Processing And Performances Of Electromagnetic-Shield Effectiveness Functionally Wood-Based Laminated Composites

In order to reduce electromagnetic pollution caused by electromagnetic radiation .The research focuses on the minimal, average and maximum value of electromagnetic -shield effectiveness(SE) in the frequency range of 9 KHz-1.5 GHz, shear strength and wood failure, which were measured according to the material electromagnetic-shield effectiveness measuring method of the military standard of electron trade of the People's Republic of China (SJ 20524-1995) and National Standard (GB/T 9846-2004) separately by conventional Larix hot pressing plywood technique with hot plate-drying, blending and soft pressing technologic process. The value of SE, shear strength and wood failure is related to three kinds of conductive powders (0 dimension ) including Cu, Ni and Graphite and two kinds conductive fibers (1 dimension ) including copper fiber(CF), stainless steel fiber(SF) with two kinds of spreading amount, 250 g/m2 and 400 g/m2, and mixing of different materials(metal, nonmetal), different dimensions (0, 1), different length (3 mm～4 mm, 6 mm～7 mm, 9 mm～10 mm)with two kinds spreading amount, 250 g/m2 and 300 g/m2. The results can be got as following: (1) SE of CF (6 mm～7 mm) exceeds 30 dB when the conductive element content is 100 g/m2 and 120 g/m2 with spreading amount of 250 g/m2; SE of CF (9 mm～10 mm) exceeds 35 dB when the spreading amount is 250 g/m2 and 300 g/m2 with conductive element content 120 g/m2; (2) The increasing of conductive element is favorable for the rising of SE, but is unfavorable for rising of shear strength. At the same time, the decreasing of shear strength is unfavorable for rising of SE. Therefore, the SE is a comprehensive result of these two respects. The impact on material SE and shear strength varies with different conductive elements. With the same conductive element content, exerting fiber is more favorable for the rising of SE than powder, but is unfavorable for the rising of shear strength; (3) Because Cu powder has been oxidized, the SE of Cu is almost 0, but the shear strength can meet the National Standard. When the proportion of Cu powder accounting for solidified adhesive is 120:125, close to 50% with spreading amount of 250 g/m2 and conductive element content of 120 g/m2, the SE of Cu is almost 0; (4) The minimum SE of the material appears in the low frequency (<300 MHz). With the rising of frequency the SE firstly rises gradually, then it drops. The wood failure is usually low, and coefficient of variation is usually large. The change of wood failure is irregular. In general, the SE rises gradually with the rising of frequency, but the shear strength drops gradually with the rising of frequency. In this testing, the shear strength exceeds 0.8 MPa of minimum National Standard GB 9846.12-2004Ⅱplywood standard except SF25 –80, and the coefficient of variation is relatively small; (5) With the same conductive element content, exerting CP is more favorable for the rising of SE and shear strength than Ni, specially with greater amount of the spreading. Exerting Ni is more favorable for the rising of SE than CP in the same Volume-filling Content; (6) The percolated threshold of average and maximum SE of Ni is equal to or greater than 16.3% with adhesive spreading 250 g/m2. The percolated threshold of average and maximum SE of Ni is 9.5%～10.9% with spreading amount of 400 g/m2. The percolated threshold of average and maximum SE of CP is 34.3%～38.5% with spreading of 250 g/m2. The percolated threshold of average and maximum SE of CP is 34.3%～38.5% with adhesive spreading amount of 400 g/m2; (7) The shear strength of SF rises gradually with the rising of adhesive spreading . The rising of adhesive spreading have both positive and negative influence on SE too, the SE is a comprehensive result of these two respects. To CF, in general, the increasing of adhesive spreading is unfavorable to the shear strength; (8) With the same conductive element content, exerting SF is more favorable for the rising of SE and shear strength than CF. But when adhesive spreading is 400 g/m2 and SF content is 80 g/m2, the density of SF in the mixed adhesive is too high. It is greatly unfavorable for the rising of shear strength. The negative influence can be reduced by rising of adhesive spreading;(9) The percolated threshold of average and maximum SE of SF is 5.7% with spreading amount of 250 g/m2. The percolated threshold of average and maximum SE of SF is 3.6% with spreading amount of 400 g/m2. The percolated threshold of average and maximum SE of CF is 12.2% with spreading amount of 250 g/m2. The percolated threshold of average and maximum SE of CF is equal to or greater than 6.5% with spreading amount of 400 g/m2; (10) With the rising length of conductive element , the average and maximum SE of SF and CF rises while shear strength drops. With the rising of adhesive spreading, the average and maximum SE of SF and CF drops while shear strength rises. (11) The decreasing of SF is unfavorable for the rising of SE while it is favorable for the rising of shear strength. The contribution of SF to SF-Ni system is greater than SF-CP system. To both CF-CP system and CF-Ni system, the decreasing of CF is unfavorable for the rising of SE. It is favorable for the rising of shear strength of CF-Ni system while is unfavorable for the rising of shear strength of CF-CP system. The contribution of CF to SF-CP system is greater than SF-Ni system. (12) The decreasing of SF is unfavorable for both the rising of SE and shear strength in SF-CF system.