DOWNCYCLING

A common problem associated with mechanical recycling is the degradation and mixing of polymers leading to loss of the characteristics that made the initial pre-recycled polymer desirable. As the plastic qualities are degraded through the recycling process, some may not be able to be returned as input to new plastics, and are used to create less valuable, limited application, plastic products.

Some examples include:

· park benches

· plastic lumber poles for gardens

· drainage pipes

· carpets

· railroad ties

· truck bed liners

· plastic roads

Most of these products have lost so many structural characteristics that they cannot be subject to recycling again after they have reached this point. It is not clear if chemical recycling can be applied to these downcycled products in an effort to recover monomers or polymers.

Concerns have been raised over recent projects attempting to integrate plastic waste into road surfaces as a means of downcycling. The roads are expected to last 4-6 years (Hahladakis et al., 2018) before deterioration, and it is not clear how the material from maintenance or demolition will be managed. Very few studies have attempted to examine the environmental and human health implications of blending plastic waste into bitumen or paving using only melted plastics.

Some of the concerns include:

· Perpetuating a linear extraction-production-disposal cycle for plastics that ‘hides’ the waste.

· Risk to road workers from inhaling toxic emissions from the melted plastic during construction, including chlorinated and brominated dioxins (from PVC and brominated flame retarded plastic respectively).

· Risk to road workers from inhaling other gases from melted plastics such as PP, PS, or PE during construction, including especially carbon monoxide, acrolein, formic acid, acetone, formaldehyde, acetaldehyde, toluene, and ethylbenzene, as well as from the vast array of additives incorporated into the plastics such as phthalates.

· The release of chemical additives and microplastics into the environment from a combination of weathering and abrasion from vehicle tires and the subsequent impact on the environment.

When downcycling plastic waste via shredding and extrusion into lowervalue products and uses, there is also a significant risk to workers engaged in the manufacturing process, where heating of the waste releases volatile compounds or particulates (and this may also apply to plastic road construction workers). The processes of shredding and heating can endanger workers from exposure to emissions such as dioxins, phthalates, and VOCs, but also to particulate-adsorbed monomers with toxic characteristics, such as Bisphenol A (BPA), styrene, and vinyl chloride monomers.