Mechanical Performance and Bond Integrity of Finger Jointed High-Density Sub-Tropical Hardwoods for Residential Decking

Abstract

Finger jointing has long been a method of extending the longitudinal span of short-length timber pieces through a tooth-like profile of a nominated length and bonded with adhesive. With the high-density hardwood resource in the sub-tropics, local industries have found it difficult to obtain adequate bond integrity for high moisture areas and outdoor applications, where a good bond is governed by the dry modulus of rupture (MOR) and the percentage of wood fibre present in the separated joint after exposure to water impregnation. This paper presents the finger joint performance in terms of MOR, stiffness (MOE), and wood fiber amount (WFA) under different variables, joint profile (10 and 20 mm long fingers) using two structurally rated adhesives (a single-component polyurethane (1C-PUR) and resorcinol formaldehyde (RF)) on spotted gum (Corymbia citriodora) and Darwin stringybark (Eucalyptus tetrodonta) jointed boards. Dry bending strength or MOR testing indicated the 20 mm joints with the PUR adhesive had the best performance across both tested species compared to the RF adhesive. The measured MOE of the joints showed the RF samples to have higher MOE (7% to 13%) than the PUR samples for both joint sizes and species. Testing of joint durability through water impregnation resulted in MOR and MOE values decreasing by up to 50% for the RF and PUR joints. Conversely, the performance of water-impregnated joints after being allowed to re-condition to a 12% equilibrium moisture content produced a regain of MOR for the PUR joints across both species of 30% to 40%. Furthermore, it was found that the WFA increased for the PUR samples between the water-impregnated samples and the re-conditioned samples.