Conifer Bark and Related Tannins as Plywood Adhesives

6 Conifer Bark and Related Tannins as Plywood Adhesives 

Research directed to the use of bark extracts from various species of pines has continued despite the marked reduction in prices for petroleum in the late 1970s and early 1980s. A number of new plywood adhesive formulations based on extracts of Pinus radiata have been described recently. Weissmann and Ayla (253) used sulfonated tannin extracts at 40% solids and fortified these extracts with a phenol-formaldehyde resin (Kauresin 260 produced by BASF) at levels to 10% to 50% by weight of solids. Both paraformaldehyde and hexamethylenetetramine were examined as aldehyde sources. Exterior grade plywood bonds were obtained at 10% and 30% fortifying levels. 

Woo (1982, personal communication) has described the exterior plywood adhesive formulation based on Tannaphen and used commercially by New Zealand Forest Products Ltd. since 1981 as Tannaphen - 56 parts, a phenol-formaldehyde fortifying resin - 21 parts, olivestone flour - 18 parts, and paraformaldehyde - 5 parts on weight of solids. These adhesives are tolerant of open assembly times of 20 minutes, closed assembly times of up to 4 hours, and veneer moisture contents of up to 15%. Both shear strengths and wood failure were comparatively high even after 5 weeks in water at 25 °C. 

Chen (46) used sodium hydroxide extracts from southern pine Pinus spp.) bark, peanut Arachis hypogea) hulls, and pecan Carya illinoensis) pith as 20% replacements for phenol-formaldehyde resins in plywood adhesives. Bond quality was highly dependent on veneer moisture content and assembly time. Exterior quality bonds (U.S. Product Standard PS-1-74) in southern pine plywood were obtained if the veneer moisture content was 7.0% with 60-minute assembly times. Phenol-formaldehyde resin replacements could be increased to 40% by peanut hull and pecan pith extracts when the extraction solutions containing 5% of sodium hydroxide were used (45). Southern pine veneer (0.32 cm) was spread at a rate of 41.5 to 42.5 g/lOOOcm of double glue line and the layups were stored in a 

Assembly time (min.) Press time (min.) Wood failure by adhesive type ( Vb)  

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