Reduction of Set-recovery of Surface densified Scots Pine by Furfuryl Alcohol

Abstract:  For wood products such as flooring and worktop, only one surface is normally exposed in their use, and the mechanical properties like hardness and wearing resistance of that surface is then important. Since mechanical properties are strongly related to the density, surface densification, i.e. transverse compression of the wood cells beneath the surface of a piece of wood with the aim to increase the density of that region, may be a method for improving hardness and wearing resistance when low-density species are used for such products. The set-recovery, i.e. the moisture-induced swelling of the densified wood cells back to their original shape, is the main obstacle in the use of densified wood products. Although there are several methods reported in literature, such as post heat-treatment, that can almost eliminate the set-recovery, but such methods are either time consuming or difficult to implement into an industrial continuous process which may do densification competitive to techniques or materials that can achieve at least the same hardness.     In the present study, furfuryl alcohol was used as pre-treatment to fix the set-recovery of surface-densified Scots pine sapwood. The main effect and interactive influence of four process parameter (impregnation time, press temperature, press time and compression ratio) on set-recovery and Brinell hardness after two wet-dry cycles were studied by a two-level full factorial design of experiments. The characterizing variables of the density profile after the surface densification and set-recovery test were carried out as a supplementary tool to learn the mechanism of this two-step modification process. According to the result, the surface densified wood with furfuryl alcohol pre-treatment can retain their dimension and keep hardness at a very high level after two wet-dry cycles. The set-recovery and hardens after two wet-dry cycles were about 20 % and 30 N/mm2, respectively. The Pearson correlation analysis shows that the correlation coefficients between set-recovery with impregnation time, press temperature, press temperature, compression ratio were -0.35, -0.52, -0.37, and 0.16, respectively. That means that for the specimens with furfuryl alcohol pre-treatment, the higher press temperature can reduce the set-recovery significantly. The longer press time and impregnation time can also reduce the set-recovery in some extent, but the influence was  low. As expected, the hardness improvement was retained with low set-recovery. The lowest set-recovery value was 14% with the corresponding hardness of 41 N/mm2 was achieved by specimens processed with 120 minutes of impregnation, 10% compression ratio, 210℃ pressing temperature, and 15 minutes of pressing time. With 20 minutes of impregnation time, 10% compression ratio, 210℃ pressing temperature, and 5 minutes of pressing time, the sample still owns twofold hardness after the set-recovery test.