Kothapalli V. S. N. Raju

Temperature dependence of viscosity of polyurethane polyol solutions: Application of rheological models†

Polyurethane polyols were synthesized by reacting the diols with polyisocyanates. Viscosity of their 50% (w/w) solutions in various solvents has been determined at different temperatures by using Haake Roto Visco RV12 Rotational Viscometer. The temperature dependence of viscosity data was solved by Levenberg Marquardts algorithm by using nonlinear regression models based on WLF, Vogel, and Arrhenius equations. The Tg values obtained from WLF and Vogel equation are comparable to each other and these equations can be satisfactorily used for the analysis of temperature dependence of viscosity data of oligomer solutions. The residuals are random and the absolute average percentage error in analyzing the viscosity data by these equations is minimum. The values of constants in WLF equation are found to be system dependent and adjustable parameters. The predicted In η values obtained from WLF and Vogel equations fit well with the plots of experimental In η values as a function of temperature.

Scratch hardness of paint coatings: effect of ageing and film thickness

Abstract The scratch hardness of organic coatings is greatly influenced by factors such as the nature of the metal substrate, the thickness of the coating and the temperature and humidity of the surroundings. Little information is available in the literature about the effect of these factors on the scratch hardness and how this property is affected during the course of weathering. Taking these aspects into consideration, 10 different commercial paints and six of their combinations (primer + topcoat) were selected for study. Tests were conducted using a power-driven automatic Erichsen scratch hardness tester, on paint coatings air-dried for 48 h and then aged in the laboratory for various periods over a span of 300 d. The information obtained from these tests was quantitative and has been used in assessing the performance of the paint coatings. It is found that some coatings which showed a fairly low scratch hardness during initial testing attained a significantly greater hardness during the course of ageing. The thickness of the coating had no significant influence on the hardness during the early periods of air-drying. However, with aged coatings, scratch hardness was observed to increase significantly with the thickness of the coating. For comparative assessments of the performance of paint coatings, scratch hardness data obtained after 48 h air-drying could not be taken as final. For a better assessment of comparative performance, it is preferable that this test be conducted on coatings aged for a period of 60–90 d.