“Knitting fans”, not “knitting grandmas” since knitting as a Sunday morning relaxing hobby is going more and more trendy these days.
What is a plastic? In chemistry jargon, we call them polymers, because they are made of monomers linked together. Several monomers form a polymer. A monomer can be imagined as a thread of wool. Many threads linked together by tight knots or simply by a messy entanglement can represent a polymer.
Globally, we distinguish two types of polymers: thermoplastics, and thermosets. You can notice that we come back to the term “plastic”. But you need to be careful, here we refer to the mechanical property of plasticity. Plasticity means “that can deform irreversibly”. A thermoplastic is a polymer, or a plastic, that can deform irreversibly when temperature is applied. A thermoset is a polymer that “sets” or “consolidates”, when temperature is applied. We will come back to that type of polymer later.
Let’s take a concrete example of a thermoplastic, ABS or Acrylonitrile-butadiene-styrene. It is the most common plastic filament used in personal 3D printers. Each plastic filament consists in very long monomers that hold together through many entanglements. Like several balls of wool messed together.
How would you disentangle balls a wool? You need to loosen it so that the threads can move and separate themselves from each other. In the case of ABS it is the same principle, only you can use heat or solvents to loosen up the monomers. Indeed, heat usually gives mobility to the elements – boiling water is really not calm water), and solvents give more volume thus more space for movement too – spaghettis are straight when dry but floating and curling in water.
Heat, or melting, is the principle mostly used in personal 3D printer to render the ABS filament liquid enough to be extruded through the nozzle. And this is reversible: by cooling down – or removing the solvent- the long chains will have less volume to move and thus molten ABS will get solid again.
During this process, you can easily imagine that if you align the polymeric chains when they are in the molten state, then cool them down, you can form a consolidated ABS filament with aligned chains.
This actually can happen naturally. It is a spontaneous process which results from some chains that like better to be aligned than to be in a messy state. This is called crystallization because like in crystals, there is a well defined arrangement of the constituents. A crystal has aligned atoms, a polymer has aligned chains. Of course this alignment can be very complex but this is not the subject here.
Crystallization can happen artificially when applying a very strong voltage for example or by shearing the polymer.
On the opposite side, you can also make sure that the chains remain messy, by example by cooling down very fast. This will form an amorphous polymer, like a glass: no crystallization domains at all.
All these modifications of the internal structure modify the properties of the plastic piece. For example, a more glassy plastic will have less crack than a more crystalline one, but a more crystalline will be stronger than a plastic one. This explains why the filament of ABS might behave differently when you reuse it. Or why it can age if you leave it is a warm area. Similar to chocolate: a chocolate bar that has been heat up then cooled down is different than the initial chocolate bar (notice the white little crystals).
Now what about the other family of polymers? They are called thermosets because they do not become plastic when heat up. They don’t melt in a solvent either. This means that if you do so, you will damage them. They burn and degrade or simply go away in a solvent and you cannot reuse them, at the opposite of the ABS. So why the difference?
To make a thermoset, you also have monomers but you link them not by entanglement but by strong knots. These knots are called chemical crosslinks. This forms a network, which can be more or less tight depending on the crosslinker, like a macramé with diagonal double half hitch or a macramé with only square knots. In opposition to thermoplastics, you need at least to mix two components, the monomer, usually also called the resin, and the crosslinker, to make a thermoset.
A very common thermoset is epoxy. It can be find in any supermarket under the name of Loctite or Araldite and is mostly used as a glue, or as a paint. As you can see from the tube, it is initially a viscous liquid, but after some treatment, typically heat and time, it sets, it cures, it becomes a solid.
So thermosets set when heated up because the chemical reaction required for the crosslinking, the formation of the knots, needs a trigger, such as heat. There are other ways to trigger a reaction but heat is the easiest. To ensure that the reaction will take place quickly, some other components can be added. These are called accelerators, or catalysts, and their role is to accelerate the curing. That way, when you buy an epoxy glue that cures in 5 minutes, the composition has been optimized such as mixing and room temperature (or slightly higher since mixing will provide heat) leads to efficient fast and strong curing.
Understanding the difference between the 2 types of polymers is crucial when you consider their applications, in particular in term of life time versus recycling possibilities. It is very painful and almost impossible to unknot all the knots from a macramé bag, but it is much easier to disentangle a ball of wool. On the other hand, knots are stronger than entanglements and are more likely to be maintained for a longer time.
Of course all these considerations need to be carefully assessed for each chemistry and use. UVs are known to affect polymers irreversibly, temperature cycles too, etc.
Hopefully, once you disentangle the ball of wool, you can use it to make a macramé bag.