Study on the Properties of Compressed Poplar Veneer
CHEN Min1, HUANG Hao1, DENG Yuhe1
1College of Wood Science and Technology, Nanjing Forestry University, Longpan Road 159, Nanjing, PR China, 210037
e-mail: huanghao-1985@hotmail.com
Keywords: Pressing parameters, veneer compression ratio, recovery ratio, veneer properties, shear strength.
ABSTRACT
This paper describes the ratio of veneer compression and recovery by different experimental factors as well as the properties of veneer with different compression set. Test results revealed that the major factor that works on compression set is the pressing pressure. The higher the pressure, the greater veneer’s compression set will be. When the pressure was 4MPa and 2MPa, the compression set reached 50% and 29%, respectively.
Pressing temperature is a secondary influencing factor. The compression set of veneer was 49% when the temperature was 180°C. Pressing time shows the lowest impact.
The pressing temperature and pressure also affected the recovery set of veneer. When the temperature and pressure were 180°C and 4MPa, the recovery set and actual compression ratio measured were 8.26% and 22.74%, respectively, and compared to those veneers had not been dealt with, the compressed veneer’s properties such as modulus of elasticity (MOE), modulus of rupture (MOR) were 4762.32MPa and 47.85MPa, both improved by 120% and 60%. The untreated veneers and compressed ones were made into plywood, and the results confirmed that the adhesive shear strength of the latter plywood was 0.72MPa, increased by 112%.
INTRODUCTION
Fast-growing tree is a kind of soft material, which will be a main supply for the wood industry. It has the characteristic of low density, hardness, mechanical properties, and easy to corrupt. Due to all these defects, the traditional wood process and utilizing methods cannot meet the demands for improving its value, it also brought about a negative impact to the effective use of poplar resources. So, how to expand the use area of low-quality poplar is a problem to solve. The project will use an environmentally friendly method of physical to enhance the properties of fast-growing tree veneer. The research work carried out has the following practical significances: 1) Using a compression method with high temperature and high pressure condition to strengthen the properties of veneer. The manufacturing process is harmless to both human and the earth because no chemical stuff is used and no harmful substances are set free. It is a physical environmental approach; 2) Improve fast-growing tree values to meet the needs of plywood.
EXPERIMENTAL METHODS
Test materials
The Poplar veneers were obtained from a plywood manufacture (Jiangsu Province, P.R.China). The veneers size was 150 x 150 x 2.5 mm and 400 x 400 x 2.5mm. Urea formaldehyde resin adhesive (UF) was obtained from the Dajiang wood industry.
Test methods
The poplar veneers were pressed at a moisture content of 10%, 20% and 30%.with a pressure of 2 MPa, 4 MPa and 6 MPa and a pressing temperature of 160°C, 180°C and 200°C. The pressing time was 2min, 4min and 6min. The pressure conditions of veneer were shown in Table 1. Their initial compression ratio (ń), recovery set (Ϊ) and actual compression ratios (Ĺ) were calculated using the following formulas:
Ń = [(H1-H2) / H1] × 100% (1)
Ϊ = [(H3-H2) / H1] × 100% (2)
Ĺ = [(H1-H3) / H1] × 100% (3)
Where H1 is the veneer thickness before compression, H2 is the veneer thickness after compression, and H3 is the veneer thickness after the recovery test.
The recovery test has three treatment conditions. First, put the compressed veneer at room temperature during a week and determine the thickness of veneer. Second, immerse the compressed veneer in water of 20± 2°C for 24 hours. Third, cook the compressed veneer in boiling water for two hours.
According to the test results of first phase, processing parameters were chosen. The test method is listed on Table 2. After compression, the veneer was cooked in boiling water for two hours, and its recovery ratio as well as the actual compression ratio was calculated. The mechanical property of veneer such as modulus of elasticity (MOE) and modulus of rupture (MOR) were measured. The shear strength of plywood was measured according to the China Country Standard of plywood GB/T 17657-1999.
Table 1: Different parameters
Table 2: Parameters of hot-press processing
RESULTS AND DISCUSSION
The effect of different parameters on compression ratio
According to the balance analysis of the orthogonal test, the parameters that have effects on compression ratio can be clearly recognized as shown in table 3 and figure 1. The applied pressure has the most effect on the compression ratio. When pressure was 2 to 4MPa, the higher the pressure was, the greater the compression ratio was, and the highest compression ratio was 50% under 4MPa. When the pressure was reaching to 6MPa, the compression ratio decreased to 40%. The following factors are the pressing time and pressing temperature. The preferable pressing time was 4 min or 6 min, from which the compression ratio can reach to about 45%. The preferable pressing temperature was 180°C. When the temperature was over it, the compression ratio would decline from 48% to 45%. The last is the moisture content. It didn’t have remarkable effect on compression ratio which of this test was nearly 40%.
Table 3: Balance analysis of orthogonal test
Figure 1: Compression ratio of different pressing parameters
The recovery ratio and actual compression ratio of veneer
Three recovery tests were carried out. The test results were represented on Fig. 2, 3 and 4. When the compressed veneer was placed in room temperature, they can absorb water from the atmosphere. The test results of compressed veneer placed in room temperature for a week show the greatest recovery ratio was nearly 32% under a pressure of 2MPa and a temperature of 160°C , while the smallest one was only 3% and the actual compression ratio was 48% with the help of preferable parameters of pressure 4MPa, pressing time 4 min and temperature 180°C. According to this test result, the recovery ratio was the smallest when the compression ratio was the biggest.
When the compressed veneer was immersed in the water of 20 °C ± 2 °C for 24 h, the biggest recovery ratio reached nearly 42%, and the smallest one was about 8%. The actual compression ratio was nearly 50% under the temperature of 180°C, pressure of 6MPa and time of 2min. Because the treatment was more violent than the former test, the recovery ratios were commonly bigger than those of the former one.
The test results represented the compressed veneer recovery ratio as the most marked when the compressed veneer was cooked in boiling water for 2 hours. The highest recovery ratio reached to nearly 50%, the actual compression ratio was only 4%. As the treatments are different, the recovery ratios of compressed veneer are different. In the three tests, the recovery ratio of compressed veneer cooked in boiling water was the highest, because the treatment was the harshest.
Figure 2: Recovery ratio and Actual compression ratio of veneers after placed in room temperature for a week
Figure 3: Recovery ratio and actual compression ratio of veneers after immersion in water of 20°C ± 2 °C for 24h
Figure 4: Recovery ratio and actual compression ratio of veneers after cooking in boiling water for two hours
The compression ratio, MOR and MOE of veneers
In accordance with the principle of "smallest recovery ratio and appropriate actual compression ratio" and the analysis of test results above, processing parameters were chosen. The poplar veneers were pressed at a pressure of 2MPa and 4MPa; a pressing temperature of 180°C and 200°C; a moisture content of 10% and a pressing time of 32 min.
Fig. 5 represents the veneer compression ratio after compression under different parameters shown on Table 2. The test results showed the highest compression ratio was about 31% under the applied pressure of 4MPa and a temperature of 180°C.
After compression, the veneer was cooked in boiling water for two hours, and its recovery ratio as well as the actual compression ratio was calculated on Fig. 6. The recovery ratio was 8% and the lower actual compression ratio was 23% under the pressing pressure of 4MPa, temperature of 180°C.
The mechanical property of veneer such as modulus of elasticity (MOE) and modulus of rupture (MOR) were measured. According to Fig. 7, MOE and MOR were 4762.32MPa and 47.85MPa respectively with an applied pressure of 4MPa and pressing temperature of 180°C, each improved by 120.5% and 60.3% compared to the untreated veneers.
Figure 5: Compression ratio of veneer
Figure 6: Recovery ratio and actual compression ratio of veneer after cooking in boiling water for two hours
Figure 7: Modulus of elasticity and modulus of rupture of veneer
0: veneer of the untreated technology; 1~4: veneer of the technology shown in Table 2
The Shear Strength of Plywood
The untreated veneers as well as the compressed ones were made into plywood, and the results of Fig. 8 confirmed that the shear strength of the plywood reached to 0.72MPa under the pressing pressure of 4MPa, temperature of 180°C , while that of the plywood made by untreated veneers was 0.34MPa, which was increased by 111.8%.
According to the test results, conclusion was derived that the compressed veneer recovery ratio was controlled and the mechanical property of veneer such as MOE and MOR were enhanced. The shear strength was improved compared to the untreated ones under the pressing pressure of 4MPa, temperature of 180°C.
Figure 8: Adhesive shear strength
CONCLUSIONS
According to the test results, the pressing pressure has the most effect on compression ratio. The following factors are the pressing time and pressing temperature. The moisture content doesn’t have a remarkable effect on compression ratio.
The veneer’s actual compression ratio can be advanced. Meanwhile, the veneer’s properties such as MOE and MOR can be improved, and the shear strength of Plywood will also be significantly enhanced when the poplar veneers were pressed at a pressure of 4MPa; pressing temperature of 180°C; moisture content of 10% and a pressing time of 32 min.
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