Have you ever wondered what the numbers after various grades of nylon mean? You know, 6 and 66 and 612 etc. I am going to explain this to you along with a few other interesting tidbits about nylon that you might not have known.
First of all, the chemical name for nylon is polyamide. For the interesting history of how polyamide started being called nylon, see my blog post on the subject here. Polyamide is a large molecule made up of repeating molecular units that link end to end like all thermoplastics. The amide refers to a specific kind of molecular bond referred to as an amide bond. The details of how this bond works is beyond the scope of this article. If you want to read more about it, you can start here. Suffice it to say that the amide bond gives the material some special properties such as great tensile strength.
Starting in the 1930s, when scientists at DuPont and at BASF in Germany started messing around with amide bonds, they discovered that there were several different ways of using amide bonds to make polymers. The polymers had similar properties with some distinct differences. The scientists called of these materials polyamides but there needed to be a way to distinguish between the different versions. The person that actually came up with the numbering system for polyamides seems to have been lost to history. In all likelihood, it was the folks over at DuPont led by Wallace Carothers who is credited with inventing nylon.
The numbering system works like this: Some versions of polyamide are made using one monomer which after polymerization forms into repeating units like this:
The dashes represent the amide bond. The “A” represents the repeating molecule. Each of the repeating units has 6 carbon atoms. This material became known as PA6 or nylon 6
The other version (the one that DuPont invented) had two monomers which after polymerization formed into repeating units like this:
The dashes again represent the amide bond. In each repeating unit, the A has 6 carbon atoms and the B also has 6 carbon atoms. This material became PA66 or nylon 66.
As a side note, it is not correct to express nylon 66 or PA66 with a slash in it like this: Nylon 6/6. It can be expressed as either Nylon 66 or PA66 or Nylon 6,6 or PA6,6. The slashes are only used when describing copolymers of nylon.
Here are some more varieties:
Again, the dashes represent the amide bond. In this case the C represents a monomer repeating unit that has 12 carbon atoms. This is PA12 or nylon 12. By the way, nylon 12 is the only nylon that can be had in clear.
Now this one:
This is nylon 612. The A repeating unit has 6 carbon atoms and the C repeating unit has 12.
Over the years, a number of different polyamides have been developed, some with one monomer and some with two. They each have slightly different properties. Here is a list of the different types of polyamide that are currently on the market:
Now, to make this even more confusing, some of these materials can be polymerized together forming copolymers. This is where the slash is used to separate the two monomers.
PA 6/66 (sometimes expressed 66/6 instead) This is nylon 6 and 66 polymerized together.
Unfortunately, it doesn’t end there. There is also something called semi-aromatic polyamide. You don’t see these much but aromatic molecules are designated by letters added to the numbers. Here are just a few:
There are dozen or so others. This seems to be a big area for research as new varieties are always coming out. Suffice it to say that if you see letters after the numbers, you are dealing with a different animal than normal nylon. These materials are very high heat and often have to be processed north of 700 degrees F. They are also very pricey and are often filled with PTFE and carbon fiber. I will write a future post on semi aromatic polyamides.
I hope this clear this up a little. Right now, an organic chemist is laughing his or her ass of while reading this. I have greatly simplified the explanation here as I always do. This blog is not for organic chemists, it is for the layperson in the industry.
The really interesting thing about all of these different types of nylon is that they have a wide variety of properties but yet they are all similar. If you burn a little piece of nylon, it has the same smell whether it is 6 66 or 12 or any other version that matter. They all also have remarkably high tensile strength. However, things like melting point and heat resistance can vary a lot.