Influence of the cure level on the monomeric friction coefficient of natural rubber vulcanizates

An experimental investigation was performed in order to correlate the changes observed in dynamic modulus and loss tangent of vulcanized natural rubber with its network structure. A set of samples of natural rubber vulcanized at 413K and 433K for different times was prepared to achieve different network structures in the compound material. These networks were analyzed by means of solid-state NMR techniques in order to distinguish the sulfur bridges formed during vulcanization. The crosslink density was calculated using swelling techniques for each cure condition. Dynamic mechanical tests were performed using a free decay pendulum in the range of temperatures between 213K and 373K and oscillating frequencies between 0.1 s(-1) and 20 s(-1). Loss tangent data were analyzed within the frame of the KMF model and the variation of the monomeric friction coefficient with the crosslink density was obtained. This variation is a consequence of the network structure formed in the different stages of the curing process. (C) 2004 Society of Chemical Industry.