Paints are generally a blend of pigments, solvents/water, resins, and various additives to give the properties we require. However, the quality of the paint can be significantly compromised by relatively small changes to the composition and it is therefore important to understand the interactions of the additives.
Challenge
PRA were approached a client, who required a scientific understanding relating to the potential impact of styrene-acrylic acid-butyl acrylate copolymer’s molecular weight, on waterborne coating properties. The PRA received a small-scale (<50 g) formula from a third party, but further optimisation from the PRA scientists was needed as they had more experience in emulsion polymerisation and scale-up.
Our Approach
A specification (essential and preferred characteristics) for the resulting copolymer and process was agreed with the customer. The supplied process was reviewed against industry standard practise, improvements suggested, and a series of small scale experiments devised to test the processes. Following completion of the small scale trials, a series of latexes was prepared using a “seed and feed” process on the 500 g scale, to the specified copolymer composition. To achieve a range of molecular weights, 2 approaches were used: i) variation of initiator concentration, ii) use of a chain transfer agent (CTA) at different concentrations. A testing schedule was devised as; • Polymer molecular weight distribution by Gel Permeation Chromatography (GPC) • Particle size distribution by laser light scattering (Mastersizer) • Glass transition temperature (Tg) by Differential Scanning Calorimetry (DSC) • Final conversion of monomer to polymer by gravimetry
Results
Molecular weight was expected to be related to initiator concentration and CTA concentration: an increase in initiator concentration or CTA concentration for the same amount of monomer should lead to a decrease in molecular weight in both cases. This trend was observed, with a range of molecular weights from 31600 to 383300 measured for the resulting polymers (300 to 3500 repeat units per polymer molecule). Particle size analysis showed the molecular weight distribution was similar for all batches, indicating that this was predominantly independent of the polymer molecular weight. As expected, Tg (polymer softening temperature) was found to reduce as molecular weight reduces. The impact of these polymer property changes is expected to be improved film formation, but poorer stain resistance and durability as the polymer molecular weight decreases. The client was able to use PRA’s expertise to develop their understanding of coating structure-property relationships and further their internal R&D programmes.
