Investigating how the dye colour is impacted when chemically separating polyester-cotton blends

Figures & data

Figure 1. The original polycotton fabric and two recovered products from the separation of the 80/20 polyester/cotton blend. Note the heightened transparency of the recovered polyester due to cotton removal.

Table 1. Descriptions for different values for the colour difference.

Colour difference range, ΔE	Description
0–0.5	No change in colour.
0.5–1	An almost imperceptible colour change.
1.0–2.0	A small change in colour.
2.0–4.0	A perceivable difference in colour.
4.0–5.0	A change in colour rarely tolerated in industry.
> 5.0	A different colour.

Figure 2. (a) Thermal gravimetric analysis (TGA) data displaying the thermal decomposition of 100% polyester fabric, recovered polyester, and the 80/20 polycotton blend, (b) thermal decomposition of treated and untreated 100% polyester, and (c) Differential Scanning Calorimetry (DSC) data showing the melt characteristics of 100% polyester and recovered polyester from the 80/20 blend.

Table 2. DSC results including the onset and midpoint of the glass transition, the onset and peak for melting temperature, and the enthalpy for 100% polyester and recovered polyester from the 80/20 polycotton blend.

Sample	Tg_onset (°C)	Tg_midpoint (°C)	Tm_onset (°C)	Tm_peak (°C)	Enthalpy (J/g)
100% polyester	75.9 ± 0.8	84.5 ± 0.8	238.2 ± 0.8	249.3 ± 1.2	49.9 ± 0.4
Recovered polyester	77.4 ± 2.0	84.0 ± 1.7	239.0 ± 1.3	251.1 ± 1.3	51.2 ± 0.8

Figure 3. FTIR spectra for (a) 100% polyester and recovered polyester from 80/20 polycotton and (b) 100% polyester, 100% cotton, and 80/20 polycotton fabric.

Table 3. Draw ratio, linear density, tenacity, elongation, and elastic modulus of fibres produced form 100% cotton and from polycotton, compared to viscose fibres.

Fibre type	Draw ratio	Linear density (dtex)	Tenacity (cN/tex)	Elongation (%)	Elastic modulus (GPa)
Viscose (Michud et al., 2016)	NA	1.8 ± 0.1	20.7 ± 1.5	19.2 ± 2.2	6.9 ± 0.5
RCF from polycotton [this study]	6	1.3 ± 0.1	21.3 ± 2.1	8.9 ± 1.6	13.7 ± 3.2
RCF from 100% cotton (Ma et al., 2020)	6	2.3 ± 0.6	21.6 ± 2.2	6.7 ± 1.4	18.1 ± 2.4

Table 4. CEILab space coordinates, colour strength values and colour difference values for polycotton 80:20 blend, 100% polyester and recovered materials.

Sample	L* (mean ± SD)	a* (mean ± SD)	b* (mean ± SD)	Colour strength (mean ± SD)	Colour difference ΔE
Polycotton 80/20 fabric	49.9 ± 0.2	−4.6 ± 0.1	−22.3 ± 0.1	3.14 ± 0.03	0
Recovered polyester fabric from 80/20 blend	50.5 ± 0.1	−3.45 ± 0.1	−25.72 ± 0.1	3.19 ± 0.02	3.7
Regenerated cellulose fibres from 80/20 blend	48.8 ± 1.2	−5.6 ± 0.4	−14.1 ± 0.7	3.07 ± 0.01	8.3
Sample	L* (mean ± SD)	a* (mean ± SD)	b* (mean ± SD)	Colour Strength (mean ± SD)	Colour difference ΔE
100% polyester fabric used in 50/50 mixture	55.8 ± 0.1	−5.3 ± 0.1	−27.1 ± 0.1	2.53 ± 0.01	0
100% polyester snippets used in 50/50 mixture	59.9 ± 0.3	−6.4 ± 0.1	−28.2 ± 0.3	2.05 ± 0.04	4.4
Recovered polyester from 50/50 mixture	58.0 ± 0.2	−5.1 ± 0.1	−22.1 ± 0.1	1.97 ± 0.02	5.4

Figure 4. Schematic showing that the discrete dye colour specific to fibre type is retained on the recovered materials. Note the dark colour of the sample labelled ‘mixed and pre-treated’ and the solution-dispersion labelled ‘dissolved’ indicating a homogenous mixture of dissolved orange cotton and undissolved blue polyester snippets.

Figure 5. SEM images for chopped snippets of (a) 100% polyester and (b) recovered polyester, (c) polycotton 80/20 fabric and (d) recovered polyester fabric, and (e) polycotton 80/20 fibre bundle and (f) recovered polyester bundle.

Figure 6. SEM images of recovered polyester fabric at low (a) and high (b) magnification displaying the residual cellulose film remaining on the polyester fibres before the DMSO rinse step was introduced. The regenerated cellulose film is labelled as ‘Cellulose’.

Michud, A., Tanttu, M., Asaadi, S., Ma, Y., Netti, E., Kääriainen, P., Persson, A., Berntsson, A., Hummel, M., & Sixta, H. (2016). Ioncell-F: Ionic liquid-based cellulosic textile fibers as an alternative to viscose and Lyocell. Textile Research Journal, 86(5), 543–552. https://doi.org/10.1177/0040517515591774

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Ma, Y., Rosson, L., Wang, X., & Byrne, N. (2020). Upcycling of waste textiles into regenerated cellulose fibres: Impact of pretreatments. The Journal of the Textile Institute, 111(5), 630–638. https://doi.org/10.1080/00405000.2019.1656355

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