The discoloration of poly(ethylene terephthalate) (PET) due to thermooxidative degradation is one of the most critical problems that occur during melt processing of the polymer and reprocessing by recycling. This phenomenon adversely affects the properties and the end use of the final product making it only suitable for low grade materials. Although the degradations of PET have been extensively investigated, surprisingly fewer studies were dedicated solely to identify the mechanism of color formation, and there is no complete understanding of the pathways as to how the colored species are formed during melt processing. Therefore, further studies are crucial to understand the cause of discoloration and characterize the discoloration process. / To prevent such undesirable degradations during melt processing and reprocessing by recycling, various stabilizers, primarily antioxidants are used. However, such stabilizers have a series of disadvantages limiting their inherent chemical efficiency. When stabilizers remain as a physical mixture, they can be depleted over time as a result of their stabilization action. Other ways of losing stabilizers are the environmental conditions in which the polymer is used. Stabilizers can be physically lost from the polymer by evaporation and leaching hence, their concentration in the polymer decreases with time, leaving it susceptible to degradation. Thus there is a distinct need to develop a new type of stabilizer, which can remain in the polymer in a bound form for the life time of the product. / In this work, research was dedicated to two aspects: (i) in-depth identification of the cause and mechanism of PET discoloration during melt processing, (ii) undertaking novel approach to eliminate the discoloration and degradation problem. / The thesis is divided into eight chapters. A brief description of PET preparation, processing and recycling relevant to this work is presented in Chapter 1. A background literature review on PET degradations (techniques used to investigate such degradations and proposed mechanisms) and stabilization has also been presented, followed by the rationale, aim and scope of this present work. / Chapter 2, details the materials and methods used in this work. A description of the analytical techniques used to investigate the discoloration of PET and efficiency of stabilizers is also given. / Chapter 3 has been dedicated to PET discoloration investigation. Discoloration has been reproduced using laboratory conditions by subjecting the polymer to controlled thermooxidative degradation at 280C in air. The physico-chemical changes on the PET surface and bulk material during melt processing in air have been investigated using various spectroscopic techniques. In addition, chemical derivatization with trifluoroacetic anhydride (TFAA) was used to label the hydroxyl groups introduced on the polymer surface by thermal oxidation. From these studies it was evident that colour formation starts initially with the hydroxylation of the terephthalic ring. Further, the formation of additional carbonyl functionalities and conjugated chromophoric systems complete the colour formation process. / Chapter 4 presents the use of some new spectroscopic techniques: Matrix-Assisted Laser Desorption/Ionization Mass Mpectrometry (MALDI-MS) and Laser Desorption/Ionization on Porous Silicon-Mass Spectrometry (DIOS-MS) to obtain detailed information about molecular weight, end groups and structure of the degraded products from the thermooxidative degradation of PET; and to gain a better understanding of the mechanism of such degradation and discoloration. The MALDI-MS spectra of the degraded polymer showed the formation of COOH end group oligomers via chain scission at the ether link present in PET. In addition, a variety of cyclic oligomers were found to form via two different mechanisms from the linear precursor of the virgin PET. Furthermore, MALDI-MS study directly on the TLC plate and DIOS-MS, enabled to identify low molecular weight compounds (not detected previously due to matrix interference) that are contributors to colour formation. / Chapter 5 reports on the novel approach for discoloration and degradation control of PET by using a nanostructured organometallic macromer trisilanol isobutyl POSS (T-POSS). Incorporating such POSS containing reactive groups into PET can lead to reactive functionalization of the polymer. The resultant material was investigated using thermal analysis and oscillatory rheology. The interaction between PET and the nanostructured additive was investigated by XPS and MALDI-MS. Thermal studies show that the additive increases the thermooxidative stability and consequently prevents discoloration of the material. Shear storage modulus and dynamic viscosity of the sample also increased, indicating better melt elasticity and easier procesability of the material. The XPS and MALDI results confirmed that the molecular level stabilization is achieved by covalent interaction of the additive and PET. / Chapter 6 details another type of stabilizer, based on epoxy-phosphites that have been developed. Their synthesis and performance in PET has also been discussed in detail. From the thermo-rheological studies it was found that the new epoxy-phosphites improved the procesability characteristics and thermal properties of the material due to reactive functionalization. Furthermore, the spectroscopic investigation showed the inhibition of terephthalate ring hydroxylation and formation of additional carbonyl functionalities pointing to enhanced discoloration stability of the material. / In Chapter 7 the effect of the new stabilizers on the thermooxidative stability of PET was further investigated using isothermal and non isothermal kinetic analysis to evaluate their performance at different temperatures and heating rates. Kinetic parameters of the degradation in presence of stabilizers were evaluated and also life time predicted. It was concluded that the polymer samples decompose via an autocatalytic mechanism in presence of oxygen. However, the degradation of the material is delayed by the addition of stabilizers, which is suggested by the higher values of the activation energies and pre-exponential factors obtained for the stabilized PET. / Thesis (PhDAppliedScience)--University of South Australia, 2006.
| Identifer | oai:union.ndltd.org:ADTP/267172 |
| Date | January 2006 |
| Creators | Ladasiu Ciolacu, Flaviu C. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | copyright under review |
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