UV-Curable Coating Composition with Low Hydroxyl Resins
Solution Overview
Problem
Current UV-curable clearcoat compositions for automotive coatings face challenges such as limited adhesion to subjacent layers under severe outdoor conditions, requiring high-energy UV radiation, and being unsuitable for complex shapes due to shadow areas, while dual cure systems have short pot-life and high solvent content, posing environmental concerns.
Innovation Solution
A UV-curable coating composition comprising resins with low hydroxyl numbers and high (meth)acrylic group functionality, isocyanate species with (meth)acrylate groups, aprotic organic solvents, photoinitiators, light stabilizers, and UV absorbers, which cures fully at ambient temperature using low-energy UV-A radiation, offering extended pot-life and improved interlayer adhesion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If UV-curable clearcoat compositions are used to replace two-component systems, then environmental problems and workplace safety issues are reduced, but adhesion to subjacent layers under severe outdoor conditions deteriorates
Solution Approach 1:
The patent employs a composite binder system combining polyurethane acrylate (providing flexibility and adhesion) with polyester acrylate (providing hardness and chemical resistance). This composite approach allows the UV-curable coating to achieve both good adhesion to basecoat layers and durability under severe outdoor conditions, while maintaining the environmental benefits of UV curing without high solvent content.
2Reliability
If high-energy UV-B and UV-C radiation is used to sufficiently cure UV-curable coatings, then curing completeness is improved, but safety measures and equipment complexity increase
Solution Approach 1:
The patent modifies the photoinitiator system parameters by selecting photoinitiators with absorption maxima in the UV-A region (320-400 nm) rather than requiring UV-B or UV-C wavelengths. This parameter change allows the use of lower energy, safer UV-A radiation while achieving sufficient curing through optimized photoinitiator-resin combinations, thereby reducing safety requirements and equipment complexity.
3Reliability
If dual cure systems are used to combine two-component and UV-curable features, then adhesion and curing speed are improved, but pot-life is shortened and solvent content increases
Solution Approach 1:
The patent extracts and eliminates the hydroxyl groups from the binder system that would otherwise participate in isocyanate crosslinking reactions. By using acrylate-functionalized polyurethane and polyester resins without hydroxyl groups, the invention removes the source of thermal crosslinking while maintaining UV-curing capability, thereby preserving long pot-life and avoiding the need for high solvent content that characterizes dual-cure systems.
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 composition achieves rapid curing at ambient temperature with low-energy UV-A radiation, maintaining high pot-life, excellent polishability, and enhanced weathering resistance, along with improved adhesion to subjacent layers, meeting stringent weathering standards like the Weather-Ometer CAM 180 test.
Implementation Method 1
UV curable coating compositions which fully cure at ambient temperature (25° C.) in a very short time, even with low energy UV radiation such as UV radiation in the UV-A wavelength range
Implementation Method 2
Since one of the main crosslinking reactions in dual cure compositions is typically the reaction between isocyanate groups and hydroxyl groups
Data Source
AI summary
Disclosed herein is a UV curable coating composition including (A) a component A, comprising Aa) one or more resins, each of these resins having a hydroxyl number in a range of from 0 to 25 mg KOH/g; Ab) one or more UV reactive monomers including one or two (meth)acrylic groups; (B) a component B, including Ba) one or more isocyanate species; (C) a component C, including Ca) one or more aprotic organic solvents which do not react with any ingredient of the coating composition; and (D) a component D, including Da) one or more photoinitiators; at least one of Db) and/or Dc), Db) being one or more light stabilizers; and Dc) being one or more UV absorbers; wherein the total molar ratio of hydroxyl groups to free isocyanate groups in the UV curable coating composition is in a range from 0 to 1:5.