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How to Address Defects in UV 3C Coatings (Part 9)

Release time:

2026-07-03 06:02

How to Address Defects in UV 3C Coatings (Part 9)

In the practical production of UV‑3C coatings, curing shrinkage marks represent a rather unique type of surface defect. They arise during UV curing when uneven shrinkage of the coating film generates textured surface irregularities—distinct from leveling issues such as orange peel or craters—and stem from volumetric changes that occur during the curing reaction. These shrinkage marks not only compromise the product’s appearance but may also adversely affect the long-term durability of the coating. Addressing this defect requires targeted measures across multiple dimensions, including coating formulation design, control of coating thickness, and optimization of the curing process. This paper outlines approaches to mitigating curing shrinkage marks by examining resin system selection, coating‑thickness management, exposure‑energy adjustment, and stress‑relief design.

I. Selection of Resin Systems and Control of Shrinkage Rates

Polymeric shrinkage is an intrinsic characteristic of UV‑curable coatings, and the volumetric shrinkage rates vary among different resin systems. During formulation, it is essential to select a resin system with a lower shrinkage rate. Epoxy systems generally exhibit lower volumetric shrinkage than acrylate systems and should be given priority when performance requirements permit. For acrylate systems, the shrinkage rate can be reduced by optimizing the types and ratios of monomers and oligomers.

High‑functionality monomers exhibit high crosslink density, but also a relatively large volumetric shrinkage. To meet requirements for hardness and wear resistance, low‑functionality monomers or toughening resins can be incorporated to reduce the overall shrinkage of the system. Additionally, an appropriate amount of inert fillers in the formulation helps lower the proportion of organic components contributing to shrinkage.

II. Control of Coating Thickness Uniformity

Differences in coating thickness are a major cause of uneven shrinkage. During processing, the coating application process must be carefully controlled to ensure consistent coating thickness across all areas. When spraying, the gun travel speed should remain steady to prevent fluctuations in coating weight caused by inconsistent speeds. The distance between the spray gun and the workpiece should also be kept constant, as variations in this distance directly affect coating‑thickness uniformity.

For workpieces with complex geometries, the coating thickness at edges and in recessed areas often differs from that on flat surfaces. Adjusting the spray path and appropriately modifying the spray angle at edges and in recesses can help reduce thickness variations across different regions, thereby minimizing uneven shrinkage caused by such disparities.

III. Proper Setting of Exposure Energy

Exposure energy is a critical factor influencing the formation of shrinkage cracks. During processing, the appropriate curing energy should be set based on the coating’s properties and the coating thickness. At higher energy levels, the polymerization reaction proceeds more rapidly, leading to concentrated release of shrinkage stresses and an increased likelihood of crack formation. Conversely, reducing the curing energy to allow the polymerization to proceed at a more moderate rate facilitates the gradual dissipation of shrinkage stresses.

For production lines that use conveyor‑type curing equipment, the conveyor speed can be adjusted to control the irradiation time. A longer irradiation time combined with a lower energy density helps dissipate residual shrinkage stresses over an extended period, thereby reducing the formation of shrinkage cracks. Setting the curing parameters requires striking a balance between crack control and curing efficiency.

IV. Application of the Segmented Curing Process

Segmented curing is an effective approach to mitigating the issue of shrinkage cracks. During processing, a combined curing strategy—low‑energy pre‑curing followed by high‑energy main curing—can be employed. The low‑energy pre‑curing stage allows the coating to achieve initial structural integrity, releasing a portion of the internal shrinkage stresses; this is then followed by high‑energy main curing to complete the remaining crosslinking reactions.

Segmented curing allows shrinkage stresses to be released in stages, thereby preventing excessive stress caused by their concentrated release at once. The duration and energy parameters of the pre‑curing stage must be optimized according to the coating’s properties, ensuring that the coating attains sufficient strength after pre‑curing to withstand subsequent shrinkage stresses.

V. Design of Stress-Relief Structures

The stress state within the coating influences the formation of shrinkage cracks. During formulation, the coating’s stress‑relief capability can be enhanced by optimizing the recipe. Incorporating resins or toughening agents with flexible segments increases the mobility of the coating’s polymer chains, allowing a portion of the shrinkage stress to dissipate as the coating cures.

In applications where the coating is relatively thick, a multi‑layer coating approach can be adopted, with each layer applied at a reduced thickness and cured separately. This results in lower shrinkage per layer, and the interlayer interfaces help to distribute stress, thereby minimizing the formation of shrinkage cracks.

VI. Integrated Process Control

Addressing curing‑induced shrinkage requires comprehensive control across multiple stages, including formulation design, coating processes, and curing conditions. In terms of formulation, select a resin system with a lower shrinkage rate to balance crosslink density and the degree of shrinkage; for coating, ensure uniform film thickness and minimize thickness variations; during curing, optimize exposure energy and adopt a stepwise curing process; and in stress management, incorporate flexible chain segments and toughening agents, while employing a multilayer coating approach.

The control of each process stage is interrelated and must be considered holistically during adjustments. In actual production, the primary source of shrinkage marks can be identified based on their morphology and distribution, allowing for targeted adjustments to the relevant process steps.

VII. Conclusion

Addressing curing‑shrinkage cracks involves multiple steps, including selecting an appropriate resin system, controlling coating thickness, optimizing exposure energy settings, and designing for stress relief. By choosing a resin system with a lower shrinkage rate, ensuring uniformity in coating thickness, carefully calibrating exposure energy to prevent localized release of shrinkage stresses, implementing a staged curing process to allow stress to dissipate in phases, and incorporating flexible chain segments to enhance stress‑relief capabilities, the occurrence of curing‑shrinkage cracks can be effectively minimized. Optimizing each of these aspects requires coordinated efforts and a holistic consideration of material properties, equipment condition, and process requirements to achieve a satisfactory outcome.

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 superior 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.
B-650A	Aliphatic polyurethane acrylate	Low viscosity, excellent matting effect, fast curing, and good wettability.
Wearable device
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-6211	Aliphatic polyurethane acrylate	Fast curing, high hardness, scratch-resistant, and free of organotin.
Hand feel
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-328M	Aliphatic polyurethane acrylate	Low gloss, low viscosity, excellent wettability, and a pleasant hand feel.
B-868	Organosilicon photocurable resin	Excellent leveling, smooth finish, fast curing, and stain resistance.
B-868H	Organosilicon photocurable resin	Excellent leveling, smooth finish, fast curing, and stain resistance.
Large-area spraying
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-374	Aliphatic polyurethane acrylate	Excellent flexibility, good leveling, resistant to abrasion and chemicals, and resistant to yellowing.
Car interior
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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-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.
Resistant to steel wool
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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 superior chemical and wear resistance.
Oil-resistant pen
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-868	Organosilicon photocurable resin	Excellent leveling, smooth finish, fast curing, and stain resistance.
B-868H	Organosilicon photocurable resin	Excellent leveling, smooth finish, fast curing, and stain resistance.
Battery casing
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-519	Self-curing polyester acrylate	Self-initiated photopolymerization performance
B-560	Polyester acrylate	Fast curing and excellent pigment wetting.
Yellowing resistance
Product Model/English Abbreviation	Product Name/Product Type	Product Features
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 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 10)

How to Address Defects in UV 3C Coatings (Part 8)