Abstract
Water-borne and emulsifier-free coatings have attracted academic and industrial interests because they are environmentally-friendly. The water barrier properties are being developed, but it is hypothesized that removal of surfactant will improve the barrier properties. In this work, water transport and sorption in acrylate copolymer coatings deposited from colloidal dispersions (emulsion polymers and secondary emulsion polymers) and from solution in an organic solvent were investigated in order to compare the water transport and sorption for the different materials and to identify the important factors, including the presence of boundaries between particles and the presence of surfactants. Solution polymer films were found to have no particle boundaries and to be more homogenous in structure compared to the other types of films. They sorbed the least amount of water from both vapour and liquid. Secondary emulsion polymer films sorbed less amount of water than the emulsion polymer films of similar chemical composition. It was found that the increase of sorbed water content in the emulsion films can be ascribed to the hydrophilic nature of the surfactants that stay at the particle boundaries, enabling the water transport. Hence, after being dialyzed, the emulsion polymer films were found to adsorb more water, probably due to poor film formation (particle coalescence). It was concluded that dissolving the particle boundaries in emulsion polymer films (by casting from organic solvent) reduced the uptake of water. We used the technique of NMR relaxometry to find that there were three 1H peaks in the different regions of the T2 spectra for films after soaking in water. These three peaks correspond to different environments: molecularly dissolved water (and mobile polymer); interfacial water; and water in voids or channels. It was found that the water environment and the extent of confinement correlate with the optical transmission through the film. A greater volume fraction of scatterers (mobile water in voids or pockets) and larger scatterers led to a loss of the optical transparency that is usually not desired in coatings applications. The water vapour diffusion through all of the films was Fickian, as indicated by a sufficient regression to the experimental data over a series of water activities ranging from 0 to 0.9. In all cases, the diffusion coefficients (D) decreased with increasing water activities. The D seemed to have marginal differences between the various samples, indicating the removal of boundaries has limited effects. D is more strongly dependent on the water activities, and hence also on the sorbed water content. Without surfactants, a smaller diffusion coefficient was found for the secondary emulsion film. This result means that the presence of surfactant speeds the diffusion of water vapour molecules. Comparing the water uptake results, the equilibrium sorption of water vapour (100 % RH) and liquid water show only small differences. We also used Rutherford backscattering spectrometry (RBS) to investigate the transport of ions through various films. The diffusion of ions into the emulsion and solution films can be described using the theory of Fickian dynamics. The solvent-borne films had a slow diffusion of ions, and the surface concentration was lower. A higher atomic surface concentration was observed in the emulsion film, possibly due to an ion exchange with the counter-ions that balance the negative charges (e.g. SO3-, NH4+) of surfactants in situ.