The effects of the addition of surfactants and the machining method on the adhesive bond quality of spotted gum glue-laminated beams

Abstract

The effects of adding surfactants to polyurethane and resorcinol formaldehyde adhesives were tested relative to the gluability of spotted gum timber for structural glue-laminated beams (Glulam). While previous attempts to improve the bond performance of this very difficult to glue timber have focused primarily on timber surface preparations, this study concentrated on lowering the adhesive surface tension through added surfactants to improve the adhesive-timber surface wetting. Accordingly, 44 glulam samples were manufactured using polyurethane and resorcinol formaldehyde adhesives, with eight surfactant formulations and two different pre-gluing surface machining methods, i.e., face milling and planing. Although the surfactants were successful in drastically lowering the adhesive surface tension and improving adhesive spreading, none of the surfactant formulations tested were successful in improving the glulam adhesive bond qualities. Overall, the surfactant formulations produced considerably higher delamination, lower shear strength, and lower wood fibre failure compared to the control samples; therefore, they are not a viable solution to improve the gluing of spotted gum. The resorcinol formaldehyde adhesive and face milling produced considerably better results compared to the polyurethane adhesives and conventional planing.

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].…”