[Science Popularization of the Society of Coatings] What are the internal factors that affect the drying efficiency of waterborne polyurethane coatings?

With the changes in national environmental protection policies, traditional organic solvent-based paints are gradually being replaced by water-based solvent paints.

The biggest feature of water-based paint is that it replaces the original organic solvent with water as the paint dispersant and thinner.

However, the volatility of water is much worse than that of organic solvents.
The natural volatilization (evaporation) of water at room temperature and pressure is minimal compared with organic solvents.
Therefore, the film forming process of water-based coatings in industrial production is Heating drying is used, and only some large-scale equipment cannot be heated and dried or manufacturers that do not require high production efficiency will use natural drying.

So what are the internal factors that affect the drying efficiency of water-based coatings? Aqueous polyurethane is a new type of polyurethane system in which water replaces organic solvents as the dispersion medium.
It is also called water-dispersed polyurethane, water-based polyurethane or water-based polyurethane.

Waterborne polyurethane uses water as the solvent, and has the advantages of no pollution, safety and reliability, excellent mechanical properties, good compatibility, and easy modification.

Polyurethane (PU) is a type of polymer compound containing repeating structural units of urethane bonds on the main chain.

Water-based polyurethane (WPU) refers to an emulsion formed by dissolving or dispersing PU in water.

Because WPU uses water as the dispersion medium, it has the advantages of non-toxic, non-flammable, and environmental protection.
It can be used on the surface of textiles, leather, paper, rubber, automobiles or furniture for decoration and protection.
It has gradually replaced it.
Solvent-based PU paint.

There are three main forms of water molecules in waterborne polyurethane coatings: free water, bound water and gel water.

Compared with solvent-based PU paint, the WPU drying process is characterized by a high temperature at which water starts to evaporate, and there is a certain degree of humidity in the external environment (the water vapor pressure of the paint film itself must be greater than the ambient water vapor pressure to evaporate).

These two characteristics make the drying rate of water-based polyurethane paints slower than that of solvent-based paints, which affects the promotion of WPU to a certain extent, so improving the drying rate of water-based paints is a research that must be done to expand the market.

What are the internal factors that affect the drying rate of waterborne polyurethane coatings? First, the influence of solid content.

The content of free water is closely related to the solid content.
The higher the solid content in the emulsion, the less free water, thus shortening the volatilization time of free water, which is beneficial to increase the drying rate of WPU.

Studies have shown that when the solid content of WPU is increased to more than 50%, the drying rate is equivalent to that of ordinary solvent-based PU.
When drying, a carbon fiber heating tube is used to heat the ambient temperature to 40℃-60℃.

In the production process, the solid content of the paint can be controlled by the annual paint.

Second, the influence of the content of hydrophilic groups.

In order to improve the dispersibility of WPU, hydrophilic ionic groups are usually introduced into the molecular chain of PU.
However, the introduction of hydrophilic groups increases the difficulty of drying WPU and reduces the drying rate.

Bound water is the water in a PU colloidal particle formed by hydrogen bonding and intermolecular force equal to PU hydrophilic group.
Its volatilization rate is much lower than free water.

This also means that the more hydrophilic groups, the more bound water in the emulsion, which is not conducive to the volatilization of water.

In addition, as the content of hydrophilic groups increases, the phase transition point of the entire coating system will be delayed accordingly, which is not conducive to coating emulsions with high solid content.
The simpler thing is that water is dissolved in the coating, and there is no free water as a dispersion.
Agent, paint cannot be used.

Therefore, under the premise of ensuring the stability of the emulsion, it is necessary to reduce the hydrophilic group as much as possible.

Third, the influence of hard segment content.

The increase of hard segment content can speed up the drying rate.

This is because as the content of the hard segment increases, the compatibility of the soft segment and the hard segment increases.
The more golden the molecular structure, the easier it is for water molecules to be discharged when the molecular chain moves.

However, the hard segment content should not be excessive.
When the hard segment content is about 46%, the drying rate will decrease.
This is because the emulsion surface is easy to crust and form a closed paint film at the beginning of drying, which is not conducive to the volatilization of the bottom layer of the paint film.
Prone to defects such as orange peel, fish eyes, and unqualified adhesion.

Fourth, the influence of neutralization degree.

The polite carboxylic acid group has weak hydrophilicity, while the carboxy anion that is neutralized to form a salt has good hydrophilicity.

Therefore, the degree of neutralization will affect the content of hydrophilic groups, so the higher the degree of neutralization, the stronger the hydrophilicity of the molecular chain, so the bound water content will increase, thereby affecting the drying rate of WPU.

At the same time, the greater the degree of neutralization, the higher the viscosity of the paint, which will cause the surface of the paint to be easily skinned, which will affect the volatilization of water in the paint film, and affect the drying rate and product quality.

Fifth, the influence of the film-forming substrate.

The surface properties of the film-forming substrate determine the force between its water molecules, so it has a great influence on the drying rate of the paint.

For example, the drying rate of the WPU water-based coating on the surface of the glass substrate is greater than the drying rate on the surface of the composite template substrate.

This is related to the surface tension of the substrate and the interaction between the emulsion and the substrate.
Wood with lower surface tension has a large force with water, which makes it difficult for water to volatilize.

In fact, the free water in the paint is physically absorbed by the substrate.
If you want to dry this part of the water, you need to provide more heat to make the water volatilize from the substrate.