Evaluation of post-consumer cellulosic textile waste for chemical recycling based on cellulose degree of polymerization and molar mass distribution

Abstract

The aim of this study is to improve the understanding of which end-of-life cellulosic textiles can be used for chemical recycling according to their composition, wear life and laundering—domestic versus service sector. For that purpose, end-of-life textiles were generated through laboratorial laundering of virgin fabrics under domestic and industrial conditions, and the cellulose content and its intrinsic viscosity and molar mass distribution were measured in all samples after two, 10, 20, and 50 laundering cycles. Results presented herein also address the knowledge gap concerning polymer properties of end-of-life man-made cellulosic fabrics—viscose and Lyocell. The results show that post-consumer textiles from the home consumer sector, using domestic laundering, can be assumed to have a similar, or only slightly lower, degree of polymerization than the virgin textiles (−15%). Post-consumer textiles from the service sector, using industrial laundering, can be assumed to have a substantially lower degree of polymerization. An approximate decrease of up to 80% of the original degree of polymerization can be expected when they are worn out. A higher relative decrease for cotton than man-made cellulosic textiles is expected. Furthermore, in these laboratorial laundering trials, no evidence evolved that the cellulose content in blended polyester fabrics would be significantly affected by domestic or industrial laundering. With respect to molar mass distribution, domestic post-consumer cotton waste seems to be the most suitable feedstock for chemical textile recycling using Lyocell-type processes, although a pre-treatment step might be required to remove contaminants and lower the intrinsic viscosity to 400–500 ml/g.

Keywords

cotton Lyocell viscose industrial laundering domestic laundering degree of polymerization

In the textile industry, current recycling strategies can be divided into mechanical and chemical processes. For natural fibers such as cotton, the main recycling process is mechanical recycling. Chemical recycling of cellulosic fibers has not developed at a global scale yet.¹ The collection of post-consumer textiles is well established today, but the main barrier toward up-scaling chemical recycling is the lack of mature and cost-effective sorting and recycling technologies. Apart from manual sorting, which is a slow and costly process, presently there are only a few research and industrial activities aiming to demonstrate automated near infra-red (NIR) sorting of textiles at an industrial scale; namely, Fibersort, SIPTex, and Resyntex.^2–5 The only chemical cotton recycling technology used today at industrial scale is patented by Lenzing,⁶ and involves dissolution of post-production cotton textile waste mixed with wood dissolving pulp in N-methylmorpholine N-oxide (NMMO) solution, followed by spinning of Lyocell-type cellulosic fibers (REFIBRA technology).⁷ A more recent Lyocell technology, termed Ioncell, is currently being developed at Aalto University and uses an ionic liquid for the dissolution of cotton fabric waste. The resulting solution is then spun into Lyocell-type cellulosic fibers via dry-jet wet spinning.^8–11 However, this technology is yet to be scaled-up to an industrial level.⁸ Both chemical recycling technologies require pure materials, and considering that most cotton-based fabrics on the market are blends with polyester, an additional step for separation of the two components is required before chemical recycling.^11–13 Another critical parameter for chemical recycling is the chain length of the cellulose fibers, determined indirectly by intrinsic viscosity measurements. It is known that long-time service and laundering of textiles results in a decreased chain length of cellulose fibers due to a combined effect of mechanical and chemical degradation.^11,14–17 The cellulose chain length affects the rheological properties of the spinning solution, which is important for the structure formation and final properties of the regenerated fibers.¹⁸ Possible ways to control the rheology of the spinning dope are either by changing the cellulose concentration in the solution or changing the degree of polymerization (DP) of the cellulose. Hence, the material purity and the cellulose chain length of the end-of-life fabrics are both important requirements for chemical textile recycling processes. For the Lyocell-type spinning technologies, optimum spinnability and economy are observed if the cellulose has an intrinsic viscosity of ca. 400–500 ml/g and, in addition, a molar mass distribution with a defined proportion of low and high molar mass fractions.⁹ Where deviations in the feedstock occur, the spinning behavior can be improved by mixing the cellulosic textile waste with other substrates, mostly wood dissolving pulp.⁹ At the beginning of the Lyocell development, mixtures of high and low molecular weight pulps were used as preferred raw materials. Since this was too expensive, Lenzing limited its feedstock to pulps with a certain molecular weight distribution. However, this practice of blending raw materials was reintroduced by Lenzing with the recycling of cellulosic textiles (Tencel REFIBRA).⁷ This also significantly reduces the need for additional pre-treatment and purification of the solvent.⁹

The aim of this study is to improve the understanding of the effects of wear life and laundering (domestic sector versus service sector) on the composition and chemical recyclability of end-of-life cellulosic textiles. The results obtained in this study also cover the current knowledge gap concerning the macromolecular properties of end-of-life man-made cellulosic fabrics—viscose and Lyocell. The experimental design of this study includes: (1) generation of end-of-life textiles through controlled laundering of virgin textile samples simulating domestic and industrial conditions, and (2) determination of the cellulose content and its intrinsic viscosity and molar mass distribution in all fabrics before and after laundering. Laboratorial laundering assures that the textile samples are subjected to the same mechanical and chemical impact, thus providing comparable data for different textiles.

Methods

Materials

The virgin textile materials used in the laboratorial laundering are summarized in Table 1.

Table 1.

List of virgin textiles used in the laboratorial laundering trials

Textile	Description
Cotton greige	Greige bed sheet for hospital use, previously characterized, supplied by a Swedish distributer company.
Cotton	Plain woven mercerized fabric for shirts supplied by Tekstina. Prepared for printing, weight 112 g/m².
Lyocell	Plain woven fabric TENCEL®MICRO, 0.9 dtex, 119 g/m², supplied by Tekstina.
Viscose	Plain woven fabric from staple fibers (Art. USTIA) supplied by Tekstina.
Cotton/Lyocell blend	Supima Cotton ELS/TENCEL®, blend 65/35, 1.4 dtex according to supplier, 125 g/m², half panama 2, moisture crosslinking, supplied by Tekstina.
Cotton/polyester blend	White hospital bed sheet with printed logotypes, supplied by a Swedish laundering service company.
Lyocell/polyester blend	White hospital bed sheet with printed logotypes and color, supplied by a Swedish laundering service company.

Unfortunately, it was not possible to receive information regarding the finishing procedure and spin finishes applied to the fabrics.

Laboratorial laundering

The virgin fabrics presented in Table 1 were cut into pieces of 15 × 50 cm and laundered in a horizontal axis, front-loading washing machine according to domestic and industrial standard laundering conditions.^19,20 Laundering under domestic conditions was performed by Swerea IVF using so-called reference detergent number 2 according to Annex J of ISO 6330:2012.¹⁹ The laundering temperature was 60℃ for all fabrics for comparable results, even though 60℃ is not recommended for laundering viscose fabrics. Tumble drying was performed at 80℃. The laundering machine was filled with 2 kg of polyester tricot garments as ballast. Samples were taken for analysis after two, 10, 20, and 50 laundering cycles. Laboratorial laundering under industrial conditions was performed by Swerea IVF, according to ISO 15797:2017.²⁰ To mimic the detergent used by the laundry companies, peracetic acid was mixed with hydrogen peroxide according to the aforementioned standard method: 835 ml hydrogen peroxide (30 wt%) + 120 ml peracetic acid (40 wt%) + 45 ml water. This detergent mixture was then added to the laundry process at a dosage of 2 g/l (Table 1, page 7 in the ISO 15797:2017 standard). A laundering temperature of 75℃ and a tumble-drying temperature of 80℃ were chosen. Usually, laundering services use a mangle to dry, but tumble drying was chosen for these laboratorial trials because it saves time and handwork. The laundering machine was filled with 14 kg of cotton/polyester blouses as ballast. Samples were taken out for analysis after two, 10, 20, and 50 laundering cycles. The viscose and cotton/Lyocell fabric samples were not submitted to laboratorial laundering under industrial conditions.

Sample preparation for chemical analysis

For a representative sample, the bed sheets or garments were torn into 3 × 3 cm pieces, mixed, and then 100 g were sampled and ground with a sieve size of 0.45 mm before chemical characterization.

Polymer quantitative analysis

The cellulose content in blended textiles was determined according to ISO 1833-11:2010, using 75% sulfuric acid for dissolution of the cellulosic fibers.²¹

Intrinsic viscosity

The intrinsic viscosity (η) of the cellulosic textile samples dissolved in cupriethylenediamine (CED) was determined in a capillary viscometer according to the standard method SCAN-CM 15:88.²² All samples were studied in duplicate. The intrinsic viscosity of the blend-textiles was determined after filtration of the polyester residue with a P3 glass filter. The polyester content was gravimetrically determined at 105℃ and the intrinsic viscosity corrected for the polyester amount. The intrinsic viscosity (η) of cellulosic solutions in CED is correlated with the average chain length and degree of polymerization (DP_v) of cellulose, according to the Mark–Houwink equations:

D P_{v} = (\frac{η}{2.28})^{\frac{1}{0.76}} if η > 420 ml / mlg g

D P_{v} = (\frac{η}{0.42}) forsmaller η

Gel permeation chromatography

Gel permeation chromatography (GPC) was performed on the cellulosic samples to determine the number-average molecular weight (M_n), the weight-average molecular weight (M_w), the weight-average degree of polymerization (DP_w), defined as M_w[kDa]/0.162, and the polydispersity index (PDI), defined as M_w/M_n, of the cellulose chains. Prior to the analyses, the samples were activated in a water–acetone–N,N-dimethylacetamide (DMAc) solvent exchange sequence. The activated samples were dissolved under gentle stirring in 90 g/l lithium chloride (LiCl) containing DMAc at room temperature. The solutions were then diluted to 9 g/l in LiCl/DMAc, filtered with 0.2 µm syringe filters, and analyzed in a Dionex Ultimate 3000 system with a guard column, four analytical columns (PLgel MIXED-A 7.5300 mm), and RI-detection (Shodex RI-101). The flow rate was 0.75 ml/min. Narrow pullulan standards (343 Da–2500 kDa) were used to calibrate the system. A correction of the molar mass distribution obtained by direct-standard calibration was performed using an algorithm to calculate cellulose-equivalent molar masses of pullulan standards:²³ MM_cellulose =q*MM^p_pullulan, with q = 12.19 and p = 0.78. All samples were measured in duplicate. The blend textiles were not analyzed by GPC since they contain polyester and, thus, were not soluble in the GPC solvent.

Results and discussion

The virgin fabrics were chemically analyzed after zero, two, 10, 20, and 50 laundering cycles under domestic and industrial conditions. Table 2 shows that the initial DP/intrinsic viscosity of the cellulose chains depends greatly on the raw material and can also differ significantly within the same fiber type (e.g. cotton). Man-made cellulosic fibers have an innately lower intrinsic viscosity with classical viscose fibers showing the lowest values. Pre-consumer Tencel fibers are around 450 g/ml and decrease depending on the history of the individual fabric. Cotton already spans a wide viscosity range in its native state. This obviously adds to the complexity of cellulosic fabrics as substrate for man-made fibers and is one of the inherent challenges to overcome in chemical textile recycling. Pre-treatment procedures have to be developed that will allow to reduce the macromolecular inhomogeneity of the incoming material to an acceptable level.¹¹ In general, it was observed that after several industrial laundering cycles the fabrics got thinner than the fabrics washed in a domestic setup. After 44 industrial laundering cycles, the Lyocell fabric started to tear in the middle, and after 50 cycles, it was too disintegrated for further laundering. This was not observed with the other fabrics tested (Figure 1).

Figure 1.

Visual appearance of different fabrics before and after two, 10, 20, and 50 cycles of domestic and industrial laundering.

Table 2.

Polymer properties of textile samples submitted to domestic laundering

Textile sample	Domestic laundering cycles	Intrinsic viscosity (ml/g)	DP_v	M_n (kDa)	M_w (kDa)	DP_w	PDI (M_w/M_n)	Cellulose (wt%)
Cotton greige	2 (ref)	1980	7358	696	2282	14,086	3.3	–
	10	1914	7037	518	2125	13,117	4.1	–
	20	1850	6729	581	2036	12,568	3.5	–
	50	1819	6581		Poorly soluble			–
Cotton	2 (ref)	996	2979	216	1143	7056	5.3	–
	10	944	2776	245	794	4901	3.2	–
	20	871	2497	155	704	4346	4.5	–
	50	812	2277	210	639	3944	3.0	–
Lyocell	2 (ref)	338	805	58	304	1877	5.2	–
	10	342	814	67	271	1673	4.0	–
	20	340	810	53	274	1691	5.2	–
	50	333	793	63	257	1586	4.1	–
Viscose	2 (ref)	153	364	36	80	494	2.2	–
	10	151	360	38	77	475	2.0	–
	20	145	345	34	73	451	2.1	–
	50	133	317	40	72	444	1.8	–
Cotton/ Lyocell blend	2 (ref)			Poorly soluble
	10			Poorly soluble
	20			Poorly soluble
	50			Poorly soluble
Cotton/ polyester blend	2 (ref)	1112	3444	–	–		–	45
	10	1099	3391	–	–		–	45
	20	1027	3102	–	–		–	45
	50	947	2788	–	–		–	44
Lyocell/ polyester blend	2 (ref)		–	Poorly soluble				60
	10		–	Poorly soluble				60
	20		–	Poorly soluble				60
	50		–	Poorly soluble				59

Tables 2 and 3 summarize the viscometry and GPC data of the laboratorial laundering samples. Samples after two laundering cycles were chosen to be the reference cases since some of the never-washed fabrics did not give reliable analytical results, probably due to effects imposed by residual textile finish. Moreover, two laundering cycles had a negligible effect on the cellulose chain length. During chemical analysis, all cellulosic samples dissolved completely in the GPC and CED solvents, except for the cotton/Lyocell blend fabric, which was poorly soluble in both solvents, and the cotton greige fabric after 50 domestic laundering cycles, which was poorly soluble in the GPC solvent.

Table 3.

Polymer properties of textile samples submitted to industrial laundering

Textile sample	Industrial laundering cycles	Intrinsic viscosity (ml/g)	DP_v	M_n (kDa)	M_w (kDa)	DP_w	PDI (M_w/M_n)	Cellulose (wt%)
Cotton greige	2 (ref)	1719	6109	600	1866	11,519	3.1
	10	1011	3038	268	945	5833	3.5
	20	608	1556	119	486	3000	4.1
	50	352	838	76	198	1222	2.6
Cotton	2 (ref)	838	2374	200	722	4457	3.6
	10	446	1035	105	299	1846	2.9
	20	323	769	71	182	1123	2.6
	50	227	540	48	115	710	2.4
Lyocell	2 (ref)	290	690	55	215	1327	3.9
	10	172	410	38	88	543	2.3
	20	126	300	29	61	377	2.1
	44^a	100	238	23	44	272	1.9
Viscose	2 (ref)	–		–	–		–
	10	–		–	–		–
	20	–		–	–		–
	50	–		–	–		–
Cotton/ Lyocell blend	2 (ref)	–		–	–		–
	10	–		–	–		–
	20	–		–	–		–
	50	–		–	–		–
Cotton/ polyester blend	2 (ref)	960	2838	–	–		–	44
	10	577	1453	–	–		–	44
	20	429	984	–	–		–	44
	50	275	655	–	–		–	42
Lyocell/ polyester blend	2 (ref)	Poorly soluble		–	–		–	61
	10	197	469	–	–		–	60
	20	142	338	–	–		–	59
	50	93	221	–	–		–	56

No more chemical analyses were performed after the 44th laundering cycle due to severe fabric disintegration.

The intrinsic viscosity data in Table 2 and Figure 2 show that 50 cycles of domestic laundering had little or no effect on the cellulose chain length of the cotton greige and Lyocell fabric samples. In the case of the cotton, viscose and cotton/polyester fabrics, intrinsic viscosity or DP_v decreased just 13–18% after 50 cycles of domestic laundering. The M_w values did not decrease significantly either. Figure 3 shows the correlation between DP_w obtained by GPC and DP_v obtained by viscometry for the fabric samples before and after laundering. In general, linearity prevailed up to the highest DP_v values, but viscosity measurements predicted lower DP values than GPC (i.e. DP_v was lower than DP_w). In the range of DP_v 700–1200, however, linearity was not evident for cellulosic fabrics laundered at 60℃. There, DP_w remained relatively stable at 1500–1800, while DP_v dropped to relatively low values (700–800). Results in Table 2 also show that there was no significant decrease in the cellulose content of blended polyester fabrics upon 50 domestic laundering cycles. Hence, repeated domestic laundering at 60℃ did not have a large impact on the cellulose chain length of textiles and the original DP level of domestic post-consumer textile waste was preserved. For further processing, mixing with wood dissolving pulp and/or a pre-treatment to reduce DP to values that provide optimum spinnability and economy for Lyocell-type processes might be required.^7–11

Figure 2.

Evolution of the intrinsic viscosity of cellulosic fabrics after two (REF), 10, 20, and 50 domestic laundering cycles at 60℃ (CO = cotton, PES = polyester).

Figure 3.

Correlation between DP_w and DP_v for cellulosic fabric samples after laundering at 60℃ and 75℃ and for wood dissolving pulps.

The intrinsic viscosity data in Table 3 and Figure 4 show that repeated industrial laundering at 75℃ had a large impact on the cellulose chain length of all fabrics tested. Indeed, intrinsic viscosity decreased by 70–80% in the five fabrics tested, and M_w decreased by 80–90% in the three fabrics tested. During the washing process in a commercial laundry machine, the samples are exposed to thermal, chemical, and mechanical stress. Each of those parameters can have a significant influence on the integrity of the cellulose polymer chains. Cellulose degradation kinetics of autohydrolysis or in the presence of catalysts, oxidants, or other reactants have been studied extensively. However, detailed mechanistic analysis of the laundry studies presented herein was impeded by the complex interplay of several factors inflicted by the nature of the samples and laundry procedures. The values of intrinsic viscosity measured after 50 industrial laundering cycles were below the optimum specification for chemical textile recycling. Nevertheless, post-consumer textile waste from the service sector can be mixed with wood dissolving pulp to obtain a proper molar mass distribution that provides good spinnability of regenerated cellulose fibers.^7–11

Figure 4.

Evolution of the intrinsic viscosity of cellulosic fabrics after two (REF), 10, 20, and 50 industrial laundering cycles at 75℃ (CO = cotton, PES = polyester).

There was no significant decrease in the cellulose content of blended fabrics upon 50 industrial laundering cycles, though. The Lyocell/polyester fabric showed a slightly higher decrease in cellulose content than the cotton/polyester fabric: from 61% to 56%, but it is reasonable to assume that post-consumer CO/PES and Lyocell/PES textiles will retain the original cellulose content stated on the label.

The Lyocell/PES fabric was poorly soluble in CED and GPC solvents after domestic laundering. It also had poor solubility after two industrial laundering cycles. Considering that the detergent used in the industrial laundering procedure was a mixture of hydrogen peroxide and peracetic acid, one possible explanation for this behavior is that a few washing cycles under oxidative bleaching conditions at 75℃ can remove or reduce the amount of crosslinking agent (textile finish) that potentially limits the solubility of the fabric. Thus, in chemical recycling of domestic post-consumer textiles, a pre-treatment step is usually required to remove crosslinking agents, reactive dyes, and other contaminants.^13,16

As mentioned before, the molar mass distribution of cellulose chains plays a significant role in the spinnability of the cellulose solutions in Lyocell-type processes.⁹ According to a previous study, the molecular weight distribution of the feedstock should have a share of 20% or higher of cellulose chains with DP_v >2000, a share of 5–10% of chains with DP_v <100, and a minimum polydispersity index of 3.4 to favor both spinnability and the mechanical strength properties of the spun fibers.⁹ Figure 5 shows the molar mass distribution curves of the cotton and Lyocell fabric samples. Tables 2 and 3 show that the PDI decreases below 3.4 for all fabrics after repeated domestic or industrial laundering cycles, except for cotton greige and cotton fabrics after domestic laundering. Thus, with respect to molar mass distribution and average DP, domestic post-consumer cotton waste seems to be the most suitable feedstock for chemical textile recycling using Lyocell-type processes, although a pre-treatment step might be required to remove contaminants and lower the intrinsic viscosity to 400–500 ml/g.

Figure 5.

Molar mass distribution of cellulose chains in cotton and Lyocell fabrics after two (Ref), 10, 20, and 50 domestic (Home) and industrial (Ind) laundering cycles.

Conclusions

The laboratorial laundering trials allow for some general conclusions. Post-consumer textiles from the home consumer sector, using domestic laundering, can be assumed to have similar, or only slightly lower, DP than the virgin textiles (−15%). Post-consumer textiles from the service sector, using industrial laundering, can be assumed to have a substantially lower DP. An approximate decrease of up to 80% of the original DP can be expected when they are worn out. In general, a higher decrease of DP was found for cotton. This is mostly due to the fact that the starting DP of cotton is higher than those of man-made cellulosic fibers. Long polymer chains are more likely to undergo statistical degradation. Furthermore, the laboratorial laundering trials showed no evidence that the cellulose content in blended polyester fabrics is affected significantly by domestic or industrial laundering taking into account that the original composition is often only vaguely described in the garment label. With respect to molar mass distribution, domestic post-consumer cotton waste seems to be the most suitable feedstock for chemical textile recycling using Lyocell-type technologies, although a pre-treatment step might be required to remove contaminants and lower the intrinsic viscosity to 400–500 ml/g. Of course, it has to be emphasized that this study only evaluated the effects of laundering on the macromolecular integrity of the fabrics. Wear abrasion and deterioration due to UV exposure will also affect the properties of waste textiles.

Footnotes

Acknowledgments

Swerea IVF is gratefully acknowledged for performing the laundering trials. Tekstina, Senstex, and Textilia are gratefully acknowledged for supplying fabrics.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was conducted within the project “Trash-2-Cash: Designed high-value products from zero-value waste textiles and fibres via design driven technologies” that received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No. 646226.

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