Effect of Combined Heat Treatment/Densification on Mechanical Properties of Poplar Wood Impregnated with epoxidized soybean oil

Densification is a mechanical modifying method to improve the mechanical properties of cultivated fast growing woods. A prevalent approach to overcome the set recovery obstacle of densified wood is heat treatment which causes decrease of mechanical properties. Heat treatment with plant functionalized oil, due to reaction of functional groups of oil with hydroxyl groups of wood, leads to an improvement of physical and mechanical properties of wood. The objective of the present research was to compensate the loss of mechanical properties through combination of densification and heat treatment in epoxidized soybean oil. In the present study, fast grown poplar wood was impregnated with raw and epoxidized soybean oil and then densified to a compression set of 30 percent. The epoxidized oil impregnated/ densified wood samples were heat treated in an oven at 180 oC for four hours and the mechanical properties of both densified and non-densified samples, impregnated with the raw and epoxidized oil were investigated comparatively. calculation of oil uptake values of raw and epoxidized soybean oil in densified and non-densified samples showed that in non-densified sample, epoxidized oil had nearly twofold uptake compared to the raw one, because of its higher polarity and higher penetrating ability. Densification led to a considerable removal of oil in both level; however epoxidized oil in densified sample had higher uptake compared to the raw one. Results of mechanical properties showed heat treatment in raw and epoxidized oil caused a 97 and 7 percent loss of flexural strength respectively. Heat treatment of densified wood with raw and epoxidized oil cause a 96 and 20 percent decrease in hardness values compared to the non-heat treated ones. Heat treating of densified wood in epoxidized oil medium led to a decrease of flexibility and impact strength as well. Wood impregnation with epoxidized oil, because of its permeation into the cell wall, led to a considerable decrease of flexural strength and hardness; however densification further increased these strengths. Heat treatment of epoxidized impregnated samples after densification led to a lower decrease of flexural strength and hardness, as compared to raw oil impregnated samples, however this effect was not observed for impact strength. Generally, heat treatment in epoxidized soy bean oil could be recommended as an effective method for decreasing the spring back and water absorption of densified wood which led to a lower reduction of mechanical strength as well.