# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
## 5 Stabilized Exposure Materials  
Pure or blended biodiesel, as a liquid fuel, needs to be contained within tanks during production, transportation, storage, and application stages. In this case, the tanks and containers materials would be exposed to biodiesel fuel for long time. However, there is a concern about the materials in direct contact with biodiesel fuel whether they are storage tanks or auto parts in the fueling system. The acidity and FFA content in biodiesel fuel may deteriorate the containers or auto part materials over time. These deteriorating factors are either inherent from the raw feedstock or formed during hydrolysis reactions in the fuel body. With the help of heat, light, metals, and peroxides, unsaturated alkyl esters can be oxidized and formed organic acids and other deteriorating factors in biodiesel fuel. Beside wearing out tanks and auto part materials, the existence of organic acids and FFA diminishes biodiesel physicochemical properties and leads to poor performance of the fuel [17; 110; 111]. Polymers are widely used in auto parts and storage tanks because of desirable properties of durability, affordability, and stability-flexibility nature. However, some polymeric materials revealed a negative effect once they were exposed to biodiesel fuel. The incompatible contact between biodiesel and polymers results in changes in physical characteristics like swelling, chemical characteristics like the structural composition, and mechanical characteristics like hardness and softness. The physical and chemical deterioration is driven by the polymers' affinity to absorb and react with biodiesel solvent. This behavior maximizes the mass and volume of the polymeric material which is known as swelling. Consequently, the chemical structure of polymers would be affected, leading to another change in the mechanical nature of polymeric material represented in hardness, stiffness, thickness, elongation, and tensile stress [31; 112]. Static immersion test is conducted to a variety of polymers in order to test the stability toward biodiesel exposure. The results revealed different stability responses depending on the polymeric material itself. In this regard, polymers can be classified into two groups, stable and unstable polymers, towards biodiesel exposure as it is explored in Tables 9 and 10. [PERSON] et al. [113] conducted the static immersion test for high-density polyethylene (HDPE) on soybean and sunflower biodiesel at 60 \({}^{\circ}\)C. The polymeric HDPE gained 5% of its initial weight after 125 days, likewise the results of [PERSON] et al. [17] where the HDPE gained 5% after 98 days at 55 \({}^{\circ}\)C when it was immersed in palm biodiesel. The same test procedures were implemented on polyamide 66 (PA66). However, the polymeric materials showed 2% reduction from the initial weight. Polyethylene (PE) and polyoxymethylene (POM) showed weight reduction as well. Around 1% of their initial weight was lost after 28 days of immersing in a mixture of biodiesel from different feedstock sources at room temperature [114]. In addition, polytetrafluoroethylene (PTFE) lost weight after