Delayed elastic strain and set-recovery evaluation in high-pressure densified hybrid poplar wood – New assessment considerations

Abstract

The deformation behavior of high-pressure (HP)-treated hybrid poplar wood, the subsequent swelling, and the equilibrium moisture content properties of HP-densified wood were evaluated using a modified delayed strain/set-recovery method of cyclic humidification-dehumidification at different relative humidity (RH) conditions. The HP treatment resulted in significant compression (densification) of the wood under different treatment conditions. For treated wood samples, the value of delayed elastic strain was relatively small when stored at 20 °C and 65% RH, which indicated that HP-densified wood possessed dimensional stability. The HP-compressed poplar yielded lower equilibrium moisture content than the control at low RH, while major increases were observed at high RH above 76%. Marginal of thickness swelling was observed under the cyclic humidification-dehumidification method in the low RH range while significant swelling occurred at high RH. Conventional methods would only show results that were appropriate for storage at high RH environments. The RH threshold for set-recovery of HP-compressed wood was between 33% and 54% for optimal use, and the extent of set-recovery increased rapidly when RH was between 85% and 95%. Such differences could not be recognized with the conventional methods. In addition, the prolonged holding time significantly decreased the RH threshold value (P < 0.05).

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“…When the densified wood was remoistened, the dimensional expansion, i.e., the combined swelling and set-recovery, in the radial direction increased sharply in the first few days, followed by only a small increase thereafter. The general set-recovery behaviour is that most of the compression deformation is totally recovered as soon as the densified wood material absorbs moisture, and this means that the time to effectuate setrecovery depends on the species, on the size of the densified wood, and on the treatments etc [39]. The set-recovery is mostly several magnitudes greater than the swelling of the wood material, but the value is highly dependent on the compression ratio.…”