Investigation of the effects of raw material density and molecular weight of phenolic binders on the properties of lightweight panels based on Paulownia wood

This thesis focuses on different production process conditions of strand boards (SBs), especially with wood species (Paulownia tomentosa) by various adhesive binders (pMDI) and their influence on the properties of strand boards. The kiri tree is an extremely fast-growing, ring-porous tree with a low density of about 280 kg m-3 and it is mainly grown in plantations. Based on that, this study focused on the production of lightweight strand boards under 500 kg m-3. In the first part of the study, lightweight strand boards were produced at a target density of 300 kg m-3 and 400 kg m-3 with three wood species including low-density kiri wood (Paulownia tomentosa), medium-density pine wood (Pinus sylvestris) and high-density beech (Fagus sylvatica L.). However, beech strands did not glue together at these low board densities because of the low compaction ratio. The cohesion of beech strand boards was insufficient for further testing, which is attributed to the lack of bonding contact points for a high-density wood strand at low board density. The higher surface area of kiri SBs only indirectly led to a steep vertical density profile and improved mechanical properties including modulus of rupture (MOR), modulus of elasticity (MOE) and internal bond strength (IB). However, as the total surface area of strand mat increases, the thickness swelling of kiri SBs increases because a higher compaction ratio causes spring-back effects. In the second phase of the research, strands were blended with low (LMW) and high molecular weight (HMW) PF resin and its mixture 50-50%. LMW acts not only as a treatment agent but also as an adhesive. SBs from kiri timber were produced with densities of 400 kg m-3 and 500 kg m-3. At target density 400 kg m-3, the modulus of rupture (MOR) and modulus of elasticity (MOE) of SBs containing LMW PF resin were slightly higher than those of HMW PF at both adhesive contents. Internal bond (IB), screw withdrawal resistance (SWR) and thickness swelling (TS) of SBs containing LMW PF were significantly higher than those of HMW PF at 10% adhesive content and the differences slightly decreased as the adhesive content increased to 20%. At 500 kg m-3 target density, MOR and MOE of SBs for both PF were approximately similar, while IB of SBs containing LMW PF was about two times as high as that of SBs bonded with HMW PF. This effect was observed at 10% adhesive content but was more pronounced with 20% adhesive content. TS of SBs containing LMW PF was reduced by about 100% (at 20% adhesive content) and 50% (at 10% adhesive content) compared to SBs containing HMW PF. The improved physical and mechanical properties of kiri SBs based on LMW PF might be associated with cell wall matrix stiffening, deeper penetration and larger resin area coverage. Further characterization of LMW and HMW PF resin granules were implemented with various methods: (1) X-ray microtomography (2) SEM scanning electron microscopy (3) water-proof and filter papers aim to investigate the mechanism of resin expansion and penetration. The enhancement in the internal bond strength and the reduction of thickness swelling of kiri SBs based on LMW PF resin is highly linked to the expansion and penetration abilities of low resin molecules, which are associated with a high compaction ratio of kiri strands resulting in better bonding quality. At the third phase of the research, an innovative lightweight VSB was manufactured by combining kiri and birch veneers on the surface and inside the panel in order to enhance the mechanical properties of the panel. The results revealed that the MOR and MOE values of VSB meet not only the requirement for OSB\4 but also the requirement for plywood. In contrast, the internal bond strength was reduced, which is attributed to no glue applied on the veneer surface resulting in a weak bonding formation between strands and veneers. The improved mechanical properties might be assigned to the bonding reinforcement between veneers and strands, moreover, the longitudinal grain of veneer was arranged parallel with the orientation of strands. The fourth phase of research investigated the compaction behavior of wood strands from different wood species associated with different molecular weight phenol-formaldehyde (PF). Strand boards were manufactured at a target density of 500 kg m-3 from kiri, beech, pine strands blended with low and high molecular weight PF resin. The IB value of kiri SBs containing LMW PF resin was ten times higher than those of pine and beech. Also, bending properties and screw withdrawal resistance increased linearly with the compaction ratio for both resins. HMW PF resin can only contribute to IB performance until a certain level of compaction ratio (CR) is reached, whereas LMW PF resin starts producing bonding contacts if the compaction ratio goes higher than 1.0 resulting in deformed wood cells on the strand surface. The deformation on the strand surface occurred causing more contact points between strands, which leads to more effective glue utilization. In the fifth phase of research, the effects of pressing time on the properties and formaldehyde emission of strand boards treated with different molecular weight PF resin were investigated. No clear impact of resin molecular weight and pressing time were observed for bending strength and water-related properties. IB of SBs based on LMW PF was significantly higher than those of HMW PF; however, IB value decreased considerably with increasing press time. The formaldehyde emission decreased linearly with the increasing press time, which might be attributed to the progressive condensation of the resin.