A new tool to screen biodegradable polymers as technically and commercially viable fertiliser coatings

Polymer coated controlled release fertilisers can mitigate nutrient pollution by aligning nutrient release to plant demand, thereby reducing losses, fertiliser application and potentially increasing yields. However, most current commercial products use polymers that do not readily degrade. This use of non-degradable plastics to coat fertilisers is being phased out, opening new opportunities to develop and commercialise truly biodegradable coating alternatives. However, the technical challenge is substantial. The coating needs to eventually fully mineralise, leaving no microplastic legacy, yet it must also maintain good mechanical and barrier properties for extended periods. This work presents the first effort to develop a polymer material selection tool to guide biodegradable polymer selection for use as fertiliser coatings based on the polymers' known properties and commercial considerations. Using a new mechanistic model, a relationship was established between the elongation required to avoid coating rupture and the water vapour permeability (WVP) of the coating. Then, a broad list of commercially available biodegradable polymers was assembled and literature data on their WVP and elongation at break collated. By comparing this data and the model outcomes, the polymers most likely to achieve long term release were shortlisted. This list was further condensed by setting a maximum polymer price and minimum global production capacity for commercial viability. We have shortlisted polycaprolactone, biodegradable polyurethane and natural rubber as strong candidates for biodegradable fertiliser coatings. However, their rate of biodegradation requires further investigation. Flexible polyhydroxyalkanoates, poly(butylene succinate) and poly(propylene carbonate) are technically promising, but not currently commercially viable.