WO2001038426A1

WO2001038426A1 - A manufactured article comprising at least one metallic element embedded in a vulcanized elastomeric material

Info

Publication number: WO2001038426A1
Application number: PCT/EP2000/011541
Authority: WO
Inventors: Giuseppina Ratti
Original assignee: Pirelli Pneumatici S.P.A.
Priority date: 1999-11-23
Filing date: 2000-11-20
Publication date: 2001-05-31
Also published as: AU1703501A

Abstract

A manufactured article comprising at least one steel element coated with a zinc/cobalt alloy, with a cobalt content below 1 %, embedded in a vulcanized elastomeric matrix, obtained from a rubber composition comprising at least one vulcanizable elastomer and one vulcanizing agent comprising sulphur or a derivative of sulphur, in which the said rubber composition comprises, as adhesion promoter, a cobalt-boron complex of formula (I), in which R', R'' and R''', which may be identical or different, are an aliphatic or aromatic C6-24 group.

Description

"A manufactured article comprising at least one metallic element embedded in a vulcanized elastomeric material"

* * * * *

The present invention relates to a manufactured article comprising at least one metallic element embedded in a vulcanized elastomeric material.

More particularly, the present invention relates to a manufactured article comprising a steel element with surface coating of an alloy comprising zinc (Zn) and cobalt (Co) embedded in a vulcanized elastomeric material, in which the said vulcanized elastomeric material comprises a bonding promoter.

It is known that some articles made of vulcanized elastomeric material are reinforced by embedding suitable metallic structures in an elastomeric matrix. In general, this metallic structure is made of steel wires and/or cords.

It is also known that the metal of choice for the said structure is steel, on account of its excellent mechanical properties. Its chemical characteristics, however, are not optimum, as they do not provide good bonding to the vulcanized elastomeric material and therefore confer little resistance to corrosion.

For that reason it is necessary to coat the steel element with a metal or a metallic alloy so as to give the steel itself the properties of corrosion resistance and bonding to the vulcanized elastomeric material. One alloy that has long been employed for coating steel reinforcing wires of manufactured articles made of vulcanized elastomeric material is brass (copper/zinc alloy with approx. 70% copper (Cu) and 30% zinc). In this case bonding is promoted by the formation of a thin layer of copper sulphide Cu_xS, from sulphur itself or from its known derivatives that are present, as vulcanizing agents, in the rubber composition.

Although brass-coated steel bonds well to the vulcanized elastomeric matrix, the bond tends to deteriorate rather quickly over time through the action of moisture/oxygen. In fact, in manufactured articles made of vulcanized elastomeric material reinforced with brass-coated steel wires, possibly in the form of cords, as in tyres for example, corrosion is induced either by moisture that permeates through the elastomer, or by water that succeeds in coming into contact with the brass-coated steel reinforcing structure through holes or splits in the elastomer.

This deterioration of the bond between brass and elastomer can be partially counteracted by adding cobalt in the form of one of its salts (US 5 356 71 1). It is considered that the role of cobalt is to cause a drop in electrical conductivity of the film of copper sulphide; there would be a correspondingly lower rate of diffusion of the Zn⁺⁺ ions and, in consequence, less growth of the ZnO/Zn(OH)₂ interfacial layer that is responsible for destroying the integrity of the Cu_xS film and hence the bond.

However, the resulting brass-coated steel suffers from the inherent disadvantages of copper, namely rapid deterioration of adhesion to the vulcanized elastomeric material and of corrosion protection.

Therefore alloys have been investigated that do not contain copper and yet display the desired characteristics of adhesion to the vulcanized elastomeric material, resistance to corrosion and capacity for wiredrawing. Tests were therefore conducted on binary alloys of zinc.

Patent EP-B-0 669409 describes a metal steel wire that has been given a surface treatment to make it suitable for use for reinforcing composite elements of elastomeric material, provided with a surface coating layer of metal alloy in which the said coating comprises a zinc/cobalt alloy with a cobalt content below 1%. In the description of the aforementioned patent it is suggested that additions of bonding promoters, in particular cobalt neodecanoate, to the elastomeric material can promote adhesion of the metallic wire to the elastomeric material (page 3, line 14). However, the aforementioned patent does not supply experimental results, nor does it mention other specific promoters.

The inventors of the present invention found, surprisingly, that by embedding the steel wire coated with zinc/cobalt alloy according to patent EP-B-0 669 409 in an elastomeric material comprising a cobalt- boron complex, adhesion to the vulcanized elastomeric material is obtained, which improves over time with regard to the adhesion: a) of steel wires coated with brass and embedded in the same elastomeric material (Comparative Example 1 , Table 7-10); and b) of steel wires coated with a zinc/cobalt alloy and embedded in an elastomeric material comprising, as adhesion promoter, cobalt neodecanoate as stated in patent EP-B-0 669 409 (see Comparative

Example 1 , Tables 1 1 -13).

In a first embodiment, the present invention thus relates to a manufactured article comprising at least one steel element coated with a zinc/cobalt alloy, with a cobalt content below 1 %, embedded in a vulcanized elastomeric matrix, obtained from a rubber composition comprising at least one vulcanizable elastomer and a vulcanizing agent comprising sulphur or a derivative of sulphur, characterized in that the said rubber composition comprises, as adhesion promoter, a cobalt- boron complex of formula (I) O-Co-O-COR'

B / \ R'OC-O-Co-O O-Co-O-COR" (I) in which R', R" and R'", which may be identical or different, are an aliphatic or aromatic C_6-24 group.

Typically, the said aliphatic group is an alkyl with from 6 to 10 carbon atoms. Preferably the aliphatic acyloxy group R'-CO-O-, R"-CO-O- and R'"-

CO-O- is derived from an aliphatic carboxylic acid selected from the group comprising n-heptanoic, 2,2-dimethylpentanoic, 2-ethylpentanoic, 4,4-dimethylpentanoic, n-octanoic, 2,2-dimethylhexanoic, 2- ethylhexanoic, 4,4-dimethylhexanoic, 2,4,4-trimethylpentanoic, n- nonanoic, 2,2-dimethylheptanoic, 6,6-dimethylheptanoic, 3,5,5- trimethylhexanoic, n-decanoic, neodecanoic, 2,2-dimethyloctanoic, 7,7- dimethyloctanoic, n-undecanoic acids and their mixtures.

Even more preferably, the said aliphatic carboxylic acid is 2- ethylhexanoic or neodecanoic acid. In its turn, the aromatic acyloxy group R'-CO-O-, R"-CO-O- and R"'-

CO-O- is preferably derived from an aromatic carboxylic acid selected from the group comprising benzoic acid, benzoic acid substituted with alkyl, alkoxyl, amino, halo, thio or hydroxyl groups, naphthoic acid, complex aromatic acids comprising conjugated ring systems and their mixtures.

Typically, the said aromatic carboxylic acid is salicylic, anthranilic, 4- chlorobenzoic, phthalic, terephthalic and cinnamic acid.

A preferred example of cobalt-boron complex of formula (I) is cobalt- boron 2-ethylhexanoate-neodecanoate. Preferably, the quantity of the said cobalt-boron complex of formula (I) is equivalent to a quantity of divalent cobalt equal to at least 0.2 wt.% relative to the weight of the said elastomer. Even more preferably, the said quantity, in terms of divalent cobalt, is between 0.2 and 1 wt.% relative to the weight of the said elastomer.

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

Preferably, the steel wire according to the present invention is coated with a zinc/cobalt alloy in accordance with the process described in patent EP-B-0 669 409.

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

According to a second embodiment, the present invention relates to a tyre comprising a reinforcing structure comprising at least one steel element coated with a zinc/cobalt alloy, with a cobalt content below 1%, embedded in a vulcanized elastomeric matrix, obtained from a rubber composition comprising at least a vulcanizable elastomer and a vulcanizing agent comprising sulphur or a derivative of sulphur, characterized in that the said rubber composition comprises, as adhesion promoter, a cobalt-boron complex of formula (I) as mentioned above.

Typically, the said reinforcing structure consists of belt strips and/or of bead wires incorporated in the beads of the said tyre. The present invention is further illustrated by the following Examples and the following Diagrams, which are given purely for purposes of illustration and are non-limitative.

Fig. 1 shows a cord, formed from wires, embedded in an elastomeric matrix according to the invention. Fig. 2 is a sectional view of a tyre according to the invention. Fig. 3 is a perspective sectional view of a conveyor belt according to the invention.

Fig. 4 is a perspective sectional view of a drive belt according to the invention. Fig. 5 is a perspective sectional view of a hose according to the invention.

More particularly, Fig. 2 shows a tyre formed from a vulcanized elastomeric matrix and a reinforcing structure which comprises at least one steel element coated with a layer of a ZnCo alloy as defined above. In a preferred embodiment, the vulcanized elastomeric matrix of the said tyre was obtained from a rubber composition comprising at least one vulcanizable elastomer, at least one vulcanizing agent comprising sulphur or a derivative of sulphur and a cobalt-boron complex of formula (I) according to the present invention. The said tyre conventionally comprises at least one carcass ply 14 whose opposite side edges are externally bent around respective bead wires 12, each incorporated in a bead 10 defined along an internal circumferential edge of the tyre, corresponding to which, the tyre itself engages on a wheel rim 13 forming part of the wheel of a motor vehicle. Along the circumferential development of the carcass ply 14, one or more belt strips 15 are applied, consisting of cords formed from steel wires coated with a film of ZnCo alloy as defined above. Externally to the carcass ply 14, in respective opposite side portion thereof, a couple of sidewalls 17 is also applied, each of which extends from bead 10 to a so-called "shoulder" zone 18 of the tyre, defined to correspond to the opposite ends of the belt strips 15. A tread 16 is applied circumferentially on the belt strips 15, the side edges of the said tread ending so as to correspond to the shoulders 18 joining the sidewalls 17. The tread 16 has, externally, a rolling surface 16a, which is to come into contact with the ground, and this can have circumferential grooves 19 spaced by transversal grooves recesses, not shown in the enclosed drawing, which define a large number of small blocks 9 variously distributed on the said rolling surface 16a.

In their turn, Figs. 3 to 5 show, respectively, a conveyor belt 20, a drive belt 30 and a hose 40, each of which is composed of a vulcanized elastomeric matrix reinforced with cords 15 formed from steel wires coated with a layer of ZnCo alloy as defined above. Preferably, in each of these manufactured articles the vulcanized elastomeric matrix was obtained from a rubber composition comprising at least one vulcanizable elastomer, at least one vulcanizing agent comprising sulphur or a derivative of sulphur, and a cobalt-boron complex of formula (I) according to the present invention.

In the following examples, the notation 2+1 x 0.28 indicates a cord formed from 2 wires with diameter of 0.28 mm around which another wire of the same diameter is wound; the notation 2+1 x 0.20 indicates a cord formed from 2 wires with diameter of 0.20 mm around which another wire with the same diameter is wound; the notation 2+3 x 0.28 indicates a cord formed from 2 strands, one consisting of 2 wires with diameter of 0.28 mm and one formed from 3 wires with the same diameter.

The other symbols have the following meanings:

N.R. = natural rubber,

C.B. = carbon black,

HMMM = hexamethoxymethylmelamine, DCBS = N,N'-dicyclohexyl-2-benzothiazylsulphenamide.

The amounts of the components of the vulcanizable rubber compositions in the examples are expressed in parts by weight.

The following examples have the purpose of illustrating the present invention though without limiting it in any way. EXAMPLE 1 Vulcanized Elastomeric Material Test specimens were prepared by embedding steel cords (of type 2+1 x 0.28, 2+1 x 0.20 and 2+3 x 0.28) with a coating of ZnCo alloy (Co content = 0.5%) with thickness of 0.3 microns, in a rubber composition with the composition shown below. This rubber composition was then vulcanized at 151 °C for 40 minutes. The embedded length of the cord in the cylinder of the rubber composition was equal to 12 mm for the 2+3 x 0.28 cords and to 6 mm for the 2+1 x 0.28 and 2+1 x 0.20 cords. The compositions of the rubber compositions thus prepared are shown in the following Tables 1 and 2.

Table 1

Table 2

(a) in the form of 2-ethylhexanoate-neodecanoate of cobalt-boron, sold by the company Rhone-Poulenc under the trade-name Mawobond™

680c.

EXAMPLE 2 Adhesion to the Vulcanized Elastomeric Material

Adhesion was measured on test specimens of vulcanized rubber composition on steel wires coated with a Zn/Co alloy (Co = 0.5%), by the method described in "Kautschk und Gummi Kunststoffe", 5, 228- 232 (1969), which measures the force required to pull a cord out of a cylinder of vulcanized rubber.

The pull-out force was measured in Newton with an electronic dynamometer.

The degree of coverage of the cord extracted from the test specimen was expressed by means of a coverage index ranging from 1 to 4 and evaluated as the percentage of cord surface that is still well covered with elastomer. The initial adhesion values (Test I) and those found after holding in a climatic test chamber at 65°C and 90% RH for 8 days (Test II) are given in the following Tables 3-6 in which the first number represents the adhesion value, as the mean from 8 tests, whereas the index in parentheses shows the degree of coverage of the cord.

Table 3

Table 4

Table 5

Table 6

COMPARATIVE EXAMPLE 1

Adhesion Tests With the same procedure as in the previous Example 1 , two groups of test specimens were prepared with: a) steel cords (of type 2+1 x 0.28, 2+1 x 0.20 and 2+3 x 0.28) with a coating of ZnCu alloy (Cu content = 67%) with thickness of 0.3 micron; and b) steel cords (of type 2+1 x 0.28, 2+1 x 0.20 and 2+3 x 0.28) with a coating of ZnCo alloy (Co content = 0.5%) with thickness of 0.3 micron.

The aforementioned cords were embedded in three comparative rubber compositions that were completely identical to the rubber compositions of the invention C, M3 and M4 except that the divalent cobalt salt (Co²⁺) present in the rubber composition was added in the form of cobalt neodecanoate instead of 2-ethylhexanoate- neodecanoate of cobalt-boron of the invention.

The said rubber compositions were denoted with the symbols D, M1 and M2, respectively.

The adhesion tests were conducted as described in the preceding Example 2.

The results obtained with the test specimens with cords of type a) are shown in the following Tables 7-10, whereas those obtained with the specimens with cords of type b) are shown in the following Tables 11 -13.

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Table 13

Claims

1. A manufactured article comprising at least one steel element coated with a zinc/cobalt alloy, with a cobalt content below 1 %, embedded in a vulcanized elastomeric matrix, obtained from a rubber composition comprising at least one vulcanizable elastomer and one vulcanizing agent comprising sulphur or a derivative of sulphur, characterized in that the said rubber composition comprises, as adhesion promoter, a cobalt-boron complex of formula (I) O-Co-O-COR'

I

B

/ \ R'"OC-O-Co-O O-Co-O-COR" (I) in which R', R" and R'", which may be identical or different, are an aliphatic or aromatic C₆-2₄ group.

2. A manufactured article according to claim 1 , characterized in that the said aliphatic group is an alkyl with 6 to 10 carbon atoms.

3. A manufactured article according to claim 1 or 2, characterized in that the aliphatic acyloxy group R'-CO-O-, R"-CO-0- and R'"-CO-O- is derived from an aliphatic carboxylic acid selected from the group comprising n-heptanoic, 2,2-dimethylpentanoic, 2-ethylpentanoic, 4,4-dimethylpentanoic, n-octanoic, 2,2-dimethylhexanoic, 2- ethylhexanoic, 4,4-dimethylhexanoic, 2,4,4-trimethyipentanoic, n- nonanoic, 2,2-dimethylheptanoic, 6,6-dimethylheptanoic, 3,5,5- trimethylhexanoic, n-decanoic, neodecanoic, 2,2-dimethyloctanoic, 7,7-dimethyloctanoic, n-undecanoic acids and mixtures thereof.

4. A manufactured article according to claim 3, characterized in that the said aliphatic carboxylic acid is 2-ethylhexanoic or neodecanoic acid.

5. A manufactured article according to claim 1 , characterized in that the cobalt-boron complex of formula (I) is cobalt-boron 2- ethylhexanoate-neodecanoate.

6. A manufactured article according to claim 1 , characterized in that the aromatic acyloxy group R'-CO-O-, R"-CO-O- and R"'-CO-O- is derived from an aromatic carboxylic acid selected from the group comprising benzoic acid, benzoic acid substituted with alkyl, alkoxyl, amino, halo, thio or hydroxyl groups, naphthoic acid, the complex aromatic acids comprising conjugated ring systems and mixtures thereof.

7. A manufactured article according to claim 6, characterized in that the said aromatic carboxylic acid is salicylic, anthranilic, 4- chlorobenzoic, phthalic, terephthalic and cinnamic.

8. A manufactured article according to any one of the preceding claims from 1 to 7, characterized in that the quantity of the said cobalt-boron complex of formula (I) is equivalent to a quantity of divalent cobalt equal to at least 0.2 wt.% relative to the weight of the said elastomer.

9. A manufactured article according to claim 8, characterized in that the said quantity, in terms of divalent cobalt, is between 0.2 and 1 wt.% relative to the weight of the said elastomer.

10. A tyre comprising a reinforcing structure comprising at least one steel element coated with a zinc/cobalt alloy, with a cobalt content below 1%, embedded in a vulcanized elastomeric matrix, obtained from a rubber composition comprising at least one vulcanizable elastomer and one vulcanizing agent comprising sulphur or a derivative of sulphur, characterized in that the said rubber composition comprises, as adhesion promoter, a cobalt-boron complex of formula (I) as described in any one of the preceding claims from 1 to 9.

11. A tyre according to claim 10, in which the said reinforcing structure is a belt strip.

12. A tyre according to claim 10, in which the said reinforcing structure is a bead wire incorporated in a bead.