Circular economy solution for flame-retardant protective clothing
The Austrian Centre of Industrial Biotechnology (acib), Graz/Austria
TU Wien, Institute of Chemical, Environmental & Bioscience Engineering, Vienna/Austria
BOKU, Institute of Environmental Biotechnology, Tulln a. d. Donau/Austria
Viscose Faser GmbH, St. Pölten/Austria
Initial situationThe need for personal protective equipment (PPE) in the industrial environment or for applications with high risk potential and their continuous development has led to the combination of a multitude of materials .
Mechanical robustness, fire resistance and chemical resistance are often essential; the textiles should at the same time limit the wearing comfort as little as possible and be durable.
Special fiber types and yarns that are optimized for the specific purposes of these fabrics, knits, etc. primarily support the complex requirements laid out by that [2, 3].
The segregation of PPE as a textile waste due to wear is associated with the loss of intact ingredients often produced with considerable energy take-up, particularly for fibers and functional additives.
Recovering and processing of these valuable substances therefore makes sense economically, and is ecologically and legally – for example with regard to waste prevention strategies as implication of Directive 2008/98/EC of the European Parliament  – expedient.
However, conventional recycling methods are unsuitable or reach their limits here.
Idea and challenges with a recycling processWhen selecting textiles as representative model substances, 2 material fractions commonly present in PPE, namely flame-retardant viscose fibers and a synthetic share with the main component aramid staple fiber, were focused on. A critical parameter is the fiber staple length, which must not be adversely affected in the process. The sole application of mechanical separation operations on a fabric structure that is still firmly conjunct does not tend to guarantee this.
The aim of recovering the synthetic fiber fraction with least possible application of force, led us to disintegrate the viscose fiber in order to loosen the fabric composite, thus enabling a subsequent mechanical separation of the synthetic part without loss of quality. A principle that has been successfully used in the textile industry for some time (e.g. for desizing, bio-polishing, and others) consists in the possibility of enzymatic hydrolysis , in which synergistic enzymes catalyze cellulose degradation. If a transfer of the process to the subject material is possible, an environmentally friendly option is available for the highly selective depolymerization of the viscose fiber. The monomer dissolved at the end of the process in an aqueous environment can then serve as a source for biofuel production or chemical synthesis.
In parallel, the flame-retardant is released successively into the reaction solution and can be used to produce new flame-retardant viscose fibers. It is crucial to the success of the process, though, to gain characteristics comparable to the commercial product for the obtained products, as well as to fetch their clean separation with at the same time careful use of resources.
Separation strategy and process developmentFirst, the conversion and the quality of the reaction product and released additive via hydrolysis of pure viscose (FR) fibers, as used in the flame-retardant-textile production, and commercially available blended fabric with a viscose (FR) fiber as the blending component were investigated (Fig. 1). Here, the flame-retardant pigment (FR), a phosphorus-based, organic solid, is homogeneously dispersed within the entire fiber volume.
Chemical hydrolyses with sulfuric acid (standard method) was used for comparison. Only from high H2SO4 concentrations and elevated temperatures, satisfactory conversion rates were achieved. Due to the hardly controllable oxidation of the reaction products and valuable substances produced, as well as the necessary high dilution to a pH range acceptable for subsequent process steps, this hydrolysis method does not appear to be feasible in terms of a sustainable recycling process.
In contrast to this is the enzyme-catalyzed degradation of the regenerated cellulose fiber. For this purpose, the material samples were incubated in buffered enzyme preparations providing a pH value of approx. 5. The required reaction times for complete chemical conversion ranged from a few up to about 24 hours. Fig. 2 shows examples of time-dependent conversions of the viscose share and light-microscopic images of the incubated blended fabric samples. The incubation temperature affected the conversion, with a temperature optimum around 50 °C, as did the provided buffer solution, where a significant influence on hydrolysis correspondent to a well-adjusted pH value was observed. A buffer strength of 50 mmol/l was sufficient. An extensive variation of the process parameters, i.e. temperature, pH value/buffer strength, enzyme concentration and reactor design, allowed for finding suitable reaction conditions. A degradation of cellulose to glucose was achieved with negligible residual levels of oligosaccharides . From the reaction solution containing the glucose, the released flame-retardant pigment was centrifuged off. The pigment was washed to remove buffer salt and adhering residues of enzyme/protein. An analysis of the chemical structure by NMR also showed that the flame-retardant was not adversely affected . From the centrifuge supernatant, on a liter scale, an enzyme-containing fraction could be separated using membrane technology and be reused. The nearly enzyme-free, glucose-containing permeate was successfully used for the fermentative production of ethanol .
© Foto: acib
Fig. 2 Conversion rates of viscose share from aramid-viscose (FR) blended fabric using different enzyme mixes (measurement by gravimetry and chromatography); reaction conditions constant at pH4.8, capacity of pH buffer 50mmol/l, incubation temperature 55 °C
Scale-up and assessment of recovered substances and products derived thereofThe developed process was adapted to process larger quantities of feed material. For this purpose, enzymatic hydrolyses were carried out using flame-retardant viscose yarn and aramid-viscose (FR) blended fabrics on a semi-industrial scale under the established conditions (Fig. 3). Released flame-retardant pigment (A) and the synthetic fiber fraction (B) which remained intact after the incubation, were separated from the reaction solution after incubation, purified and prepared for reuse.
© Foto: acib
Fig. 3 Pilot trial, overview Left: hydrolysis reactor prior to and after incubation of fabric and sample of reaction solution prior to and after sedimentation of flame-retardant pigment Right: isolated aramid fraction and recovered, washed flame-retardant pigment.
A) Flame-retardant pigment (rFR) and production of flame-retardant viscose yarnThe pigment recovered in a pilot plant was washed and the purified FR pigment dispersion was thickened. Samples were characterized by optical methods and dry mass determination. The presented quality was comparable to that of specimen of a standard dispersion type used for the production of flame-retardant viscose yarn. Laser diffraction measurements that were carried out to determine the particle size distributions showed that agglomerates that appeared under the light microscope were resolved by sonication (during the measurement). In the measurement, a mass fraction of 90 % was determined to exhibit a mean particle diameter below 16 microns, during ultrasonic treatment this dropped to 3 µm. The distributions mean regularly ranged at 2-4 (without ultrasound) or 0.5-1 µm (with ultrasound). A reference measurement of commercial FR dispersion confirmed the comparability of the particle size distributions when ultrasound was applied (Fig. 4). The recovered pigment dispersion and the commercial product were spun into flame-retardant viscose multifilament yarns (CV-rFR) using different concentration ratios and spin arrangements. Individual tensile tests on the produced FR viscose filaments finally confirmed a comparable mechanical behavior in terms of tensile strength and elongation at break .
© Foto: acib
Fig. 4 Comparison of volume-based particle size distributions of recovered flame-retardant pigment from viscose (FR) fibers and aramid-viscose (FR) blended fabric related to commercial product; laser diffraction analysis
B) Reconditioned synthetic fraction (rAR), production of aramid/viscose blended fabricThe synthetic share of the blended fabrics was on hand after incubation in the form of a mechanically loosened fabric structure. This material was washed and dried, coarsely shredded, then opened in a modern textile recycling line and isolated into staple fibers. These were re-spun, twisted with commercial, flame-retardant viscose yarn and processed into fabrics (rAR/CV-FR) of the same type as the feedstock material (AR/CV-FR) (Fig. 5).
Length measurements were performed on the volume of isolated aramid fibers. For comparison material, individual fibers were carefully dissected out of by hand before hydrolysis and prior to isolation of single fibers out of the synthetic fraction. The preservation of staple length during the isolation process was largely confirmed and the fibers were found neatly isolated. Titer, tensile strength and elongation at break in the individual process stages were determined by single-fiber tensile tests. The measurement results – from the blended fabric to the reconstructed fabric – showed a slight influence on the titer, tensile strength and staple length only after carding of the separated fibers, while a minimal remnant of undissolved fiber bundles remained visible in the thread. Stretching and reweaving steps were uncomplicated.
© Foto: acib
Fig. 5 Processing of synthetic fraction; shredding, opening of fabric and recovery of single fibers, re-spinning and twisting using FR viscose yarn, reconstruction of blended fabric
ConclusionAs a rule, when work and protective clothing lose sub-functions, these clothes are usually withdrawn. Because of raw materials that are frequently expensive, but also from an ecological point of view, recycling at the end of the textile’s useful life is desirable, yet currently not achievable. The process presented has achieved a recovery of synthetic fibers and a functional additive out of blended fabrics containing regenerated cellulose fibers. The use of energy and operating resources were at the same time kept relatively low. Characteristic properties of the recovered valuable substances were equivalent or comparable to those of the primary products. Since the core part of the process, the decomposition of cellulose, is very selective, the degradation product glucose has high purity and its use in white biotechnology is possible.
The next step will be to extend the method to a wide range of input materials, while treatment steps subsequent to the core process shall be further optimized. The process will be able to make a significant contribution to a sustainable use of resources in this range of applications.
This work has been supported by the Federal Ministry for Digital and Economic Affairs (BMDW), the Federal Ministry for Transport, Innovation and Technology (BMVIT), the Styrian Business Promotion Agency SFG, the Standortagentur Tirol, Government of Lower Austria and ZIT – Technology Agency of the City of Vienna through the COMET-Funding Program managed by the Austrian Research Promotion Agency FFG. The funding agencies had no influence on the conduct of this research.
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Lecture held at the 57th Global Fiber Congress Dornbirn, Dornbirn-GFC, September 12-14, 2018 in Dornbirn/Austria