Fillers are defined as materials that are added to a polymer formulation to lower the compound cost or to improve properties . Such materials can be in the form of solid, liquid or gas. By the appropriate selection of these materials, not only the economics but also the other properties such as processing and mechanical behavior can be improved. Although these fillers retain their inherent characteristics, very significant differences are often seen, depending on the molecular weight, compounding technique, and the presence of other additives in the formulation. Therefore, once the basic property requirements are established, the optimum filler type and the loading level for cost and performance balance must be determined [1,8,9].
The addition of fillers also requires a balance of formulation for optimum processing properties. Therefore, before making a final decision on a filled compound, it is critical to establish the following:
1. Optimum loading level for property and benefit.
2. Optimum formulation for processing and production output.
3. Economics of filled formulation.
1.2 CLASSIFICATION OF FILLERS
Fillers have been classified in many different ways, ranging from their shapes to specific characteristics. Broadly, fillers can be classified into two categories, which are as follows:
1.2.1 CLASSIFICATION based on performance
The extender fillers primarily occupy space and are mainly used to lower the formulation cost[1,8,9].
Properties of extender fillers:
In general, ideal extender filler should
· Be spherical to permit retention of anisotropic properties.
· Have an appropriate particle size distribution for particle packing.
· cause no chemical reactivity wit the polymer or the additives
· Have low specific gravity.
· Have desirable refractive index and colour.
· Be low in cost.
Functional fillers on the other hand have a definite and required function in the formulation apart from lowering the formulation cost.
However, some of the extender fillers when reduced to a finer particle size and/or surface treated would perform as functional filler. Fillers that are functional in one polymer may be merely extenders in another polymer. Such factors seriously complicate the task of establishing sharp boundary lines between extenders and functional fillers in terms of their generic composition; however, on the performance basis they can be separated as shown in the fig
Therefore, the extender fillers basically lower the formulation cost and increase the flexural modulus, whereas the functional fillers provide at least one specifically required function in the formulation.
1.2.2 Classification based on TYPE
Particulate fillers are divided into two types, inert fillers and reinforcing fillers[1,9]. The term inert filler is something of a misnomer as many properties may be affected by incorporation of such a filler. For normal uses, such fillers should be quite insoluble in any liquids that the polymer compound is liable to come into contact. Each type of filler may differ in the following ways:
· Average particle size and size distribution.
· Particle shape and porosity.
· Chemical nature of the surface.
· Impurities such as grit and metal ions.
The common observation is that the finer the particle sizes the higher the values of the tensile strength, modulus and hardness. Coarser particles will tend to give less strong compounds than the virgin material (i.e. material without the filler), but if the particle size is fine there is an enhancement of the mechanical properties and the phenomenon is called as reinforcement.
Impurities in the fillers can have serious effects on the polymer compound. Coarse particles lead to points of weakness in soft polymers and will therefore fail under stresses below that which might be expected. In general, the phenomenon of reinforcement appears to depend on three factors:
· An extensity factor – the total amount of surface area of filler per unit volume in contact with the polymer.
· An intensity factor – the specific activity of the filler – polymer interface causing chemical and/or physical bonding.
· Geometrical factors such as structure and porosity of the particles.
Rubbery fillers are often incorporated into rigid thermoplastics to improve their toughness. The increase in toughness of the material leads to significant increase in the fracture resistance of the material[1,9].
Fibrous fillers have been long used in plastic materials. Fibrous fillers are often embedded in the laminar form. The fibers have higher modulus than the resins in which they are embedded so that when the composite of resin plus the fiber is strained in the plane of the fibrous layer the bulk of the stress is taken up by the fiber. As a result of this, both the strength and the modulus are enhanced when compared to the unfilled resin[1,9].