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Common Defects and Their Causes When Using UV Coatings

Release time:

2026-03-05 07:15

Common Defects and Their Causes When Using UV Coatings

UV coatings, with their advantages such as rapid curing and environmental friendliness and energy efficiency, have found widespread application in numerous industrial fields. However, during actual coating processes, various film defects may arise due to factors including material properties, process control, or environmental conditions. Understanding the manifestations and underlying causes of these issues can help implement targeted measures during production and construction, thereby improving coating quality.

1. Surface sagging

Sagging manifests as tear-like or curtain-like raised streaks on a coating film after it has cured on a vertical surface, often occurring at the edges or recessed areas of the workpiece. This phenomenon is typically associated with low coating viscosity, excessive film thickness applied in a single coat, improper solvent evaporation rates, or improperly set spray application parameters (such as spray gun distance and paint output). In vertical surface coating applications, if the coating lacks sufficient sag resistance, gravitational forces can easily cause the coating to flow downward.

II. Pinholes and Bubbles

Pinholes refer to a dense array of tiny holes on the coating surface, resembling the marks left by a needle prick. They typically result from air bubbles trapped in the coating that fail to escape before the coating cures. Common causes include excessive coating viscosity, entrainment of air during dispersion, rapid solvent evaporation leading to premature surface closure, or improper temperature control during the infrared leveling stage. Overly thick coatings can also exacerbate this type of defect.

III. Uneven Gloss and Atomization

After the coating film has cured, if the surface gloss is lower than expected or if localized whitening or hazy, foggy appearances occur—this is particularly common in high-gloss topcoats. Such issues may be related to the compatibility between the resin and fillers in the coating formulation, curing conditions (such as insufficient UV intensity), the uniformity of application, and differences in the absorbency of the substrate material. Residual solvents or incomplete curing can also lead to a reduction in gloss.

4. Shrinkage Holes and Fish Eyes

Shrinkage defects manifest as circular or elliptical depressions on the coating surface, with slightly raised edges that resemble fish eyes. Such defects typically originate from trace amounts of incompatible substances—such as oil contamination, moisture, or silicon-based contaminants—introduced either onto the substrate surface or within the coating itself. Failure of the compressed air filtration system, inadequate cleaning of workpieces, or insufficient environmental cleanliness can all contribute to the occurrence of these defects.

5. Poor adhesion

After the coating film has cured, insufficient adhesion to the substrate may lead to phenomena such as delamination and paint chipping. The causes may include improper surface treatment of the substrate (e.g., failure to remove mold release agents, or failure to perform corona treatment or sanding), poor compatibility between the coating formulation and the substrate, or inadequate curing conditions resulting in insufficient crosslinking. In multi-layer coating systems, interlayer adhesion issues may also arise due to excessive curing of the underlying layer or an unreasonable interval between layers.

6. Incomplete curing

If the coated film still feels sticky, lacks sufficient hardness, or exhibits poor solvent resistance after UV irradiation, it indicates that the curing process has not reached the desired level. Common causes include insufficient power of the UV light source, inadequate irradiation time, poor compatibility between the photoinitiator and the wavelength of the light source, or excessive coating thickness, which prevents the deeper layers from receiving sufficient UV radiation. Improper storage of the coating, leading to a decline in the activity of the photoinitiator, is also one of the contributing factors.

7. Residual odor

The cured coating still emits a noticeable irritating odor, typically due to the incomplete reaction of volatile components in the coating or insufficient curing depth. If the formulation contains high-boiling-point diluents or unreacted monomers, these may continue to release slowly even after curing. Additionally, inadequate ventilation of the equipment can also lead to the accumulation of odors.

8. Undercutting Phenomenon

In multi-layer coatings, active diluents or solvents in the topcoat may penetrate and swell the underlying, fully cured coating film, leading to surface defects such as wrinkling, blistering, or localized flaking. This issue is often associated with poor compatibility of the coating formulations, inadequate control over the degree of curing between layers, or insufficient curing of the underlying coat.

9. Yellowing of the coating film

After curing or during use, the coating film may turn yellow, a phenomenon that is particularly noticeable in white or light-colored systems. Yellowing can be attributed to the effects of photoinitiator decomposition products, insufficient thermal stability of the resin system, or prolonged exposure of the coating film to factors such as ultraviolet radiation and high temperatures. Optimizing formulation design and curing conditions can help mitigate the tendency toward yellowing.

X. Summary

Various defects that occur during the application of UV coatings are often the result of the combined effects of multiple factors, including coating properties, application techniques, substrate conditions, and curing conditions. By standardizing substrate preparation, optimizing coating parameters, ensuring stable operation of curing equipment, and adjusting formulations accordingly, it is possible to effectively reduce the incidence of defects and improve both coating quality and the aesthetic appearance of the finished product.

Boxing Related Product Recommendation – UV Coatings
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-106	Acrylated epoxy soybean oil resin	Good flexibility, excellent pigment wetting properties, and biodegradability after curing.
B-160D	Modified epoxy acrylate	Good flexibility, yellowing resistance, and excellent adhesion.
B-241	Aliphatic polyurethane acrylate	Fast onset of mute effect, excellent skin feel, and yellowing resistance.
B-328M	Aliphatic polyurethane acrylate	Low gloss, low viscosity, good wettability, and pleasant to the touch.
B-570	Polyester acrylate	Low viscosity, benzene-free, low odor, excellent wettability, fast curing, suitable for LED UV systems.
B-609	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch-resistant, chemical-resistant
B-828	Silicone-based UV-curable resin	Good leveling, smooth finish, fast curing, and stain-resistant.
B-912	Aliphatic polyurethane acrylate	Fast curing, high hardness, good toughness, chemical resistance, and wear resistance.
BM3232/TMETA	Trimethylolpropane triacrylate	Low odor, high reactivity, low irritation, high crosslink density, low volatility
BM6263/DPHA-90	Dipentaerythritol hexaacrylate	Fast curing, high crosslink density, high hardness, chemical resistance, abrasion resistance, and water resistance.

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