Low Free MDI Reactive Polyurethane Hot Melt Adhesive
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Solution Overview
Problem
Current reactive hot melt adhesives face challenges in achieving high tensile strength, low viscosity, and short initial bond times while maintaining low levels of free diisocyanate monomers, which are crucial for regulatory compliance and industrial hygiene.
Innovation Solution
A reactive polyurethane prepolymer composition is developed, comprising a first polyurethane prepolymer from methylene diphenyl diisocyanate (MDI) and poly(1,6-hexamethylene adipate) glycol, and a second polyurethane prepolymer from MDI and polypropylene glycol or succinic acid-based polyester polyol, both with low free MDI monomer content, optionally combined with catalysts, to form a hot melt adhesive with improved properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional polyurethane prepolymers with higher free diisocyanate content are used, then adhesive strength and cure speed are improved, but regulatory compliance and industrial hygiene are compromised
Solution Approach 1:
The patent changes the chemical parameters of the polyol components (using poly(1,6-hexamethylene adipate) glycol with Mw 500-10,000 g/mol and polypropylene glycol or succinic acid-based polyester polyol with Mw 250-4,000 g/mol) and controls the NCO content (0.5-12 wt%) to achieve both high adhesive strength and low free diisocyanate content (<0.1 wt%). This parameter optimization allows the adhesive to meet regulatory requirements while maintaining performance.
2Ease of operation
If polyurethane prepolymers with low viscosity are used, then ease of application is improved, but initial bond time and adhesive strength are reduced
Solution Approach 1:
The patent creates a composite polyurethane prepolymer composition combining multiple polyol components (poly(1,6-hexamethylene adipate) glycol, polypropylene glycol, and/or succinic acid-based polyester polyol) with specific molecular weights and NCO contents. This composite approach achieves low viscosity for easy application while the specific composition ensures rapid solidification and high lap shear strength after short cure time.
3Speed
If high NCO content prepolymers are used, then cure speed is improved, but adhesive strength and processing ease are reduced
Solution Approach 1:
The patent optimizes the NCO content parameter to a specific range (0.5-12 wt%, preferably 3-11 wt%) and selects polyol molecular weights (500-10,000 g/mol for PHAG, 250-4,000 g/mol for PPG or polyester polyol) to achieve the desired balance. This parameter optimization enables high cure speed while maintaining high adhesive strength and ease of processing.
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 high lap shear strength, low viscosity, and rapid solidification, meeting regulatory requirements and enhancing adhesive performance with reduced industrial hygiene concerns.
Implementation Method 1
isocyanate terminated polyurethane prepolymers... react with surface or ambient moisture through diffusion into the adhesive in order to chain-extend
Implementation Method 2
forming an irreversible new polyurethane/urea polymer
Implementation Method 3
one or more catalysts
Data Source
AI summary
The present invention relates to a reactive polyurethane prepolymer composition, which can be used as a hot melt adhesive, comprising a first polyurethane prepolymer which is the reaction product of the reaction of methylene diphenyl diisocyanate (MDI) with poly(1,6-hexamethylene adipate) glycol (PHAG) with a molecular weight (Mw) of 500 g/mol to 10,000 g/mol and with an NCO content of 0.5 wt% to 7 wt%, preferably of 3 wt% to 7 wt% and more preferably of 5 wt% to 7 wt%, comprising less than 0.1 wt% free MDI monomer based on the total weight of the first polyurethane prepolymer, a second polyurethane prepolymer which is the reaction product of the reaction of methylene diphenyl diisocyanate with polypropylene glycol (PPG) or a succinic acid based polyester polyol with a molecular weight (Mw) of 250 g/mol to 4,000 g/mol and with an NCO content of 0.5 wt% to 12 wt%, preferably of 3 wt% to 11 wt% and more preferably of 5 wt% to 10 wt%, comprising less than 0.1 wt% free MDI monomer based on the total weight of the second polyurethane prepolymer, and optionally one or more catalysts, as well as a process for producing said reactive polyurethane prepolymer composition, a method for adhesively joining or sealing two substrates and the use as an hot melt adhesive.