Premature vulcanization

Mooney viscometer is also used to measure the time it takes, from initial exposure of the compound to a particular temperature, to the time of onset of cure at that temperature [2]. This is known as the scorch time. Scorch time is an important parameter to the rubber processor, as a short time may lead to problems of premature vulcanization. As the test is taken past the onset of cure, the rotor tears the cured rubber, and therefore this device cannot be used to investigate rheological properties after the scorch time. 

Another cure system consideration is the compound scorch behavior. Prior to vulcanization, rubber is plastic-like and can be processed into desired shapes such as tires, hoses, belts, or other articles. The time available to accomplish this processing depends laigely on the cure system and is referred to as the scorch time. If a compound cures prematurely during the processing step, it usually becomes useless scrap. Therefore, a key requirement of the vulcanization step is to minimize premature vulcanization or scorch (Fig. 4). 

Precure - Premature vulcanization taking place during the process prior to vulcanization. Similar to scorch. 

Scorch - Premature vulcanization of a rubber compound, generally due to excessive heat history. 

The problem of scorching or premature vulcanization is very acute with ultra or last accelerators. Rubber stocks are usually bad conductors of heat and therefore flow of heat to the interior of a vulcanizing stock from outside is very slow. As a result, in thick items the outer layers may reach a state of overcuring before the core or interior layers begin to cure. For such thick items, a slow accelerator (Table 2.5) is most suitable. 

The effect of the addition of small concentrations of the premature vulcanization inhibitor (PVI), N-(cyclohexylthio)phthalimide, is also given by Figure 7.11. This retarder (Coran and Kerwood, 1970) is frequently used to independently control scorch resistance with little effect on the rate of crosslinking (Coran, 1978). Before the development of N-(cyclohexylthio)phthalimide as a PVI, acidic retarders like salicylic acid, acetylsaUcylic acid, phthaUc anhydride, and benzoic acid were used. These additives improved scorch resistance but also gave greatly reduced rates of crosslink formation after the delay. Another retarder of the past was N-nitrosodiphenylamine, which is less active and not now used because of toxicological concerns. 

If MET could be taken out of the system as fast as it forms, substantial increases in processing safety would result. Such is the case when the premature vulcanization inhibitor, N-(cyclohexylthio)phthalimide (CTP), is present. This compound (Coran and Kerwood, 1970) and others Uke it react rapidly with MET to form 2-(alkyldithio)benzothiazoles, R-S-S-ET, which are active accelerators but which do not interact rapidly with the sulfenamide accelerator ... 

Carbonated nitrile rubber (XNBR) incorporates up to 10% of a third comonomer with organic acid functionality. When compared to NBR, XNBR has improved abrasion resistance and strength. XNBR can be difficult to process, and it requires special formulation to prevent sticking to mixer surfaces and premature vulcanization. 

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. 

The scorch time, which is the measure of premature vulcanization, and the optimum cure of vulcanizates increased. ... 

Another study by Sui et al. shows the vulcanization curves of the neat NR and NR composites. In Figure 21.11 it can be seen that the scorch time (measurement of premature vulcanization) decreased after adding CNTs or carbon black into the NR. This may be because when CNTs are added, they would absorb the basic accelerator species and delay the onset of vulcanisation. The reason for this is that the addition of the CNTs could improve thermal... 

Also called pile burning or premature vulcanization. 

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