DE2049538A1 - Crystalline film made from a polyester - Google Patents

Description

Registration number. 122 664

Eastman Kodak Company, 343 State Street, Rochester, New York State, United States of America

Crystalline film made from a polyester

The invention relates to a crystalline film made of a polyester, the repeating acid residues of which at least 85 mol% of terephthalic acid residues and their recurring Glycol residues at least 85 mol from 1,4-butanediol residues exist.

It is known to use polyester, e.g. B. from polyalkylene terephthalates, in particular from polyethylene terephthalate to films extrude. However, in order to obtain these polyester films in a clear, transparent form, it is necessary to use the polyester to keep in the amorphous state or to biaxially orient the amorphous film before crystallization of the polyester. Starting from the polyesters mentioned, amorphous films can easily be obtained, for example, by the quickly quench the extruded film. In this context, reference is made, for example, to US Pat. No. 3,407 and British Patent 609,797.

From the magazine Ind. Iing. Chem., 6 ±, No. 5, page 15 (1969) it is also known that practically no crystalline

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Polymers such as the linear polyolefins, polyacetals and polyamides are transparent. Rather is it is necessary to avoid crystallization when it comes to making crystal clear polymers.

It has been found that slow cooling of amorphous extrudates according to the teachings of the prior art leads to crystallization, whereby opaque films are obtained. However, amorphous foils do not have the Advantages of crystalline films. The advantages of crystalline films are based in particular on their higher dimensional stability temperature, their higher tensile strength, their higher stiffness, their increased hardness, their increased resistance to solvents, their reduced permeability to gases and the like. Disadvantageous however, it is a feature of such crystalline films that the crystalline films produced hitherto have a haze have or are even opaque or opaque.

However, there are many fields of application which require clear and transparent films, so that there is a need for consists of such polyester films which are both crystalline and transparent.

The previously known method for producing polyester films that are both crystalline and clear and transparent require that the amorphous film initially produced from z. B. polyethylene terephthalate obtained by hot melt extrusion and quenching is stretched or oriented, followed by heat treatment or heat setting treatment to give the desired crystallinity. In terms of the required

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Orientation and heat treatment or heat setting is the known method from an economical point of view very expensive. In addition, when the known method is carried out, the tensile strength of the films is reduced. It has been shown that the tensile strength of oriented, heat-set polyethylene terephthalate films, for example is lower than the corresponding tensile strength of non-oriented polyethylene terephthalate films,

The object of the invention is therefore to produce one by extrusion to indicate obtained, non-oriented polyester film which is crystalline and clear or transparent and furthermore is distinguished by an excellent tensile strength without the need for biaxial stretching and heat fixation at elevated temperatures is required.

The invention was based on the knowledge that one too crystalline Films with the properties described are obtained when using a film-forming or film-forming polyester, its recurring acid residues to at least 85 MoI-I from terephthalic acid residues and their recurring glycol residues at least 85 MoI-I consist of 1,4-butanediol residues, too a crystalline film, preferably into a film having an inherent viscosity of at least 0.8 and if the film obtained is quenched under controlled conditions, so that an unoriented, crystalline film, which is transparent or clear is obtained. It is therefore essential that, according to the invention, a crystalline one by extrusion Film is produced contrary to the teachings of the prior art that the extruded film must be amorphous.

1 0 B 8 1 7/2? 7 2

£ 049538

The invention thus provides a crystalline film made of a polyester, the recurring acid residues of which consist of at least 85 mols of terephthalic acid residues and the recurring glycol residues of at least 85 mols of 1,4-butanediol residues, which is characterized in that it is transparent and unoriented , has a heat distortion temperature of at least 80 ⁰ C and a tensile strength according to Elmendorf of at least 35 grams per 0.0254 mm.

The polyesters used to produce the films can be produced by customary known methods. this means that not only terephthalic acid is used to produce the polyester and 1,4-butanediol can be used, but rather also the corresponding condensable derivatives of terephthalic acid and 1,4-butanediol.

Films whose heat distortion temperature is at least 100 ^° C. have proven to be particularly advantageous.

As used herein, the term "not oriented" means that the film is not stretched in the solid state. A certain stretching may take place during extrusion to achieve the correct film thickness, the self-supporting, solid one The film itself, however, is not stretched.

The polyesters used to produce the crystalline film according to the invention can, as already stated, be prepared by conventional methods, for example by ester exchange of 1,4-butanediol and a dialkyl ester of terephthalic acid or by direct esterification of terephthalic

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£ 049538

thalic acid with 1,4-butanediol. Conventional esterification catalysts can be used in the preparation of the polyesters, e.g. B. tetraisopropyl titanate. Preferably, polymerization temperatures are from about 240 - 26O ⁰ C applied. If necessary, up to about 15 mol-1 of other dicarboxylic acids or dicarboxylic acid derivatives can be added for the preparation of the polyesters, and furthermore up to 15 mol-1 of other glycols.

The polyesters used to produce the crystalline films should also have inherent viscosity, such as it is required for film production.

Since there is occasionally a slight reduction in inherent viscosity during the extrusion of the polyester, a polyester film is expediently used for the production of the films Inherent viscosity is about 0.1 to 0.2 units higher than the inherent viscosity of the extruded film.

It has proven to be particularly advantageous if the polyester has an inherent viscosity of ¹ U _n J ₁ , determined at 25.degree

the equation

In, C

no inh "

where: r \ _{r is} the ratio of the viscosity of a dilute solution of 0.23 g of the polyester in 100 ml of a solvent mixture of 60 parts by weight of phenol and 40 parts by weight of tetrachloroethane to the viscosity of the pure solvent mixture and C is the concentration of the polyester in 100 ml of the solvent mixture of at least 0.8.

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Modifying dicarboxylic acids, in particular those with 3-20 carbon atoms, can be used to produce the polyesters be used, e.g. B. aliphatic dicarboxylic acids with 3-20 carbon atoms, alicyclic dicarboxylic acids of 6-20 Carbon atoms and aromatic dicarboxylic acids of 8-16 carbon atoms. Typical such dicarboxylic acids are Isophthalic acid, adipic acid, azelaic acid, dirnethylmalonic acid, Dodecanedicarboxylic acid, the isomeric cyclohexanedicarboxylic acids, sulfonyldibenzoic acid, diphenic acid, oxydibenzoic acid and methylenedibenzoic acid. Of course you can manufacture the polyester these acids also in the form of their derivatives, e.g. B. can be used in the form of their dialkyl esters.

Typical glycols that can be used to modify the polyesters are those with 2-20 carbon atoms, including aliphatic glycols with 2-20 carbon atoms and alicyclic glycols with 4-20 carbon atoms, z. B. ethylene glycol, neopentyl glycol, 1,4-cyclohexanedimethanol, 1,10-decanediol, 1,4-cyclohexanediol, 2-ethyl-2-butyl-1,3-propanediol and 1,6-hexanediol.

The polyesters can be extruded by conventionally known methods using conventional equipment. It has proved to be advantageous when the polyester at temperatures of about 265 - 295 ⁰ C are extruded, depending upon the inherent viscosity of the polyesters used.

The quenching temperature used in the manufacture of the transparent crystalline films depends somewhat on the Inherent viscosity of the particular polyester used and the strength or thickness of the film produced. All the less the inherent viscosity of the extruded polyester is so

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the quenching temperature required to produce a clear crystalline film may be lower. Correspondingly, the thicker the desired film, the higher the inherent viscosity of the polyester should be or, correspondingly, the lower the quenching temperature. Of course, the quenching temperature should be so low that crystallization is avoided. Quenching temperatures of 10-4O ⁰ C, in particular 10-15 ⁰ C, have proven to be expedient.

For example, if the inherent viscosity of the film is approximately 0.8, then clear 0.076 mm thick films can be obtained by extruding the polyester on metal rollers at approximately 20 ^° C. On the other hand, for example, the inherent viscosity of the film at about 0.9, then clear films of up to about 0.076 mm thickness can be obtained in that the molten polymer is extruded onto metal rolls at a temperature of 4O ⁰ C.

If necessary, the quenching or cooling rollers can be cooled by circulating a cooling liquid for the purpose of discharge the heat of the molten polymer can be increased, which makes it possible to also use thicker films of a certain inherent viscosity in a clear and crystalline state. For example, the inherent viscosity of the film at around 1.0, clear films up to 0.254 mm thick can be obtained when the roller temperature is around 20 ° C lies. Thicker films that are clear and crystalline can be made by lowering the temperature of the metal rollers or ErzMen by increasing the inherent viscosity. For example

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, the inherent viscosity is about 1.3 and the rolling temperature at 20 ⁰ C, then clear films have a thickness of up to 0.38 mm obtained by extrusion. On the other hand, depending on the extrusion and quenching conditions, clear, transparent films can also be obtained whose thickness is no greater than about 0.00635 mm.

By modifying the polyester with a second dicarboxylic acid, z. B. isophthalic acid and / or a second glycol, e.g. B. ethylene glycol or neopentyl glycol can be crystalline Produce even thicker films.

Although quenching of the extruded films is preferably done by extruding the polyester onto cold metal rollers, other known quenching techniques can also be used. For example, it is also possible to use the extruded To quench the film with an inert liquid using a nozzle with a cold inert gas directed at the film or by extruding into water and the like.

The fact that the clear, undrawn, non-heat-set films of the invention have a high heat distortion temperature owning is surprising. The degree of heat distortion temperature depends somewhat on the degree of Crystallinity, which is influenced by the polymer inherent viscosity and also the degree of quenching (quenching).

The higher the quenching, the lower the quenching temperature or the lower the film thickness. A lower quench allows for greater crystallinity, which in turn leads to an increased heat distortion temperature.

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Advantageous crystalline films according to the invention can have dimensional stability temperatures of up to 200 ° C., measured at 3.50 kg / cm ² . If there was no crystallinity, the heat distortion temperature would be close to the glass transition temperature, which is 22 ° C., as is known, for example, from the journal J. Polymer Sci., Part A-1, £, (1966), page 1853.

The glass transition temperature of a quenched, the extruded polyethylene terephthalate film is 69 ⁰ C, (see. The specified reference) and the heat distortion temperature (heat-distortion temperature) is within a range Λ few degrees from this value. If such a film is ^{heated to 110 °} C. in an oven, for example for one hour, the film becomes crystalline and cloudy.

The poly (tetramethylene terephthalate) films of the invention are crystalline when they have been extruded, and the sufficient crystallinity having films remain clear and transparent, even if they are heated in an oven for several hours to a temperature up to 15O ⁰ C.

The crystallinity of the inventive unoriented, transparent films of the invention can be demonstrated by X-ray refraction studies. Are films of the invention to very high temperatures, for example, heated to temperatures of 22O ⁰ C, they are hazy (hazy), this being due to increased crystallinity.

Although even higher tensile strength values, stiffness values and even higher deformation temperatures can be achieved would, if the films according to the invention stretched and heat

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would be fixed, it has been found that these additional process steps are not required; H. that technically valuable films with desired properties can also be obtained without these additional steps. This means, that the unstretched clear films of the invention have excellent properties without being oriented and heat set. In this way, for example, crystalline films according to the invention can be produced which have dimensional stability temperatures of up to 200.degree.

As has already been stated, influence one by the way Stretching and heat setting adversely affect the tensile strength of the films. Unexpectedly it has been shown that the films according to the invention have an unexpectedly high tensile strength according to Elmendorf, d. H. a tensile strength of at least 35 g per 0.0254 mm. In advantageous The tensile strength according to Elmendorf can be 200 g per 0.0254 mm or even higher if the films have an inherent viscosity of at least about 0.8.

It should be noted that, for example, the tensile strength of non-oriented polyethylene terephthalate films, which is about 35 g per 0.0254 mm, is greater than the tensile strength of oriented, heat-set polyethylene terephthalate films, which is about 25 g per 0.0254 mm. In contrast, the films of the invention, which have an inherent viscosity of at least about 0.80, have an Elmendorf tear strength of at least 35 g ¹ per 0.0254 mm. Preferably the tensile strength is at least about 50 grams per 0.0254 mm. In this context, reference is made to Table I below. '

If the inherent viscosity of the film is increased, the

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Usually neither the tensile strength nor the elongation of the film is significantly increased. On the other hand, however, it is surprising Way, the tensile strength increased significantly. With particularly advantageous inherent viscosity values above about 1.1 the tensile strength exceeds a value of 200 g / 0.0254 mm, i.e. H. there are tensile strength values obtained, which are surprisingly very high.

The crystalline films of the invention can optionally contain so-called nucleating agents, also pigments, antioxidants, Stabilizers, plasticizers, anti- * blocking agents (lubricants), lubricants, the fire ™ resistance or flammability of the films (fire retardant agents) as well as other additives that are usually incorporated into the polyesters used to make films.

Since the films of the present invention are crystalline, they have excellent resistance to solvents. In contrast to this, amorphous foils are more easily caused to swell or even dissolved by solvents. With regard to the crystallinity of the foils, it is surprising that they can easily ^{be deformed cold at about 50 °} C. to give transparent, crystalline particles. d

The foils or films according to the invention can be used wherever transparent, solid, tough, dimensionally stable, solvent-resistant or heat-resistant foils or films are required.

For example, the films according to the invention can be applied apply a wide variety of surfaces, d. H. use for coating. They can also be used in an advantageous manner

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Use photographic film supports, in the context of office copying processes, for the production of microfilms, for electrical insulation, for the production of flexible electrical circuits, also for the production of drawing and writing foils as well as measuring tapes,

also for the production of container linings, for the production of transparent windows for envelopes, for the production of packaging materials, protective covers and the like.

The following examples are intended to further illustrate the invention.

All Inherent viscosity values were determined at 25 ° C. using a solvent that was 60 parts by weight Phenol and 40 parts by weight of tetrachloroethane. The concentration of the lölye ice in 100 ml of the solvent mixture was 0.23 g.

Examples 1 - 18

The polyester used to make the polyester films was made from dimethyl terephthalate and 1,4-butanediol according to a customary esterification process at condensation temperatures of 240 - 260OC. As an esterification catalyst tetraisopropyl titanate was used.

The polyester initially produced was dried overnight in a vacuum oven at a temperature of 100 ° C. and then dried using a standard 3.81 cm extruder (length / diameter ratio 24: 1) extruded onto a chill roll cooled with water. The melting temperatures were included

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265-295 ⁰ C, depending on the Polymerinherent viscosity. The particular temperatures used for the quenching rolls and the film properties are listed in Table I below. A chill roll temperature of 4O ⁰ C was achieved in that the inflow of cold water was reduced in the quenching and in that further polymer melt quenching the heated slightly.

The following table shows the dimensional stability temperatures and tensile strengths for the clear transparent films and tensile strengths according to Elmendorf.

The properties of the foils were determined according to the usual standard methods of the American Society for Testing Materials certainly. The strength properties (tensile strength, elongation and modulus) were measured using an Instron tensile tester determined according to ASTM method D882. The deformation temperatures were at a load of 3.5 kg / cm in a convection oven according to ASTM test method D1637. Finally, the Elmendorf tensile strength was determined using ASTM test method D1922.

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Inherent temperature

TABLE I.

Polytetramethylene terephthalate film

viscosity foil rature strength Clear- Verfor 72
153
172 187
185 tensile strenght Tear
firm
speed in
kg / cm ² Module in
kg / cm ² Elongation in
% Tensile strength
in g / 0.0254 mm I. O example
No. Poly-
meT 0.71 from the
fright
rolling
in oc of foity
lien in the
mil foil
C1 mil =
0.0254mm) mung s-
temp.
⁰ C clear 200
veiled at the
Stretch
border
in kg /
cm ^ I. CO
cn
OJ
OO 1 0.75 0.80
0.80 20th 5 veiled clear 562 14100 4 50 80 2
3 0.85
0.85 0.89
0.89
0.89
0.94
0.94 10
10 1
5 clear 85
veiled 352 576
590
682 14100
14700
11900 500
480
540 110
93
140 4th
5 - *
6 ο
CD
OO
7 ->
8 -Y 0.96
0.96
0.96
1.20
1.20 1.09
1.09
1.09 10
40
40
10
10 3
3
5
10
13th clear 90
135
Schlei- 190
erig
clear 188
veiled 352
373
281 590
569 14700
14700 490
450 185
200 9 NJ
10 Ni
11 -Y 1.15
1.15
1.15 1.13
1.13
1.13 40
40
20th 5
8th
10 clear 168
SchleiTig
clear 160 373
366 576
576
640 14100
14100
14700 540
500
460 200
200
200 12 ^Μ
13th
14th 1.28
1.28
1.28 1.16
1.16 20th
20th
20th 6th
10
13th clear
dd
It 352
366
380 633 15400 460 200 15th
16 1.30
1.30 1.26
1.26 40
40 10
20th 380 647
668 14100
14700 470
450 200
200 17th
18th 1.40
1.40 40
20th 10
15th 373
380

For example, as can be seen from Example 2 of Table I, the 1 mil thick (0.0254 mm) film is the heavily quenched became clear, whereas the 5 mil CO (127 mm) thick film of Example 3 was not sufficiently quenched using the same chill roll temperature.

Clear 3 mil (0.076 mm) thick films were used in the cases of the Examples 4 and 5 were obtained. The film of Example 5 has the higher deformation temperature due to the lower quenching caused by the higher chill roll temperature. Larger crystalline spherulites were found in the case of the foil's Example 6 produced due to the higher film thickness (lower quenching), which is why the film is not entirely clear was. The lower deformation temperature of example 12 compared to example 9 is due to the greater quenching (lower roller temperature) of Example 12.

It should be noted that generally hazy films at the lower inherent viscosities, higher chill roll temperatures and higher film thicknesses occur, whereas clear, transparent films by increasing the inherent viscosity and lowering the chill roll temperature.

Example 19

The polyester films of Examples 5, 9 and 13 were at temperatures Cold formed from 50 - 60OC. Films that were stretched to a fifth of their original thickness remained clear and transparent.

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Example 20

A polyester with an inherent viscosity of 1.10, produced from 1,4-butanediol, 95 mol-1 dimethyl terephthalate and 5 mol-1 dimethyl isophthalate, was extruded as described in Example 1, but using a quenching roller at a temperature of 40 ^° C. In this way a clear film with an inherent viscosity of 0.97 and a thickness of 0.0762 mm and the following properties was obtained:

Deformation temperature:

Tensile strength at the yield point Breaking strength module

Elongation tensile strength according to Elmendorf

In a corresponding manner, 0.25 mm thick films with a deformation ^{temperature of 126 °} C. were obtained.

Example 21

Starting from 1,4-butanediol, 90 mol-1 dimethyl terephthalate and 10 mol / l dimethyl acetal, a polyester with an inherent viscosity of 1.24 was prepared and as in Example 1 described extruded. Starting from the polyester described, clear films with a thickness of 0.254 mm with a Deformation temperature of 131 ° C and a tensile strength according to Elmendorf of 200 g / 0.0254 mm.

93 ° C; 343 kg / cm; 668 kg / cm; 14 700 kg / cm; 460 I; 160 g / 0.0254 mm

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Example 22

Based on 1,4-butanediol and ethylene glycol in a ratio of 90:10 a polyterephthalate was produced with an inherent viscosity of 1.09. Nacfyflem in Example 1 Clear films 0.254 mm thick were produced from the polyester. The foils had the following Characteristics:

Deformation temperature Tensile strength at the yield point modulus of elasticity strain

120 ⁰ C; 336 kg / cm ² ; 15400 kg / cm ² ; 320 %.

Examples 23-36

A polytetramethylene terephthalate having an inherent viscosity of 1.3 was extruded into a sheet using a conventional extruder device. The film had an inherent viscosity of about 1.1. The temperature of the quenching roller was increased from 16 ^° C. to 100 ^° C. in a series of tests. In general, it has been found that film density (an indication of the degree of crystallinity of the film) and deformation temperature increase as the temperature of the chill roll increases. A veil does not occur in the film on as long as the temperature of the quenching does not exceed 77 ⁰ C. (Example 33)

The values "MD" and "TD" listed in the table below relate to the properties of the film which determines were in the "machine direction" and the "transverse direction". The machine direction is the direction in which the

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Film moves during extrusion, since the film is practically unoriented, there is a relatively small difference in the properties to the perpendicular directions.

From the following Table II it follows that the chill roll, preferably ° C is advantageously at least 6 ⁰ C for at least 11 below the temperature at which fogging of more than 1% occurs (77 ⁰ C).

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MD
TD TABLE II A 23 24 of polyester films 26th 27 28 29 MD
TD Physical Properties 16 21st 25th 38 43 49 54 Example no. MD
TD ASTM
method 1.43 1.84 32 1.87 1.89 1.71 1.68 Chill roll temperature ° ( MD
TD 1.276 1.277 1.88 1.279 1.282 1.283 1.285 Thickness of the foils in mil
CO, 0254 mm) on average 1.0 1.0 1.278 1.0 1.0 1.0 1.0. Density of the foils in g / cm D15O5 82 68 1.0 85 70 76 70 Turbidity in % D1003 92
102 74 91
96 105
103 85
94 77
72 l Transparency in % D1746 299
301 287
294 95
106 308
308 322
305 326
333 356 1
316, D1922 518
651 469
504 277
299 651
670 658
670 777
763 707
651 D882 490
560 380
410 588
710 490
510 490
520 550
570 550
550 D882 11812
11391 12210
12700 500
530 13820 1
14900 1 5670
5670 15460
15820 15180
15390 D882 48 46 12700
13470 51 93 81 118 D882 56 JV)
CD D1637 CD
cn
co
OO Tensile strength according to Elmen-MD
village in g / 0.0254 mm TD Tensile strength at the stretch
border -
in kg / cm Tensile strength in
kg / cm ^e.g. Elongation in % Modulus of elasticity in kg /
cm ² (stiffness) 2% heat deformation
Temperature ⁰ C

TABLE II B
Physical properties of polyester films

Example no. 30 3J 3_2 33 34 35 36_

Quench roll temperature ⁰ C

Thickness of the foils in mil
(0.0254 mm) on average

Density of the foils in g / cm
_ ^ Turbidity in %

ο transparency in %

cc tensile strength according to Elmen

- ^ village in g / 0.0254 mm

Z ^ Tensile strength at the yield point in kg / cm ²

^ Tensile strength in kg / cm
Elongation in %

Young's modulus in kg / cm
(Stiffness)

2 Uge heat deformation 130 149 170 172 197 199 200

Temperature ⁰ C

1
1 60 1
1 66 71 77 82 88 100 1.66 1.66 2.04 2.36 2.59 1.68 1.71 1.288 1, 288 1.291 1.293 1.293 1.293 1.289 1.0 1.0 1.0 3.0 7.0 7.0 52.0 · 68 71 72 70 59 68 26 ζ MD
TD 80
79 73
77 68
61 58
59 56
57 56
57 57 *
60 MD
TD 340
333 356
336 361
343 340
385 378
343 352
336 357
389 MD
TD 773
609 595
634 707
588 745
756 672
602 686
494 731
602 MD
TD 590
550 530
530 580
490 560
570 560
540 560
440 540
450 MD
TD 6020
5466 5466
5880 15600
15670 16090
16720 15950
17350 17490
17210 17600
17140

CD -C-

The term "cold forming" used here (see, for example, Example 19) means that the foils ^{are deformed at temperatures of about 20 to 100 °} C. by customary known methods, according to which foils and foil sheets made of synthetic resins are deformed.

As used herein, "at least 85 MoI-I" relates on the condensation of a dicarboxylic acid, it being assumed that 100 mol-I of the dicarboxylic acid or a condensable one Derivatives of dicarboxylic acid and 100 mol-I of a glycol or a condensable derivative of a glycol implemented together. This means that the polyester produced from 100 mol-I dicarboxylic acid residues and 100 mol-I consist of glycol residues.

The production of the films can advantageously be carried out continuously are done by continuously extruding a molten polyester onto a rotating quenching surface a film of the properties described is obtained if the parameters of (1) inherent viscosity, (2) film thickness; and (3) rotating quench surface temperature, respectively, to form a clear one Slide can be selected.

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Claims (11)

- 22 claims 1. Crystalline film made of a polyester whose recurring acid residues consist of at least 85 MoI-I from terephthalic acid residues and whose recurring glycol residues consist of at least 85 mol% of 1,4-butanediol residues, characterized in that it is transparent and unoriented, a dimensional stability temperature of at least 8O ⁰ C and has a tensile strength according to Elmendorf of at least 35 grams per 0.0254 mm. 2. Crystalline film according to claim 1, characterized in that the polyester has an inherent viscosity η. , determined at 25 ⁰ C according to the equation lnn _r 'inh _c where: η is the ratio of the viscosity of a dilute Solution of 0.23 g of the polyester in 100 ml of a solvent mixture of 60 parts by weight of phenol and 40 parts by weight of tetrachloroethane the viscosity of the pure solvent mixture and C the concentration of the polyester in 100 ml of the solvent mixture of at least 0.8. 3. Crystalline film according to Claims 1 and 2, characterized in that that it consists primarily of polytetramethylene terephthalate. 4. Crystalline film according to claim 1, characterized in that it has a heat distortion temperature of at least 100 ⁰ C. 5. Crystalline film according to claim 1, characterized in that it has an inherent viscosity of at least 1.1. 109817/2272 6. A crystalline film according to claim 1, characterized in that it mm possesses a tensile strength according to Elmendorf of at least 200 g / 0.0254 at about 23 ⁰ C. 7. Crystalline film according to claim 1, characterized in that the recurring glycol residues of the polyester up to 15 mol-I consist of ethylene glycol. 8. Crystalline film according to claim 1, characterized in that the recurring acid residues up to 15 MoI-! from isophthalic acid residues exist. 9. Crystalline film according to claim 1, characterized in that it has a haze of less than 1.0 l. 10. A method for producing a crystalline film according to claims 1 to 9, characterized in that the fine melt of a polyester whose repeating acid residues are at least 85 mol-I of terephthalic acid residues and their recurring glycol residues to at least 85 MoI-I consist of 1,4-butanediol residues, to one Film is extruded and the extruded, crystalline film is quenched at a temperature which is below the temperature fog of more than 1.0 l occurs in the film. 11. The method according to claim 10, characterized in that the quenching ^{temperature is at least 6 ° C, preferably at least 11 0} C below the temperature at which a fog of more than 1.0 I occurs in the film. 109817/2 2 72