UV Curable Hot Melt Adhesive Viscosity Stability
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Solution Overview
Problem
Hot melt pressure sensitive adhesives (HMPSAs) face challenges with viscosity instability during application, which limits their pot life and adhesive strength due to incompatible functional groups and unstable linkages, making it difficult to achieve balanced cohesive and adhesive properties.
Innovation Solution
The development of acrylate polymers functionalized with UV curable photoinitiators that are polymerized into the polymer backbone, providing stable hot melt viscosity and effective crosslinking upon exposure to UV radiation, as defined by specific structural formulas (I, II, and III), enhancing the adhesive's cohesion and adhesion properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If very low molecular weight polymers are used to achieve sufficient fluidity in hot melt pressure sensitive adhesives, then the adhesive flows easily during application, but the cohesive strength of the adhesive is insufficient
Solution Approach 1:
The patent changes the molecular weight parameter of the polymer from very low to moderate range, and introduces crosslinking density as a new parameter to control cohesive strength. This allows the adhesive to have appropriate fluidity during application while achieving sufficient cohesive strength through controlled crosslinking after application.
Solution Approach 2:
The patent creates a composite structure by combining moderate molecular weight polymer chains with crosslinked networks. The polymer matrix provides fluidity and basic adhesive properties, while the crosslinked structure provides cohesive strength, creating a composite material system that achieves both requirements simultaneously.
2Strength
If very high molecular weight polymers are used to improve cohesive strength, then the adhesive gains better internal bonding, but the viscosity becomes too high to be easily coatable onto substrates at common application temperatures
Solution Approach 1:
The patent optimizes the molecular weight parameter to a moderate range rather than using very high molecular weight polymers. This parameter optimization ensures the polymer maintains low enough viscosity for easy coating at application temperatures while still providing sufficient cohesive strength when combined with crosslinking.
Solution Approach 2:
The patent applies different properties to different aspects of the adhesive: moderate molecular weight for bulk fluidity and coatability, and localized crosslinked structures for cohesive strength. This local differentiation of properties allows the adhesive to be easily applied while maintaining strong internal bonding.
3Strength
If polymers with functional groups and photoinitiators are used to enable controlled crosslinking reactions upon UV exposure, then cohesive strength can be increased, but viscosity instability occurs during heating with a dramatic rise in viscosity over a short period
Solution Approach 1:
The patent optimizes the concentration parameter of photoinitiators and functional groups to appropriate levels that enable sufficient crosslinking for cohesive strength without causing excessive viscosity increase during heating. This parameter optimization balances crosslinking reactivity with thermal stability during the application process.
Solution Approach 2:
The patent uses partial crosslinking during storage and handling to maintain stability, with the understanding that full crosslinking occurs after application upon UV exposure. This partial action approach prevents premature viscosity buildup while ensuring adequate cohesive strength develops after curing.
4Ease of operation
If moderate molecular weight polymers are used to balance fluidity and cohesive strength, then the adhesive achieves usable flow properties, but viscosity instability arises from incompatible functional groups or unstable linkages such as urethane bonds
Solution Approach 1:
The patent changes the chemical composition parameters by selecting compatible functional groups and stable linkages (avoiding unstable urethane bonds). This parameter change ensures the polymer maintains viscosity stability during heating while retaining adequate fluidity for application.
Solution Approach 2:
The patent converts the potential harm of functional group incompatibility and unstable linkages into benefit by carefully selecting stable, compatible functional groups that enable both adequate fluidity and long-term viscosity stability. The moderate molecular weight polymers with stable linkages provide a foundation that prevents premature crosslinking while maintaining operability.
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 results in HMPSAs with significantly improved hot melt viscosity stability, up to 10-fold greater than prior art, and maintains UV crosslinkability, ensuring consistent adhesive performance and extended pot life.
Implementation Method 1
polymers of moderate molecular weight have been made with various functional groups, photoinitiators, which undergo controlled crosslinking reactions upon exposure to UV radiation
Implementation Method 2
Hot melt adhesives are solids at room temperature, melt at elevated temperatures, and enable easy coating on a substrate
Data Source
Figure 1
AI summary
Disclosed are photoinitiators that are ultra violet (UV) reactive. The photoinitiators can be polymerized into polymeric backbones for use in UV curable hot melt pressure sensitive adhesives. The preferred polymeric backbones are acrylic-based polymers. The photoinitiators are very UVC sensitive and have excellent hot melt viscosity stability that is many fold higher than existing photoinitiators. The subject photoinitiators feature aryl ketones linked through an ether linkage to a variety of connector sequences that allow for polymerization of the photoinitiators.