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Physical Properties of UV Resins with Poor Adhesion to Substrates
Release time:
2026-09-27 17:16
The physical properties of UV‑curable resins for difficult‑to‑adhere substrates constitute the material basis for achieving reliable adhesion on low‑surface‑energy plastics. Compared with conventional UV‑curable resins, these formulations exhibit distinctive characteristics in surface tension, cure‑shrinkage behavior, wetting performance, mechanical properties, and interfacial morphology. These attributes are interrelated and collectively determine the resin’s adhesion performance and coating quality on substrates such as polypropylene, polyethylene, and polyester.
I. Surface Tension and Wetting Properties
Surface tension is one of the key physical properties of UV‑curable resins on poorly adherent substrates, directly determining whether the coating can effectively spread across the substrate surface.
The surface tension of conventional UV‑curable resins typically exceeds the surface energy of substrates such as polypropylene and polyethylene, causing the coating to bead up on the substrate and preventing the formation of a continuous, uniform film. To address the issue of poor adhesion to these challenging substrates, UV‑curable resins can be formulated by incorporating low‑surface‑tension components, thereby reducing the coating’s surface tension to a level that approaches or falls below the substrate’s surface energy. In organosilicon‑modified systems, polydimethylsiloxane segments exhibit inherently low surface tension, while acrylic‑modified polydimethylsiloxanes can further lower the coating’s surface tension to a value sufficient to wet both polyethylene and polypropylene surfaces.
The improvement in wetting behavior can be quantitatively assessed using the contact angle. A smaller contact angle indicates better wettability and more uniform spreading of the coating on the substrate surface. For substrates that are difficult to adhere to, the contact angle of UV‑curable resins is significantly lower than that of conventional UV‑curable resins, which is a prerequisite for achieving effective adhesion.
II. Curing Shrinkage Characteristics
The curing shrinkage rate is an important physical parameter that influences the stability of interfacial bonding. During the polymerization of UV‑curable resins, the van der Waals distances between liquid monomer molecules are transformed into covalent bond lengths in the solid polymer, leading to a reduction in system volume and the development of shrinkage stresses.
This shrinkage stress concentrates at the interface between the coating and the substrate; when the stress exceeds the interfacial adhesion strength, the coating delaminates from the substrate. For substrates such as polyester, the root cause of adhesion problems often lies in the interfacial stresses generated by volumetric shrinkage. To address the challenge of poor adhesion to difficult substrates, UV‑curable resins employ molecular‑structure design to reduce curing shrinkage. The incorporation of cyclic acrylates leverages the ring structure’s ability to buffer volume changes during ring‑opening polymerization, thereby lowering the curing shrinkage rate. Monomers bearing bulky, rigid side groups hinder back‑addition reactions to chain‑end radicals during polymerization, significantly reducing both short‑chain and long‑chain branching; the resulting structural regularity further helps to mitigate shrinkage‑induced stresses.
The reduction in curing shrinkage directly diminishes stress concentrations at the interface, enabling the coating to maintain close contact with the substrate after curing.
III. Mechanical Properties
The mechanical properties of UV‑curable resins on difficult-to-bond substrates require a balance between adhesion, hardness, and flexibility.
Coating hardness is a key indicator for assessing surface scratch resistance. High‑functionality monomers can increase crosslink density and hardness, but they typically also lead to greater curing shrinkage and higher internal stresses, which can compromise adhesion. For substrates that are difficult to bond, UV‑curable resins often employ lower‑functionality resin systems, prioritizing reliable interfacial adhesion while maintaining the required mechanical properties.
Flexibility is essential for accommodating substrate deformation. Substrates such as polypropylene and polyethylene may bend or deform during use, and the coating must exhibit sufficient flexibility to follow these changes without cracking. Silicone‑modified systems and fluoropolymer‑based systems, by incorporating flexible polymer segments, provide adequate flexibility while maintaining strong adhesion.
Adhesion, a key physical property of UV‑curable resins for difficult-to-bond substrates, is typically quantified using either the cross‑hatch test or the pull‑off test. In the cross‑hatch method, a grid is scored on the coating surface and then removed with adhesive tape to assess the percentage of the coated area that has peeled off. The pull‑off method measures the maximum tensile force required to detach the coating from the substrate under a perpendicular load. High‑quality UV‑curable resins designed for challenging substrates can achieve a high level of adhesion to materials such as polypropylene and polyethylene.
IV. Interface Morphology and Microstructure
Interface morphology is an important physical characteristic for evaluating adhesion quality. By examining the coating–substrate interface with a scanning electron microscope, one can assess the penetration depth of the coating into the substrate surface, the extent of mechanical interlocking, and the distribution of defects at the interface.
UV‑curable resins on difficult‑to‑adhere substrates form a tight interfacial contact upon curing. Specific components in the coating can penetrate the substrate’s microscopic surface textures and pores, creating an interlocking structure between the substrate and the coating after curing. This interlocking architecture generates mechanical interlocking at the interface, thereby enhancing the physical adhesion.
Surface enrichment is also an important characteristic of interfacial morphology. During film formation, fluorinated chain segments selectively accumulate at the coating–air interface, forming a thin fluorinated surface layer. This surface enrichment can be characterized by X-ray photoelectron spectroscopy, which allows for the analysis of elemental composition differences between the coating’s surface and its interior.
V. Viscosity and Rheological Properties
Viscosity is an important physical parameter that affects both application performance and coating uniformity. For UV‑curable resins used on substrates with poor adhesion, viscosity must be adjusted according to the application method to suit different coating processes.
Roll coating requires the coating to possess appropriate viscosity and rheological properties, enabling it to spread evenly on the roller and transfer onto the substrate surface. Spray coating demands a lower viscosity and excellent atomization performance, allowing for the formation of uniform, fine mist droplets. Pour coating calls for stable curtain‑forming capabilities, ensuring the creation of a continuous, even coating curtain.
In terms of rheological properties, UV‑curable resins formulated for difficult‑to‑adhere substrates typically exhibit a degree of thixotropy—meaning their viscosity decreases under shear and recovers upon standing. This behavior ensures that the coating flows readily during application while maintaining a stable, non‑sagging form when at rest.
VI. Thermal Properties
Thermal properties influence the stability of coatings under varying temperature conditions. Parameters such as the glass transition temperature, coefficient of thermal expansion, and thermal stability of UV‑curable resins on poorly adherent substrates determine the coating’s performance at high or low temperatures.
The glass transition temperature indicates the temperature at which polymer chain segments begin to move. A higher glass transition temperature means that the coating can maintain its hardness and dimensional stability even at elevated temperatures. The coefficient of thermal expansion characterizes the degree of expansion or contraction of the coating as temperature changes. If the difference between the coating’s and the substrate’s coefficients of thermal expansion is too large, interfacial stresses will develop during temperature fluctuations, compromising adhesion. For substrates that are difficult to bond, UV‑curable resins are formulated to match the coating’s coefficient of thermal expansion as closely as possible to that of the substrate, thereby minimizing interfacial stresses caused by temperature variations.
VII. Conclusion
The physical properties of UV‑curable resins for difficult‑to‑adhere substrates encompass surface tension and wetting behavior, curing shrinkage, mechanical performance, interfacial morphology and microstructure, viscosity and rheological characteristics, as well as thermal properties. Reduced surface tension enables the coating to spread effectively on low‑surface‑energy substrates; diminished curing shrinkage mitigates stress concentrations at the interface; a balanced mechanical performance ensures an optimal trade‑off among adhesion, hardness, and flexibility; optimized interfacial morphology enhances mechanical anchoring; tailored viscosity and rheological behavior accommodate various application methods; and matched thermal properties minimize interfacial stresses induced by temperature fluctuations. The synergistic interplay of these physical attributes collectively underpins the reliable adhesion of UV‑curable resins to low‑surface‑energy plastics.
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.
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, excellent flexibility, and superior 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. |
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