News

Knowledge

Company News

Application Mind Map

Test Report

Technical Article

Industry News

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 5)

Release time:

2026-10-20 16:50

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 5)

Intercoat adhesion is one of the most critical defects to address when using UV‑curable resins on substrates that are difficult to bond, particularly in multilayer coating systems. Without strong interlayer cohesion between the primer and topcoat, the topcoat may delaminate or blister, compromising the coating system’s aesthetic integrity, protective performance, and service life. In applications with stringent requirements for appearance and durability—such as automotive interiors and appliance panels—poor intercoat adhesion can lead to premature coating failure during assembly, use, or environmental aging. To mitigate this issue, a systematic approach is needed, addressing both process control and surface preparation.

I. Properly Control the Degree of Primer Curing

Improper control of the primer’s degree of cure is the primary cause of intercoat adhesion problems. As the substrate for the topcoat, the primer’s surface condition directly determines whether the topcoat can effectively wet, penetrate, and form a strong bond with the primer. Both excessive and insufficient curing can adversely affect intercoat adhesion.

When the primer is over‑cured, its surface becomes excessively dense, making it difficult for the topcoat to wet and penetrate, thereby reducing interlayer adhesion. Over‑curing diminishes the mobility of molecular chains at the primer’s surface and lowers its surface energy, weakening the topcoat’s ability to spread and wet the substrate. At the same time, the overly dense surface reduces the penetration depth of the topcoat into the primer, hindering the formation of effective mechanical interlocking and intermolecular interactions between the two layers, which in turn lowers the interlayer bond strength. Under these conditions, although the topcoat may form a continuous film on the primer surface, the interfacial adhesion remains weak, making the coating prone to delamination or peeling under external forces or environmental exposure.

When the primer is insufficiently cured, its internal cohesive strength is inadequate. At this stage, the primer surface still contains a significant amount of unreacted monomers or oligomers, resulting in high surface energy, which facilitates wetting and penetration by the topcoat and promotes good interlayer adhesion. However, the primer itself lacks sufficient cohesive strength, and the coating has not yet developed a sufficiently dense crosslinked network. Under external loading, failure typically occurs within the primer rather than at the interface, manifesting as delamination of both the primer and the topcoat from the substrate, or as cohesive failure within the primer itself. Consequently, while incomplete curing can enhance interlayer adhesion, the overall coating strength remains insufficient, failing to meet performance requirements.

When addressing this issue, it is necessary to determine the appropriate curing energy and curing time based on the compatibility between the primer resin system and the topcoat, ensuring that the primer surface is neither overly dense nor exhibits insufficient cohesion due to inadequate curing. Controlling the degree of primer cure requires striking a balance between interlayer adhesion and the overall strength of the coating: at an optimal level of cure, the primer surface retains sufficient reactive activity and surface roughness to promote wetting and penetration by the topcoat, while also developing adequate cohesive strength to meet the mechanical performance requirements of the entire coating system.

II. Keep the primer surface clean

Contamination on the primer surface can also compromise intercoat adhesion, representing another significant cause of poor interlayer bonding. After the primer has cured, its surface may adsorb dust or oils from the operator’s gloves, creating a weak interfacial layer that impedes effective bonding between the topcoat and the primer.

During the coating process, after the primer has cured, it must undergo a period of cooling, conveyance, or storage before the topcoat can be applied. Throughout this stage, the primer surface is exposed to the production environment and readily adsorbs airborne dust particles. These particles adhere to the primer, forming a physical barrier that reduces the effective contact area between the topcoat and the primer. Consequently, the topcoat exhibits diminished spreading and adhesion in these regions, turning them into weak points in the intercoat bond.

Oils and greases on operators’ gloves constitute another common source of contamination. During handling, inspection, or coating application, glove‑contaminated hands may come into contact with the primer surface, transferring these residues to the coating. Such oils form a low‑surface‑energy contaminant layer on the primer, impeding the wetting and spreading of the topcoat and leading to pinholing or poor adhesion in localized areas. In regions affected by oil contamination, the lack of effective intermolecular interactions between the topcoat and the primer results in easy delamination at the interface under external stress.

When addressing this issue, prior to topcoat application, the primer surface should undergo appropriate cleaning to remove dust and oil contamination, thereby restoring its surface activity. Additionally, from the time the primer has cured until the topcoat is applied, the primer surface must be kept clean to prevent dust adhesion and oil contamination, and the interval between primer and topcoat applications should be carefully controlled to avoid prolonged exposure that could lead to changes in surface condition or the accumulation of contaminants.

III. Perform Necessary Surface Treatments

For applications with stringent performance requirements, additional processes such as sanding or plasma treatment can be applied after the primer has cured, increasing the primer’s surface roughness and surface energy to enhance the wetting and penetration of the topcoat. Such surface treatments can restore or improve the primer’s surface activity, creating favorable conditions for the topcoat’s spreading and penetration, thereby strengthening interlayer adhesion.

IV. Pay attention to the compatibility between the primer and the topcoat.

The compatibility between the primer and the topcoat also deserves attention. The resin systems, surface energies, and curing characteristics of the primer and topcoat should be well matched to prevent poor interlayer adhesion resulting from excessive mismatches. During formulation design and process planning, the primer and topcoat should be treated as an integrated system to ensure that a stable and effective bond can form at the interface.

V. Conclusion

Intercoat adhesion is a typical defect that must be addressed when using UV‑curable resins on substrates with poor adhesion in multilayer coating systems. Its root causes primarily stem from inadequate control of the primer’s cure degree and surface contamination. To mitigate this issue, approaches can be taken at both the process‑control and surface‑treatment levels: appropriately regulate the primer’s cure to ensure its surface is neither overly dense nor excessively weak in internal cohesion; maintain a clean primer surface by removing dust and oil contaminants and carefully managing coating intervals; for applications with stringent requirements, incorporate additional steps such as sanding or plasma treatment to enhance wetting and penetration of the topcoat; and ensure proper compatibility between the primer and the topcoat. By implementing these measures, intercoat adhesion can be reliably maintained to meet service specifications, thereby preventing problems such as topcoat delamination or blistering.

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 – Membrane Materials
Difficult to adhere to the substrate
Product Model/English Abbreviation	Product Name/Product Type	Product Features
B-186	Modified epoxy acrylate	Boil-resistant, excellent adhesion, high-temperature resistant, chemically resistant
B-509B	Polyester acrylate	Good adhesion, good flexibility, and excellent pigment wetting.
B-531	Polyester acrylate	Good adhesion, impact resistance, excellent flexibility, and yellowing resistance.
B-546	Polyester acrylate	Good adhesion, fast curing, and excellent flexibility.
B-590	Polyester acrylate	Good adhesion, fast curing, and excellent pigment wetting.
BM2224 (EO-HDDA)	Ethoxylation of 1,6-hexanediol diacrylate	It exhibits excellent adhesion to plastics, good dilutability, and low volatility.

Share to:

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 6)

How to Address Defects in UV Resins on Difficult-to-Bond Substrates (Part 4)