EP0640114A1

EP0640114A1 - Sulfur-vulcanized rubber composition

Info

Publication number: EP0640114A1
Application number: EP93909928A
Authority: EP
Inventors: Rabindra Nath Datta; Johannes Hermanus Wilbrink
Original assignee: Akzo Nobel NV
Current assignee: Akzo Nobel NV
Priority date: 1992-05-13
Filing date: 1993-05-06
Publication date: 1995-03-01
Also published as: FI945324A0; AU669524B2; TW222663B; JPH07506606A; AU4066093A; BR9306351A; ZA933323B; CA2135763A1; CN1085229A; FI945324A; WO1993023467A1

Abstract

A rubber composition which is the vulcanization reaction product of a rubber, sulfur or a sulfur donor, particular anti-reversion coagents, and specific additive components, is disclosed. The invention also relates to a sulfur-vulcanization process which is carried out in the presence of an anti-reversion coagent and an additive component and the use of an anti-reversion coagent in combination with an additive component in a process for the sulfur-vulcanization of rubber. The resulting sulfur-vulcanized rubbers have significantly improved physical properties.

Description

Sulfur-vulcanized Rubber Composition

This invention relates to a sulfur-vulcanized rubber composition having improved physical properties. More particularly, it relates to a sulfur-vulcanized rubber composition which is vulcanized in the presence of an anti -reversion coagent and an additive component comprising a thiol and an electron donor group. The invention also relates to a sulfur-vulcanization process which is carried out in the presence of an anti -reversion coagent and an additive component comprising a thiol and an electron donor group and the use of an anti-reversion coagent in combination with an additive component comprising a thiol and an electron donor group in the sulfur-vulcanization of rubber. Finally, the invention also relates to rubber products comprising rubber vulcanized with sulfur in the presence of an anti-reversion coagent and an additive component comprising a thiol and an electron donor group.

In the tire and belt industries, among others, better mechanical and heat resistance properties are being demanded. In order to improve these properties, it has been proposed to use other agents than sulfur in vulcanization systems or to add such an agent to sulfur-vulcanization systems. One known type of agent are the maleimides. Mai eimi de-vulcanization systems are disclosed in, "Vulcanization With Maleimides," Journal of Applied Polymer Science, Vol. 8, pp. 2281-2298 (1964).

Some articles relating to the sulfur-vulcanization of rubbers disclose the use of bismaleimides as coagents. Such articles include Chavchich, T.A., et al., Kauchuk i Rezina, vol. 4, pp. 20-3, 1981; Krasheninnikov, N.A., et al., Kauchuk i Rezina, vol. 3, pp. 10-12, 1974; Krasheninnikov, N.A., et al., Kauchuk i Rezina, vol. 3, pp. 16-19, 1975; Prashchikina, A.S., et al., Kauchuk i Rezina, No. 10, pp. 15-19, 1985. Even more recently, Japanese patent applications J6 3286-445 and J63312-333 disclosed the vulcanization of rubber with sulfur and an aliphatic bis-maleimide or N,N'-toluene bis-maleimide.

Several patent publications exist which disclose sulfur-vulcanization systems wherein maleimides are used as coagents in combination with other components. Such patent publications include Japanese patent publication J6 1014-238, European patent applications 0 191931, 0345825 and 0410 152.

However, despite the fact that some of the above publications claim to improve rubber properties with the reduction in reversion by addition of a bismaleimide, in actual practice the reduction in reversion achieved with the bismaleimides is insufficient.

It has been observed that the sulfur-vulcanization of a composition comprising a rubber and an anti -reversion coagent comprising at least two groups selected from citraconimide and/or itaconimide groups results in a substantial reduction in the reversion of sulfur- vulcanized rubber compositions, as compared to the above-mentioned maletmide systems. This concept is generally disclosed in non-prior published PCT patent application PCT/EP 91/02048. However, these sulfur-vulcanized rubber products still exhibit an initial fall in ultimate torque after vulcanization is complete and subsequently a marching cure causing a "dip" in the cure curve. This leads to rubber products whose basic characteristics do not remain constant over time.

It is therefore the primary object of the present invention to improve upon the use of anti-reversion coagents by providing an additive component in combination with the anti-reversion coagent disclosed by PCT/EP 91/02048 which will solve the problems associated with these anti-reversion coagents. It has been found that the presence of an additive component comprising a thiol and an electron donor group can significantly reduce the initial fall in torque and the subsequent marching effect, shown by a constant modulus after vulcanization over time, resulting in constant or even improved physical and mechanical properties.

For example, the heat resistance is improved by the use of the anti-reversion coagent in combination with the additive components. Also, a decrease of loss compliance (J") is established with the compositions of the present invention compared to the control and compositions comprising only anti-reversion coagent. This property leads to tires with a reduced rolling resistance. No detrimental effects on the properties have been established in using the combination of anti-reversion coagent and additive components.

Accordingly, the present invention relates to a sulfur-vulcanized rubber composition which comprises the vulcanization reaction product of a composition containing at least:

A) 100 parts by weight of at least one natural or synthetic rubber;

B) 0.1 to 25 parts by weight of sulfur and/or a sufficient amount of a sulfur donor to provide the equivalent of 0.1 to 25 parts by weight of sulfur;

C) 0.1 to 5 parts by weight of at least one anti-reversion coagent comprising at least two groups selected from citraconimide and/or itaconimide groups;

D) 0.1 to 8 parts by weight of at least one additive component which, in the presence of rubber, contains a thiol group and an electron donor group, which additive component is a compound of the formulas (I) or (II)

wherei n X is -N(R₄)(R₅) or - (0-R₉) _n;

R₄, R₅, R₈, and R₉ are i ndependently sel ected from hydrogen,

C_1-18 alkyl groups, C_2-18 al kenyl groups , C_2-18 al kynyl groups,

C_6-18 aryl groups, C_7-30 al karyl groups , C_7-30 aral kyl groups ,

C_1-18 cycl oal kyl groups, optional ly containi ng one or more hetero atoms; i n additi on R₉ may be -R₆- (SH)_m; R₅ and R₇ are i ndependently sel ected from C_1-18 al kyl ene groups,

C_2-18 al kenylene groups, C_2-18 al kynyl ene groups, C_6-18 aryl ene groups, C_7-30 al karyl ene groups, C_7-30 aral kyl ene groups, C_3-18 cycl oal kyl ene groups, optional ly contai ni ng one or more hetero atoms;

R₄ and R₅, R₄ and R₆, or R₅ and R₆ may combine to form a ring or R₄, R₅ and R₆ may combine to form a ring, in which case R₄ is nothing and R₅ is double bonded to the nitrogen, this double bond may be part of an aromatic structure; m and n are 1 or 2;

R₇ and R₈, may combine to form a ring: and

Z₁ and Z₂ are independently selected from oxygen and sulfur; or the additive component is a composition of at least two compounds of which one contains the thiol group and the other the electron donor group, the electron donor group-containing compound being an amine or an ester compound.

R₄, R₅, R₆, R₇, R₈. and R₉ may be substituted by amino, nitroso, sulpho, oxygen, nitrogen, silicon, phosphorus, sulfur, polysulfide, sulphone, sulfoxy and boron, SiO₂, amido, imino, azo, diazo, hydrazo, azoxy, alkoxy, hydroxy, iodine, fluorine, bromine, chlorine, carbonyl, carboxy, ester, carboxylate, SO₂, SO₃, sulphonamido, SiO₃, nitro, imido, thiocarbonyl, cyano, and epoxy groups.

The ring which may be formed by R₇ and R₈ may be, for example, lactone, lactam, or thiol actone. An example of the additive component being a composition of two compounds is the combination of 2-propanethiol and pi peri dine.

Although we do not want to be bound by any particular theory, we believe that the additive components retain their thiol and electron donor groups when mixed with rubber compositions, until sulfur-vulcanization begins, and further that some of the thiol and electron donor groups remain intact in the rubber compositions even after vulcanization is complete. Accordingly, compounds like mercaptobenzothiazole do not have the same properties as the additive components of the present invention since, in the presence of rubber, the thiol group changes immediately to another functional group.

More specific examples of some of the additive components of formula (I) in accordance with the present invention include, but are not limited to, the following:

2-aminothiophenol; 2-amino-ethanethiol (cysteamine); 3-amino-propanethiol; 2-amino-propanethiol; 4-amino-butanethiol; 2-amino-butanethiol; 5-amino-pentanethiol; 2-amino-pentanethiol; 6-amino-hexanethiol; 2-amino-hexanethiol; 7-amino-heptanethiol; 2-amino-heptanethiol; 8-amino-octanethiol; 2-amino-octanethiol; 2-amino-nonanethiol; 2-amino-3-phenyl-propanethiol; 2-amino-2-phenyl-ethanethiol; 2-pyridinethiol; 3-pyridinethiol; 4-pyridinethiol; 3-chloro-2-pyridinethiol; 4-bromo-2-pyridinethiol; 4-ami no-2-pyri di nethi ol ;

N,N,N',N'-tetramethyl-2,4-diamino-6-mercaptopyrimidine; 2-mercaptopyrimidine; 5-mercaptopyrimidine; 6-mercaptopurine;

2-mercapto-4-methyl pyrimidine;

N,N-di-n-butyl-2-amino-4,6-dimercapto-1,3,5-triazine; N,N,N',N'-tetra-n-butyl-2,4-diamino-6-mercapto-1,3,5-triazine; 2-(diethylamino)-ethanethiol; 2-(ethylamino)-ethanethiol; 2-(N-methyl-N-propylamino)-butanethiol; 4-mercapto-1H-pyrazolo(3,4-d)pyrimidine hemihydrate; 2-mercaptoethylether; 1,3-dimercapto-2,3-propanediol; and 1,4-dimercapto-2,3-butanediol .

More specific examples of some of the additive components of formul a

(II) in accordance with the present invention i nclude, but are not l imited to, the fol lowing: methyl-3-mercaptopropionate; ethyleneglycol -bi s-mercaptoacetate; dimethyl -mercaptosuccinate; methyl -thioglycolate; ethyl -thioglycolate; octyl -thioglycol ate; ethyl -2-mercaptopropionate; ethyl -thiosali cate; and ethyl -2-mercapto-dithiopropionate.

Japanese application J63182-355 discloses a sulfur-vulcanized rubber composition comprising bismaleimide and a triazine type compound. However, such a composition exhibits a pronounced reversion effect during vulcanization and, when vulcanized, exhibits several disadvantages regarding certain properties of the composition. In addition, it is known to use a combination of maleimide, mercapto triazine and an acid accepti ng metal compound from J6 3312-334. The same disadvantages as described above are found in such a composition.

Thioglycolic acid derivatives are known as accelerators in rubber vul canization from JP 7314 777, J4 8094-743, and EP 0 041 742. However, in none of these publications is referred to the use of a citraconimide- or itaconimide-containing anti-reversion agent, nor to any influence which the thioglycolic acid derivatives might have when used in combination with such compounds in the sulfur-vulcanization of rubber.

In addition, the use of some sulfur-containing citraconimides in sulfur vulcanization is known from U.S. patent 3,974,163. These compounds inhibit premature vulcanization of diene rubbers, optionally in the presence of vulcanization accelerators. However, the particular advantages of the compositions of the present invention are neither disclosed nor suggested by this publication.

Finally, in Canadian Patent no. 738,500 the vulcanization of rubbers in the absence of sulfur, with either bis-maleimides and bis-citraconimides, is disclosed. This process had, for its purpose, to be an alternative to sulfur-vulcanization processes. However, the rubber products made by the process of this patent suffer from the usual disadvantages of peroxide-cured rubbers such as low tensile strength and significant reductions in other important properties. This patent does not disclose the use of the bis-maleimides or bis-citraconimides in the sulfur-vulcanization of rubber.

The present invention is applicable to all natural and synthetic rubbers. Examples of such rubbers include, but are not limited to, natural rubber, styrene-butadiene rubber, butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubber, chloroprene rubber, isoprene-isobutylene rubber, brominated isoprene-isobutylene rubber, chlorinated isoprene-isobutylene rubber, ethyl ene-propylene-di ene ter-polymers, as well as combinations of two or more of these rubbers and combinations of one or more of these rubbers with other rubbers and/or thermoplastics.

Examples of sulfur which may be used in the present invention include various types of sulfur such as powdered sulfur, precipitated sulfur and insoluble sulfur. Also, sulfur donors may be used in place of, or in addition to sulfur in order to provide the required level of sulfur during the vulcanization process. Examples of such sulfur donors include, but are not limited to, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, dipentamethylene thiuram hexasulfide, dipentamethylene thiuram tetrasulfide, dithiodimorpholine, caprolactam disulfide, dialkylthiophosphoryl disulfide, and mixtures thereof.

In this text, references to sulfur shall include sulfur donors and mixtures of sulfur and sulfur donors. Further, references to the quantity of sulfur employed in the vulcanization, when applied to sulfur donors, refer to a quantity of sulfur donor which is required to provide the equivalent amount of sulfur that is specified.

The anti-reversion coagents of the present invention are characterized by the fact that they must be capable of forming cross-links bonded to the rubber by a carbon-carbon linkage. This type of cross-link is known in the rubber literature from, for example, Krasheninnikov, N.A., et al., Kauchuk i Rezina, No. 3, pp. 16-20, 1975. Such cross-links bonded to the rubber by a carbon-carbon linkage are highly desirable in rubbers, and particularly sulfur-vulcanized rubbers since such cross-links are thermally stable.

Anti-reversion coagents of the present invention include, but are not limited to compounds represented by the general formula A:

Q₁-D-(Q₂)_p (A);

wherein D, optionally containing one or more heteroatoms selected from nitrogen, oxygen, silicon, phosphorus, boron, sulphone, sulphoxy, and sulfur, is a monomeric or oligomeric divalent, trivalent or tetravalent group, p is an integer selected from 1, 2 or 3, Q₁ and Q₂ are independently selected from the formulas III and IV: and

wherein R₁, R₂ and R₃ are independently selected from hydrogen, C₁-C₁₈ alkyl groups, C₃-C₁₈ cycloalkyl groups, C₁-C₁₈ aryl groups, C₇-C₃₀ aralkyl groups and C₇-C₃₀ alkaryl groups and R₂ and R₃ may combine to form a ring when R₁ is hydrogen; B and B₁ are independently selected from the following hetero atoms: oxygen and sulfur.

The imides of the present invention are, in general, known compounds and may be prepared by the methods disclosed in, Galanti, A.V. et al., J. Pol. Sc.: Pol. Chem. Ed., Vol. 19, pp. 451-475, (1981); Galanti, A.V. et al., J. Pol. Sc: Pol. Chem. Ed., Vol. 20, pp. 233-239 (1982); and Hartford, S.L. et al., J. Pol. Sc: Pol. Chem. Ed., Vol. 16, pp. 137-153, 1978, the disclosures of which are hereby incorporated by reference.

The imide compounds useful in the present invention and represented by the formula A include, but are not limited to, the biscitraconimides wherein Q₁ and Q₂ are of the formula III, R₁=R₂=R₃=H, p=1 and B=B_ι=oxygen; the bis-itaconimides wherein Q₁ and Q₂ are of the formula IV, R₁=R₂=R₃=H, p=1 and B=B₁=oxygen; the mixed citraconimide and itaconimide wherein Q₁ is of the formula III, Q₂ is of the formula IV, R₁=R₂=R₃=H, p=1 and B=B₁=oxygen; and mixtures of the above-mentioned compounds.

More specifically, the group D mentioned in the formula A can be a monomeric divalent, trivalent or tetravalent linear or branched radical chosen from a C₁-C₁₈ alkyl, C₂-C₁₈ alkenyl, C₂-C₁₈ alkynyl, C₃-C₁₈ cycloalkyl, C₃-C₁₈ polycycloalkyl, C₆-C₁₈ aryl, C₆-C₃₀ polyaryl, C₇-C₃₀ aralkyl, C₇-C₃₀ alkaryl, oligomers of one or more of these radicals, and which radicals may optionally contain one or more of oxygen, nitrogen, silicon, phosphorus, sulfur, polysulfide, sul phone, sulfoxy and boron, all of which radicals may also be optionally substituted at one or more of the atoms in the radical with a substituent selected from oxygen, nitrogen, silicon, SiO₂, sulfoxy, boron, sulfur, phosphorus, amido, imino, azo, diazo, hydrazo, azoxy, alkoxy, hydroxy, iodine, fluorine, bromine, chlorine, carbonyl, carboxy, ester, carboxylate, SO₂, SO₃, sulphonamido, SiO₃, nitro, imido, thiocarbonyl, cyano, and epoxy groups.

More specific examples of some of the imide compounds useful in the present invention include, but are not limited to, the following: bis(2-citraconimidoethyl) sulfide; bis(2-citraconimidoethyl) disulfide; bis(2-citraconimidoethyl) polysulfide; bis(3-citraconimidopropyl) disulfide; bis(4-citraconimido-2-methyl butyl) disulfide; bis(2-citraconimidocyclohexyl) disulfide;

2-citraconimidoethyl-3-citraconimidopropyl sulfide;

1,2-bis(2-citraconimidophenylthio)-ethane;

1,2-bis(2-citraconimido-5-chlorophenylthio)-ethane; 1,4-bis(2-citraconimidophenylthio)-butene-2;

1,3-bis(2-citraconimidophenylthio)-propanone-2; α,α'-bis(2-citraconimidophenylthio)-m-xylene;

2-citraconimidoethyl-2-citraconimidophenyl sulfide; bis(4-citraconimidophenyl) disulfide; bis(4-citraconimido-3-chlorophenyl) disulfide; bis(2-citraconimidophenyl) disulfide; bis(2-citraconimidophenyl) sulfide; bis(2-citraconimidophenyl) tetrasulfide; bis(2-citraconimido-4-methoxyphenyl) disulfide; 2,4,6-tris(2-citraconimidoethylthio)-1,3,5-cyanurate; 2,5-bis(2-citraconimidoethylthio)-1,3,4-thiadiazole; bis(4-citraconimido-2,6-dimethylphenyl) disulfide; bis(2'-citraconimidoethoxyethyl) disulfide; bis(3,5-dicitraconimidophenyl) disulfide; N,N'-ethylene-bis-citraconic imide (BCI-C2); N,N'-hexamethylene-bis-citraconic imide (BCI-C6);N,N'-tetramethylene-bis-citraconic imide; N,N'-2-methyl-pentamethylene-bis-citraconic imide; N,N'-(1,3-propylene)-bis-citraconic imide; N,N'-(3,3'-oxydipropylene)-bis-citraconic imide; N,N'-(aminodiethylene)-bis-citraconic imide; N,N'-(aminodipropylene)-bis-citraconic imide; N,N'-(1,10-(4,7-dioxa)-decanediyl)-bis-citraconic imide; N,N'-(4,4'-(di-(2-methylcyclohexyl)methylene)-bis-citraconic imide;N,N'-(4,4'-dicyclohexyl-isopropylene)bis-citraconic imide; N,N'-(4,4'-dicyclohexyloxy)-bis-citraconic imide;

N N'-(4,4'-dicyclohexylene)-bis-citraconic imide; N,N'-o-phenylene-bis-citraconic imide; N,N'-m-phenylene-bis-citraconic imide(BCI-MP); N,N'-m-phenylene-bis-itaconic imide (BII-MP); N,N'-p-phenyl ene-bis-citraconic imide; N,N'-(5-chloro-1,3-phenylene)-bis-citraconic imide; N,N'- 5-hydroxy-1,3-phenylene)-bis-citraconic imide; N,N'- 5-methoxy-1,3-phenylene)-bis-citraconic imide; N,N'- α,α'-(1,3-dimethylphenylene))-bis-citraconic imide; N,N'- 4,4'-(1,10-decanediol-dibenzoate))-bis-citraconic imide (BCI-BAE10); N,N'-(4,4'-diphenyl-bisphenol-A-ether)-bis-citraconic imide; N,N'-(4,4'-biphenylene)-bis-citraconic imide; N,N'-(4,4'-diphenylmethylene)-bis-citraconic imide (BCI-DPM);N,N'-(4,4'-diphenylmethylene)-bis-itaconic imide (BII-DPM);N,N'-m-xylylene-bis-citraconic imide (BCI-MX); N,N'-(4,4'-diphenylisopropylene)-bis-citraconic imide; N,N'-(3,3'-dimethyl-4,4'-biphenylene)-bis-citraconic imide: N,N'-(3,3'-dichloro-4,4'-biphenylene-bis-citraconic imide;

N,N'-(3,3'-difluoro-4,4'-biphenylene)-bis-citraconic imide;

N,N'-(4,4'-oxydiphenylene)-bis-citraconic imide;

N ,N'-(4,4'-diphenylsulfone)-bis-citraconic imide;

N,N'-(4,4'-diphenylcarboxy)-bis-citraconic imide;

N,N'-(4,4'-(1,1-diphenylpropylene))-bis-citraconic imide;

N,N'-3,5-(1,2,4-triazole)-bis-citraconic imide;

N.N'-dodecamethylene-bis-citraconic imide;

N,N'-(2,2,4-trimethylhexamethylene)-bis-citraconic imide;

N,N'-(1,11-(4,8-dioxa-undecanediyl))-bis-citraconic imide;

N,N'-(4,4'-benzophenonediyl)-bis-citraconic imide;

N,N'-(1,4-anthraquinonediyl)-bis-citraconic imide;

N,N'-(1,3-naphthalenediyl)-bis-citraconic imide;

N,N'-(1,4-naphthalenedτyl)-bis-citraconic imide;

N,N'-(1,5-naphthalenediyl)-bis-citraconic imide;

N,N'-(1,3-cyclohexylene)-bis-citraconic imide;

N,N'-(1,4-cyclohexylene)-bis-citraconic imide;

N,N'-(5-methyl-1,3-phenylene)-bis-citraconic imide;

N,N'-(α,α'-(1,3-dimethylcyclohexylene))-bis-citraconic imide

(BCI-BAC) ;

N,N'-(α,3-(1,1,5,5-tetramethyl-cyclohexylene))-bis-citraconic imide;

N ,N'-(isophoronyl)-bis-citraconic imide;

N,N'-(dimethyltricyclododecylene)-bis-citraconic imide;

N ,N'-octamethylene-bis-citraconic imide;

N,N'-(1,2-propylene)-bis-citraconic imide;

N,N'-decamethylene-bis-citraconic imide;

N,N'-heptamethyl ene-bis-citraconic imide;

N,N'-(5-bromo-1,3-phenylene)-bis-citraconic imide;

N,N'-(1,13-(7-aza-tridecanediyl))-bis-citraconic imide;

N,N'-(1,7-(4-aza-heptanediyl))-bis-citraconic imide;

N,N'-(1,11-(3,6,9-triaza-undecanediyl))-bis-citraconic imide;

N,N'-(1,8-(3,6-diaza-octanediyl)-bis-citraconic imide; N,N'-(N,N'-di-2-ethylpiperazinyl)-bis-citraconic imide;

N,N'-(2-hydroxy-1,3-propylene)-bis-citraconic imide;

N,N',N"-(2,4,6-trihexamethylene-isocyanuratetriyl)-tris-citraconic imide (TCI-AA33); N,N'-(3,5-benzoic aciddiyl)-bis-citraconic imide;

N,N'-pentamethyl ene-bis-citraconic imide;

N,N'-undecamethyl ene-bis-citraconic imide;

N,N'-(4-(N-methylene-citraconic imide)-octamethylene-bis-citraconic imide (TCI-C9v); N,N'-nonamethylene-bis-citraconic imide;

N ,N'-(2-butyl-2-ethylpentamethylene)-bis-citraconic imide;

N,N'-polytetrahydrofuryl-bis-citraconic imide; N,N'-(Jeffamine D230^®)-bis-citraconic imide; N,N'-(Jeffamine D2000^®)-bis-citraconic imide; and N,N'-(Jeffamine ED600^®)-bis-citraconic imide.

Jef famine D230^®, Jef famine D2000^® and Jef famine ED600^® are registered tradenames of the Texaco company. The biscitraconic imides based on these amines have the following general structure:

Q₁-CH(CH₃)-CH₂-(O-CH₂-CH(CH₃))_q-O-CH₂CH(CH₃)-Q₂

Q₁ and Q₂ are as defined above, q represents from 1 up to 1000.

In addition, the bis-, tris- and tetra-itaconimides of the present invention may be the same as mentioned above, except that all citraconimide groups are exchanged for itaconimide groups. The same materials as mentioned above may be mixed imides if some of the citraconimide groups are exchanged for itaconimide groups.

The amount of sulfur to be compounded with the rubber is, based on 100 parts of rubber, usually 0.1 to 25 parts by weight, and more preferably 0.2 to 8 parts by weight. The amount of sulfur donor to be compounded with the rubber is an amount sufficient to provide an equivalent amount of sulfur which is the same as if sulfur itself were used. The amount of anti-reversion coagent to be compounded with the rubber is, based on 100 parts of rubber, 0.1 to 5 parts by weight, and more preferably 0.2 to 3 parts by weight. These ingredients may be employed as a pre-mix, or added simultaneously or separately, and they may be added together with other rubber compounding ingredients as well.

The amount of additive component to be compounded with the rubber is, based on 100 parts of rubber, 0.1 to 8 parts. More preferably, 0.2 to 2 parts of additive component per 100 parts of rubber are employed.

In most circumstances it is also desirable to have a vulcanization accelerator in the rubber compound. Conventional, known vulcanization accelerators may be employed. The preferred vulcanization accelerators include mercaptobenzothiazole, 2,2'-mercaptobenzothiazole disulfide, sulfenamide accelerators including N-cyclohexyl-2-benzothiazole sulfenamide, N-tertiary-butyl-2-benzothiazole sulfenamide, N,N'-dicyclohexyl-2-benzothiazole sulfenamide, and

2-(morpholinothio)benzothiazole; thiophosphoric acid derivative accelerators, thiurams, dithiocarbamates, diphenyl guanidine, diortho- tolyl guanidine, dithiocarbamyl sul fenamides, xanthates, triazine accelerators and mixtures thereof.

When the vulcanization accelerator is employed, quantities of from 0.1 to 8 parts by weight, based on 100 parts by weight of rubber composition, are used. More preferably, the vulcanization accelerator comprises 0.3 to 4 parts by weight, based on 100 parts by weight of rubber.

Other conventional rubber additives may also be employed in their usual amounts. For example, reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, as well as mixtures of fillers, may be included in the rubber composition. Other additives such as process oils, tackifiers, waxes, antioxidants, antiozonants, pigments, resins, plasticizers, process aids, factice, compounding agents and activators such as stearic acid and zinc oxide may be included in conventional, known amounts. For a more complete listing of rubber additives which may be used in combination with the present invention see, W. Hofmann, "Rubber Technology Handbook, Chapter 4, Rubber Chemicals and Additives, pp. 217-353, Hanser Publishers, Munich 1989.

Further, scorch retarders such as phthalic anhydride, pyromellitic anhydride, benzene hexacarboxylic trianhydride, 4-methyl phthalic anhydride, trimellitic anhydride, 4-chlorophthalic anhydride, N-cyclohexyl-thiophthalimide, salicylic acid, benzoic acid, maleic anhydride and N-nitrosodi phenyl ami ne may also be included in the rubber composition in conventional, known amounts. Finally, in specific applications it may also be desirable to include steel-cord adhesion promoters such as cobalt salts and dithiosul fates in conventional, known quantities.

The present invention also relates to a vulcanization process which comprises the step of vulcanizing at least one natural or synthetic rubber in the presence of 0.1 to 25 parts by weight of sulfur or a sulfur donor per 100 parts by weight of rubber, characterized in that said process is carried out in the presence of an effective amount of an anti -reversion coagent and an effective amount of an additive component to improve the effect of the anti-reversion coagent.

The process is carried out at a temperature of 110-220°C over a period of up to 24 hours. More preferably, the process is carried out at a temperature of 120-190°C over a period of up to 8 hours in the pre sence of 0.1 to 5 parts by weight of anti-reversion coagent and 0.1 to 8 parts by weight of additive component. Even more preferable is the use of 0.2-3 parts by weight of anti-reversion coagent with 0.2-2 parts by weight of additive component. All of the additives mentioned above with respect to the rubber composition may also be present during the vulcanization process of the invention.

In a more preferred embodiment of the vulcanization process, the vulcanization is carried out at a temperature of 120-190°C over a period of up to 8 hours and in the presence of 0.1 to 8 parts by weight, based on 100 parts by weight of rubber, of at least one vulcanization accelerator.

In another preferred embodiment of the vulcanization process, the anti-reversion coagent is selected from a compound of the formula A.

The present invention also comprises the use of an anti-reversion coagent in combination with an additive component in a process for the sulfur-vulcanization of rubber.

Finally, the present invention also includes articles of manufacture, such as tires, belts or inner tubes which comprise sulfur-vulcanized rubber which is vulcanized in the presence of the anti -reversion coagents and additive components of the present invention. More particularly, the compositions of the present invention can be used in tire treads for truck tires and off-the-road tires, in particular, for sidewalls, for tire carcasses and for steel-cord skim stocks. In belts, the rubber compositions of the present invention are particularly useful for conveyor belts and V-belts which are subjected to high loading and abrasion in service. The invention is further illustrated by the following examples which are not to be construed as limiting the invention in any way. The scope of the invention is to be determined from the claims appended hereto.

EXPERIMENTAL METHODS USED IN THE EXAMPLES

Compounding, Vulcanization and Characterization of Compounds

In the following examples, rubber compounding, vulcanization and testing was carried out according to standard methods except as otherwise stated:

Base compounds were mixed in a Farrel Bridge BR 1.6 liter Banbury type internal mixer (preheating at 50°C, rotor speed 77 rpm, mixing time 6 min with full cooling).

Vulcanization ingredients and coagents were addded to the compounds on a Schwabenthan Polymix 150L two-roll mill (friction 1:1.22, temperature 70°C, 3 min).

Cure characteristics were determined using a Monsanto rheometer MDR 2000E (range 2.5 Nm/arc 0.5°, ISO 3417): delta torque or extent of crosslinking (R∞) is the maximum torque (MH, also denoted as initial torque maximum, T_i) minus the minimum torque (ML). Scorch safety (t_s2) is the time to 2% of delta torque above minimum torque (ML), optimum cure time (tgø) is the time to 90% of delta torque above minimum.

Sheets and test specimens were vulcanized by compression molding in a Fontyne TP-400 press. Tensile measurements were carried out using a Zwick 1445 tensile tester (ISO-3712 dumbbells, tensile properties according to ASTM D 412-87, tear strength according to ASTM D 624-86).

Hardness was determined according to DIN 53505, and ISO 48 (IRHD).

Rebound resilience was measured at room temperature (RT= 23°C) according to ASTM D 1054-87.

Compression set was determined after 72 h at 23°C according to ASTM D 395-89 (ISO R 815) .

Ageing of test specimens was carried out in a ventilated oven in the presence of air or nitrogen at 100°C for 3 days or at 70°C for 14 days (ISO 188).

Heat build-up and permanent set after dynamic loading were determined using a Goodrich Flexometer (load 1 MPa, stroke 0.445 cm, frequency 30 Hz, start temperature 100°C, running time 30 min or start at room temperature (RT=23°C), running time 60 min; ASTM D 623-78).

Abrasion was determined using a Zwick abrasion tester as volume loss per 40 m path travelled (DIN 53516).

Dynamic mechanical analysis was carried out using a RDA-700 (prestrain 0.75%, frequency 15 Hz at 60°C or 1 Hz at 0° or 20°C, ASTM D 2231). Examples 1-2 and Comparative Examples A-C

Natural rubber was vulcanized using formulations listed in Table 1. Comparative Example A was a control example with no coagent or additive component.

Table 2 lists the cure characteristics of the compositions A-C, 1, and 2 obtained at 150°C and 170°C. Values in parentheses designate the values obtained for the vulcanizates cured at 170°C. TABLE 2

The vulcanized rubbers listed in Table 1 were then tested for physical and mechanical properties upon overcure.

TABLE 3

Table 3 lists the properties of the vulcanizates cured at 150 °C for t₉₀ and for 60 min, all properties measured at room temperature. Values in parentheses designate the values obtained for the vulcanizates cured at 150 °C for 60 min.

TABLE 4

Table 4 lists the properties of the vulcanizates cured at 170°C for t₉₀ and for 30 min, all properties measured at room temperature. Values in parentheses designate the values obtained for the vulcanizates cured at 170 °C for 30 min. TABLE 5

Table 5 lists the properties of the vulcanizates cured at 170°C for t₉₀ and for 30 min, all properties measured at 100°C. Values in parentheses designate the values obtained for the vulcanizates cured at 170°C for 30 min.

TABLE 6

Table 6 lists the heat build up and permanent set properties of samples cured at 150°C for t₉₀ and for 60 min and then aged for 3 days at 100°C. Values in parentheses designate the values obtained for the aged vulcanizates cured at 150°C for 60 min.

TABLE 7

Table 7 lists the heat build up and permanent set properties of samples cured at 170°C for t₉₀ and for 30 min and then aged for 3 days at 100°C. Values in parentheses designate the values obtained for the aged vulcanizates cured at 170°C for 30 min.

From the results in Tables 3-7 it is clear that compositions according to the invention show no marching effect and have constant or even improved basic properties in the ultimate vulcanizates. More particularly compositions A and C show a decrease in modulus during overcure signifying reversion. Composition B has a marching modulus, i.e. an increase in modulus during overcure. However, compositions 1 and 2 show a retention of the modulus during overcure and improvement of the ultimate vulcanizates, as is seen for example in the reduction of the heat building up and permanent set, resulting in an improved heat resistance.

Storage modulus (G'), loss modulus (G"), and loss tangent (tans) are measured by dynamic mechanical analysis.

Increased loss modulus (G") leads to an improvement of tire properties such as wet grip or skid resistance (K.A. Grosch, Nature, 197, 858, 1963). Increased storage modulus (G') and decreased loss tangent (tanδ) result in a lower loss compliance (tanδ/G'=J") which leads to improved tire properties such as reduced rolling resistance (J.M. Collins et al., Trans. Inst. Rubber Ind. 40, T239, 1964), which by consequence leads to fuel savings during service. Tables 8-11 show the results of the dynamic mechanical analyses of the compositions of the present invention 1 and 2 compared to the comparative compositions A and B.

TABLE 8

Dynamic-mechanical data of the compositions cured at 150°C for t₉₀ and for 60 min. obtained at 60°C and 15 Hz. Values in parentheses designate the properties of the compositions cured at 150°C for 60 min.

TABLE 9

Dynamic-mechanical data of the compositions cured at 170°C for t₉₀ and for 30 min. obtained at 60°C and 15 Hz. Values in the parentheses designate the properties of the compositions cured at 170°C for 30 min.

TABLE 10

Dynamic-mechanical data of the compositions cured at 150°C for t₉₀ and for 60 min. obtained at 0±1°C and 1 Hz. Values in the parentheses designate the properties of the compositions cured at 150°C for 60 min.

TABLE 11

Dynamic-mechanical data of the compositions cured at 170°C for t₉₀ and for 30 min. obtained at 0±1°C and 1 Hz. Values in the parentheses designate the properties of the compositions cured at 170°C for 30 min.

From the results in Tables 8 to 11 it is clear that the use of the compositions of the present invention in tires improves its properties. This is especially shown for the loss compliance which is decreased significantly for the compositions of the present invention, resulting in a reduced rolling resistance. The loss modulus G" remains constant or is improved slightly.

Examples 3-6 and Comparative Examples D-E

Examples 3-6 and Comparative Examples D-E are formulations for truck tire treads. The components of each formulation are given in Table 12, the cure characteristics of these formulations are listed in Tables 13-14, the physical and mechanical properties for different curing conditions are given in Tables 15-16, and the dynamic-mechanical data are listed in Tables 17-18. The rubber compounding took place in the following order. Natural rubber (NR) and butadiene rubber (BR) were separately mixed with vulcanization ingredients on a W&P GK 5E (volume 5.0 1: 70% load factor; preheating at 50°C, rotor speed 30 rpm, mixing time 6 min. for NR and 8 min. for BR). Then NR and BR were mixed together in a W&P GK 5E (volume 5.0 1: 71.2% load factor; preheating 50°C, rotor speed 30 rpm, mixing time 3 min.). Finally, coagents and additive components were mixed in on a Schwabenthan Polymix 150L two-roll mill (friction 1:1.22, temperature 50-70°C, 10 minutes).

TABLE 13

Table 13 lists the cure characteristics of the compositions D-E and 3-6 obtained at 150° and 170°C. Values in parentheses designate the values obtained for the vulcanizates cured at 170°C.

TABLE 14

Table 14 lists the torque values of the compositions D-E and 3-6 measured at tmax, 12, 36, and 60 minutes cured at 170°C.

The results in Table 14 show that composition D suffers from an initial fall in torque and continues to reverse. Composition E shows the beginning of a marching cure. However, the compositions of the present invention show a clear retention of the torque, thus no initial fall in torque and no marching. TABLE 15

Moduli of the vulcanizates cured at 150°C for t₉₀ and for 60 min. Values in parentheses designate the moduli of the vulcanizates cured at 150°C for 60 min. The moduli have also been measured for vulcanizates cured for 240 min. (see third values in parentheses).

TABLE 16

Moduli of the vulcanizates cured at 170°C for t₉₀ and for 30 min. Values in parentheses designate the moduli of the vulcanizates cured at 170°C for 30 min.

From the results in Tables 15-16 it is clear that truck tire tread compositions according to the invention show a very good retention of the modulus during overcure. TABLE 17

Heat build up (HBU) and permanent set of vulcanizates cured at 150°C for tgo, for 60 min. and for 240 min. Values in parentheses designate the properties of the vulcanizates cured at 150°C for 60 and 240 min. resp.

TABLE 18

Heat build up (HBU) and permanent set of vulcanizates cured at 170°C for t₉₀ and for 30 min. Values in parentheses designate the properties of the vulcanizates cured at 170°C for 30 min.

The data shown in Tables 17-18 clearly reflects the advantage of the composition of the present invention in reducing the heat during flexing, which results leads to an improvement in heat resistance.

Tables 19-22 show the results of the dynamic mechanical analyses of the compositions of the present invention 3-6 compared to the comparative compositions D and E.

TABLE 19

Dynamic-mechanical data of the compositions cured at 150°C for t₉₀, for 60 min. and for 240 min. obtained at 60°C and 15 Hz. Values in parentheses designate the properties of the compositions cured at 150°C for 60 min. and 240 min. respectively.

TABLE 20

TABLE 21

Dynamic-mechanical data of the compositions cured at 150°C for t₉₀, for 60 min. and for 240 min. obtained at 20°C and 1 Hz. Values in the parentheses designate the properties of the compositions cured at 150°C for 60 min. and 240 min. respectively.

TABLE 22

Dynamic-mechanical data of the compositions cured at 170°C for t₉₀ and for 30 min. obtained at 20°C and 1 Hz. Values in the parentheses designate the properties of the compositions cured at 170°C for 30 min.

From the results in Tables 19 to 22 it is clear that the use of the truck tire tread compositions of the present invention in tires improves its properties. This is especially shown for the loss compliance which is decreased for the compositions of the present invention, resulting in a reduced rolling resistance. The loss modulus G" remains constant.

Example 7 and Comparative Examples F-G

Example 7 and Comparative Examples F-G are formulations for off-the- road tire treads. The components of each formulation are given in Table 23, the cure characteristics of the formulations are listed in Table 24 and the physical and mechanical properties for different curing conditions are given in Tables 25-26.

TABLE 24

Table 24 lists the cure characteristics of the compositions F-G and 7 obtained at 150° and 170°C. Values in parantheses designate the values obtained for the vulcanizates cured at 170°C.

TABLE 25

Mechanical properties of the vulcanizates cured at 170°C for t₉₀ and for 30 min. Values in parentheses designate the properties of the vulcanizates cured at 170°C for 30 min.

It is clear from the results listed in Table 25 that the off-the-road tire composition of the present invention is best in retention of the mechanical properties after overcure, especially in the retention of the modulus, i.e. no marching occurs.

TABLE 26

Heat build up in (ΔT,°C), measured at 100°C, of the vulcanizates cured at 170°C for 8 and 30 minutes. Values in parentheses designate the properties of the vulcanizates cured at 170°C for 30 minutes

The data shown in Table 26 clearly reflects the advantage of the composition of the present invention in reducing the heat during flexing, especially when aged, which results leads to an improvement in heat resistance.

Claims

What is claimed is:

A sulfur-vulcanized rubber composition which comprises the vulcanization reaction product of:

A) 100 parts by weight of at least one natural or synthetic rubber;

D) 0.1 to 8 parts by weight of at least one additive component which, in the presence of rubber, contains a thiol group and an electron donor group, which additive component is a compound of formula (I) or (II)

X - R₆ - (SH)_m (I)

wherein X is -N(R₄)(R₅) or -(O-R₉)_n;

R₄, R₅, R₈, and R₉ are independently selected from hydrogen,

C_1-18 alkyl groups, C_2-18 alkenyl groups, C_2-18 alkynyl groups,

C_6-18 aryl groups, C_7-30 alkaryl groups, C_7-30 aralkyl groups,

C_3-18 cycloalkyl groups, optionally containing one or more hetero atoms;

i n addition R₉ may be -R₆- (SH)_m;

R₆ and R₇ are i ndependently sel ected from C_1-18 al kyl ene groups, C_2-18 al kenyl ene groups, C₂-₁₈ al kynyl ene groups, C_6-18 aryl ene groups, C_7-30 al karyl ene groups, C_7-30 aral kyl ene groups, C_3-18 cycl oal kyl ene groups, opti onal ly contai ni ng one or more hetero atoms;

R₄ and R₅, R₄ and R₆, or R₅ and R₆ may combine to form a ring or R₄, R₅ and R₆ may combine to form a ring, in which case R₄ is nothing and R₅ is double bonded to the nitrogen, this double bond may be part of an aromatic structure;

R₄, R₅, R₆, R₇, R₈, and R₉ may be substituted by amino, nitroso, sulpho, oxygen, nitrogen, silicon, phosphorus, sulfur, polysulfide, sulphone, sulfoxy and boron, SiO₂, amido, imino, azo, diazo, hydrazo, azoxy, alkoxy, hydroxy, iodine, fluorine, bromine, chlorine, carbonyl, carboxy, ester, carboxylate, SO₂, SO₃, sulphonamido, SiO₃, nitro, imido, thiocarbonyl, cyano, and epoxy groups;

m and n are 1 or 2;

R₇ and R₈, may combine to form a ring; and

Z₁ and Z₂ are independently selected from oxygen and sulfur; or which additive component is a composition of at least two compounds of which one contains the thiol group and the other the electron donor group, which electron donor group containing compound is an amine or an ester compound.

2. A sulfur-vulcanized rubber composition as claimed in claim 1, wherein the additive component is a composition which comprises 2-propanethiol and piperidine.

3. A process for the vulcanization, at a temperature of from 110 to 220°C for up to 24 hours, of a vulcanizable composition comprising at least one natural or synthetic rubber in the presence of 0.1 to 25 parts by weight of sulfur or a sufficient amount of a sulfur donor to provide the equivalent of 0.1 to 25 parts by weight of sulfur, said process being carried out in the presence of 0.1 to 5 parts by weight of at least one anti-reversion coagent comprising at least two groups selected from citraconimide and/or itaconimide groups, and in the presence of 0.1 to 8 parts by weight of at least one additive component which, in the presence of rubber, contains a thiol group and an electron donor group, which additive component is a compound of formula (I) or (II)

X - R₆ - (SH)_m (I)

wherein X is -N(R₄)(R₅) or -(O-R₉)_n;

R₄, R₅, R₈, and R₉ are independently selected from hydrogen, C_1-18 alkyl groups, C_2-18 alkenyl groups, C_2-18 alkynyl groups, C_6-18 aryl groups, C_7-30 alkaryl groups, C_7-30 aralkyl groups, C_7-30 cycloalkyl groups, optionally containing one or more hetero atoms; in addition R₉ may be -R₆-(SH)_m;

R₆ and R₇ are independently selected from C_1-18 alkyl ene groups,

C_2-18 alkenylene groups, C_2-18 alkynylene groups, C_6-18 arylene groups, C_7-30 alkarylene groups, C_7-30 aralkylene groups, C_3-18 cycloalkylene groups, optionally containing one or more hetero atoms;

R₄, R₅, R₆, R₇, R₈, and R₉ may be substituted by ami no, nitroso, sulpho, oxygen, nitrogen, silicon, phosphorus, sulfur, polysulfide, sulphone, sulfoxy and boron, SiO₂, amido, imino, azo, diazo, hydrazo, azoxy, alkoxy, hydroxy, iodine, fluorine, bromine, chlorine, carbonyl, carboxy, ester, carboxylate, SO₂, SO₃, sulphonamido, SiO₃, nitro, imido, thiocarbonyl, cyano, and epoxy groups;

m and n are 1 or 2;

R₇ and R₈, may combine to form a ring; and Z₁ and Z₂ are independently selected from oxygen and sulfur; or which additive component i s a composition of at least two compounds of which one contains the thiol group and the other the electron donor group, which electron donor group containing compound is an amine or an ester compound.

4. The use of an anti -reversion coagent compri sing at least two groups selected from citraconimide and/or itaconimide groups in combination with an additive component which, in the presence of rubber, contains a thiol group and an electron donor group, which additive component is a compound of formula (I) or (II)

X - R₆ - (SH)_m (I)

wherein X is -N(R₄) (R₅) or -(O-R₉)_n;

R₄, R₅, R₈, and R₉ are independently selected from hydrogen, C_1-18 al kyl groups, C_1-18 al kenyl groups, C_1-18 al kynyl groups, C_6-18 aryl groups, C_7-30 alkaryl groups, C_7-30 aral kyl groups, C_3-18 cycloal kyl groups, optional ly containing one or more hetero atoms; in addition R₉ may be -R₆-(SH)_m;

R₆ and R₇ are independently selected from C_1-18 al kylene groups,

C_2-18 al kenyl ene groups, C_1-18 al kynyl ene groups, C_6-18 aryl ene groups, C_7-30 al karyl ene groups, C_7-30 aral kyl ene groups, C_3-18 cycloal kyl ene groups, optional ly containing one or more hetero atoms;

R₄, R₅, R₆, R₇, R₈, ancl R₉ may be substituted by amino, nitroso, sulpho, oxygen, nitrogen, silicon, phosphorus, sulfur, polysulfide, sulphone, sulfoxy and boron, SiO₂, amido, imino, azo, diazo, hydrazo, azoxy, alkoxy, hydroxy, iodine, fluorine, bromine, chlorine, carbonyl, carboxy, ester, carboxylate, SO₂, SO₃, sulphonamido, SiO₃, nitro, imido, thiocarbonyl, cyano, and epoxy groups;

m and n are 1 or 2;

R₇ and R₈, may combine to form a ring; and

Z₁ and Z₂ are independently selected from oxygen and sulfur; or which additive component is a composition of at least two compounds of which one contains the thiol group and the other the electron donor group, which electron donor group containing compound is an amine or an ester compound, in the sulfur vulcanization of rubber.

5. An article of manufacture comprising a rubber vulcanized by the process of claim 3.