| In our daily lives and industrial applications, coating always provides excellent decoration and protection on the surface of solid materials. As a typical and complex colloidal system, coating is usually made of resins, pigments, additives as well as solvents. With the development of science and environmental regulations, global solvent-based coatings are being replaced by environment-friendly coatings, such as high solid coatings, UV curable coatings, powder coatings, waterborne coatings etc. China, as a coating power, has ranked first in annual coating production in the world since 1998. However, China is not the technology leadership of coating. Take automobile coatings for example, so far domestic car joint enterprises have almost included all world famous car manufacturers, but the corresponding car painting suppliers are all foreign companies, such as Akzo-Nobel, BASF Coatings, Nippon etc. Even if the burgeoning domestic car brand uses imported coatings as well.Chinese coating technology has been developed fast in the last thirty years, whose solvent-based coatings technique has been close to developed countries. While in waterborne coatings field we are still at the starting stage. For example, China is at a distinct disadvantage in terms of important waterborne coatings raw material, critical waterborne coatings preparation techniques and construction technology in comparison to foreign countries. As a consequence, there is much work to do for us in waterborne coatings field.Automobile coatings usually stand for the highest technology level in coating industry, meanwhile metallic coating plays an important role as the most popular topcoat today. Metallic coating often needs low solid content and high thixotropy so that it can promote the orientation process of metal powder and obtain excellent automobile appearance. Traditional solvent-based metallic coating will discharge a large amount of organic solvent in application, thus waterborne metallic coating has become the mainstream of topcoat nowadays, whose technical core is rheology control. The work presented here systematically studied the synthesis, structure control means as well as rheological properties of the common acrylic latex used in waterborne metallic coating, with the hope of laying foundations for developing waterborne metallic coating with independent intellectual property. The whole thesis can be divided into five parts:Chapter one reviewed the history, development, synthesis methods and applications of acrylic resins, recent research progress in waterborne metallic coating as well as rheology control means for waterborne coatings. In this part the project background and research design were given.Chapter two mainly focused on the synthesis of core/shell structure acrylic resin used for waterborne metallic coating. The resin was synthesized with common acrylate monomers via core/shell emulsion polymerization technique, using ammonium persulfate as initiator and lauryl sodium sulfate (SLS) and alkylphenol ethoxylate (OP-10) as the mixed emulsifiers. The influence of reaction temperature, initiator concentration, emulsifiers concentration and ratio on conversion, polymer diameter and molecular weight were investigated systematically. It was found that small temperature change made almost no influence on conversion. With initiator concentration increasing from 0.30% to 0.62%, conversion increased first and then decreased, while the polymer molecular weight decreased all the way. The optimum initiator concentration is 0.39%. The results also show that emulsifiers had a great influence on conversion and diameter:with OP-10 ratio increasing in the mixed emulsifiers, conversion decreased with diameter and its distribution increasing, and we infer that the possible reason for this phenomena is that OP-10 has poorer emulsifying capacity than SLS. When SLS/OP-10 ratio was fixed at 1/0.5, conversion increased first and then reached a platform as the whole emulsifiers concentration increased from 1.0%to 2.5%, meanwhile the diameter distribution became narrower. The optimum synthesis route is:SLS/OP-10=1/0.5, emulsifiers concentration=1.40%, initiator concentration=0.39%, reaction temperaturc=76?, conversion=96.4%. The resin obtained from the above route was later characterized by electron microscope, infrared spectrum, GPC, TG respectively. Later the resin was utilized in a waterborne metallic coating formula, and the resultant topcoat had good aluminum orientation effect and film performance, whose FI index reached 23.04.Chapter three mainly concentrated on the rheological properties study of the above alkali-swellable core/shell structure acrylic latex. The results show that methacrylic acid (MAA) was the main reason for resin thickening, which had the biggest influence on resin rheological properties. When other components in the polymer composition were constant, resin swellable ability, viscosity and modulus increased a little with increasing hydroxypropyl methacrylate (HPMA) content. When butyl acrylate (BA) content in the core portion increased, resin swellable ability increased all the time, meanwhile its modulus increased first and then decreased. We conclude that the possible reason for this is that the polymer structure intensity decreased when BA content was too high. The influence of soft/hard monomers ratio in the shell portion on resin rheological properties was the same with the core portion.Chapter four was the preparation and thickening mechanism study of a kind of alkali and cosolvent co-thickening latex used for waterborne metallic coating. As we know, alkali-swellable acrylic latex can swell and achieve rheology control capacity after it is neutralized with alkali (organic amines). However, these organic amines are easier to evaporate out of long-term storage or environmental temperature increase, thus the thixotropy and stability of the thickened dispersion are destroyed. To solve this problem, we designed and synthesized a kind of cross-linked core/shell structure acrylic latex, which could not be thickened by alkali directly. Nevertheless, when the latex was neutralized with alkali (dimethylethanolamine), it could be markedly thickened by additional coalescent agent ethylene glycol butyl ether (EGBE). The swelling behaviors, interfacial properties, microstructure as well as rheological behaviors of this thickened dispersion with alkali and cosolvent were intensively investigated by zetasizer, tensiometer, SEM, rheometer respectively. It was found that alkali could swell particles to some extent, while EGBE could not swell particles but precipitate them by changing the interfacial properties of latex. The main reason for latex thickening were the combination of alkali-swellable ability and cosolvent precipitation effect. The study also found that the rheological properties of this novel thickened dispersion can be controlled by solid content, neutralization degree and cosolvent content. The co-thickening dispersion is quite stable, which can be sheared well even after six months storage in sealed container at room temperature.In chapter five, to verify the effectiveness and universality of the alkali and cosolvent co-thickening mechanism, we introduced this thickening model into common acrylic latex system without core/shell structure. The acrylate copolymer was prepared with MAA, BA and methyl methacrylate (MMA) via emulsion polymerization technique. The latex couldn't be directly thickened by alkali at first since lots of its carboxyl groups were imbedded inside the copolymer. After the resin was neutralized with dimethylethanolamine, it could be markedly thickened with additional EGBE as well. It was found that the main reason for the resin thickening was also the combination of alkali-swellable ability and cosolvent precipitation effect. Further study of the rheological properties of the dispersion, it was found that resin thickening ability was controlled by polymer composition, cosolvent content and cosolvent type. Moreover, the results also show that this thickening model was effective in traditional styrene-acrylic latex system, and we infer the alkali and cosolvent co-thickening mechanism can be also applied in other acrylic latex systems. |
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