Tannins cold-setting lamination

Applications for cold-setting, wood-laminating adhesives initially followed the same approach (47) used for laminating resins from western hemlock (38) (i.e., reaction of tannin with phenol-resorcinol-formaldehyde prepolymers). Improvements resulted through the application of Kreibich s Honeymoon technique (48) wherein one side of the material to be bonded is treated with resin and the other with catalyst. One of the preferred systems (49) was phenol-resorcinol-formaldehyde or tannin-resorcinol-formaldehyde at pH 8 with extra paraformaldehyde on the A-side and tannin at 53% solids or tannin-resorcinol-formaldehyde at pH 12 on the B-side. Such resin systems are currently used to laminate eucalyptus or pine in most South African timber-laminating plants. 

The reactivity of resorcinol with formaldehyde is essential for developing the cohesive strength of the interlayer and its bonding characteristics. Condensed tannins are known to be very reactive with formaldehyde (7-0), so these renewable phenolic polymers are good candidates as resorcinol replacements. Indeed, condensed tannins from wattle and pine bark extracts have been successfully used in cold-setting, wood-laminating adhesives, and the former are used extensively in the commercial production of laminated timbers in South Africa (Pizzi, A., National Timber Research Institute, Pretoria, South Africa, personal communication, 1982) (10-13). 

Kreibich R E, Hemingway R W 1987 Condensed tannin-sulfonates in cold-setting wood-laminating adhesives. For Prod J 37 (2) 43-46... 

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... 

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