Wahyu Hidayat, Sugeng P Harianto, Fauzi Febrianto
Abstract – Three layers oriented strand boards (OSB) bonded with methylene diphenyl isocyanate resin were produced with the core layer orientation perpendicular to the face and back layers. Nine strand combinations from Maesopsis eminii (M), Paraserianthes falcataria (S), and Acacia mangium (A) were manufactured, namely MMM, MSM, MAM, SSS, SMS, SAS, AAA, AMA, and ASA. The results indicated that strand combination affected some properties of the board. OSB manufactured from wood with lower density had low dimensional stability, mixing with strands from higher density woods significantly improved dimensional stability of the board. On the other hand, OSB manufactured from higher density had low modulus of elasticity and modulus of rupture values due to low compression ratio, the values increased by mixing strands with lower density woods. Physical and mechanical properties of OSB made from mixed fast growing tree species met the requirement of CSA 5908 (2003) standard. The results showed the feasibility of using wood strands of mixed fast growing tree species from planted forests for OSB manufacturing, which is important to qualify appropriate raw material supply for the board industry.
Key words : OSB; Fast growing tree species; Strand combination.
The ability of forest to supply high quality of wood has kept decreasing in line with the decrease of forest area. In 2000-2005, the world suffered a net loss of some 37 million ha of forest (FAO 2006). On the other hand, demand on wood as structural components has kept increasing. Globally, wood consumption in 1990 was 0.32 m3 per person per year (Rice 1995), and it has increased by 2001 to 0.55 m3 per person per year (FAO 2001). In Indonesia, the total wood supply was around 31.49 million m3 in the year 2007, consisting of log from natural forest at 6.44 million m3, timber utilization concession at 3.06 million m3, industrial plantation forest at 20.61 million m3,state-owned enterprise at 48.03 thousand m3 and community forest and other official licenses at 1.33 million m3 (Ministry of Forestry Republic of Indonesia 2008). The species harvested from plantation forest and community forest are mainly fast growing species, such as musizi (Maesopsis eminii), sengon (Paraserianthes falcataria), acacia (Acacia mangium), etc. The existing problem when using such species is about the timber quality that is lower compared to timber extracted from tropical rain forest due to the low density and low durability. Hence, such timbers are not suitable for structural applications.
In recent years a diverse modern forest industries sector has been encouraged to adapt to the use of the species from planted forests. One of industrial products from planted forests is oriented strand board (OSB). OSB is a performance-rated structural panel engineered for uniformity, strength, versatility, workability, etc. It is utilized internationally in a wide array of applications, including building construction, flooring, partitioning, packaging, and as parts in furniture and automotive products (SBA 2004). Because it is engineered, OSB can be manufactured to meet specific requirements in thickness, density, panel size, surface texture, strength, and rigidity. This engineering process has been made OSB to become one of the most widely accepted and preferred structural panels.
Production trends in the forest products industry indicate less production of structural plywood and much more production of composite wood panels such as OSB (UNECE/FAO 2002). In Indonesia, OSB industry has not developed yet. To date, there is no OSB plant operated there. On the other hand, in USA up to 2004, there are 65 OSB industries (APA 2004). In order to utilize fast-growing tree species wood effectively and efficiently, OSB is promising to develop in Indonesia to substitute plywood and solid wood in the near future. The objectives of this study were to evaluate the effects of strand combination and strand pretreatment on the physical and mechanical properties of OSB from fast growing tree species bonded with MDI resin.
II. Materials and methods
( I ) Board Manufacturing
Strands were prepared from Maesopsis eminii (M), Paraserianthes falcataria (S) and Acacia mangium (A) wood. The trees ages range 7-15 years and the diameters range 15-25 cm. The wood densities were 0.41, 0.36, and 0.46 g/cm3 for Maesopsis eminii, Paraserianthes falcataria, and Acacia mangium, respectively. Strands were produced using a laboratory disk flaker. Their average dimensions determined from 100 strands, were 60-70 mm in length, 20-30 mm in width and 0.2-1.4 mm in thickness. Strands were screened and sorted by using a sieve before oven-dried at 80°C to a moisture content of less than 3%. Strand boards were manufactured with a target density 0.6 g/cm3 and final dimensions of 300 x 300 x 13 mm. Seven percent of liquid MDI resin content was applied to strands using a pressurized spray gun in a box-type blender. No wax or other additives were used. Hand-formed mats were pressed for 7 min at a temperature of 180°C using a maximum pressure 25 kgf/cm2. Three layers OSB panels were produced with the core layer orientation perpendicular to the face layers (layer structure was based on final oven-dried weight, and the face-core-face ratio was 1:2:1). Nine combinations of three layer structures were manufactured, namely MMM, MSM, MAM, SSS, SMS, SAS, AAA, AMA, and ASA.
( II ) Evaluation of physical and mechanical properties
Prior to testing, all boards were fully conditioned at a relative humidity (RH) of 65% and a temperature of 25°C for 1 week. All boards were tested according to Chinese National Standard (CNS) 2215 for particleboards to determine physical properties (density, moisture content (MC), water absorption (WA), thickness swelling (TS)) and mechanical properties (modulus of rupture (MOR), modulus of elasticity (MOE), internal bond strength (IB) and screw holding power (SHP)). The results of physical and mechanical properties from the tests were compared to JIS A 5908 (2003) standard. In addition, density profile and grave yard test were performed. Three replications were performed for each type of specimen.
Density and MC tests were evaluated using specimen with dimension of 100 x 100 x 13 mm. The specimens were immediately weighed and dried in the oven at 103 ± 2°C until they reached constant weight. For water absorption and thickness swelling tests, the dimension of specimen is 50 x 50 x 13 mm. The specimens were immediately weighed. Average thickness was determined by taking several measurements at specific locations. After 2 and 24 h of submersion, specimens were dripped and wiped cleaning of any surface water. The weight and thickness of specimens were measured.
Mechanical properties were tested by using a Universal Testing Machine. Evaluation of MOE and MOR were performed both in their long dimension parallel and perpendicular to the major axis of panel using specimen with dimension of 200 x 50 x 13 mm. The three-point bending was applied over an effective span of 150 mm at the loading speed of 10 mm/min. IB was evaluated using specimen with dimension of 50 x 50 × 13 mm. Steel plates were bonded to each face of the specimen using epoxy resin adhesive for about 6 h to ensure a good glue bond. The maximum load at the point of delamination was determined for each specimen. Wood SHP test was performed using specimen with dimension of 100 x 50 x 13 mm by pulled out wood screw along the vertical direction. The maximum withdrawal strength was measured. The average of maximum loads at two locations was used as wood SHP. The IB and SHP tests were performed at the loading speed of 3 mm/min. Density profiles in the thickness direction of the specimens were analyzed using a QTRS-01X tree ring analyzer (Quintek Measurement Systems, Knoxville, TN, USA) based on the relationship of X-ray attenuation and density (QMS 1999). Specimens with dimensions of 20 x 13 x 2 mm were used, and the measurement was made at 0.04 mm intervals.
The experimental design was a completely randomized factorial design. The results of the properties tested were submitted to an overall analysis of variance (ANOVA). The homogeneity of the means among combinations was tested using the Tukey’s honestly significant difference (HSD) test.
III. Results and discussion
Species is one of the most significant factors in the OSB process. It interacts virtually with every other variable that can be imagined in the process. In commercial OSB mills, supply of similar wood species as raw material sometimes could not be attained. As solution, some manufacturers mixed different raw materials to produce the boards. This study was performed to determine the effect of strand combination on the physical and mechanical properties of OSB made from mixed fast growing species without pretreatment to the strands prior to OSB manufacture.
Table 1. Effect of strand combination on the properties of OSB manufactured
Notes: Means within a row followed by the same capital letter are not significantly different at 5% significance level using Tukey’s HSD test. Numbers in parenthesis are standard deviations. Means are average of 3 replications. //: Parallel to the grain direction; ^: Perpendicular to the grain direction
Results of the physical and mechanical properties of OSB manufactured are presented in Table 1. Regarding physical properties, all properties evaluated met the requirements set forth by JIS A 5908 (2003) standard. The values of density and MC ranged from 0.54-0.63 g/cm3 and 6.72-8.81% respectively. The values of density and MC showed significant differences between strand combinations. The values of WA ranged from 5.55-13.19% and 15.31-29.81% for 2 h WA and 24 h WA tests, respectively. Furthermore, TS values ranged from 2.54-6.81% and 7.39-14.26% for 2 h TS and 24 h TS tests. The values of WA and TS showed significant differences between strand combinations.
The SSS board had lower dimensional stability compared to other strand combinations. The values for MC, WA for 2 h, and TS for 2 and 24 h of SSS board were the highest. It is suspected that Paraserianthes falcataria wood used to manufacture the board had the lowest density while compared to other species used. As stated by Hsu (1987), TS and WA were the sum of three components, i.e. reversible swelling of the wood itself, springback of compressed wood, and separation of furnish. Based on this report, a higher density of the final boards will increase the springback. A higher board density resulted on higher compression ratio. The stress inside the panel is particularly released when submitted to water immersion. It is more pronounced as the springback effect is higher.
Strand combination significantly improved dimensional stability of homogenous board. The values of MC, WA (2 h), and TS (24 h) for SSS board decreased when the strands were mixed with Maesopsis eminii (M) and Acacia mangium (A) applied in face or core layer of OSB panel as shown in Figure 1.
Figure 1. Effect of strand combination on (a) dimensional stability of SSS board and (b) MOR of MMM board
Regarding mechanical properties, MOR values were 50-137% higher in the parallel direction as compared to the JIS A 5908 (2003) standard. Results of MOR were higher in the perpendicular direction (120-220%). However, only MOR in parallel direction showed significant differences between strand combinations. Furthermore, MOE values were 18-47% higher in parallel direction as compared to the same standard. Results of MOE were higher in the perpendicular direction (32-75%). In homogenous boards, MOR for both parallel and perpendicular direction decreased in order from SSS, AAA to MMM boards. The density of woods to produce the boards were 0.36, 0.41, and 0.46 g/cm3 for SSS, MMM, and AAA boards, respectively. In general terms, the lower density woods will produce panels within the present desired density ranges, usually with strength properties superior to a higher density species (Maloney 1993). MOR values of MMM board improved when it mixed with Paraserianthes falcataria (S) and Acacia mangium (A) as shown in Figure 1.
Values of IB were 1.8-3.4-folds higher than the specification of the JIS A 5908 (2003) standard. It is an indicator of good adhesion, as well as efficient resin spreading and fine atomization. In general, internal bond failure occurs in the center line of board thickness because of the core’s low density (Sumardi et al. 2007). The homogenous boards used in this study also exhibited failure in the middle layer but only some boards from mixed strand exhibited the same failure as homogenous boards. This is suspected because of the density gradient in thickness direction of OSB from mixed strand was not so significant compared to density gradient of homogenous boards (Fig. 2).
Figure 2. Vertical density profiles of: (a) homogenous board (MMM); and (b) mixed strand board (ASA)
The results indicated that strand combination affected some properties of the board. OSB manufactured from wood species with lower density has lower dimensional stability (water absorption and thickness swelling values), mixing with higher density strands would improve dimensional stability of the board. On the other hand, OSB manufactured from higher density has low MOR and MOE values, the values increased when the strands mixed with lower density strands. Furthermore, physical and mechanical properties of OSB made from mixed fast growing tree species met the requirement of JIS A 5908 (2003) standard. The results showed the feasibility of using wood strands of mixed fast growing tree species from planted forests for OSB manufacturing, which is important to qualify appropriate raw material supply for the board industry.
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