Potting Compounds

Ensuring the isolation of electrical contacts within connectors in moisture prone environments is no trivial task. Careful selection of connectors, backshells, grommets, and drip loops can only do so much to prevent moisture ingress. There comes a point where these are insufficient to prevent fluid intrusion. In these cases, potting compounds are used. The use of potting compounds creates a moisture-tight seal, albeit with drawbacks from its use. This article reviews a specific implementation of potting compounds and goes over the testing of these compounds and potential drawbacks for their use.

The Basics

MIL-M-24041 , “Polyurethane Potting Compound for Watertight Electrical Assemblies” is a military specification that defines requirements for two-component, chemically cured polyurethane compounds used for molding, encapsulating, and potting electronic components. The primary goal of this material is to ensure reliable watertight protection and electrical insulation for critical connections in harsh environments; it is the only potting compound specification that is called out in the SAE’s Wiring System Installation Standard, AS50881. MIL-M-24041 potting compounds are typically applied to electrical connectors, cable harnesses, cable end seals, circuit boards, and other electrical components to seal out moisture and contaminants. By filling connector backshells or mold cavities with these compounds, assemblies achieve a durable environmental seal superior to what mechanical grommets or shrink boots alone can provide. Depending on how the potting compound is applied, it improves water resistance, provides cable/harness strain relief, and acts to dampen the impact of mechanical shock/vibration. In effect, MIL-M-24041 defines the material that serves as a protective coating over sensitive wiring connections, increasing their ability to withstand SWAMP environments and other hazards.

Specifics

The scope of MIL-M-24041 includes polyurethane potting materials formulated for use across a wide range of use cases. These compounds are designed for use in both sea-water environments and aerospace applications where connectors may be exposed to seawater spray, deicers, altitude, temperature extremes, and mechanical stress. The specification ensures that potted connections remain electrically insulating and physically intact from the cold of high altitudes (e.g. –60°C) up to elevated temperatures near aircraft engines (125–150°C).

Materials and Categories Covered by MIL-M-24041

MIL-M-24041 specifically covers two-part polyurethane resin systems. These are supplied as a resin and curing agent that are mixed immediately before use, then harden into a solid elastomer. Polyurethane was chosen for its flexibility, adhesion, and durability advantages over other potting materials like epoxies or silicones. Notably, the specification focuses on polyether-based polyurethanes, which are hydrolytically stable. A typical qualified material is described as a “two-component polyether polyurethane” that remains flexible at low temperatures, resists cold-flow (creep under mechanical stress), and provides excellent electrical insulation. Naturally, the potting compounds must also bond well to metals and common cable jacket materials, ensuring a tight seal to those substrates. A means to determine the adhesion of a potting compound to standard wire/cables is detailed in the AS4373 Method 102, but it is not a requirement of the AS22759 wire specification.

Classifications

Within MIL-M-24041, materials are classified into categories and types based on their formulation and packaging. The specification defines two categories of potting compound:

• Category A – Formulations cured with MbOCA (4,4’-methylene-bis(2-chloroaniline)), an aromatic diamine curing agent. MbOCA-cured polyurethanes were common for their robust properties, but MbOCA is recognized as a carcinogen. Category A materials contain this curative, and while they meet performance needs, they carry health and safety concerns.
• Category B – Formulations cured with non-MbOCA systems. These are sometimes referred to as “non-carcinogenic” polyurethane potting compounds. Category B materials achieve similar cured properties using alternative curatives that avoid the issues with MbOCA’s toxicity. Over time, Category B has become the preferred choice in new applications due to stricter health regulations.

Additionally, MIL-M-24041 distinguishes Type I vs. Type II potting compounds. Both types are two-component polyurethanes, but they differ in delivery form: Type I is the standard two-part kit (resin and hardener provided separately, to be hand-mixed), whereas Type II refers to materials supplied in a premixed and frozen format for convenience and quality control. In industry practice, Type II potting compound comes pre-proportioned and pre-mixed by the manufacturer, then frozen to arrest the cure; the end-user thaws and dispenses it with the potential of mixing errors. Both Type I and II ultimately cure to the same material; the type designation merely accommodates different application methods (field mixing vs. factory-mixed frozen). All MIL-M-24041 compounds, regardless of type/category, are room-temperature curing elastomers (though many achieve optimal properties with a post-cure heat treatment around 80°C).

Getting Back in There

Polyurethane potting compounds are designed for permanence and not for rework. Their thermoset chemistry crosslinks into a tough, solvent-resistant matrix that adheres tightly to connector shells, insulation, and inserts. Once cured, they resist heat, fuels, and most solvents, making removal extremely difficult without damaging the underlying connector or wires. For this reason, maintenance programs typically treat potting removal as a destructive procedure. In practice, the removal of potting might only be undertaken for failure analysis or when the component cannot be replaced. In other words, it is important to verify harness performance before potting.

When potting removal is necessary, the preferred method combines mechanical chipping and controlled heating. Technicians carefully expose and lift the compound under magnification, applying low heat to soften the polymer and reduce its adhesion. Chemical softeners can assist, but these must be used sparingly to avoid attacking connector inserts or wire jackets (limited testing is recommended on spare parts if they are available). Every removal step is followed by cleaning, drying, and verification through insulation-resistance and dielectric-withstand tests before any re-potting or reuse.

Ultimately, potting compounds should be viewed as structural components of the connector assembly, not removable sealants. Their mechanical strength and adhesion provide long-term protection but eliminate easy rework as a consequence. The best practice is to design systems with maintenance in mind by using potted connectors only when needed for permanent environmental sealing.

In Summary

Potting compounds are a critical part of EWIS design, by providing a means to protect critical wiring terminations in environments where full exposure is inevitable. These materials, such as those identified in MIL-M-24041, are reliable potting compounds that have been part of aerospace wiring systems for decades. The compatibility of these potting compounds with wiring system components should not be assumed and must be proven acceptable. When used, the potting compounds should be considered as a solution that cannot be reworked and either requires that the connector be cut off and the harness re-terminated (assuming there is sufficient wire/cable length) or a new harness to be installed.

To help with material compatibility testing and potting compound performance, contact Lectromec.