The film forming mechanism of water-based two-component polyurethane is different from thermoplastic polymer solution, and also different from thermoplastic polymer dispersion and thermosetting polymer solution.
From the composition point of view, it is related to both the particle coalescence mechanism of the thermoplastic polymer dispersion and the film formation mechanism of the thermosetting polymer solution's crosslinking reaction.
During the film forming process:
Volatilization of volatile matter (solvent, water);
Co-coagulation of polyol and polyisocyanate particles;
The reaction of polyisocyanates and water;
The reaction of polyols and polyisocyanates.
Its film formation is composed of a series of kinetic processes, and its occurrence sequence and development process have an important influence on the quality and performance of the film.
At the beginning of the mixing of the two components, the coalescence of polymer particles and the chemical reaction of isocyanate are important processes that affect the system. In addition to the above factors that continue to play an important role in the film formation process, the evaporation of water and solvent also becomes very important. Once the isocyanate component is mixed with the hydroxyl component, it is immediately dispersed. During the mixing stage, the polymer particles coalesce to a small degree. Many studies have confirmed that
At the beginning of the mixing of the two components, the coalescence of polymer particles and the chemical reaction of isocyanate are important processes that affect the system. In addition to the above factors that continue to play an important role in the film formation process, the evaporation of water and solvent also becomes very important. Once the isocyanate component is mixed with the hydroxyl component, it is immediately dispersed. During the mixing stage, the polymer particles coalesce to a small degree. Many studies have confirmed that the reaction of -NCO with water is much slower than the reaction with hydroxyl, which is an important theoretical basis for the design of water-based two-component polyurethane coatings.
As the water and solvent evaporate during the film formation process, the polymer particles are squeezed together, and the polymer molecular segments diffuse through the interface to cause the coalescence of the polymer particles, which also promotes the reaction of -NCO and hydroxyl groups,- -The reaction rate of NCO varies depending on the system. It usually takes a few days to react 80% of -NCO, and the performance of the membrane is generally fully established within 7 to 14 days.
Using infrared spectroscopy and thermal analysis to study the liquid chemical reaction of water-based two-component polyurethane, it is found that the reaction rate of -NCO is not only related to the hydroxyl concentration and moisture content of the system, but also related to the type and colloidal structure of the hydroxyl component. The colloidal structure of the hydroxyl component and other groups on the polymer chain have a very important effect on the reaction rate of -NCO, and the reaction rate of -NCO directly relates to the construction time limit of the coating and the CO2 problem that produces film bubbles. After the two components are mixed, the particles condense and the reaction of isocyanate is the main process; after construction, the evaporation of water and solvent is the main process.
By studying the competitive reaction of water, hydroxyl and isocyanate before construction after mixing, it was found that the reaction of hydroxyl and isocyanate occurred almost immediately, while water and isocyanate only started to react after 2h. After the two components are mixed, the viscosity of the system increases slightly in the first 1-2h, and then the viscosity gradually decreases, beginning to foam slightly, and finally foaming a lot. The foaming is caused by the formation of CO2 between water and isocyanate. As the amount of catalyst (dibutyltin) increases, the foaming time is shortened. Catalysts, defoamers and stirring speed all affect this result and also affect the product's lifespan.
In the water-based two-component polyurethane system, the polyol and curing agent are dispersed in the medium water in the form of latex particles, and the film forming process is different from the solvent-based two-component system. The initial stage is a physical drying process, which is mainly the volatilization of water. During this process, the polymer particles are close to each other and merge, and the polymer chains begin to diffuse and react with each other. The main factors affecting the process are:
The water content is affected by temperature and humidity;
The viscoelasticity of polymer polyol and curing agent will affect the fusion deformation process of particles. Viscoelasticity depends on Tg, polarity, molecular weight, solvent and plasticizer, etc.;
For anionic and nonionic stable latex particles, it is difficult to overcome the repulsive force on the particle surface.
The interdiffusion of particles is an important factor of the thermosetting two-component system, because it determines the degree of crosslinking of the crosslinking agent through the particle interface to form a network structure. Especially for high Tg, Mw quick-drying system, the shear modulus G is high, and the film forming property is poor. Using low Tg HDI trimer crosslinking agent, high Tg polymer polyol emulsion can be used. The particle size is small and the film formation is good, but it is difficult for such emulsions to achieve the balance between solid content/viscosity, and more water needs to be evaporated to achieve the fusion state, in order to obtain better film formation for high Tg and Mw emulsions Performance, by modifying the polymer surface, when the emulsion reaches a better solid content/viscosity balance, the polymer chains can be fused and diffused. In addition, increasing the interfacial tension between air and water is also beneficial to the film formation of particles. The key factor in the film formation process is the dispersion of polymer molecules during coalescence, which is related to the large-scale segment movement of molecular chains, so the polymer will be lower than Tg when the temperature is higher than the glass transition temperature (Tg) When it is easier to form a film. However, the coatings produced by polymers with lower Tg have poor strength, durability, abrasion resistance and chemical resistance. To overcome these performance deficiencies, polymers with higher Tg and higher molecular weight are used. In order to form a proper film, such dispersions need to add a solvent to swell the polymer particles to reduce Tg, or use a higher film forming temperature to increase the fluidity of the polymer. But these can cause higher VOC content or require a baking step.
Due to the existence of many side reactions, the chemical drying of the two-component waterborne polyurethane system is more complicated than the corresponding solvent-based system. The rate of these reactions depends on the reaction conditions such as temperature, catalyst, concentration, and the reactivity of polyisocyanate and active hydrogen groups. For aliphatic and cycloaliphatic polyisocyanates, excess water will compete with polyols. The reaction rate of water and such polyisocyanates largely depends on the pH value (pH>7, the reaction rate is fast). The reaction rate of carboxyl groups is slow, while amino groups are one or several orders of magnitude faster than polyols. The mixing and compatibility of polyisocyanates and polyols in the particles are very important factors. They affect the distribution of the crosslinking agent between and within the particles, as well as the uniformity of the final coating film.
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