Size defines a microplastic. As mentioned earlier, a solid particle is considered a microplastic when its main component is a synthetic organic polymer, and its size, measured as its major dimension, falls between 1 µm and < 5 mm. However, size is not the only important information about microplastic particles. Along with size, it’s common to include descriptors that help understand the material’s origin. Industrial granules, packaging foams, or remnants of fishing nets correspond to very different sources of emission, and their identification is relevant for the purpose of combating this type of pollution.
Since the beginning of the research on microplastics, several classifications based on shape or typology have been proposed. Hidalgo-Ruz and coworkers, in a well-known review article, mention the following types of microplastics: fragments, pellets (granules), filaments, films, foams, granules, and Styrofoam (a commercial name for extruded polystyrene foam). In another common reference, Lusher and her team distinguish fragments, fibers, beads, foams, and pellets. Finally, the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection (GESAMP) in their 2019 Report No. 99, while acknowledging the difficulty of standardizing the morphological classification of microplastics, proposes five categories:
1. Fragments: Irregular particles originated from the fragmentation of larger ones.
2. Foams: Approximately spherical and easily deformable particles.
3. Films: Flat and flexible particles.
4. Lines (or filaments): Particles with substantially greater length than width (high aspect ratio).
5. Pellets: Granular particles that are hard and have a smooth surface.
The subjectivity of that classification is evident, and even though GESAMP suggests creating subdivisions, such as distinguishing lines or filaments (from fishing nets, for example) from textile fibers, the separation between categories is blurred. For instance, the length-to-width ratio that distinguishes a filament from a fragment is not specified. The flexibility required for a particle to be classified as foam rather than a fragment is also not mentioned.
The shape of particles can be quantitatively described using various descriptors. The clearest one is the minimum aspect ratio (the ratio of length to diameter) required for a fibre or filament. It’s common to use a minimum value of 3 for the length/diameter ratio of fibers, which is a value inherited from asbestos legislation: “The term respirable asbestos fibres denotes asbestos fibres with a diameter of less than three micrometres and with a length-to-diameter ratio greater than 3 to 1” (International Labour Organization, C162 – Asbestos Convention, 1986).
Particle science employs techniques to accurately quantify a particle’s shape. X-ray microtomography, for instance, enables the determination of the three orthogonal dimensions of a particle: L (major dimension), I (intermediate dimension or the greatest dimension perpendicular to L), and S (dimension perpendicular to both L and I). To ensure this attribution is unambiguous, some additional clarifications are necessary, such as requiring L, I, and S to be the dimensions of the smallest orthogonal parallelepiped that encloses the particle, as shown in the following figure.
The shape description based on L, I, and S can be made dimensionless by defining three parameters: roundness, S/L; planarity, (I-S)/L; and elongation, (L-I)/L. These three parameters sum up to one, and they can be visually represented on a ternary diagram. In this diagram, spherical shapes, which exhibit greater roundness, approach the upper vertex. Flat shapes, with higher planarity, move toward the lower-left vertex. Lastly, fibers, characterized by greater elongation, converge at the lower-right vertex as shown in the figure below. (Plastic number 5 is the one shown in the preceding figure.)
A classification like this is relevant because the environmental distribution of plastic particles depends on size (measured as the major dimension) and shape, and both factors determine their behaviour in a fluid medium, whether it’s water or air. Specifically, the shape of particles strongly influences their sedimentation rate. A fibre settles at a much slower rate than a rounded particle of equal mass and volume. This explains the high mobility of fibers and their prevalence compared to fragments as we move away from urban centres. The unique characteristics of synthetic fibers will be discussed later in this blog. For more information on the topics covered here, you can refer to this article, available in the author’s version.
Science, Plastic and Environment 