Study On Composite Emulsions And Coatings Prepared By Colloidal Silica And Waterborne Polyurethane

Waterborne polyurethane paints as the environmental protection coatings are widely used,but compared with their solvent paints,there is still a large gap between them.In order to effectively improve the performance of waterborne polyurethane paint,using the characteristics of organic polyurethane and inorganic colloidal silica to prepare waterborne polyurethane/colloidal silica composite emulsions（WPU/SiO₂）and to improve the mechanical properties,water resistance,thermal stability and other shortcomings of paint films.By mixing the polyurethane prepolymer of isocyanate（-NCO）with colloidal silica during their dispersion processes,the inorganic particles had more chances to bond with the polyurethane active group（-NCO）,so as to improve the compatibility between the organic phase and the inorganic phase,solving the problem of insufficient interaction forces between the polyurethane prepolymers and the colloidal silica.The acrylate monomer was used to modify polyurethane to change the network structure of organic resins,and both the differences of organic resin structures and the properties of composite paint films were studied.The influence of the properties of colloidal silica on the composite films was further studied using colloidal silica with different particle sizes.The composite films with high colloidal silica contents were prepared by modifying colloidal silica with organosilicon,and its water resistance and application range were improved.The main results are as the following.（1）The WPU/SiO₂composite emulsions were prepared by mixing colloidal silica with isocyanate（-NCO）terminated polyurethane prepolymers,so that silica nanoparticles were evenly dispersed in the resin matrix and provided some chemical bonds with polyurethane resins,enhancing the interaction force and compatibility between the two phases.When the initial R value of polyurethane was 3,the r value of prepolymer was 1.2,the chain expansion ratio of ethylenediamine was 0.6 and the dimethylolpropionic acid（DMPA）content was 3.5wt%,the WPU/SiO₂composite film has the best performance.It was found that when the colloidal silica content was 10 wt%,the tensile strength and elongation at break increased to5.57 MPa and 407.45%,water absorption decreased to 10.88%,hardness increased from H to2H,and the storage stability of the emulsion was effectively prolonged.The strong interaction force made the glass transition temperature of the composite film was risen from 42.6℃to57.1℃,and the viscosity of the composite emulsion also increased to a certain extent.The microstructure of the composite film was smoother than that of the physical blending film,and there was no particle agglomeration.（2）The polyurethane acrylate/colloidal silica（WPUA/SiO₂）composite emulsions were synthesized by modifying polyurethane with methyl methacrylate（MMA）.WPUA/SiO₂film prepared by colloidal silica with particle size of 80 nm and the addition of 30 wt%can obtain good water resistance and mechanical properties.The water absorption rate was as low as2.67%,the pencil hardness was 3H,the elongation at break was 564.58%,and the water resistance reaches the best.When the amount of 14 nm silica was 10 wt%,the properties of the composite film were improved most obviously.The tensile strength and elongation at break were 9.93MPa and 437.43%,respectively,and the water absorption was 13.56%.However,when the content of 14 nm silica rose to 30 wt%,the performance of the film would be damaged due to the agglomeration of inorganic particles,and the storage stability was insufficient.（3）The composite films with a high silica content were prepared by modifying colloidal silica with methyl potassium silicate（PMS）.The mass of colloidal silica and polyurethane was 1.5:1.The presence of-CH₃in the infrared spectrogram of the modified silica powder proved that PMS successfully reacted with silica.SEM found that PMS modified silica formed raspberry-like particles.The best composite film could be prepared by modified colloidal silica with 3 wt%,and the water absorption decreased from 40.71%to 10.92%,significantly improving water resistance.When the mass ratio of silica to polyurethane was1.5:1,the compatibility was still good,and the transparency of the film reached 97.2%.Besides,the optical properties were not damaged,and the storage stability was also good.Compared with previous experiments,the compatibility between colloidal silica and resin with different structures was different,and resin with high soft chain content and large molecular weight has the better compatibility with silica,which could carry higher content of inorganic particles.The properties of polyurethane/colloidal silica composite film were limited by the compatibility between inorganic silica and organic resin,while the particle sizes and contents of colloidal silica affected the hydroxyl content and reactivity on the surface of inorganic particles,as well as the degree of interaction with polyurethane.In this thesis,a simple method was used to improve the compatibility of two phases in the composite emulsions,exploring the influence of particle sizes and amount of colloidal silica on the performance of the coating films.Through the modification of polyurethane and colloidal silica,film properties could be further improved,owning a wide range of prospects application.