SYNOPSISPolyester melts exhibit two discrete degradation rates depending on the moisture content in the solid state. The initial fast rate is very sensitive to the moisture content while the later slow rate is not sensitive to the moisture content. The former is attributed to hydrolysis and the latter is attributed to thermo-oxidative degradation. A combined kinetic equation which represents both hydrolysis and thermo-oxidative degration of the polyester melt is proposed. The model equation agrees well with the experimental data. A method to determine hydrolytic and thermal degradation rate constants, moisture content, molecular weight for polyesters by means of a melt viscometry is discussed.

The molecular dimensions and melt rheology of a thermotropic all-aromatic liquid crystalline polyester (TLCP) composed of p-hydroxy benzoic acid, hydroquinone, terephthalic acid, and 2,4-naphthalenedicarboxylic acid is examined. The Mark-Houwink exponent (␣) of 0.95 is estimated for the TLCP. The persistence length estimated from molecular weight (M) and intrinsic viscosity ([]) data using the Bohdanecky-Bushin equation is about 95 Å, whereas that estimated from light scattering data is 117 Å. These persistence lengths and the observed ␣ value, both higher than those for flexible polymers, suggest that the present TLCP is a semirigid polymer. The zero shear melt viscosity ( 0 ) varies with approximately M 6 for molecular weight M Ͼ 3 ϫ 10 4 g/mol; below this molecular weight, 0 varies almost linearly with M. Widely different entanglement molecular weights (M e ) are predicted, depending on the method used; the plateau modulus estimates M e of about 8 ϫ 10 5 g/mol, whereas the ratio of mean square end-to-end distance and molecular weight (͗R 2 ͘ 0 /M) predicts M e 's either too small (0.33 g/mol) or too large (2.5 ϫ 10 6 g/mol), depending on the theory used. Although the change in the molecular weight dependency of melt viscosity appears to be associated with the onset of entanglement coupling of the semirigid molecules, its origin needs further investigation.

The viscous response of a coating formulation changes over several orders of magnitude after application onto a substrate. Furthermore, the performance and application attributes for a refinish basecoat formulation are slightly different from those of an OEM basecoat. The absence of a high temperature baking step during the refinish process necessitates that the paint exhibit very good antisag behavior as well as quick 'dry to touch' without compromising the flow and leveling properties of the coating as it dries. As most paint formulations are multicomponent in nature, it is always a challenge to precisely measure the impact of specific formulation components on the collective performance attributes of the applied coating. In this work, the focus has been to develop novel techniques that can be used to demonstrate a more quantitative measure of some of the performance attributes that rheological additives like cellulose esters can provide to automotive coatings. A series of pigmented basecoat formulations were prepared which contained cellulose acetate butyrates (CABs) of varying molecular weights at different levels based on the total solids of the coating formulation. The nonvolatile content of the formulations was also varied. The viscoelastic behavior of a typical automotive basecoat formulation during the drying process was then investigated using a novel rheological technique. Complex viscosity data (including storage and loss moduli as well as tan delta) were determined as a function of drying time and then compared to the macroscopic properties typically associated with a coating film as it dries. Thermogravimetric analysis (TGA) was also used to correlate the rheology of the metallic basecoat formulations with drying behavior of the coating. The final appearance of the coating was investigated by several microscopic techniques such as Laser Scanning Confocal Microscopy, Scanning Electron Microscopy, Atomic Force Microscopy, and Surface Profilometery, and attempts were made to correlate bulk measurements like 'flop index' with the microstructure of the coating.