Introduction
In the twenty-first century, the efficient utilization of energy and water, pollution control, and waste management are critical issues that need to be addressed. With the increase in population, the demand for textiles has also risen significantly. Currently, the world consumes over a hundred million tonnes of different textile fibers, with synthetic fiber consumption accounting for more than fifty percent by weight (Figure 1). Consequently, textile waste has also increased, posing a significant challenge. Moreover, the COVID-19 pandemic has further escalated the use of protective gear, resulting in a substantial quantity of medical textile waste. Textile waste, which includes fibers, yarns, and fabrics, has been largely neglected as a waste category. However, it is essential to categorize the waste generated during textile production, as well as the waste generated at the end of a textile material's life, to effectively quantify and manage these wastes. Openly dumped textile waste creates microfibers that easily travel through waterways, leading to the presence of approximately 1.5 million trillion microfibers in the sea. These microfibers, through biomagnification, have entered the food chain, posing threats to aquatic animals and human health.
Materials such as ethanol, glucose, nanocellulose, cellulose nanocrystals, biogas, thermal and acoustic insulation, concrete and bricks, fibers, yarns, fabrics, and polymeric composites can all be produced from textile waste. Polymeric composites, in particular, have significant potential in utilizing waste textiles. However, there are various challenges involved in the collection, sorting, storage, and distribution of used textiles for recycling purposes.
Different Techniques of Textile Waste-based Composite Development
Textile waste can be utilized in four different ways as a reinforcement material for composite development:
1. Discarded Fabric as a Reinforcement
When waste textiles are obtained in fabric form from landfills, retail chains, or donations, the fabric's form and composition play a crucial role in determining the matrix material. For instance, if a fabric consists of more than fifty percent polyester, it can be directly hot-pressed to develop a composite material with sufficient mechanical properties for secondary load-bearing components. Thermoset composites can also be developed using discarded fabrics as a reinforcement, but the physical properties of the reinforcement material and the desired mechanical performance must be thoroughly understood before opting for this manufacturing route. Additionally, curing the composite materials at room temperature can reduce costs, and the break-even fiber volume fraction should be determined.
2. Shoddy as Reinforcement
Shoddy is the fibrous material obtained by shredding waste textiles. When waste textiles in fabric form cannot be directly used as reinforcement, they need to be cut into pieces and then rag-teared to form shoddy. Shoddy can be processed directly on an injection molding machine along with thermoplastic fibrous waste, such as polyester or polypropylene, to produce beads. These beads can then be reprocessed through injection or compression molding to create composite components. Another option is to produce a carded web from shoddy, which can be converted into a thermoset composite through compression molding. The mechanical performance of shoddy-reinforced composites can be further enhanced by adding filler materials such as cellulosic and non-cellulosic nanoparticles and microparticles. It is important to consider the color or hue of the shoddy used for composite production and process textile waste with similar color or hue separately to maintain the aesthetics of the final product.
3. Yarn, Woven, or Nonwoven Fabrics Developed from Waste Textiles as Reinforcement
The technology of converting shoddy into rotor-spun yarns is well-established. Rotor spinning is a fast and cost-effective method for producing yarns. It has been found that adding approximately 25% waste fibers to virgin fibers does not significantly affect the rotor yarn's tenacity, irregularity, and elongation. Recycled fiber-based yarns are already being used in applications such as bedsheets, doormats, and school blazer fabrics. However, the potential of recycled fiber-based yarns for 2D and 3D woven forms in composite development has yet to be fully realized. Nonwoven fabric manufacturing is the cheapest way to convert recycled fibers into fabric form. These nonwoven fabrics can be tailored to develop thermoplastic and thermoset composite materials according to specific requirements. For example, blending thermoplastic fibers with shoddy can produce a composite nonwoven fabric that is suitable for compression molded component production.
4. Nano/Microstructures Developed from Waste Textiles as Reinforcement
Nano or microparticles have a large specific surface area, which enhances their interface with the matrix material in a composite. This, in turn, helps distribute the applied stress in the composite material. Nanomaterials such as cellulosic nanocrystals can be extracted from waste textiles through milling or chemical hydrolysis. However, it is important to remove dyes from the textile before extracting nanocrystals. Additionally, recycled fibers like hemp, jute, and banana can be directly milled to produce nano/microparticles that can be used as fillers in composite materials. These particles have been found to enhance the mechanical performance of composites.
Outlook and Summary
The mechanical properties of fibers extracted from waste textiles can vary significantly, which directly impacts the mechanical performance of the resulting composite material. Hybridizing recycled fiber-based preforms with high modulus fibers like waste jute and hemp, or high-performance fibers like waste glass and carbon fiber, can further enhance the mechanical performance of textile waste-based composites. Engineered preform structures are crucial in improving the mechanical properties of these composites. Therefore, textile waste-based composite materials can find suitable applications in furniture materials, automotive components, biomedical applications, packaging materials, and various plastic products like magnet bars, rulers, sheets, flower pots, card cases, and bookcases. Textile waste-reinforced composites are highly beneficial in the building and construction industry for applications such as flooring, furniture, interior design, thermal and sound insulation materials, and concrete and brick reinforcement. It is puzzling why composite manufacturers have not fully realized the true potential of waste textile-reinforced composites for these applications. Textile waste-based composites have enormous potential to replace virgin natural fiber-reinforced thermoset composites in various applications, reducing the dependency on virgin natural fibers to a great extent. The choice of textile recycling routes should be based on a life cycle assessment to minimize environmental impact. Upcycling textile materials not only offers a sustainable solution for waste management but also opens up new avenues for innovation and resource conservation in the textile industry.