Pinholes are paint film defects that occurs because of the gas bubbles formed in the wet paint leaving the film during the drying process. As the film dries, viscosity of the coating increases and the tear marks can be formed during the escape of the bubbles that cannot be closed by leveling.
If the same gas bubbles rise in the film at a later stage of drying, the rising film viscosity will prevent the bubble from leaving the film. In this case, unpopped gas bubbles form on the film surface. The occurrence, severity, and shape of the defect depend on the film viscosity at which the bubble leaves the film.
Therefore, when all other conditions remain the same, no problem in thin films was observed, but after a certain film thickness limit, the problem begins to occur. This limit is called the “pinhole limit”. The observed forms of the defect in the cross section of an oven-drying paint applied in gradual thicknesses are schematized in Figure 1. (Figure.1: Schematic representation of pinhole and gas trapping defects in film sections of different thicknesses)
Three types of occurrence can cause pinhole defect:
- Air bubbles trapped in the paint during manufacture or application;
- Solvents that remain in the film when the paint starts to gel during the curing phase and evaporate by boiling because it has a low boiling point;
- Gaseous chemical reaction by-products of paint polymers during both crosslinking and interaction with the environment.
Air bubbles trapped in the paint during manufacture or application can also cause bubble or crater shaped defects in the cured/hardened paint film. Such imperfections spoil the appearance of the paint and can be points that cause deficiencies in corrosion resistance and strength.
Bubbles can form during dispersion, sub-addition, or pre-mixing processes throughout the manufacturing process. Vortexes that are created during mixing or pigment dispersion are a major cause of air entrapment during the manufacturing process. The vortex formation can cause air to be drawn into the mixer or mixer blades, which then disperse and mix into the paint or paste.
During paint application, processes such as pumping, mixing, roller application and spraying cause bubbles to form in the paint. Spraying raises several possibilities for bubble formation. As in airless spraying processes, the air may first dissolve in the paint under pressure in the circulation line and then come out by boiling during spraying. Air bubbles that cannot leave the paint during the spraying period until the paint hits the surface remain trapped in the wet paint film formed on the surface.
Also, if paint is applied to a porous surface, the paint may tend to foam.
If the viscosity of the film is low enough and the film stays fluid long enough, bubbles trapped in the paint can leave the film without causing a problem. However, rapid solvent loss, rapid curing or high initial viscosity prevent air from leaving the film and keep the bubbles in the film. The use of low-volatile solvents is sometimes effective in keeping the film soft longer time so that the foams can burst or leave the film. Defoamers and bubble breakers destabilize the bubbles and cause them to fuse with each other and leave the film. (Figure.2: An example of intense pinhole - Figure.3: Cross-section photograph of a typical pinhole caused by an entrapped solvent bubble)
The best way to avoid air trapping is to prevent foam formation in the first place. Proper dispersion and mixing practice reduce air trapping during production. It is a basic requirement that the batch size and the size of the production tool be compatible.
When the paint starts to gel during curing, the solvent still trapped in the film leaves the film with a blistered boiling if its boiling point is low. The increased film viscosity due to gelation prevents the hole formed during vigorous solvent escaping from disappearing by film spreading.
Pinhole defect due to solvent trapping is common in oven-drying paints. Pinhole can be observed at room temperature in paints containing large molecule thermoplastic polymers that give a hard film by solvent evaporation, especially in hot seasons. Cuvette, in addition to flat-looking pinhole-like images, dome images of bubbles that come very close to the surface of the film but cannot leave the film are also frequently encountered. Solvent-induced pinhole formation can be reduced by extending the flash-off time of the paint, applying gradual heating in the oven, reducing the reaction rate of the paint, or using heavy solvents. The last precaution keeps the film fluid and therefore “open” for a longer period, allowing the wounds inflicted by other solvents to be removed by the spreading of the film.
In water-based paints, where organic solvents are replaced by water, solvent-induced pinhole formation is exacerbated. Aqueous formulations of similar polymers give lower "pinhole limits" than paint formulations with organic solvents. There are three probable explanations for these differences. The first reason is that, although organic solvent paintings can be made with various solvents with varying evaporation dynamics, water is always the determining factor in the evaporation table for water-based paints. Second, it should be noted that at room temperature, the rate of evaporation of water is lower than that of many organic solvents. Furthermore, the hydroxyl groups of water form strong hydrogen bonds with the polar groups on the binder molecule, causing further evaporation delay.
Gaseous reaction products are the third because that creates pinhole defects. The mechanism is like the trapped air imperfection mechanism. A prominent example is a pore defect caused by carbon dioxide formed by reactions of polymers containing isocyanate groups with moisture or amines.
Using defoaming and anti-foaming chemicals is also beneficial in resolving pinhole defects caused by both solvent and reaction product gases. The main concern with such additives is that they are difficult to mix with the paint homogeneously, and there is a possibility of cratering defects if the mixture is not homogeneously mixed. Relatively risk-free defoamers, methyl alkyl polysiloxanes, and acrylate surface additives with limited paint compatibility can be used. (Figure.4: Bubbles in a topcoat as a result of air entrapment and/or solvent boiling.).