Crystallinity and Filaments

Know Your Filament Part 2

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 4 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 covered 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.

First up, what on Earth is crystallinity?

Crystallinity is a property of all polymers (if you don't know exactly what a polymer is, check out Part 1). Polymer chains are able to form organised 3D structures as they fold back on themselves and then stack against each other, like folded socks in a draw.

However, polymers are very rarely entirely crystalline. Much like socks in a draw, they are not always so neatly folded! Mixed in with the neatly folded polymer chains are chains that have no order or arrangement - the equivalent of socks that have been bundled up and thrown in the draw.

A polymer's degree of crystallinity is the amount of the polymer that is ordered in a crystalline structure, given as a percentage. The remainder of the polymer is the amorphous portion. It is worth noting that a perfectly crystalline plastic is all but non-existent, crystallinity generally ranges between 10% to 80%.

In the simplest terms - the more crystalline a polymer is, the more structured it is on a molecular scale.

So how does crystallinity affect a filament?

The molecular structure of a polymer is what determines many of its properties - in particular, the thermal and physical properties that are important to consider during 3D printing, such a melting temperature.

Why does order and structure (i.e. crystallinity) affect physical properties? This is due to the bonding that exists within polymers. As we mentioned in Part 1, the individual units of a polymer are linked by very strong covalent bonds. On the other hand, the bonds between separate polymer chains are relatively weak Van der Waals bonds. Van der Waals forces are short-range interactions; so the tighter the polymers can pack together, the stronger these interactions will be. The neatly folded polymers of a crystalline substance can stack against each other much better than the messy, jumbled up amorphous polymers (think back to socks in a draw).

In short: More crystallinity = More ordered structure = Closer packing of polymers = stronger forces. You can see in the figure above how introducing amorphous regions in the polymer reduces the interaction between the two molecules.

Crystallinity itself is affected primarily by the structure and regularity of the polymers. If a polymer is regular and ordered, it can pack closely against its neighbours and increase the strength of its Van der Waals forces. Additionally, some polymers are able to interact with their neighbours through other bonding types, such as Hydrogen bonds, this increases the order and therefore crystallinity of the polymer.

Crystallinity and melting temperature

One of the most profound effects of crystallinity is on the melting temperature of a polymer. Put simply, the higher the degree of crystallinity (as a percentage), the more exact the melting temperature will be. A polymer with high crystallinity will remain solid until it has reached a sufficiently high temperature, then it will rapidly turn into a liquid. This is because the bonds in the polymer are very regular so all need the same amount of thermal energy to break.

This works both directions; an amorphous polymer (with a low degree of crystallinity) will not have an exact melting temperature, instead it will have a wide range across which the material becomes soft, viscous and eventually a liquid. This temperature range is known as the glass transition temperature.

Melting point or glass transition temperature is an extremely important consideration for the polymers used in 3D printing filaments. Not only must you take into consideration the temperature you will need to set your hot-end to, you also need to consider things like environmental temperature during printing to ensure good adhesion, will you require a print enclosure and what will the final product be used for? If it goes soft over a range of temperatures will it lose its functionality?

A good example of the effect of crystallinity on melting temperature is PET vs PETG. PET has a relatively high degree of crystallinity, therefore it has quite a sharp melting point at 260°C. PETG is co-polymer of PET with significantly reduced crystallinity. PETG is considered to be amorphous and has a glass transition temperature of 230°C - 255°C.

Crystalline vs Amorphous key differences

Sharp Melting Temperature. Broad Glass Transition Temperature.
Tendency to be opaque. Tendency to be translucent.
Have fixed cleavage planes (i.e. they break along a straight, regular plane). Do not have fixed cleavage planes (i.e. they tend to break with a curved, irregular surface).
Tend to be suitable for bearing, wear and structural applications. Tend to be suitable only for structural applications.
Harder to bond well with adhesives or solvents. Easier to bond with adhesives and solvents.
Higher strength: Tend to have better tensile strength and resistance to deformation. Lower tensile strength and resistance to deformation (creep resistance).
Tend to be more brittle and less tough. Tend to be more flexible and elastic, making them tougher.

It is important to remember that in reality there is no clear division between amorphous and crystalline polymers, instead there will be higher or lower percentages of crystalline regions in the polymer, surrounded by amorphous regions. Therefore, this table is not an absolute specification, rather a generic guide through which you can better understand your filaments.

Crystallinity is an extremely important concept when it comes to 3D printing and understanding your filaments. If you can understand how the structure of your filament affects its properties, it will be far easier for you to select the perfect filament for the project you have in mind!

To make things a little easier for you, we've made this handy table that shows some common plastic types used in 3D printing, whether they're usually crystalline or amorphous divided into 3 performance grades. Some rough price ranges are given for each grade. Remember this is just a guideline, specifics will vary between manufacturers and products. It's also worth noting that some polymers, like TPE, really blur the line between amorphous and crystalline!

Always Remember: Crystallinity will vary between brands and productrs depending on resin, additives and the manufacturing process used. For example, PLA can range all the way from amorphous to very crystalline!

Useful Sources

Some useful links and sources for further reading.

Chemistry LibreTexts: 29.3: Forces Between Polymer Chains

Kavesh, S. and Schultz, J. M.: Meaning and Measurement of Crystallinity in Polymers