Curing, rubber drawbacks

Perhaps the more important drawback of this method is the fact that for a 0.5-mm-thick sample, the temperature through its thickness should instantaneously reach that of the heated slabs of the press. This assumption is not exactly true, as shown in Figure 1.6 with the curve expressing the increase in the state of cure at the mid-plane of the rubber sheet as a function of time. Thus, the S shape for the curve representing the cure (or rather the mean cure) versus time (Figure 1.1) is logical. Moreover, the scorch time [2,3], which is the time necessary for the onset of vulcanization to take place, will perhaps need another explanation. 

As shown in Chapter 4, the cure operation takes place in the mold where the rubber compound is heated to a temperature at which the reaction starts with a significant rate. But a drawback arises when the rubber, initially at room temperature, is placed in the slabs of the mold previously heated at the desired temperature, because it takes some time for the rubber to be heated to the mold temperature. Moreover, the mold may be so complex in size that it is not possible to introduce the rubber compound in a solid state. For these two reasons, the technique of injection molding is applied. 

A variety of dialkyl peroxides and tertiary butyl perbenzoate lead to high efficiency of cross-linking reactions in gum rubber, but only di-tertiary butyl and dicurnyl peroxides are capable of curing compounds containing reinforcing black fillers. Of these two, the former has the serious drawback of volatility, and hence very low efficiency. 

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... 

However, this theory has several drawbacks. It can only explain the modest increase in toughness and cannot explain the dramatic increase in fracture energy reported in many rubber-toughened epoxy systems. There are many discrepancies between reported data for improvement in toughness for thermoset resins due to the sensitivity of the toughening effect to the curing condition and inherent matrix ductility [119, 120]. The effect of G on test temperature and strain rate cannot be explained using... 

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