Hot roll glosser method with glossing temperature below free air glass transistion temperature of resin utilized

A hot roll glosser and method for the transparentising of resin-coated microencapsulated media includes a pair of heated pressing rolls, in which the roll which comes into contact with a resin surface is formed with an ultra-smooth silicone surface of low furface energy, and the glossing temperature of the rolls at the pressing nip is maintained below the glass transition temperature of the resin. Take-off rolls are disclosed for maintaining a tension in the web material exiting the nip to maintain a negative exit angle with respect to the silicone roll and to prevent adhesion and sticking to the roll.

BACKGROUND OF THE INVENTION 
This invention relates to the glossing of thermoplastic resins on 
microencapsulated receiver sheets as disclosed in the commonly assigned 
U.S. Pat. Nos. 4,399,209 and 4,416,966. 
The art of glossing polymer resins on substrate sheets is discussed in 
Cowgill, U.S. Pat. No. 2,554,663 issued May 29, 1951. Cowgill described 
the glossing of such resins in terms of the second order transition 
temperature of the resin (T.sub.2), also known as the glass transition 
temperature (T.sub.9). The second order transition temperature as defined 
by Cowgill is that temperature at which the amorphous thermoplastic resin 
material changes from a two dimensional liquid to a three dimensional 
liquid. At this point the resin is said to begin to flow more readily as 
the temperature is increased. As the temperature is increased, the 
viscosity goes down and the tackiness of the material increases. 
Cowgill believed that the setting of the coating by heating to a 
temperature well above the second order transition temperature was 
essential in order to obtain satisfactory glossing. Cowgill also showed 
that the addition of a plasticizer could be used to lower the spread 
between the second order transition temperature T.sub.2 and the actual 
glossing temperature, and suggested an oven for preheating the resin to a 
given temperature above T.sub.2, and a roll-type casting apparatus 
including a heated mirror finish roll against which the web is pressed 
face down and wrapped about 180.degree. and then peeled off face up and 
cooled. The mirror finish roll was maintained at a temperature above the 
glass transition temperature, by an actual amount dependent upon the 
amount of plasticizer employed. 
Cowgill's investigation also showed that it was possible to reduce the 
glossing temperature with increased pressure at the nip of the pressure 
rolls, but that the decrease in glossing temperature as the pressure was 
increased from 200 to 1,500 psi only amounted to approximately 10.degree. 
C., where the s, ting temperature was already 40.degree. C. above T.sub.2. 
With respect to the resin-coated receiver sheets of the kinds described in 
the above-identified commonly owned patents, it has been found that 
coalescing resin coating improves the perceived image quality by 
increasing color saturation. While Cowgill was primarily interested in 
coating paper stock such as may be used in the manufacture of cartons and 
the like, and employed primarily a resin copolymer of styrene and 
butadiene, current technology prefers the use of vinyl polymers and 
copolymers as disclosed in commonly assigned copending application Ser. 
No. 086,059 filed Aug. 14, 1987 now U.S Pat. No. 4,877,767, or phenolic 
resins as disclosed in commonly assigned copending application Ser. No. 
073,036 filed July 14, 1987 now U.S. Pat. No. 4,859,651. Such phenolic 
resins may be metallated to improve their characteristics, and 
plasticizers may be employed to lower the glass transition temperature and 
to improve surface quality. 
Such resins as currently used are obtained in a dispersed form and are roll 
and/or blade coated. Commonly the resin is finely ground or finely divided 
and dispersed in a carrier oil, and the resulting coating on the 
substrate, has a rather opaque, white, or milky appearance. In the 
processing or glossing stage, it is necessary to transparentize this 
coating and provide thereon a suitable smooth high gloss surface 
condition. 
The milky or opaque appearance of the coating, prior to transparentizing, 
is believed to be due to the reflection and refract on of light at the 
uneven surface, and additionally due to the breaking up of the light at 
the interfaces between the carrier and the dispersed particles. The latter 
may be viewed as small globules or spheres of particles under a microscope 
at between 200 and 500 power magnification. 
The copolymer has a glass transition temperature T.sub.9 or range in which 
the coating constituents, each having approximately the same index of 
refraction, coalesce into a common surface where the individual parts can 
no longer be visually or optically distinguished, and one which has a 
minimum of residual haze and a maximum of gloss, as may be measured on a 
conventional gloss meter. 
Substantial efforts have been expended to find glossing apparatus and 
methods which are compatible with the microencapsulated system, and which 
can provide satisfactory glossing at a throughput rate compatible with 
sheet development. At the present time, this rate is considered to be at 
least 10 ppm (pages per minute) for paper substrate and 5 ppm for overhead 
projection substrate (based on 81/2.times.11 inch paper). Such 
investigations have included the evaluation of hot roll as well as heated 
platen type glossing apparatus, and the commonly assigned U.S. Pat. No. of 
Stone et al, 4,807,560 issued Feb. 28, 1989 discloses and claims 
satisfactory developer sheet glossing apparatus employing a heated arched 
plate and a casting belt. 
At the same time, heated roll-type glossing apparatus was investigated, but 
as stated in U.S. Pat. No. 4,807,560, less than satisfactory results were 
obtained. A principal drawback was the tendency of the heated resin to 
adhere to the hot rollers. Such adhesion not only causes misfeeds, but 
also causes degradation to the image and surface. 
In connection with the investigation, it was discovered that the portions 
of the sheet being glossed having the lowest image density were those 
portions which required the highest glossing temperature. The reason for 
this phenomena is not fully understood, but it is believed that the dye 
precursor and oil released from the ruptured microcapsules may assist as a 
plasticizer in the film-forming characteristics of the resin. Such dye 
precursor and/or oil is much more abundant in the dense or darker areas 
than in the areas of minimum density. Therefore, it has been necessary to 
adjust the glossing temperature in accordance with the most difficult 
areas to be glossed, namely, those of minimum density. This is further 
exacerbated by the sticking problem when attempting to use roll-type 
glossers. One reason why the heated arched plate glosser of the 
above-identified patent was successful was due to the fact that the resin 
surface was formed by the surface of belt and not the plate, and was 
subsequently peeled off the belt. 
The apparatus employed in the tests of the heated roll glosser was 
published in April 1987 in Article No. 27660 in Research Disclosure, 
Emsworth Studios Inc., 260 West 39th Street, New York, N.Y. 10018. 
High temperature and high pressure roll-type glossing of latex coated 
pigment and latex coated paper stock is shown in Vreeland, U.S. Pat. No. 
3,873,345 issued Mar. 25, 1975 and Vreeland divisional Pat. No. 4,112,192 
issued Sept. 5, 1978. These prior art disclosures also describe the glass 
transition temperature phenomena T.sub.9 and employ a hot steel roll in 
the gloss calendar which is maintained at a temperature in excess of 
T.sub.9, and at a substantial nip pressure such as 400 pli (lbs. per 
linear inch). Since mineral pigmented coatings were employed, the 
transparentizing of the coating was not a necessary objective, and the 
results were measured in terms of surface gloss. Vreeland, U.S. Pat. No. 
4,624,744 issued Nov. 25, 1986 further discloses the calendering of 
uncoated papers under heat and pressure to provide a surface gloss, using 
a drum surface in excess of the glass transition temperature of the 
uncoated cellulose paper fibers. 
Very smooth and high gloss skins of silicone rubber have been applied to 
rolls for the calendering of synthetic resin materials, and the rolls have 
been manufactured as disclosed in Nauta et al, U.S. Pat. No. 4,368,240 
issued Jan. 11, 1983. The rolls as made in this reference were used in the 
smoothing and calendering of webs formed of synthetic thermoplastic 
material. 
SUMMARY OF THE INVENTION 
The applicant has found that a two-roll or nip-forming roll type 
transparentizer may be satisfactorily used to transparentize coatings as 
described in the above-identified co-pending applications in which the 
resin contacting roll has a super fine high gloss, a low energy surface, 
and in which the resin material in the nip is elevated to a temperature 
(T.sub.9) less than the free air glass transition temperature of the resin 
composite. It is believed that prior efforts to use roll-type heated 
glossers with the combination of materials as defined were unsuccessful 
because prior art teachings were followed which dictated the use of roll 
temperatures in excess of T.sub.9 and further by reason of the failure to 
recognize that an extremely low energy, high gloss roll surface can be 
used to transparentize such material at a surface temperature less than 
T.sub.9. 
Energy of a surface is a measure of its wettability. A liquid, resting on a 
low energy surface, subtends the surface at a low angle since the drop of 
liquid tends to spread. On the other hand, a surface with low surface 
energy supports the liquid, such as a drop of water, almost as a bead, and 
the angle subtended between the bead and the surface is much higher as an 
indication of low surface energy. 
The terms "surface energy" and "low surface energy" refer to the 
solid-vapor interface energy of a stable configuration of a liquid placed 
on the solid smooth surface of the roll. This energy may be expressed in 
dynes per centimeter. In such instances, the equilibrium shape of a liquid 
drop conforms to the minimum total surface and interface energy for all 
the phase boundaries present. 
The angle between the solid surface and a tangent to the liquid surface, at 
the contact point, can vary between 0.degree. and 180.degree.. If the 
solid-vapor interface energy is high, the liquid will tend to spread out 
indefinitely to eliminate the interface, and if low, the liquid tends to 
form a ball having a small interface area. By "low surface energy," the 
angle between the solid surface and the tangent to the liquid surface at 
the contact point is greater than 90.degree., usually defined as a 
non-wetting surface. This may be represented by the formula cosine theta 
equals (solid-vapor interface energy minus solid-liquid interface energy) 
divided by liquid-vapor interface energy, where theta is the angle between 
the solid surface and the tangent to the liquid surface at the contact 
point. Accordingly the angle theta may also be used as measure of the 
surface energy. 
I have discovered that the adhesion between the elastomer roll and the 
resin surface may be regulated or reduced to a controllable value by 
suitably controlling the surface energy, the surface smoothness, the 
pressure and the temperature in the nip, and the separation angle from the 
nip. I have found that highly satisfactory transparentizing can be 
performed by a hot-roll type glosser at satisfactory throughput speeds 
without permitting the resin to be cast to the surface of the elastomer 
roll, but merely passing it through the nip, and then releasing it 
immediately, by suitably controlling the exit angle. 
The backing roll should be metal, and spray coated with a release coating 
such as Teflon, It should have a low thermal mass and a high heat transfer 
ability. The nip exit conditions assure that the web does not tend to 
follow the surface of the elastomer roll. 
In connection with my investigations, I have found that transparentizing of 
overhead projection material, in which the substrate may be formed of PET, 
rather than paper, provides a more demanding test of the method and 
equipment. This is apparently due to the greater flatness of the substrate 
as compared to paper, and its greater tendency to adhere to the surface of 
the elastomer roll, which tendency is believed to be due, at least in 
part, to the exclusion of air at such surface in the nip. Further, the 
criticality in the percent of reduction of haze, as measured by residual 
haze, is more apparent in the transparentizing of projection material than 
that of opaque material, such as coated paper. Also, stress cracks in the 
coating show up as bars on the image under projection, and the employment 
of a method and apparatus which reduces or prevents adherence and sticking 
to the elastomer roll reduces stress cracking which is more critical in 
the OHP material. Exit tension may be controlled, to control the release 
angle from the nip. 
As previously mentioned, the nip temperature which provides satisfactory 
transparentizing, is less than T.sub.9 by about 10.degree. C. to 
20.degree. C. when accompanied by a pressing force of at least about 4 
lbs. per linear inch (pli) up to about 15 pli or more. Good results have 
been obtained in the 7-15 pli range. Pressures substantially in excess of 
15 pli should be avoided, to avoid sheet wrinkling and to avoid 
accelerating the wear on the rolls. The elastomer roll, under these 
conditions, should define characteristics as set out below in terms of 
gloss, hardness and surface energy. For satisfactory results, the residual 
haze should not exceed 10% and the gloss level should be at least 70% or 
more, as measured, for example, by a Hunter D-48-7 glossmeter, with a 
surface reflectance based on a scale of zero to 100. 
It is accordingly a principal object of this invention to provide method 
and apparatus for the glossing or transparentizing of resin coated 
receiver sheets or microencapsulated media material employing a heated 
roll-type glosser in which the nip temperature is less than the nominal 
glass transition temperature of the polymer resin, and in which the 
resin-contacting roll is formed with a high gloss low-energy surface. 
Another object of the invention is the provision of a roll-type 
transparentizer and method having a high throughput rate for use with 
microcapsule type media material. 
These and other objects and advantages of the invention will be apparent 
from the following description, the accompanying drawings, and the 
appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A two-roll glosser or transparentizer, in accordance with this invention, 
is illustrated in FIG. 1 as including a pair of nip-forming heated 
cylindrical rolls. These include an upper roll 10 and a lower roll 12 in 
nip-forming relation. The rolls are suitably mounted for rotation on a 
frame 13 so that the lower roll 12 is fixedly mounted on bearings 14, 
while the upper roll 10 is mounted on arms 15 in bearings 16. The arms 15 
are pivoted at pins 17 so that a pressing force, as represented by the 
arrows 18, may be adjustably applied, for controlling and adjusting the 
pressure in the nip. It is also understood that the force represented by 
the arrows 18 would normally be applied in the form of a loading spring or 
the like. A motor 19 may drive the roll 12 counterclockwise as viewed in 
FIGS. 2 and 3 with the upper roll 10 driven by friction through the nip. 
Referring to FIG. 2, the media or sheet material 20 is processed in the nip 
between the pair of rolls 10 and 12, with the upper or resin surface 22 in 
contact with the surface 23 of the upper roll 10. 
The rolls 10 and 12 are preferably internally heated, and for this purpose, 
each roll may contain an elongated internal a quartz-type IR lamp 
diagrammatically illustrated at 25 in FIG. 1. The quartz lamps are 
operated through a temperature controller which may include a pair of 
surface mounted thermocouples 26 providing a suitable feedback signal to 
the controller. 
The roll 12 may be formed with a hollow outer shell 30 carried on a pair of 
end discs 32. Preferably, the thermal mass of the shell 30 is relatively 
low and formed of material of good heat conductivity, such as aluminum. 
The outer surface of the shell 30 may be coated with non-stick material, 
such as a spray coating of Teflon, sold under the trademark SILVERSTONE by 
E. I. du Pont de Nemours and Company. 
The characteristics of the upper roll 10 are critical to the proper 
performance of the apparatus and method. Primarily, the characteristics of 
the outer surface 23 of the roll 10 are particularly critical, to prevent 
adhesion or sticking of the resin with the surface, and to provide the 
desired glossing and transparentizing of the resin. 
The surface 23 may be formed by a gloss silicone overcoat, applied to an 
elastomer underlayer which is in turn applied to a metal support shell. 
The shell is diagrammatically illustrated at 40 in FIGS. 1 and 2 and 
carries a silicone rubber coating on its outer surface, as illustrated by 
the layer 42. The main body of the layer 42 may be approximately 0.25" 
thick, formed with minimal waviness, such as by precision grinding. 
In order to provide the necessary finish, the layer 42 is surface coated 
with a finish coating of silicone on the order of 0.005" thick. 
This final surface coating defines the outer surface 23 of the roll 10. It 
is believed that the surface finish should have a smoothness in the order 
of 16 microinches or better and a very low surface energy on the order of 
25 dynes per centimeter or less. 
Another method of forming the surface 23 is to apply a gloss silicone 
overcoat, approximately 0.005" to 0.015" thick, directly to the metal core 
40 (FIG. 2). In this case, the loss of compliance in the nip can be 
compensated in the design of the roll 12 (FIG. 2). These gloss silicone 
overcoats are typically applied by spraying methods, wherein the flow 
characteristics of the silicone resin are controlled to form a high gloss 
upon heat curing. 
The force 18 is preferably adjusted to provide an optimum pressure in the 
nip which may be in the order of 7 pli. As previously noted, nip 
pressures up to about 15 pli have been found to be satisfactory. I have 
found that the pressing force materially aids in the transparentizing of 
the resin coating at temperatures less than T.sub.9, using the rolls as 
described herein. A substantially lower force provides consistent glossing 
results, but requires a higher roll temperature. A substantially higher 
force causes undue distortion and lengthening of the nip and increases the 
tendency for adherence of the resin to the surface 23, may further 
contribute to cracking of the resin, and may cause unwanted wrinkling of 
paper substrates. 
The surface temperature of each of the rolls 10 and 12 is substantially 
uniformly maintained by the internal infrared quartz lamps 25 and the 
thermocouples 26 so as to provide surface transparentizing temperatures at 
the nip which are in the range of between about 5.degree. C. and 
25.degree. C. below T.sub.9, with a preferred range of about between 
10.degree. C. to 15.degree. C. below the glass transition temperature. 
With respect to the classes of resin polymers disclosed in the 
above-identified co-owned and co-pending applications, the glass 
transition temperatures fall within a range in excess of 110.degree., and 
generally between 125.degree. C. and 130.degree. C. With such polymers, 
satisfactory transparentizing and glossing has been performed on paper 
based media material with throughput rates of 120 inches per minute and 
glossing temperatures of about 120.degree. C. Overhead projection base 
material (OHP) has been satisfactorily glossed with throughput rates of 50 
inches per minute and a surface temperature of about 110.degree. C. in the 
nip, for each of the rolls 10 and 12. In both instances, the nip pressure 
was about 7 pli. 
It has also been discovered that it is desirable to control the exit 
geometry of the web sheet material 20 to assure that the sheet does not 
follow the surface 23 of the roll 10. For this purpose, a downward exit 
angle .alpha. of some positive value is preferred, and the angle .alpha. 
may be controlled by suitably controlling the tension T in the sheet or 
web material. 
The maintenance of an exit angle .alpha., also known as a separation angle, 
is more critical in the transparentizing of OHP material than paper 
material because of the greater tendency of surface cracks to become 
apparent in the coated transparency material. Thus, if the angle .alpha. 
is too large, surface cracks can be induced. On the other hand, if the 
angle .alpha. is too small, random image defects can result from 
uncontrolled contact with the surface 23 of the upper roll 10 beyond the 
nip. 
FIG. 3 illustrates an arrangement for maintaining the separation angle 
constant by maintaining a desired and constant exit tension T. First, a 
blade 45 may be positioned at the exit side of the nip adjacent the 
out-running surface of the roll 10, to serve as a doctor or deflector 
blade to prevent unwanted excursions of the web 20 about the circumference 
of the elastomer covered roll 10 at the exit side of the nip. To minimize 
the friction, and hence the wear, of the surface 23 on the blade 45, it 
may be desirable to apply a small amount of lubricant to the surface 23. 
This lubricant can be low viscosity silicone oil, which also enhances the 
release capability of the surface. The material 20 as it leaves the nip 
may be gripped between a pair of pull rollers 50 which provide a constant 
tension to the sheet or web. The rollers 50 may be driven through some 
suitable means, such as a slip clutch, at the same or slightly higher 
speed than the surface velocity of the rolls in the nip. Good results in 
the transparentizing of both paper and OHP materials have been achieved 
with an angle .alpha. of approximately 5.degree.. 
While the methods herein described constitute preferred embodiments of this 
invention, it is to be understood that the invention is not limited to 
these precise methods, and that changes may be made without departing from 
the scope of the invention, which is defined in the appended claims.