PROFILE FORMING OF WOOD-STRAND COMPOSITES: PROCESSES, FORMING CHARACTERISTICS, AND PRODUCT PROPERTIES

Yi Wang

Successful continuous or semi-continuous profile forming of wood-strand composites requires development of the underpinning science, including a suitable binding system, forming defect management, and process-property relationships. A hybrid resin to stabilize the wood-strand preforms for subsequent profile forming and curing was devised using a mixture of poly(vinyl acetate) (PVAc) and phenol formaldehyde (PF) resins. This study examined the influence of PVAc on the cure kinetics of PF with differential scanning calorimetry. Results show that the cure kinetics of PF blended with PVAc does not differ significantly from neat PF resin for blend ratios of 1 or lower. The nth-order Borchardt Daniels model provided good prediction for the curing of adhesive system with a PVAc/PF ratio lower than 1 and neat PF resin. This blended resin system can potentially be used for continuous or semi-continuous profile forming of wood-strand composites. To gain an initial understanding, the complex state of deformations in wood-strand preforms during profiling forming was studied using single-curvature bending. Three failure types were identified based on the primary failure mechanisms in bending, buckling and shear slip. The initial failure in bending was predicted with a strength-based model with the goal of minimizing defects, such as buckling. A relationship between preform conditions, forming defects and thickness recovery was established. Profile-formed wood-strand composites showed much better shape conformance than profiled veneer-based composites. Slow closing rate and relatively high moisture content caused flat and peak-at-core vertical density profiles (VDP). Lower moisture content resulted in more uniform VDP. The internal stress state in the bend of the profile-formed V-specimens was determined with elastic curved beam theory. Delamination, dominated by radial stress, was the initial failure mode. The Chang-Springer failure criterion was used to determine the initiation of delamination failure within the bend of the V-specimens on roller support. V-specimens tested on pin support yielded greater bending strength than those on roller support. These support conditions provide lower and upper bounds for stresses in the bend of the specimen for some applications. This study adds to the limited knowledge base of profile forming of wood-strand composites and provides an insight into process-property relationships.

PROFILE FORMING OF WOOD-STRAND COMPOSITES: PROCESSES, FORMING CHARACTERISTICS, AND PRODUCT PROPERTIES

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