Finger-jointing green softwood: Evaluation of the interaction between polyurethane adhesive and wood

Abstract

Existing European standards for finger-jointing of load-bearing lumber require the wood to be dried before gluing. This article presents a study on the properties of green-glued finger joints, wet wood being bonded prior to drying. Issues to consider, in comparison to finger-jointing of dry wood, are mechanical performance of the joint, absorption of the polymer by the wood in its natural/wet state, and the chemical reactions of the adhesive on contact with water. Finger-jointed samples were tested in bending, and the glue joints analysed by optical microscopy, scanning electron microscopy and microdensitometry. A patented one-component polyurethane adhesive developed for gluing-green wood which has a moisture content usually higher than 70% was used in the study. The resulting green-glued joints showed improved strength properties in comparison to dry-jointed joints. The results confirm that green-glued joints provide a wide, continuous wood/adhesive interface from one substrate to the other. The adhesive penetrates several cells deep and the density of the wood adjacent to the joint surfaces is increased. The results also indicate that the patented adhesive forms covalent bonds to the wood substrate.

Keywords:

• Adhesion
• chemical bonds
• finger-jointing
• green wood
• maritime pine
• Pinus pinaster
• polyurethane
• specific mechanical properties
• density

The authors thank Dr P. Morlier (Professor, LRBB, CNRS/UB1/INRA) and Mr B. Källander (Research Engineer, SP Sweden) for their support, and P. Pardon (Research Engineer, Institute of Pine UB1), E. Sellier (Research Engineer, CREMEN, CNRS/UB1) and P. Rozenberg (Research Engineer, INRA Orléans) for their participation in the experiments.