Moisture-Curable Polyurethane Formulation with Urea-Based Thixotropic Additive
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Solution Overview
Problem
Polyurethane adhesive formulations face challenges in achieving high green strength and gunnability without increasing viscosity or reducing cold temperature flexibility, making it difficult to bond heavy substrates effectively.
Innovation Solution
A moisture-curable polyurethane formulation with an isocyanate-terminated prepolymer and a urea-based thixotropic additive, which includes a carrier and excess isocyanate, is developed to enhance green strength and gunnability, allowing for bonding of substrates weighing at least 500 grams without holding instruments, while maintaining suitable viscosity and flexibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If higher concentrations of urea groups are provided in the polyurethane formulation, then green strength is enhanced, but viscosity increases to undesirable levels
Solution Approach 1:
The patent applies parameter changes by precisely controlling the NCO/OH mole ratio within a specific range (about 0.5 to about 2.0) and limiting free isocyanate content to about 0.1% to about 10% by weight. These parameter adjustments optimize the formulation to achieve high green strength through controlled urea group formation while preventing excessive viscosity increase, resolving the contradiction between strength enhancement and viscosity control.
2Strength
If higher concentrations of urea groups are provided in the polyurethane formulation, then green strength is enhanced, but the formulation becomes too stiff for good wet out and adhesion
Solution Approach 1:
The patent resolves this contradiction by implementing specific parameter ranges: NCO/OH mole ratio of about 0.5 to about 2.0 and free isocyanate content of about 0.1% to about 10% by weight. These controlled parameters enable the formulation to maintain optimal flexibility for wet out and adhesion while still achieving enhanced green strength through controlled urea group concentration.
3Strength
If viscosity is increased to enhance green strength, then bond strength improves, but the formulation becomes difficult to dispense from caulking guns
Solution Approach 1:
The patent applies parameter changes by controlling the NCO/OH mole ratio (about 0.5 to about 2.0) and free isocyanate content (about 0.1% to about 10% by weight), which optimizes the balance between green strength and dispensability. Additionally, the inclusion of a thixotropic agent provides shear-thinning behavior, allowing the formulation to maintain higher viscosity at rest for strength while becoming more fluid under shear stress during dispensing, thus resolving the contradiction between bond strength and gunnability.
4Strength
If free isocyanate content is increased to enhance green strength, then adhesion improves, but the formulation becomes too viscous for application
Solution Approach 1:
The patent resolves this contradiction by implementing precise parameter control: free isocyanate content is limited to about 0.1% to about 10% by weight, and NCO/OH mole ratio is maintained within about 0.5 to about 2.0. This controlled approach allows sufficient free isocyanate to enhance green strength and adhesion while preventing excessive viscosity that would hinder application.
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 formulation achieves high green lap shear strength and gunnability, enabling effective adhesion and application from a caulking gun, with the urea-based thixotropic additive contributing to improved rheological properties and substrate bonding.
Implementation Method 1
sufficient monomeric/polymeric isocyanate is present to provide an excess of free isocyanate in the prepolymer and/or formulation
Implementation Method 2
at least one of: i) a urea-based thixotropic additive in the presence of a carrier... or ii) a urea-based thixotropic additive compound produced by the in situ reaction of the excess of free isocyanate
Implementation Method 3
a urea-based thixotropic additive in the presence of a carrier, wherein the additive comprises about 4% to about 15% of urea groups
Data Source
AI summary
A moisture-curable polyurethane formulation is provided capable of curing to form a reaction product, having high green strength and being dispensable from a caulking gun. The polyurethane formulation comprises at least one isocyanate-terminated prepolymer, at least one rheology modifier or filler, optionally monomeric/polymeric isocyanate, and at least one urea-based thixotropic additive added to the formulation or formed in situ, to provide greater than 1% of urea groups in the reaction product. A method for making a moisture-curable polyurethane formulation capable of curing to form a reaction product and methods of bonding materials together using the formulation are also provided.