In cluster 3, there is increasing research into the use of recycled plastic waste in the construction sector. Cluster 3 consists of two subclusters, “Use of recycled plastics in concrete” (average publication year, 2017.1) and “Use of recycled plastics in asphalt” (average publication year, 2017.5). Both are young research fields. The number of papers on the use of recycled plastics in concrete (2028) is higher than the number of papers on their use in asphalt (734). Citation/paper ratio of concrete is 6.4, which is higher than 3.1 of asphalt. Concrete applications are a more intensely studied research topic than asphalt applications. On the other hand, the use of recycled plastic mixed with concrete results in inferior strength and durability, and there are few reports of actual field applications. In the case of asphalt use, many studies have reported that strength, durability, and economic efficiency are improved, and there are practical examples in actual road construction.

By country, we found that China and the United States had the highest number of papers. Specifically, in cluster 1, China ranked first with a share of 12.4% of the total number of papers, and the United States ranked second with a share of 11.1%. In cluster 2, China ranked first (25.8% share of papers) and the United States second (11.0% share of papers). Our first key finding is that China and the United States are global leaders in many research fields. In contrast, countries other than Europe, the United States, and China (mostly developing countries) are in the top 10 of cluster 3, a notable divergence from clusters 1 and 2. For example, in subcluster 3-1, “Use of recycled plastics in concrete,” India ranks first, followed by Iraq (2nd), Malaysia (5th), Saudi Arabia (6th), and Algeria (10th). In subcluster 3-2, “Use of recycled plastics in asphalt,” India ranks fifth, followed by Malaysia (7th), Saudi Arabia (8th), and Turkey (9th). These studies are being actively carried out in developing countries, and it is thought that they are attracting attention due to their high economic efficiency as a recycling method. These are reverse innovation that should be considered as methods of using waste that are not suitable for recycling using other methods, even in developed countries. Our second key finding is that research on the use of recycled plastics in the construction sector is actively being conducted in developing countries.

2.1. Use of Recycled Plastics in Concrete

The first subcluster is the use of recycled plastics as raw material for concrete. Generally, concrete is mixed with aggregate (usually sand or gravel is used). By substituting plastics for a portion of sand and gravel, the plastics can be mixed into concrete as an aggregate at the concrete casting site. Mixing plastics into concrete as an aggregate suppresses heat generation and shrinkage when the concrete hardens, and helps prevent cracks. In addition, since cement paste and mortar, which are the main raw materials of concrete, are expensive, the amount of these expensive raw materials used can be reduced, and the construction cost can be suppressed by mixing plastics. Although much research has been done on the use of recycled plastics in concrete, there are few papers on their field applications. Demand for concrete is high around the world, and if recycled plastics can be used as aggregate, a large amount of plastic waste can be processed. For this reason, the existing body of research is large, and it is currently an active field of study [216–218]. As the proportion of plastics in concrete increases, mechanical properties such as compressive strength, flexural strength, tensile strength, and elastic modulus decrease [216]. Replacing 20% of the existing aggregate with plastics reduces compressive strength by 72%. However, a 5% replacement results in only a 23% decrease in compressive strength [219]. Substituting with PET at a rate of 15% decreased flexural strength by 16% for pellet-type PET and by 60% for thin-form PET [220]. When fine aggregate is replaced by 10%, the tensile strength decreases by 8.7%, and when it is replaced by 20%, the tensile strength decreases by 54% [221]. Several studies have reported that as the content of plastics increases, the ultrasonic pulse velocity (UPV), which reflects the quality of concrete, also decreases [216]. The value of UPV decreases with increasing content of PVC in concrete. However, the reduction is less than 16% if the PVC replacement rate is up to 45%. Replacing up to 85% reduces the UPV value by 30% [222]. Utilizing plastics as aggregate reduces concrete slump (i.e., reduces flowability) and results in concrete that is difficult to handle on construction sites. Replacing 20% of the fine aggregate with plastics has been reported to reduce slump values by up to 50% [223]. These characteristics are thought to be due to the low density of plastics, irregular shapes and sizes, and sharp corners of recycled plastic fragments. Plastics do not mix well with the existing aggregates, and water absorption, permeability, and carbonation of concrete enhance with increasing plastic content, adversely affecting the concrete durability [216].

Concrete made with recycled plastics is inferior to existing concrete in many respects. However, it is expected that concrete containing plastics will be used for non-structural materials that do not require high strength and applications that do not require high durability. Possible applications include highway median strips, temporary structures, and general-purpose bricks and blocks (for example, riverbanks) [216,217]. Applications in concrete pavement and sports courts are also mentioned. Concrete with plastic aggregate has a high water absorption rate, which helps with the proper drainage of rainwater. The use of additives such as superplasticizers can increase the flexibility of plastic-containing concrete and potentially improve the workability of concrete, thus reducing the challenges at construction sites. Plastic-containing concrete has a lower density, but lighter concrete could open up new uses. Furthermore, the possibility of applying new additives may complement the mechanical properties of plastic-containing concrete [216].

In solving the plastic waste problem, the use of recycled plastics as aggregate for concrete has great potential. Several issues remain, including the improvement of mechanical properties, long-term behavior change of mechanical properties, improvement of durability, development of additives to compensate for these shortcomings, elucidation of the optimal shape and size of plastics to mitigate performance degradation, and heat insulation and sound insulation properties [216–218] There are many themes in this field, which will require extensive research to resolve the numerous problems identified.

2.2. Use of Recycled Plastics in Asphalt

The second subcluster is the utilization of recycled plastics in asphalt. Asphalt is a hydrocarbon containing material with chemical similarities to plastics. There is a consensus among researchers that recycled plastics, when properly blended with asphalt under optimal conditions, significantly improves the performance and longevity of asphalt pavements [224]. For example, ethylene vinyl acetate (EVA) is a class of polymers that modifies asphalt by forming a tough, rigid, three-dimensional network that resists deformation, and virgin EVA has been used in road construction for many years [225]. The use of recycled plastics for asphalt provides a solution to the problem of waste treatment, improves the performance and economic efficiency of asphalt pavement, and may lead to cost reduction in the long term [226]. For example, it has been reported that approximately 1 ton of asphalt can be saved by constructing a 1 km long road (3.75 m wide) with asphalt using recycled plastics [227]. For these reasons, there has been increasing research into the utilization of recycled plastics in asphalt.

There are two methods of paving with asphalt-containing plastics [228]. One is the dry method, in which plastics are incorporated into hot aggregates prior to the addition of binders. This method applies in most cases to hard plastic types with high melting points such as high-density polyethylene (HDPE) and PET. The hardness and stiffness of recycled plastics particles play a role similar to the fine aggregate that is the skeleton of the asphalt mixture and contributes to its integrity [228]. Another method is the wet method, which involves adding plastics directly to the asphalt binder as a modifier before mixing it with aggregate. Low melting point plastics such as low density PE (LDPE) and PP are suitable for this method.

Since the effects and characteristics of asphalt mixtures differ depending on the type of plastics used, research has been conducted according to the type of plastic. PET is mainly used in dry processes, and when used as an aggregate substitute for asphalt mixtures, it increases stiffness and improves both rutting and fatigue resistance [229]. Conversely, it has been reported that thermal cracking and moisture resistance are impaired [230]. PET-modified asphalt can weaken the bond between aggregates and asphalt in asphalt mixtures [231]. This is due to the high stability and inert nature of plastics, and it is recommended to add an oxidizing agent to activate the plastic surface [232]. HDPE is mainly used in dry processes due to its high density and high rigidity. PS is mainly used in dry process. PS increases asphalt hardness and improves rutting resistance [233]. The addition of PS hardens the asphalt mix and improves its resistance to moisture damage, although its impact on resistance to rutting and fatigue cracking is inconclusive.

LDPE is mainly used in wet processes, which require high shear rates and high temperatures to fully dissolve the LDPE into the asphalt. Although it is generally accepted that the addition of LDPE to asphalt improves rutting, fatigue, and moisture resistance, the results of thermal cracking resistance differ from study to study [228]. PP is mainly used in wet processes, and when added to asphalt, it increases hardness and contributes to improving rutting resistance. On the other hand, PP reduces the ductility of asphalt, resulting in more air voids. One study demonstrated that increased air voids resulted in impaired rutting resistance [234].

As experimental levels of research, asphalt mixed with recycled plastics are likely to be stiffer, resulting in overall improvements in viscosity, strength, rutting resistance, and fatigue resistance. However, verification of the performance of asphalt mixed with recycled plastics ultimately needs to be confirmed by field projects that use it for road paving. Several field projects have so far been carried out in India, South Africa, New Zealand, Australia, Canada, the United Kingdom, the United States, and other countries, with positive performance results in terms of workability, constructability, and sustainability [228]. However, few field projects have studied long-term performance, and it is not clear whether the performance of asphalt mixed with recycled plastics will be sustained over longer time periods. Further research into the long-term viability of plastic-containing asphalt, as well as the effects of asphalt mixtures on parameters such as fatigue resistance, thermal crack resistance, and moisture resistance is needed.

3.Conclusions

Using bibliometrics analysis, we synthesized an overview of 35,519 publications on plastic recycling, identified emerging topics, and conducted a comprehensive review to elucidate research trends and key issues. We collected bibliographic data from academic publications related to plastic recycle. We used data collected with the query (plastic* OR chemical*) AND (recycl* OR “circular economy”) by using academic database “Web of Science”. After acquiring relevant publications, we created citation networks by treating the papers as nodes and the citations as links. We used the direct citation method. We removed irrelevant papers that were not connected to other papers in the largest component of the citation network. We divided the network into clusters using the Newman’s algorithm topological clustering method after obtaining the largest connected component. Using this algorithm, we divided clusters into subclusters according to the rule of maximizing modularity, which has been used in previous bibliometric studies.

We found that research topics on plastic recycling can be broadly classified into the following six clusters: general issues of plastic recycling; waste electrical and electronic equipment (WEEE); use of plastic waste in the construction sector; chemical recycling of polyethylene terephthalate; use for wood-plastic composites; and recycling of fiber reinforced polymers. After extracting the above clusters, we conducted a comprehensive review on each cluster as well as subclusters of the larger three clusters.

The largest cluster (cluster 1) is on general issues of plastic recycling and includes subclusters such as the biodegradability of plastics, bioplastics, pyrolysis, and life cycle assessment (LCA). Among them, the biodegradability of plastics is the youngest subcluster (average publication year, 2018.7) and the most active topic. Many studies on biodegradation of plastics derived from fossil resources are being conducted, and at the same time, research on biodegradable plastics is also attracting attention. The former is still in the research stage and has not been industrialized, while the latter, such as PLA, PHA, has been industrialized, but the production cost is extensively high. Consequently, the market share is low. In general, biodegradable plastics need to be sorted by consumers because the recycling method differs from that of other plastics. We pointed out the problem that it is difficult to distinguish the type of plastics just by the appearance, and that recycling methods have not been established. Bioplastics is the second youngest subcluster (average publication years, 2017.9), with a rapidly increasing number of papers. Definitions of bioplastics differ among papers, and we clarified that three different definitions were used. In this study, we defined bioplastics as polymers derived from renewable resources and materials or synthesized by microbial metabolism. Pyrolysis is a relatively old subcluster (average publication year, 2013.5), but has the largest number of papers in cluster 1 (number of papers, 772). The citation per paper ratio is also the largest (4.8), which makes this subcluster the central theme in cluster 1. LCA is a relatively young subcluster (average publication years, 2015.7) with the second largest number of papers in cluster 1 (number of papers, 568). The combined results of many studies on LCA reveal that mechanical recycling is superior to chemical recycling in terms of global warming potential, but inferior in terms of residual solid waste for landfill. We proposed that mechanical recycling and chemical recycling should not compete with each other, but should be used in a complementary manner depending on the type and condition of plastic waste.

In the second largest cluster (cluster 2), research regarding WEEE recycling is increasing rapidly (average publication years, 2014.8). The brominated flame retardants (BFR) used in WEEE plastics is hazardous to human health and ecosystems. Hazardous BFR waste is transported both legally and illegally to areas where labor costs are low. As much as 1818 kg of harmful brominated low-molecular-weight compounds are released into the environment every year around the world, especially in disposal sites in Asia. The treatment of BFR make recycling difficult, and considerable effort is being taken to address this. Mechanical recycling is the most desirable method for treating WEEE plastic, and most of the recycling currently performed is mechanical recycling. The separation of BFR from WEEE by chemical recycling has been intensively researched but not industrialized.

In the third largest cluster (cluster 3), there is increasing research into the use of recycled plastic waste in the construction sector. Cluster 3 consists of two subclusters, “Use of recycled plastics in concrete” (average publication year, 2017.1) and “Use of recycled plastics in asphalt” (average publication year, 2017.5). Both are young research fields. The number of papers on the use of recycled plastics in concrete (2028) is higher than the number of papers on their use in asphalt (734). Citation/paper ratio of concrete is 6.4, which is higher than 3.1 of asphalt. Concrete applications are a more intensely studied research topic than asphalt applications. On the other hand, the use of recycled plastic mixed with concrete results in inferior strength and durability, and there are few reports of actual field applications. In the case of asphalt use, many studies have reported that strength, durability, and economic efficiency are improved, and there are practical examples in actual road construction.

By country, we found that China and the United States had the highest number of papers. Specifically, in cluster 1, China ranked first with a share of 12.4% of the total number of papers, and the United States ranked second with a share of 11.1%. In cluster 2, China ranked first (25.8% share of papers) and the United States second (11.0% share of papers). Our first key finding is that China and the United States are global leaders in many research fields. In contrast, countries other than Europe, the United States, and China (mostly developing countries) are in the top 10 of cluster 3, a notable divergence from clusters 1 and 2. For example, in subcluster 3-1, “Use of recycled plastics in concrete,” India ranks first, followed by Iraq (2nd), Malaysia (5th), Saudi Arabia (6th), and Algeria (10th). In subcluster 3-2, “Use of recycled plastics in asphalt,” India ranks fifth, followed by Malaysia (7th), Saudi Arabia (8th), and Turkey (9th). These studies are being actively carried out in developing countries, and it is thought that they are attracting attention due to their high economic efficiency as a recycling method. These are reverse innovation that should be considered as methods of using waste that are not suitable for recycling using other methods, even in developed countries. Our second key finding is that research on the use of recycled plastics in the construction sector is actively being conducted in developing countries.

In order to realize a global circular economy, we proposed and discussed the principle of local waste treatment, the principle of global waste treatment, and global technology transfer. In the principle of local waste treatment, plastic waste should be handled responsibly and appropriately in the country where it is generated. According to this principle, the environmental burden associated with waste treatment may be minimized, but the economic rationality is questionable. In the principle of global waste treatment, the international trade of waste resources is allowed and requires a division of labor between developed and developing countries. Although the principle of global waste treatment has the advantage of minimizing the cost of processing plastic waste globally, there remain concerns that it may promote environmental pollution associated with improper waste processing in importing countries. We also highlighted the necessary measures to promote both principles, such as building a traceability system and transferring technology in both directions between the developed and developing countries.

We proposed that an international manifesto system which tracks the movement of plastic waste in importing countries is an effective way for buiding a traceability system and ensuring appropriate plastics waste reatment. The exporter issues a control sheet called a manifesto together with the waste to the importer (transporter). The importer describes in the manifesto when, by whom, and how the waste was transported and processed. The importer must return the manifesto to the exporter within a certain period of time. In order to ensure the accuracy of the contents of the manifesto, export companies or third-party organizations should conduct regular audits. Such manifesto system will greatly help ensure international traceability of plastic waste. The international manifesto system is our research contribution for global plastic waste treatment. Further research is required to identify the means to realistically advance in both principles.

In addition, we discussed the necessity of global technology transfer. Especially research on the use of plastics in the field of construction that is actively being conducted in developing countries. Although there are criticisms that the use of plastic waste in the construction sector is not circular, considering the economic efficiency and environmental improvement effects associated with using recycled plastics in the construction sector, plastic waste that is not suitable for recycling can be used in construction. Even in developed countries, the use of such plastics in the construction sector has a certain rationality. For this reason, the technology transfer (reverse innovation) of research in this field from developing to developed countries should also be actively promoted. In the theory of international cooperation, technology transfer in both directions between developed and developing countries is essential for realizing proper plastic waste treatment and recycling systems as well as to promote a circular economy.