Effect of polymeric cold flow improvers on flow properties of biodiesel from waste cooking oil

•The best polymeric cold flow improver was polymethyl acrylate (PMA).•PMA did not deteriorate other important fuel properties of biodiesel.•PMA changed the crystallization way of the crystals.•All the four polymeric cold flow improvers improved the viscosity index of biodiesel.•PMA could be a potential effective viscosity index improver for biodiesel. The impact of four polymeric cold flow improvers, namely, polymethyl acrylate (PMA), ethylene vinyl acetate copolymer (EVAC), poly-α-olefin (PAO), and polymaleic anhydride (HPMA) on the cold flow properties and viscosity of biodiesel from waste cooking oil was evaluated. The viscosity indexes of neat biodiesel and biodiesel treated with cold flow improvers were calculated. In addition, the influence of these cold flow improvers on total glycerol, flash point, acid value, and oxidation stability was also determined. Finally, polarizing microscopy and differential scanning calorimetry were used to investigate the low-temperature crystal morphology and crystallization behavior of the biodiesel samples. The results indicated that PMA was the best candidate for enhancing the low-temperature flow properties and viscosity index of biodiesel from waste cooking oil without deteriorating other important fuel properties of biodiesel. Upon addition of 0.04% of PMA, the pour point and cold filter plugging point of biodiesel were reduced by 8 and 6°C, respectively. PMA essentially retarded crystal aggregation at low temperature. It modified the crystallization behavior of the crystals by transforming the shape of crystals and inhibiting the formation of larger crystals, which resulted in the improvement of the biodiesel cold flow properties. The results also indicated that PMA could be used as an effective viscosity index improver for biodiesel.