What are the main reasons for the drying speed of water-based coatings?
Fast drying is the most frequently heard customer requirement for waterborne coatings. Due to the uniqueness of its molecular structure, i.e. the extremely strong hydrogen bonding between molecules, the characteristics are distinctly different from the vast majority of organic solvents. In the field of waterborne coatings, this characteristic is concentrated in the fact that, due to the high heat of evaporation of water, the evaporation rate of water is more than ten or even tens of times slower than that of common coating solvents. Moreover, due to the significant amount of water vapor in the air and the large seasonal variations, the evaporation rate of water changes accordingly. At its worst, if the relative humidity of air reaches 100%, the evaporation of water will stop, while non-water solvents are not affected by this factor.
Although, waterborne coatings face the technical challenges described above, they are bound to become an important player in the coatings field due to their environmental friendly properties. With the unremitting efforts of waterborne coating workers in the past decade or so, waterborne coating technology is becoming more and more mature. The following is a discussion of the main factors affecting the drying speed of waterborne coatings and the corresponding measures that can be taken when formulating.
1. Selection of resin.
Like all coatings, the performance of water-based coatings is largely determined by the resin chosen in the formulation. Most of the waterborne film-forming resins are emulsion systems, the film-forming mechanism of which is different from that of solvent-borne coatings. Solvent-based resins form a single-phase system with the solvent, and as the solvent evaporates, the viscosity of the system increases until it becomes solid, which is a continuous process in terms of the mechanical properties of the system. However, when the volume of emulsion particles reaches a critical value, the system suddenly changes from a state to a solid state, which is a discontinuous process. The full manifestation from surface drying to paint film performance depends on the evaporation rate of residual water in the system, the interpenetration of macromolecules in the emulsion particles, and the volatilization rate of other organic small molecules in the system. In order to optimize the system, the resin should be selected from the following aspects when making waterborne paint formulations.
a. Solid content: Usually, the higher the solid content of the emulsion, the closer it is to the surface drying critical value, the faster it dries. However, too high a solid content can also bring a series of disadvantages. Fast surface drying will shorten the painting interval and cause inconvenience in construction. Emulsions with high solid content usually have poor rheological performance due to the small spacing of resin particles and are not sensitive to thickeners, making it more difficult to adjust the spraying or painting performance of the paint.
b. Emulsion particle size: the smaller the particles of emulsion, the smaller the spacing between the particles under the same solid content, the lower the table dry critical value, the faster the drying speed. Small emulsion particles will also bring other advantages such as good film-forming properties and high gloss.
c. Resin glass transition temperature (Tg): Generally speaking, the higher the Tg of the resin, the better the performance of the final film. However, for the drying time, the trend is basically opposite. resins with high Tg usually need to add more film-forming additives to the formulation in order to facilitate the interpenetration of macromolecules between emulsion particles and promote film quality. These film-forming additives, however, require sufficient time to volatilize from the system and actually prolong the time from surface drying to full drying. So, in terms of this Tg factor, drying time and film-forming performance are often at odds with each other.
d. Phase structure of emulsion particles: depending on the emulsion preparation process, the same monomer composition may result in different particle phase structures. The widely known core-shell structure is one of the examples. Although it is not possible for all particles of an emulsion to be made into a core-shell structure, this figurative analogy is a way for people to have a general understanding of the film-forming properties of an emulsion. If the particles have low shell Tg and high core Tg, the system requires less film-forming additives and dries faster, but the hardness of the film will be affected because the continuous phase is a low Tg resin after film formation. On the contrary, if the shell Tg of the particles is high, a certain amount of auxiliaries is needed for film formation, and the drying speed of the film will be slower than the former, but the hardness after drying will be higher than the former.
e. Type and amount of surfactants: common emulsions use certain surfactants in the manufacturing process. Surfactants have an isolating and protective effect on the emulsion particles and have a great influence in the film formation process where the particles are fused with each other, especially in the initial stage, i.e. surface drying. Moreover, these unique chemicals, which have a certain solubility in both water and oil phases, dissolved in the resin will actually act as film-forming additives. Different surfactants, due to their different solubility in the resin, will have different film-forming agent roles.
2. Curing mechanism of resin.
Water-based resin film-forming curing generally has several levels of mechanism. First, the aggregation and fusion of emulsion particles, is the mechanism that all emulsion surface drying are bound to experience. Then, the volatilization of water and other film-forming additives, which allows the basic properties of the thermoplastic resin itself to be fully realized, is the second stage of curing. Finally, certain emulsions introduce a cross-linking mechanism during preparation, or cross-linking agents during coating application, to further increase the hardness of the film on top of the thermoplastic resin. The crosslinking mechanism in this last step can have a significant impact on the final speed and degree of curing of the film. Common crosslinking mechanisms include oxidative crosslinking (e.g., crosslinking of alkyd resins), Micell additive crosslinking (e.g., some self-crosslinking emulsion systems), and nucleophilic substitution crosslinking (e.g., epoxy, polyurethane, etc.). These cross-linking reactions, are affected by temperature, pH and other factors, in the formulation should balance the curing requirements of the system and other properties of the relationship.
3. The amount and type of film-forming additives.
Theoretically, the solvent of any resin is a film-forming additive. In practice, taking into account safety, cost, speed and other factors, there are only a dozen common film-forming additives, mainly some high boiling point alcohols, ethers and esters. These film-forming additives are preferred by different waterborne coating engineers. Generally, there are only two or three kinds of film-forming additives commonly used by an experienced engineer. The main consideration is the distribution of the reagent between the water and the resin and within the resin particles. Especially when the water-based resin is multi-phase resin, the selection and matching of film-forming additives is particularly important.
4. Construction environment.
At the beginning of this paper, we discussed the issue of water. Because of the characteristics of water, the construction environment of water-based paints is more demanding than that of oil-based paints, mainly because the temperature and humidity during construction should be controlled as much as possible. For general purpose formulations, high humidity should be avoided as much as possible. If it is necessary to work under high humidity, the formulation should be adjusted, or a resin with fast film formation should be selected or the site should be isolated.
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