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Fillers are relatively cheap, solid substances that are added to plastic formulations in high percentages to adjust volume, weight, and mechanical performance (Zweifel et al. 2001). Inert fillers often do not compromise the functional properties of the plastic; they are cheaper than resin and can significantly reduce the cost of the formulation. Fillers can also serve as reinforcing agents that improve the mechanical performance and durability of the plastic. They are used as powders, fibers, or nanotubes.

Filler consumption globally in 1999 was 66% calcium carbonate, 6% talc, 6% clays, 3% wollastonite (CaSiO₃), and the remaining 19% included silica, glass, asbestos, alumina, rutile, CB, and carbon nanotubes (CNTs; Civancik‐Uslu et al. 2018; Zweifel et al. 2001). Reinforcements are generally strong fibers including glass, carbon, or aramide fibers (Alam et al. 2019; Hansen et al. 2013). Fillers and reinforcements are used virtually in all polymers, but the largest fraction (i.e. over 90%) is used primarily in rubbers, PVC, and polyolefins. The improved mechanical properties in a filled plastic are derived from the interface layer between the polymer and the filler. The stronger the interfacial interactions, the better the mechanical performance of the composite (i.e. polymer/filler mix). The efficacy of the filler is also dependent on adequate dispersion in the polymer matrix. Good dispersion is achieved through extrusion, dispersing agents, or by surface treatment of the filler to improve compatibility with the polymer matrix.

Fillers can be microscopic (1 μm) or macroscopic (<100 μm) in size and have a very low propensity to leach out of the plastic. However, degradation and wear of a filled plastic can release fibers into surrounding environments either during use or after disposal (Froggett et al. 2014). In addition, the use of nano‐particulate fillers, such as CNTs or silica nanoparticles (silica‐NPs), has gained popularity over the past 20 years due to the superior material properties of nanocomposites when compared to conventional composites. Currently, commercial availability is low as these materials are relatively new. Nonetheless, production volumes are increasing with market sizes projected to grow significantly (Hendren et al. 2011).