Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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HIGH DURABILITY CARDBOARD PRODUCT AND METHOD FOR THE MANU-
FACTURING THEREOF
Object of the Invention
The present invention relates to a high durability cardboard product and a
method
for the manufacturing thereof.
Background of the Invention
The invention has been developed to provide a technically and economically
viable
replacement to the broad range of single-use plastics. The new technology will
ad-
dress the increasing demand for packaging and single-use products which are de-
rived from fully renewable raw materials and which can be fully recycled and
re-pro-
cessed in a circular economy. Currently produced single-use plastics used in a
range
of applications from packaging to eating utensils and other single-use items
are be-
ing banned in Europe and a number of other countries around the World. A
single-
use plastics directive in the European Union (EU) also prevents use of natural
poly-
mers that have been permanently modified or paper and cardboard single-use
prod-
ucts containing a barrier coating, film or lining. Other countries or regions
around
the world are implementing various restrictions on single-use plastics.
The current renewable material solution is cardboard, which is derived from
wood or
other ligno-cellulosic raw materials and is commonly used for a broad range of
packaging, storage and logistics, and single-use utensils such as plates and
cups.
The actual formulation of cardboard varies significantly depending on the
intended
end use with the most basic form being brown kraft liner box board through to
more complex white coloured liquid, grease and oxygen barrier containing board
which may also have other printed media applied to the surfaces. Many
countries
are now developing effective recycling systems to collect and re-process the
card-
board materials for re-use into new cardboard products.
A challenge with current cardboard products is the continued durability of the
card-
board when used in moist or varying humidity environments or in applications
where the cardboard may come into contact with high humidity, moisture,
liquids,
or grease. When exposed to such conditions, cardboard will lose its strength,
hold
moisture and increase the risk of microbial growth and will often no longer
provide
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the needed functionality it was intended for and limit its use as a
replacement to
conventional thermoplastic materials such as polypropylene, polyethylene, high-
den-
sity polyethylene (HDPE), and polyvinyl chloride (PVC). To address these
limitations
barriers are usually applied to the surfaces of the cardboard which prevent or
limit
the negative impacts of moisture and grease. Traditionally fossil based
polymer
films such as linear low-density polyethylene (LLDPE) has been used but in
more re-
cent times a number of advancements in bio derived films and barriers have
been in
development and are beginning to enter the market, such as polylactic acid
(PLA)
and latex. However, a number of challenges remain which still creates
limitations on
the use of cardboard as a replacement to plastics. The application of a film
coating
on the surfaces helps to protect the direct surfaces but does not limit the
risk of
moisture and grease incursion and degradation from the edges of the boards and
at
joints and if the barrier is broken the functionality is lost. In addition,
the use of a
plastic barrier increases the complexity of the recycling and reprocessing of
the
board as the very different materials need to be carefully separated before re-
pro-
cessing is possible. Products made of paper or cardboard which contain a
barrier
coating or film and or lining are also included in EU directive on single-use
products.
The present invention provides novel cardboard products produced from 100% re-
newable materials with significantly enhanced strength properties and moisture
and
liquid resistance without the use of any layer films. Additionally, the
products show
improved anti-microbial properties and are fully recyclable with existing
cardboard
materials and products.
Prior Art
The use of the combination of citric acid or similar carboxylic acids and
sorbitol or
similar polyols as a base formulation to facilitate an esterification reaction
with the
hydroxyl groups within the acid and also within wood fibres is well known and
at it
earliest incorporated to a patent US3661955 (A) with the title "Polyesters of
citric
acid and sorbitol" having a priority date of 3.11.1969 and a later and even
more rel-
evant patent CA2443901 C with the title "Cross-linked pulp and method of
making
the same" having a priority date of 11.4.2001. The Canadian patent primarily
refers
to the use of carboxylic acid or maleic acid as the cross-linking additive
which is to
be applied via various means to pulp with intended outcome of an
esterification
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reaction with the cellulosic pulp fibres to improve wet strength in the use of
hygiene
products.
The above-mentioned prior art focuses on improving the absorbency and wet
resili-
ency properties of cellulosic pulp materials and do as such solve a problem
opposite
the one of the present invention, which aims to limit liquid absorbance for
the dura-
tion of its use and maintain the needed functional strength.
Description of the Invention
The present invention discloses a novel cross-linking reaction of cellulosic
pulp ma-
terials in the form of sheets or rolls for the preparation of high-durability
cardboard
products achieving their final dimensional stability and strength after a
curing step.
A simultaneous or subsequent hydrophobation function may be employed to
further
enhance the hydrophobic properties of the product.
The novel invention relates to the use of a cellulosic material for the
preparation of
a fully bio-based high durability cardboard products. The sheet pulp material,
herein
referring to all kinds of pulp sheets, can be of a determined size or
continuous
sheets.
The aim of the invention is to provide a cardboard product with no liquid
barrier or
lining, having increased strength properties both in dry conditions and when
ex-
posed to moisture and liquid for a limited period of time, such as is the case
for sin-
gle-use cutlery, but still being fully recyclable with conventional, untreated
card-
board. To achieve full recyclability properties, it is beneficial to have a
product with
high cellulosic content and a ratio of solids content of crosslinking agents
that is op-
timised in relation to the needed strength and performance in end use. At the
same
time, the product should not be too strong so that the product is not possible
to
easily break down in a mechanical and water agitation process typically used
in
cardboard and paper recycling processes to obtain individual cellulose fibres
for new
cardboard or paper products.
Preferably the treatment is carried out on sheets, bales, or rolls of
industrially avail-
able market pulp without any other pre-processing steps. Cellulose fibres or
pulp of
different origin are suitable for the process.
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The esterification process is carried out by use of a base chemical solution
contain-
ing a reactive cross-linking acid, preferably a tricarboxylic acid, and a
water-soluble
polyol containing multiple hydroxyl groups. The ratio of the tricarboxylic
acid to pol-
yol and the solid's ratio to the base solvent are varied depending on the end
appli-
cation and the desired properties of the cardboard material.
To obtain a high-quality product and as smooth product surface as possible,
the
treated sheets which enter the forming stage should be pre-dried. A low
moisture
content minimises risk of steam bubbles occurring during the hot pressing. A
mois-
ture content of 10% or lower is preferred. Ideally the moisture content of the
sheets is below 5% and preferably less than 3% and even more preferably less
than
2%. In a continuous process the moisture content can be controlled by the
limited
exposure time to ambient humidity from exiting the sheet dryers to entering
the
forming stage. However, provision can be made for intermediate sheet drying to
flash off any absorbed humidity in the event of a process stop or if a batch
produc-
tion of treated sheets is made.
The enhanced strength properties and moisture and liquid resistance of the end
product in accordance to the present invention is achieved by a final curing
step.
The hydrophobic properties of the material can be further enhanced by the
addition
of a novel functional hydrophobation emulsion.
The emulsion comprises organic and commercially available substances with
hydro-
phobic functionality mainly derived from essential methylene groups forming a
non-
polar moiety of the molecule. The emulsion is formed in a base solvent such as
wa-
ter or an organic solvent with similar functionality, such as alcohols. A non-
ionic sur-
factant may be added as an emulsifying agent.
The cross-linking formulation and the optional hydrophobation emulsion are
synthe-
sised separately at temperatures that enables formation of a liquid
formulation, of-
ten a temperature of around 60 and higher is necessary. At instances where
the
hydrophobation additives are in a liquid form, synthesising at lower
temperatures
may be preferable. The obtained reaction formulations are applied to the pulp
sheet
to be treated either as a blend or as separate formulations using methods
known in
the art, such as by submersion, spraying or impregnation. The uptake of the
reac-
tion formulation can be enhanced by use of microwave treatment. Any excess for-
mulation is extracted and may be re-used in the process. The hydrophobation
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emulsion can be applied during the treatment step with the cross-linking
reagent,
prior to the final curing step, or as a combination of any of these.
The dry sheets may be treated in a number of methods. In one such method the
sheet enters the treatment stage in a continuous process which can be through
use
of a continuous treatment bath with conveyors carrying the sheets through the
solu-
tion and at a speed which ensures the needed residence time is achieved to
take up
sufficient solution. The residence time may vary between the pulp types and
poros-
ity and density of the sheets and can be adjusted dependent on the targeted
solids
content required for the final product which will contribute to the strength
and di-
mensional stability of the product. An alternative method of treatment is to
use a
modified continuous curtain coating line where a continuous flow of solution
is ap-
plied from above and the sheet passes through the solution curtain at the nomi-
nated speed to ensure sufficient solution uptake is achieved, the line may
have
more than one curtain coating head to ensure and optimize the application of
solu-
tion. After treatment via both methods the sheets can optionally pass through
a
mangle like device to remove excess solution with said solution recycled and
dosed
back into the main treatment solution ensuring any contaminants are filtered
out
first. The temperature of the solution is maintained preferably above 50 C and
reg-
ular agitation of the solution may be maintained to ensure good dispersion. An
al-
ternative treatment method may include dipping of a larger batch of pulp
sheets
into a tank of the solution for the required time or alternatively whole pulp
stacks
may be introduced to a high-pressure impregnation vessel where the solution is
in-
troduced to the pre vacuumed vessel to impregnate the pulp stacks and with the
option of a post vacuum step to remove excess solution before transferring to
the
drying process.
The solids content to be maintained in the sheet will vary depending on the
applica-
tion and needed strength and stability but is preferably in a range of between
5-
50% weight gain after drying. The required solids content is managed through
the
initial solution preparation step and also through residence time of the pulp
sheets
in the solution.
The preparation of the solution can be carried out with the targeted solids
content
already in the synthesis stage with the final ratio of solids to water or
alternatively it
has been found that a very high solids content synthesis can be made which may
be
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up to 90% solids to water ratio in the mixing and synthesis step and this very
high
solids solution can then be stored and dosed and mixed later to the heated
water to
achieve the targeted solids to water ratio and subsequent solids content in
the dried
pulp sheet.
The treated pulp sheet material is pre-dried before further processing.
Multiple
methods of drying can be used, including a combination of hot air circulation
and or
microwave, Infra-Red heating, hot surface contact heating combined with
specified
vacuum to remove the moisture in a drying oven which may be a continuous or a
batch process. Specific drying schedules can be used to speed the water
evapora-
tion but leaving the solids in the pulp sheets with the key objective of not
rising the
treated pulp sheet to a temperature which would prematurely initiate the start
of an
esterification reaction, preferably the temperature is below 120 C.
In a preferred embodiment, the chemically treated pulp sheet material is in
the form
of individual sheets or rolls, whereby the sheets may be applied on top of one
an-
other, optionally in a cross-layer formation, to increase the structural
properties and
dimensional stability of the final cardboard product.
The chemically treated cardboard material is preferably formed and cut into a
de-
sired shape before final curing of the end product. Preferable end products
are dif-
ferent packing materials, such as boxes and inserts shaped to hold a product
in
place. Due to the non-toxic characteristics of the reagents and raw material,
the
material is well suited for, but not limited to, end-use in the food industry.
Especially
preferable end products are eating utensils, such as cutlery. Besides single-
use
forks, knives and spoons, also other tableware, such as plates, cups, lids and
bowls
are further examples of cardboard products of the invention. The cardboard
mate-
rial can also be pre-cut into foldable boxes or wrappings that can be used for
take-
away meals, or transport boxes or the like.
The mould forming tools used in the forming step are preferably designed to
pro-
vide a very smooth surface finish to the final product which differs
significantly to
the common methods of wet pulp forming where a rough surface is only possible
as
the water is pressed out of the mould through a fine mesh which in turn gives
a
rough surface texture. It is possible to achieve a very smooth product surface
by
the process of the invention, which for applications which are inserted to the
mouth
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like a spoon or fork or drinking cup cap is far more acceptable to the
consumer than
the texture of wet pressed pulp or wooden cutlery.
It should also be noted that due to the high surface densification of the
material
and smooth surface, the fibers do not become separated or raised when the
product
is contacted with water or other liquids and the material becomes wet, as
might be
expected with wet formed pulp or untreated wood products, such as wood
cutlery.
This is especially important in the application of single-use utensils, such
as single-
use spoons, forks, knives or cup lids.
An additional development of the mould design allows for variation in the
density of
the material through targeted densification. An example is a knife product
which
needs to have sharp cutting teeth. Through the mould design it is possible to
con-
centrate a higher level of densification just at the area of the teeth which
provides
excellent cutting functionality of the knife, something which the existing
products
made of paper, wet pressed pulp and wood cannot achieve.
In addition, a designed control of the densification of the formed products in
the re-
gions where the cutting is to take place provides a more equal density at the
point
of cutting, which enhances the final cut surface of the product.
The final curing reaction is an essential stage in the process and can be used
to in-
fluence the level of reaction both with residence time and temperature.
Depending
on the end use the residence time can be changed to influence the ease of re-
pulp-
ing of the material after recycling after use.
Due to the effectiveness of the curing reaction, the invented cardboard
product, for
example in the form of cutlery, requires no additional barrier coating to be
applied
to prevent moisture induced loss in strength or fiber loosening on the
surface. This
functionality is unique compared with all the other available pulp and paper-
based
cutlery products which all require some form of barrier coating and lining to
resist
moisture and maintain functionality in use. As a result, the invented product
con-
forms with the EU Directive for single-use plastics, which states that paper
and
cardboard single-use items which do not have a barrier and lining are excluded
from
the directive.
The invented material has been tested at an independent and certified
laboratory
for content of thermoplastics and the product was found to contain no
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thermoplastics and is thus in conformance with the European directive on
single-use
plastics where paper and carboard single-use products which do not contain a
bar-
rier coating or lining are excluded from the directive. It has been further
assessed
by qualified experts in the field that the natural polymer, primarily
cellulose, has not
been permanently modified and that the invented material only leads to a
reversa-
ble surface modification occurring on the cellulose fibres and thus the
material can
be reprocessed into new cardboard products at end of use together with other
card-
board products.
Furthermore, the produced cutlery products have been tested for re-pulping
with
other cardboard products including paper plates and cups with good results.
The
product was also found to be fully suitable for home composting.
Summary of the Invention
The object of the invention is a method for manufacturing of a bio-based high-
dura-
bility cardboard product according to the characterising part of claim 1,
wherein a
cross-linking formulation comprising at least one cross-linking acid and at
least one
polyol is applied to a pulp sheet material, the pulp sheet material treated
with the
cross-linking formulation is pre-dried, a single sheet or combined multiple
sheets of
the pre-dried pulp sheet material treated with the cross-linking formulation
is pre-
heated, at least one cardboard product is shaped and cut out of the pre-heated
pulp
sheet material, and said at least one cardboard product obtained is subjected
to a
final curing step. The pre-dried pulp sheet material is consolidated under
pressure
prior to or during the cutting step, preferably during the pre-heating step
and/or the
shaping step and/or the cutting step.
Preferably the cross-linking acid and polyol is selected from a range of
agents ap-
proved in the food and pharmaceutical industries, preferably a tricarboxylic
acid se-
lected from 1-hydroxypropane-1,2,3-tricarboxylic acid, propane-1,2,3-
tricarboxylic
acid, 2-hydroxynonadecane-1,2,3-tricarboxylic acid, 2-hydroxypropane-1,2,3-
tricar-
boxylic acid, benzene-1,3,5-tricarboxylic acid and prop-1-ene-1,2,3-
tricarboxylic acid
and a polyol selected from but not limited to xylitol, sorbitol and
erythritol. A car-
boxylic acid having at least three carboxyl groups is preferred. The polyol
preferably
has at least six hydroxyl groups.
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In another preferred embodiment, an optional functional hydrophobation
emulsion
is applied to the pulp sheet material as a mix with the cross-linking
formulation,
subsequent to treatment with the cross-linking formulation and/or on the
formed
product of the pulp sheet material treated with the cross-linking formulation
prior to
curing of the cardboard product. The cross-linking formulation, the
hydrophobation
emulsion or the mix thereof may be applied to the pulp sheet material in a
pressur-
ised environment through impregnation. The reaction formulation may be in
liquid
form or possibly as a mist in semi-gaseous or atomised form. Alternatively,
the
cross-linking formulation, the hydrophobation emulsion, or the combination
thereof
is applied to the pulp sheet material in liquid form, such as by submersion,
spray or
curtain coating methods, and any excess solution removed in the process is
option-
ally re-used. In a further preferred embodiment the cross-linking formulation,
the
hydrophobation emulsion, or the combination thereof is applied to the pulp
sheet
material under microwave treatment, whereby the pulp sheet material and
formula-
tion are combined within a microwave device. Preferably the cross-linking
formula-
tion, the hydrophobation emulsion, or the combination thereof is applied to
the pulp
sheet material at a temperature from 20 C to 100 C, preferably from 80 C to
100 C.
In a further preferred embodiment, at least two pulp sheets are arranged on
top of
each other prior to shaping, cutting and curing, optionally in a cross-layer
orienta-
tion, preferably in a 90 angle with respect to the predominant fibre
direction of the
adjacent layer. The shaping and/or cutting of the product may take place at
temper-
atures ranging from 120-170 C, preferably from 140-170 C. The final curing of
the
product may take place at a temperature ranging from 120 C to 200 C,
preferably
from 150 C to 200 C.
A further object of the invention in that the cardboard product is a bio-based
high
durability cardboard product according to the characterising part of claim 12.
The
product is formed from at least one layer of a cellulosic pulp sheet material,
wherein
the pulp sheet material comprises moieties of at least one polyol and at least
one
organic cross-linking acid being at least partially cross-linked to the
cellulose struc-
ture of the pulp sheet through ester bonds. In a further embodiment of the
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invention, the cardboard product additionally comprises at least one
hydrophobic
agent. Said at least one cross-linking agent, optionally together with a
hydrophobic
agent, is preferably present in the cardboard product in a total amount of at
least 5
wt-% based on the total weight of the product, preferably 5-50 wt-%, more
prefer-
ably 10-30 wt-% and even more preferably 12-25 wt-%. The cardboard product
may be formed out of a cross-linked pulp sheet or multiple combined sheets
consoli-
dated in to one piece, whereby the main surfaces of the cardboard product are
flat
or curved surfaces of essentially even thickness, optionally with a three-
dimensional
structure for improved product strength or other functionality. The product
may
comprise areas of different density, such that functional areas may have a
higher
density than the rest of the product. In a further preferred embodiment of the
in-
vention the product is suitable for containing or handling food, such as
eating uten-
sils, boxes, containers, lids, cup lids or wrappings used during
transportation of
meals, preferably the product is single-use cutlery and tableware. The
invention also
relates to a bio-based high durability cardboard product obtainable by the
method
defined in claim 1.
Drawings
The invention is hereinafter described in detail with reference to the
following draw-
ings, wherein:
Figure 1 is a flowchart presenting the general steps of the
process of the inven-
tion as a preferred embodiment.
Figure 2 presents the weight percentage gain WPG (0/0) of different
cardboard
products and reference materials described in Examples 2 and 3.
Figure 3 presents spectra obtained by Fourier transform
infrared (FTIR) spec-
troscopy for the paper surface of a commercial paper cup used as ref-
erence (uncoated surface) and a pulp sheet material of the invention,
the specific parameters of which is presented in Example 4.
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Definitions
Pulp sheet material is within this application referring to a an essentially
flat, sheet-
like pulp material. The pulp sheet material may be in the form of sheets of a
deter-
mined size. It may also refer to long sheets of paper pulp, possibly
continuous
sheets that may be rolled or folded for easier handling. The pulp sheet
material may
be manufactured from any kind of pulp known in the art, and any combination of
pulp of different origin. Most preferably the pulp sheet material has been
delivered
without further modification from the industrial producer. Typically, the
sheet thick-
ness of the pulp sheet material is about 1-1.5 mm but may vary depending on
the
product specifications and grades of different suppliers.
Bio-based is herein to be understood as a material or a compound that is
obtainable
from a natural source or any combination of such materials or compounds.
Herein
the term bio-based also includes synthetically produced equivalents to such
corn-
pounds and mixes consisting essentially of such compounds. The term bio-based
also refers to any unprocessed or processed renewable material, especially
plant-
based materials.
The term recyclable herein refers to a product being recyclable together with
con-
ventional products produced from the same base raw material, namely pulp.
There
is no need to separate binding or functional agents prior to recycling as
these are
chosen from a range of agents that can be fully blended into the recycled
material
without significant negative effects, such as increased toxicity, formation of
harmful
components, formation of lumps, such as from plastic films or barriers, etc.
The term reaction formulation herein refers to the cross-linking formulation,
Le. the
base formulation, the hydrophobation emulsion, i.e. the functional emulsion,
or a
combination of these. The reaction formulation is water-based or prepared in
an or-
ganic solvent with similar functionality.
The term cross-linking agent and hydrophobic agent herein refers to the active
in-
gredient of the reaction formulation. The total solids content of cross-
linking agent
and or hydrophobic agent in the final product comprises both reacted moieties
of
the agents and unreacted agents in solid state.
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Detailed Description of the Invention
The pulp sheet material used as raw material in the present invention is
preferably
an industrially available market pulp in the form of a sheet either cut to
determined
size or in the form of a continuous sheet that may be roll or folded for
easier han-
dling of the material. No other pre-processing steps, such as hammer milling,
sepa-
rating, de-fibrillating or refining or air lay mat forming, are required. A
broad variety
of pulp types are suitable for the process including but not limited to thermo-
me-
chanical, cherni-thermornechanical pulp (CTMP), softwood and hardwood kraft
pulps, dissolving pulp, recycled pulp, pulp derived from alternative
agricultural ligno-
cellulosic fibres such as hemp, flax, bagasse, palm, rice stems, bamboo and
the
likes.
The pulp sheet material which is used in the current invention is preferably
supplied
in sheet form stacked on pallets, whereby the sheet may be kept in this form
through the process and is not milled or broken down in the individual fibers
during
the process unlike in paper making, wet pulp forming or alternative fully dry
meth-
ods such as hammer milling and air laying webs for post forming. The pulp is
most
commonly Bleached Kraft cellulose but is not exclusively restricted to this.
Alterna-
tive pulps from other ligno-cellulosic raw materials may be used but it will
always be
used in the form of a sheet or if available from a pulp roll.
The pulp sheet material, preferably in the form of bales or rolls of pulp
sheet are fed
into the treatment and manufacturing line (1). For the reaction process to
occur,
which typically is an esterification process, a base chemical solution is
used. This
base formulation contains at least one reactive organic cross-linking acid
where one
or more of the hydrogen atoms have been replaced by a carboxyl group and
prefer-
ably containing at least three carboxyl groups. Preferable is use of an acid
well
known and approved in the food and pharmaceutical industries such as, but not
lim-
ited to, 1-hydroxypropane-1,2,3-tricarboxylic acid, propane-1,2,3-
tricarboxylic acid,
2-hydroxynonadecane-1,2,3 tricarboxylic acid, 2-hydroxypropane-1,2,3-
tricarboxylic
acid, benzene-1,3,5-tricarboxylic acid and prop-1-ene-1,2,3-tricarboxylic
acid. A car-
boxylic acid having at least three carboxyl groups is preferred. Additionally,
the
chemical solution contains a water-soluble polyol which contains multiple
hydroxyl
groups. The polyol is preferably selected from a range of polyols obtainable
from
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natural sources, preferably widely used and approved in the food and
pharmaceuti-
cal industries such as, but not limited to, xylitol, sorbitol and erythritol.
The polyol
preferably has at least six hydroxyl groups. These primary components are
synthe-
sised in a base of water or similar functional organic solvent in a variety of
formu-
lated ratios between 1:1 to 5:1 cross-linking acid to polyol, in one preferred
embodi-
ment the cross-linking acid to polyol ratio is 3:1. The solid's ratio to the
base formu-
lation is preferably between 5 and 50% by weight depending on the end
application
and desired properties of stiffness, bending strength and moisture resistance.
The
formulation is prepared at a temperature where the cross-linking acid and
polyol are
dissolved under stirring in the solvent used. A temperature range of 60-119 C
is
preferred. The base formulation thus obtained is herein referred to as the
cross-link-
ing formulation.
The enhanced strength properties and moisture and liquid resistance of the end
product of the present invention is achieved by a curing process that may be
per-
formed using a variety of techniques. The strength and hydrophobicity
properties of
the final product can be modified, such as increasing or decreasing, by
performing a
densifying step prior to or in combination with the final curing step (6).
This densify-
ing step might be performed only partially or locally.
In order to increase the hydrophobic properties of the final product, a
hydrophoba-
tion emulsion may be added (2, 6). The hydrophobation emulsion comprises
organic
and commercially available substances with hydrophobic properties as
hydrophobic
agents, the primary constituents of which include at least one substance
selected
from fatty acid esters, fatty alcohols, other hydrophobic organic acids and
hydrocar-
bons or additional or alternative functional substances selected from a range
of pen-
tacyclic triterpenoids such as, but not limited to oleanolic acid, betulin and
betulinic
acid. Such hydrophobic agents may be derived from natural oils and waxes, in
one
preferred embodiment the hydrophobic agent is carnauba wax. The hydrophobation
emulsion is formed in a base solvent, possibly in combination with a non-ionic
sur-
factant commonly used in the art for oil in water emulsions. The base solvent
can
be water or an organic solvent with similar functionality, such as ethanol.
The cross-
linking formulation and functional emulsion are synthesised separately at
tempera-
tures enabling the formation of the formulation and the emulsion. The cross-
linking
formulation may be prepared at a lower temperature, such as from 10 C up to a
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pre-reaction temperature of the reaction process, such as of the
esterification pro-
cess, commonly below 120 C. The aim is to apply the solution to the cellulosic
pulp
sheet material in a form where the esterification process of the cross-linking
solution
is not yet initiated, thus enabling formation of cross-linking between the
cellulosic
structure, whereby the amount of available hydroxyl groups is reduced within
the
substrate. The hydrophobation emulsion usually requires a temperature where
the
hydrophobic agent is in liquid form, for most waxes the temperature should be
above 60 C. Preferable temperatures for the preparation of the reaction
formula-
tions ranges from about 60 C to 119 C, even more preferably from 80 C to 100
C.
The duration of this preparation step is often around 1 hour or more. As noted
above, special care should be taken not to rise the temperature to a level
initiating a
premature esterification reaction. The cross-linking formulation and the
functional
emulsion may be blended upon completion of the independent synthesis steps
within the same or a similar temperature range. Alternatively, the functional
emul-
sion may be applied separately to the pulp sheet material treated with the
cross-
linking formulation. The ratio of the solids content of the functional
emulsion to the
solids content of the cross-linking formulation is preferably from about 0.1%
to 15%
in the mixed formulation. Preferably, the surfactant ratio of the functional
emulsion
ranges between 0.1% and 50% by weight of the solids content of the emulsion.
Upon final synthesising of the reaction formulations, Le. the base
formulation, the
functional emulsion or the mixed formulation, the pulp sheet material is
exposed to
the combined or separate reaction formulation via a range of alternative
methods
known in the art, including submersion, spraying or impregnation in a
pressurised
environment or any combination of these. The reaction formulation is
preferably ap-
plied to the pulp sheet material at a temperature ranging from 10 C or 20 C up
to a
temperature still not initiating an esterification reaction within the
formulation, such
as 100 C or 119 C. Preferably the temperature is above 60 C, more preferably
from
80 C to 119 C, and even more preferably from 80 C to 100 C. Most preferred is
a
temperature from about 90 C to 95 C. For the mixed formulation and the hy-
drophobation solution a temperature above 60 C is often necessary to achieve
good
results. Due to the emulsion being very stable even at room temperature, also
lower
temperatures may be employed, such as the ranges given above. Herein, the term
reaction formulation refers to the cross-linking formulation, the functional
emulsion
or the mix of these two prepared as described above.
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In one embodiment, bales or rolls of pulp sheet are impregnated (2a) with the
mix
of the cross-linking formulation and the functional emulsion, only the cross-
linking
formulation, or the functional emulsion subsequent to a treatment of the pulp
sheet
material with the cross-linking formulation.
Preferably the cross-linking formulation or the mix of the cross-linking
formulation
and the hydrophobation emulsion is applied throughout the material to be
treated.
This enables the cross-linking reaction to take place through the whole cross-
section
of the pulp-sheet material, thus giving maximum stability and ensuring good
interfa-
cial properties when combining multiple sheet layers. In applications where
higher
flexibility of the cardboard is needed, the cross-linking agent may be applied
only on
the surface of the pulp sheet material or by using a cross-linking formulation
having
a lower total cross-linking agent concentration.
When the hydrophobation emulsion is applied separately, this may be added only
onto the surface, such as by spray coating after formation and cutting of the
prod-
uct shape. The surface may be treated partially or entirely, and it is also
possible to
apply the functional emulsion only on one surface of the pulp sheet. The
hydropho-
bation emulsion may be added during the initial treatment step and/or prior to
cur-
ing of the end product. An increase in water contact angle of the cardboard
material
is achieved through this additional step, which is beneficial in applications
where the
cardboard product is exposed to moisture for a prolonged time or when water
repel-
lent properties are needed.
In one embodiment where the pulp sheet material is impregnated (2a) with at
least
one of the reaction formulations, the process may be carried out at an
internal pres-
sure between 2-10 bar and a spray release of the liquid reaction formulation
at a
temperature above 80 C to a pressure chamber to create a mist-based impregna-
tion with gradual release of pressure to atmospheric pressure. In a further
embodi-
ment utilising the impregnation approach, the reaction formulation may be
formed
into a very fine mist by use of an atomisation technique, whereby the separate
or
combined formulation is introduced into the vacuum chamber at high velocity
through suitably fine nozzles which cause the fine atomisation to a mist as it
enters
the chamber which enhances the absorption of the formulation into the pulp
sheets,
bales or rolls. Conventional pressure impregnation methods may be employed as
well, wherein the reaction formulation is being applied in liquid form.
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In another embodiment, at least one reaction formulation is added in liquid
form.
(2b) Individual pulp sheets may for example be submersed in the reaction
solution.
The individual sheets or a continuous pulp sheet material can be conveyed
through
a bath containing the reaction formulation, preferably at a temperature above
60 C
and up to the pre-reaction temperature of the esterification reaction, even
more
preferably above 80 C and below 120 C, even more preferably below 100 C. The
residence time depends on the thickness of the material, often 5-30 s or 10-30
s is
sufficient. The uptake of the solution may be enhanced by a longer residence
time,
such as 1 minute or more. Excess solution is then removed from the pulp sheet
ma-
terial, for example by conveying it into a mangle, which may be mildly heated
hav-
ing a temperature around 60 C, for dispersion and homogenisation. Excess
liquid
may alternatively be removed by use of vacuum, suction or other techniques
known
in the art. At least one reaction formulation may also be applied to the pulp
sheet
material in liquid form as a spray or by a curtain coating process.
Use of microwave treatment at the point of combining the solution with the
pulp
material has been found to significantly enhance the solution uptake both in
the ini-
tial treatment stage and also in the retention of the solids post drying. Upon
reach-
ing the desired weight percentage gain (WPG), the pulp sheets or pulp rolls
are re-
moved and, if necessary, excess solution is extracted by use of, for example,
a
mangle or via vacuum or suction. This excess solution may be recycled for re-
use.
The targeted solids content to be retained in the pulp sheets or rolls will be
deter-
mined based on the final application and controlled with residence time,
tempera-
ture and possibly through regulated pressure and varying solids ratio within
the so-
lution. For most applications, a WPG of 100-160% in wet form is targeted after
ap-
plication of the combined or separate reaction formulations. By optimising the
pa-
rameters, a WPG of 200-300% may be achieved in wet form, thus resulting in a
solids content increase of around 75% in the final product.
The application of the cross-linking and optional hydrophobic agents in a
water solu-
tion, or similar functionality solvent, has been found beneficial with respect
to the
interaction between the substrate and the cross-linking agent. The cardboard
prod-
uct obtained shows a very good dimensional stability and reduced water uptake
af-
ter curing even at relatively low solid content of the cross-linking agent and
optional
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hydrophobic agent in the final product, such as around 20-30 wt-% presented in
the examples.
The pulp-sheet material should be pre-dried (3) after application of the at
least one
reaction formulation. This may be carried out by removal of any excess liquid,
for
example by use of vacuum, suction or a mangle, as mentioned above, and/or by
evaporation of the solvent. Other methods include, but are not limited to,
Infra-Red
heating, hot surface contact heating combined with specified vacuum to remove
the
moisture in a drying oven which maybe a continuous or a batch process, or any
combination of such processes. Optionally continuous drying processes commonly
used in wood veneer drying, namely "Jet Drying" whereby hot air is blown
through
pipes with outlets at high velocity and temperature targeted at the pulp
sheets as
they are conveyed through the drying oven may be used. The vacuumed heat and
moisture may be transferred via heat exchangers to recycle the heat and
collect the
condensate. Because of this there could be varying stages and temperatures and
vacuum applied during the drying process.
When the solvent is evaporated by the use of heat, the temperature range is
prefer-
ably 50-104 C. In one preferred embodiment the temperature is raised
gradually,
thus removing moisture slowly and simultaneously pre-heating the treated pulp
sheet material prior to the curing process. Preferably, the moisture content
is re-
duced to below 10% during the pre-drying step, more preferably lower than 5%.
A
higher moisture content will be likely to cause deformation of the surface
during the
forming and curing step or partial separation of combined layers. When
entering the
hot-pressing stage, the moisture content of the sheets is preferably below 5%,
more preferably below 3% and even more preferably below 2%.
Higher drying temperatures than those of the above given range may be used for
a
quicker drying step, for example 180-200 C for a certain period, as the
tempera-
ture within the substrate defines the initiation of a cross linking reaction.
Since the
thermal energy at this stage mainly is used to convert moisture into vapor,
the tem-
perature of the substrate would still remain below the esterification
temperature for
a period of time that is dependent on the moisture content and the properties
of the
substrate, such as thickness, density and heat transfer properties. When the
heating
or drying step is short enough not to rise the temperature within the
substrate
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above 120 C, the temperature of the surrounding may be higher than the
esterifica-
tion temperature without initiating an esterification reaction.
The pre-drying may for example be carried out by transferring chemically
treated
pulp sheet material to a pre-heated oven, preferably at a temperature ranging
from
50-104 C, even more preferred is a temperature of 80-100 C, where the pulp
sheet material is dried to a moisture content which is close to ambient with
the in-
door climate of the production plant. The equilibrium moisture content (EMC)
in a
relative humidity (RH) of 55-60% is estimated to be 7-9%. The targeted dry
solids
content of the cross-linking agents, optionally together with a hydrophobic
agent,
remaining in the pulp sheets or rolls after drying is determined based on the
end
application, preferably being at least 5% wt-% based on the total weight of
the
product. Often a total solids content of between 5 or 10 wt-% and 50 wt-% is
tar-
geted. More preferably the solids content is 10-30 wt-% and even more
preferably
12-25 wt-%. In some embodiments, a solids content above 50 wt-% is preferred.
The pulp sheet material, often in the form of bales of sheets or as rolls can
be
stored in the mid process to be transferred to the forming line or transported
to an-
other production unit. When no storage is needed, for example in production
lines
using a continuous process and a single sheet or roll material, the
temperature may
be raised further in the pre-drying step in order to pre-heat and eliminate
absorbed
humidity from the material before the forming step.
The individual pulp sheets or the continuous unrolled pulp sheet are conveyed
to
the consolidation and forming of the product. For high-strength end products
at
least two sheets are laid on top of each other, preferably 2-5 sheets. In
these appli-
cations, simultaneous treatment of more than one bale or roll of pulp sheet
material
may be beneficial. The individual layers of pulp sheet material may optionally
be ori-
ented in a cross-layer formation whereby the predominantly single direction
ori-
ented fibres of one sheet are turned in a different direction in the next
layer, prefer-
ably in a 90 angle, to build a cross layer structure before entering the
consolidation
and forming process. In an especially preferred embodiment, three layers of
treated
pulp sheets are stacked with the middle layer oriented at a 90 angle to the
outer
layers fibre direction prior to further processing. The primary objective of
the cross
layering of the fibre directions is to further increase the structural
properties and di-
mensional stability of the final cardboard products.
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A multilayer structure, including a double layer structure, is preferred for
increased
strength. Such a structure may be obtained by folding a single pulp sheet
prior to
shaping. A method has been developed to fold a pulp sheet in two during the
pro-
duction process so that one individual sheet can be doubled up to make a two-
layer
substrate before it enters the forming stage of the process. The sheet is
preferably
folded after the drying stage prior to entering the forming mould section of
the pro-
cess.
The treated pulp-sheet material is then pre-heated (4) to further decrease the
mois-
ture content of the material and evacuate any absorbed humidity from the mid
stor-
age as well as to raise the temperature to a pre-reaction level ready for
consolida-
tion and forming (5) of the final products. This pre-heating process may be
applied
to the pulp sheet or sheet layups by means of infra-red radiation, high-
frequency,
microwave or conventional hot air heating technologies. It may be carried out
rap-
idly to a temperature of about 80 or 90 to 110 C after which the sheets are
sub-
jected to an optional consolidation process followed by forming and cutting
(5) of
the product and a final curing processes (6). The step of pre-heating and the
step
of forming and/or cutting of the final product may be carried out
simultaneously.
Preferable is a continuous process, such as the one described in the patent
"Method
for manufacturing products made from fibre material, and disposable products
made by this method" with the application number PCT/FI2020/050511. A
hydraulic
press system may also be employed.
In a further preferred embodiment, and an advancement of the aforementioned pa-
tent, the pulp sheet material will be consolidated under pressure. The
pressure used
may range from, for example, 300 kN to 1500 kN. This further enhances the
strength and hydrophobicity properties of the product. Preferably a partial
cross-
linking chemical reaction between the reaction formulation and the hydroxyl
groups
of the cellulosic pulp fibre is initiated by temperatures ranging between 120
C to
170 C. The heating may be carried out separately or simultaneously with the
above
optional consolidation step. When the optional hydrophobation solution is
added, a
pressing temperature below the melting temperature of the hydrophobic agent is
preferred to avoid this to leach out of the material prior to curing.
Simultaneous heating and consolidation of the pulp sheet material treated with
the
at least one reaction formulation may be carried out by transferring a single
sheet
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or a multiple sheet layup, preferably as an unrolled continuous sheet, to a
heated
roller or hydraulic plate press to densify and/or consolidate said sheet or
sheet layup
by means of heated compression and densification plates or rollers, preferably
at a
temperature of about 120 C to 140 C. This step can possibly include some basic
pre-shaping and pre-activation of the chemical reactions. After this, the
densified
single or multiple consolidated sheets are transferred to a forming station,
such as a
heated roll die press or plate press. The temperature is at this stage
preferably
raised to around 140-170 C for pre-esterification. The final form shaping (8)
may
be carried out using a pressure of at least 300-1500 kN. The duration of the
pres-
sure and heat treatment is often 5-10 s, whereby only partial esterification
is initi-
ated and the cardboard material may be easily formed into desired shape. The
final
product may be formed using any known technology suitable for the material,
such
as hydraulic mould press, roll forming, and cutting with such technologies as
stamp-
ing, rotary embossing and cutting or laser cutting.
The cutting may be performed simultaneously or separately from the forming
step.
It has been found that for optimum cutting quality after the forming step the
tem-
perature of the formed sheet is maintained at a temperature close to 100 C or
at
least within a range between 80-120 C, which ensures a higher quality cut.
This
temperature range is also suitable for the above-mentioned forming steps.
Likewise,
the cutting may be performed under the above mentioned forming conditions as
well. The off-cut material can at this stage be recycled and milled for
alternative end
uses and it has also been proven that the uncured fibre once fibrillated may
be
sprinkled or dosed onto a treated pulp sheet prior to the forming step either
replac-
ing the second layer or filling the core before the sheet is folded to further
enhance
the strength properties of the product.
The product is formed in one piece out of a cross-linked pulp sheet of
essentially
even thickness, whereby the main surfaces of the cardboard product are flat or
curved surfaces of even, optionally with a three-dimensional structure for
improved
product strength or other functionality. For example, in the production of
single-use
cutlery, the gripping surface and edges that are not used for cutting or
holding food
are often folded or rounded for increased comfort and strength of the product.
Simi-
larly, for many packing materials it is preferable to form the material into a
shape
that protects an item from breaking and holding it in place. Connecting pieces
and
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shapes may also be formed allowing at least two cardboard products to be
attached
to each other or to be placed in connection to each other.
It is especially preferred that the product material shows sections of
different den-
sity, i.e., areas of higher density and lower density when compared to each
other.
The denser areas of the material typically provide increased strength of the
product
locally, without affecting the recyclability and compostability of the
product. Such
denser regions may be, for example, the cutting edge of a single-use knife or
the
points of a fork or the fork tines.
The material being pre-dried enables use of smooth-surfaced, non-draining,
press-
ing moulds, thus providing a smooth surfaced product. This is especially
important
in the application of single-use cutlery, as a smooth surface provides a good
mouth-
feel and prevents splintering of the material in the mouth.
The cellulosic pulp sheet material is finally cured through an esterification
process
taking place between the at least two carboxyl groups of the cross-linking
acid and
the hydroxyl groups of the cellulose, more specifically the surface of the
cellulose fi-
bres, as well as the polyol. This reduces the amount of available hydroxyl
groups of
the cellulose fibres within the substrate and forms a cross-linked structure
providing
improved dimensional stability within the material and enhanced durability and
hy-
drophobicity properties. This final reactive curing step (6) is applied once
the arti-
cles have been formed and/or cut out to the final shapes as described (5).
The final curing step is preferably carried out at temperatures ranging
between
150 C and 200 C, more preferably between 170 C and 190 C, for a required time
to complete the chemical cross-linking reaction and fixate any additional
functional
additives. The duration depends on the size and thickness of the product and
also
the heating medium used. A variety of heating mediums known in the art, such
as
infrared radiation (IR), high frequency or heated fan ovens, may be used for
the
curing step. For a single layer cardboard product or a product consisting of
less than
four layers, a curing time of between 10 and 30 min is often sufficient. In a
pre-
ferred embodiment, a curing time of 15 minutes is applied for a single or
double
layer product at a temperature between 170 C and 190 C. Any unreacted offcuts
from the cutting process can be diverted before the final curing step and
further
processed and formed into alternative products and finally reacted under the
same
final curing step.
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Preferably the temperature of the entire cross-section of the substrate
reaches the
above-mentioned curing temperature during the curing step. At shorter curing
times, the curing temperature may be higher than the above-mentioned range, as
the temperature within the substrate determines the degree of esterification
taking
place during the curing.
The final durable cardboard products thus obtained can then be carried forward
to a
packing station for bagging and labelling.
The cardboard product of the present invention is formed from at least one
layer of
a cellulosic pulp sheet material, wherein the pulp sheet material comprises
moieties
of at least one polyol and at least one organic cross-linking acid being at
least par-
tially cross-linked to the surface of the cellulose fibres of the pulp sheet
through es-
ter bonds, which are reversable in hydrolytic conditions. Additionally, the
cardboard
product may comprise at least one optional hydrophobic agent. Said at least
one
cross-linking agent, optionally together with a hydrophobic agent, may be
present in
the cardboard product in a total amount of at least 5 wt-% based on the total
weight of the product. Preferably in a total amount of preferably 5-50 wt-%,
more
preferably 10-30 wt-% and even more preferably 12-25 wt-%.
The product may be a single sheet cardboard material or combined multiple
sheet
cardboard material and any product produced thereof. The pulp sheet material
treated may be chosen from a variety of thicknesses. Consequently, the
cardboard
product may be a very rigid, thick cardboard, possibly of combined pulp
sheets, as
well as a paper like, very thin single sheet cardboard and anything between
these.
The cardboard product may be formed in one piece out of a cross-linked pulp
sheet
or multiple combined sheets consolidated in to one piece, whereby the main sur-
faces of the cardboard product are flat or curved surfaces of essentially even
thick-
ness, optionally with a three-dimensional structure for improved product
strength or
other functionality. In one preferred embodiment, the cardboard product of the
present invention is suitable for containing or handling food. Examples of
such prod-
ucts are eating utensils, boxes, lids and cup lids, wrappings used during
transporta-
tion of meals, preferably the product is single-use cutlery and tableware. The
result-
ing products obtained by the process as described above will have
significantly en-
hanced strength properties, improved moisture and liquid resistance, and
addition-
ally improved anti-microbial properties when compared to conventional
cardboard
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products. Furthermore, the cardboard product thus obtained is fully recyclable
and
re-pulpable with existing cardboard-based materials and products.
The stabilization of the cardboard material is in the present invention
achieved by
introducing a cross-linking acid and a polyol in a base of water to a pulp
sheet ma-
terial. The cross-linking agents react selectively with the amorphous region
on the
surface of the cellulose fibres to obtain needed strength in service but at
the same
time allow for easy re-hydrolysis, decoupling and repulping at the end of
life. In
combination with carefully chosen forming steps, this process results in
increased
stability within the cellulosic material and significantly reduced moisture
uptake,
thus providing a strong cardboard product having very high cellulosic content,
mak-
ing it both recyclable and suitable for use in single-use products.
Examples
In the following examples different cardboard pieces were prepared and tested.
The
parameters used in the tests, such as reaction times and pressures, may also
be ap-
plied to pulp sheet material treated with other reaction formulations
presented in
the description of the invention.
Example 1: Strength properties for single layer cardboard
A single ply pulp sheet was treated with a water based cross-linking
formulation
comprising citric acid and sorbitol in a ratio of 3:1 and having a total
solids content
of 18%. After a pre-drying process, the sheet was pressed at 700kN for 10
seconds
at a press temperature of 170 C after which sheet was cured at 180 C for 15
minutes.
Several strength tests were performed on the obtained treated pulp sheet
having
the dimension of 25 mm x 135 mm. A single ply pulp sheet of the same kind was
used as reference. No cross-linking formulation was applied to the reference
sheet.
A similar pressure treatment at 700 kN for 10 seconds and at a temperature of
170 C was performed for the reference. The results are presented in table 1,
show-
ing a WPG of 27% corresponding to a solid content of around 21 wt-%, a
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significantly improved bending resistance at different degrees, as well as
improved
bending stiffness and Taber stiffness compared to the reference material.
Table 1: Strength properties for the cardboard product of Example 1
Final durable card-
Measurement Unit REF board sheet
Weight g 3.3 4.2
Thickness urn 870 930
Density kg/m3 1135 1330
Bending resistance 50 mN 504 1195
Bending resistance 7.50 mN 739 1801
Bending resistance 15 mN 1276 3554
Bending resistance 300 mN 1722 5350
Bending stiffness 50 mNm 5.1 12.0
Taber stiffness 15 mNm 61.6 172.0
3-point bending*
Flexural strength MPa 25 76
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Example 2: WPG after water soaking test
Two different test sets of single ply pulp sheet were prepared using slightly
different
concentration of the cross-linking formulation. Two test pieces of cardboard
material
were prepared from each solution and the effect of pressure treatment on the
water
uptake was studied. Four reference pieces were prepared using the same heat
and
pressure treatment without the addition of the cross-linking formulation. All
card-
board pieces had a dimension of 20 mm x 100 mm and were soaked in water for 5
minutes at 23 C.
Test Set 1: Single ply pulp sheet treated with a solution of citric acid and
sorbitol in
a ratio of 3:1 and having a solids content of 15%. No pressure was applied to
test
piece la prior to curing at 180 C for 15 min. Test piece lb was pressed at
700kN
and a temperature of 170 C prior to curing at 180 C for 15 min.
Test Set 2: Single ply pulp sheet treated with a solution of citric acid and
sorbitol in
a ratio of 3:1 and having a solids content of 17.5%. No pressure was applied
to test
piece 2a prior to curing at 180 C for 15 min. Test piece 2b was pressed at
700kN
and a temperature of 170 C prior to curing at 180 C for 15 min.
The reference materials were prepared as follows. Ref la was not pressed and
not
cured. Ref 2a was not pressed but cured at 180 C for 15 min. Ref lb was
pressed
at 700kN at 170 C and not cured. Ref 2b was pressed at 700kN at 170 C and
cured
at 180 C for 15 min.
The results from the soaking test are presented in figure 2 and indicates that
the
water uptake of the material is significantly decreased by the treatment with
the
cross-linking formulation followed by the curing step. The water uptake was
further
decreased by densification treatment prior to curing.
Example 3:
A hydrophobation emulsion was prepared by emulsifying carnauba wax in an
amount of 6 wt-% in water at a temperature ranging from about 95 C to 100 C.
Cremophor0 RH 40 from BASF was used as surfactant. A cross-linking formulation
was prepared at a similar temperature by dissolving citric acid and sorbitol
in a 3:1
ratio in water to a total solids content of 20%. The functional emulsion and
the
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cross-linking formulation were mixed at a similar temperature to form a
uniform for-
mulation and a pulp sheet material of 20 mm x 100 mm was soaked in the mixed
formulation at a temperature of from about 95 C to 100 C for 1 min. The pulp
sheet material was dried at 100 C for 1 hour leading to an average WPG of 75%.
The final curing was performed at 180 C for 15 min.
The hydrophobation treatment significantly increased the wetting contact angle
of
the material and the moisture resistance. After a 5 min water soaking test the
WPG
of the hydrophobic cardboard piece was 4.4%, calculated as an average value
for
five test pieces. The test result is presented in the diagram of figure 2 as
Test set 3.
Example 4: FTIR analysis of single layer cardboard
A single ply pulp sheet was treated with a cross-linking formulation prepared
ac-
cording to the description of the invention containing citric acid and
sorbitol in a 3:1
ratio and having a total solids content of 22%. After pre-drying of the pulp
material,
a pressure of 700kN was applied for 10 seconds at a press temperature of 170
C.
The cardboard thus obtained was cured at 180 C for 15 minutes.
A FTIR-analysis was performed on the material obtained (SET4) and the spectrum
obtained was compared to a reference spectrum (Cup board). A commercial paper
cup was used as reference material and the analysis was performed on the un-
coated side of the paper cup. The results are presented in Figure 3.
The FTIR-analysis confirms the presence of carbonyl groups within the
structure of
the cardboard product of the present invention, thus indicating the presence
of es-
ter bonds within the material and a cross-linked structure.
Example 5: Recycling of the material
The produced cutlery products were tested for re-pulping with other cardboard
products including paper plates and cups. The cutlery product prepared by the
method of the invention was crushed and mixed with cup and plate stock and
soaked in a similar process used in re-pulping of waste cardboard and paper
and it
was found that the invented material was able to break back down to fibers and
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integrated with the mainstream of waste cardboard and paper pulp for
production
into new cardboard products.
Disposal of uncured or cured offcuts and waste can be recycled with normal
paper
and cardboard products.
Example 6: Home composting of the cutlery products
Trials were carried out by a professional composting company to test the
ability to
home compost the material. Several cutlery items were either broken into
pieces or
kept as whole and inserted into mesh bags before being placed into several
home
composting units. The cutlery was not in direct contact with other bio waste
typi-
cally placed into the home composters but in the same unit because of the mesh
bag. The home composting trial was very successful with the temperature inside
the
composter maintained close to the control level of just below 30 C and the
enzy-
matic breakdown of the cutlery was completed in 3-4 months with no material re-
maining in the bags at the end of the trial. It was stated that if the cutlery
had been
mixed directly with the other bio waste the composting breakdown would have
been
even quicker.
Example 7: Strength results of invented cutlery and commercially available
paper
cutlery
A three-point bending test trial was carried out on the invented cutlery to
compare
the strength properties with a commercially available paper alternative. The
results
of the bending test are presented in Table 2.
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Table 2: Strength comparison of a cardboard fork of the invention and a
commercial paper fork
Maximum load to break Ns
Invention Fork Commercial Paper Fork
Sample 1 32.527 11.711
Sample 2 32.002 14.082
Sample 3 31.661 12.082
Sample 4 32.771 12.549
Sample 5 31.918 12.545
Sample 6 32.793 10.844
Sample 7 38.421 11.483
Sample 8 32.412 9.308
Sample 9 27.212 10.885
Sample 10 30.594 10.619
The test was carried out by a certified third-party laboratory using standard
testing
procedures. The maximum force applied to break was measured in Newtons. The
results clarified that the invented material is up to three times stronger to
the point
of breaking compared with the commercially available paper forks which will
provide
greater performance to the end user.
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