Additives & Composites (Vol. 2)

Additives and composites bring even more diversity on a market already full of polymers. Yet these remain simple concepts accessible to anyone.

Polymers: Additives and composites

Have you heard of conductive filament ? Copper-filled plastic ? How can PLA be sometimes transparent and sometimes of a deep dark blue ? All of these filaments contain additives, that’s what makes them special.
Let’s dive into two related concepts: additives and composites.

How to process a polymer + additive ?

Polymers which contain additives can be processed via extrusion just like “normal” polymers, with minor differences (you can find an introduction to extrusion here. In a nutshell, extrusion is the process that transforms pellets or powders into filament or other “continuous items”. It is possible to incorporate fibers and powders in the mix in order to create composites. A simple rule of thumb is that a formulation which contains additives requires more mixing when being processed, whereas “pure” homogeneous polymer only needs heat to melt. It is possible to integrate mixing mechanisms in the design of an extruder and keep the process simple. A straightforward method is to equip the extruder with a special screw that contains a dedicated mixing section.

In both the cases of 3D printing and filament extrusion, the size of solid reinforcements is limited, mainly by the opening of the nozzle of the machines (approximately 0.4mm for standard 3D printers). In reality, this is not a major limitation: indeed, additives of small size can be distributed more homogeneously in the matter and are therefore preferred than larger additives. Fibers can be quite long but thin, therefore not causing clogging issues.

How to deal with colorants ?

It is surprisingly easy to customize the visual aspect of a 3D printing filament using colorants. For a given polymer, different masterbatches are usually available on the market. A masterbatch is made of the same polymer, but is also carries pigments of a certain color. It is then possible to add a certain percentage of masterbatch in the basic formulation, and extrude colorful filament using a mixing extruder. Depending on the intensity of the pigment, less than 5% are typically required to give the chosen color to the whole filament. One might even find it enjoyable to search for the perfect tone by mixing different masterbatches in different proportions, or to create rainbow filament. The usage of masterbatches may not be the most appropriate method for mass-production of colored spools, because it is not the most reliable process when it comes to replicating precisely the exact same color tone in dozens of spools. It is however the preferred method to experiment with colorants and create smaller batches of custom filament.

Recycling additives

When talking about composites in general, the main concern is that they are manufactured specifically with a certain infill and a set shape. It is then impossible to separate the reinforcement from the matrix. As far as 3D printing is concerned however, because the additives have to be small in size and well dispersed in the plastic, they can also be recycled with it.
The process is rather straightforward. The first step is to shred filled polymers, typically 3D printing waste. Instead of obtaining pellets or powders, the shredding step gives a regrind. When the particles become more or less as small as pellets (a few millimeters), they can simply be extruded again, almost like virgin plastic, while still containing the fillers. Certain fillers such as carbon nanotubes being rather expensive, recycling them becomes very advantageous.

You can find a case study about the making of colored PLA filament here.