Investigation of the Influence of Stearic Acid On Rubber–Brass Adhesion

Brass-coated steel cords and wires are extensively used as reinforcement materials in various rubber products such as hydraulic hoses, tires and handrails. For a convincing performance a good adhesion and adhesion retention is needed. This adhesion is achieved by a Cu_xS-ZnS interface, which is created during the sulfur-based curing process (vulcanization) of rubber by a chemical reaction between the brass surface and the rubber compound [1]. The properties of the built-up interface are influenced by the composition of both the rubber compound and the brass surface.

In this work we investigated the influence of stearic acid on the adhesion interface, by employing the so-called squalene method. Different formulations, containing squalene as model compound and all essential rubber curatives, were prepared according to Hamed et al.[2] Brass-coated steel wires were cleaned with toluene and immersed into the squalene mixture for 20 minutes at 160 °C. Subsequently the wires were removed, thoroughly rinsed with toluene and stored under inert gas. The surface of the sulfidated wires were analyzed using an optical microscope, an infinite focus microscope and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX).

Moreover, adhesion tests with cured natural rubber (NR) were performed to compare the actual pull-out forces with the results obtained from the experiments with squalene.

[1] van Ooij, Rubber Chem. Technol., 1984, 57, 421

[2] G. R. Hamed and R. Paul, Rubber Chem. Technol., 1997, 70, 541