UV-Curable Ink Formulation for LED Curing
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Solution Overview
Problem
Existing active energy ray-curable inks fail to achieve sufficient curing when used with UV-LEDs emitting UV-rays in the range of 350 to 420 nm, particularly due to poor light absorption by photopolymerization initiators and pigments, leading to inadequate curability in multicolor overprinting.
Innovation Solution
A combination of photocleavage type and hydrogen abstraction type photopolymerization initiators, including α-aminoalkylphenone, acylphosphine oxide, and tertiary amine compounds, with specific molar absorption coefficients, is used to enhance the curability of active energy ray-curable inks, optimized for UV-LED irradiation, along with a multicolor set of inks where the weight concentration of photopolymerization initiators is adjusted based on their proximity to the irradiation source.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a photopolymerization initiator having absorption in wavelength from 350 to 420 nm is used for UV-LED curing, then the ink can respond to UV-LED irradiation, but pigments in the ink also absorb light in this wavelength range, preventing sufficient curing
Solution Approach 1:
The patent divides the photopolymerization initiator system into multiple components with different absorption characteristics. Specifically, it combines initiators that absorb at different wavelengths within the UV-LED range (350-420 nm), ensuring that at least some initiators remain effective even when pigments absorb certain wavelengths. This segmentation of the initiator system allows the ink to cure reliably despite pigment interference at specific wavelengths.
2Manufacturing precision
If film thickness of inks is increased for deeper color density or multicolor overprinting, then color quality is improved, but light transmittance decreases, deteriorating curability
Solution Approach 1:
The patent changes the parameters of the photopolymerization initiator system to match the UV-LED emission spectrum. By selecting initiators with absorption peaks that align with the 350-420 nm UV-LED range and have high molar absorption coefficients, the system achieves effective curing even through thicker ink films with deeper color density. The specific parameter optimization includes choosing initiators with absorption coefficients of 10,000 L·mol⁻¹·cm⁻¹ or higher at the UV-LED wavelength.
3Manufacturing precision
If black ink and cyan ink are used in multicolor overprinting, then color quality is improved, but their large light absorption at 365 nm prevents sufficient light energy from reaching photopolymerization initiators, resulting in poor curability
Solution Approach 1:
The patent introduces a specific photopolymerization initiator system that acts as an intermediary between the UV-LED light source and the pigment-containing ink film. The initiators are selected to have high absorption efficiency at the UV-LED wavelength (350-420 nm) and appropriate energy transfer characteristics that enable them to initiate polymerization even when pigments like black and cyan inks absorb significant light. This intermediary initiator system bridges the gap between the light source and the cured ink film.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides excellent surface and internal curability of active energy ray-curable inks when used with UV-LEDs, matching the performance of traditional UV-curable inks with mercury or metal halide lamps, and ensures effective multicolor overprinting with improved printability and storage stability.
Implementation Method 1
An active energy ray-curable ink is known. An existing active energy ray-curable ink is cured by use of a light source such as a mercury lamp or a metal halide lamp, and a photopolymerization initiator matched to an emission wavelength used to cure the ink is used.
Implementation Method 2
a photopolymerization initiator having an absorption in a wavelength from 350 to 420 nm has to be used. However, pigments used in inks have, though different in magnitude of absorption thereof, light absorption in a wavelength from 350 to 420 nm in many cases.
Data Source
AI summary
An ink curable with actinic energy rays which is characterized by containing a photocleavage type photopolymerization initiator (A) comprising a-aminoalkylphenone compound (A1) having a molar absorption coefficient as measured at a wavelength of 365 nm of 100 (l/mol·cm) or more and less than 100,000 (l/mol·cm) and/or an acylphosphine oxide compound (A2) having a molar absorption coefficient as measured at a wavelength of 365 nm of 100 (l/mol·cm) or more and less than 100,000 (l/mol·cm); and a hydrogen abstraction type photopolymerization initiator (B) comprising 4,4'-dialkylaminobenzophenone having a molar absorption coefficient at a wavelength of 365 nm of 10, 000 (l/mol·cm) or more and less than 1,000,000 (l/mol·cm) and by optionally containing a tertiary amine compound (C) having a molar extinction coefficient as measured at a wavelength of 365 nm of 1 (l/mol·cm) or lower. The ink curable with actinic energy rays further includes a resin, a pigment, and a compound having an acrylate group. When used with a light emitting ultraviolet rays especially having a luminescence-peak wavelength in the range of 350 to 420 nm as an ultraviolet irradiator, the ink has excellent curability and printability in both monocolor printing and overprinting, the ink further has excellent storage stability.