A Study on the Thermal Degradation Resistance of Thermoplastic Polyurethane Coatings

In this work, a comprehensive study of thermoplastic polyurethanes (TPUs) and the influence of the hard block content on their performance was carried out. The study comprises an extensive literature survey of the chemistry, structure and properties, and degradation and stability of TPUs, an experimental section dedicated to the materials, methodologies and instrumentations involved in the execution of the study, and a results and discussion section detailing all the findings and interpretation of the investigation. TPU elastomers were prepared from 4,4’-diphenyimethane diisocyanate (MDI), monoethylene glycol (MEG), triethylene diamine (Dabco EG), Coim G-211 poly(ester) diol using the ‘one shot’ melt polymerization process. The hard-segment content of polyurethane prepared in this study was in the range of 29-48 wt %. TPU samples were thermally aged in an air oven at 100°C for 7 and 14 days. The effect of the hard-segment concentration (HSC) as well as thermal aging duration on the polyurethane structure and abrasion resistance were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA). Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and a rotary drum abrader. Endothermic peaks at low temperatures (77-102°C) were identified in DSC for various samples and attributed to the distribution of hard segment crystallites with relatively short-range order. Single or multiple endothermic peaks were also observed at higher temperatures (150-181° C) and identified as the melting transition of hard segment crystallites with long-range order. The melting peak temperature was found to increase with the HSC. The thermal stability of TPU samples was investigated using TGA, which indicated that all samples started to decompose at temperatures well above 175°C in a distinct two-step degradation mechanism. The material loss after the first stage correlates well with the HSC. The onset temperatures of degradation also increased with the increase in HSC except for one case in which samples with 47.8% HSC started degradation at lower temperature than those with 41.5% HSC. The abrasion resistance of the various materials to scrubbing stresses was also found to increase with the increase in HSC, which confirmed the increase in the microphase separation with the increase in the HSC. Mesoscopic morphological investigations using SEM detected an alternating hard and soft segment lamellar structure. However, it was observed that thermal aging at 100°C for 14 days did not cause chain degradation or apparent chemical changes to the TPU samples. FTIR confirmed that no major alteration of the urethane link took place due to aging. Indeed, microphase separation has been affected due to the effect of thermal aging on the hydrogen bonding. Increasing aging duration was found to reduce phase separation and increase phase mixing of the hard and soft domains with more pronounced effect on the higher HSC samples. That was reflected in the broadening of the FTIR characteristic peak of urethane links at 1730-1710 cm-1 for the aged samples. The same was also confirmed by having an increased rate of lost volume in the abrasion test with the increase of the aging duration