The addition of polymer fillers was required for economic reasons to reduce the cost. However, they also bring changes in the properties of polymers, so that, by technical-economic optimization, with the reduction of the cost price, the properties of the polymer can be improved, from the mechanical, thermal, conductive or chemical resistance point of view. For example, in injection, the fillers lead to a shrinkage movement, thus solving the problem of dimensional stability of the injected part.

These materials are known by the generic term filler. In the general sense, filler refers to those fillers meant to reduce the cost price. Technically speaking, the filler refers to the contribution of materials in general, both for the filling (filler extender – CaCO3, talc, kaolin, etc.) and for the reinforcement of polymers (glass fibers / spheres, aramid fibers, carbon fiber, etc.).
Advantages of using fillers:
  • Reduced cost
  • Shrink reduction
  • Density increase
  • Increasing HDT
  • Increased hardness
  • Improving surface quality
  • Facilitates printing
  • Decreasing the variation of mechanical and physical properties compared to temperature
The disadvantages are mainly related to:
  • Abrasive action
  • Material anisotropy (modification of certain mechanical properties preferentially in one direction)

The most used filling materials and their advantages are:

Calcium carbonate (CaCO3)
  • Low cost
  • Good thermal stability and resistance to aging
  • Non-toxic, odorless
  • Heat resistant up to 840 ° C
  • Used a lot in PP
  • In fine granulation it has a nuclear effect
  • The flow rate decreases
  • Reduces creep
  • Good thermal conductivity
  • Increases abrasion resistance
  • Reduces tensile and impact resistance
  • It gives a brown tint
  • Food Contact is generally not suitable
  • Quality talc is more expensive than CaCO3
Kaolin (Al2O3 • 2SiO2 • 2H2O)
  • Improves electrical properties
  • Increases chemical resistance
  • Reduces the tendency to crack
  • Improves shock resistance
  • Improves surface quality
  • Significantly increases stiffness
  • Provides high dimensional stability at high temperatures
Dolomite (CaMg (CO3) 2)
  • Good dielectric properties
  • More abrasive and less weather resistant than CaCO3
Gypsum (CaSO4 • 2 H2O)
  • Increased resistance to acids
Asbestos (Na2Fe2 + 3Fe3 + 2Si8O22 (OH) 2)
  • Stiffness increases a lot
  • Dangerous to health
Silicates – glass spheres (6SiO2 • CaO • Na2O)
  • Grain uniformity
  • Transparency
  • Increases resistance to compression and traction
  • High thermal stability
  • Does not significantly influence MFR
  • Empty spheres can decrease the density of the material
  • Usually used with a silane coupling agent
Calcium sulphate (BaSO4)
  • Flame / Smoke retarding
Barite (BaSO4)
  • Transparent
  • Print brown

The most used plastic filler is calcium carbonate, with the net cost advantage. It is non-toxic, odorless, has a low oil absorption index and does not induce color deviations.

We can better understand the changes made by carbonate in polyolefins, if we take a look at the basic properties of the two:

Property U.M. Calcium carbonate Polyolefin
Specific gravity in solid state g/cm3 2,7 0,90 – 0,96
Specific weight of the melt g/cm3 0,764 (HDPE) | 0,739 (PP)
Conductivity W/(m*K) 2,7 < 0,5
Specific heat KJ/(Kg*K) 0,9 1,8 – 2,4
Hardness Mohs 3
Oil absorption index ml/100 g 14 – 29
Refraction index 1,640 – 1,660 1,51 ( LDPE ) | 1,54 ( HDPE ) | 1,49 ( PP )
Decomposition temperature 0C 840 335 – 450 ( PE ) | 328 – 410 ( PP )
The thermal conductivity of calcium carbonate, significantly higher than that of polyolefins, induces faster melting of the mixture during processing and, subsequently, a faster cooling, thus leading to a shorter processing cycle and, consequently, to improved productivity. . On average, adding a percentage of calcium carbonate by weight results in a one percent increase in the volume processed per unit time. On the other hand, lower specific heat will reduce the energy consumption required to melt the mixture. The considerably higher decomposition temperature (directly from the solid phase) of the carbonate will give the final product a better thermal behavior (increases the thermal deformation temperature (HDT) and the Vicat softening point). Because it does not change its density significantly with temperature, it will lead to a decrease in the variation of the density of the polymer matrix compared to it, therefore to a lower shrinkage index and, implicitly, to a decrease in warpage.
The quality of such a filler depends on:
  • carbonate granulation (usually between 1 and 3 µ)
  • particle size distribution, the presence of particles with very different sizes leading to defects (if they are too large they are the germs of defects, such as breaking the balloon in film production / too small – increase the viscosity of the mixture and tend to agglomerate)
  • degree of dispersion in the polymer matrix (a good dispersion ensures homogeneity / mineral agglomerations are germs for cracks, ruptures, etc.)
  • carbonate surface treatment (usually with calcium stearate / coupling agents based on silane derivatives or organic titanates)