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Hu, D.; Zeng, X.; Lin, Y.; Chen, Y.; Chen, W.; Jia, Z.; Lin, J. Recovery of Waste-Printed Circuit Boards Non-Metallic Components. Encyclopedia. Available online: https://encyclopedia.pub/entry/48978 (accessed on 19 May 2024).
Hu D, Zeng X, Lin Y, Chen Y, Chen W, Jia Z, et al. Recovery of Waste-Printed Circuit Boards Non-Metallic Components. Encyclopedia. Available at: https://encyclopedia.pub/entry/48978. Accessed May 19, 2024.
Hu, Dechao, Xianghong Zeng, Yinlei Lin, Yongjun Chen, Wanjuan Chen, Zhixin Jia, Jing Lin. "Recovery of Waste-Printed Circuit Boards Non-Metallic Components" Encyclopedia, https://encyclopedia.pub/entry/48978 (accessed May 19, 2024).
Hu, D., Zeng, X., Lin, Y., Chen, Y., Chen, W., Jia, Z., & Lin, J. (2023, September 08). Recovery of Waste-Printed Circuit Boards Non-Metallic Components. In Encyclopedia. https://encyclopedia.pub/entry/48978
Hu, Dechao, et al. "Recovery of Waste-Printed Circuit Boards Non-Metallic Components." Encyclopedia. Web. 08 September, 2023.
Recovery of Waste-Printed Circuit Boards Non-Metallic Components
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This entry is adapted from the peer-reviewed paper 10.3390/molecules28176199

The reutilization non-metallic components from a waste-printed circuit board (WPCB) has become one of the most significant bottlenecks in the comprehensive reuse of electronic wastes due to its low value and complex compositions, and it has received great attention from scientific and industrial researchers. To effectively address the environmental pollution caused by inappropriate recycling methods, such as incineration and landfill, extensive efforts have been dedicated to achieving the high value-added reutilization of WPCB non-metals in sustainable polymer composites.

waste-printed circuit boards reutilization

1. Introduction

Recent decades have witnessed rapid development and phenomenal progress in electronic industries, which significantly increased the supply of electrical and electronic products to the public, occurring in tandem with the technological innovation in and continuous falling price of new products [1][2][3]. Moreover, the average service lives of electronic devices have greatly shortened, eventually lead to a staggering increase in electrical and electronic equipment (WEEE) and electronic waste (e-waste) [4][5][6]. It is estimated that 53.6 million tons of e-waste is generated globally per year, and this figure is expected to double by 2050 [7][8]. The environmental pollution caused by e-waste and its resource utilization have become an emerging social problem and created a major challenge to global sustainable development [9][10]. Thus, recycling and eco-friendly reutilization of e-waste has attracted extensive interest from scientific and industrial researchers.
Printed circuit boards (PCB) are integral and indispensable components used in almost all electronic devices, and they have been widely applied in various fields of the electronics industry [11][12][13]. Undoubtably, with the acceleration of electronic product renewing, the generated waste-printed circuit boards (WPCBs), including defective products used in manufacturing process and scrapped products, have also dramatically stacked up [14][15][16][17]. The management rational treatment of WPCB has, thus, become an imminent issue [18]. WPCB includes an approximately 30% metallic fraction (e.g., copper, iron, nickel, antimony, lead, and gold) and a 70% non-metallic component (e.g., thermosetting resins, glass fibers, etc.) [19][20][21]. At present, the separation and recovery of valuable metals have been widely investigated owing to their high added value and high purity. However, most of the non-metallic components used in WPCBs are usually incinerated or landfilled without any effective disposal, ultimately leading to severe resource wastage and great damage to the environment [22][23]. Therefore, exploring feasible and appropriate strategies to achieve eco-friendly reuse of non-metallic components used in WPCBs has far-reaching implications in terms of saving resources and mitigating the risk of environmental pollution. Until now, the researchers have searched for many feasible techniques for recycling WPCB non-metallic materials, such as chemical recycling (pyrolysis, depolymerization) and mechanical recycling. Among these techniques, the mechanical treatment of PCBs is considered to be a more straightforward and effective process, and as-obtained WPCB non-metallic powder could be further applied in construction products, modified asphalt, and polymer composites [24][25][26][27]. In particular, over the past few decades, scientists and engineers have dedicated great efforts to developing ecofriendly polymer composites by incorporating WPCB non-metals into various polymer matrixes, including thermosetting resin (e.g., epoxy resins, unsaturated polyester resin, phenolic resin, etc.), thermoplastic (e.g., polypropylene, poly(vinyl chloride), polyethylene, polyvinyl alcohol, polystyrene, acrylonitrile–butadiene–styrene, etc.), and rubber composites due to their wide availability, low cost, and outstanding environmental protection. It was found that the WPCB non-metallic powder could endow polymer composites with excellent mechanical properties, enhanced thermal stability, and flame retardancy. These pioneering and impressive studies have created new opportunities for the high value-added reutilization of WPCB non-metallic components.

2. Recovery and Characterization of WPCB Non-Metallic Components

Although waste-printed circuit boards only account for about 3% of all produced e-waste, the complex toxic components used in WPCBs, which involve heavy metals and brominated flame retardants, make them hazardous waste that must be treated very cautiously [28][29][30]. Moreover, considering the fact that various valuable metals, polymers, glass fiber, and toxic components were simultaneously integrated in such small volumes, the recovery and recycling of WPCBs become particularly intractable. In general, the compositions of a WPCB are divided into non-metallic components and metallic components [31]. Currently, driven by the economic interests, the recovery of metallic components from WPCBs has attracted extensive attention by using various extraction processes, including leaching, mechanical and hydrometallurgical processing techniques [32][33][34]. For comprehensive reviews of metal recovery from WPCB, readers can refer to the reviews written by Hao et al. [35], Qiu et al. [36], Lu et al. [37], and Akcil et al. [38]. While this review will focus on the recovery techniques used for non-metallic components, which account for about 70 wt% of waste PCBs and still face serious environmental and economic challenges [24]. On the other hand, to better reuse these non-metallic components of WPCB in polymer composites, the detailed characterization of WPCB non-metals is also essential, as they have a decisive influence on the properties of as-obtained sustainable polymer composites. Thus, the recovery techniques and characterization of non-metallic components from WPCBs are discussed below.

2.1. Recovery of WPCB Non-Metallic Components

Generally, the recovery strategies used for WPCB non-metallic components include chemical recycling strategies and physical recycling strategies. For chemical recycling, the WPCB are usually depolymerized into some useful molecules via pyrolysis, gasification, supercritical fluids, glycolysis, aminolysis, and alcoholysis [39]. Among these methods, pyrolysis is one of the techniques most commonly used to degrade the resins in WPCB into oils, gases, tar, and glass fibers, which is usually conducted without oxygen or using some inert gas [40][41][42]. The obtained pyrolysis gases exhibit high calorific values, and the pyrolysis oil can be further utilized as the chemical raw material or asphalt modifier. However, the pyrolysis process of WPCB recovery is challenging due to the presence of dibenzofurans, 4-methyl-Benzofluroethane, hydrogen bromide and brominated compounds in pyrolysis oils [43]. Gasification is performed in oxygen, air, or steam at a high temperature, and the gaseous product is similar to that of the pyrolysis process [44][45][46]. Recently, supercritical fluids were widely exploited as a highly efficient means of achieving the metal–non-metal separation in WPCBs, which can destroy the epoxy resin derived from WPCB and produce organic molecules [47][48][49][50]. Typically, the polymer materials can be effectively oxidized within a short time under the action of supercritical fluids, including water, methanol, and ethanol [51][52][53]. Moreover, organic solvents have been employed to dissolve the bromine epoxy resin to efficiently separate the glass fibers and polymer materials in WPCB non-metals [54][55][56]. Unlike the above chemical recycling methods, the physical recycling process is dependent on the differences in terms of physical characteristics between the metallic and non-metallic components of the WPCB. And the shape, size, and size distribution of liberated WPCB have a critical influence on their separation effectiveness. Generally, the most commonly used separation technologies of WPCB include shape separation, density-based separation, magnetic separation, electrical separation, and electrostatic separation [57][58][59]. For example, density-based separation is usually carried out to separate lighter components from other heavier products based on density difference. However, due to the simultaneous influence of particle shape, its separation efficiency is relatively poor. Electrostatic separation is another promising method used to separate the metals and non-metallic materials, and it has received widespread attention due to its low energy consumption, facile operation conditions, and environmentally friendly characteristics [60][61][62]. Eddy current-based electrostatic separator is generally used to separate thermosetting plastics and non-ferrous metals from the complex mixture via the eddy current and external magnetic field [63][64]. Low-intensity drum magnetic separators can also recover ferrous materials from the non-magnetic fraction. There is no doubt that the physical recovery method is relatively simple, effective, practical, and energy saving, which lays the foundation for the diversified applications of recycled non-metallic products derived from WPCBs. However, there are still some issues related to the physical process of recycling WPCBs. Generally, it is very difficult to completely remove all metals through the physical recycling process, and some residual metals in the as-reclaimed non-metallic components may lead to serious deterioration in the aging properties of polymer composites. Moreover, other than the residual metals, the complex compositions of non-metallic components, including various resin powders, glass fiber, metals derivatives, and even some toxic additives, make their high value-added reutilization even more challenging.

2.2. Compositions and Structure of WPCB Non-Metals

The compositions of non-metallic components in WPCB vary depending on the different types of pristine PCBs. In general, non-metallic components in WPCBs are derived from the substrates, accounting for appropriately 70% of PCBs. The choice of substrate materials is closely related to the application of PCBs. For example, epoxy resins are usually utilized in multi-layered PCBs, while the phenolic resins are used in single-layered PCBs [17][65]. As for their application to radio-frequency fields, people usually choose the low-loss Teflon substrates. To endow the PCB substrate with excellent properties, glass fibers or cellulose fibers are usually incorporated to reinforce the resins. Moreover, some inorganic materials are present, including mineral filler, alumina, and other oxides. Among these materials, the glass-fiber reinforced epoxy resins are the most used substrates. In the previous work, to realize the better reutilization of WPCB non-metallic components, the compositions and characteristics were comprehensively investigated [23]. It was found that the thermosetting resin was mainly tetrabromobisphenol A epoxy resin, and the glass fibers were largely separated from resin particles after ball milling, which shows great potential as the strength enhancer of sustainable polymer composites. Furthermore, the presence of residual metal, such as Cu and Fe, may lead to serious aging of polymer composites. Moreover, although brominated flame retardants have been banned or phased out in some countries and regions, while some new flame retardants have been developed as replacements, the brominated flame retardants still play a significant role in PCB development due to their extraordinary flame-resistant effects on combustible resins. Thus, traditional combustion of these non-metallic components in WPCB may induce the release of toxic gases, such as the dibenzodioxins, polybrominated dibenzofurans, and dioxins and furans [66][67][68]. In contrast, when the non-metallic components are further employed in polymer composites, the lifespan of a WPCB is effectively extended, and it does not pose a great threat to the environment due to it being present in low concentrations.

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Subjects: Materials Science, Composites
Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Dechao Hu
,
Xianghong Zeng
,
Yinlei Lin
,
Yongjun Chen
,
Wanjuan Chen
,
Zhixin Jia
,
Jing Lin
View Times: 140
Revisions: 2 times (View History)
Update Date: 11 Sep 2023
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