The influence of mechanical surface preparation method, adhesive type, and curing temperature on the bonding of Darwin stringybark

Abstract

Darwin stringybark (Eucalyptus tetrodonta) is one of Northern Australia’s most important commercial forest resources. The wood exhibits desirable wood properties including high strength, natural durability, and visual appeal. The production of engineered wood products (EWPs) such as glulam from this resource represents a significant commercial opportunity for the timber industry in northern Australia. However, a major challenge to overcome is the achievement of satisfactory glue bond performance. This study evaluated the effects of different surface machining preparations, adhesive types, and curing temperatures on the bonding characteristics of Darwin stringybark. The pre-gluing surface machining method significantly influenced the timber wettability, roughness, permeability and tensile shear strength of adhesive bonds. Planing resulted in the lowest wettability, roughness, and permeability, while bonded planed samples produced the poorest tensile shear strength. Alternative surface machining methods including face milling and sanding post-planing were shown to significantly improve the timber wettability, roughness, and permeability, and also to increase the tensile shear strength of bonded samples. The resorcinol formaldehyde adhesive resulted in slightly improved tensile shear strength in most cases compared to the polyurethane adhesive. There was no significant improvement in tensile shear strength with the use of elevated temperature curing.

Barriers to the Effective Adhesion of High-Density Hardwood Timbers for Glue-Laminated Beams in Australia

“…extractive content, low wettability, minimal permeability and poor adhesive pene [4][5][6][7][8]12,13].…”

“…High-density timbers have been shown in many studies to be usually more problematic to glue [4][5][6][7][8][12][13][14][15][16][17][18][19][20][21]. The key reasons that have been identified for these gluing problems are summarised below and discussed hereafter in detail.…”

“…For these two hardwood species, structural glulam manufacture is currently a challenge with all common adhesive types used for glulam, which in Australia mainly includes resorcinol formaldehyde (RF) and one-component moisture-curing polyurethanes (1C-PUR) [4][5][6][7][8]. Most of the relevant research on these high-density timbers from Australia has focused on high molecular weight adhesives, such as RF and PUR because these are two of the dominant adhesives used in structural glulam internationally, as well as in Australia [4][5][6][7][8][9][10][11]. These adhesives are also considered to provide the strongest and most durable bonds for structural, weather-exposed engineered wood products.…”

“…Considerable research has been undertaken by the Queensland Department of Agriculture and Fisheries (DAF) to develop a manufacturing protocol that results in a reliable and repeatable glue bond with these high-density timbers for glulam production [4][5][6][7][8][9][10][11]. These studies have included trials testing the latest international best-practice approaches with guidance from international adhesive companies to improve the adhesion of refractory species: including different surface machining methods (e.g., face milling, sanding, planing), chemical pre-treatments (including surface wipes and washes), adhesive primers, coupling agents, adhesion promoters, hydrophobic agents to promote dimensional stability, surface incisions, different adhesive formulations, surfactants, varying timber moisture content, different adhesive spread rates, elevated temperature curing, different press times and press conditions [4][5][6][7][8][9][10][11]. Studies to improve the adhesion of high-density and extractives rich hardwood timbers from Australia have also been reported with other alternative approaches such as flame treatments in combination with DCM (dichloromethane extraction and PEI (polyethylene imine)) graft chemicals [12], plasma treatments [13] and CSIRO Surf-Bond [14].…”

“…However, despite this intensive research effort and some notable improvements with the use of surface machining approaches, such as face milling, a commercially acceptable gluing protocol that can reliably and consistently produce compliant structural bonds has not yet been established. Studies so far have confirmed the extreme difficulties in gluing these timbers, which have been mainly attributed to their high dimensional movement, extractive content, low wettability, minimal permeability and poor adhesive penetration [4][5][6][7][8]12,13].…”