Synthesis of hybrid latexes and polymerization kinetics of functional latexes.

摘要

Hydrophilic or hydrophobic functional monomers impart unusual properties to latexes. The present work focused on assessing the effects of the type and the amount of functional monomers on the physical properties of hybrid latexes (particle size, solid content, and glass transition temperature, etc.), polymerization kinetics of core-shell latexes and mechanical properties of thermoset latex films.The first aim was to investigate the effect of hydrophobic groups such as polysiloxane on the physical properties of latexes. Polysiloxane-functionalized acrylic latexes were prepared by three different grafting techniques. In the first method, an acrylic core was prepared with the addition of a coupling agent, 3-(trimethoxysilyl) propyl methacrylate, after which a cyclic siloxane monomer (octamethylcyclotetra-siloxane, D4) was grafted onto the coupling agent. In the second method, a methacrylate-terminated polysiloxane was copolymerized with ethyl acrylate (EA) and 2-ethylhexyl acrylate (EHA) in batch emulsion polymerization. In the third method, D4 was added during emulsion polymerization of EA, EHA and 2-hydroxyethyl methacrylate. A core-shell morphology was observed in transmission electron microscopy (TEM) for the first preparation method. Microphase separation was observed by atomic force microscopy (AFM) after polysiloxane-functionalization for all latex films. Energy dispersive X-ray data indicated that only the hybrid latex by copolymerization of methacrylate-terminated polysiloxane (second grafting method) resulted in higher silicon content at the film-air interface than the film-substrate interface. In all methods, storage modulus and surface energy of latex films decreased after polysiloxane-functionalization of latexes.Secondly, the effect of polymerization of hydrophilic functional monomers with different types of surfactant on the polymerization kinetics was investigated. A semi-batch emulsion copolymerization of butyl acrylate (BA), methyl methacrylate (MMA), 2-hydroxyethyl methacrylate (HEMA), and methacrylic acid (MAA) was performed in which the concentration of HEMA (in core), MAA (in shell) and the type of surfactant (two anionic and two nonionic) were varied. New particle formation occurred throughout the polymerization even under almost starved-monomer conditions. The instantaneous rate of polymerization was inversely proportional to the concentration of HEMA and MAA. Secondary nucleation and limited coagulation were more significant when the anionic surfactant (Triton X-200) was used. In general, the smallest particle size was obtained when Triton X-200 was used. Generally, the anionic surfactant (Aerosol MA-80) yielded slower polymerization reactions which were attributed to high critical micelle concentration (CMC) compared to the other surfactants.Finally, the latexes with hydrophilic functional monomers were crosslinked to study the effects of crosslinker type on mechanical properties. The latexes with varying concentrations of HEMA, MAA and two types of surfactants (Tergitol XJ, Triton X-200) were crosslinked with five different types of crosslinkers. Melamine-formaldehyde (MF) resin was employed to crosslink hydroxyl functionalities in the core. Carboxylic acid groups in the shell were crosslinked with zinc ammonium carbonate and N, N-dicyclohexylcarbodiimide. Cycloaliphatic diepoxide and hexamethylene diisocyanate (HDI) isocyanurate were used to crosslink with hydroxyl or carboxyl functional groups in the core and the shell. The toughest films were obtained when MF resin was used as crosslinker in the tensile test. However, zinc crosslinker yielded brittle films with very low toughness and pencil hardness. The highest Young's modulus was obtained for the latex films when HDI isocyanurate or carbodiimide were used as crosslinker. In general, anionic surfactant (Triton X-200) showed higher crosslink density compared to nonionic surfactant (Tergitol XJ). This was attributed to the broader particle size distribution of the latexes with Triton X-200.Dual-cure (thermal/visible light process) amide- and acrylate-functionalized latexes were prepared via semi-batch emulsion polymerization. Thermoset latex films were prepared by blending amide- and acrylate-functionalized latexes in different fractions (50/50, 40/60, 60/40 wt/wt%). The tensile, dynamic mechanical and thermal properties of the thermoset films were evaluated. The effect of the photosensitizer (camphorquinone) concentration was also investigated on mechanical and thermal properties. Amide- and acrylate-functionalized latexes (50/50 wt/wt.) were mixed with different amounts of methanolic camphorquinone and t-butyl hydroperoxide relative to the amount of trimethylolpropane triacrylate (1, 2, 4, and 6 wt% methanolic camphorquinone and t-butyl hydroperoxide). The highest tensile modulus and elongation % was observed in amide- and acrylate-functionalized latex with 50/50 wt% blend ratio. As the concentration of photosensitizer (camphorquinone) increased, storage modulus of films increased. Two glass transition temperatures were observed for the latex films. This could be attributed to the induced phase separation after the crosslinking of acrylic functionalities followed by thermally crosslinking between acetoacetoxy and amide functionalities. (Abstract shortened by UMI.)