Characterising the Chemical Additive Content of Agricultural Plastic Mulch Film

Plastic is a prominent material finding growing use globally within agricultural supply chains, through cultivational practices to product packaging. Mulch film is one such ‘plasticulture’ application where opaque film is stretched over the soil surface to maintain a humid microclimate at the crops / fruits soil-root zone as well as protecting ungerminated seeds and small crops from pests. However, mulch film represents an unregulated chemical input source to agroecosystems, whose ecotoxicological legacy is yet to be assessed. The first step in addressing this issue is characterising the chemical content of the source material that may later be released. Amidst the polymeric base, manufacturers include a diverse multitude of chemical additives into plastic formulations to aid with manufacturing, improve functionality, retard degradation and alter aesthetics. In most cases, additives are not chemically but physically bound to the polymeric matrix meaning over time they are free to migrate and be released to the surrounding agroecosystem. Once released, these chemicals may cycle between compartments, interacting with biota or entering the food chain through uptake by plant life or livestock. To trace the extend of release and subsequent interaction of chemical additives within ecosystems, the formulation of the film must be determined. An untargeted characterisation approach has to be adopted, as additive packages remain confidential as property of the manufacturers. Microwave assisted solvent extraction and dissolution-precipitation have been implemented for two agriplastic mulch films: synthetic low density polyethylene (LDPE) and biodegradable polylactic acid (PLA; 15 %) / polybutyrate adipateterephthalate (PBAT; 85 %) blend. Extracts were analysed by GC-MS and spectral libraries used to characterise the leachable content of the films. Alternatively, thermal desorption of the additives from the polymeric base and subsequent analysis of the volatiles through py-GC/MS allows for rapid screening of low molecular weight species, with the caveat of loss in quantification accuracy. An array of chemicals have been identified including: plasticisers (phthalates, citrates, adipates), slip agents (fatty amides), antioxidants (hindered phosphates, hindered phenols), antistatics (fatty esters), lubricants (fatty alcohols, alkanes) and transformation products of both additives and polymer. Additive characterisation work is complimented by confirmation of the mulches polymeric compositions or revelation of unreported polymeric components, through techniques such as fourier transform infrared (FTIR) spectroscopy and ¹H nuclear magnetic resonance (NMR). Later investigation seeks to understand the transformation, cycling and fate of these additive targets within agroecosystems at predicted release levels, which are based on leaching studies into water and in-field measurements.