# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 2 Microplastics Types and Properties  
Plastics is a term, derived from Greek plastikos--meaning fit for molding [34], commonly used for a wide range of (semi-)synthetic polymers of high molecular weight. The most common types are: polyethylene (PE), polypropylene (PP), polyvinyl-chloride (PVC), polystyrene (PS), and polyethylene-terephthalate (PET) (Figure 1) [6; 35], possessing specific physical, chemical, mechanical, optical, and electrical properties (Table 1). Applications of polymers are determined, but not limited, by their properties. Applicative properties can be enhanced by the addition of accelerants, cross-linking additives, UV stabilizers, antidegradants, antioxidants, antiozonants, photosensizers, surfactants, pigments, flame retardants, or plasticizers with biocidal additives [9]. Generally, plastics consist of crystalline and amorphous phases, affecting mechanical properties such as strength and elasticity. At a lower amount of amorphous phase, crystallinity and density increase. With the increase in the latter property, the elastic modulus, tensile strength, stiffness, and surface hardness also increase, but impact strength decreases [43]. PE is homopolymer consisting of long hydrocarbon repeated chains of ethylene monomer (Figure 1) and is used in a variety of products, e.g., plastic bags; bottles for milk, water, shampoo, or motor oils; toys; food packaging like yogurt cans, margarine containers or cereal box liners; irrigation and drainage pipes; various medical and cosmetic products [38; 44]. It is not surprising that most commonly found plastics debris in freshwater ecosystems pertains to PE [6]. PE is classified as: (i) ultra-high, high or ultra-low molecular weight PE (UHMWPE, HMYPE or ULMWPE, respectively), (ii) high, high cross-linked, medium, linear low, or very low-density PE (HDPE, HDLPE, MDPE, LLDPE, LDPE or VLDPE, respectively), or (iii) chlorinated PE [38]. PE is a good electrical insulator with \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Types of Plastic** & **PE** & **PP** & **PVC** & **PS** & **PET** \\ \hline Density & 0.91–0.97 [9; 36], & 0.85–0.94 [9] & 1.38 [9], & 0.96–1.05 [9], & 1.34–1.39 [9], \\ (g cm\({}^{-3}\)) & 0.91–0.92 & 0.89–0.92 [36], & 1.30–1.58 [36], & 1.04–1.05 [36], & 1.29 [36], \\  & (23/4 °C) [37] & 0.94–0.97 & 1.35–1.45 & 0.90–0.91 & 1.03–1.09 \\  & (23/4 °C) [37] & (23/4 °C) [37] & (23/4 °C) [37] & (23/4 °C) [37] & (23/4 °C) [37] \\ \hline Melting & 90–110 [38], & & & none (atactic), & \\ point (°C) & 98–130 [36] & 168–175 [36] & 115–245 [39] & 240 (isotactic), & 245 [36] \\  & & & & 270 (syndiotactic), [40] & \\ \hline Glass transition & \(-\)10 [36], & 75–105 [36] & 74–105 [36], & 73 [36], & \\ temperature (°C) & \(-\)25 [36] & \(-\)20 [41] & 80–85 [41] & \(>\) 80 [41] & 70 [41] \\ \hline Tensile strength (MPa) & 8–32 [36] & 31–41 [36] & 41–52 [36] & 36–52 [36] & 48 [36] \\ \hline Crystallinity (\%) & 45–95 [9], & 50–80 [9], & High [9], & & & High [41], \\  & 50 [37] & 65 [41], & 5–15 [41], & 0 [37] & 30–40 [42], \\  & 50 [37] & 0 [37] & 0 [37] & & 0–50 [37] \\ \hline Lifespan (year) & 10–600 [37] & 10–600 [37] & 50–100 [37] & 50–80 [37] & 450 [37] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Physical and chemical properties of most used plastics. Figure 1: Chemical structures of: (**A**) polyethylene, (**B**) polypropylene, (**C**) polyvinyl-chloride, (**D**) polystyrene, and (**E**) polyethylene terephthalate. low hardness, rigidity, and melting point, and high ductility and impact strength (Table 1). It has a lower density comparing to water (Table 1); thus, it is uncommon to be present in deep layers of aquatic ecosystems [6]. PE is very persistent in the environment due to its non-reactive C-C and C-H bonds, high molecular weight, hydrophobicity (allowing sorption on sediments or activated sludge) [6], and lack of functional groups that can be \"recognized\" by microbial enzymatic systems [45].