Note: Descriptions are shown in the official language in which they were submitted.
PROCESS FOR FORMING PAPER-BASFD PRODUCTS
lH~VING C~MENT-BASED COATINGS
This invention relates to coating compositions and process for using the
5 coating compositions. More particularly, this invention relates to cemen~-containing
coating compositions which can be applied to paper products and the processes for
preparing and coating these products.
In recent years, increasing concerns have been expressed about the use
10 of plastic containers~ such as styrofoam cont~iners, in the food industry. Plastic
containers, such as those used in grocery stores and fast food restaurants to package
food, are not degradable and, as a result, are contributing to the fill-up of landfills.
In addition, when some plastic containers are manufactured or burned, harmful
compounds are released into the atmosphere which cause pollution or which damage15 the ozone layer. For example, styrofoam is usually foamed by using
chloroIluorocarbon blowing agents such as Freon 11 and Freon 12, which can damage
the ozone layer. Thus, during the manufacturing of styrofoam, chlorofluorocarbons
are usually released into the atmosphere. In addition, when styrofoam is formed
using chlorofluorocarbons, certain chlorofluorocarbon blowing agents will leak out
20 of a styrofoam product during the life of the product. Due to these environmental
concerns, there has been an increased interest in recent years in finding substitutes
for food packaging plastic containers which are degradable and which do not result
in the release of harmful substances when being manufactured or burned.
Although paper containers are degradable, a drawback to using paper
containers instead of plastic containers for food is that paper absorbs moishlre and
grease leading to premature degradation of the container. Thus, in order for paper
containers to be adequate substitutes for plastic containers, it is necessary to treat the
surface of the paper containers to render them substantially moisture-resistant.
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It is known that paper products and containers can be rendered
moisture-lesistant by coating them with various polymers. For example. United
States patent No. 3,573,125 discloses that coatings comprised of ethylene-vinyl
acetate and wax have been used for years in the packaging field to coat various
substrates, including paper. United States patent No. 3,704,157 discloses that paper
sheets adapted for packaging can be coated with a copolymer of ethylene and vinyl
acetate to impart high moisture barrier properties. United States patent No.
3,298,855 discloses a moisture-resistant wrapping paper prepared by coating paper
with blends of polyethylene latex and vinyl acetate copolymer latex. United States
patent No. 3,305,383 discloses that the moisture permeability resistance of paper
board substrates used to prepare cartons can be enhanced by employing coating
compositions containing wax and a copolymer of ethylene and vinyl acetate.
One problem in using polymcr coatings to render paper articles
lS moisture resistant is that the polymer coatings serve as a barrier to moisture once the
articles are buried in a landfill and thereby inhibit the natural and, at this point,
desirable degradation of the paper article. However, once the polymer coating breaks
down, the paper article can degrade more readily. Thus, there is a need for a
polymer coating which can be applied to paper substrates to render them moistureresistant and which will readily degrade so as to permit the degradation of the
substrate.
Various compositions containing cement are disclosed in the prior art.
For example, United States patent No. 3,063,~51 discloses a cement-based paint
comprised primarily of Portland cement admixed with a non-aqueous vehicle made
up of a small amount of resin dissolved in an ~rganic solven~. The cement-based
paint is used to waterproof masonry walls. United States patent No. 2,600,081
discloses a Portland cement-based paint that can be used to coat porous fibre-board
material. The Portland cement-based paint contains methyl cellulose.
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Several patents disclose the use of an ethylene-vinyl acetate copolymer
in conjunction with cement. For example, a United States patent No. 4,434,257
discloses a cement composition which is comprised of a mixture of a conventionalcement composition with an ethylene-vinyl acetate copolymer emulsion which contains
5 a polyvinyl alcohol and a fatty acid ester of a polyvalent alcohol. The cement composition can be applied to buildings to improve water resistance.
United States patent No. 4,441,944 discloses a cementitious
composition comprising a Portland cement, a polymeric emulsion and flyash. The
10 composition can be foamed and applied to the surface of a sheet~like insulating board
which can be made of fibrous cellulose.
United States patent No. 4,395,159 discloses a coating composition
applied to a metal substrate to protect the substrate from deterioration. The coating
15 composition comprises a cementitious material, such as Portland cement, a filler, a
polymeric material~ such as vinyl acetate polymers and copolymers, and chopped
fibres.
United States patent No. 4,844,964 discloses that Portland cement can
20 be mixed with water and an aqueous emulsion of polymers, such as ethylene vinyl
acetate copolymers, and then used to prepare a signboard.
The prior art also discloses that variolls acids may be employed to
retard the setting time for cement. For example, United States patent No. 4,892,586
25 discloses various organic acids or salts thereof which may be used as retarders for the
setting or solidification of cements. Vnited States patent No. 2,470,505 discloses that
maleic acid can be added to a cement slurry to act as a retardant to lengthen the
stiffening time. United States patent No. 4,054,461 discloses that
hydroxypolycarboxylic acid may be used as a retarder in a cement composition, and
30 United States patent No. 3,188,221 discloses that some organic acids may be used as
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retarding agents for retarding the setting time of eement. Canadian patent No.
638,274 discloses a cement product eontaining Portland eement and ~atty acids.
However, none of the above-mentioned patents discloses a eoating
5 composit;on which can be applied eO a paper substrate to render the substrate moisture
resistant during the substrate's useful life but which will readily degrade once the
substrate is discarded, sueh as in a landfill, so that the paper substrate can then
degrade and disintegrate.
It is an object of the present invention to provide a novel process for
preparing a moulded paper stock produet whieh obviates or mitigates a~ least one of
the above-mentioned defieienees of the prior art.
Accordingly, the present invention provides a process for preparing a
15 moulded paper product, the process comprising ~he steps of:
(a) providing a pulped paper feedstock;
(b) moulding the pulped paper feedstock to produee a shaped
product;
(c) drying the shaped produet;
(d) applying a coating eomposition on to at least one surfaee of the
shaped produet; and
(e) euring the coating eomposition.
The proeess generally eomprises providing a pulped paper feedstock,
25 moulding the feedstock to produced a shaped product; drying the shaped product;
coating the dried, shaped produet with a eoating eomposition; euring the eoatingcomposition; and, optionally, hot pressing the cured coated produet. Depending on
the nature of the coating composition, once the coated produet is discarded in amoisture active landfill, the eoating will begin to break down, thereby allowing the
30 paper substrate to degrade. The eoating composition suitable for use in the present
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process is not particularly restricted and is preferably degradable to some extent.
Prefereably, the coating composition is comprised of a cement, at least one of a clay
and a sulphate and an aqueous emulsion polymer. More preferably, the coating
composition further comprises at least one of an organic acid and a wax.
The choice of pulped paper feedstock for use in the present process is
not particularly restricted nor is the manner by which it is obtained. Preferably, the
pulped paper feedstock is repulped newsprint or other paper-based waste. The
following detailed description of an embodiment of the present invention is provided
10 in the context of using a pulped paper feedstock including a repulped newsprint or
other paper-based feedstock; however, it will be appreciated that the invention is not
limited solely to such a feedstock.
An embodiment of the present invention will be described with
15 reference to the attached Figure in there is illustrated a block diagram of the present
process. As shown in the Figure, waste newsprint is repulped, separated, moulded,
dried, coated, cured and hot pressed to produce a moulded paper product.
The stock which is used for coating with the composition of the instant
20 invention is preferably a moulded pulp stock which is derived from waste paper, more
preferably from waste newsprint and corrugated liner. Of course, virgin pulp andpulp screenings may also be used. In general, the waste paper is received in bullc and
manually sorted to remove glossy printed papers which are difflcult to recycle. After
sorting, the selected waste paper is transferred by conveyor into a batch repulper,
25 where it is mixed with water, rosin (which is used to size the paper) and an
emulsified wax to help reduce moisture abso~ption. Within the batch repulper, the
waste paper is beaten into a uniform consistency with a measured quantity of recycled
water to produce a pulped paper feedstock having a solids content in the range of
about 2 to about 10 percent by weight, preferably from about 3 to about 5 percent by
30 weight.
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In general, the amount of rosin used is in the range of from about 0.5
to about 2 percent by weight, preferably from about 1 to about 3 percent by weight.
The emulsified wax is added in ~he range of from about 0.5 to about 2 percent byweight, preferably from about 1 to about 3 percent by weight. The preferred
5 emulsified waxes are paraffin based, although any waxes which are compatible with
the stock solution herein rnay be used. The stock ;s maintained in the batch repulper
at a temperature which is normally .slightly above ambient conditions, generally in the
range of from about 80F to about 115F, preferably from about 90F to about
100F for about 30 to about 45 minutes or until the fibre is broken down and is in
10 the form of a pulp.
The repulped waste paper stock is then pumped into a raw stock tank
where it is held for further processing. From there, the raw stock is pumped into a
high density separator to remove debris and other contaminants. Next, the raw stock
15 may or may not be passed over a screen where lumpy material is removed. The
lumpy material may then be processed through a refiner and combined with the
acceptable material from the screen which together are then pumped to a re~ned stock
tank where they are held for further processing at a slightly lower solids content than
the raw stock storage.
The reflned material and the material from the high density separator
which is not in need of refining are pumped into a tank where they are blended with
recirculated water, aluminum sulphate in the range of up to about 1 percent by weight
to ;mprove the ability of the rosin to size the paper stock, a defoamer in the range of
25 from about 1 to about 10 parts per million by weight, and other necessary ingredients,
depending wpon the end use. Examples of other ingredients include ~llers, additives
such as clays, including kaolin, calcium carbonate, as well as polymer de-watering
enhancers such as cationic starch. The blended material produced in this step is then
suitable for use as pulped paper feedstock in the present process.
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The blended material, in this instance the pulped paper feedstock for
the present process, is then transferred to the moulding vat where additional water is
added to reduce the solids content to from about 0.3 to about 2.0 percent by weight,
preferably from about 0.5 to about 1.0 percent by weight. In the moulding vat, the
5 pulp stock is con.inuously recirculated as it is exposed to the vacuum moulder which
is designed to form the product to be utilized. In general, particularly with small
items such as food trays, the moulder will form large size assemblies containingmultiple units of the item to be ~ormed. For example, with food trays the overall size
of the assembly is about 14 x 25 inches and contains about 3-9 food trays per
10 assembly. The moulding machine itself comprises, in addition to the moulding vat,
the vacuum moulder and the transfer dye mechanism. The vacuum moulder consists
of a moulding drum, preferably having several faces cont uning dyes having the shape
of the desired product. As the drum rotates, each face in succession is dipped into
the pulp stock. A vacuum system attached to the rotating drum causes the fibres
15 contained in the pump stock to be deposited onto the forming dyes on the face of the
drum as the water in which the fibres are suspended is drawn through the dyes and
drained out. DuAng the moulding operation the fibres orAent in a mechanically-
interlocked layer to form the moulded product. The removed water is recycled forfurther use. As the moulding drum continues to rotate, at a point just prior to re-
20 entry into the vat, a rotating transfer dye mates vvith the moulded face on the drumand, under suction, removes the wet moulded product, depositing it on a conveyor
line. Moulders of the type which may be used in the present process are further
descAbed in United States patent No. 3,661,707.
The wet moulded product is then transferred into a dryer where it is
dAed for a peAod of time and under temperature conditions which are adjusted
depending upon the size, shape and end use of the particular product being moulded.
In general, for paper trays, the product is dAed at an oven temperature in the range
of from about 375F to about 500P, preferably from about 400F to about 450F,
for period of time in the range of from about 10 to about 30 minutes. Preferably, as
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the product leaves the dryer, it will have achieved a surface temperature of from
a~out 180F to about 230F.
After exiting the dryer, the product may be coated with a coating
5 composition usin~ one of several coating application ~echniques as described above.
However, the preferred method involves spray coating. In order to minimize
moisture and coating absorption, the tray assembly is reheated immediately before
coating in an oven at the same temperature described above so that the ~emperature
of the assembly is again elevated to a value in the range of from about 190F to10 about 230F. After one side of the tray assembly is coated, the assembly is then
reheated to about 190F and the second side is coated. Preferably, the back or rough
side of the tray assembly will be coated to a 3 mil dry film thickness while the front
or smooth side will be coated to about a 1.5 mil dry film thickness. While the
coating processes which are employed, as pointed out above, may vary, it is preferred
15 to use a two-step spraying process employing an air-assisted, low pressure spray
followed by an electrostatic spraying.
As described above, in addition, different coatings may be applied to
the front and to the back of the article and multiple coatings may be applied,
20 depending upon the particular end use.
After the assembly is coated, it is preferably passed through a 350F
to 500F curing oven where it is maintained for a period of from about 30 seconds
to several minutes, all again depending upon the coating composition and the desired
25 end use involved.
After curing, the assembly may then be hot-pressed using a standard
press for a period of from about 0.2 to about 2 seconds, at temperatures rangingbetween about 250F and about 400F, and at a pressure of from about 50 ~o about30 25Q psi. The purpose of hot-pressing is to smooth the coating and to add gloss.
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After the coated assembly leaves the hot press it is then, in the preferred instance,
disassembled by cutting, stacked and stored for shipment.
The prefierred coating composition suitable for use in tlle present
5 process compr~ses a cement, more preferably white Portland cement.
The preferred coating composition also preferably comprises a clay.
The clay should be of extremely fine particle size and should consist of one or more
clay minerals, including hydrosilicates of aluminum, iron or magnesium. Most
10 preferred among the clays are the kaolin clays which include kaolin;te dickite and
halloysite-endellite. Clays are well known in the art and the appropriate c]ay for use
in the preferred coating composition would be apparent to a person skilled in the art.
In addition to the clay, another component of the preferred coating
15 composition for use in the present process is a sulphate, preferably, an alkali or
alkaline earth metal sulphate, and most preferably, calcium sulphate.
The cement, clay and sulphate suitable for use in the preferred coating
composition are readily available commercially.
The preferred coating composition also contains an aqueous emulsion
polymer which, preferably, has F:DA (or other appropriate governmental) approval,
if the finished product is to be used in a food-type application. Any aqueous
emulsion polymer which will impart water resistance to a paper substrate may be
25 employed as long as it will not be degraded or otherwise break down at a relatively
high pH or in the presence of a cement. Preferably, the polymer is a carboxyl
functional emulsion polymer, such as acrylic, polyester, epoxy, vinyl, vinyl acetate,
ethylene-vinyl acetate, or epoxy ester polymer or copolymer. Most preferably, the
emulsion polymer is an ethylene-vinyl acetate aqueous emulsion copolymer.
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The ethylene-vinyl acetate copolymers which are suitable can be
prepared by copolymerizing a mixture of ethylene and vinyl acetate in the presence
of a free-radical catalyst. Suitable ethylene-vinyl acetate emulsion compositions are
readily available commercially. One example of a suitable emulsion copolymer is
5 Air~exTM 100 HS latex, available from Air Produces and Chemicals, Inc.
The preferred coating composition for use in the present process more
preferably further comprises a wax, most preferably a wax emulsion, to aid in press
release and to retard water and o~her moisture absorption. The waxes which may be
10 used herein may either be synthetic or naturally occurring. However, in order for
the wax to carry out its intended function, it is important that the wax not degrade in
the presence of other components in the coating composition. Among the waxes
which may be used are included naturally occurring waxes such as esters of long
chain fatty alcohols and acids, petroleum and`rnineral waxes. Among the common
15 waxes which rnay be used are the vegetable waxes such as carnauba wax, candelilla
wax, and hydrogenated candelilla wax. Other waxes include the synthetic waxes such
as polyethylene waxes and paraffin waxes, these latter waxes being preferred.
The preferred coating composition more preferably further comprises
20 an organic acid, such as maleic, stearic, potassium hydrogen t~rtrate or oleic acid or
any organic sugar such as corn syrup, which serves to retard the setting time of the
coating compositions. Difunctional acids are preferred. The presence of an organic
acid in the coating composition helps to prolong the pot life and to make it easier to
spray the composition onto a degradable substrate such as a paper substrate.
In addition to the above-described components, the coating composition
may contain conventional fillers, additives, thickeners, defoamers and pigments.Since the cement coating composition can be readily applied to paper substrates, such
as vacuum moulded pulp food trays, and since pigments can be readily added to the
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coating composition, the coating composition provides a relatively easy means for
colouring the surface of a paper substrate a particular colour.
Preferably, the coating composition is prepared by first combining the
5 clay, the sulphate, the acid and the aqueous emulsion polymer in water. The cement
should be added shortly before the coating composition is applied to a paper substrate
because the coating composition has a pot life of up to 24 hours once the cement is
added.
Generally, the preferred coating composition will contain from about
40 to about 90 percent by weight cement, preferably from about 50 to about 75
percent by weight; from 0 to about 20 percent by weight of a clay, preferably from
about 2 to about 10 percent by weight; from 0 to about 20 percent by weight of
sulphate, preferably from about 1 to about 15 percent by weight; and from about 6
15 to about 60 percent by solids weight of the aqueous emulsion polymer, preferably
from about 10 to about 40 percent by weight; with the plOViSO that at least one of the
clay and the sulphate are actually present in the composition. All weights are based
upon the total solids weight of aqueous emulsion polymer, cement, sulphate, and clay
in the coating composition.
In the event an organic acid or sugar is added, it should be added in
an amount in the range of from about 1 to about 20 percent based on the above
compositional weight. In the event a wax is added, it should be added in an amount
in the range of from about the 1 to about 25 percent by weight solids level based on
25 the above compositional weight.
In order for the paper substrate to disintegrate after it is discarded, it
is necessary for the coating composition to break down and degrade so that the paper
substrate can degrade. The intact cement coating serves as a protective coating ~or
30 the paper substrate thereby inhibiting premature disintegration of the paper substrate.
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Once the cement coating begins to break down and degrade, the paper substrate can
also begin to degrade.
Dis;ntegration of the coating composition, and thus of the paper
S substrate, will occur when the degradable coating is placed on at least one side of the
paper substrate. Faster disintegration, of course, will occur when both sides of the
paper substrate are coated with the degradable coating. However, it is possible to
coat one side with the degradable coating described herein and another side with a
more conventional coating or a less degradable coating. In general, with food
10 products a less degradable or non-degradable coating is placed on the side of the
paper which will be exposed to the food product and the reverse side is coated with
the composition described herein. The partially degradable compositions which may
be used in conjunction with the coating compositions described otherwise herein
simply involve the use of higher percentages of the coating composition of the
15 emulsions described herein and lesser amounts of the cement and other components.
When the cement and other components are totally removed, of course, the coatingcomposition becomes essentially non-degradable.
A particularly preferred coating composition which still maintains
20 certain degradative properties but which exhibits heightened resistance to liquids,
generally, and food liquid specifically, involves a coating composition comprising:
from about 15 to about 60 percent by weight of a cement; from about 40 to about 75
percent by weight on a solids basis of aqueo-ls emulsion polymer; from 0 to about 20
percent, preferably from about 1 to 20 percent of sulphate; and from 0 to about 10
25 percent, preferably from about 1 to 10 percent of a clay, all as described above.
The coating composition used in the present process may be applied by
conventional spraying or a;rless sprayin~, with or without electrostatic assist. In
addition, electrostatic discs may be used to apply the coating compositions during the
30 present process.
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Although not wishing to be limited to any particular theory or mode
of the degradation the cement in the preferred coating composition, it is believed that
the sulphate (e.g. calcium sulphate) in the preferred coating absorbs water and forms
a complex with the kaolin which causes the cement to form a slush and the dried
5 coating to break down. As a result of the presence of the sulphate, kaolin and cement
in the preferred coating, the coating will readily degrade and decompose within a few
years in a moisture, and preferably biologically, active environment such as a landfill,
there~y allowing the paper substrate also to degrade.
10Since the preferred coating composition for use in the present process
is substantially non-toxic, it may be used to coat paper articles for food storage, such
as vacuum moulded paper food trays used in the fast food industry. The paper
containers coated with the coating composition have a semi-lustrous finish and smooth
surface texture and are resistant to food juices`and moisture.
Embodiments of the present invention will be described with reference
to the following Examples which are provided for illustrative purposes only and
should not be used to limit the scope of the in~ention. In the following Examples,
all parts are by weight, unless otherwise specified.
Example 1
Components A and B were prepared as follows:
25Component A
24.39 parts water were blended under agitation with 0.41 parts of a
NatrosolTM 330 thickener available from Aqualon Co., 0.96 parts of TamolTM 850
surfactant available from Rohm & Haas Co., 0.30 parts of ammonium hydroxide and
0.54 parts of DrewplusTM L140 defoamer available from Drew Chemical. Also added
30with agitation were 16.8 parts of KronosTM 2020 titanium dioxide available from
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Kronos, Inc., 2.57 parts of ~itoxTM titanium dioxide available from Hitox
Corporation of America, along with ~.57 parts of burnt umber pigment and 14.98
parts of calcium carbonate. 20.74 parts of water, 127.99 parts of Air~ex 100, anEVA emulsion (aqueous emulsion polymer) available from Air Products & Chemicals
Company and 90.17 parts of MichemlubTM 368, a wax emuls;on available from
Michelman Chemical Co were then added to the mixture.
Component B
17.95 parts of water were blended with 7.11 parts of corn syrup, 3.66
10 parts of ammonium hydroxide, 9.33 parts of potassium hydrogen tartrate, 9.33 parts
of TamolTM 850, 24.70 parts of HuberTM 35, a kaolin clay (clay), 9.89 parts of
calcium sulphate (sulphate), and 1.08 parts of DrewplusTM L140. To this mixture was
added a total 156.88 parts of tap water, and 459.8~ parts of white Portland cement
type I (cement).
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Components A and B were then mixed together and exhibited a pot life
of two hours. Over this period of time the mixture could readily be applied to
reconstituted paper or cardboard ~ats by spraying as well as by other conventional
coating means.
A coating of the Component A and Component B mixture was sprayed
to a thickness of 3 mils on reconstituted paper plates using, in one case, a standard
spray gun and, in another case, an electrostatic disc. The plates were dried in a
350F oven. The plates were then placed in a platen press and heated at a
25 temperature of 100 C and under 100 psi for about 1 second. A uniform, smoothcoating resulted. The coatings degraded when p]aced in a mo;sture active landfill,
i.e., a landfill containing sufficient moisture to degrade the coating in question over
time.
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Example 2
A two-component coating composition was prepared as follows:
S Component A
69.12 parts of water were mixed with 1.50 parts of NatrosolTM 330,
2.73 parts of TamolTM 850, 0.30 parts of ammonium hydroxide, 1.54 parts of
DrewplusTM L140 defoamer. Added to this mixture under agitation were 47.62 partsof KronosTM 2020, 7.30 parts of HitoxTM titanium dioxide, 0.96 parts of burnt umber
pigment, 42.46 parts of calcium carbonate, 58.77 parts of water, 362.70 parts ofAir~exTM 100 emulsion, and 255.54 parts of wax emulsion.
Component B
3.58 parts of water were mixed with 1.52 parts of corn syrup, 0.79
parts of ammonium hydroxide, 2.0 parts of potassium hydrogen tartrate, 2.0 parts of
TamolTM 850, 5.30 parts of HuberTM 35 clay, 2.13 parts of calcium sulphate, and
0.23 parts of DrewplusTM L140. Added for viscosity control were 33.65 parts of
water. Finally 98.64 parts of white Portland cement type I were added to the
mixture.
Components A and B were blended and sprayed to a thickness of 5 mil
on a reconstituted paper plate stock, dried and cured as in Example 1. The reverse
side of the plate was sprayed, dried and cured with the blend described in Example
1.
The coated plates were then brought in contact with meat products.
The inside coating using the Example 2 composition showed excellent resistance to
the meat product. When the coated plate is exposed to a moisture active landfill, it
degraded.
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As can be seen, the Example 2 blend contains a much higher level of
emulsion than does Example 1 so as to retard food liquid penekation of the cardboard
stock. On the other hand, the mateAal prepared ;n Example 1 is much more readilydegradable because of its high concentration of cement and its relatively low amount
5 of emulsion polymer.
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