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The effect of partial substitution of cobalt adhesion promoter by post vulcanization stabilizers on steel cord to rubber adhesion.

With the advancement of tire technology, radial tires have captured 90-95% of the tire market in most of the advanced countries and steel cord has been established as the best carcass and belt material for radial tires. It has a number of advantages like higher tenacity that leads to higher load carrying capacity, higher modulus, higher fatigue life, low extension and highest durability, etc., over all kinds of presently available commercial tire cords. But due to wide variation of physical as well as chemical properties, a good marriage between rubber and steel cord is not so easy. A number of accelerators and adhesion promoters are available in the market to enhance the bonding, but most of them have failed to retain the bonding substantially after different aging conditions especially at elevated temperatures. This problem is more aggravated e.g., in a country like India due to to overloading, rough road conditions, high humidity and temperature, or in advanced countries where the tires are abused due to higher speeds which leads to higher heat development and gradual separation of steel cord from rubber. The problem is even more severe when cuts develop in the tire through which engress of moisture, water, dirt, etc., takes place and ultimately affects the adhesion between rubber and steel cords. Although cobalt boron acylate alone can take care of good adhesion in normal conditions as well as under salt aging or steam aging conditions even at elevated temperature, to maintain the physicals as well as adhesion properties in anaerobic aging condition, it is essential to have a suitable network structure which can be achieved by using specialty chemicals like post vulcanization stabilizer (PVS) such as hexamethylene-l,6-bisthio sulfate disodium salt dihydrate (HTSNa) and bis-(3-triethoxy silylpropyl) tetrasulfide (TESPT).

In addition to this, due to the price crisis and concern over the continuity of the supply of cobalt adhesion promoter, it is necessary to seek suitable alternatives. Metals which have attracted interest include nickel, zirconium, zinc, tin, iron and manganese (refs. 1 and 2). When compared with cobalt boron acylate none of these systems tested gave satisfactory adhesion to zinc coated steel wire (refs. 3 and 4). However, only a nickel system proved superior under salt aging conditions with brass coated steel wire, but the major drawback of a nickel system is its toxicological properties. The aim of this article is to study the effect of partial replacement of cobalt adhesion promoter by different post vulcanization stabilizers and to see their effect on good initial adhesion and retention of the same during anaerobic (over cure), salt and steam aging at elevated temperature.

Experimental Materials

Materials used for these experiments are found in table 1.

Mixing and processing

Compound formulations are given in table 2. The control compound does not contain any adhesion promoter or post vulcanization system (PVS). Mixing was carried out in two stages. The masterbatch was mixed in a process mixer of 225 Kg capacity (batch wt.) having a four-wing rotor, associated with two open mill mixing facilities (one A.C. driven, another D.C. driven) and divided into 13 parts for individual final batch mixing. Accelerators, insoluble sulfur, adhesion promoters/PVS-systems were incorporated in the final stage conducted in an open mill (laboratory two-roll, 33 cm x 11 cm) at about 70 [degrees]C temperature. Cure characteristics were studied by using Rheometer R-100 at 141 [degrees]C and 191[degrees]C temperatures. Samples for measuring stress-strain properties were cured at three different cure times (4-5 mins., 3 hrs. and 6 hrs.) to simulate exactly the anaerobic aging condition. H-adhesion samples with a cord embedded depth of 15.87 mm were cured for 45 mins. and 3 hrs. at a temperature of 141 [degrees]C.


Stress-strain properties were carried out in a Zwick-1445 in accordance with ASTM D-412 and D-624 respectively. H-adhesion tests for aged as well as unaged samples were performed using Instron testing m/c. as per standard test condition.

Retention of stress-strain properties was measured after aerobic aging in a multicell aging oven at 70[degrees]C. Normal as well as over-cured samples were aged for one, two and four weeks.

Aging of H-adhesion samples was performed to see the effect of different aging conditions on rubber to steel cord adhesion. H-adhesion samples were aged under the following conditions:

* Salt aging (NaCl and Fe[Cl.sub.3]);

* NaCl-salt aging. This aging was performed at three

different concentrations (1%, 3% and 5%) of standard lab - oratory NaCl-salt. Dipping time was 48 hrs. at 25[degrees]C and the samples tested after three days.

* Fe[C1.sub.3] - salt aging. Standard LR-grade Fe[C1.sub.3] - salt (1% concentration only) was used for this aging test. Salt concentration more than 1% corroded the cord so badly that cord was torn off while coming out of embedded rubber length. The test was carried out at 25[degrees]C, dipped for two hrs. and tested after three days.

* Ssteam aging. Samples were kept in an autoclave for eight hrs. at 120[degrees]C. Heating was done directly by means of steam having three Kg/[cm.sup.2] pressure. After aging, samples were taken out and kept for three days and tested using Instron universal testing m/c. as usual.

Results and discussion

Initial and aged physical properties

The initial stress-strain properties and hardness are comparable (table 3) when cobalt boron acylate is partially substituted by different PVS-systems irrespective of the nature of accelerators. Fatigue life is highest with compounds containing cobalt boron acylate and HTSNa-combination (figure 1 ). However, retention of these properties under over cured conditions e.g., under anaerobic aging condition, is best with compounds containing the above combination. Most of these properties deteriorate in the case of compounds containing the TESPT system.

Probable reason may be that during vulcanization reaction, HTSNa molecules break down and interpose one or more hexamethylene-l,6-dithio groups within polysulfidic crosslinks to form a hybrid crosslink (refs. 5 and 6). On overcure, these crosslinks revert to one with a monosulfidic link on each side of the hexamethylene group. Since the C-S bond is less liable to heat, further breakdown is not possible. As a result, there is a greater flexibility in such a link than in a single monosulfidic crosslink between polymer chains.

Secondly, in this system due to the lower number of sulfur atoms linking the hydrocarbon chains, the possibilities of formation of extra network material, which result probably in reversion, are less. Besides reversion resistance properties of TESPT, it can also act as a cross-linking agent (equilibrium cure system) (refs. 7 and 8). Hence, it imparts fresh crosslinks during reversion. But it is not that much effective like HTSNa, especially ins a compound containing a higher amount of sulfur (ref. 9). The stress-strain properties and retention of modulus after aging are best with compounds containing DCBS accelerators, cobalt boron acylate and HTSNa-combination (A2).

Unaged and aged adhesion

The H-adhesion values of normal and aged samples are shown in table 4. Significant improvement in adhesion values were observed with compounds containing different combinations of adhesion promoter and PVS. The highest values are observed with DCBS accelerator which provides the highest Mooney scorch safety (figure 2) necessary for complete wetting of the steel cord by the rubber compound before curing starts to achieve better bonding with the steel cord (ref. 10).

Salt aging

After NaCl-salt aging at different salt concentrations (1%, 3% and 5%) at 25[degrees]C temperature, the adhesion values of the compounds containing TESPT drop down drastically in most of the cases, compared to cobalt boron acylate alone or its combination with HTSNa (table 4). In the case of salt aging, cobalt boron acylate alone is marginally better than its combination with HTSNa both at normal and overcured conditions for all accelerators except DCBS, where the combination of cobalt boron acylate and HTSNa is found to be better than cobalt boron acylate, especially at over-cured conditions. This may be due to the increase in stability of the crosslink incorporated by HTSNa under the anaerobic aging condition.

Fe[Cl.sub.3 ]- salt aging

The effects of Fe[Cl.sub.3] salt aging are presented in table 4. The three compounds containing DCBS accelerator are found slightly better than compounds containing other accelerators. Cobalt boron acylate and the combination of cobalt boron acylate with HTSNa give comparable results for both 45 mins. and three hrs. cured samples. Other compounds showed only slight improvement over the controlled compound.

Steam aging

The percent retention of adhesion is the same with cobalt boron acylate and its combination with HTSNa and marginally better than the TESPT and cobalt boron acylate combination for both normal and overcured samples and especially with the DCBS accelerator.


Comparing all the accelerators, DCBS is found to be best as far as Mooney scorch, cure characteristics, initial and retention of stress-strain properties and adhesion values after aging are concerned. The incorporation of TESPT which is mainly effective in semi-efficient vulcanization systems is not encouraging for steel cord skim compound containing higher doses of sulfur (here 4.8 phr of sulfur was used).

The combination of HTSNa and cobalt boron acylate in a compound exhibit the best initial properties as well as retention of most of the desired properties under different aging conditions (salt aging (NaCl, Fe[Cl.sub.3]), steam aging, aerobic and anaerobic aging). Especially with DCBS which is exclusively used for steel cord skim compound, partial replacement of cobalt adhesion promoters by HTSNa is preferable to achieve better salt aging under anaerobic conditions for preventing separation and corrosion of steel wire used for steel belted and all steel radial tires. Because of the unique properties of cobalt adhesion promoter, it still has a major role to play in the industry. However a partial substitution by HTSNa is possible which would result in lower cobalt usage together with the possibility of cost savings.


1. The economics of cobalt; Roskill Information services Ltd. London, 1988.

2. Metal Bulletin; January 14 (1986) 24.

3. P.E R. Tate; Rubber India, November (1986)47.

4. W.J. Van Ooij, Rubb. Chem. Technol., 57 (1984) 421.

5. A. Fitch, D.G. Lloyd and A. Orband; Kautschuk + Gummi Kunstst., 42 (1989) 880.

6. W.F. Heft, B.H. To and W.W. Paris; Rubber World, 204 (1991) 18.

7. S. Wolff; Kautschuk + Gummi Kunstst., 34 [1981) 280.

8. Theoretical and practical aspects of Si-69 application in tires. Degussa information for rubber industry, 1988.

9. P.P. Chattaraj, A.K. Chandra, R. Mukhopadhyay; Kautschuk + Gummi Kunstst. 44 (1991) 555.

10. K.D. Albrecht; Rubb. Chem. Technol. 46 (1973) 981.
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Author:Bhowmick, Anil K.
Publication:Rubber World
Date:Oct 1, 1992
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