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Typical Defects of UV 3C Coatings (Thirteen)

Release time:

2026-06-30 06:40

Typical Defects of UV 3C Coatings (Thirteen)

During the application and use of UV‑3C coatings, coating film cracking is one of the most serious defects, compromising both the integrity of the coating and its protective performance. It manifests as cracks on the coating surface—fine, thread‑like in mild cases, or penetrating the entire coating in more severe instances. Cracking not only detracts from the product’s appearance but also renders the coating’s protective function ineffective, allowing external moisture and corrosive substances to penetrate directly to the substrate through the cracks. The causes of coating film cracking are multifaceted, involving factors such as the coating formulation, curing conditions, and coating structure. A thorough understanding of the manifestations and underlying causes of cracking is essential for identifying and analyzing this defect.

I. Manifestations of Cracking in the Paint Film

Cracking of the paint film manifests as linear cracks on the coating surface, with varying morphologies and degrees of severity. Mild cracking appears as fine, filamentary surface cracks that are shallow in depth and confined primarily to the top layer of the coating. Under illumination at certain angles, these filamentary cracks reveal a reticulated or linear texture.

When severe cracking occurs, the cracks penetrate the entire coating thickness, exposing the substrate or underlying coating directly. The cracks may extend as single linear paths or form a dense network. Their orientation may correlate with the stress direction, propagating along regions of stress concentration.

Cracking typically initiates in regions of stress concentration, such as the edges of a workpiece, sharp corners, and areas surrounding holes. The geometry of these regions leads to non-uniform stress distribution, making them more susceptible to stress concentrations.

II. The Influence of Matching Thinners

The selection of a suitable thinner for the primer significantly affects cracking in the paint film. When the thinner is too strong, its dissolving power may be excessive, allowing it to penetrate into the coating and alter its swelling behavior and stress distribution. If the thinner evaporates too rapidly, the coating surface dries and shrinks quickly while the interior remains wet; this differential shrinkage between the surface and the interior can lead to stress concentration.

Improper selection of thinners can also lead to abnormal interfacial bonding between coating layers. Residual thinners in the primer may compromise the adhesion and curing behavior of the topcoat, while uneven stress transfer between the primer and topcoat can increase the risk of cracking.

III. Exposure Energy and Curing Shrinkage

Exposure energy is a critical factor influencing cracking in the paint film. When the exposure energy is excessively high, the curing reaction rate accelerates, causing the coating to undergo most of its shrinkage within a short time. This rapid release of shrinkage stress leads to an accumulation of internal stresses; once these stresses exceed the coating’s tolerance, cracks will form.

Shrinkage upon curing is an intrinsic property of UV coatings. During the polymerization reaction, intermolecular distances decrease, leading to a reduction in system volume. Shrinkage rates vary among different resin systems, with acrylate-based systems typically exhibiting higher shrinkage. Higher shrinkage translates into greater internal stresses, thereby increasing the risk of cracking.

IV. Crosslink Density and Coating Brittleness

Crosslink density is an intrinsic factor that influences the crack‑resistance of a coating. At higher crosslink densities, coating hardness increases but brittleness also rises. Brittle coatings cannot dissipate or redistribute stress through molecular chain mobility; when stress becomes sufficiently concentrated, cracks initiate.

When the conversion rate is low, the coating contains a significant amount of unreacted oligomers and monomers. These residual species create weak zones within the coating, thereby reducing its overall mechanical strength. A situation characterized by high crosslink density but low conversion can result in a coating that exhibits both increased brittleness and a weakened internal structure, leading to an elevated risk of cracking.

V. Coating Thickness and Shrinkage Stress

The influence of coating thickness on cracking is primarily reflected in the magnitude of shrinkage stresses. When the coating is thicker, the degree of shrinkage is greater, leading to higher internal stress accumulation. In thick coatings, the stress distribution is more complex, and the disparity in curing levels between the surface and the substrate becomes more pronounced—factors that collectively increase the risk of cracking.

An elevated exposure temperature also contributes to cracking. Higher temperatures intensify molecular motion, accelerate the curing reaction rate, and promote the concentrated release of shrinkage stresses. Moreover, increased temperature may widen the disparity in thermal expansion coefficients between the substrate and the coating, thereby increasing thermal stresses at the interface.

VI. Influence of Solvent Residues

Solvent residues are a late‑stage factor contributing to coating cracking. Residual solvents in the coating may continue to evaporate slowly after curing, causing the coating to undergo ongoing shrinkage during subsequent service. This prolonged, gradual shrinkage generates sustained internal stresses; when these stresses accumulate to a critical level, the coating cracks.

The distribution of solvent residues is typically influenced by curing conditions and coating thickness. In thicker coatings, solvents are more difficult to volatilize completely, resulting in higher residual levels. Additionally, lower curing temperatures or insufficient curing times can also lead to increased solvent residue.

VII. Conclusion

Cracking of the paint film is a serious defect in UV‑cured 3C coatings that compromises coating integrity, with its origins rooted in multiple factors, including diluent selection, curing conditions, coating structure, and residual solvents. An excessively strong or rapidly evaporating diluent can lead to uneven internal stresses; excessive exposure energy causes concentrated release of shrinkage stresses; an overly high crosslinking density increases coating brittleness; excessive coating thickness or elevated curing temperatures exacerbate shrinkage stresses; and residual solvents induce continued shrinkage during subsequent service. These factors give rise to surface cracks—ranging from fine filaments in mild cases to full‑depth penetration in severe instances. Cracking not only detracts from appearance but also undermines the coating’s protective performance. A thorough understanding of the manifestations and underlying causes of paint‑film cracking forms the foundation for identifying and analyzing this defect.

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.

Bosheng Related Product Recommendations – 3C Coatings
General-purpose
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 superior 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
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-572	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
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Wearable device
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Hand feel
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B-328M	Aliphatic polyurethane acrylate	Low gloss, low viscosity, excellent wettability, and a pleasant hand feel.
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B-868H	Organosilicon photocurable resin	Good leveling, smooth finish, fast curing, and stain resistance.
Large-area spraying
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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
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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.
Battery casing
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B-431	Cycloaliphatic Specialty Acrylate	Yellowing-resistant, excellent wettability, low viscosity, fast curing
B-548	Polyester acrylate	Withstands high temperatures of 250–280°C.
Solid color paint
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Yellowing resistance
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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
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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 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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How to Address Defects in UV 3C Coatings (Part 1)

Typical Defects of UV 3C Coatings (12)