Investigation into multiple melt spin recycling cycles of Poly(ethylene terephthalate) (PET)

Abstract

Polyethylene terephthalate (PET) is one of the most common fibres used in textiles due to its good mechanical properties. PET bottles are often recycled into fibres for clothing; however, very few PET based textiles are recycled in a closed loop process to make new textiles. The simplest form of recycling is mechanical, but this results in a degradation of properties and often requires the addition of virgin material. Although the loss of properties is well known no data could be found to quantify this decrease. Therefore, a study was undertaken to determine the change in the properties of PET filaments as a result of multiple recycling cycles. In order to recycle the filaments they had to be cut to less than 5 mm sections to pass through the extruder. If sections were longer, they would become entangled and would not pass through the extruder evenly. Filaments were extruded and recycled four times. The load at maximum extension was seen to decrease exponentially with the recycling cycle which was believed to be a result to chain scission. Between the second and third recycling a large change in elongation at break occurred. Recycling also increased the young’s modulus of the filaments, showing that re-extrusion increases the brittleness of the filaments. This was seen to be most prevalent between the second and third recycling (appears to be a cliff edge effect) as filaments could no longer be drawn. This was seen for both bottle and film grade PET. Recycling was seen to affect the colour of the extruded filaments; with each cycle they became more yellow and opaque. The time between extrusion and drawing was identified as an important variable due to physical ageing. Recycled filaments and filaments of the lower grade were found to be more susceptible to physical ageing than virgin filaments of a higher grade. The study also incorporated modified clay into the filaments in an attempt to mitigate the loss of mechanical properties. The PET pellets were ground into powder to aid mixing with clay. The resultant fibres were then tested however, due to incomplete mixing no statistically significant change in mechanical properties could be seen.