The effect of process variables on the dimensional stability of particleboard

The work undertaken in this thesis aim at improving the dimensional stability of particleboards caused by water absorption and stress relaxation both of which often occur simultaneously in finished composite board products. Dimensional instability was quantified by measuring the board percent thickness increase resulting from: i) springbuck on press opening; ii) reversible swelling subjected to moisture or solvent absorption and, iii) irreversible swelling due to stress release The reversible swelling was measured after a complete wetting or a humidity cycle from low to high humidity levels. The irreversible swelling was assessed after a complete drying or a humidity cycle from high to low humidity levels. Chemical modification of wood aimed at reducing the reversible swelling due to moisture absorption was attempted after the sequential replacement of water in wood by solvents. P. radiata and E. Obliqua blocks saturated with water were sequentially treated with polar followed by non-polar solvents in order to replace the water in the woods. Both species showed about 30 % retention of their original (green) volume after the final stage of treatment. The partially swollen P. radiata blocks were then chemically treated with the wood cell wall-bulking and cross-linking agents, i.e. acetic anhydride and paraformaldehyde respectively. Wood thickness swell was monitored through cyclic wetting and drying tests. The results showed that the dimensional stability of neither the acetylated nor the paraformaldehyde treated sample were improved as quantified by the anti-swelling efficiency (ASE). The presence of a catalyst (ferric chloride) was necessary to enhance the cell wall cross-linking and to improve the wood dimensional stability. This work identified that a chemical reaction between the reagents and the wood is necessary for achieving dimensional stability. Stress relaxation from springbuck and irreversible recovery are inherent in wood particles which have been stressed. Much of this work focused on reducing this stress in situ during wood composite panel consolidation. Chemical conditioning of wood materials by ammonia in situ during consolidation was studied using phenolic-based resin systems. Ammonium hydroxide (source for ammonia) was found to accelerate the curing of phenol-resorcinol-formaldehyde (PRF). Accelerated curing was also found for 50:50 (w/w) mixtures of PRF and phenol-urea-formaldehyde (PUF) at low temperatures between 45 C and 75 C and in the presence of formaldehyde. The PRF:PUF (50:50) resin system showed a significantly faster cure than the PRF resin in the presence of ammonium hydroxide at all levels of temperatures investigated. The resin system also showed shorter gel times than those of the PRF at different weight portions of ammonium hydroxide additions. Preliminary chemical structural analysis of both resin systems catalysed by ammonium hydroxide using Infrared (IR) analyser showed evidence for the presence of an azomethine group which indicated that the ammonia was being incorporated into the resin structure. Single layer particleboards were manufactured at low temperatures using the PRF binder and ammonia. Gaseous injection of ammonia in situ during board consolidation was achieved by using a laboratory press fitted with perforated plattens. An optimum pressing schedule was established whereby the ammonia was injected after the mat had reaching its final thickness in a sealed press system. The effects of pressing temperature, PRF resin loading and gas pressure on board vertical density profile, MOR, MOE, IB and thickness swell were studied. The results showed that the ammonia gas-injected boards consolidated at both temperatures of 50 C and 100 C had more uniform density profiles than those of the hot-pressed boards at the temperature of 180 C, however, the MOR, MOE, IB and thickness swell were less satisfactory. The MOR, MOE, IB and thickness swell of the ammonia gas-injected boards were bench marked against the requirements for standard grade High Moisture Resistant (HIR) particleboards in accordance with the AS/NZS 1859.1: Part 1 (1997), and were found to be below the acceptable standard. Particleboards bonded with PRF:PUF 50:50 (w/w) resin catalysed by ammonia gas were also produced. The board MOR, MOE, lB and thickness swell were less superior than those bonded with PRF, although ammonia was found to accelerate the cure of both resin systems. This was thought to be attributable to a curing reaction that was too rapid between the ammonia and the resin system. The pressing time and temperature may need to be shortened and lowered respectively for board property improvement. The effect of wood particle size on the properties of ammonia gas-injected particleboards was investigated. Large size particles were found to increase the MOR and MIE irrespective of pressing methods. However, boards made from small size particles were found to have higher lB's than those of large size particles. This was thought to be due to better gas penetration. Recommendations are provided for further work to enhance the properties of ammonia-catalysed PRF boards