Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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a.
METHOD FOR. TREATING A SUBSTRATE =
Inventors: Gary P. Fugitt, Scott E. Ginther, John W. Stolarz, Robert W.
Carlson, Stanley H.
McGrew, Steven P. Metzler, Terrell J. Green
BACKGROUND OF THE INVENTION
[0002] The present disclosure relates to a method for treating a substrate
with a polymer
film-forming composition. More particularly, the disclosure relates to a paper
or paperboard
manufacturing method comprising the steps of applying a polymer film-forming
coating to a
substrate, and, bringing the polymer coating into contact with a heated
surface while the
polymer coating is still in a wet state. The resulting polymer layer has a
smooth surface with
voids (e.g., bubbles) just below the surface. In certain embodinients, the
polymer coating
may comprise a crosslinkable hydrogel, and a crosslinking solution may be
applied to the
= polymer coating on the substrate surface thereby forming at least a
partially crosslinked
polymer coating then placed into contact with a heated surface. The present
disclosure also
relates to a treated substrate product.
[0003] Paper is manufactured by an essentially continuous production process
wherein a
dilute aqueous slurry of cellulosic fiber flows into the wet end of a paper
machine and a
consolidated dried web of indefinite length emerges continuously from the
paper machine dry
end. The wet end of the paper machine comprises one or more headboxes, a
drainage section
and a press section. The dry. end of .a modem paper machine comprises a
multiplicity of =
' steam heated, rotating shell cylinders distributed along a
serpentine web traveling route under
a heat confining hood structure. Although there are numerous design variations
for each of
these paper machine sections, the commercially most important of the variants
is the
fourdrinier machine wherein the headbox discharges a wide jet of the slurry
onto a moving
screen of extremely fine mesh.
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[0004] The screen is constructed and driven as an endless belt carried over a
plurality of
support rolls or foils. A pressure differential across the screen from the
side in contact with
the slurry to the opposite side draws water from the slurry through the screen
while that
section of the screen travels along a table portion of the screen route
circuit. As slurry
dilution water is extracted, the fibrous constituency of the slurry
accumulates on the screen
surface as a wet but substantially consolidated mat. Upon arrival at the end
of the screen
circuit table length, the mat has accumulated sufficient mass and tensile
strength to carry a
short physical gap between the screen and the first press roll. This first
press roll carries the
mat into a first press nip wherein the major volume of water remaining in the
mat is removed
by roll nip squeezing. One or more additional press nips may follow.
[00051 From the press section, the mat continuum, now generally characterized
as a web,
enters the dryer section of the paper machine to have the remaining water
removed
thermodynamically.
[0006] Generally speaking, the most important fibers for the manufacture of
paper are
obtained from softwood and hardwood tree species. However, fibers obtained
from straw or
bagasse have been utilized in certain cases. Both chemical and mechanical
defiberizing
processes, well known to the prior art, are used to separate papermaking fiber
from the
= composition of natural growth. Papermaking fiber obtained by chemical
defiberizing
processes and methods is generally called chemical pulp whereas papermaking
fiber derived
from mechanical defiberizing methods may be called groundwood pulp or
mechanical pulp.
There also are combined defiberizing processes such as semichemical,
thermochemical or
thermomechanical. Any of the tree species may be defiberized by either
chemical or
mechanical methods. However, some species and defiberizing processes are
better economic
or functional matches than others.
[00071 An important difference between chemical and mechanical pulp is that
mechanical
pulp may be passed directly from the defiberizing stage to the paper machine.
Chemical pulp
on the other hand must be mechanically defiberized, washed and screened, at a
minimum,
after chemical digestion. Usually, chemical pulp is also mechanically refined
after screening
and prior to the paper machine. Additionally, the average fiber length of
mechanical pulp is,
as a rule, shorter than that of chemical pulp. However, fiber length is also
highly dependent
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upon the wood species from which the fiber originates. Softwood fiber is
generally about
three times longer than hardwood fiber.
[0008] The ultimate properties of a particular paper are determined in large
part by the
species of raw material used and the manner in which the paper machine and web
forming
process treat these raw materials. Important operative factors in the
mechanism of forming
the paper web are the headbox and screen.
[0009] Coated paper or paperboard used for printing and for packaging is
generally
required to have high level of gloss, excellent smoothness, and excellent
printability, as well
as certain strength and stiffness characteristics.
[0010] If the coated paper or paperboard has a high stiffness, it can pass
smoothly through
=
high-speed printing or packaging machines with less feeding jams. Higher
stiffness paper
=
can be advantageously used in books, magazines, and catalogues, because it
provides a feel
of hardness or heaviness similar to a hardcover book. For packaging, high
stiffness is
necessary for maintaining the structural integrity of the paperboard product
during filling and
in subsequent use.
[0011] Stiffness has close relationship to the basis weight and density of
paper. There is a
general trend that stiffness increases as the basis weight increases, and
decreases as the paper
density increases. Stiffness and other properties can be improved by
increasing basis weight.
However, this would result in a product utilizing more fibers, which add cost
and weight.
Therefore, coated paper or paperboard with high stiffness but moderate basis
weight is
. desirable. Paper with moderate basis weight is also more economical
because less raw
material (fiber) is utilized. In addition, shipping costs based on weight are
less for low basis
weight paper.
=
[0012] In addition to high stiffness, coated paper or paperboard which must be
printed is
often required to have high gloss and smoothness. For coated paper or
paperboard to have
. such quality characteristics, density typically must be increased to some
extent to allow for a
usable printing surface. Smoothness is normally achieved by calendering.
However,
calendering will cause a reduction in caliper, which typically results in a
corresponding
reduction in stiffness. The calendering process deteriorates the stiffness of
paper by
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significantly reducing caliper and increasing the density. The base sheet for
conventional
Coated board grades typically is heavily densified by calendering to provide a
surface
roughness low enough to produce final coated smoothness acceptable to the
industry. These
calendering processes, including wet stack treatment, may increase density by
as much as
20% to 25%.
[0013] Thus, the relationship between gloss and stiffness and between
smoothness and
stiffness are generally inversely proportional to each other, for a given
amount of fiber per
unit area. Packaging grades are sold based on caliper, so manufacturing
processes that reduce
the caliper (increasing the density of the board) decrease the selling price.
Processes that
cause less caliper reduction save material costs. Caliper is measured in
"points", where a
point --= 0.001 inches. For example, the conventional method for making a 10-
point board
requires the use of a board having a thickness of greater than 12 points prior
to calendering.
It would be desirable to be able to produce a finished board having
approximately the same
thickness as the starting substrate.
[0014] Improvements in the calendering process including moisture gradient
calendering,
hot calendering, soft calendering, and belt calendering slightly improved
stiffness for a given
caliper but did not change the fundamental ratio between caliper, stiffness,
smoothness, and
printing properties.
[0015] Various proposals have been made to improve the stiffness of coated
paper or
paperboard without calendering for printing. For example, several proposals
include high
softwood content in the raw stock, addition of specially engineered fibers in
the raw stock,
addition of highly branched polymers within the raw stock, and high amounts of
starch or
copolymer latex with a high glass transition temperature (commonly referred to
as "Tg")
within the coating formulation.
[0016] However, potential drawbacks to these methods of stiffness improvement
are that
although they are useful in improving paper stiffness, they could potentially
degrade the
smoothness, gloss, and/or printability of the coated paper obtained.
[0017] For the reasons mentioned above, it has been very difficult to obtain
satisfactory
paper smoothness without increasing density. Other methods can be used for
changing the
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density/smoothness relationship in paper and paperboard grades. Applying a
paper coating is
a very common way to enhance the surface properties of paper without causing
the drastic
increases in paper density typically associated with the levels of calendering
required to
obtain a certain level of smoothness. Preferably, the final coated surface
should be uniform
to provide acceptable appearance and printing properties.
[0018] Therefore, it would be desirable to provide a paper or paperboard
product having the
desired properties while maintaining the initial density of the sheet or
minimizing the
increase in density. Furthermore, it would be desirable to provide a paper or
paperboard
exhibiting improved smoothness without the concomitant increase in density
associated with
conventional methods for creating smoothness. Cast coating methods exist for
producing a
very smooth surface, but these methods are typically run at production rates
slower than the
speed of many paper machines.
SUMMARY OF THE DISCLOSURE
[0019] In one embodiment, a product is disclosed that includes a substrate
with a coating on
the substrate. The coating includes a water soluble polymer and a release
agent. There are
voids formed within the coating.
[0020] In another embodiment, a product is disclosed that includes a substrate
with a
coating on the substrate. The coating includes a water soluble polymer and
essentially no
=
elastomeric material. There are voids formed within the coating.
[0021] In another embodiment, a product is disclosed that includes a substrate
with a
coating on the substrate. The coating includes.a surface, and the surface has
a Sheffield
Smoothness of less than about 300 units. There are voids formed under the
surface of the
coating.
[0022] In another embodiment, a product is disclosed that includes a substrate
with a
coating on the substrate. The coating includes a water soluble polymer, a
release agent, and
essentially no elastomeric material. The coating includes a surface, and the
surface has a
Sheffield Smoothness of less than about 300 units. There are voids formed
under the surface
of the coating.
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[0023] In another embodiment, a process is disclosed for treating a substrate.
A wet film of
aqueous polymer solution is applied to the substrate. The aqueous polymer
solution is
immobilized by bringing it into contact with a heated surface to cause the
aqueous polymer
solution to boil, and to at least partially dry the aqueous polymer solution.
[0024] In another embodiment, a process is disclosed for treating a substrate.
A wet film of
aqueous polymer solution is applied to the substrate. The aqueous polymer
solution is
immobilized by bringing it into contact with a heated surface to cause the
aqueous polymer
solution to boil and form voids that remain in the aqueous polymer solution,
and to at least
partially dry the aqueous polymer solution.
[0025] In another embodiment, a process is disclosed for treating a substrate.
A coating of
aqueous polymer solution is applied to the substrate as a wet film. The
coating includes a
water soluble polymer and a release agent. The film is immobilized by bringing
it into
contact for less than about 3 seconds with a heated surface with a temperature
above about
150 C so as to cause the aqueous polymer solution to boil and form voids in
the film, and to
at least partially dry the film.
[0026] In another embodiment, a process is disclosed for treating a substrate.
A coating of
. aqueous polymer solution is applied to the substrate as a wet film. The
coating includes a
water soluble polymer and essentially no elastomeric material. The film is
immobilized by
bringing it into contact for less than about 3 seconds with a heated surface
with a temperature
above about 150 C so as to cause the aqueous polymer solution to boil and form
voids in the
film, and to at least partially dry the film.
aqueous polymer solution is applied to the substrate as a wet film. The
coating includes a
water soluble polymer and essentially no elastomeric material. The film is
immobilized by
bringing it into contact for less than about 3 seconds with a heated surface
with a temperature
above about 150 C so as to cause the aqueous polymer solution to boil and form
voids in the
film, and to at least partially dry the film. The coating surface after drying
has a Sheffield
Smoothness of less than about 300 units.
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[0028] In another embodiment, a process is disclosed for treating a substrate.
A coating of
aqueous polymer solution is applied to the substrate as a wet film. The
coating includes a
water soluble polymer, a release agent, and essentially no elastomeric
material. The film is
immobilized by bringing it into contact for less than about 3 seconds with a
heated surface
with a temperature above about 150 C so as to cause the aqueous polymer
solution to boil
and form voids in the film, and to at least partially dry the film. The
coating surface after
drying has a Sheffield Smoothness of less than about 300 units.
[0029] In another embodiment, a process is disclosed for treating a cellulosic
substrate. A
wet film of aqueous polymer solution is applied to the substrate. The aqueous
polymer
solution includes at least about 60% water soluble polymer by dry weight, and
up to 10%
release agent by dry weight. The aqueous polymer solution is immobilized by
bringing it into
contact for less than about 3 seconds with a heated surface with a temperature
above about
150 C so as to cause the aqueous polymer solution to boil and form voids in
the aqueous
polymer solution, and to at least partially dry the aqueous polymer solution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030j FIG. 1 is a schematic view of an apparatus for treating a substrate
with a polymer
coating in accordance with one embodiment of the present invention.
[00311 FIGs. 2 ¨ 9 are cross section micrographs showing the morphology of
samples made
in accordance with one embodiment of the invention, and having a top coating.
[0032j FIGs. 10-12 are cross section micrographs showing the morphology of
samples
made in accordance with one embodiment of the invention.
[00331 FIGs. 13-14 are surface micrographs made by scanning electron
microscope
showing the morphology of samples made in accordance with one embodiment of
the
invention.
[00341 FIGs. 15-16 are surface micrographs made by backscatter scanning
electron
microscope showing the morphology of samples made in accordance with one
embodiment
of the invention.
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[0035] FIG. 17 is a graph showing distribution of void dimensions in samples
made in
accordance with one embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0036] In describing the preferred-embodiments, certain terminology will be
utilized for the
sake of clarity. It is intended that such terminology include not only the
recited embodiments
but all technical equivalents that operate in a similar manner, for a similar
purpose, to achieve
- a similar result. The citation of any document is not to be construed as an
admission that it is
prior art with respect to the present invention. Unless indicated otherwise or
unless the
context suggests otherwise, all weights, percentages, and ratios are by
weight.
[0037] The present disclosure relates to a method for treating a substrate
with a polymer
film-forming coating. More particularly, the disclosure relates to a paper or
paperboard
manufacturing method comprising the steps of applying a polymer coating to a
substrate, and
bringing the polymer coating into contact with a heated surface while the
polymer coating is
still in a wet state. Boiling of water in the polymer coating causes voids to
form under the
surface, but the surface of the film is smooth. The paper or paperboard
produced in
accordance with certain embodiments of the present invention exhibits
desirable levels of
surface flatness and smoothness without significant densification of the base
paper. In certain
embodiments, the polymer coating may include a crosslinkable material and a
crosslinking
solution may be applied to the polymer coating on the substrate surface
thereby forming at
least a partially crosslinked polymer film-forming composition. In such cases,
the polymer
coating may typically be applied to the web first and then the cross linking
solution applied
before the treated web contacts the heated surface. For weakly cross linking
polymers, it may
be possible to provide the cross linking solution in the coating itself.
[00381 One advantage of treating a substrate with a polymer film-forming
coating in
. accordance with the present invention relates to the improvement in
smoothness and/or .
flatness that can be obtained without significantly increasing the density or
decreasing the
caliper of the sheet. The heavy calendering of the cellulose paper web
associated with
conventional techniques is not required to produce a paper having print
properties
comparable to conventional coated papers. Furthermore, even when the cellulose
paper web
is smoothed, much lower pressures can be applied to provide similar printing
properties on
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=
papers with increased stiffness. In accordance with certain embodiments of the
present
invention, the cellulose paper web is smoothed such that the caliper decreases
not more than
about 7% and typically is decreased by between about 2% and 5%. By comparison,
conventional coated papers are typically calendered before coating at much
higher pressures,
which cause an increase in density of from about 20 to 25%. In accordance with
one aspect
of the invention, the cellulose paper web may be calendered to a Parker Print
Surf
smoothness of between about 2 and 6 microns prior to application of the
polymer film.
However, substrates with higher Parker Print Surf values may be used. For
example, a
. substrate with a Parker Print Surf smoothness of about 9 microns may be
used. Parker Print
Surf smoothness is determined in accordance with TAPPI standard T 555 om-99.
[0039] FIG. 1 illustrates an apparatus 10 useful in practicing certain
embodiments of the
invention. A substrate or web 12 is subjected to treatment on one surface
thereof with
crosslinkable polymer coating 14 to form a layer of polymer coating 16 on web
12. While the
polymer coating is still wet, an optional crosslinking solution 18 may be
applied to the layer of
polymer coating 16 thereby forming a cross linked polymer coating 20 on web
12. The polymer
coating 20 is typically at least partially crosslinked. The polymer coating is
still in a wet state
before being brought into contact with hot polished drum 22 by pressing the
web 12 against the
drum surface with a press roll 24. Heat from the drum surface causes boiling
within the wet
polymer coating, so that voids form in the polymer under the surface. The
crosslinking solution
causes the polymer coating to crosslink and gel into a substantially
continuous layer or film.
Typically, the resulting film will exhibit improved strength over the base
sheet. The polymer
treated sheet may not be fully dried so it may be conveyed through a secondary
heater 26. Any
type of secondary heating device can be used that is capable of drying the
treated sheet without
adversely affecting the properties of the sheet. The treated sheet emerges
from secondary heaters
26 as a polymer film treated substrate 28 characterized by improved flatness
and smoothness.
Optionally, additional coating processes 30 (and other processes such as
coating, gloss
calendering, etc) may be used to form a coated product 32.
[0040] As shown in FIG. 1, the web wraps a substantial portion of hot polished
drum 22.
The amount of wrap may depend on operating conditions such as web speed,
moisture
=
content of the polymer film forming composition 20, temperature of the drum,
and other
process factors. It is possible that a small amount of contact time with hot
polished drum 22
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may be sufficient. Besides providing the substrate in web form, it may also be
provided in
sheet form.
[0041] The crosslinkable polymer coating and the optional crosslinking
solution may be
applied by any number of techniques, such as dip-coating, rod coating, doctor
blade coating,
gravure roll coating, reverse roll coating, metered size press, smooth roll
coating, extrusion
coating, curtain coating, spray coating and the like. The crosslinkable
polymer coating and
crosslinldng solutions may be applied by the same coating technique or
different methods
may be used for each.
[0042] One embodiment in accordance with the present invention is based on the
coagulation or gelling that occurs between polyvinyl alcohol and borax. In
accordance with
this type of system polyvinyl alcohol (PVOH) is an example of a crosslinkable
polymer and a
borax solution is an example of a corresponding crosslinker. Once the PVOH
solution 14 is
applied, at approximately 25% solids and coverage of about 5 g/m2 dry, the
crosslinker
solution 16 is applied at a rate and solution solids to give a borax coverage
of at least about
0.1 g/m2 dry. This wet, crosslinked polymer film 20 is then brought into
contact with a hot
polished drum 22 by pressing the web 12 against the drum surface with a press
roll 24. The
drum surface temperature is at least about 150 C, or in accordance with
certain embodiments,
at least about 190 C so that the coating can be dried and release from the
drum surface. The
contact time of the polymer film to the drum may be in the range of up to
about 3.0 seconds,
more particularly between about 0.5 ¨ 2.0 seconds. This is sufficient time for
the polymer
film to immobilize and solidify, giving the surface of the polymer film a flat
smooth finish
mirroring the surface of the drum. Immobilizing the polymer film includes at
least partially
drying the film. The coating is not necessarily completely dry when it leaves
the drum, so
additional drying 26 may be needed. The web then continues on through the
process and
may receive additional coating layers, for example conventional coatings,
prior to being
wound up. The polymer coating may be applied as a single layer or as two or
more layers.
Limited experiments also suggest that a polymer film may be immobilized or
solidified with
just momentary contact with the heated drum, as may be achieved by using a
press roll 24 to
press the web 12 against the hot drum 22, without any additional wrap of web
around the hot
drum. However, it is contemplated that some wrap of the hot drum may be
practiced, and
that optionally a felt 23 may be used to help press the web into contact with
the hot drum. If a
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=
felt 23 is used to help press the web into contact with the hot drum, then the
felt 23 may be
carried between the press roll 24 and the heated drum 22.
[0043] The contact between the polymer film and the hot drum causes boiling to
occur in
the polymer film, creating voids or bubbles in the film. Nip conditions should
be adjusted so
that boiling may occur. Satisfactory lab results were obtained with a
resilient press roll, a 9"
wide web, and nip loads between about 2 to about 15 pounds per linear inch.
Depending on
press roll hardness and the diameters of the hot drum and press roll,
conditions may have to
be adjusted.
=
[0044] Specific examples of crosslinkable polymers useful in certain
embodiments of the
present invention include crosslinkable hydrogels. The following crosslinkable
hydrogels are
particularly useful: starch, waxy maize, protein, polyvinyl alcohol, casein,
gelatin, soybean
protein, and alginates. One or more polymers selected from the above-recited
ones can be
used. The crosslinkable polymer typically is applied in solution form and
usually as an
aqueous solution. The concentration of the polymer in solution is not
particularly limited but
can be easily determined by one of ordinary skill in the art. For example, a
solution of about
20% starch may be used as described below. The crosslinkable polymer may be
applied to
provide a surface coverage (dry basis) of from about 3 to about 15 gsm (g/m2)
more
particularly from about 4 to about 8 gsm. In accordance with particular
embodiments of the
present invention, the crosslinkable polymer may be used in an amount ranging
from about
60% to about 100% by weight of the dry materials.
= [0045] Specific examples of crosslinkers include borates, aldehydes,
ammonium salts,
calcium compounds and derivatives thereof. The crosslinker if used typically
may be applied
in solution form and usually as an aqueous solution. The concentration of the
crosslinker in
solution is not particularly limited but can be easily determined by one of
ordinary skill in the
art. The crosslinker may be applied to provide a surface coverage (dry basis)
of from about
0.1 to about 0.5 gsm more particularly from about 0.2 to about 0.3 gsm.
[0046] The temperature of the heated surface is in excess of that typically
used for cast
coating. The higher temperature should allow for higher run speeds. It is
anticipated that
paper or paperboard produced in accordance with certain embodiments of the
present
invention may be produced at speeds in the range of about 750 to 3000 fpm,
more
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particularly from about 1500 to 1800 fpm. Although not wishing to be bound by
theory, the
higher temperature and the dwell time are selected such that the coating
composition is
heated to its boiling temperature and it appears that when the coating boils
there is an
increase in contact area between the coating and the drum. The increased
contact results in a
polymer film heated surface that exhibits improved smoothness and gloss.
Furthermore, the
treated surface is ink receptive. Boiling of the coating as it is being
smoothed on the polished
drum surface appears to significantly improve gloss and smoothness of the
finished polymer
film treated substrate.
[0047] The polymer coating on the substrate is typically pressed against the
heated surface
for a sufficient period of time to allow the coating to boil and then set to a
smooth, glossy
finish. In accordance with particular embodiments, the contact time of the
forming polymer
= film to the drum is within the range of up to about 3.0 seconds, more
particularly up to about
2.0 seconds, and most particularly up to about 0.5 seconds.
[0048] The polymer coating may also include one or more pigments. Examples of
useful
pigments include, but are not limited to, kaolin, talc, calcium carbonate,
calcium acetate,
titanium dioxide, clay, zinc oxide, alumina, aluminum hydroxide and synthetic
silica such as
noncrystalline silica, amorphous silica or finely divided silica are examples
thereof. Organic
pigments may also be used.
[0049] The crosslinkable polymer coating and/or the crosslinking solution may
further
include one or more release agents. Specific examples of release agents useful
herein
include, without limitation, waxes, such as petroleum, vegetable, animal and
synthetic waxes,
fatty acid metal soaps, such as metal stearates, long chain alkyl derivatives,
such as fatty
esters, fatty amides, fatty amines, fatty acids, and fatty alcohols, polymers,
such as
polyolefins, silicone polymers, fluoropolymers, and natural polymers,
fluorinated
compounds, such as fluorinated fatty acids and combinations thereof. One of
ordinary skill in
the art can readily determine the amount of release agent to use in a
particular application.
Typically, the coating may contain from about 0.3 to 10 percent release agent,
more
particularly from about 2 to 5 percent by weight. Instead of or in addition to
release agent in
the coating, release agent may be sprayed onto the coating surface, or applied
to the heated
drum surface. If a non-sticking surface can be provided on the heated drum,
whether by a
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release agent or other means, then application of a release agent in the
coating or onto the
coating surface may not be needed.
[0050] The polymer coating employed in certain embodiments of the present
invention,
wherein at least the aforementioned polymer is contained, is generally
prepared in the form of
an aqueous composition. An appropriate ratio between those ingredients is
different
depending on the polymer composition, the application conditions and so on,
but it has no
particular limitation as far as the treated paper produced can satisfy the
quality required for
the intended use thereof. Further, the polymer coating according to certain
embodiments of
the present invention can optionally contain additives, such as a dispersant,
a water retaining
agent, a thickening agent, an anti-foaming agent, a preservative, a colorant,
a waterproofing
agent, a wetting agent, a drying agent, an initiator, a plasticizer, a
fluorescent dye, an
ultraviolet absorbent, a release agent, a lubricant and a cationic
polyelectrolyte.
[0051] In accordance with a particular embodiment of the present invention,
the substrate is
treated with the polymer coating near a central region of the paper machine,
such as the size
press position. Furthermore, the apparatus for applying the polymer coating to
the substrate
may be positioned relative to the paper machine so as to apply the polymer
film to either
surface of the forming paper web. More than one apparatus may be employed to
apply a
polymer film to each side of the forming paper web.
[0052] These advantages allow the use of lightly calendered paper or
paperboard, thus
preserving stiffness while providing good printing properties.
[0053] The base sheet is typically formed from fibers conventionally used for
such purpose
and, in accordance with the particular embodiments, includes unbleached or
bleached lcraft
pulp. The pulp may consist of hardwood or softwoods or a combination thereof
The basis
weight of the cellulose fiber layer may range from about 30 to about 500 gsm,
and more =
particularly, from about 150 to about 350 gsm. The base sheet may also contain
organic and
inorganic fillers, sizing agents, retention agents, and other auxiliary agents
as is known in the
art. The final paper product can contain one or more cellulose-fiber layers,
polymer film
layers and, in accordance with certain embodiments, other functional layers.
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[0054] The present invention in accordance with certain embodiments, provides
one or two-
sided coated paper or paperboard for printing or packaging whose Parker Print
Surf
smoothness value after the coating and finishing processes, when measured
according to
TAPPI paper and pulp test method No. 5A, is lower than about 2-3 microns.
[0055] The paper or paperboard described herein may further be provided with
one or more
additional coatings. A top coating containing conventional components may be
provided to
improve certain properties of the paper or paperboard. Examples of such
conventional
components include pigments, binders, fillers and other special additives. The
top coating, =
when present, may be applied at much lower coat weights than conventional
coatings and yet
provide similar print properties. Accordingly, the top coating weight may be
about 4 to 9
gsm as a single coating layer or about 8 to 18 gsm as two coating layers. By
contrast,
conventional coated papers typically require about 10 to 20 gsm as a single
coating layer or
. 18 to 30 gsm as two coating layers to provide comparable surface properties.
The paper or
paperboard may also be coated on the side of the .sheet having the non-treated
surface.
[0056] Having given the teachings of the present disclosure, it will now be
illustrated by
means of specific examples which should not be considered as limiting the
scope of the
claims in any way.
[0057] A base sheet having a caliper of about 10 points, a Parker Print Surf
(PPS) value of
about 9 microns (10kg pressure with a soft backing) and a Sheffield smoothness
of about 310
can be treated in accordance with certain embodiments of the present invention
to provide a
treated sheet having improved smoothness with only a minimal decrease in
.caliper. The base
sheet may be treated by applying a PVOH solution at approximately 25% solids
to the base
sheet to provide a coverage of about 5 g/m2 dry. Next, the crosslinker
solution may be
applied at a rate and solution solids to give a borax coverage of at least
about 0.1 g/m2 dry.
The wet, crosslinked polymer film can be brought into contact with a hot
polished drum by
pressing the sheet against the drum surface. The drum surface temperature may
be at least
about 190 C. The coating would be dried and released from the drum surface.
The contact
time of the polymer film to the drum would typically be in the range of
between about 0.5 ¨
2.0 seconds. The treated sheet would have a caliper of between about 9.6 and
10.0 points, a
PPS value of about 2.4 to 3.0 and a Sheffield smoothness of about 140¨ 170.
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[0058] In a preferred embodiment, a starch solution may be used as the
polymeric material
in the polymer coating.
[0059] One aspect of the disclosure relates to a paper or paperboard
manufacturing method.
In accordance with one embodiment of the invention, the method includes
applying a
polymer coating comprising a crosslinkable hydrogel to a substrate, applying a
crosslinldng
solution to the polymer coating on the substrate surface thereby forming at
least a partially
crosslinked polymer film-forming coating and bringing the polymer film-forming
coating
into contact with a heated surface while the polymer film-forming coating is
still in a wet
state. The heated surface may be a hot polished drum having a flat smooth
finish. The
temperature of the heated surface typically is within a range of from about
150 C to about
240 C. Higher temperatures may be used, for example up to about 300 C. The
temperature
of the heated surface in accordance with certain embodiments is within a range
of from
180 C to about 200 C, and in accordance with certain embodiments is at least
about 190 C.
[0060] In accordance with particular embodiments of the invention, the
crosslinkable
polymer may be selected from the group consisting of starch, waxy maize,
protein, polyvinyl
alcohol, casein, gelatin, soybean protein, and alginates. In accordance with
certain aspects of
the present invention, the crosslinkable polymer may be used in amounts
ranging from about
60 to about 100% by weight of the dry materials.
[0061] In some manifestations, the crosslinker may be a borate or borate
derivative such as
= borax, sodium tetraborate, boric acid, phenyl boronic acid, or butyl
boronic acid. The
crosslinker may be used in amounts ranging from about 1 to about 12% based on
the
crosslinkable polymer.
[0062] The present invention is also directed to treated papers produced in
accordance with
the method described herein. The treated papers are characterized by improved
smoothness
in conjunction with relatively minor increases in density compared to the
original sheet.
[0063] As it is desirable to have the coating in a wet state when it contacts
the heated drum,
the coating may be moistened for example by applying water. One method is to
spray water
onto the coating before it contacts the hot drum. However, in certain
embodiments, it may
also be possible to operate without any additional moistening.
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[0064] In certain embodiments, starch may be used as the soluble polymer. In
certain
embodiments, starch-based coatings can be run successfully without a
crosslinker, and good
results may be obtained without gelling (also called coagulating).
[0065] A starch solution containing 2-5% of a release agent was brought into
contact with a
heated drum under conditions described above. In certain conditions, if
moistening of the
coating is desired, water alone may be used as the spray and yield a good
reproduction of the
polished surface. If the coating solids are low enough, the process works
without a
moistening water spray. A 20% solids starch coating was applied to the web and
brought into
contact with a heated drum, and gave good reproduction.
[0066] Starch coatings were also tested having 25% and 30% solids. Both of
these coatings
released from the drum without any sticking, but without good surface
reproduction. The
25% solids coating gave moderate reproduction, but the 30% solids coating was
not very
smooth. It appears that a certain amount of water present at the surface may
help to
propagate boiling throughout the entire coating. Below a certain amount of
surface water,
localized surface areas may still have sufficient boiling to give good
reproduction of the drum
surface, but other surface areas do not. Thus, without moistening of the
surface with a water
spray, as solids increase above 20%, the percentage of the area that
reproduces the smooth
drum surface decreases with increasing coating solids, until at about 30%
coating solids, little
or no surface smoothness reproduction is achieved. If sufficient water is
sprayed on the
surface of the 30% solids coating before it contacts the heated drum, complete
surface
reproduction can be obtained. We would expect this relationship to also be
affected by
rawstock absorptivity, coat weight, coating viscosity and process speed. It
should be possible
to establish the effects of these parameters by further experiments.
[0067] The examples described above were run with a chrome surface on the
heated drum.
. The examples described below were run after the drum was resurfaced with
a tungsten
carbide coating. In each of these examples, several "runs" were made to
collect the data. A
run consists of the drum being heated to approximately 190 C, the spray level
being set,
coating being applied to the web by a metered rod method, optionally followed
by moistening
spray (which optionally may contain a cross linking agent), and then by the
web being
brought into contact with the drum at 35 fpm. The drum temperature during a
run varied
16
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between 180 C and 190 C. During a run, the only variable that was changed was
the coating
weight applied by the metering rod. Changes in coating type, coating solids,
or spray level
were made in different experimental runs on the equipment. Coat weight was
measured by
differential weight and is reported as bone-dry. Some of experiments were run
with cross
linker in the coating itself, for example when a material such as starch was
used, which does
not strongly cross link.
[0068] Example 1
100691 A minimally pressed base sheet with a basis weight of 111 lb/3000 ftz
was used as a
substrate on which to apply and treat simple coating compositions. The first
coating was
95% by dry weight CELV01:203S polyvinyl alcohol (PVOH) and 5% Emtal 50 VCS, a
triglyceride used as a release agent. The coating solids were 20% by weight.
The coating was
applied by a metering rod. The Table is a list of samples and test conditions.
Sample 1.1 was
made by spraying the coating with a crosslinking solution containing 3% by
weight borax and
1% by weight of a sulfonated Castor oil as a release agent. The spraying rate
was 48
milliliters per minute. The sample replicated the drum well and released from
the drum
without sticking. Significant improvements in smoothness were obtained with
minimal loss
of caliper. For sample 1.2, the conditions were the same except that no borax
was used in the
spray solution. Without the borax to crosslink the polyvinyl alcohol, the
coating did not
release from the surface, and part of the film remained on the drum surface.
This experiment
clearly showed the benefit of crosslinking the polyvinyl alcohol.
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TABLE
Samples and Test Conditions
Release
Coating Coating Moistening Spray from
Sample Material Solids Spray Rate Replication Drum
95w% PVOH, lw% castor oil, 48
1.1 5% triglyceride 20w% 3w% borax mllmin good Yes
95w% PV01-1, 48
1.2 5% triglyceride 20w% lw% castor oil ml/min N/A No
95w% CMC, lw% castor oil, 48 not as good as
1.3 ¨ 1.4 5% triglyceride 7w% 3w% borax ml/min PVOH (1.1) Yes
95w% CMC, 48 better than with
1.5 5% triglyceride 7µ0/0 lw% castor oil mVmin borax (1.3,1.4)
Yes
=
=
95w% starch lw% castor oil, 46
2.1 5% triglyceride 20w% 3w% borax ml/min good Yes
95w% starch 46
2.2 ¨ 2.5 5% triglyceride 20w% lw% castor oil ml/min good
Yes
not quite as good
95w% starch as with spray
2.6 ¨ 2.7 5% triglyceride 20w% no spray 0 (2.2-2.5) Yes
95w% starch
3.1-32 5% triglyceride 23w% no spray 0 good Yes
95w% starch
=
3.3 ¨ 3.4 5% taiglyceride 25.7w% no spray 0 90-95% Yes =
95w% starch 48
3.5 ¨3.6 5% triglyceride 25.7w% lw% castor oil ml/min 100%
Yes
95w% starch
3.7 5% triglyceride 30w% no spray 0 poor Yes
95w% starch 48
3.8 5% triglyceride 30w% lw% castor oil ml/min mottled Yes
3.9- 95w% starch 98
3.12 5% triglyceride 30w% lw% castor oil ml/min 100% Yes
3.13- 95w% starch
3.14 5% trigl yceri de 17.5w% no spray 0 100% Yes
95w% starch .
3.15 5% triglyccridc lOw% no spray 0 poor
=
[00701 In another run, carboxymethyl cellulose (CMC) was substituted for the
polyvinyl
alcohol to compare polymer performance. The carboxymcthyl cellulose was
FINFIXX 30,
which could only be run at 7% solids due to coating viscosity. The coating was
formulated
with 95% polymer and 5% Emtal. Samples 1.3 and 1.4 are two different coat
weights
sprayed with 48 ml/min of borax spray. The coating replicated the drum surface
well and
=
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released completely from the drum. Smoothness was improved with minimal loss
of caliper,
but smoothness was not as good as for polyvinyl alcohol. For the run that
produced Sample
1.5, no borax was used in the spray. The coating replicated the drum surface
well and
released completely from the drum. Smoothness was improved by removing the
borax. This
showed that a non-crosslinked coating could replicate and release from the
drum, which
indicates that materials other than crosslinkable materials can be used in
this process.
[0071] Example 2
= [00721 A minimally pressed base sheet having a basis weight of 111
lb/3000 ft2 was used as
a substrate on which to apply and treat simple coating compositions. The first
coating was
4;=
95% by dry weight CLEER-COTE 625 starch (a viscosity modified waxy corn
starch) and
=3(-
5% Emtal 50 VCS, a triglyceride used as a release agent. The coating solids
were 20% by
weight. The coating was applied by a metering rod. Sample 2.1 was made by
spraying the
coating with a crosslinking solution containing 3% by weight borax and 1% by
weight of a
sulfonated castor oil as a release agent. The spraying rate was 46 milliliters
per minute. The
sample replicated the drum well and released from the drum without sticking.
Significant
improvements in smoothness were obtained with minimal loss of caliper. Samples
2.2,2.3,
2.4 and 2.5 were made with different coat weights of the same coating, but the
spray did not
contain borax. All samples replicated the =dace well and released completely
from the
drum. Samples 2.6 and 2.7 were run without any spray at all. The samples
replicated the
surface well and completely released. Smoothness values were not quite as
good, but
samples still had significantly improved smoothness with minimal reduction in
caliper. This
= demonstrates that the process can work without any moistening spray.
100731 Example 3
[0074] . This experiment was a continuation of Example 2 exploring the effect
of coating
solids. Samples 3.1 and 3.2 were run at 23% coating solids without any
moistening spray.
Good replication and release were obtained. For samples 3.3 and 3.4, coating
solids were
increased to 25.7%, again applied with no moistening spray. Complete release
was obtained,
but incomplete replication of the surface was achieved. Based on visual
inspection, only
about 90-95% of the surface replicated the drum. For samples 3.5 and 3.6, this
same 25.7%
solids coating was run and a moistening spray of 48m1/min was applied. The
surface
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replication was complete and the smoothness values were greatly improved. For
Samples 3.7
through 3.12, a 30% solids coating was used. When no moistening spray was used
(3.7),
complete release was achieved, but only a small percentage of the surface was
replicated.
When 48 mllmin of moistening spray was used (3.8), the replication was greatly
improved,
but the surface was still mottled with areas of poor replication. When the
moistening spray
was increased to 98 ml/min (3.9, 3.10, 3.11 & 3.12), the replication was
complete and
smoothness was greatly improved with minimal reduction in caliper. Next, the
coating solids
were lowered. At 17.5% coating solids (3.13, 3.14) with no moistening spray
applied, good
release and complete replication were obtained. At 10% solids (3.15) with no
moistening
spray applied, the low coating viscosity led to reduced coat weight and
increased coating
absorption into the sheet, so poor replication occurred.
100751 Samples of the smooth products, produced using starch as the polymer
coating, at
20% solids, were top coated with a conventional pigmented clay coating (about
two-thirds
clay and one third carbonate, with a latex binder, applied in a single coat of
approximately 10
lb/ 3000ft2) applied over the smooth polymer layer. These samples then were
cross sectioned
to examine the morphology of the coating layer. Cross sectioning was done by
freezing the
samples in liquid nitrogen, then cracking the samples in two (freeze
fracturing). The cracked
edges of the samples (e.g., the cross sections) were then viewed under a
microscope.
[0076] Micrographs revealed that voids exist in the polymer coating layer, as
shown in
FIGs. 2 through 9, which include measurement bars to indicate their scale. For
FIGs. 2-5, the
microscope magnification was 1000, and the measurement bars are 20 microns
long. In FIG.
2 as an example, the structure as shown includes a paperboard substrate 110.
The substrate
thickness generally extends below the area of the micrograph. Because of the
freeze
fracturing process, the substrate 110 as shown in the micrographs is sometimes
separated or
partly separated from polymer layer 120. Therefore the upper boundary of
substrate 110 may
be only approximately shown by the bracketed distance denoting the substrate.
10077] In these samples, the polymer coating layer 120 had been applied onto
substrate
110, and dried against a heated drum, as described previously. Then a top
coating 130 was
applied and dried. The term "polymer coating" is used here to describe that
layer applied as
described above, then contacted while wet against a heated drum. The term "top
coating" is
CA 02643891 2008-08-22
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used to describe the outer layer, which was applied as one layer. Obviously
the "top coating"
could be applied in more than layer and could be of coating materials other
than those used
here.
[0078] Voids 121 are evident in the polymer coating layer 120, as seen in
FIGs. 2-9. FIG. 2
for example shows several voids 121 in polymer coating layer 120, with the
voids appearing
to be approximately 5 to 20 microns in lateral dimension. It is assumed that
their size going
"into" the fractured sample is in approximately the same range. The voids
typically appear to
be somewhat "flattened" in the "vertical" direction, that is, going into the
sample thickness.
The voids also appear to have "walls" that are relatively smooth, and
generally thin. These
thin walls are most apparent as seen between adjacent voids. Where a void wall
is adjacent to
the top coating 130, its thickness may be difficult to see but its presence
may be deduced by
the smooth lower contour of the top coating 130 adjacent to the void.
[0079] FIG. 3 is an example micrograph showing several voids 121 in the
polymer coating
layer. The voids appear to extend over an area equivalent to more than half
the coated surface
area. The polymer coating layer is not well defined in this micrograph.
[0080] FIG. 4 is an example micrograph showing several voids 121 in polymer
coating
layer 120. The walls of the voids appear to be relatively thin, as evidenced
by a somewhat
translucent appearance in the walls of two of the voids.
[0081] For FIGs. 5-9, the microscope magnification was 500 and the measurement
bars are
50 microns long. FIG. 5 shows several voids 121 in polymer layer 120, with
individual
measurement bars showing dimensions of the selected voids, for example, moving
generally
from left to right, measurements of 10.5 microns in vertical distance, 36
microns in lateral
distance, 10.6 microns in vertical distance, and 36.3 microns in lateral
distance. Again the
. voids appear to extend over an area equivalent approximately half the
coated surface area.
[0082] FIG. 6 shows another sample with similar measurement bars, for example,
moving
generally from left to right, measurements of 8.66 microns in vertical
distance, 32.1 microns
in lateral distance, 11.8 microns in vertical distance, and 22.7 microns in
lateral distance.
Measurements such as these in FIGs. 5 and 6 were collected for use-in the
graph discussed
later in FIG. 17.
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[0083] FIG. 7 shows voids 121 in polymer layer 120, including several showing
a generally
flattened aspect. The voids appear to extend over an area equivalent to nearly
all the coated
surface area. FIG. 8 shows another sample with similar widespread voids 121.
The wall
areas of several voids are visible. FIG. 9 shows yet another sample where the
voids
121appear to extend over an area equivalent to nearly all the coated surface
area.
[0084] Other samples of the smooth products, produced using starch as the
polymer
coating, at 20% solids, were not top-coated. These samples were cross
sectioned to examine
the morphology of the coating layer. Cross sectioning was done by freezing the
samples in
liquid nitrogen, then cracking the samples in two (freeze fracturing). The
cracked edges of
, the samples (e.g., the cross sections) were then viewed under a microscope
as shown in FIGs.
to 12, which include measurement bars to indicate their scale. The microscope
magnification was 1000, and the measurement bars are 20 microns long. FIG. 10
shows the
polymer layer 120, which contains voids 121 and has a very smooth outer
surface. The
polymer layer is on paperboard substrate 110, and one of the cellulose fibers
112 is denoted.
The substrate thickness generally extends below the area of the micrograph.
[0085] FIGs. 11 and 12 show additional micrographs of samples that were
polymer coated
but not top-coated. Again the smoothness of the polymer layer 120 is evident,
as are the
underlying voids 121. The walls of the voids often coincide with the surface
of the polymer
coating.
[0086] FIG. 13 (at 200x magnification) and FIG. 14 (at 500x magnification)
show the
surface of samples as seen under a scanning electron microscope. These samples
were not
given top coating 130: The larger string-like structures 112 are cellulose
fibers of the
substrate 110. The smaller cell-like structures 122 that appear as a fine
network or mesh are
individual voids in polymer layer 120. The polymer layer here appears
essentially
transparent, except for the walls of the voids.
. [0087] FIGs. 15 and 16 show the surface of samples as seen under a
backscatter scanning
electron microscope. These samples were not given top coating 130. The larger
string-like
structures 112 are cellulose fibers of the substrate 110. The smaller cell-
like structures 122
that appear as a fine network or mesh are the walls of individual voids in
polymer layer 120.
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The polymer layer here appears essentially transparent, except for the walls
of the voids. The
voids appear to be distributed over the entire surface.
[0088] FIG. 17 is a graph showing the distribution of void sizes based on
approximately 90
measurements each of void width (lateral dimension) and height (vertical
dimension in the
micrographs). The measurements show an average void width (measured in the
direction
parallel to the thickness of the sample) of about 19 microns, with a standard
deviation of
about 9 microns. The measurements show an average void height (measured in the
direction
going "into" the sample thickness) of about 10 microns, with a standard
deviation of about 4
microns.
[0089] These void dimensions appear to be representative of the samples
studied here.
However, they are not meant to be limiting as changes in materials or
processing conditions
might give other dimensions.
[0090] It is hypothesized that steam bubbles create these voids while the
coating is in
contact with the heated drum, and that the bubbles may provide a force to help
keep the
coating in contact with the drum. The resulting voids typically help bridge
the gap between
an otherwise rough substrate layer 110, and the smooth surface of the heated
drum. Thus the
dried polymer coating has a smooth replicated surface, which is smoother than
the substrate
layer 110. It appears that many or most of the voids remain intact when top
coating 130 is
applied. Therefore the top coating may end up smoother because of the
relatively smooth
underlying polymer layer 120. This is seen as an advantage achieved by the
invention.
Besides the hypothesized influence of the voids on help creating a smooth
replicated surface,
the voids may also contribute to a lower density in the product.
[00911 The conditions in the nip between press roll 24 and hot drum 22 may
influence
whether voids form in the polymer coating. Depending on press roll hardness,
and the
diameters of the press roll and hot drum, it may be necessary to adjust the
nip loading (for
example, the PLI loading on the nip) in order to achieve boiling in the nip
which creates the
voids.
[0092] The polymer-coated paper or paperboard created by this process may be
used
wherever a smooth substrate or finished product is desired. The polymer-coated
paper or
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paperboard may be used as-is (e.g., as shown in FIGS. 10-16), or it may be
used as a
substrate for additional coatings or other treatments to be applied (for
example the top coating
130 shown in FIGs. 2-9, or other coatings) thereon. Additional finishing
materials or
processes may be applied to the polymer-coated paper or_paperboard, with or
without
additional coatings. For example, one or more additional coatings may be
applied, as is
typical with base coating, top coating, and triple coating of conventional
paper or paperboard
substrates. Calendering processes may be applied, before or after optional
additional coating.
For example one or more additional coatings may be applied, followed by a
gloss calendering
step.
[0093] Methods of making and using polymer-coated material in accordance with
the
invention should be readily apparent from the mere description of the material
and process as
provided herein. No further discussion or illustration of such material or
methods, therefore,
is deemed necessary..
[0094] While preferred embodiments of the invention have been described and
illustrated, it
should be apparent that many modifications to the embodiments and
implementations of the
invention can be made without departing from the spirit or scope of the
invention. Although
the preferred embodiments illustrated herein have been described in connection
with a paper
or paperboard substrate, these embodiments may easily be implemented in
accordance with
the invention in other structures, including without limitation textiles, non-
woven fabrics,
fibrous materials, polylactic acid substrates, and porous films.
[0095] It is to be understood therefore that the invention is not limited to
the particular
embodiments disclosed (or apparent from the disclosure) herein, but only
limited by the
claims appended hereto.
24
