The difference between pinholes and shrinkage cavities and their treatment methods

Pinholes and craters are common defects in powder coatings. Their presence not only affects the product's appearance but also severely impacts its mechanical and protective properties. Corrosion of the protected material often begins at these pinholes, ultimately rendering the powder coating unusable. Therefore, minimizing pinholes and craters during powder coating application is of practical importance.

Pinholes occur when gases in the powder coating, during the melting and curing process, pass through a high-viscosity, nearly sealed elastic resin layer to the coating surface and fail to escape, resulting in small, needle-like holes. These defects form on the paint film surface due to rapid solvent evaporation during application or air bubbles failing to level out after drying.

Pinholes are surface defects in coatings, mainly including depressions, pinholes, and edge shrinkage or thickening. Surface depressions can be either circular or hexagonal. The depressions appearing on the coating surface are caused by surface tension gradients. Variations in coating composition and temperature lead to uneven surface tension, causing fluid to flow from areas of low surface tension to areas of high surface tension. This results in depressions on the fluid surface, also known as the Maragoni effect. Ultimately, this manifests as a circular pinhole with raised edges and a sunken center, or a Bernardo vortex with raised edges and a sunken center, resembling a hexagonal groove. The center of a pinhole contains a substance with low surface tension, creating a surface tension difference between it and the surrounding coating. This difference is the driving force behind pinhole formation, causing the surrounding liquid to flow away from the source of the pinhole and form a depression.

1. Several Characteristics of Pinholes

1.1 Shape Characteristics

1.1.1 With a Core – The pinhole has a core particle in the center, appearing as a fibrous or granular structure. This pinhole is large and deep, exposing the substrate.

1.1.2 Without a Core – The pinhole has no core particle in the center and is mostly located on the surface of the coating.

1.2 Distribution Characteristics

1.2.1 Uniform Distribution – Pinholes are evenly distributed throughout the coated workpiece;

1.2.2 Characteristic Distribution – Locally highly concentrated, even appearing as comet-like shapes;

1.2.3 Anisotropy – When the workpiece has an upward-facing plane, pinholes are more numerous on the upper surface.

1.3 Temporal Characteristics

1.3.1 Pinhole problems are prone to occur during spring and autumn when temperature fluctuations are significant;

1.3.2 Pinholes are prone to occur when compressed air drying equipment malfunctions;

1.3.3 Pinholes are prone to occur when the spraying equipment is restarted after a long downtime;

1.3.4 After pinhole problems occur, cleaning the raw material from the equipment will alleviate or resolve the problem.

1.4 Causes of Formation

Pinholes form during the coating application and drying process due to both problems with the coating itself and issues with the cleanliness of the substrate. The presence of low surface tension droplets in the coating itself, or low surface tension areas on the coated surface due to contamination, creates uneven surface tension. Under the influence of this surface tension difference, the coating flows from areas of low surface tension to areas of high surface tension, resulting in concave cavities. We consider this uneven surface tension as the internal cause of cavitation formation. However, certain properties of the coating itself, such as viscosity, thixotropy, drying speed, and film thickness, can intensify or weaken the flowability of the coating fluid, thus exacerbating or reducing the degree of cavitation. We consider these factors as external causes of cavitation. Internal causes are the inevitable conditions for cavitation, while external causes can appropriately control or exacerbate its degree.

After coating application, during the drying and film-forming process, surface solvent evaporation, increased surface polymer concentration, and increased powder coating viscosity all lead to surface tension and density exceeding the bulk density, resulting in depressions. Generally, thicker liquid coatings (>4mm) are primarily driven by density gradients, while thinner liquid coatings are controlled by surface tension gradients (Figure 1), forming pinholes.

2. Characteristics of Pinholes

Pinholes are caused by the expulsion of gas (air or gases produced by reactions, etc.), while pinholes are caused by poor wetting, such as a small surface tension difference between the substrate and the topcoat, or substrate contamination. Some pinholes are also caused by contamination from matting agents or texture agents in the topcoat. These are generally uniformly distributed in the coating film, appearing irregularly in time and place, and are small, dot-like pinholes.

2.1 Causes: Factors related to the coating formulation and manufacturer. Poor varnish refining, improper solvent selection and mixing ratios, poor dispersion of pigments and fillers, and the introduction of air bubbles and moisture during coating production.

2.2 Low storage temperature reduces resin miscibility, increases viscosity, or causes localized precipitation, easily leading to particle or pinhole defects (especially in asphalt coatings).

2.3 Prolonged vigorous stirring introduces air into the coating, generating numerous bubbles.

2.4 High ambient humidity during application, malfunctioning oil-water separators in the spraying equipment, and unfiltered air allow moisture to be carried in and out of the air hose during spraying, causing pinholes or even blisters on the coating surface. Excessive spraying pressure or distance disrupts the solvent balance of the wet coating. Excessive force during brushing or excessively high roller speed during roller application prevents bubbles from escaping.

2.5 During the initial film-forming stage after coating, rapid solvent evaporation, or application at high temperatures, especially high-temperature baking, can prevent the coating from filling gaps, resulting in a series of small holes, i.e., pinholes.

2.6 Improper surface preparation of the object to be coated, such as coating on oily surfaces, high moisture content of steel, and insufficient drying of putty and coating materials. Applying the coating too thickly in one coat prevents the solvent from evaporating in time, trapping it within the coating layer. After a period of time, when the solvent evaporates, pinholes form.