Shellac as an Ingredient of Rubber Compositions
Authors: Scott J. R. 
Published: February 2011

1. Shellac, even in quite large proportions, can readily be incorporated into raw rubber by the normal process of mixing on a roll mill. It does not assist the breakdown of the rubber during mastication, nor does it show the useful stiffening effect possessed, e.g., by benzidine and p-aminophenol. The effects of shellac on plasticity, indeed, are simply what would be expected of a solid that melts to a viscous liquid at about 80° C. The elastic recovery of masticated rubber after deformation is not increased by admixture of moderate proportions of shellac. Shellac greatly accentuates the calender grain produced by passing unvulcanized rubber between the rolls of a mill or calender. This grain, however, disappears during vulcanization. 2. Shellac does not increase the danger of scorching; indeed, there is evidence that it may exert a useful antiscorching effect, which merits further investigation. 3. Shellac tends to interfere with vulcanization, the attainment of optimum physical properties being retarded and the vulcanizate made weaker and softer. The extent of this interference depends greatly on the composition of the mix, particularly the accelerator. It is probably due largely, though not entirely, to the acidity of the shellac, since it can be reduced by adding a base, e.g., magnesium oxide, or by chemically modifying the shellac to reduce its acidity. Although the effect is always present, it is not serious in loaded mixes such as are used for most manufactured products, provided a suitable accelerator is chosen. Best results have been obtained with accelerators of the metallic dithiocarbamate class; mercaptobenzothiazole also is good, but diphenylguanidine and especially butyraldehydeaniline generally give poor results. Shellac can be used in conjunction with alkali-reclaimed rubber just as well as with new rubber. 4. In mixings containing large proportions of fillers or reinforcing agents, e.g., china clay and carbon black, addition of a relatively small proportion of shellac makes the vulcanizate much harder. As the unvulcanized stock is, if anything, softened by this addition, the use of shellac enables very hard vulcanizates to be produced without the processing difficulties that would arise from an attempt to obtain the hardness by using fillers alone. Tests with high carbon black stocks and with china clay flooring or tiling stocks have demonstrated the advantages of this procedure. In the former, abrasion resistance was somewhat impaired by using shellac, but there was evidence of an improvement in flex-cracking resistance, an effect that deserves further investigation. It is considered that the unexpectedly great hardening effect of shellac in mixes of this type should be more fully studied, in view of its potential utility. 5. Shellac is generally inert towards the aging of vulcanized rubber, and in no case has it been found detrimental. 6. The results obtained in this investigation do not support the claims that have been made to the effect that shellac improves the resistance of vulcanized rubber to swelling by gasoline, benzene, etc., and to absorption of water. 7. Dewaxed shellacs have given better results than TN shellac, as they produce less discoloration in mixes containing zinc oxide, and in some cases give better physical properties. Hard lac resin possesses the same advantages, particularly in the latter respect, but is difficult to disperse, as it does not readily melt. A sample of lac bleached by sodium hypochlorite interfered so seriously with vulcanization that it would probably be unsuitable for use in rubber. 8. Addition of TN shellac, sulfurized lac or hard lac resin to an unloaded (rubber-sulfur) ebonite has not been found to improve the mechanical or electrical properties; indeed, shellac would appear to be unsuitable when a low radio-frequency power loss is required. 9. Mixtures of rubber and shellac containing large proportions of the latter have thermoplastic properties which suggest their use, with or without the addition of fillers, vulcanizing agents, and accelerators of the hardening of shellac, as moulding materials, adhesives, impregnating agents, binders for fibrous materials, etc., and also, in view of the good electrical properties of rubber and shellac, as electrical insulants, particularly to replace gutta-percha. Promising results have been obtained with unvulcanized rubber-shellac mixtures as adhesives for making plywood. As the present research was concerned primarily with the use of shellac in vulcanized rubber products, these unvulcanized mixtures have not been extensively studied. They are considered to merit a more extended investigation, as they do not appear to have received any attention from scientific investigators hitherto.