AIR- AND SOLAR-DRYING CHARACTERISTICS OF BAKAU POLES

Transcription

1 AIR- AND SOLAR-DRYING CHARACTERISTICS OF BAKAU POLES K.S. Gan & R. Zairul Amin Forest Research Institute Malaysia Kepong, Selangor Darul Ehsan Malaysia Abstract The shortage of timber supply for manufacturing solid wood products and the global demand for ecolabelling are a pressing issue. As such, there is a need to look for alternative, suitable timber species for timber products. One of the potential species is bakau. With the availability of bakau, the material can be potentially used for solid products development if it can be dried successfully. The study reports on the air- and solar-drying characteristics of bakau poles. Bakau minyak (Rhizophora apiculata), y old with diameter 10 to 18 cm, were obtained from the well-managed Matang Mangrove Reserve of Peninsular Malaysia. The results indicated that drying bakau poles in solar dryer could reduce the drying time, down to 12% MC compared to air drying. However, the quality of air-dried bakau poles is better than when solar dried. Pole size did not significantly influence the drying rate but significantly affected the MC variation within poles. Surface-check development during drying can be reduced by making grooves along the pole length. Based on this study, air drying of bakau pole down to 22% MC followed by solar drying to approximately 12% is the optimum procedure to produce quality bakau poles in a shorter time. INTRODUCTION The shortage of timber supply for manufacturing solid wood products and the global demand for ecolabelled products or products made of timber from sustainably managed forests are a pressing issue. As such, there is a need to look for alternative, suitable timber species for timber products which may be similar or even better in terms of performance and appearance. One of the potential species is bakau which has been identified as fast-growing, able to survive in open conditions and does not require large tracts of planting space (Srivastava et al. 1988). Mangrove forests cover about ha in Malaysia with 57 percent found in Sabah, 26 percent in Sarawak and 17 percent or ha in Peninsular Malaysia. Some of these forests are gazetted and managed sustainably by the Forestry Department to ensure sustainable supply of bakau wood. Currently, Malaysia practises a clear-felling cutting system of to y rotation to produce charcoal and poles. 142

2 More than 60 main bakau species are found in the mangrove forest but the most common species in Peninsular Malaysia are bakau minyak (Rhizophora apiculata), bakau kurap (R. mucronata) and bakau pasir (R. stylosa). The bakau tree can achieve up to m height with diameter of 59 cm after y. Thinning is usually carried out 15 and y after planting with diameter of cm. The log is usually cut at breast height approximately 1.3 m, into 1.6-m lengths where about eight to nine logs can be obtained from each tree. Bakau is classified as a medium to heavy hardwood species with basic density ranging from 685 to 960 kg m -3. The sapwood is light yellow-brown to light red-brown and the heartwood is grey-brown to purple-brown but not easily differentiate from the sapwood. Texture is fine and even with interlocked grain. The timber is also difficult to resaw, planing is slightly difficult and planed surfaces are moderately smooth. Nailing property is rated poor. The wood is rated as non-durable when exposed to humid condition, except in the ground, as it is susceptible to fungi and therefore, needs to be protected to prolong its service life. Bakau has another drawback feature, which is that it cracks when dried too fast. In Malaysia, bakau wood is commonly used for manufacturing black charcoal, as firewood and piling structure in construction. The timber is also suitable for parquet flooring and garden furniture because of its high density. Bakau wood is, however, not explored commercially. Nik Adlin et al. (10) estimated that bakau chair generates an income of RM6.25 per kg of bakau log as compared to 26 sen generated by charcoalmaking. With the availability of bakau, the material can be potentially used for solid products development if it can be dried successfully. Bakau usually has a diameter of 6 to cm and the green moisture content is usually to 60%. The diameter of bakau for charcoal in generallly is not a critical issue. However, for wood products, larger diameter is usually desired as it affects the recovery and drying quality. Either for indoor or outdoor uses, the wood must be dried to a moisture content as close as possible to that it has in service to reduce movement during application. The main objective of this study was to determine the drying characteristics of bakau poles. The specific objectives were to investigate the effects of log size and grooves on the drying characteristics (drying rate and quality) of air- and solar-dried bakau poles and to recommend the optimum strategy to dry timber in a shorter time with low drying degrade. MATERIALS AND METHODS Four hundred (0) poles of -y-old bakau minyak (R. apiculata) were obtained from Matang Mangrove Forest Reserve in Kuala Sepetang, Perak, and their physical properties were determined. The basic density was determined by dividing the ovendry weight of each samples by its green volume. The oven-dry weight was obtained by drying the samples in an oven at 103±2 o C until constant weight was attained. The volume was obtained by measuring the transverse (radial and tangential) and longitudinal dimensions. The poles, 2.5 m long, were end-coated to minimize the effects of end-drying. Bark was removed from the poles. These bakau poles were divided into two diameter classes, 10±2 and 16 ±2 cm. Grooves or slots were made on half of these two groups of pole size. Grooved and solid poles of each diameter class were then air-dried under shed and solar dried (Figures 1 and 2). Air-dried poles were solar dried to further reduce their moisture content. In total, eight drying runs were conducted. 143

3 Figure 1 Air drying with stickers under shed Figure 2 Solar dryer (greenhouse type) Five sample logs from each drying run were used to monitor the drying progress (Figure 3). At the end of the drying trial, three one-inch length discs were cut from each sample log four inches from both ends. The two discs from both ends were oven-dried for the evaluation of the final MC. The third disc was cut into several sections for determination of the MC difference between core and shell. The size and number of checks were also noted after drying. All logs were visually evaluated for the degree of checks as shown in Figure mm 1 m 25 mm 1 m 25 mm Butt SD AD Top Slot or groove 2 cm N2 N1 Disc for density determination T L S1 S2 R Disc for moisture content determination Figure 3 Preparation of sample log 144

4 Description 1 Random short hairline end- and surface-checks 2 Hairline end- and surface-checks appearing continuously (> cm length) 3 Medium end-checks (< 2 mm width), medium surface checks (< 2 mm width, < cm length) 4 Medium end-checks (< 2 mm width), medium surface checks (< 2 mm width, > cm length) 5 Large end-checks (> 2 mm width), large surface checks (> 2 mm width, > cm length) along the log length Figure 4 Defects classification: degrees of end- and surface-checks RESULTS AND DISCUSSION Physical properties of bakau minyak The physical properties of two sizes of bakau minyak are presented in Table 1. The basic densities, radial shrinkages, longitudinal shrinkages and volumetric shrinkages of these two sizes of bakau were not significantly different. However, green MC and tangential shrinkages for small-size (10 ± 2 cm diameter) bakau were slightly higher and lower from big-size (16 ± 2 cm diameter) bakau respectively. Figure 5 shows the MC and density variation across the diameter. The density of big size-size poles appears to increase from the pith to the middle of the disc before decreasing towards the bark, while for small-size poles there is no clear pattern observed due to insufficient number of specimens. The MC for both sizes tends to increase from the bark towards the pith. Lim and Gan (1998) reported that this pattern of variation is commonly observed in Malaysian hardwood timbers. Table 1 Green moisture content and basic density values of bakau minyak 10 ± 2 cm (small) 16 ± 2 cm (big) Mean ± SD Range Mean ± SD Range Green MC (%) 39.7 ± 4.6 a ± 4.9 b Basic density (kg m -3 ) 817 ± 27 a ± 31 a Tangential shrinkage (%) 9.6 ± 0.7 a ± 0.8 b Radial shrinkage (%) 5.2 ± 1.4 a ± 0.9 a Longitudinal shrinkage (%) 0.5 ± 0.5 a ± 0.4 a Volumetric shrinkage (%) 14.2 ± 1.7 a ± 1.1 a Note: SD = standard deviation. Means with different superscript letters differ significantly (p=0.05). 145

5 Basic density (kg/m 3 ) 3 ) Sample 1 8 Sample 2 8 Sample Sample Distance from pith (mm) MC (%) Sample 1 Sample 2 Sample 3 Sample Distance from pith (mm) (a) 10 ± 2 cm Basic Basic density density (kg/m (kg/m 3 ) 3 ) Distance from pith (mm) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 (b) 16 ± 2 cm Figure 5 Average basic density values and MC variations with position from the pith for 10±2 cm and 16±2 cm bakau minyak Drying time and drying rate Figure 6 shows the drying curves of bakau poles. Bakau poles were air-dried from about % MC to approximately 21% MC in 102 days (3 months). The poles then were solar-dried for another 128 days to 11%. The total drying time for drying poles from air to solar drying was 254 days (8.5 months). For timber totally dried in solar dryer, MC of about 15 to 17% can be reached in 102 days, which is approximately 4 percent lower than when air dried. The difference in MC is probably due to fact that the EMC in the solar drier is lower than outside. In air- and solar drying trials, it was found that there was no significant difference in drying rate between grooved and solid poles and between pole sizes (Table 2). However, when air-dried bakau was shifted to the solar dryer, drying rates of smallsize poles were higher than of big-size poles at early stages of 22 to 12%. In all drying trials, small-size poles had about 1 to 2% MC lower than big-size poles. MC (%) Distance from pith (mm) Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 146

6 MC(%) dmc/dt = 0.41%/day (a) Solar drying dmc/dt = 0.14%/day Drying time (day) Big Solid Big Slot Solid Slot (b) Air followed by solar drying Figure 6 Drying curves of bakau poles indicating average drying rates (dmc/dt) at different stages of drying MC(%) AD dmc/dt = 0.24%/day dmc/dt = 0.19%/day Drying time (Days) (days) SD dmc/dt = 0.02%/day dmc/dt = 0.02%/day Big Solid Big Slot Solid Slot Table 2 Drying rates at different stages of drying Drying rate, dmc/dt (% MC/day) Solid Slotted Solid Slotted AD Stage 1: to % MC Stage 2: % to 22% 0.28 (0.05) a 0.22 (0.03) a 0.13 (0.01) b 0.14 (0.02) b Big 0.24 (0.02) a 0.22 (0.05) a 0.13 (0.01) b 0.14 (0.02) b AD followed by SD Stage 1: 22 to 12% MC Stage 2: 12% to 11% 0. (0.02) c 0.19 (0.01) c 0.01 (0.00) e 0.01 (0.00) e Big 0.10 (0.01) d 0.09 (0.01) d 0.02 (0.01) e 0.02 (0.01) e SD Stage 1: to % MC Stage 2: 25% to 17% MC 0.47 (0.03) f 0.37 (0.09) f 0.14 (0.00) g 0.13 (0.01) g Big 0. (0.08) f 0.42 (0.06) f 0.13 (0.02) g 0.13 (0.00) g Note: Number in parentheses indicates standard deviation. Means with different superscript letters differ significantly (p=0.05). MC distribution Assessment of MC variation within air-dried poles (Figure 7) indicated that the differences of MC between shell and core were about 3.6% for small- and 9.2% for big-size poles, which are considered high. For solar-dried poles, the differences were approximately 1.3% and 4.7% for small- and big-size poles respectively. 147

7 MC(%) Solid 15.0 Slotted 10.0 Big Solid 5.0 Big Slotted Distance from pith (cm) (a) Air drying MC(%) Solid 10.0 Slotted Big Solid 5.0 Big Slotted Distance from pith (cm) (b) Solar drying Figure 7 MC variation within air- and solar-dried bakau poles Drying quality The occurrence of surface-checks in air-dried bakau poles was quite low compared with that in solar-dried poles (Figure 8). For air-dried bakau, all the small-size poles fall under 1 while for big-size poles, more than 50% of the poles were graded as 1. For big-size poles, quality of grooved poles was slightly better than for solid ones. However, when the air-dried poles were moved to the solar dryer, the quality of grooved poles was downgraded from 1 to 2. It was observed also that more than 50% of the solid poles were downgraded to 5. Percentage Percentage Solid Slotted Big Solid Big Slotted (a) Air drying Solid Slotted Big Solid Big Slotted Percentage (b) Solar drying 4 5 Solid Slotted Big Solid Big Slotted (i) (ii) (iii) (iv) (c) Air- followed by solar-drying Figure 8 Quality grades 148 (d) Surface- and end-checks in air- followed by solar-dried bakau poles; (i) Big solid; (ii) Big, (iii), (iv) solid

8 For solar-dried bakau, it was found that the big-size poles, whether grooved or solid poles, tended to crack more than small-size poles. Grooved poles always had better quality compared to solid poles. In general, the use of grooves reduces the occurrence of surface- and end-checks of solar-dried and air- followed by solar-dried bakau poles. Table 3 shows the patterns of checks that were observed in air- followed by solardried bakau poles. Table 3 Patterns of check that occurred in air- followed by solar-dried bakau poles Surface-checks End-checks Large checks (2 5 mm width) along the slot 1 large check, 1 to 4 small checks solid At least 1 large check (2 5 mm width) and 2 to 3 small checks (2 3 mm width) 1 large check, 1 to 4 small checks Big Checks along the slots (2 10 mm width) 1 large check, 2 to 5 small checks Big solid At least 1 to 2 large checks (5 10 mm width) and 2 to 3 small checks (2 3 mm width) along the pole length 2 large checks, 3 to 7 small checks 149

9 Table 4 Summary of results AD SD AD followed by SD Big solid Big solid Big solid Big solid Big solid Big solid Initial MC (%).8±2.4 * 38.1± ± ± ± ± ± ±1.7.8± ± ±1.9 a 41.2±2.0 Final MC (%) 21.5±0.6.9± ± ± ± ± ± ± ± ± ± ±0.3 Average shell MC 19.5± ± ± ± ± ± ± ±0.1 N/A N/A N/A N/A Average core MC 24.2± ± ±1.6.3± ± ±0.1.9± ±0.8 N/A N/A N/A N/A Drying time (days) Quality (%) * Standard deviation N/A = not available

10 CONCLUSION Based on this study, drying bakau poles in solar dryer could reduce the drying time, down to 12% MC compared to air drying. However, the quality of air-dried bakau poles was better than that of solar-dried poles. Pole size did not significantly influence the drying rate but significantly affected the MC variation within poles. Surface-checks due to development of radial stress during drying can be reduced by making grooves along the pole length. These grooves may be hidden during application using rubber seal. To increase drying rate and minimize drying defects, it is recommended that the bakau poles be air dried first down to approximately 22% MC before further drying in the solar dryer to achieve 12% MC which is suitable in most indoor applications. ACKNOWLEDGEMENTS This study was part of the Planting of Mangroves and Other Suitable Species on the Shorelines of the Country project. The authors would like to thank Shamsudin Ibrahim for the financial allocation and Tariq Mubarak for supplying the bakau poles. Technical support from the Sawmill and Wood Drying Laboratory staffs of FRIM is greatly acknowledged. REFERENCES Lim, S.C. & Ga n, K.S Density variation of Malaysian-grown teak. Journal of Tropical Forest Products 4(2): Nik Ad l i n, N.M.S., Wa n Ta r m e z e, W.A. & Kh a i r u l, M. 10. Designing with bakau timber. FRIM in Focus June 10. Forest Research Institute Malaysia. Sr i v a s t a v a, P.B.L., Sa w, L.G. & As h a r i, M Progress of crop in some Rhizophora stands before first thinning in Matang Mangrove Reserve of Peninsular Malaysia. Pertanika 11(3):