Synthesis And Properties Of The Alkoxysilane-Modified Acrylic Latex Via Miniemulsion Polymerization

Due to high Si-0 bond-energy, low surface-energy and chain-flexible characteristics of silicone polymer, siloxane modified acrylic latex rapidly presents to be high performance out-wall coating materials in terms of water proof, weather resistant, stain resistant. However, alkoxysilane monomers are inherently susceptible to premature hydrolysis and condensation reactions in the aqueous phase during polymerization and storage. The extensive premature hydrolysis and condensation can lead to the issue of the colloidal instability and short shelf life. Also, such a reaction results in a poor film formation of the latex.A series of the stable alkoxysilane-modified acrylic latexes with various alkoxysilane fractions was synthesized via miniemulsion polymerization, in which monomer droplet nucleation is dominant. That means the particles directly form from monomer mini-droplets, unlike conventional emulsion polymerization. A small fraction of alkoxysilane side groups were incorporated into acrylic main chains via copolymerization. The hydroxyl oligomer alkoxysilane was then grafted to the main chains via the condense reaction of the hydroxyl groups with hydrolyzed alkxoysilane groups. The effect of sonication time, surfactant system, initiator, pH value, main monomer (acrylic) type, and siloxane type on the colloidal stability, particle size and particle size distribution were investigated. The obtained latex of MMA/BA/MPMS/ hydroxyl oligomer alkoxysilane was analyzed by Dynamic Light Scattering, TEM, Solvent Extraction, and FT-IR. And the effect of siloxane type on the properties of copolymer film was also studied.It was found that, merely using miniemulsion polymerization is insufficient to obtainstable silicone-modified polymer latex in some cases. When the sonication time is 15min, coexistence of reactive polymeric surfactant Latemul S-180Aand SDS are used as a composite surfactant system, initiator is APS and pH value is let alone, the colloidal stability of the obtained latex is better, and the particle size is smaller. The selection of the main monomer, which can fast consume alkoxysilane comonomer, is critical to obtain the stable latex. The alkoxysilane monomer type also plays a role in the colloidal stability during miniemulsion copolymerization. y-Methacryoxypropytrimethoxysilane (MPMS) is a better alkoxysilane comonomer than vinyltriethoxysilane (VTES) is in terms of colloidal stability during polymerization and the hydrophobic property of the resulted polymer coating.When MPMS(1.5wt%) was used and the content of hydroxyl oligomer alkoxysilane was lower than 15wt%, the colloidal stability is very good either during the polymerization, and the particle is obviously smaller than that of control pure acrylic latex. The obtained latex had a long shelf life. The TEM photograph shows that the surface of particle is smooth, and the particle size distribution is narrow. The results of FT-IR and Solvent Extraction show that the oligomer alkoxysilane can well be grafted onto the acrylic polymer chain with a grafted degree around 50%, and the gel percentage around 90%.The incorporation of alkoxysilane dramatically increases the hydrophobic property of the resulted coating. The water contact angle increases from 75 to 95 degree with 10wt% oligomer alkoxysilane and further up to 105 degree with 20wt% oiligomer alkoxysilane.