Mass transport mechanism for the formation of latex-modified epoxy coatings by evaporation from aqueous dispersions

Abstract

The transport mechanism for the evaporation of dispersing liquid during the solidification of an epoxy dispersion that had been stabilized to prevent crack formation with a latex dispersion was studied. Aqueous dispersions consisting of an experimentally determined ratio of epoxy resin and nitrile latex were evaporated at 35°C. When the dispersion was evaporated under controlled conditions without forced air flow, a flexible and adherent polymer material formed. The mechanism for coalescence was related to the loss in weight of dispersing liquid during an initial zero order kinetics stage. This was followed by a rate-controlled Fick's law diffusion through the developing coating with subsequent evaporation to the atmosphere. Experimental measurements are compared with theoretical predictions. The rate constant for the zero order time frame is 0.086 ± 0.02 hr -1 . In the second time frame, Ficks's law evaporation rate constant is 0.046 ± 0.017 cm·hr -1 with a diffusion coefficient of 0.00092 ± 0.00051 cm 2 ·hr -1 at 35 ± 1°C and RH 35 ± 7%. Applications for evaporation kinetics are discussed.T he objective of this work is to examine the solution to Fick's law diffusion equation for the formation of polymer coatings by evaporation from water or other aqueous dispersions. Deposition of polymers from waterborne dispersions by dispersing liquid evaporation has application to coating substrates. 1-13 The overall deposition process consists of a number of steps including water transport from the dispersion to the atmosphere, 1-10 coalescing of the dispersed polymer, 4,6,8 evaporation of dispersing water 1,2,4 or dispersing liquid, and solid formation by particle movement within the coalescing coating. 4,6,[9][10][11] The mechanism for water transport is initially surface evaporation, followed by diffusion-controlled mass transport 1-6 through the coating. The initial evaporation is manifested by a linear increase in the amount of liquid evaporated over a relatively short time frame. This is followed by a nonlinear increase where the mass of water evaporated decreases for a longer time.Croll 1 studied heat and mass transfer during drying by measuring the weight loss of latex coatings versus time with a digital balance connected to a microcomputer. Evaporation was controlled at 22 ± 1°C and 50 ± 2% RH; air velocity was 1.8 m·sec -1 or ambient condition. The kinetic data were interpreted as a mechanism consisting of two drying stages: water loss due to evaporation from the surface, followed by additional water loss that is limited by transport of water through a coalescing coating. In a subsequent publication, Croll 2 described an initial stage with a short time frame when the flux is 0.85 of the flux of pure water. In the second stage, water evaporation is limited by the concentration in a wet reaction layer. A derived equation reproduced the drying curves. Adesanya, Nanda, and Beard 3 monitored the moisture weight by a computer system during drying of samples of yellow poplar wood with precise dimensions at temperatures ...