Novel Methodology to Form Micro- and Nano-Plastics from Agricultural Plastic Materials and their Dimensional, Thermal, and Chemical Characterization
A. F. Astner, D. G. Hayes, H. M. O'Neill, B. R. Evans, S. V. Pingali, V. S. Urban, and T. M. Young.  2019.  BioEnvironmental Polymer Society Annual Meeting, Greenville, SC, 5-7 June 2019.

Abstract:
Microplastics (Ps) and nano plastics (nPs) release into agricultural fields by increased use of agricultural plastics in specialty crop production is of great concern due to ecotoxicity to organisms that provide beneficial service to the soil such as earthworms, and the potential ability of microplastics and nano plastics to enter the food chain. Most studies of terrestrial microplastics employ idealized microplastics as models, such as polystyrene spheres. Herein, a methodology was developed to produce Ps and nPs from mulch films consisting of the biodegradable polymer polybutyrate adipate-co-terephthalate (PBAT) and from low-density polyethylene (LDPE), a common component of agricultural plastics. PBAT (after being exposed to cryogenic soaking, a step that promotes embrittlement, i.e., mimics environmental weathering) and LDPE were subjected to mechanical milling and then fractionated into P fractions of 840 m, 250 m, 106 m, and 45 m, with recovery of the latter two fractions being 2% and 1%, respectively, for each polymeric material. The 106 m fraction was subjected to wet grinding to produce nPs of average particle size 366.0 nm and 389.4 nm for PBAT and LDPE, respectively. The particle size distributions for Ps was described by a two-parameter Weibull distribution, while nPs possessed bimodal distributions, with each sub-distribution also described by the two-parameter Weibull. Chemical property analyses demonstrated the absence of significant changes upon size reduction, except for slight depolymerization via hydrolysis and an increase of crystallinity that accompanied embrittlement during the cryogenic treatment; therefore, the methodology does not impose artifacts into the chemical properties. This study demonstrates that Ps and nPs can form from agricultural plastics at the interface of soil (or remaining plastic fragments in the soil) and water in the presence of shear.