Effect of Moisture on Curing Rate of GR-S

1. Moisture content of uncured GR-S compounds must be reckoned with because of its effect on curing rate and the serious consequence of overcure. 2. Excessively dry polymer (below about 0.15 per cent water) will, in general, be slow curing (Figure 2). 3. Mixed stock with less than 0.5 per cent water will, in general, be slow curing and erratic in curing rate (Figure 5). 4. The normal moisture range of carbon black does not affect the curing rate of GR-S (Figure 4). 5. The normal moisture content of carbon black is not available to the polymer for stabilizing its curing rate. 6. Mixed stock which is dry and slow curing may be conditioned to stable curing rate by storage under humid conditions. A moisture content of 0.5 to 1.0 per cent is indicated. This moisture content seems also to iron out differences in curing rate between polymers. 7. Since conditioning of mixed stocks is not always feasible on the factory scale, the stabilization of cure by direct water addition during mixing should be given consideration. 8. Laboratory-scale tests on a GR-S tread compound indicate that an addition of about 2.5 to 5 per cent water (on the polymer) during mixing will result in a mixed-stock moisture content giving minimal cure variation for normal-curing and slow-curing (very dry) polymers for periods of stock layover up to about 3 weeks. 9. Any additions of water as suggested above will, on the factory scale, require adjustment, since moisture retention is a function of the compounding ingredients, mixing cycle and temperature, storage time, and humidity conditions actually obtaining.