Case study - 'Waste not, want not'

Background

Polymers have become an integral part of modern life, and now make up between 20-25 per cent of municipal waste. For example, polyethylene (PE) is one of the worlds most popular polymers, with a global demand for 43.4 million metric tonnes in 1996 alone.

However in view of their poor biodegradability, most polymers are considered unsuitable for landfill disposal and incineration means that any useful chemical content of the waste is lost. Utilisation of waste plastics as fuels or chemicals is a more attractive method of disposal, both economically and environmentally.

Evolved Gas Analysis

The sophisticated evolved gas analysis facility has been used to identify the major gases evolved during the catalysed and uncatalysed degradation of polyethylene (PE) when it is heated under controlled conditions.

The heart of the system is a sensitive balance which records the weight lost as a sample is heated at a fixed rate. The gases evolved from the sample are passed through two transfer lines. The first takes the sample directly to a mass spectrometer which smashes the gas molecules apart and looks at the resulting charateristics breakdown pattern. The second takes the gases to a trap where they are collected and then seperated, identified and quantified using a gas chromatograph linked to a second spectrometer.

Both detection systems, which are very powerful in their own right, become awesome when used in combination.

Results

Figure 1 shows the weight loss curves for the catalysed and uncatalysed degradation of PE. Some information about the weight of material lost can be derived from these curves.

However from this data alone it is not possible to obtain any information about the range and potential usefulness of products formed.

Figure 2a The chromatographic separation of the products formed from PE with and without catalyst is shown in figures 2a-c.

Each peak in figure 2a represents a product formed during the uncatalysed thermal degradation of PE, products of which are long straight chain hydrocarbons containing zero, one or two double bonds

Figure 2b shows how the distribution of products, each of which can be identified by their characteristic mass spectrum, changes when K10 is used as a catalyst for the degradation.

This catalyst is not active enough to alter the straight chain hydrocarbons but converts the components containing double bonds into useful cyclic compounds.

The information contained in Figure 2c shows that the zeolite ZSM-5, a very active. catalyst which is commonly used to convert alcohol to gasoline, transforms the usually inert straight chain molecules from PE into short chain hydrocarbons and cyclic compounds.

From the results presented here it is clear that by careful selection of an appropriate catalyst, a range of different products can be obtained from the thermal degradation of PE. Therefore, the energy potential of plastic wastes can be harnessed for useful purposes thus making the waste a viable raw material.

Discussion

Evolved gas analysis is a powerful analytical technique which will benefit a wide range of companies, large and small, whether they are involved in routine quality assurance, failure analysis or the development of new products and materials. The combination of sophisticated analysis techniques in this system provides a unique state-of-the-art facility which has enormous potential in a wide range of market sectors including

• environmental pollutants and contaminants
• oils and petrochemical
• paints, coatings and adhesives
• catalysts
• polymers and plastic/resins
• ceramics and composites
• fibres, fabrics, rubbers and foams
• packaging laminates
• aqueous solutions and dispersions
• soaps and surfactants
• body fluids
• food analysis