Epoxy adhesive used in chemical process equipment must survive direct contact with process fluids that would degrade most polymer materials within days or weeks. Acids, bases, solvents, oxidizers, fuels, and process gases all attack polymer adhesives through different mechanisms — swelling, hydrolysis, oxidation, or dissolution — and the epoxy chemistry that resists one class of chemical may be completely unsuited to another. Selecting epoxy adhesive for chemical process service requires knowing what chemicals the bond will contact, in what concentration and at what temperature, and matching that service condition to the chemical resistance profile of the specific adhesive formulation. Generic “chemical resistant epoxy” claims on product data sheets are not sufficient — material compatibility must be verified for each specific chemical or fluid in the application.

How Chemicals Attack Epoxy Adhesive

Swelling. Organic solvents — ketones, esters, chlorinated solvents, aromatic hydrocarbons — absorb into the cured epoxy network and physically swell the polymer. Swelling increases volume, reduces modulus, and degrades adhesion to substrates. For adhesive bonds, a swollen adhesive that has also plasticized and weakened the polymer matrix may release from the substrate under much lower load than the dry-state adhesion would predict.

Hydrolysis. Water-based acidic and basic solutions attack epoxy through hydrolysis of the ester groups in the cured polymer backbone. Alkaline conditions are particularly aggressive; sodium hydroxide solutions attack epoxy faster than equivalent concentration acids. Hydrolysis degrades the polymer chain structure and reduces both cohesive strength and adhesion.

Oxidation. Strong oxidizing agents — nitric acid, hydrogen peroxide, chromic acid, hypochlorites — attack the organic polymer through oxidation reactions that break carbon-carbon and carbon-nitrogen bonds in the polymer chain. Oxidative attack is often rapid and aggressive, and most standard epoxy formulations have poor resistance to strong oxidizers.

Temperature amplification. All chemical attack mechanisms accelerate with temperature. An epoxy that resists dilute acid at ambient temperature may degrade rapidly in the same acid concentration at 60°C. Chemical resistance data should be obtained at the actual service temperature, not at ambient, for process equipment applications.

Epoxy Chemistries with Different Chemical Resistance Profiles

Not all epoxy adhesives have the same chemical resistance — the curing agent chemistry as much as the epoxide resin determines what the cured material can withstand.

Amine-cured epoxy: Good general chemical resistance. Resists dilute acids, fuels, oils, and many solvents adequately. Weaker against strong alkalines (the amine curing agent residue in the polymer is susceptible to base attack), and moderate resistance to solvents depending on filler level and cross-link density.

Anhydride-cured epoxy: Generally superior to amine-cured systems against alkaline attack. Higher cross-link density from anhydride cure provides better solvent resistance. Commonly used for chemical process applications.

Novolac epoxy: High cross-link density systems based on epoxy novolac resins achieve superior chemical resistance compared to standard bisphenol-A epoxy. Novolac-based adhesives show improved resistance to acids, bases, and solvents. Used for the most demanding chemical service environments.

Furan-modified or phenolic-modified epoxy: Modified systems for specific chemical process applications with particularly aggressive chemicals. These formulations are specialty products for specific industrial applications.

If you need chemical resistance data for specific fluids and concentrations at your service temperature, Email Us — Incure provides chemical immersion test data and formulation recommendations for chemical process adhesive applications.

The Chemical Resistance Test Protocol

Published chemical resistance tables in adhesive data sheets are generated by immersing cured adhesive specimens in the listed chemical for a defined period (typically 7 or 30 days) at a defined temperature, then measuring property retention. The properties measured are typically weight change (swelling or extraction), hardness, and tensile or shear strength. A material showing less than 10% change in all properties is typically rated as having good resistance; more than 25% change indicates poor resistance.

For critical applications, these tabulated ratings are a starting point, not a final answer. The immersion test on a pure adhesive specimen does not capture the effect of chemical attack on adhesion at the adhesive-substrate interface, which may be more vulnerable than the bulk adhesive. Testing the actual bonded joint — lap shear specimens — in chemical immersion at service temperature for extended periods (90 to 180 days) and measuring strength retention after immersion is the appropriate qualification approach.

Substrate Compatibility in Chemical Service

The adhesive-to-substrate interface in chemical service must also be chemically stable. An epoxy adhesive that resists the process chemical in bulk may still fail at the interface if the substrate oxide is attacked by the chemical. Aluminium in acidic or alkaline process fluids corrodes at the oxide layer that the epoxy bonds to; if the adhesive bond edge is exposed to the process fluid, corrosion at the interface progresses inward regardless of bulk adhesive stability.

Edge protection. For chemical process equipment where bond edges contact the process fluid, covering the bond perimeter with a chemically resistant sealant or lining is required. The sealant must resist the same chemical environment. For highly aggressive chemicals, a continuous fluoropolymer or rubber lining over the bonded joint edge may be necessary.

Substrate selection. In the most aggressive environments, substrates that are intrinsically resistant to the process chemical — stainless steel, glass fibre composite — combined with appropriate epoxy chemistry eliminate the substrate corrosion concern at the interface.

Application Examples in Chemical Processing

Flange bonding for process piping. Epoxy adhesive bonded pipe flanges on non-metallic (FRP composite) process piping distribute load across the full flange face rather than concentrating it at bolt holes. The adhesive in this application contacts the process fluid if the gasket fails; novolac-based epoxy provides adequate resistance for many process chemicals.

Lining panel bonding in storage tanks and reactors. Chemically resistant lining panels bonded to carbon steel vessel walls with structural epoxy protect the vessel from internal corrosion. The bond between the lining panel and the steel must resist the operating temperature and internal pressure while the lining surface contacts the process chemical.

Equipment repair. Epoxy repair compounds for patching corroded process equipment must resist the same chemical environment that caused the original corrosion. Vinyl ester or novolac epoxy repair compounds provide broad chemical resistance for general process equipment applications.

Contact Our Team to discuss chemical process adhesive selection, chemical resistance testing, and bond design for your specific process fluid and operating condition.

Visit www.incurelab.com for more information.