13C CP/MAS NMR as a Direct Probe of the Wood-Phenol Formaldehyde Adhesive Bondline

Robert G. Schmidt, Charles E. Frazier

Abstract


Resole phenol formaldehyde resin was synthesized using 13C labeled paraformaldehyde. Wood flake model composites were prepared with the labeled resin, cured for various times and temperatures, and then characterized by 13C CP/MAS NMR. The relative degrees of adhesive cure were determined from the corrected peak areas of the methylene and hydroxymethyl carbon signals. Similar cure information was provided by measuring rotating frame proton spin-lattice relaxation times. However, cure measurements via relaxation times were less sensitive because of the effects of spin diffusion within the cured resin. Relaxation measurements revealed that wood induces heterogeneity in cured PF, whereas neat-cured PF is normally more homogeneous. This method provides cure and morphological information for intact bondlines. The procedure is readily applicable to pilot or industrial scale manufacturing processes, allowing the direct evaluation of resin cure within a wood-based composite.

Keywords


Phenol formaldehyde;adhesion;CP/MAS NMR;cure

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