Water is not merely a plasticizer in adhesive systems — it is also a reactive chemical that participates in bond-breaking reactions. Hydrolysis, the cleavage of chemical bonds by water molecules, attacks specific linkage types in adhesive polymers and converts strong covalent bonds into weaker or non-existent ones. In industrial environments where adhesive joints face water, steam, humidity, or aqueous cleaning solutions, hydrolytic damage is a significant cause of premature bond failure.
The Chemistry of Hydrolysis in Adhesive Polymers
Hydrolysis is the reaction of water with a functional group that cleaves the group into two fragments, each incorporating part of the water molecule. The general reaction adds water across a bond: one fragment receives a hydroxyl group (–OH), the other receives a proton (–H). The reaction requires that the target bond be thermodynamically susceptible to this cleavage — which means the reaction products must be more stable than the starting material — and sufficient water activity and temperature to drive the reaction forward.
In adhesive polymers, the hydrolysis-susceptible bonds are:
Ester linkages (in polyesters, polyacrylates, and some epoxy hardeners): Water cleaves the ester bond to form an alcohol and a carboxylic acid. This reaction is catalyzed by acid and base, meaning that acidic or alkaline environments accelerate ester hydrolysis dramatically. Hydrolysis of ester bonds reduces molecular weight, generating fragments with lower mechanical performance, and introduces hydrophilic end groups that attract more water, accelerating further degradation.
Urethane linkages (in polyurethane adhesives): The urethane bond — formed by reaction of isocyanate and hydroxyl — can hydrolyze to produce an amine and carbon dioxide gas. At moderate temperatures and modest humidity, urethane hydrolysis is slow. But in hot water, steam, or aggressive alkaline environments, urethane hydrolysis proceeds at industrially significant rates, generating CO2 bubbles within the bondline (causing blistering) and reducing molecular weight.
Siloxane linkages (in certain modified silicone adhesives): Si–O–C linkages in silicone-organic hybrid systems, or silicone-to-substrate bonds through silane coupling agents, are susceptible to hydrolysis. The Si–O–Si backbone of pure silicone is generally stable in water, but interfacial siloxane bonds formed by coupling agents can hydrolyze, undermining the key chemical link between adhesive and substrate.
Amide linkages (in nylon-based or polyamide adhesives): Hydrolysis of amide bonds generates an amine and a carboxylic acid. Polyamide adhesives are particularly vulnerable in high-temperature water or steam environments.
Where Hydrolysis Damage Is Most Severe
The rate and extent of hydrolysis depend on temperature, pH, and water availability. The fastest hydrolysis conditions in industrial settings are:
Hot aqueous cleaning. Caustic wash systems, steam cleaning, and hot water rinses expose adhesive bonds to rapid hydrolysis. Alkaline cleaning solutions — particularly those with NaOH or KOH at elevated temperature — aggressively cleave ester and urethane linkages. Food processing equipment that undergoes daily hot caustic cleaning cycles accumulates hydrolytic damage rapidly in adhesive bonds used for assembly.
Steam autoclave sterilization. Medical devices and food equipment sterilized by steam autoclave face combined high temperature (121–134°C) and 100% relative humidity. This is among the most demanding hydrolysis environments encountered in practice, and only a narrow range of adhesive chemistries (certain silicones, some high-Tg epoxies) survive repeated autoclave cycles without significant strength loss.
Immersion in water or aqueous process fluids. Adhesives in continuously immersed service — sealing pumps, bonding components in tanks, or joining materials in water treatment equipment — experience sustained water activity at the adhesive surface and through the bondline. Even adhesives rated for wet environments show some hydrolysis-driven degradation over service lifetimes of years or decades.
Humid tropical climates. High ambient temperature combined with sustained high relative humidity drives moisture into adhesive bonds continuously. While less aggressive than liquid water immersion, the cumulative hydrolytic damage over years of service in tropical climates can degrade bonds that were designed for temperate environments.
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Consequences of Hydrolytic Damage
The mechanical consequences of hydrolysis depend on which bonds are cleaved and at what rate, but several failure patterns are common:
Cohesive strength reduction. Molecular weight reduction in the adhesive bulk decreases tensile and shear strength. A polyurethane adhesive that has undergone significant urethane hydrolysis may test at 30–50% of original lap shear strength after accelerated aging, even if the joint still appears visually intact.
Blistering and delamination. Carbon dioxide generated by urethane hydrolysis, or water vapor accumulating between the adhesive and substrate, can blister the bond. These blisters are visible as domed regions in film adhesives or surface irregularities in thick bonds. Blister growth under subsequent thermal or pressure cycling eventually propagates into full delamination.
Interfacial hydrolysis and adhesion loss. If the coupling agent or interfacial bonding layer hydrolyzes preferentially, the adhesive detaches from the substrate while the bulk adhesive retains most of its cohesive strength. This presents as a clean interface failure — adhesive cleanly removed from the substrate surface — rather than cohesive fracture. Interfacial hydrolysis is common in silane-coupled glass or metal bonds exposed to sustained wet service.
Progressive embrittlement. In some adhesive systems, particularly crosslinked thermosets, early-stage hydrolysis plasticizes the adhesive (reducing Tg and modulus), while extended hydrolysis followed by drying produces a re-crosslinked or restructured polymer that is more brittle than the original formulation. This two-stage behavior complicates prediction of failure timing.
Selecting Adhesives for Hydrolysis Resistance
Hydrolysis resistance requires selecting adhesive chemistries that lack hydrolysis-susceptible linkages in their backbone, or where these linkages are shielded from water access.
High-Tg epoxy systems cured with aromatic amine or anhydride hardeners have relatively low ester content and good hydrolysis resistance compared to ester-containing epoxy formulations. The dense crosslink network limits water diffusion into the bulk adhesive.
Silicone adhesives with Si–O–Si backbones resist hydrolysis far better than carbon-backbone polymers. For high-temperature, high-humidity service, silicone is often the chemistry of choice despite lower room-temperature strength compared to structural epoxies.
Polyimide adhesives offer exceptional hydrolysis resistance at high temperatures due to the imide ring structure’s stability in water. They are used in aerospace and electronics applications where hot, wet durability is mandatory.
Substrate surface treatment using hydrolysis-stable silane coupling agents — titanate or zirconate coupling agents rather than organosilanes in extremely alkaline environments — improves interfacial resistance to hydrolysis-driven adhesion loss.
Incure’s Hydrolysis-Resistant Products
Incure develops adhesive formulations with backbone chemistries and coupling agent systems optimized for humid and aqueous service conditions. Products for autoclave, immersion, and humid industrial environments are formulated and validated for hydrolytic stability.
Contact Our Team to discuss your wet service conditions and identify Incure products with the hydrolytic stability your application requires.
Conclusion
Hydrolysis damage in industrial adhesive bonds is a chemical degradation process driven by water reacting with ester, urethane, amide, and interfacial siloxane linkages. The severity of damage depends on temperature, pH, and water availability — with hot aqueous cleaning and steam sterilization representing the most demanding environments. Preventing hydrolytic failure requires selecting adhesive chemistries inherently resistant to these reactions, applying appropriate surface treatments to protect the interface, and validating durability through wet aging tests that replicate actual service conditions.
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