WO2003070820A1

WO2003070820A1 - Steel cord adhesion promoting accelerators

Info

Publication number: WO2003070820A1
Application number: PCT/EP2003/001260
Authority: WO
Inventors: Rabindra Nath Datta; Auke Gerardus Talma; Nicolaas Maria Huntink
Original assignee: Flexsys B.V.
Priority date: 2002-02-22
Filing date: 2003-02-06
Publication date: 2003-08-28
Also published as: AU2003205749A1

Abstract

The present invention relates to a composition comprising a rubber, sulfur, a steel cord bonding agent, and a steel cord adhesion promoting accelerator comprising a pryrimidine derivative of Formula I or a pyrazine derivative of Formula II: (Formula I and Formula II); wherein R1, R2, and R3 independently represent hydrogen or a C1-C12 alkyl group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms, R1 and R2 or R2 and R3, providedthat R2 and R3 are on adjacent carbon atoms, may, together with the carbon atoms to which they are attached, form a 5- or 6- membered ring, said ring optionally being substituted with a C1-C6 alkyl group, said ring or group optionally containing one or more oxygen, nitrogen and/or sulfur atoms, x is 1 or 2, provided that when x is 2, y is 0, R4 represents hydrogen or a C1-C6 alkyl group, y is 0 or 1, z is 1 to 4, and R5 represents hydrogen or a mono-, di-, or tetravalent C1-C18 hydrocarbon group, optionally containing one or more oxygen, nitrogen, phosphorous and/or sulfur atoms, if y is 1, R4 and R5 may, together with the nitrogen atom to which they are attached, form a 5- or 6-membered ring, said ring optionally containing one or more oxygen, nitrogen and/or sulfur atoms, with the providso that the composition does not contain a non-thiazole sulfenamide accelerator. Preferably, a pyrimidine derivative of Formula I is used. Preferably, said pyrimidine or pyrazine derivative is in a granulated form. The invention also relates to a process comprising vulcanizing said composition and to rubber articles, such as pneumatic tyres, comprising the rubber vulcanizate obtained by said process.

Description

STEEL CORD ADHESION PROMOTING ACCELERATORS

The invention relates to a composition comprising a rubber, sulfur, and a steel cord bonding agent. The invention also relates to a process for improving rubber to steel cord adhesion comprising vulcanizing said composition and to rubber articles comprising the rubber vulcanizate obtained by said process.

Adhesion between rubber and steel cord is an important factor in determining the durability of tyres, conveyor belts, hoses, and other rubber-based articles of manufacture which typically use brass plated steel cord as reinforcement.

In considering rubber to steel cord bonding, much attention has been paid in the art to the influence and efficacy of steel cord bonding agents; their effect on adhesion formation and retention has led to significant advances with regard to the durability of the rubber-steel cord compound as, for example, in so-called steel cord skim or belt compounds. However, whilst these bonding agents provide enhanced adhesion, they can be detrimental to the long-term properties of the cured rubber compound. For example, cobalt salts such as cobalt naphthenate, cobalt stearate, cobalt neodecanoate, and cobalt-containing organometallic compounds such as Manobond 680C are known to improve rubber to steel cord adhesion characteristics, but as a result of the pro-oxidant nature of these materials detrimental effects on long-term compound properties can occur. It would therefore be desirable to be able to reduce the amount of steel cord bonding agent in the rubber compound without negatively influencing rubber to steel cord adhesion.

A further problem is that steel cord skim compounds are typically cured with a cure system comprising insoluble sulfur and a benzothiazole-2-sulfenamide accelerator, for example, N,N-dicyclohexyl-benzothiazole-2-sulfenamide

(DCBS), giving a rubber vulcanizate not having the desired properties with regard to modulus and retention of hardness upon ageing of the vulcanizate, nor with respect to resistance to the negative effects of humidity/steam on rubber to steel cord adhesion.

The present invention comprises a composition comprising a rubber, sulfur, a steel cord bonding agent, and a steel cord adhesion promoting accelerator comprising a pyrimidine derivative of formula I or a pyrazine derivative of formula II:

formula I

formula II wherein

R¹, R², and R³ independently represent hydrogen or a C1-C₁2 alkyl group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms,

R¹ and R² or R² and R³, provided that R² and R³ are on adjacent carbon atoms, may, together with the carbon atoms to which they are attached, form a 5- or 6- membered ring, said ring optionally being substituted with a Cι-C₆ alkyl group, said ring or group optionally containing one or more oxygen, nitrogen and/or sulfur atoms, x is 1 or 2, provided that when x is 2, y is 0,

R⁴ represents hydrogen or a Cι-C₆ alkyl group, y is 0 or 1 , z is 1 to 4, and

R⁵ represents hydrogen or a mono-, di-, tri-, or tetravalent Cι-C₁₈ hydrocarbon group, optionally containing one or more oxygen, nitrogen, phosphorous and/or sulfur atoms, if y is 1 , R⁴ and R⁵ may, together with the nitrogen atom to which they are attached, form a 5- or 6-membered ring, said ring optionally containing one or more oxygen, nitrogen and/or sulfur atoms, with the proviso that the composition does not contain a non-thiazole sulfenamide accelerator.

Applicant's WO 01/70870 discloses the use of a number of pyrimidine derivatives according to formula I as hardness stabilizers in sulfur-vulcanizable rubber compositions further comprising a rubber, a sulfur vulcanizing agent, and a non-thiazole sulfenamide accelerating agent. In this disclosure, the pyrimidine derivatives are always used in combination with one or more non- thiazole sulfenamide accelerators. In accordance with the present invention, the pyrimidine derivatives of formula I and the pyrazine derivatives of formula II act as slow cure accelerators, and they should not be used in combination with a non-thiazole sulfenamide accelerator.

Incidentally, US 3,839,303 discloses the inhibition of premature vulcanization of natural or synthetic diene rubbers by the inclusion in the, vulcanizable composition of accelerating agents, such as thiazole accelerators and a compound comprising certain pyrimidinesulfenamides, such as N-cyclohexyl- 4,6-dimethyl-2-pyrimidinesulfenamide, a pyrimidine derivative in accordance with the above-depicted formula I. The agents of this reference are formulated so as to be effective in inhibiting premature vulcanization in the vulcanizable composition to which they are added. There is no disclosure or suggestion in this document regarding their use in steel cord-reinforced rubber compounds. According to the present invention, it has been found that by adding appropriate amounts of certain pyrimidine or pyrazine derivatives to a sulfur-vulcanizable rubber composition comprising natural rubber or other rubbers, vulcanizates - from which e.g. pneumatic tyres can be made - having improved rubber to steel cord adhesion properties can be obtained. In particular, these pyrimidine and pyrazine derivatives have the effect of improving the retention of adhesion of rubber to brass plated steel cord following ageing of the rubber-steel cord compound. Accordingly, their use allows for employing a reduced amount of the conventionally used steel cord bonding agent. Additionally, the use of a pyrimidine derivative of formula I or a pyrazine derivative of formula II has the effect of stabilizing the hardness properties of the rubber vulcanizate, e.g., during the service life of a pneumatic tyre, without inhibiting or slowing vulcanization, i.e. increasing "scorch" time, in the production of the tyre, as disclosed in WO 01/70870. Thus, hardness stabilization is achieved without slowing of the vulcanization process, thereby avoiding loss in production efficiency.

In this application, the abbreviation "phr" means the number of parts by weight per 100 parts by weight of rubber. In the case of a rubber blend, it is based on 100 parts by weight of total rubber.

Either natural rubber (NR), styrene-butadiene rubber (SBR) or a blend of NR and SBR or NR and SBR with one or more other rubbers can be used in accordance with the present invention, it being understood that for the purposes of this invention the term "rubber" means an elastomer containing a hydrocarbon unit which is a polymer with some unsaturated chemical bonds. Typically, natural rubber (NR) is used. The type of rubber or mixture of rubbers will have some effect on the precise amount of pyrimidine or pyrazine derivative appropriate to achieve retention of rubber to steel cord adhesion without inhibition of the vulcanization. In accordance with the present invention, sulfur is employed. Any grade or form of sulfur may be used in accordance with the present invention. The amount of sulfur to be compounded with the rubber usually is at least 0.1 phr, preferably at least 1 phr, more preferably at least 2 phr. Typically, the upper limit is 10 phr, preferably 8 phr. Preferably, an insoluble form of sulfur (e.g. Crystex^® HS OT20) is used in accordance with the present invention.

In accordance with the present invention, any conventional steel cord bonding agent may be used, such as a bonding agent comprising a cobalt salt (see the examples described above) or a dithiosulfate such as hexamethylene-1 ,6- bisthiosulfate disodium salt (e.g. Duralink^® HTS). Depending on the application and the performance requirements, the cobalt salt may be used alone or in combination with a resin system. A typical resin system consists of resorcinol or a resorcinol donor and a methylene donor, such as hexa(methoxymethyl)- melamine (e.g. Resimene^® HMMM). Duralink^® HTS typically is used in combination with 1 ,3-bis(citraconimidomethyl)benzene (e.g. Perkalink^® 900). Another bonding agent is 2,3,5,6-tetrachIoro-1 ,4-benzoquinone (TCBQ), which typically is used in combination with resorcinol donor R-6. In accordance with the present invention, such bonding agents are used in conventional, known quantities.

Typically, the amount of the steel cord adhesion promoting accelerator employed in accordance with the present invention will be at least 0.1 phr, preferably at least 0.5 phr, most preferably at least 1.5 phr. A practical upper limit is 10 phr, preferably 5 phr, more preferably 3 phr, most preferably 2 phr. The derivatives of formulae I and II are either known in the art or can be prepared from suitable starting materials by one of ordinary skill in the art using conventional methods and equipment.

In the context of this specification, the one or more oxygen, nitrogen, phosphorous and/or sulfur atoms may be present as separate atoms in a hydrocarbon chain or ring or they may form functional groups such as -SP(S)(O-)₂, -SC(S)N-, -SC(S)N-, -C(O)NH-, -C(O)O-, and -C(O)OH groups. Such groups may be present within a hydrocarbon chain or ring or be attached to a hydrocarbon chain or ring as pendant groups.

In particular, if y is 0, R⁵ may represent a -SP(S)(OR⁶)₂, -SC(S)N(R⁶)R⁷ or -SC(S)OR⁶ group, wherein R⁶ independently represents a C₁-C₁₂ alkyl group or a C₆-Ci₂ aryl group, R⁷ represents hydrogen or a Cι-C₆ alkyl group, and R⁶ and R⁷ may, together with the nitrogen atom to which they are attached form, a 5- or 6-membered ring, said alkyl or aryl groups or said ring optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

Examples of suitable hydrocarbon groups include C₁-C18 alkyl, C₃-C₈ cycloalkyl, C₆-Ci₈ aryl, C₇-C₁₈ alkylaryl, and C7-C18 aralkyl groups, optionally containing one or more oxygen, nitrogen and/or sulfur atoms. Preferred groups are C₁-C₁₂ alkyl, C₃-C₈ cycloalkyl, C₆-Cι₂ aryl, C7-C12 alkylaryl, and C₇-C ₂ aralkyl groups, optionally containing one or more oxygen, nitrogen and/or sulfur atoms. More preferred groups are C Cβ alkyl and C₃-C₆ cycloalkyl groups, optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

Preferably, in the derivatives of formulae I and II, R , R², and R³ independently represent hydrogen or a Cι-C₆ alkyl group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms; x is 1 ; R⁴ represents hydrogen; y is 1 ; z is 1 or 2; and R⁵ represents a mono- or divalent Cι-C₁₈ hydrocarbon group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

More preferably, R¹, R², and R³ independently represent hydrogen or a methyl group; x is 1 ; R⁴ represents hydrogen; y is 1 ; z is 1 ; and R⁵ represents a monovalent C-1-C18 hydrocarbon group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms. It is preferred that x, y, and z in formulae I and II are 1. Most preferably, R⁵ represents a monovalent C1-C12 hydrocarbon group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

Examples of suitable starting materials for making di-, tri-, and tetravalent derivatives of formula I or II include ethylenediamine, diethylenetriamine, triethylenetetramine, 2,2'-oxybis(ethylamine), 2,2'-thiobis(ethylamine), and cystamine. An example of a suitable starting material for making derivatives of formula I or II wherein R⁴ and R⁵, together with the nitrogen atom to which they are attached, form a 6-membered ring containing an oxygen atom is morpholine. Examples of suitable R⁵ groups include t-butyl, cyclohexyl, 2- benzothiazoyl, and 4,6-dimethyl-2-pyrimidyl groups. These groups can be incorporated into formula I or II by using as starting materials, for example cyclohexylamine or 4,6-dimethyl-2-pyrimidinethiol.

In accordance with the present invention, a pyrimidine derivative of formula I, a pyrazine derivative of formula II or a mixture thereof may be used. Preferably, a pyrimidine derivative of formula I is used in accordance with the present invention. A particularly preferred pyrimidine derivative according to formula I is N-cyclohexyl-4,6-dimethyl-2-pyrimidinesulfenamide (CDMPS) having the following formula:

In a preferred embodiment of the present invention, the steel cord adhesion promoting accelerator in accordance with the present invention is used in a formulated form. The pyrimidine or pyrazine derivative may be formulated into any form which is suitable for use in rubber compounding, using any conventional formulation method known to a person of ordinary skill in the art, for example by means of a granulator or extruder. The skilled person will have no trouble in selecting and making suitable formulations of said derivative for use in rubbers.

Suitable formulations of the pyrimidine or pyrazine derivatives in accordance with the present invention include oil-coated formulations, such as processing oil- and silicone oil-coated formulations, granulated formulations, and master- batches - i.e. formulations in rubber - of the pyrimidine or pyrazine derivative. Preferably, a pyrimidine or pyrazine derivative in a granulated form is used. Any granulated form may be used in accordance with the present invention, thus the size and the shape of the granulate are of no importance. Typical granulated forms include pellets, beads, and rods.

Preferably, the granules contain an inert filler, such as a clay, a talc, a chalk, a carbon black or a mixture thereof. Any inert filler may be used in accordance with the present invention; "inert" meaning that it does not have any negative influences on the compounding of the rubber composition and/or the properties of the rubber vulcanizate. Suitable clays include kaolin, zeolite, montmorillonite, and kaolinite. These clays are available in different forms and grades, e.g., sodium montmorillonite which is bentonite. Suitable carbon blacks include surface-modified carbon blacks. Suitable examples of inert fillers to be used in accordance with the present invention include kaolin, talcum powder, and ordinary chalk as available under the name Precarb.

Granules typically also contain one or more anionic, cationic, amphoteric or non-ionic surfactants. Any surfactant may be used in accordance with the present invention. Preferably, a non-ionic surfactant is used. Suitable examples of non-ionic surfactants include ethoxylated alcohols, such as ethoxylated nonylphenols. The granules preferably also contain a binder, i.e. typically a wax-like compound having a melting point or melting range in the range of 40 to 160°C, preferably 40 to 120°C, most preferably 50 to 80°C. Suitable examples of binders include paraffins, polyethyleneglycols, vaseline, waxes including mineral waxes (e.g. paraffin wax), vegetable waxes (e.g. Camauba wax), animal waxes (e.g. beeswax), and synthetic waxes (e.g. polyethylene and polyisobutylene), and mixtures thereof.

A preferred granulated form of the pyrimidine or pyrazine derivative to be used in accordance with the present invention comprises a clay, an ethoxylated alcohol, and a paraffin. A most preferred granulated form comprises kaolin, an ethoxylated nonylphenol, and a paraffin.

Preferably, the granulate to be used in accordance with the present invention has a low friability, is easily dispersible in the rubber compound, and does not cake. Suitable methods for determining these properties are known to a person of ordinary skill in the art.

The amount of pyrimidine or pyrazine derivative in the granulate may vary within a wide range. It typically ranges between from 10 to 99 wt%, preferably 20 to 80 wt%, more preferably 40 to 60 wt%, based on the total, weight of the granulate. The inert filler, surfactant, and binder are used in their conventional amounts and these may also vary within a wide range. The amount of inert filler in the granulate typically varies between from 0 to 80 wt%, preferably 0 to 60 wt%, more preferably 40 to 60 wt%, based on the total weight of the granulate. The amount of surfactant typically varies between from 0.1 to 5 wt%, preferably 0.1 to 2 wt%, more preferably 0.1 to 0.5 wt%, based on the total weight of the granulate. The amount of binder typically varies between from 0.1 to 50 wt%, preferably 0.1 to 10 wt%, more preferably 0.1 to 2 wt%, based on the total weight of the granulate. A particularly effective composition in accordance with the present invention comprises natural rubber, sulfur, a steel cord bonding agent, and N-cyclohexyl- 4,6-dimethyl-2-pyrimidinesulfenamide.

Conventional rubber additives may also be included in the sulfur-vulcanizable rubber composition in accordance with the present invention. Examples include reinforcing agents such as carbon black, silica, clay, whiting and other mineral fillers, processing oils (e.g. Ingralen 150), tackifiers, waxes, antidegradants (e.g. Flectol^® TMQ, Santoflex^® 6PPD, and Q-Flex^® QDI), pigments (e.g. titanium dioxide), resins, plasticizers, factices, vulcanization activators, such as stearic acid and zinc oxide, and antireversion agents (e.g. Perkalink^® 900). These conventional rubber additives may be added in amounts known to the person skilled in the art of rubber compounding. The reader is also referred to the Examples that are described below.

For further details on these typical rubber additives and vulcanization inhibitors, reference is made to W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.

The present invention further relates to a process for improving rubber to steel cord adhesion comprising vulcanizing the invention composition which is described above. The vulcanization process of the present invention can be carried out using means and equipment that are well known to a person skilled in the art. Suitable vulcanization procedures are described in W. Hofmann, Rubber Technology Handbook, Hanser Publishers, Munich 1989.

A typical method comprises preparing a masterbatch consisting of rubber, carbon black, a steel cord bonding agent, a vulcanization activator, and a processing oil, in an internal mixer such as a Banbury mixer or a Werner &

Pfleiderer mixer, and subsequently adding a vulcanization system comprising sulfur and the steel cord adhesion promoting accelerator in accordance with the present invention to the masterbatch either in a low-temperature mixer or on a two-roll mill, i.e. the productive stage of mixing. The uncured rubber composition is then vulcanized by heating, e.g., in a compression mould.

The invention vulcanization process typically is carried out at a temperature of 110-200°C, preferably 120-190°C, most preferably 140-180°C, for a period of time of up to 12 h, preferably up to 6 h, more preferably up to 1 h, most preferably up to 30 min.

The composition of the present invention is useful in the manufacture of many articles, including pneumatic tyres, e.g., for passenger cars and trucks, and industrial rubber goods, which comprise the rubber vulcanizate obtained by the process of the present invention.

The invention is illustrated by the following Examples.

EXAMPLES

Examples 1-2 and Comparative Example A

A masterbatch of rubber, carbon black, stearic acid, zinc oxide, processing oil, antidegradants, and the cobalt salt steel cord bonding agent was made in an internal mixer. The sulfur and the steel cord adhesion promoting accelerator were mixed on a two-roll mill at approx. 50-70°C. Rubber compounds were vulcanized by compression moulding at 150°C for a period of time equal to t_go. After cooling the vulcanized rubber sheets for 24 h, test pieces were cut and analyzed.

The rheological properties were determined on a Monsanto Rheometer MDR2000E, arc 0.5°, 150°C/60 min. Scorch time (t_s2) is the time to increase the torque 2 dNm above the minimum torque (M_L). Optimum vulcanization time (t₉₀) is the time at 90% of the maximum torque (MH). Delta torque (Delta S) is the difference between the minimum and the maximum torque.

The rubber test pieces were aged in a hot air circulation oven for 2 days at 100°C to simulate hardening during use, for example, as a tyre, and with hot air for 2 days at 100°C, with steam for 8 h at 120°C, and with 5% NaCl salt water for 2 days at 90°C. In order to test rubber to brass plated steel cord adhesion, steel cord of specification 3+9+15*0.175+1/3.5 (Bekaert), Cu content of 63%, was used. The rubber to steel cord adhesion characteristics were determined in accordance with ASTM D-2229-85.

The hardness stabilization characteristics were determined by calculating the so-called modulus stabilization (MS), i.e. Mod 100 stab and Mod 200 stab.

The modulus stabilization is the ratio of the modulus at elongation 200% (Mod200) of the aged rubber test pieces to the unaged ones and is expressed as a percentage by multiplying this ratio by 100%. The lower the ratio Mod200_aged/Mod200unaged, the better the modulus retention or hardness stabilization. The Mod200 was obtained from tensile stress-strain tests which were performed in accordance with ISO 37-1994 (dumb-bell type 2).

The control shown in Table 1 is Comparative Example A, which is a composition containing no steel cord adhesion promoting accelerator and which was cured using a conventional 2-benzothiazolesulfenamide accelerator, i.e. Santocure^® DCBS. In Example 1 , a conventional amount of a cobalt salt steel cord bonding agent was used, whereas in Example 2, a reduced amount of said bonding agent was employed.

The data in Tables 2-4 shows that the use of a pyrimidine derivative of formula I in accordance with the present invention, i.e. CDMPS in Examples 1 and 2, provides stabilization of compound properties, such as modulus and hardness stabilization, following oxidative ageing of the rubber compound, and an improved retention of adhesion of rubber to brass plated steel cord after ageing under various conditions, notably after steam ageing. Furthermore, it was found that only half of he conventional amount of cobalt salt was required, while still maintaining an improved retention of rubber to steel cord adhesion.

Table 1 : Steel cord skim rubber compositions

Table 2: Rheological properties at 150°C/60 min

Table 3: Properties of the vulcanizates, cure at 150°C/t_! 90

Table 4: Steel cord adhesion, cure at 150°C/1.7tι 90

Claims

1. A composition comprising a rubber, sulfur, a steel cord bonding agent, and a steel cord adhesion promoting accelerator comprising a pyrimidine derivative of formula I or a pyrazine derivative of formula II:

formula

formula II wherein R¹, R², and R³ independently represent hydrogen or a C-i-C-^ alkyl group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms, R¹ and R² or R² and R³, provided that R² and R³ are on adjacent carbon atoms, may, together with the carbon atoms to which they are attached, form a 5- or 6-membered ring, said ring optionally being substituted with a C-ι-C-₆ alkyl group, said ring or group optionally containing one or more oxygen, nitrogen and/or sulfur atoms, x is 1 or 2, provided that when x is 2, y is 0, R⁴ represents hydrogen or a CrC₆ alkyl group, y is 0 or 1 , z is 1 to 4, and R⁵ represents hydrogen or a mono-, di-, tri-, or tetravalent Cι-Cι₈ hydrocarbon group, optionally containing one or more oxygen, nitrogen, phosphorous and/or sulfur atoms, if y is 1 , R⁴ and R⁵ may, together with the nitrogen atom to which they are attached, form a 5- or 6-membered ring, said ring optionally containing one or more oxygen, nitrogen and/or sulfur atoms, with the proviso that the composition does not contain a non-thiazole sulfenamide accelerator.

2. A composition according to claim 1 , characterized in that R¹, R², and R³ independently represent hydrogen or a C-_t-C₆ alkyl group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms; x is 1 ; R⁴ represents hydrogen; y is 1 ; z is 1 or 2; and R⁵ represents a mono- or divalent C-i-C-is hydrocarbon group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

3. A composition according to claim 1 or 2, characterized in that R¹, R², and R³ independently represent hydrogen or a methyl group; x is 1 ; R⁴ represents hydrogen; y is 1 ; z is 1 ; and R⁵ represents a monovalent Cι-C-ι₈ hydrocarbon group, optionally containing one or more oxygen, nitrogen and/or sulfur atoms.

4. A composition according to any one of claims 1-3, characterized in that R⁵ represents a t-butyl, cyclohexyl, 2-benzothiazoyl or 4,6-dimethyl-2-pyrimidyl group.

5. A composition according to any one of claims 1-4, characterized in that the composition comprises a pyrimidine derivative of formula I.

6. A composition according to any one of claims 1-5, characterized in that the pyrimidine derivative of formula I has the formula:

7. A composition according to any one of claims 1-6, characterized in that the amount of the pyrimidine or pyrazine derivative is from 0.1 to 10 phr.

8. A composition according to any one of claims 1-7, characterized in that the pyrimidine or pyrazine derivative is in a granulated form.

9. A composition according to claim 8, characterized in that the granulate comprises an inert filler, a surfactant, and a binder.

10. A composition according to claim 8 or 9, characterized in that the granulate comprises a clay, an ethoxylated alcohol, and a paraffin.

11. A composition according to any one of claims 8-10, characterized in that the granulate comprises kaolin, an ethoxylated nonylphenol, and a paraffin.

12. A process for improving rubber to steel cord adhesion comprising vulcanizing the rubber composition according to any one of the preceding claims at a temperature of 110-200°C for a period of time of up to 12 hours.

13. A rubber article of manufacture, such as a pneumatic tyre, comprising the rubber vulcanizate obtained by the process according to claim 12.