Preparation And Properties Of Cathodic Electrophoretic Coating Of Waterborne Polyurethane

Polyurethane (PU) coating is one of the high-grade coatings. It is interested in the developing of waterborne polyurethane because ofthe toxicity of solvent-based polyurethane. Waterborne polyurethane coating has become a hot topic in the PU research field due to its nontoxic, nonpollution, nonflammable, easy to store, easy to use, and good properties as solvent-based polyurethane. The research of waterborne polyurethane is relatively backward in China, and mostly remains in laboratory studies.Its performance is poor, such as low solid content, poor water-resistance, poor heat-resistance, low hardness of film. In order to overcome the shortcomings, TDI-based cathodic electrophoretic coating of waterborne polyurethane (T-CEWPU) and IPDI-based cathodic electrophoretic coating of waterborne polyurethane (I-CEWPU) were prepared with TDI or IPDI and PCLD as mainly raw material, MDEA as hydrophilic chain extender in this thesis. The synthesis of cathodic electrophoretic coating of waterborne polyurethane was studied. The effects of molar ratio of NCO/OH, MDEA content and chain-extending agent on the properties of coating and its film were also investigated. In this thesis, the most significant innovation is that cathodic electrophoretic coating of waterborne polyurethane with 60% solid content is prepared, and its pencil hardness of the film is up to 5H by dealing with a curing agent during the electrophoresis process. The results are as follows.The experimental results showed that the emulsion particle size of T-CEWPU decreased at first then increased with the increasing NCO/OH molar ratio in prepolymer. Its pencil hardness of the film also increased. When the NCO/OH mole ratio of prepolymer is 3.0, the T-CEWPU with best overall properties was prepared. For I-CEWPU cathodic electrophoretic coating, both the particle size and film adhesion increased at fist then decreased, the pencil hardness of the film increased. When the NCO/OH mole ratio of prepolymer is 2.2, the I-CEWPU with best overall properties was prepared.The viscosity of T-CEWPU emulsion increased with the increasing content of hydrophilic chain extender MDEA, the pencil hardness and water resistance of film decreased, whereas the particle size of I-CEWPU emulsion decreased, and its pencil hardness increased at first then decreased. It showed that the average particle size of I-CEWPU emulsion decreased, the pencil hardness increased and the film adhesion unchanged with the increasing neutralization degree of MDEA.The T-CEWPU cathodic electrophoretic coating prepared by 1,4-butanediol or diethylene glycol as chain-extending agent have smooth surface of film, and the film adhesion is between level 1and level 2, but the film prepared by 1,4-butanediol has higher pencil hardness. In the I-CEWPU cathodic electrophoretic coatings prepared by 1,4-butanediol, ethylene glycol or diethylene glycol, the coating prepared by diethylene glycol has the smoothest morphology of wet film.The I-CEWPU-2 cathodic electrophoretic coating with 60% solid content was prepared by improved process. The drainage of wet film of I-CEWPU-2 cathodic electrophoretic coating was decreased with the adding of flow agent, which contribute to high-temperature flow. The wet film surface of I-CEWPU-2 cathodic electrophoretic coating has no obviously change when the amount of curing agent SA30 is low. However, when the amount of curing agent SA30 is over 1wt%, film adhesion decreased with the increasing amount of curing agent, appearance of film as an orange peel even with a lot of bubble-like was formed. Curing agent SA30 is also to improve the heat-resistance of I-CEWPU-2 cathodic electrophoretic coating. Curing agent YA75 reduced the thermal stability of I-CEWPU-2 cathodic electrophoretic coating.