Attapulgite Clay – a Filler for Exterior Plywood Adhesives

The clay mineral attapulgite, and acicular hydrous magnesium aluminum silicate occurring in large deposits in northwestern Florida and southwestern Georgia, has unique gelling, adsorptive, and absorptive characteristics which make it potentially useful as the primary filler-extender in plywood resin adhesives. The water-holding capability of the clay may prevent excessive dry-out during long assembly times. It also develops desirable thixotropic properties in various emulsions. Four adhesive resin formulations were developed using two types of attapulgite clay substituted for 50 and 100 percent of the furafil in a phenolic plywood adhesive. The thixotropic indices for furafil-filled and 100 percent attapulgite-substituted adhesive mixes were approximately 1.2 and 2.8 respectively. Test results from experimental Douglas-fir plywood panels bonded with these resin formulations demonstrated that there was no significant difference in glue bond quality between furafil- and attapulgite-filled adhesives. Closed assembly times ranging from ten minutes to four hours also had no significant effect on glue bond quality. Prepressing at five minutes and at two hours generally developed poor panel consolidation, while prepressing at one-half to one hour developed excellent panel consolidations with all resin formulations. Several in-plant trials have also shown that a attapulgite clay is feasible to use as the primary adhesive filler in commercial plywood production.

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