**1. Introduction**

Microplastics (MP) are defined as smaller pieces of synthetic plastic polymers, which are between 1 μm and 5 mm [1, 2]. MPs are used in many areas such as personal care products and synthetic textile products. However, MPs, also formed by the breakdown of plastic waste in the environment, are a global concern in today's world, where they are ubiquitous in all environmental environments [3]. They are commonly found in air, soil, and water [4–6]. MPs are highly discussed due to their adverse effects on the ecological environment, social economy, and human health. However, despite their

negative effects on the environment, they are frequently used in fields such as medicine and industry. MPs are divided into two groups as primary and secondary [7]. The primary group is the industrial production of plastic microbeads of different sizes. These products are widely used in areas such as raw materials in the manufacturing industry, personal care products, and sandblasting media [8, 9]. The secondary MPs are formed both during the use of products and when plastic wastes are broken down into smaller sizes depending on weather conditions that are exposed to light, heat, and mechanical stress [10]. However, the properties of plastics such as thermal, mechanical, electrical, and physical may vary depending on the production processes and the raw material used [11, 12]. Therefore, their mechanical and chemical properties may vary. Zhang et al. [13] reported that the glass transition temperature (Tg), melting temperature (Tm), and tensile strength values of the polylactic acid (PLA) filament, which was purchased from the NatureWorks company, were 61°C, 153°C, and 84 MPa, respectively. Olam and Tosun [14] showed that Tg, Tm, and tensile strength values of PLA, which was purchased from the PLA Max company, were 66°C, 160°C, and 53 MPa, respectively.

Plastics are used effectively in almost all industries, including construction, packaging, textiles, transportation, education, electricity, electronics, consumer products, and industrial machinery [15, 16]. Since the use of plastics is increasing day by day, their production is also increasing [17, 18]. According to ASTM D883 80c, plastics are divided into two groups; they are thermoset plastics and thermoplastics according to their chemical and mechanical properties [19, 20]. Thermoplastics are resins that liquefy when heated and harden when cooled [21]. Thermosets, on the other hand, do not reform under heat and pressure after they are produced [22]. Thermosetting plastics are alkyds, amine, silicones, allylics, phenolics, epoxies, urethanes, and polyester [23]. Thermoplastics are polypropylene (PP), polyamide (PA), polycarbonate (PC), highdensity polyethylene (HDPE), low-density polyethylene (LDPE), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), polystyrene (PS), polyether ether ketone (PEEK), polyphenylene sulfide (PPS), polyetherimide (PEI), polyamide imide

**Figure 1.** *Distribution of the global plastics production by type in 2021 [27].*

*Mechanical and Thermal Properties of HDPE/PET Microplastics, Applications, and Impact… DOI:http://dx.doi.org/10.5772/intechopen.110390*

(PAI), acrylic (PAA), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), polyvinyl chloride (PVC), polyurethanes (PUR), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (Teflon) [24–26]. PP, LDPE, PVC, HDPE, and PET constituted significant proportions in the world plastic production in 2021 (**Figure 1**).

HDPEs and PETs, which have a significant usage rate today, have an important ratio among the microplastics existing in the environment. The presence of mHDPE and mPET is commonly detected by scanning electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy [28, 29]. Removal of MPs is mostly done by coagulation, filtration, adsorption, oxidation, and photocatalysis methods. However, these methods are rarely used [30, 31]. There is a need to increase the prevalence of the use of the methods and to develop new methods. Due to their small size, MPs can easily enter the environment and living organisms through air and contact [32]. If these wastes are disposed of, it is obvious that it will have many negative effects on the environment and life.
