Ultraviolet-curable coatings and their development trends

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1 Introduction 4 K" X: G" y! b: l' O" w After years of development, the coating industry has now seen the emergence of environmentally friendly coatings, represented by water-based coatings, radiation-cured coatings, high-solid-content coatings, and powder coatings. Especially radiation-curable coatings are regarded as the top choice among environmentally friendly coatings, as they contain no volatile organic solvents, cause no environmental pollution, have no residual solvents after the coating dries, and possess excellent performance characteristics. This article provides a review of UV-curable coatings, aiming to discuss their structural composition, curing mechanisms, as well as coating design and application techniques. It also introduces the latest developments in the synthesis of the key components of UV coatings, namely prepolymers and reactive diluents, in order to offer designers and users a comprehensive overview of relevant theories and recent advancements. / J* n" t$ P4 2 UV coating curing mechanism! E# i) ]6 v1 s( R% TAccording to Grotthuss’ law, a necessary condition for photochemical reactions is the effective absorption of light by the substance. Light absorbed by a substance can initiate photochemical reactions, but not all light absorbed by a substance is capable of causing such reactions; only light with sufficiently high energy can stimulate the substance and lead to photochemical reactions. Clearly, the relationship between a substance’s absorption of light, the energy absorbed by the light, and the substance’s own chemical structure is a key factor determining whether a photochemical reaction can occur. ; ~3 R9 r+ i5 j: H; B The photoequivalence law states that, in a primary reaction, the number of molecules (or atoms) that are activated is equal to the number of photons absorbed. A photon is the most fundamental unit of energy in light, and its energy is given by δ=hy=hcλ. To activate 1 mole of molecules, 6.02×1023 photons need to be absorbed. The photon energy absorbed by 1 mole of a substance is called 1 Einstein, that is: μ′=йhc/λ9 }; i! ~8 }9 n" Y' Z) t; if nanometers are used as the unit for wavelength, then μ′=1.196×105/λ (kJ/mol). The wavelength range of ultraviolet light is 200-400 nm, and according to the photochemical equivalent law, the Einstein quantum value for ultraviolet light is 598-299 kJ/mol. The photon energy in this range is almost higher than the bond energies of the vast majority of organic compounds; from the perspective of chemical thermodynamics, ultraviolet light can essentially induce photochemical reactions in most organic substances. However, further research has shown that photochemical reactions can occur effectively only when the wavelength of the radiation matches the excitation energy levels of the material. Substances that are capable of effectively absorbing ultraviolet light and can be easily excited to form active atoms or active atomic groups are generally referred to as photosensitizers. 0 l: B( t3 a; e% x8 Z Chemical kinetic studies show that the mechanism by which ultraviolet light promotes the curing of UV coatings is free radical chain polymerization. First is the light-initiation stage ； Next is the chain growth reaction stage; during this stage, as the chain growth proceeds, cross-linking occurs and the material solidifies to form a film ； Finally, the chain radical will achieve chain termination through coupling or disproportionation. Among the three consecutive stages of UV curing, the photoinitiation stage is the slowest, and it serves as the controlling factor for the overall speed of the photocuring reaction; therefore, the photoinitiation activity of the photosensitizer directly affects the drying speed of UV-cured coatings. $ F4 \2 D) D( S Based on the comprehensive analysis, the performance of UV-cured coatings is related to the intensity and wavelength of UV light, as well as to the structural composition of the UV coatings – factors such as prepolymers, reactive diluents, and pigments. c. ~7 E: y6 Z+ E) Composition of UV-curing coatings: y5 E+ F2 L% N/ J( {5 b/ h5 W UV-curing coatings are mainly composed of prepolymers, reactive diluents, photosensitizers, pigments and fillers, as well as various additives; the properties and functions of each component are described separately. 5 D3 F5 S1 c, n’ E! V3.1 Prepolymer 9 D. Z4 Z- @- @8 ^3 W) u2 A p Prepolymers or photopolymerizable resins are one of the active structural components of UV-curable coatings; their properties are closely related to the manufacturing process of the UV-curable coatings, the application techniques, as well as various physical and chemical properties of the resulting coating film. There are currently two main types of photopolymer prepolymers used in coatings: one type is unsaturated polyesters ； Another category is acrylic resins. The properties of common prepolymers are shown in Table 1. + @; \* c, X’ H& X$ n% I* } Table 1 Common prepolymers for UV-curable coatings* U’ V9 f% Y2 k/ k Prepolymer Advantages Disadvantages& X% R b$ I7 o Unsaturated polyester Low cost, fast curing, good wettability of pigments and fillers, high toughness Low gloss, poor resistance, prone to emulsification j5 M. N1 g$ R Polyester acrylate Low cost, good wettability of pigments and fillers, good overall performance Poor chemical resistance 5 M: H4 V, @: c8 {& F$ k Polyether acrylate Fast curing, high adhesion, good gloss, good flexibility Poor resistance, prone to emulsification. J( s. H7 a7 D- \. A Epoxy acrylate Low cost, fast curing, good adhesion, high gloss Poor flexibility, prone to emulsification 6 J0 M9 U* b8 W! k( `5 m4 ^3 B: d Polyurethane acrylate Fast curing, high gloss, excellent overall properties in terms of flexibility and wear resistance High cost, high viscosity