Additives & Composites (Vol. 1)

The world of additives and composites is even broader than the world of polymers.

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.

What is an additive ?

An additive, or filler, can be defined as a secondary material or substance (usually in the form of solid particles) which is added to the primary plastic to modify its properties. A variety of solid fillers can be added to the formulation of a polymer: other polymers, metals, ceramics, glasses, wood, and more. This can be a very interesting development strategy, insofar as it can turn common or inexpensive plastics into easy-to-process engineering composites. 

What is a composite ?

There are several families of materials: metals, ceramics, glasses, woods, plastics. A composite is a material made of at least two distinct materials, usually from different families. Mix carbon fibers and epoxy resin to craft a leg prosthesis, and you’ll get a composite. In the case of solid additives and polymers, the plastic matter is called the matrix; it contains the filler, additive, or reinforcement. Solid additives must often be of rather small dimensions, since they must be distributed in the polymer. This is especially true when working with thin shapes: pellets, filament.

The most common shapes of additives are:

  • Fibers: they usually increase the strength of the material in their direction. In a filament, fibers usually end up aligned in the direction of the filament.
  • Powders: metals, ceramics, and pigments are usually added in the form of fine powders. This allows for a very homogeneous distribution.
  • Sheets: sandwich composites are made of different layers. The idea is to have surfaces resistant to impact and efforts, held together in a certain shape by a very light core for flexibility. This does not apply to filament extrusion though.

Almost all polymers contain additives, but not necessarily solid reinforcements. Stabilizers, which make plastics more easily processable and heat-resistant, are a good example of chemical additives. Some stabilizers increase the polymer’s resistance to UV, or to flames, or other exterior factors. In that case, the term “composite” is not being used. But at the end of the day, all additives have the same purpose: enhancing the properties of a given material.

Applications Additives, or fillers, bring even more variety and customization options on a market already full of polymers. They give manufacturers the power to face new challenges and make very specific parts for all kinds of applications. Mechanical strength can be enhanced thanks to glass or carbon fiber. Additives can also be used for very simple aesthetics purposes, like giving a flashy color to a filament using colorants, or making it look shiny by adding metal particles. Customizing polymer filaments with additives naturally brings 3D printing up another notch. It pushes its limits back by making complex parts intrinsically special, allowing for an even wider range of applications. For a materials expert, the best solution to a physical problem is : the right material filled with the right additives, given the right shape with the right process. In most cases, additives improve the properties of a material without making it much harder to process. Here are a few examples of fillers which can be added to a plastic to give it new exciting properties:

  • Pigments
  • Color
  • Wood - warm touch
  • Carbon fibers
  • Mechanical strength
  • Glass fibers
  • Mechanical strength
  • Carbon black
  • Electrical conductivity

In a nutshell, do not feel limited by the number of materials on the market: assemble different materials and make up your own for unexpected applications ! For more information about how additives affect polymer processing, read 'additives & compounds volume 2'.