Mechanistic investigation of rubber-brass adhesion: Effect of formulation ingredients

It is very customary to use adhesion-promoting resins in the belt compounds' formulation to enhance the adhesion between rubber and brass-coated steel cords. Conventionally, two-component adhesion-promoting resins, i.e., HexaMethoxy-MethylMelamine (HMMM) + Resorcinol Formaldehyde (RF) precondensed resin, are commonly used in the tire industry to enhance the initial and aged adhesion between rubber and brass-plated steel cord. However, one-component adhesion-promoting resins were developed in an attempt to eliminate resorcinol from the formulation of belt compound. This study was undertaken to unravel the role of these newly developed one-component resins in enhancing the initial as well as aged adhesion performance.; Initial experiments were conducted using a squalene liquid rubber modeling approach in the laboratory to study the effect of resins on the chemistry of the vulcanization reaction and their effect on the adhesion interface. Based on the GPC, SEM/EDX and TOF-SIMS data, a mechanism was proposed to explain the possible role played by resins in the improvement of initial and aged adhesion performance between rubber and brass-coated steel tire cords.; Actual natural rubber compounds were later cured to confirm the proposed mechanism derived from the initial squalene-based studies. The proposed mechanism based on squalene studies, which theorized the possible migration of resins towards the rubber-brass interface, was found to be unfeasible in the actual natural rubber compounds, primarily because of the substantial increase in the molecular weight of the rubber matrix. Thus, a new approach was undertaken and the effect of resins on the mechanical properties of the cured natural rubber compounds was studied. Similarly, the initial, heat and humidity aged adhesion studies were performed using these natural rubber compounds. The effect of resin incorporation on the crosslink density distribution was studied using C-13 NMR analyses of the cured rubber compounds.; In order to understand the effect of resins on the rubber-brass adhesion interface, brass-plated steel cords and highly polished brass coupons were sulfidized against the natural rubber compounds. The cured rubber was removed using a unique approach, i.e., ortho dichlorobenzene solvent cleaning method. The sulfidized brass cords were tested for their corrosion-resistance properties using electrochemical (DC polarization) techniques. Furthermore, the effect of resins on the crystal structure of copper sulfide adhesion layer was understood by analyzing the sulfidized brass coupons using synchrotron source grazing incidence angle X-ray diffraction (GIXRD) method. A better understanding of the effect of one-component resins on the rubber-brass bonding was achieved. Based on the results obtained in this research work, a new mechanism of action for the one-component resins is proposed that explains the effect of these resins in improving the rubber-to-brass adhesion performance, especially after aging.