# Utilization of Polymeric Materials toward Sustainable Biodiesel Industry: A Recent Review
## 4 Cold-Flow Improvers  
The flow behavior of oils is known as cold-flow properties. This scientific term includes three properties which are cloud point (CP), pour point (PP), and cold filter plugging point (CFPP). Cloud point is the lowest temperature at which small crystal shapes with hazy color start formation. Regarding pour point, it is the lowest temperature at which the liquid oil resists pouring and stops tilting. Lastly, the cold filter plugging point is defined as the ability of specific oil volume to pass through particular wired mesh during given time. All the three characteristics of cold-flow properties are usually measured based on standardized methods of the American Society for Testing and Materials (ASTM), e.g., ASTM D2500, ASTM D97, and ASTM D6371 for CP, PP, and CFPP, respectively [26; 90]. Cold-flow properties of biodiesel are greatly influenced by fatty acids composition and existence of micro-impurities. The basic units of biodiesel fuel are the fatty acids alky esters. Those fatty acids were classified based on saturation situation into saturated fatty acids (SFA) and unsaturated fatty acids (UFA) which includes monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA). UFA are usually sourced from vegetative first-generation feedstocks whereas the SFA are obtained from second and third generation feedstocks. Melting point is one of the main features that can distinguish between the two types of fatty acids. SFA usually have melting points between 4.3-58.6 \({}^{\circ}\)C which are much higher than the UFA melting points that range between \(-1\) to \(-52\)\({}^{\circ}\)C. Accordingly, the components and ratio of fatty acids are the main reason behind the cold-flow properties. The higher the SFA contained in biodiesel fuel, the higher cold-flow properties [91; 92]. Beside the fatty acids' composition, micro-impurities influence cod-flow properties as well. It was reported that some biodiesel contain monoacylglycerlevels and steryl glucosides compounds. These two compounds have high melting points, reaching 70 \({}^{\circ}\)C for monoacylglycerolglycerols and exceeding 240 \({}^{\circ}\)C for sterol glucosides. So, they are solidified fast and form crystals shapes in liquified biodiesel fuel [93; 94]. Poor cold-flow properties impact negatively on diesel engine systems, particularly in cold weather regions. Start-up problems are usually related to the cold-flow properties. Such problems arise when the diesel engine is turned off over night or stops working for long time. As a result of cold weather, biodiesel fuel solidifies in pipes and filters. Consequently, the fueling system will be plugged. Besides, poor cold-flow properties of biodiesel lead to poor performance of the diesel engine since the fuel supplies are hindered. In addition, engine damage is another concern related to the poor CP of biodiesel. The crystal particles formed during lower temperatures may make the fuel lose its flowability and engine loses its operability. Based on that, treating the poor cold-flow properties is an essential procedure for biodiesel in particular and fuel in general to come up with standardized physicochemical properties and well-operated performance [95; 96]. There are three practical approaches to enhance the poor cold-flow properties of biodiesel, i.e., blending with conventional diesel fuel or other similar fuels having good cold-flow properties, winterization, and using additives. Literature reports acceptable cold-flow properties for biodiesel fuel when it was blended with fossil fuel diesel, kerosene, ethanol, and other good cold-flow biodiesel [95; 97]. Winterization is a thermochemical fractionation process used to remove undesirable parts of oils. It is conducted based on the differences in melting points where the fuel is subjected to specific low temperature points till crystallized shapes are formed. Then, the solidified parts, which are responsible for the poor cold-flow properties, are filtered and avoided [98].