An Investigation Into the Thermal Degradation of Poly(3-Hydroxy Butyric Acid)

Poly(hydroxybutyrate), or 'Biopol' as it is commercially known, is a naturally occurring, biodegradable thermoplastic, produced by bacterial culture. It can be processed and used in similar ways to conventional thermoplastics, offering the same versatility and convenience, whilst also being much more 'environmentally friendly'. Thus, as a marketable product it has sparked great interest, but to become a commercial success a number of intrinsic difficulties have to be overcome. Although workable, pure PHB has relatively poor mechanical properties with respect to other thermoplastics, and it ages, worsening this situation. It is also difficult to work with because it decomposes at its melt temperature, again depressing its physical properties. The aim of this project was to explore the possibility of using additives to improve the thermal stability of poly(hydroxybutyrate) to enable easier processing of the material. To enable this, new techniques were developed to examine the kinetics and mechanism of degradation, along with methods for determining the effect of additives. By screening a large number of additives, a number of compounds have been identified that demonstrate a stabilising effect, reducing the rate of degradation. The new methods have included quantitative and kinetic measurements by FT-IR upon degrading systems by analysing either the molten polymer, using a heated mirror, or examining the volatile gases in a heated gas cell. A number of modifications to the TVA (thermal volatilisation analysis) apparatus were explored. Internal thermocouples have been shown to be able to detect cold ring formation, and a thermal volatilisation mass spectrometry system (TVMS) was constructed and used to great effect. A technique to quantitatively determine the degradation products from PHB by gas chromatography, was devised, successfully identifying PHB chain fragments up to six monomer units long. This operated by chemically forming derivatives of these degradation products The effectiveness of the additives was examined in a number of ways. Many techniques were discarded, either being too unwieldy or of poor reproducibility. It turned out that weight loss measurements were the most straightforward and reliable, and proved to be effective for rapidly assessing additives. Kinetic measurements were also made using this technique. Molecular weight determinations were performed on degraded samples prepared using the same apparatus, allowing further comparisons and kinetic measurements to be made. A system was designed that made it possible to record IR spectra of a degrading film of the polymer, in real time, that was being heated on a reflective surface. Loss of material from the polymer and changes in its chemical nature could be observed easily, and allowed further comparison of additives. In summary, a number of new and novel techniques have been successfully developed to assess the kinetics and mechanism of degradation, and a number of additives have been identified that demonstrate stabilising effects