3 Types of Waterbased Resins and Film Formation Mechanism

30th May 2024

What is water-based resins?

Water-based resin is a polymer compound that can be dissolved or dispersed in water to form a liquid substance. Compared with traditional oil-based resins, water-based resins are more environmentally friendly because they do not contain harmful volatile organic compounds and do not release harmful gases. In addition, water-based resins have many other advantages, such as easy processing, easy coating, and non-flammable. Therefore, water-based resins have been widely used in coatings, adhesives, inks and other fields.

3 types of water-based resins

1. Water-soluble resins have a certain solubility in water, and the resin liquid is usually transparent in appearance. Hydrophilic groups are introduced into the resin to increase its solubility in water. The characteristic is that the coating film has a good appearance, but due to the small molecular weight, the drying, water resistance and solvent resistance are poor. For example, water-based cellulose derivatives and water-based modified polybutadiene are all water-soluble resins.

Because the polymer will completely dissolve in water, the viscosity of the system will become very large, so water-soluble resins cannot be used as film-forming substances, and the resins used in water-based coatings are partially miscible or insoluble in water.

2. Water-dispersible resins are dispersed in water in the form of tiny soft colloid particles, with a dispersed particle size of 0.01-0.1um. These resins are generally transparent or translucent milky white in aqueous solution. The coating of water-dispersible resins is dense after curing, with good gloss and transparency, and good water resistance, solvent resistance and corrosion resistance. These resins are very water-soluble, and the water resistance of the coating after drying is much better than that of water-soluble resins. If they are mixed with cross-linking agents and cured, the water resistance of some varieties can be comparable to that of solvent-based resins.

3. Water-emulsion resins are dispersed in aqueous solution in the form of smaller soft colloid particles, with a dispersed particle size greater than 0.1um, and are milky white and opaque in aqueous solution. Emulsion resins have a large molecular weight, so they have better mechanical properties, drying, water resistance and corrosion resistance. This type of resin needs to add a large amount of emulsifier during synthetic emulsification, which will have a certain impact on the water resistance of the final coating, and its water resistance is usually not as good as that of water-dispersible resins.

Film formation mechanism of waterbased resins

The film formation process is relatively complex, and it has to go through a process from dispersed polymer particles to mutual aggregation to form a whole. After its construction, the water evaporates, and the spherical particles must fuse with each other to form a continuous coating film. The film formation is divided into the following processes:

1. The particles gradually approach and fill the process
The spherical particles in the latex paint remain dispersed by the double electric layer and shielding stability. After the water-based paint is applied, the water gradually evaporates, and the polymer particles and pigments and filler particles that were originally dispersed by electrostatic repulsion and steric stabilization gradually approach, but can still move freely. At this stage, the volatilization of water is similar to the volatilization of pure water, which is a constant rate volatilization.

2. Particle fusion process
As the water evaporates further, the protective layer adsorbed on the surface of the polymer particles is destroyed, the exposed particles contact each other, the gap between them becomes smaller and smaller, and the volume of the paint film shrinks. When the water evaporates, the driving force will also disappear. When the capillary diameter is large, due to the capillary efficiency, the capillary pressure is higher than the deformation resistance of the polymer particles, the particle stability is destroyed and deformed, and finally agglomerates and fuses into a continuous coating film.
This process is the key to whether the film can be formed. If the glass transition temperature (Tg) of the emulsion polymer is high (in order to make the coating have good mechanical properties, weather resistance and stain resistance, the Tg value is generally not too low), it is difficult to deform at a low ambient temperature, which will hinder the fusion process and lead to the failure of film formation. At this time, a film-forming aid is needed to assist in film formation. The film-forming aid can make the latex particles swell and soften, so it is easy to fuse them together to form a continuous film.

3. The process of polymer chain segments interdiffusion, penetration, cross-linking and film formation
As time goes by, the additives remaining in the water gradually diffuse into the coating film, and make the long chain segments of polymer molecules interpenetrate and diffuse, entangle to form a uniform coating film with good performance. As the film-forming additives gradually evaporate from the paint film, an ideal coating film with excellent performance is finally formed.