Poly (ethylene terephthalate) thermo-mechanical and thermo-oxidative degradation mechanisms Wanderson Roma ˜o a , Marcos F. Franco a , Yuri E. Corilo b , Marcos N. Eberlin b , Ma ´ rcia A.S. Spinace ´ a , Marco-A. De Paoli a, * a Laborato ´rio de Polı ´meros e Reciclagem, Instituto de Quı ´mica, Universidade Estadual de Campinas, Caixa Postal 6154,13084-971 Campinas, SP, Brazil b Laborato ´rio ThoMSom de Espectrometria de Massas, Instituto de Quı ´mica, Universidade Estadual de Campinas, 6154,13084-971 Campinas, SP, Brazil article info Article history: Received 26 December 2008 Received in revised form 16 April 2009 Accepted 22 May 2009 Available online 30 May 2009 Keywords: Poly(ethylene terephthalate) Thermo-oxidative degradation Thermo-mechanical degradation Diethylene glycol 1 H NMR MALDI-TOF MS abstract 1 H NMR and MALDI-TOF MS measurements were used to study the thermo-mechanical and thermo- oxidative degradation mechanisms of bottle-grade PET (btg-PET). In the thermo-oxidative degradation, the concentration of low molar mass compounds increased with time and the main products were cyclic and linear di-acid oligomers. In the thermo-mechanical degradation, the main-chain scission reactions affect the stability of the cyclic oligomers. One of the most important bottle-grade PET co-monomers is diethylene glycol (DEG), which is a ‘‘reactive site’’ in the thermal degradation of btg-PET. The DEG co-monomer was shown to be the precursor to colour changes in btg-PET, owing to the attack by molecular oxygen on the methylenic protons adjacent to the ether oxygen atoms of DEG. This behaviour was observed in the thermo- oxidative degradation process in which the degradation of DEG causes the release of hydroxyl radicals in the polymeric matrix, thereby producing mono- and di-hydroxyl substituted species. This was also observed in the thermo-mechanical degradation process. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Owing to its extensive use over the past two decades, poly(- ethylene terephthalate), PET, is considered to be one of the most important polymers. Its excellent properties, such as: tensile and impact strength, chemical resistance, clarity, processability, colour- fastness and reasonable thermal stability [1]; promoted its use in a variety of applications. The global production of PET at the end of the 1990s was ca. 24 million tons used mainly for production of textile fibres (67%), followed by blow-moulded bottles (24%), engi- neering polymers (9%) and biaxially oriented films [2]. A variety of co-monomers are used to tailor the PET polymer properties for specific applications. For example, bottle-grade PET (PET-btg) is produced using a low concentration of co-monomers to decrease the thermal crystallization rate and improve ductility, processability and clarity. Some of the more common co-mono- mers used are: diethylene glycol (DEG), isophthalic acid (IA) and cyclohexanedimethylene glycol [3]. DEG is the main co-monomer used in PET-btg production and its ether groups provide more flexibility to the relatively stiff PET backbone, thereby slowing down its thermal crystallization rate. This type of copolymerization reduces the hazing observed upon cooling of blow-moulded objects such as PET bottles [4–7]. These co-monomers can also be formed as a by-product during processing. The DEG is a by-product of PET manufacture and, depending upon the polymerization conditions, the concentration of DEG in the final polymer changes from 1 to 4 mol% [6–11]. The concentration of DEG has important consequences for both chemical and barrier properties of the PET polymer. The barrier properties are very important for PET bottles. Hartwig [12] and Hiltner et al. [13,14] reported that gas permeability decreases with the increase in the crystallinity degree of btg-PET owing to the lack of free volume in the crystalline structure. Crystallinity is not a desirable property during the processing of the PET bottle. To produce pre-forms with the desired clarity, the thermal crystallization rate during the injection stage should be minimized. DEG units are known to be reactive sites along the PET chain, leading to the thermal and thermo-oxidative degradation of PET. Studies [2,15] showed that the thermo-oxidative degradation occurs preferentially at the ether link of DEG, leading to a possible pathway to colour formation in PET. The ether links are first affected by the thermal oxidation, forming hydroxyl radicals (  OH). These radicals react with the aromatic groups yielding di-hydroxyl * Corresponding author. Tel.: þ55 19 3521 3075; fax: þ55 19 3521 3023. E-mail address: mdepaoli@iqm.unicamp.br (M.-A. De Paoli). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2009.05.017 Polymer Degradation and Stability 94 (2009) 1849–1859