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 it into two fragments, each incorporating part of the water molecule — one fragment receiving a hydroxyl group (–OH), the other a proton (–H). The reaction requires that the target bond be thermodynamically susceptible to this cleavage, meaning the products are more stable than the starting material, plus sufficient water activity and temperature to drive it forward.
In adhesive polymers, the hydrolysis-susceptible bonds are:
Ester linkages (polyesters, polyacrylates, some epoxy hardeners): water cleaves the ester bond into an alcohol and a carboxylic acid, a reaction catalyzed by acid and base — so acidic or alkaline environments accelerate it dramatically. The result is reduced molecular weight and hydrophilic end groups that attract still more water, accelerating further degradation.
Urethane linkages (polyurethane adhesives): the urethane bond can hydrolyze into an amine and CO₂ gas. Slow at moderate temperature and modest humidity, but in hot water, steam, or aggressive alkaline environments it proceeds at industrially significant rates, generating gas bubbles that blister the bondline and reduce molecular weight.
Siloxane linkages (modified silicone adhesives): the Si–O–Si backbone of pure silicone resists hydrolysis well, but interfacial siloxane bonds formed by silane coupling agents can hydrolyze, undermining the key chemical link between adhesive and substrate.
Amide linkages (nylon-based or polyamide adhesives): hydrolysis generates an amine and a carboxylic acid, and polyamide adhesives are particularly vulnerable in high-temperature water or steam.
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 bonds to rapid hydrolysis. Alkaline cleaning solutions — particularly NaOH or KOH at elevated temperature — aggressively cleave ester and urethane linkages, and food processing equipment running daily hot caustic cleaning cycles accumulates hydrolytic damage rapidly. The alkaline mechanism here is the same one covered in more depth in chemical attack on adhesives in harsh industrial conditions, where hydrolysis is one of several concurrent degradation pathways rather than the only one.
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 bonds continuously. Less aggressive than liquid immersion, but the cumulative damage over years of tropical service can still degrade bonds designed for temperate environments — the combined heat-and-moisture attack detailed in why adhesives fail in high-humidity, high-heat 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 bulk decreases tensile and shear strength — a polyurethane adhesive with significant urethane hydrolysis may test at 30–50% of original lap shear strength after accelerated aging, even while the joint still looks visually intact.
Blistering and delamination. CO₂ from urethane hydrolysis, or water vapor accumulating between adhesive and substrate, blisters the bond — visible as domed regions in film adhesives or surface irregularities in thick bonds — and blister growth under subsequent thermal or pressure cycling propagates into full delamination.
Interfacial hydrolysis and adhesion loss. If the coupling agent hydrolyzes preferentially, the adhesive detaches from the substrate while the bulk retains most of its cohesive strength — a clean interface failure rather than cohesive fracture, common in silane-coupled glass or metal bonds under sustained wet service.
Progressive embrittlement. In crosslinked thermosets, early-stage hydrolysis plasticizes the adhesive (lower Tg and modulus), while extended hydrolysis followed by drying can produce a re-crosslinked, more brittle polymer than the original — a two-stage behavior that complicates predicting 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, and screening candidate formulations with a standardized immersion method such as ASTM D570 (Water Absorption of Plastics) gives a fast, comparative first pass before committing to full hydrolytic aging studies. Backbone selection here follows the same aromatic-versus-aliphatic logic used to resist oxidative degradation in high-temperature adhesives — aromatic structures resist both attack mechanisms better than aliphatic ones, for related structural reasons.
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 is a chemical degradation process driven by water reacting with ester, urethane, amide, and interfacial siloxane linkages, with severity set by temperature, pH, and water availability — hot aqueous cleaning and steam sterilization are the most demanding environments. Preventing hydrolytic failure means selecting chemistries inherently resistant to these reactions, protecting the interface with appropriate surface treatment, and validating durability through wet aging tests that replicate actual service conditions.
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