Aids in Vulcanization of Lignin-Natural Rubber Coprecipitates. Lead, Copper, and Bismuth Oxides

No difficulty is experienced in the vulcanization of natural rubber containing lignin added as a dry powder, but this method of addition results in relatively low tensile strengths. When lignin solution was added to latex, however, the masterbatch obtained by coprecipitation was difficult to vulcanize. For example, if conventional acceleration was used, i.e., mercaptobenzothiazole with zinc oxide, the test slabs were soft, tacky, and logy, and could be removed from the mold only with difficulty. Although hydrogen sulfide might be a necessary intermediate in the vulcanization reaction, published experimental evidence shows that an excessive concentration of hydrogen sulfide retards vulcanization, even in the presence of powerful organic accelerators. In order to test the possibility that the retardation of cure with lignin master batches might be due to excessive hydrogen sulfide developed during vulcanization, owing to reaction of sulfur with lignin, various metallic oxides were tried which might react with hydrogen sulfide to form insoluble sulfide, and so reduce the hydrogen sulfide concentration. Lead oxides, copper oxide, and bismuth oxide were successful with lignin masterbatches, particularly when used with a dithiocarbamate accelerator. A lignin compound containing litharge, curing in 20 minutes at 282° F, had a tensile strength about 800 pounds per square inch above that obtainable with MPC carbon black. The Bashore resilience figure for the lignin compound was about double that for MPC carbon black. The aging qualities of vulcanizates containing lead oxide and lignin appear to be good, and even copper oxide, which is normally considered to be detrimental, ages satisfactorily, provided an antioxidant is included. A lignin compound containing 7 parts of copper oxide on 100 parts of rubber and 2 parts of antioxidant showed a drop of less than 20 per cent of its tensile strength after 2 days in an air oven at 90° C.