Impact of Multiple Reprocessing on Properties of Polyhydroxybutyrate and Polypropylene

Abstract

Biobased plastics have the potential to be sustainable, but to explore their circularity further, current end-of-life options need to be broadened. Mechanical recycling is one of the most accepted methods to bring back plastics into the loop. Polyhydroxybutyrates (PHBs) are biobased and biodegradable in nature with promising properties and varied applications in the market. This study focuses on their potential for mechanical recycling by multiple extrusion cycles (E1-E5) and multi-faceted characterization of the virgin (V) and reprocessed materials from E1 to E5. The behavior is compared to polypropylene (PP) as a reference with a similar property profile, which has also been reprocessed five times. The thermal properties of both series showed a stable melting point and thermal decomposition temperature from thermal analyses (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)). However, a steady increase in the degree of crystallinity was observed which could counterbalance the decrease in molecular weight due to repeated extrusion measured by gel permeation chromatography and resulted in similar values of tensile strength across the cycles. The strain at break was impacted after the first extrusion, but no significant change was observed thereafter; the same was observed for impact strength. Even in scanning electron microscopy (SEM) images, virgin and E5 samples appeared similar, showing the stability of morphological characteristics. Fourier transform infrared spectroscopy (FTIR) results revealed that no new groups are being formed even on repeated processing. The deviation between the PHB and PP series was more predominant in the melt mass flow rate (MFR) and rheology studies. There was a drastic drop in the MFR values in PHB from virgin to E5, whereas not much difference was observed for PP throughout the cycles. This observation was corroborated by frequency sweeps conducted with the parallel plate method. The viscosity dropped from virgin to E1 and E2, but from E3 to E5 it presented similar values. This was in contrast to PP, where all the samples from virgin to E5 had the same values of viscosity. This paper highlights the possibilities of mechanical recycling of PHB and explains why future work with the addition of virgin material and other additives is an area to be explored.

Keywords: PHB; PP; biobased plastics; circular economy; mechanical recycling; polymer processing; reprocessing.

Figure 1

Schematic of followed mechanical recycling pathway.

Figure 2

Melt flow rate (MFR) of PHB virgin–E5 samples (a) and of PP E1–E5 (b).

Figure 3

Frequency sweep curves for (a) PHB and (b) PP.

Figure 4

Complex viscosity vs. angular frequency curves for (a) PHB and (b) PP.

Figure 5

FTIR spectra of PHB after different reprocessing cycles.

Figure 6

FTIR spectra of PP after different reprocessing cycles.

Figure 7

SEM images of PHB-virgin sample (a) and PHB-E5 sample (b).

Figure 8

SEM images of PP-virgin sample (a) and PP-E5 sample (b).

Figure 9

Representative DSC curves for the 2nd heating and cooling cycle of (a) PHB virgin–E5 and (b) PP virgin–E5.

Figure 10

Representative TGA curves of (a) PHB and (b) PP for all processing cycles.

Figure 11

Tensile strength and strain at break of (a) PHB and (b) PP for virgin material and after 1–5 reprocessing cycles (E1–E5).

Figure 12

Young’s modulus of (a) PHB and (b) PP for virgin material and after 1–5 reprocessing cycles (E1–E5).

Figure 13

Charpy notched impact strength of (a) PHB and (b) PP for virgin material and after 1–5 reprocessing cycles (E1–E5).