In general, fillers are relatively cheap, solid substances that are added in fairly high percentages to plastics, paints, and paper to adjust volume, weight, costs, or technical performance. To reflect the diversity of materials used, fillers may be defined as solid materials that may be able to significantly affect certain properties of a base material as a result of their physicochemical constitution. Naturally, the fillers must interact with the base material, i.e., the filler surface has to more or less come in close contact with the matrix.
Fillers for plastics can basically be divided into inactive (extender fillers) and active fillers; the latter are frequently also referred to as functional fillers. Inactive fillers are used mainly to reduce costs, while functional fillers bring about a special change in properties so that the compound largely meets the requirements demanded of it; however, in reality, there is no filler that is fully inactive and reduces costs only.
Fillers have been used since the early days of plastics, and the dramatic growth of the polymer industry would not have been possible without the advantageous properties fillers impart to polymers. Famous examples include the use of wood flour in phenolic resins (Bakelite) and carbon blacks in rubbers.
In most cases, the plastic modified by functional fillers is more expensive than the matrix polymer. This is because of the often costly method of incorporation, the higher stabilizer costs, the need for special additives, and finally, the greater handling and logistics costs compared with unfilled polymers. Furthermore, as the specific gravity of most fillers is substantially higher than that of polymers, the specific volume of the compound is decreased, which causes higher costs for the end user, who is purchasing by weight but selling the end product by volume. The selection of the optimal filler is very critical and depends on the properties required in the intermediate or end product. In developing a particulate-filled composite, the formulator needs to answer the following questions :
• What property benefits are being sought?
• What deleterious changes may also occur and can they be tolerated?
• How easy is the filler to handle and how might it affect processing?
• Are special additives needed?
• What is the true cost of using the filler; is it justifiable and are there more cost-effective alternatives?
World filler consumption in plastics (1999) totals approximately 10 million tones, with 66% accounted for by calcium carbonate alone (Table 1) Fillers are used in virtually all polymers, but the largest proportion (over 90%) is restricted to a
relatively small number of plastic types, e.g., rubbers, PVC, and polyolefins.
Table 1 World filler consumption, 1999
Fillers can affect nearly every property of a polymer when incorporated: surface, color, density, shrinkage, expansion coefficient, conductivity, permeability, and mechanical and thermal properties. The effectiveness of a filler depends on its type, incorporation method, loading, and surface treatment.
The presence and/or arrangement of the filler particles within the matrix can even affect intrinsic properties of the matrix, e.g., crystallinity (nucleation) and glass transition temperature (hindered conformal molecular motion). However, in most cases, formulators are seeking improvement in mechanical and thermal properties.
The theory of reinforcement has been quite well developed and a number of equations as a function of filler parameters, such as aspect ratio and packing fraction, have been presented.
All of them have been derived from the simple rule of mixtures; however, it is sometimes difficult to get accurate values of the filler parameters. This is discussed in detail later.
The stiffness of a composite is affected by the module of both filler and matrix, the filler loading, the aspect ratio, the packing fraction, the filler-polymer interaction, and the orientation of the particles within the matrix. Usually, as soon as the stiffness increases,the composite becomes more brittle, and subsequently, toughness decreases; however, depending on the type of polymer and filler, there are exceptions, e.g., glass fiber filled polyamides. The impact strength of a composite is very sensitive to coarse particles and/or agglomerates, because they can act as micro-notches and/or reduce the load bearing effective cross section area. Improvements can be achieved through applying a surface treatment on the filler, which generates either primary (matrix-filler) or secondary (coatingmatrix), flexible, energy-absorbing bonds.
The best choice of a filler for a particular application depends on the desired properties of the composite; however, some basic principles should be taken into account:
• The filler must retain its structure during processing and remain inert, insoluble, thermally stable, with no volatiles, no phase transitions, no catalytic activity, and low additive adsorption.
• The filler must be compatible to the matrix (readily wettable) and should be non-abrasive.
• The handling should be easy; with high bulk density, low moisture, low dust, non-toxic.
• The filler must be (locally) available in sufficient amounts at moderate prices and in constant quality.
To estimate a specific property of a given polymer-filler system, it is sufficient to apply the simple rule of mixtures as a first step
Because fillers affect nearly all matrix properties, the formulator should focus first of all on the main effect needed, e.g., reinforcement. After this the formulation can be adjusted by changing filler loading, using filler mixtures, adding additives, varying the matrix, and treating the filler surface, among others
2 Tensile Properties
3 Impact Properties
4 Thermal Properties
5 Tribological and Surface Properties
6 Electrical Properties
7 Optical Properties
8 Acoustic Properties
11 Organoleptic Properties