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The difference between UV resin for difficult-to-bond substrates and ordinary UV resin

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2026-09-28 17:00

The difference between UV resin for difficult-to-bond substrates and ordinary UV resin

In the application systems of UV‑curable materials, conventional UV resins and UV resins for difficult‑to‑adhere substrates serve different substrate types. Conventional UV resins are designed for high‑surface‑energy plastics such as ABS and PC, as well as traditional substrates like wood and paper, achieving adhesion through standard interfacial wetting and physical anchoring. In contrast, UV resins for difficult‑to‑adhere substrates are formulated specifically for low‑surface‑energy plastics such as polypropylene, polyethylene, and polyester, requiring specialized molecular design and formulation strategies to address interfacial wetting, chemical anchoring, and shrinkage‑stress management. These two categories differ significantly in their target applications, surface tension, curing‑shrinkage characteristics, adhesion mechanisms, and formulation compositions.

I. Differences in Applicable Substrates

The substrate compatibility of conventional UV‑curable resins is primarily limited to materials with relatively high surface energies. Engineering plastics such as ABS, PC, and PMMA typically exhibit surface energies in the range of 40–50 mN/m, which closely match those of standard UV‑curable resins, enabling excellent wetting and spreading. Porous substrates like wood and paper, on the other hand, rely on the coating penetrating the fiber interstices to form mechanical anchoring, making them less sensitive to surface energy. A common characteristic of these substrates is that their surfaces possess sufficient polarity or micro‑roughness, allowing for effective interfacial adhesion with conventional UV‑curable resins.

UV resins for difficult-to-adhere substrates are specifically formulated for materials with low surface energy. Polypropylene and polyethylene typically exhibit surface energies below 30 mN/m, significantly lower than the surface tension of conventional UV resins. Although polyester materials have slightly higher surface energies than polyolefins, their high crystallinity and chemical inertness also pose substantial adhesion challenges. Thermoplastic polyurethanes and other similar materials present comparable adhesion difficulties. A common characteristic of these substrates is the absence of reactive functional groups on their surfaces, making it difficult to form chemical bonds with standard UV resins.

II. The Difference Between Surface Tension and Wettability

The surface tension of conventional UV‑curable resins typically falls within the range of 35–45 mN/m, a value that exceeds the surface energies of polypropylene and polyethylene but remains lower than those of ABS and PC. On ABS and PC substrates, such resins exhibit excellent wetting and spreading. However, on polypropylene and polyethylene, they fail to wet effectively; the coating retracts into droplets, preventing the formation of a continuous, uniform film.

UV‑curable resins for difficult‑to‑adhere substrates reduce the coating’s surface tension to below 30 mN/m by incorporating low‑surface‑tension components, bringing it close to or even lower than the surface energy of polyolefin substrates. The introduction of organosilicon‑modified systems and fluorine‑containing modified resins further lowers the coating’s surface tension, enabling effective wetting and uniform spreading on polypropylene and polyethylene surfaces. Enhanced wettability is the foundation of adhesion; only when the coating achieves full contact with the substrate surface can subsequent mechanical interlocking and chemical bonding occur efficiently.

III. Differences in Curing Shrinkage Characteristics

The curing shrinkage of conventional UV‑curable resins is typically quite high, with volumetric shrinkage in acrylate systems reaching 14%–24%. For substrates such as ABS and PC, the interfacial stresses arising from this shrinkage remain within acceptable limits, as the interfacial adhesion between the substrate and the coating is sufficient to withstand the shrinkage‑induced stresses.

UV‑curable resins intended for substrates with poor adhesion must have their curing shrinkage carefully controlled to minimize interfacial stresses. Substrates such as polyesters are particularly sensitive to shrinkage‑induced stresses; excessive shrinkage can lead to delamination of the coating from the substrate. Incorporating cyclic acrylates leverages the volume‑buffering effect of their cyclic structures during polymerization, thereby reducing curing shrinkage. Monomers like isobornyl acrylate, which possess bulky, rigid side groups, can significantly suppress both short‑chain and long‑chain branching during polymerization; the resulting structural regularity helps to lower shrinkage‑related stresses.

IV. Differences in Adhesion Mechanisms

The adhesion between conventional UV‑curable resins and substrates relies primarily on mechanical interlocking and intermolecular forces. On substrates such as ABS and PC, the coating can penetrate the substrate’s surface micro‑pores, creating mechanical interlock, while polar functional groups in the resin form hydrogen bonds or other intermolecular interactions with the substrate’s surface functional groups. For wood and paper substrates, the coating penetrates the fiber gaps, establishing deep‑penetrating anchorage.

The adhesion mechanisms of UV‑curable resins on difficult‑to‑adhere substrates are more complex. For polyolefin substrates, the lack of reactive functional groups and surface micro‑porosity makes it challenging for mechanical interlocking and intermolecular forces alone to provide sufficient adhesion. To address this, UV‑curable resins for such substrates typically incorporate chlorinated polyolefin‑based adhesion promoters, which leverage a molecular structure similar to that of the substrate to promote interfacial wetting and physical entanglement. Additionally, reactive phosphates, silane coupling agents, and reactive metal salts are employed to establish chemical anchoring by forming interactions with trace polar sites on the substrate surface, thereby enhancing interfacial bonding.

V. Differences in Formulation Composition

The formulation of conventional UV resins is relatively simple, consisting primarily of polyurethane acrylate or epoxy acrylate prepolymers, acrylate‑based reactive diluents, photoinitiators, and standard additives. The key to formulation design lies in balancing hardness, flexibility, cure speed, and application performance.

Formulations for UV‑curable resins intended for difficult‑to‑adhere substrates must incorporate additional adhesion‑promoting components on top of the aforementioned base. The inclusion of chlorinated polyolefin‑based promoters, phosphate ester‑type promoters, and fluorine‑ or organosilicon‑modified resins, among other ingredients, increases formulation complexity. At the same time, formulation design must balance multiple conflicting trade‑offs, such as adhesion versus cure‑induced shrinkage, hardness versus flexibility, and storage stability versus reaction activity. The dosage of adhesion promoters must be carefully controlled: too low a level results in insufficient adhesion, while excessive amounts may compromise storage stability or adversely affect other performance attributes.

VI. Differences in the Degree of Dependence on Preprocessing

Conventional UV‑curable resins exhibit relatively low dependence on substrate pretreatment. Substrates such as ABS and PC can be directly coated after cleaning, without the need for additional surface activation. For wood and paper substrates, adequate adhesion can be achieved through sanding.

Although UV‑curable resins for difficult‑to‑adhere substrates have improved adhesion through molecular design, they still require substrate pretreatment in certain applications. Corona or flame treatment can increase the surface energy of polyolefin substrates, but the effectiveness of such treatments diminishes over time. By establishing stronger interfacial bonding, these specialized UV‑curable resins can partially mitigate the loss of adhesion caused by the decay of pretreatment effects; however, their performance still demands more rigorous pretreatment than that required for conventional UV‑curable resins.

VII. Conclusion

The differences between UV resins for difficult-to-bond substrates and conventional UV resins are evident in aspects such as the range of compatible substrates, surface tension and wetting performance, curing‑induced shrinkage characteristics, adhesion mechanisms, formulation composition, and the degree of reliance on surface pretreatment. Conventional UV resins are designed for substrates with high surface energy and achieve adhesion through standard wetting and physical anchoring; in contrast, UV resins for difficult-to-bond substrates address the challenges of low‑surface‑energy materials by reducing surface tension, controlling residual stresses, incorporating chemical anchoring mechanisms, and optimizing formulation components. These two types serve distinct application scenarios, and selection should be guided by the substrate type and specific adhesion requirements.

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Bossin 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	Excellent adhesion, impact resistance, good flexibility, and yellowing resistance.
B-546	Polyester acrylate	Good adhesion, fast curing, and excellent flexibility.
B-590	Polyester acrylate	Excellent adhesion, fast curing, and good pigment wetting.
BM2224 (EO-HDDA)	Ethoxylation of 1,6-hexanediol diacrylate	Excellent adhesion to plastics, good dilutability, and low volatility.

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