Quantifying urea permeability through biodegradable PHBV/PBAT thin films

Biodegradable polymer-coated fertilisers have gained increasing attention due to environmental concerns associated with the use of non-degradable polymer coatings. However, limited data are available on the rate of urea permeation through these biodegradable alternatives. In this study, urea permeability coefficients were determined for biodegradable polymer films made from poly(butylene adipate- co -terephthalate) (PBAT), poly(3-hydroxybutyrate- co -3-hydroxyvalerate) (PHBV), and their blends (25, 50, and 75 wt% PHBV). The permeability coefficient for the PHBV thin film was found to be (1.0 ± 0.3) x 10 −9 cm 2 /h, while that for PBAT was (1.9 ± 0.3) x 10 −8 cm 2 /h. However, no consistent trend in permeability was found with varying blend composition. The degree of porosity and film morphology were analysed using micro-computed tomography and Raman spectroscopy. Raman mapping revealed phase separation in all blends, with complex mixtures of droplet-matrix and co-continuous morphologies present to varying degrees, influencing the urea permeation pathway. Additionally, a significant increase in porosity was observed in the 50:50 blend compared to others, which is consistent with its higher permeability coefficient (1.1 ± 0.5 × 10 −7 cm 2 /h), since coatings with structural defects allow urea to diffuse more readily through the polymer layer, leading to higher permeability and an increased urea release rate. Modelling of urea release based on permeability coefficients, excluding factors such as biodegradation, indicates that a 30 μm PHBV coating on a urea granule would significantly delay release, with 80 % of urea projected to be released over approximately 25 years. By contrast, granules coated with PBAT would take just under one year to release 80 % of urea under these conditions.