Performance and thermal behavior of wood plastic composite produced by nonmetals of pulverized waste printed circuit boards
Introduction
Recycling of waste printed circuit boards (PCBs) is an important subject not only from a standpoint of the protection of the environment but also from the recovery of reusable materials. The UN Environmental Programme estimates that the world generated 20–50 million tonnes of electronic waste each year, while PCBs formed about 3% by weight of the total amount of electronic waste [1], [2]. Mechanical–physical process is drawing more attention compared with hydrometallurgy and pyrometallurgy [3], [4]. The mechanical–physical approach involves first a crushing process, aiming to strip metal from the base plates of waste PCBs. The extreme differences in properties such as the density and electrical conductivity between metals and nonmetals provide an excellent condition for the separation of them. Various techniques including density-based separation, jigging and corona electrostatic separation are used to separate metals from nonmetals [5], [6]. Metals such as Cu, Al and Sn, are sent to recovery operations. However, significant quantities of nonmetals in PCBs (up to 70%) present an especially difficult challenge for recycling. The nonmetals of PCBs consist of thermoset resins powder and glass fibers [7]. The kind of thermoset resins varies with different PCBs. Common resins include difunctional epoxy resins such as bisphenol A, multifunctional epoxy resins such as phenol and creosol based epoxy novolacs and BT epoxy blends [8]. Thermoset resins cannot be remelted or reformed due to their network structure. Incineration is not the best method for treating nonmetals because of inorganic fillers such as glass fiber, which significantly reduce the fuel efficiency. In addition, the combustion of electronic waste in the presence of copper from PCBs may lead to higher emissions of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) into the environment, causing even worse environmental pollution [9]. Disposal in landfill is the main method for treating nonmetals of PCBs, but it may cause secondary pollution and resource-wasting. To obtain the resource utilization of nonmetals, many researchers have carried out many studies on the recycling of nonmetals for molding electronic components [10], phenolic compound [7] and modifying asphalt [11].
Wood plastic composite (WPC) is a kind of composite materials, consisting of thermoplastic resins, wood flour and small amounts of additives. Among the thermoplastic resins used, virgin and recycled polyethylene are the most common with an estimated 83% market share followed by polyvinyl chloride (9%) and polypropylene (7%) [12]. WPC is durable and low maintenance compared to wooden products, and hence ideal for nonstructural applications. Currently, most WPC is made with polyethylene for use in exterior building component, including decking, fencing, industrial flooring, landscape timbers and railing [13]. Wood flour is the most common organic filler used in WPC, with the aim to decrease cost and enhance performance of WPC. Demand for WPC has been steadily increasing in the past decade, with a market demand of 1.95 billion kg in 2006 [14]. Thus, WPC producers are forced to seek other nonwood sources to supply the increasing raw material requirement and protect timber resources. Bourne et al. studied the effects of cotton gin waste as a lignocellulosic substitute on the mechanical properties of WPC. It was found that mechanical properties of extruded cotton gin waste WPC were within the range of reported values for commercial WPC products [15].
To our knowledge, there is little published information about reusing nonmetals reclaimed from PCBs as a filler of WPC. The objective of this study is to investigate the possibility of using nonmetals in WPC production. Mechanical properties and dimensional stability of WPC with nonmetals were investigated. Furthermore, FTIR spectroscopy and thermal degradation of WPC were also studied.
Section snippets
Materials and formulations
Thermoplastics used in the study were a kind of recycled high-density polyethylene (HDPE). Wood flour used in the study was derived from various scrap wood from wood processors. The moisture content of wood flour was about 8%, and the particle size was less than 0.15 mm. The nonmetals were obtained from two step crushing and corona electrostatic separating [6]. The particle size of the nonmetals was less than 0.07 mm. WPC were prepared by mixing recycled HDPE (35%), wood flour, nonmetals and
Mechanical properties of WPC
The mechanical properties of WPC panels were shown in Table 1. Values of flexural strength and tensile strength of WPC with nonmetals (N-15-WPC and N-40-WPC) were slightly greater than those of control specimens (N-0-WPC). This is because the glass fibers in nonmetals reinforced the properties of composites. In addition, N-15-WPC possessed the highest flexural property, with a flexural strength of 25.8 MPa. This result is expected because the nonmetals and wood flour in N-15-WPC are better
Conclusions
A study was conducted to test whether the nonmetals from PCBs could be a feasible substitute of wood flour in the production of WPC. The addition of nonmetals in WPC improved the flexural strength and tensile strength, and reduced screw withdrawal strength. N-40-WPC showed best dimensional stability. The thickness swelling and thickness swelling of N-40-WPC were only 0.34% and 0.12%, respectively and after 24 h immersion in water. Thermal degradation of WPC mainly included two steps: the first
Acknowledgments
This work was supported by the National High Technology Research and Development Program of China (863 program 2006AA06Z364), Shanghai Tongji Gao TingYao Environmental Science & Technology Development Foundation.
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