Copolymerization Of Vinly Acetate/Butyl Acrylate Emulsion And Preparation Of Redispersible Polymer Powders

Re-dispersible polymer powder is a kind of polymer powder obtained by spray drying using polymer emulsion and can be redispersed in the water. The glass transition temperature of copolymer emulsion preparation for dispersible polymer powders varies from -5 to 10℃, the most commonly used emulsion is vinyl acetate/ethylene (VAE) copolymer. In order to satisfy the growing demand of the market, there will be urgent need for a kind of copolymer emulsion that can be synthesized under available pressure and used for preparation of redispersible polymer powder. In this paper, Vinyl acetate (VAc) /buty1 acrylate (BA) random copolymer emulsions were prepared by seeded emulsion polymerization following semi-continuous feeding method. The obtained emulsions could be used for spray drying to get redispersible polymer powders. Main research works are as follows:In the emulsion copolymerization system of VAc/BA, dodecyl sulfonic acid sodium salt (SDS) and OP-10, polyvinyl alcohol (PVA1788) and ammonium persulfate (APS) were employed as emulsifier, protective colloid and initiator. The dependence of concentration of emulsifier and protective colloid on stability of emulsions was studied. What's more, the dependence of different monomer ratios on glass transition temperature (Tg) of emulsions was investigated. The copolymerization process consists of three reaction stages of pre-polymerization stage, monomer-dropping stage, and temperature constant stage. The structure and Tg values of the copolymers obtained at three different reaction stages were characterized by Fourier Transform Infrared Spectroscopy (FT-IR),13C-NMR and Differential Scanning Calorimetry (DSC) to analyze the composition and sequence structure of the copolymers under the three reaction stages, and the sequence distribution was calculated by first-order Markov statistical model. The obtained results showed that the emulsion with good stability was prepared under the following conditions:the concentration of protective colloid 4.0%, the emulsifier concentration 3.0%, and the ratio of ionic emulsifier to non-ionic emulsifier 1:1. The final VAc/BA copolymer was an apparently random copolymer with only one glass transition temperature. Different ratio of monomers would affect the glass transition temperature of emulsions. When the ratio of VAc to BA increased from 50/50 to 80/20, the glass transition temperature of copolymer can be controlled between -20.0℃and 16.6℃.However, the composition and sequence structure of the copolymers changed obviously due to the monomer concentration change during different polymerization stages. The copolymers composing of BA-rich unit and VAc-rich unit segments were main products with two Tg values and high average sequence length of BA (lb) during the pre-polymerization because of rich monomer concentrations. The random VAc/BA copolymers with only one Tg values, lowerlb, and lower average sequence length of VAc (la) were achieved during the monomer-dropping stage while the monomer concentration was controlled by the monomer-dropping rate.Polyviny1 alcohol (PVA0588) was employed as release agent to experience spray drying using VAc/BA copolymer emulsion. Spray drying process were conducted at various temperatures of drying medium, emulsion with different glass transition temperature and solid content, and the effect of these parameters on the properties of redispersible polymer powders were investigated. The result showed that emulsion with higher glass transition temperature is suitable to experience spray drying. The operation conditions are as follows:inlet temperature of 130℃, atomizing disc running of 15000 r·min-1,solid content of emulsion 20%,outlet temperature of 90-100℃.PVFM was employed as new protective colloid to prepare VAc/BA copolymer emulsion. The dependence of concentration of PVFM on the viscosity of emulsion, conversion rate of monomers and resistance to water were studied. On the other hand, the effect of PVFM on process of spray drying and properties of powders were also investigated. The results showed that the viscosity of emulsion would be enhanced by the usage of PVFM. The emulsion with moderated viscosity.as well as high conversion rate of monomers was obtained when the concentration of PVFM ranged from 2.0% to 3.0%. The resistance to water was improved due to the ring structure of PVFM. What's more, the conglutination phenomenon of polymers in the spray drying process has been improved and the solubility and redispersion of powders were also improved.