title: The Influence of Surface Modification Processes on the Adhesive Bonding of High-Density Timbers from Australia
Creators: name: Leggate, William W
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Surface Modification Processes
subject: Engineered wood products
subject: High-Density Timbers

description: Wood products continue to be highly sought after by society because of their multiple sustainability, performance and utility benefits. Wood is widely regarded as the world’s most environmentally friendly building material and now close to 4 billion m3 are removed from around 1.15 billion hectares of the world’s forests each year (FAO 2020; 2021b). However, the global forest resources from which wood products are sourced have changed considerably over time in terms of species mix as well as generally diminishing log size and quality. Engineered wood products (EWPs) have emerged as the timber industry’s chief response and key solution to changing forest resources and markets. EWPs offer numerous sustainability and performance advantages compared to traditional wood products and alternative building materials such as steel and concrete. One EWP of major importance internationally is glue-laminated (glulam) beams. Market opportunities for glulam manufactured from various Australian hardwood timbers are growing, particularly because their superior structural properties create niche market applications where softwood glulam is inferior. Furthermore, in the past, large dimensioned, solid (non-glued) sawn timber products, were able to be easily obtained from the processing of large, high quality, native forest logs. However, given the smaller sized and poorer quality hardwood log resource now accessible to the Australian timber industry, it is now difficult to obtain large high quality solid sections. Instead, smaller sawn piece sizes are required to be laminated together using adhesives to form equivalent or larger dimensioned glulam. Australia possesses many very unique timbers that are internationally renowned for their extremely high density, superior strength, hardness, natural durability and attractive aesthetic appeal. These special characteristics have served these timbers well for their traditional markets, however, they also create barriers for gluing and manufacture of EWPs, especially sawn laminated EWPs such as glulam. Two high-density timbers of major commercial importance to Queensland, spotted gum (Corymbia citriodora) and Darwin stringybark (Eucalyptus tetrodonta), typify these problems, with glulam samples frequently failing to comply with the adhesive bond quality requirements of the Australian standard for structural glulam because of excessive glue line delamination. The gluing difficulties with many Australian hardwood timbers such as spotted gum and Darwin stringybark (and similar timbers internationally) have usually been attributed to their high density, particular wood extractives composition and high dimensional movement. Amongst the many approaches that could be deployed in attempts to improve wood adhesion for these timbers, pre-gluing timber surface modification processes are favoured options that can offer a cost-effective and readily adoptable commercial manufacturing solution. Major improvements in wood adhesion through the application of alternative mechanical surface preparation methods such as face milling have been demonstrated in international studies, however, are not yet adequately tested on difficult-to-glue high density Australian timbers. This study investigates the influence of different timber surface modification processes on the permeability, wettability and ultimately the gluability of commercially important high-density timbers from Queensland, Australia. Its primary objective is to contribute to the development of optimal adhesion protocols for these timbers for glulam production. Permeability and wettability are two key wood properties that have been shown to have a major influence on wood gluability. In addition to spotted gum and Darwin stringybark, a high-density softwood timber, southern pine, from Queensland plantations was also included in the study. Particular surface modification processes investigated included planing, face milling, sanding post-planing and modifying the timber surface wettability through the addition of surfactants. The study focused on the use of resorcinol formaldehyde (RF) and one component polyurethane (1C-PUR) adhesives as the dominant adhesives used in commercial structural glulam manufacture in Australia. Conventional planing produced the lowest surface wettability compared to the other surface machining methods. Improved wettability was achieved using face milling and sanding treatments post-planing. For spotted gum and Darwin stringybark, planing also produced the lowest permeability, whereas face milling resulted in the highest permeability. Particular face milling configurations and sanding treatments post-planing also significantly improved the tensile shear strength of the glued wood joints compared to planing. Semi-industrial scale tests also demonstrated the superior delamination resistance, improved block shear strength and favourable wood fibre failure results using face milling compared to planing. It was hypothesized during these studies that the performance differences were likely attributed to differences in the resulting board surface roughness, fibrillation, and sub-surface cellular damage that resulted from the different machining approaches. This resulting influence effected the wood permeability, adhesive penetration, glue line thickness, etc., and ultimately the bond integrity and performance. Despite these positive improvements, they were insufficient to yield the bond performances stipulated by the relevant Australian standard. Spotted gum and Darwin stringybark were shown to have extremely low permeability compared to southern pine, with liquid permeability in spotted gum and Darwin stringybark being so low that it couldn’t be measured using the protocols and equipment that commonly measure liquid permeability in other timber types. The surface of spotted gum and Darwin stringybark timber was also shown to be significantly less wettable compared to southern pine. The addition of surfactants to 1C-PURs and RF adhesives was trialled to improve the timber surface wetting by the adhesive and potentially improve the adhesive penetration. While this significantly reduced the surface tension of the adhesives and improved surface wetting, the approach was not successful in producing acceptable delamination results according to the standard. RF adhesive formulations tended to produce better results compared to 1C-PUR adhesives, although both were unsuccessful in achieving compliant adhesive bonds in the semi-industrial trials. This study adopted micro-computer tomography (MicroCT) and microscopy to assess key adhesive bond criteria. This research showed that there was an extremely high frequency of voids in the glue lines for all species, which would negatively impact bond strength and durability. For the two hardwood species, the frequency of voids was lower with face milling compared to the other surface machining methods for samples assessed after accelerated weathering. Face milling also resulted in significantly greater adhesive penetration compared to planing for spotted gum and Darwin stringybark, which matched the permeability results. Adhesive penetration was much deeper in southern pine compared to spotted gum and Darwin stringybark. The minimal adhesive penetration in spotted gum and Darwin stringybark is linked to the extremely low wood permeability. The very limited adhesive penetration observed in these high-density hardwood timbers (and in the latewood of southern pine) is suggested to be one of the most important barriers to the effective adhesion of these timbers. Further research is needed to establish an optimum gluing protocol for these high-density timbers. This should focus on methods to improve adhesive penetration; determine any influence of/and find solutions to problematic extractives; and to reduce the high movement of timber laminates in service that creates damaging stresses at the glue line and leads to delamination.