Production and Properties Development of Polyhydroxyalkanoates-Natural Rubber Blends

Abstract

Polyhydroxyalkanoates (PHAs) are microbial thermoplastic that are completely degraded in various natural environments. The production of PHAs is limited by the expensive carbon source and recovery processes. PHAs-producing bacteria that can use biodiesel liquid waste (BLW) as a carbon source were studied. Pseudomonas mendocina PSU produced 79.7 wt% poly(3-hydroxybutyrate) (PHB) when cultured in mineral salts medium (MSM) containing 2% (v/v) BLW for 72 h. The weight average molecular weight (Mw) and number average molecular weight (Mn) were 8.39 x104 and 4.60 x 104 , respectively. PHAS-producing bacteria SB01 and SB02 were isolated from Sirindhor peat swamp forest in Thailand. Strain SB01 cultured in MSM containing BLW produced 91.98 wt% PHB at 78 h. When palm oil was used as a carbon source, this strain produced 87.50 wt% PHB at 84 h. Strain SB02 cultured in MSM containing 2% BLW produced 97.95 wt% PHB at 78 h. When palm oil was used as a carbon source, this strain produced 96.60 wt% PHB at 84 h. This study shows the potential use of a cheap substrate to produce a high amount of PHAS. Moreover, these two strains produced high molecular weight PHB which is more flexible than PHB from P. mendocina PSU. Mw and Mn of PHB from SB01 were 2.3 x 105 and 1.3 x 105 when BLW was used as a carbon source. M, and M, of of PHB from SB01 were 2.8 x 105 and 2.1 x 105 when palm oil was used as a carbon source. M, and M, of PHB from SB02 were 2.5 x 103 and 2.0 x 10' when BLW was used as a carbon source. M., and M, of PHB from SB02 were 7.9 x 103 and 4.3 x 103 when palm oil was used as a carbon source. PHAS-producing bacteria SB01 and SB02 were identified as Burkholderia seminalis and Burkholderia contaminans, respectively according to the 16S rRNA sequence analysis. Even though these two strains are very promising, but Burkholderia spp. have also emerged as important human opportunistic pathogens and the risks associated with their uses remain unclear. Therefore, P. mendocina PSU was selected for further study. For the development of the biomaterials, PHB properties of P. mendocina PSU were improved by both copolymer production of PHBV and blending methods. Poly(3- hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) copolymer was produced. Effects of sodium propionate, which is the precursor for PHBV production, and nitrogen sources were investigated. P. mendocina PSU produced 38.6 wt% PHBV with 13 hydroxyvalerate (HV) mol% at 72 h when 0.3% sodium propionate and 0.5 g/L ammonium sulphate were added. The properties of PHBV were better than PHB because of high molecular weight and low melting temperature. PHA recovery processes by biological method using mealworms and chloroform extraction were investigated. M. and M, from biological method were higher than those of the chloroform extraction. Melting temperature (T) and purity of PHAs were comparable. This study confirmed the ability of mealworms to extract PHAs from various kinds of bacterial cells. In the attempt to improve the properties of PHB, PHB was then blended with modified epoxidized natural rubber 25 (ENR25) by solution blending, melt blending by internal mixture and melt blending by twin screw extrusion. Unfortunately, PHB and modified ENR25 could not mix well together. Biodegradation of all polymers from this study were determined by soil burial test and treating with the rubber degrading bacteria consortium. PHB and PHBV from P. mendocina PSU and rubber glove showed higher degradation rate than other materials when buried in the soil. The rubber degrading bacteria consortium degraded PHB- ENR25 blend films better than PHB and PHBV, NR and ENR25. This study has successfully established a low-cost process to synthesize biodegradable polymer. However, the polymer properties are needed to be improved further.