# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
###### Abstract
Large fertile areas, greater quantities of fertilizers, and significant water resources were dedicated to grow the feedstock of first-generation biodiesel. At the same time, the food shortage problem is encountered by many countries all around the world [13]. Therefore, expanding biodiesel production from first-generation feedstock is considered one of the main reasons behind food shrink and increasing food prices. The other three biodiesel feedstock generations seem more sustainable. Despite being preferable environmentally and economically, second (inedible and wastes oils) and third (algae) generations of biodiesel feedstocks have drawbacks in characteristics and performance. The high acidity in the form of free fatty acids (FFA) is one of the distinguished characteristics of the second and third generation feedstocks lipids [14]. A higher content of FFA in the oils is an indicator for the rancidity of feedstock since it is almost waste. They come from oxidation reactions, moisture, effect of lipase enzyme, high temperature, and hydrolysis reactions within the feedstock itself [15]. FFA hinders the synthesizing fatty acid methyl esters during the transesterification reaction due to soap formation [16]. Another disadvantage of second-generation feedstock oil is its water content. The water exists naturally in the feedstock composition or finds its way through the purification process and absorption from the atmosphere. Water content in the oils promotes hydrolysis reaction which leads to a greater FFA content within the oil. Moreover, if the water content sneaks to biodiesel, it may be responsible for damaging some auto parts, particularly polymers due to attributing hydrolysis reaction, oxidation, and FFA formation [17]. Beside the existence of FFA and moisture, the higher content of saturated fatty acids (SFA) constitutes another barrier toward utilizing biodiesel fuel from second-generation, in particular animal-based feedstocks. The higher content of SFA leads to poor cold-flow properties in biodiesel fuel, which plugs the fuel in engine system and impacts negatively on the engine and its performance. Thus, a pretreatment process is needed for second-generation feedstock before subjecting it to biodiesel production. Recently, polymers materials were introduced into biodiesel production to enhance the physicochemical properties and fuel performance, particularly for the second-generation feedstocks. Polymers are defined as series of molecules linked together by chemical bonds to form macromolecules. They exist naturally, e.g., proteins, wood, and rubber, and synthetically, e.g., nylon, polyethylene, and polyester [18]. They were introduced almost in every aspect of life including products of medicine, food, furniture, auto parts and so on. The biofuel industry was, and still is, one of the expanding sectors to introduce polymers materials to enhance the properties of fuel products. Polymeric biodiesel was synthesized biologically with the help of micro-organism activity in presence of polymeric feedstock [19; 20; 21; 22]. In addition, polymers were composited to reduce the higher content of FFA existed in second-generation feedstock oils. Moreover, they catalyze the esterification and transesterification reactions during production line of biodiesel fuel [9; 10; 23; 24; 25]. Beside catalysis, polymers were used as additives to improve the cold-flow properties and enhance the fuel performance [26; 27; 28; 29; 30]. Moreover, stable polymers were investigated as storage containers and auto parts in direct contact with biodiesel [17; 31; 32]. So, reviewing those studies would help to build a collective knowledge and solid background to valorize polymers in biodiesel production industry. Herein, recent studies regarding utilizing polymeric materials toward biodiesel industry were reviewed and depicted in Figure 1 as polymeric biodiesel, polymeric catalysts, cold-flow improvers, and stabilized exposure materials Biologically synthesized biodiesel from polymeric materials was explored.