GB1579681A - Stabilized thermoplastic polyester compositions - Google Patents

Description

(54) STABILIZED THERMOPLASTIC POLYESTER COMPOSITIONS (71) We, BASF AKTIENGESELLSCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following Statement: The present invention relates to a stabilized thermoplastic polyester molding composition.

Linear thermoplastic polyesters, e.g. polyethylene terephthalate and polybutylene terephthalate, can be processed to give shaped articles by injection molding, extrusion, film extrusion, blow-molding, extrusion-blowing and injection blow stretching. The fact that during melt processing these polyesters undergo thermal degradation has been disclosed.

This degradation lowers the molecular weight and hence the toughness of the materials, thereby lowering the quality of the shaped article produced. This thermal degradation takes place more rapidly with polybutylene terephthalate than with polyethylene terephthalate.

The degree of thermal degradation increases with increasing processing temperature and increasing dwell time. However, in many cases it is desirable to use high processing temperatures since this gives a low melt viscosity and hence better flow. This is particularly important if, for example, articles of complicated shape, with long flow paths and thin walls, are to be manufactured by injection molding.

In addition to the thermal degradation observed when processing polyester compositions, a thermo-oxidative degradation is observed if shaped articles made from polyesters are exposed to temperatures above about 1200C in air for prolonged periods. Because of their partially crystalline structure, polyethylene terephthalate and polybutylene terephthalate are particularly suitable for injection molding. Their high melting points of 250"C and 222"C, respectively, allow them to be used above the glass transition temperatures of 80 and 55"C without an excessive decrease in the modulus of shear.

A large number of compounds which retard or stop the thermal degradation of linear polyesters are disclosed in the literature. Examples of conventional stabilizers are monocarbodiimides and polycarbodiimides, isocyanates, isothiocyanates or blocked monoisocyanates and polyisocyanates which liberate monoisocyanates or polyisocyanates at the high processing temperatures. Inter alia, sterically hindered phenols and alkylated aromatic phenylenediamines have been disclosed as stabilizers against thermo-oxidative degradation.

However, these conventional stabilizers suffer from various disadvantages: phenols and phenylenediamines provide virtually no stabilization against thermal degradation and in addition diamines cause discolorations. Stabilizers based on isocyanates are not safe from the point of view of factory hygiene, because of their toxicity, and can also cause discolorations.

Carbodiimides are relatively expensive products and rather ineffective as stabilizers against thermal degradation.

We have now found an alternative stabilizer effective to improve stability to sustained heat exposure and to provide stability during processing.

According to the present invention there is provided a stabilised thermoplastic polyester composition which contains, as a stabilizer, from 0.1 to 3% by weight of an ester of a polyhydric aromatic hydroxy compound in which each hydroxyl group is directly attached to an aromatic nucleus and an aliphatic carboxylic acid of 2 to 5 carbon atoms all the hydroxyl groups being esterified. Such esters retard both the thermal and the thermo-oxidative degradation of the linear thermoplastic polyesters.

The most important polyester for which this invention may be used is polybutylene terephthalate; others are polyethylene terephthalate and the corresponding copolyesters which may either contain other dicarboxylic acids, eg. isophthalic acid, naphthalenedicarboxylic acids or adipic acid, as dicarboxylic acid components in addition to terephthalic acid, or contain other diols, eg. 1,3-propanediol, 1,3-butanediol, neopentylglycol, 1,4dimethylolcyclohexane, 1,6-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol or tripropylene glycol or polyethers, eg. polyethylene oxides, polypropylene oxides or polytetrahydrofurans of molecular weights from 150 to 2,000,asdiocomponents in addition to 1,4-butanediol and/or ethylene glycol.

Examples of polyhydric aromatic hydroxy compounds are hydroquinone, pyrocatechol, resorcinol, pyrigallol or phloroglucinol. In addition, the polyhydric hydroxy compounds of condensed aromatic systems, eg. of naphthalene, phenanthrene or anthracene, are suitable starting materials for the stabilizers of the invention. Finally, the polyhydric aromatic hydroxy compounds may also be derived from the category of the diphenyls, diphenyl oxides and arylalkanes,eg. diphenyl, diphenyl oxide, diphenylmethane, triphenylmethane, triphenylchloromethane or diphenyldimethylmethane.

Suitable carboxylic acid components of the stabilizers of the invention are above all acetic acid and propionic acid.

The preferred stabilizer is hydroquinone diacetate. The stabilizers are employed in amounts of from 0.1 to 3%, prefcrably from 0.3 to 2%, by weight based on the weight of the molding composition.

Since the voltatility of the stabilizers proposed according to the invention is not negligible, the stabilizers, if added during the polyester condensation, are advantageously only added to the melt shortly before the polyester is discharged from the reaction apparatus. In a preferred process, the stabilizers are mixed with the polyester in a single-screw or twin-screw extruder.

If desired, they may be added conjointly with other additives, such as reinforcing fillers, eg.

glass fibers, glass beads, talc, chalk or kaolin, non-reinforcing fillers, flameproofing agents, flow assistants and mold release agents or other polymers, eg. polymers which improve the impact strength.

In another preferred process, a concentrate of more than 3%, preferably of from 5 to 15% by weight strength of the stabilizer proposed according to the invention in part of the polyester is prepared, and this concentrate is then added to the main part of the polyester to be stabilized, by means of more gentle mixing processes, eg. by using static mixing elements.

Accordingly the invention also provides a concentrate for addition to a thermoplastic polyester, comprising a thermoplastic polyester and more than 3% by weight of an ester stabiliser as defined above.

The Examples which follow illustrate the invention and its significance: In the Examples the compounds employed as stabilizers were admixed with polybutylene terephthalate, in the stated proportions, in a twinscrew extruder at 2400 C. The relative viscosity of the polybutylene terephthalate was 1.595 measured on an 0.5%strength solution, in a mixture of phenol and o-dichlorobenzenc in the weight ratio of 3:2, at 25"C. Standard test bars of size 4 x 6 x 50 mm were manufactured, under various conditions, from the granules manufactured above, using a screw injection molding machine.To test the stability during processing, these standard test bars were produccd at increasing melt temperatures of 240 , 2600, 280 and 290 , with otherwise constant machine conditions, and with a mold temperature of 60"C. The dccrease in the relative viscosity and in the impact strength of specimcns with a hole were used as a measure of the stability of processing. The impact strength of specimcns with a hole was used because it is more sensitive to molecular weight degradation than is the notched impact strength.

The impact strength of specimens with a hole was measured by a method based on DIN 52,453. A hole of 3 mm diameter was bored in the center of the 6 x 50 mm surface of the test bars, and a double notch was thus obtained. The specimen was then struck in a pendulum impact tester in the lengthwise direction of the hole and the impact strength was measured in kilojoule/m2. This test permits accurate differentiation between the impact strengths of different test specimcns.

To test the thermo-oxidative stability, standard test bars were injcction-molded at a melt temperature of 255"C under the conditions described above. The decrease in the impact strength of specimens with a hole, as a function of the storage time in a through-circulation dryer at 140"C, was used as a measure of the thermo-oxidative stability. The results are shown in Table 2.

Both on testing the processing stability and on testing the thermo-oxidative stability it is found that the polybutylene terephthalate compositions containing hydroquinone diacetate, the stabilizer according to the invention. behave substantially more advantageously than the comparative samples without such stabilizer.

TABLE 1 Test of processing stability by measuring the decrease in the relative viscosity # rel and in the impact strength of specimens with a hole, aL (measured in kilojoule per m2) as a function of the processing temprature T ( C) Example Hydroquinone #rel at T = aL at T = diacetate added 240 260 280 290 240 260 280 290 1 Comparative sample 1.590 1.582 1.510 1.490 49 45 20 7 2 0.3% 1.590 1.582 1.512 1.505 48 44 34 8 3 0.5% 1.590 1.585 1.515 1.510 49 44 37 9 4 1.0% 1.590 1.590 1.548 1.535 48 46 36 12 5 2.0% 1.600 1.595 1.595 1.590 49 45 38 17 4a 1.0% of 1.552 1.496 1.488 1.470 35 21 7 4 hydroquinone TABLE 2 Test of thermo-oxidative stability by measuring the decrease in the impact strenght of specimens with a hole, aL (measured in kilojoule per m2) as a function of the storage time in days at 104 C Example 6 7 8 9 10 9a %by weight of 0.3 0.5 1.0 2.0 1% of hydroquinone hydroquinone Comparative sample diacetate Storage time in days aL aL aL aL aL aL 0 42 39 40 39 40 28 3 27 40 40 40 40 18 6 36 39 41 39 40 10 10 33 37 37 37 38 8 14 27 34 35 38 37 6 17 22 31 35 36 37 6 22 18 30 33 35 37 5 30 16 30 31 33 35 38 12 26 27 21 33 50 10 24 25 30 33 65 8 20 25 27 30

Claims (7)

WHAT WE CLAIM IS:

1. A stabilized thermoplastic polyester molding composition which contains, as a stabilizer, from 0.1 to 3% by weight of an ester of a polyhydric aromatic hydroxy compound in which each hydroxyl group is directly attached to an aromatic nucleus and an aliphatic carboxylic acid of 2 to 5 carbon atoms, all the hydroxyl groups being esterified.

2. A composition as claimed in claim 1, in which the stabilizer is hydroquinone diacetate.

3. A composition as claimed in claim 1 or 2, in which the polyester is polybutylene terephthalate.

4. A composition as claimed in any of claims 1 to 3, which contains from 0.3 to 2% by weight of the ester used as a stabilizer.

5. A concentrate for addition to a thermoplastic polyester to form a composition as claimed in any of claims 1 to 4 comprising a thermoplastic polyester and more than 3 % by weight of an ester as defined in claim 1 or 2.

6. A stabilized thermoplastic polyester molding composition made up substantially as described in any of the foregoing Examples 2 to 5 or 7 to 10.

7. Moldings made from molding compositions as claimed in any of claims 1 to 6.