Updated: Aug 10, 2020
Image source: Apium, Germany
After discussing in the previous article phenomena in processing of high-end thermoplastics with a focus on PEEK & ULTEM™, let's move on to the structure-properties relationship and applications of these materials, as they are the leading polymers for industrial printers.
These polymers have unique properties and high thermal resistance, But why? What makes them superior to other polymers? Why are they different one from another? The answer lies in the molecular structure. Molecular structure and process determine the microstructure, which affects final properties.
Molecular structure - simplified
Former chemistry knowledge might help but common sense and imagination would be enough to understand the WHY. Let’s take a look on the repeating unit of these polymers compared to other common polymers that have lower transition temperatures. As shown in the left side of table 1, High end polymers have a more bulky, complexed, and rigid structure compared to the lean and relatively simple structure of the commodity & engineering polymers shown on the Right side. The unique structures enhance the unique thermal, mechanical & chemical properties.
Let’s review the unique FST retardancy properties (Flame, Smoke, Toxicity), which are sought-after in aerospace, automotive and other applications. These high-performance polymers have a combination of high mechanical properties and inherent FST properties, while other polymers require Flame-Retardant (FR) additives that usually damage mechanical properties. The presence of aromatic rings (shown in table 2) in the molecules’ backbone increase rigidity and stabilize the polymer. Upon combustion, these polymers condense into chars and reduce flammable gas release, which makes them great candidates for aerospace interiors.
Comparing commodity to high end polymers is obvious, but what are the differences within the high-end family?
It is clear (at least for us, polymer geeks), that PEI is the most rigid polymer of the three, hence, has the highest Tg (217°C ), but how can one differentiate between PEEK & PEKK?
Both PEEK & PEKK are members of the PAEK (Poly Aryl Ether Ketone) family and have Ether & Ketone functional groups. An Ether functional group (table 2 right) is more flexible than a Ketone functional group (table 2 left), which makes the PEEK molecules more mobile with a Tg of 143°C , whereas, PEKK holds a Tg of 155-160°C .
Now things get a little bit more complicated. PEI is amorphous, PEEK & PEKK are Semi-crystalline, so what promotes crystallinity? While Tg is about chain mobility, crystallinity is about folding or alignment of molecules to form a lamella. Flexibility and chain mobility of the PEEK molecules enable folding & alignment to form a crystal more easily than PEKK, whereas, PEI is so rigid and bulky that it cannot form a lamella. But don’t get me wrong here. Flexibility doesn’t always mean crystallinity. Yes, Polymers are complicated.
While having higher Tg, PEKK has lower degree of crystallinity and slower crystallization rate than PEEK, hence, a lower melting & crystallization temperatures. PEKK’s crystallinity and melting point can be tuned, so you’ll find several (not too many, only few make them) PEKK and other PAEK grades with Tm ranging between 300°C-335°C. As stated in previous articles, PEKK and other low melting PAEK’s enable lower processing temperatures while overcoming the challenges that the fast crystallization of PEEK poses.
Crystallinity - more than just mechanical performance
High performance semi-crystalline polymers are usually associated with high mechanical properties at elevated temperature even above Tg. Higher crystallinity enhances higher thermal stability, stiffness and strength, but also decrease toughness and impact resistance, but let’s discuss another property enhanced by crystallinity- Low permeation of fluids.
Permeation is derived from two main constituents, solubility and diffusion. Solubility is related to the absorption of the fluid by the polymer, whereas, diffusion is related to the ability of smaller molecules to penetrate between the polymer molecules. Crystalline domains are tightly packed, so it is hard for smaller molecules, gas or liquids, to diffuse through and weaken the intermolecular bonds. For that reason (and more..), PEEK, as a semi-crystalline polymer, would be a good choice for gas & oil applications.
Applications of high-end polymers in 3D printing
Reviewing the structure -properties relationship helps to understand where these polymers can be implemented. The combination of AM advantages, such as design flexibility, together with high performance materials, creates new opportunities. Here are some examples:
Replacing metal components
The high thermal resistance, mechanical properties and good chemical resistance together with the design flexibility of additive manufacturing, make these polymers a suitable replacement for metal parts in automotive and aerospace industries. These polymers are often reinforced with carbon fibers (chopped or continuous) to increase strength and reduce weight.
Tooling & molds
Additive manufacturing with high end polymers enables development of tool inserts for injection molding due to high thermal stability and mechanical properties. This enables cost-effective and faster experimental design iterations.
Additive manufacturing enables tailored implants to patients. Replacing metal with bio-compatible PEEK enables CT & MRI scanning, less heating & cooling due low thermal conductivity and less damage to neighboring tissues.
My following article in this series will discuss the warpage mechanisms in additive manufacturing of semi-crystalline polymers and the available solutions.
* ULTEM™ is a brand name of SABIC