A study of pre-treatments in the drying of regrowth Eucalyptus obliqua L'Herit

This study investigated the potential of pre-treatment techniques to produce high quality back-sawn boards from regrowth messmate stringybark (Eucalyptus obliqua L'Herit), which is very prone to collapse and checking. Saturated urea solution, one percent sodium hydroxide, four percent acetic acid, water, urea formaldehyde resin (UF), and polyvinyl acetate glue (PVA) were used. In Chapter 4, the experiment of chemical soaking pre-treatments showed that soaking in sodium hydroxide solution for up to 15 days caused almost double increase of boards' drying rate. However, this technique and soaking in acetic acid solution caused more collapse and checks. In Chapter 5 and 6, the surface coating experiment confirmed that coating with UF resin did not significantly improve the drying properties of the boards. In contrast, PVA coating prevented surface checking and considerably reduced edge checking and collapse. The check value of these P boards was only a quarter the check value of control boards. In Chapter 6, the pre-treatments trial proved that significant reductions of shrinkage and collapse were achieved mainly by urea soaking for eight weeks (SSW boards). Both tangential and radial shrinkages of S8W boards were less than half the shrinkages of control boards. The collapse free boards of S8W were almost double amount of that in control boards. However, this pretreatment caused more surface checks. One-day urea soaking followed by eight weeks close stacking (C treatment) reduced checking. The check free boards of C, W, and P were 38%, 25%, and 88%, while that of the control, S8W, and S2W boards were the same, 19%. The average drying rates of W, S8W, and S2W boards were respectively 1.3 times, 0.4 times, and 0.6 times the drying rate of the control boards. The average drying rates of P and C boards were almost the same as that of control boards (1.7 x 10-2 %/hour, in a four month drying trial). This study indicates the prominent benefit of PV A surface coating in improving the quality of dried back-sawn timber of regrowth messmate stringybark. Further research, combining PV A coating and different drying process, such as high temperature drying, is needed to produce high quality dried timber in a much shorter time. This study also recommends water soaking when collapse and check prone timbers cannot be kiln-dried immediately after sawing.