Curing behavior of a UV‐curable inkjet ink: Distinction between surface‐cure and deep‐cure performance

Jonas Simon,Andreas Langenscheidt

doi:10.1002/app.49218

Abstract

AbstractSafety requirements and the need of low‐migration UV inks have received increasing attention in the packaging industry. Crucial for the development and design of low‐migration UV inkjet inks for migration‐sensitive applications is the polymerization degree. In this study, curing‐behavior of a black, high purity packaging ink (HPP‐ink) was monitored using ATR‐FTIR spectroscopy. UV irradiation of HPP‐ink led to changes in specific absorption bands of the FTIR spectra due to crosslinking reaction of double bonds. Changes in absorptions bands at 1,408 and 1,321 cm−1 permitted the determination of CC conversion of acrylic and vinyl double bond, independently of one another. In addition, a method was developed which allows the investigation of surface‐cure and deep‐cure behavior, separately.

Highlights

For packing applications and labeling with the inkjet printing process, the industry is predominantly using UV curable printing inks
The development of lowmigration UV curable inkjet inks requires to increase the degree of double bond conversion by using polyfunctional and highly reactive monomers combined with photoinitiators which are either implicitly less sensitive to migrate or supported by sufficient toxicological data to allow relatively high amounts to migrate.[11]
When the UV dosage is assigned to a conveyor velocity, ATR-Fourier transform infrared (FTIR) spectroscopy permits to determine the actual degree of conversion at a certain amount of irradiated energy

Summary

Introduction

For packing applications and labeling with the inkjet printing process, the industry is predominantly using UV curable printing inks. If the ink is not sufficiently cured, free unreacted monomers with lowmolecular-weight tend to migrate through the substrate, unless the substrate is an absolute migration barrier.[7,8] In addition, photoinitiators used in standard UV inkjet inks are low-molecular-weight molecules and thereby are not designed for migration-sensitive applications.[9,10] Low-migration inks for alternative printing techniques, such as flexo and offset, circumvent the problem by using multifunctional polymerizable compounds and photoinitiators with a molecular weight of above 1,000 Da, thereby limiting the risk of migration.[11] For inkjet printing, the viscosity shall be low enough to allow for firing through the orifice of the print head nozzles, and it is impossible to use only high-molecularweight ink compounds.[1] The development of lowmigration UV curable inkjet inks requires to increase the degree of double bond conversion by using polyfunctional and highly reactive monomers combined with photoinitiators which are either implicitly less sensitive to migrate or supported by sufficient toxicological data to allow relatively high amounts to migrate.[11]

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Conclusion