Adhesive Bonding of Low Moisture Hickory Veneer with Soy-based Adhesive

Low moisture veneer and regions of sapwood within hickory engineered wood flooring bonded with soy-flour adhesive are thought to be factors leading to potential performance deficiencies. The goal of this research was to gain a broader understanding of bonding low moisture hickory veneer with soy-based adhesive. Soyad® is of particular interest due to its novel cross-linking chemistry. Impacts of moisture content and wood region (heartwood versus sapwood) were analyzed with dry and wet shear bond strength tests, measurement of percent wood failure, lathe check characterization, and adhesive bondline thickness and penetration depth measurement. Impact of wood region and type (hickory versus red oak) was assessed by comparing wood buffering capacity and delamination following three-cycle water soaking.

Dry and wet shear strength values met expectations for engineered wood flooring yet percentage wood failure results were uniformly very low for all combinations of moisture levels and wood regions. In contrast, delamination following wet and dry cycling was minor and within minimum requirements for all specimens tested. The influence of moisture level, wood region and type were inconsistent; statistically significant relationships were not evident within the moisture range studied. However, different wood regions and types exhibited differing veneer buffering capacities that had potential to interfere with pH requirements of Soyad®. Additional study of buffering capacity and resin cure is recommended to determine the significance of the buffering capacity results found in this study.