Study On Preparation Of Composite Latexes Modified By Inorganic Nanoparticles And Applications In Waterborne Coatings

Under the volatile organic compounds (VOC) regulations strengthened from the viewpoint of environmental protection, waterborne paints has been one of the important trends of development in 21st century. Although there are various kinds of waterborne resins in domestic and oversea markets, their overall performances are not as good as solvent-based resins in some aspects, especially in film hardness and water resistance, therefor, there application and development are restricted. So the research of waterborne resins with excellent properties has been an important topic to be solved eagerly. One of strategies to improve the properties of waterborne resins is to embed inorganic particles into polymeric matrices. This new class of polymeric materials combines the properties of inorganic particles with the processability and flexibility of organic polymer matrix. Following this line, systemic researches about nano-TiO₂ / styrene-acrylate composite latices obtained by in-situ emulsion polymerization have been done in this thesis. Not only polymer latexes with excellent properties could be obtained, but also the systemically theoretical and experimental bases about preparing the inorganic/organic composite material have been supplied. Styrene-acrylate latexes have been widely used in waterborne paints due to its unique properties, such as water-solubility, non-toxicity, non-pollutant, high weather resistance, high stain resistance and low production costs, so styrene-acrylate emulsion polymerization system was chosen in this thesis. The main research content and achievements around the modification of inorganic nanoparticles, synthesis techniques of composite latexes, characterization of latex properties and particles structure, emulsion polymerization kinetics and nucleation mechanisms, and applications of the composite are shown as following. A process for modification of nanometer TiO₂ powders by silane coupling agent (WD-20) was studied. The modification effects have been investigated by FTIR, TEM and TGA. The results showed that the coupling agent was grafted successfully on the surface of nano-TiO₂ powders, and the graft ratio was 3.9%. The modified nano-TiO₂ powders could be dispersed very well in organic medium, and had been bonded polymeric functional group on their surface, that provides conditions for in-suit emulsion polymerization of composite latex.The polymerization system and technique of nano-TiO₂ / styrene-acrylate composite latexes were studied, including the selections of monomers, emulsifier and polymerization technique. In order to obtain harder and better water-resist film of styrene-acrylate latex, copolymerization system with five monomers, in which St, MMA and AN acted as hard monomers, BA as soft monomer, and AA as functional monomer, was chosen. According to the feeding manners of monomers and inorganic nanoparticles, various polymerization processes were tested and two polymerization techniques were established finally. One is continuous starved feeding polymerization system stabilized by co-emulsifier (SDS as anionic, and OP as nonionic emulsifier), the other is the surfactant-free semi-continuous seeded polymerization system stabilized by reactive emulsifier (SE-10N).The physical properties of resultant composite latexes were tested. The results showed that all the physical properties of composite latexes were superior to normal styrene-acrylate latex without nanoparticles. Their coating film had a hardness up to H and 2H grade, respectively. The water resistance of surfactant-free composite latex was superior to surfactant composite latex, and the reserve stabilities of the two kinds of composite latexes were more than 1 year. The thermal gravimetric analysis results showed that the composite latex film presented higher thermal stability than the latex film unmodified by the nanoparticles. As a whole the surfactant-free latex possessed better properties than surfactant latex. Furthermore, the composite latexes also owned perfect antibacterial characteristic. The antibacterial efficiency to both E.coli and staphylococcus aureus were near 100% within 24 hours.The structures and the morphologies of the composite latexes particles have been characterized. The TEM photograph and the light scattering experiments show that the particles size of surfactant-free composite latex was bigger than that of surfactant-free latex with no nano-TiO₂ particles added, and some smaller than that of surfactant composite latex, which were 80nm, 50-70nm and 90-110nm, respectively. The light scattering experiments also showed that there was a single latex particle peak in resultant composite latex, and there was no smaller inorganic nanoparticle peak. It dicated that nano-TiO₂ particles had been embedded in latexes particles and the composite latexes exhibited a core-shell structure with nanometer TiO₂ as core and organic polymer as shell. The TEM images showed that the surfactant composite latex particles presented normal as well as exceptional core-shell structure, and the surfactant-free composite latex particles presented exceptional core-shell structure mostly.The apparent kinetics of surfactant-free emulsion polymerization in the presence of modified nano-TiO₂ powders has been invested. The apparent kinetics equation is R_p= k [M]^0.35[I]^0.18[E]^0.97, and the apparent active energy ofthe polymerization system is 90.8kJ·mol^-1. The polymerization state in the semi-continuous process has been studied. If the polymerization state is retained staving steadily, the time of feeding monomers should be longer than 140min.This thesis reports the nucleation mechanism of polymer latex modified by inorganic nanoparticles. For the continuous technique, the modified nano-TiO₂ particles adsorbed emulsifier are the predominant particle nucleation loci at the beginning of polymerization, and then the micelle nucleation model is dominant. For surfactant-free semi-continuous seeded polymerization system stabilized by reactive emulsifier, in the stage of seeded emulsion polymerization, the modified nano-TiO₂ particles adsorbed emulsifier are the predominant particle nucleation loci, and the micelle nucleation model is dominant when dropping shell monomers.The applications of nanoparticles composite used in preparing waterborne coatings have been developed. Nanometer waterborne wood coating, nanometer waterborne anticorrosion coating and nanometer antibacterial green latex paint were empoldered. The test reports show that all of properties of the three coatings are superior to that of current products.This study will be beneficial to the further investigation and development of polymer latexes, functional waterbrone coatings and application technologies about nano materials.