What are the factors that affect the drying of water-based acrylic resin?

Factors affecting the drying of water-based acrylic resin:

1. Selection of resin:

Aqueous emulsion is a two-phase system. With the evaporation of water, the viscosity of the system does not change much at first, but when the volume of emulsion particles accounts for a critical value of the total system volume, the system suddenly changes from liquid to solid, which is an inexorable phenomenon. A continuous process, this critical point is the start of water-based paint tack, so the tack-free time of water-based paints is shorter than some solvent-based paints. The whole performance 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 evaporation rate of other small organic molecules in the system. In order to optimize the system, when making water-based paint formulations, the resin should be selected from the following aspects:

A. Solid content:

In general, the higher the solids content of an emulsion, the closer it is to the tack-free threshold, and the faster it will dry. However, too high solid content will also bring a series of unfavorable factors. Too fast surface drying will shorten the brushing interval and bring about inconvenience in construction. Emulsions with high solid content usually have poor rheological properties due to the small spacing between resin particles and are not sensitive to thickeners, which makes it more difficult to adjust the spraying or stucco performance of coatings. b. Emulsion particle size: The smaller the emulsion particles, the smaller the distance between particles under the same solid content, the lower the critical value of surface dryness, and the faster the drying speed. The small particles of the emulsion will also bring other advantages such as good film formation and high gloss.

B. Emulsion particle size:

The smaller the particles of the emulsion, the smaller the spacing between the particles under the same solid content, the lower the critical value of surface dryness, and the faster the drying speed. The small particles of the emulsion will also bring other advantages such as good film formation and high gloss.

C. Resin glass transition temperature (Tg):

In general, the higher the Tg of the resin, the better the final film-forming properties. However, for drying time, the trend is basically the opposite. For resins with high Tg, it is usually necessary to add more film-forming auxiliaries in the formulation to facilitate the mutual penetration of macromolecules between emulsion particles and promote film-forming quality. And these coalescents need enough time to evaporate from the system, which will actually prolong the time from surface dry to full dry. Therefore, in terms of this Tg factor, drying time and film-forming properties are often contradictory.

D. Phase structure of emulsion particles:

Depending on the preparation process of the emulsion, the same monomer composition may form different particle phase structures. The well-known core-shell structure is one such example. Although it is impossible for all particles of an emulsion to be made into a core-shell structure, the metaphor of this image is that people can have a popular understanding of the film-forming properties of the emulsion. If the shell Tg of the particles is low and the core Tg is high, the system requires less film-forming aids and dries faster. However, since the continuous phase after film formation is a low-Tg resin, the hardness of the paint film will be affected to a certain extent. On the contrary, if the shell Tg of the particles is high, a certain amount of auxiliary agent is required for film formation, and the drying speed of the film will be slower than that of the former, but the hardness after drying will be higher than that of the former.

E. Type and dosage of surfactant:

Common emulsions use certain surfactants in the manufacturing process. Surfactant has the effect of isolating and protecting the emulsion particles, and has a great influence on the film-forming process of the particle fusion, especially in the initial stage, that is, when the surface is dry. Moreover, these unique chemicals have a certain solubility in water and oil phase, and the part dissolved in the resin actually acts as a coalescence aid. Different surfactants, due to their different solubility in resins, have different effects as film formers.

2. Curing mechanism of resin:

Waterborne resin film-forming and curing generally has several mechanisms. First, the aggregation and fusion of emulsion particles is a mechanism that all emulsion surface dryness must experience. Then, the volatilization of water and other film-forming aids makes the basic properties of the thermoplastic resin itself fully manifest, which is the second stage of curing. Finally, some emulsions introduce a cross-linking mechanism during preparation, or introduce a cross-linking agent when the coating is used, so that the hardness of the film is further improved on the basis of thermoplastic resin. The cross-linking mechanism of this last step will have a great influence on the final curing speed and degree of the film. Common crosslinking mechanisms include oxidative crosslinking (such as crosslinking of alkyd resins), Michael addition crosslinking (such as some self-crosslinking emulsion systems), and nucleophilic substitution crosslinking (such as epoxy, polyurethane, etc. ). These cross-linking reactions are affected by factors such as temperature and pH, and the relationship between the curing requirements of the system and other properties should be balanced when formulating.

3. Dosage and type of film-forming aids:

In theory, any resin solvent is a coalescent. In practice, considering factors such as safety, cost, speed, etc., there are only a dozen common film-forming additives, mainly some high-boiling alcohols, ethers and esters. These coalescents will have their own preferences for different waterborne coating engineers. Generally, an experienced engineer only has two or three types of coalescents commonly used. The main considerations are the distribution of the reagents between the water and the resin and within the resin particles. Especially when the water-based resin is a multiphase resin, the selection and matching of the film-forming aids are particularly important.

4. Construction environment

The construction of general formula should try to avoid high humidity environment. If it must be applied under high humidity, the formulation should be adjusted, or a resin with fast film formation should be selected or the site should be isolated.