Analysis of Dispersed Particle Structure Formed from Resins of Anionic Electropaint by Acid-Base Titration

Abstract

An electropainting film is formed by the chemical reaction of dispersed particles with protons (or hydroxide ions) which are generated by water electrolysis. It is important to analyze a structure of dispersed particles in polymer resin solution, in order to understand the film formation. In the present paper, we have analyzed three suspension solutions from the material balance by using ultrafiltration (UF) technique.
We found that pH of the suspension is determined by the molar concentration ratio of carboxyl groups oriented to water phase to 2-dimethylaminoethanol (DMAE) molecules. Actual neutralization degree for the dispersed particles (α₀) could be easily estimated from pH of the suspension by using a pH curve of simple DMAE-MAA (methacrylic acid) solution. All the suspension systems resulted in α₀>1. Consequently, the carboxyl groups oriented to water phase on the particle surface are ionized and dissociated completely, and hence the particles form ion pairs with conjugated acid of DMAE. In addition, because free DMAE molecules exist excessively in the vicinity of the dispersed particles, they govern conductivity and pH. Dissociation state of free DMAE molecules changes buffer capacity at an electrode interface. When α₀ becomes greater value, buffer capacity increases. In this case, existence of free DMAE molecules affects deposited weight.