Liquid photopolymer, a functional material that cures rapidly under ultraviolet light, is widely used in printing and packaging, electronic devices, and optical components due to its high hardness, wear resistance, chemical corrosion resistance, and environmental friendliness. However, its adhesion to different substrates is significantly affected by the substrate's surface energy, chemical inertness, and microstructure, requiring targeted surface treatment techniques to optimize interfacial bonding performance. The following systematically elaborates on key technological pathways for improving liquid photopolymer adhesion from three dimensions: physical modification, chemical modification, and composite modification.
Physical modification, by controlling the surface morphology and roughness of the substrate, enhances the mechanical locking effect and is a fundamental means of improving adhesion. Corona discharge treatment utilizes a high-frequency, high-voltage electric field to ionize air, generating high-energy electrons and ions that bombard the substrate surface, causing the molecular chains on the material surface to break and form a microporous structure. For example, after corona treatment, the surface roughness of polyethylene film increases significantly, allowing the liquid photopolymer coating to penetrate deep into the pores and form a mechanical anchor, resulting in a significant improvement in adhesion. Plasma treatment generates active particles (such as oxygen free radicals and nitrogen ions) through glow discharge, which etch and activate the substrate surface. This technology is applicable to metals, glass, and polymer materials. It introduces polar groups and increases surface roughness, significantly improving the wettability between liquid photopolymer and substrate. Flame treatment uses high-temperature oxidation to generate oxygen-containing functional groups on the substrate surface. Simultaneously, thermal stress causes surface shrinkage, forming microcracks that provide more bonding sites for the coating. This is suitable for thermoplastic materials such as polypropylene and polycarbonate.
Chemical modification introduces polar groups or active sites to build chemical bonds between the substrate and liquid photopolymer, which is a core strategy for improving adhesion. Chemical oxidation uses strong oxidants (such as the potassium dichromate-sulfuric acid system) to corrode the surface of polyolefins, generating polar groups such as hydroxyl and carboxyl groups, enhancing surface polarity and wettability. For example, after chemical oxidation treatment, the surface tension of polyethylene is significantly increased, improving its compatibility with acrylate monomers in liquid photopolymer and significantly enhancing adhesion. Silane coupling agent treatment generates silanol groups through hydrolysis, forming covalent bonds with hydroxyl groups on the substrate surface. Simultaneously, the organic groups in the silane molecules can undergo physical entanglement or chemical reaction with the liquid photopolymer. For example, after coating an aluminum alloy surface with a silane coupling agent, a "molecular bridge" is formed between the liquid photopolymer coating and the substrate, significantly improving peel strength. Acid etching treatment uses strong acids (such as hydrochloric acid and sulfuric acid) to micro-corrode the metal substrate, generating a uniform microporous structure and exposing active metal atoms. Monomers in the liquid photopolymer can penetrate the pores and polymerize in situ, forming a mechanical-chemical dual bond.
Composite modification combines physical and chemical methods to achieve synergistic enhancement of adhesion through multi-scale interface optimization. Plasma-chemical grafting combined treatment first activates the substrate surface with plasma, then initiates a grafting polymerization reaction using ultraviolet light, covalently bonding functional monomers (such as acrylic acid and glycidyl methacrylate) to the substrate surface. This technology can introduce polar segments onto the surface of difficult-to-adhere materials such as polytetrafluoroethylene, significantly improving the adhesion and durability of the liquid photopolymer coating. Nanostructured composite coatings achieve superhydrophobic or superhydrophilic surface control by constructing nanoscale rough structures (such as nanowires and nanoparticles) on the substrate surface and combining them with chemical modification. For example, after depositing an array of silica nanoparticles on a glass surface, a liquid photopolymer coating can penetrate and solidify along the nanostructure, forming a high-strength interfacial bond and simultaneously imparting anti-reflective or self-cleaning functions to the coating.
Special substrates require tailored processing technologies. Fluoropolymers (such as polytetrafluoroethylene) have extremely low surface energy and strong chemical inertness, requiring chemical etching with sodium naphthalene solution to remove surface fluorine atoms, generating a carbonized layer and introducing double bonds to provide active sites for liquid photopolymer grafting. Liquid crystal polymers (LCPs) have a smooth surface due to their high molecular orientation, requiring plasma treatment combined with an electroplated transition layer (such as a titanium/copper composite layer) to enhance adhesion through the synergistic effect of the metal-polymer interface. Bio-based materials (such as cellulose and chitosan) require enzymatic hydrolysis or oxidation to generate reactive groups such as aldehydes and carboxyl groups, which then undergo condensation reactions with the amino or hydroxyl groups in the liquid photopolymer to form stable covalent bonds.
Optimization of process parameters is crucial for surface treatment effectiveness. Corona treatment requires control of power, processing time, and electrode spacing to avoid over-discharge leading to thermal damage to the substrate. Plasma treatment requires adjustment of gas composition (e.g., oxygen and argon ratio) and discharge power to balance surface activation and etching rates. Chemical grafting requires control of monomer concentration, initiator dosage, and reaction time to ensure uniform graft layer thickness. Furthermore, environmental humidity, temperature, and coating application processes (e.g., spraying pressure and curing energy) all affect the final adhesion performance.
Enhancing the adhesion of liquid photopolymers to different substrates requires selecting appropriate surface treatment technologies based on substrate characteristics. Physical modification strengthens mechanical anchoring by constructing microscopic rough structures, chemical modification achieves chemical bonding by introducing polar groups or active sites, and composite modification achieves synergistic enhancement through multi-scale interface optimization. In the future, with the in-depth development of nanotechnology, plasma chemistry, and biomimetics, liquid photopolymer surface treatment technology will evolve towards higher precision and multifunctionality, providing superior interface solutions for high-end manufacturing.
