Moisture content gradient and defect development of Acacia mangium Willd. during drying process
Keywords:
Acacia mangium Willd, defect development, moisture content gradientAbstract
This study investigated the moisture content gradient and the defect development of Acacia mangium Willd. during the conventional drying. Two batches of fast drying with drying gradient U = 4.5 - 5.0 and slow drying with drying gradient U = 2.0 - 2.5 were conducted to determine the moisture content gradient according to timber thickness by the ovendry slicing method and the defect development by AS/NZS 4787:2001 and AS 2082:2007 at before drying, MC of 50%, MC of 20% and after drying. The results showed that the moisture content gradient had a significant correlation to the defects development during drying process and their values were different in the two drying batches. The moisture decrease in the surface layer of the timber in the fast drying batch is faster and rapidly below the fibre saturation point (FSP) in the period from the start of drying to MC of 50%, causing the moisture difference between the core and the surface (highest 140%) was larger than that in the slow drying batch (highest 100%). This was the reason why the degree of surface check, end check, collapse and distortion in the fast drying batch are higher and more frequent than that in the slow drying batch. The moisture content gradient tended to decrease with continued drying as the surface layer started to dry slowly and the core layer continued to dry fastly and reached 24% and 14% respectively in the fast and slow drying batches at the end. Therefore, the checks tend to gradually close. The degree of collapse in the fast drying batch (4-6 mm) was much greater than that in the slow drying batch (within 0.5 mm), making the quality grade of the dried wood in the fast drying batch reached a low level (grade E according to AS/NZS 4787:2001). The distortion development was almost absent in the slow drying batch, but was a tendency to increase during the fast drying batch (warping height increased by 2-3 mm at the end of drying). The moisture content gradient in the fast drying batch was always higher than that in the slow drying batch at all 4 test times, which led to the degree of wood defects in the fast drying batch were always larger than those in the slow drying bat
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