Note: Descriptions are shown in the official language in which they were submitted.
% ~ 2 ~
PATENT
UCC- 62 4
D08241
ME;T~OD OF ~RING COATED PAPER AND PAP13R BOARD
UTILIZING IMPULSE DRYING ..
This invention relates to the manufacture of
coated paper products, and in particular, to
manufacturing coated papers having improved properties
and reduced manufacturing costs.
BACRGROUND OF TXE INVENTION
The production of high quality printing paper
typically includes various finishing operations which
include coating the paper with pigments and binder
followed by smoothing the paper by passing it through
either a series of nips or one nip formed by rolls that
may or may not be heated. The coating is generally
applied by coaters of conventional designs, such as blade
coaters or roll coaters. One or more coating
applications can be applied to one or both sides of the
paper. Typically, a coated paper is then introduced into
equipment which imparts smoothness and gloss to the
surface. Such equipment includes supercalenders, machine
calenders and gloss calenders.
Coated paper is commonly supercalendered by
passing it through a series of nips under high load at
elevated temperatures. The paper usually enters the
supercalender stacks at a relatively high moisture
content of about 7-9%. The nips of the supercalendering
equipment provide a shearing action which results in a
smooth paper with a high gloss. The high pressure
associated with supercalendering compresses and densifies
the paper, which typically limits the process to
production of low caliper paper. For this reason, paper
board for high quality printing is not usually
PATENT UCC-624
supercalendered. Supercalendering also has the effect of
reducing opacity, which is undesirable. The complexity
of the supercalendering process also ad~s significantly
to the processing time and the manufacturing cost of the
paper.
~ ost of the final product properties of coated
papers~ such as smoothness and gloss, are achieved in the
finishing section of the paper machine or in a subsequent
coating operation off-line from the main paper production
equipment. The degree of smoothness of the paper is
generally a function of the density. Examples of
artisans attempting to improve smoothness while limiting
the amount of densification are disclosed in U.S~ Patent
4r 596,633, to Attwood, June 24, 19~6; and U.S~ 4,624~74~,
to Vreeland, November 27, 1986, which are hereby
incorporated by reference. These patents refer to
methods of prodùcing a smoother paper surface without the
densification associated with conventional calendering
techniques. However, both disclose processes which are
performed after the web is already dried to about 80%
solids or more.
Attwood teaches a process that requires rewetting
the paper or paper board prior to pressing the moist
surface against a smooth polished cylinder, similar to
gloss calendering. Vreeland discloses as a finishing
step ~he process of high temperature gloss calendering at
low moisture contents to attain high gloss and
smoothness. Neither of these disclosures significantly
simplifies the paper-making operation so as to produce
higher quality papers with less complexity, and, at best,
corrects inherent problems concerning lack of smoothness
of the base paper as formed~
Another technique for regulating the surface
properties and consistency of paper, referred to by those
PATENT UCC-624
in paper-making arts as "impulse drying", has received
considerable attention of late. U.S. 4,324,613, to
Wahren, April 13, 1982; U.S. 4,738,752, to Busker et al.,
April 19, 1988; Burton, et al., "The Instantaneous
Measure of Density Profile Development During Web
Consolidation," Journal of Pul~ and Pa~er Science, Vol.
13, No~ 5, pp. J145-J149, September/ 1987, all of which
are hereby incorporated by reference. Impulse drying
employs high temperatures, high pressures and moderately
long residence times to remove water much more
efficiently than conventional pressing,with large energy
savings. Impulse drying both dries and densifies the
fibers of the sheet by compressing a web of fibers
between heated nip rollers.
Press drying and/or impulse drying of liner board
and paper board has been researched by various
organizations for the purpose of taking advantaye of the
strength increases and water removal efficiencyO See
U.S. 4,624,744, to Vreeland; and U.SO 4,692,212, to
Swenson et alO, September 8, 1987, the latter of which is
also incorporated by reference.
Most of the work in the area of impulse drying
technology has centered ~round heavy weight packaging
grades requiring high strength. However, one artisan has
suggested that the increased surface smoothness of
impulse dried papers is suitable for writing and printing
papers such as newsprint. Lavery, "Impulse Dryiny of
Newsprint," Journal of Pulp and Paper Science, Vol. 13,
No. 6, pp. J178-J184, November, 1987, also hereby
incorporated by reference. Nevertheless, the effect on
coated printing papers has not been investigated and
prior speculation about the use of impulse dried paper for
coating applications has revealed doubts about whether
PATENT UCC-624
the coating could adhere to the impulse ~ried surface
because of its low permeahility.
Accordingly, there exists a need for paper and
paper board with sufficient low permeability and high
surface smoothness for producing high quality coated
paper products with gloss and smoothness comparable to
supercalendered sheets, but with less densification and
less applied coatingO There is also a need for a base
paper having low surface porosity and absorbency, with
high surface strength and good optical properties. There
is also a need to reduce or simplify the process steps
required to make high quality coated paper so as to
improve production efficiencyO
SUMMARY OF THE ~NVENTION
This invention provides a method of making coated
paper or paper board by providing a web of paper-making
fibers, preferably having a consistency of at least about
20% solids, and impulse drying this web at a temperature
of at least the glass transition temperature of the
paper-making fibers at the level of moisture in the
fibers, to produce an impu~se dried sheet preferably
having a consistency of at least about 50~ solids and a
Parker Print Smoothness of less than about 6.5 microns
upon completion of drying. This novel method further
includes surface coating the impulse dried sheet to
obtain a coated sheet having improved properties. The
surface properties of the impulse dried paper reduce the
finishing requirements necessary for producing an
improved coated sheet.
Accordingly/ a new process for the manufacture of
coated paper and paper board is provided which permits
the high gloss and smoothness of supercalendered coated
-
-- 4
~3~2~.
PATENT UCC-624
papers without the loss in bulk or opacity associated
with supercalendering. This invention makes use of the
properties imparted to a base sheet during impulse drying
and uses these properties to make high quality coated
paper in an unexpected and improved manner. Preferably
the invention includes calendering, such as gloss
calendering, the coated sheet to provide a product with
high gloss and better smoothness at a lower average
density than a supercalendered sheet. The process
enables a relatively thin coating of about 3-5 lbs/
3000 ft2 and moderate gloss calendering conditions to
produce a coated paper product similar to a No. 3 coated
offset paper, but with higher bulk and better smoothness.
The process also results in substantial savings in
manufacturing costs through the reduction of energy
usage, materials and time required to produce the
product.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying dra~ings illustrate comparative
test data for paper produced pursuant to this invention
under laboratory conditions.
FIG. 1~ is a graphical depiction of Gurley
porosity versus bulk, comparing a standard sheet to
samples prepared with impulse drying techniques;
FIG. 2: is a graphical depiction of Parker Print
surface smoothness versus bulk for a standard sheet
versus various impulse dried sheets; and
FIG. 3: is a graphical depiction of Letterpress
Smoothness versus bulk for a standard sheet versus
various impulse dried sheets.
- 2 ~ 2 ~
PATENT UCC--6 2 4
DETAILED DESCRIPTION OF THE: INVENTION
The preferred operable embodiments of this.
invention will now be discussed. In the preferred
method, a coated paper or paper board is manufactured by
first providing a web of paper-making fibers having a
consistency of at least about 20% solids. This web is
then impulse dried at a temperature of at least about the
glass transition temperature of the paper-making fibers
to produce an impulse dried sheet having a consistency of
at least about ~0% solids and a Parker Print Smoothness
of less than about 6.5 microns upon completion of dryingO
Following impulse drying, the impulse dried sheet is
surface coated to obtain a coated sheet~ As used herein,
the term l'glass transition temperature~ of the paper-
making fibers refers to the temperature at which the
cellulose fibers begin to form an amorphous, glass-like
substance and bond to one another. If more than one type
of furnish is employed, the l'glass transition
temperature" would constitute a weighted average of the
individual glass transition temperatures for each type of
furnish. The glass transition temperature of the furnish
is lowered as the moisture content is increasedO so the
glass transition temperature at the furnish consistency
is the preferred temperakure of the fibers during impulse
drying.
In another method embodiment of this invention, a
web of paper-making fibers is provided having a
consistency of about 20-30% solids. This web is then
impulse dried at a temperature of at least about the
glass transition temperature of the paper-making fibers
to produce an impulse dried sheet having a consistency of
about 65-99% solids and a Parker Print Smoothness of
about 3.0-5.5 microns. The impulse dried sheet is then
-- 6 --
PATENT UCC--62a~
surface coated with a coating of about 3-5 lbs. per 3000
s~ fto to obtain a wet coated sheet. The wet coated
sheet is then dried to produce a substantial~y dry coated
sheet, and the dry coated sheet is then gloss calendered
at a temperature of about 250-350F and a nip pressure of
about 300-700 pli.
This invention also provides coated paper and
paper board comprising an impulse dried surface having a
coating of about 3-12 lbs. per 3000 sq. ft. Ihe coating
comprises a gloss calendered finish having a gloss of
about 64-68~, a Parker Print Smoothness of less than
about 1.5 microns~ a bulk of greater than about .9 cm3/g,
and a caliper of above about 3 milsO
This invention also provides fine coated paper
comprising an impulse dried surface having a coating of
about 3-5 lbs/3000 ftZ. This coating comprises a gloss
calendered finish having a gloss of about 64-68%, a
Parker Print Smoothness of less than about 1.0 microns,
and bulk of about 1.07-1.19 cm3/g, a caliper of about 3.2-
3.8 mils, and a basis weight of above about 30 lbs/3000
sqO ft.
The preferred processing techniques for operating
the method of this invention will now be described. As a
preliminary step, the paper-making fibers of this
invention are formed into a paper web of at least about
20% solids, and more preferably 20-30% solids. The
moisture level is typical of paper webs formed on a
fourdrinier paper machine just before conventional
pressing. The sheets are then carried to the impulse
drying apparatus using drier ~elts for absorbing some of
the water from the we~ during the impulse drying step.
The apparatus for the impulse drying step o~ this
invention can include a heated drier drum capable o~
reaching temperatures up to about 700F. Preferably the
. . .
7 --
PATENT UCC-624
impulse drying step dries the web at a temperature of
about 300-900F, and more preferably abou-t ~00-600F. The
heated drier drum is preferably in contact with another
drum for forming a nip capable of reaching nip pressures
of about 0.3-7 MPa. ~xposure times generally can run
from about 15-100 milliseconds, although about 20-60
milliseconds is preferredO Reference is made to ~.S.
4,324,613, which describes impulse drying techniques more
fully.
In accordance with the present invention, using
the above procedure, the sheets were impulse dried using
one nip at 600F and two different pressures, 287 and 445
psi, and two different dwell times, 98 and 50
milliseconds. Specific conditions for each of the
samples are listed in Table I below.
Upon impulse drying, the solids content increased
to over 69% for each of the samples. In contrast,
pressing conditions of conventional processes increase
the solids content only slightly from about 25% to about
31~ During impulse drying, the fibers in the web
consolidate more completely than in conventional
processing, and -the structure is effectively locked in.
The increased solids level after impulse drying further
reduces deformation of the fibers from additional process
stepsO
During web formation and drying, the fibers of
the web shrink and deform much more in conventional
pressing and drying than in impulse drying/ since they
are relatively unrestrained. With impulse drying~ the
web is preferably held against a heated steel roll at
high temperature and pressure and for a relatively short
time, making the fibers highly constrained as they dry.
This permits the fibers to consolidate and the structure
is locked in.
-- 8 --
PATEN~ UCC--6 2 4
3 ~
0
~3 'n
~ ~ o\C
~ ~D ~ ~ ~D
o
H
~ a) ~ OD O O O
E~
~_
S~
b ~ _ o o o o o o o
~ ~o o o o o o o o
.,1 ~
a~
H v~
PATENT UCC-624
Cross-section micrographs comparing conventional
unrestrained drying to impulse drying established that
the impulse dried fibers are well bonded, more dense, and
exhibit a very flat and smooth surface where the web has
been heated. This difference in structure a~fects the
physical properties to a great extent. These benefits
include greater surface strength, better smoothness, and
higher density than conventionally pressed papers. For
board grades, the density of the fibers towards the
heated surface during impulse drying is greater than the
underlying fibers.
Although the density of the sheet is hiqher, the
fibers of the sheet are less deforma~le. Upon final
calendering after the cooling step, the impulse dried
sheet is less dense than a conventional pressed, coated
and calendered paper.
The smoothness of an uncalendered impulse dried
sheet as measured by conventional Parker Print Smoothness
("PPS") analysis was found to be smoother than an
uncoated machine calendered conventional sheet,
I'Jamestown Xerographic" manufactured by Union Camp
Corporation, as revealed by the data in Table II.
TABLE II
Parker Print gmoothness o~ Impulse Dried Sheet vs.
Commercial Uncoated Paper~
Parker Print Smoothness in microns
Sam~le Impulse Dried Side Untreated Side
1 4.4 8.2
2 4.3 8.2
3 403 7.7
Jamestown
Xerographic N/A 5.6
: `
- 10 -
hl ~ ~ 3 g~ 2 ~L
PATENT UCC-624
The impulse dried side of the sample sheet was also much
smoother than the untreated side of these sheets.
Generally speaking r the uncalendered impulse dried sheets
of this inv~ntion will have a Parker Print Smoothness of
less than about 6.0 microns, and preferably about 3-5.5
microns.
The surface coating step of the preferred method
can include conventional apparatuses such as air knife
coating or blade coating. Preferably, about 1-12
lbs/3000 ft2, and more preferably 3-5 lbs/3000 ft2 of
coating material is employed for coating a single side of
the paper or paper board of this invention. Both sides
may also be coated. The coatings can contain standard
pigments and binders typically used in coating paper. A
typical composition is about 80 wt.% No. 1 kaolin clay
and about 20 wt.% fine ground calcium carbonate as a
pigment with styrene butadiene as a binder. This coating
was not optimized for properties but will provide a
glossy~ smooth product. The coating solids should be
relatively hiqh to minimize the amount of water applied
to the sheet. I'he coating may be applied at e.g. 5 lbs/3000 5q.
ft. to the impulse dried side of the sheets. This amount
of coating is less than the usual amount applied in
standard coating practices for high quality papers.
To get a smooth coated surface, blade coat~rs are
typically employed by manufacturers of coated paper.
Blade coaters can compensate for the roughness of a base
sheet and can cover irregularities if e~ough coating is
applied. An air knife coater, on the other hand, follows
the contours of the substrate surface. Air knife coaters
are commonly used as a top coat after a smooth surface
has already been laid down by a blade coater for coated
board grades. Since the surface of such impulse dried
sheets is already smooth, a smooth coated surface can be
-- 11 --
3 ~ ~
PATENT UCC-624
obtained even using an air knife coater with a single
coating application. Even better quality is obtained if
a blade coater is used on an impulse dried base sheet.
Following coating, the coated sheet will be wet
and thereafter dried. The coating drying operation
preferab-ly employs hot air blown onto the coated surface.
Alternatively, infrared driers can be used to quickly set
the coating. Infrared is usually used under special
circumstances, and generally results in better qualities
since it reduces the penetration of coating into the
sheet by shorteniny the time the coating is wet.
However~ the low surface porosity of an impulse drled
sheet already minimizes coating penetration, an~ thus the
need for infrared dryiny can be reduced or eliminated.
Furthermore, drying the coating on an impulse dried sheet
is easier since the coating is held out on the surface,
rather than absorbed. Less energy and/or less time is
used to dry the coatings since rewetting of the
underlying base sheet is minimized. The efficiency of
coating driers decreases dramatically if the underlying
base sheet is too wet. The base sheet can become too wet
if a high amount of coating is required needed to co~er a
rough base shee~ to obtain high quality. These
inefficiencies can be reduced or eliminated if the base
sheet is impulse dried.
The most preferred method of this invention
further includes gloss calendering the substantially dry
coated sheet to produce a fine paper. Gloss calendering
is a common finishing operation which produces high gloss
sur~ace finishes without the densification associated
with supercalendering. The technique uses heated rolls
preferably having a temperature of at least about 225F
and more preferably about 250-350F with moderated nip
pressures of about 250-1000 pli, more preferably 300-
- 12 -
PATENT UCC-624
700 pli, and primarily affects only the uppermost surface
of the paper, which is usuall~ coated. Because there is
ess densification, better opacity resultst but the sheet
is ~enerally not as smooth as supercalendered
products. Gloss calendering equipment, however, can be
used in-line with the main paper production equipment,
while supercalendering generally is not. Coated impulse
dried sheets that were tested pursuant to this invention
were gloss calendered using a soft rubber covered backing
roll at about 300F and about 300 pli. The calendering
conditions were thus relativel~ mild for producing glossy
coated paper. These conditions were typical of coated
board produced on-machine. As a result, while the
caliper of the coated impulse dried sheets was not
reduced very much at all, a very glossy smooth surface
similar to a supercalendered sheet was unexpectedly
obtained.
Alternativelyl the preferred calendering step of
this invention can be accomplished with machine
calendering. The coated paper passes through a nip
~etween two hard rolls at high pressure and moderate
temperature. The machine calender densifies the web and
is used to control the caliper of the sheet. Machine
calendering also produces smoothness but generally with
little gloss. The equipment is fairly standard for
finishing operations for paper and paper board and is
usually used in-line with the main paper production
equipment.
The properties of three experimental sheets made
in accordance with this invention were compared to a
commercially available No. 3 coated paper which was
supercalendered and blade coated on two sides with
approximately 7-8 lbs/3000 sq. ft. of coating per side.
A summary of the properties of the experimentàl impulse
- 13 -
2~3~
PATENT UCC-624
dried sample sheets and the commercial product are listed
in Table III below. The experimental samples were as
glossy, much smoother, and were less dense than the
commercial sample. The experimental samples also
possessed excellent ink holdout. secause these
propertIes are better, and di~ferent, the coated impulse
dried sheet proved to be higher in quality than the more
expensive ~oO 3 coated paperO
Further experiments were conducted employin~ more
conventional techniques and consistencies. A 12'i wide
web was formed on a web former using 40 dry pounds of
bleached chemical pulp containing high amoullts of
southern hardwood, 25-30% solids. Specifically, the
furnish contained 85% hardwood and 15% pine; 80 pulp
brightnessr and 370 S/R freeness. The pulp was reslushed
in a laborator~ beater and delivered directly to the head
box. The web former used was basically a head box, a
Fourdrinier wire with vacuum-assisted drainage, a small
one nip press roll and a spindle for winding up the wet
web~ The targeted basis weight was about 75 lbs/3000
ft2 .
After forming, the wet web was impulse dried
using consistencies, dwell times and temperatures which
were chosen b~ considering the limitations of impulse
dryers and the technical feasibility of these parameters
in practical applications. The samples employed impulse
drying parameters which included nip temperatures of 400,
500, and 600F; nip dwell times of 20 and 50 milliseconds;
and an average nip pressure of 400 pli. These
experiments used one nip on one side of the web surface.
Additionally, experiments were run which included, in the
~irst instance, the use of 400F for two nips on both
sides of the web surface, and, in the second instance, a
first nip of 600F and a second nip of 400F. A control
- 14 -
~ v ~
PATENT UCC-- 6 2 4
~
m,~
a) --
.~1
0 ~ ) O
~ _ ~
H
~; ~
E'
~ V~
h a) u~ a
,~
.C O ~ S~
X ~0 ¦ o ,,
. I
0 ~3 E ~ o
P~ (~
d
oo ~
0
O
U~
-- ~
hO o~ . ~::
a~ ~ ~ ~
~ o ..
a~
* 0
aJ h
~ ~ a) .
Q. .
O U~
o 0 o
a, E~ u~
PATENT UCC-624
was run at 100F, 20 milliseconds, and 400 pli average nip
pressure.
The nip pressure was held constant throughout the
trial at an average pressure of about 400 pli. This
pressure is not unlike conventional pressin~.
Experiments showed that higher nip pressures do not
significantly improve smoothness and may interfere with
runability.
After impulse drying~ it was noted that the
impulse dried web was significantly drier, hut because
the dwell times were shorter and the basis weight was
higher~ the same degree of drying found with handshee-ts
was not observed in this experiment. Drying the webs was
completed on a laboratory drum dryer. The ingoing
~5 consistency for impulse drying generall~ was between
about 32-34% and the outgoing consistency varied with the
conditions listed in Table IV below.
For the one nip experiments, the wire side of the
web was toward the heated roll and the felt was on the
top side. A liner board felt was used in these
experiments so the felt side of the ~heet was much
rougher than ideal, which was subsequently reflected in
~he smoothness parameters for this side. For the two ni~
experiments, the wire side was toward the first heated
nip and the top side was toward the second heated nip.
In these experiments, felt marking was observed on both
sides of the sheet because the sheet was in contact with
a felt at some point. Felt marking was more severe on
the wire side of the sheet since it was the last side to
3~ have a felt on it.
The uncoated impulse dried webs were subjected to
both physical and print lab testing. Scattering
coefficients were determined and used as a measure of how
well bonded the fibers in the sheet became with impulse
- 16 -
3~2~
PATENq~ UCC - 6 2 4
o ~ C~ o
~ C,
W
P ~
P- Q)
H
a H
U
E~
H
H ~ 0 0 0
a) ,,, o o o o o o o
E~"Iooooooo
Q. .1 o o o o o o o
O O O O O O O
o ~I d' ~ It~ 1~) ~O ~O
- 17 -
PATENT UCC-624
drying. The scattering coefficient for the impulse dried
sheets increased about linearly with bulk. It was
determined, therefore, that better bonding is usually
found for denser sheets and the scattering coefficient
decreases wi'h increases in bonded area. Unexpectedly,
however,~ the avera~e bulk of the impulse dried sheet was
found to be lower than for conventional pressed and dried
sheetsO Impulse dryingl therefore, increases the
1exibility for obtaining different properties which
depend on bulk.
Sheet porosity was found to change significantly
with impulse drying compared to standard pressing and
drying as described in FIG. 1. Gurley porosity was shown
to increase as the sheet is densified. The increase was
different but not as high with two nip impulse drying.
With two heated nips in subsequent operations, water is
probably forced through the first impulse dried side
causing it to be more open. The impulse dried sheets
were found to be much less open than the control and
standard pressed sheets made from this type of furnish.
The data for the standard conditions was obtaine~ from
pressed and calendered handsheets.
The change in Gurley porosity with bulk for
impulse dried sheets suggests that the paper is unique
and would be suitable base stock for coating. Because of
the lower porosity, the coating penetration into the
sheet should be much less and this property has many
advantages.
Surface smoothness, as measured by Parker Print
Smoothness, was determined to be much better for the
impulse dried sheets than for conventionally pressed and
dried sheets, as illustrated in FIG. 2. See 3. R.
Parker, TAPPI Journal, Vl.64, No. 12, pp. 56-58 ~1981
which further describes this procedure and which is
- 18 -
2 ~
PATENT UCC-6~4
herein incorporated by reference. This curve shows that
the impulse dried conditions produce products which were
all smoother than the controlO Typical machine
calendered ~erographic paper has a PPS of about 5.6. The
impulse dried base stock is nearly as smooth as a
finished sheet but retains a higher bulk. Increasing the
temperature and dwell time of the nip helps smoothness.
Therefore, it is further noted that smoothness
development with the impulse dryer responds similarly to
gloss calendering. ~he process can be considered as a
one-step process for pressing, drying, surfacizing, and
finishing.
When smoothness is measured by Letter Press
Smoothness (I'LSS''), the same conclusions about smoothness
development with impulse drying were reached, see FIG. 3.
Letter Press Smoothness is the measure of a depth of
black ink, in microns, sufficient to obtain complete
covexage of the paper surface. This standard was
compared to the conventionally wet pressed, dried and
machine calendered sheets of si~nilar furnish. Compared
to the control estimations, the bulk-smoothness
relationship of the impulse dried sheets was much
improved over the standard methods of producing base
stock. These samples were coated by an air knife coated
with 5 lbs/3,000 ftZ of coating.
The impulse dried base sheets of this invention
are not limited to fine paper production and can be used
as a starting material for a multitude of paper productsO
The base sheet can be treated with a pigmented size press
application to produce machine finished pigment grades or
treated with a light blade coating to produce light
weight coated paper. The base sheet could alternatively
be coated using any formulation or coating equipment to
achieve the quality of any supercalendered coating grade
-- 19 --
PATENT UCC-624
of paper, such as No. 2, or No. 1, etc. Additionally,
the base sheet can be treated by more than one coating
application to produce, for example, double coated and
triple coated grades. The base sheet can alternatively
be coated and finished by other art recogni.zed processes
for achieving smoothness and/or gloss. In sum, this
invention can be used to manufacture products which
typically use coating to impart a specialized surface
finish in which keeping the coating on the surface and
keeping that surface as smooth as possible is a desired
effectO
- 20 -
