Microfiber shedding from nonwoven materials including wipes and meltblown nonwovens in air and water environments

Characteristics of nonwoven commercial products and meltblown nonwovens

Commercial nonwoven product compositions were estimated based on various analysis methods: TGA, DSC, and FT-IR. The details are included in the supplementary materials (Fig. S1-S11). The melting temperature and decomposition characteristics can be used to characterize the types of materials in the nonwoven. The melting temperature can be compared to known standards for common polymers used in nonwoven. The thermal decomposition measured by TGA can be used to evaluate materials that do not have a melting temperature, ex. cellulose (Fig. S1–S3). FT-IR spectra (Fig. S3) can be used as further confirmation of the composition. SEM and optical microscope were also used to characterize the morphology of the fibers (Fig. S4-S11). Specially, cellulosic fibers, including natural cellulosic fiber and regenerated cellulosic fiber, could be distinguished by their shape in the microscopic images: ribbon shape of the natural fibers (including wood pulp fibers and cotton fibers) and the round shape of the regenerated fibers. Table 2 shows the results of this analysis.

The majority of the nonwoven materials were composed of more than one type of fiber. Cellulosic fibers were found to be a common material used in the products. In all cases, cellulose is also combined with a synthetic polymer. This may be attributed to the ability of the synthetic material to provide wet strength to the wipes. Among synthetic fibers, polyester and polypropylene fibers were most commonly found. There are only two commercial products that were made of a single synthetic material: I1 and I2. Both products are industrial wipe products, which may be more focused on the nonwoven strength.

More than half of the products were determined to be made by carding and hydroentangling or thermal bonding. Since the products are wet wipes, the manufacturing process seemed to be focused on the water absorbency and softness. These properties are important product properties for wet wipes. The cellulosic fiber is hydrophilic with many hydroxyl groups in its structure.

Table 3 shows the characteristics of the meltblown series of samples, including fiber width, basis weight, fabric density, and thickness. The fiber widths were determined using the optical microscopic images (Fig. S7). Compared to the nonwoven commercial products, meltblown nonwovens were composed of finer synthetic fibers.

The physical properties of nonwoven commercial products and meltblown nonwovens were analyzed with the textile softness analyzer (TSA). The detailed results of TSA, including roughness, softness, and compliance of commercial products are illustrated in the supplementary information (Table S1-S3). The TSA was also used to characterize the softness and compliance of meltblown lab samples. The softness can correlate with the increased bulkiness of the sample among other factors. Figure 3 shows the TSA results for the lab-made meltblown nonwovens. The TS7 value measured by the TSA is inversely related to the softness and the TS750 value is related to the roughness. Lower TS7 and TS750 values indicate improved softness and smoother surface. The TS7 and TS750 values correspond to the sound created by the fan blades of the instrument brushing across the sample. Also, a compliance value was measured that represents the deformation of the material when a 600 mN load is applied. Higher values represent a more flexible material.

TSA results of meltblwon nonwovens depending on DCD. a TS7 (the noise peak representing the softness of specimens), b TS750 (the noise peak representing the roughness of specimens), c compliance, and d fabric density of meltblown nonwovens (all data represent the average values of the three repetitions; overall results are in the supplementary information)

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The physical properties of the meltblown nonwovens were more affected by the airflow rate compared to the DCD (Fig. 3). With the higher airflow rate, the meltblown nonwovens had softer and less rough surfaces with a higher compliance. This can be related to the fiber width changes caused by the airflow rate. It has been well known that the meltblown manufactured with higher airflow rate is composed of finer fibers, which is also confirmed in the present study (Fig. 4).

Fiber width versus airflow rate of meltblown nonwovens. The throughput was 15.5 kg/hr/m

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DCD also affected the physical properties of meltblown nonwovens, even though the effect of DCD was less pronounced than the effect of airflow rate. Also, when DCD was higher, the surface became softer and appeared to have less roughness. Also considering that the compliance values increased with the DCD, it seemed that meltblown nonwovens with higher DCD were more flexible materials. Even though the grammages of the nonwovens were the almost same, the thickness of the nonwovens increased with the increase of DCD (average 304 um (SD ± 47um) for DCD 150 mm; 350 um for DCD 200 mm; 377 um (SD ± 45um) for DCD 250 mm (SD ± 94um)), and in turn, the density of the web decreases with the increase of DCD (Fig. 3d).

Waterborne microfiber generation

The microfiber shed in a water environment from selected products was tested as a function of agitation time (Fig. 5) in the Launder Ometer. Samples P3, P6, and P7, which showed the lowest, moderate, and highest microfiber generation at 16 min, were tested versus time. The P6 sample, which showed the lowest microfiber generation, did not show a significant difference in microfiber generation as a function of time. However, samples P3 and P7, which generated more microfibers at the 16 min condition, showed a different behavior with time. The particle generation increased sharply at early times (before 5 min), and then plateaued. The results indicate that for some products microfibers may be generated with a relatively small amount of mechanical action which may be found in wastewater treatment plants during pumping as an example.

Waterborne microfiber generation of commercial nonwoven products depending on the duration of mechanical action. (■: P6, ●: P3, and ▲:P7)

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Table 4 shows the overall result of microfiber generation for commercial nonwoven products. Compared to the textile fabrics investigated under the same experimental conditions (Zambrano et al. 2019), more microfibers in general were generated from commercial nonwoven fabrics. According to Zambrano et al. (2019), generally less than 3 mg of microfibers are generated per gram of fabric at room temperature with or without detergent present. However, in the present study, most of the commercial nonwovens generated over 10 mg of microfibers per gram of wipes. The structural difference between the textile fabrics and the nonwoven fabrics are a probable cause of this difference. The textile fabrics in the previous study were made of yarns and they were interlocked by knitting. However, commercial nonwovens in the present study were bonded by different technologies, such as hydroentangling, thermal bonding, and needle-punching, and there might be more free fibers exposed to the surface without strong attachment to the material.

The microfiber generation result was further compared to their raw materials and web structures (Fig. 6). When examining the microfiber generation by fiber type (i.e., regenerated/natural cellulosic fibers, synthetic, or both), the majority of the samples were made of more than two kinds of fibers. The nonwoven products only with synthetic fibers or regenerated cellulosic fibers generated the lowest amount of microfibers; however, the nonwoven containing natural fibers generated relatively greater amounts of microfiber (Fig. 6a). This result corresponds to the previous research that the textile fabrics made of synthetic fibers generated fewer microfibers than those with natural cotton fibers (Zambrano et al. 2019). It may be said that synthetic fibers or regenerated cellulosic fibers are more uniform compared to natural fibers, so they have less chance to be broken than natural fibers.

Six out of the 15 commercial products were formed by carding technology, and even in the carding group, the amount of microfiber generation was very different between samples (Fig. 6b). Carding is one of the nonwoven web formation processes, and in the carding process, the bales of staple fibers were separated into fiber clumps, and the fibers were combed, pulled, teased apart, and rearranged to form a nonwoven web between teeth of the rotating cylinders. (Wilson 2010). Since various kinds of fibers can be carded, the properties of carded nonwovens can be very different depending on raw materials and bonding technology. Commercial nonwovens formed by wetlaid, SPS, and CoForm showed a relatively higher amount of microfiber generation, and this was related to the raw materials of the commercial nonwovens; those three web forming technologies are usually used with natural cellulosic fibers, such as wood pulp fibers. In terms of bonding applied to the commercial nonwovens, thermal bonded nonwovens generated the most amount of microfibers (Fig. 6c). In case of thermal bonded nonwovens in this research, most samples were bonded by point, stick, or grind bonding, and no area bonding was observed. Thus, even after the bonding, there should be non-bonded areas locally, and from those non-bounded areas, microfibers are expected to be shed. Also, hydroentangling seemed to make the nonwovens resistant to microfiber generation; hydroentangled nonwovens generated the least amount of microfibers.

In the present study, 102 × 102 mm2 (4 × 4 in2) sizes of wet wipes were used to investigate their microfiber shedding. When we calculated the number of microfibers based on the surface area, it was shown that 60 to 4,000 microfibers were generated from 1 cm2 of nonwoven wet wipes. In the previous study (Lee et al. 2021), the amount of microfiber generated from wet wipes in the water conditions was about 8 microfibers per cm2 of wet wipes, which was much lower numbers compared to the present result. This difference is attributed to the mechanical actions applied in the present study. While 25 metal balls were shaken with the nonwoven materials in the water for 16 min, the wet wipes in the previous research were immersed for one hour. Since in the real environment, it is a rare situation that wet wipes were immersed in the water without any shear/impact from water or other materials. Therefore, this result suggests that in the real world, the risk of microfiber generation from the nonwoven materials, such as wet wipes, was much higher than previously estimated.

As shown in the cellulosic fiber group and hydroentangling group, the materials investigated in the present experiment were randomly collected commercial products and the factors or characteristics of the materials were not well balanced across the spectrum of products. From the controlled manufactured nonwoven fabrics, it was also expected to better explore other characteristics of nonwoven structures, including basis weight, density, or thickness, affecting micro-particle generation. Also, most of the nonwovens in the commercial products were formed by carding, so the result could only represent the carded nonwovens, which is only a part of nonwoven variations. Furthermore, recent interest has been focused on the microfiber generation of the facial mask with the occurrence of pandemic (COVID-19), and its material which is generally made of meltblown nonwovens. However, the microfiber generation of meltblown nonwovens has not been fully investigated. Therefore, further study was done to investigate the effects of different factors on micro-particle generation with known well-controlled meltblown nonwoven manufacturing process variables.

The meltblown nonwovens generated a microfiber weight from 0 to 2 mg/g in the Launder Ometer experiments and a number of microfibers from 170 to 8750 count/g (Table 5).

In melt spinning, it is known that longer die-to-collector distance (DCD) creates a bulkier sheet (Zhang et al. 2018). The airflow rate and polymer throughput are also closely related to the fiber morphology. Thus, the relationships between fiber and web properties and microfiber generation were investigated with the pilot plant meltblown materials relative to these known processing variables.

Figure 7a shows that the number of microfibers generally increases as air flow rate increases. If the airflow rate increases, the fiber diameter decreases because the polymer is pushed by the air as shown in Fig. 4 (statistically significant, p < 0.005). However, the number of microfibers and the fiber width did not show statistically significant relationships. In Fig. 3, the increased air flow rate increased the softness, decreased the roughness, and increased the compliance of the meltblown nonwovens. Therefore, it may be said that the meltblown nonwovens had thinner fibers from the high airflow rate, which made the meltblown nonwoven vulnerable to microfiber generation; however, the microfiber generation of meltblown nonwovens cannot be explained only with the single factor such as fiber width, and other structural properties also combine to effect the microfiber generation.

Although nonwovens are generally not intended for home laundering (washing and drying), oftentimes these materials are inadvertently home laundered. It is of interest to estimate the microfibers shed during home laundering. Previous research in this group has shown in comparisons of the Launder Ometer and actual home wash laundering that the microfibers shed in home washing for a variety of fabrics is about 1/40th that of the Launder Ometer results (Zambrano et al. 2019). In general, the microfibers shed herein for nonwovens in a water environment using the Launder Ometer equipment resulted in a range of 0–1,150,000 number of microfibers or 1–65 mg of microfibers generated per gram material. It is thus estimated that 0.025–6.6 mg of microfibers would be shed during normal home wash laundering of the nonwovens based on the previous research results (Zambrano et al. 2019).

Airborne microfiber generation

It is known that for fabrics that microfibers are shed during normal use in air (Pauly et al. 1998; Dris et al. 2017), and thus it was deemed useful to evaluate nonwovens microfiber generation in air (dusting). Samples of nonwovens were shaken back and forth within a TDA and the generated fibers were collected and analyzed. Unlike waterborne microfibers, the weight of airborne microfiber generated could not be measured, the results were so low that weight measurements with our filtration method were unreliable. Thus, the number of microfibers was measured by FQA (Table 6). Similar to waterborne microfibers, airborne microfiber generated less from the nonwoven products made of synthetic fibers and/or regenerated cellulosic fibers (Fig. 8a); however, the tendency was less distinct compared to the result of waterborne microfiber generation. The airborne microfiber showed a similar amount between different groups of web formation; however, the bonding technologies have a more significant influence on airborne microfiber generation (Fig. 8b and c). The nonwoven products only bonded by a single technology, especially nonwoven products only bonded by thermal, generally generated higher amounts of microfibers than those bonded with double-bonded technology (Hydroentangling and thermal bonding, others (hydroentangling or needle-punched with chemical binders)).

When the number of microfibers was calculated based on the surface area, 0–35 microfibers were generated from 1 cm2 of commercial nonwovens (wet wipes). In the previous research (Lee et al. 2021), about 1 microfiber per square centimeter of wet wipes was generated by rubbing with gloved hands. This suggests that shaking behavior as well as rubbing during the use of wipes can cause a significant amount of microfibers, and the amount of microfiber generated can be very different depending on the characteristics of nonwoven webs.

The same experiment was done with the meltblown nonwovens (Table 7); however, the average number of microfibers was much lower (525 microfibers per gram material) compared to the commercial nonwoven products (1160 microfibers per gram material). Also, half of the meltblown nonwovens showed statistically zero microfibers per gram material (lower or the same microfiber values relative to the control condition in which the TDA was run without a sample). Therefore, it may be said that the meltblown nonwovens generate a significantly lower amount of microfibers in the air environment than the commercial products in general.

Microfibers from commercial nonwovens in the water condition were much higher than microfibers from the textile fabric cotton and polyester materials, whereas the meltblown nonwovens generated a similar amount of microfibers as the textile fabrics (Table 8). Commercial nonwovens containing natural cellulosic materials generated significant amounts of microfibers due to relatively irregular fiber morphologies and the weak properties of cellulosic fibers, especially in the water environment. However, considering the previous result that showed the rapid aquatic biodegradation of microfibers from the cotton and rayon textile materials (Zambrano et al. 2019, 2020, 2021b), it is expected that the environmental effects of the cellulosic components of the commercial nonwovens are not significant even though the number shed is higher than synthetics that persist and do not biodegrade in the environment. The tissue paper completely disintegrated in the water, releasing all of the fibers into the water, as expected.