IE54474B1 - Stabilised compositions based on alpha-olefin polymers - Google Patents

Abstract

1. Compositions which are stabilised against oxidative degradation and are based on alpha-olefine polymers containing a phenolic primary antioxidant and belonging to the group of the alkylated mono-, bis- and thiobisphenols, characterised in that they additionally comprise a secondary antioxidant chosen from among phenolic polyesters derived from a polyfunctional aliphatic or aromatic carboxylic acid and an alkylated polyphenol.

Description

The present invention relates to stabilised compositions based on alpha-olefin polymers. It relates more particularly to compositions based on ethylene polymers and stabilised towards oxidative degradation by a phenolic primary antioxidant.

It is known that all polyolefins have the disadvantage of degrading as a result of oxidation phenomena, which are acclerated under the effect of various factors and more particularly during the heat treatments involved 1C in their processing. This degradation leads to an increase in the colouration and a reduction in the mechanical properties of the articles made from these polyolefins.

To overcome this disadvantage, stabilisers called primary antioxidants are incorporated into the polymer; these are most frequently phenolic antioxidants chosen from amongst alkylated monophenols, bisphenols and thiobisphenols, which effectively oppose the thermal degradation of the polymer during its processing. However, these primary antioxidants frequently have the disadvantage of being too volatile and of rapidly being eliminated from the polymer by evaporation during the manufacture of articles made from the stabilised compositions. It is for this reason that it has become conventional to use conjointly at least one secondary antioxidant which cannot be· eliminated by evaporation and which stabilises the polymer towards long-term oxidative degradation.

Conventional secondary antioxidants, which frequently act synergistically with the primary antioxidants, are, for example, thioesters, most frequently thiodipro30 pionic acid derivatives, and organic phosphites, most frequently aromatic phosphites.

Another frequent disadvantage of phenolic primary antioxidants, and more particularly of bisphenols, is the undesirable colourations which they impart to polyolefins.

One particular cause of this problem, to which no completely satisfactory solution has hitherto been found, is the formation of coloured complexes between these anti2 oxidants and the metal derivatives originating from the catalysts used for the polymerisation and remaining in the polymer as residues, and the interaction of these antioxidants with other conventional additives for polyolefins, such as antistatic agents.

The aim of the present Invention is to provide new stabilised compositions based on alpha-olefin polymers, which contain a phenolic primary antioxidant and which nevertheless no longer have the disadvantage mentioned above.

For this purpose the invention relates to compositions which are stabilised against oxidative degradation and based on alpha-olefin polymers containing phenolic primary antioxidant belonging to the group of alkylated mono-, bis- and thiobisphenols, wherein they additionally comprise a secondary antioxidant chosen from amongst phenolic polyesters derived from a polyfunctional aliphatic or aromatic carboxylic acid and an alkylated polyphenol.

The phenolic primary antioxidant used in the compositions according to the invention belongs to the group comprising alkylated monophenols, bisphenols and thiobisphenols. Examples of alkylated monophenols which may be mentioned are 2,6-di-tert.-butyl-p-cresol and n-octadecyl (3',5'-di20 tert.-butyl-4-hydroxyphenyl)-propionate. Examples of alkylated bisphenols which may be mentioned are 2,2'-methylene-bis-(4-methyl-6tert.-butylphenol) and di-(2,6-di-tert.-butyl-4-methylphenyl)pentaerythritol. Examples of alkylated thiobisphenols which may he mentioned are 4,4'-thio-bis-6-tert.-butyl-m-eresol, 4,4'-thio-bis25 6-tert.-butyl-o-cresol and thiodiethylene-bis-(3,5-dl-tert.-butyl-4hydroxy)-hydrocinnamate. Amongst the phenolic antioxidants listed above, the alkylated bisphenols are preferred and, amongst these, particular preference is given to 2,2,-methylene-bis-(4-methyl-6-tert.-butylphenol), which combines a moderate volatility with good stabilising properties In the processing of the polymer. However, the latter is also the phenolic primary antioxidant which, when used by itself, imparts the highest degree of colouration to polyolefines. Of course, the use of several different phenolic primary antioxidants also falls within the scope of the invention. 3. 5447 4 The secondary antioxidant used in the compositions according to the invention is chosen from amongst phenolic polyesters. The term phenolic polyesters is understood as denoting compounds derived from a polyfunctional ali— phatic or aromatic carboxylic acid and from an alkylated polyphenol. The polyfunctional carboxylic acid is preferably an aromatic dicarboxylic acid' such as terephthalie acid. The alkylated phenol is preferably an alkylated bi spheno l. 2,2'-Methylene-bi s-(4-methyl-6-tert.-butyl10 phenol) terephthaLate is very particularly preferred. It is generally obtained by reacting terephthaloyl dichloride with an excess of 2,2'-methylene-bis~(4-methyl-6-tert.-butylphenol) .

According to a preferred embodiment of the invention, the stabilised compositions according to the invention also comprise a tertiary antioxidant chosen from'amongst thioesters and organic phosphites. Thioesters which can be used are, on the one hand, aliphatic esters of thiodipropiom'c acid, and, on the other hand, esters derived from aliphatic polyols and from alkyIthiopropionic acids. Examples which may be mentioned of aliphatic esters of thiodipropionic acid are dilauryl, distearyl and dimyristyl thiodipropionates. Examples which may be mentioned of esters derived from aliphatic polyols and from alkylthio25 dipropionic acids are octamethylene-bis-(3-dodecylthiopropionate), bis-(ethylene-3-octadecylthiopropionate) sulphide and pentaerythritol tetrakis-(3-thiododecylpropionate).

Organic phosphites which can be used as tertiary antioxidants are, on the one hand, trialkyl and trialkyl30 aryl phosphites, and, on the other hand, cyclic diphosphites derived from pentaerythritol. Examples which may be mentioned of trialkyl and trialkylaryl phosphites are trinonyl, tri-(nonyIphenyI) and tri-C(2,4-di-tert.-butyl5-methyl)-pheny13 phosphites. An example which may be mentioned of a cyclic diphosphite derived from pentaerythritol is distearylpentaerythritol diphosphite.

Amongst all the tertiary antioxidants listed above, the esters derived from aliphatic polyols and from alkyl4 thiodipropionic acids are preferred, and, amongst these, more particular preference is given to pentaerythritol tetrakis-(3-thiododecylpropionate).

According to another preferred embodiment of fclie in5 vention, the stabilised compositions according to the invention comprise, in addition to the antioxidants defined above, a stabilising compound which is the calcium salt of a saturated aliphatic monocarboxylic acid. Examples of such salts which may be mentioned are the stearate, palmitate and 2-ethylhexanoate. The preferred salt is calcium stearate because it contributes most effectively towards opposing the production of colourations in the stabilised compositions according to the invention. The primary, secondary and tertiary antioxidants, together with the stabilising compound if appropriate, are used in the customary stabilising amounts. These amounts are usually between 0.001 and 10% by weight of the alpha-οlefin potymers forming part of the composition, preferably between 0.005 and 5% and more particularly between 0.01 and 0.5%.

The alpha-olefin polymers forming part of the compositions according to the invention are polymers containing at least 50 mol % and preferably 75 mol % of olefins with terminal unsaturation, which contain from 2 to 18 and preferably from 2 to 6 carbon atoms in their mole25 cule, such as ethylene, propylene, but-1-ene, pent-1-ene, methylbut-1-enes, hex-1-ene, 3-methylpent-1-ene, 4-methylpent-1-ene and vinyI cyclohexene. The polymers are preferably homopolymers of such alpha-olefins or copolymers of these alpha-olefins with one another or with diole30 fins containing from 4 to 18 carbon atoms, such as unconjugated aliphatic diolefinSj for example hexa-1,4-diene, or such as alicyclic di olefins having an endocyclic bridge, for example dicyclopentadiene. The best results are obtained with the polymers of this type which contain at least 90 mol % and preferably at least 95 mol % of ethylene.

The stabilised compositions according to the invention can also be based on mixtures of two or more polymers such as described above.

The polymers which can be used can be prepared by the known methods for the polymerisation of alpha-olefins, and in particular by the low-pressure processes. In pai— fi cular, the ethylene polymers can be prepared in the presence of superactive catalyst systems based on organometallic compounds and on constituents resulting from the chemical reaction of magnesium compounds with titanium eompou.nd,j.

Thanks to the use of these superactive catalyst systems, the polymers obtained generally contain less than 100 ppm (parts per million) and frequently even less than 50 ppm of titanium, in the form of derivatives originating from these constituents, without having had to undergo a special purification treatment after poLymerisation.

In addition to the polymer, the primary, secondary and, if appropriate, tertiary antioxidants and also the stabilising compound if appropriate, the stabilised compositions according to the invention can contain other customary additives such as, for example, other antioxidants such as tetrakisphenols, amines, other polymers, reinfoi— cing agents, pigments, lubricants, fillers, antistatic agents and the like.

The primary and secondary antioxidants and also the other additives can be incorporated into the polymer in any manner which is in itself known, for example by impregnating the polymer, in the form of powder or granules, with a solution of the antioxidants, or alternatively by mixing a solution or suspension of the polymer with a solution of the antioxidants. The incorporation can be effected, for example, either in a mixer or mill from which the solvent is evaporated off, or by malaxation on heated rollers or by extrusion of a mixture of the polymer and the stabilising composition.

The stabilised compositions according to the invention are suitable for processing by any of the conventional plastics-converting processes, and more particularly by extrusion. These compositions are suitable for the manufacture of sheets, films, narrow strips, fibres and filaments, injection mouldings and hollow bodies, and more particularly tubes and pipes.

In addition to their excellent yellowing resistance, the stabilised compositions according to the invention have an excellent resistance to Long-term oxidative degradation, making it possible for the objects made therefrsrr. t; feature and retain an excellent resistance to cracking under tension.

The invention is illustrated by the example below.

Examp le A stabilised composition is prepared by dry-mixing the following constituents: - 100 parts by weight of an ethylene polymer marketed by the socidte anonyme SOLVAY 8 Cie under the name ELTEX B 4002 P; 0.01 part by weight of 2,2’-methylene-bis-(4methy l-6-tert.-butyLphenoI): marketed by the Soci6te Fran^aise d'Organosynthese under the name HBP 5; 0.02 part by weight of 2,2*-methylene-bis"(4methyl-tert.-butylphenol) terephthalate: marketed by the Soei4t6 Franjaise d’Organosynthese under the name HPH 12; 0.03 part by weight of pentaerythritol tetrakis(3-thiododecylpropionate): h2"^^2"i'0®-ch2’^4c marketed by the firm Shipro Kasei Kaisha under the name SEEHOX 412 S; - 0.2 part by weight of calcium stearate.

This composition (referred to as composition 1 below) is converted to granules at 220°C in a singlescrew extruder under protection by nitrogen.

To assess the degree of colouration (yellowing) of composition 1 as a result of its conversion to granules, the granules of this composition were subjected to a measurement of their yellow index (according to ASTM Standard Specification D 1925-70T).

The results of these measurements are shown in the table below, which also collects, by way of comparison, the results of the measurements of the yellow index of the polymer without additives (non-granular); - of the yellow index of the polymer treated with only 0.01% by weight of the product MSP 5 (composition 2R); and of the yellow index of the polymer treated with 0.01% by weight of the product MEP 5, 0.03% by weight of the product SEENOX 412 S and 0.2% by weight of calcium stearate (composition 3R).

TABLE COMPOSITION POLYMER WITHOUT ADDITIVES COMPOSITION 1 (invention) COMPOSITION 2R (comparison) COMPOSITION 3r (comparison) yellow index -9.7 -8 +11.1 -6.6 These results show that only the composition according to the 25 invention (composition 1) is capable of retaining a colouration virtually identical to its initial colouration; a composition containing only the product MBP 5 as the phenolic primary antioxidant (composition 2R) yellowed extensively after the granulation step; finally, a composition not containing phenolic polyester as the secondary antioxidant according to the invention (composition 3R) is also incapable of keeping its initial colouration.

Claims (11)

1. Compositions which are stabilised against oxidative degradation and are based on alpha-olefin polymers containing a phenolic primary antioxidant and belonging to the group of the alkylated mono-, his- and 5 thiobisphenols, wherein they additionally comprise a secondary antioxidant chosen from among phenolic polyesters derived from a polyfunctional aliphatic or aromatic carboxylic acid and an alkylated polyphenol.

2. Stabilised compositions according to claim 1, wherein the secondary antioxidant is chosen from among the phenolic polyesters derived from an 10 aromatic dicarboxylic acid and an alkylated bisphenol.

3. Stabilised compositions according to claim 2, wherein the secondary antioxidant is 2,2’-methylene-bis(4-methyl-6-tert.-butylphenol) terephthalate.

4. Stabilised compositions according to any one of claims 1 to 3, 15 wherein they further comprise a tertiary antioxidant chosen from among the thioesters and the organic phosphites.

5. Stabilised compositions according to claim 4, wherein the tertiary antioxidant is pentaerythritol tetrakis-(3-thio-dodecylpropionate).

6. Stabilised compositions according to any one of claime 1 to 5, 20 wherein they further comprise a calcium salt of a saturated monocarboxylic aliphatic acid.

7. Stabilised compositions according to claim 6, wherein the calcium salt of the saturated monocarboxylic aliphatic acid is calcium stearate.

8. Stabilised compositions according to any one of claims 1 to 7, 25 wherein the phenolic primary antioxidant is a bisphenol.

9. Stabilised compositions according to claim 8, wherein the phenolic primary antioxidant is 2,2’-methylene-bis(4-methyl-6-tert.-butylphenol).

10. Stabilised compositions according to any one of claims 1 to 9, wherein the alpha-olefin polymer is a polymer containing at least 90 mole 30 % of ethylene.

11. Compositions which are stabilised against oxidative degradation and are based on alpha-olefin polymers substantially as described herein with reference to the Example.