Aqueous Coating Composition for Low-Temperature Curing
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Solution Overview
Problem
Current aqueous coating compositions for automobile bodies face challenges in achieving sufficient chipping resistance and low-temperature curability, particularly when applied to both steel and resin components, as they require high heat for curing, which can lead to reduced crosslinking density and performance issues like peeling and damage.
Innovation Solution
A one-component aqueous coating composition comprising an aqueous resin with hydroxyl and carboxyl groups, a water-dispersible blocked polyisocyanate compound, a hydrophilicized carbodiimide compound, and an aqueous polyurethane resin, which allows for low-temperature curing (70-100°C) while maintaining excellent coating film properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a high curing temperature is used to ensure sufficient crosslinking density and chipping resistance, then coating film performance is improved, but resin members with low heat resistance may be damaged
Solution Approach 1:
The invention changes the chemical parameters of the coating composition by introducing a carbodiimide compound that enables low-temperature crosslinking reactions. This allows the curing process to proceed effectively at temperatures suitable for resin members (below their heat resistance limits) while still achieving sufficient crosslinking density and chipping resistance through the unique chemistry of the carbodiimide group
Solution Approach 2:
The invention creates a composite curing system combining multiple components: aqueous resin, polyisocyanate compound, and carbodiimide compound. This composite material approach enables the coating to achieve both adhesion (through polyisocyanate) and crosslinking (through carbodiimide) at low temperatures, resolving the contradiction between performance and thermal sensitivity
2Shape
If the first base coating film is made thinner to secure smoothness in three-wet coating, then surface quality is improved, but chipping resistance deteriorates
Solution Approach 1:
The invention changes the chemical composition parameters of the base coating to include carbodiimide compounds that provide enhanced crosslinking efficiency. This allows thinner film formulations to achieve the necessary mechanical strength and chipping resistance that would normally require thicker films, thereby maintaining both smoothness and durability
3Strength
If a two-component coating composition is used to achieve crosslinking, then coating performance is improved, but coating material stability deteriorates due to premature reaction
Solution Approach 1:
The invention introduces water as an intermediary that triggers the carbodiimide crosslinking reaction. The carbodiimide compound remains stable in the aqueous coating composition during storage and application, but upon contact with water (either ambient moisture or added water), it activates and forms crosslinks. This intermediary mechanism allows maintaining stability during handling while achieving crosslinking during curing
Solution Approach 2:
The coating composition utilizes water already present in the aqueous system (from the aqueous resin or ambient humidity) to trigger the crosslinking reaction. This self-service approach eliminates the need for separate component mixing while still achieving effective crosslinking, maintaining both stability and performance
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 ensures effective curing and enhanced coating film properties, including chipping resistance and water resistance, even at lower temperatures, suitable for both steel and resin components, thereby improving the durability and consistency of the coating.
Implementation Method 1
a water-dispersible blocked polyisocyanate compound (B), a hydrophilicized carbodiimide compound (C)... a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at -20°C
Implementation Method 2
a hydrophilicized carbodiimide compound (C)... the acrylic resin having a number-average molecular weight of 10,000 to 80,000... a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at -20°C
Implementation Method 3
the aqueous polyurethane resin (D) has a glass transition point (Tg) of -50°C or less, and a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at -20°C
Data Source
AI summary
The objective of the present invention is to provide an aqueous coating composition which has excellent curability at low temperatures. The present invention provides an aqueous coating composition which contains an aqueous resin having a hydroxyl group and a carboxyl group (A), a water-dispersible blocked polyisocyanate compound (B), a hydrophilicized carbodiimide compound (C), and an aqueous polyurethane resin (D), wherein the aqueous resin having a hydroxyl group and a carboxyl group (A) has a hydroxyl value of 80 to 200 mgKOH/g and an acid value of 10 to 40 mgKOH/g in terms of resin solid content, the aqueous polyurethane resin (D) has a glass transition point (Tg) of -50°C or less, and a cured film of the aqueous polyurethane resin (D) has an elongation at break of 400% or more at -20°C.