U.S. patent application number 12/156050 was filed with the patent office on 2008-09-25 for method for treating a substrate.
This patent application is currently assigned to MeadWestvaco Corporation. Invention is credited to Robert W. Carlson, Gary P. Fugitt, Scott E. Ginther, Terrell J. Green, Stanley H. MCGREW, Steven P. Metzler, John W. Stolarz.
Application Number | 20080230001 12/156050 |
Document ID | / |
Family ID | 39797951 |
Filed Date | 2008-09-25 |
United States Patent
Application |
20080230001 |
Kind Code |
A1 |
Fugitt; Gary P. ; et
al. |
September 25, 2008 |
Method for treating a substrate
Abstract
A method for treating a substrate is described. In accordance
with one aspect, the method includes applying a polymer coating to
a substrate, and bringing the polymer coating into contact with a
heated surface in a pressure nip while the coating is still in a
wet state. Optionally the polymer coating may include a
crosslinkable material, and a crosslinking agent may be used to
promote crosslinking. The polymer coating replicates the heated
surface. A product produced in accordance with the described method
is also disclosed. The product is characterized by having
subsurface voids within the coating.
Inventors: |
Fugitt; Gary P.; (Pittsboro,
NC) ; Ginther; Scott E.; (Willow Spring, NC) ;
Stolarz; John W.; (Circleville, OH) ; Carlson; Robert
W.; (Raleigh, NC) ; MCGREW; Stanley H.; (North
Charleston, SC) ; Metzler; Steven P.; (Clayton,
NC) ; Green; Terrell J.; (Raleigh, NC) |
Correspondence
Address: |
MEADWESTVACO CORPORATION
1021 MAIN CAMPUS DRIVE, CENTENNIAL CAMPUS
RALEIGH
NC
27606
US
|
Assignee: |
MeadWestvaco Corporation
Glen Allen
VA
|
Family ID: |
39797951 |
Appl. No.: |
12/156050 |
Filed: |
May 29, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
PCT/US07/19917 |
Sep 13, 2007 |
|
|
|
12156050 |
|
|
|
|
PCT/US2007/004742 |
Feb 22, 2007 |
|
|
|
PCT/US07/19917 |
|
|
|
|
60776114 |
Feb 23, 2006 |
|
|
|
60957478 |
Aug 23, 2007 |
|
|
|
Current U.S.
Class: |
118/258 |
Current CPC
Class: |
Y10T 428/249978
20150401; B05D 3/0209 20130101; Y10T 428/249979 20150401; Y10T
428/31 20150115; D21H 19/16 20130101; D21H 19/34 20130101; B05D
2203/22 20130101; B05D 2252/02 20130101; B05D 3/12 20130101; B05D
3/0254 20130101; B05D 2401/20 20130101; D21H 23/56 20130101 |
Class at
Publication: |
118/258 |
International
Class: |
B05C 1/08 20060101
B05C001/08 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 22, 2007 |
US |
PCT/US07/04742 |
Sep 13, 2007 |
US |
PCT/US07/19917 |
Claims
1. An apparatus for treating a substrate, comprising: an applicator
for applying to the substrate an aqueous coating as a film, a drum
having a diameter between about 24 inches and about 84 inches, a
press roll capable of forming a nip with the drum, and an energy
source for achieving a drum temperature above the boiling point of
the aqueous coating, wherein the applicator, drum, and press roll
are configured to allow the substrate to travel through the nip at
a speed between about 300 fpm and about 3000 fpm, and wherein the
nip provides a dwell time between about 1 millisecond and about 60
milliseconds.
2. The apparatus of claim 1, wherein the nip dwell time is between
about 4 milliseconds and about 25 milliseconds.
3. The apparatus of claim 2, wherein the nip dwell time is between
about 6 milliseconds and about 10 milliseconds.
4. The apparatus of claim 1, wherein the nip provides a nip average
pressure between about 70 psi and about 700 psi.
5. The apparatus of claim 1, wherein the drum has a diameter
between about 60 inches and about 72 inches.
6. The apparatus of claim 1, wherein the substrate travels through
the nip at a speed between about 600 fpm and about 2000 fpm.
7. The apparatus of claim 1, wherein the press roll has a diameter
between about 12 inches and about 72 inches.
8. The apparatus of claim 7, wherein the press roll has a diameter
between about 24 inches and about 48 inches.
9. The apparatus of claim 1, wherein the aqueous coating is a wet
film.
10. The apparatus of claim 1, wherein the substrate is in web
form.
11. An apparatus for treating a substrate, comprising: an
applicator for applying to the substrate an aqueous coating as a
film, a drum having a diameter between about 24 inches and about 84
inches, a belted shoe device forming a nip with the drum, and an
energy source for achieving a drum temperature above the boiling
point of the aqueous coating wherein the applicator, drum, and
belted shoe device are configured to allow the substrate to travel
through the nip at a speed between about 300 fpm and about 3000
fpm, and wherein the nip provides a dwell time between about 1
millisecond and about 225 milliseconds.
12. The apparatus of claim 11, wherein the nip dwell time is
between about 4 milliseconds and about 150 milliseconds.
13. The apparatus of claim 12, wherein the nip dwell time is
between about 6 milliseconds and about 60 milliseconds.
14. The apparatus of claim 11, wherein the nip provides a nip
average pressure between about 70 psi and about 700 psi.
15. The apparatus of claim 11, wherein the drum has a diameter
between about 60 inches and about 72 inches.
16. The apparatus of claim 11, wherein the substrate travels
through the nip at a speed between about 600 fpm and about 2000
fpm.
17. The apparatus of claim 11, wherein the nip length in the
direction of substrate travel is between about 0.1 inches and about
18 inches.
18. The apparatus of claim 17, wherein the nip length in the
direction of substrate travel is between about 1 inches and about
12 inches.
19. The apparatus of claim 11, wherein the aqueous coating as a wet
film.
20. The apparatus of claim 11, wherein the substrate is in web
form.
Description
REFERENCE TO RELATED APPLICATION
[0001] This patent application is a continuation of and claims
priority of copending International Patent Application Number
PCT/US07/19917 filed Sep. 13, 2007 which claims priority of U.S.
Provisional Application Ser. No. 60/957,478 filed on Aug. 23, 2007
and which is a Continuation-in-Part of copending International
Patent Application Number PCT/US07/04742 filed on Feb. 22, 2007
which claims priority from U.S. Provisional Application Ser. No.
60/776,114 filed on Feb. 23, 2006. All of the listed applications
are hereby incorporated by reference in their entireties.
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 embodiments, 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. The present disclosure also relates
to a method for treating a substrate with a polymer film-forming
composition, and bringing the substrate into contact with a heated
surface in a pressure nip.
[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.
[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.
[0005] 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.
[0007] 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 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 (for a given caliper), and decreases
as the paper density increases (for a given basis weight).
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 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 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.
[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.degree. 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.degree. 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.
[0027] 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.degree. 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.
[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.degree. 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.degree. 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.
[0030] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, bringing the film into
contact with a heated surface in a nip, the nip local pressure
initially increasing and the film being heated with no vapor
formation, the nip local pressure then decreasing and the coating
film boiling and forms voids in the film, the film being at least
partly dried.
[0031] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, the coating including a
water soluble polymer and a release agent, bringing the film into
contact with a heated surface in a nip, the nip local pressure
initially increasing and the film being heated with no vapor
formation, the nip local pressure then decreasing and the coating
film boiling and forms voids in the film, the film being at least
partly dried.
[0032] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, bringing the film into
nipped contact for less than about 3 seconds with a heated surface
having a temperature above about 150C, the nip local pressure
initially increasing and the film being heated with no vapor
formation, the nip local pressure then decreasing and the coating
film boiling and forms voids in the film, the film being at least
partly dried.
[0033] In another embodiment, an apparatus is disclosed for
treating a web substrate, comprising a coating applicator, a drum
having a diameter between about 24-84 inches, a press roll forming
a nip with the drum having a nip dwell time between about 1-60
milliseconds, the web substrate travels through the nip at between
about 300-3000 fpm, and an energy source for maintaining the drum
temperature above the boiling point of the coating.
[0034] In another embodiment, an apparatus is disclosed for
treating a web substrate, comprising a coating applicator, a drum
having a diameter between about 24-84 inches, a belted shoe device
forming a nip with the drum having a nip dwell time between about
1-225 milliseconds, the web substrate travels through the nip at
between about 300-3000 fpm, and an energy source for maintaining
the drum temperature above the boiling point of the coating.
[0035] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, heating the film under a
pressure with no vapor formation, and reducing the pressure so that
the film boils and forms voids that remain in the film.
[0036] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, wherein the coating
includes a water soluble polymer and a release agent, heating the
film under a pressure with no vapor formation, and reducing the
pressure so that the film boils and forms voids that remain in the
film.
[0037] In another embodiment, a process is disclosed that includes
applying a coating film to a substrate, bringing the film into
contact for less than about 3 seconds with a heated surface having
a temperature above about 150.degree. C., wherein the contact
comprises a nipped contact with the heated surface, heating the
film under a pressure with no vapor formation, and reducing the
pressure so that the film boils and forms voids that remain in the
film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] 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.
[0039] 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.
[0040] FIGS. 10-12 are cross section micrographs showing the
morphology of samples made in accordance with one embodiment of the
invention, and having no top coating.
[0041] 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, and having no top
coating.
[0042] 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, and having no
top coating.
[0043] FIG. 17 is a graph showing distribution of void dimensions
in samples made in accordance with one embodiment of the
invention.
[0044] FIG. 18 is a detail view of an apparatus for treating a
substrate with a polymer coating in accordance with certain
embodiments of the present invention.
[0045] FIG. 19 is a graph showing a relationship between heat
transfer rate and temperature difference.
DETAILED DESCRIPTION OF THE INVENTION
[0046] 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.
[0047] 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 due to intimate
contact with the heated surface. 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.
[0048] 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 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%. Depending on the properties of the
substrate, the caliper decrease may be less. If the web has been
heavily precalendered the caliper decrease may be between 0 and 5%
By comparison, conventional coated papers and paperboards 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.
[0049] FIG. 1 illustrates an apparatus 10 useful in practicing
certain embodiments of the invention. A substrate 12 is subjected
to treatment on one surface thereof with crosslinkable polymer
coating 14 to form a layer of polymer coating 16 on substrate 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 substrate 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.
[0050] 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 may be sufficient. Besides providing the
substrate in web form, it may also be provided in sheet form.
[0051] 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 crosslinking solutions
may be applied by the same coating technique or different methods
may be used for each.
[0052] 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/m.sup.2 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/m.sup.2 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.degree. C., or
in accordance with certain embodiments, at least about 190.degree.
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 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.
[0053] 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 15 to about
30 pounds per linear inch (PLI). Depending on press roll hardness
and the diameters of the hot drum and press roll, conditions may
have to be adjusted.
[0054] 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/m.sup.2) 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.
[0055] 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.
[0056] 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 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 and it appears that when the coating boils it remains for
a time in contact with the drum. The contact results in a polymer
film 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.
[0057] 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.
[0058] 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.
[0059] 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 release
agent or other means, then application of a release agent in the
coating or onto the coating surface may not be needed.
[0060] 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.
[0061] 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.
[0062] These advantages allow the use of lightly calendered paper
or paperboard, thus preserving stiffness while providing good
printing properties.
[0063] The base sheet is typically formed from fibers
conventionally used for such purpose and, in accordance with the
particular embodiments, includes unbleached or bleached kraft 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] A base sheet having a caliper of about 10 points, a Parker
Print Surf (PPS) value of about 9 microns (10 kg 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/m.sup.2 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/m.sup.2 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.degree. 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.
[0068] In a preferred embodiment, a starch solution may be used as
the polymeric material in the polymer coating.
[0069] 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
crosslinking 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.degree. C. to about 240.degree. C. Higher temperatures may be
used, for example up to about 300.degree. C. The temperature of the
heated surface in accordance with certain embodiments is within a
range of from 180.degree. C. to about 200.degree. C., and in
accordance with certain embodiments is at least about 190.degree.
C.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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).
[0075] 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.
[0076] 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.
[0077] 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.degree. 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 between 180.degree. C. and
190.degree. 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.
EXAMPLE 1
[0078] A minimally pressed base sheet with a basis weight of 111
lb/3000 ft.sup.2 was used as a substrate on which to apply and
treat simple coating compositions. The first coating was 95% by dry
weight CELVOL 203S polyvinyl alcohol (PVOH) and 5% Emtal 50 VCS, a
triglyceride used as a release agent (CELVOL is made by Celanese).
The coating solids were 20% by weight. The coating was applied by a
metering rod. Table 1 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.
TABLE-US-00001 TABLE 1 Samples and Test Conditions Release Coating
Coating Moistening Spray from Sample Material Solids Spray Rate
Replication Drum 1.1 95 w % PVOH, 20 w % 1 w % castor oil, 48 Good
Yes 5% triglyceride 3 w % borax ml/min 1.2 95 w % PVOH, 20 w % 1 w
% castor oil 48 N/A No 5% triglyceride ml/min 1.3-1.4 95 w % CMC, 7
w % 1 w % castor oil, 48 not as good as Yes 5% triglyceride 3 w %
borax ml/min PVOH (1.1) 1.5 95 w % CMC, 7 w % 1 w % castor oil 48
better than with Yes 5% triglyceride ml/min borax (1.3, 1.4) 2.1 95
w % starch 20 w % 1 w % castor oil, 46 good Yes 5% triglyceride 3 w
% borax ml/min 2.2-2.5 95 w % starch 20 w % 1 w % castor oil 46
good Yes 5% triglyceride ml/min 2.6-2.7 95 w % starch 20 w % no
spray 0 not quite as good Yes 5% triglyceride as with spray
(2.2-2.5) 3.1-3.2 95 w % starch 23 w % no spray 0 good Yes 5%
triglyceride 3.3-3.4 95 w % starch 25.7 w % no spray 0 90-95% Yes
5% triglyceride 3.5-3.6 95 w % starch 25.7 w % 1 w % castor oil 48
100% Yes 5% triglyceride ml/min 3.7 95 w % starch 30 w % no spray 0
poor Yes 5% triglyceride 3.8 95 w % starch 30 w % 1 w % castor oil
48 mottled Yes 5% triglyceride ml/min 3.9-3.12 95 w % starch 30 w %
1 w % castor oil 98 100% Yes 5% triglyceride ml/min 3.13-3.14 95 w
% starch 17.5 w % no spray 0 100% Yes 5% triglyceride 3.15 95 w %
starch 10 w % no spray 0 poor 5% triglyceride
[0079] In another run, carboxymethyl cellulose (CMC) was
substituted for the polyvinyl alcohol to compare polymer
performance. The carboxymethyl cellulose was FINNFIX 30 (made by
Noviant, a division of Huber), 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 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.
EXAMPLE 2
[0080] A minimally pressed base sheet having a basis weight of 111
lb/3000 ft.sup.2 was used as a substrate on which to apply and
treat simple coating compositions. The first coating was 95% by dry
weight CLEER-COTE 625 starch (a viscosity modified waxy corn
starch, made by A. E. Staley, a division of Tate & Lyle) 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. 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 surface 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.
EXAMPLE 3
[0081] 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 48
ml/min was applied. The surface 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 ml/min 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.
[0082] 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/ 3000 ft.sup.2) 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.
[0083] 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.
[0084] 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 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.
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
[0090] 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 121 appear to extend over an area
equivalent to nearly all the coated surface area.
[0091] 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. 10
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.
[0092] 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.
[0093] FIG. 13 (at 200.times. magnification) and FIG. 14 (at
500.times. 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.
[0094] 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. The polymer layer here
appears essentially transparent, except for the walls of the voids.
The voids appear to be distributed over the entire surface.
[0095] 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.
[0096] 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.
[0097] It is difficult to directly observe or measure processes
within the nip, but it appears 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 ends up smoother because of the relatively smooth
underlying polymer layer 120. This is seen as an advantage achieved
by the invention. Besides the influence of the voids on help
creating a smooth replicated surface, the voids also contribute to
a lower density in the product.
[0098] The conditions in the nip between press roll 24 and hot drum
22 influences 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.
[0099] Based on results of our experiments, the replication process
seems to occur in the following manner, as depicted in FIG. 18.
Polymer coated substrate 220 enters the nip between press roll 224
and hot polished drum 222. A nip pressure profile 250 exists
between the hot polished drum and the press roll. The nip pressure
profile has an ingoing portion 252 and an outgoing portion 254. The
shape of nip pressure profile 250 is meant as an example only. A
nip local pressure exists at any point on the profile, and the nip
local pressure may vary going through the nip, as shown by the nip
pressure profile 250. For example, it may increase on the ingoing
side of the nip, then decrease on the outgoing side of the nip. A
nip average pressure 256 also exists.
[0100] On the ingoing side of the nip, a high heat transfer rate
occurs due to the intimate contact between the hot polished drum
222 and the polymer coated substrate 220, and due to a high
temperature difference between the hot drum and the polymer coated
substrate. Also on the ingoing side of the nip, the pressure is
increasing and thus does not allow the coating to vaporize, but
instead imparts superheat to the liquid phase. On the outgoing side
of the nip, heat transfer is still very high, but the pressure is
decreasing and at some point in the outgoing nip, the liquid phase
can flash (boil) to vapor and create a high volume of voids 221 in
the coating layer, which helps replicate the surface of the hot
polished drum 222. Because the liquid was superheated, that is,
heated beyond its atmospheric-pressure boiling point, there is
sufficient energy not only to vaporize liquid and create bubbles or
voids, but also enough energy to sufficiently dry the polymer
coating (such as around the bubbles, for example in the walls of
the bubbles) so that upon leaving the nip, the coating with its
voids and smooth surface retains its structure. The vapor, as its
escapes from the coating layer, may thus help dry the coating.
Shortly after leaving the nip, the web 230 releases from the hot
drum and the surface replication process is complete. In a
preferred embodiment, water in the coating is vaporized to form the
voids. However, other embodiments may utilize liquids other than
water to vaporize in the nip and form voids in the coating.
[0101] Preferably the pressure in the nip is great enough to
promote a high rate of heat transfer, and lead to a superheated
condition in the coating. However, if the pressure is too great, it
may lead to a reduction in caliper, which is not desired. Excess
pressure might possibly force coating into the substrate to the
extent that there is poor surface replication. Thus it appears that
there are optimum ranges of temperature (high enough to provide
enough heat for vaporizing and drying), pressure (high enough to
promote high heat transfer and superheating, but not so high as to
drive too much coating into the substrate), and nip dwell time
(high enough to allow sufficient heat transfer to occur).
[0102] The process depicted in FIG. 18 occurs within a certain
range of hot drum temperatures and nip widths (related to time in
the nip). The basic equation describing the heat transfer is: q=h
(T.sub.s-T.sub.sat), where q is the heat transfer rate, h is the
heat transfer coefficient, T.sub.s is the temperature of the heated
surface, and T.sub.sat is the saturated temperature of the liquid.
T.sub.sat is a function of pressure. For convenience, the
temperature difference (T.sub.s-T.sub.sat), is sometimes termed the
delta T (.DELTA.T).
[0103] FIG. 19 shows an exemplary graph of heat transfer q from a
heated surface to a liquid undergoing phase change. The heat
transfer behavior as depicted in FIG. 19 is a well known
phenomenon. The log-log graph shows heat transfer rate q vs.
temperature difference .DELTA.T. The notation "C" denotes a maximum
or "critical" heat transfer rate. For boiling water, this maximum
heat transfer rate may occur at a .DELTA.T of about 50.degree. C.
It is understood that the critical heat transfer rate for a polymer
coated substrate in a pressure nip may differ from this particular
A T, but the general shape of the graph, and the underlying
physics, may still apply.
[0104] At a .DELTA.T sufficiently below that associated with the
critical heat transfer rate, heat transfer may be insufficient to
supply the energy required to vaporize enough water from the
coating to achieve surface replication. Furthermore, as shown in
FIG. 19, as .DELTA.T increases past the point "C" associated with
critical heat transfer rate, enough vapor may be formed at the hot
surface to begin to inhibit heat transfer, due to a reduced heat
transfer coefficient.
[0105] For operation at about 800 fpm, and coat weights (dry basis)
between about 2.8 to 3.6 lb/3000 ft.sup.2, a heat transfer rate
from about 9 to 10 kilojoules per square foot gives acceptable
results. This appears to be about the amount of heat to dry the
coating. As coat weight increases, the amount of heat required per
square foot would be expected to increase accordingly.
[0106] Because of the heat transfer behavior illustrated in FIG.
19, it is expected that there will be an optimum temperature range
around point "C" for achieving best surface replication. This
temperature range may depend on pressure within the nip. It appears
that the optimum hot drum temperature is in the range of
200-260.degree. C. (400-500.degree. F.) for a typical aqueous
polymer solution or pigmented coating. If a material used in the
coating changes the vapor pressure (for example, ammonia or an
alcohol) then the temperature range might be lowered.
[0107] For the process to run over a range of speeds from benchtop
speeds to production speeds of 2000 fpm or more, it is advantageous
to know, and even to control, the time in the nip during which the
process may replicate the drum surface onto the coating. The
classic Hertzian equation for nip width between two rolls is
w = 2 L .PI. * d 1 * d 2 d 1 + d 2 * ( 1 - v 1 2 E 1 + 1 - v 2 2 E
2 ) . EQUATION 1 ##EQU00001##
[0108] w=Nip width (inches)
[0109] L=Nip load (pounds per linear inch, PLI)
[0110] E.sub.1, E.sub.2=Moduli for rolls 1, 2 (psi)
[0111] d.sub.1, d.sub.2=Diameters of rolls 1, 2 (inches)
[0112] v.sub.1, v.sub.2=Poisson's ratio for rolls 1, 2
[0113] As someone skilled in the art would recognize, certain of
these variables, such as the moduli or the Poisson's ratios, may be
influenced by temperature. This in turn, according to Equation 1,
would influence the nip width. Those skilled in the art will also
recognize that to provide a better fit to particular conditions,
the equation may be modified, for example based on empirical data,
or an alternative equation may be used.
[0114] In the embodiments described herein, the nip dwell time
appears to be an important parameter. Along with delta T, it plays
an important role in determining the amount of energy transferred
in the nip. The hot drum temperature T.sub.s and the nip pressure
may be controlled to achieve operation near point "C" on the heat
transfer curve, in order to maximize energy transfer. By
appropriately adjusting pressure in the nip, boiling may initially
be inhibited (for example on the ingoing side of the nip where the
pressure is increasing) so that a very high heat transfer rate
occurs, allowing an excess of energy to be transferred to the
coating. This excess energy may be described as "superheat." As the
substrate moves forward and the pressure decreases on the outgoing
side of the nip, the saturation pressure T.sub.sat (which is a
function of pressure) rapidly decreases and superheated water
flashes as steam.
[0115] The delta T may preferably be optimized. For example to run
at a certain speed, parameters such as roll diameter and hardness,
and nip load, may be chosen to obtain sufficient dwell time.
Operating conditions may be chosen to achieve a desired delta T,
for example, to operate close to a target value, such as 50.degree.
C.
[0116] Equation 1 (or a similar equation or equations) can be used,
along with physical properties and dimensions of the rolls, to
determine suitable operating conditions to give an appropriate time
in the nip. Because of the interactions of the different variables,
some trial and error may be required to optimize the process. When
work with a bench scale apparatus has determined a suitable
combination of hot drum temperature, nip pressure, and dwell time
for making acceptable product with the desired void-containing
coating, then theory may be used to determine approximate operating
conditions for a larger scale apparatus.
[0117] As an example, assume that a bench scale apparatus makes
acceptable product using a hot drum temperature of about
220.degree. C. (425.degree. F.), for a specific nip pressure and
nip dwell time. For a first approximation, it may be assumed that
equivalent conditions may produce satisfactory product on a larger
apparatus such as production equipment. Preferably the larger
apparatus will be capable of running with a hot drum temperature of
about 220.degree. C. Knowing the desired operating speed, and the
suitable nip dwell time, a target nip width may be calculated.
[0118] Having determined the target nip width, an equation such as
Equation 1 (or other suitable theoretical, empirical, or otherwise
derived equation) may be used to directly, indirectly, iteratively
or otherwise determine one or more sets of operating conditions for
the production equipment that will result in the desired target nip
width. Among the factors to consider are the diameters of the hot
drum and the press roll, the hardness of the press roll (or the
hardness and thickness of its cover), and the operating ranges
available for loading the nip (e.g. the PLI range of the
apparatus). These factors may apply to the existing in-place
equipment, or to replacement equipment that may be used instead.
For example, using Equation 1, a list of candidate press roll
covers may be created, which each by virtue of their respective
thickness and hardness are suitable for providing the target nip
width. An appropriate one of the candidate press roll covers may
then be chosen, for example based on availability, durability at a
given temperature, surface properties, etc.
[0119] The contact time of the polymer coated substrate with the
hot drum surface includes the nip dwell time and may also include
additional time, preferably after leaving the nip, during which the
substrate is in contact with the hot drum surface.
[0120] Results for experiments running on pilot equipment are
summarized in Table 2. The hot drum had a diameter of 46'' with a
tungsten carbide surface polished to a 2 micron finish. The press
roll has a diameter of 38'' with a 30 Shore D soft covering. The
web width was 36'' and nip load about 570 PLI. This would provide
an estimated average nip pressure of about 500 psi. The base
substrate was a bleached board with a nominal basis weight of 204
lb/3000 ft.sup.2. Coatings were applied to the web using a rod
coater prior to contact with the heated drum. Example A used a
coating made up of 97% by weight CLEER-COTE, (made by A. E. Staley,
a division of Tate & Lyle) low viscosity starch and 3% of a
homogenized vegetable oil release agent (triglyceride Emtal 50
VCS). The coating solids were 17.1%. Example B used a PG270 (made
by Penford Products) medium viscosity starch at 97% by weight with
3% vegetable oil release agent. Although the caliper of the Example
B basesheet was less than the caliper of the Example B treated
board, this could be due to variability in the board.
[0121] The heated drum temperature was about 450.degree. F. (about
230.degree. C.) at a web speed of 800 fpm. At higher web speeds,
the temperature was lower. As operating conditions are adjusted,
the aqueous polymer coating may be optimized for the new
conditions. For example, it appears that as speed increases,
coating solids may be decreased slightly to provide best
results.
[0122] The pilot conditions were run over a range of dwell times,
depending on nip width and web speed. For example, good results
were obtained at about 200 fpm with a nip dwell time of about 27
milliseconds, and at about 900 fpm with a nip dwell time of about 6
milliseconds. Less satisfactory results were obtained at about 1500
fpm with a nip dwell time of about 3 milliseconds. These dwell
times correspond to a nip width of about 1.1 inches. On bench scale
equipment, good results were obtained at about 25 fpm with a nip
dwell time of about 60 milliseconds, corresponding to a nip width
of about 0.3 inch. While speeds above about 1500 fpm have not been
tested, it is possible that higher speeds might require longer
dwell times, for example due to other operating factors, such as
web moisture.
[0123] While certain of the conditions above gave satisfactory
results, it is envisioned that the process may be adjusted to run
under a variety of conditions. For example it is envisioned that
operating conditions might be adjusted so as to permit satisfactory
results at nip pressures between about 70 psi and about 700 psi, or
more particularly between about 150 psi and 550 psi.
[0124] Various heat sources may be used to heat the hot drum. For
example, the heating may be by electrical resistance heating,
electrical induction heating, gas-fired heating, hot oil heating,
combinations of these, or other heating methods as are known in the
art.
[0125] While the work described herein used a cylindrical hot drum
and a cylindrical press roll forming a nip therebetween, it is
contemplated that the process may also be practiced with other
geometries capable of providing a heated pressure nip through which
the substrate may pass. Other geometries that may be capable of
providing a heated pressure nip may be embodied, for example, in
belted shoe press or belted shoe calender equipment. The process
disclosed herein was not tested using a belted shoe device, and
suitable operating conditions would need to be determined. Belted
shoe devices may have somewhat longer nips than would be found
between a drum and press roll, and correspondingly longer nip dwell
times. For these devices, dwell times may range from about 3
milliseconds (for example, with a two-inch shoe running at 3000
fpm) to about 225 milliseconds (for example, if an 18 inch shoe
were available running at 400 fpm).
[0126] Certain descriptions of the embodiments herein use the terms
"paper" or "paperboard" to describe the substrate. These terms are
not meant to limit the type of substrate, as it is envisioned that
the methods here may be suitable for various substrates including
without limitation either paper or paperboard. 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 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.
[0127] 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.
[0128] 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.
[0129] 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.
TABLE-US-00002 TABLE 2 Samples and Test Conditions Parker PrintSurf
Drum Sheffield 10 kg Temper- Coat Weight Caliper Smoothness
Smoothness Speed ature (lb/3000 ft.sup.2) (0.001'') (Sheff units)
(microns) (fpm) (.degree. F.) Example A Basesheet 19.0 281 6.9 3.5
18.4 143 3.4 800 455 3.6 18.5 125 3.5 900 432 3.6 18.3 123 3.5 1000
421 3.6 18.5 145 3.6 1100 419 3.7 18.4 128 3.7 1200 413 3.7 18.6
117 3.8 1300 406 3.8 18.5 133 3.9 1400 399 Example B Basesheet 18.2
249 6.6 2.2 18.5 165 3.5 800 451 2.2 18.6 143 3.7 1000 434 2.2 19.0
158 4.0 1200 419 2.3 18.9 168 4.6 1400 407
* * * * *