PAEK Polymers: PEEK, PEKK, PEK and PEKEKK for Machined Parts

The polyaryletherketones (PAEK) are semi-crystalline polymers made of aromatic rings joined by ether (E) and ketone (K) links. PEEK, with two ethers per ketone, is the member almost every machined part uses. PEK, PEKK and PEKEKK carry more ketone links, which raises the glass transition and the melting point, at the cost of fewer stock forms and higher prices.

Polymer	Link sequence	Ether : ketone	Tg, °C (typical)	Melting point, °C (typical)	Status for machined parts
PEEK	E E K	2 : 1	143	343	Standard. Widest stock range, grades and documentation
PEK	E K	1 : 1	about 152	about 373	Specialty, limited stock
PEKK	E K K	1 : 2	160 to 165	305 to 360, set by the T/I ratio	Growing in aerospace and additive manufacturing; limited stock
PEKEKK	E K E K K	2 : 3	about 160 to 165	about 385	Specialty, rare as stock
Low-melt PAEK	Proprietary	varies	about 145 to 150	about 305	Composite tapes and laminates, not usual as machining stock

Typical values from resin maker data and reference sources, rounded; DSC conditions vary. PEKK values from one maker’s 6000, 7000 and 8000 series. Confirm with the grade’s datasheet.

Why the ratio matters

Ketone links make the chain stiffer; ether links make it more flexible. The more ketone links per repeat unit, the higher the glass transition (Tg) and the melting point (Tm). Processing temperatures for the family run from about 350 to 430 °C, so a higher melting member also needs hotter moulding and extrusion, which is one reason fewer stock shapes are made from it.

For a machined part the useful number is Tg. Above it, the amorphous regions soften, stiffness falls and expansion jumps; PEEK at 143 °C is the reference. PEKK and PEKEKK push Tg to about 160 to 165 °C. That margin of about 20 °C matters for a few loaded parts that run just above PEEK’s Tg. It rarely matters below that, and it never replaces a filled grade for a part that must carry load near 250 °C, where glass or carbon fibre keeps PEEK’s heat deflection at 310 to 320 °C.

PEKK and the T/I ratio

PEKK is made from terephthalic (T) and isophthalic (I) units. The ratio changes how fast and how much the polymer crystallises:

Kepstan series (example)	T/I ratio	Tg, °C	Tm, °C	Behaviour
6000	60/40	about 160	about 305	Slow crystallising, close to amorphous in fast processes
7000	70/30	about 162	about 332	Intermediate
8000	higher T content	about 165	355 to 360	Fastest crystallising, highest crystallinity of the range

As published by the maker. Other PEKK suppliers use their own grades.

The tunable melting point is why PEKK is popular in additive manufacturing and composite processing: a slower crystallising grade bonds better between layers. For machined parts the effect is indirect: PEKK stock can have a different crystallinity profile through its thickness than PEEK stock, so annealing and DSC checks follow the maker’s guidance, not PEEK habits. The head to head for machined parts is in PEEK vs PEKK.

Crystallinity

All PAEK polymers are semi-crystalline, but how much crystal forms depends on the cooling rate. Extruded PEEK stock is typically in the region of 30 to 35% crystallinity when properly processed; a thick section cooled fast at the surface can differ between skin and core. Crystallinity sets stiffness, chemical resistance, creep and dimensional stability after machining. Two practical consequences:

• Annealing after rough machining relieves stress and can complete crystallisation, which is why tight parts are roughed, annealed and finished. See annealing PEEK parts and the annealing calculator.
• Where a specification controls crystallinity, the check is DSC. The method is explained on the DSC crystallinity page.

Grades within PEEK

Inside PEEK itself, grades differ by melt viscosity (molecular weight), by filler and by documentation:

Variable	Range	Effect on machined parts
Melt viscosity	Low flow to high viscosity grades	Higher viscosity grades are tougher; stock makers usually extrude a standard viscosity
Filler	Unfilled, glass fibre, carbon fibre, PTFE and graphite blends, barium sulfate, carbon nanotubes	Sets stiffness, wear, electrical behaviour and tool wear
Colour	Natural, black, a few colours	Affects electrical data and some compliance letters
Documentation	Industrial, food contact, medical, implant	Sets the paperwork and the supply route
Classification	ASTM D8033 cell classification for PEEK moulding and extrusion materials	Lets a drawing name properties instead of a brand

The grade pages cover the main families: PEEK grades hub, with detail on unfilled, CF30, bearing grade and ESD PEEK. Full property ranges are on the PEEK data page.

Supply

PEEK resin is made by several producers in Europe, the US and Asia, including Victrex, Syensqo (the former Solvay specialty polymers business) and Evonik, plus a growing number of producers in Asia. Stock shapes are extruded or compression moulded by stock makers from these resins. PEKK resin comes from fewer producers, with Arkema among the best known, and PEK and PEKEKK are niche. For a machined part, the stock range decides as much as the polymer: a PEEK rod in the needed diameter is usually available from several distributors, a PEKK rod may not exist in that size.

When to choose each

Need	Choice
General high temperature, chemical, steam or wear part	PEEK, unfilled or filled
Part runs loaded just above 143 °C and a filled grade is not wanted	Consider PEKK, check stock
Program already uses PEKK in printed or moulded parts	PEKK for matching material
Load near 250 °C	Filled PEEK, or PAI and polyimide; see the high performance plastics comparison
Cheaper alternative below 170 °C	PEI or PPSU, also in the comparison

How the pages in this hub relate

This page maps the family. The detail pages below take one question each: how PEEK and its relatives compare with the other high-performance plastics, and later how PEKK and the filled grades behave as machined parts. Comparisons with single materials stay in the PEEK vs cluster.