Scorch times

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

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

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. 

MHR = maximum stiffness during cure minus minimum stiffness, both in a rheometer. To convert /cm to Ibfin., divide by 0.0175. Too = Scorch time. 

Unlike a plastic blend where the properties largely depend on the properties of the individual component and the compatibUizer used, those of a rubber blend depend on the solubility and diffusivity of the curatives, reaction rates, scorch time, etc. Figure 11.16 gives relative cure rate and scorch time for a number of accelerators. Hence, in designing a rubber blend, aU these parameters have to be taken into consideration in order to obtain good properties along with good processability. 

FIGURE 11.16 Schematic diagram of cure rate versus scorch time for various commercial accelerators. (From Zhao, J., Ghebremeskel, G.N., and Peasley, J., A Smdy of EPDM/SBR Blends, presented at a meeting of the Ruhher Division, American Chemical Society, Chicago, IL, April 1999, Paper No. 16.)... 

The cure data show that Perkalink 900 does not affect scorch time or time to optimum cure (Table 14.51). Also, tensile data following overcure (at 150°C and 170°C) and aging at 100°C indicate a trend of improved strength characteristics (Figure 14.20). 

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. 

An early cure instrument which combined in one operation the functions of a Mooney viscometer and of a curometer or vulcameter, i.e., it measures in one quick test the plasticity (viscosity) of the (uncured) mix, its scorch time and cure rate. Now superseded by instruments such as the moving die rheometer. 

Sulphur vulcanisation of rubber is catalysed by the presence of alkali materials. This activation of the vulcanisation system can result in unwanted short scorch times. The addition of weak acids to the rubber compound results in retardation of the crosslinking mechanism. The common materials used for retardation are... 

Scorch resistance, in vulcanization, 22 811 Scorch time, in vulcanization, 22 803 SCORE cleaning system, 24 22 Scotch-Marine boiler, 22 319 Scouring, 9 171, 183, 189, 192, 197 of fibers, 22 180 in wool processing, 26 384-385 Scoville Heat test, 23 159 Scrap... 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

Mix number3 Mixing temperature (°C) Organoclay loading (phr) Roc (dNm) Scorch time t2 (min) Optimum cure time t9 (min)... 

Mix Maximum rheometric torque (Roo) (dNm) Scorch time (t2) (min) Cure time (min) 100% modulus (MPa) Tensile strength (MPa) Elongation at break (%) Hardness (Shore A)... 

The cure characteristics for the compounds were measured according to the procedure described in Sect. 4.1.2. From the curemeter data shown in Table 13 it becomes clear that the SBR compounds containing coated sulfur show longer scorch times (ts2) and optimal curing times (t90) than the compound with uncoated sulfur. The only exception is for sample PPASg-4, which gives a similar optimal curing time to uncoated sulfur (Sg). 

Sulfur sample Scorch time Optimal vulcanization Min. torque Max. torque... 

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