If you’re anything like this author, you got a C- in high school chemistry and never looked back. With a newfound interest in the topic, I’m hoping to revisit the molecular science of some of the most popular materials in 3D printing to understand them—not just in terms of applications and physical properties, but chemical makeup.
According to its “Polymer Additive Manufacturing Markets and Applications: 2020-2029” report, SmarTech Analysis expects polymer 3D printing to create as much as $11.7 billion in revenue in 2020, driven by sales of polymer AM hardware and all associated material families. In particular, the polyaryletherketone (PAEK) family of plastics is one of the most important in the additive space due to the high strength, temperature resistance and chemical resistance. This group includes polyether ketone ketone (PEKK) and polyether ether ketone (PEEK). PAEKs differ from their lower cost alternative, polyetherimide (PEI), which is most recognizable under the SABIC brand ULTEM and often discussed in the same context as PAEK polymers, such as the pioneering VICTREX PEEK brand.
Due to the above properties, PAEKs are well known in high performance industries, such as aerospace, oil and gas, and automotive. Their ability to undergo sterilization without breaking down makes them well suited for medical applications as well, including medical implants like spinal cages.
To learn about the chemistry of the PAEK family, we reached out to Victrex, who is a leading provider of PAEK polymers for 3D printing. John Grasmeder, chief scientist at Victrex, was able to provide us with an explanation of what gives PAEK plastics their unique physical properties at the molecular level.
“It’s not so much about the atoms themselves, which are principally carbon, hydrogen and oxygen, but the molecular structure which these atoms form. PolyArylEtherKetones’ molecular structures comprise the following building blocks:
“The aryl and ketone groups are fairly rigid and provide stiffness which means high strength combined with resistance to heat. The ether groups provide some degree of flexibility, for good toughness, and like the aryl and ketone groups are unreactive, so all three building blocks provide resistance to chemical attack,” Grasmeder said.
The number and arrangement of ether and ketones in the polymer chains of PAEK plastics (PAEK vs PEEK vs PEKK, etc.) determines the glass transition temperatures and melting point of the plastic, as well as its heat resistance and processing temperature. The higher the ratio of ketones to ether, the more rigid, thus increasing the glass transition temperature and melting point.
In particular, PAEK has a continuous operating temperature of 250 °C (482 °F) and can even handle loads for a short period of time at temperatures of over 300 °C (572 °F). When burned, PAEK puts out a low amount of heat and its fumes are the least toxic and corrosive. PAEK also has good chemical resistance. The material does not break during an unnotched Izod impact test, has a tensile strength of 85 MPa (12,300 psi), a Young’s modulus of 4,100 MPa (590,000 psi) and yield strengths of 104 MPa (15,100 psi) at 23 °C (73 °F) and 37 MPa (5,400 psi) at 160 °C (320 °F).
PEKK possesses all of the same properties as PAEK mentioned above. However, there are up to three different types of PEKK, each with different melting points and crystallinities. There is 60/40 PEKK which has a low melting point but is amorphous, an intermediate 70/30 PEKK with an intermediate melting point and some crystallinity, and an 80/20 PEKK which has a high melting point and is more semi-crystalline. Whereas one type can exhibit greater compression strength than PEEK, other varieties either have a wider processing window or can be printed at a lower temperature than PEEK with better layer adhesion.
Victrex’s PAEK for 3D printing was developed to offer a unique combination of a low melting point (like 60/40 PEKK) but with semi-crystallinity like PEEK. This combination of stiffness and flexibility locates PAEK plastics into the semi-crystalline category of thermoplastics. Grasmeder explained how these characteristics manifest in terms of physical performance. “The presence of a significant degree of crystallinity ensures that the polymers are more resistant to friction, wear, heat, creep (long-term deflection under temperature and load), fatigue (repeated application of a cyclic stress) and resistant to chemicals. Without crystallinity, these properties would be severely compromised,” Grasmeder said.
Whereas PAEK polymers are semicrystalline, PEI is amorphous, meaning that the polymer chains are random—they have no particular order or arrangement. In turn, PEI is less expensive, has a lower impact strength, and a lower usable temperature.
While PAEKs can be processed using pretty much any thermoplastics processing technology, they present a particular issue for 3D printing, which all polymers face: interlayer bonding. Due to the weakness of the bonds between layers in printed objects, the Z-axis of these parts lack the same strength exhibited in the X- and Y-axes. In turn, PEEK developed and used for injection molding can’t simply be taken and used for fused filament fabrication (FFF).
For this reason, Victrex has recently released a PAEK-based filament, VICTREX AM 200, that is meant to be “optimized specifically for AM.” Grasmeder explains that it is intended to “address this weakness in 3D printing PEEK.” For some time before the FFF material, Victrex was already well known for its development of PAEKs for use in laser sintering. Specifically, powdered PAEKs were meant to have improved thermal stability for recycling unsintered powder, to lower the costs of printing with the material by ensuring high reusability.
Due to the characteristics described, PAEK family of materials and PEI are becoming increasingly attractive alternatives to metal parts that can be potentially made at a lower cost. This is in part being driven by the third generation of FFF systems that, after the expiration of some Stratasys patents, incorporate heated build chambers and high temperature printheads necessary for 3D printing PAEK plastics and PEI. In particular, PAEK manufacturers are targeting aircraft interiors and some structural components, such as brackets, for light-weighting.
As we have noted, chemical companies are increasingly looking to take advantage of the shift from fossil fuels to renewable energy in global infrastructure, with oil companies looking to shift revenue streams to petroleum-derived plastics. However, from an ecological perspective, this doesn’t address the ecological issues associated with the extraction of the ingredients for these materials or the lifecycles of these materials. Therefore, it’s important to consider the possibilities of biopolymer alternatives.
Grasmeder said that, while there are not yet any biopolymer equivalents to PAEKs, they may exist in the future.
“At present to my knowledge there are no commercially available PAEKs which are wholly based on renewable sources but PAEKs have their own sustainable credentials in the form of lightweighting and recyclability potential in applications,” Grasmeder explained. “As an aside, the molecules used to make PAEK that are mentioned above are ultimately manufactured from basic chemical building blocks, such as benzene and toluene. There are already bio-based routes to these building blocks being developed and scaled. It’s a reasonable assumption that as the bio-based chemical industry expands, the bio-based raw materials that we need to make bio-based PAEK may become available sometime in the future.”
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