Silane-Crosslinking Adhesive Composition for Wood Bonding
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Solution Overview
Problem
Conventional wood adhesives, such as polyvinyl acetate and isocyanate-crosslinking PU adhesives, face issues with slow curing speed, limited moisture resistance, and safety concerns due to isocyanate toxicity, while silane-crosslinking adhesives have low tensile shear strength and cannot meet European standards like DIN EN 204, stress group D4.
Innovation Solution
A composition comprising silane-terminated polyurethane prepolymers with urethane and polyester units, combined with silane-terminated polyethers and a curing catalyst, which accelerates curing in the presence of atmospheric moisture, resulting in a harder, more resistant adhesive with improved tensile shear strength and elasticity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If isocyanate-crosslinking PU adhesives are used to achieve high mechanical strength and moisture resistance, then the adhesive strength and reliability are improved, but the curing speed remains moderate and storage stability deteriorates due to side reactions
Solution Approach 1:
The patent changes the chemical parameters of the adhesive system by replacing isocyanate crosslinking with silane crosslinking. The silane-terminated polyurethane prepolymer contains alkoxy silane groups that hydrolyze and condense to form crosslinked networks, providing high adhesive strength while enabling faster curing through controlled moisture reaction without the side reactions associated with isocyanate systems.
Solution Approach 2:
The patent extracts and eliminates the isocyanate component from the adhesive system, replacing it with silane-terminated polyurethane. This removes the harmful isocyanate groups while maintaining the crosslinking functionality through silane chemistry, thereby improving safety and storage stability while achieving the desired mechanical properties.
2Reliability
If isocyanate-crosslinking adhesives are used to achieve high mechanical strength, then the reliability under stress is improved, but the safety deteriorates due to isocyanate toxicity and sensitizing effects
Solution Approach 1:
The patent extracts and removes the toxic isocyanate groups from the adhesive formulation, replacing them with non-toxic silane-terminated polyurethane. The silane crosslinking mechanism provides the necessary bond strength without the health hazards associated with isocyanate exposure, including sensitization and carcinogenicity risks.
Solution Approach 2:
The patent converts the potentially harmful isocyanate crosslinking mechanism into a beneficial silane crosslinking system. The silane groups provide the same structural crosslinking function that gives PU adhesives their high strength, but without the toxicological drawbacks, effectively turning a harmful approach into a safe one.
3Productivity
If silane-crosslinking adhesives are used to achieve non-toxic and fast-curing properties, then the safety and curing speed are improved, but the tensile shear strength is low and cannot meet DIN EN 204 D4 standards
Solution Approach 1:
The patent creates a composite adhesive system combining silane-terminated polyurethane prepolymer with specific molecular weight polyether units (≤4000 Daltons) and silane crosslinkers. This composite structure integrates the fast-curing advantage of silane chemistry with enhanced mechanical strength through optimized molecular architecture and crosslinking density, achieving both high curing speed and compliance with DIN EN 204 D4 standards.
Solution Approach 2:
The patent optimizes the molecular weight parameter of the polyether units in the prepolymer to a maximum of 4000 Daltons, which balances flexibility and strength. Additionally, the silane content and crosslinking density are adjusted to achieve the required tensile shear strength while maintaining fast curing characteristics, thereby meeting the stringent DIN EN 204 D4 requirements.
4Strength
If polyvinyl acetate dispersions are used as wood adhesives, then the adhesion to wood is good, but the setting speed is very long and moisture resistance is limited
Solution Approach 1:
The patent replaces the physical drying and coalescence mechanism of polyvinyl acetate with a chemical crosslinking mechanism based on silane hydrolysis and condensation. This chemical substitution enables rapid network formation and bond development, dramatically reducing setting time while simultaneously providing superior moisture resistance through the crosslinked structure.
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 solution achieves significantly higher tensile shear strength, faster curing, and enhanced resistance to hot water, meeting DIN EN 204 D4 standards, while being non-toxic and reducing viscosity, thus addressing the limitations of existing adhesives.
Implementation Method 1
alkoxysilane-functional prepolymers first hydrolyze upon contact with atmospheric moisture and then harden through a condensation reaction
Implementation Method 2
alkoxysilane-functional prepolymers first hydrolyze upon contact with atmospheric moisture and then harden through a condensation reaction
Data Source
AI summary
The invention relates to compositions (K) containing A) 100 parts by weight of a silane-terminated polyurethane prepolymer (STPU) whose backbone contains polyurethane units and units (E) selected from among polyether and polyester units, where the individual polyether and polyester units (E) have an average molar mass of not more than 4000 dalton and the at least one end group has the general formula (1) -L1- (CH2)y-SiR2 3-x(OR1)x (1), B) from 1 to 200 parts by weight of a silane-terminated polyether (STPE) whose backbone comprises polyether units having an average molar mass of from 5000 to 30 000 dalton and has end groups of the general formula (2) -L2- (CH2)m-SiR4 3-n(OR3)n (2), C) from 0 to 10 parts by weight of a curing catalyst (HK) which accelerates the curing of the compositions (K) in the presence of atmospheric moisture, where L1, L2, R1, R2, R3, R4, x, y, m and z are as defined in Claim 1. The compositions (K) according to the invention are very resistant to tensile shear after curing and are used as adhesives (K).