Steric and anchimeric effects on the hydrolysis of oligoesters and their influence on end-use polyurethane coatings

Polyesters are used in a wide range of industries due to the ease of handling, breadth of formulation, good balance of end properties, and cost. However, the performance of polyesters is affected by the sensitivity of ester groups towards water. Based on this setback, the objective of this dissertation was the study of the different parameters affecting the hydrolysis of polyesters. Model oligoester compounds synthesized with different diacids and diols were used to analyze the influence of the chemical structure. The findings of this analysis were further applied to the hydrolysis of polyester-urethane (PU) films. One of the main objectives was to prove that the hydrolysis of the soft-segment of polyester-urethane films was reproduced by model compounds. Additionally, in order to observe the reproducibility of the degradation mechanism in an outdoor setting, weathering studies were also performed on different PU films.;This research began with a hydrolysis study of model oligoester compounds. The oligoesters were end-capped with phenyl isocyanate in order to eliminate end-group effects and to mimic the chemical structure of the soft-segment of PU coatings. The study focused on two key effects: the steric and the anchimeric. Different oligoesters containing two (binary), three (ternary) and four (quaternary) different monomers were used for this study. The monomers included a combination of diacids (adipic acid; 1,4-cyclohexanedicarboxylic acid; isophthalic acid; terephthalic acid; and phthalic acid) and polyols (1,2-ethylene glycol; 1,3-propenediol; 1,4-butanediol; 1,5-pentanediol; 1,6-hexanediol; neopentyl glycol; 1,4-cyclohexanedimethanol; 2-methyl-1,3-propanediol; and trimethylolpropane). The binary systems were synthesized using one diacid and one diol. These systems allowed the control of the chemical structure with different steric and anchimeric effects. Hydrolysis rate constants were obtained from titration measurements. It was observed that the hydrolysis of oligoesters based on 1,4-cyclohexanedicarboxylic acid was influenced by steric effects. On the other hand, the lack of steric hindrance and the flexibility of the chain were key factors triggering the hydrolysis of adipic acid-based oligoesters.;Thermodynamic studies were performed in an attempt to verify the relationship between steric and anchimeric effects in the initial stage of the hydrolysis of model compounds. In the case of closely related reactions, plots of enthalpy (DeltaH‡) and entropy of activation (DeltaS ‡) can prove that the reactions undergo the same reaction mechanism. This relationship is known as the compensation effect or the isokinetic relationship. This study showed that the hydrolysis of adipic acid-based oligoesters was influenced by anchimeric effects during the first stage of degradation. On the other hand, the beginning of the hydrolysis of sterically hindered compounds containing neopentyl glycol was influenced by steric effects.;After establishing the reaction mechanism of binary systems, the hydrolytic stability of different copolyesters was evaluated. The oligoesters were prepared from adipic acid (AA) and isophthalic acid (IPA), with different polyols. The experiments included the investigation of phenyl isocyanate end-capped oligoesters and the resulting polyurethane films. The presence of IPA had a direct impact on the hydrolytic stability of the oligoesters due to the disruption of intra- and intermolecular catalysis. Ponderal analysis demonstrated that the hydrolytic stability of oligoesters is a non-additive property and revealed the importance of the ester block over the chain composition.;Accelerated weathering studies were performed on PU films in order to correlate hydrolytic stability to the degradation of coating properties on outdoor conditions. Coating properties were measured at different intervals during the accelerated weathering test. Similarly to the hydrolysis of end-capped oligoesters, polyurethane films containing AA-binary oligoesters showed poor weathering performance. On the other hand, binary IPA-oligoester systems showed the best hydrolytic stability of all the oligoester systems. Weathering of PU films caused not only the degradation of the ester groups (through hydrolysis) but also the degradation of urethane groups, through photo-oxidation.;Overall, three different scenarios of degradation were proposed. The first scenario of hydrolysis was a function of the steric hindrance. Build up of hydroxyl functionalities gave rise to the second scenario of hydrolysis. Intramolecular catalysis was the predominant mechanism of degradation. Subsequent degradation of the molecule leaded to the hydrolysis of small molecules (3rd scenario). The hydrolysis of PU films occurred through end-group scission, emphasizing the importance of the ester block over the oligoester composition. The presence of flexible ester blocks with low steric hindrance was the main cause of hydrolytic degradation. A good correlation between hydrolysis of oligoesters and PU films confirmed the initial assumption that end-capped oligoesters mimic the hydrolytic stability of the soft segment of polyester-urethanes.