AbstractStarch xanthates with degrees of substitution (D.S.) ranging from 0.08 to 0.58 were reacted with high or low molecular weight polyethylenimine (PEI, two of six times the stoichiometric amount) to form starch poly(ethylenimino thiourethanes). Aqueous thiourethane solutions were mixed with commercial styrene–butadiene and acrylonitrile–butadiene latices, and the solids were coprecipitated by adding 1MZnSO4and 1NH2SO. The master batches (filtered, dried at 70°C), having 15 to 50 parts of starch (equivalent) per 100 parts of rubber (phr), were masticated and compounded in a sulfur‐accelerator recipe and press cured to yield vulcanizates of improved physical characteristics compared to control vulcanizates. About 0.22 D.S. produced best results. With this D.S. xanthate and a 3.5 ratio of 100,000 MW PEI, a 25 phr starch loading gave maximum tensile strength (2720 psi) in a styrene–butadiene vulcanizate. Hardness generally increased with increasing D.S. and starch contents. The thiourethane decreased compression set and increased abrasion resistance. The former was lowest and the latter highest at ca. 0.20 D.S. and 25 phr starch. Both set and abrasion were lowest with thiourethane prepared from xanthate made in a “Roto‐feed” process rather than a “Ko‐Kneader” operation. Rebound was not appreciably changed, and increase of volume swelling in water was small upon thiour
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