As a standalone material with no filler reinforcements, PEEK (PolyEtherEtherKetone) comfortably holds its own as one of the most robust polymers around. With the exception of Polyimide (aka Kapton), PEEK has no viable rivals on pure strength and high-temperature capabilities. A specific gravity of only 1.3 makes it a darling of the aerospace industry, while a chemical inertness that nearly matches that of PTFE makes it highly sought after in nuclear applications and refineries.
That said, there is always room for improvement. The same fillers that give materials such as PTFE or PPS enhanced properties also work well on PEEK. As such, with the aim being to maximise strength and durability, carbon is a key filler for PEEK.
Carbon (Coke) Powder vs Carbon Fibre
Before we proceed, a distinction must first be made between powdered carbon – which is effectively finely ground coal, and carbon fibre – which is a result of milling more complex carbon materials to yield miniscule, high-strength strands that go on to reinforce the polymer into which they are blended. While carbon fibre is far more expensive than conventional carbon powder, its impact too exceeds that of its poorer cousin by a significant margin.
Thus, when we refer to carbon filled PEEK, it should be understood here that we are speaking specifically about carbon fibre.
PEEK, PEEK+Carbon, and HPV PEEK
In this article, we will primarily be comparing three different grades.
Unfilled PEEK is the polymer in its virgin form. As stated above, even in this form – where the PEEK takes an even tan colour – the material is robust and capable of withstanding high loads and temperature.
PEEK+Carbon or Carbon-filled-PEEK is usually a blend of PEEK and either 15% or 30% carbon. For the purpose of this comparison, we will be looking at PEEK+30% carbon (CF30), as we have accurate data for the same.
PEEK HPV material is a special blend of PEEK popularised by the brand Ketron (Quadrant). HPV PEEK was designed to have a composition of 70% PEEK, 10% carbon, 10% graphite, and 10% PTFE. This unique blend offers a large boost to the strength of the base material – thanks to the carbon reinforcement – while also bringing better wear and friction properties due to the graphite and PTFE. HPV PEEK – also referred to as bearing grade PEEK – is both difficult to blend and prohibitively expensive in comparison to regular PEEK. However, given its fantastic properties, most OEMs are happy to pay the price.
Comparing Properties
As you can see from the table below, the addition of carbon and other fillers has an appreciable impact on the properties of the material.
Apart from an increase in tensile strength of ~35%-207% for HPV and CF30 respectively, there are increases of anywhere from over 300% to over 500% on other metrics such as Young’s Modulus, Flexural Modulus, and Flexural Strength.
In addition to pure strength, what can also be observed are lower coefficients of thermal expansion and higher deflection temperatures under load – both indicative of a more dimensionally stable material in high-temperature applications.
HPV – which has a lower coefficient of friction and can go as low as 0.05 under lubricated conditions – is ideal for wear applications and dry-running applications.
|
|
Unfilled PEEK |
PEEK+30% Carbon |
PEEK HPV |
Unit |
Test |
|
Tensile Strength |
97 |
201 |
133 |
Mpa |
ASTM D638 |
|
Young's Modulus |
3650 |
19700 |
11000 |
Mpa |
ASTM D638 |
|
Flexural Modulus |
3860 |
17500 |
10500 |
Mpa |
ASTM D790 |
|
Flexural Strength |
152 |
317 |
221 |
Mpa |
ASTM D790 |
|
Coefficient of Linear Thermal Expansion |
4.3 x 10-5 |
5.2 x 10-6 |
2.2 x 10-5 |
cm/cm/°C |
ASTM E831 |
|
Deflection Temperature Under Load |
162 |
315 |
291 |
°C |
ASTM D648 |
|
Coefficient of Friction |
0.35 |
- |
0.25 |
|
ASTM D3702 |
Machining properties
As manufactures of high-precision machined components, a lot of our experience ultimately ends with understanding how well the part adheres to close dimensional tolerances.
Both bearing grade PEEK and Carbon reinforced PEEK are highly stable both during and post-machining. While special tools are needed to handle the material, tolerances of as low as 10 microns can be achieved on the machined parts. Further, the low coefficients of expansion allow for parts that are machined in one part of the world to travel elsewhere with no loss of dimensional integrity due to changing climates and environments.
Conclusion
While Carbon filled PEEK (CF30) is clearly the winner on pure strength, bearing grade PEEK (HPV) forms a decent middle ground between PEEK and CF30 and is an excellent choice for bearing applications. While it has traditionally been an expensive grade, in-house blending techniques have allowed for significant cost optimisations, allowing the price to rest only slightly above that of unfilled PEEK.
Whatever the requirement, it is evident that any application where strength is a key criteria would benefit from the reinforcement of PEEK with carbon.
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