Natural rubber sulfur-free curing

The butadiene-styrene rubber (SBR), or butadiene-acrylonitrile rubber (NBR) and elastomeric graft copolymers were found particularly valuable for impact modification of PP. The PP alloys (with 5-20 wt.% of an elastomer) were reported to have advantageous properties for blow molding of bottles free from brittleness and stress cracking. Blends with natural rubber (NR) require sulfur-curing [4]. Blending with NBR dramatically increased the modulus, but the material was brittle [5]. 

These are chemicals whose function is to produce a crosslinked, thermosetting plastic from an initially linear or branched polymer. For example, a vinyl monomer, such as styrene, a free-radical initiator, and sometimes a promoter (e.g., cobalt naphthenate, to speed up the reaction) can be dissolved in a low molecular weight unsaturated polyester resin, which forms crosslinks by an ordinary addition mechanism involving the double bonds in the polyester (Example mprb2.3). Curing is also important in forming tires, as sulfur is compounded with natural, butadiene, or isoprene mbber to form a crosslinked rubber. 

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