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The film forming essence of water-based industrial paint is: gradually volatilization of water+cosolvent → extrusion, fusion, deformation and coalescence of lotion particles to form a continuous film. Cosolvent is not simply “volatilized”, it is a temporary plasticized lotion, auxiliary particle fusion film forming agent; Its evaporation speed directly changes the film-forming window, viscosity evolution, surface tension gradient, particle fusion rhythm, and ultimately manifests in appearance.

First of all, distinguish: water is the main solvent, and the boiling point of cosolvents is generally higher than that of water. Many of them remain in the paint film after water evaporates and continue to play a plasticizing role.

1、 Bottom principle: volatilization rate determines

Ideal order (good appearance):

1. Water evaporates in large quantities first → the viscosity of the system slowly increases and there is time to level off;

2. When the moisture reaches the later stage, a part of the cosolvent will be retained to soften the lotion particles so that the particles can be fully deformed and fused;

3. Finally, the co solvent slowly evaporates, and the paint film gradually hardens and solidifies.

If the co solvent evaporates too quickly/slowly, the timing will be disrupted and the appearance will be affected.

2、 The co solvent evaporates too quickly

List of Problems

1. The cosolvent of flow adjustment, orange peel, brush mark and roll mark runs out quickly, and lotion particles lose plasticization, become hard and elastic; Before leveling, the particles have already “frozen” and the surface texture is directly fixed.

2. The surface solvent of pinholes and bubbles evaporates rapidly, and the surface is first sealed to form a film; The water and residual solvents in the lower layer need to run out and be blocked by the hard skin, breaking the paint film and forming pinholes.

3. Early water white/mist white solvents evaporate too quickly, resulting in insufficient particle fusion, multiple micropores inside the membrane, and white light scattering; Even if the water resistance can be restored in the later stage, the newly formed film will still be foggy and the luster will not improve.

4. If the paint dries too fast, it is easy to dry. When the paint drops after spray are flying to the workpiece, a large amount of solvent volatilizes, and the drop particles become dry and lose their plasticity. When they fall on the substrate, the dry powder accumulates, which is rough and dull.

Typical scenario: Adding fast solvents such as isopropanol and ethanol can make it dry quickly when sprayed, but the orange peel and gloss are unstable.

3、 The co solvent evaporates too slowly

List of Problems

1. The surface dries slowly, sticks to the hands, and becomes dusty. In the later stage of film formation, a large amount of co solvents remain in the paint film for a long time, continuously plasticizes the resin, and the paint film becomes soft for a long time; If there is dust in the workshop, it will stick to the dust and the appearance will be dirty.

2. If the solvent stays for too long in the flow, thick edges, and tear marks, the viscosity of the system will be low for a long time, and the coating at the thick film and corner positions will continue to flow, resulting in flow.

3. In the later stage, bubbles, sweating, and sticking of the surface have already appeared dry, and the internal solvent cannot be released for a long time; Subsequent heating/exposure to sunlight causes the internal solvent to migrate outward again, resulting in bubbles and a feeling of oil on the surface.

4. Long term presence of solvents for coloring and floating, and sufficient time for pigments and fillers to settle and migrate; The dispersion stability of different pigments in solvents varies, which can lead to color stratification and discoloration.

5. Water resistance delay risk: Long term residual solvents in the paint film are equivalent to internal plasticizers, which have poor initial water resistance and are more prone to whitening when soaked in water.

4、 Deeper layer: The difference in volatilization rate brings about [surface tension gradient → Bernard vortex]

The surface tension of different solvents varies:

Fast solvents preferentially evaporate from the surface → the local composition of the surface changes, and the surface tension fluctuates between high and low;

Generate surface tension difference, driving spontaneous convection of paint (Benard circulation);

Circulation carries pigments and fillers with it, forming concave convex patterns, floral patterns, and circular orange peels.

Key point: It is not the absolute value of surface tension, but the change of surface tension over time during the volatilization process, and the volatilization rate of the cosolvent is the largest variable.

5、 Addendum: Matching with the evaporation rate of water is the key

Solvent based paint: continuous evaporation of solvents, single system; Water based paint is a binary volatile system with a very important indicator: relative volatilization rate ratio

The evaporation rate of co solvents is approximately equal to that of water: when both run together, there is not enough solvent to soften the particles in the later stage, resulting in poor fusion and low gloss;

Co solvent is significantly slower than water (ideal combination): water goes first, co solvent stays behind to ensure particle fusion, and then slowly evaporates; That’s also why the mainstream choice for industrial water-based paint is alcohol ethers rather than alcohols.

6、 Practical formula rules

1. High gloss topcoat: Priority is given to medium and slow volatile cosolvents to ensure sufficient agglomeration, high gloss, and fine texture; Control should not be too slow to prevent sticky ash from flowing and hanging;

2. Fast drying anti rust primer: Paired with a portion of fast solvent to enhance surface dryness and prevent dust sticking, but the proportion of fast solvent should not be too high, otherwise pinholes and orange peel may occur;

3. Thick coating system: Reduce ultra fast volatile solvents, avoid premature sealing of the surface layer, and prevent high incidence of pinholes;

4. Low temperature and high humidity construction environment: Water evaporation itself is slow, and extremely slow co solvents cannot be added, which are prone to sagging and re sticking;

5. High temperature baking line: Caution should be exercised when using slow solvents, as the release rate of solvents during baking can suddenly change, leading to bubbles and pinholes.