Functional fillers are widely used in plastics and the trend is for ever-increasing use of fillers driven by several factors. Firstly, fillers allow for plastic materials with a much wider range of properties including properties not normally associated with plastics, such as high electrical conductivity or thermal conductivity. Functional fillers can be used to reinforce plastics, for example, fillers like talc and glass fibre have allowed polypropylene to compete successfully in the engineering polymers market. Almost all plastics are made from petroleum feedstock and the price of plastics inexorably increases as an abundance of oil and gas declines. In contrast, mineral fillers are abundant and filler prices climb at a much slower pace, with the consequence that filled plastics become more commercially attractive year after year. The same resins are available to all but by understanding fillers and other additives well, one can craft formulations that outperform.
While there are whole books devoted to this subject, it can be rather hard to learn from them because they contain too much information and it is spread over hundreds of pages. I know because I own most of them and have helped write a few. The books are great if you are already an expert and are looking for more detail on a narrow topic, but are of little help for anyone else. Therefore, this article will give an overview of the whole topic, in a concise manner, with links to more detailed pages on individual topics. Furthermore, it is written from a practical standpoint, with an intentional avoidance of equations and other theoretical nonsense, which does nothing to improve understanding of materials or how to design them.
Particles can have many shapes, but for simplicity, we can put them in three groups.
Round or cubic particles where calcium carbonate (calcite) or glass spheres are typical examples. That shape of particle increases stiffness somewhat, decreases strength slightly and has least effect on impact resistance and elongation to break. An advantage is that properties are altered equally in all three directions (x, y & z) and there is no tendency for warpage.
Needle shaped particle and fibers typified by wollastonite and glass fiber, can reinforce plastics if the aspect ratio is high enough and if they are well bonded to the surrounding polymer. The key property is aspect ratio, which simply means the largest dimension of the particle divided by the smallest dimension. So, for a needle 10 microns long and 1 micron wide, the aspect ratio is 10:1. Higher aspect ratio means better reinforcement. What does reinforcement mean exactly? It’s a term used a lot, but rarely does anyone say what it means. The best definition I have seen is that a reinforcement increases both stiffness and strength.
Platy particles for example talc and mica, can also reinforce and the higher the aspect ratio, the more effective they are. They reinforce along both of the long dimensions of the plate, whereas a fiber only reinforces along its one long dimension. So, in that sense, plates are more effective than fibers of equivalent aspect ratio. Plates also have the benefit of giving barrier properties, that is gasses and liquids cannot easily penetrate through plastics filled with high aspect ratio platy fillers. In recent years, high aspect ratio talc and mica have gained traction due to high reinforcement effectiveness. Platy fillers also lead to more isotropic shrinkage, i.e. less warpage and are often added to fiber filled composites to alleviate the high warpage that fibrous reinforcements can create.
Aspect ratio gives reinforcement but usually poor impact resistance and lower elongation to break. This implies that fibers and plates act as flaws. The reason is that to have high aspect ratio, you need to have one (for fibers) or two (for plates) long dimension(s). That long dimension is likely to be over the 10-20 micron limit where stress concentrations are high, facilitating crack initiation and failure. There is one more important point to make about aspect ratio. The aspect ratio that matters is not the one from the datasheet which describes the as-supplied powder. What does matter is the aspect ratio of the particles inside the final composite material because that is what affects mechanical properties, barrier and so on. It is therefore vital to ensure the particles are not broken down, decreasing aspect ratio, during handling and compounding. The more aspect ratio that can be preserved, the better the final properties.
Speciality functional fillers
The fillers covered above are of most importance in terms of volume used but many other types are selected to achieve particular effects.
Barium Sulfate (Barite or Blanc Fixe) – BaSO4
White in powder form, it occurs naturally as the mineral barite or the corresponding synthetic version is referred to as Blanc Fixe. The two main attributes are high density (4.0-4.5 gcm-3) depending on purity and radio-opacity. It is therefore added to polymers to make them heavy, e.g. for sound damping or to make them x-ray visible for implants to give on example. It is highly unreactive (inert). The particles are blocky in shape and therefore non reinforcing.
Magnetite – Fe3O4
A black lustrous powder with unusual properties. Often used for its high density (5.2 gcm-3 for pure grades). It also has high thermal conductivity and quite high electrical conductivity. It is x-ray opaque and can be used to block radiation. Magnetite is non reinforcing due to the low aspect ratio of the particles. The many unusual properties are detailed here (magnetite page) and in an encyclopedia chapter I authored.
Halloysite – Al2Si2O5(OH)4
Halloysite is an aluminosilicate and member of the Kaolin group of minerals. It has identical chemistry to kaolinite but instead of platy particles, the silicate sheets are rolled up in a scroll-like fashion to create nanotubes. These are typically 50nm in diameter and a few microns long to give an aspect ratio of 10-20:1 and thus moderate reinforcement. Although many potential applications have been postulated, commercialization in polymers has been very slow to take off. More information on halloysite can be found on a dedicated page ( halloysite page).
Polyhedral oligomeric silsesquioxanes
These are technically molecules that look like particles and have therefore been described as “molecules”. The centre of the molecule is a silica-like cage which is surrounded by organic groups which impart solubility. This means it is possible to add rigidity but with perfect compatibility and the option to chemically bond to the matrix polymer. Applications are starting to take off in the last couple of years. More information on these unique additives can be found on a separate page.
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