CA1038584A - Tubular film of polyethylene terephthalate and process for the production thereof - Google Patents

Abstract

Abstract of the Disclosure This invention relates to a biaxially stretched and longitudi-nally afterstretched thin-walled tubular film of polyethylene terephthal-ate having a tensile stress of 40 to 60 kg/mm2 and a tear resistance of 40 to 70 kg/mm2 in the longitudinal direction, and an elongation at break of 350 to 800% in the transverse direction. The invention also relates to a process for the production of the film.

Description

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TUBULAR FILM OF POLY~THYLENE TEREPHTHALATE AND
PROCESS FOR THE PRODUCTION THEREOF
This invention relates to a biaxially stretched and longitudinally ,. afterstretched tubular film of polyethylene terephtnalate which is dis~
tinguished from~prior art films by improved physic~l properties. The invention further relates to a process for the production of the tubular -~ film. ;
Tubular films of polyethylene terephthalate are produced accord~
ing to known processes as described, for example, in British Patents Nos. 787,479; 811,066, and 843,113, in such a manner that the amorphous :~ lo tubular film obtained by the extrusion of polyethylene terephthalate is : ~ .j . i .
subjected to biaxial stretching at temperaturës above the second order transition point of the polyethylene terephthalate. According to the disclo-~;i sure of Briti9h Patent No. 811~066, aertain physical properties, particularly ;. ,~ . . ~
the tensile strength, of the biaxially stretched tubular film can be f~rther improved when biaxial stretching is followed by another longltudinal stretching ~,,s ~. . .
~ at temperatures between the second order transition point and the melting s`l~ temperature range.
i~s~s~ According to those known processes for the production of tubular `
films of polyethylene terephthalate, biaxial stretching generally is carried ;j 20 out 9imultaneou91y, longitudinal stretching occurring between two pairs of ~ rollers with speeds which are different by the desired longitudinal stretching `~ factor and circumferential stretching occurring by simultaneously inflating ~i the tubular film by~means of a gas. According to the known processes for ~ ~`
-~ longitudinal stretching following biaxial stretching, the tubular film must l be inflated again to counteract neck-in and the formation of longitudinal ``~ creases during longitudinal stretching and to prevent the two tube walls from .,.-. . , sticking together when they ,-, .

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:' contact each other~ Whereas in~lation of the tubular film for biaxial stretching can be performed relatively easily according to known meth-ods, the introduction of a gas cushion during longitudinal stretchlng following biaxial stretching is much more difficult. According to a gen~
erally known method, inflation of the tube can be performed by introduc-~ ,.ing a gas by means of an injection needle. E~ut for this purpose, the tube must be pierced for which reason other methods which do not damage the tube have been sought.
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According to a method mentioned in the known British Patents ~
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the gas is introduced by means of a hotlow needle which is passed from the extruder head through the pairs of rollers; for this purpose, however, at least one roller of the pair of rollers must have an indenta-.
tion around its circumference corresponding to the cross-section of the hollow needle. Apart from the fact that the rollers must be provided - with accurately machined grooves and additionally, for achievlng a satisfactory seal, must be lined with elastic materials, this process -of introducing gas can be used only when the total tube production pro-cess proceeds in a straight line.
For longitudinal stretching following biaxial stretching, accord- -. . ~ .
ing to another method, the gas for inflating the tube is introduced by means of special pairs of squeeze rollers. Working in only one line is avoided thereby but the technical equipment, particularly for the special ;~ ~
'~ ~ manufacture of the pairs of squeeze rollers, is very expensive and main-taining a certain air pressure in the tube during longitudinal stretching can be achieved only with difficulty due to the always occurring pres-. "' ' ':
~;- sure fluctuations .
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By the above processes, particularly by the process of British Patent No. 811,066 which is most similar to the invention, tubular films .~.:: .

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K-22~8 1~)3BS84 can be o})tained which have relatively thin walls but the tensile stress and the tear resistance in the longitudinal direction as well as the elongation at break in the transverse direction are not optimally coor- ~
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' dinated to one another. In particular, thin-walled stretched tubular films are desirable which have a small diameler and which have higher :j :
; I values than have the hitherto known tubular fiLlms, particularly as re- ~ ~
'~ gards the tear resistance in the longitudinaL clirection, the tensile ~ ;
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stress, and the elongation at break in the transverse direction and, furthermore, preferably do not shrink in the transverse direction or do so only to an extremely limited extent also upon temperature stress.
Thus, a need has existed for developing a film of the above ~ -~
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kind.
. . . .
The present invention providss a biaxially stretched and lon-gitudinally afterstretched thin-walled tubular film of polyethylene . .. . .
. terephthalate, having a tensile stress of 40 to 60 kg/mm2 and a tear rssistance of 40 to 70 kg/mm in the longitudinal direction, and sn ~i elongation at break of 350 to 800% in the transverse direction. The .. . .
tensile stress, the tear resistance, and the elongation at break were .
determined according to DIN 53455 (German Industrial Standard). Pre-ferably, the film has a wall thickness of 15 to 50~m and a diameter of 12 to 150 mm. Shrinkage in the transverse direction, measured at 90 C, of a further preferred type of the film is in the range of ~ 5%, ~ j -; ~ particularly of < 2%.
The film of the invention is particularly suitable for filling with pasty goods, such as with cements, colors or the like, as it is stressed by fllling particularly as regards the tear resistance in the .. . .
longitudinal direction and elongation in the transverse direction.
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The present invention also relates to a process for the pro-duction of a tubular film of polyethylene terephthalate in which longi- ~;
tudinal stretching following biaxial stretching for improving the physical properties of the tubular film can be carried out without inflation of the tubular film.
This is achieved by biaxial stretching in the range of the sec-ond order transition point of an extruded amorphous tubular film of poly~
ethylene terephthalate and subsequent longitudinal stretching at temp-eratures between the second order transition point and the melting temperature range. Biaxial stretching is carried out at a stretching ratio of 3.3 to 4.2 in the longitudinal direction alld 4.0 to 4.7 in the trans- ~;
verse direction at temperatures from 84 to 86C., and further longitudinal stretching is carried out without the introduction of a gas cushion at a stretching ratio of 1.05 to 2.5 at temperatures from 148 to 152C.
It was surprising that, under the conditions of temperatures and stretching ratios employed in accordance with the invention, longi- ~:
~ tudinal stretching following biaxial stretching of an extruded polyethyl-,.~J; ene terephthalate tube could be performed without inflating the tube, ,~ whereas the introduction of a gas cushion during longitudinal stretching according to the known processes is considered absolutely necessary for :~ ~
l preventing creasing.

Suitable polyethylene terephthalates are those produced by `~ polycondensation of terephthalic acid with aliphatic diols according to ~

~ conventional processes. The second order transition point of such ~ ~ --~ polyethylene terephthalates is in the range of about 70 to 80 C.

- The process of the invention is performed in such a manner :;
~ that the amorphous tubular film obtained according to known methods by ,~. -1' ~ extrusion - e.g. by extruding the polyethylene terephthalate melt ',' :,,` :

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over a cooled, cylindrical calibrating mandrel and drawing-off the amorphous tubular film by means of a pair of rollers - i9 first subjected to biaxial stretching at a longitudinal stretching ratio of 3.3 to 4.2, preferably of 3.3 to 3.7, and a circumferential stretching ratio of 4.0 to 4.7, preferably of 4.2 to 4.5, and at a temperature of the tubular film of 84 to 86 C. The biaxially stretched tubular fil~n is further longitudinally stretched without the introduction of a gas cushion at a longitudinal stretching ratio of 1 . 05 to 2.5, preferably of 1. 1 to 2 . 0, ~y the temperature of the tubular film being 148 to 152 C, preferably 150C.
It is advantageous for achieving the improved physical prop~
. . ~ , . .
1~ erties of the film to exactly follow the stretching ratios and tempera-:.~ , , tures of the process given for biaxial stretching and subsequent longitudinal stretching.
,1 Biaxial stretching is advantageously carried out, according to known methods, simultaneously between two pairs of rollers. The longitudinal stretchlng force is effected by a second pair of rollers arranged in the direction of extrusion, which, for achieving the neces- `~
, ,, sary longitudinal stretching ratio, run at a correspondingly higher speed than the first pair of rollers closer to the extruder. The trans-verse stretching force is effected by a gas pressure, e.g. air pressure, which is applied to the tube by introducing a ~as. The introduction of .~ , . .
~, the gas into the interior of the tube may be, for example, from the extruder head through a feed line in the calibrating mandrel, the width ;i of the first pair of rollers belng smaller than the width of the ilattened - tubular film -: :J
i, The longitudinal and transverse stretching ratios to be main- ~;

tained in accordance with the invention can be easily adjusted by using t.`

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a corresponding internal pressure and a corresponding speed relation ~:
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between the first and second pairs oi rollers. BiaxiaL stretching also -may be performed consecutivety. In this case, however, it is neces~
sary for the success of the process of the invention that first longitudi~
~j nal stretching and then transverse stretching be perEormed.
"5 Longitudinal stretching following biaxial stretching is also performed according to known methods, e.g. between the mentioned second pair of rollers and a third pair of rollers which run at a higher speed corresponding to the required stretching ratio. Simultaneous inflation of the tube during this longitudinal stretching for preventing creasing or sticking of the tube walls to each other is not necessary ln accordance with the invention. The tube runs without creasing, completely planar through the longitudinal stretching zone and the ";~ mutual contact of the inside walls of the tube does not result in sticking.
;~ ~ During biaxial stretching as well as durlng subsequent lon~
gitudinal stretching, the tube runs through a heating zone in which it is heated to the required temperatures. Heating may be ;performed, for example, by means of hot air passed in a su1table device to the ;
tubular film or by means of lnfrared radiators. Exact control of the film -~, 20 temperatures is possihle by means of a radiation pyrometer, for exampte.
The process of the invention may be performed continuously or intermittently.l A tube biaxially stretched according to the condi~
tions of the invention may be subjected to longitudinal strëtching in~
a second process step not necessarily immediately~following biaxial stretching. ~ ;
~ ~ The process, furthermore, may be performed at a right angle `~ ~ or another angle to the extruder. `

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An exemplary arrangement for the performance oE the process of the invention is illustrated in the accompanying drawing in a dia-:~
grammatic cross~sectional view, without the invention being limited -to the embodiment shown.
-~ The polyethylene terephthalate melt extruded fr~m the extruder 1 through the annular die 2 is calibrated on an internal cooling mandrel

3 to form an amorphous tubular film ~preliminary tube) VS, which is flattened by the pair of rollers 4 and drawn-off thereby at a speed V .
The amorphous tube passes through the heating zone 5 in which it is ~ `
heated to the biaxial stretching temperature. The longitudinal compo-nent of the stretching force is provided by the pair of squeeze rollers 6 which impart to the tube the speed V2, which corresponds to the de-sired longitudlnal stretching ratio;V2 thus is correspondingly hlgher than V1. Transverse stretching is effected by means of gas pressure lntroduced via the feed llne 7 Into the interior of the tube. The blax-ially stretched tubular film BS, flattened by the pair of rollers 6 and drawn-off at the speed V2, first runs over the two guide rollers 8 and 9 and then over the heated rollers 10 and 11 I the speed V3 of which is ' so adjusted that the film remains tightened between the parts 6 and 10.
By means of the heated rollers 10 and 11, in particular by , .:
means of the heating device 12, the tubular film is heated to the longi-:, '.', , - ~ :
- tudinal stretching temperature. Longitudinal stretching is performed 3 ~ by means of the pair of rollers 13 and 14 which are cooled and run at the speed V4, V4 being correspondingly hlgher than V3. The two rubber , rollers 15 and 16 ensure that the tube closely contacts the rollers be- ~ -. . : ~
`~ tween which longitudinal stretching is performed. As for biaxial ~ stretching, heating of the tube to the required temperature is effected ,.~
by an infrared radiator; the tube temperature is controlled by means of ;;' a radiation pyrometer.

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After longitudlnal stretching, the tube passes over the guicle roller 17 and ls wound onto the roll 18. `~
The process of the lnvention for the produc~ion of the tubular fllm from polyethylene terephthalate therefore is very efficient because it can be performed ln a simple manner and with less apparatus than the `~ known processes. Whereas, accordlng to the known processes, longi--, tudinal stretching for partially improving the physical properties of a . biaxially stretched ~ube of polyethylene terephthalate can be performed ~ I ~
~; only with technlcally complicated lnflatlon of the tube, inflation ls not , necessary ln the present process. Furthermore, the tubes have physical ~, properties the lnterestlng physlcal characteristlcs of which are superior ;~ to those achlevable accc)rdlng to known processes. The resultlng ~ ,-3 values, whlch are superlor to those of the prior art, can be seen from the table below.
In the following examples the production of tubular films of polyethylene terephthalate ~ccordlng to the lnventlon ls described and -;
`:~ the measured physlcal propertles of the tubes are compared with the -j values of tubular films of polyethylene terephthalate obtained according .; to the most slmllar process of Britlsh Patent No. 811, 066.
Example 1 Polyethylene terephthalate wlth a second order transltion temperature of 76C, a crystalllzatlon temperature of 132C, a melt~
lng point of 260C (calculated on a heating speed of 2C/min.), and a vlscoslty of 1, 800 poises at 275C was fed as a granulate to an extruder which melted the product at 270C . After extruslon from an annular die ;
,~3 mounted downstream, the melt was hardened on a calibratlng mandrel ~diameter 14.5 mm) connected with the die and cooled with water to form an amorphous tube of a dlame~er of 14.5 mm. The output speed was ~' lO m per mlnute. The amorphous tube hardened on the coollng mandrel ~;! :
~^1, was drawn-off by a flrst palr of rollers at a speed of lO
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m per minute and flattened. The flatt0ned tube, further conveyed by a : .
: ~ second palr of rollers, was heated to 85 C between the two pairs of ' ~ ."
rollers by means of infrared radiation and then biaxially stretched.
~ Stretchlng in the longitudinal direction was performed at a ratio of 3.6, . since the second pair of rollers further conveyed the tube at a speed of .
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: ~ 36 m per minute, and transverse stretching was performed at a ratio of , .i 4.5, since the tube was expanded to a diameter of 65 mm by means of . ~ , an internal gas pressure of 0.2 bar introduced via the calibrating man-;- . drel. The thus biaxially stretched tube flattened by the second pair of rollers, after passing the guide rollers, was conveyed over a third pair ~ ., "
.-., of rollers running at a slightly higher speed than the second pair of ;
. 1 rollers, for maintaining the tension, and over a fourth pair of rollers, .longitudlnal stretching being performed between the thircl and fourth ~ pairs of rollers. F'or thls purpose, the tube was heated to 150C be-: ~ tween the third and fourth palrs o:E rollers and .longitudinally stretched . -, at a ratio of 1.28, since the fourth pair of rollers drew-of the tube at ~ a speed of 46 m per mlnute . After the latter stretching, the cooled ~ ::
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. tube was wound onto a roll.
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` 20 The process was repeated analogously to the procedure of ~ :
', E~ample 1 but at a .longitudinal stretching ratio of 1.83 for.longltudinal :;
stretchlng following biaxial stretchlng. The internal gas pressure for :~ biaxial stretching was 0.3 bar.
i ExamPle 3 ~
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. ., The process was repeated analogously to the procedure of :. Example 1 with the use of a calibrating mandrel of a diameter of 22 : ' mm, but at a longitudinal stretching ratio of 1.56 for longitudinal ::, ~, .

stretching following biaxlal stretching. The internal gas pressure for . biaxial stre tchlng was 0 . 2 bar .
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Example 4 The process was repeated analogously to the procedure of Example 1 with the use of a calibrating mandrel of a diameter of 9.5 mm, but at a transverse stretching ratio of 4 . 25 for biaxial stretchlng .. :
and a longitudinal stretching ratio of 1.96 for longitudinal stretching following biaxial stretching. The internal gas pressure for biaxial stretching was 0.55 bar.
Example 5 The process was repeated analogously to the procedure of i Example 1 but with the use of a polyethylene terephthalate of a vis~
cosity of 4,000 poises at 275C and a calibrating mandrel of a diame-ter of 42 mm, but at a transverse stretching ratio of 4.3 for biaxial ` stretching and a longitudinal stretchlng ratio of 1.35 for subsequent longitudinal stretchlng. The internal gas pressure for blaxial stretch-lng was 0.1 bar.
In aLl cases, shrinkage of the films produced according to ; the foregoing examples was below 2% in the transverse direction, .,.1 . measured at 90C, 15 sec. in water.

In the Table below, the physical properties of the tubes pro-duced in Examples 1 to 5 are shown and compared with the values '~J7 achieved according to the prior art.
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As can be seen from the Table, the tensile stress and the tear - resistance in the longitudinal direction as well as the elongation in the ~` transverse direction are markedly increased compared to comparable films produced according to the prior art, whereas the other ~iven values ~: .
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Claims (9)

WHAT IS CLAIMED IS:

1. A biaxially stretched and longitudinally afterstretched thin-walled tubular film of polyethylene terephthalate having a tensile stress of 40 to 60 kg/mm and a tear resistance of 40 to 70 kg/mm2 in the longitudinal direction, and an elongation at break of 350 to 800%
in the transverse direction.

2. A film according to claim 1 having a wall thickness of 15 to 50µm.

3. A film according to claim 1 having an external diameter of 10 to 150 mm.

4. A film according to claim 1 having a shrinkage at 90°C in the transverse direction of < 2%.

5. In the process for the production of tubular films of polyethyl-ene terephthalate by biaxial stretching in the range of the second order transition point of an extruded, amorphous tubular film, subsequent longitudinal stretching at temperatures between the second order tran-sition point and the melting temperature range, and cooling, the improvement comprising performing the biaxial stretching at a stretching ratio of 3 .3 to 4.2 in the longitudinal direction and 4.0 to 4.7 in the transverse direction at a tube temperature of 84 to 86°C, and performing the longitudinal afterstretching without the introduction of a gas cushion at a stretching ratio of 1.05 to 205 and at a tube temperature of 148 to 152 C.

6. A process according to claim 5 in which the longitudinal stretching of the biaxial stretching is performed at a ratio between 1:
3.3 and 3.7.

7. A process according to claim 5 in which the circumferential stretching of the biaxial stretching is performed at a ratio between 1: 4.2 and 4.5.

8. A process according to claim 5 in which the longitudinal after-stretching is performed at a ratio between 1: 1.1 and 2Ø

9. A process according to claim 5 in which after biaxial stretch-ing, the film is cooled.