Enhanced processability of poly-hydroxy-alkanoates by thermal stabilization and nucleation

Poly-Hydroxyalkanoates, PHAs, are considered as one of the most promising family of bio-plastics technologies. PHAs are thermoplastic, semi-crystalline, aliphatic polyesters produced by bacteria and fully biodegradable.;Poly-Hydroxy-Butyrate, PHB, a common PHA, and its copolymers, is bio-plastic comparable in properties to Polypropylene, Polyethylene, and even Polystyrene. However, with no modification PHBs possess inherent weaknesses like: limited thermal stability, slow recrystallization and, poor toughness that limit its use in many applications were biodegradation and sustainability can be an advantage.;In this work, P3HB-co-4HB copolymer was studied with respect to its thermal stability, recrystallization from the melt, rheological and mechanical properties.;Two types of nano modifiers were characterized with the aim to overcome the above mentioned drawbacks. Nano structured Poly-Oligomeric-Silsesquioxane, POSS, and Hyperbranched Polymers, HBP, were introduced into PHB through melt mixing.;The first one, POSS, is an inorganic/organic hybrid nano-particle. Its unique core---shell structure, was found to impart good plasticization effect, leading to molecular weight retention under high shear. The Plasticizing effect was compared to a those achieved with Citrate Oil Plasticizer, CTP. It was concluded that the mechanism for molecular weight retention is based on physical interactions rather than chemical chain extension.;The second nano-modifiers, solid and liquid Hyperbranched polymers, HBP, were examined for the effects on PHB.;The solid HBP, was melt mixed into PHB copolymer, up to 5wt%. The most significant effect of the solid HBP was the increase in the recrystallization rate. In the presence of the Solid HBP, the half time recrystallization was more than 46 times shorter; Tc, increased by up to 22°C, and the degree of crystallinity increased from 30 to 42%.;The liquid HBP, comprised of fatty acid modified second pseudo generation, Hyperbranched polyester, was incorporated into PHB, up to 5wt%. It was concluded that plasticization effects and the resultant molecular weight retention were the decisive phenomenon, similar to the effect obtained with POSS. CTP, was found to dissolve the liquid HBP. Hence, it was used for plasticizing and as a dispersing agent for the HBP.;The Zero Shear Viscosity of the PHB/HBP compounds indicated a plasticization effects, and supported the conclusions drawn. Furthermore, when introduced together with CTP, the Zero Shear Viscosity increased above the level when only HBP was used. High viscosity was shown to conform to a higher level of molecular weight retention. The modification of PHB with HBP/CTP solution led to a molecular weight higher by 18%, compared to un-modified PHB.;Furthermore, with the introduction of CTP an increase in the rate of recrystallization by close to 200 times was obtained, accompanied with an increase of Tc, by 25°C and doubling of the degree of crystallinity. SEM images supported the hypothesized mechanism.;Finally, the presence of a low Tg, liquid HBP, improved the toughness and extensibility of the PHB. Experimental results indicated that the addition of CTP as co-modifier led to a synergistic effect increasing both the energy to break and elongation to break by more than 100%. Modulus retention was exhibited in the ternary PHB/HBP/CTP blends.;The results obtained in this work are of great importance for PHB's processability and it acceptance in the plastics industry. The significant drawbacks, related to processability and performance of PHB were addressed and consequently were remedied by the introduction of long-chain grafted POSS and HBP, polyesters.