Development and Evaluation of Yarns Made from Mechanically Recycled Textiles

Abstract

Mechanical textile recycling presents a sustainable alternative to linear “take–make–waste” models in the fashion industry. This study intended to develop yarns using textile-to-fiber mechanically recycled fibers. ReSpool mechanically recycled wool, cotton, polyester, silk, and rayon fibers from pre-consumer and post-consumer textiles were acquired and blended with new fibers at varying ratios (100% ReSpool fibers, 85% ReSpool fibers, and 65% ReSpool fibers) to make batts, which were spun into yarns. The yarns’ size (Tex), strength (breaking force and tenacity), elongation, and moisture regain were evaluated. ReSpool recycled fibers from both pre-consumer and post-consumer textiles can be used to produce yarns that have appropriate strength for weaving and knitting. It was possible to produce yarns from 100% ReSpool recycled wool, polyester, and silk fibers, but ReSpool recycled cotton and rayon fibers must be blended with new fibers to produce yarns. There was no significant difference among the percentage of ReSpool recycled polyester and cotton fibers in the yarns on the strength and elongation of the yarn. It is recommended to use the higher percentage of ReSpool recycled fibers in yarn development to maximize recycled material utilization.

1. Introduction

The textile and apparel industry currently follows a linear model of take–make–waste [1] where garments may be worn less than 10 times before being thrown away [2]. In the U.S., 17.03 million tons of textile waste were generated with only 14.7% were recycled in 2018 [3]. While an estimated 500 billion U.S. dollars in value is lost each year due to clothing underutilization and the lack of textile recycling [4], current technologies are not scalable enough to make recycling cost effective at the commercial level [5].

Textile waste includes pre-consumer waste such as textile waste from the textile and apparel manufacturing process and deadstock fabrics and post-consumer waste that are discarded textile products by end-consumers [6]. Textiles can be recycled in thermal, chemical, and mechanical routes. Both thermal and chemical recycling of textile waste require high material purity. However, most garments and textile products are produced from a blend of fiber materials, which causes a challenge in thermal or chemical recycling, especially for post-consumer textile waste.

Mechanical textile recycling is also called fiber recycling, which takes apart fabrics through shredding or cutting and preserves fibers [7]. Mechanical recycling is lower-cost, scalable, and the most established recycling process for textiles [7]. Compared to thermal and chemical recycling, mechanical recycling is more flexible on fiber content: fabrics made from any fiber type and fiber blend can be shredded and recycled [6]. Shredding creates shorter fiber length, which reduces fiber quality. New fibers are required to be added to mechanically recycled fibers in the textile production, so the quality of the textile is not significantly reduced [8,9]. Johnson et al. reported that mechanically recycled fiber content would be 20–30% and the new fiber content would be 70–80% in textile production to assure fabric quality [8]. Aronsson and Persson indicated that the mechanically recycled fiber content could be 50% in yarn production without compromising the mechanical properties of yarns [9]. To produce textiles that require certain fiber length, it is more appropriate to use fibers from mechanically recycled pre-consumer textile waste, which does not experience wearing and tearing and has higher quality than post-consumer textile waste [8].

The mechanically recycled fibers can be used to produce yarns and nonwoven fabrics. Fibers obtained from mechanically recycled textile scraps can be further spun into yarns. There existed a positive correlation between recycled fiber length and yarn tenacity [10]. Fibers shorter than 4–5 mm are lost as waste during carding in yarn production, while fibers shorter than 12–15 mm do not contribute to yarn strength and only increase the bulk of the yarn [11]. Nonwoven production does not require high fiber length and quality as yarn production, which makes it more suitable for mechanically recycled fibers. About 95% of mechanically recycled fibers are not re-spun into yarns but processed directly into nonwovens [12]. Wazna et al. shredded pre-consumer acrylic and wool textile waste to develop needle-punched nonwoven material for building insulation, which has competitive thermal properties as conventional insulation material [13].

Yarn development plays a critical role in shaping downstream impacts, such as textile performance, yarn quality and durability, and product integrity. Beyond being a structural component of textiles, yarn is a strategic intervention point for improving sustainability throughout the value chain. Stressing yarns during the weaving process causes yarn deformations, which may result in yarn breakage and stoppage of the weaving machine [14]. Warp yarns undergo greater stress than filling yarns during the weaving process, so warp yarns must have a certain minimum strength while filling yarns can be quite weak [15]. Lappage tested eight 31 Tex and 37 Tex yarns with breaking forces of 199 to 304 g and tenacity of 8.06 to 9.90 g/Tex (0.896 to 1.100 g/denier) in weaving trials and found the warp peak tension was in the range of 77.6 to 108.8 g (0.0776 to 0.1088 kgf) [16]. A weaving process simulator to simulate different weaving conditions of shed height, reed densities, and dynamic tensile forces found that the mean tensile forces of warp yarns were in the range of 90 to 130 cN (0.92 to 1.33 kgf) [14].

Shcherban et al. measured spun yarns’ tension in knitting and found that the tension ranges were 5.93 to 51.98 cN (0.061 to 0.530 kgf), 3.7 to 44.29 cN (0.038 to 0.452 kgf), 3.79 to 46.48 cN (0.039 to 0.474 kgf), and 3.72 to 43.75 cN (0.038 to 0.446 kgf) for 29 Tex cotton yarn, 28 Tex wool yarn, 30 Tex flax yarn, and 29 Tex viscose yarn, respectively [17]. Liu and Miao analyzed yarn tension on a tricot knitting machine and found that the highest tension was about 60 cN (0.612 kgf) [18].

ReSpool is a transdisciplinary partnership and circular textile ecosystem model established in 2022 to recycle textile and apparel waste into new products. It integrates academic, industrial, governmental, and nonprofit partners to create regional, transferable systems for collection, sorting, recycling, and manufacturing. ReSpool advances textile sustainability through proprietary technologies such as a textile-to-fiber shredder, alongside innovative fiber-to-fiber processes and collaborative design and workforce development, turning discarded textiles into yarns, nonwovens, and fabrics for high-value applications [19,20].

While conventional textile shredders often produce short, low-value outputs, The Waypoint Forward Fiber Shredder is a patent-pending mechanical textile recycling technology developed by the University of Minnesota, Duluth and commercialized through ReSpool. It shreds textile waste into spinnable fibers (ReSpool fibers) long enough to be reused in yarns, nonwovens, and mat development [20,21,22]. ReSpool fibers are part of ReSpool’s textile-to-textile recycling system. ReSpool fibers can be combined with new fibers (e.g., wool, polyester, cotton) to enhance spinnability, durability, and comfort in textile development. One limitation of spinning mechanically recycled fibers is that shredding often yields short fibers only suitable for lower quality products. ReSpool fibers are engineered to retain sufficient length and quality for higher-value textile applications [20,23]. The purpose of this research was to develop yarns from ReSpool mechanically recycled fibers and evaluate size, strength, and elongation of the yarns.

2. Materials and Methods

Figure 1 is the schematic diagram to summarize the yarn development and testing methods in this research.

Figure 1. Methods in yarn development and testing.

2.1. Materials Sourcing and Preparation

The research collaborator in the University of Minnesota, Duluth sourced 100% denim cotton, 100% polyester, 100% wool, and 100% silk pre-consumer textiles and 100% cotton and 100% rayon post-consumer textiles. The textiles were shredded into ReSpool recycled fibers using a proprietary textile-to-fiber shredder [21]. We received these ReSpool recycled fibers from the University of Minnesota, Duluth for yarn development. We also purchased 100% new staple polyester fibers, 100% new cotton fibers, and 100% new wool fibers for yarn development.

The ReSpool shredded fibers were carded on a Strauch Drum Carding Machine (Hickory, NC, USA). Pure ReSpool polyester, wool, and silk fibers could be carded to create 100% ReSpool recycled fiber batts, but pure ReSpool cotton and rayon fibers could not be carded by themselves. Therefore, new cotton or wool fibers were added to ReSpool cotton and rayon fibers. A total of 20 g fibers were used in each batt development. Three ReSpool recycled fiber to new fiber ratios, i.e., 100% (20 g) ReSpool fibers, 85% (17 g) ReSpool to 15% (3 g) new fibers, 65% (13 g) Respool to 35% (7 g) new fibers, were investigated in this research. The blending of ReSpool and new fibers assured that the blend and the resulting yarn would be either biodegradable (e.g., biodegradable ReSpool cotton or rayon fibers being blended with new biodegradable wool or cotton fibers) or recyclable (e.g., ReSpool polyester fibers being blended with new polyester fibers). The fiber compositions for batt and yarn development that included ReSpool recycled fibers are in Table 1. For comparison, batts were also made by 100% (20 g) new staple polyester and 100% (20 g) new wool fibers for yarn development.

Table 1. Fiber compositions for batt and yarn development that included ReSpool recycled fibers.

ReSpool Fiber Origin	ReSpool Fiber Type	New Fiber Type	Ratio of ReSpool to New Fibers
			100% Respool	85% ReSpool and 15% New	65% ReSpool and 35% New
Pre-Consumer Textile	Polyester	Polyester	Yes	Yes	Yes
	Wool	N/A	Yes	No	No
	Silk	N/A	Yes	No	No
	Cotton	Cotton	No	Yes	Yes
	Cotton	Wool	No	Yes	Yes
Post-Consumer Textile	Cotton	Cotton	No	Yes	No
	Rayon	Cotton	No	Yes	No

Note: “Yes” means yarn was developed and “No” means yarn was not developed.

2.2. Yarn Development Process

When blending recycled fibers with new fibers, the new fibers were carded first. The recycled fibers were carded on top of the new fibers, creating a two-layered batt. The batt was removed from the machine and carded one more time to mix the new and recycled fibers into one cohesive batt, for a total of three times carded per batt. After carding, the batts were spun into 1-ply yarn on an Electric Eel Wheel Spinner. Two 1-ply yarns were further spun into a 2-ply yarn on the Electric Eel Wheel Spinner.

2.3. Yarn Testing

For the yarn samples, durability properties (tex, tensile strength, tenacity, and elongation) were tested. Before testing, the yarns were conditioned at 21 °C and 65% relative humidity in an Environmental Chamber (Lunaire, Model No. CEO910-4, Thermal Product Solutions, New Columbia, PA, USA) in accordance with ASTM D1776 method (Standard Practice Conditioning and Textile Testing). The weight of eighteen inches of yarn was measured (with three replications), and the tex (g/km) and denier (g/9 km) data were calculated. Tensile strength and elongation were measured in accordance with ASTM D2256 (Standard Test Method for Tensile Properties of Yarns by the Single Strand Method) using a H5KT Benchtop Materials Tester (Tinius Olsen, Horsham, PA, USA) with 3 or 5 replications. Tenacity data were calculated by tensile strength and denier data (gf/denier). Moisture regain was measured in accordance with ASTM D2654 (Standard Test Methods for Moisture in Textiles) using an Environmental Chamber (Lunaire, Model No. CEO910-4, Thermal Product Solutions, New Columbia, PA, USA), an oven (Stabil-Therm Constant Temperature Cabinet, Blue M Electric Co., Blue Island, IL, USA), and a digital balance (Model ME203E, Mattler-Toledo Ltd., Lancaster, UK) with 3 replications. Scanning electron microscope (SEM) images were taken from a Zeiss Auriga 60 High Resolution Focused Ion Beam & Scanning Electron Microscope.

2.4. Statistical Analysis

t-tests and analysis of variance (ANOVA) were used in data analysis. The data were analyzed using IBM SPSS Statistics version 31.

3. Results

3.1. Yarns

Figure 2 shows examples of the yarns developed from pre-consumer ReSpool recycled fibers. Figure 3 shows examples of the yarns developed from post-consumer ReSpool recycled fibers. Because of a hand spinning device used in the yarn development, the yarns have big and uneven diameters. Staple fibers were obtained from the shredding, which resulted in the fuzzy edges of the yarns.

Figure 2. Samples of yarns developed from pre-consumer ReSpool recycled fibers (a): 100% ReSpool polyester; (b): 100% ReSpool wool; (c): 85% ReSpool denim cotton/15% new wool; (d): 85% ReSpool denim cotton/15% new cotton; (e): 65% ReSpool denim cotton/35% new cotton; (f): 65% ReSpool polyester/35% new polyester.

Figure 3. Samples of yarns developed from post-consumer ReSpool recycled fibers (a): 85% ReSpool cotton/15% new cotton; (b): 85% ReSpool rayon/15% new cotton.

Figure 4 shows the SEM images of the cross-section of the yarn made from 85% ReSpool denim cotton and 15% new wool (sample shown in Figure 2c). From Figure 4b,c, we identified that the smaller diameter fibers were ReSpool cotton fibers and bigger diameter fibers were new wool fibers. The yarn cross-section in Figure 4a showed that the ReSpool denim cotton fibers were bundled together because short pieces of cotton yarns existed in the ReSpool cotton fibers from the shredding. The new wool fibers were distributed across the yarn, indicating the cardings can successfully blend the ReSpool cotton and new wool fibers.

Figure 4. SEM images of the cross-section of yarn made from 85% ReSpool denim cotton/15% new wool fibers (a): cotton and wool fibers are in bundles and new wool distributed across the yarn; (b): small-diameter fibers show convolution in cotton; (c) big diameter fibers show scales in wool and small-diameter fibers show convolution in cotton.

3.2. Yarn Testing Results

The yarn size (tex), strength (breaking force and tenacity), and elongation testing results for the yarns are shown in Table 2.

Table 2. Yarn size, strength, and elongation testing results.

No	Origin of Fiber	Fiber Content and Ratio	No. of Plies	Tex Mean ± SD	Breaking Force (kgf) Mean ± SD	Tenacity (gf/Denier) Mean ± SD	Elongation (%) Mean ± SD
1	Pre-consumer textile	100% ReSpool polyester	2	1899.97 ± 264.81	1.72 ± 0.88	0.097 ± 0.036	17.60 ± 8.20
2		100% ReSpool wool	2	1886.12 ± 278.85	1.41 ± 0.26	0.083 ± 0.010	23.97 ± 1.99
3		100% ReSpool silk	2	1065.91 ± 107.87	2.66 ± 0.64	0.283 ± 0.089	11.57 ± 1.05
4		85% ReSpool polyester/15% new polyester	1	366.00 ± 49.41	0.04 ± 0.03	0.014 ± 0.010	5.75 ± 2.61
5		85% ReSpool polyester/15% new polyester	2	922.28 ± 121.43	0.96 ± 0.17	0.115 ± 0.008	16.83 ± 3.11
6		85% ReSpool denim cotton/15% new cotton	2	3187.52 ± 323.46	2.14 ± 0.52	0.074 ± 0.011	23.43 ± 3.91
7		85% ReSpool denim cotton/15% new wool	2	2824.44 ± 425.55	2.37 ± 0.51	0.095 ± 0.030	21.07 ± 2.55
8		65% ReSpool polyester/35% new polyester	2	1278.07 ± 60.39	1.83 ± 0.42	0.158 ± 0.031	23.10 ± 8.15
9		65% ReSpool denim cotton/35% new cotton	2	3388.01 ± 148.33	2.96 ± 0.77	0.097 ± 0.026	25.00 ± 1.85
10		65% ReSpool denim cotton/35% new wool	2	2450.42 ± 352.63	3.39 ± 0.97	0.161 ± 0.073	23.10 ± 2.82
11	Post-consumer textile	85% ReSpool cotton/15% new cotton	2	2022.75 ± 342.19	1.17 ± 0.40	0.065 ± 0.025	15.74 ± 3.08
12		85% ReSpool rayon/15% new cotton	2	2048.56 ± 161.74	1.00 ± 0.24	0.054 ± 0.012	17.76 ± 3.45
13	New fiber	100% new polyester	2	839.17 ± 55.95	5.38 ± 0.60	0.718 ± 0.122	29.70 ± 0.12
14		100% new wool	2	897.49 ± 86.67	3.07 ± 0.18	0.382 ± 0.033	10.27 ± 1.11

3.3. Yarns Made from Pre-Consumer ReSpool Fibers

3.3.1. Comparison of Different Plies of Yarns Made from ReSpool Fibers

t-tests were conducted to compare the size (tex), strength and elongation between different plied yarns (one-ply and two-ply) made from 85% ReSpool polyester fibers and 15% new polyester fibers (No. 4 and 5 in Table 2). The results are in Table 3. Two-ply yarns made from ReSpool recycled fibers were significantly bigger, stronger (both breaking force and tenacity), and had significantly higher elongation than one-ply yarns made from ReSpool recycled fibers (all p values < 0.05). One-ply yarns are too weak for fabric production, so two-ply yarns were developed and evaluated in the following experiments.

Table 3. Comparison of different plied yarns made from ReSpool polyester fibers (85% ReSpool/15% new).

	One-ply 85% ReSpool Polyester/15% New Polyester Mean	Two-ply 85% ReSpool Polyester/15% New Polyester Mean	p-Value for t-Test (Means are Equal)
Tex	366.00	922.28	0.002
Breaking Force (kgf)	0.04	0.96	0.001
Tenacity (gf/ denier)	0.014	0.115	<0.001
Elongation (%)	5.75	16.83	0.009

3.3.2. Comparison of Yarns Made from New and ReSpool Fibers

The researchers developed yarns from 100% ReSpool polyester fibers (No. 1 in Table 2), 100% new polyester fibers (No. 13 in Table 2), 100% ReSpool wool fibers (No. 2 in Table 2), and 100% new wool fibers (No. 14 in Table 2). t-tests were conducted to compare the size (tex), breaking force, tenacity, and elongation between yarns made from new and ReSpool fibers, and the results are shown in Table 4. Yarns made from new fibers are significantly smaller and stronger (both breaking force and tenacity) than yarns made from ReSpool fibers (all p values < 0.05). The shredded ReSpool fibers were randomly oriented, while the new wool fibers were slivers and new polyester fibers were also more parallel. After the same numbers of carding, the new fibers had higher parallelism than the ReSpool fibers. In addition, the ReSpool fibers contained some short yarns, which had a bigger diameter than new fibers. Therefore, the yarns made from new fibers are significantly smaller than yarns made from ReSpool fibers. For elongation, there is no significant difference between yarns made from new and ReSpool polyester (p = 0.068), but yarns made from ReSpool wool fibers have significantly higher elongation than yarns made from new wool fibers (p < 0.05). The sliver form of new wool fibers and higher parallelism of new wool fibers in batts were the possible reasons that yarns made from new wool fibers had significantly lower elongation.

Table 4. Comparison of yarns made from new and ReSpool fibers (polyester and wool).

	100% Polyester			100% Wool
	New Mean	Respool Mean	p-Value for t-Test (Means are Equal)	New Mean	Respool Mean	p-Value for t-Test (Means are Equal)
Tex	839.17	1899.97	0.002	897.49	1886.12	0.004
Breaking Force (kgf)	5.38	1.72	0.004	3.07	1.41	0.001
Tenacity (gf/denier)	0.718	0.097	0.001	0.382	0.083	<0.001
Elongation (%)	29.70	17.60	0.068	10.27	23.97	<0.001
Moisture Regain (%)	--	--	--	4.82	4.82	0.99

We compared the moisture regain of yarns developed from 100% new and ReSpool wool fibers and the results are in Table 4. There was no significant difference in moisture regains between yarns made from 100% new and ReSpool wool fibers (p = 0.99).

3.3.3. Comparison of Yarns Made from 100% ReSpool Polyester, Wool, and Silk Fibers

One-way ANOVA was conducted to compare the size (tex), strength and elongation of yarns made from 100% ReSpool polyester (No. 1 in Table 2), 100% ReSpool wool (No. 2 in Table 2), and 100% ReSpool silk (No. 3 in Table 2) fibers. It was found that there was no significant difference among the ReSpool fibers breaking force (p = 0.122), and elongation (p = 0.057). After the shredding, all ReSpool polyester, wool, and silk fibers are staple fibers with similar length, which could be the reason that yarns made from ReSpool polyester, wool, and silk fibers have similar breaking force. Because the p-value of the elongation comparison was slightly higher than 0.05, a Tukey HSD post hoc test was conducted. It was found that there was no significant difference in elongation between ReSpool polyester and ReSpool wool fibers (p = 0.320), and ReSpool polyester and ReSpool silk fibers (p = 0.353), but there was a significant difference between ReSpool wool and ReSpool silk fibers (p = 0.048). There existed significant differences among the ReSpool fibers in tex (p = 0.007) and tenacity (p = 0.008). A Tukey HSD post hoc test found that there was no significant difference in tex between ReSpool polyester and ReSpool wool fibers (p = 0.997), but there were significant differences between ReSpool polyester and ReSpool silk fibers (p = 0.011) and between ReSpool wool and ReSpool silk fibers (p = 0.011). A Tukey HSD post hoc test found that there was no significant difference in tenacity between ReSpool polyester and ReSpool wool fibers (p = 0.947), but there were significant differences between ReSpool polyester and ReSpool silk fibers (p = 0.016) and between ReSpool wool and ReSpool silk fibers (p = 0.011).

3.3.4. Comparison of Yarns Made from Different Percentages of ReSpool Polyester Fibers

One-way ANOVA was conducted to compare the size (Tex), strength and elongation of yarns made from 100% ReSpool polyester fibers (No. 1 in Table 2), 85% ReSpool polyester fibers (No. 5 in Table 2), and 65% ReSpool polyester fibers (No. 8 in Table 2). It was found that there existed a significant difference among the percentage of ReSpool polyester fibers in tex (p = 0.001). A Tukey HSD post hoc test on tex found that there was no significant difference between yarns made from 65% and 85% ReSpool polyester fibers (p = 0.097), but there existed significant differences between yarns made from 65% and 100% ReSpool polyester fibers (p = 0.010) and between yarns made from 85% and 100% ReSpool polyester fibers (p = 0.001). Adding highly parallel new polyester fibers in the yarn development would reduce the diameter of the yarns. There was no significant difference among the percentages of ReSpool polyester fibers in breaking force (p = 0.209), tenacity (p = 0.089), and elongation (p = 0.519). The percentage of ReSpool polyester fibers in the yarn did not affect the strength and elongation of the yarn.

3.3.5. Comparison of Yarns Made from Pre-Consumer ReSpool Cotton Fibers

Two-way ANOVA was conducted to compare the size (tex), strength, and elongation of yarns made from ReSpool denim cotton fibers (No. 6, 7, 9, 10 in Table 2). There were two independent variables, i.e., percentage of ReSpool cotton fibers and new fiber type. The percentage ReSpool cotton fibers had two levels: 85% and 65%; and the new fiber type had two levels: cotton and wool. The p-values of the two-way ANOVA tests are in Table 5. For all four independent variables (tex, breaking force, tenacity, elongation), there was no significant interaction between the percentage of ReSpool cotton fibers and new fiber type (all p values > 0.05), and there was no significant difference between the percentage of ReSpool cotton (all p values > 0.05). For strength (breaking force and tenacity) and elongation, there was no significant difference between the types of new fibers added (all p values > 0.05). For tex, there existed a significant difference between types of new fibers added (p = 0.009). Adding new wool fibers in the yarn development would make the yarn finer than adding new cotton fibers. The percentage of ReSpool cotton fibers and the type of new fiber added in the yarn did not affect the strength and elongation of the yarn.

Table 5. Two-way ANOVA results for comparison of yarns made from pre-consumer ReSpool denim cotton fibers (% of ReSpool fibers and type of new fibers).

	p-Value for Interaction	p-Value for Main Effect of % of ReSpool Fibers	p-Value for Main Effect of New Fiber Type
Tex	0.169	0.660	0.009
Breaking Force (kgf)	0.804	0.058	0.450
Tenacity (gf/denier)	0.404	0.117	0.104
Elongation (%)	0.892	0.310	0.235

A t-test was conducted to compare the moisture regain of made from different percentages (85% and 65%) of ReSpool denim cotton fibers and new cotton fibers (15% and 35%) (No. 6 and 9 in Table 2). It was found that there was no significant difference (p = 0.27) in moisture regain between yarns made from 85% ReSpool denim cotton/15% new cotton (mean = 1.54%) and 65% ReSpool denim cotton/35% new cotton (Mean = 1.97%).

3.4. Yarns Made from Post-Consumer ReSpool Fibers

3.4.1. Comparison of Yarns Made from Post-Consumer ReSpool Cotton and Rayon Fibers

t-tests were conducted to compare the size, strength, elongation, and moisture regain between yarns made from 15% new cotton fibers and 85% post-consumer ReSpool cotton and rayon fibers (No. 11 and 12 in Table 2). The results are in Table 6. There existed no significant difference in size (tex), strength (breaking force and tenacity), elongation between yarns made from 15% new cotton fibers and 85% post-consumer cotton and rayon fibers (all p values > 0.05). There existed a significant difference in moisture regain between yarns made from 15% new cotton fibers and 85% post-consumer cotton and rayon fibers (p = 0.007).

Table 6. Comparison of yarns made from post-consumer ReSpool cotton and rayon fibers (85% ReSpool/15% new cotton).

	85% Post-Consumer ReSpool Cotton/15% New Cotton Mean	85% Post-Consumer ReSpool Rayon/15% New Cotton Mean	p-Value for t-Test (Means are Equal)
Tex	2022.75	2048.56	0.883
Breaking Force (kgf)	1.17	1.00	0.460
Tenacity (gf/denier)	0.065	0.054	0.403
Elongation (%)	15.74	17.76	0.357
Moisture Regain (%)	1.09	3.15	0.007

3.4.2. Comparison of Yarns Made from Pre-Consumer and Post-Consumer ReSpool Cotton Fibers

t-tests were conducted to compare the size, strength, elongation, and moisture regain between yarns made from 15% new cotton fibers and 85% pre-consumer ReSpool denim cotton fibers (No. 6 in Table 2) and post-consumer Respool cotton fibers (No. 11 in Table 2). The results are shown in Table 7. Yarns made from pre-consumer ReSpool denim cotton fibers had a significantly bigger diameter than yarns made from post-consumer ReSpool cotton fibers (p = 0.003). The breaking force and elongation of yarns made from pre-consumer ReSpool denim cotton fibers were significantly higher than yarns made from post-consumer ReSpool cotton fibers (both p values < 0.05). There existed no significant difference in tenacity (p = 0.571) and moisture regain (p = 0.330) between yarns made from pre-consumer and post-consumer ReSpool cotton fibers.

Table 7. Comparison of yarns made from pre-consumer and post-consumer ReSpool cotton fibers (85% ReSpool/15% new cotton).

	85% Pre-Consumer ReSpool Denim Cotton/15% New Cotton Mean	85% Post-Consumer ReSpool Cotton/15% New Cotton Mean	p-Value for t-Test (Means are Equal)
Tex	3187.52	2022.75	0.003
Breaking Force (kgf)	2.14	1.17	0.005
Tenacity (gf/denier)	0.074	0.065	0.571
Elongation (%)	23.43	15.74	0.021
Moisture Regain (%)	1.54	1.09	0.330

4. Discussion

4.1. Quality of Yarns Made from ReSpool Fibers

Mechanical textile recycling through shredding fabrics to fibers made staple fibers, so all of the yarns developed in this research were spun yarns. We developed one-ply yarns from 85% ReSpool polyester and 15% new polyester, but its 0.04 kgf breaking force and 0.014 gf/denier tenacity were too weak for weaving and knitting. Therefore, mechanically recycled ReSpool fibers must be made into multi-ply yarns for future weaving and knitting.

The lowest mean breaking force of all two-ply yarn samples made from ReSpool recycled fibers was 0.96 kgf (No. 5 in Table 2, 85% pre-consumer ReSpool polyester, 15% new polyester). The breaking forces of all two-ply yarn samples were higher than the peak warp tension of 77.6 to 108.8 g (0.0776 to 0.1088 kgf) in the weaving process [16], the highest tension of 51.98 cN (0.530 kgf) in circular knitting [17], and 60 cN (0.612 kgf) in tricot knitting [18]. The two-ply yarn samples made from pre-consumer and post-consumer ReSpool recycled fibers have sufficient tensile strength to avoid yarn breakage in weaving and knitting. Researchers suggested that recycled fibers from pre-consumer textiles could be used for yarn production while recycled fibers from post-consumer textiles was best used for producing nonwovens or for building insulations due to lower quality fibers after wear [8]. ReSpool recycled fibers from post-consumer textiles can also be used to produce yarns with appropriate strength for weaving and knitting. Though the two-ply yarn samples’ breaking forces were higher than the warp tension in the weaving process, it is recommended to use two-ply yarns made from ReSpool fibers as the filling yarns in weaving since filling yarns can be quite weak [15].

Because a hand yarn spinning device was used in this research, all of the two-ply yarn samples developed in this research have much higher diameters (tex) than commercial yarns used in the literature [14,16,17]. This resulted in the much lower tenacity of the yarns developed from the ReSpool recycled fibers than commercial yarns; however, the breaking forces of the yarns made from ReSpool recycled fibers are comparable to or higher than the yarns in the literature [14,16].

4.2. Yarns Made from Pre-Consumer ReSpool Fibers

Using the same yarn spinning device, yarns made from 100% ReSpool recycled wool and polyester fibers (No. 1 and 2 in Table 2) are significantly larger in diameter (tex) and weaker (both breaking force and tenacity) than yarns made from 100% new wool and polyester fibers (No. 13 and 14 in Table 2). Yarns made from 100% new wool fibers also have significantly higher elongation than yarns made from 100% ReSpool recycled wool fibers. The quality of yarns made from ReSpool recycled fibers is lower than that of yarns made from new fibers. Fabrics made from ReSpool fiber yarns will be thicker and weaker than those made from new fibers. Yarns made from 100% ReSpool and new wool fibers have similar moisture regain, indicating that ReSpool recycled wool fibers maintain the moisture management property.

For yarns made from ReSpool polyester and denim cotton fibers, the percentage of ReSpool fibers (100%, 85%, and 65% for ReSpool polyester fibers; 85% and 65% for ReSpool denim cotton fibers) in the yarns did not significantly affect the strength (both breaking force and tenacity) and elongation of the yarn. There also existed no significant difference in size (tex) between 85% and 65% ReSpool fiber content for both ReSpool polyester and denim cotton fibers. Yarns made from 85% ReSpool denim cotton/15% new cotton had similar moisture regain to yarns made from 65% ReSpool denim cotton/35% new cotton. Yarns made from a high content of ReSpool recycled fibers have a similar quality as yarns made from a low content of ReSpool recycled fibers. Researchers suggested that up to 50% mechanically recycled fibers can be used in yarn production without affecting the mechanical properties of the yarn [9]. The percentage of ReSpool recycled fibers in yarn production can be much higher than 50%. To maximize the utilization of the ReSpool fibers made from textile waste, it is recommended to make yarns made from 85% ReSpool recycled fibers.

It is not feasible to use 100% ReSpool cotton fibers from pre-consumer denim textiles to make batts. Therefore, it is necessary to add new fibers to the ReSpool recycled cotton fibers to make batts and produce yarns. Adding new cotton or wool fibers (both of them are biodegradable) to ReSpool denim cotton fibers can assure the biodegradability of the yarn. For ReSpool denim cotton fibers, adding new cotton and wool fibers in the spinning process made no significant difference in the yarn’s strength (both breaking force and tenacity) and elongation. Adding new wool in yarn spinning would make the yarn’s size (tex) smaller than adding new cotton. To reduce the cost of the yarns made from ReSpool denim cotton fibers, it is recommended to blend them with new cotton fibers rather than new wool fibers unless a small diameter is required for the yarn.

4.3. Yarns Made from Post-Consumer ReSpool Fibers

There were no significant differences in size (tex), strength (breaking force and tenacity), elongation between yarns made from 15% new cotton fibers and 85% post-consumer ReSpool cotton and rayon fibers. New cotton yarns are stronger than new rayon yarns [24]. Mechanical recycling through textile-to-fiber shredding weakens both cotton and rayon. As a result, yarns made from ReSpool recycled cotton fibers and ReSpool recycled rayon fibers have similar strengths. Yarns made from 85% post-consumer ReSpool rayon and 15% new cotton fibers have a significantly higher moisture regain than those made from 85% post-consumer ReSpool cotton and 15% new cotton fibers. This can be attributed to the higher moisture regain of rayon than cotton [24].

The breaking force and elongation of yarns made from 85% pre-consumer ReSpool denim cotton fibers and 15% new cotton fibers were significantly higher than yarns made from 85% post-consumer ReSpool cotton fibers and 15% new cotton fibers. ReSpool fibers made from pre-consumer textile waste are a better starting material to produce yarns than ReSpool fibers from post-consumer textile waste. This corresponds to the literature, as textile wear reduces the fiber quality [8]. Because there were wear and tears for the post-consumer textiles, the post-consumer ReSpool fibers might have smaller diameter than pre-consumer ReSpool fibers, which resulted in yarns made from post-consumer fibers had significantly smaller tex as in Table 7. Because of the difference in yarn diameters, there was no significant difference in tenacity between yarns made from pre-consumer and post-consumer ReSpool cotton fibers. To increase the breaking force of yarns made from post-consumer ReSpool cotton fibers, more plies (e.g., three-ply rather than two-ply) can be applied. Both yarn size and breaking force could be similar between two-ply yarns made from 85% pre-consumer ReSpool denim cotton fibers and 15% new cotton fibers and three-ply yarns made from 85% post-consumer ReSpool cotton fibers and 15% new cotton fibers.

5. Conclusions

We successfully developed yarns using a high percentage of mechanically recycled ReSpool wool, silk, polyester, and cotton fibers. ReSpool recycled fibers from both pre-consumer and post-consumer textiles can be used to produce yarns that have appropriate strength for weaving and knitting. There were a few key issues to prepare the yarns from ReSpool recycled fibers: sufficient cardings to align the fibers, adding new fibers when needed in batt and yarn development, and spinning into multiple plies to achieve appropriate yarn strength. It is possible to produce batts and yarns from 100% ReSpool recycled wool, polyester, and silk fibers, but ReSpool recycled cotton and rayon fibers must be blended with new fibers to produce batts and yarns. Due to the mechanical strain in the mechanical recycling process and short fiber length, 95% of mechanically recycled textile fibers are used in low value nonwoven production instead of high value yarn production [12]. ReSpool recycled fibers can be used in yarn production to develop high value products from textile waste.

There was no significant difference among the percentage of ReSpool polyester and cotton fibers (100%, 85%, and 65% for ReSpool polyester fibers; 85% and 65% for ReSpool cotton fibers) in the yarns on the strength (both breaking force and tenacity) and elongation of the yarn. It was recommended to use the higher percentage of ReSpool recycled fibers, e.g., 100% polyester and 85% cotton, in yarn development to maximize recycled material utilization.

To assure the further recycling or composting of the end-of-use yarns made from ReSpool fibers, it is recommended to blend ReSpool fibers with appropriate new fibers, e.g., blending ReSpool polyester fibers with new polyester fibers for recycling and blending ReSpool cotton fibers with new cotton or wool fibers for composting (biodegradation). Adding new cotton or new wool fibers to ReSpool cotton fibers did not make significant differences in the yarn’s strength (both breaking force and tenacity) and elongation.

Yarns made from post-consumer ReSpool cotton fibers’ breaking force, elongation, and size were significantly smaller than yarns made from pre-consumer ReSpool denim cotton fibers. Yarns made from pre-consumer ReSpool denim cotton fibers have higher quality than yarns made from post-consumer ReSpool cotton fibers. Increasing ply numbers of yarns made from post-consumer ReSpool cotton fibers can increase the yarn strength without increasing yarn size.

There were a few limitations of this research that should be incorporated in future investigation. We used a hand spinning device to produce yarns. Whether the mechanically recycled fibers can be spun by industrial machines that have much higher rotation speeds and production rates will be further investigated. Since the ReSpool recycled fibers were from textile waste, we do not know whether and what types of dyes and finishes were used on the textiles. The unknown chemicals that were applied to the starting textile materials and existed in the yarns might affect the test results. Developing and testing yarns made from pre-consumer textiles with known dyes/finishes or no dyes/finishes will be further investigated.