Introduction

The reduction of making waste in the world and the return of recyclable materials is highly focused by researchers, and due to the increasing need of the international community, attention to solving this crisis is more than before, such that the proposed new methods pay more attention to the economic aspect (AliAkbari et al. 2020). Today, plastics are an integral part of our modern life and have been widely used due to their low cost of production, ductility, molding in different sizes, and some other unique properties (Barnes et al. 2009; Jambeck et al. 2015; da Costa et al. 2016; De Sá et al. 2018). Their applications include packaging, agriculture, electronics, and construction (Idumah et al. 2019). Presumably, the main reason for the plastic waste crisis is their very long life cycle, which makes it necessary to recycle or reuse them (Li et al. 2019a; Li et al. 2019b). The production of polymers in 2018 is reported to be around 359 MTs, and it is predicted that in the next 30 years, the production of these materials will triple (Lebreton and Andrady 2019; Tournier et al. 2020). It is estimated that PET accounts for 18% of the world production (Leng et al. 2018) and 7.4% of European plastics production (Europe 2018). A small amount of this PET waste is recycled, and the rest is left without recycling, regardless of their destructive effects (Chinchillas-Chinchillas et al. 2020). Also, about 1 million plastic bottles are wasted every minute and are estimated to double in the next 20 years (Magnier et al. 2019), and that many of these bottles are made of PET. As the high level of concern about PET waste becomes clear, it is necessary to apply global laws and mechanisms to reduce the pollution of these plastics. Maybe the most important concepts in this regard to reduce plastic waste are the life cycle assessment (LCA) and circular economy (CE) (Lonca et al. 2020), which we will explain in the next paragraph about this concept. Figure 1 summarizes the applications of plastics, production, general classification, and the concept of LCA.

Fig. 1
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A summary of the applications of plastics, their production, general classification, and the concept of LCA

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Recycling non-degradable plastics significantly reduces this waste environmental damage by reducing the accumulation in the environment and reducing the need for oil mining (Bataineh 2020a). Life cycle assessment (LCA) is an objective process for assessing the environmental effects associated with certain products, processes, or activities that are performed in the process of recycling PET waste, and shows the recycling results in a significant reduction in biological impact, reducing greenhouse gas (GHG) emissions, and fossil fuel consumption, and eventually, compared with other PET disposal schemes (Saleh 2016; Zhang and Wen 2014; Nakatani et al. 2010; Gomes et al. 2019). High recycling rates lead to high net environmental benefits, so the use of PET waste to product manufacture is increasing that social and environmental values have led to this growth (Foolmaun and Ramjeeawon 2013; Zhang et al. 2020). LCA is an effective method for environmental and economic analysis and management if combined with life cycle costing (LCC) analysis. By considering the systematic quantity of inputs and outputs of targeted products and processes, LCA and LCC can significantly help improve decision-making, products, and policies (Hong et al. 2018; Ye et al. 2018). About the concept of circular economics of plastics and especially PET, this theory should link dynamic research with the prediction of social, environmental, and economic consequences. Also, this theory should provide rational solutions to current misguided policies, and achieve a successful circular economy about to plastics, and studied possible complementarities between chemical and mechanical recycling properties. Fortunately, in recent years, many models have been proposed to achieve the stated goals related to PET (Cámara-Creixell and Scheel-Mayenberger 2019; Majumdar et al. 2020; Sardon and Li 2020; Shi et al. 2020; Velásquez et al. 2020; Bora 2020).

PET production reaction is carried out by ethylene glycol and terephthalic acid or dimethyl terephthalate monomers (Bai et al. 2020). During this reaction, poly-(ethylene terephthalate) is produced as the main product and water/methanol as the byproduct. The reaction is accelerated in the presence of a suitable catalyst (metal oxide or acid). Because the reaction is reciprocating, according to the Le Chatelier principle, by removing the byproducts under vacuum and high temperature, the reaction can be inclined towards polymer production. The reaction steps are as follows: (1) Due to the reaction of monomers, a low-viscosity pre-polymer is produced. (2) Polymer viscosity increases through a melt phase of additional condensation-reaction (> 280 °C). (3) Under vacuum, the esterification reaction products such as H2O or ROH and further monomers are removed. (4) The melt is expelled into PET pellets (low viscosity). (5) Further condensation is done during a solid-state post-condensation (SSP) mechanism that makes crystalline pellets (Duh 2002; Mendiburu et al. 2020; Wang et al. 2019; Welle 2011; Ravindranath and Mashelkar 1984). PET is produced in 4 commercial grades, which are fiber (textile, and technical and tire cord), film (biaxially oriented PET film, and sheet grade for thermoforming), bottle (water bottles and carbonated soft drink), and monofilament (Gharde 2020; Naz et al. 2020; Bethke et al. 2020; Anjum et al. 2020). A schematic of the PET production and the processes performed can be seen in Fig. 2.

Fig. 2
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PET production

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Commonly, the technology of recycling can be categorized into 4 classes, namely primary, secondary, tertiary, and quaternary approaches (Kumar 2020). Product recycling back into the first state is primary recycling or closed-loop recycling. When the recycled product has less physical, mechanical, and chemical properties and even new applications, it is secondary recycling or open-loop recycling. If the process of recycling is done by pyrolysis, gasification, and hydrolysis and waste change to simple chemicals or fuels, it is tertiary recycling. When the heat energy from the incineration of solid waste materials is used in the recycling process, it is quaternary recycling (Esi and Baykal 2020). Recycling post-consumer waste PET bottles and conversion to recycled PET (R-PET) fibers are secondary recycling (Ronkay et al. 2020).

Generally, there are two methods for PET recycling: mechanical and chemical. In general, in the chemical method (16% of recycling), the reverse of the polymerization reaction, i.e., depolymerization, occurs and the primary monomers are obtained (Al-Sabagh et al. 2016; Scremin et al. 2019; Dębowski et al. 2019). Chemical recycling is less used because it causes destructive changes in properties such as mechanical, thermal, and electrical conductivity (El Essawy et al. 2017). The mechanical method, which accounts for 84% of recycling, includes collection, sorting, washing, and shredding (Ragaert et al. 2017; Maris et al. 2018), and due to the disadvantages of chemical recycling, mechanical recycling and products from mechanical recycling use are the best solution for managing this waste (de Lima et al. 2020). Also, incineration and landfill are also two unprincipled methods to prevent the accumulation of PET in some areas, which imposes high environmental damages. Incineration of PET releases large amounts of greenhouse gases and toxic substances into the atmosphere, which is contrary to the goals of a low-carbon economy (Zander et al. 2018; Zheng and Suh 2019; Song and Hyun 1999).

Given the environmental hazards of PET waste and the acceptable performance of mechanical recycling, we want to study on PET mechanical recycling in recent years. The difference between this review article and with most of the studies in the use of recycled fibers is that this article is specifically dedicated to recycled PET fiber applications, and our aim in this paper is to elucidate the high potential of recycled PET fibers in various products.

Products Made from R-PET Fibers

The polyester used in bottles can also be applied to produce fibers, especially filament yarn, although this is a new issue and has become an interesting topic for environmentalists (Abbasi et al. 2020). Bottles and containers of PET by a mechanical process change to fibers and other products which is a simple, cost-effective, and environmentally friendly process (Albini et al. 2018). Figure 3 shows the products that are outcome by PET recycling.

Fig. 3
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Recycled PET products

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In the recycling process, small flakes from the bottles go into the dryer and after drying, they enter the extruder, then after the extrusion process, they turn into yarn and fabric (Montava-Jorda et al. 2020). Doan et al. (Doan et al. 2020) prepared R-PET fibrous membrane by electrospinning and applied them as an oil-water separator. Nonwoven fabrics, air filters, and smoke filters are another product from different applications of recycled PET fibers. Because of increasing worries about environmental air pollution, filtration is one of the best applications for ultra-thin R-PET fibers. R-PET nonwoven fabrics due to their porous structure, mechanical properties, and low cost of production are used in dust filtration (Strain et al. 2015). Also, the flakes can be cut and used to reinforce concrete. Recycled PET fibers have the potential to replace virgin PET (V-PET) fibers, and further research can uncover the potential of these fiber applications. For this purpose, in the following, we reviewed three applications of recycled PET fibers.

Concrete

The construction sector and cities in Europe are responsible for 50% of greenhouse gas emissions, and the cement and steel industries account for 10–12% of total greenhouse gas emissions (Favier et al. 2018). In recent years, new approaches to building materials have been developed to reduce greenhouse gases (Zhao et al. 2020; Rasmussen et al. 2020). About cement production, low-carbon approaches can reduce greenhouse gas emissions by up to 80% (Giesekam et al. 2016). The use of recycled materials in concrete production can greatly contribute to the goals of low carbon and circular economics (Nasr et al. 2020); it is also important to reduce the cost of concrete production (Mariri et al. 2019). One of the techniques to reduce the hazardous impact of PET waste on the environment and reduce the costs of building material is to recycle it as a building material as an alternative to sand or fibers added to concrete (Adnan and Dawood 2020). Recycled fibers used in concrete can be prepared in two ways: (1) after collecting used bottles, they are washed, dried, and cut to specific dimensions (de Luna and Shaikh 2020). (2) Pellets R-PET bottles are melted, 20–100 fibers are then extruded from the nozzle, and are drawn into fibers (Ochi et al. 2007). Finally, the fibers produced in both methods are used in the concrete production stage.

The data in Table 1, which are collected in connection with the use of recycled PET fibers in concrete, provide information about the physical and mechanical properties of concretes containing these fibers.

Table 1 Addition of recycled PET fibers to cement and concrete
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Concrete has relatively low tensile strength, low ductility, and is prone to cracking (De Silva and Prasanthan 2019). The information in Table 1, in addition to confirming the improvement of the mechanical and physical properties of concrete, in the presence of PET fibers, clarifies the fact that the fibers prevent cracking or expansion of cracks. It has also been proven that the presence of fibers prevents corrosion processes in reinforced concrete structures. It can be said that the main function of fibers is to improve the strength and durability of the structure by preventing the formation of microcracks that occur naturally in the early stages of the life of the structure, which is known as the “sewing effect” (Foti 2019). In general, concerning the studied parameters, PET fibers increased flexural strength, ductility, and unconfined compressive strength (UCS) and decreased drying shrinkage of mortar, total porosity, and compressive strength. This technique is attractive, both economically and environmentally. Also, the information presented in Table 1, that was collected from recent studies, confirms the improvement of the physical and mechanical properties of the resulting concrete. The important effects of PET fibers on concrete can be seen in Fig. 4 comparatively. Given the attractive advantages mentioned, it can be hoped that recycled PET fibers will be widely used in concrete. Of course, to achieve this goal, in the future, efforts must be made concerning the following: esthetic improvement of these building materials, better physical and mechanical performance, reduction of production costs, more attention to research and development units, and so on.

Fig. 4
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Comparison of increasing and decreasing properties of recycled PET fibers on concrete

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Nonwoven

Various definitions have been given for nonwoven fabrics (Karthik and Rathinamoorthy 2017a). Nonwoven fabrics, unlike woven fabrics, are made directly from short or long fibers by various methods; for example, one of the general and traditional nonwoven fabrics is felt (Müller and Saathoff 2015; Jhang et al. 2020). The structural differences between nonwovens and woven fabrics are shown in Fig. 5 (Karthik and Rathinamoorthy 2017a). Nonwovens can be produced with 3 processes: dry-laying, wet-laying, and extruded polymer-laying (Pourmohammadi 2013). The raw materials for the dry-laying method are textiles; for the wet-laying method they are paper materials; and finally, in the extruded polymer-laying method, melted plastics are used (Durga and Kalra 2020). Spunbonded nonwovens are mainly produced in 3 steps: (1) filament spinning, (2) web formation, and (3) web bonding (Ding et al. 2020). To use recycled PET bottles in the extruded polymer-laying method of nonwoven production, first, the used bottles are cut into flakes, and after washing and drying, they are transferred to the extruder, and the melted PET is used to produce nonwoven fabric in this method. In Fig. 5, you can see the differences between knitted, nonwoven, and woven fabrics.

Fig. 5
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The structural differences between nonwoven and woven fabrics

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Fusible nonwoven fabric has been so popular since the mid-twentieth century that it accounted for 80% of the market in the early twenty-first century. At first, these products were connected with the help of binders, and then in the 1960s, the first binder-free nonwoven was produced. In the 1970s, the production of nonwoven began in Kaiserslautern and the industry gradually expanded around the world (Karthik and Rathinamoorthy 2017b). One of the attractions of producing nonwoven fabrics versus woven fabric is its low production cost (Jeon 2016). The other is the ability of nonwoven fabrics to be expanded, followed by the expansion of the market for nonwoven fabrics (). The applications of nonwoven fabrics are very wide, such as sound absorber (Özkal and Cengiz Çallıoğlu 2020), apparel (Anderson 2005), medical textiles (Mothilal et al. 2019), automotive textiles (Atakan et al. 2018), filters (Chauhan et al. 2019), sanitary masks (Opálková Šišková et al. 2020), and packaging (Lin et al. 2018).

The main chemical fibers used in the production of nonwoven fabrics are rayon viscose, polyester, polyamide, and polypropylene. PET is the main raw material of fiber polymers and recently microfiber nonwoven added to the applications of PET (Albrecht et al. 2006). Today, it can be said that all PET waste is recycled. Three quarters of the total production costs of PET fibers are related to its raw materials; so one of the main ways to reduce the production costs of these fibers is to use recycled PET materials (Altun and Ulcay 2004). The same is true of the fibers used in nonwoven fabrics. Hence, from an economic and environmental point of view, the use of recycled PET in the production of nonwoven fabrics has attracted a lot of attention. In Table 2, we summarized the recent researches on the producing nonwoven fabrics from recycled PET fibers.

Table 2 Use of recycled PET fibers to produce nonwoven fabrics
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Based on the results in Table 2, nonwoven fabrics prepared from recycled PET can be applied to filter fabrics, automotive interiors, vehicle seat cover, flexible stab-resistant hybrid fabric, etc., and lead to reduce environmental pollution. Also, due to reducing the cost of raw materials in the production of these products, the production cost is reduced. As can be seen from the comparison chart in Fig. 6, the use of PET recycled fibers can bring attractive benefits to manufacturers.

Fig. 6
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Comparison of increasing and decreasing properties of recycled PET fibers on nonwoven

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Yarn

From 1998 to 2013, the consumption of textile fibers per person enhanced by approximately 1.5 times, and by 2050 will be twice. Nearly 63% of the textile fibers are made from petrochemical materials, and polyester is the most popular fiber in the textile industry (Majumdar et al. 2020). Recycling reduces the storage and transportation of wastes and makes new economic and environmental trends (Sarioğlu et al. 2020). In the last years, R-PET production has enhanced dramatically, but just 30% of PET bottles were recycled. R-PET fibers are 20% cheaper than other ones with similar physical properties (Abbasi et al. 2020). Therefore, their use in the textile industry, that has a major role in the trade of any country, has been considered. Recent studies on the use of recycled PET in yarn production are shown in Table 3. The recycling process of polyester staple fibers from the post-consumer PET bottle was indicated in Fig. 7.

Table 3 Use of recycled PET fibers to produce yarn
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Fig. 7
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Process of producing recycled polyester fibers from R-PET

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There is little literature on the properties of R-PET spinning and yarn production (Abbasi et al. 2020). In Table 3, we summarized the research conducted to produce yarn and fabric from recycled PET.

According to the results in Table 3 and by comparing mechanical and thermal properties of R-PET and V-PET for yarns, it can be concluded that the use of recycled PET yarns in the production of more stable and environmentally friendly fabrics will be useful. Due to the cost-effectiveness of these materials, studies conducted in this path can be attractive for the textile industry with different applications. If necessary, these fibers can be blended with other polymers and create the required properties of each application. Figure 8 shows a comparison of the advantages and disadvantages of using recycled PET in yarn production.

Fig. 8
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Comparison of advantages and disadvantage of recycled PET fibers on yarn

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Summary and Outlook

Integrated recycling systems for plastics are essential, especially in situations where export and landfilling are not available (Sheldon and Norton 2020). In Fig. 9, you can see the price differential in the form of virgin PET minus recycled PET flakes based on S&P Global Platts’ reports (Platts, and P. Global, S&P Global Platts 2018). This chart shows the approximate price difference between virgin and recycled PET from 2008 to 2019, and surprisingly in 2019, the price of recycled PET is higher than the virgin one. This is because of the easier access to virgin PET. The high volume of extraction of gas and light petroleum liquids, as well as greater access to new ethylene crackers, has led to a drop in the price of virgin PET, but on the other hand, the ban on imports of Chinese mixed waste in 2018 has caused a relative shortage of recycled PET (Lee 2019; Sears n.d.). Assuming the price of virgin PET is lower, using recycled PET is recommended because, despite the lower cost of materials used, the use of virgin PET brings more environmental and health costs (Engel and Scott 2020; Dubé et al. 2020; Lourenço et al. 2020).

Fig. 9
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Approximate price difference in the form of virgin PET minus recycled PET flakes

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PET has the ability to be recycled over and over again by washing, drying, and melting and using it in the production of new products (Gu et al. 2020). Simon et al. (Simon et al. 2016) examined the life cycle impact of beverage packaging systems and, after reviewing 20 times PET recycling, concluded that the first seven cycles caused a considerable decrease in GHG emission, but the further enhancement in the number of recycling does not yield considerable environmental advantages.

The life cycle assessment (LCA), by evaluation of energy and material consumptions, emissions in the environment, and disposal of wastes, can be a helpful way to the determination of the potential advantages of recycling works (Martin et al. 2020). Many studies have been published on the LCA of recycling post-consumer PET and have reported that better environmental gains can be achieved from mechanical recycling compared to landfill and incineration with energy recovery (Wäger and Hischier 2015; Wäger et al. 2011; Al-Maaded et al. 2012). Bataineh (Bataineh 2020b) studied the LCA of recycling post-consumer PET and showed that the total energy requirements for the recycled PET flake are 14–17% of the virgin PET flake. The life cycle impact difference between resin made of recycled PET and virgin PET mainly is the result of reducing virgin PET production (Ren et al. 2020). Finally, it can be concluded that PET recycling presents more considerable environmental benefits than single-use virgin PET and can improve eco-efficiency (Mahmud and Farjana 2020).

In this review article, the goal is to summarize recent studies on the practical results of returning post-consumer PET to the production cycle. To implement the macroenvironmental goals, it is very important to pay attention to economic issues. In this review, both aspects, economic and environmental, are discussed, because as mentioned in the previous sections of this review, relatively high value-added products were obtained from a wide range of applications of recycled PET fibers, which in most of the work done did not show a decrease in the quality of the product or even improved the quality with special techniques used.

In three separate sections, the use of recycled PET fibers in the production of concrete, nonwoven fabrics, and the yarn is discussed. In the concrete sector, PET recycled fibers are used without heat treatment and special process, but in the manufacture of nonwoven fabrics and yarn, recycled fibers need to be heat-treated in a special process of production of these two categories of products.

To improve the properties of products or to obtain new properties in recycled PET fibers, additives must be used or blended with another polymer (Chinchillas-Chinchillas et al. 2020; Deng et al. 2019; Sarioğlu et al. 2020; Thomas and Moosvi 2020b). Therefore, in the future, it may be possible to get better properties from products made from recycled PET fibers with new and better auxiliary compounds or to increase the amount of use of recycled PET fiber without lowering the quality, which is in line with economic and environmental goals. Also, the use of modern and innovative process techniques can further expand the industrialization of these products.