(210b) Biodegradable Polymer Films As Mulch for Sustainable Agriculture

Polyethylene mulches are used in agriculture to conserve water, prevent pests, and enhance crop yields. Simultaneously, their ongoing use contributes millions of tons of microplastic contamination annually. The present work is motivated by the urgent need to reduce microplastic dissemination through development of biodegradable mulches (BDMs) with comparable functionality as polyethylene films. The objective of this work is two-fold: firstly, to develop a comprehensive methodology informed by research, industry guidance, and legislative requirements; and secondly, to optimize the performance of biodegradable mulch films through this methodology. Achieving these objectives requires a multidisciplinary approach that integrates materials science and polymer chemistry. European Standard (EN) 17033: Plastics – Biodegradable mulch films for use in agriculture and horticulture – Requirements and test methods, released in 2018, is an industry standard that recommends properties for all mulch films irrespective of the type of soil or crop they will be used on. Techniques for measuring these properties generally follow standardized protocols such as those published by ASTM or ISO. While EN 17033 provides a valuable framework for quality control of BDMs, characterization techniques used in literature to develop BDMs vary widely and often rely on methods not included in EN 17033, demonstrating a need for additional guidance. This work utilizes biodegradable polymers polylactic acid and polybutylene adipate terephthalate and film processing techniques such as hot melt pressing and biaxial stretching to achieve both objectives.

In preparing a reproducible, consistent polymer blend with respect to composition, thickness, and behavior, lab scale films required hot melt mixing and thermal processing of polymer blends over solvent casting. Furthermore, given that films must meet 350% strain and 18 MPa stress at break, mechanical properties are the first screening analysis. Our studies revealed that mechanical behavior is dependent on sample geometry with dog bone shapes providing more accurate and reproducible results during uniaxial testing despite rectangular shapes being recommended by EN 17033. Rigorous mechanical testing with rectangular or dog bone specimens and different gripping systems show that for highly ductile thin film samples, preventing slipping during testing and proper sample geometry are both critical to reproducible and meaningful results. Rectangular specimens were susceptible to larger strain fields and slipping in the gripped region which resulted in premature failure, slipping out of the grip during testing, and noise in the data.

Further characterization of BDMs encompasses a wide range of analyses including microscopy, thermal analyses, mechanical testing, water interactions, and degradation phenomena. Thermal analyses, including techniques such as differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), assess polymer crystallinity, thermal stability, and impurities. Uniaxial tensile testing is used to evaluate strength and ductility. Water absorption assay probes water interactions, artificial weathering tests ensure stability in the field, and greenhouse degradation studies to track degradation and chemisorption of antibiotics after incorporation into soil. These properties reflect critical aspects of all stages of consumer mulch use: suitable mechanical properties for application onto soil, adequate soil-air interface stability during ultraviolet light and water exposure, and timely biodegradation after incorporation into soil. To achieve our first objective, this work makes recommendations regarding characterization and sample preparation. For example, we recommend DSC over X-ray diffraction for crystallinity assessment because DSC can distinguish between crystallinity induced by processing versus composition.

With respect to our second objective, preliminary results indicate that both composition and film processing parameters affect bulk film properties such as strain and stress at break failure. For this reason, we are employing a full factorial design of experiment to optimize both composition and processing parameters (i.e., operating temperature, draw ratio) based on desired water absorption, mechanical properties, film stability, and film biodegradation. Preliminary results with a 90% PBAT/10% PLA blend show that biaxial stretching is a feasible strategy for improving mechanical properties without the addition of potentially toxic additives such as compatibilizers and plasticizers. Specifically, elongation and stress at break for the 2x2 and 4x4 stretched samples were 984.6±203.1%, 16.4±2.1MPa and 563.9±215.5%, 24.2±6.4MPa respectively. Results demonstrate that the stretching condition can be optimized where EN 17033 criteria and other properties will be satisfied. Future work will expand this optimization methodology to composites with nanoparticles serving as nutrients, which will increase the practical value of the blends while decreasing their overall cost to end-users. The objectives presented herein advance sustainable agriculture for a growing human population without exacerbating the ongoing microplastic public health crisis. By leveraging materials engineering principles, systematic characterization techniques for these highly ductile thin films, and adherence to regulatory standards practiced in industry, this research aims to expedite adoption of BDM technology and contribute to a more sustainable future for global agriculture.