Ultem vs Polysulfone: Which High-Performance Plastic Wins?

Which Material Should You Choose?Choose Ultem PEI (Polyetherimide) if:Ultem offers 60°C higher HDT and inherent flame retardance vs PSU's superior steam sterilizability and lower costChoose Polysulfone PSU if:Superior performance in extreme...

Ultem PEI and Polysulfone PSU are the two dominant amorphous high-temperature engineering thermoplastics for medical devices, pharmaceutical processing equipment, and electrical/electronic components. Both are transparent amber, steam-sterilizable, and dimensionally stable — leading to frequent confusion about which to specify. The choice fundamentally depends on whether the application is temperature-driven (choose Ultem) or chemical/steam-driven (choose PSU).

Property Comparison

Glass Transition Temperature (Tg): Ultem: 217°C / PSU: 185°C. Ultem has a 32°C Tg advantage — translating to a 55°C HDT advantage under load. HDT at 1.82 MPa: Ultem: 201°C / PSU: 174°C. Any application requiring structural integrity above 175°C (lead-free soldering, high-temperature sterilization above standard autoclave 134°C, engine compartment electronics) must use Ultem. Steam Autoclave Resistance (500 cycles at 134°C/2.2 bar): PSU: Excellent — retains >90% tensile strength after 500 autoclave cycles. Ultem: Moderate — retains 75-85% tensile strength; PEI undergoes microcracking from the combined thermal+moisture cycling, and the drop in elongation at break is more pronounced than PSU. For repeated steam sterilization, PSU is the better choice. Flame Retardance: Ultem: Inherent V-0 at 0.25 mm (no additives — the aromatic imide structure chars and extinguishes). PSU: V-2 at 1.5 mm without additives; requires FR additives to achieve V-0. For electrical/electronic applications where UL 94 V-0 is a non-negotiable requirement, Ultem's inherent FR is a major advantage. Chemical Resistance: PSU is significantly more resistant to hot water and steam; Ultem is slightly more resistant to non-polar solvents (fuels, oils). Both are attacked by polar organic solvents (acetone, MEK, methylene chloride) — they craze and crack within minutes to hours.

Decision Matrix

Choose Ultem When: (1) HDT >180°C is required (lead-free soldering, high-temperature connectors); (2) Inherent UL 94 V-0 flame retardance without additives is required; (3) Higher stiffness is needed (flexural modulus 3.3 GPa vs 2.7 GPa — Ultem is ~20% stiffer); (4) The part will see short-term exposure to 200°C+ (Ultem survives brief excursions to 210°C; PSU distorts). Choose PSU When: (1) Repeated steam autoclave sterilization (500+ cycles) is required — medical and pharmaceutical processing equipment; (2) Hot water/steam chemical resistance is the primary environmental concern; (3) Lower cost is important (PSU is approximately 30-40% less expensive than Ultem); (4) The part requires excellent clarity and transparency retention after thermal cycling (PSU maintains higher light transmission and lower yellowing after repeated autoclave cycles than Ultem); (5) The part will be solvent-bonded (PSU can be solvent-bonded with methylene chloride; Ultem has poor solvent-bonding characteristics and is typically joined by ultrasonic welding or adhesive bonding).

Comparison at a Glance

Equivalents & Cross-References

Related Diagnostics & Materials

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb. Material Property Data for Engineering Thermoplastics. matweb.com
• ISO 1043. Plastics — Symbols and Abbreviated Terms. iso.org