The Challenge
A PRA client wanted to understand the relationship between polymer molecular weight and thermal properties for a series of styrene-acrylic copolymers prepared by free radical emulsion polymerisation. Various aspects of the project, such as copolymer composition and solids content, were specified by the client, however they were seeking advice from PRA on industry best-practice for the polymerisation methodology – our “industry know-how”. One of the requirements was that the copolymer composition should be consistent throughout the radius of the latex particles. This can be a challenge, as some monomers react faster than others – leading to poor uniformity.
PRA’s Approach
A range of process types were tested, and we settled on a “seed and feed” method, where a pre-emulsion is prepared using a small portion of the monomer mixture and reacted to form an initial “seed” particle. A slow and continuous “feed” of the remaining monomer mixture then takes place. This approach ensures the monomer composition can be controlled throughout the reaction, and results in a good particle size distribution for coatings.
6 latexes were prepared on a half-litre scale, using 2 approaches to control the molecular weight of the polymer within the particles: i) variation of initiator concentration or ii) inclusion of a chain transfer agent (CTA) in different concentrations.
These latexes then underwent testing for the following properties:
- Particle size distribution – this affects coating rheology and surface appearance of paint. Ideally it would stay as consistent as possible.
- Glass transition temperature (Tg) – a change from brittle/glassy properties to rubbery properties occurs at this characteristic temperature. It is a crucial parameter to ensure that latex particles coalesce during drying of a coating to form strong films on a surface.
- Polymer molecular weight – this affects film forming, and coating durability.
The Outcome
The molecular weight could be effectively controlled by both adjusting either the initiator or the CTA concentrations. The particle size distribution was not strongly affected by these changes, except for the lowest initiator concentration. The thermal analysis of the polymers fit the expected trend of lower Tg at lower molecular weight.
We supplied our client with the latexes to test in their own applications, and they were able to use these samples and data to better understand their existing formulations and provide a platform for future developments.
Polymer latexes with systematic variation of chemical/physical structure are not always available from commercial sources. Contact [email protected] for more information on how PRA can help you with latex synthesis and testing to increase your knowledge of how such changes can impact your products.
