Effect of screw speed and plasticizer on the torque requirement in single screw extrusion of starch based plastics and their mechanical properties

555-559Rod-shaped plastics have been prepared from blends of corn starch (69%) and plasticisers (31%), in the form of glycerol and water in different proportions (ranging from no glycerol to full glycerol) using a single screw extruder (compression ratio 1:1) (Length: Diameter ratio of 20:1) operated at specified constant temperatures, viz., 70, 110 and 110C at feed, mixing (plasticizing) and metering zones of the barrel, respectively. The screw speed was varied between 20 and 80 rpm. The torque generated during extrusion was recorded and then ultimate tensile strength (UTS) and elongation at break (EB) were measured after storing the extrudates for nine months at ambient condition. There was no significant difference in torque requirement for extrusion for use of zero to 9% glycerol content in the blends; however, for glycerol content greater than 9%, the developed torque gradually increased over the full range of rpm used. The range of torque was from 4.5 to 28 Nm, and for each blend it passed through a minimum at 60 rpm. Ultimate tensile strength followed a decreasing trend with increase in glycerol content in the plasticizer mix. However, elongation at break followed an increasing trend for increase in glycerol content up to 24%, and thereafter followed a decreasing trend with further increase in glycerol content. For glycerol content less than 24%, screw speed had a strong and direct bearing on UTS, but for EB, screw speed had no specific trend