UV-Curable Inkjet Overprint Varnish Oxygen Shielding
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Solution Overview
Problem
UV-curable inkjet fluids face challenges with oxygen inhibition, leading to poor surface cure and high concentrations of photoinitiators, which increase costs and affect ink viscosity and properties, particularly in food packaging applications.
Innovation Solution
Applying a UV-curable overprint varnish (OPV) over a partially cured UV-curable ink layer, followed by further UV irradiation, to shield the ink from atmospheric oxygen and enhance radical generation and monomer conversion, allowing for full cure with reduced photoinitiator concentrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If high concentrations of photoinitiators are used to overcome oxygen inhibition, then sufficient radical flux is achieved and cure response is improved, but ink cost increases and viscosity is negatively impacted
Solution Approach 1:
The patent introduces an overprint varnish layer as an intermediary between the UV-curable ink and atmospheric oxygen. This varnish layer acts as a physical barrier that excludes oxygen from the ink film, eliminating the need for high photoinitiator concentrations to overcome oxygen inhibition. The varnish layer mediates the interaction between the ink and environment, allowing cure at lower photoinitiator levels.
Solution Approach 2:
The overprint varnish creates an inert environment by forming a protective barrier that excludes atmospheric oxygen from the UV-curable ink. This inert environment prevents oxygen inhibition of the polymerization reaction, enabling effective curing with reduced photoinitiator content and lower ink costs.
2Reliability
If UV-curable ink is cured under nitrogen blanket to exclude oxygen, then oxygen inhibition is overcome, but equipment complexity and operational cost increase
Solution Approach 1:
Instead of using complex nitrogen blanket systems, the patent employs an overprint varnish layer as a simple intermediary barrier. This varnish layer physically excludes oxygen from the ink film without requiring specialized equipment or inert gas generation systems, thereby eliminating the need for complex press designs while still overcoming oxygen inhibition.
Solution Approach 2:
The patent extracts the oxygen exclusion function from complex equipment systems and transfers it to a simple overprint varnish layer. By taking out the oxygen barrier function and embedding it in the varnish coating, the system eliminates the need for nitrogen blankets and specially designed presses, simplifying both equipment and operation.
3Reliability
If high photoinitiator concentrations are used, then sufficient radical flux is generated, but unreacted residues and migration potential increase
Solution Approach 1:
The overprint varnish serves as a mediator that protects the UV-curable ink from oxygen during curing. This protection enables complete polymerization reactions with lower photoinitiator concentrations, thereby reducing unreacted residues and photoinitiator migration to levels below detectable limits, ensuring food safety compliance.
Solution Approach 2:
The patent changes the photoinitiator concentration parameter from high levels (typically 10% or more) to lower levels (reducing migration potential). The overprint varnish enables this parameter change by providing oxygen exclusion, allowing effective curing at lower photoinitiator concentrations and reducing harmful migration to food contact surfaces.
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
This approach enables UV-curable inkjet fluids to achieve acceptable cure responses under ambient conditions with lower photoinitiator levels, reducing unreacted residues and contamination risks, while maintaining fast printing speeds and suitable viscosities for various printing technologies.
Implementation Method 1
UV-curable inkjet fluids having the potential to be printed on food packaging... UV-curable (free radical) inks and coatings... under the action of UV-light to produce a sufficient radical flux
Implementation Method 2
One of the major issues associated with UV-curable inks, especially UV-curable inkjet fluids, is that of oxygen inhibition. Due to the particularly low viscosity of inkjet fluids, and to a lesser extent other UV-curable (free radical) inks and coatings, such fluids are especially prone to oxygen inhibition, which slows the cure response of the ink.
Data Source
AI summary
A process for preparing printed matter is disclosed which comprises applying first a UV-curable ink containing photoinitiators followed by an overprint varnish and then UV curing such as the amount of photoinitiators is less than 6% by weight of the total weight of the ink. The present invention relates to a printing process by applying first a UV-curable ink followed by an overprint varnish (OPV) and curing.