Vulcanization, accelerated-sulfur characteristics

This type of vulcanization-system design was reported by McCall (1969). He found that by judiciously balancing the levels of accelerator, sulfur, and DTDM, he could obtain good vulcanization characteristics, good thermal stability, good flex life, and superior retention of flex life. Others have reported on more recent work on the effects of crosslink type on reversion (Datta et al., 2007 Fan et al., 2001). 

The hydrophilic nature of silica also affects the cure characteristics of rubber compounds, the properties of vulcanized rubber and also the compatibility with non-polar rubber such as natural rubber (NR). Silica retards the vulcanization as it reacts with zinc-accelerator-sulfur complex. These drawbacks can be overcome through the use of silane coupling agents. The most common silane coupling agent used is bis(3-triethoxysilylpropyl) tetrasulfide (TESPT). A silane... 

The irreversible binding of accelerator moieties by desulfuration or by some other reaction limits the formation of zinc accelerator complexes and subsequently hinders desulfuration of both pendent groups and crosslinks. Although direct evidence for this pathway being responsible is lacking, it does appear that with NR at high temperatures, or with butadiene-based rubbers at all temperatures, removal of the accelerator by combination with the polymer gives the system many of the characteristics of an unaccelerated sulfur vulcanization" (see Section 4.7). 

A very important time in the creation of a network polymeric composition is the choice of oligomer, because its chemical composition and structure determine characteristics of the created material. This is true also for RubCon, the liquid phase of which consists from rubbers with various microstructures of polymeric chains. Liquid rubbers in projected compositions are capable, if acted on by special sulfur-accelerating systems, to be vulcanized with formation of space-linked net polymers, the space net of which mainly determines the positive properties of the hard base of the RubCon composite. 

One of the curing characteristics that we would like to control is reversion, which can occur in compounds containing natural rubber. There is more than one approach to reducing the amount of reversion. We can use sulfur donors or increase the ratio of accelerator concentration to sulfur concentration, or we could carry out the vulcanization at a reduced temperature for a longer period. However, these approaches give rise to effects that will have to be compensated. Another approach is use of additives such as certain bisimides, e.g., N,N -m-phenylene-biscitraconimide and N,N -/m-phenylene-bismaleimides (Datta et al., 1997, 1998 Datta and Ivan, 1995) or trialkoxysilylalkylpoly-sulfides such as bis-(3-triethoxisilylpropyl)-tetrasulflde (Tan and Wolff, 1985). 

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