**2. What is plastic?**

Plastics are defined as synthetic organic polymers typically made from petrochemicals.

Specifically, synthetic polymer molecules consist of many monomers which react in different ways. Many simple hydrocarbons, such as ethylene and propylene, can be transformed into polymers by adding monomers to the growing chain. The combination of these monomers creates various kinds of polymers. Other substances can be added to polymers to give the final product some desired characteristics [1].

Sometimes, the term plastic is also used to indicate blends with different synthetic polymers or other low molecular weight compounds such as additives, UV or thermal stabilizers, flame retardants, dyes, antioxidants, plasticizers, etc. [2].

Because plastics are considered chemically, physically, biologically stable and resistant materials, once in the environment, they can undergo degradation upon exposure to different factors, such as sunlight, water, and wind, and break down into tiny plastic particles known as microplastics. After fragmentation, they can be transported by wind and water due to their lightweight [3].

Thus, once in the environment, microplastics accumulate and persist. Consequently, they are ubiquitous in terrestrial, fresh water and marine environments [4]. The source of microplastics includes wastewater treatment plants, landfills, automotive tires, pre-production plastic pellets, synthetic clothing, road signs, and paint [5], (**Figure 1**).

Among all sectors, the textile one is considered one of the major sources of microplastic pollution.

Textile processes are responsible for 20% of global water pollution and the washing of synthetic garments contributes to about 35% of the global release of primary microplastics. These materials are not retained during the filtration systems of wastewater treatment plants (WWTPs) and, therefore, enter the marine ecosystem directly [6].

These microplastics occur in different forms (e.g., cylindrical, spherical, etc.) and partly escape the filtration systems of WWTPs, reaching seas and oceans directly. In this respect, the identification and classification of fiber fragments are necessary to spot any weak points in the textile production process and in the life cycle of synthetic garments. The release of microplastics can occur during the different processes and use phases, including spinning, weaving, finishing (gauzing, finishing, dyeing), packaging, wear, washing, drying, and finally, at the end of life, landfill disposal [7].

**Figure 1.** *Example of microplastic sources in water system (https://unsplash.com).*

#### **Figure 2.**

*Plastics spread in the environment (https://unsplash.com).*

Hartline et al. have estimated that a WWTP plant with a 94% of removal percentage and considering 0.35 m3 of wastewater per person a day would produce for 100,000 people about 1.02 kg of microfilaments (**Figure 2**) [8].

The term microplastics was first coined by R. Thomson in 2004 by observing micrometer-sized plastic fragments in marine sediments and then refined by Arthur et al. by setting size limits above 5 mm [9, 10]. Later in 2011, Cole et al. divided microplastics into two categories: primary ones produced at a micro size and secondary ones that only reach that size through fragmentation and degradation due to environmental biodegradation effects. In 2016, nano-sized particles were also included in the definition of microplastics GESAMP [11].

Although the definition of microplastics is still being debated, the current one follows the European Chemical Agency (ECHA), "a material composed of solid polymeric-containing particles to which additives or other substances may be added. The family of microplastics includes synthetic-based particles, such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyamides (PA), polyethylene terephthalate (PET), polyvinyl chloride (PVC), polyacrylonitrile (PAN), polymethylacrylate (PMA), elastomers and silicone rubber with particles ranging from 1 nm to 5 mm and

#### **Figure 3.**

*Optical images obtained with an optical microscope coupled to MicroFTIR of a) polypropylene microparticle and b) polyester microfilaments.*

fiber lengths ranging from 3 nm to 15 mm and a length-to-diameter ratio of > 3" [12]. The difference between particles and filaments is reported in **Figure 3**.

### **2.1 Textile fibers**

Fibers are a class of materials consisting of a fibrous structure whose length is thousands of times higher than its diameter. Fibers are the units from which all textile materials are made. They are incredibly important to textile production as they have properties that allow them to be spun into yarn or directly made into fabric. This means they must be strong enough to hold their shape, flexible enough to be shaped into a fabric or yarn, elastic enough to stretch, and durable enough to last. Textile fibers also have to be a minimum of 5 millimeters in length as shorter ones cannot be spun together. Textile fibers are generally classified as natural or man-made. An outline is reported in **Figure 4**.

Natural fibers are further subdivided into animal (e.g., wool, mohair, cashmere, angora, silk), vegetable (e.g., cotton, flax, kapok, jute, hemp), or mineral (e.g., asbestos), as shown in **Figure 4**. Animal fibers are typically obtained from the coats or fleeces of animals, or in the case of silk, the raw material is the extruded filaments of the silkworm cocoon [13]. Vegetable fibers grow as seeds, leaves and bast fibers, whereas mineral ones are mainly asbestos fibers. In **Figure 5** an optical picture of animal (a) and vegetable (b) fibers is reported.

Wool fibers have the form of elliptical cylinders. The range diameter of around 20 μm is typical of merino wool, the most commonly used for clothes. It shows a scale

**Figure 4.** *Classification of textile fibers.*

**Figure 5.**

*Optical microscopy image of merino loose wool (a) and cotton fibers (b) obtained in transmission mode. (500 X).*

#### **Figure 6.**

*Optical microscopy image of PA 6 (a) and PA 6.6 (b) fibers obtained in transmission mode (500 X).*

structure with an irregular profile and a stopped one due to overlapped cuticle cells, like a tiled roof. Cotton fibers show a flat band structure with corkscrew-like twisting. The convolution frequently varies between 3.9 and 6.5 per mm and the number of reversals per mm ranges between 1.0 and 1.7. The longest cotton fiber is 2.8 cm and can be found in Scottish thread. The section of the fibers shows variable shapes such as elliptical, oval, and kidney with a well-defined central lumen parallel to the outer wall [14].

Man-made fibers are any fiber that is derived from an artificial process. The fibers made from chemical synthesis are called synthetic fibers, e.g., Polyamide 6 (PA 6), Polyamide 6.6 (PA 6.6), polyester (PET), polyacrylic (PAN), cholorofibers (PVC), and aramids (kevlar), while the fibers generated from natural polymer sources are called regenerated fibers or natural polymer fibers e.g.: Viscose, Rayon, cellulose triacetate, etc. [13]. An optical micrograph of synthetic fibers is reported in **Figure 6**.

The fibers are uniaxially oriented during the melt, dry, or wet spinning process, which gives the fibers high tenacity and strength. Typically, they appear as smooth filaments, cylindrical or slightly elliptical.

**Figure 6** and b shows the morphology of two nylon fibers (PA6 versus PA 6.6). [15]. They are generally semicrystalline polymers extruded and drawn in various cross-sectional shapes, which can be circular, kidney-shaped or three-lobed with smooth edges. In **Figure 6b**, the fiber shows the presence of fillers.

#### **2.2 Why do fibers from clothes pollute?**

Man-made fibers have tripled their market share from 23% in 1965 to nearly 72%. In addition, synthetic fibers have continued to grow to 75%, while cellulosic fibers, for example, have remained constant at about 6.4% [15]. Compared to natural ones, synthetic fibers do not depend on animal breeding or cultivation and are not affected by environmental factors such as seasonality and climate change.

Polyester is considered the best fiber in terms of production cost, raw material quality, and ability to improve performance and properties. Polyester fibers have reached 85% of the market share of the synthetic sector [16].

Moreover, in recent years, synthetic fibers have become the main protagonists of fast fashion (a clothing industry that produces low-quality and low-priced clothing and constantly launches new collections in a short time), generating large amounts of waste from unsold, unwanted and/or landfilled goods.

Furthermore, synthetic textiles are estimated to be responsible for a global discharge of between 0.2 and 0.5 million tons of microplastics into the oceans yearly [17]. Synthetic fragments can enter the aquatic environment during use, machine washing and drying of garments, or through leaching of waste material (pre-consumer, post-consumer) that accumulates in landfills.

**Figure 7.** *Source of microplastic fibers release during textile life-cycle.*

According to [18] approximately 35% of microplastics released into oceans globally originate from washing synthetic textiles, as shown by their incidence in freshwater and saline environments, near urban centers, in sewage sludge and its by-products, in wastewater treatment plant effluents, in sediments and in some biota such as invertebrates, birds, and fish.

Although wastewater from washing machines is considered the main transport route for synthetic microfilaments, air can be a possible way, too. The fibrous fragments are comparable to dispersed solid particles suspended in the air. Several researchers have pointed out that textile products, especially during manufacture, packaging, drying and use, can release microplastics. Furthermore, synthetic textiles used for upholstered furniture can release fibrous microplastics through friction and abrasion. Many works have shown that the amounts released are comparable to those produced during a washing machine cycle [19–21], as shown in **Figure 7**.

In recent years, concerns have grown about the environmental and health impacts associated with microplastic pollution. Textiles made of fibers of natural origin shed micro fragments, too. All fibers undergo a biodegradation process in water. However, natural fibers (e.g., cotton) are completely degraded in the aqueous matrix, whereas in the case of synthetic fibers, there is no complete degradation but fragmentation

#### **Figure 8.**

*Example of fiber material released from: a) the synthetic fabric during a 40-minute washing cycle (Wash & Wear) in laundry machines; b) tumble dryers (60-minute drying time).*

into smaller filaments or particles that can reach nanometric dimensions. Another finding from the experimental data is that PET fibers are the most commonly found in the environment, followed by PAN, PP, and PA fibers (**Figure 8**) [22, 23].

#### **2.3 Environmental impact**

In recent years, a growing concern about microplastic environmental pollution and health impacts has emerged. Several studies have shown a certain degree of chronic exposure to microplastic pollution is an integral part of contemporary life [24]. Due to their shape, microplastic can be ingested by all living organisms, from plankton to humans. Furthermore, another source of concern is the potentially toxic chemicals that they can contain, such as additives, monomers, catalysts and other by-products. Once microplastics have been released into the environment, due to their fragmentation, degradation and chemical contents, they can reach the biota and consequently enter the food chain. In addition, microplastics have characteristics such as size, shape, polymer composition and even color that can potentially be more important than their concentration in the environment to induce adverse effects, making it more challenging to identify their impact on organisms. In addition, fibrous microplastic fragments in terms of size (length and diameter) geometry, physical properties and surface characteristics may be responsible for the levels of biological interfaces with tissues and cause pathology. Small microplastics can easily penetrate cells and organs and carry a considerable content of harmful substances due to the high surface area unit they possess [24, 25].

#### **2.4 Microplastic textiles: related problems**

Nowadays, estimating and measuring the quantities of microplastics, particularly those with fiber shapes, is challenging. Estimating the number of released microplastics is highly uncertain because of the lack of standardized sampling and measurement methods. Furthermore, the obtained data are not fully shared by the scientific community and are not validated with inter-laboratory tests. At present, the experimental and the analytical protocols under study are mainly focused on the determination of microplastic with particle shapes, leaving out fiber-shaped ones.

Indeed, microplastic textile standard methods are rarely used in the study cases. Existing methods for preparing MFs (microfilaments) are focused on cutting or cryogenically grinding synthetic filaments, resulting in a wide distribution of fiber lengths [26]. Some scientists have prepared nylon, polyethylene terephthalate (PET) and polypropylene (PP) microplastic fibers with pre-determined lengths (40, 70 or 100 μm) using a cryotome protocol. They proved that this method effectively produces tens of thousands of MFs suitable for testing [27].

Despite these promising results, the proposed analytical techniques have several drawbacks since they are limited in counting and separation.

Thus, a novel approach to counting and identifying fibrous microplastics is becoming fundamental. For this reason, a standardized analytical method and its subsequent validation must be obtained.

A possible solution to this lack could be the use of appropriate standard microfilaments. The more specific issues are microplastic cut-off size, sample type, sampling procedure, laboratory sample processing, identification techniques and reporting units. Therefore, a new routine for qualitative and quantitative microplastic analysis with fiber shape could be established to have a standardized analytical method to compare different lab results.
