Effect Of Heat Treatment On Properties Of Blue-stained Masson Pine Wood And Its Evaluation Based On Cluster Analysis

Masson pine(Pinus massoniana) is one of the most common plantation species in China. Masson pine wood is particularly vulnerable to blue stain fungus. Plus, its dimensional stability and decay resistance are not desirable. These features lead to a limited application in furniture and other interior decoration uses. Wood heat treatment can not only improve wood properties like dimensional stability and durability, but also enhance color homogeneity of blue-stained wood.The present research conducted 20 steam heat treatments at five temperature(170oC, 185 oC, 200 oC, 215 oC, 230oC) and four duration(2 h, 4 h, 6 h, 8 h) levels. Blue stained masson pine wood specimens with dimension of 500(L)×70(T)×30(R) mm3 were prepared for heat treatments. In this study, the effect of heat treatment on the color change of blue-stained and normal area was investigated and color homogeneity before and after heat treatment was evaluated. Changes of physical and mechanical properties and durability of wood were also studied. Efforts were also made to clarify the brittleness, adhesion of coating films and crystallinity of heat-treated wood. Based on the mentioned wood properties, cluster analysis was applied to divide the levels of heat treatment and evaluate the effects of heat treatment on comprehensive wood properties. The main findings and conclusions are summarized as follows:(1) With a higher temperature and longer duration, heat treatments declined color differences( Δ L*, Δ a* and Δ b*) between stained and normal area. When treated in temperature higher than 200 oC for no less than 4 hours, standard deviation of color parameters of blue stained wood was close to that of normal wood. In case that heat treatments lower than 200 oC and duration no more than 8 hours, the differences between stained and normal area were still significant. To make blue stains invisible to naked eye, the minimum treatment intensity should be 200 oC for 6 hours.(2) Equilibrium moisture content(EMC) reduced linearly with treating temperature. After treated at 230 oC for 8 hours, EMC reduced by 58.2%. Specimens after treated with an intensity below 230oC/6 h have an anti-shrinkage efficiency less than 10%. In comparision, specimens treated at 230 oC for 8 hours have an anti-shrinkage efficiency of 38%. In order to improve durability of masson pine wood to highly resistant class, the minimum treatment intensity should be 200 oC for 8 hours.(3) When treated at 170 and 185 oC for no more than 4 hours, specimens showed higher strength than untreated wood. The brittleness of untreated masson pine wood was 22.26%, and increased to 33.64%~89.01% after heat treatment. Control group has no specimens of brittle failure, while more than half specimens showed brittle failure after being treated at 230 oC for 6 hours. After heat treatment, adhesion for water-based acrylic paint was not affected and was in class 0. As heat treatment intensity increases, adhesion for polyurethane coatings decreased, but few specimens can reach class 3 or worse classes(4) As correlation analysis and factor analysis revealed, ten property variables were highly correlated, which include air-dry volumetric shrinkage(SH-air), oven-dry volumetric shrinkage(SH-ov), air-dry volumetric swelling(SW-air), water-swollen volumetric swelling(SW-wa), decay resistance(DR), brightness(L*), modulus of rupture(MOR), EMC, oven-dry density(Den) and chromatic coordinates on the blue-yellow axis(b*). Chromatic coordinates on the green–red axis(a*) and modulus of elasticity(MOE) showed relatively poor correlation with other variables. According to the distances of variables in factor loading graph, five representative varibles were selected: MOR, EMC, a*, Den and MOE and it can include the main information for heat treated wood property.(5) Twenty treatments can be divided into 4 classes by cluster analysis as follows. Super-mild heat treatments hardly changed wood property and durability was still nonresistant class. Mild treatments improved durability to moderately resistant class and barely reduced mechanical properties. At moderate treatments, wood durability ranged from moderately resistant to highly resistant classes and strength loss was less than 24%. In the severe treatments, durability improved to resistant or highly resistant classes, but strength decreased more than 27% and brittleness was too high. The result of cluster analysis based on selected 5 variables was similar to that based on 12 variables.