**2. Sources of polymeric recycled materials**

Being part of the European Green Deal, the new Circular Economy Action Plan points out textiles and plastics as two of the key value chains that will be addressed as a matter of priority [6]. The textiles' system is characterized by significant greenhouse gas emissions and a high use of resources: water, land and a variety of chemicals [7, 8]. Textile industry is the third consumer of plastics after Packaging and Building & Construction Industries, representing a quarter of the world carbon dioxide budget [9–11]. Every year, 80 billion new garments are produced for fastgrowing fashion industry [12], and apparel business utilizes over 97% natural and synthetic based (principally plastic) virgin materials, only 12% of which is recycled into another item after disposal [13–15]. Moreover, apparel industry represents up to 2% of worldwide oil request, and in this manner a portion of the 300 million tons of plastic is created consistently. The production of fibers, their finishing processes and the chemicals for the required functional properties add up to about 20% of worldwide modern water contamination credited to the sector [13–15]. There is no need to say that effective recycling of such wastes will bring ample environmental and economic benefits such that about 7.5 million cubic yards of landfill space, 17 million tons of CO2, and 4.2 trillion gallons of water can, for example, be saved [12, 16, 17]. In addition to these, it should also be noted that the world population of 7.6 billion people is anticipated to reach 8.6 billion in 2030, 9.7 billion in 2050 and almost 11 billion in 2100, thus resulting in a further increased consumption of textiles and apparel, and in turn that of synthetic (polymeric) fibers [18].

The cause of the plastic pollution is mostly due to the fact that plastic, being generally landfilled or incinerated, does not decompose in the environment, continually accumulating in the waterways, agriculture soils, rivers and oceans, massively contributing to global warming [10, 11], causing damages to biodiversity and ecosystem services, and leading to social and economic drawbacks [19, 20]. Plastics have tended to replace traditional materials such as wood, glass, etc. because of their lower weight, flexibility, and simple processing. They can be made from a single polymer, or multiple layers of different polymers, or other materials. Generally, plastics can be classified into petrochemical-based or bio-based, depending on the material from which they are made. Petrochemical-based plastics can be separated into thermoplastics and thermosets.

#### *An Evaluation of Recycled Polymeric Materials Usage in Denim with Lifecycle Assesment… DOI: http://dx.doi.org/10.5772/intechopen.99446*

Thermoset plastics are permanently cross-linked together and therefore difficult to be reformed whereas thermoplastics can be remelted and reformed, and thus are the most commonly used plastics in the economy [21]. Bio-based plastics are, however, derived from biomass (e.g., starch, sugar, and vegetable oils) excluding materials from geological formations or fossilized, as defined by the European Standard EN 16575 [22–24]. Bio-based plastics can be classified into two groups, namely polymers made entirely from biomass and polymers made partly from biomass [23]. Furthermore, all plastics, regardless of whether they are petrochemical or bio-based, can be designed to behave in two distinct ways: biodegradable and non-biodegradable. Biodegradable plastics can be decomposed in the environment by the activity of microorganisms (bacteria or fungi) into water, carbon dioxide (CO2), methane (CH4), and biomass (e.g., growth of the microbial population), though their biodegradability can vary based on the plastic's inherent and designed properties [25, 26], climatic and process specific conditions, and degradation speed [21, 27, 28]. Biodegradable plastics may also be compostable, which are capable of undergoing biological decomposition in a compost site as part of an available program, and the resultant breakdown fragments are completely used by the microorganisms under the certain conditions certified by the international standards ISO 17088, EN 13432 (Europe), ASTM D400, and D6868 (United States) [28, 29]. It should, though, be noted that not all biodegradable plastics are compostable, but all compostable plastics are biodegradable [30].

A slower development within the field of recycling of plastics in terms of the methods employed, created added value, properties of recycled polymers, etc., causes some problems regarding the inclusion of such polymeric materials in the economic cycle [30, 31]. Among typical examples of waste stream products, are there short-life packaging materials (bags, bottles, etc.), used goods (computers, cell phones, etc.), demolition materials from buildings, and disposables. About 1 million plastic bottles, for example, are wasted every minute and are estimated to double in the next 20 years [32].

With about 13% of the market share, textiles are an important source of manmade polymers. Nearly 63% of the textile fibers are made from petrochemical materials such as nylon, acrylic, polyester, and polypropylene, and these fibers' production and fate give rise to significant carbon dioxide (CO2) emissions [33]. From the perspective of plastics, the scope of this section is, therefore, limited to textiles made of synthetic fibers (**Figure 1**). Polyester is one of the most popular fiber in the textile industry [34], which is followed by polyamide (nylon).

#### **Figure 1.**

*Comparison of the environmental impacts of the manufacturing of 1 kilogram of dyed, woven fabric (black = worst, white = best) (adapted from [35]).*

Polyester alone had a market share of around 52% of total global fiber production, and approximately 58 million mt of polyester was produced in 2019. Polyamide, on the other hand, accounted for 5.6 million mt and approximately 5 percent of the global fiber market in 2019 [36]. In the last years, recycled PET (rPET) production has enhanced dramatically, but only 30% of PET bottles were recycled [37, 38]. In 2019, the estimated rPET share of polyester staple fiber was as high as around 30 percent whereas that of polyester filament was at around 6 to 7% [36]. Recycled PET fibers have the potential to replace virgin PET (v-PET) fibers, and these fibers can be blended with other polymers to create the required properties for each relevant application. But more research appears to be needed to uncover the further potential of rPET fiber based applications [31, 39–42].

Polyamides (PAs), also known as nylons, are other polymeric materials which are widely used in many engineering applications including textile fibers. This is due to their excellent mechanical properties, chemical resistance, wear resistance, dimensional stability, low friction, etc. and ease of processing. Unfortunately, these useful properties are also the ones causing significant environmental consequences. In fact, nylon accounts for about 10% of the debris, mostly in the form of fishing nets in oceans. Contamination is also another issue as far as nylon is concerned. This is mainly because of the fact that nylon is melted at lower temperatures, meaning some contaminants, i.e. non-recyclable materials and microbes or bacteria, can survive. Therefore, all nylon waste must be cleaned thoroughly before a recycling process [41].

As a final note, there is a number of textile companies that have successfully applied the various recycling technologies to produce commercially available raw polymeric materials as is presented in **Table 1** [43, 44]. Sportswear brands are in particular increasingly using recycled synthetic fibers. Most use rPET made from PET bottles [45, 46], but some brands work with recovered ocean plastics, recycled nylon made from discarded fishing nets, and/or with recycled elastane. Also, there are several brands looking for plant-based fibers such as lyocell, Tencel to replace polyester [47, 48].
