Wood Plastic Composite Produced by Nonmetals from Pulverized

Dec 9, 2009 - The nonmetals were used to replace wood flour in the production of WPC, ..... In Resource Recovery from Recycling; United States Geologi...
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Environ. Sci. Technol. 2010, 44, 463–468

Wood Plastic Composite Produced by Nonmetals from Pulverized Waste Printed Circuit Boards JIE GUO, YINEN TANG, AND ZHENMING XU* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China

Received September 23, 2009. Revised manuscript received December 1, 2009. Accepted December 1, 2009.

Nonmetals reclaimed from waste printed circuit boards (PCBs) are used to replace wood flour in the production of wood plastic composite (WPC). To evaluate property durability against weather exposure, the effects of accelerated aging process on the properties of WPC are investigated. The results show that filling of nonmetals in WPC improves the flexural strength and tensile strength, and reduces screw withdrawal strength. Before hollow WPC with 15% nonmetals (H-15-WPC) underwent aging process, H-15-WPC had a flexural strength of 25.8 MPa, a tensile strength of 9.8 MPa, a charpy impact strength of 3.4 kJ/m2, and face/edge screw withdrawal strength of 121/115 N/mm. It is found that flexural strength of H-15WPC decreases linearly with the increase of accelerated aging cycles, and the effects of aging test on tensile and impact strength of H-15-WPC are minor. For solid WPC, the accelerated aging test decreases screw withdrawal strength slightly. All the results indicate that nonmetals of waste PCBs can be reused as an alternative for wood flour in WPC products rather than resorting to their landfill or combustion.

Introduction Printed circuit boards (PCBs) contain nearly 28% metals, including Cu, Al, Sn, etc. The purity of precious metals in PCBs is more than 10 times higher than that of rich-content minerals (1, 2). The major economic drive for recycling of electronic waste is from the recovery of metals. Recycling of PCBs is an important subject not only from the treatment of waste but also from the recovery of valuable materials as the amount of deserted PCBs is dramatically increasing. 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, and then different methods to separate metals from nonmetals (5). 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. Nonmetals of PCBs mainly consist of thermoset resins and reinforcing materials. 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, * Corresponding author phone: +86 21 54747495; fax: +86 21 54747495; e-mail: [email protected]. 10.1021/es902889b

 2010 American Chemical Society

Published on Web 12/09/2009

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 (6). Disposal in landfill is the main method for treating nonmetals of PCBs, but it may cause secondary pollution and resource-wasting. Wood plastic composite (WPC) is a unique development in the wood products industry, an emerging renewable material class based on performance, process, and product design innovation (7). WPC consists of thermoplastic resins, wood flour, and small amounts of additives. Currently, most WPC is made with polyethylene, both recycled and virgin, for use in exterior building components (8). 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 (9). In recent years, wood flour supply for making WPC has become scarce and expensive. Thus, WPC producers are forced to seek other nonwood sources to meet the increasing raw material requirement and protect timber resources. Bourne 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 (10). To our knowledge, there is little published information about reusing nonmetals reclaimed from PCBs as a filler of WPC. In our previous studies, nonmetals were obtained after a two-step crushing and electrostatic separation (11). Then nonmetals were used to produce phenolic molding compound (12) and unsaturated polyester molding compound (13), and to modify asphalt (14). To extend the practical fields of the nonmetals, a novel technique has been developed to reuse the nonmetals of waste PCBs. The nonmetals were used to replace wood flour in the production of WPC, thus protecting wood resources, avoiding land-filling or incineration of the nonmetals, and reducing environmental load. Furthermore, effects of the accelerated aging process on the properties of WPC were also investigated.

Materials and Methods Materials and Formulations. Thermoplastics used in the paper were a kind of recycled high-density polyethylene (HDPE). Wood flour used in the study was less than 0.15 mm. The particle size of the nonmetals was less than 0.07 mm (see Figure S1 in the Supporting Information). Four batches of WPC were produced according to different formulations and dies as shown in Table 1. The nonmetals were added to the raw materials mixture at weight fractions of 15% and 40%. The additives included silane coupling agents (2%), wax (1%), zinc stearate (0.5%), and pigment (1.5%). Preparation of the WPC. The preparation process of WPC products is shown in Figure 1. The recycled HDPE, nonmet-

TABLE 1. Formulations and Types of WPC formulations (wt %) sample

HDPE

wood flour

nonmetals

additives

type

H-0-WPC S-0-WPC H-15-WPC S-40-WPC

35 35 35 35

60 60 45 20

0 0 15 40

5 5 5 5

hollow solid hollow solid

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FIGURE 1. Flowchart of preparation of WPC products.

TABLE 2. Accelerated Aging Conditions step

aging condition

1 2 3 4 5 6 7

immerse in water at for 49 ( 2 °C 1 h expose to steam and water vapor at 93 ( 2 °C for 3 h store at -18 ( 2 °C for 20 h heat at 99 ( 2 °C in dry air for 3 h expose again to steam and water vapor at 93 ( 2 °C for 3 h heat in dry air at 99 ( 2 °C for 18 h repeat steps 1-6

als, wood flour, and other additives were premixed by a highspeed mixer, compounded by a parallel corotating twin-screw extruder (TSE), and pelletized to composite granules. Then, the composite granules were used for profile extrusion by a conical counter-rotating TSE. Along with the conical counterrotating TSE, the die was an important part of the WPC product extrusion system. The die regulated the dimensions and shapes of the extruded part. The die was heated using cartridge heating elements. Hollow and solid deck boards were produced using different dies. After the die came the cooling tank, which was used to “freeze” the extruded profile in its linear shape. After the cooling tank, the WPC profile went through a cutoff saw that could cut the WPC products to the desired lengths. Average temperatures in the extruder barrel and die were maintained at 175 and 180 °C. The extruder pressure was maintained at approximately 9.5-11.5 MPa. The speed of the extruder was set at approximately 600 mm per minute. Accelerated Aging Process. Durability against weather exposure is critical for WPC since they are often used in outdoor environments with high humidity levels. Determining the effects of aging on WPC products in real time is a lengthy process that would severely delay market introduction of WPC products. Therefore, the six-cycle aging process is used for accelerated aging of wood-based products as specified in American Society for Testing Materials (ASTM) D1037, sections 118-124, and the details are given in Table 2. Each of the six cycles contains six individual exposure steps. After accelerated aging, additional time is needed to recondition the specimens before property testing. For the aging process, the specimens were treated at -18 °C, instead of -12 °C as specified in the standard, to simulate an even more severe winter temperature. For each material, specimens were randomly selected and tested after 1, 2, 3, 4, 5, or 6 cycles of accelerated aging. 464

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Measurement of Properties. Field emission scanning electron microscopy, FEI SIRION 200, was used to analyze the dispersion of fillers into the resin matrix. Prior to the analysis, the surfaces of the specimens were sputter coated with a thin layer of gold. Flexural strength is maximum bending stress developed in a specimen just before it cracks or breaks in a flexure test. Flexural tests were performed using the 3-point bending method according to ASTM D 790 standard. Tensile strength measures the force required to pull a specimen to the point where it breaks, and is measured in units of per unit area or pascals (Pa). Test samples were cut from composite panels. Tensile tests were carried out using an universal mechanical tester in accordance with ASTM D 638 standard. Charpy unnotched impact strength is defined as the amount of energy absorbed in fracturing an unnotched specimen at high velocity and is expressed as Kilojoule per square meter. Impact strength was assessed with a universal pendulum impact tester at room temperature according to ISO 179-1982 standard. Screw withdrawal strength is defined as the peak load required to pull a standard screw from the panel specimen and is expressed as newton per mm of screw penetration. Face and edge screw withdrawal strength were tested according to EN 320-1993. Specimens’ shapes for screw withdrawal strength were 150 × 50 × 30 mm. The screws used in the paper are a kind of cross recessed pan head tapping screw with diameter of 4.2 mm and length of 38 mm. A 2.7 mm diameter pilot hole was drilled 19 mm into each specimen by an electric drill. Then a screw was hand-driven 15 ( 0.5 mm into the specimen. The screws were perpendicular to the face and edge plane. In addition, the screws were driven prior to the aging exposure. Having been subjected to each of these accelerated aging environments, the specimens were conditioned at room temperature for 48 h before the mechanical tests.

Results and Discussion Production of the WPC. Manufacturing WPC is a complex and continuous process, including premixing, compounding, extruding, cooling, and cutting. A schematic illustration of nonmetals of waste PCBs filling the WPC is shown in Figure 2. However, compounding and extruding were the two main steps among them. In the compounding step (sometimes called “meltblending”), wood flour, nonmetals, and additives were combined with molten thermoplastic to produce a homo-

FIGURE 2. Schematic illustration of production of WPC products.

FIGURE 3. SEM photograph of composite granules. geneous composite material. Generally, the early stage of melting occurred through the formation of a melt film at the heated wall of the barrel. In a common version of melting mechanism, the melt film produced at the barrel wall was wiped by the traversing screw flight, causing polymer to collect and circulate at the rear of the channel. The solid bed of polymer granules or powder further forward in the channel was pressurized against the barrel surface. Extensive shear deformation of polymer occurred at the solid-melt interface generating considerable heat and making a substantial contribution to the overall melting process. A central requirement for most compounding situations is intimate blending of the components. Dispersive blending involved the breakdown of agglomerates of solid particles, generally in a deforming viscous polymer melt, by forcing the mixture to pass through high shear zones. Agglomerate rupture occurred when hydrodynamic forces exerted on the particles exceeded the cohesive surface attraction within the agglomerate (15). In addition, compounding could achieve the function of removing most of the moisture and converting the molten mixture into a processable formscomposite granules. Composite granules were obtained after compounding, and the morphology of the ingredients in the granules is shown in Figure 3. Voids and poor wetting of glass fibers were seen. The composite granules were intermediate products, and needed further extruding. The composite granules were extruded by a counter-rotating TSE. With proper screw design, the screw segments in the TSE consisted of three sections: melting, compression, and metering. The function of the melting section was to convey the granules forward from the hopper and convert them into molten mixture. The compression section, in which the depth of the screw flight decreased, was designed to compact and mix the molten mixture to provide a more or less homogeneous melt to the metering section, the function of which was to pump the molten mixture out through the die. Solid and

hollow WPC panels could be made using different dies as shown in Figure S2. A series of downstream and auxiliary equipment was used to cool and cut the WPC. Finally, the WPC products were obtained. Mechanical Properties of the WPC. WPC is a kind of filler-reinforced composite, and the fillers such as wood flour and nonmetals can enhance the mechanical properties of WPC. Generally, the strength of fiber-reinforced composites depends on the properties of constituents and the interface interaction. For unaged WPC, the addition of nonmetals in H-15-WPC and S-40-WPC improved the flexural strength and tensile strength compared to H-0-WPC as shown in Figure 4a and b. This is likely because the glass fibers in nonmetals possessed better reinforcement than wood flour. Effects of the number of cycles of the ASTM D 1037 accelerated-aging exposure test on the mechanical properties of WPC are shown in Figure 4. The flexural strength of H-15WPC decreased linearly as the number of accelerated aging cycles increased. Before the aging test, H-15-WPC possessed the highest flexural property, with a flexural strength of 25.8 MPa. After a six-cycle aging process, the flexural strength decreased to 20.5 MPa. The reductions were mainly caused by the aging process. In repeated soaking, steaming, and drying cycles, any stresses built into the panels on manufacturing were released. Hydrogen bonds within wood particles and between them were broken at high moisture content, and covalent bonds were broken as well due to swelling stresses (16). The flexural strengths of WPC with nonmetals (H-15-WPC and S-40-WPC) were slightly greater than those of control specimens (H-0-WPC). This is because the glass fibers in nonmetals reinforced the properties of composites. In addition, when considering the flexural properties, homogeneity of the overall composite needs to be taken into account. This is mainly because in bending, the convex side of the specimen is extended and the concave side is compressed (17). Similar to the flexural strength, the decrease in tensile strength can be attributed to additional fracture sites caused by adding wood flour and nonmetals. For tensile loads, the entire cross section resists the tensile force transmitted through the specimen. For S-40-WPC, tensile strength showed a significant drop after 2 aging cycles as shown in Figure 4b. The 40 wt % nonmetals content contributed to high content of glass fibers. The addition of many glass fibers in the S-40WPC increased the probability of filler agglomeration that create regions of stress concentrations. When the samples underwent the aging process, water vapor penetrated into the inner part of the panels, destroyed the interface adhesion, leading to deformation and bond breakage of microcosmic structures. So S-40-WPC panels were destroyed more easily by the severe aging test than H-0-WPC and H-15-WPC. Unlike the flexural or tensile strength of composites, a balance in properties between the matrix and fiber is required to obtain good impact strength. For H-0-WPC and S-40WPC, impact strength fluctuated with an increasing trend as the number of aging cycles increased. This result is expected VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Variation in mechanical properties of WPC with number of aging cycles. because the soaking and steaming process increases the water absorption of WPC, and high water absorption leads to high impact strength. The effects of the aging process on the impact strength of the H-15-WPC were minor. This result was expected because the nonmetals and wood flour in H-15WPC were better encapsulated by thermoplastic matrix, and better encapsulation led to better resistance to the aging process, which was evident from the SEM study in the forthcoming section.

FIGURE 5. WPC panels with screws before and after aging process. 466

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Screw Withdrawal Strength. The WPC panels need to be fastened by the screws when they are used in practical applications. So screw withdrawal strength is an important property of WPC panels. At present, WPC products such as deck boards, landscape timbers, and park benches are mainly used in the exterior environment. So it is interesting to evaluate the effects of aging process on the screw withdrawal strength of WPC panels. Figure 5a and b show the S-40-WPC panels with screws before and after six aging cycles. When the screws underwent soaking and steaming, the exposed part of the screws was rusted. For all specimens, the material tended to fail locally around the screw threads along the entire length of the screw (18). However, after the screws were pulled out from the panels, the unexposed part of the screw was well protected from erosion as shown in Figure 5c. Effects of the number of cycles of the ASTM D 1037 accelerated-aging exposure test on face and edge screw withdrawal strength of S-0-WPC and S-40-WPC are shown in Figure 6. It is found that accelerated aging did not appear to reduce the overall screw withdrawal strength considering the mean values changed by less than 15%. However, mean values of screw withdrawal strength of aged WPC panels were lower than those of unaged panels. The loss in screw withdrawal capacity must have resulted from aging exposure. This result is expected because water steam and dry heat can infiltrate the holes and rupture the interfacial adhesion during the aging process after the inner structure of WPC panels is destroyed by drilling and screwing. In addition, the decrease of edge screw withdrawal strengths of aged S-0-WPC panels is the most as shown in Figure 6b. This indicates that the edge surface of S-0-WPC was vulnerable to weather exposure. Fastener performance of WPC depends on the inherent properties of individual components of the formulation, the interactions among the individual components, and the end use environment. The fastener properties of the S-0-WPC were greater than those of S-40-WPC. The mean values of screw withdrawal strength of S-0-WPC and S-40-WPC varied from 130 to 150 N/mm, and from 105 to 121 N/mm, respectively. The use of nonmetals in combination with the plastic materials decreased the screw withdrawal strength of WPC panels. While drilling and hand-driving the screws into the panels, the microcosmic material structures between glass fibers and matrix in S-40-WPC were severely deteriorated, leading to breakdown of glass fibers and generating many holes inside the panels. This lowered the friction between the screw and the hole thus causing a lower withdrawal load. Morphology after Flexural Fracture. WPC consist of many ingredients, and adhesion among the ingredients can be destroyed when suffering fracture. Cracks can be easily generated between two different polar ingredients, such as hydrophilic wood flour and hydrophobic HDPE. SEM photographs of different WPC after flexural fracture are shown in Figure 7. Figure 7a shows the fracture morphology of H-0WPC. Wood fibers inserted in the matrix. The fracture was glossy, and no deep cracks were seen. Figure 7b shows the SEM photograph of H-15-WPC. Fillers including wood flour

FIGURE 6. Face (a) and edge (b) screw withdrawal strength of WPC after accelerated aging.

FIGURE 7. SEM photographs of WPC after flexural fracture: (a) H-0-WPC; (b) H-15-WPC; (c) S-40-WPC. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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and nonmetals were well encapsulated by resin matrix, and dispersed homogeneously in the composite, leading to stable properties. The adhesion between glass fibers and matrix was very poor, and deep holes are seen in Figure 7c. The amount of short glass fibers in WPC increased as the content of nonmetals increased. The increasing amount of glass fibers decreased the flow ability of composite and reduced dispersion of ingredients, leading to poor interfacial adhesion. In addition, the agglomeration of glass fibers also decreased the property of S-40-WPC. It also explains why S-40-WPC possessed poorer screw withdrawal strength than S-0-WPC. Nonmetals, a byproduct generated from recycling of waste PCBs, can be successfully used to replace wood flour to produce WPC. This offers a novel method to treat nonmetals and achieve complete recovery of reusable resources. So there is no doubt that the practical application of producing WPC with nonmetals can be realized in the future.

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.

Supporting Information Available Recycling process of waste PCBs and the ingredients of nonmetals; photos of solid and hollow WPC panels. This information is available free of charge via the Internet at http://pubs.acs.org.

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