Self-Crosslinking Polyurethane Dispersion for Waterproof Coatings
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Solution Overview
Problem
Solvent-based polyurethane coatings used in waterproofing applications pose health risks due to flammability and the presence of free isocyanate monomers, and aqueous polyurethane dispersions suffer from low water resistance and long drying times, leading to inadequate mechanical performance in humid environments.
Innovation Solution
Aqueous self-crosslinking polyurethane dispersions are developed by partially esterifying anionic carboxylate groups with epoxysilanes and incorporating water-insoluble transition metal compound particles as crosslinking agents, allowing for adjustable crosslinking density and improved water resistance without destabilizing the dispersion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solvent-based polyurethane coatings are used to achieve durable waterproofing, then water resistance and mechanical performance are improved, but health risks increase due to flammability and free isocyanate monomers
Solution Approach 1:
The patent changes the fundamental parameter of the coating system from solvent-based to water-based, replacing harmful organic solvents with water as the dispersion medium. This eliminates flammability and isocyanate monomer exposure while maintaining polyurethane film-forming capabilities through aqueous dispersion technology
Solution Approach 2:
The patent creates a composite system combining aqueous polyurethane dispersion with silane crosslinking agents and transition metal catalyst particles. This composite approach enables water-based formulation while achieving crosslinked network structure for enhanced water resistance and mechanical performance
2Object-affected harmful factors
If aqueous polyurethane dispersions are used to eliminate health risks, then safety is improved, but water resistance and mechanical performance deteriorate due to long drying times and high water uptake
Solution Approach 1:
The patent incorporates crosslinkable silyl groups pendant along the polyurethane chains in advance, before film formation. These pre-positioned crosslinking sites enable rapid crossreaction upon water evaporation, transforming the film-forming process from simple coalescence to chemical crosslinking, thereby achieving fast drying and high water resistance
Solution Approach 2:
The patent changes the molecular architecture of polyurethane from linear chains to branched/crosslinked structures by incorporating pendant silyl groups. This structural parameter change enables the formation of a three-dimensional crosslinked network that provides exceptional water resistance and mechanical strength while maintaining aqueous processability
3Reliability
If crosslinking silyl groups are introduced as end groups to improve water resistance, then durability is enhanced, but crosslinking density cannot be increased sufficiently
Solution Approach 1:
The patent applies local quality by placing silyl crosslinking groups at specific locations along the polyurethane chains (pendant groups) rather than uniformly at chain ends. This localized placement at multiple positions per chain enables significantly higher crosslinking density and more extensive crosslinked network formation, enhancing both durability and water resistance
4Reliability
If more carboxylate groups are esterified with epoxysilanes to increase crosslinking density, then water resistance improves, but dispersion stability decreases due to reduced anionic charges
Solution Approach 1:
The patent applies partial esterification, where only a portion of the carboxylate groups are reacted with epoxysilanes. This partial action leaves sufficient unreacted carboxylate groups to maintain anionic charge density and electrostatic repulsion for dispersion stability, while still providing enough crosslinkable silyl groups to achieve high crosslinking density and water resistance in the cured film
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 resulting coatings exhibit enhanced water resistance and mechanical performance equivalent to solvent-based systems, with fast drying times and a shelf life of several months, while being non-toxic and free from flammability risks.
Implementation Method 1
The epoxy group of the silane esterifies with the carboxylate groups of the anionic polyurethanes, thereby decreasing the overall anionic charge density of the dispersed polymer particles and covalently bonding alcoxysilyl groups to the polymer
Implementation Method 2
The remaining non-reacted carboxylate groups provide stability to the silyl-functionalized PU dispersion
Implementation Method 3
using specific crosslinking particles as an additional crosslinking agent, and thereby further increase the durability of the final coating
Implementation Method 4
even several weeks after application, film-formation and water evaporation
Data Source
AI summary
The present invention is drawn to an aqueous polyurethane dispersion comprising: —anionic polyurethane particles comprising both free pending carboxylate groups of formula —COO−M+ where M+ is a cation resulting from neutralization of carboxylic acid groups with a base, and pending carboxylate ester groups resulting from esterification of free carboxylic acid groups with an epoxysilane, —crosslinking particles made of water-insoluble transition metal compounds. It is also drawn to a method of coating a substrate comprising applying such an aqueous polyurethane dispersion onto a substrate and letting it dry, preferably without applying any heat or radiation.