Environmentally-friendly thermal and acoustic insulation materials from recycled textiles

Journal of Environmental Management 251 (2019) 109536

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Review

Environmentally-friendly thermal and acoustic insulation materials from recycled textiles

T

Shafiqul Islam, Gajanan Bhat∗ Department of Textiles, Merchandising and Interiors, University of Georgia, Athens, GA, USA

ARTICLE INFO

ABSTRACT

Keywords: Textile waste Recycling Sustainability Acoustic insulation Thermal insulation materials Composites

Continuing increase in energy consumption and environmental pollution are some of the main challenges of the 21st century. One of the approaches to overcome these challenges is to increase the use of recycled materials and environmentally-friendly approaches to manufacturing. Thermal and acoustic insulation in buildings and transport vehicles from recycled textiles can play an important role in energy savings and reduction of environmental pollution. Textiles contribute a significant amount to the waste stream since most of these valuable fiber products are discarded after use. These discarded but valuable textiles can be recycled to produce several products including thermal and acoustic insulation materials. In this paper, a comprehensive review of the current state of textile waste generation and its environmental effects, and present progress of using industrial and post-consumer recycled textiles in insulation materials is provided. Mechanism of acoustic and thermal insulation materials of textile fibers are also reviewed. Existing research of some textile waste used as building insulation materials, method of conversion of textile waste into building insulation materials, comparative analysis of different insulation materials and life cycle assessment of textile fibers are assessed.

1. Introduction One of the biggest challenges of the 21st century is to maintain sustainability in all levels of energy resources and environmental context (Palmer, 2002). There is a steadily increasing concern in the energy and environmental sectors (Wazna et al., 2018). In the energy sector, concern is mainly due to the imbalance between the consumption and limited resources of energy (Rüttinger and Feil, 2010). In the environmental sector, concern is due to the rapid increase of world population with their consumption rate, and gradual increase in tendency to discard materials to landfill as waste before the end of lifetime of the products (Bergstrom and Randall, 2016). Building and automobile sectors are considered as main consuming sectors of global energy. At the global level, it is estimated that buildings consume about 40% of the world global energy, 25% of the global water, and 40% of the global resources (Asdrubali et al., 2015). Buildings are responsible for onethird of CO2 productions (Pérez-Lombard et al., 2008), approximately two-thirds of halocarbon and 25–33% of black carbon emissions (ÜrgeVorsatz et al., 2015). Similar results are also observed in Europe. In Europe, buildings are accountable for the consumption of 40% energy (Directive, 2010), and for the emission of 36% of the total CO2 (del Mar Barbero-Barrera et al., 2016). Therefore, reducing energy consumption in buildings is becoming a major concern, and is challenging policies in ∗

the present world (Iwaro and Mwasha, 2010). Thermal insulation in building materials can play a vital role in reducing energy consumption. Using efficient insulation materials can help to save energy by minimizing the losses and gains of heat during heating and cooling of building (Al-Homoud, 2005). Based on literature, a good insulation could save about 65% of energy consumption in domestic buildings (Hadded et al., 2016) and could save over one hundred times of the impacts of carbon footprint from material usage and disposal, irrespective of the type of materials used (Schmidt et al., 2004). Several ways to reduce CO2 emission during construction and renovation of buildings have been described. Some of the efficient techniques are improving building design to save energy, increasing use of sustainable building materials such as reuse or recycling (Meggers et al., 2012), increasing use of renewable energy (solar, wind, hydropower, bioenergy), integrating solar system with buildings to supply energy to buildings ((Mills, 2004; Schlaich, 1995; Kennedy, 2011), reducing electricity consumption (as a result indirect emission) by using more efficient instruments, utensils and lighting, and capturing and storing of CO2. Carbon capture and storage methods have the potential to reduce CO2 emission by more than 80% (Leung et al., 2014). Several researchers are studying the capturing methods of CO2 (Abdel-Rehim et al., 2006; Pervaiz and Sain, 2003; Zhang et al., 2018, 2019).

Corresponding author. E-mail addresses: [email protected] (S. Islam), [email protected] (G. Bhat).

https://doi.org/10.1016/j.jenvman.2019.109536 Received 1 April 2019; Received in revised form 30 July 2019; Accepted 4 September 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Environmental Management 251 (2019) 109536

S. Islam and G. Bhat

However, this technology is not fully commercially proven yet (Leung et al., 2014). CO2 is captured by combustion process followed by transport and storage. There are mainly three different combustion processes, namely, post-combustion (CO2 was removed from the flue gas after combustion has taken place), pre-combustion (fuel, coal or natural gas, is pre-treated before combustion) and oxyfuel combustion (oxygen, instead of air, is used for combustion). Combustion is a very costly process. According to the U.S. National Energy Technology Laboratory, post-combustion capture of CO2 would increase the cost of electricity production by 70% (Elwell and Grant, 2006). However widely used technique is to use building insulation materials that reduce CO2 emission by reducing energy consumption (Gummer et al., 2013). Redesigning concrete materials by using waste materials that would otherwise go to landfills have great potential to minimize the emission of CO2 (Huntzinger and Eatmon, 2009). Replacing concrete materials by lightweight composite materials with high strength and thermal insulation properties can significantly reduce the emission of CO2 (Meggers et al., 2012). It was observed that solid wall insulation is the most cost-effective approach to reduce CO2 emission when applied to large electrically heated buildings and used in conjunction with building construction or renovation works (Gummer et al., 2013). On the other hand, the gradual increase of noise pollution due to urbanization, industrialization, increased use of vehicles, electrical and mechanical appliances in home and industry, become a major health and environmental concern (Zach et al., 2012). Noise pollution are responsible for several health effects including sensorial, stress, high blood pressure, coronary heart disease, and stroke (Passchier-Vermeer and Passchier, 2000). In most cases, the diagnosis is not immediate which leads to further worse situations. A study estimated that about 65% of European citizens are exposed to noise levels which may lead to several adverse effects on health (Wallace et al., 2015). Besides health problem, noise pollution also disturbs people's work and reduces work efficiency (Chen et al., 2010). Sound insulation is certainly one of the techniques that can be used to reduce the harmful effects of noise. Several countries in Europe have regulations that buildings must install soundproof materials to reduce the adverse effect of noise pollution (Reixach et al., 2015). These regulations have further increased the demand for efficient and low-cost sound insulation materials. At present, the commonly used building insulation materials are produced from synthetic materials including glass fiber, mineral wool, and plastics (Patnaik et al., 2015). It was estimated that around 60% of manufactured thermal insulation materials in buildings come from mineral or inorganic fibrous materials (glass and stone wool), 30% are from foam materials (expanded polystyrene, extruded polystyrene, less widespread polyurethane), and the remaining 10% are from other nontraditional or composite materials (wool-wood insulations, gypsumfoam, etc.) (Ardente et al., 2008). In 2011 mineral wool and plastic shared 52% and 41% of the world thermal insulating materials market (Asdrubali et al., 2015). These types of materials can cause a diverse effect on the environment due to their non-renewable and non-disposable properties (Asdrubali et al., 2015). Glass fiber-based materials are obtained from the silica sources (Patnaik et al., 2015) which have carcinogenic effects on human body (Papadopoulos, 2005). Similarly, sound insulation materials are also composed of porous synthetic materials, including rock wool, glass wool, polyurethane or polyester, which are generally based on petrochemicals (Patnaik et al., 2015) and have an adverse effect on human health and environment. Due to these adverse effects of insulation materials, the demand for environmentfriendly insulation materials is increasing (Berardi and Iannace, 2015). Nowadays, awareness has been increasing in the use of environment-friendly and healthy materials. These understandings have motivated people towards the use of natural and recycled materials (Asdrubali et al., 2012; Glé et al., 2011; Iannace et al., 2012). As a result, desire for using building insulation materials made from harmless natural materials and recycled products is gradually increasing. Few of these materials are already present in the markets, but a vast

majority of the materials are in research or development stage (Asdrubali et al., 2015). Several authors are studying natural materials and fibers to develop thermal and acoustic insulation materials (Binici et al., 2014; Desarnaulds et al., 2005; Fatima and Mohanty, 2011; Fouladi et al., 2011; Zhou et al., 2010) due to their positive environmental effect, low carbon footprint (Pervaiz and Sain, 2003), and low hazardous effect to health. However, some researchers have expressed doubt on the actual sustainability of natural fibers as some of the natural fibers are produced using toxic chemicals (Berardi and Iannace, 2015). Again, during the production of insulation materials, special chemical treatments are done to increase the resistance of natural materials against fire, fungi, and parasites (Berardi and Iannace, 2015). Therefore, using recycled fibers, either natural or synthetic, might be the better option to produce sustainable insulation materials. Using waste textiles for acoustic and thermal insulations can help the development of energy performance, reduction in consumption of non-renewable resources, reduction of strain on the environment, and improvement of the health of humans and other living beings (Zach et al., 2016a). Again, every year millions of tons of clothing and textiles are discarded to landfill, which cause serious environmental pollution. In USA about 16 million tons (14.52 million mt) of textile waste were generated in 2014, of these 10.46 million tons (9.5 million mt) were sent to the landfill (Leblance, 2018). These huge amounts of landfilled textile wastes cause series of environmental pollution by contaminating groundwater and forming greenhouse gases upon decomposition (Dissanayake et al., 2018). Decomposed biodegradable textile waste produces methane, a powerful greenhouse gas that contributes to global warming (Wang, 2010). On the other hand, due to non-biodegradable and toxic nature, harmful effects of the landfilled synthetic materials on environment are immeasurable. Hence, use of recycled textiles for building insulation has multiple advantages. Using recycled materials can help reduction of fiber production by extraction or agricultural method which contribute to the highest amount of carbon footprint in apparel sector (Dissanayake et al., 2018), and can help get rid of environmental adverse effects due to landfilling of textiles waste. The purpose of this review paper is to provide the readers with a realistic picture of the present status and the potential for recycled textiles to be used as insulation materials. At first, the mechanism of acoustic and thermal insulation is discussed followed by the present condition of textile waste and its environmental effects, conversion of textile waste into building insulation materials, method of conversion, present research on some common textile waste used as building insulation materials. Comparative analysis of different insulation materials, and life cycle assessment of materials are also reported. 2. Acoustic insulation 2.1. Sound absorption of porous materials In order to achieve good acoustic insulation properties, the materials should be porous in structure. Porous materials allow sound to enter in their matrix and to dissipate. When sound waves enter into porous materials, air molecules within the pores vibrate that transform sound energy into thermal and viscous heat (Berardi and Iannace, 2015). At low frequencies, this energy is dissipated by the isothermal process (limited amount) but at high-frequencies, energy is lost by the adiabatic process (Berardi and Iannace, 2015, 2017). As a result, a low amount of sound is absorbed in low frequency, but absorption is higher at high frequency, resulting in low sound absorption coefficient at low frequencies. Textile fibrous products which are porous in structure can absorb sound energy and can be used as good sound-absorbing materials (Patnaik, 2016). Some of the useful parameters that can influence sound absorptions are the diameter, length, and the regularity of the 2

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fibers. Textile fibers with high density can increases the sound absorption value in the mid to high-frequency ranges (Koizumi et al., 2002). Thicker materials comparatively have lower air permeability but higher sound absorption properties (Küçük and Korkmaz, 2012). The non-woven structure has good sound absorption properties at the mid and higher frequency range, but low sound absorption properties at lower frequencies (100–400 Hz) (Seddeq et al., 2013).

including porosity, tortuosity, fiber density, viscoelasticity, thermal loss and air movement (Tang and Yan, 2017). Johnson-Allard microstructural model (Allard and Champoux, 1992; Allard et al., 1994; Johnson et al., 1987) can explain the acoustic properties of porous and fibrous materials filled with air at room temperature. This model could predict the acoustic absorption coefficient more accurately than empirical models (Tang and Yan, 2017).

2.2. Acoustic absorption models

2.3. Measurement of sound absorption

Several researchers have studied the relationship between noise reduction coefficient and various macro/micro-structural parameters of fibers (Nute and Slater, 1973a, 1973b). Both empirical and theoretical models have been developed to describe the acoustic absorption properties (Tang and Yan, 2017). These models have been mainly developed based on fiber length, fiber diameter, porosity, bulk density, and thickness. Empirical models were built applying regression analysis or curve fitting procedures by measuring the characteristics of wave impedance and propagation constants (Arenas et al., 2014; Bonfiglio and Pompoli, 2013; Garai and Pompoli, 2005). Delany and Bazley used a nonacoustical parameter of airflow resistivity for predicting the acoustical characteristics and many other factors and proposed the following simple model for fibrous absorbent materials (Delany and Bazley, 1970).

According to literature, researchers use several parameters (transmission loss, absorption coefficient, scattering coefficient, and sound reduction index) to describe acoustic properties of insulation materials. The sound reduction index (R) is used to measure the acoustic properties of the real sized sample including windows, door, wall, and ventilator. It can be measured in decibels following the international standard (ISO 10,140) by using the following equation (ISO 10140-2, 2010).

zc =

0c

1 + 0.0571

kc =

/ c 1 + 0.0978

0f

0f

0.754

j 0.087 0.7

j0.189

0f

R = L1

0.732

0.595

(2)

=1

where, zc is characteristic wave impedance, kc is characteristic propagation constant, ρ0c is characteristic impedance (Pa s/m), ω is the angular frequency (ω = 2πf), c is the sound speed (m/s), σ is the flow resistivity (Rayl/m), and f is the frequency (Hz). Delany and Bazley models were developed over a specified frequency range (10 < f/σ < 1000) and with a porosity of the material close to 1. Therefore, this model can be applied with the materials which have a high fraction of pores. Furthermore, this model was developed by measuring fibers whose diameters were between 1 and 10 μm, but some natural fibers' diameters are around 20–50 μm (Berardi and Iannace, 2015). Hence, Delany and Bazley's model has limitation for use with structures produced from thicker fibers (Attenborough, 1982; Garai and Pompoli, 2005; Horoshenkov et al., 1998). Garai and Pompoli modified the Delany–Bazley model by experimenting with thicker polyester fibers (20–50 μm diameter) and proposed the following model (Garai and Pompoli, 2005).

TL =

zc =

0c

1 + 0.078

kc =

/ c 1 + 0.121

0f

0f

0.623

j 0.074 0.53

j 0.159

0f

(5)

r2

(6)

20 log

(7)

where, r is the reflection coefficient and τ is the transmission coefficient. There are several international standards to measure these acoustic properties including ISO 10,534–2 (ISO 10534-2, 1998), ISO 354 (2003), and ASTM C423-17 (ASTM C423, 2017astm:2017). Sound absorption properties could also be measured using a single rating system including noise reduction coefficient (NRC) and sound absorption average (SAA) (Asdrubali et al., 2015). NRC is the arithmetic average of sound absorption coefficients at four 1/3 octave frequencies (250, 500, 1000, and 2000 Hz) to the nearest multiple of 0.05. SAA is also the arithmetic average of sound absorption coefficients at twelve one-third octave bands from 200 to 2500 Hz to the nearest multiple of 0.01. 3. Thermal insulation

0.66

Heat transfers from higher temperature to lower temperature by conduction, convection, and radiation processes. Thermal insulation is the property of a material to reduce heat flow or transfer. Heat transfer by radiation can be reduced by using absorbing or reflecting surfaces (Pelanne, 1977). Heat transfer by conduction can be controlled by using less conductive materials and breaking the continuous structure (Simmler et al., 2005). Transfer of heat through fibrous materials depends on the number of fibers, packing geometry, contact between fibers and temperature differences (Viskanta, 1965). Heat transfer through conduction and radiation can be reduced by increasing the thickness of fibrous assemblies (Zakriya et al., 2017). Thicker webs for the same areal density entrap a higher amount of air that reduces conduction (Anjaria, 1988). Thicker webs also create a tortuous path that increases the absorption or scattering of radiation and reduce heat transfer (Anjaria, 1988; Pelanne, 1977). As thermal insulation properties of materials depend on porosity

(3) 0f

S A

where, L1, L2 are the energy average sound pressures in the source and receiving room respectively (db), S is the area of the free test opening in which the test element is installed (m2), and A is the equivalent sound absorption area in the receiving room (m2). But impedance tube is used to measure the acoustic properties of small size samples. By using impedance tube several parameters of sound can be calculated including absorption coefficient (α) (Alcaraz et al., 2017) and transmission loss (TL) by applying the following equations (Jung et al., 2008).

(1) 0f

L 2 + 10 log

0.571

(4)

Garai–Pompoli model gives more accurate result when using natural fibers with larger diameters (Berardi and Iannace, 2015). Several other authors including Dunn-Davern (Dunn and Davern, 1986), Miki (Miki, 1990a, 1990b), Ramis (Ramis et al., 2010) and Yoon (2013) also proposed models to predict noise reduction coefficient. But the reliability of these models is limited for the interpretation of the behavior of natural materials due to high irregularity of fibers’ diameters, pores, and structures (Berardi and Iannace, 2017). Microstructural models are established on the microscopic physical characteristics of acoustic wave propagation through fibrous materials. The model is also developed by considering some factors of fibers 3

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S. Islam and G. Bhat

Fig. 1. Worldwide production of textile waste (a), and of textile fibers (b), based on the data from various sources cited.

(Smith et al., 2013) and tortuosity (ratio of the open pores length and the thickness) of that material (Hadded et al., 2016), textile fabrics that have a huge fraction of interconnected voids (Stanković et al., 2008) have become good choice to produce thermal insulation materials. Fibrous insulation materials produced by non-woven techniques possess adequate small void spaces with entrapped air layers which are ideal to prevent convective heat transfer (Woo et al., 1994). The layering of several fibrous webs increases air layers and thickness without adding proportional weight that increases the thermal insulation properties of those materials (Barker and Heniford, 2011). Thermal insulation property of a material is normally measured by thermal conductivity (λ) or thermal transmittance (U-Value). Thermal conductivity can be defined as the rate at which heat is transferred at unit length of a material in a direction perpendicular to the surface of a unit cross-sectional area as a result of the temperature gradient. It is quantified by using the units of W/mK. Thermal transmittance is the rate at which heat is transferred through one square meter of a material with a 1 K temperature difference. It is denoted by W/m2K. If heat flow, Q, (W) passed in 1 h through an area (A) of 1 m2 of the fabric with a thickness of L (m) at a temperature difference (T1-T2) of 1 °C, then conductivity factor λ (W/m°C) can be expressed by following equation (Sukigara et al., 2003).

= QL / At (T1

T2 )

4. Textile waste context 4.1. Present condition of textile waste Consumption of apparel and textile products has been increasing rapidly due to the increase of world population, consumer's buying power, and changing consumption and fashion patterns (Marshall and Farahbakhsh, 2013; Tojo et al., 2012). Fast fashion textile industries are responsible for high production and consumption of apparel and textiles. About 20% of global wastewater is produced by fashion industry. Fashion brand Burberry had burned about $40 m of unsold clothes, accessories, and perfume instead of selling it off cheaply, in order to protect the brand's value (Cooper, 2018). Within 15 years from 2000 to 2014, global clothing production has increased by 100%. At present on the average, a person buys 60 percent more clothing items and uses half of the time compared to 15 years ago. These changing consumption behaviors are generating a huge amount of textile waste (Leblance, 2018). In the US, from 2000 to 2014 the amount of textile waste has increased by 71.1% while overall other solid waste has increased by 6.17%. Only 16.2% of this textile waste has been recycled (EPA, 2016), and most of the wastes are disposed into landfill. It is estimated that around 92 million metric tons (mt) of textile waste is produced annually and this waste is likely to increase by about 60% from the year 2015–2030 (GFA, 2017). In 2014, more than 16 million tons (14.52 million mt) of textile waste was produced in the United States, of which 2.62 million tons (2.38 million mt) were recycled, 3.14 million tons (2.85 million mt) were combusted for energy recovery, and 10.46 million tons (9.5 million mt) were discarded to the landfill (Leblance, 2018). Similarly, in Europe among 5.8 million mt of wastage textile, only 1.5 million mt were recycled and rest of the 4.3 million mt were sent to landfill (Al-Homoud, 2005; Asdrubali et al., 2015; Hadded et al., 2016). A recent report estimated that the present wastage of textiles in Europe has increased to about 16 million mt per year (European Commission, 2017). In addition to these, a huge amount of waste is produced in the textile industry during the manufacturing process. These wastes are composed of short fibers, yarns, threads, cutting waste, fabric scraps, and rejected fabric in quality control sections (del Mar Barbero-Barrera et al., 2016). Fig. 1 shows a summary of textile waste around the world. It seems that post-consumer textile wastes are landfilled as it is the most convenient method. But things are changing in the developed and densely populated cities where it is difficult to find an open space to discard waste. In USA, it is estimated that the average landfilling cost of waste per ton (0.907 mt) was USD 50.3 in 2017 and present increase in rate of landfilling cost is about 6 percent which will be much higher in

(8)

Specific heat resistance (r) is the characteristic inverse of λ and expressed by the following equation (Abdel-Rehim et al., 2006).

r = 1/ = At (T1

T2)/ QL

(9)

Thermal resistance, Rth, of textile fabrics, is the ratio of actual thickness (L) of sample in meter to the thermal conductivity (k) in W/ m2°C.

Rth = L / k

(10)

Thermal conductivity is considered the most important parameter to evaluate thermal insulation material. A material with a thermal conductivity lower than 0.07 W/mK can be regarded as a thermal insulator (Asdrubali et al., 2015). Thermal conductivity can be measured by following international standard ASTM C518 (2017) and thermal transmittance can be estimated by following ISO 6946 (2017). Thermal insulation value of textile materials can be determined by three common methods, Disc or plate methods (The lee's disc method), constant temperature method, and the cooling method. The lee's disc method is commonly practiced in the laboratories (ElNashar and Bashkova, 2014). 4

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the future (Thompson et al., 2018). The condition is worse in the northeastern states of the US, where landfilling cost was 74.75 USD per ton (0.907 mt) in 2017 (Thompson et al., 2018).

preceding modification and transferring to a new consumer (Fortuna and Diyamandoglu, 2017). Recycling means converting the textile waste into new (textile or non-textile) products with or without damaging the previous one (Sandin and Peters, 2018). Recycling routes of textile waste can be mechanical, chemical, thermal or combination of mechanical, chemical and thermal processes. Based on the disassembling process, recycling can be referred to as fabric recycling (also referred as reuse), fiber recycling (fabric dissembled up to fiber), and polymer recycling (break it down to polymer) (Sandin and Peters, 2018). Based on end products, recycling can be classified into two categories. One is recycling within the production process which is related to the production of new items that are similar to the textiles. Another approach is recycling out of the production process which refers to producing a different item like developing a composite material for building insulation (Rubino et al., 2018; Zonatti et al., 2016). Cost of recycling also varies with recycling routes, dissembling process, and categories of recycling. Again, industrial data of recycling cost is hardly found in the literature. However, we can indirectly get an idea about recycling cost to produce insulation materials and it is very cheap. For example, at present, average price of 1 kg cotton fiber is $1.39 (Markets Insider., 2019) and average price of 1 kg cotton yarn is $2.80 (TEXTILEBEACON, 2019). Therefore, production cost of 1 kg cotton yarn should be less than $1.40 (as yarn price $2.80 includes fiber price, profit, and other costs). General production process of yarn from fibers includes blow-room, carding, drawing, roving, and spinning. On the other hand, most of insulation materials from waste textiles are produced by nonwoven techniques which require less expensive carding process followed by needlepunching. Hence recycling of waste textiles requires fewer steps that the five steps of yarn production. Therefore, it is possible to get an idea about recycling cost which would be roughly one-fourth of the yarn production cost. Also, one has to take into consideration the savings from landfilling cost of the material. Although recycling cost of textile waste is low and it is estimated that up to 95% of textile waste could be recycled into different valuable products (Dissanayake et al., 2018) but still the rate of recycling is relatively low. This may be due to the diversity of fibrous waste and structure (Serra et al., 2017). Textile waste varies with colors, types, compositions and properties, which make it difficult to find an appropriate recycling technique (Briga-Sa et al., 2013). In order to promote the recycling rate, a Directive (2010) has been passed in European parliament. Integrated knowledge and effort are required to increase the rate of recycling (del Mar Barbero-Barrera et al., 2016; Mulinari et al., 2011) and for producing economically feasible products. Producing acoustic and thermal insulation materials by textile waste is one of the effective sustainable recycling processes. Recently researchers also determined that the physical properties of textile waste are very much comparable to that of the conventional building insulation materials (Zach et al., 2012). For this reason, textile wastes are recommended as feedstock to building insulation materials by several researchers (El Wazna et al., 2017). The recycling of textile waste into building insulation materials has potential benefits in environmental, health, social and economic sectors. The use of high quality thermal and acoustic insulation materials can reduce strain on the environment, energy consumption, space required for landfill, virgin fibrous materials, greenhouse gases, pollution (noise, air, water, land); can save petroleum, fuel, and natural resources; and can improve the healthiness of human habitat (del Mar Barbero-Barrera et al., 2016; Pichardo et al., 2017; Zach et al., 2016a). The recycling of textile waste as an insulation material could also develop a new model of the circular economy (Rubino et al., 2018). By producing acoustic and thermal insulation materials, it is possible to get rid of the harmful effects of discarded textile waste, and a new market or economy can grow for selling and buying these discarded textiles. In addition, by producing low-cost thermal insulation materials, energy required for heating and cooling of our buildings and automotives can be reduced.

4.2. Environmental effect Rapid increase in generation of textile waste has been creating a serious problem on the environment and on public health. It is estimated that the annual environmental effect of a family's clothing is equivalent to the amount of water needed to fill 1000 bathtubs and the amount of carbon dioxide that is emitted due to run a modern car for about 6000 miles (Leblance, 2018). The emission of CO2 is predicted to increase by more than 60% to nearly 2.8 billion mt per year by the year 2030 (GFA, 2017). Studies have estimated that if half of the people in UK use their clothes 9 months longer than their present use time, then the carbon footprint and water footprint will decrease by about 8% and 10% respectively (WRAP, 2017). Based on these results one can easily understand that the textile waste has huge adverse effect on the environment. Both natural and synthetic fibers are responsible for this harmful effect. Textile waste from natural cellulosic fibers produces methane gas after degradation which is a powerful greenhouse gas and is responsible for global warming (Wang, 2010). Formation of greenhouse gases may also pollute ground water. In 2017, 30 people died in Srilanka due to the exposure of high concentration of methane gas which was produced by careless landfilling of waste (Dissanayake et al., 2018). Textile waste from organic animal fibers like wool produces ammonia, which is responsible for creating toxicity in land and water (Jayasinghe et al., 2010). Synthetic textile waste which comes from petroleum-based resources has a more adverse effect on the environment due to its nonbiodegradable and toxic characteristics. Textile waste that is discarded to landfill also produces leachate after decomposition which has the potential to pollute both surface and groundwater sources (Jha et al., 2004). Therefore, just by discarding textile waste we are polluting our environment in various ways, at the same time contributing to depleting our valuable natural resources. 4.3. Sources of textile waste Textile waste comes from a variety of sources, starting from fiber producers to end-users. Although most of the textile waste comes from household sources, waste from production lines are also increasing. Based on sources, textile waste can be classified as pre-consumer or post-industrial and post-consumer (Serra et al., 2017). Pre-consumer waste or post-industrial waste is generated during the manufacturing process of apparel and textiles. These types of wastes include short fibers, combers noils, yarn waste, rejected fabrics due to manufacturing or dyeing faults, garments cutting waste, rejected garments, trimming and end lots from surplus production (Rubino et al., 2018). It is estimated that a total 10–20% textiles are wasted during the manufacturing process (Domask, 2007). Post-industrial textile wastes are considered virgin or clean waste as the materials are discarded without being used (Dissanayake et al., 2018). Post-consumer textile waste is referred to any types of clothing or textiles that are no longer used by the consumer due to damage, wear, out of fashion or any other problems in the materials that end the willingness to use the products by the consumer. These post-consumer textile waste can be used for different purposes. It is estimated that more than 70% of useful life remains at the time of discarding the clothes (SATCOL, 2019). 4.4. Reuse or recycle of textile waste Textile waste can be utilized in different applications by reuse or recycling of the waste (Briga-Sa et al., 2013). Reuse of textile waste means extending the serviceability of textiles with or without some 5

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5. Life cycle assessment

and polyamide were determined at different production stages (Roos et al., 2016). Some other researchers also estimated the carbon footprint and water footprint of textiles. Cotton consumption is accountable for 2.6% use of global water (Chapagain et al., 2006). Worldwide water consumption for the production of cotton fiber from 1997 to 2001 was 256 GM3 (256 × 1030 L) (Chapagain et al., 2006). One of the ways to reduce water consumption is to replace cotton fibers with recycled cotton. Roos et al. found that in a Swedish factory by using recycled cotton fiber over one-year carbon footprint and water consumption can be reduced by around 2.4 × 106 tons equivalent CO2 and above 900 billion liters of waters respectively. Similarly, by using recycled polyester, carbon footprint and water consumption can be reduced by around 2.3 × 106 tons equivalent CO2 and above 1000 billion liters of waters respectively (Roos et al., 2016). Van der Velden et al. did LCA analysis of textiles made of cotton, polyester, nylon, acrylic, or elastane. It was observed that acrylic and polyester have least effect on environment and cotton has the highest effect (van der Velden et al., 2014). Woolridge et al. conducted life cycle assessment at different stages of textile manufacturing of cotton and polyester fibers including fiber, spinning, knitting, dyeing, and garments sections. It was found that by using one tonne of cotton and reused polyester garments, a net energy of 64,951 kWh and 89,811 kWh can be saved (Woolridge et al., 2006). After considering several factors including extraction of resources, manufacture of materials, electricity generation, clothing collection, processing, distribution and final disposal of wastes, it was found that by replacing 1 kg of virgin cotton with 1 kg of second-hand clothing approximately 65 kWh energy could be saved. And for one kg of polyester, the savings in the amount of energy is about 90 kWh (Woolridge et al., 2006). Production of 1ton of recycled polyester garments required only 1.8% energy in comparison to that of the production of 1ton polyester garments from virgin materials. Similarly, producing of 1ton cotton clothes needed only 2.6% energy in contrast to production of 1ton cotton clothes from virgin materials (Woolridge et al., 2006).

Life Cycle Assessment (LCA) is a systematic way to assess or estimate the environmental impact of a product in its complete lifetime from production to disposal (Woolridge et al., 2006). It covers every phase from raw materials to disposal (Bhat, 2007). This is also referred to as cradle to grave approach (Asdrubali et al., 2015). Normally, design and development of a product are not included in calculation of LCA as it is considered that design and development do not significantly impact the environment. But some researchers have disagreed with this and showed that design and development of a product can enormously affect other stages of life cycle (Rebitzer et al., 2004). For estimation of LCA of a product, the emissions to air, water, and land are assessed during production, use, and disposal of the product. After that, the assessed amount is related to the possible environmental effects including resource depletion, ozone depletion and global warming (Woolridge et al., 2006). A complete LCA analysis can be accomplished by following some international standards, ISO 14040 (2006) and ISO 14044 (2006). LCA analysis is normally done by some consultancy companies or research institutes (Esteve-Turrillas and de la Guardia, 2017). Several researchers have evaluated the effect of cotton cultivation on environment both by conventional and organic agriculture methods (Baydar et al., 2015; Murugesh and Selvadass, 2013a) and the effect of different dyeing and finishing processes on the environment (Murugesh and Selvadass, 2013b; Yuan et al., 2013) using LCA. Van der Velden et al. have studied LCA of different textile fibers, polyester, nylon, acrylic, and cotton, and determined the comparative effects of these fibers on the environment (van der Velden et al., 2014). But in the case of recycled textile fibers, only a limited number of researchers did some LCA study (Morley et al., 2006; Woolridge et al., 2006). Roos et al. studied the life cycle assessment (LCA) of Swedish apparel sector to investigate the contribution to carbon footprint (Fig. 2) at different stages of clothing and textiles from fiber production to end of life (Roos et al., 2016). Here carbon footprints of cotton, polyester,

Fig. 2. The carbon footprint of the Swedish apparel sector over one year (reprinted with permission) (Adapted from Roos et al., 2016).

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6. Conversion of waste to acoustic and thermal insulation

approach (Shoshani and Wilding, 1991; Shoshani and Yakubov, 1999, 2000). Nonwovens are ideal methods for producing insulation materials due to their unique fiber orientation and porous structure (Wazna et al., 2018). General steps used in nonwovens techniques start from the fiber or polymer that is recycled from textile waste. The recycled fibers are then transformed into webs with different areal density. Web formation is a process where loose fibers are arranged in a sheet structure by laying down of fibers in a number of ways including dry-laid webs, wet-laid webs, spunbonding, and film casting. Both staple and filament fibers can be used to produce a fiber web depending on the technique used (Belal, 2009). After web formation, there needs to be some kind of bonding between the fibers to increase the strength and stability of the materials. There are mainly three different bonding processes, chemical (latex or chemical reagents), thermal or heat, and mechanical bonding (needle punching or hydroentanglement) (Belal, 2009). Selection of web formation and bonding process depends on several factors including fiber type, required strength, density, thickness and the desired end-use properties of the produced nonwovens. General steps of drylaid webs (carding) and needle punched bonds are presented in the following scheme (Fig. 4). Wazna et al. used the nonwoven technique to produce insulation materials from acrylic and wool wastes (Wazna et al., 2018). After collecting textile wastes, they were shredded into fibers. These fibers were opened and cleaned as short-staple fibers with a flow rate of 5 kg/ min. Then, fiber webs with controlled thickness were produced by a dry-laid process where fibers were introduced into the card, and folded into 10 times. Wazna et al. used the needle punching technique for bonding (Wazna et al., 2018) which is one of the oldest methods of making nonwoven fabrics. A barbed needle is used which is driven upward and downward through fiber webs. These actions of needles interlock the fibers and help hold the structure together by frictional forces. In this method, bonding is done without using any toxic binders (Rubino et al., 2018). Wazna et al. produced four different nonwoven thermal insulation materials by varying waste materials and got excellent insulation performance, with the thermal conductivity values in the range of 0.03476–0.04877 W/mK (Wazna et al., 2018). Patnaik et al. also used a similar nonwoven technique to produce thermal and acoustic insulation materials from waste wool and recycled polyester fibers (RPET) (Patnaik et al., 2015). In that research, a bale opener was used to convert the waste textile into individual fibers, followed by cross lapper to produce a cross-lapped web, and a needle punching machine to bond the fiber web. In addition, the authors included a mixture of di-ammonium phosphate and sodium tetraborate as a fire retardant (5% by weight) and silicon (1% by weight) with the fiber web to increase the fire retardancy and moisture resistance properties of the insulation materials. The average thermal conductivity and sound absorption coefficients of their materials were 0.032–0.035 W/mk and 0.42 to 0.58 (at 1000–2000 Hz) respectively (Patnaik et al., 2015). Zach et al. produced thermal and acoustic insulation materials using recycled cotton, polyester and flax fibers (Zach et al., 2016a). In that research also, a nonwoven technique was used, but instead of using the carding process, airlay method was used for web formation. In airlay method, fibers can be separated by suspending them in an air stream, and then blowing the fibers onto a moving belt to form a uniform layer of fiber web (Belal, 2009). The production capacity in airlaying method is higher than that of the carding process (Zach et al., 2016a). After forming layers of fibers as webs, bonding was done by the hot-air or mechanical bonding process. Zach et al. designed five different mixtures of the sample by combining polyester, flax, cotton, and bicomponent fibers of polyester with lower sheath melting temperature. The average thermal conductivity and absorption coefficients of their materials were 0.037–0.049 W/mk and 0.7 to 0.9 (at around 1000 Hz) respectively (Zach et al., 2016a).

Conversion of textile waste into thermal and acoustic insulation materials is not a straight forward process. As the type of textile waste is different including industrial or post-consumer, synthetic and natural, therefore the conversion process is also varied. In general, at first, the pre-consumer textiles or post-consumer garments are accumulated from various textile industries and consumers. After that, the wastes are preciously categorized according to their fiber type, quality, and color. Then these wastes are cut into small pieces, shredded and carefully opened back into fibers without breaking the length of the fibers. The obtained fibers are sorted again based on the characteristics of fibers including length, strength, and count (Esteve-Turrillas and de la Guardia, 2017). But for synthetic fibers, the process can go back up to polymer formation. Then these fibers or polymers may be used to produce thermal or acoustic insulation materials by a nonwoven processing technique including web formation, web bonding, and finishing. According to desired end properties, different fibers can be blended together. In some cases, nonwoven materials may undergo high heat and pressure to produce composite materials. However, all researchers do not follow the same steps. Researchers are continuously developing novel methods to optimize processes for maximum benefits. Some researchers directly used scrap fabric, then join them together and produce insulation materials (Jordeva et al., 2014; Trajković et al., 2017). Several others used nonwoven techniques and prepared composites by applying heat and pressure (Shoshani and Wilding, 1991; Shoshani and Yakubov, 1999, 2000). Some researchers used waste textile fibers and mixed them with building materials including lime and cement (Algin and Turgut, 2008; Balasubramanian et al., 2006; Binici et al., 2009b, 2010). 6.1. Direct method In this method, fabrics are directly used to produce insulation materials without converting fabrics into fibers. In some cases, the opening of fabrics into fibers may lead to loss of structure and mechanical strength of fibers. Synthetic fabrics like polyester may get melted due to the heat generated from the applied mechanical force during the conversion of fabrics into fibers. On top of that, sometimes fiber recycling may not be economically beneficial due to higher time and energy required. In these cases, fabrics are directly used for the manufacture of insulation materials (Trajković et al., 2017). Several researchers investigated the use of waste fabrics directly to produce insulation materials (Jordeva et al., 2014), developed thermal insulation materials by cutting textile wastes and then stabilizing the wastes by stitching. Trajković et al. directly used polyester apparel, cutting them to construct the insulation structures (Trajković et al., 2017). In this research, three different types of polyester woven clothes were collected, then cut into small species (both irregular and exactly small species) by using rotary blades and vertical knife, and marked them A, C, and D (Fig. 3). They also collected polyester knitted (70/25/ 5 PES/cotton/Lycra®), converted them into partial fibrous form (for comparison) and marked as B (Fig. 3). Subsequently, they encased the cutting waste by using 100% polypropylene non-woven fabric and prepared insulation materials. By applying this method, very good thermal and acoustic properties were achieved with the thermal conductivity ranging from 0.0520 to 0.0603 W/mK and NRC ranging from 54.71 to 74.77%, and no significant difference with sample B to others (Trajković et al., 2017). 6.2. Nonwovens techniques Nonwovens being the most commonly used techniques for preparing insulation materials, several researchers have tried to produce acoustic and thermal insulation materials from waste textiles by this 7

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Fig. 3. Wasted polyester (a) Woven cut pieces at different size (b) Knitted fabric with fibrous form (c) Woven cut pieces at average dimension 6 × 4 inches (d) Woven cut pieces at average dimension 8 × 4 inches (reprinted with permission) (Adapted from Trajković et al., 2017).

insulation by using a recycled Kevlar/Nylon/low-melting polyester nonwoven, which was reinforced with recycled polypropylene selvages (Lin et al., 2016). Similar to the previously discussed methods Lin et al. also produced Kevlar/Nylon/low-melting PET nonwoven fabrics through the opening, blending, carding, lapping and needle-punching processes. Then these nonwoven fabrics were passed through 1-mm-gap Twin-Roller Hot-presser at 160 °C at a speed of 0.5 m/min. After that, 10 wt% of two-layer PP nonwoven selvages were kept between Kevlar/ Nylon/low-melting PET nonwoven fabrics and again needle punched. Finally, the composite layers were hot-pressed at different temperatures. Lin et al. found the best puncture resistance of their hot-pressed composite layer was at a temperature of 180 °C. Also low thermal conductivity and good sound absorption coefficients were observed for their five layers composite materials which were 0.047 W/mK and above 0.9 at 1000 Hz respectively (Lin et al., 2016). Dissanayake et al. used nylon/spandex (NS) and polyurethane (PU) to produce insulation materials by using a compression molding technique (Dissanayake et al., 2018). At first, NS and PU fabric wastes were collected and shredded into very small pieces (2 mm × 2 mm) (Fig. 5a and Fig. 5b) by using a commercial shredding machine. Then NS and PU were mixed at different ratios. After that circular insulation materials were produced (Fig. 5c) with varying thickness using a compression molding method where the shredded samples were pre-heated and compressed by high pressure. The best thermal conductivity observed was at the composition of 60:40, NS: PU (%W/W) with a thermal conductivity value of about 0.0953 W/mK (Dissanayake et al., 2018).

6.3. Preparing composite with heat and pressure/compression molding Insulation materials from textile waste can also be produced as composites by applying heat and pressure. In most of the cases, fiber webs are initially produced by a nonwoven technique. After that heat and pressure are applied to different layers or mixture of fibers to form the composite. When the cellulosic fibers are used in the mixture or layers of the composite, extra care should be taken so that the high temperature does not degrade the cellulosic fiber. If the melting temperature of the synthetic binder fiber is higher than that of the cellulosic fiber, then the plasticization and hydrolysis process are done to reduce the melting temperature of the synthetic fiber. Palakurthi developed a composite material from recycled PET and cotton (Palakurthi, 2016). As the melting temperature of PET (260 °C) is higher than the degradation temperature of cotton (146 °C), she treated PET with plasticizers (2 Phenyl phenol, Benzyl Butyl Phthalate, Diallyl Phthalate, Benzoic Acid) and alkali (Dimethyl Sulfoxide, Tetramethyl ammonium hydroxide) to reduce the melting temperature of PET (Palakurthi, 2016). Ramamoorthy et al. also investigated reusing of discarded cotton/PET blend fabrics as reinforcement in composites (Ramamoorthy et al., 2014). Three compression-molding concepts were discussed and evaluated. In the first method, the melting temperature used was higher than that of polyester fabric with or without using a plasticizer. The drawback of this method was that cotton fiber degraded due to high temperature. In the second method, a bio-based resin from soybean oil was used as a matrix. And in the third method, a thermoplastic core-sheath type bi-component fiber was used that was carded, and needle punched to form a nonwoven fabric. Ramamoorthy et al. placed this nonwoven between layers of recycled cotton/PET fabrics and subjected to compression molding to form composites. Compression molding temperatures were used in such a way so that the sheath of the bicomponent fiber melts and acts as a matrix. Ramamoorthy et al. were able to produce a composite successfully by using method two and three without degrading the cotton fiber. It was observed that the composite produced by the third method showed improved mechanical properties with 4 times and 2.2 times higher tensile strength from composite type 1 and 2 respectively (Ramamoorthy et al., 2014). Lin et al. produced composite materials for acoustic and thermal

6.4. Mixing with building materials Textile waste has been used with building materials (lime, cement, and others.) for decades. Textile fibers show a higher homogeneity in their performance than agro-based fibers (Reddy and Yang, 2005). Fabric weight is about 1/30 to the brick, steel or concrete which can be used as a low-cost reinforcement material (Binici et al., 2012). Recently researchers observed that textile waste used in mud-brick structures (Binici et al., 2005, 2007, 2009a) or other ways to produce building have several other advantages including earthquake-resistance (Binici

Fig. 4. Schematic diagram of (a) web formation (dry-laid) and (b) bonding (needle punch) (Adapted from Belal, 2009).

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Fig. 5. (a) Shredded PU, (b) Shredded NS, and (c) molded sample (reprinted with permission) (Adapted from Dissanayake et al., 2018).

et al., 2005), low thermal conductivity (Binici et al., 2007) and good sound insulation properties (Binici et al., 2009a). Along with these properties, textile fibers can increase strength, performance, and durability of the building materials (del Mar Barbero-Barrera et al., 2016; Zakaria et al., 2017). The conversion of textile waste to thermal and acoustic insulation materials is very simple when used with building materials. Here conversion method is completely different from nonwoven and composite techniques. In these cases, there is a need to mix the textile waste in appropriate ratio with the cement, lime, water, bricks and other building materials. Several research articles, investigating the use textile waste with building materials (Algin and Turgut, 2008; Balasubramanian et al., 2006; Binici et al., 2009b, 2010), have already been published. Del Mar Barbero-Barrera et al. used textile fiber waste with natural hydraulic lime to produce boards with higher acoustic and thermal insulation properties (del Mar Barbero-Barrera et al., 2016). Initially, textile wastes (mainly cotton) were collected from industry (Fig. 6a), and without any further processing, these wastes were mixed with natural hydraulic lime and water in three different ratios. After several days of curing and drying the samples were tested (Fig. 6b). Del Mar BarberoBarrera et al. observed the best thermal conductivity of 0.14 (W/mk) which is much lower than that of a similar board made with wood–gypsum (0.189–0.277 W/mK) (Bekhta and Dobrowolska, 2006) and newspaper sandwiched aerated lightweight concrete panels (0.300 W/ mK–0.600 W/mK) (Ng and Low, 2010). But the acoustic absorption coefficient was about 0.2 at 2000 Hz which is comparatively low. Higher compression strength was also observed after using textile waste fiber (del Mar Barbero-Barrera et al., 2016). Binici et al. used textile waste (cotton), sunflower stalk, and stubble fibers to produce sound and thermal insulation materials for building applications. In this research, insulation boards were produced by two approaches: one is by using plaster and another by using epoxy as a binder (Binici et al., 2014). In the first method, the mixture of textile waste and sunflower stalks were grounded. Then these materials were

applied on the wall (mud bricks, concrete bricks, and red bricks) along with plaster binder. In the second method, sunflower stalks, cotton waste, and textile waste fiber were mixed with an epoxy binder at different ratios and produced as layers. After that, high pressure was applied on these layers to produce insulation boards. Binici et al. found lower thermal conductivity (0.0728 W/mk) and high sound insulation properties compared to the wall without these insulation materials (Binici et al., 2014). 7. Thermal and acoustic insulation from different textile wastes 7.1. Polyester Polyester is the most widely used fiber in apparel and textiles. In 1990 global polyester fiber production was 8.67 million mt (statistica, 2019) and present production has increased more than 5 times compared to 1990. In 2017, the annual production of polyester fiber was around 53.7 million mt which is about 51 percent of the global fiber production (Textile Exchange, 2018). Although Polyester waste is the leading element of the clothing industry waste, its recycling rate is very low. In 2017 only 14% of waste has been recycled (Fig. 7), and a huge amount of this synthetic fiber has been discarded to landfills as waste (Textile Exchange, 2018). Several researchers are studying approaches to minimize the environmental effects by recycling polyester waste into different materials including acoustic and thermal insulation materials. Trajković et al. used polyester cutting waste during apparel manufacturing to produce thermal and acoustic insulation materials (Trajković et al., 2017). Here polyester cutting waste was directly used as an insulation blanket for roofing and buildings’ internal walls. A good insulation property was observed with the thermal conductivity between 0.0520 and 0.0603 W/mK and the noise reduction coefficient ranging from 54.71 to 74.77%. Trajković et al. also found that their insulation materials had good resistance against fire and were less degradable in moist condition (Trajković et al., 2017). Lee et al. developed sound absorbing nonwoven materials using

Fig. 6. Textile waste fiber and produced board (reprinted with permission) (Adapted from del Mar Barbero-Barrera et al., 2016).

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2018). In 2017, the average production of cotton around the world was 25.8 million mt (Fig. 7) which was about 24.5% of the global textile fiber market (Textile Exchange, 2018). According to the Ellen MacArthur Foundation, less than one percent of all clothing is recycled back into clothing (Textile Exchange, 2018). Therefore, a huge amount of cotton fiber is landfilled as discarded apparel. In addition to this, a massive amount of virgin cotton fiber is discarded as waste during cotton yarn production. Several researchers proposed the use of cotton waste for the production of thermal and acoustic insulation materials. Some companies have started commercial production of insulation materials form cotton waste. Bonded Logic incorporated commercially produced Echo Eliminator™ Acoustic Panels from recycled cotton fibers. The noise reduction coefficient of their products was 0.8–1.15, measured according to ASTM C 423 standard test method (Bonded Logic, 2019). Küçük and Korkmaz produced nonwoven composites by mixing cotton and polyester fibers at different ratios. It was observed that a mixture of 70% cotton and 30% polyester had an excellent sound absorption coefficient in the mid-to-high frequency ranges (Küçük and Korkmaz, 2012). Del Rey et al. also observed similar results of sound absorption properties for recycled cotton and polyester composites (del Rey et al., 2015). It was also noticed that the absorption coefficient and airflow resistivity values of recycled cotton polyester composites are very similar to those of commercially available sound absorptive materials in the market (del Rey et al., 2015). Sedlmajer et al. studied the thermal conductivity of insulation mats produced from recycled polyester, cotton, and bicomponent fibers at different ratios. It was determined that mats with higher cotton fiber ratio showed better insulation properties (Sedlmajer et al., 2015). Binici et al. produced lightweight composite materials from cotton waste and fly ash and it was observed that the composites with higher amount of cotton waste showed better thermal conductivity (Binici et al., 2012). Some researchers examined the effect of mixing cotton waste with building materials to improve the insulation properties. Aghaee et al. focused on increasing the thermal insulation of building materials with cotton waste. In this research, lightweight perlite and concrete panels were produced for building a partition. Aghaee et al. embedded the waste textile fibers consisting of cotton fibers and woven meshes of glass fibers, then confined with textile meshes embedded in the central part of perlite lightweight concrete. The perlite porosity and the textile fiber core reduced the thermal conductivity to 0.3 W/mK (Aghaee and Foroughi, 2013a, 2013b). Binici et al. also used textile waste (cotton), sunflower stalk, and stubble fibers to produce sound and thermal insulation material for building applications (Binici et al., 2014). 44 samples were produced by mixing these materials at different ratios. Among 44 samples, some noticeable desirable thermal conductivity values were 0.0871 and 0.0893 produced by a combination of sunflower stem/cotton waste/epoxy (36.36/36.36/27.27) and sunflower stem/sunflower stalk fiber/cotton waste/epoxy (37.04/7.41/24.69/ 30.86) respectively (Binici et al., 2014). Rajput et al. mixed recycled cotton waste and paper mill waste with cement bricks. Cotton fiber waste increased the porosity of cement bricks from 0.18 to 0.29, and improved the thermal insulation of the building. Thermal conductivity value decreased from 0.32 to 0.25 W/mK (Rajput et al., 2012). Binici et al. combined cotton waste with fly ash and analyzed the potential use of this combination for manufacturing low-cost and lightweight composites as building materials. Here cotton waste and fly ash were mixed with cement and a model house was built. Then the thermal insulation properties of this model house were compared with that of a control sample that was made without using cotton waste and fly ash. Binici et al. determined that the thermal insulation properties of their model house were superior to that of the control one (thermal conductivity was 29.3% lower). The compressive strength, flexural strength, unit weight, and water absorption properties of their developed materials also fulfilled the required standards (Binici et al., 2010; Binici and Aksogan, 2015).

Fig. 7. Comparison of produced and recycled fibers.

Fig. 8. Influence of temperature and bulk density on thermal conductivity of polyester mat (reprinted with permission) (Adapted from Drochytka et al., 2017).

recycled polyester fibers (Lee and Joo, 2003). Here, the influence of fiber diameter, and fiber orientation angles on sound absorption coefficient were also investigated. Although there was no significant influence of fiber orientation on the absorption coefficient, it was noticed that with the increase in fiber diameter, the absorption coefficient of the nonwoven mats had increased (Lee and Joo, 2003). Drochytka et al. also investigated polyester waste and found that bulk density (Fig. 8) had influence on both sound absorption and thermal conductivity (Drochytka et al., 2017). It was observed that with the increase of bulk density, sound absorption coefficient increases, and thermal conductivity decreases. But the influence of bulk density is more prominent for thermal insulation. Drochytka et al. also studied the effect of temperature on thermal insulation properties. It was observed that the thermal conductivity of polyester mat significantly decreases with decrease in temperature (Fig. 8) (Drochytka et al., 2017). Alcaraz et al. analyzed the effect of microfiber layers on sound absorption properties by adding microfiber layers with the polyester nonwoven mats (Alcaraz et al., 2017). It was observed that the sound absorption coefficient had significantly increased with the addition of microfiber layers (Alcaraz et al., 2017). Valverde et al. developed insulation from textile industry scraps, consisting of polyester and polyurethane. It was noticed that the insulation panel showed the lowest thermal conductivity 0.041 W/mK at a density of 396 kg/m3 (Valverde et al., 2013). 7.2. Cotton Cotton is one of the most cultivated non-food agricultural products mainly used in apparel production (Asdrubali et al., 2015). At present cotton is the second largest produced fiber after polyester, and in the case of natural fibers, it is the largest produced fiber (Textile Exchange, 10

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Fig. 9. (a) Sound absorption coefficient of polyester, flax, and cotton mixtures at different frequencies (b) Average absorption coefficient of polyester, flax, and cotton mixtures at five different ratios (reprinted with permission) (Adapted from Zach et al., 2016a).

Zach et al. produced thermal and acoustic insulation materials by combining recycled cotton, polyester and flax fibers (Zach et al., 2016a). In this research, five different mixtures of the samples were designed by combining polyester, flax, cotton, and bicomponent fiber of polyester with lower sheath melting temperature and tested thermal conductivity and sound absorption properties (Fig. 9). The average thermal conductivity and sound absorption coefficients of their materials were 0.037–0.049 W/mk and Mixture 2 (40% cotton, 40% polyester, and 20% bicomponent) had the highest sound absorption coefficient (Zach et al., 2016a).

et al., 2011). The value of the absorption coefficient at medium frequency 1000 Hz–2000 Hz is very high as well as uniform. The absorption coefficient of wool is about 0.9 at around 800 Hz–2000 Hz. (Bhat, 2016; Oldham et al., 2011). Berardi and Iannace used inverse method to predict the acoustical properties of sheep wool and obtained higher sound absorption coefficients (above 0.95) for wool fiber at 1500 Hz–2000 Hz (Berardi and Iannace, 2017). Zach et al. studied the thermal conductivity of wool fiber at different thickness, temperature (Fig. 11), and moisture. It was observed that thermal conductivity of sheep wool fiber decreased with the decrease of temperature, moisture, and density and with the increase of thickness (Zach et al., 2012). Sometimes, wool fiber is mixed with other materials (binder) in order to ensure sufficient cohesion between the fibers to create thermal insulation materials by using the thermal bonding method. Hassanin et al. developed insulating panels by mixing of Tetra Pak waste and wool yarn waste at different ratio. Several percentages of Tetra Pak were mixed with wool waste and then insulation materials were produced by hot pressing. It was found that Tetra Pak with 15% waste wool had 3% lower thermal conductivity compared to Tetra Pak without wool fiber. Air trapped within the wool fibers improved the thermal insulation properties of Tetra Pak and wool fiber waste composite (Hassanin et al., 2018). Patnaik et al. used waste wool and RPET and combined them at different ratios, 100% waste wool, 100% RPET, and 2 layers of 50/50 proportion of waste wool and RPET to develop new insulating materials for building industry applications (Patnaik et al., 2015). In this research

7.3. Wool Wool fiber is one of the mostly used animal fibers in the world since ancient time due to its excellent thermal insulation properties. In 2017, world wool fiber production was 1.2 million mt, 95% of which were sheep wool. At present, around 22,000 mt of wool has been recycled (Fig. 7) and the rest of the fiber has been discarded as waste (Textile Exchange, 2018). Waste wool can be collected from different sources. Along with discarded textiles, short virgin wool fibers can be obtained during shearing the sheep-hair. As short wool fiber is not suitable for apparel production, it is discarded as waste (Patnaik et al., 2015). Wool is generally composed of many different amino acids, which form long chains. The coiled springs of wool molecular chains contribute to fiber resilience, which is suitable for sound absorption (Bhat, 2016; Oldham et al., 2011). The sheep wool has excellent sound-absorbing properties (Fig. 10) due to its micro-cavities as well (Berardi and Iannace, 2015; Oldham

Fig. 10. The absorption coefficient of sheep wool (reprinted with permission) (Adapted from Berardi and Iannace, 2015). 11

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Fig. 11. Influence of temperature on thermal conductivity at different thickness of wool fiber (reprinted with permission) (Adapted from Zach et al., 2012).

different properties including thermal insulation, acoustic absorption, moisture absorption, fire retardancy, and biodegradation behavior were studied. It was observed that the best thermal insulation, acoustic absorption, moisture absorption, and fire retardancy properties were for 50/50 waste wool/RPET mat. The 2-layer 50/50 waste wool/RPET mats absorbed more than 70% noise in the frequency range of 50–5700 Hz (Patnaik et al., 2015).

Gounni et al. analyzed recycled textile materials (acrylic spinning wastes) to develop thermal insulation properties of building materials (Gounni et al., 2018). The results showed that the density, air permeability, and thermal conductivity of the materials were 10.583 kg/m3, 1100 L/m2/s and 0.03827 W/mK respectively. These results indicate that those developed materials can be competitive thermal insulation commercial materials to increase the thermal performance of building walls (Gounni et al., 2018).

7.4. Acrylic

7.5. Jute

Acrylic fiber is considered as artificial wool fiber and it replaces wool fiber especially in hand-made knitted and hosiery garments due to their bulkiness and high elastic properties (Bajaj et al., 1996). These fibers have very good insulation properties. Several researchers conducted their investigation to develop thermal and acoustic insulation materials from acrylic waste fibers. Briga-Sa et al. tried to find out the potential applicability of woven fabric waste (WFW), and a waste of this residue, named woven fabric subwaste (WFS) (both were 100% acrylic) to develop alternative thermal insulation building materials (Briga-Sa et al., 2013). A double wall was built with the air-box in the middle, and wastage WFW and WFS materials were kept between the double wall in an air box about a density 440 kg/ m3 and 122.5 kg/m3 respectively. It was found that the application of the WFW and WFS in the external double wall increases its thermal behavior by 56% and 30%, respectively. Briga-Sa et al. also showed that the thermal conductivity value of WFW is very similar to that of some of the commercially available materials including polystyrene (EPS), extruded polystyrene (XPS) and mineral wool (MW), granules of clay and vermiculite or expanded perlite (Briga-Sa et al., 2013). El Wazna et al. evaluated the potentiality of textile waste in the application of building insulation material by nonwoven techniques (El Wazna et al., 2017). In this study, four nonwoven based insulation materials (A1, A2, W1, and W2) were produced using acrylic and wool waste. The thermal conductivity of A1, A2, W1, and W2 were 0.0350 W/mK, 0.0335 W/mK, 0.0348 W/mK and 0.0339 W/mK respectively. These thermal conductivity values indicate that acrylic and wool waste showed good insulating properties compared to the traditional insulation materials like glass wool and mineral wool. El Wazna et al. also measured the air permeability and investigated the dependency of the thermal conductivity and air permeability on porosity and density of materials. It was found that the thermal conductivity and air permeability of non-woven based insulation materials decrease with increasing porosity, but increase linearly with density (El Wazna et al., 2017).

Jute is the second largest natural fiber after cotton which is used mainly in industrial applications (Datta et al., 2004). At present, about 3.2 million mt of jute fiber is produced annually (Friedrich and Breuer, 2015) 95% of these fibers come from India, Bangladesh, China, Nepal and Thailand (Maity et al., 2012). A huge amount of these jute fibers become waste and are discarded to the landfill either as slivers during jute fabric production process or as used cloth, jute bags, and in other end products (Friedrich and Breuer, 2015). Jute fibers have very good mechanical properties compared to many other natural fibers including sisal, coir, and ramie (Mohanty et al., 2005). Jute fiber has a porous structure (Shaikh and Channiwala, 2006). There is a hole in the center of jute fiber cell and the cell has a closed longitudinal void as a lumen (Fig. 12) (Gassan et al., 2001). Krach and Advani found that the shape and distribution of the voids in jute fibers determine their thermal conductivity (Krach and Advani, 1996). Therefore, these fibers have very good potential to be used as efficient thermal and acoustic insulation materials (Paul and Mukhopadhyay, 1977). Devireddy and Biswas observed very high insulation properties of jute fibers with the thermal conductivity value of 0.036 W/mK (Devireddy and Biswas, 2016). Mohanty and Fatima developed jute felt which had a thermal conductivity of 0.064 W/mK in the temperature range from 50 to 80 °C. It was suggested that the jute fiber can be used as insulation up to a temperature of 260 °C which can be further improved by treatment with suitable chemicals (Mohanty and Fatima, 2015). Bujoreanu et al. presented an experimental study on sound absorption coefficients by combining jute waste with some other waste materials such as rubber particles, polypropylene, crumbled plastic, wood flour, and cord fabrics, with different backing plates. It was observed that the samples that included jute fiber waste and plasterboard backing had better sound absorption properties than others (Bujoreanu et al., 2017). 12

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Fig. 12. SEM images of a Jute fiber (reprinted with permission) (Adapted from Francucci et al., 2012).

7.6. Nylon

7.7. Polypropylene

Synthetic nylon has excellent strength and other mechanical properties. In 2017, 5.73million mt of nylon was produced in the world which consisted of 5.4 percent of world fiber production market (Textile Exchange, 2018). A huge amount of this synthetic fiber has been discarded as waste both in landfills as well as in water. Dissanayake et al. used the post-industrial textile waste of nylon fiber mixed with spandex (NS) and polyurethane (PU) waste to produce thermal insulation materials. It was observed that the combination of 60% NS and 40% PU gave the best thermal insulation properties (Dissanayake et al., 2018). Tiuc et al. did some experimental study by mixing textile waste (nylon, modal and acrylic fiber) with polyurethane foam at different ratios (Tiuc et al., 2016). Polyurethane foam was considered the best thermal insulation material (Kapps and Buschkamp, 2004) and it is widely used in industry for its mechanical, electrical, thermal and acoustic properties (Verdejo et al., 2009). Tiuc et al. found that the composite material with 40% textile waste and 60% rigid polyurethane foam had better sound absorption properties than other compositions. The noise reduction coefficient of composite materials is almost twice that of 100% rigid polyurethane foam (Fig. 13) (Tiuc et al., 2016).

Polypropylene is a thermoplastic polymer fiber used in a wide variety of applications, especially in the health sector. Global polypropylene fiber production in 2014 was around 6 million metric tons which comprise about 8.5% of all textile fiber production in 2014 (CISION, 2015). Polypropylene is extensively used in nonwoven fabric production. Polypropylene occupies 63% of total fibers used in the nonwoven sector (Lin et al., 2016). Again, lots of nonwoven products, especially health care products are single-use products. Therefore, a huge amount of polypropylene fiber is discarded as waste. Polypropylene is the lightest of all fibers. It is 34% and 20% lighter than polyester and nylon fibers. Due to its low specific gravity, this fiber yields the largest volume at a given weight and provides good bulk and cover. This unique property of polypropylene fiber helps it to retain heat for a longer time. The thermal conductivity of polypropylene fiber is the lowest in comparison to any natural or synthetic fiber (Syntech Fibers, 2013). Several researchers are trying to use waste polypropylene fibers to produce functional fabrics (Bhat and Messiry, 2019; Chen et al., 2004, 2007, 2008; Lou and Lin, 2004; Lou et al., 2011) or sound absorption

Fig. 13. The sound absorption coefficient of textile waste, where 100RPF means 100% rigid polyurethane and 60 RPF means 60% rigid polyurethane and 40% textile waste (nylon, modal, and acrylic) and so on (reprinted with permission) (Adapted from Tiuc et al., 2016).

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Table 1 Summary of thermal insulation properties of various materials. Materials

Thickness, t (m)

Density, ρ (kg/ m3)

Thermal conductivity, λ (W/mK)

Relative thermal conductivity, air (0.026) = 1

Ref.

Recycled polyester (RPET) Polyester (85% waste + 15% BiCO) Kenaf fiber with PET

0.016 0.010

62.50 80 100

0.035 (0.003) 0.0467–.0487 0.040

1.35 1.80–1.87 1.54

100% wool

0.010

25

0.040

1.54

Sheep wool 100% wool waste from carpet 50% Coring wool 50% Recycled polyester (CWP) Cotton (recycled) 40% COT 2, 40% PES, 20% BiCO Sunflower stem sponge/cotton waste/epoxy (36.36/36.36/27.27) 100% Acrylic (spinning waste) 60% nylon/spandex (NS)_40% polyurethane (PU) Jute (68%) + binder (20%) +shives (12%) Flax (68%) + + binder (20%) +shives (12%) 75% FLAX, 25% BiCO Technical hemp (64%) + binder (20%) +shives (16%) Hemp fibers + cellulose fibers (60:40) Banana and polypropylene (pp) fiber Polyurethane foam Glass wool

0.040 0.030 0.016

40 45 62.50 25–45 38.9

0.034–.039 0.0311–0.0339 0.032 (0.004) 0.039–0.044 0.0361 0.0871

1.31–1.50 1.20–1.30 1.23 1.50–1.69 1.39 3.35

Patnaik et al. (2015) Drochytka et al. (2017) Pennacchio et al. (2017) Pennacchio et al. (2017) Zach et al. (2012) El Wazna et al. (2017) Patnaik et al. (2015) Innotherm (2019) Zach et al. (2016a) Binici et al. (2014)

14.571

0.043–0.0486 .0953

1.65–1.87 3.67

0.0812 0.0774 0.087 0.0796

26.1 32.1 22.8 29.6

0.0458 0.0429 0.0495 0.0475

1.76 1.65 1.90 1.83

10

30–60 980–1040 30–80 92.5

0.046–0.047 0.157–0.182 0.02–0.027 0.040

1.77–1.81 6.04–7.00 0.77–1.04 1.54

36 80–200 24–112

0.040–0.045 0.025–0.035 0.035–0.032

1.54–1.73 0.96–1.35 1.23–1.35

Mineral wool (MW) Rockwool Fiber glass

0.067

0.010

Wazna et al. (2018) Dissanayake et al. (2018) Korjenic et al. (2011) Korjenic et al. (2011) Zach et al. (2016a) Korjenic et al. (2011) Reif et al. (2016) Paul et al. (2008) Al-Homoud (2005) Pennacchio et al. (2017) El Wazna et al. (2017) Zou (2008) Al-Homoud (2005)

polypropylene), using the needle punched nonwoven technique (Seddeq et al., 2013). It was observed that the composite materials showed good sound absorption coefficient at mid and high frequencies. It was also found that the sound absorption coefficient improved by increasing the thickness of the materials and adding air space behind the sample (Seddeq et al., 2013).

(Seddeq et al., 2013) or thermal insulation materials. Lin et al. produced puncture-resisting acoustic and thermal insulation composite materials by using recycled Kevlar/Nylon/low-melting polyester nonwovens which were reinforced with recycled polypropylene nonwoven selvages (Lin et al., 2016). In this research, a nonwoven composite was produced by inserting PP nonwoven selvages with a different direction between Kevlar/Nylon/low-melting polyester nonwoven fabrics. It was found that after the addition of PP selvages static puncture resistance, sound absorption and thermal insulation of composites increased by 37.35 N, 0.2 (above 2224 Hz) and 0.026 W/mK respectively. Their 5layer composite materials showed lowest thermal conductivity of 0.047 W/mK and sound absorption coefficient of above 0.94 at a frequency of higher than 1890 Hz (Lin et al., 2016). Zhao et al. mixed polypropylene fiber with cement, aggregate (expanded perlite, clay ceramsite, and slag) and admixture to produce porous sound-absorbing concrete slabs to examine the reduction of railway noise (Zhao et al., 2014). It was observed that porous soundabsorbing concrete slabs can significantly reduce railway noise at different train speeds. The reduction of noise is about 4.05 dB when the train speed is less than 200 km/h (Zhao et al., 2014). Bhat and Messiry studied the effect of microfiber layers on acoustical absorptive properties of nonwoven fabrics, and found good sound absorption properties of polypropylene microfiber melt blown nonwoven fabrics over a wide range of frequency (Bhat and Messiry, 2019). Some researchers blended polypropylene fiber with natural materials and fibers during manufacturing of sound absorption materials to get enhanced benefits. Thilagavathi et al. blended polypropylene fiber with banana, bamboo, and jute fibers by needle punched nonwoven technique and found that bamboo/polypropylene nonwoven had the highest noise reduction coefficient (Thilagavathi et al., 2010). Veerakumar and Selvakumar observed that kapok and polypropylene nonwoven fabrics have excellent noise reduction properties over a wide range of frequency (Veerakumar and Selvakumar, 2012). Seddeq et al. produced acoustic insulation materials by blending natural textile fibers (jute, cotton, and wool) with synthetic textile fibers (polyester and

7.8. Other materials Polyethylene fiber wastage is used by several researchers to produce insulation materials. Ricciardi et al. developed insulation materials of three layers (Ricciardi et al., 2014). Two external layers of recycled polyethylene fibers and an internal one of waste paper are glued and pressed together. The obtained mean value of thermal conductivity of two samples was 0.034 and 0.039 W/mK which gives much better performance than traditional insulating materials (Ricciardi et al., 2014). There are some other waste fibers which are also used in producing thermal and acoustic insulation materials. Sometimes researchers and manufacturers collect textile wastes from industry which is the combination of different types of fibers and which is very difficult and time consuming to separate one type of fibers from another. In this case, researchers used mixed waste materials as a textile waste to produce thermal and acoustic insulation materials. Hadded et al. used recycled textile waste materials and developed two samples of waste linter and tablecloth. It was found that the thermal conductivity of waste linter and tablecloth were 0.039 W/mK and 0.033 W/mK, respectively, which means that this material will have the potential to be used in building insulation (Hadded et al., 2016). Cosereanu et al. simulated the thermal behavior of a sandwich structure three layered wooden wall. The core is filled with different textile waste, wood fiber, and wood chips. It was concluded that the best thermal insulating composites are those that use acrylic lacquer as a binder (Coșereanu et al., 2012). Curtu et al. also showed that along 14

15

0.639 0.543 0.721 78.9 89.2 106.1

0.063 0.079

0.60 0.643 0.698 91.5 100.7

0.55

.45 0.89

0.081 0.147

.84

1.0

.95 .93

.68 .96 .91

0.044 0.047 0.048 0.048

.63

1.01

0.089 0.104

.25 .11

.79 .31

10.05 10.10 41.24 10.00 9.91 10.10 40 Jute/PP Jute/PET/PP Flax/Bico.(75/25) Flax/PP Kenaf/PP Kenaf/PET/PP Rigid polyurethane foam/textile waste (60/40) PLLA Ramie fiber/PLLA

81.4 65.6 130

.08 58.82 48.06

17 25.4 50.63 25.47 10.00 Dorper wool/recycled polyester (50/50) Recycled cotton Cotton/PES/Bico. (40/40/20) Cotton/PES/Bico. (40/40/20) Cotton/PET/PP Jute fiber Jute felt

40 40

.10

.13

.33

.55

.71

.77 0.85 .90 .76 .84 .50 .55 .15 249.4 80 46

.35 .15

1600 1250 1000 800 500

Polyester cutting waste wastage polyester fiber/bicomponent fiber (binder) (80/20) Sheep wool

Along with combination of several information from different sources, some analysis of these data was also done. Two factor (Thickness and Density) analysis of variance was used to check the effect of thickness and density on the response variable thermal conductivity and NRC. From the statistical analysis, it was found that thickness has the most significant influence on both thermal conductivity and sound absorption coefficient.

Thick-ness (mm)

8.3. Analyzing the thermal and acoustic data

Insulation materials

Table 2 Summary of acoustic insulation properties of various materials.

Density (kg/ m3)

Similar to thermal insulation properties, comparison of acoustic insulation properties of different materials are summarized in Table 2. A complete comparison of acoustic insulation properties of different materials studied by several researchers are included in supplementary materials (see Supplementary Table 2). The absorption coefficient of both recycled textile fibers and natural materials are listed for better comparison. But the comparison of acoustic properties is not straight forward like thermal properties since acoustic properties depend a lot on frequency. Different researchers measured the acoustic properties at different frequency ranges. For better comparison, noise reduction coefficient (NRC) was calculated by finding the average of the sound absorption coefficients at 250, 500, 1000, and 2000 Hz and rounding the value to the nearest multiple of 0.05. By closely analyzing the data it is evident that the recycled polyester, wool, cotton, and jute fibers have very good sound absorption properties at medium to high frequency. But sound absorption properties are not so good at low frequency.

250

8.2. Acoustic insulation comparison

125

Sound absorption coefficient (α) at different frequency Hz

2000

3000

0.765

0.75

.80

Trajković et al. (2017) Drochytka et al. (2017)

Avg./weig-hted α

Comparison of thermal insulation properties of different materials is summarized in Table 1. A thorough comparison of thermal insulation properties of different materials studied by several researchers are included in supplementary materials (see Supplementary Table 1). At first, thermal conductivity of various textile wastes, and its composites were tabulated. After that thermal conductivity of natural materials and present commercially available materials are listed for making a comprehensive comparison. Along with thermal conductivity, thickness and density of the materials were also recorded to make the comparison more realistic. For most of the materials, thickness and density were not found in the literature. It is observed that lowest thermal conductivity value (0.032 W/mK) is shown by corring wool and 50/50 corring wool/ recycled polyester. Thermal insulation materials prepared from recycled wool, polyester, cotton, jute have shown much better thermal insulation properties than natural materials with lower density. Even thermal insulation properties of recycled textile fibers have exhibited better properties than some commercially available insulation materials including glass wool and mineral wool. Specific relative thermal conductivities of all materials with respect to the thermal conductivity of air (considering air thermal conductivity, 0.026 W/mk as 1) was also computed so that it is easier to compare the thermal conductivity among different materials.

0.60 0.35

8.1. Thermal insulation comparison

.55

8. Comparative analysis of thermal and acoustic insulation materials

.75 .60

NRC (250–2000 Hz)

Ref.

with composite materials binder also affects the acoustic insulation properties (Curtu et al., 2012). Here acoustic materials were produced by mixing wood and textile waste bonded with different binders including acrylic copolymer, clay solved, gypsum solved and formaldehyde. The influence of the type of binder on the acoustic behavior of the samples was investigated. It was found that the samples prepared with the acrylic copolymer as a binder showed better sound absorption properties (Curtu et al., 2012).

Berardi and Iannace (2017) Patnaik et al. (2015) (Bonded Logic, 2019) Zach et al. (2016a) Zach et al. (2016a) Parikh et al. (2006) Oldham et al. (2011) Fatima and Mohanty (2011) Parikh et al. (2006) Parikh et al. (2006) Zach et al. (2016a) Parikh et al. (2006) Parikh et al. (2006) Parikh et al. (2006) Tiuc et al. (2016) Chen et al. (2010) Chen et al. (2010)

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Fig. 14. Influence of density (a) and thickness (b) on thermal conductivity of some recycled textile fibers.

Thermal conductivity was plotted against density (Fig. 14a) and thickness (Fig. 14b). It was observed that with the increase in density thermal conductivity decreases and with the increase of thickness thermal conductivity increases. Wool fiber showed lower thermal conductivity at a density of around 40–70 kg/m3. Also, relationships of noise reduction coefficient (NRC) against thickness (Fig. 15a) and density (Fig. 15b) were evaluated. NRC values increased with the increase in thickness and density. Cotton fibers show higher NRC than other fibers at low thickness and density levels. From statistical analysis, it was observed that the interaction of thickness and density has a significant effect on thermal conductivity and NRC. 3D graphs of thermal conductivity (Fig. 16a) and NRC (Fig. 16b) against thickness and density are shown here. Lowest thermal conductivity was observed at different combinations of thickness and density (like for 40 mm thickness lowest thermal conductivity is around the density of 50 kg/m3 and for 60 mm thickness lowest thermal conductivity is around 60 kg/m3). Similarly, higher NRC values were observed around 40 mm thickness with low density and around 50 mm thickness with high density. Since the data for these analyses were obtained by different researcher at different conditions, one has to be careful with this interpretation. However, the data were randomly selected for analysis to minimize the effect of intrinsic variables.

9. Other properties of insulation materials Along with good thermal and acoustic properties, a commercially successful insulation material should have some other important properties including fire and water resistance. Fire resistance of a material is its ability to withstand the effect of fire. Fire resistance properties of insulation materials can be evaluated by several parameters including temperature increase, mass loss rate, heat release, and smoke production (Asdrubali et al., 2015). Insulation materials produced from conventional materials like recycled glass, rock wool have very good resistance to fire and are classified as non-combustible materials (Asdrubali et al., 2015). But natural fibers are less resistant to fire than mineral fibers. It was found that insulation materials produced from recycled cotton have poor resistance to fire (Innotherm, 2019). Fire resistance properties of natural fibers can be increased by special chemical treatment or by blending natural fibers with synthetic thermoplastic fibers which have better resistance to fire. Fatima and Mohanty measured three parameters, limiting oxygen index (LOI), flame propagation and smoke density to find the fire resistance properties of 2.5% and 5% natural rubber (NR) latex jute composite. From test results, it was observed that 5% NR latex jute composite gave best fire resistance properties with higher LOI (30.2) value, low flame propagation rate (9.77 mm/min) when mixed

Fig. 15. Influence of thickness (a) and density (b) on noise reduction coefficient of some recycled textile fibers.

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Fig. 16. Influence of both density and thickness on thermal conductivity (a) and noise reduction coefficient (NRC) of recycled textile fibers.

(Korjenic et al., 2011). Mechanical properties and volume of insulation materials also change with the change of moisture (Zach et al., 2016b). For good insulation properties, moisture absorption of materials should be around 2% (Patnaik et al., 2015). However, moisture absorption of some natural textile fibers is much higher than 2%. Therefore, insulation materials produced from organic textile fibers should always be separated from the source of moisture. But in all cases, it is difficult or impossible to separate insulation materials from the source of moisture. Wool fiber has very good insulation properties, but it is very sensitive to moisture. Moisture absorption of wool fiber can be reduced by spraying 1% silicon (by weight) on wool fiber (Patnaik et al., 2015). As synthetic fibers have higher resistance to moisture, blending of recycled synthetic and natural fibers to minimize moisture absorption may have higher environmental benefit instead of treating with toxic chemical. Zach et al. produced insulation materials by blending of 40% cotton, 40% recycled polyester, 20% bicomponent fibers and found very good resistance to moisture (0.6% moisture absorbed at 80% RH and 23 °C) (Zach et al., 2016a). Patnaik et al. observed that the recycled polyester/waste wool mats have adequate moisture resistance under high humidity conditions (Patnaik et al., 2015).

with 1% sodium phosphate, and low smoke density rating (9.89%) (Fatima and Mohanty, 2011). Fatima and Mohanty also compared the flame resistance parameters of 5% NR latex jute composite with other fibers. LOI value of 5% NR latex jute composite is higher than that of some other natural fibers including cellulose (19), and wool (25). Smoke density rating of 5% NR latex jute composite (9.89%) was lower than the smoke density rating of fiber glass (20.55%) (Fatima and Mohanty, 2011). Berardi and Iannace produced insulating materials by treating hemp fibers with soda or boron salts to improve the fire retardancy. It was also found that treated hemp fibers did not contain any toxic substance and it did not entail any risk to health during the useful life of insulation panels (Berardi and Iannace, 2015). Thermoplastic synthetic fibers like recycled polyester have comparatively better fire resistance properties. Thermoplastic fibers shrink during burning, melt and drip on contact with a flame that cause the fibers to stop burning (Trajković et al., 2017). Trajković et al. produced insulation materials from polyester waste and checked fire resistance properties. Test result showed that there was no flame on insulation materials which indicate that recycled polyester fiber is a good option to produce insulation materials (Trajković et al., 2017). Wool fibers are inherently less flammable than most other fibers (Schindler and Hauser, 2004). Patnaik et al. studied insulating materials made from waste wool and recycled polyester fibers and checked the flame resistance properties. Here waste wool and recycled polyester fibers were treated with low-level fire retardants of di-ammonium phosphate and sodium tetraborate (5% by weight). Samples showed good fire resistance properties which can prevent fire hazards up to 400 °C. Washed wool showed better fire resistance than recycled polyester fibers (Patnaik et al., 2015). However, there are serious side effects of some flame-retardant finishing when applied at a high percentage. One of the adverse effects is that some of the flame-retardant finishes, especially halogenated compounds, are highly toxic (Schindler and Hauser, 2004). Researchers are developing several approaches to minimize side effects. Sol-gel and layer-by-layer chemistries are considered as sustainable fire-retardant finishing on textile materials (Horrocks et al., 2018). Deposition of silicon-based species on surfaces can significantly improve the flame retardancy (flash fire resistance) of textile fibers (Carosio et al., 2011; Horrocks et al., 2011; Tata et al., 2012). Horrocks et al. successfully produced environmentally sustainable flame-retardant textiles by using novel atmospheric plasma/UV laser technology (Horrocks et al., 2018). Another required property is that insulation materials should have adequate resistance to moisture and humidity. Insulation materials, especially produced from organic natural fibers may be damaged by biological corrosion (degradation by bacteria, attacked by mildew and fungi) when exposed with high humidity for a long time or come in contact with liquid water (Korjenic et al., 2011). On top of tha,t thermal insulation properties of materials decrease with increase in humidity

10. Conclusions In this review paper, an extensive overview of thermal and acoustic insulation materials produced from textile waste has been compiled. The mechanism of thermal and acoustic insulation and measurement process by following the international standards are discussed. The effect of discarded textiles on environment and health, and ways to minimize it by the recycling process have been summarized. Also included is a thorough overview of the transformation of textile waste into insulation materials. The present study of textile waste as insulation materials by different researchers is comprehensively covered and the insulation properties of different materials are compared. Although the present market is completely dominated by some conventional synthetic insulation materials there is a potential to replace these conventional materials by recycled textiles. In some cases, thermal and acoustic insulation materials produced from waste textiles show much better results than the currently available and dominating products in the market. In addition to that by discussing LCA, it is specifically reported as to how much environmental effect can be minimized by using recycled textiles. However, the research of using textile wastes as an insulation material is still in its initial stages. Research has been done only on some of the common fibers. Majority of the investigated materials are not entirely characterized. Structure of insulation materials may influence the insulation properties. But there is limited study about the structure of textile-based insulation materials. Multiple layers with varying 17

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structures like different layers with different properties may also have an effect on insulation properties. Similarly, it is not clear whether the surface area, fiber morphology, fiber cross-section, diameter, and length have significant influence on insulation properties or not. There is limited information about some other important factors of insulation materials like fire resistance, water resistance, cleaning process, resistance against pest, dust, fungi, and bacteria. Natural organic materials like natural fibers are sensitive to high moisture. Natural fibers are likely to absorb moisture into the internal porous structure at increased humidities, and on the other hand, can release moisture gradually at lower humidity. Again, insulation materials produced from recycled textiles are porous in structure. These pores act as capillary tubes and moisture at surface can enter at the inner part of the insulation materials. Prolonged exposure of materials at high humid conditions may cause biological corrosion including degradation by bacteria, mildew, and fungi acting on the material. As a result, mechanical properties change, and thermal insulation properties decreased due to humidity. It is still not clear how to minimize the effect of humidity by maintaining environment-friendly properties of insulation materials, although several possible approaches have been discussed. One of the most important things for commercialization is the cost and performance benefit of the product. Several authors only mentioned that the cost will be low but there is no specific information about the cost. In every study, researchers mentioned the environmental advantages of using recycled textiles but very few authors include the LCA analysis. Based on above discussions, it can be concluded that textile waste has the potential to be used as an environmentally friendly insulation material, but still some of the limitations need to be addressed. Well focused research is required to overcome these limitations and successfully commercialize products made from recycled textiles in the market.

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