Overview
PSU (polysulfone) is the amorphous high-performance thermoplastic that sits in the middle of the sulfone polymer family and on a value curve just below PEI. The chemistry: aromatic ether and sulfone groups in the backbone give PSU its high glass transition (~187°C), inherent flame resistance, and exceptional hydrolytic stability — the combination of properties that defines the “sulfone polymer” family.
Solvay’s Udel is essentially synonymous with PSU. The dominant machined-stock variant is unfilled Udel P-1700, supplied as translucent amber rod, plate, and sheet. The medical-grade variant Udel P-1700 NT11 (and the newer Solvay Eviva line) carries USP Class VI biocompatibility certification and is one of the most commonly specified materials for autoclavable medical instruments.
The defining properties:
- Hydrolytic stability through hundreds of steam autoclave cycles. PSU tolerates 135°C / 0.2 MPa steam sterilization repeatedly with minimal property loss — the principal medical-industry value proposition.
- Translucent amber appearance — semi-transparent in thin sections, useful for visual inspection of fluid-handling components and for visible-light medical diagnostics.
- Continuous service to 150–160°C — sufficient for almost all autoclave, food, and pharmaceutical equipment applications, well above PC’s ~115°C limit.
- IPA tolerance — unlike PC, PSU does not ESC under isopropyl alcohol wipe-down. A practical advantage for medical and pharmaceutical cleaning protocols.
- Inherent UL94 V-0 without flame retardant additives.
- USP Class VI medical grades broadly available — wider regulatory coverage in medical applications than PEI for blood-contact and long-term implant-adjacent service.
The cost positioning: roughly 35–50% of PEI / Ultem in equivalent machined stock, roughly 25–35% of PEEK. This makes PSU the default high-performance plastic specification when:
- Continuous service is 100–160°C
- Autoclave sterilization or hot-water exposure is required
- USP Class VI biocompatibility matters
- Cost matters more than PEI’s higher heat tolerance
- Translucent amber appearance is acceptable or desired
The PSU / PEI / PPSU decision
This is the high-performance amorphous plastic selection question. All three are translucent amber, all three are autoclavable, all three have USP Class VI medical grades — but they differ on impact, temperature, and cost.
Pick PSU when:
- Continuous service is 100–160°C
- Cost is the principal driver among the sulfone polymers
- Standard autoclave sterilization at 135°C is the maximum stress
- Moderate impact resistance is sufficient
Pick PEI / Ultem when:
- Continuous service exceeds 160°C (PEI rated to 170°C)
- Highest dielectric strength is required
- Aerospace flame certification (Ultem 9085) is needed
- The premium over PSU is acceptable
Pick PPSU / Radel R when:
- Impact resistance is the design driver (notched Izod ~13 ft·lb/in vs PSU’s 1.3 ft·lb/in)
- Repeated drop-and-handling abuse is expected (dental tools, surgical instruments, baby bottles after BPA migration concerns)
- Highest autoclave cycle count is needed (PPSU tolerates 1000+ cycles in service)
- Cost premium over PSU (typically 2–3×) is acceptable
The practical default for cost-sensitive medical and food applications is PSU; PEI takes over above 160°C continuous; PPSU takes over when impact toughness is critical.
Machining notes
PSU is one of the more forgiving high-performance plastics to machine — closer to polycarbonate in chip behavior than to PEEK or PPS. The amorphous structure means no semi-crystalline transition behavior to manage, and the high Tg (~187°C) means frictional heat doesn’t cause surface softening at typical machining temperatures.
Practical recipe for unfilled Udel P-1700 stock:
- Sharp carbide tooling, polished edges
- Speed: 250–500 SFM
- Feed: 0.005–0.015 in/rev
- Coolant: recommended for production, air-blast OK for light cuts
- Polished cutting edges critical for visual-quality surface
Unlike PC, residual machining stress in PSU does not cause ESC under most common cleaning chemistries — IPA wipe-down, water-based detergents, and typical autoclave conditions are fully tolerated. This is one of PSU’s quiet practical advantages over PC for medical applications.
For tight-tolerance critical parts, anneal at 175°C for 1–2 hours per inch of section thickness after heavy machining to relieve residual stress. Surface finishes of 16 Ra are readily achievable; 8 Ra with finishing passes. Optical-quality finishes possible with diamond flycutting — PSU is used for some optical waveguide applications.
Glass-filled PSU (Udel GF-130) is abrasive but manageable with carbide. Expect 40–60% reduction in tool life versus unfilled.
PSU bonds well with structural epoxies, cyanoacrylates, and UV-cure adhesives. Surface preparation is usually a light abrasion and IPA clean — no plasma treatment required. Solvent welding works with methylene chloride but is impractical because the same chemistry causes ESC under stress.
The autoclave story
PSU’s signature application is autoclavable medical instruments. The combination of properties:
- Tolerates 135°C / 0.2 MPa steam autoclave cycles repeatedly
- USP Class VI biocompatibility in medical grades
- Translucent amber appearance allows visual inspection
- IPA wipe-down compatibility for between-use cleaning
- Gamma / e-beam / ETO sterilization additionally supported
makes PSU the workhorse material for surgical instrument handles, sterilization trays, dental tools, surgical staplers, fluid-handling manifolds in dialysis and blood-contact devices, and reusable medical device housings.
The “hundreds of cycles” claim is industry-standard. PSU instruments survive hospital sterilization workflows for years — the limiting factor is usually mechanical wear from use, not material degradation.
Where PSU is not sufficient for autoclave service:
- Very high cycle counts (1000+ continuous cycles) where small mechanical-property loss matters — step up to PPSU
- Repeated drop-and-handling abuse where impact toughness is the failure mode — step up to PPSU
- Service temperatures above 160°C (rare in autoclave applications) — step up to PEI
Failure modes worth designing around
Aromatic and chlorinated solvent ESC is the dominant chemical failure mode. Methylene chloride, toluene, xylene, ketones, and certain esters cause crazing and brittle fracture under stress. The vulnerable chemistry list overlaps significantly with PC, but PSU is somewhat less sensitive — particularly to alcohols, which are well-tolerated by PSU but cause PC ESC.
Notch sensitivity — unfilled PSU has moderate unnotched toughness but notched Izod is only ~1.3 ft·lb/in. Sharp internal corners are crack initiation sites in impact-loaded service. PPSU (Radel R) is the impact-resistant alternative when toughness matters more than cost.
UV degradation in unstabilized PSU — yellowing within months of direct sunlight, embrittlement within a year or two. PSU is generally not specified for outdoor applications without UV stabilization or carbon filling.
Long-term creep at service-temperature upper bound — continuous service at 150–160°C under sustained mechanical load causes slow creep deformation over months to years. Design margins for sustained load near the upper service-temperature limit.
Concentrated hot caustic above 60°C eventually hydrolyzes the polysulfone linkage. Short caustic CIP cycles are fine; continuous high-pH service is not appropriate. For aggressive caustic, step up to PEEK or fluoropolymers.
Variant selection guidance
- Udel P-1700 (unfilled) — the default. Machined stock, general engineering use, semi-transparent applications.
- Udel P-1700 NT11 / Solvay Eviva (medical) — USP Class VI biocompatible. Sterilizable medical instruments, blood-contact devices. Premium pricing.
- Udel GF-130 (GF30) — structural grade. Connector housings, dimensional-critical electrical components.
- Mindel A-670 (mineral-filled blend) — dimensional stability and lower cost. Housings, structural parts requiring isotropic shrinkage.
- PSU membrane grade (Udel UV-3000P) — ultrafiltration and RO membrane fabrication. Outside the typical machined-stock workflow.
Applications by industry
- Medical and surgical (the largest PSU market) — sterilizable instrument handles, sterilization trays, surgical staplers, dental tools, dialysis components, blood-contact devices. USP Class VI medical grades dominate this space.
- Pharmaceutical processing — equipment components, fluid handling manifolds, autoclavable fixtures, sight glasses. IPA tolerance and autoclave compatibility are key drivers.
- Food and dairy processing — components in pasteurization equipment, milking machinery, dairy fluid-handling. 3-A Sanitary Standards compliance for direct food contact.
- Coffee and beverage equipment — internal components in espresso machines and commercial coffee equipment. Tolerates hot water, steam, and frequent cleaning cycles.
- Water treatment — ultrafiltration and reverse-osmosis membrane support layers (cast porous PSU membranes are a major commercial product). NSF 61 hot-water plumbing fittings.
- Laboratory equipment — autoclavable bottles, fluid handling, sample containers. Replaces glass where impact resistance matters and PC’s hydrolysis vulnerability is disqualifying.
- Plumbing and fluid handling — hot-water valve bodies, manifolds, fittings. NSF 61 compliance for drinking-water service.
- Electrical — limited compared to PEI; used in elevated- temperature electrical components where the lower dielectric strength is acceptable and the cost advantage matters.