Toward Greener Multilayer Packaging Material Solutions Based on Microbial Protein and Polyhydroxyalkanoate

Plasticized microbial (single cell) proteins (MPs) can be used to produce ductile and flexible plastic films with good oxygen barrier properties. However, as with other hydrogen-bond-forming oxygen barrier materials, like ethylene-vinyl alcohol copolymer (EVOH), they need to be protected from moisture because moisture decreases the oxygen barrier properties. Here, we solved the problem by producing three-layer laminate films that are fully biobased and biodegradable. Two different MP films (originating from a mixed microbial culture and Delftia tsuruhatensis biomass) were sandwiched between two different moisture-shielding polyhydroxyalkanoate (PHA) films (a poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) and a poly-(3-hydroxybutyrate-co-3-hydroxyhexanoate) material). The low-temperature melting features of the PHAs made them suitable for lamination through hot-pressing with the MPs. Liquid-water-resistant and UV-blocking laminates could be obtained, where the individual layers were also possible to delaminate as a possible recycling solution, where the MP layer could potentially be used as a fertilizer and the PHA mechanically recycled into similar or other products or composted. The laminates showed, in the best cases, an oxygen permeability of 2 cm³ mm/(m² day atm) and a water vapor permeability below 0.1 g mm/(m² day). All in all, the concept is promising as a sustainable biobased alternative to today's fossil-based EVOH-laminate packaging solutions.