Other Wattle Tannin-Based Adhesives

Wattle tannins have been used as fortifiers in starch-based adhesives for corrugated carton manufacture in order to improve the moisture resistance of boxes used in shipment of fruit, for example (144, 217). Addition of 4% wattle tannin based on resin solids substantially improved the durability of these cartons. Wattle tannin urethane-based varnishes with excellent durability have been developed by Saayman (214). Other adhesive applications based on reactions of wattle tannins with isocyanates have been developed for specialty applications such as the bonding of aluminum (181). 

5 Conifer Bark and Related Uannins as Particleboard Adhesives 

These adhesives performed equally well when used for bonding of oriented strandboards made from Douglas-fir flakes. In addition, composites made from oak or southern pine flakeboards to which southern pine veener was bonded on the face and back passed the 6-cycle durability test of the American Plywood Association when the core was bonded with the peanut hull-based adhesives, even when the 0.64-cm cores were pressed for only 75 seconds. Chen s work on these adhesives is described further in a series of four U.S. Patents (41-44). 

New Zealand Forest Products Ltd. continued their research on the use of radi-ata pine tannins and had a 1 ton/day pilot plant operating by 1974 and a full-scale plant in operation from mid-1981 to 1987. These tannin extracts were marketed under the trade name Tannaphen (Woo, 1982, personal communication). The extracts were used in bonding of both plywood (described below) and particle board. The Tannaphen extract (100 parts) is combined with a defoamer (0.5 parts), paraformaldehyde (6-10 parts), and parafin wax (10 parts) it is sprayed on wood chips at a loading of 10% to 12% by weight of solids on the face and 7% to 9% by weight of solids on the core particles. Panels 20 mm thick are pressed for 7 minutes at 170°C to obtain panels with the properties shown in Table 10.3.4. These panels were marketed primarily as a flooring material in New Zealand. While not classified as exterior panels, these Tannaphen-bonded boards re- 

It would seem that condensed tannins would have good potential for use in adhesives for oriented strand- or flakeboard particularly because of their known tolerance to comparatively high moisture content furnish and because of the potential for more rapid cure rates. At current prices for phenol-formaldehyde resins in North America, interest in these types of adhesives is not particularly strong. However, considering the rapid expansion of this industry, good potential exists if prices for phenol-formaldehyde resins should increase substantially. It should be recognized that amounts of resins applied to oriented strand- or flakeboards in North America are much lower that those used in the above-described exterior particleboard adhesive applications. 

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Problems and facts that in the author s personal experience arise in the industrial application of tannin-based adhesives for timber sometimes indicate lack of correspondence with laboratory practice and results. These are often problems related to unusual characteristics of the adhesive itself, or of its application technique, which could not be noticed during research under laboratory conditions, but the existence of which could easily jeopardize successful implementation of laboratory technology into industrial practice. Correcting the credibility gap between research focus and industrial usage is seen as a critical step toward market expansion for these new products. Important considerations are consistency of tannins, extracts and adhesives properties due to the natural raw material variability formulation in cold-setting adhesives and application conditions (such as wood moisture and adhesive-content or pressing time) in particleboard adhesives. These problems have been overcome in use of wattle tannin-based adhesives as shown by a visual comparison of tannin-, phenolic-, and melamine-bonded particleboards exposed to the weather for 15 years and the growing use of tannin-based adhesives in other countries. 

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... 

Most of the recent efforts to develop uses for the condensed tannins have centered on their application in wood adhesives. Reviews by Pizzi (182, 186) and others (15, 78, 87, 93, 208) provide references to several hundred papers and patents on this subject. Despite world-wide research efforts on other sources of tannins, particularly since the 1972-1973 petroleum shortage, the mimosa or wattle tannins extracted from the bark of black wattle Acacia mearnsii) remain the major source of condensed tannins exploited commercially for adhesive manufacture. Of the approximately 100000 tons of wattle tannin produced annually, only about 10000 tons are used in wood adhesives, predominantly in South Africa but also in Australia and New Zealand (186). The extensive use of wattle tannins by the wood products industry of South Africa is impressive indeed, as these tannins have partly replaced phenol and resorcinol usage in adhesives for bonding of particleboard, plywood, and laminated timbers (182, 186, 213). Three factors have contributed to the success in use of wattle tannin-based adhesives, namely the comparatively high costs of phenol and resorcinol in the Southern Hemisphere, their resorcinolic functionality and low molecular weight and, perhaps most importantly, the commitment by the research and industrial communities of these countries to reduce the reliance of the forest products industry on petroleum-based adhesives. 

Natural phenolic compounds are used as both replacements for substantial portions of synthetic phenol in plywood adhesive resins and as glue mix additives to improve performance 4 to 6% is added, based on phenolic resin solids. They bring about improvements in assembly time tolerance and flow with no significant change in adhesion. Glue mix additions of wattle tannin or other condensed flavonoid tannin extracts with or... 

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