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Typical Defects of UV 3C Coatings (Part 3)
Release time:
2026-06-25 23:10
During the application of UV 3C coatings, bubbles and pinholes are common defects that compromise both the coating’s density and its surface smoothness. Bubbles appear as circular blisters of varying sizes on the coating film, while pinholes are tiny depressions left behind after the bubbles rupture. Although their appearances differ, both are closely linked to entrapped gases within the coating. In the context of 3C electronic products, where surface quality is paramount, the presence of bubbles and pinholes not only detracts from aesthetics but also undermines the coating’s protective performance.
I. Appearance of Bubbles and Pinholes
Blisters appear as circular or elliptical, raised cavities on the surface of the paint film, ranging in size from pinhead‑sized to one or two millimeters in diameter. When numerous small blisters are densely distributed, the nail surface takes on a pitted appearance; larger blisters, by contrast, form distinct bulges. The cavities within the blisters may contain air, solvent vapor, or water vapor. Under illumination, the raised portions of the blisters exhibit conspicuous highlights, contrasting sharply with the surrounding smooth surface.
Pinholes are the result of bubble rupture and appear as tiny depressions on the paint film surface. They typically have a small diameter, with irregular fracture patterns at their edges. Under illumination, the edges of pinholes cast shadows, while the interior reflects dimly. Pinholes not only compromise the aesthetic appearance but also reduce the coating’s density, creating pathways for moisture and contaminants to penetrate.
The difference between a blister and a pinhole lies in the integrity of the coating: in a blister, the coating remains continuous, with only localized bulging; in a pinhole, the coating has been breached, exposing the underlying substrate. Pinholes have a more severe impact on the coating’s protective performance, as external substances can penetrate directly through the pinhole to reach the substrate.
II. Moisture and Oil Contamination in Compressed Air
Moisture and oil contaminants in compressed air are major sources of bubble and pinhole defects. During the spraying process, compressed air is mixed with the coating; if the compressed air is not adequately dried and filtered, its moisture and oil will be carried into the coating.
When moisture enters the coating, it vaporizes upon heating during the curing process, causing a rapid expansion in volume and forming bubbles within the coating. Oil contaminants not only generate bubbles themselves but also impair the coating’s leveling and curing, making it even more difficult for these bubbles to escape. Moisture and oil in compressed air typically originate from the air compressor itself; if the compressed‑air system lacks effective drying and filtration equipment, or if such equipment is not maintained regularly, the problem will persist.
III. Coating film thickness is too high
Excessive coating thickness is another major factor contributing to bubbles and pinholes. When the coating is too thick, the surface layer first comes into contact with ultraviolet light and cures rapidly, forming a hard skin. This hard skin seals the coating’s surface, preventing solvents, air, or reaction by-products from escaping.
These gases trapped within the coating expand during subsequent curing, forming bubbles. If the bubbles rupture before curing, they leave behind pinholes; if they fail to rupture, they remain entrapped inside or on the coating’s surface, resulting in visible bubble defects. In thick coatings, bubbles tend to be larger in size, as a greater volume of gas becomes enclosed within the thicker layer.
IV. Porosity and Moisture on the Workpiece Surface
The condition of the workpiece surface also influences the formation of bubbles and pinholes. When the substrate surface contains microscopic pores, air or solvents in the coating can become trapped within these voids. During curing, the gases in the pores expand upon heating and, as they escape, give rise to bubbles or pinholes on the coating surface.
When the substrate has a high moisture content, the water vaporizes during curing and migrates from the interior of the substrate toward the surface. As this vapor passes through the coating, it can form bubbles or pinholes on the coating surface. The hygroscopicity of plastic substrates varies depending on the material type; substrates stored in humid environments typically exhibit higher moisture levels and are more prone to such defects.
V. Air and Solvents in Coatings
The air and solvents contained within the coating itself are also sources of bubbles and pinholes. During processes such as stirring, pumping, and spraying, air can become entrained in the coating. If these bubbles are not adequately released before curing, they will become trapped within the coating film.
Solvents in coatings evaporate during the curing process; if the surface layer cures too rapidly, solvent vapors cannot escape in time, leading to bubble formation. Different types of solvents have varying evaporation rates, and both excessively fast or excessively slow evaporation can increase the risk of bubble defects.
VI. Conclusion
Bubbles and pinholes are common defects in UV‑cured 3C coatings that compromise coating density and surface smoothness. Their formation stems from multiple factors, including compressed air quality, spray film thickness, substrate condition, and the coating formulation itself. Moisture and oil contaminants in the compressed air can be entrained into the coating during spraying and vaporize upon curing, creating bubbles. When the spray film is excessively thick, the surface layer cures rapidly, sealing the surface while gases trapped beneath cannot escape. Pores and residual moisture on the substrate surface allow gases to vent when heated. Additionally, entrained air and solvents within the coating may leave behind gaseous residues during curing. These mechanisms give rise to internal or surface voids, manifesting as visible bubbles or pinholes after curing. In the context of 3C electronic products, where stringent requirements are imposed on coating density and protective performance, the presence of bubbles and pinholes not only detracts from appearance but can also undermine the coating’s protective efficacy. A thorough understanding of the characteristics and root causes of these defects is essential for their identification and analysis.
Disclaimer: The above content has been compiled from publicly available sources and is provided for reference only. If any infringement occurs, please contact us, and we will address it promptly.
Bossin Related Product Recommendations – 3C Coatings | ||
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Product Model/English Abbreviation | Product Name/Product Type | Product Features |
B-102 | Bisphenol A epoxy acrylate | High hardness, high gloss, chemical resistance, contains 15% TMPTA. |
B-151 | Modified epoxy acrylate | Low halogen, yellowing-resistant, excellent plating performance, and strong adhesion. |
B-165 | Modified epoxy acrylate | Good flexibility and strong adhesion |
B-216 | Aliphatic polyurethane acrylate | Fast curing, high fullness, and excellent toughness. |
B-368 | Aliphatic polyurethane acrylate | Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance. |
B-574C | Polyester acrylate | Low viscosity, low odor, excellent wettability, suitable for LED UV. |
B-601 | Aromatic polyurethane acrylate | High hardness, scratch resistance, chemical resistance, and excellent cost-effectiveness. |
B-6019 | Special functional group acrylate | Good leveling, excellent wetting, resistant to boiling water, and excellent color dispersion. |
B-609 | Aliphatic polyurethane acrylate | Fast curing, high hardness, scratch resistance, and chemical resistance. |
B-615A | Aliphatic polyurethane acrylate | Fast curing, excellent toughness, wear resistance, and chemical resistance. |
B-619W | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance. |
B-6380N | Special functional group acrylate | Excellent adhesion to plastics, strong hiding power, and improved paint film appearance. |
B-919B | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, and outstanding chemical and wear resistance. |
Matte | ||
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B-572 | Polyester acrylate | Low viscosity, low odor, excellent wettability, suitable for LED UV. |
B-650A | Aliphatic polyurethane acrylate | Low viscosity, excellent matting effect, fast curing, and good wettability. |
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B-6211 | Aliphatic polyurethane acrylate | Fast curing, high hardness, scratch-resistant, and free of organotin. |
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B-328M | Aliphatic polyurethane acrylate | Low gloss, low viscosity, excellent wettability, and a pleasant hand feel. |
B-868 | Organosilicon photocurable resin | Good leveling, smooth finish, fast curing, and stain resistance. |
B-868H | Organosilicon photocurable resin | Good leveling, smooth finish, fast curing, and stain resistance. |
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B-374 | Aliphatic polyurethane acrylate | Good flexibility, excellent leveling, resistant to abrasion and chemicals, and yellowing‑resistant. |
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B-6063 | Special functional group acrylate | High molecular weight, low curing shrinkage |
B-6210 | Aliphatic polyurethane acrylate | Low viscosity, chemical resistance, environmental resistance, and dual photothermal curing. |
B-6263 | Special functional group acrylate | Fast curing, high build, boil‑water resistant, and excellent toughness. |
B-916 | Aliphatic polyurethane acrylate | Low viscosity, solvent resistance, chemical resistance, and steel-wool resistance. |
B-919B | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, and outstanding chemical and wear resistance. |
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B-910A2 | Aliphatic polyurethane acrylate | Low viscosity, yellowing resistance, chemical resistance, and steel-wool resistance. |
B-916 | Aliphatic polyurethane acrylate | Low viscosity, solvent resistance, chemical resistance, and steel-wool resistance. |
B-919B | Aliphatic polyurethane acrylate | Fast curing, high hardness, excellent toughness, and outstanding chemical and wear resistance. |
Oil-resistant pen | ||
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B-868 | Organosilicon photocurable resin | Good leveling, smooth finish, fast curing, and stain resistance. |
B-868H | Organosilicon photocurable resin | Good leveling, smooth finish, fast curing, and stain resistance. |
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B-548 | Polyester acrylate | Withstands high temperatures of 250–280°C. |
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B-519 | Self-curing polyester acrylate | Self-initiated photopolymerization performance |
B-560 | Polyester acrylate | Fast curing and excellent pigment wetting. |
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B-151 | Modified epoxy acrylate | Low halogen, yellowing-resistant, excellent plating performance, and strong adhesion. |
B-160D | Modified epoxy acrylate | Good flexibility, yellowing resistance, and excellent adhesion. |
B-216 | Aliphatic polyurethane acrylate | Fast curing, high fullness, and excellent toughness. |
B-296 | Aliphatic polyurethane acrylate | Fast curing, chemical resistance, yellowing resistance, impact resistance |
B-431 | Cycloaliphatic Specialty Acrylate | Yellowing-resistant, excellent wettability, low viscosity, fast curing |
Monomer Recommendation | ||
Product Model/English Abbreviation | Product Name/Product Type | Product Features |
BM3231 (TMPTA) | Trimethylolpropane triacrylate | High crosslink density, high hardness, high gloss, and excellent wear resistance. |
BM3235 (PET3A) | Pentaerythritol triacrylate | Fast curing, high crosslink density, high hardness, and excellent chemical resistance. |
BM3380 (3EO-TMPTA) | Pentaerythritol triacrylate | More flexible and less irritating than TMPTA. |
BM4241 (DiTMPTA-80) | Bis(2,3-dihydroxypropyl) tetraacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
BM4242 (Di-TMPTA) | Bis-trimethylolpropane tetraacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
BM6261 (DPHA-80) | Dipentaerythritol hexaacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
BM6263 (DPHA-90) | Dipentaerythritol hexaacrylate | High crosslink density, high hardness, chemical and wear resistance, and water resistance. |
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