By Qiniso Mbhamali, Zahra Naghizadeh and Brand Wessels at the Department of Forest and Wood Science, Stellenbosch University
South African Pine wood that has been treated with CCA or boron can form excellent bonds when using PUR adhesive.
With the increasing movement towards environmentally friendly and sustainable building materials in the construction industry, engineered wood products have become a preferred structural material.
Glued laminated timber (glulam or GLT), laminated veneer lumber (LVL) and cross-laminated lumber (CLT) have become popular options, as they have lower environmental impacts when compared to mineral based building products such as concrete and steel.
However, mass timber structures are susceptible to bio-deterioration especially in tropical and subtropical regions resulting in reduced service life of these structures. Preservative treatment before gluing is the most effective way to preserve timber structure against biological agents.
Post-treatment is not possible as very large laminates cannot be treated after production due to dimensional constraints of treatment facilities. However, pre-treatment also presents complications as some wood preservative deposits have been reported to change wood surface properties and interfere with bond formation and lead to poor bond quality. Furthermore, several studies have reported that increasing concentration levels of preservative drastically affects bond strength and durability.
In a study at Stellenbosch University the effect of CCA and Boron treatment on the bond strength and durability of polyurethane (PUR) joints was evaluated. Different treatment concentrations were evaluated for both CCA (2% and 4%) and the boron-based disodiumoctaborate tetrahydrate (DOT) treatment (1.67% and 3.3%).
Shear strength results
All groups in the experiment met the average requirements of the European standard EN 14080 (2013) for shear strength (Figure 1) and wood failure percentage. Interestingly, the 4% CCA treated specimens displayed superior shear strength in comparison to the 2% CCA specimens. This was contrary to what has been reported, that with increasing CCA concentration levels, the adhesion between the adhesive and wood laminates often decreases.
Both the CCA and boron treated delamination test blocks met the requirements of EN 14080 (2013), as the average total delamination did not exceed 10% in length in any of the groups (Figure 2), with 4% CCA showing the highest total delamination out of all the concentration levels tested (Figure 3-left). The 1.68% boron treated samples were found to have a better resistance to delamination in comparison to 4% CCA treated samples. As expected during impregnation cycles it was observed that boron leached out of the test blocks (Figure 3-right). This was due to the high-water solubility of boron, which limits the use of DOT as it cannot be used to treat wood products exposed to exterior conditions or frequent wetting or areas of high moisture content.
The results highlight that South African pine timber treated with CCA (2% and 4%) and DOT (1.67% and 3.3%) and bonded with a one-component PUR adhesive can be successfully used to produce engineered wood products as the shear strength and delamination test results met the EN 14080 (2013) standard requirements.