Scorch safety

The same behaviour occurs with the scorch-retarded version of ZDMA and is illustrated in Table 8.1 with EPDM rubber. In this example, a 40% increase in TSj is obtained with Saret 634, the scorch-retarded version of ZDMA, without any significant loss in maximum torque (MHF). MHF is the difference between the minimum torque (Ml) and the maximum torque (M ) obtained for a rubber compound by oscillating disk rheometer (ODR). It is a measure of the crosslink density of a compound and is part of the curing characteristics of a compound. 

with the Saret metallic coagents it is possible to gain the benefit of increased crosslink density and faster cure rates, while maintaining good scorch safety. 

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Fig. 2. Cure curve from oscillating disk rheometer where A represents scorch safety B, cure rate C, state of cure D, optimum cure time and E, reversion.

Fig. 4. Effect of heat history on processiag (scorch) safety where A shows the time after processiag and storage and B shows vulcanisa tion without...

Accelerator type Scorch safety Cure rate Cro ss-link length... 

In one experiment the effect of ppd assay was correlated to scorch safety. As the ppd degrades Hberate free amine, scorch time decreases and cure rate is faster. The degradation products apparentiy serve to activate the cure, since both the induction time, and cure time, decrease with decreasing ppd assay. However, the effect on unaged properties is minimal. 

Extruded Articles. In extmded article compounding, the most important parameters are scorch safety and flow characteristics (53). The bisphenol cure system again offers the best scorch resistance of the available fluorocarbon elastomer cure systems. Good flow characteristics can be achieved through proper selection of gum viscosities. Also, the addition of process aids to the formulation can enhance the flow characteristics. Typical formulations for extmsion grade fluorocarbon elastomers are given iu Table 7. 

The comparisons between accelerator classes are shown in Figure 14.5. Figures 14.6 and 14.7 show comparisons of primary accelerators CBS, TBBS, MBS, and MBTS in NR and SBR, respectively. Major differences in scorch safety, cure rate, and modulus development are observed. 

A-l,3-dimethylbutyl-A -phenyl quinone diimine (6QD1) has been introduced as a multifunctional additive for diene rubbers and provides an advantage in mixing characteristics (functions as peptizer and improves scorch safety) as well as improved performance (better antioxidant activity than paraphenylenediamine antidegradants) of the end products [36]. 

The Echo S cure system has been commercialised by BFGoodrich and this is said to give improved scorch safety and reduced mould fouling over the use of thioureas. Inorganic acid acceptors other than those based on lead are recommended for use with the Echo S cure system. 

Zinc peroxide is used as the curative for carboxylated acrylonitrile-butadiene rubber (XNBR) compounds. It confers better scorch safety than does zinc oxide. It is usually added in a masterbatch form. 

Balance of fast cure and scorch safety Mold release... 

Mixing mill temperature can be 85 °C to 90 °C. In Banbury mixing the cycle will be 5 minutes. Dumping should be done before the temperature goes up to 110 °C, especially if a litharge cure is used. From a scorch safety point of view, masterbatches of litharge can... 

Unlike the other instruments we have discussed, the ODR gives useful information concerning not only a compound s processing behavior and scorch safety but also average and peak cure rate, final state of cure, and reversion resistance properties. Figure 28 shows a schematic of the ODR. The ODR is described in ISO 3417, BS 903, Part A60-2, and ASTM D2084 [106]. Since its introduction in 1963, over 5000 ODRs have been produced throughout the world. 

FIGURE 7.4 The effect of heat history (processing) on scorch safety. 

Waxes act as internal lubricants in rubber and increase the scorch safety somewhat. Unfortunately, waxes have a number of shortcomings. First, they are ineffective under dynamic stress conditions. This is likely due to a lack of adhesion between the wax film and the rubber and to the inextensibility of the wax bloom. 

The level and consistency of bond strength are influenced by the nature of the rubber factors include the mix viscosity, level of filler, scorch safety and, in the case of sulphur-vulcanized rubbers, the amount of elemental snlphur in the vulcanizing system. 

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