Sulfur peak stress

Figure 4. EPR signals of wheat flour extruded at 185 X die temperature, 16% moisture, 225 g/min mass flow rate, and 500 rpm screw speed, then ground for analysis by various methods. Sulfur peaks in EPR signals show that shear stress (grinding) produces sulfur radicals via scission of S-S bonds.

With the RFL adhesive, a somewhat different kinetic profile was recorded. Certainly, a similar peak in peel strength was observed after around 10 minutes of vulcanization. But pull values were still increasing significantly after 40 or 60 minutes. This slower buildup in peel strength can be attributed to sulfur migration from the SBR into the RFL adhesive layer. The net physical effect could be a smoother gradient of crosslink densities for better stress distribution prior to rupture — a process not unlike annealing. 

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