Synthesis Of Polyacrylate Emulsion With High Hydroxyl Value And Its Application In Two-Component Waterborne Polyurethane Coatings

Polyurethane?PU?becomes increasingly popular in lots of fields such as automotive coatings,adhesives,plastics because it owns excellent properties of good solvent and chemical resistance,good abrasion resistance,hardness and flexibility,etc.Due to the increased strict legal requirements and demands for higher-performance,formaldehyde free,thermosetting waterborne coatings,two-component waterborne polyurethane?2K-WPU?coatings have attracted great attention during the past decade years.2K-WPU is mainly composed of waterborne hydroxyl polyols and hydrophilic modified polyisocyanate curing agents.But the polyacrylic emulsion with high hydroxyl value generally has many disadvantages such as poor stability,poor emulsification ability to curing agent,uneven distribution of hydroxyl groups and insufficient crosslinking density.In order to enhence the properties of 2K-WPU,the hydroxyl polyacrylate emulsion was usually modified by organic siloxane.However,the hydrolysis and condensation of siloxane monomers easily occur in the aqueous phase,which has seriously affected on stability of hydroxyl functional resin.And the siloxane-modified films often show the phase separated structure due to the siloxane's low surface energy and poor compatibility with polymers,which causes the inferior performance of 2K-WPU film in the siloxane poor domains.In this paper,we synthesized the core-shell polyacrylic emulsion with high hydroxyl value and a novel hydroxyl polyacrylate latex modified by octavinyl polyhedral oligomeric silsesquioxane?OvPOSS?to overcome the above mentioned defects and improve the properties of 2K-WPU film.2K-WPU coatings were prepared by using the as-prepared emulsion as hydroxyl component and hydrophilic modified polyisocyanate as curing agent.The appearance and properties of the 2K-WPU films were also characterized and studied.The core-shell hydroxyl polyacrylic emulsion with high hydroxyl value?128.7mg KOH/g?was synthesized via core-shell emulsion polymerization with reactive emulsifier and anionic emulsifier as hybrid emulsifier.The effect of particle types,core shell mass ratio,hydroxyl monomers types and distribution and glass transition temperature on the emulsion polymerization stability and the particle sizes was investigated.The results indicated that the high hydroxyl value polyacrylic emulsion with hard core/soft shell had better appearance,smaller average particle size and narrow size distribution.With the increasing hydroxyl monomers on shell,the particle size decreased.When the core/shell mass ratio was 1/1,the latex particle possessed two obvious glass transition temperatures.Nonetheless when the core/shell mass ratio was 2/1 and the proportion of hydroxyl monomers to shell was 58%,the emulsion polymerization was failed.The polyacrylic emulsion with high hydroxyl value prepared by high hydrophilic 2-hydroxyl acrylate owned high viscosity and was easily aqueous nucleation.This particle had larger average size and broaden distribution.The particle size of polyacrylic emulsion prepared by 2-hydroxylpropyl acrylate was 107nm.The octavinyl polyhedral oligomeric sisesquioxane?OvPOSS?was synthesized via hydrolysis and condensation of vinyltriethoxysilane.The structure and appearance were determined by FTIR,XRD,¹H NMR and ²⁹Si NMR.The novel hydroxyl polyacrylate latex?OvPOSS/HPAE?was synthesized via semi-continuous seed emulsion polymerization.The results of FTIR and XRD showed that the OvPOSS had been successfully embedded into the HPAE emulsion.OvPOSS had a homogeneous dispersion in the composite matrix.The appearance and stability of OvPOSS/HPAE were very well.With the increasing modified OvPOSS,the particle size became small.The incorporation of OvPOSS restrained the mobility of the hybrid polymer chains,causing the increased Tg and thermodynamic stability.The particle size of 1.0wt%OvPOSS/HPAE was 88nm.The Tg increased to 21.6?.Two different 2K-WPU coatings were prepared by core-shell polyacrylic emulsion with high hydroxyl value and OvPOSS/HPAE and hydrophilic modified polyisocyanate curing agents according to–NCO/-OH molar ratio of 1.5/1.The compatibility,hardness and the water resistance of 2K-WPU films were investigated.The results showed that compared with soft core/hard shell type,the hydroxyl polyacrylic emulsion with hard core/soft shell had better compatibility with curing agent.The better appearance film with higher hardness could be obtained.More hydroxyl monomer on shell promoted the crosslinking reaction between–NCO group and–OH group,forming the tough cured film.When core shell mass ratio was 1/1,the hardness of 2K-WPU films was 0.86 and the water absorption was 4.0%.The film prepared by primary hydroxyl monomers?HPA?had shorter drying time and rapid establishment of early hardness.The pendulum hardness of film is of 0.79.The glass transition temperature had positive impact on the hardness of 2K-WPU film.The incorporation of OvPOSS did not reduce the compatibility between the HPA and polyisocyanate.OvPOSS had a homogeneous dispersion in the composite matrix and no obvious phase separation was observed.And the application performance of modified 2K-WPU film such as hardness,water and chemical resistance was improved.