**3.1 Properties of PET**

PET is a thermoplastic with high resistance to most solvents, weak acids, and bases; strength; gloss; high impact resistance; and hardness [73]. It is also resistant to many other chemicals such as hexane, methanol, sulfur dioxide, and solutions with low acid concentrations [74–76]. The physical and chemical properties of PET are examined in **Table 3**; it is a valuable hydrocarbon containing 62% C, 4% H, and 34% O and has a high calorific value. PETs are self-extinguishing and have very low gas permeability, good adhesion and weldability, high hardness, good refractive index, and good resistance to ultraviolet [89]. They have very low O2 and water permeability


#### **Table 3.**

*Physical and thermal properties of PET.*

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

compared to most plastics (PS, PVC, etc.) [97]. The thermal conductivity of PET is also very low compared to PLA, ABS, HDPE, PP, and PA. Although it is a fairly transparent and colorless material, it usually appears opaque and off-white as the thickness increases. Depending on the thermal and process conditions, it can be found in semicrystalline or amorphous form. Because of this, it may appear dull, white, or glassy. It depends on process parameters such as crystal structure, processing temperature, cooling rate, and stretching. Crystallization is an important parameter that affects the properties of polymers. The benzene ring in the main chain of PET provides both slow crystallization during cooling and hardness. This adversely affects the spinning process of high-speed fibers [98]. Glass transition temperature (Tg) is an important value that affects the material properties and potential applications of a polymer [99]. Mechanical properties (deformation modulus, etc.) and physical properties (density, volume, specific heat, etc.) of polymers in glass transition state are not. Due to the Tg temperature, PET loses its glassy property and becomes viscous at 67–80°C; its cold crystallization (Tcc) is in the range of 115–140°C, melting temperature is 248–250°C, and enthalpy of melting (∆Tm) is 35–50 J/g; its hot crystallization temperature (Tc) is 194–205°C, and its enthalpy (∆Tc) is 29–55 J/g (**Table 3**) [100]. PETs begin to thermally degrade at temperatures above 340°C [101]. Although the melting temperature (Tm) of PET is not as high as PEEK (334°C), it is higher than that of PP (170°C), LDPE (134°C), PS (106°C), and PVC (199°C) [102].

PETs have excellent mechanical properties, creep resistance, fatigue resistance, friction resistance, and dimensional stability over wide temperature ranges (**Table 4**) [99, 110]. However, the disadvantages of slow crystallization rate, machining difficulties, high molding temperature, and poor impact


#### **Table 4.** *Mechanical and electrical properties of PET.*

performance limit their use [111]. The storage modulus of PET, which is the plastic deformation energy of a polymeric material, is 2000–4200 MPa at 25°C and 242 MPa at 80°C [112]. Tensile strength, flexural strength, Young's modulus, elongation at break, impact strength, flexural modulus, and hardness of PET are approximately 40–60 MPa, 55–100 MPa, 1000–3500 MPa, 19–46 MPa, 4.6 kJ/m<sup>2</sup> , 2000–3500 MPa, and 96, respectively (**Table 4**). PET's tensile strength (26 MPa) at 77 K is higher than PA (14.5 MPa), PC (13.5 MPa), Teflon (4.3 MPa), and PVC (9.5 MPa), and its elongation at break value is higher than theirs [113].

Dielectric is an important property of insulating materials. When an ever-increasing voltage is applied to an insulating material, it eventually reaches a point where its electrical properties deteriorate, causing a drop in resistance, and it begins to lose dielectric strength. PET shows a dielectric property of 150–200 kV/cm (**Table 4**).

### **3.2 PET applications**

PET has wide use in many fields (packaging, textile, medical, etc.) due to its satisfactory properties [114–116]. In fact, PETs that have completed their useful life (waste) are ground to micron sizes and used as concrete additives [117]. Micro PETs are generally produced and used in spherical and fibrous structures [118]. They are used in protective fabrics, filters, wound dressings, drug delivery, and scaffolds [119–121]. Because of PET's properties such as biocompatibility, high uniformity, mechanical strength, and chemical resistance, it has been successfully used in vascular prostheses for large vascular grafts and in various surface modification methods to improve the cell adhesion properties (**Figure 4a**) [127]. PETs are also used in a variety of biomedical applications such as implants, heart valves, sutures, scaffolds, surgical nets, and urinary and blood circulation catheters [122, 128]. Micro PET fibers are the most widely used synthetic fibers in textile yarn production, and their consumption is expected to be 50 Mton/year by 2050 (**Figure 4b**) [129, 130]. However, although PET's usage has become widespread in many special applications (**Figure 4a** and **b**), it is mostly used in food and beverage packaging (**Figure 4c** and **d**) [131].

**Figure 4.**

*PET applications (a) vascular prostheses [122, 123] (b) carpet [124] (c) beverage bottles [125] (d) food packaging [126].*

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

### **4. The formation of mHDPE and mPET in nature**

HDPE microplastics (mHDPE) and PET microplastics (mPET) are discharged into the environment in two ways [132]. The first of these is that they are fabricated by producers in <5 mm dimensions in the form of granules, powders, or pellets for later use in applications [133]. During the production process and during their use in the application areas, they are discharged into the environment through the disposal in air, wastewater, and other waste. The second is formed by the degradation of HDPEs and PETs under environmental conditions, which have reached the end of their useful life and are collected directly into the environment as waste or in municipal waste collection areas [134]. In nature, HDPE and PETs are resistant to chemical degradation and take decades for theenvironmental residues to decompose completely [135]. This degradation is the reduction of molecular weight as a result of chemical changes in the structure of the polymer [136]. Waste HDPE and PET exposed to sunlight undergo photooxidation as a result of the absorption of high-energy wavelengths of the ultraviolet (UV) spectrum [137]. As long as decomposition continues in the presence of oxygen, temperature-dependent thermo-oxidative reactions may occur. In addition, degradation may occur due to both biological and mechanical stresses. As a result, these decomposed HDPE and PET wastes become brittle and gradually break up into smaller pieces of micron size [138].

Photodegradation is one of the common degradation processes of polymers, which provokes cross-linking and chain scission reactions [139]. In the photooxidation mechanism that occurs during the UV-irradiation period, first carbonyl groups are formed; then, vinyl and hydroxyl/hydroxyperoxide groups are formed, and these chemical changes can be observed by FTIR spectroscopy [140, 141]. In the FTIR spectra of HDPE, its peaks are occurred at 3300–3600 cm−1 of hydroxyl groups (∙OH), 1700–1800 cm−1 of carbonyl groups (>C〓O), and 1600–1650, 989 and 908 cm−1 of vinyl groups (∙C〓C∙) [142, 143]. The effect of UV irradiation on polymers, carbonyl, vinyl, and hydroxyl groups is studied as an indicator of polymer basic scission [144]. Since the main photooxidation product groups of HDPE are carbonyl and vinyl, the effect of UV radiation can be examined by looking at the carbonyl index and vinyl index [145]. When HDPEs in thin film form are exposed to UV irradiation at 280 nm at a light intensity of 500 W/m<sup>2</sup> at 25°C and 50% constant relative humidity, the carbonyl and vinyl indexes increase over time [139, 142]. After 30 days, the carbonyl and vinyl indexes of HDPE increase significantly [61, 140, 144]. As a result, it can be said that waste HDPEs, which are exposed to UV irradiation in nature, start to decompose after 30 days and turn into smaller particles. However, the carbonyl index of PET does not change significantly over time [146]. It can be said that PET is more resistant to UV irradiation than HDPE. In short, waste PETs take more time to decompose in nature by UV irradiation compared to HDPE.

The degradation of plastics, which exist as waste in nature, by using microorganisms is of great interest. Biological agents (bacterial and fungal species) and their metabolic enzymes, which are abundant in nature, have different degradation abilities for natural and synthetic polymers [147]. The biodegradation process of HDPE in nature is quite slow [148]. Therefore, it is necessary to recover HDPE wastes by appropriate methods such as physical, chemical, and biological processes [149]. Moog et al. [150] highlighted the potential for biodegradation of waste PETs and stated that PET substrates under varying conditions have enzyme functionality, secretion, and production of recombinant proteins. Shabbir et al. [151], in their experimental

study with PE, PET, and PP microplastics, stated that there was weight loss in all MPs, indicating structural, morphological, and chemical changes, and confirmed this situation with SEM and FTIR analyses. Farzi et al. [152] investigated the biodegradation of mPETs by streptomyces bacterial species at laboratory scale and stated that the degradation is slow compared to physicochemical methods, and additional physical/ chemical methods should be applied to increase the degradation rate. The biodegradation of 500 m particle size HDPE under laboratory conditions is approximately 10% in 24 days [153].

Mechanical degradation of polymers is typically limited to chain scission [154]. As given in **Tables 2** and **4**, due to the good mechanical properties of PET and HDPE such as tensile strength and elongation at break, they are slow to decompose as waste by mechanical forces in nature.

However, although the mechanical, biological, and chemical degradation times of mPET and mHDPE, which exist as waste in nature, are long separately, this time becomes shorter when it is considered that they occur together. As the use of PET and HDPE increases day by day, it creates more waste in nature, and accordingly, more microplastics are formed.

### **4.1 The effect of mHDPE and mPET on nature and life**

Plastics are the most useful synthetic polymers used in packaging industries, agriculture, household applications, and many similar applications [155]. The unpredictable use of these synthetic polymers leads to an ever-increasing accumulation of solid waste in the natural environment [156]. This causes soil and water pollution at alarming rates, affecting the natural system and creating various environmental hazards [157]. Plastics, which are resistant to environmental influences, are seen as an environmental threat. MPs, both leaching into the environment in micron sizes during the production and usage processes by the manufacturers and occurring as small-sized plastic particles by the degradation of plastics found as waste in the environment, can lead to potential ecotoxicological effects [158, 159]. In general, the densities of microplastics found in nature vary in the range of 0.8–2 gcm−3. The microplastic particle has the average weight of 12.5 μg, volume of 0.011 mm3 , and density of 1.14 gcm−3 [160]. Therefore, MPs can enter the body through airborne inhalation and food intake, which exist in many places in atmospheric environments today. When inhaled, they may cause inflammation or other biological responses in the lung [161]. In addition, they can cause health effects such as genotoxicity, which is due to the desorption of pollutants associated with polycyclic aromatic hydrocarbons (PAH); reproductive toxicity, which is due to the plastic itself and additives (plasticizers, dyes, etc.); mutagenicity; and carcinogenicity [161, 162]. Khalid et al. [163] stated that there are various inorganic and organic chemicals absorbed on MPs and that this poses a greater threat to living things than to MPs. In addition, MPs affect some plant community structure [164]. Issac et al. [165] stated that PE (about 54%) is the most abundant microplastic floating in the ocean.

Cheng et al. [166] investigated the effect of HDPE (25 μm) and PP (13 μm) microplastics on earthworms (*Metaphire guillelmi*) using Nile red fluorescent staining and observed ingestion by earthworms exposed to HDPE and PP microplastics. PP microplastics significantly reduced bacterial diversity and changed the bacterial community structure in the soil. Bringer et al. [167] exposed oyster embryos to different sizes of mHDPE for 24 hours and observed that the mHDPEs bind to the locomotor eyelashes of oyster D-larvae, influencing the swimming activity and development

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

of oyster D-larvae. Kim et al. [168] conducted an experimental study using the *nematode caenorhabditis elegans* and zebra fish to determine the effects of mHDPEs on human health. They stated that it affected *caenorhabditis elegans* reproduction and zebra fish larval death at a concentration of more than 200,000 particles/mL. Jemec et al. [169] observed that mpETs, formed by abrasion and washing of textiles, were ingested by daphnia magna and increased the death rate of daphnia magna in their guts. Shen et al. [170] observed that the higher the concentration of mPETs, the more pronounced was the negative effect on *Drosophila*, with reduced egg production of the female flies and lower lipid, glucose content, and starvation resistance of the male flies. Najahi et al. [170] studied the effects of mPETs on human bone marrow mesenchymal stromal cells and adipose mesenchymal stromal cells. They found that it caused an approximate 30% reduction in proliferating cells associated with the onset of senescence or an increase in apoptosis.

Existing studies on mPETs and mHDPEs show that there is a lot of evidence that these wastes have a negative impact on living life. However, this does not cover all living things. Weber et al. [171] investigated the effect of the freshwater invertebrate amphipod *Gammarus pulex* exposed to mPETs for 48 hours and observed that the survival, development, metabolism, and nutritional activity of *Gammarus pulex* did not change significantly depending on the amount of mPET and the age of *Gammarus pulex*.

### **5. Conclusions**

Microplastics are pollutants that accumulate in large quantities in the environment with each passing day and cause significant pollution. In addition to leaking into the environment during the production and usage processes of MPs, they can be formed by the mechanical, thermal, and biological decomposition of plastics, which are discharged into the environment after the completion of their useful life. HDPE and PETs, which have significant usage and application areas among plastics, have a very long life in the natural environment due to their chemical and mechanical resistance. In the current studies, it was seen that mHDPEs and mPETs indirectly or directly affect the habitat characteristics of living things and basic ecosystem functions. mHDPEs and mPETs can affect the living organism in which they are infested directly with their function and properties, as well as with the impurities they absorb. Although mPETs and mHDPEs do not cover all living organisms in the world, they adversely affect life and the environment. However, more scientific studies are needed to predict this situation.

### **Conflict of interest**

The authors declare no conflict of interest.

*Advances and Challenges in Microplastics*
