Recycling is one answer to the growth in plastic waste but treating “wash water” used to clean recycled plastic before re-use can produce safer recycled plastics, according to a new study.
The U.S. Environmental Protection Agency (EPA) has set a goal to achieve a plastic recycling rate of up to 50% by 2030 which would involve an 8- to 10-fold increase in recycling efforts to meet the challenge.
Plastic packaging waste streams generally contain different polymers and impurities including paper, organic residues, adhesives, metals, glass and inks. The contaminants and the additives present in these polymers include plasticizers, antioxidants, colorants, light and heat stabilizers, lubricants, antistatic agents, slip compounds, heavy metals and other chemicals.
A new study by Iowa State University researchers examines various techniques for washing post-consumer plastics before they are re-used and identifies potential chemicals of concern. The research was supported by IAFNS’ Food Packaging Safety & Sustainability Committee.
The study focuses on contaminants in post-consumer polypropylene wash water with the aim of making recycling processes more effective, sustainable and safer by lowering potential exposure to harmful chemicals. Polypropylene is a versatile plastic that is used for a wide range of applications including food packaging and consumer products.
Mechanical recycling is one of the most widely used processes for plastic waste, enabling reuse of raw materials. But this involves sorting and washing to reduce the presence of impurities. Cold water, sometimes in combination with caustic or surfactant chemicals, is used for washing off debris and residues from raw plastics slated for recycling. Hot water can be applied to remove more stubborn contaminants that stick to plastic waste.
However, during the process, wash water can become contaminated with various substances including oils, greases, suspended solids and organic pollutants. These can be potentially detrimental to human health and the environment if not treated and addressed. Effective wash water treatment can produce safer recycled plastics.
According to the paper, “there appear to be limited international or national legal quality requirements for water used in post-consumer plastic waste washing.” Polypropylene was selected for this study as it’s a material frequently used in food packages like tubs, carryout bags and stackable crates.
The study also looks at different washing methods and their potential to generate contamination in closed-loop wash water systems. Finally, it presents a framework for future research to understand potential wash water risks from recycling processes.
Results from the study indicated that DCHP (Dicyclohexyl phthalate) and DEHP (di(2-ethylhexyl) phthalate) were the most prominent high-concern contaminants in polypropylene wash water, while other phthalates and bisphenols were not detectable. The researchers also concluded that surfactant concentration changes as more material was processed indicating that monitoring is required to maintain cleaning performance and minimize potential health and environmental risks.
According to the authors, “These findings suggest that implementing surfactant-caustic washing and ultrasonication applications can bring a higher cleaning effect in recycled plastic but generate more contaminated wash water, requiring further treatment.”
Removal of contaminants from post-consumer recycling feedstocks is necessary to manufacture safe and high-quality packaging materials. This study demonstrated that some contaminants can be transferred to the wash water and mitigation strategies are needed to maintain overall sustainability of plastics recycling.
The paper is available here.