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Typical Defects of UV 3C Coatings (10)

Release time:

2026-06-29 06:32

Among the various defects in UV 3C coatings, poor adhesion is a serious issue that compromises coating reliability. It manifests as the paint film easily peeling off the substrate—so much so that it can be removed with a simple tape test—indicating insufficient interfacial bonding strength between the coating and the substrate. Adhesion is the foundation upon which a coating’s protective function depends; inadequate adhesion directly results in the failure of the coating’s protective performance. Understanding the characteristics and root causes of poor adhesion is crucial for quality control and defect identification.

I. Manifestations of Poor Adhesion

Poor adhesion is primarily characterized by insufficient interfacial bonding strength between the layers of the coating system. Poor adhesion between the primer and the substrate manifests as complete delamination of the coating, including the primer, from the substrate surface, with a smooth delamination interface and no residual substrate material remaining. Poor adhesion between the topcoat and the primer results in peeling or flaking of the topcoat layer from the primer surface.

In the grid‑cut test, samples with poor adhesion exhibit extensive coating delamination upon tape removal. The extent of delamination reflects the strength of adhesion: the larger the delaminated area, the poorer the adhesion. Furthermore, the condition of the substrate surface after delamination can help identify whether the failure occurred at the coating–substrate interface or within the coating itself.

In practical applications, coatings with poor adhesion may exhibit peeling or flaking under normal service conditions. The edge regions are typically the most common sites of adhesion failure, where the coating first begins to lift at the edges and gradually propagates inward.

II. Improper Substrate Preparation

Improper substrate preparation is the primary cause of poor adhesion. During injection molding, plastic substrates often retain release agents on their surfaces. These release agents are low‑surface‑energy substances that form an isolating film on the substrate; when a primer is applied, this film prevents direct contact between the primer and the substrate.

Oil contamination is also a common issue. The substrate surface may be contaminated with oils originating from processing equipment, operators, or the storage environment. Such oil layers likewise exhibit low surface energy, making it difficult for the primer to wet and adhere to them. Additionally, dust particles adhering to the substrate surface reduce the effective contact area between the primer and the substrate.

For low‑surface‑energy substrates such as PP and PE, the inherent surface energy is relatively low, making it difficult for primers to achieve direct wetting and adhesion. On untreated low‑surface‑energy substrates, coating adhesion is generally inadequate.

III. Influence of Curing State on Adhesion

The curing state is a critical factor influencing adhesion. When curing is incomplete, the coating exhibits insufficient crosslink density and low cohesive strength. During adhesion testing, failure under external load may occur within the coating rather than at the interface, manifesting as cohesive failure of the coating.

When over‑cured, the coating exhibits an excessively high crosslink density, leading to increased internal stresses arising from curing shrinkage. If these internal stresses exceed the interfacial adhesion between the coating and the substrate, the coating may delaminate from the substrate surface. Moreover, excessive curing can render the coating brittle; such a brittle coating is more prone to failure at the interface under external loading.

IV. Compatibility Between Coatings and Substrates

The compatibility between the coating and the substrate also affects adhesion. Since different substrates vary in their chemical composition and surface characteristics, it is essential to select a coating system that matches each specific substrate. A coating formulation suitable for ABS may not be appropriate for PC or PP substrates.

The hardness and film thickness of the primer also affect adhesion. When the primer is excessively hard, the large difference in elastic modulus between the coating and the substrate leads to stress concentration at the interface, reducing adhesion. If the primer film is too thick, internal stresses within the coating increase; if it is too thin, the adhesion base is insufficient.

When the primer contains silicone‑based additives, excessively low surface tension can compromise the adhesion of subsequent coating layers. Silicone‑based substances tend to accumulate at the coating interface, forming a weak interfacial layer that reduces interlayer adhesion.

V. Analysis of Adhesion Failure Locations

The location of adhesion failure helps identify the root cause of poor adhesion. When failure occurs at the coating–substrate interface, the fracture surface is smooth and no coating residue remains on the substrate, indicating insufficient bonding between the primer and the substrate—likely due to inadequate substrate preparation or an excessively low substrate surface energy.

When failure occurs within the coating, residual coating material can be observed on the delamination surface, indicating insufficient cohesive strength of the coating, which may be attributable to incomplete curing or inadequate intrinsic strength of the coating itself.

When failure occurs at the interface between the primer and the topcoat, it indicates insufficient intercoat adhesion, which may be attributable to surface contamination of the primer, excessive curing of the primer, or paint incompatibility.

VI. Conclusion

Poor adhesion is a critical defect in UV‑cured 3C coatings that compromises coating reliability, with its root causes spanning substrate preparation, curing conditions, and coating compatibility. Residual release agents and oil contaminants on the substrate surface impede primer adhesion, while primers applied to low‑surface‑energy substrates without proper treatment often fail to meet adhesion specifications. Incomplete curing results in insufficient cohesive strength, whereas over‑curing increases internal stresses, weakening interfacial bonding. Additionally, coating–substrate incompatibility, inappropriate primer hardness and film thickness, and the presence of silicone‑based additives in the primer can all contribute to adhesion issues. Analyzing the location of adhesion failure helps identify the underlying failure mechanism. A thorough understanding of the manifestations and causes of poor adhesion lays the foundation for its effective identification and analysis.

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-368	Aliphatic polyurethane acrylate	Good toughness, excellent leveling, excellent bend resistance, and excellent heat resistance.
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B-619W	Aliphatic polyurethane acrylate	Fast curing, high hardness, excellent toughness, wear resistance, and chemical resistance.
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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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Typical Defects of UV 3C Coatings (Part 11)

Typical Defects of UV 3C Coatings (9)