VTT develops cylindrical extruder to turn problematic waste into pellets

Scientists from the VTT Technical Research Centre of Finland have developed a cylindrical extruder to turn problematic textiles and plastics, as well as food waste, into pellets, which they believe will revolutionise the processing of recyclable materials.

The first prototype has already exceeded the industrial steering group’s expectations during initial testing and VTT is now looking for a partner to commercialise the technology.

An extruder is a device that is capable of melting, mixing and extruding paste through a shaped nozzle. The plastics industry uses extruders to make pipes and profiles, and they also have various applications in the food and feed industry.

Research scientists tested the prototype with pieces of plastic film, mixed plastic waste, textiles and bread. As well as recycling waste, the device can produce long fibre composites and can be utilised in food and feed processing.

When VTT research scientists discovered that materials can rotate around the device’s hollow cylinder, the research centre got to work developing their own version for processing problematic recyclable materials.

“Many textile recycling processes are only suitable for products containing homogenous fibres. However, textiles are often made of a mix of fibres, and many products are comprised of different layers,” said Pirjo Heikkila, VTT senior scientist.

“The new extruder opens up a revolutionary opportunity to recycle mixed textiles and materials without having a separate fibres or components.

“We have successfully tested the device for recycling pillows without removing the filling in the course of a project called Telaketju, with funding from Business Finland.”

The diameter of the extruder screw determines the size of the feed throat and also the kinds of materials that the device is capable of processing. The first prototype has a screw diameter of 30cm, instead of the 3-4cm typically found in conventional devices of the same output.

This large diameter, combined with a shallow screw channel, makes it possible to mix different components of problematic, porous and lightweight materials and to make the mixed mass compatible with the next stage of the production process.

The design enables accurate temperature control combined with efficient mixing and long residence time, particularly considering the size of the device. This can be beneficial when processing materials such as food and feed. Long fibres can also be processed without cutting them, which is useful when processing textiles or when mixing fibre composites.

VTT’s first prototype is less than 2m long and weighs 1.5 tonnes, making it compact enough to be easily transported and mounted upright if necessary. Due to its simple design, the device is said to be cheaper to make than traditional mixing twin-screw extruders.

Research scientists have been testing the prototype’s performance with pieces of plastic film, mixed plastic waste, various kinds of textiles and bread. In addition to recycling, the device has been used to produce long fibre composites, and it can also be utilised in food and feed processing.

“Benefits include it being cheaper to make than traditional mixing twin-screw extruders, is easy to transport, and the design enables accurate temperature control combined with mixing and long residence time and long fibres can be processed without cutting them,” said Tomi Erho, VTT principal scientists.

“Commercialising the device would create completely new possibilities both in terms of waste processing and novel material combinations.”