Hemlock and Spruce Tannins: An Odyssey

Abstract

In North America, the only significant chemical utilization of bark has been that of hemlock, which served as the major raw material for leather tanning for more than a century. This chapter traces the history of leather manufacture from its beginning as a colonial cottage industry through its emergence at the end of the nineteenth century as a major industrial activity based largely on a local eastern hemlock resource. The later availability of domestic (chestnut) and imported (quebracho and wattle) extracts caused a geographic shift in the industry to the source of hides rather than bark. Substitution of chromium salts for vegetable tannins also contributed to the demise of hemlock tannin utilization except for the production of sulfonated tannins from western hemlock used for oil well drilling, water treatment, agricultural trace metal treatments, etc. Modern chemical investigations of hemlock tannin composition began in 1954 with the advent of paper chromatographic separation techniques, which eventually showed that tannins were polymeric cis and trans procyanidins terminated by catechin and epicatechin. Recent work demonstrates the need for studies of polyphenolic polymers from morphologically distinct zones of bark. In spite of yet-to-be solved chemical structure problems in the field of spruce/hemlock tannins, utilization is not being held up. The major need is for the development of extraction technology that will remove the organic solvent-insoluble procyanidin polymers without major structural alteration. The basic requirement appears to be a system that cleaves procyanidin polymers (without causing condensation with adjacent lignin molecules) and solubilizes them in an aqueous system. The best potential for use is as a resorcinol substitute in cold-setting adhesives, since domestic vegetable tannins are unlikely to again be of any major consequence in leather manufacture in North America.

This chapter is based on Dr. Hergert’s address in acceptance of the first North American Tannin Conference Award. Most of the research by the author and presented in this chapter was conducted at the ITT Rayonier research laboratory, Shelton, Washington.