A brief explanation of polymers.

I’m currently a 4th year student at Monash University in Melbourne, Australia. This year im finishing off my undergrad course majoring in Materials Science and doing a minor in chemistry.

As part of one of my current units im studying adhesives, including hot melt adhesives, which is essential the basis of a reprap. So partly for my own revision and partly for anyone interested, here is a crash course in all things plastic in the most simple terms I can. If you already know a thing or two about plastics it doesn’t hurt for a refresher.

Polymers

All plastics used for reprap based extrusion consist of polymers. Polymers are repeating units of atoms that form a chain, the length of this chain is known as the polymers molecular weight. Longer chains equal larger molecular weight polymers.

A schematic representation of a single, uncross linked polymer chain.

A polymer does not have to form a straight line (think spaghetti analogy) , but can have many points along its length where it can bond with other chains to form a cross linking structure.

A schematic representation of a cross linked structure.

The length of a polymer chain, its atomic make up  and the level of cross linking can all be altered to give a huge range of different plastics. Each polymer chain also has a slight attraction to the next chain, giving it a subtle ‘stickyness’ at the atomic level.

Everything is moving: Thermal (kinetic) Vibration

If a polymer chain is above absolute zero (0 kelvin) then it can be thought of as never stationary. It is constantly vibrating and wiggling about due to thermal vibration, just as all atoms are. The level of vibration depends on the temperature, higher the temperature, the more it jumps around. Its key that when ever you imagine a a polymer, you think of it as jumping and wriggling around. If it was not for the atomic ‘stickyness’ mentioned above, then a plastic would act more like a rubber, in that it may have little load baring strength (yield strength) and can be deformed with out fracturing. This is because the polymers would be able to slide past each other but with out physically breaking the chains.

Glass transition temperature (Tg)

When you heat a plastic, the amount of thermal vibration (jumping about at the atomic level) increases. Increase it too much and it will start to overcome that weak attractions between each polymer chain (‘atomic stickyness’). This point is know as the glass transition temperature (Tg) for that particular type of plastic. At this point the plastic will lose the majority of its strength and can be deformed with out fracturing if pressure is applied. This is a perfect state for pushing the plastic through a nozzle (extruding) on a reprap device! How much pressure it takes to deform a plastic that’s above its Tg depends on the length of the polymer chain, its make up and its level of cross linking. The longer the chain and the greater the cross linking, the more pressure that will be required to extrude it. If you continue to heat a thermoplastic (see below) any  crystals in the polymer will also start to break down, allowing for the plastic to flow. *

Many thanks to nophead for pointing out a few mistakes I made in my original description of glass transition temperatures.

Thermosetting and Thermoplastics

A plastic can be one of two types, thermosetting or a thermoplastic. A thermoplastic is a plastic that can be melted, and thus, extruded by a reprap. The important thing to remember about thermoplastics is that as long as its not heated too high, the process is reversible. Some typical examples include ABS and HDPE. When heated, a thermoplastic will reach its Tg and if the right amount of pressure is supplied it will start to deform. This also means it can be recycled. Generally, thermoplastics have only moderate chain chain lengths and relatively low levels of cross-linking.

A thermosetting plastic on the other hand is of little interest to the reprap community, as once its formed (solidified), it can not be melted. However, stereolithography and other variations do use thermosetting plastics for 3D printing. An example of a thermset plastic is cured epoxy resin. **

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This is obviously just the tip of the iceberg, but if you can understand these basics then you will be able to gain a deeper understanding of why an extruder behaves the way it does for different plastics and temperatures. Obviously I’m only a student still, so if something in my simplistic description above doesn’t seen right be sure to let me know.

*An interesting side note is that rubbers are always above their Tg at room temperature and that’s why they are so elastic. If you take a slab of rubber and place it against another slab of rubber it will actually begin bind with it as the vibrating polymers intertwine at the interface. Given a long enough time and the right rubber it would bond perfectly together with no sign they were once two separate pieces.

**The reason a thermoset wont remelt is that it has a strong networked cross linked structure. It is so highly cross linked (10 to 50%) that it can resist thermal vibration up to very high temperatures. When you do heat it hot enough the strong bonds that keep the polymer chains together start to break. This leads to a break down of the plastic as a whole which is not reversed when cooled.