A coupling agent, or better a polymeric coupling agent is a polymer that attaches
an inorganic filler to the polymer matrix.
Typical fillers are Calcium Carbonate, Glass fibers, Talc, or flame retardants such
as ATH (Aluminumtrihydrate) or Mg(OH)2 (Magnesiumhydroxide).
The purpose of adding fillers is either to lower the cost of the polymer (CaC03,
Talc), make it tougher or stiffer (glass fibers, CaCO3) or make it flame
retardant so that it does not burn when it is ignited (ATH, Mg(OH)2).
In any case the addition of the filler will reduce the elongation at break, the
flexibility and in many cases the toughness of the polymer because the fillers will
be present up to very high levels. (E.g. ATH; 20% Polymer, 80% Filler).
The reason for this is that the fillers in most cases are not compatible with the
polymers, this means that the fillers do not like the polymers very much and will
even repel them.
In order to overcome the drawbacks of the addition of fillers coupling agents have
to be added in order to reduce the repellency of the polymers and fillers respectively.
As a result the polymer will like the filler more, the filler will adhere better
to the polymer matrix and the properties of the final mixture (e.g. elongation,
flexibility, solubility of the filler in the polymer will be enhanced)
PP Hopolymer + 30% Glass Fiber
5% Polymeric coupling
Such coupling agents have to be on one hand compatible with the polymer (ideally
they have to be the same chemistry of the polymer) and on the other hand they have
to react/interact or better glue to the filler.