The concept of microplastics is reasonably well established. As mentioned in other posts, microplastics are plastic particles whose major dimension falls within the range of 1 to less than 5000 microns (5 mm). However, it is important to provide some clarifications regarding what constitutes a plastic for regulatory and environmental purposes, as the term ‘plastic’ corresponds more to common language than to scientific terminology. As indicated later, a plastic is a subclass of polymeric materials. A polymer (Article 3.5 of the REACH Regulation and ECHA Guidance for Monomers and Polymers) is a substance composed of at least 50% by weight of molecules formed by the covalent bonding of (at least three) constituent units called monomers, that may also include a variety of simple molecules acting as plasticizers, stabilizers, or other functions.
Polymers can be thermoplastics, thermosets, and elastomers. Thermoplastics (including all synthetic fibers) can be melted and indefinitely moulded without suffering substantial degradation- Thermosetting polymers have their molecules linked in a three-dimensional network and cannot be melted. Elastomers, like thermosets, have their molecules crosslinked, as in vulcanized rubber, but instead of being rigid, they exhibit elastic behaviour without undergoing permanent deformations. Typical examples of thermoplastics are polyolefins (polyethylene or polypropylene), polystyrene, or PVC, while epoxy resins and polyurethanes are thermosets. Some old classifications of polymers only include thermoplastics and thermosets, excluding elastomers, but the development of thermoplastic elastomers, especially since the 1970s, has blurred the boundaries between these materials and suggests their unified treatment.
Plastic materials, regardless of its shape and in the context of environmental pollution and related fields, must be water insoluble. (This excludes some polymers like povidone or polyethylene glycol.) This limitation can be found in the restriction proposal developed in 2020 by the European Chemicals Agency (ECHA) concerning intentionally added microplastics in products placed on the community market. The report was jointly prepared by the Committee for Risk Assessment (RAC) and the Committee for Socio-economic Analysis (SEAC) at the request of the European Commission in order to adopt the measures required by the REACH Regulation (Annex XV). In addition to clarifications about particle dimensions, the report indicates that the regulation excludes natural polymers that have not been chemically modified, biodegradable polymers (according to specified criteria in Table 22 of the background document), and those with solubility > 2 g/L. Natural polymers are excluded from regulation as defined in the REACH Regulation, Article 3.40, provided they have not undergone a modification of their original structure (though certain chemical or physical treatments are allowed, for instance, to remove unwanted substances).
There is a variety of polymeric materials that, despite originating from natural polymers, cannot be considered as such because they have undergone chemical modifications. The most well-known example of this family of semi-synthetic polymers is regenerated cellulose. This material is obtained from cellulose fibres (such as cotton), which are treated with carbon disulphide in a basic medium to form cellulose xanthate. Cellulose xanthate is then mechanically processed, and cellulose subsequently reconstituted in an acidic medium (while there are other methods to regenerate cellulose, the one mentioned is the most commonly used in commercial applications). Rayon and cellophane belong to this group of materials. Rayon, referred to as viscose in Europe, is the most common semi-synthetic polymer. Its silky shine made it very popular as a silk substitute since the late 19th century. (Nylon is another classic example of synthetic silk substitute, although nowadays, polyester is used for that purpose.) Once cellulose is regenerated, the resulting material is difficult to distinguish from cotton or other cellulosic fibres using spectroscopic techniques. Vulcanized natural rubber is another example of a semi-synthetic polymer, although it has long been replaced by fully synthetic elastomers. Another notable group consists of cellulose nitrates, which include celluloid and gun cotton.
The typology of polymeric materials thus encompasses three types of substances: (1) natural polymers, (2) artificial materials based on natural polymers that have undergone chemical transformations or industrial processing, and (3) purely synthetic polymers. In the context of environmental sciences, the term ‘plastic’ exclusively refers to synthetic polymers, which are those in which the constituent monomers have been obtained through chemical synthesis (from materials of any origin, fossil, or non-fossil).
The previous classification includes a dual typology of artificial materials: semi-synthetic polymers (such as rayon) and industrially processed (but not chemically modified or treated) natural polymers. This is because natural polymers are not marketed in their raw state as obtained from nature. Instead they reach our hands after undergoing various industrial processes that generally involve the incorporation of a wide range of chemical additives. Without straying from the textile industry, the list of chemical substances involved in fabric manufacturing processes is quite extensive. It includes bleaching agents, softeners, lubricants, antistatics, emulsifiers, coagulants, defoamers, dyes, and biocides, among others. Many of them are entirely synthetic compounds that become incorporated to different degrees into the fabric, either incidentally or intentionally. Some dyes are toxic, such as those based on quaternary ammonium compounds which, being cationic, can interact with the negatively charged cell membranes. Azo dyes, common in many fabrics and generally considered non-toxic, are also hazardous as some of their metabolization and degradation products do exhibit toxicity to various organisms.
In essence, both plastics themselves and semi-synthetic polymers, along with industrially processed natural materials (thereby transformed into artificial materials), should be regarded as anthropogenic pollutants, either due to their inherent properties or to the substances accompanying them as additives or remnants of the reagents used in their production. Their effect to the natural environment is still poorly understood, but it is certain that they reveal an impact of human activity on the natural environment.
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