Chemical and physical changes in PET fibres due to exhaust dyeing : Issues in thermo-mechanical recycling of dyed PET textiles

Abstract: Polyethylene terephthalate (PET) is the most used fibre in the textile industry. PET is also used in other products, e.g. soft-drink bottles and food packaging. Approximately 60% of the globally produced PET is intended for production of textile fibres and the demand for polyester fibres have steadily increased over the last decade. Yet, most of the recycled PET fibres are produced from discarded bottles and not discarded textiles even though the generation of textile waste is increasing year by year. The importance of finding efficient recycling routes for discarded PET textiles is obvious. In thermo-mechanical recycling the thermoplastic characteristic of PET is utilized to re-melt and re-form PET waste into new valuable products. Today, this is used for bottle-to-fibre recycling but not for fibre-to-fibre recycling. The main research question asked in this Master thesis is if the process of exhaust dyeing compromise the possibility to recycle PET textiles through remelt spinning. It is believed that PET degradation through hydrolysis may occur during dyeing. The degradation behaviour of PET has been widely studied. However, degradation during exhaust dyeing has not been investigated.   The process parameters temperature, time and number of dyeing cycles have been investigated. Also, possible effects of different auxiliary chemicals have been studied. Dyeing and characterisation of two PET fabrics with filaments of different titer was performed in order to investigate if the filament titer is also a parameter to consider.   Tensile testing and surface characterisation through demand absorbency test showed that the filament titer seems to affect how the tensile and moisture related properties change due to dyeing. Differential scanning calorimetry showed that the crystallisation rate is affected by the dyeing process. This can be an effect of formation of shorter PET chains during dyeing. The auxiliary chemicals have been shown to be the most critical factor in changes of the crystallisation behaviour. Fourier-Transform infrared spectroscopy indicated that chain scission has occurred during dyeing.   The results have shown that the exhaust dyeing process causes changes in tensile properties, moisture related properties, degree of crystallinity as well as crystallisation behaviour. DSC and FTIR results indicate chain scission. Based on the results it cannot be concluded if the changes are large enough to compromise the possibility to recycle PET textiles thermo-mechanically. Further research is required in order to correlate the observed changes with possible problems in thermomechanical recycling of dyed PET textiles.