Properties of filled and reinforced plastics
The main difference between inactive and active or reinforcing fillers is their influence on physical and mechanical properties. Modulus of elasticity and stiffness are increased to some extent by all fillers, even the spherical types such as CaCO3 and glass spheres. On the other hand, tensile strength can only be improved by fibre reinforcement. Also the temperature of deflection under load(HDT) can not be increased to the same extent as by fibre reinforcement. Fillers in platelet form, such as talc or mica, produced a marked improvement in these properties.
The use of extender fillers can result in the following changes in the properties of thermoplastics
* Increase in density
* Increase in modulus of elasticity, as well as in compressive and flexural strength (stiffening)
* Lower shrinkage
* Increase in hardness and improvement in surface quality
* Increase in HDT
* Less temperature dependence of mechanical and physical properties
* Cost reduction
Reinforcing fillers produce the following improvements in thermoplastics:
* Increase in Tensile strength at break and compressive, shear and flexural strength
* Increase in modulus of elasticity and stiffness of the composite material
* Increase in HDT and decreasing temperature dependence of mechanical properties
* Lower shrinkage
Two discrete phases are always present in reinforced plastics. The discontinuous filler phases should exhibit higher tensile strength and higher modulus of elasticity than the polymer matrix, whereas the continuous polymer phase should possess higher elongation at break than the fibre. For this reason, fibres are suitable as reinforcing agents.
When the fibre reinforced material is subjected to a tensile load, local tensile stresses are transferred to the polymer/fibre interface by shear forces and distributed over the fibre surface. For this purpose, the fibre must adhere well to the polymer and possess a specific length, since otherwise it slips out of the matrix material. The higher the modulus of elasticity of the matrix polymer, the smaller can be the minimum length of the fibre. Adhesion can be considerably improved by coupling mechanism between the filler and the plastic.
Calcium Carbonate Filled PP
Calcium Carbonate (CaCO3) can be classified as
* Mineral ground or natural
* Precipitated or synthetic
Naturally occuring calcium carbonate is found as chalk, limestone, marble and is the preferred variety for filler incorporation into PP.
A typical composition of filler grade calcium carbonate is shown below
CaCo3 : 98.5-99.5%
MgCO3: Upto 0.5%
Fe2O3 : Upto 0.2%
Other impurities include silica, alumina and aluminium silicate, depending on location, source of the ore.
Typical mineral properties are
Density : 2.70 g/cc
Moh;s hardness : 3
Degree of whiteness : 85-95%
Oil absorption : 9-21g/100 g. powder
Specific surface area : 1-15 m2/gm
Loadings of calcium carbonate in PP typically run from 10 to 50%, although concentrations as high as 80% have been produced. The filler is available in a variety of particle sizes and size distributions can be coated or uncoated. Generally speaking, large particle size, greater than 5 um CaCO3 are less expansive, but they reduce the impact strength of the PP compound. Smaller particle sizes (less than 1 um) cost more and are more difficult to compound, but provide superior impact strength and improved surface appearance. CaCO3 is usually selected as a filler when a moderate increase in stiffness is desired. Minimal sacrifice in impact strength can be tolerated.
Other effects of the mineral filler are to increase the density of the PP compound, reduce shrinkage which can be helpful in terms of part distortion and the ability to mould in tools designed for other polymers. At typical levels 10-50%, the viscosity of the compound is not significantly affected by the CaCO3. The main secondary additive employed in CaCO3 formulations is a stearate. The stearate acts as a processing aid, helping to disperse the finer-particle size CaCO3. It also helps to prevent the absorption of stabilizers into the filler. Finally, as an added benefit, it acts to cushion the system, resulting in improved impact. (Figure 1.)
Figure 1. (a) Effect of filler level on falling weight impact strength of PP at 23 deg. C.