MOR and MOE of Plastic OSB with Poplar Strand
LIAN Hailan1, FANG Qin1, MENG Hui1, HUA Yukun1
1 Nanjing Forestry University, Nanjing 210037, Jiangsu, P.R. China
e-mail: Doctorlian1971@yahoo.com.cn
Keywords: Fast-growing poplar, Plastic OSB, MOR, MOE, oriented angle
ABSTRACT
In this study, effects of strand density, resin content, hot pressing time and orientation angle on MOR and MOE of plastic OSB with thin and long strand from fast-growing poplar were investigated.
The strand length was fixed at 200mm. The results showed that a higher orientation angle could lead to lower MOE and MOR of plastic OSB. The board density has significant influence on the MOR (but the MOE was different), influenced by the hot press time. A plastic OSB with a density of 0.75g/cm3 was produced using a hot press time of 1.4min/mm and a resin content of 350g/m2, resulting in higher values of MOR (56.4-103 MPa) and MOE (7808-13455 MPa).
INTRODUCTION
Fast-growing poplar has been one of the main raw materials with its fast –growing ability, wide adaptability, light weight, easy processing, and it could be widely managed. The poplar plantation wood has become the main material for our panelboard industry( Kai et al., 1994, Hua 2004). But it is difficult to be processed because this species has a low density, low strength, is easily harmed by bugs ,epiphyte and mildew. Also, it has small diameter logs, thinnings and plantations (Zhao et al. 2001 ) How to use this resource to produce high quality wood composites is being focused on.
Structural plywood, as a main product in the plywood family, was mainly used in container floorings, carriage board and concrete models. In comparison with normal plywood (Bai 2008), the structural plywood has more layers, is thicker (12mm), and it also has different physical properties and a different production process. OSB has superior performances and relatively lower production costs (Li et al. 2006) It maintains the natural properties of wood with high mechanical strength and it has a good dimensional stability when exposed to moisture. Highly workable (easy to saw, drill, nail etc.), OSB could be exempted from the phytosanitary restrictions. OSB was rapidly developed and is used in buildings, furniture and packaging materials. Today, as substitution of the structural plywood, the utilization of OSB is 70% and structural plywood is 30% (Jiang and Cao 2008) in the construction sector of residential and commercial buildings in North America. The domestic particleboard is produced with normal particles, the length of strands is 75 to 100 mm, it was not suitable for structural materials because of the shortages of point bonding from resin spraying, uneven forming and high amount of holes, and especially the creep would be serious in humid environmental conditions (He 1998). Wood is naturally an elastic-plasticity material and under certain conditions, its mechanical strength properties could be improved and its structure retained when the density improved by a slight plastic-compresssion. The plastic OSB as a plastic-panel, is made of thin and long strands from fast growing wood, impregnated with PF resin, by static or mechanical oriented hot pressing.
He (1998) made plastic-OSB to avoid the shortage of the normal OSB by impregnate gluing, thus improving its properties. But this kind of gluing consumes high amounts of resin and it is costly. Another problem is how to optimize the resin distribution on the strand surface, where results are unknown. However, brush gluing could improve the efficiency of resin application. The purpose of this study was to evaluate the feasibility of manufacturing plastic OSB using fast-growing poplar with 200 mm long strands and to optimize appropriate technology parameters from both manufacturing technology and plastic OSB properties in laboratories.
MATERIALS AND METHODS
Strand preparation
The poplar panel comes from a Jianhu wood-based panel mill, Yancheng city, Jiangsu Province, with a density of 0.38g/cm³. The parts of the panel with nature bug, colors, knurr and cracks on the surface were wiped off and cut with a knife into long and thin strands with a dimension of approximately 200mm (length), 10mm (width) and 1.5mm (thickness), The initial moisture content of these strands was 15.4%. and they were dried at 70°C in a dryer to a MC about 8.0%.
Resin blending
The Phenol-formaldehyde liquid resin (the solid content was 43%) was brushed on each face of each test strands.
Strand forming
The strands were enclosed in perforated Teflon pouches and carefully distributed by hand through the depth of the hand-formed mat at the predetermined location. The mat size was 400 x 250 mm. We manually controlled the homogeneous thickness with only one joint on each layer.
Hot-pressing
The nominal thickness of plastic OSB was 12 mm, and the hot pressing temperature was 150°C, the utmost pressure was 9MPa. The hot pressing time was determined according to the experimental design.
Property test
All boards were conditioned at 20+3°C and 65+1% relative humidity (RH) for one week, and then cut for routine performance assessments, i.e. bending modulus (MOE) and strength (MOR), according to the Chinese standard GB/T4897-2003.
The process chart was given in Fig.1
Figure 1: The process chart of OSB
Experimental Design
L9(34) mathematical orthogonal method was adopted to arrange the experimental design. Three factors were strand density, resin content and hot-press time. The level of testing factors was shown in Table 1.
Table1: Levels of experimental factors of orthogonal experiment
Note: The resin contents were 16%,18% and 20% based on oven-dry weight of strands
Orientation angle specimen preparation
Sample boards of 500 x 500 x 12mm were manufactured at the end of the orthogonal experiment and cut from the sample boards according to Chinese standard GB/T4897—92 (shown in fig. 3.), with an orientation angle of 0, 10 or 20° respectively to the size of 290mm, 50mm and 12mm. and four density samples, with a size of 50 x 50 mm.
Figure 2: Orientation angle samples.
Note: .
0 A、 0 B —— orientation angle was 0°；
10A、 10B——orientation angle was 10°；
20A、 20B——orientation angle was 20°；
RESULTS AND DISCUSSION
Optimal parameters of plastic OSB
Structural plywood commonly using as outside engineering, except need to favorable weatherability, long-term durability, earthquake resistance and impact resistance, but also request of Modulus of MOR and MOE. MOR is an important property of OSB as component, which indicates the utmost capability to resist rupture under outer force. MOE is an index of stiffness indicating the relationship between stress and strain. In wood technology, the modulus of rupture was representative to the degree of strength or elasticity of wood, it was the maximum surface stress in a bent beam at the instant of failure, and it was expressed the ability of withstand the distortion (Li 1994). The modulus of rupture is the ratio of the bending moment at the point of failure to the moment of resistance. And the modulus of elasticity can be used to predict bending strength, as the bearing structural materials, the design stress or the loading of OSB should be limited in the range of critical elasticity or the ultimate creep, and avoid causing plastic deformation (Wang 1986) .
Table 2: Results of orthogonal experiment
In the experiment, set pressing temperature at 150°C constantly, exam the OSB property influenced by different densities, pressing time and resin amount, to find the proper processing parameter for long strand OSB production, the effect of factors on MOR and MOE of longer strands Plastic OSB was studied to evaluate the feasibility of manufacturing structural products.
Table 3: Range analysis result of orthogonal experiment
The experiments and results were seen in Table 2, and the range analysis result was listed in Table 2 , Table 3 and Fig. 3-4.
From the table we could found the fact density was similar with the design density, the other factors had fewer influence on the density.
Figure.3: Relation between levels of testing factors and MOR
Discussion on the factors of each variable
The result of this experiment indicated that, among the three levels of three factors, board density had the most significant effect on various board properties and then the hot-press time. The MOR was highly correlated with the panel density. MOR was improved with the increase of the density, as shown in Fig.4, higher density means more wood strands in one unit and develops a closer contact among strands, thus a more effective resin bonding and a higher shear strength. Meanwhile, higher density also means more solid skeleton substantial (cellulose) in unit, more effective area to bear force, thereby a higher MOR and MOE. When the density changed from 0.68g/cm3 to 0.78g/cm3, the MOR was improved by 48.4%.
The next factor was the hot-press time, longer time and suitable temperature made PF resin cure more completely. The effect of resin level was least, with the increased resin level, the bonded area in the board would be enlarged and the glue drops among strands would increase. Moreover, the contact of strands with glue would change from point to area to develop a better bonding, MOR was raised. But when the resin content from 350 g/m2 to 400 g/m2, the negative influence was found, because the excessive glue layer causes the difference of expansion to the interfacial inner stress and caloric stress would be greater, the number of air bubbles and the other disfigurement increased, the probability of early destroyed increased, then the MOR was lower (He 1998).
Figure 4: Relation between levels of testing factors and MOE
The results also demonstrated that the hot-press time plays an important role in the MOE, a more interesting phenomenon appeared in the experiment, that two different changes occurred with the raised time. MOE was first decreased and then increased, this may be explained by the principle of PF resin cure. A longer time and suitable temperature made the PF resin cure more completely, and at the same time the plastic of wood was increased, the mat easy to be pressed tightly, the strand closer contact, and better bonding developed in higher density board, MOE was increased. However, if the time became too long and the excessive cure of resin and possible damage to wood strand in face layer would have a negative effect on MOE, in this experiments limits, the negative influence of longer time wasn’t found.
The influence of board density and resin level on MOE and MOR were the same.
According to the above optimal parameters, it was obvious that the 9th group was the best in 9 experiments. The final technological parameters were board density 0.75 g/cm3, hot press temperature 150°C, hot-press time 17 min and the resin level 350 g/m2. Its MOR (which was 103 MPa) and MOE (13455 MPa) reached or exceeded the nation’s longitudinal specified value of Plywood for container flooring. Compared to the Industrial standard LY/T 1580-2000 of the People’s Republic of China about OSB properties, it can be seen that the MOR is 2 times and the MOE is 1.6 times compared to the normal OSB. Therefore, the plastic-elastic of plastic OSB made of 200mm strand is better than that of normal OSB.
Table 4: Result of test for plastic OSB
Influence of orientation angle on MOR and MOE
Table 5: Sample orient angle with average MOE
Figure 5: The relation between angle and MOE & MOR.
The reference to evaluate the performance of forming equipment is the orient result. It can be evaluated by measuring the orient angle of the strand. Oriental forming can change the strength ratio in the longitudinal and tangential direction of the board. To determine the proper angle, we need to cut it into different angles and perform strength test to find a good angle. The experiment, according to the orthogonal experiment result, is to make extend strand OSB sample size as 500 x 500 x 12mm, density was 0.75g/cm3, PF amount was 400g/m2, on both sides. Press temperature was 150°C and pressing time 17 min. We cut the sample with 0°,10°,20°, two of each, so that we can see the influence to the board by different angle. The results are indicated in Table 5 and Fig 5.
We could see that the bigger angle, the smaller the MOR. The MOR ratio for 0°, 10° and 20° is 1, 0.86 and 0.75, respectively. When the angle changed between 0° and 10°, there was no big change on MOE, but when the angle changed between 10° and 20°, the MOE decreased observably. The MOE ratio for 0°,10° and 20° is 1, 0.97 and 0.73, respectively. Considering above aspects, the strand forming angle should be in the range of 0~10°.
Figure 6: Load-deflection curve
In addition, from the Fig. 6 of load-deflection curve we could see that when the sample was broken, it still retained for some time. Still could bear rather high force. We could imagine that it is suitable to be a good structural board as it maintains high retentiveness after impact.
CONCLUSION
From the experiment results, the following conclusions were achieved:
Regarding the MOR, density is a main factor, then pressing time, and then the resin amount. As for MOE, pressing time is the main factor, then density, then resin amount. For 200mm strands from fast-growing poplar, to produce 12 mm OSB, the proper process condition is: density 0.75 g/cm3, PF amount 350 g/m2 on double sides, pressing temp 150°C, pressing time 17 min.
The smaller the angle, the better the elastic-plastic property, so the angle should be controlled between 0° and 10°.
It is suitable to be used as structural material, as it still maintains high retentiveness after impact.
The brush-gluing could make good use of the limit resin to form a continuous film on the strand surface. Its plastic mode for the extend strand increase the OSB MOR and MOE. But it is difficult to be carried out in industrial production. Therefore further research should adopt impregnate-gluing, control the resin amount and internal & external distribution, discuss the feasibility of extend OSB production with fast-growing poplar.
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