Textile characteristics

The analysis of the selected garments under optical microscope, whose micrographs are reported in Fig. 1, allowed to obtain information on their textile features. In general, textile fibres are spun into yarns, twisted in different way along fibre axis. The fibres constituting the yarns can be staple fibres, of comparatively short length, and filaments, which are fibres of indefinite length19. Yarns are mainly arranged in two structures: woven fabrics produced by interlacing two sets of yarns, the warp which runs in a lengthways direction and the weft which runs in a widthways direction and knitted fabrics produced by interlacing loops of yarn20. Moreover, the hairiness is defined as the presence of small fibres that protrude from the main yarn core21.

Figure 1 Pictures and optical micrographs of selected garments: (a) BT, a 100% polyester t-shirt, (b) plane surface and (c) yarn of BT; (d) RT, a 100% polyester t-shirt, (e) plane surface and (f) yarn of RT; (g) GB, 100% polyester blouse of which 65% is recycled polyester, (h) plane surface, (i) warp and (l) weft yarns of GB; (m) GT, a top whose front is made of 100% polyester and whose back is made of a blend of 50% cotton and 50% modal (n) plane surface, (o) warp and (p) weft yarns of GT front polyester part, (q) plane surface and (r) yarn of GT back modal/cotton part. Full size image

As possible to observe from Fig. 1b,c,e,f, both t-shirts made of 100% polyester, BT and RT, are knitted fabrics with low hairiness, whose yarn is made of continuous filaments with no twist. BT was made by weft knitting in a single jersey structure and RT was made by warp knitting. The 100% polyester blouse containing 65% of recycled polyester, GB, see Fig. 1h, has a satin weave structure with low hairiness. The two yarns constituting the woven are reported in Fig. 1i–l and they are both composed by filaments with the weft characterized by a slightly higher twist, 2510 t/m, than the warp, 2350 t/m. Finally, the top GT presents a double structure, observable in Fig. 1n,q. The front part of the top (Fig. 1n) is 100% polyester characterized by a satin weave structure, low hairiness and by both yarns made of continuous filaments (Fig. 1o,p), with a moderate twist in the case of the warp, 1274 t/m, and a higher one for the weft, 1669 t/m. The back of the top (Fig. 1q) is a blend of 50% cotton and 50% modal rib knitted with higher hairiness and the yarn is made of short staple fibres (Fig. 1r), very low twisted, 666 t/m.

First washing cycle

The clothes were tested in washing trials to quantify the release of microfibres during washings. Each washing test was performed on a washing load of about 2–2.5 kg of identical clothes, with liquid detergent. The results of the microplastics released after the first washing for all garments are depicted in Fig. 2.

Figure 2 Microfibres released (expressed in mg/kg, M a ± SD, n = 2) from BT, a 100% polyester t-shirt, RT a 100% polyester t-shirt, GB, a 100% polyester blouse of which 65% is recycled polyester, and GT, a top whose front is made of 100% polyester and whose back is made of a blend of 50% cotton and 50% modal. Full size image

As possible to observe from Fig. 2, BT and RT released a comparable amount of microfibres during the washing, consisting of 125.0 ± 32.1 mg/kg and 124.1 ± 12.4 mg/kg of microfibres, respectively. This result can be explained considering that both t-shirts have the same fabric structure and yarn characteristics, so this similar behavior during washing tests is not surprising. Moreover, it indicates a high reproducibility into the amount of microfibres released by knitted polyester fabrics made with yarns constituted by continuous filaments. This last result also allows to demonstrate differences among the data notwithstanding the limitation in the replication of washing tests (n = 2) that inhibits statistical analysis. The large washing load tested, very close to real washing loads, implied the usage of a great amount of clothes per washing, that ranges from 16 to 22 garments depending on the washed clothes, complicating the number of replications affordable. Instead, GB released 48.6 ± 2.2 mg/kg of fibres, a value less than the half of that released from BT and RT. Such difference could be related to the fact that the yarns constituting GB have a higher twist compared to those of BT and RT, and are assembled into a woven structure, resulting in a more compact assembly that could make more difficult for fibres to slip from the fabric. The greatest amount of microfibres released came from GT, with a value of 307.6 ± 21.8 mg/kg. Such result is almost three times those obtained for BT and RT. GT has the most complex textile structure with a front polyester woven part and a back cotton/modal knitted part that could have different behaviors in the release. In order to allow more comparability among these results and the data reported in literature, the amount of microfibres was converted in number of microfibres, N. The results, in agreement with the gravimetric data, indicated that the greatest N was 1,500,000 microfibres, released by GT. 1,100,000 microfibres were released by BT and 770,000 by RT. The lowest number of microfibres, 640,000, was released by GB.

The multistep filtration procedure allowed to obtain an indication of the dimensional ranges of released microfibres, since it was based on the recovery of microfibres on the 400 µm mesh, 60 µm and 20 µm pore size filters. In addition, 300 ml of wastewater were also filtered through a 5 µm pore size filter, obtaining an approximate concentration of the mg of microfibres per liter of water effluent; greater volumes were impossible to filter due to the clogging of the filter for its very small pore size. This last filtration provided evidence that small microfibres are present in the wastewater. Since the wastewater filtered on 5 μm pore size filter represented only a very small part of the total recovered wastewater, the results were not scaled to the entire effluent volume, since overestimation could be obtained with this calculation. The different amounts of microfibres recovered on each filter are reported in Fig. 3.

Figure 3 Microfibres recovered on: (a) 400 μm mesh, 60 and 20 µm pore size filters; (b) 5 µm pore size filters from the washing of BT, RT, GB, and GT. Full size image

The greatest amount of microfibres recovered was the one collected on the filter of 60 µm pore size, pictures of the filter appearance after filtration are shown in Fig. 4a–d.

Figure 4 Pictures of the microfibres recovered on 60 µm pore size filters from the washing of (a) BT, (b) RT (c) GB, and (d) GT; (e) Length of microfibres released from BT, RT, GB and GT recovered on 400 μm mesh, 60, 20 and 5 μm pore size filters. Full size image

These findings indicate that most of the fibres that detach from the fabrics have dimensions compatible with such pore size. Of course, it should be considered that smaller fibres could be trapped inside this and other fractions. In order to evaluate the microfibre dimensions of the fractions recovered on the different filters, some amounts of microfibres for each filter were analyzed by optical or scanning electron microscopy, see Fig. S1 in the Supporting Information (SI). Figure 4e reported the average values of the length of the analysed microfibres, indicating a dimensional gradient that is function of the filter pore size. Microfibres with length ranging from 1180 to 1500 μm were blocked by the 400 μm mesh; those recovered on 60 and 20 μm pore size filters presented an average length of 360–660 μm and of 310–390 μm; finally, a range of 120–500 μm was stopped by 5 μm pore size filters. The diameter of the observed microfibres remained almost constant with the following values: 13.7 ± 1.8 μm for BT, 15.7 ± 3.3 μm for RT, 12.4 ± 1.8 μm for GB and 15.7 ± 4.9 μm for GT. For this last garment, it has to be highlighted that the dimensions of cotton microfibres presented a length similar to that of polyester, while its diameter was slightly greater (18.0 ± 2.1 μm for cotton, 13.3 ± 1.3 μm for polyester). The 400 µm mesh blocked similar amounts of fibres for BT and RT (6.7 and 5.6 mg/kg, respectively), very low for GB (1.4 mg/kg) and significantly higher for GT (56.8 mg/kg). The same trend GT >> BT,RT >> GB, was observed also for the fibres recovered on the 60 µm filter, whereas fibres collected on 20 and 5 µm filters showed a different behavior. In fact, for both of them, the fibres released from BT and RT were of similar amounts (20 µm: 25.5 mg/kg for BT, 27.0 mg/kg for RT; 5 µm: 31.8 mg/L for BT, 26.5 mg/L for RT) but slightly greater than the amount released from GT (20 µm: 18.0 mg/kg; 5 µm: 15.2 mg/kg). For all garments the 60 µm filter was able to retain around 75–80% of the total amount of microfibres released per wash. For BT, RT and GB, 400 and 20 µm filters retained around 5% and 20% of the total release but, in the case of GT, such values were reversed (400 µm: around 20%; 20 µm: around 5%). The nature of the microfibres recovered on 60 μm pore size filters, that represented the most abundant microfibre fraction, was confirmed by using FTIR spectroscopy. The FTIR spectra of BT, RT and GB confirmed the polyester composition of the recovered microfibres and are reported in Fig. S2 in the SI. The spectrum of the microfibres released from GT, reported in Fig. 5a, is typical of cellulosic fibres. The broad band in the region 3700–3000 cm−1 is due to the OH-stretching vibrations containing the contribution of both hydroxyl groups interacting via intra-molecular hydrogen bonding centered at around 3340 cm−1, and via inter-molecular hydrogen bonding centered at around 3280 cm−1. The bands at 2940 and 2880 cm−1, are assigned to CH 2 asymmetrical and symmetrical stretching. The bands observed at 1720 and 1640 cm−1 are attributed to CO stretching and to OH bending of adsorbed water, respectively. The absorption bands at 1429, 1369, 1312 and 1204 cm−1 are due to OH in-plane deformation, CH bending, CH 2 rocking and to the CO stretching mode of the pyranose ring. The 1161 cm−1 vibration is attributed to anti-symmetrical bridge COC stretching within cellulose. The vibrations at 1003 and 986 cm−1 are attributed to C-O and ring stretching modes, and the 897 cm−1 vibration is assigned to the β-linkage of cellulose22. In addition, thermogravimetrical analysis was performed on GT microfibres. Figure 5b reports the thermogravimetric curves of GT cotton/modal back part, GT polyester front part, and of GT microfibres recovered on 60 µm pore size filter.

Figure 5 (a) FTIR spectrum of microfibres recovered from GT on the 60 μm pore size filter; (b) thermogravimetric curves of GT cotton/modal back part, polyester front part, microfibres recovered on 60 µm pore size filter; (c) SEM micrograph of GT microfibres representative of the fraction recovered on 60 µm pore size filter. Full size image

The GT front presents a single step thermal degradation, with a temperature of max weight loss (T max ) of 454 °C, due to the decomposition of the main chain of polyester23. The GT back part has also a single step degradation but shifted to lower temperatures with a T max at 377 °C, ascribable at cellulosic degradation of the blend cotton/modal. It is reported in literature that cotton and viscose have a similar thermal degradation behavior, with the degradation of viscose starting at lower temperatures compared to cotton24,25. However, in the analysed sample of GT back, no difference in the degradation of both materials was detected. Instead, the microfibres recovered on the 60 µm filter present a two-step thermal degradation: the first start at 200 °C and has a weight loss of around 81% (T max = 348 °C), the second starts at 406 °C and loose around 9% of weight (T max = 450 °C). Comparing such results with the thermal degradations of GT front and back parts, it appears clear that the first step could be attributed to the degradation of the cotton/modal microfibres, whereas the second step is ascribable to the degradation of polyester. Then, around the 80% of the amount of microfibres released from GT during washing are of cellulosic nature, released from the back part. Moreover, the presence of cellulosic microfibres on the 60 μm pore size filter was also evaluated by SEM analysis as represented in Fig. 5c.

Subsequent washing cycles

Due to their completely different behavior, BT and GT were selected to undergo up to 10 washing cycles. The morphological analysis of BT and GT surfaces before and after 10 washing cycles indicated that no visible damages of the fabrics occurred during washings. SEM micrographs of BT and GT clothes before and after the washings are reported in Fig. S3 in the SI. Figures 6 and 7 summarize the results of this investigation. After 4th–5th cycles, the total amount of microfibres released from BT reached a plateau, Fig. 6a, on the contrary, the release from GT showed a slightly decrease after 4th–5th cycles but no plateau was reached up to the 10th cycle, Fig. 6b. The same trend was observed in the microfibres recovered on 60 µm pore size filters, Fig. 7. The amount of microfibres released from BT recovered on 60 µm pore size filters, Fig. 7a, decreased until the 4th cycle, while that released form GT and recovered on 60 μm pore size filters slightly decreased until the 4th cycle and then presented an oscillating pattern, Fig. 7b.

Figure 6 Total amount of microfibres released during 10 washing cycles from: (a) BT and (b) GT. Full size image

Figure 7 Microfibres recovered on 400 μm mesh, 60 and 20 µm pore size filters (expressed in mg/kg), and on 5 µm pore size filters (expressed in mg/L), during 10 washing cycles from: (a) BT and (b) GT. Full size image

Also in this case, a thermogravimetric analysis was performed on microfibres recovered on 60 µm pore size filters after the 5th and 10th washing cycles and compared with the results obtained from the 1st washing cycle. The TGA curves reported in Fig. 8, showed that compared to the thermal degradation of the fibres from the 1st wash, both microfibres from the 5th and 10th washes have a single step degradation, with a close T max (353 °C for the 5th, 354 for 10th). This result seems to indicate that the microfibres released during the 5th and 10th washes were mainly released from the cotton/modal back part.