Original article date: Apr. 18, 2014
Updated by Jochum Beetsma on Oct. 12, 2022
Introduction
During film formation, a paint must transform from a liquid material to a closed, strong and beautiful coating. To give optimum protective and aesthetical properties, a coating should be smooth and without craters, pinholes, cracks etcetera. Defects can ruin the desired properties of coatings. A huge amount of time and money is invested in preventing and resolving defects. Substantial savings can be made when knowledge, being both simple and available, is used. Preventing or resolving a defect is easier and faster when the phenomena, that cause the defect, are known and understood. A wide variety of coating defects occur.1,2 In the following, only a few of them are briefly discussed.
Crawling
Crawling, also called retraction or pulling back, is a uniform de-wetting effect: the surface energy of the solid substrate (ℽsg) is lower than the surface tension of the liquid paint (ℽlg).3 The resulting effect is that the liquid paint does not fully wet the substrate: the liquid retracts and forms droplets, leaving part of the substrate naked.
Wetting becomes more critical 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, implying that the surface of the substrate is not able to interact strongly with a material brought on top of it. Unmodified plastics, for example, are materials of low surface energy. Crawling can be prevented in a number of ways. A pre-condition that has to be met, in order to obtain complete wetting, is that the substrate is clean. In some cases, the substrate must be modified in such a way that the surface energy of the substrate is raised, implying that the surface of the substrate is changed from hydrophobic to more hydrophilic. Substrate pre-treatment is often applied to plastics prior to coating.4 Another approach, often used to prevent crawling, is to lower the surface tension of the liquid paint by adding an additive called wetting agent. However, the use of wetting agents can worsen key properties, like hardness, recoatability and foam sensitivity, of the system.
Cratering
Cratering is an undesired phenomenon that can start at the paint-air interface or at the substrate-paint interface. Cratering is the result of flow of liquid during film formation, caused by gradients in surface tension gamma (ℽ). These gradients develop when a material is present that has both a low surface tension (implying that it is hydrophobic) and a limited miscibility with the paint.
A system strives to attain a state of minimum energy. Therefore, a liquid will flow from an area with low surface tension to cover an area with higher surface tension.
Droplets or particles of a hydrophobic contaminant, like lubricating oil, defoamer or gel particles, are often the initiators of craters in a coating. The formation of craters can be prevented when the presence of incompatible hydrophobic contaminants is prevented. When there is cratering problem, it is a good idea to take a look at the defoamer that might be present in the system.5 Another point of attention is to check whether or not the substrate is contaminated.
Foam
Foam bubbles can already be present in the paint before application starts. Foam can also be introduced during the application process. Especially during brushing, rolling and spraying, air can be brought into the paint. A formulator wants to develop a paint that loses its foam as soon as application has finished and film formation proceeds.
Two aspects strongly dominate the formation, stabilization and release of foam. First, foam bubbles can be stabilized by additives that have a surfactant (soap) structure. This problem is most often experienced in waterbased 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 is not able to level out when the viscosity of the film is too high.
The prevention of bubbles and pinholes in films, caused by foam, can be arranged in a number of ways. First, the use of surfactants should be minimized, especially in waterbased systems. Secondly, the introduction of air in the paint during production, handling and application should be prevented as much as possible. Finally, the rheology of the system should be such that the air, that was introduced, can easily escape during the first stage of film formation. Please be aware that the use of a defoamer, an approach that cannot always be prevented, increases the risk of cratering.5
Cracking
A thin film on a substrate can break when the film suffers from stress. Breakage of a coating is called cracking. A coating loses its protective properties in the areas where the film has broken: within the cracks, 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 case of wooden substrates. Cracking is often accompanied by loss of adhesion. Water molecules, for example, can migrate through a coating at the cracks, to reach the substrate-coating interface. Then, water can induce loss of adhesion either directly, by interfering with the intermolecular forces between substrate and coating, or indirectly, as a result of corrosion caused by the water being present at the interface.
Most often stress builds-up in coatings because of shrinkage taking place during film formation. Several phenomena can give film shrinkage. 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 during film formation, resulting in stresses being frozen-in. Secondly, evaporation can proceed too fast. In many cases, there is a competition between cure and evaporation during film formation giving stress.
A high loading of solid particles (pigments and/or fillers) can also give stress in films.
Finally, stresses can occur when the substrate is not stable in its dimensions. An example is the dimensions of wood changing because of variations in humidity.
Specifically, waterbased systems can suffer from internal stress, thus giving cracking. Waterbased wall paints, for example, are often based on binder emulsions. During film formation, the water evaporates from the film and the binder droplets merge. Stress can build-up in such a system during film formation, especially when a high amount of solid particles is present. In such a loaded system, the merging of the binder droplets can become critical and the film can crack because of the stress that builds-up during film formation.
Cracking is highly undesirable from both protective and aesthetical point of view. As with many defects, several options are available to tackle a cracking problem. What solution must be chosen depends upon the cause(s) of cracking of the coating.
References
- Article Overcoming Paint Film Defects: Causes and Remedies, Ron Lewarchik, 16 June 2017
- Article Coating Film Defects – Part 2, Marc Hirsch, 30 September 2016
- Article Surface Tension and Surface Energy, Jochum Beetsma, 27 September 2019
- Article Plasma Processing of Plastic Surfaces, Andy Pye, 30 December 2016
- Article Defoamers, An Introduction, Marc Hirsch, 29 October 2021
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Simple and clear explanations, thank you.
Hello
Does the addition of DAPRO® U-99 to a coating formulation improve the coating runnability & fluidity (the rheology). my current formula had a defect that the lost drop flow so slowly and dry forming a skim (cosmetic defect).
I’m looking for a resolution to eliminate Orange Peel. I have an UV Curable coating with low surface area. I use a commercial grade solvent screen printing ink. Once the ink is screen printed, Orange Peel is observed. The ink has slow evaporating solvents, so even with the slower drying process of the ink, the Orange Peel does not level out. Any feed-back would be helpful.
Dear Lam Dang,
have excellent experiences for the described application in polyaspartic systems, using Pat-Add AF 70 as defoamer! Just small dosages show impressive speed of de-aeration
Pls suggest how to control popping on abs part
First thing to do: describe the CAUSE of the popping problem as clear as possible, then sum-up possible solutions and then pick the most suitable solution.
For example, in case popping is caused by vapour bubbles that are formed in the system during heating-up in the oven, a solution might be to SLOWER heat-up the object.
Sorry
i wanted to say cratering problem not crawling:
We have problem with Cratering in oven curing liquid paints without using defomer and no other external contamination!!
Can any one helps to solve this problem
Dear Aabkari,
The cause of the cratering problem must be found. This can be done by ‘undressing’ the paint formulation. So, start with the binder system + water/solvents only and apply this undressed paint (preferably on glass) and cure the film. The film should be clear and free of defects.
If you see craters and/or other defects like haze: further undress the paint, for example: leave some solvents out.
If the undressed paint is clear and free of defects: start adding the other components and see where it goes wrong.
This should give you insight in the cause of the problem.
Success with your challenge!
Greetings, Jochum Beetsma.
Hello,
If you are still looking for information on the orange peel defect, came across another article mentioning possible solutions, “Overcoming Paint Film Defects: Causes and Remedies” by Ron Lewarchik. Maybe it can help.
Best regards.
Dear Dr. Beetsma,
What can you tell about the occurrence of Snail trails in water-based flexible outdoor paints ?
The causes and remedies ?
Thanks,
Snail trails on exterior wall paints are caused by watersoluble materials that are within the coating. These materials migrate to the surface and dissolve in water that comes from rain or dew. The mixture runs down because of gravity, the water evaporates and the materials stay behind on the surface of the coating as trails. Two sorts of snail trails can roughly be distinguished. Trails that result from watersoluble salts in the coating are matt. Glossy trails result from watersoluble oligomers, mostly surfactants or rheology additives.
To solve the problem, it must be found out which paint component causes the defect. This component should be excluded from the formulation.
Note: snail trails are caused by materials that are within the coating. Surface pollution that is caused by watersoluble salts that migrate from the substrate, through the coating to the surface is called efflorescence. Efflorescence mostly does not give trails but it results in a matt regular deposition on the surface of the coating.
Hi Dr. Beetsma,
How could you explain the formation of pinholes in a hydrophobic coating formed on a hydrophilic substrate?
Mostly, pinholes in a coating are caused by small air bubbles that are entrapped in the coating, so-called microfoam. There are a few things to check. The first thing to do is finding out if there is an additive in the paint formulation that stabilizes foam, like surfactant. Such additives should be removed from the formulation as much as possible. Secondly, a suitable defoamer should be used in the formulation to enable the release of foam during film formation.
Hi
We have bad coating film(waves, like orange peel , also staging if paint more diluted), but only on the vertical surface.
this problem occur with solvent based Acrylic polyurethane paint. solvent based paint
the product viscosity is 85ku
we added leveling agent byk 320 and surface tension reducer Byk 306, we added also Byk 410, the problem remain the same
It appears as if you might have a sagging problem: running of paint on a vertical surface, caused by gravity.
You might try rheology additive that consists of clay particles, for example a Bentone product. You can find those products on the UL Prospector website.
First you judge the polarity of your paint and then select the right Bentone product.
Dear Mr. Jochum
Thank you for your explanations on paint defects.
Pls explain to us on orange peels on substrate. The reason and how to avoid either during paint formulation and production and application.
Thank you
Dear Sahara paint co:
The film defect orange peel is caused by a contamination that is not compatible with the paint. This contaminant can be present on the substrate, for example because of insufficient cleaning. Another possibility is that the contamination is within the paint, for example gel particles from the binder system. Also, using the wrong defoamer can result in orange peel. Orange peel can be avoided by assuring that no incompatible materials are present, neither on the substrate nor within the paint.
Thank you for reading,
Jochum
Your help required regarding this technical issue and give me the best possible solutions :
Using Solvent Base Screen Printing Ink And after Drying appears bubbles or small dots in the printing Area (Yellow Ink)
Thanks for your kind cooperation.
Good day, Ibrar:
First it is important to find out what it is that you see: bubbles, dirt/gel particles, . . .
If bubbles cause the defect, please realize that bubbles are formed when force is applied to a system that is in contact with air/gas. This ‘force’ can be stirring, brushing, rolling, spraying, screen printing etcetera.
For how long the bubbles remain in the system depends, for example, on viscosity. The higher the viscosity, the more difficult it is for bubbles to escape.
There are several possible routes to prevent/minimize bubbles, for example: use a defoamer or adjust the rheology of your system.
There are other possible causes for bubble formation in a system. An example is the formation of gas during crosslink reaction. A notorious example in paint industry is the formation of CO2 gas in 2-component polyurethane systems, when isocyanate crosslinker reacts with water molecules that are present in the system (or on the substrate). The release of air from the pores of a porous substrate can also give bubbles: when liquid paint penetrates the pores, the paint substitutes the air in the pores.
I advise to first find out what it is that you see and then find the cause for the defect.
Thanks for reading!
Jochum