Oct. 5, 2011 Chemical engineer Ms Inês Mariz has presented guidelines for obtaining better quality and more efficient latex, in a PhD thesis defended at the University of the Basque Country (UPV/EHU). This involves a strategy which facilitates obtaining a more concentrated material without it losing its handling properties. In this way it is able to cover the substrate in question with fewer layers than is usual, it dries quicker, its formulation is more flexible and enables savings in storage and in transport.
The thesis is entitled High solids content low viscosity latexes with small particle size. A number of articles have also been published based on the research, including the one in Polymer Journal.
The research by Ms Mariz is based on the synthesis of latex with a high solid content. Latex is an aqueous emulsion and so, with this synthesis, the idea is to concentrate the greatest percentage of solid material possible within this emulsion, in order to obtain a more efficient and better quality material. Nevertheless, this concentration has to be limited as, otherwise, the latex will be too viscous and difficult to handle. Viscosity can be reduced by increasing the size of the particles used for the synthesis, but previous studies show that the largest ones are not those most propitious for forming films. As a consequence, the objective of this thesis was to set out a strategy for producing latex with high solid content and low viscosity, but with particles of a size less than 350 nanometres.
Unimodal and bimodal
Ms Mariz undertook several trials before obtaining the desired particle size distribution (PSD); i.e. that which maximises the packing (the compacted organisation) of the particles, always respecting the determined range of sizes.
In the first place, this mentioned synthesis with unimodal PSD (all particles of equal size) was investigated. Provoking polymerisation in a semi-continuous reactor, acrylic latexes were obtained which, with particles less than 350 nanometres, had a 61 wt% (concentration index) of solids content and with a reasonable viscosity. It was also verified that this type of latex stays stable with an ionic surfactant concentration (substance that facilitates and stabilises emulsions) less than 1 wt% with respect to the compound.
Subsequently, the synthesis of latex with high solids content with bimodal PSDs was tackled, i.e. those with particles of different sizes. In this way, the smallest particles cover spaces that are left between the largest particles, thus increasing the solids content and maximising packing.
The synthesis of unimodal latex is easier because the growth of the particles is known a priori. But, as regards bimodal particles, Ms Mariz had to design a strategy that would make this equally predictable. Thanks to this strategy, it was known a priori what polymerisation reaction (initiated in semi-continuous reactor) formula was required for obtaining the desired result: that is, a bimodal latex which, with particle sizes less than 350 nanometres, has high solid content (up to 70 wt%) and the lowest possible viscosity.
Finally, water-based paints with latex of different PSDs have been formulated and it has been shown that particles sized less than 350 nanometres produce better results. Concretely, the latex with high solids content and small particle size has given rise to paints with enhanced properties, such as higher brilliance and elasticity and less drying time. They also show a low content in volatile organic components, given that a greater concentration of solids in the aqueous emulsion of the latex means, at the same time, a lower content of solvents.
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