Atomic force and scanning electron microscopic studies of effect of crosslinking system on properties of maleated ethylene–propylene–diene monomer rubber composites filled with short melamine fibers



This article reports the development of new composites based on maleated ethylene–propylene–diene monomer (EPDM) rubber and melamine fiber. The compositions are cured using three different types of curing systems: conventional covalent crosslinking based on sulfur and an accelerator, fully ionic crosslinking based on zinc oxide/zinc stearate, and mixed crosslinking involving both covalent and ionic bonds. A dry bonding system comprising resorcinol, hexamethylene tetramine, and hydrated silica, popularly known as HRH, is found to increase the mechanical properties of the composites cured using both the sulfur/accelerator and mixed crosslinking systems. However, incorporation of the dry bonding system and/or melamine fiber adversely affects the strength of the composites if the linkages are fully ionic. Among the three crosslinking systems employed, composites prepared by using the mixed crosslinking system give higher tensile strength and modulus. Aging at 150°C for 48 h causes improvement in the strength and modulus of the composites cured by using the sulfur/accelerator system, particularly at higher fiber loading. However, in the case of an ionic system, both the modulus and strength drop with aging; in the case of the mixed crosslinking system, the modulus increases, but there is a drop in strength on aging. Rheological studies of the uncured compounds show that the presence of zinc stearate in the formulations of mixed and ionic crosslinking systems results in reduced viscosity and a smoother extrudate as compared to the sulfur crosslinking system. An atomic force microscopy analysis of the roughness reveals that the fiber surface roughness is reduced because of aging for the ionic crosslinking system. This is attributable to the formation of a zinc stearate layer on the fiber surface, which also reduces the adhesion between the fiber and the matrix, thereby reducing the hysteresis after aging. However, for the sulfur and mixed crosslinking system, the roughness of the fiber surface is increased and there is a concomitant increase in hysteresis. © 2003 John Wiley Periodicals, Inc. J Appl Polym Sci 89: 1211–1229, 2003