WO2007144609A1

WO2007144609A1 - Adhesion promoters for improved rubber to metal bonding

Info

Publication number: WO2007144609A1
Application number: PCT/GB2007/002193
Authority: WO
Inventors: William Stephen Fulton; John Michael Hilton
Original assignee: Omg Uk Limited
Priority date: 2006-06-12
Filing date: 2007-06-12
Publication date: 2007-12-21
Also published as: GB0611570D0; TW200806682A

Abstract

A metal complex, wherein an atom Q, chosen from boron, aluminium, titanium, zirconium, silicon or alkyl- or aryl- silicon or dialkyl-, diaryl- or alkylaryl-silicon, bears groups of formula -OMX and -OMY, wherein M is a divalent metal, X is a residue of an aromatic carboxylic acid or an aromatic aryloxy carboxylic acid and Y is a residue of a terpenic acid.

Description

ADHESIONPROMOTERS FORIMPROVED RUBBERTO METAL

BONDING

The present invention relates to metal complexes comprising a combination of aromatic and terpenic acids and their use as rubber-metal adhesion promoters.

Background

It is often desirable to form an adhesive bond between rubber and a metal. For example, it is well known that the adhesive bond formed between brass- or zinc- coated steel cords and rubber compound during vulcanisation is an essential characteristic governing the performance and durability of car and truck tyres. The rubber-metal interface is prone to deterioration, particularly at high temperature, high humidity and high salinity. For example, such conditions will reduce the reinforcement by steel cord to cause a concomitant reduction in the life of a tyre.

It is known to use metal-organic compounds (adhesion promoters) to improve and maintain a durable bond at the rubber-metal interface. EP0148782 describes an adhesion promoter comprising the reaction product of a nickel or cobalt salt of a carboxylic acid and an alkaline earth metal borate. US4234496 describes the use of metal organic compounds comprising boron or phosphorus, a divalent metal such as cobalt, and aliphatic monocarboxylic acid residues as rubber compound adhesion promoters. GB2330581 describes metal organic compounds comprising cobalt or nickel linked through oxygen atoms to aluminium, silicon or titanium and having a combination of aromatic and aliphatic residues, and the use of such compounds as rubber compound adhesion promoters. It was shown in EPO 150840 that the efficacy of adhesion could be improved by incorporation of resin acid residues together with aliphatic carboxylic acid residues into a cobalt-boron complex.

Although such complexes are recognised by those proficient in the art as efficient rubber-metal adhesion promoters, there is still a need to produce complexes that increase the retention of adhesion after ageing at high temperature and high humidity. It is the aim of this present invention to provide a metal complex that meets these requirements.

Summary of the invention

In one aspect of the invention there is provided a metal complex, wherein atoms or groups Q, chosen from boron, aluminium, titanium, zirconium, silicon or alkyl- or aryl- silicon or dialkyl-, diaryl- or alkylaryl-silicon, bear groups of formula -OMX and -OMY, wherein M is a divalent metal, X is a residue of an aromatic carboxylic acid or an aromatic aryloxy carboxylic acid and Y is a residue of a terpenic acid.

The invention also provides a process of preparing a metal complex, comprising heating together a mixture in the required proportions of

(i) the acids XH and YH which give rise to the residues X and Y as defined above, (ii) a source of the metal M,

(iii) a compound of formula Q(OR) γ₅ wherein V is the valency of Q, OR is a residue of a C_1-4 alcohol and Q is as defined above, and

(iv) an acid capable of forming a volatile ester with the C_1-4 alcohol residue present in the said compound, and distilling off the said volatile ester, wherein (i), (ii) and (iv) are pre-reacted before the compound (iii) is added; and a metal complex obtainable by said process.

In another aspect, the present invention provides an elastomeric composition comprising an unvulcanised rubber polymer and the metal complex.

In a further aspect, the metal complex of the present invention is used as a rubber- metal adhesion promoter. The present invention also provides (a) a process of adhering rubber polymer to steel coated with copper, zinc or brass, and (b) vulcanised rubber reinforced with steel coated with copper, zinc or brass, wherein the rubber comprises the metal complex as an adhesion promoter. Detailed description of the invention

Metal complex

The metal complex is a mixture of individual molecules. This mixture will be made up of individual molecules, some having formula Q(OMX)γ, others Q(OMY)v, and still others Q(OMX) _n(0MY) _m, wherein V is the valency of Q, n and m are integers and n+m=V. Accordingly, another way of representing the metal complex is by an average formula (I)

Q[OMX] _x[0MY] _y (I)

wherein (x+y) = V, the valency of Q and neither x nor y is zero, such that overall the metal complex bears a mixture of groups X and Y. The preferred average values of x and y are 0.2V < x < 0.8V and 0.2V < y <0.8V.

Thus, the molar ratio of X to Y in the average metal complex is preferably from 1:4 to 4:1. More preferably, the molar ratio X:Y is from 1 :2 to 2: 1 and most preferably 1 :2 to 1 :1.

By way of illustration, a boron-cobalt complex having an average formula B[OCoX]₂[OCoY] could arise from 2 individual molecules of formula B(OCoX) ₃ and 1 molecule of formula B(OCoY) ₃. Alternatively, the same average formula could arise from 3 individual molecules of formula B(OCoX)₂(OCoY), or from 2 molecules of formula B(OCoX)(OCoY) ₂ and 2 molecules of B(OCoX) ₃.

When Q is alkyl- and/or aryl-substituted silicon, the alkyl group is preferably one having 1 to 4 carbon atoms, especially methyl, while the aryl group is preferably phenyl. Typical examples include methylsilicon, dimethylsilicon, diphenylsilicon and methylphenylsilicon.

In a preferred embodiment, Q is trivalent, i.e. V=3; for example boron, aluminium, zirconium or mono-alkyl- or mono-aryl-silicon. Most preferably, Q is boron. M may be any divalent metal, for example, cobalt, manganese, barium, calcium, cerium, copper, iron, magnesium, nickel, strontium or zirconium. Preferably, the divalent metal M is cobalt or nickel. More preferably, M is cobalt.

In an especially preferred embodiment, Q is boron and M is cobalt.

Preferably, X is a residue of an aromatic carboxylic acid of 7 to 15 carbon atoms. The aromatic carboxylic acid may be, for example, benzoic acid, an alkyl-, alkoxy-, amino-, halogen-, thio- or hydroxyl-substituted benzoic acid, such as 3,4,5- trimethoxybenzoic acid, salicylic acid, or a more complex aromatic acid comprising a conjugated ring system. Alternatively, X is a residue of an aliphatic carboxylic acid substituted with an aryl or aryloxy group, wherein the residue has 8 to 15 carbon atoms. The aryl group may be mono-, di- or tri- substituted, for example 3,4,5- trimethoxyphenylacetic acid. An example of an aryloxy substituted acid is a phenoxysubstituted acid, such as phenoxyacetic acid or phenoxypropionic acid.

Y is preferably a residue of a cyclic terpenic acid, more preferably a resin acid. Resin acids usually contain two functional groups: a carboxyl group and double bonds, and are in the form of a 3-ring fused system, with the empirical formula

Examples of such acids are abietic acid, neoabietic acid, dehydroabietic acid and palustric acid. Thus, in one embodiment, Y is a residue of a pure resin acid such as abietic acid, neoabietic acid, dehydroabietic acid or palustric acid. In another embodiment, Y is a residue derived from a mixture of resin acids. Such a mixture may be obtainable from naturally occurring resin acids. Examples of natural sources of resin acids are wood rosin, such as pine oleorosin, tree sap, fossil coal, copal resins, gum rosin, such as Portuguese gum rosin, and tall rosin, and disproportionated or hydrogenated products thereof. Resin acid mixtures comprising such products are commercially available.

The metal complex of the present invention may be associated with microcrystalline wax and/or process oil and/or a hydrocarbon resin and/or a resorcinol/formaldehyde resm.

Process of preparing metal complex

A metal complex, such as those described above, may be prepared by a process comprising heating together a mixture in the required proportions of (i) the acids XH and YH which give rise to the residues X and Y as defined above, (ii) a source of the metal M, such as the oxide, hydroxide or carbonate, (iii) a compound of formula Q(OR) _v, wherein V is the valency of Q₅ OR is a residue of a C_M alcohol, e.g. n-butanol, and Q is as defined above, and

(iv) an acid capable of forming a volatile ester with the C_1-4 alcohol residue present in the said compound, and distilling off the said volatile ester, preferably under reduced pressure {i.e. typically less than 10OkPa). All of the acids (i) and (iv) and the metal source (ii) must be pre-reacted before the compound (iii) is added. The reaction temperature is typically in the range 5O⁰C to 25O⁰C.

Examples of the compound (iii) of formula Q(OR) v are n-butylorthoborate, aluminium triisopropoxide, tetrabutyl titanate, tetraethyl orthosilicate and tnethyltriethoxysilane, when Q is respectively boron, aluminium, titanium, silicon or methyl-silicon.

By volatile ester is meant an ester that can be distilled off under the prevailing temperature and pressure conditions. Thus, examples of the acid (iv) are C_1-8 aliphatic acids. Acetic acid or propionic acid are particularly preferred.

Preferred Q, M, X and Y are as described above for the metal complex.

The present invention also provides a metal complex obtainable by the above process, i.e. the material which is obtained when (i), (ii) and (iv) are pre-reacted, the ester (iii) is added, and the volatile ester is distilled off. Elastomeric composition

The metal complex may be any metal complex as described above, including those obtainable by the above-described process. The metal complex may also be associated with other components as described above before incorporation into the elastomeric composition.

Preferably, the amount of the metal complex present in the composition is sufficient to provide at least 0.1 parts by weight of the divalent metal M per 100 parts by weight of the rubber polymer (0. lphr). More preferably, the amount in terms of the divalent metal M is at least 0.2phr. In one embodiment, the metal complex provides 0.1 to 0.5phr of the divalent metal, preferably 0.2phr.

The unvulcanised rubber polymer is not specifically restricted and, for example, natural rubber, styrene-butadiene copolymerised rubber, butadiene and isoprene rubber can all be used alone or in combination. Thus, cis-polyisoprene, styrene- butadiene, nitrile rubber or a blend of any such rubber with polybutadiene are suitable. In one embodiment, the rubber polymer consists of natural rubber. In another embodiment, the rubber polymer comprises a blend of natural rubber with cis-polyisoprene and/or polybutadiene rubber.

The elastomeric composition of the present invention may further comprise one or more conventional rubber compounding ingredients such as pigments, fillers, extenders, accelerators, antioxidants, vulcanising agents, process oils, tackifiers and other ingredients for use in the production of a fully compounded rubber stock. Additives which may be used include, for example, zinc oxide which may be used in an amount, for instance, from 2 to 10 parts by weight per 100 parts of rubber, and stearic acid at a concentration, for instance, from 0.5 to 5 parts by weight per 100 parts by weight of rubber. Other additives include antioxidants or antidegradants, for example N-(l,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, typically used at a concentration from 0.5 to 5 parts by weight per 100 parts by weight of rubber; and accelerators, for example, N,N-dicyclohexyl-2-benzathiazole sulfenamide and N₅N'- diphenylguanidine. In the compositions of the invention, these are typically used individually in amounts from 0.1 to 3 parts by weight per 100 parts by weight of rubber depending on the nature of the rubber, the amount of sulphur and the properties required in the cured rubber. If sulphur is used as the vulcanising agent, the amount is typically in the range from 0.3 to 6 parts by weight per 100 parts by weight of rubber.

Typically, the elastomeric composition can also contain other traditional ingredients, for example resorcinol and hexamethoxymethylamine.

Use as rubber-metal adhesion promoter

The metal complexes of the present invention may be used as rubber-metal adhesion promoters, for example by incorporation into elastomeric compositions such as those described above. The metal complexes are particularly suitable for adhering a rubber compound to steel coated with copper, zinc or brass. The steel may, for example, be in the form of a copper-, zinc- or brass-plated cord or wire. In a preferred embodiment, the metal complexes of the present invention are used for adhering a rubber compound to brass-coated steel cord. It is particularly preferable that the brass has a copper content of 63 to 64 % by weight.

Thus, the present invention also provides a process of adhering rubber compound to steel coated with copper, zinc or brass, which process comprises incorporating the metal complex into the rubber prior to vulcanisation and then vulcanising the rubber and steel together.

Vulcanisation can be carried out using conventional procedures. For example, when forming a tyre for a passenger vehicle, the mixture is places in a tyre mould and vulcanised at high temperature, for example from 170 to 180⁰C, under pressure for a given time. The metal complex is typically incorporated such that it provides at least 0.1 parts by weight of the divalent metal M per 100 parts by weight of the rubber (0. lphr). More preferably, the amount in terms of the divalent metal M is at least 0.2phr. In one embodiment, the metal complex provides 0.1 to 0.5phr of the divalent metal, preferably 0.2phr.

The rubber compound suitably comprises any rubber polymer as described above for the elastomeric composition, for example, natural rubber, styrene-butadiene copolymerised rubber, butadiene or isoprene rubber used alone or in combination. The rubber compound may further comprise one or more conventional rubber compounding ingredients or other traditional ingredients, as described above.

The present invention also provides vulcanised rubber reinforced with steel coated with copper, zinc or brass, wherein the rubber comprises the metal complex as an adhesion promoter between the rubber and metal. Preferably, the rubber is reinforced with brass-coated steel cord.

Typical examples of a manufactured article according to the present invention are a tyre, a conveyor belt, a drive belt or a hose comprising said vulcanised rubber.

The metal complexes of the present invention improve in particular the retention of adhesion of rubber to metal after ageing under conditions of high humidity and high temperature.

EXAMPLE

Preparation of Adhesion Promoter

Xylene (4.0kg), resin acid (Portuguese gum rosin, a mixture of isomers of terpenic acid; 6.01kg) and o-toluic acid (1.67kg) were placed in a reaction vessel. The mixture was stirred and heat was applied to bring the reaction mixture to 50⁰C - 6O⁰C to dissolve the acids. Cobalt hydroxide (2.87kg) was added with mixing to give a slurry, followed by the addition of propionic acid (2.45kg). The temperature was raised gradually to 180⁰C while distillation of water of reaction took place. Vacuum distillation was used to complete the distillation. Vacuum and temperature were maintained until the distillation was complete.

N-butyl orthoborate (2.36kg) was added slowly. The mixture was kept at 18O⁰C and maintained at this temperature until the distillation of n-butyl propionate ceased. Then the product could be cooled to room temperature.

The final product (10.48kg) was a deep blue brittle solid which could be flaked or shaped as desired.

Adhesion Properties

The present invention was demonstrated by comparing the adhesion properties of belt formulations as shown in Table 1 in which the amount of cobalt boron complex was adjusted to give 0.2 parts by weight of cobalt per 100 parts by weight of rubber. The properties evinced by this addition were compared to a control compound containing a known cobalt boron complex (See Table 3).

Ingredient Amount (phr)

Natural rubber 100

Carbon black (N326) 55

Process oil 3

Zinc Oxide 8

Stearic acid 0.5

Antioxidant (6PPD)^a 2

Cobalt adhesion promoter 0.2 (as Co metal)

Accelerator (DCB S)^b 0.8

Sulphur 4 a) N-(l-3-dimethylbutyl-N'phenyl-p-phenyIene diamine b) N,N-dicycIohexyI-2-benzathiazole sulphenamide

Table 1. Compound formulation

The adhesion promoters were added to the rubber compound during mixing in a 1.5 litre internal laboratory mixer and sheeted off on a 2-roll laboratory mill. All compositions were vulcanised to T99 plus 5 minutes at 16O⁰C.

Adhesion testing was performed using a static block pull test in accordance with ISO 5603 1986 (Method 1) using an embedment length of 10mm. Wire coverage was also assessed. This is a visual estimation of the percentage of rubber that is left on the wire after it has been pulled out during the adhesion test. A sample of 10 wires per test piece was viewed through a low powered microscope and a visual estimate of the coverage expressed as percentage or given a numerical ranking from 0 to 10, wherein 0 is no wire coverage and 10 is 100% wire coverage.

Typical brass-coated steel tyre cord of the construction 2+2x0.25mm was used; each cord had brass plating with an average copper content of 63.5%. Measurement of adhesion was made in the initial stage, i.e. without ageing and after ageing under conditions listed in Table 2. The results are given in Table 3, where adhesion values are quoted in kgf (and N), as well as wire coverage (given in the range 0 - 10).

Ageing Conditions

Steam 121⁰C for 16 hours

Humidity 7O⁰C, 95% RH for 7, 14 days

Heat Overcure

Green humidity 4O⁰C, 95% RH for 7 days

Salt 5%NaCl solution for 7 days

Table 2. Ageing Conditions

The results given in Table 3 show that rubber compounds containing the cobalt boron complexes of the present invention exhibit substantially improved adhesion of rubber to brass-coated steel cord in comparison with a well-known cobalt boroacylate compound. This improvement is particularly marked under conditions of high temperature and high humidity.

* Manobond 680C®

Table 3. Adhesion Results

Claims

1. A metal complex, wherein an atom Q, chosen from boron, aluminium, titanium, zirconium, silicon or alkyl- or aryl- silicon or dialkyl-, diaryl- or alkylaryl-silicon, bears groups of formula -OMX and -OMY, wherein M is a divalent metal, X is a residue of an aromatic carboxylic acid or an aromatic aryloxy carboxylic acid and Y is a residue of a terpenic acid.

2. A complex according to claim 1, wherein the molar ratio of X: Y is from 1:4 to 4:1.

3. A complex according to claim 1 or claim 2, wherein Q is boron.

4. A complex according to any one of the preceding claims, wherein M is cobalt or nickel.

5. A complex according to claim 4, wherein M is cobalt.

6. A complex according to any one of the preceding claims, wherein X is a residue of an aromatic carboxylic acid of 7 to 15 carbon atoms, or a residue of an aliphatic carboxylic acid substituted with an aryl or aryloxy group, wherein the residue has 8 to 15 carbon atoms.

7. A complex according to any one of the preceding claims, wherein Y is a residue of a cyclic terpenic acid.

8. A complex according to claim 7, wherein Y is a residue of a resin acid.

9. A process of preparing a metal complex, comprising heating together a mixture in the required proportions of

(i) the acids XH and YH which give rise to the residues X and Y, (ii) a source of a metal M, (iii) a compound of formula Q(OR) y, wherein V is the valency of Q, OR is a residue of a C₁^ alcohol and Q is as defined in claim 1, and (iv) an acid capable of forming a volatile ester with the C_1-4 alcohol residue present in the said compound, and distilling off the said volatile ester, wherein (i), (ii) and (iv) are pre- reacted before the compound (iii) is added and X, Y and M are as defined in any one of claims 1 to 8.

10. A metal complex obtainable by a process according to claim 9.

11. A complex according to any one of claims 1 to 8 or claim 10, which is associated with microcrystalline wax, process oil, or hydrocarbon resin or resorcinol/formaldehyde resin.

12. An elastomeric composition comprising an unvulcanised rubber polymer and a complex according to any one of claims 1 to 8 or claims 10 to 11.

13. A composition according to claim 12, which comprises the complex in an amount to provide from 0.1 to 0.5 parts by weight of the divalent metal M per 100 parts by weight of the rubber polymer.

14. A composition according to claim 12 or claim 13 wherein the rubber polymer consists of natural rubber.

15. A composition according to claim 12 or claim 13 wherein the rubber polymer comprises a blend of natural rubber with cis-polyisoprene and/or polybutadiene rubber.

16. A composition according to any one of claims 12 to 15, further comprising one or more of a pigment, filler, extender, accelerator, antioxidant, vulcanising agent, process oil and a tackifier.

17. Use of a complex according to any one of claims 1 to 8 or claims 10 to 11 as a rubber-metal adhesion promoter.

18. Use according to claim 17, for adhering a rubber compound to steel coated with copper, zinc or brass.

19. Use according to claim 18, for adhering a rubber compound to brass- coated steel cord.

20. A process of adhering rubber compound to steel coated with copper, zinc or brass, which process comprises incorporating a complex according to any one of claims 1 to 8 or claims 10 to 11 into the rubber prior to vulcanisation, and then vulcanising the rubber and steel together.

21. Vulcanised rubber reinforced with steel coated with copper, zinc or brass , wherein the rubber comprises, as an adhesion promoter, at least one complex according to any one of claims 1 to 8 or claims 10 to 11.

22. Vulcanised rubber according to claim 21 , wherein the rubber is reinforced with brass-coated steel cord.

23. A tyre, conveyor belt, drive belt or hose comprising vulcanised rubber according to claim 21 or claim 22.