Investigating the Effect of Recycled Cotton Included Fabrics on the Thermal Behaviour by Using a Female Thermal Manikin

1. Introduction

As a result of world population growth, consumption, and therefore the amount of waste, increases day by day, whereas the resources to make primary synthetic fibers are becoming less and less. Therefore, the recovery of waste and the development of related technologies have become inevitable. Obtaining secondary raw materials from recyclable wastes both reduces the raw material requirement of the industry and contributes to the economy. The biggest advantage of recycling is that it helps conserve natural resources. In recycling, both nature is protected and raw material costs are reduced in terms of production and additional energy efficiency is provided [1]. However, recycling is an important concept, not only for energy and water saving, but also for a sustainable life in the world [2].

The increased awareness of the clothing industry in terms of fashion and design has also changed the production processes in the industry [3]. The apparel products are designed and manufactured quickly and cheaply; therefore, textile consumption and the need for fiber and fabric are constantly increasing. Until 2025, it is estimated that total fiber production will increase by 3.7% every year [4]. To solve this increasing problem in the textile and apparel industry, it is important to use recycled materials [4], which give an added value to the products [5].

In the literature, there have been many studies on recycled materials, such as recycled polyester (PES) fibers as well as recycled cotton (r-Co) fibers for manufacturing recycled fibers, included garments. The studies on recycled PES fibers are more common [5—], whereas the studies on r-Co fibers are limited due to the lower quality of r-Co fibers. Possible cotton (Co) reuse applications depend on the quality and properties of waste Co sources. The quality of pre-consumer waste Co is mostly easier to assess than the quality of post-consumer Co affected by different wear and treatment conditions and unknown fiber blends [10]. These materials in different qualities are re-manufactured for the automotive, aeronautic, home building, furniture, mattress, coarse yarn, home furnishings, paper, apparel, and other industries [11]. Recycled Co even found a place in the smart textiles research field. Chen et al. [12] developed flexible sensors by combining natural rubber latex with carbonized waste Co fabrics.

The process of collecting and recycling Co fiber waste has significant importance in order to obtain r-Co fibers that are appropriate to use in garment manufacturing. The studies mostly focused on the recycling of Co fiber waste obtained during yarn production. Liu et al. [13] studied the production of regenerated cellulosic fibers from Co waste. Halimi et al. [14–15], Hasani and Tabatabaei [16], Wanassi et al. [17], and Kılıc et al. [18] investigated the recycling of Co fibers from waste Co yarns.

Some studies also exist in the literature which investigate the mechanical recycling process for Co fibers from fabric waste. Kurtoğlu et al. [2] aimed to investigate the use of recycled garments obtained by evaluating fabric scraps, and they concluded that recycled garments obtained from fabric scraps could be used in the apparel manufacturing industry. Telli and Babaarslan [5] aimed to produce denim from recycled yarns; they recovered Co fibers derived from the raw materials in the denim fabric production process and the recovered PES fibers recovered from PET bottle waste. In the study conducted by Utebay et al. [19], the Co textiles collected from pre-consumer knitted waste were sorted regarding the fabric tightness and the applied finishing treatments.

Only limited studies have been encountered in the literature in terms of clothing comfort properties of recycled fibers, including garments. In the research conducted by Vadicherla and Saravanan [9], the thermal comfort properties of single jersey fabrics produced by recycled PES and Co blended yarns at different blending ratios, frequency and loop length were investigated. Dhanapal and Dhanakodi [20] studied the moisture management properties of socks made from recycled PES, virgin Co, and its blends. Gun et al. [21] and Celep et al. [22] analyzed the thermal conductivity, the thermal resistance, the thermal absorptivity, and the air permeability values of fabrics, including r-Co fibers.

In these limited studies, thermal comfort properties of recycled fibers, including fabrics, were only evaluated in two-dimensional fabric forms; however, investigating the thermal insulation of clothing should also be considered. When the fabric is transformed to the garment form and worn by the individual, the thermal comfort properties could vary according to the environmental conditions and even to the garment fit. Therefore, thermal manikins, which measure the heat flux over the whole body surface area [23], are inevitably used to evaluate the clothing insulation due to their advantages, namely, simplicity and repeatability of the tests [24]. In this research, besides investigating the thermal resistance and air permeability properties of virgin as well as r-Co fabrics, effective clothing insulation was evaluated using the thermal manikin method.

2. Experimental

2.2. Methods

Before performing the tests, all samples were conditioned at least 24 hours in standard atmospheric conditions (constant ambient temperature of 20 ± 2 ºC, relative humidity of 65 ± 2%).

2.2.1. Yarn Tensile Strength

The obtained yarns’ tensile properties were evaluated using Zwick Roell Z010 testing equipment. The breaking force (cN/dtex) and elongation percent (%) at the break of the yarns. Ten measures were performed for both samples according to ISO 2062:2009 standard [25].

2.2.2. Air Permeability

Air permeability is an important property that determines the ability of air to flow through the fabric. Ten repetitions were conducted for each fabric using the Textest FX 3300 air permeability instrument (Textest Instruments AG, Switzerland) at a pressure of 100Pa in 20 cm² measurement area according to ISO 9237:1995 standard [26].

2.2.3. Thermal Resistance and Absorptivity

Thermal resistance of a fabric is described as the ability of a fabric to resist heat flow. The thermal resistance values were measured using the Alambeta instrument (Technical University of Liberec, Czech Republic), which presents the thermal absorptivity, the thermal conductivity, and the fabric thickness values as well. All tests were performed with five repetitions.

2.2.4. Effective Clothing Insulation

In order to calculate the effective clothing insulation of fabrics, the tests with a female thermal manikin (PT-Teknik, Denmark) were conducted for all samples. For this purpose, long sleeve rounded neck shirts were produced from each fabric (SN-1, SN-2, SN-3, and SN-4). The thermal manikin has a similar size and configurations as an adult woman. It comprises 20 sections and can only sense dry heat transfer. The thermal manikin was dressed in the manufactured shirts for each test and throughout the period of tests, the thermal manikin was hung inside a climatic chamber, which is able to achieve desired climatic conditions. All tests were performed at constant ambient temperature of 20 ± 0.5 °C and relative humidity of 60 ± 5%, where standard atmospheric conditions were attempted in order to make comparisons with the obtained thermal resistance data. The constant skin temperature mode was chosen for the tests and 33 ± 0.2 °C as skin temperature of thermal manikin was set regarding to ISO 9920:2007 standard [27].

The thermal manikin was placed with its legs hanging straight and the arms hanging straight at its sides (Figure 1). The testing time for each shirt was 30 minutes and the data were observed and recorded with the software-installed computer (Figure 1). The effective clothing insulation values (I_cle) were calculated regarding to the global method. This method assumes that the thermal manikin has only one segment; thus, first, the sum of all heat losses in weighted areas for 20 sections was calculated. Afterwards, the thermal clothing insulation (I_T) was calculated according to the following equation ISO 15831 [28]; [29–30]:

(1) IT=∑i(fixtsi)−t0∑(fixQsi),

where f_i is the relationship between the surface area of the segment “i” of the manikin (A_i) and the total surface area of the manikin A (f_i = A_i/A); t₀ is the air temperature within the climatic chamber [°C]; t_si is the skin surface temperature of the body segment “i” of the manikin [°C]; and Q_si is the sensible heat flux of the manikin obtained by area (W/m²).

Figure 1.

The shirt worn by thermal manikin (left) and an image of the software-installed computer during the test (right).

It is a fact that the I_T does not show the actual thermal insulation value of the sample due to the insulation property of the air layer. Therefore, the thermal insulation of the boundary air layer (I_a) needs to be considered, and it is determined by testing a nude manikin at the same climatic conditions. The I_cle, consisting of the difference between I_T and I_a is calculated by Eq. (2),

(2) Icle=IT−Ta.

3. Results and Discussion

In this research, the tensile strength and elongation percent of yarns were measured; the results are presented in Table 2.

The tensile strength of PES yarns is higher than that of Co yarns, and as the amount of PES in Co/PES blends increases, the tensile strength and elongation values increase [31,32]. Supportively, the elongation value of 45% r-Co/55% PES yarn is higher than the elongation value of 50% Co/50% PES yarn (Table 2), which is thought to be the result of the increase in the amount of PES content in 45% r-Co/55% PES yarn. On the contrary, the tensile strength of 50% Co/50% PES yarn is higher than the tensile strength of 45% r/Co-55% PES yarn. Although 45% r-Co/55% PES yarn contains a higher percentage of PES, it is thought that this is due to the lower breaking strength value of r-Co compared to virgin Co [31].

The thermal resistance was measured using the Alambeta instrument as well as a thermal manikin. Thermal resistance of a fabric is described as the ability of a fabric to resist heat flow. Consistent with the literature, the fabrics knitted with r-Co had greater thermal resistance results than the fabrics knitted with virgin Co for both single jersey and rib constructions. As is known, thermal resistance shows a linear ratio with fabric thickness; therefore, the increase in fabric thickness increases the thermal resistance [21], and it is generally accepted that the fabric thickness is one of the main factors influencing thermal resistance [33]. Table 3 shows the thermal test values for all samples. In support of the existing literature, in Table 3, the fabric thickness values were in linear manner with thermal resistance values (Figure 2). Moreover, in the research conducted by Celep et al. [22], it was found that the single jersey fabrics knitted with r-Co have greater thermal resistance results, and it was suggested that the garments manufactured with r-Co-included fabrics would be used in cold environmental conditions.

Figure 2.

The thermal resistance and the fabric thickness values of samples.

Thermal absorptivity characterizes the warm-cool feeling of a garment during the first contact of a fabric with the human skin [34]. The higher the thermal absorptivity of the fabric, the cooler the feeling. Regarding Table 3, r-Co-included single jersey fabric had the greatest thermal absorptivity; namely, it had the coolest feeling during the first contact with the skin. In terms of knitting structure, single jersey fabrics had higher results than 1×1 rib fabrics due to their more uniform, flat, and smooth fabric surface. The surface property of a fabric has great importance in thermal absorptivity, i.e., a higher contact area increases the cooler feeling [35].

Air permeability is the ability of air to flow through the fabric, and in addition to the fabric thickness, it is affected by the fabric porosity [36]. Regarding the air permeability results, it was observed that SN-3 and SN-4 had higher air permeability values than SN-1 and SN-2 fabrics because of the knit structures (Table 3), similar with the existing literature highlighting that rib fabrics have higher air permeability than single jersey fabrics [37]. When the effect of recycled fibers on air permeability tests were evaluated, it was seen that the fabrics including recycled fibers led to a decrease in the air permeability values. Moreover, although SN-1 is the lightest fabric, its air permeability value is the lowest compared to other fabrics. This is due to the fact that the recycled fibers cause unevenness in the yarns [2], which creates an extra filling effect between the loops forming the fabric. Regarding the statistical analyses (Table 4), it was observed that using recycled yarns had a significant effect on the air permeability values for rib knitted structure, whereas there was not any significant difference for the jersey knitted structure.

The ANOVA statistical analysis shows that there were not significant differences (p > 0.05) between the single jersey fabrics containing r-Co fibers on the thermal resistance and the air permeability (Table 4). On the contrary, significant differences (p < 0.05) were observed between the 1×1 rib fabrics. In addition, the statistical analysis (Table 5) showed that the knitted structure had a statistically significant effect on air permeability values and on thermal resistance of r-Co-included fabric. The fabric manufactured from virgin Co yarns did not show significant differences regarding the knitted structure.

Table 6 presents the results of the shirts manufactured for all samples obtained with the thermal manikin tests. The total thermal insulation (I_T) and the effective thermal insulation (I_cle) values were calculated using the global method. According to the obtained results, I_cle of virgin Co shirts were greater than r-Co shirts for both single jersey and rib constructions.

The thermal insulation of a fabric is of limited value for evaluating the thermal property when it is constructed into an ensemble. The thermal manikin enables the evaluation of the thermal insulation properties of garments by measuring the heat flux over the whole body surface area. As observed in this study, the thermal insulation of a fabric and a garment vary. Figure 3 shows the thermal resistance and I_cle results of all samples. It can be clearly seen that, including r-Co affected I_cle results of garments in a way opposite to thermal resistance results of fabrics.

Figure 3.

Effective clothing insulation values of shirts and the thermal resistance values of fabrics.

Table 7 presents the t-test results of effective clothing insulation values of the knitted fabrics considering the raw material. When the I_cle results were compared with respect to the case of including r-Co, it was seen that in both knitted types, single jersey and rib, the I_cle values showed significant differences.

4. Conclusion

Because of the importance of sustainability, recycled yarns have been widely used in the apparel industry. They are generally blended with PES or other textile fibers. In this study, in order to investigate the usability of fabrics, including recycled fibers, in the apparel industry, r-Co-contained fabrics were obtained and tested. Ne 30/1 yarns were produced with the blend ratio of 45% r-Co/55% PES. Afterwards, in order to compare with the recycled yarns, Ne 30/1 50% PES/50% Co yarns, which are widely used in the textile industry, were included in the study. Single jersey and rib fabrics were knitted by using these yarns under the same production conditions. The thermal properties were tested using a thermal manikin, an Alambeta instrument, and an air permeability instrument.

Regarding the obtained results, it was seen that, despite 45% r-Co/55% PES yarn containing a greater amount of PES, the r-Co caused a decrease in tensile strength value. Moreover, the fabrics, including recycled fibers, led to a decrease in air permeability values, whereas there were increases in thermal resistance values. It is a fact that the recycled yarns have higher hairiness and so a higher fabric thickness, which is thought to be the main reason for the decrease in air permeability values and the increase in thermal resistance values.

Regarding the thermal manikin test results, it was observed that, including r-Co affected I_cle results of garments in the opposite way of thermal resistance results of fabrics. Therefore, it can be concluded that, in order to evaluate thermal insulation of a garment in real-use conditions, thermal manikin tests of garments are more effective than thermal resistance tests of fabrics.