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Typical Defects of UV Resins on Difficult-to-Adhere Substrates (Part 2)

Release time:

2026-10-08 16:39

Typical Defects of UV Resins on Difficult-to-Adhere Substrates (Part 2)

Curing shrinkage is an intrinsic property of UV‑curable resins during the polymerization process; however, on substrates with poor adhesion, the interfacial stresses generated by shrinkage are more likely to induce adhesion failure. This issue is particularly pronounced on substrates such as polyesters, which are highly sensitive to shrinkage‑induced stresses. During curing, the coating undergoes volumetric contraction, and the resulting shrinkage stresses concentrate at the coating–substrate interface. When these stresses exceed the interfacial bond strength, the coating delaminates from the substrate. A thorough understanding of the mechanisms underlying this defect and the factors that influence it can facilitate the implementation of targeted control strategies in practical production.

I. Mechanism of Shrinkage During Curing

The curing process of UV‑curable resins involves the polymerization of liquid monomers under ultraviolet irradiation, transforming the liquid into a solid polymeric network. During this process, the intermolecular distances shift from the range governed by van der Waals forces to the covalent bond length; as molecular spacing decreases, the system’s volume contracts, resulting in curing shrinkage. This shrinkage is an intrinsic characteristic of polymerization and cannot be entirely eliminated; it can only be managed through careful formulation design.

The internal stresses generated by thermal contraction accumulate within the coating and become concentrated at the coating–substrate interface. The interfacial adhesion strength between the coating and the substrate must withstand these contraction stresses; when the stress exceeds this interfacial strength, the coating delaminates from the substrate, resulting in adhesive failure.

II. Causes of Stress Concentration at Interfaces

The root cause of interfacial stress concentration lies in the constraint relationship between the coating and the substrate. During curing, the coating undergoes volumetric shrinkage that is restrained by the substrate, preventing it from contracting freely; this contraction tendency gives rise to internal stresses. The greater the stiffness of the substrate, the stronger the constraint on the coating, and the higher the interfacial stress.

Polyester and similar substrates are highly sensitive to shrinkage stresses because these materials exhibit high stiffness, imposing strong constraints on the coating. Moreover, polyester substrates have limited surface chemical activity, resulting in relatively weak interfacial adhesion with the coating and insufficient resistance to shrinkage stresses. When the shrinkage stress exceeds the interfacial bond strength, the coating delaminates from the substrate.

The magnitude of interfacial stress also depends on the coating thickness and crosslink density. Thick coatings exhibit greater shrinkage during curing, leading to higher accumulated shrinkage stresses. In systems with high crosslink density, a more compact network structure forms during polymerization, typically resulting in a larger shrinkage rate and, consequently, increased interfacial stress.

III. Delayed Failure Caused by Stress Relaxation

Adhesion issues caused by curing shrinkage do not necessarily become apparent immediately after coating application. Even when the interfacial bond strength is sufficient to withstand the initial shrinkage stresses, residual stresses may gradually relax over time during subsequent service.

Residual stresses persist within the coating over long periods and, under external influences, may gradually weaken interfacial adhesion. Temperature variations induce thermal expansion and contraction in both the coating and the substrate, generating cyclic shear stresses at the interface. Humidity changes cause the coating to swell upon absorption of moisture and to contract upon drying, also inducing stress cycles at the interface. Under the combined action of these factors, the coating may progressively delaminate or lift off after a period of service, manifesting as delayed failure.

IV. Factors Affecting Shrinkage Stress

Coating thickness is a critical factor influencing shrinkage stress. Thick coatings exhibit substantial shrinkage during curing, leading to higher accumulated stresses. Reducing the thickness of each coating pass or adopting a multi‑layer, thin‑coat approach can decrease the shrinkage per layer and thereby mitigate interfacial stresses.

Crosslink density also influences shrinkage stress. In systems with a high proportion of high‑functionality monomers, the crosslink density is greater, and the shrinkage rate is typically higher as well. Provided that hardness requirements are met, moderately reducing the crosslink density can help mitigate shrinkage stress.

The substrate’s stiffness and surface condition also influence the actual manifestation of shrinkage stresses. A stiffer substrate exerts stronger constraints on the coating, leading to more concentrated interfacial stresses. Substrates with well‑executed surface treatments exhibit higher interfacial adhesion strength, enabling them to withstand greater shrinkage stresses.

V. Areas for Improvement

To address interfacial stresses caused by curing shrinkage, improvements can be pursued through both formulation design and process control.

In terms of formulation, incorporating low-shrinkage monomers is a common solution. Cyclic acrylates buffer volumetric changes during polymerization, thereby reducing the cure‑shrinkage rate. Monomers with bulky, rigid side groups can significantly minimize short‑chain and long‑chain branching during polymerization, and their more regular molecular architecture helps to lower shrinkage stresses. The addition of low-shrinkage monomers can mitigate interfacial stresses, but it requires striking a balance between adhesion and hardness.

From a process standpoint, staged curing is an effective control strategy. First, pre-cure with lower energy to achieve initial shaping of the coating and minimize liquid‑phase flow time; then, complete deep curing using higher energy. Staged curing allows shrinkage stresses to be released in stages, thereby reducing stress concentrations at the interface. Controlling coating thickness and avoiding single‑coat applications of excessive thickness also helps to mitigate shrinkage stresses.

VI. Conclusion

Interfacial stress caused by curing shrinkage is one of the typical defects in UV‑curable resins applied to substrates with poor adhesion. Its origins lie in the inherent shrinkage associated with the polymerization reaction, the substrate’s constraint on the coating, and factors such as coating thickness and crosslink density. Shrinkage stress concentrates at the interface; when it exceeds the interfacial bond strength, adhesion failure occurs. Residual stresses may be gradually released during subsequent service, leading to delayed failure. Incorporating low‑shrinkage monomers, employing a stepwise curing strategy, and carefully controlling coating thickness can effectively mitigate the adverse effects of shrinkage stress on adhesion. In practical production, it is essential to strike an appropriate balance between adhesion and other performance attributes—such as hardness—based on the substrate characteristics and product 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.

Bosheng Related Product Recommendations – Membrane Materials
Difficult to adhere to the substrate
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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	Excellent adhesion, impact resistance, good 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.

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Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 3)

Typical Defects of UV Resins on Difficult-to-Bond Substrates (Part 1)