Quantitative and qualitative analysis of dyes in pulp from recycled textiles

Abstract: To decrease the environmental footprint, the textile industry needs to become more circular. One company that is trying to close the textile lifecycle loop is the Swedish company Renewcell. In the Renewcell process, discarded clothes and other textile waste are turned into a dissolving pulp that can be used for production of new textiles. For this to work, it is important to generate a non-colored dissolved pulp. Therefore, it is of great interest to be able to optimize the bleaching process and evaluate different starting materials. Today, there are different methods that have been used for quantification of specific dyes and for identification of raw materials in textiles. However, there is no complete method that can be used for all types of dyes and raw materials. This master thesis will therefore investigate the bleaching step and the raw materials used in Renewcell’s recycling process. The aim with this project is to develop a method, to be able to analyse the presence of dyes qualitatively and quantitatively in the dissolved pulp from the recycled cotton-based textiles. To do so, different chemical analysing methods were used. These include kappa number measurements, UV-VIS and FTIR. Nine different types of jeans, one yarn, three raw materials (cotton, PET and elastane), and two pure dyes (indigo and black sulfur dye) were selected for this project. The result from the kappa number measurements shows that only the blue jeans, blue yarn and black jeans could be bleached and measured by using the original kappa number method at 25 degrees C. But when the same procedure was done manually and the temperature was increased to 70 degrees C, almost all materials could be bleached and measured. Therefore, the conclusion is that this method can be a successful quantification method. However, the temperature correction equation needs to be developed further to be able to quantify the exact amount of dyes. The kappa number was also compared with the absorbance factor, i.e. the k-value from the UV-VIS results. A correlation could be seen for the blue jeans, blue yarn, and black jeans. So, the UV-VIS method could also be a possible method for quantification of dyes in textiles. For the qualitative analyses, FTIR was used. The results showed that the raw materials could be identified by comparing the resulting FTIR spectra with a reference spectrum. For the jeans and yarn materials, indigo dye was identified for the blue jeans, blue yarn, and black jeans. However, for the other materials, the amount of dyes was too low to be able to draw conclusions about the chemical structure. With further development, these methods could be used as successful qualitative and quantitative methods for analysing dyes in textiles. This would generate positive consequences in several ways. The usage of bleaching chemicals could be optimized, and the quality of the final textile material could be improved. This would benefit both the manufacturer of these recycled textiles (economically) and the consumer who will buy the final product (quality). Indirectly, this would help to decrease the environmental footprint from the textile industry.