Lignin for Reinforcing Rubber

Lignin, as made from the waste black liquor of the sulfate wood pulp process, is an effective reinforcing agent for synthetic or natural rubbers when incorporated into latex by the coprecipitation or master-batching procedure. GR-S, reinforced in this manner with 38.5 volumes of lignin, yields vulcanizates having a tensile strength of 2900 pounds per square inch and a tear resistance of 380 pounds per inch, values exceeded only with channel black. With 77 volumes loading the tensile strength of 2800 pounds per square inch and tear resistance of 550 pounds per inch are higher than those for any other pigment tested. In abrasion resistance lignin vulcanizates are between carbon blacks and the common inorganic fillers. Shore hardness values are close to those for channel black. Lignin-reinforced rubbers weigh appreciably less per unit volume, owing to the low specific gravity, 1.3, of lignin. The brown color of lignin permits a wide color range, without sacrifice of mechanical properties, by blending with white pigments. Lignin coprecipitates require only a brief milling time for complete mastication. Details are given on the preferred procedure for preparing master batches of lignin and GR-S. Other suggested incorporation procedures made possible by the colloidal properties of lignin are reviewed.