# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
## 8 Conclusions  
Polymers are used in a wide spectrum of our daily life. Alternative energy industry is one of the flourishing sectors that distinguishes the current and coming era of human history. In this context, polymers were introduced in bioenergy industry. The current review focused on the vaporization of polymers in the biodiesel industry. Polymers were utilized in biodiesel production based on four approaches, i.e., polymeric biodiesel, polymeric catalysts, cold-flow improvers (CFIs), and stabilized exposure materials. Polymeric biodiesel is produced biologically from polymeric materials known as polyhydroxyalkanoates (PHAs), e.g., medium chain length PHAs (mcl PHAs) and poly-(R)-3-hydroxybutyrate (P3 HB), which are sourced from materials rich in carbon. Those polymers are usually esterified to synthesize hydroxyalkanoates methyl ester (HAME) and hydroxybutyrate methyl ester (HBME) which are the polymeric biodiesel. Despite the similarity with vegetative and animal-based biodiesel, more studies are needed to develop the production techniquessince the current method consumes much time and produces little yield with low cetane number. Polymers offer efficient acidic, basic, homogeneous, and heterogeneous catalysts for free fatty acids (FFA) esterification and triglycerides transesterification. In this review, polymeric catalysts were classified into three groups chemically synthesized, naturally derived, and waste derived polymeric catalysts. The last two groups are privileged with ecofriendly, reusable, non-toxic, and low-costs traits. Besides catalysis, polymers were utilized as CFIs for both pure biodiesel, such as polyolefins, ethylene/vinyl acetate copolymers, polyacrylates and related copolymers, and maleic anhydride copolymers and their derivatives, and blended biodiesel/diesel, such as Polyolefin (copolymers of C9-C22 \(\alpha\)-olefins), EVA copolymer, poly(methyl acrylate), and poly(acrylic acid) and poly(tetradecyl methacrylate). The activity of polymers as CFIs comes from their capability to modify biodiesel agglomeration where the formed crystals flow easily due to the needle-like shape. Lastly, polymers are used in products directly in contact with biodiesel fuel, e.g., storage tanks and auto parts. Static immersion test is the tool used to verify the stability of polymeric materials toward biodiesel exposure. Teflon, Viton A401-C, Viton GFLT, nitrile rubber, polychloroprene, fluoro-Viton A, and polyoxymethylene (POM) are examples of polymers that showed insignificant change toward biodiesel exposure. However, high-density polyethylene (HDPE), polyamide 66 (PA66), polyethylene (PE), polytetrafluoro-roethylene (PTFE), ethylenepropylenediene monomer (EPDM), nylon 6/6, nitrile rubber, and high-density polypropylene showed instability once they were in direct contact with different sources of biodiesel fuel. Indeed, expanding the utilization of polymers toward biodiesel industry is promoted in accordance with the revolution of green chemistry.  
Conceptualization, F.B., M.I.A. and S.A.Z.; validation, M.I.A. and S.A.Z.; data curation, F.B.; writing--original draft preparation, F.B.; writing--review and editing, M.I.A. and S.A.Z.; visualization, F.B.; supervision, M.I.A.; funding acquisition, M.I.A. and S.A.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by ministry of higher education (grant number FRGS/1/2022/STG05/USM/02/9), Universiti Teknologi Malaysia under the Industrial-International Incentive Grant (Vot 03M71), and a grant from Takasago Thermal Engineering Co., Ltd., Japan (4B424).  
Not applicable.  
The authors declare no conflict of interest.