Characterisation of selected mechanically opened cotton fibres: A study on the feasibility of spinning the staple fibres into recycled yarn

With the upcoming EU directive and the recent Finnish waste legislation it has become necessary to find ways to collect, sort and recycle textile waste. Textile waste is now collected nationwide in Finland, with reusable items directed to second hand sales, and non-reusable waste directed into recycling centres. Two mechanical opening lines were opened in Paimio in 2021 with a shared theoretical opening rate of 50 tonnes of fibre per year. While the theoretical recycling rate is high, the problem arises with the poor reputation of recycled fibres on the textile market. It has been reported that fibres should have a certain tenacity and length to suit the yarn production needs. Telavalue is a Business Finland (BF) project that aims to create value for the textile waste and offer solutions for circularity of the textile economy. The project is coordinated by VTT, LAB University of Applied Sciences and Turku University of Applied Sciences and executed in partnership with other research centres, companies, and other organisations. The commercial actors in the textile industry are interested in the quality of recycled fibres for future textile solutions. This study aims to show the value in fibre-to-fibre recycling and in mechanically opened (MO) fibres further processed into yarn by staple fibre spinning.
This study utilises both quantitative and qualitative methods to characterise six different types of MO cotton fibres: PRE-KNIT, PRE-WOVEN, POST-B2B, POST-n, POST-s and POST-ea. The cotton types include both pre- and post-consumer derived fibres. Pre-consumer fibres are obtained from garment production of knitwear (KNIT) and denim (WOVEN). The post-consumer textiles are collected from workwear companies (B2B) and from municipal waste collection with either neutral (n) sorting and opening, softener (s) used in the opening, or with up to 10% elastane (ea) and other fibres allowed in sorting. A reference of 100% virgin cotton, REF-CO, was used. The fibres were obtained from a fibre collection belonging to the Telavalue BF.
In general, the results showed shortened fibre lengths for the mechanically opened fibres. While the REF-CO average length was calculated to be 18.8 millimetres, the longest mechanically opened fibres were 25-27 mm long. The shortest fibre contents were established for the sample PRE-KNIT, with average length of 8.6 mm. Although the fibre lengths were weak, the fibre uniformity indexes (UI) of the MO fibres were from 54 to 59% and the UI of REF-CO was 64%. The PRE-WOVEN and POST-B2B were made up of a greater content of long fibres than the other MO samples. The linear densities were near equal to the values of REF-CO. Tenacities at break were generally greater for post-consumer samples than any other samples. POST-ea was found with greatest elongation at break and PRE-KNIT with the weakest. The surfaces of the fibres were inspected with scanning electron microscopy (SEM). The inspection showed some damaging of the MO fibres. However, this damage was sparse and only visible on individual fibres. Additionally, a 50-year-old bedsheet was hand-opened into fibres to look out for surface wear. There was some surface opening on those fibres, as well as on some post-consumer fibres. Moreover, the post-consumer samples were dirtier than REF-CO or the pre-consumer samples. SEM imaging revealed other fibres than cotton in the samples. Furthermore, a Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC) were applied to investigate the composition of the cotton qualities. It was discovered that alongside REF-CO, PRE-KNIT and PRE-WOVEN did not have any melting points between 0 to 300 ºC whereas all post-consumer samples showed peaks. A cross-examination with FT-IR hinted that the post-consumer samples include e.g., polyester, which is a common fibre in denim garments.
A discussion of a classification of mechanically opened cotton fibres is included as a result of the experimental work. The multiple factors affecting MO fibre properties make it challenging to draw conclusions without neglecting some properties.