Effect of crystallization conditions on the physical properties of a two-layer glassine paper/polyhydroxybutyrate structure

Polyhydroxybutyrate (PHB) is a hydrophobic, biodegradable biopolymer, which can be a potential substitute for currently used synthesized polymers in the packaging industry. However, its utility is often limited by its brittleness and poor mechanical properties, mainly because of its through-thickness fractures. In this study, we laminate and crystallize PHB on glassine paper by hot-pressing and tune the crystallization conditions to minimize cracking. Glassine paper is impermeable to PHB granules and allows the formation of a distinguishable bilayer of PHB. It was found that glassine paper serves as a soft substrate, which increases the number of nucleation sites of the spherulites and prevents growth of the cracks in the neighboring PHB layer. Quenching the films to the crystallization temperature was found to minimize cracking enough to reduce the water vapor transmission rate to \(15\)–\(25\,{\mathrm{g}}\,{\mathrm{m}}^{-2}\,{\mathrm{day}}^{-1}\), irrespective of the crystallization temperature; however, the mechanical properties improved only at the crystallization temperatures below 77 °C, perhaps due to the local stress in the existing cracks at higher crystallization temperatures. The optimum crystallization conditions were found to be quenching the film in an ice bath and crystallization at room temperature, by which we obtained mechanical strength and Young’s modulus of \(80\,\text {MPa}\) and \(2.5\,\text {GPa}\), respectively, and a water vapor transmission rate of \(20\,\text {g}\,\text {m}^{-2}\,\text {day}^{-1}\). Our results suggest a simple and cost-effective method to produce PHB films with enhanced mechanical and barrier properties.