Blocked Polyisocyanate Crystallization Stability via Composite Blocking
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Solution Overview
Problem
Polyisocyanates based on linear aliphatic diisocyanates containing isocyanurate groups, blocked with secondary monoamines, suffer from lack of crystallization stability and tend to solidify, making them unsuitable for long-term storage and practical applications.
Innovation Solution
A process involving a reaction between a polyisocyanate component, a branched aliphatic diol, and a secondary amine, where the polyisocyanate component includes a mixture of linear aliphatic and cycloaliphatic polyisocyanates, results in completely solidification-stable, non-crystallizing blocked polyisocyanate crosslinkers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If polyisocyanates based on linear aliphatic diisocyanates containing isocyanurate groups are blocked with secondary monoamines, then low baking temperatures are achieved, but the solutions exhibit very high tendency to solidify through crystallization
Solution Approach 1:
The patent applies parameter changes by modifying the polyisocyanate composition parameters - specifically using a mixture of linear aliphatic and cycloaliphatic polyisocyanates in a defined ratio (val ratio 2.0:1.0 to 5.9:1.0), and controlling the blocking agent composition (secondary amine 30-70 equivalent% combined with CH-acidic ester and/or 1,2,4-triazole 30-70 equivalent%). These parameter adjustments resolve the contradiction by preventing crystallization while maintaining low deblocking temperature capability.
Solution Approach 2:
The patent employs composite materials by creating a mixed polyisocyanate system combining linear aliphatic polyisocyanates (providing low baking temperature) with cycloaliphatic polyisocyanates (providing storage stability). This composite approach allows the blocked polyisocyanate solution to remain stable for at least 6 months without crystallization while still achieving low baking temperatures through the secondary amine blocking mechanism.
2Ease of operation
If blocked polyisocyanates are stored in conventional paint solvents, then they can be used for coating applications, but they crystallize and solidify over extended periods
Solution Approach 1:
The patent changes the compositional parameters of the blocked polyisocyanate system to achieve both ease of operation and extended storage duration. The specific val ratio of linear to cycloaliphatic polyisocyanates (2.0:1.0 to 5.9:1.0) and the combined blocking agent system create a formulation that remains soluble in conventional paint solvents for at least 6 months without crystallization, enabling both practical application and long-term storage.
3Temperature
If exclusively secondary amines are used as blocking agents, then particularly low baking temperatures are achieved, but the blocked polyisocyanate solutions are not stable over extended periods
Solution Approach 1:
The patent modifies the blocking agent composition parameters by combining secondary amines (30-70 equivalent%) with CH-acidic esters and/or 1,2,4-triazoles (30-70 equivalent%). This compositional parameter change allows the system to maintain low baking temperatures (benefit of secondary amines) while achieving storage stability for at least 6 months (reliability requirement), resolving the contradiction between temperature performance and storage reliability.
Solution Approach 2:
The patent uses a composite blocking agent system combining secondary amines with CH-acidic esters and/or 1,2,4-triazoles. This composite blocking strategy allows the secondary amine component to provide low baking temperature capability while the CH-acidic ester and/or 1,2,4-triazole components contribute to storage stability, preventing crystallization in the blocked polyisocyanate solution.
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 blocked polyisocyanates are storage-stable for at least 6 months without crystallization, making them suitable for coil coating applications and providing coatings with good yellowing resistance.
Implementation Method 1
Blocked polyisocyanates, which can be obtained by reacting isocyanate groups with so-called blocking agents
Implementation Method 2
At higher temperatures, the blocking agent is cleaved, releasing the isocyanate group for crosslinking with the polyol component
Implementation Method 3
releasing the isocyanate group for crosslinking with the polyol component
Data Source
AI summary
The invention relates to a process for the preparation of a blocked polyisocyanate, comprising a reaction A) of a polyisocyanate component with B) at least one branched aliphatic diol and with C) at least one secondary amine having aliphatic, cycloaliphatic and/or araliphatic substituents, characterized in that component A) contains at least one linear aliphatic polyisocyanate A1) which has at least isocyanurate and/or iminooxadiazinedione structures, and at least one cycloaliphatic polyisocyanate A2), wherein A1) and A2) are present to one another in an val ratio of 2.0 to 1.0 to 5.9 to 1.0 and component B) is present in an amount of more than 2 wt.-%, based on the total amount of components A) and B) is used and component C) is used in an amount which corresponds to at least 95 mol-% of the isocyanate groups still present after the reaction of components A) and B), as well as the blocked polyisocyanates.