# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 2 Microplastics Types and Properties
PP is a crystalline thermoplastic polymer with methyl-group on every subunit of polymer backbone (Figure 1); this improves its mechanical properties and thermal resistance. Linear PP has various structural isomers: isotactic, syndiotactic, and atactic. Isotactic PP has a greater degree of crystallinity than the other two; thus, it is less susceptible to biodegradation. A high degree of tacticity (iso- or syndio-) provides higher crystallinity, better mechanical properties, and chemical stability. A low degree of tacticity means lower hardness, strength, density, rigidity, stability, and melt fluidity. Commercial PP is approximately 90% isotactic with 60-70% crystallinity [43]. Comparing to HDPE, PP has some similar properties (permeability of gasses and vapor is almost the same), although differs in hardness, tensile strength, elasticity, transparency, and gloss (higher than HDPE). PP is highly resistant to water, inorganic chemicals, organic solvents, and lubricants; degradation occurs by strong oxidants [43]. PP is also a very good electrical insulator. Accordingly, it has numerous applications: as packaging material; medical and electronic equipment; furniture production; textile and automotive industries [6; 10; 38; 44; 46]. Hydrophobic surface and high molar mass of PP limit its biodegradation [37; 47]. The insertion of hydrophilic groups onto the polymer surface via physical or chemical processes (e.g., degradation) is highly required to enable the attachment of microorganisms to such partially destructed polymer surface [48]. Accordingly, microorganisms can use low-molecular-weight fragments (oligomers), dimers, or monomers as carbon and energy sources [49], providing biodegradation. However, small oligomers may also diffuse into the organism and get assimilated, providing adverse effects [37]. PVC structure (Figure 1) contains heteroatom: chlorine, on every second C-atom of polymer backbone. PVC usage in Europe for packaging only exceeds 500,000 tons annually [43; 44]. Due to its extreme versatility, PVC is irreplaceable for medicine equipment (blood bags, tubs for catheters, surgical gloves) and wall and floor coverings [46]. It is also used for construction purposes: piping, electrical cable insulation, doorframes, and windows [6; 44]. PVC comes in two forms: (i) the rigid, which is used for food packaging, pharmaceutical, and medical products, and (ii) the flexible, which is used as a film for food wrappings due to its stretchability [43]. PVC is an excellent water and oxygen barrier; thus, packaged food may last longer ensuring its taste. PVC is resistant to chemicals but very sensitive to photodegradation [6]. Since its inception, PVC is a known polymer with exceptional resistance to biodegradation, making it an environmental issue; from production to waste disposal. PS has an amorphous structure with a phenyl ring linked on every second C-atom of polymer backbone (Figure 1). It is well-known in its expanded form as Styrofoam and is used for packaging, bowls, containers, rigid trays, lids, and tumblers [43; 44; 50]. Global production of PS in 2013 was -21 MT [50]. PS is hard, stiff, durable, and transparent thermoplastic polymer, which is available in four forms: (i) general-purpose PS (GPPS), (ii) high impact PS (HIPS), (iii) PS foam, and (iv) expanded PS foam (EPS) [50; 51]. Its foamed form is lightweight (it floats on the water surface), bulky, and recyclable [51]. PS is brittle and flammable with relatively low melting temperature and with a glass transition temperature of \(>\)100 \({}^{\circ}\)C; thus, it softens in boiling water. PS is poor oxygen and vapor barrier and sensitive to UV irradiation (it yellows) [6; 43]. PET (Figure 1) is, along with PE, one of the main food packaging materials. It is very common in the textile industry as well, where is commonly used in the following forms: (i) fiber filling in insulated clothing, furniture, and pillows, (ii) fine filaments in artificial silk, and (iii) large-diameter filaments in carpets production [7; 43].