Polymers, Copolymers and 3D printing Know Your Filament Part 1
Anyone with an interest in 3D printing will be well aware of the mountain of jargon, science, engineering and often down-right confusion that surrounds 3D printing filaments. If you've ever wondered what crystallinity has to do with melting temperature or what on earth a 'block-copolymer' is then fear not. In this 3 part series of short articles, we're going to lay-out exactly what goes into a filament and why it's important for 3D printing, as well as debunking some common misconceptions about filaments.
In part 1 we'll cover what polymers and copolymers are and their importance in 3D printing.
In part 2 we'll take a look at crystallinity and why it's so important to your filament.
In part 3 we'll briefly discuss what goes into a filament and how it's made.
So what exactly is a polymer?
Polymers are essential components of our lives; sugars in our food, the fuel in our car and the plastics that are so ubiquitous to our everyday, including those used in 3D printing. Polymers are essentially chains of many repeating subunits, called monomers. Monomers are joined together through a process called polymerization, which can occur through many mechanisms. The type, size and number of monomers will determine the nature and properties of the polymer. Polymers that consist of only one type of monomer are more accurately called 'homopolymers' however they are generally referred to simply as 'polymers'. Polymers with two or more monomer types are called 'co-polymers'.
Polymers can be synthetic (meaning they are synthesized from petroleum oil or its by-products) or bio-based (meaning they are based on naturally occurring substances that are extracted for human use, for example starch). Both synthetic and naturally occurring polymers are used in 3D printing filaments and each has its own advantage.
Polymers are not limited to only one type of monomer. Two or more monomer types can react together to polymerise, when this happens the product is known as a copolymer and the result is some of the most commercially important polymers. Copolymers are very commonly used in 3D printing.
Put very simply, a copolymer is merely a polymer with multiple types of monomers. There are many types of copolymer with very distinct properties based on the distribution of the different monomers.
Polymer Description Structure Homopolymer All monomers are the same type. A - A - A - A - A - A - A - A - A - A Alternating Copolymer Regularly alternating monomers. A - B - A - B - A - B - A - B - A - B Periodic Copolymers Monomer pattern that is repeated (e.g. [ABBAB]~n~. A - B - B - A - B - A - B - B - A -B Random Copolymer There is no pattern to the monomer distribution. A - B - A - A - A - B - B - A - A - B Block Copolymer The monomers are separated into distinct sub-units with distinct properties. A - A - A - A - A - B - B - B - B - B Branched Copolymers The primary chain is one type of polymer, with branches of the other type.
Many of the most familiar names in 3D printing are copolymers, such as Acrylonitrile Butadiene Styrene (ABS) and Glycol-modified Polyethylene Terephthalate (PETG). The distinct advantage of using copolymers is that it combines the desirable properties of each of the monomers, it can also be used to alleviate the undesirable properties that some monomers have. For example, Cyclohexane dimethanol is copolymerised with PET to reduce the printing temperature of PET and make easier to print with, resulting in one of the most popular polymers in 3D printing, PETG.
How are polymers made?
Polymers are made through a mechanism called polymerisation. Polymerisation is a blanket term for the process of reacting monomers together in a chemical reaction, to link them and produce a chain. The bonds that form between monomers in a polymer chain are strong covalent bonds. How a polymer is made can influence its physical properties. Whilst there are countless specific methods of polymerisation, they can usually be divided into two general families.
Addition polymerisation: Is a process of polymerisation that couples monomers that have 'multiple bonds'. Under specific conditions, often in the presence of a catalyst, the multiple bonds of the monomer open up, allowing it to chemically bond with its neighbour. Some common addition polymers found in 3D printing are polypropylene (PP), polystyrene (PS) and polyvinyl alcohol (PVA).
Condensation polymerisation: Also known as step polymerisation, is the process by which two monomers are bonded to each other via the loss of another small molecule, often water. Each monomer provides part of the small molecule and are joined together when the small molecule leaves. Common condensation polymers found in 3D printing are Polyethylene terephthalate (PET) and Nylon 6,6.
So there you have it, a (very brief) insight into polymers and copolymers. Hopefully this article has helped you to better understand the materials at the very core of 3D printing. In the next article, we will start to look at how the structure of polymers (specifically their crystallinity) affects their properties and why this is so important in 3D printing.
Some useful links and resources for further reading.