GB2036000A - Process for the preparation of polyhydric alcohol esters of 3- alkylthiopropionic acid - Google Patents

Abstract

A polyhydric alcohol 3- alkylthiopropionate polyolefin stabiliser capable of enhancing the resistance to deterioration and minimising the formation of undesirable colour and odour of a polyolefin resin when heated at 150 DEG C. and higher, is obtained by heating a reaction mixture containing at least one polyhydric alcohol having 3 to 6 alcoholic hydroxyl groups and 3 to 20 carbon atoms, the number of carbon atoms being at least equal to the number of alcoholic hydroxyl groups, and for each mole of alcoholic hydroxyl group supplied by the polyhydric alcohol from 0.6 to 1.5 moles of a beta - mercaptopropionic acid compound selected from beta -mercaptopropionic acid and the lower alkyl esters thereof and from 0.6 to 2 moles of at least one alpha -olefin having 6 to 40 carbon atoms, and an amount effective to initiate reaction of a reaction initiator selected from azonitriles and organic peroxides, and recovering the polyhydric alcohol 3- alkylthiopropionate polyolefin resin stabiliser from the mixture. Stabilised polyolefin resins are also described.

Description

SPECIFICATION Process for the preparation of polyhydric alcohol 3-alkylthiopropionate ester The invention is conerned with a new process for the preparation of polyhydric alcohol 3alkylthiopropionate ester stabilisers for polyolefin resins.

Polyhydric alcohol 3-alkylthiopropionate ester stabilisers have been disclosed previously but their preparation has conventionally required the use of relatively expensive starting materials and given products which, when used to stabilise polyolefin resins, have contributed to the formation of undesirable yellowish and beige discolorations, as well as unpleasant odours when the stabilised resins were processed or heat-aged at elevated temperatures.

Thioether carboxylic acid esters and their use for stabilising polymers have been disclosed by M.

Minagawa et awl., in U.S. Patent Specification No. 4,106,629 and, for the sake of brevity, it is sufficient here to refer to Minagawa's discussion of the art.

Trihydric and higher valent polyhydric alcohol 3-alkylthiopropionate ester stablisers have conventionally been prepared as disclosed, for example, by Dexter U.S. Patent Specification No.

3,758,549, by the reaction of a polyhydric alcohol, such as pentaerythritol, with methyl 3laurylthiopropionate by transesterification and subsequent purification of the product, such as by chromatography on a column of alumina. Dexter disclosed the preparation of the desired 3alkylthiopropionic acid ester intermediate by reaction of the alkyl mercaptan with methyl acrylate and purification of the intermediate by high vacuum distillation, so that Dexter's process comprises two synthesis steps and two purification steps, which present considerable difficulty in large scale operations.

Minagawa, in Japanese Kokai No. 75/106881, disclosed stabilised resin compositions containing 3-alkylthiopropionate esters of alcohols containing a nitrogen-hete;ocyclic ring, for example tris-(2hydroxyethyl isocyanurate) and optionally a phenolic antioxidant.

Minagawa's esters are described as being synthesised by esterification of the nitrogenheterocyclic alcohol with a 3-alkylthiopropionic acid, without saying how the required 3alkylthiopropionic acid intermediates were to be obtained.

Disclosures of the preparation of 3-alkylthiopropionic acids, their lower alkyl esters and their metal salts include Gribbins' U.S. Patent Specification No. 2,416,052, according to which mercaptans are added to acrylonitrile, followed by acid or alkaline hydrolysis of the nitrile group; Leistner's U.S. Patent Specifications Nos. 2,680,107 and 2,723,965, according to which an alkyl mercaptan is added to methyl acrylate, using sodium mercaptan as a catalyst, the resulting methyl ester is hydrolysed to the sodium salt of the acid by heating with sodium hydroxide, followed by acidification to isolate the acid; and Yamane's U.S.Patent Specification No. 3,741,909, which discloses stable organic materials prepared by mixing with a compound of the general formula:

wherein R is a hydrocarbon radical containing 5 to 22 carbon atoms, X is an alkylene radical containing up to 5 carbon atoms, M is a non-alkali metal and n is an integer of from 1 to 5, with a phenol-type or amine type antioxidant.

This compound is synthesised by the following process: UV irradiation R.CH=CH2 + HSCH2COOH

50-700C: 1.5 hrs. R.CH2.CH2.S.CH2.COOH

R.CH2.CH2.S.CH2.COONa

(R.CH2.CH2S.CH2CO0)2Ca An object of the present invention is to provided a simplified, one-step process for preparing a polyhydric alcohol 3-alkylthiopropionic acid ester polyolefin resin stabiliser capable of stabilising the resin against oxidative degradation for a long period of time.

Another object of the present invention is to provided a polyhydric alcohol 3-alkylthiopropionic acid ester having a high polyolefin resin-stabilising effect with no need of undergoing extensive or repeated purification steps.

Thus, according to the present invention, there is provided a process for preparing a polyhydric alcohol 3-alkylthiopropionate polyolefin resin stabiliser capable of enhancing the resistance to deterioration and minimising the formation of undesirable colour and odour of a polyolefin resin when heated at 1 500C. and higher, comprising heating a reaction mixture containing at least one polyhydric alcohol having 3 to 6 alcoholic hydroxyl groups and 3 to 20 carbon atoms, the number of carbon atoms being at least equal to the number of alcoholic hydroxyl groups, and for each mole of alcoholic hydroxyl group supplied by the polyhydric alcohol from 0.6 to 1.5 moles of a s-mercaptopropionic acid compound selected from ,B-mercaptopropionic acid and the lower alkyl esters thereof and from 0.6 to 2 moles of at least one or-olefin having 6 to 40 carbon atoms, and an amount effective to initiate reaction of a reaction initiator selected from azonitrile and organic peroxides, and recovering the polyhydric alcohol 3-alkylthiopropionate polyolefin resin stabiliser from the reaction mixture.

A preferred technique for recovering the stabiliser from the reaction mixture involves the use of a combination of solvents comprising an aromatic hydrocarbon boiling in the range of 80-1 900C. and a lower aliphatic alcohol, from which the stabiliser having the desired properties is obtained in excellent yield.

Polyolefin resins stabilised with the polyhydric alcohol 3-alkylthiopropionates prepared according to the present invention have an excellent heat stability and a retention of good colour and odour properties and include homopolymers and copolymers of a-olefins having 2 to 6 carbon atoms, especially polypropylene and polyethylene.

Excellent stabiliser compositions for polyolefin resins comprise the stabiliser prepared according to the present invention, together with at least one known polyolefine stabiliser, such as a phenol and/or an alkaline earth metal salt of a monocarboxylic acid having 6 to 24 carbon atoms. Such stabliser compositions typically contain from 10 to 95 parts by weight of polyhydric alcohol 3 alkylthiopropionate prepared according to the present invention, from 5 to 90 parts by weight of a phenol and from 0 to 50 parts by weight of an alkaline earth metal salt.

The a-olefin starting materials for the process according to the present invention all have the characteristic terminal =CH2 group, and the carbon atom adjacent the terminal =CH2 carries 1 or 2 alkyl radicals. Accordingly, the a-olefin can be represented by the general formula RR'C=CH2, in which R is an alkyl radical, R' is an alkyl radical or a hydrogen atom, and the sum of the number of carbon atoms in R and R' is from 4 to 38 and preferably from 8 to 28.Examples of a-olefins which can be used include hex-1-ene, 2-methylpent-1-ene, 4-methylpent-1-ene, hept-1-ene, oct-1-ene, 2- ethylhQx-lrene,,nofi-l -one, ec-1 -ene, 2,4,4-trimethylpent-1 -ene, dodec-1 -ene, hexadec-1-ene, eicos-1-ene, tetracos- 1 -ene and octacos- 1 -ene.

The ,B-mercaptopropionic acid compound can be represented by the general formula HSCH2CH2CO2R", wherein R" is a hydrogen atom or a lower alkyl radical such as i-butyl, n-butyl, sbutyl, t-butyl, ethyl, isopropyl, methyl and n-propyl.

Polyhydric alcohols used according to the present invention have 3 to 6 alcoholic hydroxyl groups and 3 to 20 carbon atoms, not more than one hydroxyl group being linked to any one carbon atoms.

Examples of polyhydric alcohols which can be used according to the present invention include trimethylolpropane, trimethylolethane, glycerol, tris-(2-hyd roxyethyl)-isocya nurate, pentaerythritol, ditrimethylolpropane, dipentaerythritol, mannitol, sorbitol, inositol and the like.

The reaction initiator (azonitrile or organic peroxide) is used in a concentration effective to initiate the reaction, for example 0.001 to 5 per cent by weight of the a-olefin and p-mercaptopropionic acid compound together and preferably 0.05 to 2 per cent. Larger amounts of reaction initiator can be used but are wasteful and uneconomical.

Preferred azonitrile initiators include 2,2'-azobis-(2-methylpropionitrile), 2,2'-azobis-(2- methylbutylronitrile), 2 ,2'-azobis-(2,4-dimethylvaleronitrile) and and 1,1 '-azobis-(1 -cyclohexanenitrile).

Preferred organic peroxides have 1 or 2 peroxide (-00-) group and 4 to 40 carbon atoms and include t-alkyl and aralkyl peroxides, such as t-butylhydroperoxide, cumyl-t-butyl peroxide, 2,5- dimethyl-2,5-di-(t-butylperoxy)hexane, di-t-butylperoxide and dicumyl peroxide; monoperesters, such as t-butyl peracetate, t-buty!pero,xyiaobutyrate, t-butylperbenzoate, t-butylperpivalate, t-butylper-2 ethylhexoate, t-butylperoxyneodecanoate, t-butylperlaurate and mono-t-butylperoxymaleic acid; diperesters, such as 2,5-bis-(2-ethylhexanoylperoxy)-2,5-dimethylhexane, di-t-butylperoxyphthalate and 2,5-bis-(benzoylperoxy)-2,5-dimethylhexane; aromatic diacyl peroxides, such as 2,4 dichlorobenzoylperoxide, benzoyl peroxide and o-toluoylperoxide; ketone peroxides, such as methyl ethyl ketone peroxide, cyclohexanone peroxide and 1,1-bis-(t-butylperoxy)-3,3,3-trimethylcyclohexane; and peroxycarbonate esters, such as di-sec-butylperoxydicarbonate, di-t butylcyclohexylperoxydica rbonate and t-butylpe roxyisopropyl carbonate.

A particularly preferred group of organic peroxide initiators consists of aliphatic diacyl peroxides, such as acetyl peroxide, acetylpropionyl peroxide, acetyl 2-ethylhexanoyl peroxide, 3,3,5- trimethylhexanoylperoxide, lay royal peroxide, octanoyl peroxide, decanoyl peroxide, stearoyl peroxide, propionyl hexacosanoyl peroxide and succinic acid peroxide.

In the reaction of the a-olefin with the /3-mercaptopropionic acid compound and polyhydric alcohol, the proportions of the reactants are one double bond of an a-olefin with one SH group of a ,3- mercaptopropionic acid compound and one carboxylic acid or ester group derived from a mercaptopropionic acid compound with one alcoholic hydroxyl group of a polyhydric alcohol.

In carrying out the reaction, the reactants can be mixed in these proportions but it is sometimes advantageous to use an excess of one reactant and, if desired, to remove and recover the unused portion of such reactant.

When the reactants are mixed in such proportions that there is a modest excess of p- mercaptopropionic acid compound relative to the amounts of a-olefin or a modest excess of polyhydric alcohol relative to the amount of P-mercaptopropionic acid compound, the product of the process of the present invention can include unobjectionable and sometimes even beneficial minor amouns of constituents having free mercapto groups and/or alcoholic hydroxyl groups, in addition to thioether and carboxylic ester groups.Accordingly, in carrying out the process of the present invention, the reactants can be mixed in proportions ranging from 0.6 to 1.5 moles of ,B-mercaptopropionic acid compound and 0.6 to 2 moles of cw-olefin per mole of alcoholic hydroxyl supplied by the polyhydric alcohol. A preferred range of proportions in which the reactants are mixed is from 0.9 to 1.1 moles of p-mercaptopropionic acid compound and 0.9 to 1.1 moles of a-olefin per mole of alcoholic hydroxyl group.

The reaction is preferably carried out at a temperature within the range of from 20 to 200cm. and more preferably of from 50 to I 500C.

Once initiated, the reaction of a polyhydric alcohol and c-olefin with a p-mercaptopropionic acid compound is exothermic. A convenient way to control the reaction is to warm the polyhydric alcohol and a-olefin and a quantity of the initiator to a temperature where reaction can be initiated, such as 40--600C., to remove the heat source and to add the s-mercaptopropionic acid compound at a rate such that the reaction is sustained until completed by the exothermic effect or by externally supplied heat, if necessary.

Another useful method is to premix the polyhydric alcohol, ,l-mercaptoprnpionic acid compound and c-olefin to 40--600C. and to add quantities of reaction initiator from time to time, with further heating as needed, until analysis shows that the consumption of the mercaptopropionic acid compound is complete. If desired, any unreacted starting materials can then be removed before isolating the desired product.

The reaction can be helped to completion by removal of the by-product water or lower alkanol, by the use of an esterification catalyst or by both of these expedients in combination. Water or lower alkanol produced during the esterification can be removed by distillation, assisted by the application of a vacuum, by sparging with a gas or by the use of a boiling inert solvent.

Esterification catalysts which can be used include organic sulphonic acids, inorganic acids, bases and multivalent metal and organo-metaliic compounds, for example sulphuric acid, hydrochloric acid, boric acid, ethanesulphonic acid, sodium methoxide, potassium bicarbonate, zinc chloride, aluminium, titanium and zirconium butoxides, dimethyltin dichloride, dibutyltin oxide and di-n-octyltin dioxide. As little as 0.001% of esterification catalyst is helpful and up to about 5% can be used by weight of the reactants.

The polyhydric alcohol 3-alkylthiopropionate produced by the process of the present invention can be isolated from the reaction mixture in which it is obtained by conventional techniques, such as stripping volatile reactants and by-products, preferably under reduced pressure, from the desired product; crystallisation from an appropriate solvent, directly or after stripping; or separation from undesired impurities by enriching the desired product in one of a pair of immiscible liquids, for example the solvent pair hexane and 80% aqueous methanol.

A particularly preferred and convenient method for recovering polyhydric alcohol 3alkylthiopropionate according to the present invention comprises the use of a solvent combination of an aromatic hydrocarbon having an atmospheric boiling point in the range of from 80 to 1 900C. and a lower aliphatic alcohol. Examples of aromatic hydrocarbons include benzene, toluene, ethylbenzene, o-, m- and p-xylene, cumene, --cymene, trimethylbenzenes, ethylmethylbenzenes, diethylbenzenes and mixtures thereof. Examples of lower alcohols include the isomeric butyl, ethyl, methyl and propyl alcohols and mixtures thereof. An especially preferred solvent combination comprises toluenes and methanol. The combined solvents can be used in admixture or sequentially, usually first the hydrocarbons and then the alcohol.

Examples of polyhydric alcohol 3-alkylthiopropionates prepared by a process of the present invention include the following: 1 ,3 ,5-Tris-(n-h exylthiopropionyloxyethyl)-i socya nu rate

1,3,5-Tris-(n-dodecylthiopropionyloxyethyl)-isocyanurate

1 ,3-Bis-(n-octadecylthiopropionyloxyethyl)-5-hydroxyethyl isocya nurate

Pentaerythritol tetra kis-(3-n-octylthiopropionate)

Pentaerythritol tetrakis-(3-n-dodecylthiopropionate)

Pentaeryth ritol tetrakis-(3-n-octad ecylthiopropionate)

Pentaeryth ritol tris-(3-n-docosanylthiopropionate)

Glyceryl tris-(n-decylthiopropionate)

Glyceryl mono-(3-n-hexadecylthiopropionate)

The following Examples are given for the purpose of illustrating the present invention: EXAMPLE 1 Synthesis of pentaerythritol tetrakis-(3-laurylthiopropionate).

40.3 g. (0.24 mole) Dodec-1-ene, 21.2 g. (0.2 mole) 3-mercaptopropionic acid, 6.8 g. (0.05 mole) pentaerythritol, 0.68 g. p-toluenesulphonic acid and 0.07 g. azobisisobutyronitrile were put into a flask and heated, with stirring, at 800C, in an atmosphere of nitrogen for 4 hours. The resulting water and excess dodec-1 -ene were distilled off in a vacuum. After cooling, toluene was added, and the solution washed with water and then with a 10% aqueous solution of sodium bicarbonate. After drying, the toluene was removed. 54.9 g. (yield 94.7%) of a white powder (m.p. 44-470C.) were obtained by adding methanol and collecting the crystalline product.

EXAMPLE 2 Synthesis of pentaerythritol tetra kis-(3-stea rylthiopropionate).

52.9 g. (0.21 mole) Octadec-l -ene, 21.2 g. (0.2 mole) 3-mercaptopropionic acid, 6.8 g. (0.05 mole) pentaerythritol, 0.75 g. p-toleuensulphonic acid and 0.75 g. lauryol peroxide were put into a flask and heated, with stirring, at 11 00C. for 5 hours. 75 ml. Toluene were added and the reaction mixture further heated, while stirring, at reflux temperature for 2 hours. After cooling, the reaction mixture was washed with water and with a 10% aqueous solution of a sodium bicarbonate. After drying, the solve'nt was removed under vacuum. 70.1 g. (yield 93.7%) of white powder (m.p. 64.0-66.00C.) were obtained by adding isopropanol and collecting the crystalline product.

EXAMPLE 3 Synthesis of tris-(2-hydroxyethyl)-isocyanurate tris-(3C 12-14 a alkylthiopropionate).

43.4 g. (0.24 mole) of mixed ct-olefin (C1214, Dialen 124; Mitsubishi Chemical), 24.0 g. (0.2 mole) methyl 3-mercaptopropionate, 17.5 g. (0.67 mole) tris-(2-hydroxyethyl) isocyanurate, 0.8 g. sodium methoxide and 0.08 g. azobis-isobutyronitrile were put into a flask and heated, with stirring, at 800 C. in an atmosphere of nitrogen for 3 hours and then at 1000C. for 5 hours. The resulting methanol and excess of -olefin were distilled off in a vacuum. After cooling, toluene was added and the mixture washed with water, dried and stripped of toluene. 57.2 g. (yield 91.2%) of a white powder (m.p.

67-690C.) were obtained by adding methanol, triturating and collecting the crystalline product.

EXAMPLE 4 Synthesis of pentaeryth ritol tetrakis-(3--C,,~,, alkylthiopropionate).

53 g. (0.5 mole) 3-Mercaptopropionic acid, 117.8 g. (0.51 mole) of mixed -olefin (C1618, Mitsubishi Dialen 168), 17.0 g. (0.125 mole) pentaerythritol, 1.9 g.p-toluene-sulphonic acid and 0.19 g. azobisisobutyronitrile were put into a flask and heated, with stirring, at 80 to 900 C. in an atmosphere of nitrogen for 5 hours. The resulting water was distilled off in a vacuum.After cooling, toluene was added, the mixture was washed with water and then with a 10% aqueous solution of sodium bicarbonate, dried and concentrated by distilling off the toluene.162.9 g. (yield 92.3%) of a white powder (m.p. 61 .5-540C.) were obtained by adding methanol to the highly concentrated residue and collecting the crystalline product.

EXAMPLE 5 Synthesis of tris-(2-hydroxyethylXisocyanurate tris-(3-stearylthiopropionate) 172.3 g. (0.33 mole) Octadec-1-ene, 31.8 g. (0.3 mole) 3-mercaptopropionic acid, 26.1 g. (0.1 mole) tris-(2-hydroxyethyl)isocyanurate, 0.23 g. azobisbutyronitrile and 2.3 g.p-toluenesulphonic acid were put into a flask and heated, with stirring, at 800C. in an atmosphere of nitrogen for 5.5 hours. The resulting water and excess of octadecene were distilled off in a vacuum. After cooling, adding toluene, washing with water, drying and removing the toluene, 11 8.2 g. (yield 92.3%) of a white powder (m.p.

74.5-76.50C.) were obtained by adding methanol to the residue and collecting the solid product.

EXAMPLE 6 Synthesis of pentaerythritol tetrakis-(3-C20~24 alkylthiopropionate).

63.2 g. (0.21 mole) of C20-24 a-olefin mixture, 6.8 g. (0.05 mole) pentaerythritol, 21.2 g. (0.2 mole) 3-mercaptopropionic acid, 0.9 g.p-toluene-sulphonic acid and 0.9 g. t-butyl peroxyneodecanoate were put into a flask and heated, with stirring, at 1 10--1 150C. in an atmosphere of nitrogen for 3 hours. 0.45 9. t-Butyl peroxyneodecanoate was added and the reaction continued for a further 3 hours.

After cooling, adding toluene, washing with water, drying and removing the solvent, 73.9 g. (yield 87.4%) of a white powder (m.p. 66--680C.) was obtained by recrystallisation from heptane.

COMPARATIVE EXAMPLE Synthesis of pentaerythritol tetrakis-(3-laurylthiopropionate) (of U.S. Patent Specification No.

3,758,549).

123 g. (0.6 mole) n-Lauryl mercaptan and 0.5 g. sodium methoxide were put into a flask and, while maintaining the temperature at 25 to 300C., 86.1 g. (1.0 mole) methyl acrylate were added dropwise over a period of approximately 50 minutes. After completion of the addition, the mixture was further stirred at 25 to 3O0C. for 17 hours and then vacuum distilled to give 122.3 g. (yield 70.8%) methyl /3-laurylthiopropionate. The distillation temperature was 1 52 to 1 53.50C./0.3 to 0.4 mm.Hg.

57.69. (0.2 mole) of the ester obtained, 6.89. (0.05 mole) pentaerythritol and 0.259. sodium methoxide were put into a flask and heated, with stirring, at 100--1 1 00 C. in an atmosphere of nitrogen for 7 hours. After cooling, toluene was added and the solution passed through a bed of alumina. The toluene was removed and 55.4 9. (yield 95.5%) of a white powder (m.p. 47--49 OC.) were obtained by adding methanol and collecting the crystalline product. Total yield: 67.6% for the two steps.

Polyolefin resins which can be stabilised with polyhydric alcohol 3-alkylthiopropionates prepared by a process of the present invention include polymers of olefins having 2 to 6 carbon atoms, such as polyethylene, polypropylene, polybut- 1 -ene, poly-3-methylbutene, poly-4-methylpentene, polyhex- 1 - ene and copolymers of these olefins, particularly copolymers of ethylene with propylene, but-1-ene and of hex-1 -ene, as well as blends of two or more of these polyolefins, and ar-olefin copolymers, such as ethylene-vinyl acetate copolymers, ethylene-ethyl acrylate copolymers and ethylene-propylene-diene terpolymers.They also include olefin resins and copolymers cross-linked by heating with a peroxide or by exposure to ionising radiation and foamed polyolefins which are foamed by a blowing agent.

Stabiliser compositions comprising a polyhydric alcohol 3-alkylthiopropionate prepared by a process of the present invention, together with a polyolefin resin stabiliser, such as a phenol, an organic phosphite, and/or an alkaline earth metal salt of a monocarboxylic acid having 6 to 24 carbon atoms can be formulated and marketed in liquid, solid and paste form. An inert solvent can be used to facilitate handling. The components can also be formulated as a uniform mixture by heating together, until a homogeneous melt is formed, at temperatures up to about 1 600C. for 1 to 4 or more hours, if necessary, and allowed to solidify and comminuted by grinding or flaking.

A stabiliser prepared according to the present invention can be added to the polyolefin resins, such as mentioned above, to improve their oxidative stability in an amount of from 0.01 to 5 weight parts, preferably 0.05 to 3 weight parts, per 100 weight parts of the resins.

Incorporation of a phenol in the stabiliser composition of the present invention produces an excellent synergistic effect, enhancing the effectiveness of the polyhydric alcohol 3-alkylthiopropionate stabiliser.

Examples of phenols include 2,6-di-t-butyl-p-cresol, 1 ,1 ,3-tris-(2-methyl-4-hydroxy-5-t- butylphenyl)-butane, n-octadecyl-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate and 1,3,5-tris-(3',5'di-t-butyl-4'-hydroxybenzyl)-isocyanurate. A comprehensive disclosure of phenols is given in U.S.

Patent Specification No. 3,997,551.

Examples of organic phosphites include triisodecyl phosphite, tri-n-octadecyl phosphite, phenyl di2-ethylhexyl phosphite and di-n-octadecyl pentaerythritol diphosphite. A comprehensive disclosure of useful organic phosphites is also given in U.S. Patent Specification No. 3,997,551.

Examples of alkaline earth metal salts of monocarboxylic acids having 6 to 24 carbon atoms include calcium 2-ethylbutyrate, strontium caproate, barium benzoate, calcium-p-t-butylbenzoate, strontium laurate, barium myristate, calcium palmitate, strontium benhenate and barium linoleate.

Additional alkaline earth metal salts of monocarboxylic acids having 6 to 24 carbon atoms include those mentioned in U.S. Patent Specification No. 3,869,423. The concentration of the alkaline earth metal salt, based on 100 parts of polyolefin resin, can be from 0 to about 1 part by weight.

The preparation of stabilised polyolefin resin compositions according to the present invention is easily accomplished by conventional procedures. A heated two-roll mill, for example, is a convenient compounding means for blending stabiliser compositions of the present invention with polyolefin resins.

EXAMPLE 7 100 parts by weight of unstabilised polypropylene resin (Profax (Registered Trade Mark) 6501), 0.2 parts by weight of calcium stearate, 0.1 part by weight pentaerythritol tetrakis-P-(3,5-di-t-butyl-4- hydroxyphenyl)-propionate and 0.3 parts by weight of sample compound were compounded by grinding and mixing for 10 minutes. The compounded mixture was kneaded on a two-roll mill at 1 800C. for 6 minutes to give a rough sheet and then a polished sheet of 1.0 mm. thickness was prepared by compression moulding at 1800 C. and 200 kg/cm2 for 5 minutes. Test pieces of 10 x 20 mm. were cut off from this sheet and a heat-ageing test was carried out on aluminium foil in a Geer oven at 1 600C. in an air atmosphere.The colour of the sheets after 64 hours exposure to fluorescent light was also measured as a yellowness fraction by means of a Hunter Colorimeter. The results are shown in the following Table:- TABLE

160"C. Oven beginning 64 hour time of light deterioration exposure No. Sample compound in hours yellowness CONTROL 7-1 Dilauryl thiodipropionate 680 0.16 7-2 Pentaerythritol tetrakis-(3-laurylthio- 1390 0.10 propionate) of comparative Example EXAMPLE 7-1 Pentaerythritol tetrakis-(3-laurylthio- 1420 0,08 propionate) of Example 1 7-2 Pentaerythri tol tetraki s-(3-steary I thio- 1370 0.10 propionate) of Example 2 7-3 T.H.E,I.C. tris-(3-alkylthiopropionate) 1330 0.10 of Example 3* 7-4 Pentaerythritol tetrakis-(3C,6~,s alkyl- 1400 0.09 thiopropionate) of Example 4 7-5 T.H.E.l .C. tris-(3-stearyl thi opropi onate) 1350 0.09 of Example 5* 76 Pentaerythritol tetrakis-(3-C204 alkylthio- 1340 0.10 propionate) of Example 6 *T.H.E.I.C. = tri s-(2-hydroxyethyl) isocyanurate.

The results of these tests show that polyhydric alcohol 3-alkylthiopropionate stabilisers prepared by a process of the present invention are far superior in effectiveness to standard commercial thiodipropionate and equal or better when compared with a polyhydric alcohol 3-alkylthiopropionate made by a conventional process of synthesis.

Claims (18)

1. A process for preparing a polyhydric alcohol 3-alkylthiopropionate polyolefin stabiliser capable of enhancing the resistance to deterioration and minimising the formation of undesirable colour and odour of a polyolefin resin when heated at 1 500 C. and higher, comprising the steps of heating a reaction mixture containing at least one polyhydric alcohol having 3 to 6 alcoholic hydroxyl groups and 3 to 20 carbon atoms, the number of carbon atoms being at least equal to the number of alcoholic hydroxyl groups, and for each mole of alcoholic hydroxyl group supplied by the polyhydric alcohol from 0.6 to 1.5 moles of a jB-mercaptopropionic acid compound selected from p-mercaptopropionic acid and the lower alkyl esters thereof and from 0.6 to 2 moles of at least one a-olefin having 6 to 40 carbon atoms, and an amount effective to initiate reaction of a reaction initiator selected from azonitriles and organic peroxides, and recovering the polyhydric alcohol 3-alkylthiopropionate polyolefin resin stabiliser from the mixture.

2. A process according to claim 1, wherein ,B-mercaptopropionic acid compound is /3mercaptopropionic acid.

3. A process according to claim 1, wherein the p-mercaptopropionic acid compound is methyl ,B- mercaptopropionate.

4. A process according to any of the preceding claims, wherein the reaction initiator is an aliphatic diacyl peroxide having 4 to 40 carbon atoms.

5. A process according to any of claims 1 to 3, wherein the reaction initiator is lauroyl peroxide.

6. A process according to any of claims 1 to 3, wherein the reaction initiator is 2,2'-azobis-(2methylpropionitrile).

7. A process according to any of the preceding claims, wherein the polyhydric alcohol is pentaerythritol.

8. A process according to any of claims 1 to 6, wherein the polyhydric alcohol is tris-(2hydroxyethyl)isocyanurate.

9. A process according to any of the preceding claims, wherein the reaction mixture contains, as an additional ingredient, a catalytically effective amount of an esterification catalyst selected from cation exchange resins, organic sulphonic acids, inorganic acids, organotin compounds and metal alkoxides.

10. A process according to any of claims 1 to 8, wherein the reaction mixture contains, as an added ingredient, a catalytically effective amount of a basic esterification catalyst.

11. A process according to any of the preceding claims, wherein the polyhydric alcohol 3alkylthiopropionate resin stabiliser is recovered by the use of a solvent combination comprising the aromatic hydrocarbon and a lower aliphatic alcohol.

12. A process according to claim 11, wherein the aromatic hydrocarbon is toluene and the aliphatic alcohol is methanol.

13. A process according to claim 1 for preparing a polyolefin stabiliser, substantially as hereinbefore described and exemplified.

1 4. A polyolefin resin stabiliser, whenever produced by the process according to any of claims 1 to 13.

1 5. A polyolefin resin, whenever stabilised with a stabiliser according to claim 14.

1 6. A stabilised polyolefin resin according to claim 15, wherein the stabiliser is used in an amount of from 0.01 to 5 parts by weight per 100 parts by weight of resin.

17. A stabilised polyolefin according to claim 16, wherein the stabiliser is used in an amount of from 0.05 to 3 parts by weight per 100 parts by weight of resin.

18. A stabilised polyolefin according to any of claims 1 5 to 1 7, wherein there is additionally present a phenol stabiliser and/or an organic phosphite and/or an alkaline earth metal salt of a monocarboxylic acid containing 6 to 24 carbon atoms.

1 9. Stabilised polyolefins according to claim 1 5, substantially as hereinbefore described and exemplified.