Wattle Tannin-Based Plywood Adhesives

The development of wattle tannin-based plywood adhesives occurred concurrently with the particleboard adhesives and, although quite different adhesive properties are required to bond these two types of furnish, many of the basic premises on which their development were based are the same. A series of wattle tannin-based plywood adhesives that provide exterior bond qualities have been described in detail by Pizzi (182, 186). All recent formulations use low-molecular-weight polymers, as first suggested by MacLean and Gardner (131), rather than formaldehyde for crosslinking agents. 

Wattle tannin-based plywood adhesives were also crosslinked with 10% to 20% (by weight of solids) of either a phenol-formaldehyde resol or a phenol-resorcinol-formaldehyde polymer with approximately 20% resorcinol, to which paraformaldehyde was added to provide crosslinking between the resorcinol and 

A-ring of the tannin (194). Resin synthesis conditions varied in the approach to addition of formaldehyde. For example, the tannins could be crosslinked by reaction of a phenol-resorcinol-formaldehyde resin carrying little or no methylol functionality by addition of paraformaldehyde or by reaction of a phenol-formaldehyde prepolymer carrying a comparatively high methylol functionality. Adhesive formulations were similar to those described above. Typically, 100 parts of a 55% solids content water solution of wattle tannin were combined with 0.25 parts of defoamer, 7 parts of paraformaldehyde and 9 to 10 parts of coconut shell powder the pH was adjusted to 6.5 to 7.4 by addition of 40% NaOH. These adhesives also provided exterior quality glue lines. They are exceptional in their tolerance to high moisture content veneer and permit fast curing rates, subjects of particular interest in the plywood industry today. 

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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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