Additives for Identification of Plastics

Much interest has been expressed in the possibility of developing safe and reliable additive systems which could act as markers in waste plastic products, reflecting X-ray or infra-red signals and triggering separation mechanism. There are major problems to be overcome, however, in developing a system that can operate on a fully commercial scale (for example, mounted over a conveyor belt and accurately scanning many tonnes of mixed plastics waste per hour). 

An organic marker system which can be added to a polymer molecule has been developed by Eastman Chemical. The company claims to have demonstrated the feasibility in the laboratory, but there is much more work needed to develop a commercial application. If successful, it is envisaged that such a marker would provide a relatively economical means of identification and therefore separation of different types of plastics, which could be operated on a continuous industrial scale. The system is said to work both with transparent and opaque plastics. 

Chlorine and bromine components, heavy metals and various types of fillers can be identified in plastics, by means of a number of technologies being developed for identification of plastics in post-consumer waste. 

The vital factor is that any method of automatic selection (and separation) must be able to operate on a large scale, over large volumes of plastics waste, probably conveyorized. Elements of high atomic weight, such as chlorine and bromine can be identified rapidly by means of X-ray fluorescence. This technique could be used to separate compounds containing chlorinated or brominated flame retardants, but to date it has been used in practice only for separating PVC bottles from other plastics. 

Transparent bottles can be separated from opaque green-coloured bottles by detecting the difference in transparency with the aid of transmission sensors which determine the transmission of waves of a specific length, so making it possible to separate PET from HPDE, 

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