In a circular economy where discarded materials are recycled in resource efficient manners, a successful shift of textile waste recovery, from incineration to high quality recycled textile fibers, is necessary. Textile material recycling has been identified as a bottleneck in the Swedish textile waste management and large scale recycling plants are requested globally. To overcome this hurdle, the challenge of fiber separation from blends needs to be solved which is addressed herein by enabling recycling technology for separation of fiber blends containing nylon and elastane. To shift textile recycling

In a circular economy where discarded materials are recycled in resource efficient manners, a successful shift of textile waste recovery, from incineration to high quality recycled textile fibers, is necessary. Textile material recycling has been identified as a bottleneck in the Swedish textile waste management and large scale recycling plants are requested globally. To overcome this hurdle, the challenge of fiber separation from blends needs to be solved which is addressed herein by enabling recycling technology for separation of fiber blends containing nylon and elastane. To shift textile recycling from current low value products to a circular economy, the key issue of fiber blends must be addressed and it is thus clear that the lack of chemical recycling techniques, in particular for fiber blends, is substantial.

The ambition with the project was to answer the question: Which technical methods is required to separate nylon (polyamide) and elastane (polyurethane) from other fiber types, and enable recycling of these textile blends. The goal is thus to develop positive strategies for separation of different fibers from a fiber blend prior to chemical recycling, according to a circular economy.

Work package 1:

Thermomechanical separation The work on thermomechanical separation was led by Dr. Christina Jönsson (Swerea IVF). Most of the work within this work package was delivered as a diploma work by Ida Marklund, at Swedish School of Textiles, 2017. The background knowledge is based on studies with the Vinnova funded project ‘Spill till guld’, wherein one of three areas involved waste from textile production.

Work package 2:

Enzymatic design This biotechnology track has been led by Dr. Per-Olof Syrén at SciLifeLab/KTH (Stockholm, Sweden) and the work has been conducted as a joint effort between BOKU- University of Natural Resources and Life Sciences in Vienna and SciLifeLab/KTH. Dr. Doris Ribitsch at BOKU recently contributed to the discovery of the first known polyurethane (e.g. elastane) cleaving enzyme. KTH/BOKU together has also developed the first designer enzyme capable of nylon hydrolysis to be further developed for the design of enzymes suited also for the recycling of elastane in textiles.

2.3.1 Thermomechanical separation

Most of the work conducted on thermomechanical separation was published as a Diploma Thesis by Ida Marklund, June 2017.The materials studied in this work can broadly be divided into three primary groups:

• Model system: known material composition was created to map how different amounts of elastane affect the thermomechanical processing and the properties of the nylon6 and elastane-recyclate. In total, four different ratios are included in the model system; 0, 5, 10, and 15% elastane. A pure PA6 quality (0% EL) was processed in the same way as the blends to provide comparable reference values. For further details see Diploma Thesis, Ida Marklund, 2017.

• Post-industrial waste: Two different warp knitted, Nylon6 fabrics; containing 10% and 22% Elastane respectively, correspond as post- industrial waste. The type of Elastane was not specified. The fabrics were provided by Boob design and are normally used in their swimwear collection.

• Post-consumer waste: Bags with sorted post-consumer waste, containing polyamide and elastane garments, were provided by Human Bridge. The waste included swimwear, sport socks, underwear and a great variation of panty hose and active wear.

Thermomechanical processing

All four ratios (0, 5,10 and 15%) of elastane were processed both at 240°C and 280°C. The elevated temperature, 280°C, was used to simulate the process temperature for PA6.6. The three fractions of post-consumer waste (swimsuits, active wear and pantyhose) were cut into smaller pieces by hand, before being grinded in a Rapid granulator, G150-42-DT. The milled garments were fed by hand into zone three of the extruder. A melt filter was attached at the extruder’s outlet to separate out the other fibres contaminating the polyamide and elastane blend. A first trail with a 250 μm filter was performed, before it was changed to 500 μm filter.

Characterisation of the thermomechanical study

A number of different characterisation methods were used, for example Fourier transform infrared spectroscopy (FTIR), Thermogravimetric analysis (TGA) and Differential scanning calorimetry (DSC). As well as microscopic methods. Mechanical tests were performed on all specimens as they were produced through injection moulding, measuring tensile strength. In order to get a better understanding of elastane, various kinds of elastanes and TPUs were characterized with FTIR, DSC and TGA.

Enzymatic design

Within the enzymatic design track in Re:Mix, in-house computational enzyme design tools developed in Syréns laboratory at KTH/SciLifeLab were utilized to develop an engineered enzyme towards polyamide man-made materials [14]. Specifically, this project allowed us to pursue our hypothesis that hydrolytic enzymes constitute highly suitable starting scaffolds to introduce polyamidase activity by enzyme engineering. With their solvent exposed active site, we reasoned that cutinases would constitute suitable starting templates to achieve optimized transition state (TS) stabilization by enzyme design[9] to afford polyamide plastic hydrolysis.

Thermomechanical separation

The results from the mechanical separation are below presented as compounding, filament production or injection molding. For the different collected waste material (post and pre consumer waste), compounding to pellets was successful for only some materials. The results from these materials were compared to a model system to identify challenges for post-consumer waste. In summary the results showed:

• Pellets could be produced from all materials

• Injection moulding successful in most materials

• Meltspinning only successful with some materials

In summary, parameters of importance were identified as follows:

• Dependency on drying condition of the material

• Dependency of material content

• Gas formation during processing

• Initial attempts of melt filtration clogged the filter system

Conclusion

Thermomechanical separation

All the materials with elastane were possible to compound both at 240°C and 280°C, but in the next process step (inject moulding or fibre spinning) it was more difficult at 280°C, especially with mixtures with more than 10% elastane. More gas formation and a crackling sound were constant at the higher admixtures of elastane.

Enzymatic design

This project resulted in gained knowledge with respect to biotechnologically-assisted depolymerization of man-made materials. This pre-study has allowed for preliminary material analysis after enzymatic hydrolysis by MALDI, from which we conclude that further optimization of our experimental setup is needed. This is planned for future joint projects. We have shown that enzyme design is a promising method to afford hydrolysis of pure synthetic polymers.

Next steps

Next step within the thermomechanical track is to evaluate the possibilities with melt filtration more with the materials from the model system to have a more controlled setup. In addition to this it will be important to search for other coupling agents on the market that could help in the process to make elastane more compatible with polyamide in the recycling processes. To increase the shear stress during compounding could be another way to reduce the particle size if it is not possible to separate the elastane from the polyamide.

Within the enzymatic track this pre-study shows the importance of a multidisciplinary approach in tackling complex research problems. The knowledge generated will constitute a cornerstone for further collaboration on catalyst development, with a future focus on textile materials. An important aspect of future research will be the analysis of the performance of designer enzymes in the hydrolysis of material blends, e.g. mixtures of polyamides with other fiber types.