UV-Curable Clear Coat Resin for Water-Based Base Coats
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Solution Overview
Problem
UV-curable coating compositions face issues with oxygen inhibition and initial yellowing, leading to inadequate curing and surface properties, particularly in multilayer coatings with water-based base coat layers, which restrict their use in clear coats and topcoats.
Innovation Solution
A process involving a water-based color-imparting or special effect base coat layer followed by a clear coat composition containing compounds capable of free-radical polymerization, photoinitiators, and triaryl phosphine derivatives, cured using high-energy radiation, which optionally includes additional chemical curing reactions to enhance curing and prevent yellowing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If UV-curable coating compositions are used for clear coats and topcoats, then curing time is reduced and hardness is improved, but oxygen inhibition occurs leading to inadequate surface curing and reduced scratch resistance
Solution Approach 1:
The patent uses a composite binder system combining polyester resin with both (meth)acryloyl groups for UV curing and epoxy groups for chemical crosslinking. This dual-functional composite material enables simultaneous UV curing and chemical crosslinking, achieving complete surface cure while maintaining fast curing speed and high hardness.
Solution Approach 2:
The patent changes the chemical parameters of the binder by incorporating epoxy-functional groups into the polyester resin structure. This parameter change enables the resin to undergo both free-radical polymerization (UV curing) and chemical crosslinking reactions, resolving the oxygen inhibition problem while maintaining fast cure times.
2Productivity
If UV-curable clear coat compositions are applied over water-based base coat layers, then production efficiency is improved, but initial yellowing occurs restricting use in clear coats and topcoats
Solution Approach 1:
The patent employs a composite binder system where epoxy-functional polyester resin undergoes chemical crosslinking in addition to UV curing. This composite material structure provides yellowing resistance while maintaining fast cure times, enabling use over water-based base coats without initial yellowing issues.
Solution Approach 2:
The patent converts the potential harm of water-based base coats (which can cause yellowing when covered with UV-curable clear coats) into a benefit by using the epoxy crosslinking mechanism. The chemical crosslinking prevents the yellowing reaction, transforming a problematic combination into a successful multilayer system.
3Reliability
If chemically modified resins with allyl ether groups are used to overcome oxygen inhibition, then surface curing is improved, but additional process control difficulties arise
Solution Approach 1:
The patent changes the chemical structure of the polyester resin by incorporating both (meth)acryloyl groups and epoxy groups into the same molecular structure. This parameter change enables dual-curing mechanism without requiring complex process control, as the chemical crosslinking occurs automatically during UV curing.
4Reliability
If work is performed in inert gas atmosphere to exclude oxygen, then oxygen inhibition is avoided, but process complexity and cost increase
Solution Approach 1:
The patent uses a composite binder system that combines UV-curable (meth)acryloyl groups with chemically crosslinking epoxy groups. This composite material enables complete curing without oxygen exclusion, eliminating the need for inert gas atmospheres and associated process complexity.
Solution Approach 2:
The patent converts the harmful effect of atmospheric oxygen (which inhibits free-radical polymerization) into a beneficial outcome by incorporating epoxy crosslinking. The chemical crosslinking mechanism is insensitive to oxygen, allowing complete curing in atmospheric conditions without requiring inert gas environments.
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 process results in fully cured coatings with improved solvent and humidity resistance, allowing rapid further treatment like polishing, and significantly reduces or eliminates initial yellowing, enabling their use as clear coats or light-pigmented topcoats without elaborate processing methods.
Implementation Method 1
Binders used in the coating compositions curable by high-energy radiation are generally those containing olefinically unsaturated groups, in particular (meth)acryloyl groups, and cure by free-radical polymerization initiated by UV radiation
Implementation Method 2
it is also already known to use so-called dual-cure systems in which free-radical polymerization initiated by UV radiation is combined with a further chemical cross-linking mechanism
Data Source
AI summary
This invention relates to a process for multilayer coating of substrates, comprising the steps: I) applying a layer of a water-based color-imparting and/or special effect-imparting base coat coating composition to a substrate, II) applying a layer of a clear coat coating composition curable by means of high energy radiation and optionally curable by at least one additional chemical curing reaction, and III) curing said clear coat layer by exposing it to high energy radiation, wherein the clear coat coating composition comprises: A) at least one compound capable of free-radical polymerization having at least one olefinically unsaturated group, B) at least one photoinitiator and C) at least one organic phosphine derivative.