Extrusion and reaction

‘Extruders,’ says Hadj Benkreira of the School of Engineering, Design and Technology at the University of Bradford, ‘are fantastic machines.’

Often neglected in chemical engineering training, they are widely used as mixers, especially in the plastics sector and increasingly for formulation of pharmaceuticals.

Benkreira, however, is looking at other possible applications, and believes he can use extruders for a far wider range of uses, even as reactors.

The great strength of extruders is that they incorporate a huge number of operations into a single process — melting, mixing, pumping and shaping, for example. Benkreira’s work focuses on improving the extrusion hardware ‘to make them safer, cleaner and cheaper’.

One key innovation is the rolling die. Previously, Benkreira explains, extruder dies have been stationary, merely acting as the nozzle through which the melted solid is pushed.

Benkreira’s team devised a die consisting of two rollers, whose rotation is powered, and which can either oppose or drag the flow of material from the extruder. ‘In the forward mode, the rollers drag the extrudate out at low pressures and the output rate is increased.

This mode of operation can be exploited to extrude “normal” viscosity materials at lower temperatures,’ he said. ‘In the counter-flow mode, very large pressures can be developed in the die without having to reduce the gap.’

One application of this is in the manufacture of low density plastic foams. The ability to extrude at lower temperatures means that supercritical CO2 can be used as the foam blowing agent rather than environmentally damaging HCFCs.

A further advantage of this is that sCO2 acts as a plasticiser even in very small proportions, improving processing characteristics and making it possible to extrude recycled plastics, which tend to degrade at high temperatures. Moreover, he adds, sCO2 is cheap.

Pressure needs to be high to ensure that the sCO2 dissolves in the molten plastic, which would normally mean that the extruder die would have to be small. This, Benkreira says, would limit the thickness of the product. But using a rolling die to oppose the flow of the plastic allows pressure to be raised to a high level while still allowing thick foams to be extruded.

The versatility of the device means that other materials could be used; in particular, Benkreira says, the extruder could be used as a reactor. This might involve melting a solid in the extruder barrel and adding a liquid reagent or catalyst. The walls of the barrel or the screw component could also be coated with a solid catalyst, he suggests.