PEEK vs PEKK, what is the difference?

FFF of PAEK polymers

What is the difference between PEEK and PEKK? Both are semi-crystalline thermoplastics, but their degree of crystallinity highly depends on the way they are processed.

Some of the polymers that belong to the PAEK family (polyaryletherketones), have shown to be 3D printable with FFF (Fused Filament Fabrication) technology. The most popular are PEEK and PEKK (Polyetherketoneketone), because they both show extraordinary properties in terms of mechanical, thermal, and chemical resistance.

Should you print PEEK or PEKK? To understand it, it is better to take a step back and explain the differences between semi-crystalline and amorphous materials first.

Differences between semi-crystalline and amorphous polymers

According to the arrangement of macromolecules in space, two main categories of polymers can be distinguished:

• semi-crystalline;
• amorphous

The main differences between semi-crystalline and amorphous polymers are firstly the arrangement of the molecular chains and, consequently, how that affects the behaviour of the polymer under heat:

• Amorphous polymers have disorganized molecular chains intertwisted and randomly oriented. This property leads to having a range of temperatures at which they will melt, therefore not precisely defined temperatures. The disordered arrangement of the polymer chains does not induce preferential behaviors in particular directions, thus leading to a more isotropic behavior. In general, amorphous materials look translucent and they have lower fatigue and stress resistance but better impact resistance.
  
  Some examples of polymers with an amorphous morphology are:
  - polycarbonate (PC)
  - polyetherimide (PEI or Ultem®)
  - acrylonitrile butadiene styrene (ABS)
  - polystyrene (PS)
  - polysulfone (PSF)

• On the contrary, the semicrystalline have an ordered molecular structure with a precise melting temperature. When exposed to a rise in temperature, the crystalline domains give the material greater stiffness - even at temperatures above the glass transition - thus allowing the polymer to maintain its mechanical properties over a wider temperature range than its amorphous counterpart. The extrusion process through a nozzle generates a preferential orientation of the macromolecules in a direction parallel to the extrusion flow, thus making the material tend to be anisotropic. This anisotropy can be observed by analyzing the shrinkage of the material. In the direction parallel to the flow, this shrinkage will be more than in the transverse direction.

  Due to their shrinkage that leads to lower dimensional stability, semi-crystalline materials introduce greater difficulties in production processes.

  Among the semi-crystalline polymers there are:
  - polyamides (PA)
  - polyethylene (PE)
  - polyacetal (POM)
  - polyethylene terephthalate (PET).

When reinforced, both amorphous and semi-crystalline materials show considerable improvements in strength and stiffness way beyond their glass transition temperature (Tg), due to the higher thermal deflection temperature (HDT).

3D Printing PEEK for Fused filament fabrication (FFF) technology

Fused filament fabrication (FFF) is an Additive Manufacturing technology that extrudes continuous filaments of thermoplastic polymers to 3D print parts. 
Spools of filament act as a feed for the extrusion head. The filament, passing through the hot end, is heated beyond the melting temperature and subsequently extruded through the nozzle. The head moves on the X and Y directions to deposit the filament, while the build plate moves on the vertical Z axis when a new layer needs to be extruded.

To print any filament, it is necessary to have an extrusion temperature above the melting point (Tm). By heating the working chamber to temperatures above the glass transition, molecular mobility is increased, thus favoring the diffusion of macromolecules at the interface between adjacent layers and stimulating coalescence between the strands, thus maximizing adhesion between the layers. Since these values are quite high with PEEK (Tm=338°C, Tg=146°C), it needs particular attention: good print quality and repeatability cannot be achieved with hobbyist printers. 
Roboze PEEK filament is printed with a very hot extruder head, up to 450°C and a heated chamber that can heat up to 180°C.

Having a temperature-controlled environment - i.e. a heated chamber - is necessary otherwise the PEEK will cool too quickly, thus inducing deformation phenomena induced by shrinkage and inhibiting the formation of crystalline domains. To increase the percentage of crystallinity in the workpiece, PEEK can be annealed. This is a heat treatment in which the material is heated to high temperatures (generally around 200 ° C) for a couple of hours, depending on the geometry, and then cooled very slowly to minimize residual stresses.

Advantages of Fused filament fabrication (FFF) technology

The heated chamber is fundamental to ensuring an even cooling rate across the part that leads to full crystallization and reduction of residual stresses and shrinkage. This is what happens with PEEK on board of Roboze Argo printers. 3D printing PEEK with FFF (Fused Filament Fabrication) technology has many advantages: here’s 8 reasons why you should 3D print PEEK.

Differences between 3D printing PEEK and PEKK

Looking at the backbone of the two polymers, one can notice that, compared to PEEK, PEKK has an additional ketone group that replaces one ether group. This ketone group increases the glass transition temperature because it is a stiffer bond than the ether linkage. 
The main difference between the 3D prints of the two main members of the PAEK family, PEKK (Polyetherketoneketone) and PEEK, is in the degree of crystallinity. Since PEKK has a lower crystallisation rate, it can be often treated as an amorphous polymer, depending on the production process.

PEEK, on the other hand, has a much higher crystallization rate, which allows, under suitable conditions, to obtain crystalline components through additive manufacturing processes. The difference can be seen with the naked eye: crystalline PEEK has a dull beige color while amorphous PEEK has a more translucent brown color.

Now that we know what PEEK is, we can also add that:

While PEEK can reach a very high level of crystallinity (up to 40%), PEKK can be commonly found as either amorphous or semi-crystalline. According to the information gathered on the market, PEKK is amorphous when 3D printed.

One can notice the difference with the naked eye: crystalline materials look opaque while amorphous ones look translucent.

• 3D printed PEKK: While semi-crystalline materials have higher thermal and chemical resistance properties, a temperature-controlled production environment that minimizes the shock terms that arise during solidification, thus allowing slow cooling of the polymer melt, PEKK is commonly printed with lower chamber temperatures in its amorphous morphology because, due to its slow crystallization rate, below the cooling rate, it does not have enough time to crystallize during the printing process. The material morphology also affects the printing behavior. For instance, untreated amorphous PEKK has a use temperature of about 150°C and it is chemically inert to a smaller range of chemicals compared to PEEK. Its semi-crystalline version would have better characteristics but requires extensive post-processing which adds complexity to the production steps;
• 3D printed PEEK: after PEEK is melted and extruded, it starts to cool down, forming crystalline parts in the deposited plastic. At a microscopic level, the formation of these ordered domains, as opposed to the amorphous parts in which the molecules arrange themselves randomly, leads to a shrinking rate of up to 2% that makes printing difficult. This difficulty occurs during the solidification phase of the material in which the different shrinkage between semi-crystalline and amorphous areas causes deformation and detachment of the piece from the printing platform.

Although PEKK belongs to the same family of polymers, it is easier to print because it is a more amorphous material, so the shrinking effect is limited to about 0.01%. However, the processing conditions used to produce PEEK or PEKK parts can influence their crystallinity, hence their properties. In general, more crystalline materials, like PEEK, have better mechanical, chemical, and thermal properties compared to amorphous materials like 3D printed PEKK. Indeed, PEEK has obtained NORSOK M-710 certification for the Oil & Gas industry, thanks to its very high chemical resistance. Unlike amorphous 3D printed PEKK that reaches a continuous use temperature (CUT) of 150°C when untreated, PEEK’s CUT is much higher, 260°C, due to its crystalline nature.

Despite printing PEEK might be generally considered difficult, it gets easy if you have Roboze Argo production systems. Featured with the heated chamber that guarantees an homogeneous thermal flow and the HVP extruder that is in charge of the deposition of such viscous filament, Argo printers handle PEEK’s shrinkage excellently, ensuring a positioning accuracy of 10 μm. The degree of crystallinity of PEEK achieved with Argo printers is very high, ensuring the best material quality in terms of mechanical, chemical and thermal properties. 

Both PEEK and PEKK have their pros and cons and whether one or the other should be used, must be determined by the user and the application.

Table: 3D printed PEEK vs 3D printed PEKK, as built.

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