Scorch Scorchy

Functionally, accelerators are classified as primary or secondary. Primary accelerators provide considerable scorch delay, medium fast cure, and good modulus development. Secondary accelerators, on the other hand, are usually scorchy and provide very fast cure. There are a wide variety of accelerators available to the compounder including accelerator blends this number well over 100. In order to rationalize the extensive range of materials it is useful to classify them in terms of their generic chemical structure listed below and shown in Figure 14.4. 

Premature vulcanization of a rubber compound (called scorch) can occur if the reaction temperature of the vulcanizing ingredients is reached before the desired time. If this temperature is reached in the mixing process before the proper viscosity and level of dispersion is obtained, then the addition of accelerators and vulcanizing agents will cause scorchiness and poor processability. 

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. 

Other sulfur donors such as tetramethylthiuram disulfide (TMTD) can also be used however, TMTD is much more scorchy (imparts less scorch safety time) compared to DTDM. 

Diamines give effective vulcanizates, but those leading to the best properties are very scorchy [19,21]. Derivatives of diamines designed to stiffen the amine fimction, so as to reduce the scorching tendency, are by far the most widely used curing agents [19,21]. The most common examples are the carbamate and bis-cinnamylidene derivatives of the hexamethylene diamine (HMDA) ... 

ACM polymers tend to be scorchy and slow curing, thus requiring a post-cure to obtain the optimum physical properties. While the NPC (No Post-Cure) system available from Zeon Chemicals is very fast curing, it is also very sensitive to process temperatures hence the compound must be kept under 90°C after the curative has been incorporated to prevent scorch. If the compound is handled properly, the bin stability and scorch resistance are excellent. 

Quinoid cures are, however, likely to be scorchy and to produce vulcanisates with lower than normal physical strength. Stearic acid and other acidic materials are omitted because they increase the risk of scorch. Antioxidants and antiozonants cannot be used because they react with the oxidising agent in the curing system. 

Figure 4 compares five different classes of accelerators in NR compounds. Thiurams and dithiocarbamates are fast but relatively scorchy. Thiazoles (e.g., MBTS) and guanidines (e.g., DPG) are scorchy, slow accelerators and seldom used alone. On the other hand sulfenamides (e.g., TBBS) exhibit excellent scorch safety and moderate cure rate with a good torque development. 

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