CA1151828A - Process for the production of flat thermoplastic sheets having improved dimensional stability - Google Patents
Process for the production of flat thermoplastic sheets having improved dimensional stabilityInfo
- Publication number
- CA1151828A CA1151828A CA000341455A CA341455A CA1151828A CA 1151828 A CA1151828 A CA 1151828A CA 000341455 A CA000341455 A CA 000341455A CA 341455 A CA341455 A CA 341455A CA 1151828 A CA1151828 A CA 1151828A
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- Prior art keywords
- film
- cooling
- process according
- heat
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Abstract
ABSTRACT OF THE DISCLOSURE
Disclosed is a process for the production of biaxially stretched flat sheets of thermoplastic material of improved dimensional stability, comprising the steps of: extruding the thermoplastic material from a slot die to form a sheet; cooling the extruded sheet to form a pre-film; heating the pre-film to an appropriate stretching temperature; stretching the heated pre-film in orthogonal directions to form a stretched film; heat-setting the stretched film; cooling the heat-set film; and during the cooling step guiding the film in such a manner that it converges in at least one of the transverse and longitudinal directions.
Disclosed is a process for the production of biaxially stretched flat sheets of thermoplastic material of improved dimensional stability, comprising the steps of: extruding the thermoplastic material from a slot die to form a sheet; cooling the extruded sheet to form a pre-film; heating the pre-film to an appropriate stretching temperature; stretching the heated pre-film in orthogonal directions to form a stretched film; heat-setting the stretched film; cooling the heat-set film; and during the cooling step guiding the film in such a manner that it converges in at least one of the transverse and longitudinal directions.
Description
-~ ~ 5~82~3 PROCESS FOR THE PRODUCTION OF FLAT THERMOPLASTIC SHEETS
HA~ING IMPROVED DIMENSIONAL STABI~ITY
BACKGROUND OF THE INVENTION
The present invention relates to a process for the production of biaxially stretched flat sheets of thermo-plastic material which have a better dimensional stability than hitherto known films and which simultaneously possess a high strength.
The production of biaxially stretched ~lat sheets of thermoplastic materials has been part of the prior art for many years. According to a typical prior art process, a melt of the selected thermoplastic material is extruded from a slot die and is solidified, preferably on a chill roll. The resulting pre-film is then heated to the stretching temperature and is stretched, first in one direction and then in a direction perpendicular to the first stretching direction, and finally the stretched film is cooled and wound up. The film may be stretched first in the longitudinal and then in the transverse direction, but reverse stretching processes are also known.
Further, it is part of the present state o~
the art to stretch the film simultaneously in ~he longi-tudinal and in the transverse directions, with so-called
HA~ING IMPROVED DIMENSIONAL STABI~ITY
BACKGROUND OF THE INVENTION
The present invention relates to a process for the production of biaxially stretched flat sheets of thermo-plastic material which have a better dimensional stability than hitherto known films and which simultaneously possess a high strength.
The production of biaxially stretched ~lat sheets of thermoplastic materials has been part of the prior art for many years. According to a typical prior art process, a melt of the selected thermoplastic material is extruded from a slot die and is solidified, preferably on a chill roll. The resulting pre-film is then heated to the stretching temperature and is stretched, first in one direction and then in a direction perpendicular to the first stretching direction, and finally the stretched film is cooled and wound up. The film may be stretched first in the longitudinal and then in the transverse direction, but reverse stretching processes are also known.
Further, it is part of the present state o~
the art to stretch the film simultaneously in ~he longi-tudinal and in the transverse directions, with so-called
2~ simultaneous-stretching tenters being used for this purpose. Films prepared in this manner shrink in the longitudinal and in the transverse directions, and the degree of shrinkage is determined by the conditions of manufacture and the thermoplastic material used. Such '' ': ~ :
: :
~ ~5~
films are commercially used as shrink films, e.g., for the manufacture of shrink-on packaging material.
In addition to the above-described films, there is a need for films having a very low shrinkage, i.e., an improved dimensional stability, under the influence of heat. Such films are required for many purposes, e.g., as drawing and release films, as base films for photo-graphic purposes, for sound recording and computer tapes, for printed circuits, and the like.
In practical operation, such films are prepared by subjecting biaxially stretched films in a tenter to a heat-setting process, i.e., a thermal treatment at ele-vated temperature, whereby their shrinkage is reduced.
This process is also referred to as "-thermal fixationl' of the film.
Although the shrinkage of the film is reduced ;
to a certain degree by heat-setting, a certain residual shrinkage is always retained, which depends on the thermoplastic material used and the conditions prevailing during heat-setting. This residual shrinkage cannot be altogether removed even if the heat-setting period is prolonged and/or the temperature is increased, so tha-t it is impossible to go beyond a certain limit.
SUMMARY OF THE INVENTION
It is therefore an object of the present in-vention to provide a process for the manufacture of biaxially stretched and heat-set flat sheets of thermo-plastic material.
It is also an object of the invention to provide such a process wherein the residual shrinkage of films produced by conventional processes is further reduced without substantially changing the desirable physical properties of such films, such as their tensile strength, elongation at break and the like.
~ ~5~
In accomplishing the oregoing objects, there has been provided according to the present invention a process for the production of biaxially stretched flat sheets of thermoplastic material of improved dimensional stability, comprising the steps of: extruding the thermo-plastic material from a slot die to form a sheet; cooling the extruded sheet to form a pre-film; heating the pre-film to an appropriate stretching temperature; stretching the heated pre-film in orthogonal directions to form a stretched film; heat-setting the stretched film; cooling the heat-set film; and during the cooling step guiding the film in such a manner that it converges in at least one of the transverse direction and the longitudlnal direction. Thus, the film can be guided during the cooling step in such a manner thai it converges in the transverse direction, or in such a manner that it converges in the longitudinal direction, wherein the speed of the film is reduced in comparison to the film speed during the heat-setting step. Preferably, the film is guided during the cooling step in such a manner that it converges in both the transverse and in the longitudinal directions, and the speed of the film is reduced in comparison to the film speed during the heat-setting step. The degree of converging is typically between about 1 and 5~.
~:~5~8;~8 DETAILED DESCRIPTION OF PREFERR~D ~:MBODIMENTS
The present invention provides a process for the production of biaxially s-tretched flat sheets of thermo-plastic material of improved dimensional stability, wherein the thermoplastic material is extruded from a slotdie and cooled to form a pre-film. The pre-film is then heated to an appropriate stretching temperature and is stretched in orthogonal directions, either in stages or simultanèously, and the stretched pre-film is finally heat-set, cooled and possibly wound up. The characterizing feature of this process is that, after heat-setting, the film is prefer-ably guided in such a manner during the cooling step that it converges in the transverse direction, while the feed speed of the ~ilm is reduced as compared with the speed in the heat-setting zone.
Surprisingly, it was found that, by the process according to the invention, it is possible to reduce the lower limit for the shrinkage of biaxially stretched and heat-set thermoplastic films beyond the values achieved by a process according ~o the present state of the art.
Films made by the present process have an improved dimen-sional stability and thus are particularly suitable for the above-mentioned applications.
According to the purpose for which the film is produced, it is possible by the present invention to re-duce the shrinkage of the film in the transverse or in the longitu*inal direction. It is preferred, however, to improve the dimensional stability simultaneously in the transverse a in the longitudinal direc~ion. The shrink-age in the transverse direction may be reduced by guidingthe film in such a manner that it conver~es. This may be achieved, e.g., by using converging guide means in a clip tenter or a similar device, while simultaneously cooling the film. The film web is cooled on at least two rolls arranged one after the other in the longi~udinal direction, which are maintained at diminishing temperatures and rotate at diminishing speeds, as seen in the feed direction of ''.~ '' ' ' :
~L~5~8;~3 the film. Preferably, however, an arrangement of several rolls is used around which th~ film i5 looped, and it is also provided that the film is further cooled in the gaps between the rolls, preferably by means of cooling gas jets.
Alternatively, it is possible to introduce the film into a converging tenter wherein the distances be-tween clips diminish in the feed direction of the web, with cooling being preferably effected by jets of cooling gas. In principle, this manner of guiding the film represents a reversal of the guiding system used for stretching a film simultaneously in the longitudinal and in the transverse directions.
As a further means ~or cooling the ~ilm in accordance with the present invention, the edges of the film may be thermally insulated after the film leaves the heat-setting zone, and further heat may be simul-taneously applied to the edges, if desired, while the portion of the film extending between the edges is ~0 cooled, e~g., by means of jets of cooling gas. Prefer-ably, the edges of the film are sealed off during this operation, so as not to interfere with the cooling of the central area. Immediately following the cooling zone, the edges of the ~ilm are trimmed. The film is guided along a convergent path.
Because the latter process can be conducted in a relatively simple manner technically, it represents the preferred embodiment.
Alternatively, it is possible to perform the process in such a manner that the edges of the film are trimmed after the heat-se-tting operation and that the film is then guided unsupported and without any tension during the cooling step. Films produced in this manner show an improved dimensional stability, but their degree of planarity is not particularly good. Therefore, they can be used for such purposes where good planarity pro-perties are not of particular importance. For this reason, this last-mentioned method is intended to be used for . .
~153l~3Z~il special purposes onlyO
On the other hand, when -the film is guided and cooled/ e.g., by means of a suspension dryer with jets arranged on each side, or when porous rollers are used into which gas is introduced, no deterioration of the planarity of the film is observed.
Although the process according to the present invention may be applied to all stretchable films having a residual shrinkage after heat-setting, it is prefexably used for polyester and polypropylene films, because these films are particularly frequently employed in the pre~
viously mentioned fields of application.
The in-vention will be further illustrated by reference to the following e~amples, which are merely illustrative and in no sense to be construed as limiting.
Several polyethylene terephthalate films having various degrees of stretching are produced by different processes and heat-set, in some cases by the longitudinal/
transverse process, with air heated to 230C, and in other cases by the transverse/longitudinal process, using air heated to 200C. ~fter leaving the heat-setting zone, the edges of the film are maintained, to a depth of 5 cm, at a certain temperature by means of infrared radiation, while the area between the edges is cooled to 25C by means of an air jet. During this process, the marginal zones are sealed off by means of upright metal sheets.
The results obtained are compiled in the following table:
~ 15~Z~3 . - 7 - : .
U~
F~ ~
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~ ~3~ I o I o I o o I o IY 3 C~ ~ H ¦r--i ¦ `
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_ oU~ oUl O U~ ~ ~1 E~ i~, i~1--i 0 r-i O r-i O O ~ ~
Z ~
O U~
zæ .' C~ Z ca ~
H O ~1 0 a~ O ~ O C) d ~ d :~ d ~ P~ d o~
o o C~
U~
U~ H
H ~ ~ O O O O O O O t~l :
, ~ O
.
~0 @ 04 Z
.,1 ~3 oo 00 0 0 0 0 O
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ll ll u~ ul O O O O ~ a~
.
.'~
~5~82~
The shrinkage is determined at 150C within 15 minutes, using square samples of an edge length of 10 cm.
In all cases, the ~ransverse shrinkage amounted ko 0 per cent, because the tenter is adjusted so that the film is guided with a convergence of 2.5 per cent.
A 50 ~m thick polyethylene terephthalate film is produced, at a longitudinal stretching rate ~1 = 3.5 and a transverse stretching rate ~t = 3.8, and the resulting film is then heat-set at 230C. After heat-setting, the edges of the ~ilm are trimmed and the film is cooled while running unsupported. Shrinkage in the longitudinal di-rection and in the transverse direction is 0 per cent in each case, but the planarity of the film is clearly deteriorated.
A film is produced as described in E~ample 2 and, a~ter heat-setting is passed at a speed of 30 m/min. over a roll the circumferential speed o~ which is also 30 m/min.
The temperature of the roll is adjusted to 200C. Then the film is passed to a roll having a temperature of 30C, and in the gap between the two rolls, the film is addi-tionally cooled by blowing with cooling air o~ 25C.
If the speed of the second roll is also 30 m/min., a shrinkage of 1.0 per cent is determined (150C, 15').
If the circumferential speed of the second roller is reduced to 29.7 m/min. or 29.4 m/min., the shrinkage can be red~lced to 0.6 per cent or even to 0 per cent.
The transverse shrinkage is also 0 per cent.
A 30 ~m thick, biaxially stretched polypropylene film ~1 = 6;~t = 8) is heat-set at 160C. The process described in Example 1 is repeated, except that ~he edges of the film are maintained at a temperature of 110C. A
shrinkage in the longitudinal direction (130C, 15') of 1.5 per cent is determined, ~hile the transverse shrinkaye ~:lS~
g amounts to 0 per cent. In this case, the tenter has a convergence of 4 per cent. If the film is not cooled according to the invention, a longitudinal shrinkage of
: :
~ ~5~
films are commercially used as shrink films, e.g., for the manufacture of shrink-on packaging material.
In addition to the above-described films, there is a need for films having a very low shrinkage, i.e., an improved dimensional stability, under the influence of heat. Such films are required for many purposes, e.g., as drawing and release films, as base films for photo-graphic purposes, for sound recording and computer tapes, for printed circuits, and the like.
In practical operation, such films are prepared by subjecting biaxially stretched films in a tenter to a heat-setting process, i.e., a thermal treatment at ele-vated temperature, whereby their shrinkage is reduced.
This process is also referred to as "-thermal fixationl' of the film.
Although the shrinkage of the film is reduced ;
to a certain degree by heat-setting, a certain residual shrinkage is always retained, which depends on the thermoplastic material used and the conditions prevailing during heat-setting. This residual shrinkage cannot be altogether removed even if the heat-setting period is prolonged and/or the temperature is increased, so tha-t it is impossible to go beyond a certain limit.
SUMMARY OF THE INVENTION
It is therefore an object of the present in-vention to provide a process for the manufacture of biaxially stretched and heat-set flat sheets of thermo-plastic material.
It is also an object of the invention to provide such a process wherein the residual shrinkage of films produced by conventional processes is further reduced without substantially changing the desirable physical properties of such films, such as their tensile strength, elongation at break and the like.
~ ~5~
In accomplishing the oregoing objects, there has been provided according to the present invention a process for the production of biaxially stretched flat sheets of thermoplastic material of improved dimensional stability, comprising the steps of: extruding the thermo-plastic material from a slot die to form a sheet; cooling the extruded sheet to form a pre-film; heating the pre-film to an appropriate stretching temperature; stretching the heated pre-film in orthogonal directions to form a stretched film; heat-setting the stretched film; cooling the heat-set film; and during the cooling step guiding the film in such a manner that it converges in at least one of the transverse direction and the longitudlnal direction. Thus, the film can be guided during the cooling step in such a manner thai it converges in the transverse direction, or in such a manner that it converges in the longitudinal direction, wherein the speed of the film is reduced in comparison to the film speed during the heat-setting step. Preferably, the film is guided during the cooling step in such a manner that it converges in both the transverse and in the longitudinal directions, and the speed of the film is reduced in comparison to the film speed during the heat-setting step. The degree of converging is typically between about 1 and 5~.
~:~5~8;~8 DETAILED DESCRIPTION OF PREFERR~D ~:MBODIMENTS
The present invention provides a process for the production of biaxially s-tretched flat sheets of thermo-plastic material of improved dimensional stability, wherein the thermoplastic material is extruded from a slotdie and cooled to form a pre-film. The pre-film is then heated to an appropriate stretching temperature and is stretched in orthogonal directions, either in stages or simultanèously, and the stretched pre-film is finally heat-set, cooled and possibly wound up. The characterizing feature of this process is that, after heat-setting, the film is prefer-ably guided in such a manner during the cooling step that it converges in the transverse direction, while the feed speed of the ~ilm is reduced as compared with the speed in the heat-setting zone.
Surprisingly, it was found that, by the process according to the invention, it is possible to reduce the lower limit for the shrinkage of biaxially stretched and heat-set thermoplastic films beyond the values achieved by a process according ~o the present state of the art.
Films made by the present process have an improved dimen-sional stability and thus are particularly suitable for the above-mentioned applications.
According to the purpose for which the film is produced, it is possible by the present invention to re-duce the shrinkage of the film in the transverse or in the longitu*inal direction. It is preferred, however, to improve the dimensional stability simultaneously in the transverse a in the longitudinal direc~ion. The shrink-age in the transverse direction may be reduced by guidingthe film in such a manner that it conver~es. This may be achieved, e.g., by using converging guide means in a clip tenter or a similar device, while simultaneously cooling the film. The film web is cooled on at least two rolls arranged one after the other in the longi~udinal direction, which are maintained at diminishing temperatures and rotate at diminishing speeds, as seen in the feed direction of ''.~ '' ' ' :
~L~5~8;~3 the film. Preferably, however, an arrangement of several rolls is used around which th~ film i5 looped, and it is also provided that the film is further cooled in the gaps between the rolls, preferably by means of cooling gas jets.
Alternatively, it is possible to introduce the film into a converging tenter wherein the distances be-tween clips diminish in the feed direction of the web, with cooling being preferably effected by jets of cooling gas. In principle, this manner of guiding the film represents a reversal of the guiding system used for stretching a film simultaneously in the longitudinal and in the transverse directions.
As a further means ~or cooling the ~ilm in accordance with the present invention, the edges of the film may be thermally insulated after the film leaves the heat-setting zone, and further heat may be simul-taneously applied to the edges, if desired, while the portion of the film extending between the edges is ~0 cooled, e~g., by means of jets of cooling gas. Prefer-ably, the edges of the film are sealed off during this operation, so as not to interfere with the cooling of the central area. Immediately following the cooling zone, the edges of the ~ilm are trimmed. The film is guided along a convergent path.
Because the latter process can be conducted in a relatively simple manner technically, it represents the preferred embodiment.
Alternatively, it is possible to perform the process in such a manner that the edges of the film are trimmed after the heat-se-tting operation and that the film is then guided unsupported and without any tension during the cooling step. Films produced in this manner show an improved dimensional stability, but their degree of planarity is not particularly good. Therefore, they can be used for such purposes where good planarity pro-perties are not of particular importance. For this reason, this last-mentioned method is intended to be used for . .
~153l~3Z~il special purposes onlyO
On the other hand, when -the film is guided and cooled/ e.g., by means of a suspension dryer with jets arranged on each side, or when porous rollers are used into which gas is introduced, no deterioration of the planarity of the film is observed.
Although the process according to the present invention may be applied to all stretchable films having a residual shrinkage after heat-setting, it is prefexably used for polyester and polypropylene films, because these films are particularly frequently employed in the pre~
viously mentioned fields of application.
The in-vention will be further illustrated by reference to the following e~amples, which are merely illustrative and in no sense to be construed as limiting.
Several polyethylene terephthalate films having various degrees of stretching are produced by different processes and heat-set, in some cases by the longitudinal/
transverse process, with air heated to 230C, and in other cases by the transverse/longitudinal process, using air heated to 200C. ~fter leaving the heat-setting zone, the edges of the film are maintained, to a depth of 5 cm, at a certain temperature by means of infrared radiation, while the area between the edges is cooled to 25C by means of an air jet. During this process, the marginal zones are sealed off by means of upright metal sheets.
The results obtained are compiled in the following table:
~ 15~Z~3 . - 7 - : .
U~
F~ ~
E~
~ ~3~ I o I o I o o I o IY 3 C~ ~ H ¦r--i ¦ `
~E~
H
_ oU~ oUl O U~ ~ ~1 E~ i~, i~1--i 0 r-i O r-i O O ~ ~
Z ~
O U~
zæ .' C~ Z ca ~
H O ~1 0 a~ O ~ O C) d ~ d :~ d ~ P~ d o~
o o C~
U~
U~ H
H ~ ~ O O O O O O O t~l :
, ~ O
.
~0 @ 04 Z
.,1 ~3 oo 00 0 0 0 0 O
~J
ll ll u~ ul O O O O ~ a~
.
.'~
~5~82~
The shrinkage is determined at 150C within 15 minutes, using square samples of an edge length of 10 cm.
In all cases, the ~ransverse shrinkage amounted ko 0 per cent, because the tenter is adjusted so that the film is guided with a convergence of 2.5 per cent.
A 50 ~m thick polyethylene terephthalate film is produced, at a longitudinal stretching rate ~1 = 3.5 and a transverse stretching rate ~t = 3.8, and the resulting film is then heat-set at 230C. After heat-setting, the edges of the ~ilm are trimmed and the film is cooled while running unsupported. Shrinkage in the longitudinal di-rection and in the transverse direction is 0 per cent in each case, but the planarity of the film is clearly deteriorated.
A film is produced as described in E~ample 2 and, a~ter heat-setting is passed at a speed of 30 m/min. over a roll the circumferential speed o~ which is also 30 m/min.
The temperature of the roll is adjusted to 200C. Then the film is passed to a roll having a temperature of 30C, and in the gap between the two rolls, the film is addi-tionally cooled by blowing with cooling air o~ 25C.
If the speed of the second roll is also 30 m/min., a shrinkage of 1.0 per cent is determined (150C, 15').
If the circumferential speed of the second roller is reduced to 29.7 m/min. or 29.4 m/min., the shrinkage can be red~lced to 0.6 per cent or even to 0 per cent.
The transverse shrinkage is also 0 per cent.
A 30 ~m thick, biaxially stretched polypropylene film ~1 = 6;~t = 8) is heat-set at 160C. The process described in Example 1 is repeated, except that ~he edges of the film are maintained at a temperature of 110C. A
shrinkage in the longitudinal direction (130C, 15') of 1.5 per cent is determined, ~hile the transverse shrinkaye ~:lS~
g amounts to 0 per cent. In this case, the tenter has a convergence of 4 per cent. If the film is not cooled according to the invention, a longitudinal shrinkage of
3.5 per cent is measured.
A film produced as described in Example 4 is subjected to the treatment described in Example 3. The temperature of the first roll is adjusted to 150C, and that of the second roll to 30C. The first roll rotates at a circumferential speed of 50 m/min., which corresponds to the feed speed of the film. If the circumferential speed of the second roll is also adjusted to 50 m/min., a shrinkage in the longitudinal direction of 3.5 per cent is measured, while the transverse shrinkage is 0 per cent.
If the circumferential speed is lowered to 49.0 m/min., or 48.3 m/min. respectively, the shrinkage is reduced to 1.5 per cent and 0 per cent, respectively.
A film is produced as described in Example 4 and subiected to the treatment described in Example 2.
The shrinkage in the longitudinal direction and in the transverse direction is 0 per cent, but the planarity of the film is clearly inferior.
Although a reduction of the shrinkage from, e.g., 1.0 per cent to 0.5 per cent may not seem very impressive, such an improvement of the dimensional stability of the film is of decisive importance if the film is to be used in the above-mentioned fields of application.
A film produced as described in Example 4 is subjected to the treatment described in Example 3. The temperature of the first roll is adjusted to 150C, and that of the second roll to 30C. The first roll rotates at a circumferential speed of 50 m/min., which corresponds to the feed speed of the film. If the circumferential speed of the second roll is also adjusted to 50 m/min., a shrinkage in the longitudinal direction of 3.5 per cent is measured, while the transverse shrinkage is 0 per cent.
If the circumferential speed is lowered to 49.0 m/min., or 48.3 m/min. respectively, the shrinkage is reduced to 1.5 per cent and 0 per cent, respectively.
A film is produced as described in Example 4 and subiected to the treatment described in Example 2.
The shrinkage in the longitudinal direction and in the transverse direction is 0 per cent, but the planarity of the film is clearly inferior.
Although a reduction of the shrinkage from, e.g., 1.0 per cent to 0.5 per cent may not seem very impressive, such an improvement of the dimensional stability of the film is of decisive importance if the film is to be used in the above-mentioned fields of application.
Claims (14)
1. A process for the production of biaxially stretched flat sheets of thermoplastic material of improved dimensional stability, comprising the steps of:
extruding the thermoplastic material from a slot die to form a sheet;
cooling the extruded sheet to form a pre-film;
heating the pre-film to an appropriate stretch-ing temperature;
stretching the heated pre-film in orthogonal directions to form a stretched film;
heat-setting the stretched film;
cooling the heat-set film; and during said cooling step guiding the film in such a manner that it converges in at least one of the transverse direction and the longitudinal direction.
extruding the thermoplastic material from a slot die to form a sheet;
cooling the extruded sheet to form a pre-film;
heating the pre-film to an appropriate stretch-ing temperature;
stretching the heated pre-film in orthogonal directions to form a stretched film;
heat-setting the stretched film;
cooling the heat-set film; and during said cooling step guiding the film in such a manner that it converges in at least one of the transverse direction and the longitudinal direction.
2. A process according to Claim 1, wherein the film is guided during said cooling step in such a manner that it converges in the transverse direction.
3. A process according to Claim 1, wherein the film is guided during said cooling step in such a manner that it converges in the longitudinal direction, and the speed of the film is reduced in comparison to the film speed during the heat-setting step.
4. A process according to Claim 1, wherein the film is guided during said cooling step in such a manner that it converges in both the transverse and in the longitudinal directions, and the speed of the film is re-duced in comparison to the film speed during the heat-setting step.
5. A process according to Claim 1, wherein said guiding step comprises cooling the film on at least two rolls which, when viewed in the direction of feed, are driven at mutually decreasing speeds and maintained at mutually decreasing temperatures.
6. A process according to Claim 1, wherein said guiding comprises introducing the film into a converging tenter, the clips of which are arranged at distances which diminish in the direction of feed, and simultaneously cooling the film.
7. A process according to Claim l, 2 or 4, wherein said guiding comprises convergently guiding the film, thermally insulating the edges of the film, cooling the area between the edges, and trimming the edges of the film after the film leaves the cooling step.
8. A process according to Claim 1, wherein said guiding comprises holding the edges of the film in clips, trimming the edges and suspending the film during further transport, and wherein said cooling comprises simultaneous-ly directing jets of gas on both surfaces of the film.
9. A process according to Claim 5, further com-prising supplying additional cooling to the film in the gap between the rolls, said additional cooling comprising progressively decreasing temperatures.
10. A process according to Claim 6, wherein said additional cooling comprises directing a gaseous medium against the film.
11. A process according to Claim 1, wherein the film comprises a polyester film or a polypropylene film.
12. A process according to Claim 1, wherein said orthogonal stretching is carried out in first one direction and then the other.
13. A process according to Claim 1, wherein said orthogonal stretching is carried out simultaneously in both directions.
14. A process according to Claim 1, wherein the degree of converging is between about 1 and 5%.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000341455A CA1151828A (en) | 1979-12-07 | 1979-12-07 | Process for the production of flat thermoplastic sheets having improved dimensional stability |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000341455A CA1151828A (en) | 1979-12-07 | 1979-12-07 | Process for the production of flat thermoplastic sheets having improved dimensional stability |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1151828A true CA1151828A (en) | 1983-08-16 |
Family
ID=4115778
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000341455A Expired CA1151828A (en) | 1979-12-07 | 1979-12-07 | Process for the production of flat thermoplastic sheets having improved dimensional stability |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1151828A (en) |
-
1979
- 1979-12-07 CA CA000341455A patent/CA1151828A/en not_active Expired
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