News

Knowledge

Company News

Application Mind Map

Test Report

Technical Article

Industry News

Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 5)

Release time:

2026-10-09 17:11

Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 5)

In multi‑layer coating systems, intercoat adhesion is a common issue and one of the most critical defects to address when applying UV‑curable resins to substrates with poor adhesion. Without strong intercoat bonding between the primer and topcoat, the topcoat may delaminate or blister, compromising the coating system’s aesthetic integrity, protective performance, and service life. For applications such as automotive interiors and appliance panels—where both appearance and durability are paramount—insufficient intercoat adhesion can lead to premature coating failure during assembly, use, or environmental aging. Therefore, it is essential to systematically analyze the root causes and implement targeted corrective measures.

I. Improper control of primer curing degree

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 limits 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 reduces 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 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; consequently, the topcoat wets and penetrates more readily, leading to relatively 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 of the primer itself. Thus, while insufficient curing can promote interlayer adhesion, the overall coating strength remains inadequate, failing to meet service requirements.

Controlling the degree of primer cure requires striking a balance between intercoat adhesion and the overall strength of the coating. When the cure is moderate, the primer surface retains sufficient reactivity 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. Effects of Contamination on the Primer Surface

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 operators’ gloves, creating a weak interfacial layer that impedes effective adhesion 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 cratering 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.

The presence of a weak interfacial layer significantly reduces interlayer adhesion; even when the primer is properly cured, surface contamination can still lead to topcoat delamination or blistering. Therefore, between primer curing and topcoat application, it is essential to keep the primer surface clean, preventing dust accumulation and oil‑based contamination.

III. Areas for Improvement

To address interlayer adhesion issues, improvements can be made through process control and surface treatment.

In terms of process control, appropriately managing the degree of cure of the primer is critical. Based on the compatibility between the primer resin system and the topcoat, it is necessary to optimize the curing energy and curing time to ensure that the primer surface is neither overly dense nor exhibits insufficient internal cohesion due to inadequate curing. At the same time, the coating interval between the primer and the topcoat should be carefully controlled to prevent prolonged exposure, which could lead to changes in surface condition or the accumulation of contaminants.

In terms of surface preparation, the primer surface should be thoroughly cleaned prior to topcoat application. Dust and oil contamination can be removed by dusting or degreasing, thereby restoring the primer’s surface activity. For applications with stringent requirements, additional processes such as sanding or plasma treatment may be performed after the primer has cured, increasing the primer’s surface roughness and surface energy, and enhancing the wetting and penetration characteristics of the topcoat.

In addition, the compatibility between the primer and the topcoat 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 differences in their formulations.

IV. 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 contamination of the primer surface. Overcured primers develop an excessively dense surface, hindering wetting and penetration by the topcoat; undercured primers exhibit insufficient cohesive strength; and surface contamination—such as dust or oils—creates a weak interfacial layer. To mitigate these issues, it is essential to carefully regulate the primer’s cure during application and maintain a clean primer surface, thereby ensuring that intercoat adhesion meets service‑life requirements.

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.

Boxing 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:

Typical Defects of UV Resins on Difficult-to-Bond Substrates (VI)

Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 4)