Modification of Thermal and Mechanical Properties of Polyhydroxybutyrate (PHB) through Addition of Bio-based Plasticizing Materials and Additive Manufacturing Processing

Abstract: The non-degradable polymers currently used for commercial production and application may be inexpensive; however, their excessive use is leading to unprecedented amounts of plastic waste and extensive environmental damage. The demand for biologically-derived, biodegradable polymers is therefore increasing, as these materials are an environmentally attractive alternative to polymers of petrochemical origin. The biopolymer polyhydroxybutyrate (PHB) is bio-based, biodegradable, and biocompatible, giving it a significant advantage over fossil fuel-based polymers in regards to environmental impact. It has great potential for applications in areas such as food packaging, pharmaceuticals, and biomedical engineering; however, PHB has a narrow thermal processing window and current processing methods are very expensive and lead to poor mechanical properties, such as low flexibility and high brittleness. In this work, PHB materials and methods for thermal processing were investigated, so as to determine whether or not the material can be processed efficiently. The effects of processing PHB through extrusion, hot-pressing, and fused deposition modelling (FDM) – a relatively cheap and efficient additive manufacturing process commonly known as 3D printing – on its morphology, thermal, and mechanical properties were studied. Additionally, the introduction of epoxidized canola oil (eCO) as a green plasticizer, polylactic acid (PLA) as a polymeric plasticizer, and zinc acetate as a possible reaction catalyst to help the blending between PHB and PLA, into the PHB polymer matrix was studied to determine if the processability and flexibility drawbacks could be improved. Pure PHB pellets were melted and extruded into blended samples (containing different amounts of PHB, PLA, eCO, and catalyst) using a twin-screw micro-extruder system. This material was then hot-pressed into 0.5 mm thick polymer sheets and tested. In addition to these samples, another set of experiments was performed where the same PHB pellets were melted and extruded into neat filaments of 1.75 mm diameter. These filaments were fed into a desktop FDM/3D printer equipped with a hotend extruder and print bed at controlled, elevated temperatures to directly deposit (i.e. print) filaments into tensile specimens. Scanning electron microscopy (SEM), stereomicroscopy, confocal microscopy, and Fourier-transform infrared spectroscopy were used to study the morphology and structure of the resulting samples. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) measurements were used to study the thermal properties, stability, extent of degradation, and crystallinity of the final products. Tensile testing was also used to assess the mechanical properties of the products. Negligible changes in thermal properties and stability were observed following the extrusion, hot-pressing and FDM processes, and neat PHB samples from the two final processes (i.e. hot-pressing and FDM) have comparable properties to solvent cast thin film samples of PHB (created in another work using acetic acid) while also being in the range of observed mechanical properties for bulk PHB. These results indicate that PHB can be effectively extruded, hot-pressed, and patterned using FDM printing, with little loss in properties. For FDM processing specifically, which is easy to use, cost-efficient, and commercially scalable, significant mechanical property improvements can result with optimization of the printing process. Similarly, the addition of bio-based plasticizing agents eCO and PLA (together or separately) gave no significant changes in thermal properties or stability (as compared to neat PHB samples) at the processing and analysis temperatures chosen. The addition of 10 wt% eCO into neat PHB produced the most promising results, as the oil introduced positive plasticizing effects on the PHB matrix (i.e. increased flexibility with an expected decrease in strength). With the addition of PLA (25 wt%) and the blending catalyst (0.3 wt%), an improvement in flexibility and a decrease in strength parameters were also observed as compared to pure PHB. Finally, eCO may also give promising plasticizing results when added to a PHB/PLA blend (in a 3:1 ratio of PHB:PLA), as the addition of 5 wt% eCO produced equivalent results as compared to the sample without oil. However, the addition of 10 wt% eCO gave a statistically significant decrease in both strength and flexibility, as compared to the PHB/PLA blended sample, providing evidence that an optimum point of plasticizing material had been surpassed. By fine-tuning the amount of eCO and PLA in the PHB matrix, it is possible to create a bio-based polymer product more suitable for commercial use and more beneficial for the environment.