Film Formation from Dispersions - Preparation and Mechanisms

Improved environmental consciousness and harsh legislation concerning volatile organic compounds (VOCs) during the 1980s led to an increase in the development in the field of polymer colloids. Solvent born coatings were replaced by polymer dispersions in aqueous solution, so called latex dispersions. As a consequence, numerous research programs concerning polymerization processes (e.g. emulsion polymerization), monomer systems, stabilization systems, drying properties and final properties of the solid polymer were started. The development of latex dispersions is still in progress. The main focus in this thesis was to study the drying process, i.e. the film formation, of an emulsion polymerized methyl methacrylate-co-methyl acrylate (MMA/MA) dispersion, containing varying concentrations of sodium dodecylsulphate (SDS) and sodium persulfate (NaPS). Additional studies were also performed with the purpose to develop a new polymerization process, wherein the polymer particles were only stabilized by hydroxyethyl cellulose. The drying process was analyzed using a sorption balance (SB) by continuously recording the water loss rate for small samples (approximately 10 mg) applied as droplets on glass surfaces. The average evaporation rate ( ) was controlled by varying the relative humidity and temperature. The convex shape geometry of the samples led to a horizontal flux of water and particles towards the sample edges. It was found that, at the initial stage of the drying process, there existed a linear dependence of the global mass transfer coefficient and the exposed surface area of the samples. The outlined method facilitated the scaling and the comparison of results obtained from drying experiments with different sized dispersion samples. For all samples a shift in occurred at a critical volume fraction of polymer (?pc), at which the rate decreased significantly. It was found that ?pc was a function of , the particle stabilization and the temperature, and that the later affected the viscoelasticity and the water permeability of the polymer. Three types of mechanisms where found when the temperature was changed: 1) For samples dried at temperatures slightly above the polymer glass transition temperature (T Tg + 30 K, skinning occurred at even lower solids contents. However, the water vapor permeability for the formed film displayed an increase with temperature and the transition in became diffuse leading to decreasing with an increasing film thickness. Under constant drying conditions ?pc could also vary due to differing concentrations of sodium dodecylsulphate (SDS) and sodium persulfate (NaPS). These variations were attributed to particle stabilization in the dispersions. The increased stabilization that was obtained by the addition of surfactants to dialyzed samples prevented skin formation even at high evaporation rates and thus the ?pc values were high for SDS-containing samples. Further it was found that the drying profile of the samples differed with and with the particle stabilization. For dialyzed samples, at high , the particles accumulated in the vicinity of the edges of the droplet and formed a donut-like solid latex film, while samples containing NaPS formed cone-shaped latex films and samples dried at low or containing SDS formed flat latex films. Hydroxyethyl cellulose (HEC) is a renewable resource that, when it has an appropriate molecular weight, can be used as a reactive colloidal stabilizer. The degradation of hydroxyethyl cellulose (HEC) using sodium persulfate (NaPS) as a free radical generator was studied at 60 °C, 70 °C and 80 °C with varying NaPS/HEC ratios. It was found that the NaPS/HEC ratio must be kept low in order to maintain the control of the degradation process, since a too high NaPS/HEC ratio would lead to the degradation rate of HEC being too fast, and the molecular weight distribution being bimodal. Additionally, the decomposition rate of NaPS was found to be independent of pH in the range between pH 2 and 7