Conifer bark tannins

Tannins. Herb Hergert s introduction (Chapter 12) to the use of condensed tannins in adhesives is especially interesting because he provides some reasons why commercial success is lacking in the use of condensed tannins from conifer barks despite substantial effort worldwide to parallel the South African success in the use of wattle tannins. Much of the problem in the use of conifer bark tannins remains centered on our inadequate understanding of the fundamental chemistry of these polymers. For example, Lawrence Porter (Chapter 13) provided the first measurements of the viscosities of solutions of purified condensed tannin isolates of known molecular weight and the reactions of these polymers with formaldehyde. It is incredible that this has not been done previously considering the hundreds of papers that have been published on tannin use in wood adhesives. Further evidence for the comparatively limited knowledge... 

Progress in the use of condensed tannins in adhesive formulations might be expected to be more rapid than is the case for lignins because of the impetus provided by the commercialization of wattle tannin-based adhesives and because of the extraordinarily high reactivity of tannins in reactions with formaldehyde. This reactivity offers an opportunity to substitute tannin for resorcinol (currently priced at about 1.80/lb) instead of phenol (about 0.40/lb). Now that wattle tannins have been successfully introduced, their application can be expected to continue to expand. The situation remains difficult, on the other hand, for use of conifer bark tannins in adhesives. Herb Hergert is certainly... 

Hemingway R W, McGraw G W 1978 Formaldehyde condensation products of model phenols for conifer bark tannins. J Liq Chrom 1 163-179... 

Table 10.3.2. Properties of wood-face, bark-core, three-layer particleboards bonded with conifer bark tannins (173)...

With the exception of the early work by Herrick and Conca (95) there has, until recently, been surprisingly little research directed to the use of conifer bark tannins in cold-setting wood-laminating adhesives. Current prices for the resorcinol that generally makes up about 25% of the weight of cold-setting phenolic resins are about 1.80 to 1.85/lb. Therefore, comparatively expensive reactions and processing costs can be economically feasible in the development of these types of... 

Taking advantage of the many aromatic hydroxyl functions in conifer bark tannins, Hartmann (83) used ground bark as a polyol for reaction with isocyanates to prepare urethane foams with particularly good flammability resistance. Most uses for conifer bark tannins that involve reactions with the hydroxyl functions center on their complexation with cations. When sulfonated, condensed tannins can also be used as water-soluble heavy metal complexes. One of the more interesting of these applications is the development of water-soluble heavy-metal micronutrient complexes that have been used to correct iron deficiency in citrus... 

Conifer Bark and Related Tannins as Plywood Adhesives... 

Condensed tannins, on the other hand, occur in the bark of all conifers and hardwoods examined to date, and they are frequently present in the wood. They are primarily responsible for the tan to brown color of wood after it is exposed to air. In their purest form, condensed tannins are colorless, but they become colored very readily once isolated because of their propensity to oxidize to quinones. The primary characteristic of the water-soluble condensed tannins (4) is dehydration/oxidation to intensely colored anthocyanidin pigments (5) when refluxed in butanol and hydrochloric acid (Figure 2). For this reason, there has been a tendency to refer to these compounds as proanthocyanidins in the last few years. Prior to that, they were referred to as leucoanthocyanidins (i.e., the colorless chemical form of anthocyanidins). All references earlier than the late 1950 s, when the structure of these substances was just beginning to be understood, used the term condensed tannin. 

When P. radiata bark is extracted by sulfite-carbonate, the solution viscosities are much lower. For example, Woo (30) reported a viscosity of 1,600 mPa-s for a 45% solution of Tannaphen, a commercial tannin extract from P. radiata bark that contains approximately 70% proanthocyanidins. When extracted with sulfite-carbonate, the proanthocyanidins will be partly depolymerized 31), which will cause a fall in viscosity. Whether the very high viscosities observed for aqueous extracts by Yazaki and Hillis 29) are due to the P, radiata proanthocyanidins being of much higher molecular weight than other conifer tannins or due to complexation of the proanthocyanidins with the polysaccharide fraction 32) remains to be shown. 

White Pine. Deal pine Northern pine Weymouth pine, The dried inner bark of Pinus strobus L, Pinaceae. Constit. Conifer in glycoside, couiferyl alcohol, tannin, oleo-resin, volatile oils. 

Hergert H L 1960 Chemical composition of tannins and polyphenols from conifer wood and bark. Forest Prod J 10 610-617... 

Thylor R J, Shaw D C 1983 Allelopathic effects of Engelmann spruce bark stilbenes and tannin-stilbene combinations on seed germination and seedling growth of selected conifers. Can J Bot 61 279-289... 

Condensed tannins are excellent clay dispersants. Sulfite extracts from conifer tree barks are very effective in reducing the viscosity and increasing the gel strength of muds used in well drilling (72, 92, 145, 246, 248). Condensed tannins still face strong competition from lignosulphonates for this application, particularly because of the comparatively low thermal stability and salt tolerance of the tannin (96). However, reaction of tannins with chromium increases their thermal stability considerably to permit their use in muds for wells drilled as deep as 6000 feet (208). Sulfonated condensed tannin derivatives have found use as dispersants in other specialty applications, such as ceramic clays, pigments, carbon black, and pesticides (93, 96). 

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