Damodaran, Subin (2016)
Quantifying the effects of Processing and detaeriorating environments on polypropylene by infrared microscopy.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

Polypropylene (PP) is a highly versatile polyolefin, which is being used in a wide variety of applications. Depending on the final application, the chemical and morphological properties of PP have to be tailored. The current research work was performed with a particular focus towards the morphological changes in PP triggered by welding. Welding is a widely applied method for joining plastics which, however, generates a complex morphological polymer structure at the weld. The effect of processing on the morphology of PP was studied for welds between plates of PP homopolymer (PP-H). Due to the complex interplay of heating and cooling rates as well as the mechanical forces during welding, the PP chains rearrange themselves in different patterns and forms different zones in the weld, which were qualitatively analyzed by polarized light microscopy (PLM). Three distinct zones, which differ in their crystal structure and size, were observed in PLM, namely the injection molded plate with the original morphology, the weld seam and the weld core. The different cooling and heating rates cause the polymer to crystallize at different rates. Quantitative studies on these effects raised upon welding are a prerequisite to modulate the end use properties of a welded PP. Hence, the orientation of polymer chains was quantified by infrared microscopy (µFT-IR) across PP welds. Thus it could be shown that the polymer chains were oriented along machine direction (MD) at the plates and normal direction (ND) at the weld seams. One of the major applications of PP is the sector of pipes which are being used to transport various kinds of liquid media. Disinfection of water by strong oxidants due to health reasons is a routine process in the current situation. Yet, the transportation of this disinfected water can cause severe damages to PP. A detailed research on the deterioration rates and the mechanisms underlaying when PP is in contact with hot water and chlorinated water with different chlorine concentrations and at different temperatures was carried out. The pipes chosen for the durability studies were a random copolymer of propylene with ethylene (PP-R) nucleated with α- and β-nucleating agents (PP-R1 and PP-R2 respectively). The pipes were stabilized with 1,3,5-tris (3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-methylbenzene (AO-13), Pentaerythritol tetrakis (3-3,5-di-tert-butyl-4-hydroxyphenyl) propionate (AO-18) and the processing stabilizer tris(2,4-di-tert-butylphenyl) phosphite (PS-2). The characteristic ester carbonyl absorption in the infrared spectrum at 1740 cm-1 was used to quantify AO-18. However, the characteristic bands of AO-13 overlapped with the bands of AO-18 and PS-2. To overcome this problem a new method has been developed for µFT-IR to quantify AO-13 from a mixture with AO-18 and PS-2. The quantification results of AOs obtained from µFT-IR were well supported by Extraction→HPLC. PP is vulnerable to deterioration in the absence of stabilizers. Thus, monitoring the stabilizer loss is an alternative approach to determine the durability of polymers in reactive environments. Degradation/extraction of AOs is highly dependent on the morphology of the material. In that case, the extrusion of polymer melt into the form of hollow cylindrical pipes generates a distinct morphological profile across the pipe wall. Using µFT-IR it could be shown that the polymer chains in PP-R1 were orientated along the extrusion direction due to the fast crystallization of the injected melt triggered by α-nucleation. The diffusion of small molecules through these kinds of oriented networks is faster than randomly oriented ones. In PP-R2, the β- nucleation led to a random orientation of polymer chains at the inner surface which gave impermeability towards the small molecules. This difference in the penetration characteristics of PP-R1 and PP-R2 manifested in different deterioration rates of the pipes. The faster penetration of the media caused the formation of a higher amount of macroradicals in PP-R1. Hence a higher amount of AOs was required to stabilize these macroradicals to prevent the polymer chain scission. In order to understand the influence of temperature on the degradation of the polymer, ageing was carried out at two temperatures 95 and 110 °C. Out of these experiments, the degradative processes of the polymer, namely the loss of antioxidants and reduction in molar mass of the polymer, proceeded more rapidly at 110 °C. Experiments with different chlorine concentrations showed that deterioration was more pronounced at higher chlorine concentrations. The depletion of the two AOs, AO-18 and AO-13, differs in their rates. In detail it was found that AO-18 is lost faster to the inner medium than AO-13, irrespective of ageing condition and temperature. The degradation of AO-18 can be caused via two mechanisms: The hydrolysis of the ester bond and via the intended donation of phenolic hydrogen to the macroradicals. On the contrary AO-13 is susceptible towards degradation solely by the quenching of macroradicals. Therefore, an accelerated degradation of AO-18 was observed irrespective of the ageing temperature.

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