Cationic UV-LED Varnish for Security Documents
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Solution Overview
Problem
Current cationic UV-LED radiation curable coatings for security documents suffer from inadequate adhesion, limited physical resistance, and high shrinkage during curing, while also causing excessive fluorescence when exposed to UV light, which interferes with the detection of luminescent security features.
Innovation Solution
A cationic UV-LED radiation curable protective varnish comprising 65-90 wt% cycloaliphatic epoxide, 1-10 wt% diaryl iodonium salt, 0.01-5 wt% non-ionic surfactant, and a specific photosensitizer with a 2-keto-thioxanthone moiety, optimized for curing efficiency and minimal fluorescence under UV light excitation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If cationic UV-LED radiation curable coatings are used to improve adhesion and physical resistance, then adhesion and mechanical resistance are increased, but shrinkage during curing becomes excessively high
Solution Approach 1:
The patent modifies the chemical composition parameters of the cationic photoinitiator system by combining specific ratios of iodonium salts and sulfonium salts with corresponding sulfide compounds. This parameter optimization enables effective curing with UV-LED radiation while controlling the shrinkage that occurs during the polymerization process, thus maintaining both adhesion strength and dimensional stability.
Solution Approach 2:
The patent employs a composite photoinitiator system comprising multiple cationic photoinitiators (iodonium salts and sulfonium salts) working in conjunction with sulfide compounds. This composite approach allows the coating to achieve both high adhesion and mechanical resistance while minimizing shrinkage, as different components compensate for each other's limitations during the curing process.
2Strength
If conventional cationic UV radiation curable coatings are used to achieve optimal curing properties, then adhesion and mechanical resistance are improved, but fluorescence under UV light becomes excessive and interferes with security feature detection
Solution Approach 1:
The patent carefully selects and optimizes the chemical structure parameters of the cationic photoinitiators and their concentrations to minimize fluorescence emission. By using specific iodonium salts and sulfonium salts with appropriate substituents and controlling their ratios, the coating achieves effective curing while reducing UV-induced fluorescence that would interfere with luminescent security feature detection.
Solution Approach 2:
The patent addresses the fluorescence issue by selecting photoinitiator systems that, while effective for curing, have minimal fluorescence in the UV range. The harmful effect of fluorescence is converted into a beneficial outcome where the curing process remains efficient but the interference with security detection is minimized through careful molecular structure selection.
3Ease of manufacture
If free-radical UV radiation curable coatings are used for simplicity, then the curing process is straightforward, but adhesion properties are insufficient and physical resistance is limited
Solution Approach 1:
The patent replaces the free-radical mechanism with a cationic photopolymerization mechanism using UV-LED radiation. This substitution maintains the simplicity of the curing process (exposure to UV light) while fundamentally improving the chemical bonding mechanisms that result in superior adhesion and physical resistance properties.
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 varnish achieves optimal curing properties with reduced fluorescence, ensuring the longevity and authenticity of security documents without impairing the detection of UV-excitable luminescent features.
Implementation Method 1
Cationic UV-LED radiation curable protective varnishes are varnishes which are cured by the action of radiation, in particular of UV-LED radiation, on one or more cationic photoinitiators contained by said varnishes, wherein said cationic photoinitiators liberate cationic species, such as acids, which in turn initiate the polymerization of the monomer so as to form a cured binder
Implementation Method 2
activation by UV-Vis light of one or more cationic photoinitiators, which liberate cationic species, such as acids, which in turn initiate the polymerization
Implementation Method 3
from about 0.01 wt-% to about 5 wt-% of a non-ionic surfactant
Data Source
AI summary
The present invention relates to the technical field of varnishes for protecting security documents, such as banknotes, against premature detrimental influence of soil and/or moisture upon use and time. In particular, the present invention provides a cationic UV-LED radiation curable protective varnish comprising:a) from about 65 wt-% to about 90 wt-% of either a cycloaliphatic epoxide, or a mixture of a cycloaliphatic epoxide and one or more cationically curable monomers other than the cycloaliphatic epoxide;b) from about 1 wt-% to about 10 wt-% of a diaryl iodonium salt;c) from about 0.01 wt-% to about 5 wt-% of a non-ionic surfactant; andd) a photosensitizer of general formula (I)wherein the weight percents are based on the total weight of the cationic UV-LED curable protective varnish, and a process for coating a security document with said cationic UV-LED radiation curable protective varnish.