Research On Preparation, Redispersion, Stabilization Mechanism And Application Of Acrylate Redispersible Latex Powder

Modern architectural coatings consist of many additives, including the defoaming agent, viscosity-increasing agent, etc. These additives are the source of volatile organic compounds (VOCs) produced from coatings. As environmental policy becomes stricter, these coatings are developing toward waterborne, high solid content and powder based coatings. Redispersible latex powder, a type of polymer powder, provides large improvements in the prepation of dry powdered coatings. When redispersible latex powder is dispersed in water, it is stable and has the same properties as the original latex. Research on preparing redispersible latex powder and developing dry powdered coatings combined with other powder materials shows great significance both in theory and practice.Based on the requirements of redispersible latex powder on spray drying and coating on application, firstly acrylate latex was synthesized with the configuration of a?soft core and hard shell?, nextly the redispersible latex powder was prepared using the spray drying method. A spray drying model was established, and influencing factors on the powder's redispersibility and stabilization mechanism of reconstituted latex were investigated. Dry powdered coating was developed in which the latex powder serves as the major constituent of the film forming material. Using emulsion polymerization technology on the core shell configuration, the magnetic composite microsphere (Fe3O4/PMAA) was prepared. The following describes the main focus of this work.Acrylate latex with the“soft core and hard shell”configuration was synthesized to prepare redispersible latex powder. Through investigation of the effect of the glass transition temperature (T_g) of the core and shell layer on the appearance and redispersibility of powder, the configuration parameters of the latex particles were determined. Furthermore, by investigation on the effect of the polymerization process, emulsifier, initiator, seed monomer and functional monomer on coagulation rate, solid content, viscosity, particle size and distribution, the optimized polymerization process was established. By Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), it was determined that the latex particles have a clear core-shell configuration and two T_g s (-25?and 45?). In addition, the acrylate has good film forming properties.The redispersible latex powder was prepared by spray drying. Through investigation on the effects of latex viscosity, solid content, silicon sol and protective colloid (PVA) amount on redispersibility, water content, and residual content of the powder, the pretreatment parameters were determined. Moreover, by investigating the effect of drying temperature and atomizer speed on water content, residual content, redispersibility and film forming ability, the optimized spray drying parameters were established. By FTIR, SEM, TEM, DSC and UV-visible spectrometer, and comparison with the original latex, it was determined that the molecular configuration of redispersible powders does not change, while the particles have a spherical shape of uniform size slightly larger than original particle. The reconstituted latex is stable and the film has T_gs (-25?and 50?). All of the analyses show that after the powder is dispersed in water, it can return to its orginal conditions. The dynamic governing equations of mass and energy were utilized to establish a spray drying model for redispersible latex powder. By coupling the two equations and running a code developed in Matlab, the dynamic change of temperature and water content in the water droplet can be obtained.The stabilization mechanism of the reconstituted latex was also investigated. 4%⁵% (of total monomers) MAA was added during polymerization, and the acrylate latex was neutralized to a pH above 9.0. The carboxyl group distributed on the surface of the particles was transformed into carboxylic ions in order to protect these groups during spray drying. In this way, the powder has good redispersiblity and the reconstituted latex has good stability. The particle also has a high zeta potential as much of the carboxylic ions were distributed on the particle surface. The particles also demonstrated a strong electrostatic repulsion force as well as hydration force caused by the interparticle electrostatic effect. PVA served as a protective colloid for the particles. After the powder was dispersed in the water, the particles were surrounded by PVA whose hydroxyl group bonded with the water and long chain poly-ethylene groups were absorbed on the surface of particles. These poly-ethylene groups provide enormous steric force and help to prevent particles aggregation. The electrostatic force, hydration force and steric force together stabilized the particles in the reconstituted latex.Based on redispersible latex powder, a dry powdered coating was developed using other inorganic materials and cellulose ether. The redispersible powder has a large influence on film forming ability, water resistance and abrasive resistance of the coating. Cellulose improves the working ability of the coating. Taking the formula amount of dry-powdered coating as follows, redispersible latex powder is 30% (mass percentage of total powder), cellulose ether is 0.6% (mass percentage of total powder), the dry-powdered coating possesses good film forming ability, good water and abrasive resistance, and good workability.Considering polymer technology with core shell configuration, based on Fe₃O₄ modified by oleic acid, the Fe₃O₄/PMAA composite microsphere was prepared by miniemulsion under sonication. The sonication power was determined by investigations on the effect of sonication power on droplet size and poly dispersing index (PDI). The effect of emulsifier amount on microsphere size was also investigated. When the emulsifier concentration was below or close to its critical micelle concentration (CMC), the monomer droplets easily aggregate with each other and the composite microsphere has a wide size distribution. When the emulsifier concentration was much higher than CMC, a large amount of micelle was formed. MAA will form cores inside the micelle and cause the magnetite particles to be exposed. The suitable emulsifier concentration is slightly higher than its CMC resulting in a composite microsphere with good dispersibiliy and a size of about 100 nm.