Rethinking engineered wood product manufacturing: opportunities for using partially seasoned timber feedstocks

The manufacture of engineered wood products (EWPs), such as glue laminated beams (glulam), has traditionally been reliant on using seasoned timber feedstocks. Reducing the timber’s moisture content (MC) before use provides many benefits including pre-shrinking the timber, reducing product weight and improving the stability of the finished product. Seasoned timber feedstocks have also been critical for the effective use of commonly adopted adhesive systems, many of which demand low timber MC levels (< 12%). However, timber seasoning can be an expensive and a time-consuming process. Polyurethane type adhesives are increasingly being adopted potentially offering greater flexibility with feedstock MC, including the potential use of partially seasoned or high MC timber. This study explored the feasibility of producing finger jointed timber (FJs) and glulam from partially seasoned sawn white cypress pine (Callitris glaucophylla) using a single-component polyurethane adhesive. Boards were initially grouped by capacitance MC readings (cap) for manufacture into low-MC (LMC_cap; < 15%), random-MC allocation (RMC_cap; 15–60%), and high-MC (HMC_cap; > 60%) categories, then reclassified based on oven-dry (OD) validation to LMC_OD < 15%, RMC_OD 15–30%, and HMC_OD > 30%. FJ bending strength testing showed HMC_OD samples initially exhibited lower strength values compared to LMC_OD range samples, however, conditioning of the HMC_OD samples to a target MC of 12% resulted in similar performance to LMC_OD samples. Microscopy analysis of the resulting gluelines showed greater adhesive penetration for HMC_OD samples, a feature often linked with improved adhesion. However, despite the improved penetration, delamination results were highest delamination for the HMC_OD grouping. Glulam block shear strength testing yielded lower strength values for the HMC_OD samples.