Vulcanization surface scorching

Sometimes the surface of a rubber sheet might have been scorched or pre-vulcanized due to high processing temperatures and storage under improper storage conditions. To avoid this, modify the compound suitably. The preferred temperature for storing the in process stock of rubber can be 20°C to 24°C. 

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

Vulcanization kinetics of the composites vulcanization are very similar regardless refractory filler type, excluding the mix containing surface modified montmorillonite (M-MMT) (Table 8.2). In its case values of torque were significantly lower than for other samples minimum (M j ), optimum (M ) and maximum (M ) torque values, respectively, from 14 to 27%, from 34 to 40% and from 35 to 41%. This effect can by explained by plasticization of silicone matrix by quaternary ammonium salt present in M-MMT. Also values of scorch (tp2) and vulcanization (t ) time... 

---