Development of polylactic acid-polyhydroxybutyrate blends for packaging applications
Creators: name: Kong, Dehao
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401212
subject: Non-Newtonian fluid flows (incl. rheology)

description: Until recent years, the packaging materials which are based on non-renewable fossil resources have been used widely. However, the toxic or noxious components released during manufacture and their non-degradability cannot be ignored. Therefore, the environmental friendly and renewable materials are currently receiving more attentions by researchers and commercial packaging materials producers. In flexible packaging industry, a biodegradable material, polylactic acid (PLA) has significant potential as it offers high clarity and stiffness, together with excellent printability. On the other hand, poly (hydroxybutyrate) (PHB), the best-known poly(hydroxyalcanoate) (PHA) synthesized by controlled bacterial fermentation, presents high crystallinity which is required for film blowing application. However, the main drawback of PHB is that its melting temperature is about 170–180 °C, which is close to the degradation temperature. This presents a relatively small processing window for melt extrusion. Nevertheless, the melting temperature of PHB can be lowered far below its thermal decomposition temperature to make this material much easier to process either through chemical modifications or blending with another polymer. Since PLA and PHB present similar melting temperatures, they can be blended in the melt state to produce a new biodegradable material that combines the advantages of both polymers. This thesis reports on research conducted to improve PLA’s properties to satisfy final applications such as packaging. PLA was blended with PHB in various concentrations (0, 10, 20, 25, 30, 40, 50, 60, 70, 80, 90 and 100 wt%). The main focus of this work is to analyse the rheological and thermal properties of these PLA/PHB blends. The thermal results indicated that the PLA/PHB blends are immiscible but existed molecular interaction between two polymers over the entire blends range. The remarkable separate peaks of the melting temperature in the blends correspond to the melting temperature of pure PLA and PHB. The crystallization of PLA was enhanced in the PLA/PHB blends as the PHB content was increased up to 25 wt%. This could be due to PHB acting as a nucleating agent in PLA and high crystalline structure of PHB. Dynamic rheological tests were carried out for all PLA/PHB blends mentioned earlier to study the effect of the PHB concentration into PLA at three different temperatures (180, 190 and 200 ℃). As expected, increasing temperature was led to a decrease of the storage and loss moduli as well as dynamic viscosity values of all PLA/PHB blends. The time-temperature superposition (TTS) principle, based on the Arrhenius approach, was used to collapse all three rheological curves at 3 different temperatures into a master curve successfully. TTS was enabled the rheological properties of PLA/PHB blends to be extended on either side of the original frequency range tested depending on the selected reference temperature. It was observed that PLA/PHB Blends containing low PHB content (0-40 wt%) produced higher dynamic viscosity and elastic properties compared to pure PLA. However, for PLA/PHB Blends containing higher PHB content (&gt; 60 wt%), their rheological properties were between that of the two pure materials. At low PHB content, the PLA/PHB blends showed positive-deviation, according to the mixing rule. They also showed typical liquid-like, temperature-dependency and shear-thinning behaviour as well. The optimum PLA/PHB blends, based on the rheological and thermal properties, were found to be those containing 20-25 wt% PHB content. These blends (PLA75/PHB25 and PLA80/PHB20) can be considered for use in packaging applications.