Recent progress in reactive surfactants in emulsion polymerisation

Abstract

In the synthesis of latexes for use in waterborne coatings, the benefits of using reactive surfactants are now well known. Improvements are obtained in the stability of the latexes, due to the fact that they are not desorbed from the particle surface. The film properties are also better, chiefly if the films are exposed to humidity, where the water rebound is decreased significantly. This lecture summarises some recent progress obtained in that field through a European programme, including the participation of 10 academic and industrial laboratories. Four topics are considered. The first one is dealt with a series of anionic surmers (polymerizable surfactants) prepared upon reacting a polymerisable alcohol with either maleic, or succinic, or sulfosuccinic anhydride. One of these products, resulting from the addition of Hydroxyethylmethacrylate on maleic anhydride is now available commercially, From maleic anhydride one get bifunctional surfmers with 2 polymerisable groups, while the two other anhydride lead to monofunctional surfmers. All these products are engaged in seeded core-shell polymerisations, resulting in film-forming latexes quite stable during and after the polymerisation. However, because they are simply anionic, they do not provide steric stabilization and the latexes flocculate upon addition of strong electrolytes or in freezing tests, except if they are engaged in miniemulsion polymerisations initiated with KPS. The second topic is concerned with a series of nonionic block copolymer surfmers. A hydrophilic sequence of ethylene oxide units, produced upon ring opening living anionic polymerisation, is followed by a hydrophobic sequence of propyleneoxide, and the living polymer is killed with a polymerisable group attached on a reactive halogen atom. A variety of polymerisable groups have been used: styrenic, methacrylic, vinylic, allylic and maleic. The HLB balance is controlled through the lentgh of the hydrophilic and hydrophobic sequences. The more reactive, associated with an adequate HLB balance allows to prepare core-shell latexes with an excellent steric stabilisation at solid contents up to 40%. It is not the case for the less reactive maleic or allylic, the behaviour of which is close to that of a non reactive surfactant with the same structure. Transurfs, with an addition-fragmentation mechanism, is the subject of the third topic. A first study was carried out by the group of Eindhoven, who produced a new surfactant on the basis of a MMA dimer condensed on a long chain bromoalcohol and then sulfonated. This transurf was engaged in a MMA emulsion polymerisation, and compared with SDS; the polymerisation rate was lower and the molecular weight was broader. Optimisation of the incorporation of the transurf was achieved upon using starved feed conditions. We have carried out styrene miniemulsion using controlled RAFT process with a surfactant belonging to the first family reported above together with a RAFT agent. Finally we have used a RAFT agent derived from the nonionic block copolymer surfactants.