U.S. patent number 3,873,345 [Application Number 05/331,815] was granted by the patent office on 1975-03-25 for method of finishing coated paper.
This patent grant is currently assigned to Scott Paper Company. Invention is credited to Jay H. Vreeland.
United States Patent |
3,873,345 |
Vreeland |
March 25, 1975 |
Method of finishing coated paper
Abstract
Production of high-gloss coated paper with heated calendering
apparatus through the use of a coating composition comprising
pigment and a hard latex binder.
Inventors: |
Vreeland; Jay H. (Yarmouth,
ME) |
Assignee: |
Scott Paper Company
(Philadelphia, PA)
|
Family
ID: |
23295491 |
Appl.
No.: |
05/331,815 |
Filed: |
February 12, 1973 |
Current U.S.
Class: |
428/514; 427/361;
428/537.5; 427/391 |
Current CPC
Class: |
D21H
25/14 (20130101); Y10T 428/31906 (20150401); Y10T
428/31993 (20150401) |
Current International
Class: |
D21H
25/00 (20060101); D21H 25/14 (20060101); B44d
001/44 () |
Field of
Search: |
;117/65.2,76P,155UA |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lusignan; Michael R.
Attorney, Agent or Firm: Weygandt; John A. Kane; John W.
Claims
What is claimed is:
1. In a method of finishing by heated calender means a paper web to
which an aqueous coating composition has been applied, the steps
of:
1. applying to the surface of at least one side of the paper web an
aqueous composition comprising paper coating pigment and a binder
consisting essentially of a polymer latex having a glass transition
temperature greater than 100.degree.F
2. drying the coating at a temperature below glass transition
temperature of the polymer
3. finishing the coating by hot calendering at a temperature above
glass transition temperature of the polymer.
2. The method in accordance with claim 1 wherein the binder has a
hardness/temperature relationship as represented by the curve shown
in FIG. 1.
3. The method in accordance with claim 1 wherein the binder is
selected from the group consisting of vinyl acetate,
styrene-isoprene and acrylic polymer latices.
4. The method in accordance with claim 1 wherein the coating is
finished by gloss calendering.
5. The method in accordance with claim 1 wherein the coating is
finished by supercalendering.
6. A coated paper web made in accordance with claim 1.
7. A coated paper web made in accordance with claim 4.
8. A coated paper web made in accordance with claim 5.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of producing high-gloss
coated papers with heated calendering apparatus through the use of
coating compositions comprising pigments and synthetic polymeric
latices.
2. Description of the Prior Art
High-gloss coated papers can be produced by the application of an
aqueous coating composition containing pigment and thermoplastic
binder and subsequently developing gloss in the coated paper by
means of hot calendering, such as by supercalendering or gloss
calendering. A supercalender comprises a "stack" of alternating
steel calender rolls and resilient backing rolls, with means for
heating the steel rolls or heat being generated by friction.
Usually the paper web is threaded through the stack, thus wrapping
the heated roll for a considerable portion of its circumference.
Alternatively, the paper web may be passed directly through the nip
formed by the heated roll and the backing roll by the use of fly
rolls. The use of a super-calender involves relatively high nip
pressures, generally in the range of 1,000 to 2,000 pounds per
linear inch (pli) and typically 1,200-1,600 pli. The temperatures
of the steel finishing rolls are generally 140.degree. to
180.degree.F. While supercalendering suffers from the disadvantage
of compacting and densifying the coated paper web during the
calendering operation, it has the advantage of greater speed and
lower cost of equipment relative to gloss calendering which is
almost invariably done in-line with the coating operation.
The method of finishing coatings on paper by means of hot
calendering known in the paper-coating art as "gloss calendering"
involves the production of a glossy surface on paper or related web
materials by contacting the surface of a coated substrate with a
polished finishing drum under temperature conditions sufficient to
cause a temporary condition of plasticity in the surface to thereby
obtain a high degree of finish or gloss without unduly compacting
the substrate. This higher bulk compared to that obtained by
supercalendering leads to increased brightness and opacity, which
are desirable properties in coated printing papers, and permits the
use of a lighter basis weight paper to provide a given caliper. In
gloss-calendering of paper, an aqueous coating composition is
applied to a paper web, the web is calendered in the nip formed
between a gloss-calender drum and the resilient backing roll and
the paper web is removed from contact with the drum as it emerges
from the nip. The pressure conditions in the gloss-calender nip are
generally lower, e.g., 500-900 pli, than in the supercalender, and
the temperature conditions are typically higher --
275.degree.-350.degree.F.
Aqueous paper coating compositions generally comprise a mineral
component, which is preferably predominantly clay, but may also
include other mineral pigments such as titanium dioxide, zinc
sulfide, or calcium carbonate, and a thermoplastic binder. It has
recently been suggested (in U.S. Pat. No. 3,583,881 granted June 8,
1971) that so-called "hard" polymers be employed as the binder
material in a paper coating composition in order to obtain a high
gloss coated paper product. By "hard" is meant that the
thermoplastic polymer has a relatively high apparent second order
transition temperature or inflection temperature, more commonly in
the paper coating art called the glass transition temperature
(T.sub.g), which may be found by plotting Young's modulus of
rigidity against temperature.
Each polymer has its own "glass transition temperature" (T.sub.g);
this term is well known in the art and is generally used to define
or describe a temperature above which the polymer has acquired
sufficient thermal energy for molecular rotational motion or
considerable torsional oscillation to occur about the majority of
bonds in the main chain. This term is also used to define a
"minimum film forming temperature" of polymer latices minimum which
the polymer particles are capable of being coalesced by surface
tension upon evaporation of water to form a film. In effect, then,
the term "glass transition temperature" or "minimum film forming
temperature" describes a type of internal "melting" point for
polymer latices, but not a phase change, at and about which the
polymer preserves the outward appearance of a solid but at the same
time behaves more like a viscous liquid in its ability to undergo
plastic flow and elastic deformation. For the purposes of this
invention, the term "glass transition temperature" may be used
interchangeably with and defined as the "minimum film forming
temperature" of a polymer latex. In actuality, this "transition"
occurs over a small range of temperatures rather than an exact
point.
From this definition it may be seen that if the temperature T is
taken as room temperature (68.degree. to 77.degree.F or 20.degree.
to 25.degree.C), then any polymer having a Tg substantially greater
than T, for example, 90.degree.F or 32.degree.C, will be a non-film
former at T, while any polymer with a T.sub.g substantially below
T, for example 60.degree.F or 16.degree.C, will be a relatively
good film former at T. In the art to which the present invention
pertains, the expression "hard" refers to binders having a Tg
substantially greater than room temperature and generally a Tg in
excess of 100.degree.F or 38.degree.C.
It is stated in the above-mentioned patent U.S. Pat. No. 3,583,881,
that in order to prevent sticking of the coating to the hot steel
roll used in the gloss calender, the mean Tg value of all polymer
components of the binder must be at least 43.degree.C. It is
further stated that the drying of the coating is effected at an
elevated temperature to assure fusion of the polymer therein, the
temperature being about 20.degree. to 60.degree.C higher than the
Tg of the polymer incorporated in the coating.
Another U.S. Patent which is directed to the production of
high-gloss papers by coating the papers with a composition
comprising a pigment and a "hard" binder, U.S. Pat. No. 3,634,298
granted Jan. 11, 1972, presents a theoretical explanation as to why
"hard" binders promote gloss. The pigment is oriented during the
drying cycle by surface tension effects which bring the pigment
matrix to minimum volume. This is also the point of maximum
orientation and highest gloss in the unfinished condition. Binders,
however, freeze the pigment in a random orientation as soon as
pigment binding occurs, which prevents maximum orientation and
gloss before calendering. This binding action commences when the
emulsion is broken in the drying cycle and film formation occurs.
As the minimum film forming temperature is increased, the time
until coalescense in drying is increased, thereby increasing the
time for orientation to occur. By this reasoning, gloss is a
function of the glass transition temperature.
As will become apparent hereinafter, such a conclusion is only
partially correct. Both of the above-mentioned patents, moreover,
teach that the binder be coalesced during the drying cycle in order
that it hold the pigment in place. U.S. Pat. No. 3,583,881, column
4, lines 18 through 22; U.S. Pat. No. 3,634,298, column 1, lines 69
through 72.
SUMMARY OF THE INVENTION
The present inventor has found that by ignoring the teaching of the
prior art, namely that the binder should be fused or coalesced
during the drying step, and, instead, by carefully drying the
coated paper web below the Tg temperature, (thus avoiding
coalescense), a much higher level of gloss can be produced upon
subsequent hot calendering.
In accordance with the method of the present invention, a paper web
is coated with a composition containing a hard polymeric binder,
the coating is dried at a temperature below that at which the
binder coalesces, and the coating is calendered at a temperature
above the coalescence temperature. Because the binder in the
coating is unfused or only partially fused, the pigments in the
coating are unbound and hence the surface is much more moldable and
capable of producing a much higher level of gloss than if it had
been heated above the Tg temperature before the web reached the
calender nip.
A further feature of the invention is the discovery of the
importance, in the use of hard binders, of the relationship between
hardness and temperature. The shape of the curve produced when
hardness is plotted against temperature has been found to have
meaning for the ease and effectiveness of hard binders in enhancing
gloss.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph depicting, for a representative hard binder of
the present invention, the shape of the curve formed when Shore A
hardness is plotted against temperature.
FIG. 2 is a graph depicting, for a representative soft binder, the
shape of the curve formed when Shore A hardness is plotted against
temperature.
FIG. 3 is a graph depicting, for another hard binder, the shape of
the curve formed when Shore A hardness is plotted against
temperature.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, it is to be noted from FIG. 1 that
the region of T.sub.g is very short and the slope of the curve in
the transition from rigidity to softness is very steep. Thus,
polymers having the hardness/temperature characteristics
represented by the shape of the curve shown in FIG. 1 are preferred
for the practice of the present invention and examples thereof are
illustrated in Examples 1, 2, 4 and 5 below. Such polymers are
preferred because they begin to flow more readily upon contacting a
heated finishing surface and likewise more quickly freeze as the
web cools upon leaving the finishing surface because the transition
back to hardness occurs over a very narrow temperature range. This
rapid return to hardness allows the coating to maintain the high
polish or gloss achieved in the nip.
The curve shown in FIG. 2 is representative of a soft polymer, such
as that illustrated in Example 3 below, having a T.sub.g below the
temperature range shown in the figure. In the temperature region
shown, the curve exhibits only the flatness characteristic of the
polymer's hardness/temperature relationship in the region well
beyond the T.sub.g of the polymer. It may be seen that the hardness
of the binder represented in FIG. 1, at a sufficiently high
temperature, actually drops below that of the soft binder
represented in FIG. 2 at the same temperature. Thus, it is
postulated that the better gloss achievable with hard binders is in
part attributable to the fact that these hard binders, at a
temperature above T.sub.g, are actually softer than soft binders
and for this reason produce better results even if coalesced or
fused prior to finishing.
In the case of the binder represented in FIG. 3, while the region
of T.sub.g is very short and the slope of the curve in the
transition area is very steep, the change in hardness is less than
that depicted in FIG. 1. The curve shown in FIG. 3 indicates that
even when this polymer is in its "soft" condition, it is not as
easily molded as the polymer represented in FIG. 1. Thus, the type
of material represented by the curve in FIG. 3 is less preferred
for use in the present invention than that shown in FIG. 1, as the
material is harder at the finishing temperature and thus less
easily molded.
While the curves shown in FIGS. 1-3 describe the polymer itself,
the hardness/temperature characteristics of a composition
containing pigment and the binder can also be measured. The shape
of the resulting curve can be correlated to the gloss achieved by
the use of the coating composition.
The gist of the present discovery is that the effectiveness of a
hard binder in producing high gloss when utilized in paper coating
compositions is not purely a function of the chemical composition
of the binder or of its glass transition temperature, but primarily
of the sharpness and depth of the transition in hardness at
T.sub.g, the glass transition temperature. The preferred binders
for use in the present invention are hard at temperatures below
T.sub.g and very soft at temperatures above T.sub.g.
In accordance with the present invention, an aqueous coating
composition comprising paper coating pigment in a binder consisting
essentially of a polymer latex having a T.sub.g greater than
100.degree.F is applied to a paper web and dried under conditions
such that the temperature of the surface of the coating remains
below T.sub.g and the coated paper web is finished at a temperature
above T.sub.g. As will be herein observed, the temperature of the
air employed to dry the coating may be heated to a temperature
substantially above T.sub.g. Although the phenomenon is not
generally recognized, the actual temperature of the surface of a
paper web during drying, for example in a high velocity air dryer,
has been found by the present inventor to be about 100.degree.F
below the temperature of the air circulating in the dryer. The
explanation of this temperature difference is that the coating is
cooled by the evaporation of water from it (evaporative cooling).
The temperature of the surface of the web does not approach the
temperature of the circulating air until substantially all the
moisture is evaporated from the coated web. As will be understood
by one of ordinary skill in the art, the coated web need not be
reduced to such a degree of dryness in order to be calendered.
Accordingly, it will be seen that rapid drying can be effected by
utilizing very hot air yet the drying can be carried out in
accordance with the present invention, i.e., at a temperature below
T.sub.g, by removing the coated web from the dryer before
evaporative cooling has ceased.
The expression "consisting essentially" as used herein indicates
that the binder is effectively all of the hard, thermoplastic type.
The binder component may contain a minor proportion of binder other
than the hard, thermoplastic type, provided that the amount is less
than that necessary to bind the pigment. In general, this necessary
amount is less than 5% based on 100 parts of pigment, but may vary
depending on the amount of binder which is absorbed by the paper
web. If an amount of soft polymer sufficient to bind the pigment is
used, maximum gloss will not be obtained.
A further feature of the invention is the development of several
unique and desirable features in the finished coating. It is
believed that the final film structure produced in the practice of
the invention is different from that produced in the usual soft
binders. The hot finishing operation may not completely coalesce
the latex particles to a solid impervious film but rather leaves
what may be characterized as a "microporous" film in which the
latex particles are flowed together but form a semi-continuous
structure.
Such a film, in addition to being strong enough to securely bind
the pigment, provides certain very desirable attributes to paper
coatings. For example, one of the functions of a coated paper is to
provide a smooth surface for subsequent printing with gloss ink. A
necessary property of this coating is that it also possesses a fine
pore structure which can cause the ink to set, via the capillary
action of the coating upon the ink or certain more mobile portions
of it. This behavior reduces the tendency of printed images to
"set-off" or be transferred to the back side of the subsequent
sheet deposited on the pile as the sheets are collected on the
printing press. The result of the present invention is to produce a
coated paper with excellent "ink setting" abilities compared to
conventionally coated papers.
Another advantage of such a structure occurs in the production of
papers for web offset printing. In order for a paper to have
adequate blister resistance, it must possess sufficient porosity to
allow the escape of the sheet moisture, which is being driven off
by the high temperatures of the oven dryers. The practice of the
present invention results in a product of outstanding blister
resistance compared to a similar coated paper made with
conventional soft binders.
MODE OF OPERATION OF INVENTION
To assist those skilled in the art to practice the present
invention, the following modes of operation are suggested by way of
illustration. The base stock or fibrous cellulosic substrate to be
coated in accordance with the present invention can be one of a
wide variety of types depending upon the use for which the product
is intended. It can be an internally sized or a surface-sized stock
and can vary in weight from a light-weight paper, such a magazine
weight, to paperboard weight. As will be understood by those of
ordinary skill in the art, to provide a finish of higher quality,
it is preferred to prime coat the surface to be finished. The
composition of such a prime coat, or even its presence, is not
considered to be a critical feature of the present invention.
While the description herein mentions the coating of one surface of
the paper, both faces may be so coated if desired. "Ream" as used
herein represents 3,300 square feet. All weights, unless otherwise
indicated, are on a dry weight basis. All gloss readings given
herein were made at 75.degree. in accordance with TAPPI T-480
ts-65.
The principles, features and advantages of the invention will be
further understood upon consideration of the following specific
examples.
EXAMPLE 1
An internally sized paper stock was prime coated on both sides with
a conventional aqueous paper coating composition of starch and clay
in an amount per side of 2.5 pounds per ream (3.8 grams per square
meter).
A coating composition comprising the following was prepared:
Parts by Weight Mineral Pigment 100 Tetrasodium pyrophosphate
(dispersant) 0.15 poly (vinyl acetate) emulsion.sup.(1) 20 Tg =
120.degree.F (49.degree.C) Water in an amount sufficient to produce
a coating composition having 65% solids .sup.(1) "Gelva E-900"
supplied by Monsanto
and applied at 1,500 feet per minute to one side of the
prime-coated web by means of a flooded-nip trailing-blade coater at
a rate to provide 7 pounds per ream (10 grams per square meter).
Immediately following the application of the coating, it was dried
by passing it through an air cap containing rapidly circulating air
heated to about 235.degree.F (113.degree.C) under conditions such
that the temperature of the coating, due to evaporative cooling,
did not exceed 104.degree.F (40.degree.C).
The dried sheet was passed through four successive gloss calender
nips with fly rolls being used to guide the web between the nips.
The calendering drum temperature was 320.degree.F (160.degree.C)
and the nip pressure was about 500 pounds per linear inch (8,929
Kg/m). The surface coating had a gloss of 77-78 following the four
nip gloss calendering operation.
If the coated paper of this example is dried at a temperature above
Tg, for example 302.degree.F (150.degree.C), conditioned at 45%
relative humidity and finished as above, the gloss reading is
approximately 10 units lower.
If the coated and dried paper web of this example is finished by
supercalendering instead of by gloss calendering, the surface of
the finished coating has a gloss of 80. If the coated paper of this
example is dried at a temperature above Tg, for example
302.degree.F (150.degree.C), conditioned at 45% relative humidity
and finished by supercalendering the gloss reading is about 74.
EXAMPLE 2
A coating composition comprising the following was prepared:
Parts by Weight Mineral Pigment 100 Tetrasodium pyrophosphate 0.15
Styrene-isoprene copolymer emulsion.sup.(2) Tg = 122.degree.F
(50.degree.C) 20 Water in an amount sufficient to produce a coating
composition having 65% solids .sup.(2) "XD 3709.08" supplied by Dow
Chemical Company at 44% solids
and applied to the prime coated base stock of Example 1 in the same
manner and weight as in Example 1. Following the application of the
coating, the paper was then dried and finished as in Example 1. The
surface of the coating had a gloss of 76 following the calendering
operation.
If the coated paper of this example is dried at a temperature above
Tg, for example 302.degree.F (150.degree.C) conditioned and
finished as above, the gloss reading is approximately 10 units
lower.
EXAMPLE 3
A coating composition comprising the following was prepared:
Parts by Weight Mineral Pigment 100 Tetrasodium pyrophosphate 0.15
Styrene-butadiene latex.sup.(3) 18 Tg = 64.degree.F (18.degree.C)
Water in an amount sufficient to produce a coating composition
having 62% solids .sup.(3) "620" supplied by Dow Chemical
Company
and applied to the prime-coated base sheet of Example Example 1 in
the same manner and weight as in Example 1. Following the
application of the coating, the paper was then dried (above
T.sub.g) and finished as in Example 1. The gloss of the gloss
calendered paper was 50.
As previously pointed out, the present invention provides a coated
paper with excellent "ink setting" abilities compared to
conventionally coated papers. This ink setting property can be
described by a test which is not a standardized test but is
nevertheless useful for comparison purposes and may be described as
follows. A prescribed amount of a specific commercial printing ink
is applied as a uniform film to the paper to be tested. At regular
intervals of time after the application of the ink to the paper,
the inked sample is pressed against a surface (the composition of
which remains constant at each interval) under uniform conditions.
The density of the ink transferred to the surface is measured
optically and represents the amount of set-off. The end point of
the test occurs at that time when the density of the ink
transferred reaches a prescribed low value. The time between
application of the film of ink to the paper to be tested and the
time when the density of the ink set-off reaches the acceptable
value is called the "ink set" time.
By way of illustration, the gloss calendered seet described in
Examplel 1 gave an ink-set time of 275 seconds as measured by tthe
above described test; whereas the gloss calendered sheet of Example
3 yielded a value of 1,075 seconds.
Also as stated above, the present invention provides a product of
outstanding blister resistance compared to papers coated with
compositions containing conventional soft binders. Blister
resistance can be illustrated by performing the following
comparison. A uniform film of ink is applied to each side of the
sheet to be tested, which has been conditioned at standard relative
humidity and temperature conditions. To simulate conditions on
commercial web off-set printing presses, the inked sheet is then
immediately passed through a hot oven to test for its tendency to
blister. Provision is made for monitoring the surface temperature
of the sample and blistering is judged visually.
By way of illustration, the gloss calendered sheet of Example 1
showed no evidence of blistering when so tested at a temperature of
300.degree.F. (149.degree.C.), whereas the gloss calendered sheet
of Example 3 showed pronounced blistering at 288.degree.F.
(142.degree.C.).
EXAMPLE 4
A coating composition comprising the following was prepared:
Parts by Weight Clay 100 Tetrasodium pyrophosphate 0.15 acrylic
polymer emulsion.sup.(4) 16 T.sub.g = 154.degree.F (68.degree.C)
Water in an amount sufficient to produce a coating composition
having 63% solids .sup.(4) "B-83" supplied by the Rohm and Haas
Company at 44% solids
and applied to the prime-coated base stock of Example 1 by means of
a flooded-nip trailing-blade coater at a rate to provide 8 pounds
per ream (12 grams per square meter). Following the application of
the coating, the stock was divided into two parts. One part was
air-dried at room temperature overnight, and the other oven dried
for one minute at 302.degree.F (150.degree.C).
After drying, each part was finished by gloss-calendering,
Gloss-calendering consisted of passing the dried, coated sheet
through three successive gloss-calender nips. The calendering drum
temperature was 245.degree.F (118.degree.C) and the nip pressure
was 400 pounds per linear inch (7,148 Kg/m). The surface of the
air-dried coating had a gloss of 63 following the calendering
operation and the surface of the oven-dried coating a gloss of
57.
EXAMPLE 5
A coating composition comprising the following was prepared:
Parts by Weight Clay 100 Tetrasodium pyrophosphate 0.05 Acrylic
polymer emulsion.sup.(5) 16 T.sub.g = 217.degree.F (103.degree.C)
Water in an amount sufficient to produce a coating composition
having 63% solids .sup.(5) "B-85" supplied by the Rohm and Haas
Company
and applied to the prime-coated base stock of Example 4 in the same
manner and weight as in Example 4. Following the application of the
coating, the paper was then dried and finished as in Example 1.
The surface of the air-dried coating had a gloss of 64 following
the calendering operation and the surface of the oven-dried
coating, a gloss of 56.
It is apparent that other variations and modifications may be made
without departing from the present invention. For example, the
newly introduced "plastic" pigments, typically polystyrene spheres,
may be substituted for mineral pigments. Accordingly, it should be
understood that the forms of the present invention described above
and shown in the accompanying drawings are illustrative only and
not intended to limit the scope of the invention.
* * * * *