Additives and fillers: Getting the balance right

One of the greatest things about plastic is the ability to manipulate its properties. This allows a great deal of tailoring to make the material fit for purpose.

It might be that cost reduction is optimised or maybe it is strength that is the driving factor. However, there is no perfect material and the process of getting the best mix of properties is a delicate trade off of cost, processability, finish, strength and increasingly end of life, amongst many others.

Engineers generally have strong opinions about what it is they want, and expect, from a material. What is less clear to most engineers, however, is how to go about it.

Plastic is generally a 'recipe' consisting of a mix of primary binder with other additives and fillers that are added in different quantities to get to the desired properties. Getting this mix right can be a bit of a black art. So how can you ensure that you are getting the properties you need at the best possible price when specifying a material for an application?

"You have got to have an informed discussion," says Professor Malcolm Fox, research and development manager at Nylacast. "The worst outcome is when people just buy straight out of a catalogue. You have got to understand and discuss with suppliers the nature and physical properties of the material."

With deadlines approaching and increasing pressure to deliver, it is not unusual to use a material from a previous set of drawings to limit risk. However, there is a greater risk that this can become the norm. Innovation must always be up there as a key driver for engineering and technology firms.

The world of plastic manufacture and the use of fillers and additives can be fast moving. If something was not suitable a few years, or even months ago, it doesn't mean that it is still necessarily the case. And vice-a-versa.

It is a world that is not always straight-forward and material behaviour is not always necessarily what you think it might be. For example, some 'recipes' do not scale well, and while they might work perfectly on smaller components, manufacturing larger ones might cause some unforeseen problems.

Additives used in plastic manufacture are mostly used for two main reasons. The first is to add some kind of physical property to the mixed material. That might be water absorption, better mechanical strength, or perhaps a superior surface finish. The second is that additives are generally cheaper than the primary binders, which yields an overall lower unit cost.

"Additives bond in to the material unlike fillers which don't physically interact with the main substrate of the polymer," says Professor Fox. "Additives and fillers are usually dispersed – hopefully uniformly – to give improved physical characteristics or additional properties such as water absorption or self-lubrication."

These functional additives can be used to induce a number of different physical properties in a plastic based material. Finely dispersing carbon or a metallic powder in a material allows a conducting path. You can also disperse additives that will stop bacteria growing so they can be used in the medical industry.

Additionally some can have particles of oil dispersed throughout. This is often what is used with plastic 'lubricant free' bearings. As the surface layer of the bearing is worn down, it slowly and evenly releases an oil film which keeps the surface lubricated.

Nano fillers

Perhaps one of the most interesting and exciting prospects in the near future is the use of nano particulates such as graphenes being used as an additive material. There has been some success in the laboratory with excellent results.

The aircraft industry in particular is keen to use graphene as an additive in epoxy resin. If used as an additive within a polymer composite matrix, graphene could increase its compressive strength, allow higher temperature resistance and reduce moisture uptake. Additionally because of its superior electrical conductive properties it could also mitigate damage from lightning strikes without the need to incorporate metal materials in to the structure. Graphene's thermal conductivity could also have applications for improved aircraft de-icing systems.

"It is future additives such as graphene or nano particles being used as common additives that is really exciting," says Professor Fox. "The problem is stopping the nano particulates in polymers like nylon and PET from clumping together. This turns them essentially in to larger capsules of nano particles, which of course is no good. The problem is dispersing them uniformly. Everybody is keen to exploit the potential, or the perceived potential, of nano particles. The trouble is keeping them nano."

Justin Cunningham

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