Thiol-Terminated Polymer Sealant Adhesion via Thiol-Ene Curing
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Solution Overview
Problem
Aerospace sealant compositions face challenges in achieving strong adhesion to aircraft substrates, such as fuel tank materials, due to the rapid crosslinking reaction caused by actinic radiation curing, which may not allow sufficient time for conventional adhesion promoting agents to react with the substrate effectively.
Innovation Solution
Compositions comprising a thiol-terminated polymer and a sulfur-containing ethylenically unsaturated silane, which react via a thiol-ene mechanism upon exposure to actinic radiation, ensuring effective adhesion by incorporating a photoinitiator and optimizing the stoichiometric ratio of thiol groups to ethylenically unsaturated groups.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional adhesion promoting agents (epoxy, thiol, or amino functional silanes) are used to bond sealant to metal substrates via hydrolyzable groups, then strong adhesion is achieved, but the curing time becomes excessively long due to the slow hydrolysis and condensation reactions required for silane crosslinking
Solution Approach 1:
The invention changes the chemical reaction mechanism from slow hydrolysis/condensation to rapid free-radical polymerization. By using ethylenically unsaturated silane compounds with C=C bonds that can undergo free-radical polymerization, the curing time is dramatically reduced while maintaining adhesion strength. The silane group provides adhesion promotion, while the ethylenically unsaturated group enables fast curing through radical mechanisms initiated by peroxides or radiation.
Solution Approach 2:
The invention creates a composite functional silane molecule combining adhesion-promoting silane groups with fast-curing ethylenically unsaturated groups. This dual-functional silane compound simultaneously provides both strong metal substrate adhesion (via Si-O-metal bonds from hydrolyzed silane) and rapid crosslinking (via free-radical polymerization of C=C bonds), resolving the time-strength contradiction.
2Loss of time
If fast-curing ethylenically unsaturated compounds are used to achieve rapid crosslinking, then curing time is reduced, but adhesion strength decreases because adhesion promoting agents do not have sufficient time to react with the substrate
Solution Approach 1:
The invention merges the adhesion-promoting function of silane groups with the fast-curing capability of ethylenically unsaturated compounds into a single molecular structure. The silane-containing ethylenically unsaturated compound contains both Si-OH groups (for metal adhesion) and C=C bonds (for rapid polymerization), allowing both functions to operate simultaneously rather than sequentially.
Solution Approach 2:
The invention changes the reaction kinetics by introducing free-radical polymerizable groups that cure much faster than conventional silane hydrolysis. The ethylenically unsaturated groups react rapidly with free radicals from peroxide initiators or radiation, achieving crosslinking in minutes or seconds while the silane groups concurrently form adhesion bonds with the metal substrate.
3Reliability
If thiol-terminated polymers are used to achieve fuel resistance through crosslinking, then fuel resistance is improved, but the composition becomes less storage stable due to the high reactivity of thiol groups
Solution Approach 1:
The invention extracts the highly reactive thiol groups from the polymer backbone and replaces them with less reactive but still crosslinkable ethylenically unsaturated groups. The thiol-terminated polymer provides the base polymer matrix with desired properties, while the added ethylenically unsaturated silane compounds provide the crosslinking functionality without the storage stability issues of free thiol groups.
Solution Approach 2:
The invention uses short-living reactive ethylenically unsaturated groups that rapidly consume themselves during curing to form the crosslinked network. These groups are highly reactive during application (enabling fast cure) but are consumed in the reaction, leaving a stable cured product without residual reactive groups that would compromise storage stability.
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 enables the formation of cured sealants with strong adhesion to aircraft substrates, maintaining flexibility and resistance to elevated temperatures, while allowing for rapid curing and storage stability.
Implementation Method 1
Compositions that cure when exposed to actinic radiation, such as ultraviolet radiation, as can be the case with the reaction of thiol functional compounds with ethylenically unsaturated compounds (sometimes referred to as 'enes')
Implementation Method 2
These free radicals lead to crosslinking via a thiol-ene reaction, which can often be completed within seconds
Implementation Method 3
aerospace sealant compositions often include adhesion promoting ingredients, such as epoxy, thiol, or amino functional silanes, which are known to bond an organic composition, such as a sealant composition, to an inorganic substrate, such as certain metals, via a hydrolyzable group that forms a metal-siloxane bond (Si-O-metal)
Data Source
AI summary
Disclosed are compositions that include: a) a thiol-terminated polymer; and b) a sulfur-containing ethylenically unsaturated silane. Related products, such as sealants, that include polymers derived from such compositions, are also disclosed.