**2. Recycling of textile materials**

*Waste in Textile and Leather Sectors*

performances, and operating electrical-powered equipment. Both outdoor and indoor noise pollutions are damaging for the people and animals. According to the ISO 25267 standard, sound pressure levels in living room, kitchen, and bedroom are recommended to be lower than 30, 40, and 20 dB, respectively. Subjecting to a noise level of 85 dB and higher for a long time can lead to increased blood pressure,

Materials used for controlling the noise can be generally categorized as those appropriate for treatment at source, those for declining along the noise path, and those for receiver treatment. The classification can be done by absorbing materials, scattering or altering the sound energy on impact; barrier materials, blocking the sound waves; vibration isolation, damping the materials for reducing radiated sound; and silencers and passive and active materials, quelling the sounds [6].

Generally, it is not possible to handle most of the noise problems at source, and the decrease in noise emission is mostly undertaken by noise isolation systems. Active and passive noise control methods are available for noise control. Active noise control is the total noise reduction process using a secondary audio resource, while passive noise control is supplied by sound absorbers and sound insulation materials. Active sound control is managed by using complex digital signal processing techniques to delay the harmful sound waves [7]. Sound absorption is made in passive control method, and knowing acoustic properties of these absorbent materials is helpful for determining the suitable usage fields of the products such as

The performance of insulation materials should be evaluated in terms of multiscale principles, respecting physical features, protection level of health and environment, and applicability as building elements and their cost. The basic requirements on insulating products are good thermal and sound insulation characteristics. Noise reduction coefficient of 0.5–0.9 and thermal conductivity coefficient of 0.030–0.045 W/mK are the main characteristics of the most conventional

Nonwoven textile materials are widely used for sound insulation and absorption applications due to their porous structure in recent times. These materials are used as sound absorbers, sound diffusers, noise barriers, and sound reflectors. For the sound transmission through friction, the sound wave should penetrate into the absorbent material. The loss of sound energy in textile materials is influenced by various physical issues such as fiber type, fiber diameter, material thickness, density, bonding method, air resistance, and porosity. The nonwoven surface forms a large inner surface in the form of a flexible fiber skeleton of a fabric composition,

For sustainable development of the textile industry, solutions for both decreasing waste and reducing noise have been searched for years. Caused by the reduction of ecological supplies and the rising costs of waste removal, the recycling or reprocessing of textile waste is becoming progressively significant all over the world in terms of economy, environment, and sustainability [12, 13]. Both natural and synthetic fibers together with recycled materials are used to construct textile-based acoustic materials. The use of textile surfaces produced from recycled materials has spread to a wide range of areas, and its use as sound insulation material has also

In this study, recycling nonwoven materials are considered in terms of sound insulation characteristics; the measurement methods of acoustic features of textile materials are explained; and the sound absorption properties of nonwoven fabrics produced from both pure and recycled polyester and polypropylene fibers are

raised stress level, and long-lasting hearing injury [4, 5].

sound barriers, walls, and road surfaces [8].

providing these required physical conditions [3, 10, 11].

insulating materials [9].

become widespread.

**86**

compared.

The growth of worldwide population, the rapid development of technology, the change of living standards and fashion trend, the improvement in marketing activities, and reduced time that the clothing is used before disposal result in serious increment in textile production and consumption. Along with these key factors for the rapid development in the textile industry, an overconsumption of clothing and an extreme use of resources and energy have started to become crucial problems and significantly responsible for the generation of textile waste as well.

In order to reuse the textile articles, the end-of-life textiles should be collected separately from the remaining waste, and reusable and nonreusable clothing have to be separated from each other. Textiles intended to be used in the second-hand market should be cleaned, dried, and not worn. According to Directive 2008/98/EC (European Parliament and Council of the European Union, 2008), recycling means any recovery action in which waste materials are reprocessed into products, materials, or substances for the original or other purposes. This description separates recycling from reuse required preparation by the word reprocess. Recovery, on the other hand, denotes any operation, either processing waste by replacing materials used to fulfill a function, or replacing waste being prepared to fulfill that function, in the plant or in the wider economy [14, 15].

Textile wastes released in and after the production of textile products have a large volume, and their disposals have become one of the most important threats to the ecological system. Apart from this, the raw materials used in this process are natural and limited resources; high water consumption and water pollution from this process as well as high energy consumption and the resulting carbon emissions have exceeded 1.2 billion tons per year [16]. Shortage of natural resources and increase of raw material cost have increased the importance of recycling. Due to the environmental troubles, the uses of biodegradable and recyclable material in many products become significant [5]. Theoretically, 97% of textile waste can be recycled [9]. In addition, waste storage areas are decreasing, and all these factors have created a serious threat to the environment and increased awareness in global world. As a result, the concepts of "recycling" and "sustainability" have gained great importance in the textile industry and have been among the priority state policies in the world.

Recycling in textiles can be defined as using the waste materials as raw resources to produce new products. According to the production flow of the new product produced by recycling textile wastes, recycling is evaluated in two groups such as open loop recycling and closed loop recycling. Open loop recycling characterizes the usage of recycled material in different purposes from the used material, whereas the closed loop recycling is the production of the used materials in the same product [17].

Textile recycling material can be also categorized into two groups such as preconsumer and post-consumer wastes. Pre-consumer waste contains materials from the various production stages of textile industries, and these wastes are remanufactured for the automotive, aeronautic, home building, furniture, mattress, coarse yarn, home furnishings, paper, apparel, and other industries. Post-consumer waste is described as any type of discarded garment or household articles, which are worn out, damaged, outgrown, or have gone out of fashion. The post-consumer textiles are "opened" mechanically or chemically so as to return it to a fibrous form, and they can be able to process new products for renewed consumption [18].

Textile recycling technologies are realized in four different processes. Primary recycling is the recycling of a product to its initial state. In secondary recycling,

waste is included in a different application area than its original form and transformed into a new product having lower levels of physical, mechanical, and chemical properties. In tertiary recycling, pyrolysis occurs by turning gas into simple chemicals or fuels by gasification and hydrolysis. The fourth recycling process is realized by utilizing the heat released by incineration of solid wastes. The recycling method of the fibers differs on the type of fibers, for instance, synthetic fibers are chemically recycled, whereas all others are mechanically recycled [19]. Recycling of synthetic fibers such as polyester and polyamide is included in primary recycling technologies and is evaluated within the scope of the "closed loop recycling" class. Today, the most applied recycling is "open loop recycling," which belongs to the secondary recycling technology class [13, 20].

Since recycled materials demonstrate good sound absorbing characteristics, these materials are becoming an interesting alternative to typical materials for functional applications. Acoustic barriers and acoustic ceilings, passenger vehicle noise absorbers, and wall claddings are some of the implementation of noise control functions of the nonwoven fabrics. Nonwoven fabrics made from recycled fibers are having more advantageous in terms of environmental pleasantness compared to traditionally utilized polyurethane foams produced by environmental harmful manufacturing methods and cannot be recycled [21]. Lower product cost, good processing, and environmental protection are some of the benefits of using recycled polyester nonwovens compared to conventional sound absorbers. Former studies about the noise absorption of nonwovens have indicated that the noise absorption coefficients of these materials in the high frequency range (*f* > 2000 Hz) are equivalent to that of the traditional sound absorbers such as rockwool and glass fiber [22].
