Assessment of PHB with varying hydroxyvalerate content for potential packaging applications

Abstract

Poly-(hydroxybutyrate) (PHB) is biodegradable aliphatic polyester that is produced by a wide range of microorganisms. Basic PHB has relatively high glass transition and melting temperatures. To improve flexibility for potential packaging applications, PHB is synthesized with various co-polymers such as poly-(3-hydroxyvalerate) (HV) leading to a decrease of the glass transitions and melting temperatures. In addition, the HV broadens the processing window since there is improved melt stability at lower processing temperatures. In this study, PHB synthesized with different valerate contents (5%, 12%, and 20%) and varying in molecular weights were characterized. All PHBV materials displayed a glass transition between −10 and 20 °C. The two melting transitions found for Aldrich 5%, 12%, and Tianan 20%, resulted from crystals formed during cooling of the samples. The complex viscosity decreased with increasing temperature due to a decrease in molecular weights of the samples. These results suggest that processing the co-polymer below 160 °C would be beneficial with low screw speed. The mechanical results indicate all PHBV materials had high elastic modulus and flexural strength with low tensile strength and elongation at break. The WVTR results indicated the polymer to be very hydrophilic, resulting in higher water transmission rates.

Introduction

Bio-based materials for packaging applications are being sought to replace synthetic, non-degradable, thermoplastics due to rapid growth in municipal waste, consumer awareness and stricter government regulations [1], [2], [3]. Currently, the potential for aliphatic polyesters derived from fermentation such as poly-(ε-caprolactone) (PCL), poly-(l-lactic acid) (PLA), and poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) can be used to alleviate demands on diminishing landfill space as they are fully biodegradable due to their hydrolyzable backbones [1], [2], [3]. One potential biopolymer family is that of polyhydroxyalkanoates (PHA’s) that are biosynthesized from variety of microorganisms. Amongst PHA’s, poly-(3-hydroxybutyrate) (PHB) is studied most frequently and is easiest to produce [4]. The PHB polymer, being saturated polyesters, behaves similarly to conventional thermoplastics. However, it has relatively high glass transition and melting temperatures, leading to increased brittleness that results in a poor processing window and higher cost; such factors limit PHB polymer use [4], [5].

To overcome these drawbacks, the PHB homo-polymer can be plasticized internally by bacterial fermentation using 3-hydroxyvalerate, 4-hydroxybutyrate, 3-hydrohexonoate, or 3-hydroxypropionate [5], [6]. In this study, poly-(3-hydroxyvalerate) (HV) units were incorporated into PHB, resulting into poly-(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The added co-polymer decreased the glass and melting temperature as well as broadened the processing window, since there was improved melt stability at lower processing temperatures [1], [7]. The final structure was a nine-carbon monomer with hydroxyl and carboxyl end groups, thereby, allowing the plastic to be tailored to specific applications that are similar to conventional thermoplastics [2].

Previous researchers have assessed the PHB polymer properties utilizing only one concentration of hydroxyvalerate unit and a single manufacturer. For example Verhoogt et al., studied thermal, rheological, and mechanical properties of Biopol PHB with 12% HV units (M_w of 539 kDa), while Wang et al. studied Tianan PHB with 1% HV units (M_w of 130 kDa) and determined similar characterization as Verhoogt [3], [4]. Although hydroxyvalerate contents varied between these studies, the effect of molecular weight on thermal and mechanical properties of polymer with similar hydroxyvalerate content was not discussed. For many polymers, the molecular weight affects thermal, mechanical, and rheological properties related to processing in an extruder or injection molding units, thus it is paramount to characterize the effects of molecular weight and valerate content on material properties of PHBV. Therefore, the aims of this study were to first characterize the thermal, mechanical, rheological, and barrier properties of PHB synthesized with different valerate contents, and second to compare these properties in PHBV with similar hydroxyvalerate content but different molecular weight.