Introduction to polymers
The term “polymer” refers to a rather simple concept, which can be easily explained with a few drawings and simplified chemistry.
Families of materials
Polymers, or plastics (both words can be used as synonyms) are one of the 5 main families of materials:
- Ceramics and glasses
- Composites (usually a combination of a polymer and an additive from another family)
Materials from the same family have a similar behavior. For example, metals and ceramics are much more resistant thermally and mechanically than polymers, but also a lot harder to process. In most cases, polymers are rather inexpensive and easy-to-process, lightweight materials used for a variety of commodity applications and some high performance applications.
From a simple chemical perspective
Why is ABS softer than steel ? The physical (mechanical, thermal) behavior of polymers differentiates them from the other families and comes from their unique chemical nature. In other terms, their macroscopic properties depend on their microscopic structure. Understanding the macro/micro links is the key to predict and control polymers and all materials.
Plastics are considered organic matter, which means that the majority of their mass is carbon-based. At a microscopic level, unlike other materials, polymers consist of large groups of atoms called macromolecules. What differentiates polymers from “normal” matter is that they are made of molecules of great size called macromolecules, instead of much smaller molecules or even atoms.
Thinking of plastics as solids made of long molecules called “chains”, can often simplify scientific explanations. Let us have : long molecules sliding against one another like a fluid.
For instance, the fact that the molecules are mostly carbon-based justifies the light weight of plastics, and their great size is the source of their durability. While remaining chemically similar, polymers have an incredible diversity for a limitless range of applications.
Categories of polymers for different applications
The family of polymers can be divided in subcategories. A commonly used method is to classify plastics depending on their application level:
- commodity plastics for basic applications, like PET used for bottles
- engineering plastics for specific or slightly unusual cases, like TPU used for cable jacketing
- high performance plastics which offer a wide range of outstanding properties, including structural applications above 200°C with PEEK
The Pyramid of Polymers shows with simplicity what the different tiers of plastics are, with the most common examples.
High performance polymers obviously tend to be more expensive than the lower tiers of plastics, but they are not the only solution. Of course the best material varies depending on the constraints that come with a specific application. Surprisingly performant materials can be discovered by mixing certain polymers with certain additives, like carbon fibers.
Working with plastics
Most polymers are thermoplastics: polymers capable of being melted/solidified via heating/cooling. Melting a polymer means heating it until its macromolecular chains can move freely, this polymer can then be given a new shape. That is exactly what extrusion, injection molding, and 3D printing do: melting the polymer with heat and giving it a new shape (then cooling it down, of course).
Filament extrusion and 3D printing go very well together: the extrusion step reshapes granules or powder into filament, the 3D printing step then reshapes the filament into any other item.
If you want to dig deeper into the extrusion process, you can find an article about how it works here.
If you want to learn how a plastic can be melted and remelted again, you can find a recycling study here.