Stabilizer for synthetic resins

Abstract

A stabilizer for synthetic resins containing as an effective component a mixture comprising a phenol type compound (I) represented by the formula (I), ##STR1## wherein R.sub.1 represents a C.sub.1 -C.sub.3 alkyl group, and at least one sulfur type compound (II) selected from the group consisting of compounds represented by the formula (II-1), ##STR2## wherein R.sub.2 represents a C.sub.4 -C.sub.20 alkyl group, and compounds represented by the formula (II-2), ##STR3## wherein R.sub.3 represents a C.sub.3 -C.sub.18 alkyl group, and R.sub.4 and R.sub.5 independently represent a hydrogen atom or a C.sub.1 -C.sub.6 alkyl group, the weight ratio of phenol type compound (I) to sulfur type one (II) being 1 to 0.5-15.

Description

The present invention relates to a stabilizer for synthetic resins giving excellent stability to synthetic resins.

Various kinds of synthetic resin such as polyolefins (e.g. polyethylene, polypropylene), styrene type synthetic resins (e.g. polystyrene, impact-resistant polystyrene, ABS), engineering plastics (e.g. polyacetal, polyamide), polyurethane, etc. are widely used in various fields. It is however well known that when these synthetic resins are used alone, their stability becomes a problem, for example they deteriorate by the action of heat, light and oxygen on processing or use, thereby showing a remarkable reduction in mechanical properties accompanied by phenomena such as softening, embrittlement, surface crack, discoloration and the like.

It is also hitherto well known that, in order to solve this problem, various kinds of phenol, phosphite and sulfur type antioxidants are added or used in the course of production and processing of synthetic resins. For example, it is well known that phenol type antitoxidants such as 2,6-di-tert-butyl-4-methylphenol, 2,2'-methylenebis(4-methyl-6-tert-butylphenol), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol), n-octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, pentaerythritorl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and the like are used alone, or these phenol type antioxidants are used in combination with phosphite type antioxidants such as tris(nonylphenyl)phosphite, distearyl pentaerythritol diphosphite and the like, or sulfur type antioxidants such as dilauryl thiodipropionate, dimyristyl thiodipropionate, distearyl thiodipropionate and the like.

But, this method is not yet well satisfactory in terms of thermal and oxidation stability, thermal discoloration resistance, sublimation and the like.

Also, a stabilizer comprising pentaerythritol tetrakis[3-(3,5-dialkyl-4-hydroxyphenyl)propionate] and pentaerythritol tetrakis(3-alkylthiopropionate) is proposed in Japanese Patent Application Kokai (Laid-open) No. 20337/1984. This stabilizer is fairly superior to the conventional ones, but it is not always well satisfactory in terms of the thermal and oxidation stability, thermal discoloration resistance, etc. so that there has been a demand for the appearance of stabilizers of higher performance.

The present inventors extensively studied to solve these problems, and as a result, found that a mixture comprising a particular phenol type and particular sulfur type compounds in a particular proportion gives synthetic resins excellent thermal and oxidation stability which can never be forecast from the conventional antioxidant-combination technique. The present inventors thus attained to the present invention.

An object of the present invention is to provide a stabilizer for synthetic resins containing as an effective component a mixture comprising a phenol type compound (I) represented by the formula (I), ##STR4## wherein R.sub.1 represents a C.sub.1 -C.sub.3 alkyl group, and at least one sulfur type compound (II) selected from the group consisting of compounds represented by the formula (II-1), ##STR5## wherein R.sub.2 represents a C.sub.4 -C.sub.20 alkyl group, and compounds represented by the formula (II-2), ##STR6## wherein R.sub.3 represents a C.sub.1 -C.sub.18 alkyl group, and R.sub.4 and R.sub.5 independently represent a hydrogen atom or C.sub.3 -C.sub.6 alkyl group, the weight ratio of phenol type compound (I) to sulfur type one (II) being 1 to 0.5-15.

The phenol type compound of the foregoing formula (I) used in the present invention can be produced by the common ester interchange reaction of 3,9-bis(2-hydroxy-1,1-dimethylethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane with 3-(3-alkyl-5-tert-butyl-4-hydroxyphenyl)propionic acid, its acid halide, acid anhydride or lower alkyl ester.

In the phenol type compound represented by the formula (I), R.sub.1 represents a methyl, ethyl or propyl group, but a methyl group is preferred in terms of the thermal and oxidation stability.

In the compounds represented by the formula (II-1), a substituent R.sub.2 is preferably a C.sub.6 -C.sub.18 alkyl group, most preferably a dodecyl group in terms of the thermal and oxidation stability.

Typical examples of such compound are shown in Table 1.

TABLE 1______________________________________ ##STR7##Compound No. R.sub.2______________________________________II-1-1 C.sub.6 H.sub.13II-1-2 C.sub.12 H.sub.25II-1-3 C.sub.18 H.sub.37______________________________________

In the compounds represented by the formula (II-2), a substituent R.sub.3 is preferably a C.sub.8 -C.sub.18 alkyl group and substituents R.sub.4 and R.sub.5 are preferably a hydrogen atom or a C.sub.1 -C.sub.3 alkyl group in terms of the thermal and oxidation stability.

Typical examples of such compound are shown in Table 2.

TABLE 2______________________________________ ##STR8##Compound No. R.sub.3 R.sub.4 R.sub.5______________________________________II-2-1 C.sub.8 H.sub.17 CH.sub.3 HII-2-2 C.sub.12 H.sub.25 C.sub.4 H.sub.9 HII-2-3 C.sub.12 H.sub.25 CH.sub.3 HII-2-4 C.sub.18 H.sub.37 CH.sub.3 HII-2-5 C.sub.12 H.sub.25 H HII-2-6 C.sub.18 H.sub.37 H CH.sub.3______________________________________

The stabilizer for synthetic resins of the present invention comprises, as described above, a mixture of a phenol type compound (I) represented by the foregoing formula (I) and at least one sulfur type compound (II) selected from compounds represented by the formulae (II-1) and (II-2), and the mixing ratio of phenol type compound (I) to sulfur type compound (II) is 1 to 0.5-15, preferably 1 to 1-10, more preferably 1 to 2-6 in weight ratio.

When the weight ratio of sulfur type compound (II) to phenol type one (I) is less than 0.5, the intended effect is difficult to sufficiently obtain. Even if the weight ratio exceeds 15, a sufficient effect enough to correspond to that ratio is difficult to obtain, which becomes also disadvantageous economically.

In using the stabilizer for synthetic resins of the present invention, the amount of the stabilizer blended with synthetic resins is generally 0.01 to 5 parts by weight, preferably 0.05 to 1 part by weight based on 100 parts by weight of the synthetic resin. In using the stabilizer, the phenol type compound (I) and sulfur type one (II) may separately be blended with the synthetic resin without mixing the both compounds in advance.

For blending the stabilizer with synthetic resins, the well-known apparatus and methods for incorporating stabilizers, pigments, fillers, etc. in synthetic resins may be used almost as such.

In applying the stabilizer for synthetic resins of the present invention, other additives such as ultraviolet absorbers, light stabilizers, antioxidants, metal deactivators, metal soaps, nucleating agents, lubricants, antistatic agents, flame retardants, pigments, fillers and the like may be used together with said stabilizer.

Particularly, the light fastness of synthetic resins can be improved by using ultraviolet absorbers, hindered amine type light stabilizers, etc. such as 2-hydroxy-4-methoxybenzophenone, 2-hydroxy-4-n-octoxybenzophenone, 2-(2-hydroxy-5-methylphenyl)benzotriazole, 2-(2-hydroxy-3-tert-butyl-5-methylphenyl)-5-chlorobenzotriazole, 2-(2-hydroxy-3,5-di-tert-butylphenyl)-5-chlorobenzotriazole, 2-(2-hydroxy-3,5-diamylphenyl)benzotriazole, [2,2'-thiobis(4-tert-octylphenolate)]-butylamine nickel complex, 2,2,6,6-tetramethyl-4-piperidyl benzoate, bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate, bis(1,2,2,6,6-pentamethyl-4-piperidyl) 2-(3,5-di-tert-butyl-4-hydroxybenzyl)-2-n-butylmalonate, 1-[2-{3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxy}ethyl]-4-[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxy]-2,2,6,6-tetramethylpiperidine, dimethyl succinate, 1-(2-hydroxyethyl)-4-hydroxy-2,2,6,6-tetramethylpiperidine polycondensation product, a reaction product of dihaloalkylene with N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)alkylenediamine, a reaction product of 2,6-dichloro-1,3,5-triazine with N,N'-bis(2,2,6,6-tetramethyl-4-piperidyl)alkylenediamine, etc. together with the stabilizer of the present invention.

Also, the color of synthetic resins can be improved by adding a phosphite type antioxidant. This antioxidant includes for example distearyl pentaerythritol diphosphite, tris(2,4-di-tert-butylphenyl)phosphite, tris(2-tert-butyl-4-methylphenyl)phosphite, bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, tetrakis(2,4-di-tert-butylphenyl)-4,4'-biphenylene diphosphite and the like.

Thus, by using the stabilizer for synthetic resins of the present invention, the stability of synthetic resins is markedly improved. As such synthetic resins, there are given for example low-density polyethylene, high-density polyethylene, linear low-density polyethylene, chlorinated polyethylene, EVA resin, polypropylene, polyvinyl chloride, methacrylic resin, polystyrene, impact-resistant polystyrene, ABS resin, AES resin, MBS resin, polyethylene terephthalate, polybutylene terephthalate, polyamide, polyimide, polycarbonate, polyacetal, polyurethane, unsaturated polyester resin and the like. The stabilizer of the present invention is particularly effective on polypropylene.

Next, the present invention will be illustrated in detail with reference to the following examples, which are not however to be interpreted as limiting the invention thereto.

PRODUCTION EXAMPLE 1

(Production of the compound I-1)

0.82 Gram (0.015 mole) of sodium methoxide was added to a mixture of 30.2 g (0.121 mole) of methyl 3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate and 15.3 g (0.0503 mole) of 3,9-bis(2-hydroxy-1,1-dimethylethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane, and the whole mixture was heated. After carrying out reaction at 140.degree. C. for 3 hours, the reaction was continued for further 7 hours at 140.degree. to 160.degree. C. under a reduced pressure of 50 mmHg.

After completion of the reaction, the reaction product was dissolved in toluene, and the toluene solution was washed with a dilute aqueous hydrochloric acid and then with water. Thereafter, toluene was removed by evaporation under reduced pressure to obtain 42.9 g of a crude product of 77% in purity. The product was purified by column chromatography on silica gel to obtain 24 g of a 99%-purity, colorless and glassy product, 3,9-bis[2-{3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (compound I-1).

m.p. 45.degree.-55.degree. C.

Elementary analysis (for C.sub.43 H.sub.64 O.sub.10): Found: C, 69.61% H, 8.98%. Calculated: C, 69.70%; H, 8.71%.

Mass analytical value (FD-mass): Molecular ion peak 740.

Proton NMR (CDCl.sub.3 /TMS): .delta.(ppm) 0.92(12H s), 1.40(18H s), 2.21(6H s), 2.7(8H m), 3.4(8H m), 3.93(4H s), 4.17(2H s), 4.67(2H broad s), 6.85(2H board s), 6.96(2H broad s).

PRODUCTION EXAMPLE 2

(Compound I-2)

A mixture of 33.6 g (0.121 mole) of methyl 3-(3-tert-butyl-4-hydroxy-5-isopropylphenyl)propionate, 15.3 g (0.0503 mole of 3,9-bis(2-hydroxy-1,1-dimethylethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane and 0.82 g (0.015 mole) of sodium methoxide was reacted, after-treated and purified in the same manner as in Production example 1 to obtain 29 g of a 98%-purity, colorless and glassy product, 3,9-bis[2-{3-3-tert-butyl-4-hydroxy-5-isopropylphenyl)proprionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (Compound I-2).

m.p. 47.degree.-57.degree. C.

Elementary analysis (for C.sub.47 H.sub.72 O.sub.10): Found: C, 70.93%; H, 9.41%. Calculated: C, 70.82%; H, 9.10%.

Mass analytical value (FD-mass): Molecular ion peak 796.

Proton NMR (CDCl.sub.3 /TMS): .delta.(ppm) 0.91(12H s), 1.26(12H d), 1.40(18H s), 2.7(10H m), 3.4(8H m), 3.92(4H s), 4.19(2H s), 4.73(2H broad s), 6.91(2H broad s), 6.93(2H broad s).

PRODUCTION EXAMPLE 3

(Compound AO-1)

A mixture of 35.3 g (0.121 mole) of methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, 15.3 g (0.0503 mole of 3,9-bis(2-hydroxy-1,1-dimethylethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane and 0.82 g (0.015 mole) of sodium methoxide was reacted, after-treated and purified in the same manner as in Production example 1 to obtain 21 g of a 99%-purity white crystal, 3,9-bis[2-{3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane (Compound AO-1).

m.p. 98.degree.-100.degree. C.

Elementary analysis (for C.sub.49 H.sub.76 O.sub.10): Found C, 71.76%; H, 9.63%. Calculated C, 71.33%; H, 9.28%.

Mass analytical value (FD-mass): Molecular ion peak 824.

Proton NMR (CDCl.sub.3 /TMS): .delta.(ppm) 0.91(12H s), 1.42(36H s), 2.7(8H m), 3.4(8H m), 3.93(4H s), 4.21(2H s), 5.05(2H broad s), 6.97(4H broad s).

EXAMPLE 1

The following blend was mixed on a mixer for 5 minutes and then melt-kneaded at 180.degree. C. on a mixing roll to obtain a compound. This compound was formed into a sheet of 1 mm in thickness on a hot press kept at 210.degree. C. to prepare a test piece of 40.times.40.times.1 mm in size. This test piece was placed in a Geer oven kept at 160.degree. C. and measured for a period of time required for 30% of its area to become brittle. This period of time was taken as a thermal brittleness induction period and used for evaluation of the thermal and oxidation stability.

______________________________________Compounding: Part by weight______________________________________Unstabilized polypropylene resin 100Calcium stearate 0.1Test compound variable______________________________________

Also, a test piece prepared in the same manner as above was tested for thermal discoloration in a Geer oven kept at 160.degree. C. The thermal discoloration was evaluated by a color difference, .DELTA.YI, between the test piece above and the antioxidant-free test piece before the thermal ageing. The result is shown in Table 4.

Hereupon, the symbols of test compound in Table 4 show the following compounds:

TABLE 3__________________________________________________________________________ ##STR9##Compound No. X__________________________________________________________________________I-1 ##STR10##I-2 ##STR11##AO-1 ##STR12##AO-2: Pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)propionate]AO-3: Pentaerythritol tetrakis[3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionate]AO-4: 1,3,5-Tris(2,6-dimethyl-4-tert-butyl-3-hydroxybenzyl)-isocyanulateAO-5: Dilauryl thiodipropionateAO-6: Distearyl thiodipropionate__________________________________________________________________________ TABLE 4__________________________________________________________________________ Phenol type Sulfur type Thermal stabilizer stabilizer brittleness Part Part induction .DELTA.YI by by period 0 48 504Example No. Kind weight Kind weight (H) (H) (H) (H)__________________________________________________________________________Present 1 I-1 0.05 II-1-1 0.2 1620 -1.6 -15.6 -7.5Example 2 " " II-1-2 0.1 1460 -1.3 -14.7 -5.5 3 " " " 0.2 1870 -1.9 -16.3 -8.5 4 " " " 0.3 2300 -1.9 -16.4 -8.5 5 " " II-1-3 0.2 1650 -1.8 -16.0 -8.1 6 " " II-2-1 " 1640 -1.6 -15.8 -7.0 7 " " II-2-2 " 1670 -1.6 -15.8 -7.2 8 " " II-2-3 " 1790 -1.7 -16.0 -8.0 9 " " II-2-4 " 1860 -1.9 -16.2 -8.4 10 " " II-2-5 " 1790 -1.7 -15.9 -8.0 11 " " II-2-6 " 1850 -1.8 -16.0 -8.2 12 I-2 " II-1-2 " 1850 -1.8 -16.2 -8.3 13 " " II-2-4 " 1840 -1.8 -16.1 -8.2 14 " " II-2-5 " 1760 -1.6 -15.7 -7.9Compara- 15 I-1 " AO-5 " 400 -1.0 -11.1 --tive 16 " " AO-6 " 580 -1.2 -11.4 1.4example 17 I-2 " AO-5 " 370 -1.0 -11.0 -- 18 " " AO-6 " 440 -1.1 -11.3 -- 19 AO-1 " II-1-2 " 310 -0.9 -10.8 -- 20 " " II-2-4 " 300 -1.0 -10.9 -- 21 " " AO-5 " 340 -0.9 -10.7 -- 22 " " AO-6 " 390 -1.1 -11.0 -- 23 AO-2 " II-1-2 " 400 -1.0 -12.9 -- 24 " " II-2-4 " 440 -1.2 -13.1 -- 25 " " AO-5 " 450 -1.3 -13.2 -- 26 " " AO-6 " 750 -1.3 -13.8 1.5 27 AO-3 " II-1-2 " 1600 -1.5 -14.7 -1.7 28 " " II-2-4 " 1580 -1.5 -14.5 -1.5 29 " " AO-5 " 460 -1.4 -14.0 -- 30 " " AO-6 " 620 -1.4 -14.1 2.1 31 AO-4 " II-1-2 " 1200 0.5 -7.5 3.7 32 " " II-2-4 " 1150 1.0 -6.7 4.5 33 " " AO-5 " 430 1.2 -5.3 -- 34 " " AO-6 " 590 0.8 -5.1 12.2 35 -- -- -- -- 5 0.0 -- --__________________________________________________________________________

EXAMPLE 2

The test compound in Table 5 was bead-peptized together with an anionic surface active agent to prepare a suspension, and a prescribed amount, as shown in Table 5, of the suspension was added to a graft ABS latex. The mixture was, as usual, salted out with an aqueous magnesium sulfate solution, filtered, washed with water and dried to obtain an ABS resin powder which is a test sample. The thermal and oxidation stability of the ABS resin powder was evaluated by the methods described below. The result is shown in Table 5.

1. The degree of discoloration of the ABS resin powder after thermal ageing in a 180.degree. C. Geer oven was observed.

2. The oxygen absorption induction period (I.P.) in a 170.degree. C. oxygen atmosphere was measured using an oxygen absorption induction period measurement apparatus.

3. The ABS resin powder was repeatedly extruded on a small extruder (screw: D=20 mm.phi., L/D=25; strand die: D=3 mm.phi., L/D=10) under the following condition. The degree of discoloration of the ABS pellet after the 4th extrusion was evaluated by a color difference, .DELTA.YI, between said ABS pellet and the antioxidant-free ABS pellet after 1st extrusion.

______________________________________Extrusion condition:______________________________________Number of revolution: 40 rpmTemperature: C.sub.1 C.sub.2 C.sub.3 D 220.degree. C. 240.degree. C. 260.degree. C. 280.degree. C.______________________________________

4. The ABS pellet after 4th extrusion obtained by the method in 3 above was compression-molded under a condition of 180.degree. C..times.10 min. to prepare No. 1 test piece specified by JIS K 7111. Thereafter, the Charpy impact test was carried out according to JIS K 7111 using a Charpy impact tester. In Table 5, AO-7 shows the following compound: AO-7 : 2,6-Di-tert-butyl-4-methylphenol.

TABLE 5__________________________________________________________________________ Present exampleTest compound 1 2 3 4 5 6 7 8 9__________________________________________________________________________Phenol typestabilizerI-1 0.5 0.5 0.5 0.5 0.5 0.5I-2 0.5 0.5 0.5AO-1AO-2AO-7Sulfur typestabilizerII-1-2 0.5 1.0 0.5II-2-4 0.5 1.0 0.5II-2-5 0.5 1.0 0.5AO-5Degree ofdiscolorationAfter 30 min Pale Pale Pale Pale Pale Pale Pale Pale Pale yellow yellow yellow yellow yellow yellow yellow yellow yellowAfter 60 min Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish brown brown brown brown brown brown brown brown brownI.P. (min) 215 270 210 260 215 265 210 205 210.DELTA.YI 10.9 10.3 11.0 10.6 10.8 10.4 11.0 11.3 11.0Value of Charpy 21.3 21.8 20.5 21.1 20.8 21.3 21.1 21.6 20.3impact test(Kgf .multidot. cm/cm)__________________________________________________________________________ Comparative exampleTest compound 10 11 12 13 14 15 16 17 18__________________________________________________________________________Phenol typestabilizerI-1 0.5I-2 0.5AO-1 0.5 0.5 0.5 0.5 0.5AO-2 0.5 0.5AO-7Sulfur typestabilizerII-1-2 0.5 1.0 0.5 1.0II-2-4 0.5II-2-5 0.5AO-5 0.5 0.5 0.5Degree ofdiscolorationAfter 30 min Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish Yellowish brown brown brown brown brown brown brown brown brownAfter 60 min Brown Brown Brown Brown Brown Brown Brown Brown BrownI.P. (min) 145 140 130 140 130 130 130 145 155.DELTA.YI 14.2 14.6 15.5 15.6 15.3 15.5 15.7 14.5 14.7Value of Charpy 11.8 11.4 11.1 12.2 11.0 11.0 10.8 11.7 12.9impact test(Kgf .multidot. cm/cm)__________________________________________________________________________ Comparative exampleTest compound 19 20 21 22 23 24 25 26 27 28__________________________________________________________________________Phenol typestabilizerI-1 NoI-2 addi-AO-1 tionAO-2 0.5 0.5 0.5 0.5 0.5AO-7 0.5 0.5 0.5 0.5Sulfur typestabilizerII-1-2 0.5II-2-4 0.5 0.5II-2-5 0.5 1.0 0.5AO-5 0.5 1.0 0.5Degree ofdiscolorationAfter 30 min Yellowish Yellowish Yellowish Yellowish Yellowish Deep Deep Deep Deep Deep brown brown brown brown brown brown brown brown brown brownAfter 60 min Brown Brown Brown Brown Brown Black- Black- Black- Black- Black- ish ish ish ish ish brown brown brown brown brownI.P. (min) 145 145 155 145 155 150 150 150 140 10.DELTA.YI 14.3 14.5 14.8 14.6 14.9 14.5 14.6 14.8 15.0 15.9Value of Charpy 11.6 11.6 13.0 11.4 13.5 12.4 12.5 12.4 11.7 7.2impact test(Kgf .multidot. cm/cm)__________________________________________________________________________ Note: The amount added is the weight of the test compound (converted to solid basis) per 100 parts by weight of the solid matter of the resin.

Claims

1. A stabilizer for synthetic resins containing as an effective component a mixture comprising a phenol type compound (I) represented by the formula (I), ##STR13## wherein R.sub.1 represents a C.sub.1 -C.sub.3 alkyl group, and at least one sulfur type compound (II) selected from the group consisting of compounds represented by the formula (II-1), ##STR14## wherein R.sub.2 represents a C.sub.4 -C.sub.20 alkyl group, and compounds represented by the formula (II-2), ##STR15## wherein R.sub.3 represents a C.sub.3 -C.sub.18 alkyl group, and R.sub.4 and R.sub.5 independently represent a hydrogen atom or a C.sub.1 -C.sub.6 alkyl group, the weight ratio of phenol type compound (I) to sulfur type one (II) being 1 to 0.5-15.

2. A stabilizer for synthetic resins as described in claim 1, wherein the phenol type compound (I) is 3,9-bis[2-{3-(3-tert-butyl-4-hydroxy-5-methylphenyl)propionyloxy}-1,1-dimethylethyl]-2,4,8,10-tetraoxaspiro[5.5]undecane.

3. A stabilizer for synthetic resins as described in claim 1, wherein the sulfur type compound (II-1) is tetrakis(3-dodecylthiopropionyloxymethyl)methane.

4. A stabilizer for synthetic resins as described in claim 1, wherein the sulfur type compound (II-2) is 3,9-bis(2-dodecylthioethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane.

5. A stabilizer for synthetic resins as described in claim 1, wherein the sulfur type compound (II-2) is 3,9-bis(2-octadecylthiopropyl)-2,4,8,10-tetraoxaspiro[5.5]undecane.

6. A stabilizer for synthetic resins as described in claim 1, wherein the synthetic resin is a polyolefin resin.

7. A stabilizer for synthetic resins as described in claim 6, wherein the polyolefin resin is polypropylene.