How to Prevent Coating Film Defects?

Crawling, also called retraction or pulling back, is a uniform de-wetting effect. The surface energy of the solid substrate is too low to let the liquid paint, having a certain surface tension, cover that substrate. The resulting effect is that the liquid paint does not fully wet the substrate, but it retracts and forms droplets, leaving part of the substrate naked, Figure 1.
 

Causes of crawling

Wetting becomes more critical in two cases:
 

• when the surface energy of the substrate is low (and/or)
• when the surface tension of the liquid paint is high.

A substrate of low surface energy is called hydrophobic. This implies that the surface of the substrate is not able to interact strongly with the material that is applied to it. Unmodified plastics, for example, have a low surface energy.

 

Ways to prevent crawling
 

Crawling can be prevented in a number of ways. 
 

1. A pre-condition that must be met, to obtain complete wetting, is that the substrate is clean.
2. In some cases, the substrate must be modified to raise its surface energy. This means that the substrate is changed from hydrophobic to hydrophilic. Plastics are often pre-treated before paint is applied.
3. Another approach, used to prevent crawling, is to lower the surface tension of the liquid paint by adding wetting agents. However, this can worsen key properties of the system, like hardness, recoatability, and foam sensitivity. Avoid such issues by selecting the right wetting agent for coatings.

Cratering is an interfacial phenomenon. It can start either at the coating-air interface or at the interface between the substrate and paint.

 

Causes of cratering

Cratering is the result of the flow of liquid that is caused by dewetting and/or by gradients in surface tension, g. Such gradients build up when materials are present in the liquid paint that have both a low surface tension (implying that they are hydrophobic) and limited miscibility with the paint.

A system strives to attain a state of minimum energy. For this reason, a liquid will flow from an area with low surface tension to cover an area with higher surface tension, Figure 2.

 

Droplets of hydrophobic contamination, like lubrication oil or defoamer, are most often the initiators of craters. 

 

Ways to prevent cratering
 

The formation of craters can be prevented in the following ways: 
 

1. The presence of incompatible hydrophobic contaminants is prevented.
2. Look for a defoamer that might be present in the paint.
3. Check if the substrate is clean.
4. Use anti-crater aids to solve cratering defect.

Flooding is a film defect originating from the evaporation of solvents. This results in a non-uniform distribution of pigments over the dried film. Strong gradients in temperature and composition build up when the evaporation of solvent proceeds fast. 

 

Causes of flooding

The phenomenon, being a direct result of the gradients, is the flow of liquid from the bulk of the wet film to the paint-air interface. The temperature of the paint is lowest near the paint-air interface because solvents evaporate at this interface. 

Mobile pigments that are not well-stabilized can be dragged along in the liquid flow. Pigment separation, from the bulk of the film to the paint-air interface, takes place when pigments differ in mobility. A gradient in pigment composition is formed throughout the coating, Figure 3.
 

Ways to prevent flooding

Flooding becomes visible when pigments differ in color. Approaches that be used to prevent flooding include:
 

1. Use solvents that have a lower speed of evaporation.
2. Improve the colloidal stability of the pigment particles by using suitable dispersants, possibly in combination with adjustment of the rheology of the paint.

Foam can already be present in the paint before the application starts. It can also be introduced during the application process. Especially during brushing, rolling, and spraying, air can be brought into the paint. A coating formulator wants to develop a paint that loses its foam as soon as the application has finished and film formation proceeds.

 

Causes of foaming
 

Two aspects strongly dominate the formation, stabilization and release of foam. 
 

• Foam bubbles can be stabilized by additives that have a surfactant (soap) structure. Such foam problems are experienced most strongly in water-based paints.
• The second important aspect is the rheology of the paint. Despite the viscosity being high, the foam bubbles might be able to move to the surface of the film where they can break. However, the resulting hole possibly cannot level out when the viscosity of the film is too high, Figure 4.
   

Figure 4: Foam - The holes, that were left in the film when air was releasedduring film formation, do not level out

Ways to prevent foaming
 

The prevention of foam in films can be arranged in several ways. 
 

1. The use of surfactants should be minimized.
2. Introducing air in the paint during production, handling, and application should be prevented as much as possible.
3. Finally, the rheology of the system should be such that the air, that was brought into the paint, can escape in a short time during the first stage of film formation.
4. Be aware that the use of defoamer, being an approach that cannot always be prevented, increases the risk of cratering.

A thin solid film on a substrate can break when the film suffers from stress. In coating technology breakage of a coating is called cracking, Figure 5.
 

Figure 5: Cracking - A coating breaks because of internal stress

Effects of cracking

A coating loses its protective properties in the areas where the film has broken; at those spots there can be direct contact between the substrate and the environment. This can result in corrosion when the substrate is metal or in rot in the case of wooden substrates.

In many cases, cracking of a coating is accompanied by loss of adhesion. For example, water can migrate through a coating at the cracks getting into contact with the substrate-coating interface. When doing so, water can induce a loss of adhesion.

 

Causes of cracking

Stress due to shrinkage

Stresses in coatings most often build up because shrinkage takes place during film formation. Film shrinkage can basically be caused by two processes.
 

1. First, the chemical reaction taking place during film formation, called crosslinking, can be fast. UV-systems, for example, cure via radical polymerization, being a fast crosslink chemistry. During the cure of a UV system, a high crosslink density is generated in a short time of film formation giving rise to stresses being frozen in.
2. Secondly, evaporation can proceed too fast. In many cases, there is a competition between evaporation and cure during film formation.

Stress due to high loading of solid particles

Specific water-based systems can suffer from internal stress causing cracking. Water-based wall paints are often based on a binder dispersion. During film formation, the water evaporates from the film and the binder particles merge.

Stress can easily build up in such a system, especially when a high number of solid particles, pigments, and/or fillers, is present. It is said that such a system is highly loaded with particles. In such a loaded system, the merging of the binder particles can become critical and stress can build up in the film. The film can break because of the stress.

Finally, stresses can occur when the substrate is not stable in its dimensions. The dimensions of wood, for example, change because of variations in humidity.

Cracking is highly undesirable for both protective and esthetical reasons. As with many defects, several options are available to tackle a cracking problem. The best solution is always based on the main cause of the cracking.