Coating Composition Using Carboxylic Acid to Control Curing Rate
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Solution Overview
Problem
Existing NCO-SH based coating compositions are influenced by external factors such as ambient moisture content or light, leading to inconsistent curing rates and difficulties in applying large surface coatings, with limited pot life and curing time.
Innovation Solution
A coating composition comprising polythiols, polyisocyanates, a base compound, and a carboxylic acid compound, where the carboxylic acid is used in excess to control the curing rate, reducing CO2 formation and allowing for smooth film formation, independent of moisture or light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of moving object
If moisture-activatable base compounds are used to delay crosslinking and increase pot life, then the coating can be applied for longer periods, but the curing rate becomes dependent on ambient moisture content and the coating is more intensively catalyzed at its surface than below the coating surface
Solution Approach 1:
A carboxylic acid compound is introduced as an intermediary substance that reacts with the moisture-activatable base compound to form a carboxylate salt. This intermediary reaction controls the activation of the base catalyst, preventing direct and uncontrolled reaction with moisture. The carboxylate salt acts as a buffered catalyst that provides consistent curing rates throughout the coating thickness, eliminating the surface-intensified catalysis problem while maintaining extended pot life.
2Reliability
If photolatent base is used to control curing, then the base is activated only when needed, but the freshly applied layers need to be irradiated with actinic radiation and some spots of the surface may be more difficult to irradiate resulting in low curing speed on shadow spots
Solution Approach 1:
The patent replaces the optical activation mechanism (photolatent base requiring actinic radiation) with a chemical activation mechanism using moisture-activatable base compounds. This substitution eliminates the need for external radiation equipment and complex irradiation processes, allowing uniform curing across large surfaces including shadow spots. The moisture in the air or substrate naturally activates the base catalyst throughout the coating, providing consistent curing without equipment complexity.
3Reliability
If acid compounds are used combined with base catalysts in NCO-OH curing systems, then the acids are removed by reaction with the isocyanates, but CO2 formation occurs and smooth coating films cannot be obtained
Solution Approach 1:
The patent changes the chemical parameters of the acid compound selection, specifically choosing carboxylic acids with pKa values between 2.5 and 5.0. This parameter selection ensures that these acids react preferentially with the moisture-activatable base compound rather than with the isocyanate groups. By controlling the acid strength parameter, the system achieves curing rate control through carboxylate salt formation without significant CO2 evolution that would disrupt film smoothness.
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 a controllable curing rate with a pot life of at least 30 minutes and a curing time of less than 3 hours, providing consistent results on various surfaces without the need for external activation, and can be used in both solvent-borne and solvent-free formulations.
Implementation Method 1
The crosslinking chemistry is typically catalyzed by base catalysts
Implementation Method 2
the acids are removed by reaction with the isocyanates, resulting in CO2 formation
Data Source
AI summary
Coating composition comprising one or more polythiols, one or more polyisocyanates, and a base compound, e.g., an amine catalyst, and a carboxylic acid compound, e.g., cyanoacetic acid and/or a halogenated acetic acid, such as dichloroacetic acid or thfluoroacetic acid. The molar ratio of the acid compound to the basic compound is preferably at least 2:1, e.g. at least 10:1.