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Hou, J., Fu, F., Lu, K., and Chen, L. (2015). "Highly conductive fiberboards made with carbon and wood fibers," BioRes. 10(4), 6348-6362.

Abstract

Carbon fibers (CFs) were mixed with wood fibers using the solution blend method to make highly conductive fiberboards. The microstructure, conductivity, shielding effectiveness (SE), and mechanical properties of fiberboards filled with CFs of various lengths and contents were investigated. The uniform distribution of CFs formed an excellent, three-dimensional conductive network. The CF-filled fiberboards exhibited evidence of percolation and piezoresistivity. A greater content of shorter CFs was necessary to realize the effects of percolation. The corresponding thresholds of fiberboards containing CFs of 2, 5, and 10 mm in length were 1.5%, 0.75%, and 0.5%, respectively. The volume resistance of fiberboards tended to be stable as the external pressure increased to 1.4 MPa. The volume resistivity of fiberboards reached equilibrium when the CF content was 10%. The fiberboards with greater than 10% CF content exhibited a SE of 30 dB above the average, yet they met the requirements for commercial application. The mechanical properties of fiberboards were investigated, and CFs were found to enhance the modulus of rupture (MOR) and modulus of elasticity (MOE). Therefore, it was concluded that fiberboards containing CF of 5 mm in length exhibited the best performance between percolation threshold and steady CF content.



Full Article

Highly Conductive Fiberboards Made with Carbon and Wood Fibers

Junfeng Hou,a Feng Fu,a Keyang Lu,a,* and Lanzhen Chen b

Carbon fibers (CFs) were mixed with wood fibers using the solution blend method to make highly conductive fiberboards. The microstructure, conductivity, shielding effectiveness (SE), and mechanical properties of fiberboards filled with CFs of various lengths and contents were investigated. The uniform distribution of CFs formed an excellent, three-dimensional conductive network. The CF-filled fiberboards exhibited evidence of percolation and piezoresistivity. A greater content of shorter CFs was necessary to realize the effects of percolation. The corresponding thresholds of fiberboards containing CFs of 2, 5, and 10 mm in length were 1.5%, 0.75%, and 0.5%, respectively. The volume resistance of fiberboards tended to be stable as the external pressure increased to 1.4 MPa. The volume resistivity of fiberboards reached equilibrium when the CF content was 10%. The fiberboards with greater than 10% CF content exhibited a SE of 30 dB above the average, yet they met the requirements for commercial application. The mechanical properties of fiberboards were investigated, and CFs were found to enhance the modulus of rupture (MOR) and modulus of elasticity (MOE). Therefore, it was concluded that fiberboards containing CF of 5 mm in length exhibited the best performance between percolation threshold and steady CF content.

Keywords: Highly conductive fiberboard; Percolation effect; Negative pressure coefficient of resistance; Three-dimensional conductive network; Shielding performance; Mechanical properties

Contact information: a: Research Institute of Wood Industry, Chinese Academy of Forestry, Key Lab of Wood Science and Technology of State Forestry Administration, Beijing, 100091, P. R. China; b: Bee Research Institute, Chinese Academy of Agricultural Sciences, P. R. China;

* Corresponding author: luky@caf.ac.cn

INTRODUCTION

Radiation from various devices may cause serious electromagnetic interference (EMI), which will significantly influence the performance realization of other electromagnetic (EM) devices (Togt et al. 2008; Alhusseiny et al. 2012). Electromagnetic interference can result in information-compromising emanations and potential health hazards to humans and other organisms. Electromagnetic shielding has been regarded as an effective method to prevent EM radiation from passing through blocking media (or shields) (Razavi and Halaj-Aminhosseim 2010). Currently, the products used for EM shielding are metals composed of some other materials by filling, coating, and laminating to form composite materials that are a mixture of polymers and carbons.

Wood-based EM shielding composites have attracted the public’s interest because of their availability, cost, and renewability. In general, wood elements (i.e., wood fibers, veneers, and flakes) are composited with conductive materials (i.e., metallic conductive fillers and carbon conductive fillers) by filling (Liu et al. 2007; Yuan and Fu 2014), coating, laminating (Yuan et al. 2014), and electroless plating (Hui et al. 2014) to prepare wood-based composites with excellent shielding performance. Among the wood-based EM shielding composites, conductive material-filled fiberboards have become one of the main research trends because of their simplicity, performance dependability, and tremendous prospects in the engineering sector.

Carbon fibers (CFs) are characterized by their superior electrical properties, light weight, high strength, and high tensile modulus, and have been widely used in diverse multifunctional composites (Taipalus et al. 2001; Zhang and Liu 2009; Wang et al. 2013; Yuan et al. 2014). Several experiments have been conducted to investigate the mechanical properties (Matsumoto and Nairn 2009; Yang et al. 2012), electrical conductivity (Shi 2011), and shielding performance (Yuan et al. 2013) of CF-filled composites. In these studies, most of the research has focused on mechanical improvement by laminating the CF layer. There has been minimal research on the conductivity (Zhang et al. 2011) and EM shielding of CF-filled fiberboard. Zhang (2013) used CFs instead of powder to mix with isocynate resin. The SE of the charcoal composites reached 28.62 dB when the CF content was 50% of the fiberboard. However, it was also found that the CFs was not uniformly distributed and formed an excellent conductive network in the charcoal composites. No research has studied the effect of CF length and content on conductivity and shielding performance in CF-filled fiberboards. Therefore, research in this area needs to be conducted to predict and analyze CF-filled fiberboards with excellent conductivity and shielding performance.

Unlike previous research, which blended fiber directly with resin (Zhang et al. 2013), CFs and wood fibers in this study were blended using the solution blend method. The hybrid fibers were uniformly mixed with diphenyl-methane-diisocyanate (MDI) to prepare highly conductive fiberboards. The conductivity and shielding effectiveness (SE) of the CF-filled fiberboards of various lengths and contents were analyzed. The mechanical properties of fiberboards were also determined following the GB/T17657 (2013) standard. The purpose of this study was to develop a fiberboard product for the construction sector having excellent conductivity, shielding performance, and mechanical properties.

EXPERIMENTAL

Materials

Carbon fibers were obtained from the Weida Composite Material Co., Ltd, Nanjing, China. The lengths of the CFs were 2, 5, and 10 mm, and the diameters ranged from 7 to 10 μm. The density of the CFs ranged from 1.6 to 1.76 g/cm3, and the volume resistivity was 1.5×10-5 Ω•cm. The tensile strength and modulus of CFs were 3.6 to 3.8 GPa and 240 to 280 GPa, respectively. Wood fiber was provided by the Fenglin Wood Industry Group Co., Ltd, Nanning, China. Diphenyl-methane-diisocyanate (MDI, I-BOND® MDF EM 4330) was provided by Huntsman, Shanghai, China. The density at 25 °C was 1.23 g/cm3. The color of the resin was dark brown, and the viscosity at 25 °C was 275 cps. In addition, its solid content was 100%.

Methods

Surface pretreatment of CFs

The CFs was soaked in alcohol with a concentration of 75% for 8 h to dissolve the sizing agent on the surface of the fibers. Then, the CFs were rinsed using distilled water and oven-dried in a circulation oven at 103 ± 2 °C. The surface morphology of the CFs before and after the pretreatment was investigated by scanning electron microscope (SEM; S-4800, Hitachi Limited, Tokyo, Japan), as shown in Fig. 1. The number of grooves increased on the surface of the CFs after the pretreatment process. The depth and width of the grooves increased and resulted in a stronger interfacial bonding surface between the CFs and the MDI.

Fig. 1. The CF surface morphology (a) before and (b) after surface pretreatment

Preparation of hybrid fibers

A blender with a capacity of 200 L was used to mix the CFs and wood fibers uniformly. The uniformity of the hybrid fibers remarkably affected the fiberboard performance. Hybrid fibers were prepared at a rotational speed of 600 rpm for 20 min using the solution blend method (Guo et al.2003). Then, the fibers were dried at a temperature of 103 ± 2 °C to a moisture content of 10% to 12%. The CF (2 mm) content in the hybrid fibers was 0.25%, 0.50%, 0.75%, 1.0%, 1.25%, 1.50%, 1.75%, 2.0%, 5.0%, 10%, and 20%. The CF content (5 and 10 mm) in hybrid fibers was 0.25%, 0.50%, 0.75%, 1.0%, 2.0%, 5.0%, 10%, and 20%. These hybrid fibers were used to prepare fiberboards, and their corresponding performances were investigated.

Preparation of highly conductive fiberboards

A standardized procedure was followed to prepare 360 mm × 340 mm × 3 mm fiberboards with densities of 0.65 g/cm3. The hybrid fibers were held to between 10% and 12% moisture content. The MDI content was 10% (calculated using the weight of the hybrid fiber), and it was uniformly applied to the hybrid fibers. The fiberboards were hot-pressed at 150 °C for 6 min with a target thickness of 3 mm. The thickness of the fiberboards was determined using a thickness gauge.

Volume resistivity test

Figure 2 shows the setup for the insulation resistance and DC low-resistance tests. The insulation resistance of the fiberboards was investigated using a three-electrode method, according to the ASTM-D257 (2007) standard, and DC low-resistance was investigated using the four-electrode method, in accordance with the MIL-DTL-83528C (2001) standard. An insulation resistance tester (TH2683, Changzhou Tonghui Electronic Co., Ltd. Jiangsu Province, China) was used to measure the insulation volume resistance, ranging from 105 to 1013 Ω. A digital DC resistance tester (TH2512, Changzhou Tonghui Electronic Co., Ltd. Jiangsu Province, China) was used to measure the electrical resistance, ranging from 1.0 μΩ to 2.0 MΩ. The volume resistivity (ρ) in Ω.cm was determined using Eq. 1,

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where A is the smallest cross sectional area of a section of the sample between the probe electrodes (cm2), L is the distance between the two electrodes in cm, or 2.45 cm, and R is the volume resistance of the specimens in Ω. Specimens with dimensions of 5 × 5 mm2 were cut from the fiberboards, and four replicate specimens of each type were measured. Prior to testing, the entire specimens were conditioned at 20 °C and 65% relative humidity (RH) for 24 h, according to the ASTM D1037 (2006) standard. The external pressure selected in this study ranged from 0.2 to 2.1 MPa.

Fig. 2. Setup for measuring the (a) insulation resistance and (b) DC low-resistance

Electromagnetic shielding measurement

The SE was measured using the coaxial cable method (also referred to as transmission line method), according to Chinese standard SJ20524 (1995). The setup consisted of a vertical flanged test device with the input and output connected to a Hewlett-Packard (HP; USA) 7401A EMC Analyzer. The frequencies of the transmitted signals ranged from 100 kHz to 1.5 GHz. Standard circular specimens, with diameters of 115 ± 0.5 mm, were cut from the fiberboards (Fig. 3). Six replicate specimens of each type were prepared and tested.

Fig. 3. The setup and specimen sizing for the electromagnetic shielding effectiveness test

Microstructural characterization

The CFs’ contribution in the plane and cross-sections of fiberboards were investigated using a three-dimensional analysis system digital microscope (VHX-1000, KEYENG Corp., Itasca, IL, USA) and scanning electron microscope (SEM; S-4800, Hitachi Limited, Tokyo, Japan), respectively.

Mechanical and physical properties

The modulus of rupture (MOR), modulus of elasticity (MOE), and thickness swelling (TS) were measured in accordance with GB/T17657 (2013). Flexural tests were carried out using a three-point static bending test to determine the MOR and MOE of highly conductive fiberboards. Before testing, the dimensions (i.e., length, width, and thickness) and weight of each specimen were measured. Prior to testing, the specimens were conditioned at 20 °C and 65% RH for 24 h. For the thickness swelling test, conditioned samples of each type were soaked in water at room temperature for 24 h. Specimens were removed from the water, patted dry and measured again. A total of six specimens for the bending tests (MOR and MOE) and eight specimens for the TS test were measured for the final analysis. The presented values for the MOR, MOE, and TS tests are the mean values.

RESULTS AND DISCUSSION

Microstructure Analysis of the Fiberboard

Figures 4 and 5 depict the CF distribution in the fiberboards. A two-dimensional conductive network was observed in the fiberboards with CF contents ranging from 0.5% to 10%, and a three-dimensional conductive network was seen as the CF content increased to 20%. Results reported by Chiarello et al.(2005) showed a similar conclusion in the electrical conductivity of self-monitoring, CF-reinforced cement (CFRC).

Fig. 4. CF distribution in the plane of fiberboards. The black fibers are CFs, and the brown fibers are wood fibers

Fig. 5. CF distribution in the cross-section of the fiberboards

Piezoresistive Behavior of the Fiberboards

The volume resistance of the CF-filled fiberboards (10 mm, 5%) under various external pressures is shown in Fig. 6. CF-filled fiberboards exhibited a phenomenon of negative pressure coefficient of resistance in piezoresistivity. In general, the electrical conductivity of the highly conductive fiberboards was determined using the conductive network formed throughout the matrix. The conductive network depends markedly on the external loading (Wang and Chung 2000). The results reported by Wen and Chung (2007b) exhibited a similar conclusion. The distance between CFs decreased as the external pressure increased, especially in the low external loading area. The contact number and region of the CF increased, which contributed to a decline in the volume resistance, especially in the low-pressure region. When the external pressure was above 1.4 MPa, the volume resistance declined and reached a plateau; there was no further decrease with increasing external pressure. The same conclusion was obtained for other CF-filled fiberboards. A continuous CF polymer-matrix composite was prepared by Wang et al. (2013), and it exhibited Piezoresistivity when tension was applied to the fiber orientation.

Fig 7

Fig. 6. The volume resistance of fiberboards under external pressure

Conductive Uniformity of the Fiberboards

Figure 7 shows the normality analysis for the volume resistivity of the CF-filled fiberboards (2 mm, 5 mm, and 10 mm; the CF content was 5%).