| The essential character of cationic emulsion is that emulsion particle surface or polymer itself is positively charged. Comparing with the film prepared from redispersible emulsion powder of original anionic styrene-acrylic emulsion, the film formed by cationic styrene-acrylic emulsion had better water resistance and mechanical performance. Therefore, preparation of cationic styrene-acrylic emulsion used for redispersible emulsion powder is very important in practical application.This research was mainly divided into two parts: first, cationic styrene-acrylic emulsion with core-shell structure was synthesized by particle design and process optimization; Next, redispersible emulsion powder was prepared based on the cationic styrene-acrylic emulsion by spray drying process. The emulsions, their films and corresponding redispersible emulsion powder were characterized with Differential Scanning Calorimetry (DSC), Transmission Election Microscopy (TEM) and so on.The particle structure of"soft core and hard shell"was designed with methyl methacrylate (MMA) and styrene (St) as hard monomers, butyl acrylate (BA) as soft monomer, acrylamide (AM) as functional monomer. The effects of polymerization temperature, the amount of emulsifier, the amount of cationic monomer [2-(methacryloyloxy) ethyl] trimethyl-ammonium chloride (DMC) on polymerization reaction system, performances of emulsion and redispersible emulsion powder were investigated, and then the best formulation and process to prepare cationic styrene-acrylic emulsion were obtained. The best formulation and process were as following: the synthesis reactions were carried on at 80℃by semi-continuous seeded emulsion polymerization. The content of seed monomers was 10 wt% based on total monomers. Cetyl trimethyl ammonium bromide (CTAB) was used as emulsifier and the amount was 2 wt% based on total monomers. Azobisisobutryamide chloride (AIBA) was selected as initiator and the amount was 0.53 wt% based on total monomers. The mass ratio of DMC to AM was 1:1 and the amount of DMC was 2 wt% based on total monomers, and then the uniform and stable emulsion with small particle size and narrow particle size distribution could be obtained. Its main performance parameters were as following: the particle size was 105nm; the viscosity was 225 mPa·s; the solid content was 36.6%; pH value was 5.0 ~ 6.0;ξpotential was 45.1 mV. DSC results showed that glass transition temperature (Tg) of core and shell of the obtained polymer were -17℃(theoretical value -27℃) and 65℃(theoretical value 77℃) respectively. Pictures of Transmission Electron Microscopy (TEM) showed that the emulsion particles had clear core-shell structure.Redispersible emulsion powder was prepared from cationic styrene-acrylic emulsion by spray drying process, and factors such as pretreatment technology, the inlet temperature of spray drying tower, feeding flow, dosage of protective colloid and so on were considered. The results indicated that: without adjusting pH value of styrene-acrylic emulsion, when inlet temperature of spray drying tower was 120℃, the rotational speed of feeding pump was 30 rpm, the content of protective colloid PVA-205 was 1 wt% based on emulsion, The redispersible emulsion powder with low water content and good redispersibility was successfully prepared. Scanning Electron Microscopy (SEM) pictures showed that redispersible emulsion powder particles had smooth surface and presented globular shape with an average particle size about 10~30μm. Redispersible emulsion powder had good fluidity without agglomerating, and it could be redispersed into water. |
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