An automotive power supply potted with standard polyurethane survives 18 months before thermal cycling cracks its solder joints. The identical design, potted with high-temperature epoxy instead, survives five years or more. Material family choice matters as much as formulation quality within that family.

Three families compete for high-temperature potting duty — epoxy, silicone, and polyurethane — and each brings a distinct set of trade-offs that no amount of formulation tweaking fully erases.

Epoxy: The High-Temperature Default

Epoxy’s glass transition temperature runs 150–240°C depending on formulation, with thermal conductivity from 0.3 W/m·K unfilled up to 3 W/m·K with conductive filler, and a CTE of 40–60 ppm/°C. At $20–100/lb it’s also the most cost-effective family at scale. Its strengths are thermal stability well above 150°C continuous, excellent mechanical strength for constraining components under stress, and strong resistance to oils, fuels, and solvents — which is why it dominates automotive under-hood and industrial high-heat applications.

Its weaknesses are equally specific: standard epoxy is brittle and cracks under vibration or mechanical shock without 8–12% elastomer toughening added in, which increases both cost and formulation complexity. Unfilled, low-cost variants absorb 1–3% moisture (high-quality formulations cut that to under 0.5%), and large pours generate enough exotherm to risk cure defects without careful process control.

Silicone: Built for Flexibility and Weathering

Silicone’s Tg range (120–220°C) varies more widely than epoxy’s, with thermal conductivity of 0.3–2 W/m·K and CTE from 30–80 ppm/°C depending on filler — at a premium of $40–150/lb. Its defining trait is inherent elasticity, even unfilled, which absorbs vibration and thermal-cycling strain without cracking. Environmental resistance is outstanding: silicone tolerates UV, ozone, and weathering better than either alternative, retaining properties in outdoor deployments for a decade or more, and its shrinkage (under 1%) is the lowest of the three families, meaning less residual cure stress to begin with.

The trade-offs run the other direction on temperature: standard silicone’s Tg (120–160°C) is marginal for continuous operation above 140°C without a high-temperature-specific formulation, and softness that helps with vibration hurts mechanical support — components move more under thermal cycling than they would in a stiffer matrix. Silicone also absorbs more moisture than its reputation suggests (0.5–2%), and its naturally low surface energy can mean poor adhesion to some substrates unless the formulation includes adhesion promoters.

Polyurethane: Lower Cost, Lower Ceiling

Polyurethane’s Tg (80–160°C) is the lowest of the three families, with thermal conductivity of 0.2–0.8 W/m·K, CTE of 50–100 ppm/°C, and a mid-range cost of $25–60/lb. It shares silicone’s flexibility and impact resistance, adds genuinely hydrophobic moisture resistance from its closed-cell structure, and holds dielectric strength consistently across its (narrower) temperature range.

But most polyurethane formulations top out below 150°C continuous, disqualifying them from the high-heat applications this series otherwise covers, and their CTE — the highest of the three families — generates the most thermal-cycling stress per degree of temperature swing. UV exposure yellows and degrades unprotected polyurethane within 12–24 months, and its chemical resistance lags epoxy and silicone against industrial solvents. It remains a reasonable choice for consumer electronics and moderate-temperature industrial equipment where cost matters more than headroom.

Side-by-Side Comparison

Deflection under heat and load — the property behind most Tg-adjacent field failures — is measured per ASTM D648, the standard test for deflection temperature of plastics under flexural load, and is worth requesting from any supplier alongside a headline Tg number. Where flame rating matters for the enclosure, UL 94’s flammability classifications for plastic materials apply across all three families, though epoxy and silicone formulations reach V-0 more consistently than unmodified polyurethane.

Choosing by Application

Automotive engine bay duty (150°C continuous, thermal cycling, vibration, oil exposure) calls for high-temperature elastomer-toughened epoxy — sufficient Tg, mechanical strength, and oil resistance with a long field track record. Outdoor industrial electronics (−40°C to +100°C, humidity, UV, vibration) favor silicone, specifically a high-temperature grade, for its weathering and flexibility advantage. Power supplies around 100W at 120°C with cost sensitivity do best with high-temperature thermally conductive epoxy — adequate Tg margin, useful conductivity, and reasonable volume pricing. Consumer IoT devices operating indoors at moderate temperature are well served by standard polyurethane or low-cost epoxy, where headroom isn’t the limiting factor. Aerospace electronics (−55°C to +85°C, thermal cycling, humidity, extreme reliability requirements) typically specify high-temperature silicone or aerospace-grade epoxy, chosen for validated cycling performance and environmental stability rather than cost.

Some suppliers also offer silicone-epoxy hybrids that combine silicone’s flexibility with epoxy’s thermal ceiling, addressing Tg limitations while retaining vibration damping — worth asking about if your application sits between two of the categories above. Incure’s comparison of high-temperature potting compound against standard epoxy and guide to choosing potting for electronics above 150°C go deeper into that decision.

Cost-Per-Year-of-Service

At 150°C continuous, standard non-toughened epoxy typically lasts 1–2 years at $20/lb; elastomer-toughened, low-CTE high-temperature epoxy stretches that to 5–7 years at $60–80/lb, working out to $9–16 per year of service. High-temperature silicone runs 5–8 years at $80–120/lb, or $12–24 per year — comparable once field-failure cost is included. The lesson holds across all three families: higher material cost is routinely justified by extended service life, particularly in automotive warranty contexts where field failures cost far more than the material savings that led to them. Incure’s checklist of what to look for in potting compounds for 150°C applications covers the specification details that determine which side of that math you land on. For applications where the choice depends on whether a component needs heat conducted away or shielded from it, see our guide to choosing between thermally conductive and insulating potting.

Email Us with your temperature range, vibration exposure, and cost target, and Incure will identify which family — or hybrid — fits.

Incure formulates across all three families, including thermally conductive and elastomer-toughened variants, so the choice comes down to your application’s technical requirements rather than what one supplier happens to stock.

Contact Our Team to confirm whether epoxy, silicone, or polyurethane is the right fit for your thermal application.

Visit www.incurelab.com for more information.