Stabilisation and recycleability

Fillers often affect the stability of polymers via a variety of mechanisms. Although this is recognised, at least to some extent, it has not been studied as thoroughly as the stability of unfilled polymers. As the stability and recycleability can be critical issues, hopefully this subject will receive more attention in the future. 

Thermoplastics are usually processed in the molten state, at temperatures in the range 150-350 °C depending on the melting point and viscosity of the polymer. There are many standard stabilisation packages on the market, often containing a process stabiliser and a long-term stabiliser. Most of the stabilisers are synthetic, although recently, a natural hindered phenol, a-tocopherol (vitamin E) was found to be effective in polyolefins [141-143] and has been commercialised. For further information, the reader can consult books explicitly dedicated to stabilisation of polymers [132, 144]. 

By far the most important stabilisers are the hindered phenols, which are used in a wide range of polymers including the polyolefins, (e.g., PE and PP), polyamides, polycarbonate and PET. These stabilisers are effective both during processing at high temperature and for long-term use under ambient conditions. For increased effectiveness, they are usually combined with other stabilisers to attain an optimised combination of stabilisation and other properties such as discoloration. Often, the antioxidant is physically lost, primarily by extraction or volatilisation, rather than by chemical consumption [145]. The trend is therefore to use higher molecular mass antioxidants [132,139, 146]. 

Fillers may affect the stability of polymers via a number of mechanisms. The two most important ones are discussed here. Those are the catalysis of degradation by the filler s surface and the indirect lowering of stability that occurs when the filler surface adsorbs, and thereby deactivates, the antioxidants. 

---