Polymer chemists have come up with a variety of materials with a fantastic range of properties but they only become useful commercial plastics and rubbers thanks to a battery of chemicals mixed with the basic polymer.
Colourants speak for themselves but some additives can significantly prolong the life and performance of plastics in harsh environments. Mineral fillers, from fine powders to fibres, dramatically change the stiffness and, in some cases, the strength, while at the same time reducing mould shrinkage and improving dimensional stability. Other additives improve the way that thermoplastics behave during melt processing. Rubbery additives enhance the impact resistance while others tinker with the electrical properties.
In some cases the additive is effective at levels way below 1% while fillers can be used up to loadings of 60% and beyond. Mixing is the key to how effective an additive can be. In all cases the additive has to be uniformly distributed in the polymer matrix and in most cases it has to be well dispersed. Exceptions are impact modifiers and fibre fillers. The effectiveness of an impact modifier falls off if the dispersed particles are smaller than about 4 micron. Reducing the length of a fibrous filler reduces the resulting strength properties.
Converters should be aware that, during melt processing, it is possible to inadvertently undo the benefits of some additives by using excessive shear conditions.
Written by Dr.Charlie Geddes for Hardie Polymers