CA2053981A1 - Use of maleic acid semi-esters and fumaric acid semi-esters and their salts for reducing nitrosamine formations in sulphur vulcanization - Google Patents

Abstract

The use of maleic acid semi-esters and fumaric acid semi-esters and their salts for reducing nitrosamine formation in sulphur vulcanization A b s t r a c t Maleic acid semi-esters and fumaric acid semi-esters and their salts, in particular their zinc salts, are capable of reducing nitrosamine formation in the sulphur vulcanization of rubbers.

Le A 28 006 - Foreign Countries

Description

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The use of maleic acid semi-esters and fumaric ac1d semi-esters and their salts for reducing nitrosamine formations in sulphur vulcanization This invention relates to the use of maleic acid semi-esters and fumaric acid semi-esters and their salts, in particu1ar their zinc salts, for reducing the nitrosamine content in rubber vulcanizates produced by sulphur vulcanization.

When rubbers are mixed with vulcanizing systems containing compounds having chemically bound nitrogen (e.g. sulphur donors or accelerators selected from thiuramic compounds or vulcanization accelerators selected from thiazole compounds) there is a risk of unwanted formation of nitrosamines, and this risk is even greater when these rubber masses undergo vulcanisation. Owing to the carcinogenic effect of nitrosamines, it would be desirable to be able to produce rubber masses and vulcanizates containing little or no nitrosamine.

Whereas sulphur donors, for example, can often easily be replaced by elementary sulphur, no nitrogen-free vulcan-ization accelerator has yet been found which is aseffective as the substances hitherto used~

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So long as no such highly effective vulcanization accelerators free from nitrogen are available, it is also of interest to find additives which either suppress nitrosamine formation or absorb the nitrosamines by a chemical reaction. One such compound which has been described is ~-tocopherol (Kautschuk + Gummi-Kunststoffe 43 (1990) 95-106). The high cost of ~-tocopherol, however, would stand against its use on a large technical scale in the rubber industry.

It has now surprisingly been found that maleic acid semi-esters and their salts are eminently suitable for this purpose.
The present invention thus relates to the use of maleic acid semi-esters and fumaric acid semi-esters corresponding to the following formula HC - COOHHC COOH
HC COORROOC!CH
(Ia) (Ib) and their salts, wherein R denotes an organic group having 1 to 18, preferably 4 to 18 carbon atoms, for reducing the nitrosamine content of rubber vulcaniz-ates produced by sulphur vulcanization.
The preferred maleic acid semi-esters and fumaric acid semi-esters include semi-esters of C1-C22, preferably C4-C18 alcohols. The alcohol components of the semi-esters may be aliphatic, cyclo-aliphatic or aromatic;
they may contain (cyclo)olefinic C=C double bonds and halogen substituents, e.g. chlorine Le A 28 006 3 ~c~s~

substituents, and they may be linear or branched.
Preferred alcohol components include methanol, isopropan-ol, n-iso- and tert.-butanol, hexanols, octanols, stearyl alcohol, allyl alcohol, cyclohexanol, benzyl alcohol and phenol; the mono-n-butyl-, octyl-, ethylhexyl- and allyl esters of maleic acid are particularly preferred.

The cations of the semi-ester salts 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 as they also have a positive influence on the Mooney viscosity, the compression set, the resistance to hot air ageing and the strength of the vulcanizates.

The maleic acid semi-ester and fumaric acid semi-ester (salt)s to be used according to the invention may be put into the process in quantities of from 0.5 to 8% by weight, preferably from l to 6, in particular from 1.5 to 4% by weight, based on the rubber.

Rubbers suitable for the use according to the invention include synthetic rubbers as well as natural rubbers.
Preferred synthetic rubbers have been described, for example, by W.Hofmann, in Kautschuk-Technologie, Gentner Verlag, Stuttgart 1980. They include inter alia the following:

EPDM - ethylene/propylene/diene terpolymers 25 IIR - butyl rubber BR - polybutadiene Le A 28 006 4 ~53~

ABR - butadiene/acrylic acid-Cl-C4-alkyl ester copolymers having acrylic ester contents of from 5 to 60, preferably from 15 to 50% by weight 5 CR - polychloroprene IR - polyisoprene SBR - styrene/butadiene copolymers having styrene contents of from 1 to 60%by weight, preferably from 20 to 50% by weight NBR - butadiene/acrylonitrile copolymers having acrvlonitrile contents of from 5 to 60, preferably from 10 to 50% by weight, and in particular 15 HNBR - hydrogenation products of NBR.

The rubbers generally have Mooney visco6ities (according to DIN 53 523) of from 5 to 140, preferably from 10 to 120, in particular from 20 to 80 (ML 1 + 4) 1009C and glass temperatures below 200C, preferably below 0C, determined by the torsion vibration test according to DIN
53 445.

EPDMs include rubbers in which the ratio by weight of ethylene to propylene groups is in the range of from 40:60 to 65:35 and which may have from 1 to 20 C=C double bonds per 1000 carbon atoms. The following are examples of suitable diene 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, Le A 28 006 5 z~

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. s-ethylidene-2-norbornene, 5-butylidene-2-norbornene, 2-methallyl-5-norbornene, 2-isopropenyl-5-norbornene and tricyclodienes.

The non-conjugated dienes, hexadiene-(1,5), ethylidene norbornene and dicyclopentadiene are preferred. ~he diene content in EPDM is preferably from 0.5 to lO~ by weight, based on the EPDM.

Such EPDM rubbers are described, for example, in DE-OS
2 808 709.

~he term "butyl rubber" used in the context of this invention includes isobutene copolymers of from 95 to 99.5~ by weight, pre~erably 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 in parti¢ular i~oprene. Butyl rubber ls produced on a large technical scale almost exclusively as the isobutene/iso-prene copolymer by cationic solution polymerisation at a low temperature; see e.g. Kir~-Othmer, Encyclopedia of Chemical Technology, 2nd Edition, Volume 7, page 688, Interscience Publishers, New York-London-Sydney, 1965, and Winnacker-Xuchler, Chemische Technologie, 4th Edition, Volume 6, pages 550-555, Carl Hanser Verlag, Munich-Vienna, 1962.

The polybutadienes include polybutadiene rubbers contain-ing from 20 to 100%, preferably from 30 to 100%, o~ the cis-1,4-structure, which may be obtained e.g. by butadiene polymerisation with the aid of catalysts based on lithium, Le A 28 006 6 nickel, titanium, cobalt, rare earths or uranium.
Polychloroprenes are chloroprene polymers which in addition to containing polymerised 2-chloroprene units may contain up to 30~ by weight, preferably up to 20% by weight, based on the chloroprene polymer, of copolymerised units of other ethylenically unsaturated monomers, in other words polychloroprenes such as those described, for example, in "Methoden der Organischen Chemie" (Houben-Weyl), Volume E20/2, 842-859, Georg Thieme Verlag, Stuttgart - New York 1987.

Preferred ethylenically unsaturated "other monomers" which can be copolymerised with chloroprene include compounds having 3 to 12 carbon atoms and 1 or 2 copolymerisable C=c double bonds per molecule. The following are examples of preferred "other monomers": Styrene 2,3-dichlorobuta-diene, 1-chlorobutadiene butadiene, isoprene, acrylic acid, methacrylic acid, acrylonitrile and methacrylo-nltrlle. The most important comonomers are 2,3-dichloro-butadiene and 1-chlorobutadiene.

Preferred styrene/butadiene copolymers are those contain-ing from 18 to 60% by weight, pre~erably ~rom 20 to 50% by weight of styrene incorporated by polymerisation. Solution and emulsion polymers are particularly preferred.

Nitrile rubbers and hydrogenated nitrile rubbers include nitrile rubbers such as those described e.g. in Ullmanns Encyclopadie der technischen Chemie, Volume 13, Verlag Chemie, Weinheim-New York 1977, pages 611-614, and fully or partially hydrogenated nitrile rubbers such as those described e.g. in Die Angewandte ~akromolekulare Chemie 145/146 (1986) 161-179.

The term "nitrile rubber" is used to denote butadiene/-Le A 28 006 7 ~3 J31 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 98.5%, preferably from 94 to 98%, of the C=C double bonds capable of being hydrogen-ated are hydrogenated while the C--N triple bonds of the nitrile groups are not hydrogenated. 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, 33 29 974, EP-A 111 412 and FR-PS 2 540 503.

Detailed descriptions of sulphur vulcanization systems are found in "Vulkanisation und Vulkanisationshilfsmittel" by W.Hofmann, Verlag Berliner Union GmbH, Stuttgart 1965, and in "Vulcanization of Elastomers" by Alliger and Sjothun, Reinhold Pub. Corp. New York 1964. Examples of suitable sulphur donors include thiuramic polysulphides such as dipentamethylene thiuramic tetra- and -hexasulphide and tetramethyl thiuramic disulphide; amine disulphides such a~ dimorpholyl disulphide; sodium polysulphides and thioplasts.

Pre~erred sulphur vulcanization systems contain a) sulphur or sulphur donors, b) optionally vulcanization accelerators and c) optionally one or more activators.

Component a) is generally used in a quantity corresponding to from 0.2 to 3.0% by weight of sulphur (calculated as the amount of sulphur liberated in the case o~ sulphur donors), based on the rubber. Sulphur modified polychlo-roprene may also function as sulphur donor.
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The vulcaniztion accelerator b) is generally used in quantities of from 1 to 3.5% by weight, based on the rubber. Preferred vulcanization accelerators b) include thiazole accelerators, e.g.
2-mercaptobenzothiazole, dibenzothiazyl disulphide, benzothiazyl-2-cyclohexylsulphenamide ( CBS ), benzothiazyl-2-tert.-butylsulphenamide (TBBS), N-morpholinothio-2-benzothiazole (MBS), benzothiazyl-2-diisopropylsulphenamide ~DIBS), benzothiazyl-2-tert.-amylsulphenamide (AMZ), benzothiazyl-dicyclohexylsulphenamide (DCBS~ and morpholino-thiocarbonyl-sulphenemorpholide (OTOS).

Further examples of-preferred vulcanization accelerators b) include diphenylguanidine ( DPG) and di-o-tolylguanidine (DOTG); thiurams such as thiuramic mono- and disulphides;
dith~ocarbamates and thiophosphates and their derivatives and salts, e.g. their zinc salts.

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

Fillers such as carbon black, plasticizers, age resistors and/or processing auxiliaries may be added to the rubbers in the usual guantities before vulcanization.

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.

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

Vulcanization may be carried out at temperatures of from 100 to 200QC, preferably from 130 to 180~C, optionally under a pressure of from 10 to 200 bar. The vulcanizates may be tempered by storage at elevated temperatures after they have been vulcanized.

"Vulcanized" in the context of this invention means that less than 10% by weight, preferably less than 5% by weight, based on the rubber, can be extracted in the course of 10 hours' extraction with toluene in a Soxhlet apparatus.

Le A 28 006 10 2~r3 Examples List and abbreviations of the test methods:

Vulcameter Frank-Vulkameter System Bayer, heating for l minute, measuring range 3/20 mV, operating time at 150~C: 30 min, feed rate 600 mm/h ts/120~C or Prevulcanization time from vulcameter measurement at 120~C and 130sC, 130~C (min) Time until the shear modulus curve rises by 15 mm above the minimum tgo (min) Heating time, time required for 90% of the shear modulus (Bayer-Vulkameter) to be reached at the end of the operating time 15 tgo - tS Reaction time, difference between the (min) heating time tgo and the prevulcanization time ts measured at the same temperature, e.g. 150~C
Shear modulus Final value-initial value o~ the shear 20 ~F ~N) modulus from the vulcameter test Stepwise 4 mm claps, pres6 heating in several time heating stage6 M300 (MPa) Tension at 300% elongation, DIN 53 504 F (MPa) Tear resistance, DIN 53 504, Standard ring R I
D (%) Elongation at break, DIN 53 504, Standard ring R I
H (Shore A) Shore A hardness, DIN 53 505 E (%) Recoil elasticity DIN 53 512 30 W (N) Ring structure according to Pchle (in-house method) Compression Based on DIN 53 517, constant de~orma-set ~%] tion, cylinder 10 mm in height, 10 mm in diameter Le A 28 006 ll 2~3~

Determination of the nitrosamine content was carried out according to Franck, Kunststoffe-33 Lfg. August 1984, pages 37 et seq.
.
A. HNBR Rubber mass The rubber used was a hydrogenated acrylonitrile/butadiene copolymer having an acrylonitrile content of 33.7~ by weight, a degree of hydrogenation 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).

The rubber was masticated for 0.5 minutes in a laboratory kneader at 50sC and sulphur, stearic acid, zinc oxide ~(R) zink oxide aktiv of Bayer AG), styrenised diphenyl-amine (R)Vulkanox DDA of Bayer AG), zinc-methylmercapto-benzimidazole ((R)Vulkanox ZMB2 of Bayer AG), carbon black(Cor~x N550 o~ Degussa/Wesseling) and zinc-ethyl hexyl maleate (~or quantities, see Table 1) were then added and the components were kneaded until homogenised (4.5 minute~) .

After the rubber mass had cooled to about lOO~C on a roller, an accelerator system consisting of tetramethyl thiuramic disulphide (~R)Vulkacit Thiuram C of Bayer AG) and benzothiazyl-2-cyclohexyl sulphenamide ((R)Vulkacit CZ/MG of Bayer AG) was added.

The properties of the mixtures obtained and their vulcanizates are listed in Table 2.

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Table 1: Composition .
HNBR loO loO lOo Sulphur 0.51 0.51 0.51 Stearic acid Zinc oxide 2 2 2 Vulkanox DDA
Vulkanox 0.4 0.4 0.4 20 Carbon black 45 45 45 Vulkacit 2 2 2 Thiuram C
25 Vulkacit 0.5 0.5 0.5 CZ/MG
Zinc-ethyl _ 1.5 3 hexylmaleate Le A 28 006 13 XC~3 J~l.

Table 2: Properties Mooney viscosity MLl+4/1209C 76 74 72 Mooney Scorch/
130sC (min) 18.3 21.7 20.9 Vulcameter 1609C
ts (min) 5.2 4.7 4.7 t80 (min) 8.9 7.6 7.7 Fmax (N) 50 4 52 9 521:9 Press vulcanization: 30 minutes/160sC (S2 rods according to DIN 53 502 and 53 504) 20 Tensile strength (MPa) 29.4 28.6 27 Elongation at break (%) 500 500 490 Tenslon S10O (MPa) 3.9 4.1 3.9 Shore hardness (A) 73 72 73 Recoil elasticity (%) 42 43 43 Tear propagation resi~tance according to DIN 53 515 15.1 19.8 20.6 Nitrosamine content (ppb) be~ore vulcanization 28 29 32 a~ter vulcanization 185 45 35 Le A 28 006 14 B. NBR rubber mass The rubber used was an acrylonitrile/butadiene copolymer having an acrylonitrile content of 28% by weight and a Mooney viscosity of 45 (ML 4) 1009C ((R)Perbunan N 2807 NS
of Bayer AG).

The rubber was masticated for 0.5 minutes in a laboratory kneader at 50UC and a 50% by weight sulphur paste ((R)Struktol SU 105 of Schill & Seilacher/Hamburg), carbon black ((R)Corax N 326 or (R)Durex O of Degussa/Wesseling), styrenised diphenylamine ((R)Vulkanox DDA of Bayer AG), polymeric 2,2,4-trimethyl-1,2-dihydroquinoline ((R)Vulkanox HS of Bayer AG), plasticizer (ether thio-ether; (R)Vulkanol OT of Bayer AG), processing auxiliary based on highly active silica ((R)Aflux S of Rheinchemie Rheinau GmbH, Mannheim), zinc oxide (zinc white RS of Zinkweiss-Forschungsgesellschaft, Oberhausen), stearic acid and zinc-n-octylmaleate were then added and the components were kneaded until homogenised (4.5 minutes).

A~ter the rubber mas~ had cooled to about 100C on a roller, tetramethylthiuramic disulphide ((R)Vulkacit Thiuram/C of Bayer AG) was added~ Composition and experimental findings are shown in the following Tables 3 and 4.

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Table 3: Composition Composition [Parts by weight]

Sulphur paste 0.6 Carbon black Corax N 326 30 Caron black Durex O 65 Vulkanox DDA 1.5 Vulkanox HS
Plasticizer 6 Aflux S 3 Zinc oxide 5 Stearic acid 25 Vulkacit Thiuram/C 2.5 Zinc-n-octyl maleate see Table 4 Le A 28 006 16 2g;

Table 4: Properties Vulkameter without zinc with 4% zinc-n-n-octyl- octyl maleate maleate ts 1209C [min] 18.0 14.7 ts 150 9 C [min] 2.8 2.3 tgo 150sC tmin] 10.0 11.5 tgo-tS 1509C [min] 7.2 9.2 Fmax tN~ 53 46 Nitrosamine content 47 36 before vulcanization t ppb ~

after vulcanization 153 89 ~ppb]

C. EPDM ~ubber mass The rubber used was an EPDM (~R)Buna AP 331 containing ethylidene norbornene a~ termonomer, Mooney-Viscosi~y: 70 ~ML 1+4) 1009C, product of Huls AG, Marl).

The rubber was masticated for- 0.5 minutes in a laboratory kneader at 509C, and sulphur, stearic acid, zinc oxide ((R)Zinkoxyd aktiv of Bayer AG), styrenized diphenylamine ((R)Vulkanox DDA of Bayer AG), zinc-methylmercaptobenz-imidazole ((R)Vulkanox ZMB 2 of Bayer AG), carbon black ((R)Corax N 550 of Degussa, Wesseling) and zinc-mono-n-octylmaleate were then added and the components were kneaded until homogenized.
After the rubber mass had cooled to about 1009C on a roller, an accelerator system consisting of tetra-~e A 28 006 17 3~,$1 methylthiuramic disulphide ((R)Vulkacit Thiuram C ofBayer AG) and benzothiazyl-2-cyclohexyl-sulphenamide ((R)Vulkacit CZ/MG of Bayer AG) and optionally N-tri-chloromethylsulphenyl-benzene sulphanilide ((R)Vulkalent E
of Bayer AG) was added.

Compositions and properties of the vulcanizate are shown in Tables 5 and 6 below.

Table 5:. Composition EPDM 100 loo lOo Sulphur 0.51 0.51 0.51 Carbon black 45 45 45 Zlnc oxide 5 5 5 Stearic acid 0.5 0.5 0.5 Vulkanox DD~ 1 1 1 Vulkanox ZMB 2 0.4 0.4 0.4 Vulkacit Thiura~l C 2 2 2 Vulkacit CZ/MG 0.5 0.5 0.5 20 Zn-n-octyl ~aleate - 3 3 Vulkalent E

Le A 28 006 - 18 Table 6: Properties Mooney viscosity 77 75 74 (ML 1+4) 1209C
Vulcameter 1609C
tS [Min.] 3.4 2.4 2.2 t80 tMin-] 8.6 8.1 8.0 tgo tMin.] 13.5 12.8 12.1 tmax tMin.] ,2 9 425.3 2.3 min tN3 50 7 44.1 40.9 Fmax-Fmin [N] 47.8 41.8 38.6 Press vulcanisation: 30 min/160sC
F ~MPa] 15.3 12.1 12.6 D (%) 495 545 605 S 100 tMPa~ 2.6 2.2 2.1 25 S 200 tMPa] 5.7 4.2 3.9 S 300 tMPa] 9.1 6.6 6.1 H 239C (Shore A) 66 67 63 E 239C ~%) 59 58 56 WW* DIN 53 515 [N/mm] 13.2 16.1 16.8 Compression set (Sample l ~ody II, according to DIN 53 517) 70 h/1009C 43.0 47.1 46.5 70 h/1259C 66.4 70.8 69.1 Nitro~amine [ppb]
before vulcanization 18 14 15 45 after vulcanization 24 9 8 *WW = tear propagation resistance Le A 28 006 19

Claims (4)

1. The use of maleic acid semi-esters and fumaric acid semi-esters corresponding to the formula (Ia) (Ib) and their salts, wherein R denotes an organic group having 1 to 22 carbon atoms for reducing the nitrosamine content of rubber vul-canizates produced by sulphur vulcanization.

2. Use according to Claim 1, in which R denotes an organic group having from 4 to 18 carbon atoms.

3. Use according to Claims l and, 2, in which the semi-ester is maleic acid-n-octyl ester, -ethyl hexyl ester, -allyl ester or -cyclohexyl ester.

4. Use according to Claims l to 3, in which the semi-ester salts are zinc salts.

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