Key technologies of reactive diluents and photoinitiators in UV curing inks
As the core innovative technology of the modern printing industry, ultraviolet (UV) curing ink has been widely used in high-precision fields such as packaging printing, electronic circuits, and medical devices due to its instant curing, zero volatile organic compound (VOC) emissions, high gloss and excellent physical properties.
Its curing mechanism relies on the photoinitiator absorbing ultraviolet light of a specific wavelength to produce active free radicals or ions, triggering the polymerization and cross-linking reaction of the prepolymer and the active diluent to form a three-dimensional network cured film. In this system, the synergistic effect of the active diluent and the photoinitiator directly determines the construction adaptability, curing efficiency and final performance of the ink.
Reactive diluents play a dual role as diluents and crosslinkers in UV curing inks. Their selection and proportion have a significant impact on the viscosity, curing speed, shrinkage rate and performance of the cured film of the ink. The addition of reactive diluents can significantly reduce the viscosity of the prepolymer, so that the ink reaches a viscosity range suitable for printing. The dilution capacity (i.e., the effect of reducing viscosity) of different monomers is related to their molecular weight and structure.
The functionality of the reactive diluent directly affects the curing speed and crosslinking density. Multifunctional monomers contain multiple polymerizable double bonds and have high reactivity, which can significantly accelerate the curing speed and increase the crosslinking density. As the number of reactive diluent functional groups increases, the curing time of UV ink is shortened, the crosslinking density of the ink film is increased, and the initial wear resistance is also improved.
Photoinitiator is the "trigger" of UV curing system. Its type and dosage have a direct impact on the curing speed, curing degree and performance of the cured film of the ink.
Different types of photoinitiators have different absorption spectra and initiation activities.For different UV light sources (such as mercury lamps, LED lamps) and ink color systems, it is necessary to select a matching photoinitiator combination. UV LED light sources mainly emit long-wave ultraviolet rays of 365nm, 385nm or 395nm. At this time, initiators with strong absorption in the long-wave region (such as TPO, Irgacure 819, etc.) should be selected to effectively initiate curing.
For black or dark inks, since the pigment absorbs most of the UV light, it is necessary to use an initiator with high initiation efficiency and increase its dosage, or use a composite initiation system to ensure that the ink is fully cured in the deep layer.
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