating to the molecular weight are exceptions. Dissolved starch
can also
have a smaller molecular weight than granular starch. According to an
embodiment,
5 short-chained starch comprising a molecular weight NAN of less than
500,000 g/mol can
also be used as dissolved starch.
After deducting an optionally comprised filler component, the portion of the
dissolved
starch in the coating compound in % by weight is preferably < 50, more
preferably <
40, more preferably < 30, more preferably < 20, more preferably < 15, more
preferably
10 <10, more preferably < 7, most preferably < 5.
According to one embodiment, the lower limit of the portion of the dissolved
starch in
the coating compound in % by weight, after deducting an optionally comprised
filler
component, is 0. The lower limit can also be 0.5% by weight, more preferably
1% by
weight, more preferably 2% by weight, most preferably 3% by weight, in each
case
15 after deducting an optionally comprised filter component.
Water content of the coating compound
Water is important for adjusting the viscosity of the coating compound and for
the
gelatinization after converting the coating compound into a barrier coating.
The higher
the water content of the coating compound, the lower the viscosity thereof,
and the
20 lower the required gelatinizing temperature. On the other hand, a high
water content
makes drying more difficult, because more water must then be removed from the
barrier coating.
The upper limit for the water content of the coating compound in % by weight,
after
deducting an optionally comprised filler component, is 90, preferably 85, more
preferably 80, more preferably 75, more preferably 72, more preferably 69,
most
preferably 66.
The lower limit for the water content of the coating compound in % by weight,
after
deducting an optionally comprised filler component, is 25, preferably 30, more
CA 02923974 2016-03-10
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preferably 35, more preferably 40, more preferably 44, more preferably 47,
more
preferably 51, most preferably 53.
A portion of the water of the coating compound can be bound in the solidified
barrier
layer. The remaining water is lost in response to drying the packaging
material as well
as in response to the subsequent storing. The maximum water content of the
barrier
layer in the finished packaging material directly after the production is
maximally 25%
by weight, preferably maximally 20% by weight, more preferably maximally 15%
by
weight, more preferably maximally 10% by weight, more preferably maximally 7%
by
weight, more preferably maximally 5% by weight, more preferably maximally 3%
by
weight.
Solids content of the coating compound
In response to the coating of paper, the person of skill in the art tends to
refer to solids
content instead of to water content, wherein it goes without saying that the
two
characteristics are directly interlinked. The solids content defines the sum
of all of the
components, including possible fillers, except for water.
The solids content of a sample can be determined by means of simply
considering the
difference of the sample prior to and after drying. For this purpose, the
samples are
dried, for example, in a standard drying cabinet for 10-60 minutes at 130 C.
To
determine the solids content of the coating compound on the substrate, thus in
the
packaging material, a piece of coated substrate is used as sample.
The lower limit for the solids content of the coating compound in 'Yo by
weight, after
deducting an optionally comprised filler component, is 10, preferably 15, more
preferably 20, more preferably 25, more preferably 28, most preferably 34.
The upper limit for the solids content of the coating compound in % by weight,
after
deducting an optionally comprised filler component, is 75, preferably 70, more
preferably 65, more preferably 60, more preferably 56, more preferably 53,
more
preferably 49, most preferably 47.
CA 02923974 2016-03-10
22
Plasticizer
On principle, all of the plasticizers listed in the prior art for starch as
well as any
mixtures thereof are possible as plasticizer. A small plasticizer content
leads to a
brittleness of the barrier layer in the case of low humidity, while a high
plasticizer
content leads to adhesives and to a soft material of little expansion in the
case of high
humidity.
Plasticizers can be used individually or in mixtures of different
plasticizers. Preferably,
polyols are used, such as, for example, glycerin, sorbitol, maltitol,
erythritol, xylitol,
mannitol, galactitol, tagatose, lactitol, maltulose, isomalt, maltol etc., but
also various
sugars, such as saccharose/sucrose, maltose, trehalose, lactose, lactulose,
galactose,
fructose etc., as well as mono- and oligosaccharide. Glycerin is particularly
preferred
as plasticizer. Water is also a plasticizer for starch, but is not counted as
plasticizer
here and will be considered separately.
The upper limit for the plasticizer content of the coating compound in % by
weight,
based on starch (granular starch plus dissolved starch) plus plasticizer, is
70,
preferably 60, more preferably 55, more preferably 50, more preferably 46,
most
preferably 42.
In one embodiment, the lower limit for the plasticizer content of the coating
compound
in % by weight, based on starch (granular starch plus dissolved starch) plus
plasticizer,
is 0. In such embodiments, the coating compound according to the invention,
except
for unavoidable contaminants, thus does not comprise a plasticizer. According
to
preferred embodiments, the lower limit in % by weight is 5, preferably 10,
more
preferably 15, more preferably 20, more preferably 25, more preferably 28,
more
preferably 31, more preferably 32.5, most preferably 33.5, in each case based
on
starch (granular starch plus dissolved starch) plus plasticizer.
The limits for the plasticizer content of the barrier coating correspond to
the limits for
the plasticizer content of the coating compound.
CA 02923974 2016-03-10
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23
In preferred embodiments, plasticizers comprising a maximum melting
temperature of
150 C (for the anhydrous plasticizer), preferably 125 C, particularly
preferably 110 C,
more preferably 95 C, most preferably 70 C are used. The portion of these
plasticizers
in the total plasticizer content in % by weight is > 50, preferably > 70,
particularly
preferably > 80, particularly preferably > 90. As the melting temperature of
the
plasticizer decreases, the plasticizing effect thereof increases.
In a preferred embodiment, plasticizers comprising a molar mass in g/mol of >
90,
preferably > 120, preferably > 140, preferably > 150, most preferably > 160
are used.
The portion of the plasticizers, which fulfill this condition, in the total
plasticizer content
in % by weight is > 10, preferably > 20, particularly preferably > 30, most
preferably >
40. The ability of the plasticizer to migrate decreases as the molar mass
thereof
increases, the plasticizer then has a reduced tendency to migrate from the
barrier
coating into the paper. The barrier coating loses flexibility as a result of
the migration of
the plasticizers.
In preferred embodiments, a combination of at least 2 plasticizers is used,
preferably
of at least 3 plasticizers, wherein at least 5%, preferably at least 10%, most
preferably
at least 15% of the individual plasticizers are represented in the
combination. 2
isomeric plasticizers are hereby considered to be different plasticizers. The
tendency
of the individual plasticizers to crystallize can be reduced by the
combination of
plasticizers. The plasticizing effect disappears in response to
crystallization.
Thickening agents
A thickening agent or a mixture of thickening agents can be added to the
coating
compound, so as to adjust the viscosity of the coating compound to a desired
value,
and so as to obtain the homogeneity of the coating compound, in particular so
as to
prevent the sedimentation of the granular starch and so as to thus simplify
the
production process of the barrier coating. In addition, thickening agents are
used
advantageously for modifying the mechanical characteristics of the barrier
coating, in
particular to increase the flexibility thereof.
CA 02923974 2016-03-10
24
On principle, all hydrophilic substances and mixtures thereof, which increase
the
viscosity, are possible as thickening agents, in particular hydrophilic
polymers and of
those, preferably those from plant-based sources. Preferably, this polymer
contains
polar groups, such as, for example, hydroxyl groups, carboxyl groups or also
ionic
groups, such as carboxylate- or sulfonate groups.
Hydrocolloids and rubbers, such as galactomannans, such as guar rubber or
locust
bean gum; cellulose derivatives, in particular cellulose ethers; pectins, in
particular
rhamnogalakturonanes and protopectins; dextrans; xanthan; zymosan;
hydrocolloids of
seaweed, such as alginates, agar-agar, agarose, carrageen and carrageenans;
furcellaran; hydrocolloids of lichens, such as lichenins und isolichenins, or
hydrocolloids as exudates of woods, such as tragant (astragalus rubber),
karaya
rubber, rubber arabicum, kutira rubber; inulin; latex; chitin; chitosan;
gellan; collagen;
gelatin; casein as well as any combinations thereof, are examples for
thickening
agents.
Dissolved starch can be used for the same functionality as the thickening
agents, but
is not counted as thickening agent in the context of the present invention and
will be
considered separately.
In preferred embodiments, the maximum portion of thickening agent of the
coating
compound, as well as in the barrier coating, in % by weight, based on starch
plus
thickening agent, is 50, more preferably 40, more preferably 30, more
preferably 20,
more preferably 10, more preferably 5, more preferably 2.5, most preferably
1.5.
If present, the minimum portion of thickening agent of the coating compound,
as well
as in the barrier coating, in % by weight, based on the starch plus thickening
agent, is
0.01, preferably 0.05, more preferably 0.1, more preferably 0.3, more
preferably 0.6,
more preferably 0.8, most preferably 1Ø Due to the fact that the thickening
agent
represents an optional component, the minimum portion can also be 0% by
weight.
According to preferred embodiments, xanthan is used as thickening agent,
because
xanthan can prevent the sedimentation of suspended starch particles in a
particularly
effective manner. A maximum portion of <2.5% by weight of xanthan as
thickening
CA 02923974 2016-03-10
..
agent of the coating compound and of the barrier coating turned out to be
particularly
advantageous in further embodiments. The minimum portion of xanthan in % by
weight
is 0, preferably 0.01, more preferably 0.05, more preferably 0.1. Xanthan can
in
particular be used in combination with one or a plurality of any of the above-
mentioned
5 granular starches. In particular, any combination with the other
components, thus
dissolved starch, plasticizers, fillers, and additives are also comprised
according to the
invention.
In a further preferred embodiment, water-soluble cellulose derivatives are
used, such
as, for example, methylcellulose, ethylcellulose, hydronfethylcellulose,
10 carboxymethylcellulose, hydroxyethylmethylcellulose or
hydroxypropylmethylcellulose.
Preferably, polyvinyl alcohol (PVA) is furthermore used, because PVA does not
only
create viscosity and stabilizes the coating compound, but also improves the
mechanical characteristics of the barrier coating, in particular the
flexibility thereof.
Preferably, PVA comprising hydrolysis degree of > 70%, more preferably of >
75%
15 more preferably > 80%, most preferably > 85% is used. A hydrolysis
degree of < 99%,
more preferably of < 98%, most preferably < 96% is furthermore preferred.
According
to DIN 53015, a 4% solution of the PVA at 20 C preferably has a viscosity in
mPas of
> 3, preferably > 5, more preferably > 7, more preferably > 10, most
preferably > 15.
In a preferred embodiment, the maximum portion of PVA in % by weight, based on
the
20 dry coating compound after deducting the optionally comprised filler
component, is 40,
more preferably 30, more preferably 20, more preferably 15, more preferably
10, more
preferably 7, more preferably 5, particularly preferably 4. The minimum
portion of PVA
in % by weight is 0, preferably 0.1, more preferably 0.3, more preferably 0.6,
more
preferably 1, more preferably 1.5, more preferably 2, most preferably 3. PVA
can in
25 particular be used in combination with one or a plurality of any of the
above-mentioned
starches. In particular, any combination with the other components, thus
dissolved
starch, plasticizers, fillers and additives are also comprised according to
the invention.
The above-mentioned ranges for xanthan and PVA are to obviously be understood
such that, if one component or both components is/are present, they form a
portion of
the thickening agent or that the thickening agent includes only xanthan and/or
PVA.
CA 02923974 2016-03-10
' 26
The value ranges for xanthan and PVA are thus not to be understood as additive
to the
above-mentioned general value ranges for the thickening agent.
The limits for the portion of thickening agent of the barrier coating
correspond to the
limits for the portion of thickening agent of the coating compound.
Filler
All components, which are virtually insoluble in water or which are present in
the
coating compound as well as in the barrier coating in the form of particles,
such as, for
example, pigments, glass particles, soot particles, mineral particles, such as
titanium
dioxide, talc, carbonates, are identified as filler component of the coating
compound or
of the barrier coating, respectively. It is important to note here that starch
particles and
also the birefringent starch granules are never counted as being fillers in
the context of
the present invention. For the most part, the filler component is
mathematically
deducted in the formulations, because it is not significant for most of the
functional
components of the formulation, whether or not a filler component is present.
Unless
otherwise specified and unless obvious otherwise, the % by weight information
relating
to the composition of the barrier coating in each case refer to the portions
without the
filler component.
The portion of the filler component in `)/0 by weight in the case of the dry
barrier coating
is < 70, preferably < 50, more preferably < 30, more preferably < 20, more
preferably <
10, more preferably < 5, most preferably < 3. The filler component must also
be heated
in response to the gelatinization and thus requires a higher energy input in
this step.
The flexibility of the barrier coating is mainly reduced in the case of higher
portions of
filler.
The limits for the portion of filler of the barrier coating correspond to the
limits for the
portion of filler in the coating compound. The portion of the filler(s) is not
considered
when calculating the 100% by weight of the components of the coating compound.
In
fact, the filler is added additionally to the 100% by weight of the other
components
when formulating the weight. However, the filler is considered when
determining the
solids content / the surface weight of the barrier layer.
CA 02923974 2016-03-10
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27
If present, the minimum portion of the filler component in % by in the case of
the dry
barrier layer is 0.1, more preferably 0.2, more preferably 0.5, more
preferably 0.8,
particularly preferably 1Ø Due to the fact that the filler component
represents an
optional component, the minimum portion can also be 0% by weight.
Additives
For example, the following additives can be used as further components of the
coating
compound: surface-active agents, such as, for example, ionic or non-ionic
tensides,
wetting agents, antifoam agents, stabilizers, dyes, further polymers in
addition to those
already mentioned, biozides, pH-regulators, thixotropic agents.
The portion of the additive or additives, respectively, based on the dry
coating
compound, is 0 to maximally 5% by weight. The portion is preferably < 3, more
preferably < 2, more preferably < 1, most preferably < 0.7. If present, the
minimum
portion of the additive or additives, respectively, is preferably 0.1% by
weight.
The limits for the additive content of the barrier coating correspond to the
limits for the
additive content of the coating compound
Lecithin und fatty acids
The coating compound according to the invention and thus the barrier layer can
further
optionally contain lecithin and/or fatty acids.
The lecithin is preferably soy lecithin. Lecithin reduces the water
sensitivity of the
barrier layer, in particular the ductility is increased in response to low
humidity.
The lower limit of the lecithin portion in the coating compound or the barrier
layer in %
by weight, based on the starch, is 0, preferably 0.01, more preferably 0.05,
more
preferably 0.1. The upper limit of the lecithin portion in the coating
compound or the
barrier layer in `)/0 by weight, based on the starch, is 10, preferably 7,
more preferably
5, more preferably 4, more preferably 3.
CA 02923974 2016-03-10
28
Edible fatty acids are preferred in the case of the fatty acids. Stearic acid
is particularly
preferred. Fatty acids reduce the water sensitivity of the barrier layer, in
particular the
adhesiveness is increased in the case of high humidity.
The lower limit of the fatty acid portion in the coating compound or the
barrier layer in
% by weight, based on the starch, is 0, preferably 0.01, more preferably 0.05,
more
preferably 0.1. The upper limit of the fatty acid portion in the coating
compound or the
barrier layer in % by weight, based on the starch, is 10, preferably 7, more
preferably
5, more preferably 4, more preferably 3.
The following compositions of the coating compound turned out to be
particularly
suitable for providing a barrier layer with regard to the mechanical
characteristics
and/or the carrier effect of the barrier layers obtained therefrom as well as
with regard
to the processability of the coating compounds:
Preferably, the starch is a hydroxypropylated pea starch, comprising a
molecular
weight in million g/mol in the range of preferably 1 to 20, preferably of 2.5
to 10. The
plasticizer content in % by weight, based on starch and plasticizer, is in the
range of 0 -
45%, preferably of 15 to 37%, the starch content, based on the formulation
without an
optionally comprised filler, is in the range of 25 to 65% by weight,
preferably of 30 to
50% by weight. Preferably, the coating compound contains a portion of PVA of 1
to
30% by weight, preferably 1 to 20% by weight, more preferably 1 to 10%.
Preferably,
the barrier layer is applied in two passes, wherein a surface weight (dry) of
3,
preferably 4, more preferably 5 to 15 g/m2, is applied for each pass.
Preferably, the
plasticizer is glycerin.
A hydroxypropylated tapioca starch or a hydroxypropylated potato starch can
also be
used instead of the hydroxypropylated pea starch.
According to a further preferred embodiment of the coating compound, the
starch is a
hydroxypropylated tapioca starch comprising a molecular weight in million
g/mol in the
range of preferably 1 to 20, preferably of 2.5 to 10. The plasticizer portion
in % by
weight, based on starch and plasticizer, is in the range of 0 ¨ 45%,
preferably of 15 to
37%, the starch portion, based on the formulation without an optionally
comprised filler,
CA 02923974 2016-03-10
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29
is in the range of 25 to 65% by weight, preferably of 30 to 50% by weight. The
coating
compound preferably contains a portion of PVA of 1 to 30% by weight,
preferably 1 to
20% by weight, more preferably 1 to 1 to 10 '3/0. The barrier layer is
preferably applied
in two passes, wherein a surface weight (dry) of 3, preferably 4, more
preferably 5 to
15 g/m2 is applied for each pass. Preferably, the plasticizer is glycerin.
According to a further preferred embodiment of the coating compound, the
starch is a
hydroxypropylated potato starch comprising a molecular weight in million g/mol
in the
range of preferably 1 to 20, preferably of 2.5 to 10. The plasticizer content
in % by
weight, based on starch and plasticizer, is in the range of 0 - 45%,
preferably of 15 to
37%, the starch content, based on the formulation without an optionally
comprised
filler, is in the range of 25 to 65% by weight, preferably of 30 to 50% by
weight. The
coating compound preferably contains a portion of PVA of 1 to 30% by weight,
preferably 1 to 20% by weight, more preferably 1 to 10%. The barrier layer is
preferably applied in two passes, wherein a surface weight (dry) of 3,
preferably 4,
more preferably 5 to 15 g/m2 is applied for each pass. Preferably, the
plasticizer is
glycerin.
Packaging material
The packaging material according to the invention has several layers and
comprises a
planar substrate as carrier layer and at least one barrier layer, which is
applied to the
planar substrate, and which is composed and structured as explained above.
According to a preferred embodiment, the finished, dried barrier layer is
solid and non-
tacky.
The packaging material according to the invention can be obtained by means of
the
method according to the invention, which will be described in detail below.
Substrate
The barrier coating is applied to a planar substrate, which is suitable as
packaging
material. Papers are preferred packaging materials. Papers, which are possible
as
CA 02923974 2016-03-10
. .
substrate for the barrier coating, have a surface weight in g/m2 of preferably
< 800,
more preferably < 600, more preferably < 500, more preferably < 400, most
preferably
<380. On the other hand, the surface weight thereof in g/m2 is preferably >
30, more
preferably > 50, more preferably > 70, more preferably > 90, more preferably >
110,
5 most preferably > 120. In summary, paper is understood to be paper in a
narrower
sense, as well as cardboard and pasteboard. When reference is made hereinafter
to
paper, pasteboard or cardboard is to also always be comprised. Particularly
preferably,
papers, which are used as food packaging, in particular in the form of folding
boxes,
are used as substrate for the barrier coating.
10 The barrier coating is applied to the rear side of the papers, which
follows from the fact
that a barrier is to be obtained against the interior of the packaging. The
rear sides of
papers, which are suitable for packaging, are typically rough, while the front
sides,
which are mostly provided for printing, are smooth. However, papers comprising
rather
smooth rear sides are preferably used, because smooth surfaces can be refined
15 significantly more easily with good barrier coatings. A smoother rear
side can be
obtained, for example, by means of a precoat. The precoat refers to a layer,
which is
applied to the paper substrate before the barrier layer. The precoat can
already be
applied at the paper manufacturer.
On the front side of the substrate, thus on the outside of the finished
packaging, the
20 packaging materials according to the invention can have further
coatings, which are
known to the person of skill in the art, or can be printed. In any event, the
barrier layer
does not form the outside of the finished packaging, but is arranged in such a
manner
that it establishes a border between the planar substrate, thus the carrier
material, and
the interior of the packaging and thus the packaged good contained in the
packaging.
25 If desired, the packaging material according to the invention can
comprise further
layers.
According to a further aspect, the present invention also relates to
packaging, in
particular folding packaging, which comprise the packaging material according
to the
invention or which can be made therefrom, respectively. The starch-based
barrier layer
30 does not form the outside of the packaging hereby.
CA 02923974 2016-03-10
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31
Method
The method according to the invention for producing the multi-layer packaging
material
comprises the following steps:
a) providing a
planar substrate, which is preferably chosen from the group
consisting of paper, pasteboard and cardboard,
b) providing a coating compound,
c) applying the coating compound of step b) to at least one side of the
planar
substrate and forming a layer on the planar substrate, and
d) increasing the
temperature for drying and solidifying the coating
compound, which has been applied and shaped to form a layer.
According to the invention, the coating compound is a suspension with granular
starch
suspended therein. This suspension is gelatinized and dried on the substrate
in situ. All
of the coating compositions or coating compounds, respectively, according to
the
invention, which are explained in the present application, can preferably be
used or
utilized, respectively, in the method according to the invention.
Application of the coating compound
Methods for applying liquids, such as suspensions, to planar substrates are
generally
known. The coating can take place in different ways: e.g., the barrier coating
can be
coated, printed, cast, sprayed, rolled or applied in a planar and even manner
in a
different way. The layer thickness required for forming an effective barrier
can thereby
be applied in one or a plurality of passes.
In a preferred embodiment, the barrier or coating compound, respectively, is
applied by
coating or casting. Known and suitable coating methods are the blade coating,
doctor
coating and size press, for example. A particularly suitable casting method is
the
curtain coating, wherein particularly good barrier coatings can be obtained
with
comparatively small application quantities.
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32
Preferably, the barrier compound is produced / applied in more than one coat.
Particularly preferably, the coating is produced in 2 coating passes, between
which the
substrate web does not necessarily need to be rolled up again. Surprisingly, a
better
barrier is obtained, when a barrier layer of a certain surface weight of,
e.g., 20 g/m2 is
applied in 2 passes, wherein, e.g., 10 g/m2 are applied in each case, instead
of in one
pass, in which 20 g/m2 are applied.
In the case of a smooth paper, a sufficient barrier coating can be obtained in
1 or 2
coating passes. In the case of a rough paper, 2 to 4 coating passes might be
necessary.
The upper limit for the surface weight (dry compound) in g/m2 for an
individual
application is preferably 30, preferably 25, more preferably 20, more
preferably 18,
more preferably 16, more preferably 15, most preferably 14. The thinner the
application, the less the blister formation and pinholes can be expected and
the easier
the application compound can be dried.
The lower limit for the surface weight (dry compound) in g/m2 for an
individual
application is preferably 3, preferably 4, more preferably 5, more preferably
6, more
preferably 7. The thicker the application, the better the coverage of the
paper and the
more effective the barrier.
In a preferred embodiment, a fist barrier layer or a first application for the
barrier layer,
respectively, is obtained in-line with a paper machine. That is, the first
application is
obtained directly following the production of the paper on the fresh paper as
rear side
coating, without the paper web having been rolled up first. The second
application is
then carried out at a different location, after the paper web comprising the
first
application has been rolled up.
In a preferred embodiment, a precoat, which, on the one hand, serves the
purpose of
preventing the water from the coating compound of the following barrier layers
from
penetrating into the paper, the pasteboard or the cardboard, and, on the other
hand, to
smooth the surface, which is to be coated can be applied to the paper, the
pasteboard
or the cardboard, or can have been applied ahead of time, for example by the
paper
CA 02923974 2016-03-10
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6
33
manufacturer. In particular in the case of paper, which is highly absorbent,
such a
precoat offers significant advantages, because the water of the coating
compound is
available better through this for the gelatinization of the granular starch.
Preferably, the
precoat also has characteristics, which reduce the migration of aliphatic and
aromatic
hydrocarbons.
In the case of a precoat, compounds can be applied, which are used in the
paper
industry to improve the surfaces, in particular to reduce the surface
roughness, to
improve the printability and the machine operation. A mineral precoat, for
example a
carbonate precoat, can thus be used. On the other hand, a starch-based
compound
can also be applied in the case of a precoat.
In a particularly preferred embodiment, this precoat is produced by means of a
precoat
compound of dissolved starch. With reference to the starches and starch types,
which
are suitable for this, the statements made above to describe the granular
starch apply
accordingly. A starch-based precoat differs from the barrier layer according
to the
invention in that the precoat does not have all of the features of the barrier
layer
according to the invention, thus for example no gelatinized starch particles
within the
specified quantity range.
The preferred weight average of the molecular weight distribution KA, of the
dissolved
starch used in the precoat in g/mol is > 500,000, preferably > 1,000,000,
preferably
>2,000,000, preferably > 2,500,000, particularly preferably > 3,000,000, most
preferably > 4,000,000.
The above statements in the paragraphs "solids content of the coating
compound" and
"plasticizer" also apply with regard to solids content and plasticizer portion
of the
precoat compound from dissolved starch.
The upper limit for the surface weight of the precoat in g/m2 is 30,
preferably 25, more
preferably 20, most preferably 15.
The lower limit for the surface weight of the precoat is 1, preferably 2, more
preferably
3 and most preferably 5 g/m2.
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34
One or a plurality, preferably 2, 3 or 4 layers of the coating compound
according to the
invention can then be applied to the precoat as explained in detail above.
Speed
In the case of the heavier papers, as they are preferably used as substrate
for the
barrier coating, the speed, at which the paper web, which is to be coated, is
moved at
the paper manufacturer, is approximately 100 ¨ 1,000 m/min, which corresponds
to a
speed of 1.7 to 17 m/s. Most of the methods lie in the range of 200 ¨ 800
m/min. In
response to the further processing of papers, such as, for example, in the
case of a
coater, lower speeds are also used in parts. The barrier coating according to
the
invention can be produced in a continuous process at the speeds, which are
standard
in the industry.
Temperature of the coating compound
It is advantageous, when the coating compound is applied to the substrate,
which is to
be coated, preferably the paper, at an increased temperature. The higher this
temperature, the lower the temperature increase, which is required in response
to the
subsequent gelatinization, and the quicker the gelatinization can be reached.
In a preferred embodiment, the temperature in C of the coating compound in
response to application to the substrate, preferably paper, is > 20, more
preferably >
30, more preferably > 35, more preferably > 40, most preferably > 45.
If the temperature of the coating compound is too high, it will already
solidify prior to
the application as a result of the gelatinization, and can then no longer be
used for a
coating. The upper limit of the application temperature depends on the
composition of
the coating compound, but clearly follows for the person of skill in the art
from the
above-mentioned demands on the viscosity and thus the coatability of the
coating
compound.
When the coating compound is heated up slowly and when the viscosity thereof
is
measured, the viscosity of the coating compound decreases continuously, until
a
CA 02923974 2016-03-10
viscosity minimum is reached at the temperature Tv, whereby the viscosity
initially
increases slowly, then very quickly. The viscosity minimum thereby
characterizes the
ideal temperature of the coating compound. The application is made easier by
means
of the low viscosity or a solids content, which is as high as possible, can
still be
5 processed, respectively. On the other hand, only a minimum temperature
increase is
then still required for gelatinizing the coating compound. The viscosity
minimum of the
coating compound depends primarily on the starch, which is used. For native
starches,
Tv in the case of potato starch and tapioca starch is 60 C, in the case of
corn starch it
is 75 C, in the case of wheat starch it is 80 C, in the case of waxy corn
starch and in
10 the case of pea starch, it is 65 C. If the starches are substituted,
such as, for example
hydroxypropylated, the specified temperature are reduced by 10 C.
Preferably, the temperature of the coating compound in response to application
in C
in a range of Tv comprising the following upper limits is: around < 11, more
preferably
< 7, more preferably < 5, most preferably < 3 above Tv, and the following
lower limit: <
15 20, more preferably <15, more preferably < 11, more preferably < 7, more
preferably <
5, most preferably < 3 below Tv.
Pretreatment of the paper
An advantageous effect can be obtained, when the coating compound is applied
to a
preheated paper, wherein the paper can be heated by means of infrared heaters,
for
20 example. The heat from the paper can then also be used to heat up the
applied
coating compound.
In a preferred embodiment, the paper is thus heated up, so that the side
facing the
coating has a temperature in C of > 30, more preferably > 40, more preferably
>50,
more preferably >60, more preferably >70, most preferably > 80 immediately
prior to
25 applying the coating.
A particularly advantageous effect can be obtained, when the side of the
paper, which
faces the coating, is heated up directly prior to the application of the
coating to such an
extent that at least the lowermost, that is, the portions of the granular
starch of the
applied coating compound directly facing the paper, already gelatinize at
least partially
CA 02923974 2016-03-10
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,
36
as a result of the heat from the paper and thus solidify. The penetration of
water from
the coating compound into the paper is made more difficult through this,
because the
water is bound much stronger in the gelatinized layer than in the applied
suspension.
During the production process, the gelatinized portions at the boundary
between
applied coating compound and paper surface act as boundary layer against a
penetration of water into the paper. The subsequent drying is significantly
facilitated
through this, the process can be accelerated, and the blister formation is
counteracted.
In a particularly preferred embodiment, the surface of the paper facing the
coating is
heated to a temperature above Tv in C of > 5, more preferably of > 10, more
lci preferably of > 15, more preferably of > 20, more preferably of > 25,
more preferably of
> 30, most preferably of > 35.
Device for the in situ gelatinization of the coating compound
In response to the in situ gelatinization, the coating compound applied to the
paper
substrate is heated up on the paper. Preferably, the coating compound is
heated to
more than 50 C, more preferably to at least 60 C, more preferably to at least
70 C,
more preferably to at least 80 C, more preferably to at least 90 C, more
preferably to
at least 100 C.
It turned out that the IR radiant heaters, which are common in the drying
devices of the
paper industry, are not sufficient for the in situ gelatinization of the
coating compound
according to the invention, because the heat energy is introduced too slowly
and
because at least a part of the water, which is required for the
gelatinization,
evaporates, before a sufficient gelatinization can be obtained. This effects
particularly
the uppermost layer of the barrier coating, which dries out quickest. Due to
the
common heating aggregates, a suitable barrier coating can thus not be
embodied.
According to the present invention, this problem in response to the in situ
gelatinization
of the starch compound is solved in that, upstream of the generally common
dryer,
means are arranged, with the help of which steam, preferably water vapor, is
applied
to the coated surface. A high energy is released quickly as a result of the
condensation
of the steam on the coated surface, and is distributed to the coating compound
as a
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37
result of the positive heat conductivity of water. It turned out that suitable
steam can in
particular be applied by means of a steam shower. In the paper industry, steam
showers are used for re-wetting paper webs. In terms of the present invention,
a
corresponding design can be used, if it fulfills the following criteria.
fn a preferred embodiment, the steam, which is supplied to the steam shower,
has a
temperature in C of > 105, more preferably > 110, more preferably > 115, more
preferably > 120, most preferably > 125. The upper limit of the steam
temperature in
C is preferably < 200, preferably < 180, more preferably < 170, even more
preferably
< 160, most preferably < 150.
In a preferred embodiment, the steam, which escapes from the steam shower, has
a
temperature in C of > 100, more preferably > 101, more preferably > 103, most
preferably > 105, so that the steam condensates only when it has reached the
substrate.
In a preferred embodiment, the steam, which is emitted from the steam shower,
has a
temperature in C of < 150, more preferably < 140, more preferably < 130, more
preferably < 125, more preferably < 120, most preferably < 115. If the
temperature is
too high, the steam might not cool down sufficiently so as to condensate on
the
substrate. Preferably, this steam is dry saturated, that is, it is close to
the condensation
limit, but does not contain a condensate.
In a preferred embodiment a steam quantity in g/m2 of >0.02, more preferably
>0.04,
more preferably >0.06, more preferably >0.07, more preferably >0.08, most
preferably
>0.09 is deposited or condensed, respectively, on the substrate for each g/m2
of the
coating compound. As the steam quantity increases, the gelatinization is
accelerated
and the gelatinization degree increases. Water, which is applied to the
coating
compound prior to the application of the steam, e.g. by means of a spray
device, is
also counted to belong to the coating compound here.
In a preferred embodiment, a steam quantity in g/m2 of < 50, more preferably
<25,
more preferably <10, more preferably <5, more preferably <3, more preferably
<1,
more preferably <0.7, most preferably <0.5 for each g/m2 of the coating
compound. As
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38
the steam quantity decreases, the quantity of water condensate, which must be
dried
off subsequently, is reduced and the drying is thus accelerated and
simplified. Water,
which is applied to the coating compound prior to the application of the
steam, is also
counted as belonging to the coating compound here.
The steam is to thereby be distributed across the entire coating surface as
evenly as
possible. The technical requirements for the design of a steam chest, which
fulfills
these conditions, are known to the person of skill in the art.
Thanks to the above-mentioned teaching relating to compositions and method
parameters, it was a surprise that it was possible to obtain a complete
gelatinization
and thus a solidification of the coating compound on the substrate within a
few
hundredths of a second, thus an in situ gelatinization, wherein a low-viscous
aqueous
liquid is virtually converted abruptly into an elastic solid, and the water of
the coating
compound is bound therein.
In that the in situ gelatinization of the starch was made possible within such
a short
time period by means of applying a suitable water vapor, the problem that
water from
the coating compound substantially penetrates into the porous paper, was also
solved
or was at least significantly eased at the same time. On the one hand, such
water,
which is absorbed by the paper, is missing in response to the gelatinization
and, on the
other hand, represents a problem in response to the drying of the barrier
coating. A
comparatively long and efficient drying section would be required to dry off
such water,
which is located on the bottom, and, even in the case of relatively gentle and
slow
drying, this water, below the barrier coating, which has already solidified,
easily forms
steam bubbles, which break open the barrier coating by the formation of
bubbles /
blisters and which thus substantially destroy the barrier coating.
Spray device
It turned out that particularly good results can be obtained, if the coating
compound on
the paper is sprayed with water prior to the application of steam, which can
occur, for
example, by means of a spray bar, which includes a row of spray nozzles, via
which
the required water is distributed finely and evenly on the coated web. A water
film,
CA 02923974 2016-03-10
39
which is as even as possible and which facilitates the subsequent
gelatinization and
which leads to particularly good barrier coatings, is to thereby be created on
the
coating compound, most likely in that the uppermost layer of the coating
compound is
protected against drying out too soon.
In a preferred embodiment, a water quantity in g/m2 of <10, more preferably of
von <7,
more preferably of <5, more preferably of <3, most preferably <1 is thereby
sprayed on
for each m2 of substrate. The finer the water droplets, which are sprayed on,
the less
water is required.
In a preferred embodiment, the temperature of the water, which is sprayed on,
in C is
> 30, more preferably > 40, more preferably > 50, more preferably > 60, more
preferably > 70, most preferably > 80.
As the temperature of the water, which is sprayed on, increases, the coating
compound is heated up and a quicker gelatinization is realized subsequently.
In the
case of higher temperatures, the uppermost layer of the coating compound is at
least
partially gelatinized and thus solidified as a result of the warm or hot
water, which is
sprayed on. The water, which is sprayed on, then forms a particularly
effective
protective film against drying out, because it can barely penetrate into the
coating
compound. Such a water film on the surface can then be dried very easily and
quickly
by means of the common drying methods, which are used after the steam
application.
In particular, blister formation can be prevented through this, even in the
case of quick
drying.
Drying
The paper, which is coated with the coating compound, can be dried by means of
the
drying methods, which are common in the paper industry. Mainly infrared
heaters and
hot air hoods are used thereby. It is common thereby for that side of the
paper to be
treated with IR or hot air, on which a layer, which is to be dried, has been
applied. In a
preferred embodiment, however, at least a part of the drying method is carried
out in
such a manner that the other side of the paper (to which no layer, which is to
be dried,
has been applied) is treated with IR or hot air.
CA 02923974 2016-03-10
In a further preferred embodiment, both sides are simultaneously treated with
IR or hot
air in response to at least a part of the drying process.
The advantage of these preferred methods is that the formation of blisters and
pinholes can be suppressed more easily and that better barriers are thus
available.
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41
Brief description of the figures
The invention will be described in more detail below by means of exemplary
embodiments in connection with the drawing:
Figure 1 shows, schematically, a coating system according to the prior
art, as it is
common in the finishing of paper, and
Figure 2 shows, schematically, a device according to the invention, in the
case of
which the device parts, which are known from Fig. 1, for example, for
rolling and guiding the paper and for drying have only been omitted in the
illustration for simplification purposes.
Analytical methods
Determining the application weight
A paper having a known surface is dried in a circulating air oven at 130 C for
15 min
and is then weighed. The surface weight of the untreated paper can be obtained
in
g/m2 from the weight and the known surface.
A coated paper having a known surface is dried in the same manner and is
weighed,
and the surface weight of the coated is thus obtained in g/m2. The surface
weight of
the coating is obtained in g/m2 from the difference of the two surface
weights.
Determining the molecular weight Mw
Mw is understood to be the weight average of the molecular weight
distribution.
If the starch is present in the form of powder, the starch is suspended in
water with a
concentration of 3% of dry substance. This suspension is then heated up in a
mini
autoclave to 150 C while stirring and is held there for 20 min. The solution
obtained in
this manner is then cooled down to approximately 60 C, is thinned to 0.3%, and
is
filtered with a 0.005 mm membrane filter. The filtered solution is then
measured with
CA 02923974 2016-03-10
42
GPC-MALLS (gel permeations chromatography using multi-angle laser light
scattering).
If the starch is to be analyzed with regard to its molecular weight in a
barrier layer on a
paper, the following 2 methods can be used to obtain the starch solution.
1) The starch is scraped off the paper by means of a scalpel or it is abraded
using a
fine abrasive paper. The powdery material obtained thereby can be transferred
into a
solution, which is present in the form of powder, in the same manner as the
one
described above for starch.
2) In the alternative, the barrier layer can be analyzed together with the
paper. For this
purpose, the material is cut into pieces of approx. 2*2 mm and is suspended in
the
autoclave and is stirred at room temperature overnight. The same process as
described above for starches in the form of powder is then carried out. In
response to
the filtration, however, a coarse filter is used first, so as to filter out
the insoluble paper
components. Due to the fact that common papers can already include starch even
without a barrier layer, a reference measurement is made, if necessary, with
the .
uncoated paper or a reference measurement is made with the paper, from which
the
barrier layer was removed mechanically, respectively. An assessment can then
be
made from the reference measurement, which components are to be attributed to
the
barrier and which components are to be attributed to the paper during the GPC-
MALLS
analysis.
An Alliance 2695 separation module from Waters, a DRI detector 2414 from
Waters, a
MALLS detector Dawn-HELEOS from Wyatt Technologie comprising a wavelength of
658 nm and a K5 flow-through cell were used for the measurements. A SUPREMA
gel
column set was used for the GPC column, exclusion limits S30000 with 10E8-
10E6,
S1000 with 2E6 ¨ 5E4, S100 with 1E5 ¨ 1E3. Eluent: DMSO with 0,09m NaNO3.
Temperature: 70 C. Evaluation: Astra software 5.5Ø18. A refractive index
increment
dn/dc of 0.068 was used for the calculation.
Recovering starch particles from the coating of the packaging material
CA 02923974 2016-03-10
43
The produced barriers contain gelatinized starch particles. These starch
particles can
be separated from the soluble components (these are in particular
plasticizers, soluble
starch, thickening agents, if applicable), for example by dissolving the
barrier at 70 C
for 30 min at a stirring speed of less than 60 revolutions per minute and the
quantitative portion thereof at the barrier can thus be measured.
Recovery method No. 1
In a preferred embodiment, the minimal portion in % by weight of the starch in
the
barrier, which can be recovered after dissolving the barrier at 70 C for 30
min and a
stirring speed of less than 60 revolutions per minute, is 30, preferably 40,
more
preferably 50, more preferably 55, more preferably 60, more preferably 65,
particularly
preferably 70%.
Recovery method No. 2
In a further preferred embodiment, the portion of the compound is determined,
which
can be recovered after dissolving the barrier at 70 C for 30 min and a
stirring speed of
less than 60 revolutions per minute, and based on the compound of the barrier,
is
determined. The determination according to this definition is simpler than the
determination according to recovery method No. 1, because it can also be
applied,
when the composition of the barrier is not known exactly. The minimum portion
in % by
weight of the compound, which can be recovered, is 25, preferably 35, more
preferably
40, more preferably 45, most preferably 50.
MODES FOR CARRYING OUT THE INVENTION
Figure 2 shows, schematically, the arrangement of the devices for producing
the
starch coating according to a preferred embodiment of the present invention.
On
principle, it is not important, how the starch-containing coating compound is
applied to
the paper web 10. A blade coater 11 comprising a roll coater (profiled or
smooth) 12
turned out to be advantageous, but other application devices can also be used.
The
application by means of a curtain coater also turned out to be advantageous.
According to the invention, a spray device 13 for applying the water, and a
steam
CA 02923974 2016-03-10
44
shower 14, which, in addition to the IR heater, serve to
agglutinate/gelatinize the
applied, non-agglutinated starch and to form it into a homogenous film, is
arranged
between application aggregate 11 and IR dryer 15. The position of spray device
and
steam chest should thereby be located in the direct vicinity of the
application
aggregate.
Example 1:
A suspension consisting of 37% by weight (percent by weight based on the dry
weight
is mentioned hereinafter in each case, unless specified otherwise) of a
hydroxypropylated pea starch (Mw= 19,000,000 g/mol), 18% by weight of glycerin
(99.5%) and 45% water is homogenized at room temperature in a mixer comprising
an
anchor stirrer, and is provided in a coating device, in which a sedimentation
of the
particles is prevented by continuously circulating the compound. The
suspension has a
viscosity of 200 mPas at 24 C, a measured solids content of 52% and a pH of
7.7. The
rear side of a standard folded box cardboard comprising a surface weight of
230 g/m2
and a roll width of 60 cm serves as substrate. The starch compound was applied
via a
combo blade coating system by means of a profiled roll doctor (profile C35,
0.8 bar
doctor pressure) at a machine speed of 150 m/min. Immediately after the
application,
water was sprayed onto the coated surface in a planar manner at a flow rate of
12
g/m2 by means of a spray bar and the starch coating was gelatinized
immediately
afterwards with saturated steam, which was brought to the surface by means of
steam
shower. The steam thereby had a temperature of approx. 120 C and was operated
such that the flow rate was approx. 30 g/m2 of steam. After the steam chest,
the paper
web was dried by means of a gas-IR heater and subsequent hot air drying
according to
the settings, which are common in the paper finishing. The drying system
consisted of
4 gas IR radiant heaters, followed by 3 drying hoods comprising hot air. The
output
and temperature of the drying units was adjusted such that the paper surface
at the
measuring points between the drying elements showed a temperature, which was
as
high as possible, but not more than 110 C.
Drying conditions:
CA 02923974 2016-03-10
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 70% hoods
number of heating 4/4 2/8 1/8 0/8 temperature C 180 - 180
blocks
on/max.
It was then possible to roll up the cardboard and it did not stick. The
surface weight of
the dry coating was determined by means of differential weighting to be 10
g/m2.
A quick test with spray oil showed a significantly reduced wetting of the
cardboard
5 surface with oil as compared to the uncoated cardboard surface.
Example 2:
As example 1, but two coatings a 10 g/m2 were applied. A test with spray oil
showed
an even more reduced wetting of the coated cardboard surface. The barrier
effect as
compared to MOH (mineral oil hydrocarbons) was determined with the help of a
10 migration measurement with less than 35 C atoms through the coated
cardboard to be
>90%.
With regard to the migration measurement, it is to be noted that a standard
measuring
method does not exist at this time, but the results of different measuring
methods
match surprisingly well. The percentages refer to the ratio, by which the
quantity of
15 MOSH and MOAH was reduced as compared to the uncoated raw cardboard. A
description for a method can be found in: K. Fiselier, K. Grob in 'Packaging
Technology and Science', 2012 Vol.25, issue 5, p.285-301).
Example 3:
At 30-35 C, a suspension was mixed from 34.9% by weight of starch
20 (hydroxypropylated tapioca starch, Mw=19,000,000 g/mol), 13.7% by weight
of glycerin
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46
(99.8%), 3.5% by weight of polyvinyl alcohol (comprising a viscosity of 8
mPas,
hydrolysis degree 88%) and 47.9% by weight of water with a total viscosity of
1670
mPas at 34 C. The suspension was placed into a combo blade coating device,
where it
was circulated so as to ensure the homogeneity.
The suspension was spread onto a standard folded box cardboard comprising a
surface weight of 350 g/m2 at a web speed of 120 m/min by means of a 20 mm
smooth
doctor at a doctor pressure of 0.6 bar. Directly after spreading,
approximately 18 g/m2
of water was applied in a planar manner by means of a water spray bar. With
the help
of a steam shower, 117 C hot, saturated steam with a flow rate of approx. 35
g/m2 was
blown onto the 60 cm wide paper web immediately afterwards. After the steam
shower,
the web surface had a temperature of approx. 90 C. The coating was then moved
through a drying system comprising 4 gas IR radiant heaters, followed by 3
drying
hoods comprising hot air. The output and temperature of the drying units was
adjusted
in such a manner that the paper surface displayed temperatures, which were as
high
as possible, but not more than 110 C, at the measuring points between the
drying
elements.
Drying conditions:
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 80% hoods
number of heating 4/4 4/8 1/8 2/8 temperature C 170 200 180
blocks
on/max.
It was then possible to roll up the cardboard and it did not stick. The
surface weight of
the dry coating was determined by means of differential weighting to be 13
g/m2.
A cover coat with the same suspension was applied once again under the same
conditions to the roll, which had already been coated, only increasing the
doctor
pressure to 1.5 bar. The applied surface weight was measured to be 8.5 g/m2.
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47
The total coating had a surface weight of 21 g/m2.
Two samples of the coated cardboard were in each case subjected to a migration
test
for MOSH/MOAH, wherein one sample was first grooved in the center (1x90 ). The
test showed that the barrier effect in the case of the grooved sample was only
reduced
by approx. 1.5% as compared to the non-grooved cardboard.
Example 4:
Prior to the coating with a suspension of particulate starch, a precoat was
produced
with dissolved starch. The starch solution had the following composition in %
by
weight: 17.8% starch (slightly degraded hydroxypropylated pea starch,
Mw=4,500,000g/mol), 9.5% glycerin (99.8%), 72.7% water. The mixture was mixed
cold
and was cooked in the jet cooker, so that a clear solution was created. Said
clear
solution had a viscosity of 1180 mPas (Brookefield viscometer) at 55 C.
The precoat was applied on the combo blade by means of a roll doctor (20 mm
smooth
doctor, 0.8 bar doctor pressure) at a machine speed of 350 m/min to a folded
box
cardboard weighing 230 g/m2. The coating weight after drying was determined to
be
9.6 g/m2.
Drying conditions:
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 85% hoods
number of heating 4/4 7/8 6/8 5/8 temperature C 160 150 150
blocks
on/max.
A cover coat was applied to this precoat with a suspension consisting of 34.5%
by
weight of starch (HP tapioca starch, Mw=19,000,000g/mol), 18% glycerin
(99.8%),
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3.5% PVA (with a viscosity of 8 mPas, hydrolysis degree of 88%) and 44 % of
water.
At 29 C, the suspension had a viscosity of 1510 mPas and a pH-value of 7Ø It
was
applied by means of a 20 mm smooth doctor at 2.5 bar contact pressure and a
machine speed of 150 m/min. Approx. 7 g/m2 of water were sprayed on by means
of a
water bar. The adjustments of the steam chest were the same as in Example 3.
Immediately after the steam chest, the coated web surface had a temperature of
89 C.
After drying, the coating weight was determined to be 8.6 g/m2.
Drying conditions:
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 80% hoods
number of heating 4/4 4/8 3/8 1/8 temperature C 155 145 145
blocks
on/max.
Example 5: (comparative example)
A starch solution comprising the following composition in % by weight was
produced by
means of cooking in a jet cooker: 25.7% starch (degraded, hydroxypropylated
pea
starch, Mw=170,000 g/mol), 11.8% glycerin (99.8%), 62.5% water. The solution
had a
Brookefield viscosity of 990 mPas at 33 C and a pH of 7.9. A coat of 14 g/m2
was
applied by means of a profiled roll doctor (C40) at a machine speed of 200
m/min and
was dried.
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Drying conditions:
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 95% hood
Number of heating 4/4 8/8 5/8 0/8 temperature C 250 250 250
blocks
on/max.
A cover coat comprising a surface weight of 9.5 g/m2 was also applied to this
precoat
by a curtain coater at 170 m/min. The starch solution used for this purpose
had the
following composition in % by weight: 24.4% starch (degraded, hydroxpropylated
pea
starch, Mw=170,000 g/mol), 11% glycerin (99.8%), 1% polyvinyl alcohol
(comprising a
viscosity of 40mPas, hydrolysis degree of 98%), 63.6% water. The solution had
a
Brookefield viscosity of 920 mPas at 45 C and a pH von 7.9.
Drying conditions:
IR heater: 1 2 3 4 Hot air drying 1 2 3
output: 60% hoods
number of heating 8/8 3/8 2/8 2/8 temperature C 280 280 255
blocks
on/max.
In the test, the coating showed only a slight barrier effect as compared to
hydrocarbons. In SEM pictures of the surface, the surface showed a good
coverage
with the barrier layer, but several tears in the layer. The tear formation was
attributed
to the high degree of degradation of the used starch.
CA 02923974 2016-03-10
Example 6: (comparative example)
A suspension consisting of 34.5% by weight of starch (HP tapioca starch,
Mw=1.9*107
g/mol), 18% glycerin (99.8%), 3.5% PVA (comprising a viscosity of 8 mPas,
hydrolysis
degree of 88%) and 44% of water was mixed. At 32 C, the suspension had a
viscosity
5 of 1810 mPas and a pH of 7Ø The suspension was applied to a standard
folded box
cardboard comprising a surface weight of 250 g/m2 in the combo blade coater by
means of a smooth doctor 20 mm at 1.5 bar doctor pressure. A steam chest was
not
used, the web was guided through the drying system immediately after the
coater and
was dried with the following settings:
10 Drying conditions:
IR heater: 1 2 3 4 hot air drying 1 2 3
output: 80% hoods
number of heating 4/4 3/8 2/8 0/8 temperature C 150 130 140
blocks
on/max.
An insignificant improvement of the barrier effect as compared to the raw
cardboard
was shown in the spray test. SEM pictures of the coated surfaced showed that
the
granular structure of the starch was still visible and that the starch
compound did not
15 form a film.
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LIST OF REFERENCE NUMERALS
1 raw paper roll
1' coated paper roll
2 raw paper web
2' paper web (coated)
3 guide rollers
4 coating device
5 application roller
6 coating sump
7 doctor
8 dryer
9 hot air dryer
10 paper web
11 application aggregate/blade coater
12 roll doctor
13 spray device
14 steam shower
15 IR dryer
