| It was found that the nucleation mechanism of styrene (St)/methyl methacrylate (MMA) surfactant-free emulsion polymerization in poly(ethylene glycol) (PEG) aqueous solution was very different from the traditional emulsion and traditional surfactant-free emulsion polymerization. It is academic valuable for understanding the nucleation and growth to study this process. And a new method to make emulsion product with excellent stability, environmental friendly, nanometer scale particle, narrow size distribution is provided. Various physical and chemical properties in the process of St/MMA surfactant-free emulsion polymerization in PEG aqueous solution and the effectson the stability of polymerization system were studied. With researching the effect of PEG on the polymerization system, a new nucleation mechanism, solution-reprecipitation to nucleation-aggregate-separate dispersion nucleation mechanism, was proposed. The surfactant-free emulsion was applied in water born varnish with excellent comprehensive performance.It was found that there were many differences between St/MMA surfactant-free emulsion polymerization with and without PEG. The polymerization rate with PEG was obviously lower than that without PEG. As the size of emulsion particle polymerized with PEG changed from small to large, and then from large to small again, the viscosity of emulsion could increase sharply.With maximum bubble pressure method, the dynamic surface tension of St/MMA surfactant-free emulsion polymerization was studied. It was found that a copolymer with better surface activity obtained in PEG aqueous solution, which is lower than aqueous solution of PEG and copolymer obtained without PEG, but higher than that of monomer and oligomer, can be formed when the conversion is just 5%.The effects of molecular weight and content of PEG, content of initiator, ratio of monomers, temperature and agitation speed on the stability of St/MMA surfactant-free emulsion polymerization were researched. PEG 1000 is easy to detach from particle for its short chain length and is unable to make steric hindrance effectively. PEG8000 makes tangle and even bridging easily for its overlength chain. The molecular weight of PEG ranging from 2000 to 4000 is suitable. The system without PEG could get stable particle with more than 300nm. As the content of PEG2000 was less than 1%, the system is so unstable that the copolymer was easy to aggregate. As the content is more than 5%, the system could get stable particle withl less than 100nm. When KPS was less than 0.4%, the system could not proceed stably, and more than 0.6%, the polymerization system was stable, and the polymerization rate increase gradually with more content of initiator. The polymerization system with only St or MMA or St/MMA<1/4 could not proceed stably, when the ratio of monomers was suitable, the increasing of content of MMA could affect the change trend of polymerization rate of system. As the temperature was lower than 70℃, the stable system could not be attained, and increasing temperature could advance size change process, speed up the polymerization rate. The low agitation speed could make polymerization system unstable.Studing the change of composition of ternary phase system of monomer/PEG/ copolymer with polymerization process, modeling the dispersion of copolymer in aqueous and monomer phases with or without PEG at various conversion, it was found that PEG made St-MMA copolymer entry monomer phase easier, and bring excess PEG into monomer phase in return, which could precipitate the copolymer solved in monomer phase, and reform the particle with a new size distribution and particle pile. The adsorbed PEG will prevent particles from coagulating through steric hindrance. The packed monomer phase results in increase in viscosity, which will break the monomer drops into tiny droplets by agitation. When the viscosity of the system reaches maximum, the monomer is just in the swollen particles, and water will replace the monomer to fill the space around the particles. Further consumption of the monomer reduces the volume of the particles, thereby increasing their surface potential, which push the copolymer particles away from the particle pile. A new nucleation mechanism for St/MMA surfactant-free emulsion polymerization in PEG aqueous solution, solution-reprecipitation to nucleation-aggregate-separate dispersion nucleation mechanism, was proposed.The method of St/MMA surfactant-free emulsion polymerization was applied in the water born varnish. A water born varnish with excellent filming performance, anti-blocking, gloss and rub resistance was obtained with the modification of adding soft monomer butyl acrylate, crossing monomer hydroxy ethyl acrylate and wax. |
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