Overview
PEKK (polyetherketoneketone) is PEEK’s tunable cousin. Same PAEK backbone, similar chemical resistance, similar electrical insulation, similar maximum service temperature (~250–260°C continuous). What’s different is crystallization behavior — PEKK’s terephthaloyl/isophthaloyl ratio is a knob that polymer chemists at Arkema can turn to dial in fast, medium, or slow crystallization for a specific manufacturing process.
That tunability is the reason PEKK exists as a commercial alternative to PEEK. It pays off in three places where PEEK’s fast, sharp crystallization is a manufacturing headache:
- Thermoforming — PEEK has almost no usable rubbery plateau, making sheet sag unpredictable. PEKK 7000-series’s slower crystallization gives thermoformers a real processing window.
- FDM additive manufacturing — PEEK’s fast crystallization causes delamination, warping, and shrinkage between layers. PEKK 6000-series prints pseudo-amorphous, with layer adhesion and dimensional control closer to high-temp engineering plastics than to PEEK.
- Continuous-fiber composites — PEKK 7000-series matrix consolidates with prepreg fiber under more forgiving conditions than PEEK, enabling larger and more complex CF/PEKK composite structures.
Outside those processing-driven advantages, PEEK is generally cheaper, more available, and supported by a broader range of suppliers. The selection logic: pick PEEK by default; switch to PEKK when the process — additive, thermoforming, or continuous-fiber consolidation — specifically rewards PEKK’s tunable crystallization.
The T/I ratio — Kepstan’s defining variable
Arkema’s published Kepstan grade architecture:
| Series | T/I Ratio | Melt °C | Tg °C | Crystallization | Primary Process |
|---|---|---|---|---|---|
| 8000 | 80/20 | 358 | 165 | Fast / semi-crystalline | Injection molding, extrusion |
| 7000 | 70/30 | 332 | 162 | Medium | Composites, thermoforming, film |
| 6000 | 60/40 | 305 | 160 | Slow / pseudo-amorphous | FDM/AM, powder coating |
The higher the T (terephthaloyl) fraction, the faster and more complete the crystallization. 8000-series PEKK crystallizes nearly to completion at typical injection-mold cooling rates; 6000-series remains effectively amorphous through most processing conditions and requires an explicit annealing step to develop crystallinity.
Practical consequence: Tg-limited applications are the principal service ceiling for 6000-series parts that haven’t been annealed. A 6000-series FDM part run at 200°C continuous will creep and soften unless it’s been annealed (typically 1–2 hours at 200°C) to develop crystallinity. Annealed properly, the same part is good to 250°C.
Machining notes
PEKK machines similarly to PEEK with a few real differences:
- Slightly lower cutting speed (250–450 SFM vs PEEK’s 300–500 SFM)
- More sensitive to dull tooling — surface defects appear sooner as the edge wears
- 8000-series machines harder (more crystalline) than 7000-series at room temperature; 6000-series amorphous stock machines softer but is dimensionally less stable
- Stress-relief anneal at 180–200°C for 1 hr/inch after heavy roughing is more important than for PEEK
PCD tooling is mandatory for CF-filled PEKK in production runs. Carbide works for prototype quantities at much-reduced tool life.
Stock availability is the real machining constraint: rod and plate PEKK shapes are less broadly distributed than PEEK. Drake Plastics and a handful of specialty distributors handle the US market. Lead times can run 6–12 weeks for non-stocked sections.
Variant guidance — which Kepstan grade
- Kepstan 8000 series (8002/8003) — pick for injection-molded parts where high-temperature mechanical performance is the requirement and cycle time matters. Closest behavior to PEEK; effectively a PEEK drop-in for IM.
- Kepstan 7000 series (7001/7002/7003) — pick for thermoforming, continuous-fiber composite consolidation, or any process that needs a wider crystallization window than PEEK provides. The aerospace composites grade.
- Kepstan 6000 series (6002/6003) — pick for FDM additive manufacturing, powder coating, or applications where amorphous PEKK’s transparency and ease of processing matters more than maximum thermal performance. Plan to anneal if service temperature approaches Tg.
- CF/PEKK (continuous-fiber tape and short-fiber compounds) — pick for primary or secondary aerospace structure, replacing carbon/epoxy with thermoplastic-matrix composite that can be reshaped, welded, or recycled.
- Oxford OXPEKK / OsteoFab medical grade — pick for FDA-cleared patient-specific cranial and maxillofacial implants. Established pathway since 2013; cost premium is significant.
Process advantages — where PEKK earns its premium
FDM and high-temperature additive manufacturing is the strongest PEKK story. The PAEK family’s chemical resistance and 250°C continuous service are unmatched among FDM-printable polymers — and PEEK FDM is notoriously difficult (see PEEK file). PEKK 6000-series prints with significantly fewer warping and layer-adhesion problems because the amorphous state during printing avoids the rapid crystallization shrinkage that wrecks PEEK prints. Annealing recovers most of the mechanical and thermal performance after printing.
Continuous-fiber thermoplastic composites is the second major PEKK win. CF/PEKK unidirectional tape (Kepstan 7000-series matrix) consolidates at lower temperatures and with better interlayer adhesion than CF/PEEK, enabling larger and more complex aerospace structural parts. Airbus A350 and several US defense programs incorporate CF/PEKK.
Aircraft interior FST compliance (FAR 25.853) is met natively by unfilled PEKK — flame, smoke, and toxicity performance is among the best of any thermoplastic. Aircraft interior ducts, brackets, and non-structural panels are the volume application.
Failure modes worth designing around
Crystallization mismatch is the dominant in-service issue. An FDM or thermoformed part that hasn’t been annealed to develop intended crystallinity will creep, soften, and fail above Tg (~160°C). Always verify cure/anneal state for parts running above 150°C.
Grade ambiguity is the procurement trap. “PEKK” without grade qualification means almost nothing — 6000, 7000, and 8000 series behave differently in processing, mechanical, and thermal performance. Specify Kepstan grade (or supplier-specific equivalent) on drawings and POs.
Chemical resistance is essentially PEEK-equivalent — concentrated sulfuric and nitric at elevated temperature attack both. Below those extremes, both materials handle nearly every industrial chemistry.
UV degradation on unfilled grades — surface yellowing and embrittlement on continuous outdoor exposure beyond ~1 year, same as PEEK. Carbon-filled or pigmented for outdoor service.
Applications by industry
- Aerospace structural composites — CF/PEKK unidirectional tape for fuselage skins, wing components, and access panels. The leading thermoplastic composite matrix for primary structure on commercial aircraft programs (A350, F-35 components).
- Aircraft interior — ducts, brackets, panel components meeting FAR 25.853 FST without halogenated flame retardants. Lighter than metal, processable as thermoformed sheet.
- Additive manufacturing — Oxford Performance Materials and several industrial FDM vendors use PEKK 6000-series as their preferred high-temperature feedstock. FAA-certified printed brackets and ducts installed on commercial aircraft.
- Medical — Oxford OXFAB / OsteoFab cranial and maxillofacial implants. Patient-specific 3D-printed PEKK skull plates have been in clinical use since 2013. Lower-density radio-translucent alternative to titanium.
- Oil and gas — downhole sensor housings, electrical connectors in HPHT (high-pressure, high-temperature) wells. PEKK’s superior CO₂ and H₂S barrier vs PEEK is a real advantage in sour-service wells.
- Electronics — high-temperature connectors and insulators where FST compliance and chemical resistance matter more than cost.