Common Materials Used in Exhaust Hangers

The selection of material for exhaust hangers directly influences the system’s vibration dampening, alignment, and overall durability. Four primary materials dominate the market, each with distinct chemical resistance profiles.

Rubber (Natural and Synthetic)

Rubber remains the most widely used material due to its excellent vibration absorption and low cost. Natural rubber offers high resilience but suffers from poor resistance to oils, greases, and ozone. Synthetic rubbers such as EPDM (ethylene propylene diene monomer) provide improved weathering and ozone resistance but still degrade when exposed to hydrocarbon-based chemicals. Nitrile rubber (NBR) offers superior oil resistance but is less flexible at low temperatures. The failure mode for rubber hangers typically involves swelling, cracking, or loss of elasticity after prolonged contact with petroleum products or exhaust condensates.

Steel (Carbon and Stainless)

Steel hangers—often zinc-plated carbon steel or stainless steel (grades 304 or 316)—are valued for their mechanical strength and load-bearing capacity. Carbon steel is susceptible to rust when exposed to moisture, road salts, and acidic exhaust gases. Stainless steel resists corrosion far better, but can still suffer from pitting in chloride-rich environments (e.g., winter road salt). The primary chemical threat is chloride-induced stress corrosion cracking in austenitic stainless steels under tensile load. Coatings such as e-coat or powder coating provide a barrier, but scratches or chips expose the base metal.

Polyurethane

Polyurethane (PU) hangers offer a balance of flexibility, abrasion resistance, and chemical tolerance. They resist oil, grease, and fuel better than natural rubber, and can withstand a broader temperature range. However, polyurethane can hydrolyze (break down) when exposed to hot, humid conditions with acidic byproducts. The durometer (hardness) affects performance: softer grades absorb vibration well but may swell in solvent-rich environments; harder grades are more dimensionally stable. Polyurethane is often favored in off-road and heavy-duty applications where chemical exposure is extreme.

Silicone

Silicone elastomers are known for their exceptional thermal stability (up to 250°C continuous) and broad chemical inertness. They resist oils, solvents, ozone, and UV radiation far better than most organic rubbers. However, silicone has lower tear strength and can be more expensive. It is the material of choice for high-performance or turbocharged vehicles where exhaust temperatures exceed the limits of rubber and polyurethane. The main chemical vulnerability is to concentrated acids and alkalis, though such exposures are rare in automotive contexts.

Chemical Exposure Mechanisms

Understanding how chemicals degrade hanger materials helps engineers predict failure and choose appropriate replacements. The four primary attack mechanisms are:

Swelling and Plasticization

When a polymer absorbs a compatible chemical (e.g., oil into rubber), it swells and softens. This plasticization reduces modulus and can cause the hanger to sag, allowing the exhaust system to contact the undercarriage. Volumetric swelling greater than 20% typically renders a hanger non-functional. ASTM D471 provides standardized methods for measuring volume change in chemical immersion tests.

Plasticizer Extraction

Many rubber compounds contain plasticizers to maintain flexibility. Contact with certain solvents (e.g., gasoline, brake cleaner) can leach these plasticizers out, causing the material to become brittle and crack under vibration. The loss is often irreversible and accelerates with temperature.

Oxidation and Ozone Attack

Oxygen and ozone (O3) attack unsaturated bonds in natural rubber and some synthetics, causing chain scission and cross-linking. The result is surface crazing known as weathering cracks. This effect is exacerbated by heat and UV exposure. EPDM and silicone have saturated backbones and resist ozone much better.

Corrosion and Galvanic Action

Metallic hangers suffer from electrochemical corrosion when exposed to electrolytes such as road salt brine or acidic exhaust condensate. Galvanic corrosion can occur when dissimilar metals (e.g., stainless steel hanger touching a carbon steel exhaust pipe) are coupled in the presence of an electrolyte. Proper isolation using rubber isolators is essential to prevent accelerated corrosion.

Specific Chemical Effects on Hanger Materials

The table below (conceptual) summarizes typical chemical threats and their effects on each material. The real-world performance depends on chemical concentration, temperature, and exposure duration.

  • Motor Oil and Grease: Cause swelling and plasticization of natural rubber and EPDM. Polyurethane and silicone exhibit minimal volume change. Steel hangers are not chemically affected but may become coated with oil that traps dirt and accelerates rust.
  • Road Salt (Sodium Chloride, Calcium Chloride): No direct effect on elastomers, but salt solutions drastically accelerate corrosion of carbon steel. Stainless steel (especially 316) resists well, but pitting can occur if the protective oxide layer is damaged. Salt also increases the conductivity of moisture on surfaces, promoting galvanic currents.
  • Exhaust Fumes and Condensate: Exhaust gases contain acidic components (sulfuric acid from sulfur in fuel, nitric acid from combustion). These acids attack rubber and polyurethane, leading to surface degradation and embrittlement. Silicone handles acidic exhaust better than other polymers. Condensate can also cause acidic corrosion on steel hangers, especially during short trips where the system does not heat up enough to evaporate the moisture.
  • Industrial Pollutants (Sulfur Dioxide, Nitrogen Oxides, Ozone): In urban or industrial environments, airborne pollutants accelerate the oxidation of natural rubber and styrene-butadiene rubber (SBR). Ozone levels often correlate with UV radiation, causing severe cracking in unprotected rubber. EPDM and silicone are recommended for such conditions.
  • Fuel and Solvents (Gasoline, Diesel, Methanol, Ethanol): Direct contact with fuel causes catastrophic swelling in natural rubber and EPDM. Polyurethane and nitrile rubber have moderate resistance; silicone is generally excellent. However, high-ethanol fuels (E85) can attack certain types of silicone, so material certifications are important. Steel hangers are unaffected by fuel itself but may suffer corrosion from water-alcohol mixtures.

Selecting the Right Material for Your Environment

Choosing an exhaust hanger material requires evaluating four primary factors: chemical exposure profile, operating temperature range, mechanical load (static and dynamic), and cost constraints.

High-Temperature Environments (Turbocharged, Performance, or Heavy-Duty)

When exhaust temperatures consistently exceed 150°C (302°F), organic rubbers degrade quickly. Silicone hangers are the standard choice, capable of handling up to 250°C continuously and 300°C intermittently. Some high-temperature polyurethane formulations can endure up to 120°C, making them borderline for stock systems but inadequate for performance applications. Steel hangers are always acceptable for temperature but require corrosion protection.

Cold Climate and Road Salt Exposure

For vehicles operating in northern climates with heavy winter road salt application, stainless steel (grade 304 or 316) hangers offer the best corrosion resistance. If rubber isolators are used, they should be made of EPDM or silicone, as natural rubber can crack in subzero temperatures and is more vulnerable to salt spray. Polyurethane is also a good option if its durometer is selected to maintain flexibility at -40°C.

Chemical Plant or Industrial Fleet Applications

Vehicles that operate near chemical processing plants, refineries, or waste treatment facilities face extreme exposures. Silicone hangers combined with stainless steel brackets provide the widest chemical resistance envelope. For less aggressive environments, EPDM or polyurethane may suffice. It is critical to obtain chemical resistance data from the material supplier, typically in the form of a compatibility chart (e.g., Parker O-Ring Handbook or manufacturer-specific guides).

General Passenger Vehicles (OEM Replacement)

Most original equipment hangers use EPDM rubber or blends, which offer a reasonable balance of cost, vibration damping, and longevity under normal driving conditions. If the vehicle is kept in a moderate climate and not subject to off-road or chemical spills, EPDM will provide 5–10 years of service. Upgrading to silicone or polyurethane is unnecessary unless problems arise.

Protective Measures and Maintenance

Even the best material choices will fail prematurely if not protected from harsh chemical exposure. The following practices extend hanger life:

Protective Coatings and Linings

Steel hangers can be protected with zinc plating, e-coat, PTFE (Teflon), or ceramic coatings. Zinc plating provides sacrificial protection at scratches; e-coat offers uniform coverage but can be damaged by impacts. PTFE reduces friction and chemical adhesion, making it easier to clean off contaminants. Ceramic coatings are best for high-temperature areas but are expensive. Elastomeric hangers can sometimes be over-molded with a fluoropolymer layer, though this increases cost significantly.

Regular Inspection and Cleaning

During routine maintenance, inspect hangers for swelling, cracking, or corrosion. If oil or grease accumulates on rubber hangers, it should be wiped off promptly with a mild solvent (e.g., isopropyl alcohol, not gasoline). Metal hangers should be checked for rust and coatings intact. Many fleet operators use a 6-month inspection cycle in high-corrosion environments.

Proper Installation and Alignment

Misaligned exhaust systems place tensile and torsional loads on hangers, increasing stress and making them more vulnerable to chemical attack. Always verify that hanger brackets allow free movement while supporting the weight. Use rubber isolators to break galvanic paths between steel components. Replace any hanger that shows visible damage—cheap insurance vs. a broken system.

Material Upgrades and Retrofits

If the original hangers fail prematurely, consider upgrading material rather than replacing with identical parts. For instance, switching from natural rubber to EPDM or silicone in an oil-rich environment can triple service life. Stainless steel retrofits for carbon steel brackets eliminate rust. Costs are modest (typically $5–20 per hanger) relative to the labor of replacing a failed exhaust system.

Real-World Case Studies

Two examples illustrate the importance of material selection.

Case Study 1: Municipal Snowplow Fleet – A city fleet reported chronic exhaust hanger failures on its plow trucks. Original rubber hangers would crack within one winter season. Investigation revealed heavy exposure to calcium chloride brine. Replacing the hangers with silicone isolators on stainless steel brackets extended life to over three years. The initial cost increase of $15 per vehicle was recouped within the first year through reduced downtime.

Case Study 2: Automotive Testing Laboratory – An engine test cell used natural rubber hangers to suspend exhaust extraction piping. Repeated exposure to unburned fuel and combustion byproducts caused extreme swelling in two weeks. Switching to polyurethane hangers solved the issue, albeit with slightly stiffer vibration isolation. The lab now uses polyurethane exclusively in fuel-rich test environments.

Conclusion

The effect of chemical exposure on exhaust hanger materials is a critical but often overlooked factor in vehicle reliability. Each material—rubber, steel, polyurethane, and silicone—offers distinct advantages and vulnerabilities. By understanding the mechanisms of chemical attack and selecting the appropriate material for the specific chemical, thermal, and mechanical environment, fleet operators and vehicle owners can dramatically reduce maintenance frequency and prevent system failures. Protective coatings, regular inspection, and proactive upgrades further ensure long service life. For those responsible for automotive systems and materials science education, applying these principles in design and maintenance decisions leads to safer, more cost-effective vehicles. Consult chemical resistance charts (e.g., DuPont Chemical Resistance Guide) and manufacturer specifications to make informed choices tailored to your operating conditions. More detailed engineering guidelines can be found in SAE standard J2528 on exhaust system mounting. Further reading on polymer degradation is available via the ASTM E116-17 standard for evaluating chemical compatibility of materials.