Process for making a cellulose triacetate photographic film base

The invention provides a method of making a cellulose triacetate photographic film base which comprises providing a casting composition of cellulose triacetate, plasticizer, and solvent having a cellulose triacetate and plasticizer concentration of from 36% to 72% by weight, casting the coating composition at a temperature of from 40.degree. C. to 63.degree. C. unto a surface maintained at from -70.degree. C. to 21.degree. C. for a residence time from 3 to 8 seconds, in accordance with the equation Tr/Tg<1.9, where Tg and Tr are the glass transition temperature in degrees Kelvin and the temperature of the casting composition in degrees Kelvin at a point adjacent to the surface where the film base is separated from the surface.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to a process for producing cellulose triacetate 
photographic film base by solution casting and more particularly to a 
process for producing cellulose triacetate film base at increased speed or 
thickness, and reduced demand for drying equipment. 
The preparation of cellulose ester film and particularly triacetate film 
includes the steps of forming a dope solution by dissolving cellulose 
triacetate in a suitable solvent such as dichloromethane. The dope 
traditionally has a concentration of cellulose triacetate in the solvent 
of from about 15 to about 35 percent by weight. The dope is filtered 
through one or more filtration devices, and then extruded through a die as 
a film which is cast onto a moving surface such as a highly polished wheel 
or continuous band where the dope is "cured" by evaporation of the 
solvents from the exposed side. The dope stays on the band or wheel long 
enough to allow it to build sufficient strength to separate it from the 
band or wheel, and it is then subsequently dried through air sections and 
finally wound up as a roll of film. 
One of the rate limiting steps in this process is the curing time necessary 
for the web to become strong enough to separate it from the band or wheel 
without delamination. If the web is removed from the band or wheel 
prematurely, the cohesive forces holding the web together are less than 
the adhesive forces holding the web to the casting surface. This will 
prevent the web from being separated from the casting surface properly, 
and will result in part or all of the web going under the die a second 
time, causing web brakes and other casting difficulties. 
If the cohesive forces within the web can be increased while the web is on 
the casting surface, then the web can be separated from the casting 
surface sooner. This would result in a faster process if the surface 
curing time is held constant, or a smaller casting surface if the speed is 
held constant. The latter option is extremely important, since the bands 
and wheels used in today's cellulose triacetate film manufacturing 
processes are extremely large and expensive. The web must often cure on 
the band or wheel for 60-120 seconds. If one is inadvertently damaged, 
replacement costs would be considerable. It would therefore be highly 
desirable to reduce the necessary size of the casting surface so that 
replacement and implementation costs can be reduced. 
As a result, there is a need for a process for producing cellulose 
triacetate film which utilizes decreased curing time on the casting 
surface without causing delamination of the web. 
SUMMARY OF THE INVENTION 
The invention provides a method of making a cellulose triacetate 
photographic film base which comprises providing a casting composition of 
cellulose triacetate, plasticizer, and solvent having a cellulose 
triacetate and plasticizer concentration of from 36% to 72% by weight, 
casting the coating composition at a temperature of from 40.degree. C. to 
63.degree. C. on to a surface maintained at from -70.degree. C. to 
21.degree. C. for a residence time from 3 to 8 seconds, in accordance with 
the equation Tr/Tg&lt;1.9, where Tg and Tr are the glass transition 
temperature in degrees Kelvin and the temperature of the casting 
composition in degrees Kelvin at a point adjacent to the surface where the 
film base is separated from the surface. 
Extensive experimentation has shown that delamination between the film base 
and the casting surface occurred when the Tr/Tg ratio is between 1.9 and 
2.2. If the ratio is higher than 2.2, delamination occurs in all cases. If 
the ratio is lower than 1.9, delamination does not occur. Therefore, a 
ratio of Tr/Tg ratio of less than 1.9 guarantees a process where 
delamination on the casting surface will not occur. The Tg is determined 
by the Fox equation, where: 
##EQU1## 
W.sub.i and Tg.sub.i are the mass fraction and the Glass transition 
temperature of the ith component, respectively, w.sub.i can be determined 
in a number of ways, including Nuclear magnetic resonance, Near infrared 
spectroscopy, and the like. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
In the preparation of cellulose triacetate photographic film base in 
accordance with this invention, a very concentrated dope solution is first 
prepared by mixing and dissolving cellulose triacetate (CTA) and a 
plasticizer such as triphenyl phosphate, dibutyl phthalate, tricresyl 
phosphate, butyl stearate, tetraethylene glycol dimethyl ether, and the 
like, in a dichloromethane solvent to which may be added other ingredients 
such as alcohols including ethanol, butanol, propanol, methanol, mixtures 
thereof, and the like; ketones including acetone, methylethyl ketones, and 
the like; cyclic alkanes such as cyclohexane, methylcyclohexane, 
cyclopentane, and the like. The plasticizer to cellulose triacetate ratio 
is 13-17% by weight. This cellulose triacetate/plasticizer is mixed with 
solvents to a concentration greater than 36%. The solvents are comprised 
of dichloromethane, present in ranges from 75% to 85% by weight, methanol 
in ranges from 6% to 15%, either butanol or cyclohexane in ranges of from 
5% to 10%, and acetone in ranges of from 0% to 3%. This dope solution is 
pumped under a pressure of 7 to 50 atmospheres through a heater where it 
is heated to a temperature of from about 40.degree. C. to about 63.degree. 
C. and then through a fine filter where particles and slugs of gelatinous 
material are removed. The filtered material still under the indicated 
pressure and within the temperature range indicated is cast onto a highly 
polished rotating roll or continuous belt or band surface maintained at a 
temperature of from -70.degree. C. to 21.degree. C. After 3-8 seconds the 
web can be removed from the surface without delamination and dried in 
accordance with the conventional techniques known in the art of preparing 
cellulose triacetate photographic film base. In a preferred method, the 
solids consist of 85% CTA and 15% triphenyl phosphate. It is preferred 
that this solid mixture be combined with a solution consisting of 78% 
dichloromethane, 12% methanol, and 7% cyclohexane or butanol, with the 
most preferred being butanol, and 3% acetone. It is preferred that the 
pressure of the solution as it is passing through the heat exchanger and 
the filtering device be maintained under a pressure preferably of from 
about 10 to 45 atmospheres and most preferably from about 15 to 40 
atmospheres. The temperature of the dope solution prior to being cast upon 
the highly polished surface is preferably within the range of from 
45.degree. C. to 63.degree. C., and most preferably at a temperature of 
from about 55.degree. C. to 63.degree. C. The solution is then cast on to 
a surface which is at a temperature preferably of from about -70.degree. 
C. to 8.degree. C., and most preferably at a temperature of from about 
0.degree. C. to 8.degree. C. 
The dope or casting composition is prepared in the mixer by dissolving 
cellulose triacetate/plasticizer in the solvent dichloromethane or mixture 
of dichloromethane with other ingredients. From the mixer, it is pumped 
through a coarse filter and then pumped through a heat exchanger wherein 
the pressure is maintained within the range of 7 to 50 atmospheres and the 
temperature of the dope is raised to 40.degree. C. to 63.degree. C. The 
dope is next filtered through a fine filter to remove any gelatinous 
particles or other matter. The fine filter generally has openings of 
approximately 30 micrometers. This general procedure follows that of U.S. 
application Ser. No. 08/308,011 filed Sep. 16, 1994 now U.S. Pat. No. 
5,529,737 which is incorporated herein by reference. 
Finally, the dope still under the pressure indicated above and within the 
temperature range indicated above, is cast by means of a conventional die, 
including coat hanger dies, horseshoe dies, and the like onto the polished 
roll at the temperature described above. After 3-8 seconds, the web is 
removed from the surface of the roll and conveyed into air or nitrogen 
dryers, and finally wound into a roll. 
The invention is further illustrated by the following examples:

EXAMPLE 1 
A solution consisting of 39.5% CTA and 6.6% triphenyl phosphate was mixed 
with 1.5% acetone, 6.3% methanol, 4.3% butanol, and the balance 
dichloromethane stirred at ambient temperature in a mixer and pumped out 
of the mixer through a coarse filter, the media having openings of 30 
.mu.m and then pumped at a pressure of 41 atmospheres to a heat exchanger 
where the temperature was raised to about 63.degree. C. The heated 
solution was then pumped through a fine 30 micrometer absolute depth media 
filter. A film of thickness such that the final, dried web was 118 .mu.m 
was extruded out of a coat hanger die onto a cooled stainless steel drum 
with a surface temperature of 1.degree. C. for 7.5 seconds. The Tg and Tr 
were determined to be 158.8.degree. K., and 284.9.degree. K. giving a 
Tr/Tg ratio of 1.79. This web was readily removed from the casting surface 
without delamination. 
COMATIVE EXAMPLE 2 
Example 1 is repeated, except that the cooled drum temperature was raised 
to 21.1.degree. C. The Tg and Tr are determined to be 158.8.degree. K. and 
304.9.degree. K., respectively, giving a Tr/Tg ratio of 1.92. This web 
could not be cleanly removed from the casting surface, and delamination of 
the cellulose triacetate web resulted. 
COMATIVE EXAMPLE 3 
Example 1 was repeated except the temperature of the cooled drum is 
7.2.degree. C., and the residence time on the drum was 5.2 seconds. The Tg 
and Tr were determined to be 157.6.degree. K. and 305.3.degree. K. 
respectively, which yields a Tr/Tg ratio of 1.94. The web could not be 
cleanly removed from the casting surface, and delamination of the 
cellulose triacetate web resulted. 
EXAMPLE 4 
Example 1 was repeated except the % components of the inlet solution were 
changed to 49.8% CTA, 6.3% Triphenyl phosphate, 1.35% acetone, 5.7% 
methanol, 4.1% butanol, with the balance being dichloromethane. The Tg and 
Tr were determined to be 165.9.degree. K. and 286.0.degree. K. 
respectively, which yields a Tr/Tg ratio of 1.72. This web was easily 
separated from the cooled drum surface. 
EXAMPLE 5 
Example 1 is repeated, except the % components of the inlet solution were 
changed to 44.4% CTA, 7.0% TPP, 0% acetone, 4.7% methanol, 2.7% 
cyclohexane, and the balance dichloromethane was cast onto the cooling 
drum at 7.2.degree. C. for 4.7 seconds. The Tg and Tr were determined to 
be 166.1.degree. K. and 198.2.degree. K. respectively, which yields a 
Tr/Tg ratio of 1.8. This web was easily separated from the cooled drum 
surface.