**3. Degradation pathways**

Plastic wastes undergo environmental weathering resulting in the formation of microplastics or even smaller fragments of nanoplastics. These degradation pathways can be classified into abiotic and biotic processes [62–64].


#### **Table 1.**

*Properties and common applications of different types of plastic found in the marine environment [55, 56, 58–61].*

## **3.1 Abiotic degradation pathway**

Abiotic factors include mechanical forces that are responsible to damage the plastic wastes physically, temperature increase (thermal degradation), chemical degradation, and light irradiation (leading to photodegradation) [12].

#### *3.1.1 Mechanical degradation*

Mechanical degradation refers to the action of external forces caused by wind current, ocean waves, or physical wear and tear resulting in breakdowns of plastics [12]. Plastic litters on coastal shorelines are exposed to collision and abrasion with beach rocks and sands as a consequence of motion caused by wind and ocean circulations. In the colder zones, repetitive freezing and thawing of ice can cause the degradation of the plastics accumulated in the ice and eventually result in their flow back into the marine habitat [65, 66]. One example of mechanical degradation of plastic is wear

#### **Figure 1.**

*Characterization of microplastics based on source, size, type, and shape.*

and tear on the road as a result of friction caused by the moving car tires [67]. Tire, road, and brake wear happens because of the mechanical forces being exerted on the brake pads, tire threads, and the road surface, resulting in material stressing and fatigue [68].
