Probing the Hydroxymethylated Resorcinol Coupling Mechanism With Stress Relaxation

The isothermal stress relaxation of dry wood is well described by the Kohlrausch-Williams-Watts relationship. This provides a direct measurement of the relaxation time and cooperativity for amorphous wood-polymer segments. The insights afforded by this approach are demonstrated for wood treated with hydroxymethylated resorcinol, HMR, coupling agent. HMR significantly stiffens wood against stress relaxation as revealed by large increases in the measured relaxation time and coupling parameter. In contrast, phenol impregnation has no effect on the coupling parameter or the relaxation time. This reveals that the simple bulking of the wood cell wall does not explain the action of HMR. Instead, this suggests that HMR chemically crosslinks the cell wall. Consequently, one aspect of the HMR coupling mechanism may involve covalent crosslinking and stabilization of amorphous regions against water swelling and other mechanical stresses

Solid-state NMR Analysis of Adhesive Bondlines in Pilot Scale Flakeboards

This work demonstrates the application of solid-state NMR to the analysis of adhesive bondlines in pilot scale flakeboards. A comparison to laboratory scale experiments is also made. Phenol-formaldehyde resin is easily detected by using labeled formaldehyde. However, resin washout can occasionally prevent detection in pilot scale composites. The relative degree of resin cure is determined by measuring corrected signal areas and also by measuring proton longitudinal relaxation in the rotating frame. Such relaxation measurements were effective in laboratory scale experiments, but were much less useful for pilot scale tests. The degree of phenol-formaldehyde polymerization was not affected by changes in wood furnish moisture content; the range of furnish moisture was 13 and 24%. This suggests that phenol-formaldehyde moisture intolerance is not related to polymerization retardation by water. This work demonstrates the feasibility of performing detailed bondline analyses on pilot and possibly industrial scale composites

Improvements In The Fracture Cleavage Testing Of Adhesively-Bonded Wood

Previous researchers have used the contoured dual cantilever beam, DCB, to demonstrate the value of fracture testing for bonded wood. However, use of the contoured specimen is laborious and stringent, preventing the routine application of this powerful test. A simplified method for mode I fracture testing of adhesively-bonded wood is presented here. Two significant improvements are shown: 1) data analysis using a shear corrected compliance method derived from beam theory, and 2) the flat DCB geometry. The shear corrected compliance method is both simple and robust, accounting for variations in wood modulus that often confound traditional shear mode tests. The flat DCB geometry greatly simplifies sample preparation, eliminating difficulties associated with the preparation, calibration, and wood selection that are required with the composite contoured DCB. Real-time crack length measurements required for the flat geometry are routine using digital hardware. The sensitivity and simplification of the method are presented in hopes of promoting the wider adoption of fracture testing for bonded wood