infection | CONTROL

An important aspect for any plastic material is how much it will be affected by temperature, since this will dictate whether the particular plastic item will sustain exposure to a washer disinfector cycle or a steam steriliser cycle without melting. Many plastic materials used in dental practice are made from low-fusing thermoplastic materials where the polymer is formed into its final shape using low temperature heat. As a result of this, when exposed to hot water (65°C) or to a steam steriliser cycle (134°C), the material will revert from its shape into an unformed mass of polymer, making it impossible to reuse. This design feature is used deliberately to prevent single use plastic items being reprocessed.

Common plastic materials that are used in dentistry include polypropylene (in surgical masks and respirators), polyethylene and polyethylene terephthalate (PET) in packaging and in containers of various types and polyvinylchloride (PVC) in rigid plastics. The plastics used in packaging and in PPE have the shortest working life, because of single use. As a particular example, during the COVID-19 pandemic, it was estimated that hundreds of millions of facemasks were used, of which around 80% ended up in landfill, with the remainder most likely to have been incinerated as medical waste.

Understanding bioplastics

Abioplastic is typically made from a renewable resource (such as plants), by extracting natural polymers (starch, proteins, natural rubber, etc), or by using monomers from plants as building blocks to create polymers. Depending upon their type, bioplastics can contribute to circular economies by using renewable plant-based non-fossil resources and then having improved end-of-life outcomes through reuse or biodegradation. In each case, an assessment of the entire life-cycle of the bioplastic using the LCA approach is necessary.

The challenges of introducing more bioplastics into dentistry include their greater cost, lower efficiency for manufacturing than fossil-based plastics, limited recycling opportunities and a lack of composting facilities. There are environmental impacts associated with agricultural production. An inherent challenge with bioplastics is competition with food production when considering agricultural land use.

Bioplastics are attracting considerable interest because they have a lower carbon footprint than fossil-based plastics. Some of them are compatible with existing recycling streams, while others may undergo biodegradation if the environmental characteristics for degradation are suitable (particularly the presence of certain microorganisms and the appropriate temperature).

Some waste plastics can be mechanically recycled using heat and force, to create plastics of new shapes and sizes. This is often referred to as downcycling, since mechanical recycling often results in reductions in tensile strength or other physical characteristics. While mechanical recycling produces limited options compared to chemical recycling, it is far less complex and expensive to undertake.

Chemical recycling involves breaking plastic polymers into their component

Table 1. A “ready reckoner” for understanding different types of plastics and bio-plastics

Group 1. Non-biodegradable durable plastics that are chemically polymerised, durable and largely resistant to hydrolysis

Fossil-based: polystyrene (PS), polyethylene terephthalate (PET), polyvinylchloride (PVC), polyethylene (PE), polypropylene (PP)

Biologically based: Bio-PE, Bio-PP, Bio-PET, Bio-polycarbonate, bio-polyurethane, polyethylene furanoate (PEF)

Group 2. Biodegradable plastics that are susceptible to hydrolysis

Fossil-based: polybutylene adipate-co-terephthalate (PBAT), polyvinyl alcohol (PVA), Polybutylene succinate (PBS)

Biologically based: cellulose, polylactic acid (PLA), bio-PBS, polyhyroxyalkanoates (PHA)

Understanding microplastics

Most commonly, microplastic particles are created by the degradation of plastics, from exposure to ultraviolet light or from abrasion by contact with other items. The typical upper size limit for microplastic particles is around 1 mm. As these particles become smaller, their surface area increases and hence also does their ability to bind and carry various contaminants.

At the global scale, a growing concern is the generation of microplastics from plastic waste which has ended up in the ocean because of poor waste management practices. Microplastics in the ocean may reduce the ability of phytoplankton to capture carbon dioxide and sediment this into deep ocean soil.

Understanding plastic recycling

For effective recycling of plastics, proper waste stream segregation is essential, to make sure that the recycled plastic is free of contamination.

monomers so that those can be used to rebuild new polymers. It is not used widely and currently accounts for less than 1% of all plastic recycling. The process uses solvents and catalysts that present their own environmental impacts. For plastics such as polyethylene and polypropylene, pyrolysis can be achieved by exposing these to high temperatures (200 to 800°C) in the absence of oxygen. This recreates hydrocarbon oil or gas, that can then be used for constructing new polymers.

Which polymers are biodegradable?

As shown in Table 1, a range of biodegradable plastics exist. The key types of interest for dentistry are summarised as follows:

• Polylactic acid (PLA) is made through the condensation of lactic acid which has been generated from the fermentation of sugar. Because of its brittle nature, it is typically blended with other biological polymers to create useful materials. As one example, single use cutlery has been developed using PLA.