Translated Abstract
There are several high-temperature baking procedures during the automobile coating process, which are the main reason for energy consumption and the decrease of coating efficiency. A general and primary solution is the development of low-temperature fast drying automobile coatings. However, due to the inaccurate characterization method of coatings drying speed and lack of profound knowledge on the drying speed of automobile coatings, the study of low-temperature fast drying automobile coatings still needs the guidance of basic theory and clear research strategy, which greatly inhibits the development of low-temperature fast drying automobile coatings. In order to solve the problems of characterization method and basic theory of the drying speed of automobile coatings, taking acrylic-polyurethane coating which is widely used in automobile coatings as an example, the following work are carried out in this dissertation: The characterization methods of the drying speed of acrylic polyurethane coatings are discussed and pendulum hardness is proposed to characterize the drying speed of acrylic polyurethane coatings the evolution of pendulum hardness of acrylic polyurethane coatings during curing is discussed and a new theory is established to analyze the drying speed of coatings based on the perspective of pendulum hardness. Fox and Arrhenius equation are used to analyze the factors influencing the drying speed of acrylic polyurethane coatings and the research strategy of developing low-temperature fast drying thermoset coatings is proposed Secondary amine compounds are introduced into acrylic polyurethane coatings to improve the drying speed The effect of non-functional monomers on the curing reactivity of acrylic polyols are explored and acrylic polyols with high curing reactivity are prepared through the optimization of vinyl monomers. The following conclusions are obtained:1. This research demonstrates that pendulum hardness is the reflection of the viscoelasticity of acrylic-polyurethane coatings and the relationship between pendulum hardness and coatings structure is established. The relationship between pendulum hardness and glass transition temperature of coatings is investigated. It is found that pendulum hardness tends to be zero when the coating is in viscous state and shows linear relationship to glass transition temperature in rubbery state and tends to be constant in glassy state.2. Compared to polish time and pencil hardness, pendulum hardness has higher precision which can describe the curing process of coatings detailedly. The evolution of pendulum hardness can be divided into three stages during the curing process of acrylic-polyurethane coatings (the low increasing stage, the fast increasing stage and the stable stage) which are accordant with the solvent volatilization, thermodynamic control and diffusion control of the curing reaction.3. Analysis based on gelation theory indicates that the molecular weight and crosslinking density of cured acrylic polyurethane coatings depend on the content of functional groups of acrylic polyols and curing agent. The crosslinking structure will only be formed when the relative molar content of hydroxyl and isocyanate groups are in a certain range. When the molar ratio of hydroxyl and isocyanate groups is 1:1, the crosslinking density will be the largest.4. A new theory is established to analyze the drying speed of acrylic-polyurethane coatings based on the angle of pendulum hardness and the principles of the development of low-temperature fast drying automobile coatings are proposed. It is found that the drying speed of automobile coatings is determined by the initial hardness and curing speed of coatings through the analysis of the evolution of pendulum hardness of coatings during curing. Fox and Arrhenius equation are used to analyze the factors influencing the drying speed of acrylic polyurethane coatings. It is pointed out that in order to develop low-temperature fast drying acrylic polyurethane coatings, one should consider the following three aspects: increasing the initial hardness of coatings, decreasing the activation energy or increasing the collision factor of the curing reaction.5. Secondary amine compounds with high curing reactivity are introduced to decrease the activation energy of curing reaction and the drying speed of acrylic polyurethane coatings is improved. The results demonstrate that the activation energy of reaction between polyaspartic ester PAE-1420 and isocyanate group is obviously lower than acrylic polyols and the modified acrylic polyurethane coatings show proper pot life, faster drying speed and excellent performance of coating film when the load of PAE-1420 is between 20 and 30wt%.6. It is firstly pointed out that the steric effect of non-functional monomers is important to the curing reactivity of acrylic polyols. Results of gelation time, reaction conversion and gel fraction all demonstrate that the curing reactivity of acrylic polyols increase first and then decrease with the increase of glass transition temperature, which can be attributed to the steric hindrance of non-functional monomers on the curing reactivity of acrylic polyols. Results also indicate that curing reactivity of acrylic polyols made from monomers with small substituent group (such as styrene) is higher and becomes lower for acrylic polyols made from monomers with big substituent group (such as butyl acrylate). Analysis based on collision theory demonstrates that the steric hindrance influences the steric factor and further determines the curing reaction rate. Acrylic polyols with high curing reactivity are prepared by the optimization of momomers.
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