Characterization of Polyhydroxybutyrate-Based Composites Prepared by Injection Molding

Abstract

The waste generated by single-use plastics is often non-recyclable and non-biodegradable, inevitably ending up in our landfills, ecosystems, and food chain. Through the introduction of biodegradable polymers as substitutes for common plastics, we can decrease our impact on the planet. In this study, we evaluate the changes in mechanical and thermal properties of polyhydroxybutyrate-based composites with various additives: Microspheres, carbon fibers or polyethylene glycol (2000, 10,000, and 20,000 MW). The mixtures were injection molded using an in-house mold attached to a commercial extruder. The resulting samples were characterized using microscopy and a series of spectroscopic, thermal, and mechanical techniques. We have shown that the addition of carbon fibers and microspheres had minimal impact on thermal stability, whereas polyethylene glycol showed slight improvements at higher molecular weights. All of the composite samples showed a decrease in hardness and compressibility. The findings described in this study will improve our understanding of polyhydroxybutyrate-based composites prepared by injection molding, enabling advancements in integrating biodegradable plastics into everyday products.

Keywords: biodegradable plastics; compounding; injection molding; polyhydroxybutyrate; polymer composites.

Figure 1

Custom split-die injection mold with a cavity size of 25.4 × 6.35 mm used to form samples for compression and hardness testing.

Figure 2

Shore A hardness probing sites along the sample surface.

Figure 3

Compression testing configuration.

Figure 4

Sample process of preparing PHB based composites, through injection molding, to characterization.

Figure 5

PHB composite sample surfaces photographed at 1×, 20×, and 100× for the analysis of surface changes. The 20× and 100× photographs were taken under full ring lighting conditions.

Figure 6

PHB/microsphere (3%) composite sample surface with measured pore diameters of fully formed microspheres. Image taken at 100× under partial-ring lighting conditions.

Figure 7

(a) IR spectra of PHB composites in the 500 to 4000 cm⁻¹ frequency range. Absorption bands at 1280 and 1720 cm⁻¹ are representative of the PHB content. (b) IR spectra in the 2750 to 3100 cm⁻¹ range, for emphasizing on the effects of the PEG content at the 2930 cm⁻¹ peak.

Figure 8

(a) TGA curves of PHB composites. The inset shows the degradation temperature, T_d across samples. (b) Comparison of T_d for PHB control against PHB composite samples.

Figure 9

Shore A hardness comparison of PHB composite samples against the control sample. The bar shows the mean hardness (±standard deviation) of five measurements on each sample.

Figure 10

Stress/displacement diagram of PHB composites under 1 MPa compression for the analysis of maximum displacement.