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.
So far in this series, we’ve covered the PAEK family of plastics and a specific type of polyurethane from Carbon. Here we look at the most ubiquitous plastic in powder bed fusion (PBF): polyamide (PA). Also called nylon, the material is known for its silky quality, generally strong physical performance, wide range of operating temperature, and its abrasion resistance.
The first commercially successful synthetic thermoplastic polymer, nylon was developed first by DuPont in 1935 in the form of nylon 6,6. Soon, a variation called nylon 6 was invented in 1938 by IG Farben, the industrial giant made notorious for its manufacture of the Zyklon B that fueled the Nazi gas chambers, but now less recognizable as the European companies BASF, Bayer, Sanofi and Agfa. The plastic first hit the commercial market in the form of toothbrush bristles and nylon stocks and then became a key ingredient to making parachutes and parachute cord.
To learn more about this plastic and its role in the 3D printing industry, we reached out to Sylvia Monsheimer, head of New 3D Printing Technologies at Evonik. Evonik, which recently launched a new brand name called INFINAM to encompass all of its 3D printing products, is the largest maker of Nylon 12 in the AM market according to SmarTech Analysis’s latest report, Polymer Additive Manufacturing Markets and Applications: 2020-2029.
Nylons were developed to replace silk, an organic form of polyamide. The key to replicating this material is the amide groups that link together repeating monomer units that include carbons. The number of carbons in each unit dictates the formula’s name, with nylon 6 featuring six carbons in each unit and nylon 12 featuring 12. The numbers can also clue you into how the materials’ physical characteristics.
“The smaller the number in the name, the stiffer the material and the higher the crystallinity and the melting point, due to the carbonamide groups and hydrogen bridges which act as a link between the polymer chains,” Monsheimer explained.
She further described how the longer the carbonamide chains, the more ductile the material becomes:
“The carbonamide groups and the hydrogen bridges associated therewith are links between the polymer chains and allow crystalline structures. They also play a role in the stiffness and ductility of the material. Polyamide 12 has carbonamide groups at every 11th methylene group, the other polyamides accordingly to the number in their names. For PA12, the distance between these links is larger and the number is smaller, so the ductility is higher than for other polyamides with smaller numbers in their names. The links are also responsible for good thermal and chemical resistance.”
Forms of polyamide with two different numbers, such as nylon 6,6, are made up of two different types of monomer units featuring that number of carbons. Nylon 6,6 is composed of hexamethylenediamine units linked to adipic acid units, with six carbons in each. Monsheimer put it this way:
“If the name contains one number, the polymer is made from one building block with two different end groups each. The different end groups like each other and combine to form a longer chain. The polyamides with two numbers in the name are made from two different building blocks, the first containing two identical end groups and the other also containing identical end groups but of a different type from the first. Again, the different end groups like each other and combine, but that leads to a polymer chain made from the two, alternating building blocks alternating. You can easily imagine that crystalline structures differ between the different macro molecules.”
The end groups also play an important role in the way that polyamides are particularly suited for working with additives, according to Monsheimer.
“Each polyamide chain comprises two different end groups, which, in principle, can react with other additives. So, polyamides can be better functionalized than other polymers, which makes them well-suited for modifications and the introduction of new properties. However, many of the typical additives, like stabilizers, lubricants and flowing agents, typically do not react chemically with the polymers they modify.”
The semi-crystalline nature of the material is also important, as Nylon offers a temperature range, above the glass transition temperature but below its melting point, where the mechanical properties are ductile and stable. This behavior is something that is highly desirable in a number of applications. This also impacts what type of 3D printing technology is best suited to handle nylon.
While we do see nylon a great deal in fused deposition modeling (FDM) and similar extrusion 3D printing technologies, it is definitely the workhorse polymer of PBF processes. Monsheimer explained that this is due to the fact that semi-crystalline materials are better suited to PBF as a whole:
“PBF technologies usually work without shear and pressure (compared to injection molding) and, to achieve dense parts, the polymer has to show a very good melt flow above its melting point (which is the melting point of the crystalline areas in this case). That is a property offered by semi-crystalline materials. Moreover, these materials can easily be finetuned by adjusting the molecular weight and functionality for PBF applications. The flow is good because it is far above the glass transition temperature. On the other hand, the majority of PBF machines has a limit for build chamber heating, which limits the choice of materials.”
In contrast, amorphous polymers are better suited to FDM.
“An amorphous material is much more viscous above, but close to, its melting point (and, here, the melting point is the glass transition temperature and the proximity to this leads to high viscosity). This makes it flow too poorly, so that this class of material is rarely capable of building parts with good mechanical properties in powder based AM processes,” Monsheimer said.
As noted in our previous entries in this series, it is quite apparent that we need to shift from a fossil fuel-based society to one that relies on renewable resources. A part of that shift will naturally include polymers derived sustainably. According to Monsheimer, there is still significant work that needs to be done to develop polymers for PBF processes that are not manufactured from petroleum.
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