CO₂-Assisted Foaming of PLA–Cellulose Acetate Blends for Peat-Free Horticultural Substrates

This study investigates carbon dioxide assisted (CO₂) foaming of polylactic acid (PLA)–cellulose acetate (CA) blends to develop biodegradable substrates for horticultural applications. With increasing restrictions on peat-based growing media, PLA offers a bio-based alternative, but its limited water retention and porosity necessitate modification. PLA was compounded with varying CA contents via hot-melt extrusion and foamed using CO₂ at pressures of 800, 1000, and 1200 psi. The resulting foams were evaluated for water-holding capacity (WHC), air-filled porosity (AFP), foam expansion, density, biodegradability, pore morphology, pH, electrical conductivity (EC), and seed germination performance. Among all formulations, the PLA80–CA20 blend provided the most favorable combination of foamability, structural stability, and horticultural performance. Samples foamed at 800 psi exhibited the highest WHC (77.33%), high AFP (59.46%), and controlled expansion (5.51%), reflecting well-developed and interconnected pore structures. Increasing foaming pressure led to matrix densification, reduced porosity, and lower water retention. Scanning electron microscopy confirmed a pressure-dependent decrease in pore size from 171.77 μm at 800 psi to 47.25 μm at 1200 psi. Biodegradation testing showed that PLA80–CA20 degraded significantly faster than virgin PLA, achieving approximately 50% mineralization within 80 days. All foamed substrates maintained pH and EC values within acceptable ranges for plant growth, and successful seed germination was observed across all samples. These results demonstrate that CO₂-assisted foamed PLA–CA blends, particularly those processed at 800 psi, are viable compostable alternatives to peat-based substrates and provide a scalable route for tailoring biodegradable polymer foams for sustainable horticulture.