(82g) PLA/PBAT/MCC/HA Composite Optimized for Use As Biodegradable Mulch

The ongoing global use of poly(ethylene) mulch in agriculture contributes millions of tons of microplastic pollution annually. An alternative is needed which yields the benefits of water conservation, pest/weed prevention, and enhanced crop yield without exacerbating environmental harm. The present work is motivated by the urgent need to reduce microplastic dissemination through the development of a biodegradable mulch formulation with comparable functionality to conventional poly(ethylene). Current biodegradable options on the market exhibit premature breakage which limits competition with poly(ethylene). The objective of this work is to utilize a biodegradable hydrophobic polymer matrix of poly(lactic acid) (PLA) and poly(butylene adipate terephthalate) (PBAT). This blend composition is being optimized using a design of experiment with hydrophilic and nutrient rich additives microcrystalline cellulose (MCC) and hydroxyapatite (HA). The objectives are to enhance both crop yield and biodegradation once tilled into the soil as a mulch. These additives are expected to expedite biodegradation in soil and provide phosphorous for crops. A systematic methodology is being used to optimize the composite and meet criteria outlined in European Standard (EN) 17033: Plastics – Biodegradable mulch films for use in agriculture and horticulture.

The research applies an extreme vertices design of experiment with three replicates to evaluate the effects of each component on the films’ properties before and after accelerated aging. The design space informed by literature was constrained with 70-95% PBAT, 5-30% PLA, 0-10% MCC, and 0-10% HA by weight. The ratio of PBAT/PLA is a key constraint with a PBAT/PLA mass ratio of 9:1. However, because additives, changes in polymer grades, and processing can change the dynamics of the composite, our design space included a wide range of PBAT/PLA ratios from 7.8:1 to 9.5:1 to capture a ratio that would result in a material competitive to poly(ethylene).

Preliminary results indicate that HA and MCC can be added in concentrations less than 10% while still satisfying mechanical properties. Increasing concentrations of HA, PLA, and MCC negatively affect elongation and stress at break although MCC has a more significant impact. In contrast, PBAT concentrations are positively correlated with both elongation and strength at break for this design space. Ongoing work seeks to determine how these mechanical properties change after exposure to a growing season of solar irradiation and to elevated temperatures to address the films durability in the field. Ongoing work will also assess water absorption and thermal properties (e.g. crystallinity, volatile solids) for our films’ commercial viability as mulch.