Biothermoplastics from Renewable Resources

Keywords

Bacterial polyester • Poly(3-hydroxybutyrate) • PLA • Core-sheath fiber • Knit­ted socks

Polyhydroxyalkonates are a diverse family of biopolyester produced by bacteria as energy and carbon storage materials. Poly(3-hydroxybutyrate) (PHB) is the most common type of PHA that is commercially used. PHB is an thermoplastic material with a melting temperature of about 180 °C and glass temperature that is below room temperature. Structure and properties of PHB are highly dependent on the conditions prevailing during fiber production. For instance, slow cooling from the melt produced large spherulites and rapid cooling results in amorphous state [01Yam]. It was suggested that PHB assumed orthorhombic or а-form or the P-zigzag form depending on the annealing conditions. PHB crystallized into ortho­rhombic form when annealed under high tension and into p-zigzag form when annealed under high tension [01Yam]. Based on X-ray diffraction patterns, it was found that the amorphous molecules transformed into orthorhombic crystal when annealed without tension and when annealed under tension, the amorphous regions were stretched and crystallized into the p-form [01Yam].

Polyhydroxybutyrate-valerate (PHBV) is a copolymer of PHB that is less stiffer but tougher than PHB. However, the low crystallization rate of PHBV makes it difficult to produce fibers. To overcome this limitation, PHBV was blended with PLA to produce core-sheath fibers. PHBV with a viscosity average molecular weight of 490 kDa was extruded to obtain pellets with lower molecular weight of 260 kDa and PLA was reduced to a molecular weight of about 90 kDa, and the two polymers were blended and extruded into fibers. Table 65.1 lists the conditions used and the properties of the fibers obtained. As seen in the table, it was not possible to obtain fibers with PHBV as the sheath and PLA as the core due to poor processabil­ity of PHBV. Tensile properties of the fibers were dependent on the draw ratio and to the amount of PLA in the blend. Biocomponent fibers with PLA as the sheath and

Table 65.1 Processing conditions and properties of PHBV-PLA blend fibers [12Huf] Core Sheath Temperature (°С) Draw ratio Strength (g/den) Strain {%) Modulus (GPa) Polymer Blend (wt%) Polymer Blend (wt%) Core Sheath PLA 100 — — 220 — 1.1 0.56 ±0.2 — 28.8 PLA 100 — — 220 — 4.5 3.1 ±0.2 29 ±2 47.2 PHBV 59 PLA 41 165 195 3 1.2 ±0.1 30±3 37.6 PHBV 62 PLA 38 165 195 3 1.4 ±0.2 26 ±9 42.4 PHBV 66 PLA 34 165 195 3 1.0 ±0.2 31 ± 10 38.4 PHBV 69 PLA 31 165 195 3 1.1 ± 0.2 29 ±7 36 PHBV 35 PLA 65 170 190 3 2.0 ±0.2 38 ± 9 44 PHBV 36 PLA 64 170 190 3.5 2.6 ±0.4 28 ±5 56.8 PHBV 29 PLA 71 170 190 3.5 2.3 ±0.2 34 ±6 46.4 PHBV 22 PLA 78 170 190 3.5 2.3 ±0.2 41 ± 6 51.2 PLA 49 PHBV 51 185 175 1.5 0.6 ±0.08 125 ±29 23.2 PLA 100 PHBV — 185 — 6 3.3 ±0.4 13 ± 3 52.8 PLA 100 PHBV — 185 — 6 3.3 ±0.5 17±4 48.0 PHBV 27 PLA 73 175 185 5.5 2.4 ±0.2 23 ±2 42.4 PHBV 29 PLA 71 175 185 5.5 2.7 ±0.2 23 ±2 48.0

PHBV as the core had tensile strength of 2.7 g/den and modulus of up to 56.8 g/den. In vitro biocompatibility studies did not show any toxicity and cells grew along the length of the fibers. A decrease in fiber strength by about 33 % was observed 4 weeks after incubation [12Huf]. Blends of PHBV and PLA were prepared and extruded into fibers between 210 and 235 °C. Blend fibers containing 5 and 10 % PHBV were knitted into socks [11Piv] shown in Fig. 65.1. Increasing take-up speed improved the tensile properties and addition of PHBV above 10 % led to a decrease

in tensile strength. SEM images (Fig. 65.2) of the fracture surface showed two distinct regions suggesting that the blends were incompatible even with a low PHBV content of 5 %.

References

[01Yam] Yamane, H., Terao, K., Hiki, S., Kumura, Y.: Polymer 42, 3241 (2001)

[11Piv] Pivsa-Art, S., Srisawat, N., O-Charoen, N., Pavasupree, S., Pivsa-Art, W.: Energy Procedia 9, 589 (2011)

[12Huf] Hufenus, R., Reifler, F. A., Maniura-Weber, K., Spierings, A., Zinn, M.: Macromol. Mater. Eng. 297, 75 (2012)