This research project is an investigation on conventional and low environmental impact strategies for enhancing the decay resistance and dimensional stability of oriented strand board (OSB).
The specific objectives were to identify the best step for preservative addition in the manufacturing process to give good decay resistance without causing significant deterioration in the physical and mechanical properties (part 1, chapters 2 - 7) and to examine the efficacy of heat treatment applied at a
single step in improving the dimensional stability and reducing decay in strandboards ( chapters 8-11 ).
In the first part of the study the decay resistance of 11 types of commercial OSB was examined in a soil block test with Coniophora puteana, Coriolus versicolor and Pleurotus ostreatus. It was concluded that although the boards were sufficiently well bonded to withstand wetting, minimal brown rot decay caused
marked loss in their residual internal bond (IB) strength.
Subsequently the effect of point of preservative addition on the mechanical and physical properties of strandboard was investigated. PF bonded Scots pine "un-oriented" strandboards treated at five points of manufacture (diffusion treatment of "green" strands, vacuum treatment of dry strands, spray treatment,
post manufacture heat and cold quench (HCQ) and post manufacture vacuum treatment) with Tanalith 3485 ( copper azole borate) were made under laboratory conditions to achieve five different preservative retentions. The best resuIts were achieved with the diffusion treatment of "green strands" and the vacuum treatment of dry strands. Results with the vacuum treatment of the manufactured boards were so deleterious that further study with this method was not attempted.
Tests were then made on boards made and treated at the four remaining treatment points. The physical and mechanical properties, preservative distribution, preservative losses, and decay resistance of the boards before and after leaching of these boards were evaluated. Decay tests employed agar block test conditions with the above fungi plus Postia placenta and unsterile conditions (soil bed fungus cellar).
Vacuum treatment was found to be the best treatment for strandboard preservation. Although this treatment was the only method that gave protection after leaching, leaching caused greater preservative losses than expected. Diffusion treatment showed similar results but was less effective after leaching.
Boards made from spray treated strands showed high strength losses, although weight losses were reduced to close to zero at high preservative retentions. However after leaching it failed to brown rot even at the highest retentions. HCQ preservation resulted in significant strength losses, provided non-uniform
preservative distribution, and only achieved control in leached boards where full penetration was achieved. Overall, the best results were achieved with those methods achieved strand penetration and attempted proper chemical fixation of the preservative i.e. vacuum and diffusion.
In the initial part of second part of the study, the effect of heat treatment on the mechanical and physical properties of commercial OSB strands was evaluated. Heat treatments in the range 200 - 260°C were applied under inert atmospheric conditions to wood strands. The results of this study showed that heat
treatments reduced the dimensional stability (thickness swelling, TSw) of heat-treated wood strands proportional to the increase of temperature. Deterioration of the mechanical properties (modulus of rupture, MOR and modulus of elasticity, MOE) occurred for treatments of 250°C and above. Water absorption was marginally reduced only at the highest temperatures.
Subsequently heat treatment was also applied in the hot press by increasing the pressing temperature and pressing time. The effect of press-heat treatment on the physical and mechanical properties of strandboards was investigated. The results showed that heat treatment is an efficient method for the improvement of dimensional stability and to a lesser degree the decay resistance of OSB. However this is not an economically viable method because prolonged pressing times are required.
This thesis presented strong evidence for two potential major improvements in OSB manufacture. Preservation is feasible using pre-manufacture strand treatments by vacuum potentially by diffusion.
Dimensional stability of OSB can be achieved by heat treatments at various stages of manufacture. Decay resistance of OSB can also be partially improved using heat treatments. However, additional protection needs to be applied using conventional preservatives.
The production of a decay resistant and dimensionally stable OSB was not achieved within the time available within this study but the high retention vacuum treated board gave results which were close to this. It is likely that a successful product for ground contact could be produced by a combination of
preservative, heat treatment and an enhanced bonding system.