CA2052635A1

CA2052635A1 - Sulphur vulcanizable rubber compositions with improved degree of vulcanization

Info

Publication number: CA2052635A1
Application number: CA002052635A
Authority: CA
Inventors: Joachim Thormer; Thomas Scholl
Original assignee: Individual
Current assignee: Bayer AG
Priority date: 1990-10-05
Filing date: 1991-10-02
Publication date: 1992-04-06
Also published as: DE4031566A1; JP3150732B2; DE59106986D1; EP0479083A1; EP0479083B1; JPH04264145A

Abstract

Sulphur vulcanizable rubber compositions with improved degree of vulcanization A b s t r a c t Salts of partial polycarboxylic acid esters act as promoters in the sulphur vulcanization of rubbers which are low in C=C double bond content.

The vulcanizates obtained are found to have reduced compression set and high tension values, in particular after ageing by heat.

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Description

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Sulphur vulcanizable rubber compositions with improved degree of vulcanization ~his invention relates to vulcanizable rubber compositions containing sulphur as vulcanizing agent, a thiuramic (and optionally a thiazole) accelerator and the salt of a partial di- or tricarboxylic acid ester residue, prefer-- ably of a succinic acid semi-ester and/or glutaric acid semi-ester, as vulcanization promoter, to a process for the preparation of these rubber compositions by mixing the components and to-the use of these rubber compositions for the production of vulcanizates.

Vulcanization accelerators are, as is known, substances which shorten the vulcanization time or enable vulcaniza-tion to be carried out at a lower temperature; see - Ullmanns Encyclopadie der technischen Chemie, 3rd Edition, Urban & Schwarzenberg, Munich-Berlin 1957, pages 383 et seq.
Additional accelerators may be used to enable the full effect of the vulcanization accelerators to be obtained.

So-called "EV-Systems" ("EV" = efficient vulcanization) are frequently used for sulphur vulcanization, in which case the vulcanization accelerators used are in most cases Le A 27 838 - Foreign Countries ~2~

thiuramic compounds such as tetraalkylthiuramic monosulphides (e.g. tetramethylthiuramic monosulphide), tetralkylthiuramic tetrasulphides (e.g. tetramethyl-thiuramic tetrasulphide) and, preferably, tetralkyl-thiuramic disulphides (e.g. tetramethylthiuramicdisulphide, hereinafter referred to as TMTD). These thiuramic compounds are also referred to as ultra accelerators owing to their powerful action; in particular the vulcanizates produced with the aid of these acceler-ators have a high resistance to hot air.

Thiuramic compounds have the property that either they orthe reaction products produced from them during vulcaniza-tion tend to bleed. This has the undesirable effect of producing a deposit on the vulcanizate. The tendency to bleed depends inter alia on the nature of the rubber and on the concentration of the thiuramic compound (see W. Hofmann in Gummi-Asbest-Kunststoffe 9 (1986), pages 422 et seq).

Vulcanizates of rubbers which have a low CzC double bond content, such as EPDM, butyl rubber and especially nitrile rubber, whose C~C double bonds are selectively but not completely hydrogenated so that they are still available for sulphur cross-linking, frequently have a low degree of cross-linking, which i5 recognised by the low tension values and moderate pressure deformation residue, generally known as compression set (hereinafter referred to as DVR =Druckverformungsrest), determined by the deformation under pressure in the heat, e.g. in hot air or hot oil. The phenomenon of bleeding mentioned above limits the amount by which the compression set can be increased by using larger quantities of thiuramic compound.

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It has now surprisingly been found that a vulcanizing system containing a small quantity of sulphur (optionally in the form of a sulphur donor), thiuramic accelerators, optionally thiazole accelerators, and the salt of a partial di- or tricarboxylic acid ester as vulcanization promoter has an excellent degree of cross-linking with high tension values and low compression set values, in particular after ageing at elevated temperatures. The product is also found to be more easily processible and to 1~ display ;mproved hot-air a~eirlg.
With the aid of the vulcanization promoter the quantity of sulphur can be reduced without any essential properties of the vu1canizate being adversely affected. The reduced quantity of sulphur is advantageous with regard to ageing and also allows an improvemen~ in the covulcanization of rubbers having a low content of double bonds with diene rubbers, such as for example that of partially hydrogenated NBR wi th normal NBR .
The present invention thus relates to vulcanizable rubber compositions based on rubber having a low C=C double bond content and containing ~rom 0.2 to 1% by weight of sulphur, from 1 to 3.5% by weight of thiuramic accelerator and optionally up to 2~ by weight, preferably from 0.2 'o 1~ by weight, of thiazole accelerator, and from 0.1 to 8% by weight, preferably from 0.3 to 6, most preferably from 0.8 to 4% by weight of the salt of a partial di- and/or tricarboxylic acid ester, the percentages being based in each case on the quantity of rubber to be vulcanized.

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The invention further relates to a process or the preparation of these rubber compositions by mixing the components and to their use for the production of vulcanisates.
Rubbers with a low C=C double bond content for the purpose of this invention comprise those having iodine numbers of from 2 to 35, preferably from 3 to 30, in particular from 5 to 25. Determination of the iodine numbers is generally carried out by the addition of iodochloride in glacial acetic acid according to Wijs, DIN 53 241, Part 1.
The iodine number defines the quantity of iodine in grams which is chemically bound by 100 g of substance. Examples of preferred rubbers include EPDM, butyl rubber and especially hydrogenated nitrile rubber.

~he rubbers preferably have glass transition temperatures below O~C, in particular below -lO~C.

The term "EPDM" stands ~or ethylene/propylene/diene terpolymers. EPDMs comprise rubbers in which the ratio by weight o~ ethylene to propylene groups is in the range o~
~rom 40:60 to 65:35 and which may contain from 1 to 20 C-C
double bonds per 1000 carbon atoms. The following are examples o~ suitable dlene monomers in the EPDM:
Conjugated dienes, e.g. isoprene and butadiene-(1,3), and non-conjugated dienes having 5 to 25 carbon atoms, e.g.
1,4-pentadiene, 1,4-hexadiene, 1,5-hexadiene, 2,5-dimethyl-1,5-hexadiene and 1,4-octadiene; cyclic dienes, e.g. cyclopentadiene, cyclohexadiene, cyclooctadiene and dicyclopentadiene; alkylidene and alkenyl norbornenes, e.g. 5-ethylidene-2-norborne, 5-butylidene-2-norbornene, 2-methallyl-5-norbornene, 2-isopropenyl-5-norbornene and tricyclodienes.

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The non-conjugated dienes, hexadiene-(1,5), ethylidene norbornene and dicyclopentadiene, are preferred. The diene content of the EPDN is preferably from 0.5 to 10% by weight, based on the EPDM.
s EPDM rubbers of this type are described e.g. în DE-OS
2 808 709.

The term "butyl rubber" used in the context of this invention covers isobutene copolymers of from 95 to 99.5%
by weight, preferably from 97.5 to 99.5% by weight, of isobutene and from 0.5 to 5% by weight, preferably from 0.5 to 2.5% by weight, of copolymerisable diene, e.g.
butadiene, dimethylbutadiene, pentadiene-(1,3) and especially isoprene. Butyl rubber is produced almost exclusively as an isobutene/isoprene copolymer on a large technical scale by cationic solution polymerisation at a low temperature; see e.g. Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 7, page 688, Interscience Publisher, New York-London-Sydney 1965 and Winnacker-Kuchler, Chemische Technologie, 4th Edition, Volume 6, pages 550-555, publishers Carl Hanser Verlag, Munich-Vienna 1962.

~he preferred hydrogenated nitrile rubbers are based on butadiene/acrylonitrile copolymers having a copolymerised acrylonitrile content of from 5 to 60% by weight, preferably from 10 to 50% by weight. "Hydrogenated" means in this context that from 90 to 99%, preferably from 95 to 98.5%, and in particular from 96 to g8.5 % of the C=C-double bonds which are capable of being hydrogenated are hydrogenated. The degree of hydrogenation may be determined IR spectroscopically.

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The hydrogenation of nitrile rubber is known; US-PS

3 700 637, DE-OS 25 39 132, 30 46 008, 30 46 251, 32 27 650 and 33 29 974, EP-A 111 412 and FR-PS 2 540 503.
Hydrogenated nitrile rubber is particularly distinguished by its comparatively high resistance to oxidation.
s Rubbers suitable for the process according to the invention generally have Mooney viscosities (DIN 53 523) of from 10 to 150, preferably from 25 to 80 (ML 1~4)/lOO~C.
;'' The preferred thiuramic accelerators include, for example, the above-mentioned tetraalkylthuramic mono- and poly-sulphides in which the alkyl groups generally have 1 to 4, preferably 1 or 2 carbon atoms, but the substituents may also be cycloaliphatic, aromatic or araliphatic.

Preferred thiazole accelerators which may also be used according to the invention include in particular 2-mercaptobenzothiazole, dibenzothiazyl-disulphide, benzothiazyl-2-cyclohexylsulphenamide (CBS), benzothiazyl-2-tert.-butylsulphenamide (TBBS), ~-morpholinothio-2-benzothiazole (~BS~, benzothiazyl-2-diisopropylsulphenamide (DIBS), benzothiazyl-2-tert.-amylsulphenamide ~AMZ), benzothiazyl-dicyclohexylsulphenamide (DCBS) and morpholino-thiocarbonyl-sulphenomorpholide ~OTOS).

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The preferred partial di- and tricarboxylic acid esters (the semiesters in the case of dicarboxylic acid esters;
the diesters in the case of tricarboxylic acid esters) whose salts are used according to the invention as vulcanization promoters include the esterification products of aliphatic C4-C10-dicarboxylic acids (preferab-ly adipic acid, in particular succinic acid, glutaric acid), of cycloaliphatic C8-C12-dicarboxylic acids (preferably tetrahydrophthalic acid, hexahydro-phthalic acid) of aliphatic C6-C12-tricarboxylic acids (preferably citric acid) and of C8-C14-benzene di- and tri-carboxylic acids (preferably phthalic acid, isophthalic acid, terephthalic acid, trimesic acid, trimellitic acid) with C1-C18, preferably C4-C18 alcohols. The alcohol components may be aliphatic, cycloaliphatic, araliphatic or aromatic;
they may contain (cyclo)olefinic C=C-double bonds as well as halogen substituents, e.g. chlorine substituents, and they may be linear or branched. Preferred alcohol components include methanol, isopropanol, n-, iso- and tert.-butanol, hexanol, octanol, decanol, dodecanol, stearyl alcohol, cyclohexanol, benzyl alcohol and phenol, Succinic acid monooctylesters and glutaric acid monobutyl-esters are particularly preferred.

The cations of the salts of partial esters to be used according to the invention are preferably derived from alkali metals and alkaline earth metals and zinc, the zinc salts being particularly preferred.

Vulcanization auxiliaries and, if required, activators, fillers such as carbon black, plasticizers, age resisters and/or processing aids may be added to the rubbers in the usual quantities before vulcanization.

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The most important inorganic activators are metal oxides, in particular zinc oxide. Magnesium oxide or calcium hydroxide is also used in individual cases.

The processing auxiliaries used may be, for example, fatty acids, e.g. stearic acid.

Mixing of the components may be carried out in convention-al mixing apparatus.

The mixing apparatus used are preferably those convention-ally used in the rubber industry, such as kneaders, rollers, internal mixers and mixing extruders, which generally operate with shear rates of from 1 to 1000 sec~1, preferably from 1 to 200 sec~1.

Vulcanization may be carried out at temperatures from 100 to 200~C, preferably at 130 to 180QC, optionally under a pressure of from 10 to 200 bar.
The excellent vulcanization properties are generally obtained even without tempering but may often be further improved by tempering.

The vulcanizates obtainable according to the invention are excellent materials for transmission belts or toothed belts, for which their high recovery capacity is a great advantage. They also have excellent properties as sealing materials of all kinds and may be used inter alia for the inner or outer layers of tubes. They may also be used for rubber-izing textiles, for lining cavities and as insulating materials and sheaths for cables. The vulcani~ation systems can also be used for the production of friction linings.

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Exammles The rubber used for the following Examples was a hydrogen-ated acrylonitrile/butadiene copolymer having an acrylo-nitrile content of 33.7% by weight, a degree of hydrogena-tion of 96.4~, based on the C=C-double bonds originally present, and a Mooney viscosity of 67 (ML 1 ~ 4~ lOO~C
((R)Therban 1707 S of Bayer AG).

100 Parts of rubber were masticated in a laboratory kneader at 50~C for O.S minutes and 0.51 parts of sulphur, 1 part of stearic acid, 2 parts of zinc oxide, l part of octylated diphenylamine ((R)Vulkanox OCD of Bayer AG), 0.4 parts of zinc methylmercaptobenzimidazole ((R)Vulcanox ZMB2 of Bayer AG), 45 parts of carbon black (Corax N550 of Degussa/Wesseling) and varying quantities (see Table 1) of the zinc salt of succinic acid octyl semiester were then added and the mixture was kneaded until homogeneous (4.5 minutes).

After the rubber mass had been cooled to about 100CC on a roller, an accelerator system consisting of 2 parts of tetramethylthiuramic disulphide (~R)Vulkacit Thiuram C of Bayer AG) and 0.5 parts of benzothiazyl-2-cyclohexyl-sulphenamide (~R)Vulkacit CZ of Bayer AG) were added.

The properties of the resulting mixtures are listed below:

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Therban 1707 S lOo 100 100 Sulphur 0.51 0.51 0.51 5 Vulkanox OCD 1 1 1 Vulkanox ZMB2 0.4 0.4 0.4 10 Carbon black N 550 45 45 45 Zinc oxide 5 5 5 15 Stearic acid 0.5 0.5 0.5 Vulkacit Thiuram C 2 2 2 Vulkacit CZ 0.5 0.5 0.5 Zinc succinic acid octyl semiester _ l.S 3 ======z====================== ======= ====== =========
Mooney viscosity MLl+4/120C 78 76 76 25 Mooney-Scorch at 130C (min) 18.5 18.6 17.3 Vulkameter 160C
tlo ~min) 4,5 4.8 4.8 t8n ~min) 7.9 8.2 9.0 Fm~n ~N) 2.1 2.1 2.0 Fmax ~N) 5~4 58.6 57.6 35 Vulcanization 30 min./1609C ~ 2 rods, DIN 53 02~53504) Tensile strength ~MPa) 29.7 29.8 30.1 Elongation at break ~%) 490 480 480 40 Tension S100 ~MPa) 3.8 4.2 4.3 Shore hardness 72 72 73 45 C.S.*,Probek. II 70 h/100C 59.2 47.7 44.4 C.S.*, Probek. I, 70 h/100C 44.8 30.3 25.2 with temp. 6 h/150C ~%) relative elongation~) after 50 hot air ageing(l0 days/1509C) 41 54 52 * Compression set Le A 27 838 11

Claims

1. Vulcanizable rubber compositions based on rubber having a low C=C double bond content, containing from 0.2 to 1% by weight of sulphur, from 1 to 3.5% by weight of thiuramic accelerator, optionally up to 2% by weight of thiazole accelerator, and from 0.1 to 8% by weight of a salt of partial di-and/or tricarboxylic acid ester, the percentages being based in each case on the rubber to be vulcanized.

2. Rubber compositions according to Claim 1, in which the alcohol component of the partial ester contains from 1 to 18 carbon atoms.

3. Rubber compositions according to Claim 1, in which the alcohol component of the partial ester contains from 4 to 18 carbon atoms.

4. Rubber compositions according to Claim 1, in which the acid component of the partial ester is selected from aliphatic C4-C10-dicarboxylic acids.

5. Rubber compositions according to Claim 1, in which the acid component of the partial ester is selected from C8-C14-benzene di- and tricarboxylic acids.

6. Rubber compositions according to Claim 1, in which the acid component of the partial ester is selected from cycloaliphatic dicarboxylic acids.

7. Rubber compositions according to Claim l, wherein zinc salts of the partial di- and/or tricarboxylic acid esters are used.

8. A process for the preparation of the rubber compositions according to Claim 1 to 7 by mixing the components.

g. Use of the rubber masses according to Claims I to 7 for the preparation of vulcanizates.

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