High-tenacity polyester fiber recycling: A challenge tackled with innovative technology

rPES yarn (Foto: Starlinger)

Vienna, Austria — To produce man-made fibers and filaments, several polymers such as PES (polyester), PP, PA, and others, are available. The choice depends on the application: PES fiber, for example, is used as staple fiber (PSF – Polyester Staple Fiber), filament (POY – Partially Oriented Yarn, DTY – Drawn Textured Yarn, FDY – Fully Drawn Yarn), technical yarn and tire cord, monofilament, as well as nonwoven and spunbond. Due to its characteristics polyester is used for the production of various apparel products, that after use must be collected and recycled.

The growing popularity of polyester fibers in garment production becomes evident in the increase of the annual polyester fiber and filament production from 20 million t in 2002 to more than 48 million t in 2015, and is forecast to reach around 55 million t by 2020. This also means that the amount of scrap generated during polyester fiber and fabric production is growing accordingly.

More and more fiber producers and plastics recyclers have started to recycle fiber and fabric scrap from industrial production and post-consumer collection. rPET produced both from fiber/fabric scrap and post-consumer PET bottle collection is currently being used at a rate of 14.4 percent in polyester fiber production, mostly for polyester staple fiber. China is by far the greatest user of rPET in fibers, processing almost 80 percent of the 7 million tons produced worldwide in 2015.

But there are some challenges in recycling of high-tenacity fibers and fabrics:

  • Different production steps cause different levels of humidity: Special precautions must be taken
  • Material comes in bales, lumps, fabric etc…: Due to the exceptionally high strength of the material it is very tough and requires shredding in a single-shaft cutter
  • Requirements for color values: They allow no oxidative or thermal degradation

The reuse rate lies usually around up to 30 percent, in the same application it can reach up to 100 percent.

Selecting the correct recycling equipment

The input material and intended use of the regranulate are important factors for choosing the right recycling equipment. High-tenacity PES fiber recycling applications demand special machine features such as pre-shredding, drying before extrusion, high-vacuum degassing and fine melt filtration. Also, solid stating for IV increase in the produced pellets may be necessary to achieve the characteristics required for reuse.

Starlinger’s recycling lines for PES fiber recycling – the recoSTAR dynamic (the successor model of the recoSTAR basic recycling line) and recoSTAR universal recycling lines – have been installed for a number of high-tenacity fiber recycling applications worldwide.

A wide range of applications: recoSTAR dynamic technology

High automation, increased energy efficiency and a wide range of applications are the key highlights of the recoSTAR dynamic technology. In the smart feeder, a rotating disc at the bottom cuts the input material and creates friction. This frictional process heats and dries the mixed material, densifies it and brings it close to the melting point before it is fed into the extruder. Six extruder sizes from 150 – 2,600 kg/h (output with degassing) are available.

With intelligent features: recoSTAR universal recycling

Versatile, accessible, and equipped with intelligent features: The recoSTAR universal recycling line is perfect for processing hard-to-grind materials such as bales, fibers, fabrics and start-up lumps. In the active shredder arranged parallel to the extruder a hydraulic pusher presses the material against a water-cooled rotating shaft and thus provides efficient crushing. In combination with the sophisticated software and the dosing screw, this kind of material preparation allows the processing of materials with different bulk densities without having to change process parameters. Five extruder sizes from 150 – 1,300kg/h (output with degassing) are available.

Both lines feature high-vacuum degassing for PES recycling to ensure the effective removal of volatile contaminants such as additives and finishing agents. Concerning filtration, reuse and the required filtration fineness at the production line are determining factors; standard filter fineness is 50 µm, fine filtration (25 µm) with cascade filter system is also possible. The type of pelletizer is mostly a question of which pellets shape of the rPET is preferred – both (automatic) strand pelletising or underwater pelletising are possible. Special features are inline viscosity and colour measurement. Optionally, crystallisation and solid-stating of PET pellets and flakes can be included.

Author: Elfriede Hell, General Manager, Starlinger recycling technology

Source: Starlinger & Co. Ges.m.b.H.