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How to Address Defects in UV 3C Coatings (Part 13)

Release time:

2026-07-06 16:52

How to Address Defects in UV 3C Coatings (Part 13)

During the application and use of UV 3C coatings, coating film cracking is one of the most serious defects, compromising both the integrity and protective performance of the coating. It manifests as cracks on the coating surface—fine, thread-like in mild cases, or penetrating the entire coating in more severe instances. Such cracking not only detracts from the product’s appearance but also renders the coating’s protective function ineffective. The causes of film cracking are multifaceted, involving factors such as diluent selection, curing conditions, coating structure, and residual solvents. Addressing this defect requires targeted measures across several key areas, including appropriate diluent selection, precise control of curing energy, careful management of coating thickness, and effective control of solvent residues. This paper outlines strategies for mitigating film‑cracking defects, focusing on diluent compatibility, optimization of curing conditions, regulation of coating thickness, and management of solvent evaporation.

I. Rational Selection and Matching of Thinners

The selection of an appropriate thinner has a significant impact on paint film cracking. During application, it is essential to choose the right type and dosage of thinner based on the coating system and the specific application conditions. The solvent power of the thinner should be compatible with the coating; excessively strong thinners may penetrate into the coating, altering its swelling behavior and stress distribution. Additionally, the thinner’s evaporation rate should be moderate: if it evaporates too rapidly, the coating surface will dry and shrink quickly, and the differential shrinkage between the interior and exterior can lead to stress concentration.

The thinners for the primer and topcoat should be selected to match their respective systems. Residual thinner in the primer can adversely affect the adhesion and curing of the topcoat, while uneven stress transfer between the primer and topcoat can increase the risk of cracking. When using compatible thinners, follow the paint manufacturer’s recommendations and avoid substituting or mixing them arbitrarily.

II. Optimized Control of Curing Energy

Excessive exposure energy is a major factor contributing to cracking in the paint film. During processing, the curing energy should be set appropriately based on the coating’s characteristics to prevent excessive energy from causing concentrated release of shrinkage stresses. The curing energy settings should follow the manufacturer’s recommended range and be fine-tuned through testing to determine the optimal parameters for actual production.

For coatings of different colors, the curing energy must be adjusted accordingly. Dark-colored coatings absorb UV light more strongly and may require higher energy levels, whereas light-colored and transparent coatings absorb less UV radiation; excessive energy can increase the risk of cracking. In actual production, when switching between colors, the curing energy should be fine-tuned based on the color’s shade.

Stepwise curing is an effective approach to mitigating cracking. Low‑energy pre‑curing allows the coating to achieve initial structural integrity, partially relieving shrinkage stresses, while high‑energy main curing completes the remaining crosslinking reactions. This staged curing process enables shrinkage stresses to be released in phases, thereby preventing excessive stress caused by their abrupt, concentrated release.

III. Rational Control of Coating Thickness

The impact of coating thickness on cracking is primarily reflected in the magnitude of shrinkage stress. During processing, coating thickness must be carefully controlled to prevent excessive thickness, which can lead to overly high shrinkage stresses. When spraying, gun travel speed and paint output should be kept consistent to ensure uniform coating thickness.

For applications requiring a thicker coating, a multi‑layer thin‑coat approach can be used instead of a single thick coat. Each layer is applied at a relatively low thickness and cured separately; this results in reduced shrinkage per layer, and the interlayer interfaces help to distribute stress, thereby minimizing the risk of cracking.

IV. Control of Solvent Residues

Solvent residues are a late-stage factor contributing to cracking in the paint film. During application, it is essential to ensure that solvents within the coating fully evaporate before curing. After applying both the primer and topcoat, sufficient leveling time should be allowed to enable the solvents to escape prior to cure. The duration of this leveling period should be adjusted according to the coating thickness and ambient conditions.

The curing temperature should be appropriate; at lower temperatures, the solvent evaporates more slowly, leading to increased residual solvent. The curing time should be sufficient to ensure that the coating has enough time during curing to complete solvent evaporation. For thick coatings, the leveling or curing time may be extended as needed to minimize solvent residues.

V. Balancing Crosslink Density and Coating Flexibility

When the crosslink density is too high, the coating becomes more brittle, increasing the risk of cracking. During formulation, it is advisable to strike a balance between crosslink density and flexibility. Provided that hardness and wear resistance requirements are met, flexible‑chain resins or toughening agents can be incorporated to enhance molecular chain mobility and improve crack‑resistance.

The uniformity of the curing temperature is equally important. When the temperature is too high, molecular motion intensifies, accelerating the curing reaction and leading to the concentrated release of shrinkage stresses. The temperature distribution within the curing equipment should be kept uniform to prevent localized overheating that could cause stress concentrations.

VI. Integrated Process Control

Addressing cracking in paint films involves multiple steps, including the selection of thinners, control of curing energy, management of coating thickness, and mitigation of solvent residues. Regarding thinners, choose formulations and dosages that are compatible with the coating; for curing, keep the energy level from being too high and employ staged curing; in terms of coating application, avoid excessive thickness by applying multiple thin coats; and in solvent management, ensure adequate leveling and curing while minimizing residual solvents.

The control parameters at each stage are interrelated and must be adjusted in a comprehensive manner. In actual production, the primary cause of cracking can be identified based on its morphology and location, allowing for targeted adjustments to the relevant process steps.

VII. Conclusion

Addressing cracking in paint films involves multiple factors, including the proper selection of thinners, control of curing energy, management of coating thickness, and mitigation of solvent residues. By choosing thinner types and dosages that are compatible with the coating formulation, regulating curing energy to prevent the concentrated release of shrinkage stresses, applying multi‑layer, thin coats to manage overall film thickness, and ensuring adequate solvent evaporation prior to curing, the occurrence of paint‑film cracking can be effectively minimized. Optimizing each of these steps requires coordinated efforts and a holistic consideration of material properties, equipment condition, and process requirements to achieve an optimal level of coating integrity.

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
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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.
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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 wettability, 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 superior chemical and wear resistance.
Matte
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B-572	Polyester acrylate	Low viscosity, low odor, excellent wettability, suitable for LED UV.
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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 superior chemical and wear resistance.
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B-910A2	Aliphatic polyurethane acrylate	Low viscosity, yellowing resistance, chemical resistance, and steel-wool resistance.
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Oil-resistant pen
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B-296	Aliphatic polyurethane acrylate	Fast curing, chemical resistance, yellowing resistance, impact resistance
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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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