Film embossing process

A film embossing process of roughening a lateral edge of a thermo-plastic film by mechanically deforming said lateral edge. The lateral edge is subjected to the film embossing process at a temperature of from the glass transition temperature (Tg) of the film to the temperature of (tg+60.degree. C.). Subsequently the film is subjected to heat treatment at a temperature higher than the temperature of said film embossing process by at least 10.degree. C. to 30.degree. C.

BACKGROUND OF THE INVENTION 
This invention relates to a film embossing process of roughening a lateral 
edge of a thermo-plastic sheet material (hereinafter referred to as "a 
film" when applicable), so that the film can be readily handling during 
the manufacture. 
In manufacturing films, embossing a lateral edge of the film, or a physical 
deformation is carried out in a step of the manufacture of films. This 
tends to improve the roll winding characteristic when the film is wound. 
It also facilitates the handling of the film when it is thereafter 
subjected to various processes. 
This process is called "a film embossing process". In this process, a film 
is inserted between two rollers with small teeth, and the rollers abut 
through the film against each other, to thereby roughen a portion of the 
film. This process has merit that if the film embossing process is carried 
out with the film heated, the variation with time of the roughened portion 
of the film is relatively small. 
When the film, having a lateral edge is roughened as described above, and 
wound in the form of a roll, the roughened lateral edge near the center of 
the roll is subjected to great pressure. This is especially true in the 
case where the film thus wound is stored for a long period of time after 
the manufacture. In such a condition a greater pressure will be applied to 
the roughened lateral edge near the center of the film roll. As a result, 
the thickness of the roughened portion of the film is decreased with time, 
and accordingly the object of roughening the lateral edge of the film 
cannot be achieved. 
Furthermore, in roughening the lateral edge of a film this film embossing 
process, "wavy pleats" are created in the lateral edge of the film 
depending on a pressure applied thereto. As a result the effect of the 
film embossing process, which could otherwise be expected in handling the 
film, is often decreased. In a hot process where a film or an extruded 
material is heated, "wavy pleats" are significantly created. 
In order to eliminate the creation of the "wavy pleats", a method has been 
proposed by Japanese Patent Application Publication No. 16064/1972, in 
which a film is locally heated to form a contracted portion immediately 
before the film is subjected to a film embossing process. This tends to 
cancel the elongation which is caused in the film embossing process. 
However, this method suffers in practice from difficulties because it is 
necessary to provide a technique for locally heating the film. Moreover, 
it is difficult to locally heat the film and is necessary to control the 
amount of contraction according to the variation of a force applied in the 
film embossing process. In order to exactly cancel the elongation caused 
during the film embossing process, and it is also necessary to change the 
heating temperature whenever the films to be processed are changed, 
because the degree of contraction depends on the kinds of film. 
SUMMARY OF THE INVENTION 
Accordingly, an object of this invention is to provide a film embossing 
process in which all of the above-described difficulties accompanying a 
conventional film embossing process have been eliminated. 
It is another object of the invention to define a process whereby the 
variation with time of the roughened portion of a film is reduced, and no 
"wavy pleats" are created. 
The objects of the invention can be achieved by carrying out a film 
embossing process at a temperature of from the glass transition 
temperature Tg of a film to be processed to the temperature of 
Tg+60.degree. C., and subsequently by subjecting the film to heat 
treatment at a temperature 10.degree. to 30.degree. C. higher than the 
temperature of the film embossing treatment.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In a conventional cold roughening process, the deformation of roughening a 
film includes elastic deformation. Accordingly, with the lapse of time the 
roughness is gradually reduced, that is, the thickness of the roughened 
portion of the film is gradually reduced. On the other hand, in the hot 
roughening process of this invention carried out in a temperature range of 
from Tg to Tg+60.degree. C., a film is roughened by plastic deformation, 
and therefore its elapsed time variation is reduced. If the temperature is 
higher than the Tg+60.degree. C., in general, plastic flow results with a 
film although it depends on the kinds of film. Accordingly, it is 
sometimes difficult to sufficiently increase the thickness of the film by 
roughening it. 
It has been found that the "wavy pleats", which are formed in the lateral 
edge of a film in the conventional embossing process, are eliminated by 
subjecting it to heat treatment at a temperature 10.degree.-30.degree. C. 
higher than the temperature at which the embossing processes are carried 
out. 
It has been found that it is essential that the temperature of this 
after-heat-treatment be higher than the temperature of the film embossing 
process in order to eliminate the "wavy pleats". But, if it is excessively 
higher, then the desired thickness of the roughened portion of the film is 
decreased. That is, the temperature of the after-heat-treatment should be 
higher by 10.degree.-30.degree. C. The time required for this 
after-heat-treatment depends on the temperature utilized. However, in 
general, the higher the temperature of the after-heat-treatment, the 
shorter the time required for the after-heat-treatment and it is 
preferable that the time of the after-heat-treatment is approximately 30 
to 600 seconds. However, it should be noted that it is not preferable to 
perform the heat treatment for a long period of film at a high 
temperature, because the heat treatment will decrease the thickness of the 
roughened portion of the film. 
Thermo-plastic films utilizing this invention may be any film having a 
glass transition temperature higher than room temperature, or 30.degree. 
C., preferably 50.degree. C. Such thermo-plastic films are, for instance, 
films made of polystyrene, polycarbonate, polyethylene-terephthalate, 
cellulose acetate, cellulose acetate butylate, and nylon. These films may 
be subjected to surface treatment or surface coating to the extent that 
the glass transition temperature Tg thereof is not affected. 
The thickness of a film, which is to be embossed by the film embossing 
process according to the invention, is not strictly limited. However, it 
is preferable that the thickness of the film is in the range from 30.mu.to 
200.mu.. 
The resultant thickness of the portion of a film, which is embossed by the 
film embossing process, depends on the kind and handling characteristic 
thereof. However, it is desirable that the increased thickness is in the 
range of 10-60% of the thickness of the film. 
As described above, in the film embossing process according to this 
invention, after the film embossing process is carried out at a 
temperature of from the glass transition temperature Tg of the film to the 
temperature of Tg+60.degree. C., the film is subjected to heat treatment 
at a temperature 10.degree.-30.degree. C. higher than that of the film 
embossing process. Hence the elapsed time variation of the roughened 
portion of the film is reduced and "wavy pleats" are not created. Thus, 
the film winding characteristic and the film handling characteristic can 
be remarkably improved. 
In order to demonstrate the effects of this invention, an example and 
comparison examples will be described below: 
EXAMPLE 
Under the following conditions, the lateral edge of a film was subjected to 
film embossing process: 
(1) Film 
Polyethylene-terephthalate film oriented biaxially (the thickness being 
100.mu., the glass transition temperature being 78.degree. C.) 
(2) Conditions in the Film Embossing Process A temperature of 100.degree. 
C., and a pressure of 1.5 Kg/cm.sup.2. 
(3) Conditions in the After-Heat-Treatment For 120 seconds at a temperature 
of 130.degree. C. 
(4) Results 
The variation with time of the thickness of the roughened lateral edge of 
the film, as indicated by the curve A in FIG. 1 was substantially 
negligible. No "wavy pleats" were created. 
COMISON EXAMPLE 1 
The same film embossing process as the above-described Example was carried 
out, but the after-heat-treatment was not performed. 
(1) Results 
The variation with time of the thickness of the roughened lateral edge of 
the film was substantially negligible, similarly as in the Example, but 
the "wavy pleats" were significantly created. Thus, the film could not put 
in to practical use. 
COMISON EXAMPLE 2 
The film embossing process was carried out under the same conditions as the 
Example except that the temperature of the film embossing process was 
changed to 30.degree. C. 
(1) Results 
The variation with time of the roughened lateral edge of the film was 
finally reduced to 50% of its initial value, as indicated by the curve B 
in FIG. 1. However, the degree of creation of the "wavy pleats" was 
relatively small.