Rheology of Melts and Solutions Part 1 – Viscosity and Its Measurement

By Clifford K. Schoff

You may wonder what an article on melts and solutions is doing in a paint and coatings journal. The solution as­pect is not difficult to explain because we often deal with resins in solution, and letdowns, varnishes, and clears es­sentially are solutions. Also, more and more paint components will be incor­porated as solutions as paint manufac­turing by dispensing metered amounts of solutions and dispersions into con­tainers becomes commonplace.

In addition, dilute solution viscosities are useful for characterizing polymers. Regarding melts, readers will know that powder coatings melt and flow on bak­ing, but they may be surprised to learn that many other coatings also behave like polymer melts. For example, high ­solids coatings show considerable flow even after all solvent is lost, particularly at elevated temperatures.

The analytical aspects of all this come with the techniques used to measure rheological behavior and the questions that the results should answer: Does the paint or component meet specifications? Does the paint have the correct rheologi­cal characteristics for the application?

Why is the paint experiencing flow prob­lems? Why is the appearance not up to standard? What are the characteristics of the polymers and/or oligomers in the resin?