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.

In recent years, environmental authorities in Sweden are reporting about high content of antimony in waste water that is discharging from polyester textile dyeing industries. It is known from available scientific publications that, antimony and its compound is harmful for both human and environment. While dyeing of polyester textiles have a commercial importance and in regards to the environmental issues, the industries are looking for the factors those results in high concentrations of antimony in their dyeing waste water. Dyeing of polyester textile requires high-temperature application in association with dyes and process aid chemicals. The waste water that is being produced after dyeing contains a complex mixture of chemicals, where antimony is one of that mixture. To comply with the industries interest, this master thesis work involved the exhaust dyeing of polyester fabrics/yarns and analyzing the dyeing waste water, to determine the amount of antimony diffusion. According to literature studies, the antimony compounds are widely used as catalyst for polyethylene terepthalate (PET) polymerization and hence antimony is present in polyester textiles. The entire experimental work intended to understand the variation of antimony concentration and the factors that are causing high antimony diffusion from polyester textiles during dyeing. The materials which are polyester yarn and fabrics were collected from 3 different dyeing industries of Sweden and the materials were in 9 different types. From material analysis (before dyeing) it has been found there were variations in antimony concentration among the materials. The process parameters that have varied during exhaust dyeing were dyeing temperature, cycle time and process aid chemical (leveling agent) adding options. With the variation in process parameters, the dyeing has performed and the dyeing waste waters have analyzed through inductively coupled plasma sector field mass spectroscopy (ICP-SFMS). The expectations from the experiments were, under specific dyeing process and with same antimony concentration, the materials varying in filament/fiber diameter; big diameter filament/fiber will diffuse less antimony compared to the small diameter filaments. Also, process-wise the antimony diffusion ratio among the materials will accordingly follow the Fick’s diffusion model. To face the environmental issues with sustainability, entire thesis work could provide concentrated knowledge’s with literature evidence for the dyeing industries. As literature study indicates, the dyeing temperature, temperature ramp set and cycle time play major factor while comparing for the expected diffusion ratio. As a gentle process parameter, comparatively lower temperature and cycle time results lower antimony diffusion. The use of levelling agent could be reduced to a level with the appropriateness while added for dyeing. Also, a strong follow-up is needed in the supply chain, for lowering the initial antimony content in the materials. Overall, the findings of this thesis work also keep an importance to do further research on the polyester textile, as during the experiments most of the materials haven’t reacted accordingly as they were expected to react with the Fick’s diffusion model.