(Kevin P. Jones and Kristina Lawson [TARRC] )

In ecological terms, the effectiveness of a recycling policy can be assessed only if it recognizes the energy inputs into the raw materials, the energy requirements to process those materials, the energy associated with the use of these materials in service, the energy requirements for recycling these materials, and the energy involved in ultimate disposal. By its holistic nature an environmental policy cannot address a single segment in the lifetime of a material, or product manufactured from that material. If a material, like most elastomers, is "difficult to recycle" then this is an inescapable down side in its use, and one which has to be offset against other environmental gains, such as a noise-free environment. The Royal Commission on Environmental Pollution (1) recommended to the Departments of Trade and Industry and Environment that vehicle manufacturers and dismantlers need to develop a cradle-to-grave strategy for recycling.

Renewable resources

A key factor in an ecological approach is to divide resources into renewable, and non-renewable categories, particularly those based on fossil fuel. The former includes everything grown, although account has to be taken of energy inputs: it would be possible to grow Hevea in greenhouses at high latitudes, but the energy required would be prodigious. Some countries, such as Sweden which employ hydro-power, further sub-divide power into green and non-green forms. Most countries do not enjoy this free-form of energy in sufficient quantities to make meaningful distinctions. Increasing fossil fuel consumption, and the associated rise in atmospheric carbon dioxide levels, is recognized as a global environmental problem. At its worst it endangers the very existence of certain low lying nations, such as Bangladesh, but is also associated with other environmental ailments, including acid rainfall in Scandinavia and asthma in children.

If a product or system contains natural rubber then this is likely to be an environmental advantage as natural rubber not only comes from a renewable resource, but the presence of the rubber tree acts as a carbon dioxide sink for the use of that rubber in service and if necessary through its disposal via combustion. In theory, natural rubber can be produced with extremely low inputs of fossil fuel provided that maximum use is made of (1) solar energy and (2) human effort: air dried sheet produced by smallholders is a good example of this strategy. As smallholder farming is normally self-sufficient, such effort is unlikely to be anything other than beneficial in global ecological terms. Unfortunately, the attractiveness of the smallholder life-style is being challenged by a drift towards urban cultures in the major rubber growing areas, and it is probable that higher energy inputs will be required to harvest natural rubber in the not too distant future and this may partially reduce the eco-friendliness of natural rubber.

It has long been recognized that the major rain forests which are located near to the Equator are a major sink for carbon dioxide, especially that produced by burning fossil fuels. Table 1 lists some estimates made by Wan Abdul Rahaman2 of the biomass available in virgin tropical forest, in comparison with that available from Hevea plantations at various stages in the life-cycle of the rubber tree. Furthermore, it has been estimated that the global Hevea biomass is capable of fixing 90 million tonnes of carbon per annum. Thus, natural rubber enjoys a fundamental ecological advantage over other elastomers.

The modest energy input for natural rubber in comparison with several typical synthetic elastomers is shown in Table 2: natural rubber enjoys a considerable advantage3. It is probable that the energy requirements for some of the synthetics have been reduced since the data were gathered, and there may have been some slight increase in the energy requirements for natural rubber, but the natural rubber data make no allowance for the products manufactured from rubber tree wood, once natural rubber production has ceased. Such products are a very considerable environmental bonus: it has been estimated by Chapman4 that the energy content of wood (in general) is about 6GJ/tonne as compared with 38GJ/tonne for steel and around 100GJ/tonne for most thermoplastics.

There is a small penalty in energy terms for using natural as compared with synthetic rubber. Some of this is caused by having to store natural rubber in heated rooms during winter in the northern hemisphere, some is caused by natural rubber being slightly more difficult to mix. Nevertheless, the additional energy required to process natural rubber is far less than the difference between the energy requirements for the more efficient processors and their less efficient rivals. Furthermore, the overall energy requirements for rubber processing (of 20GJ/tonne, or lower in Scandinavia) are very considerably less than the initial energy inputs for manufacturing synthetic elastomers.

End of life cycle

A few natural rubber products are. without question, conceptually green, although in one case, there has been some pressure from environmentalists to restrict a specific activity relating to its use. These products are mainly manufactured from latex and include medical gloves, household gloves, condoms and balloons. One of the great strengths of most latex products, especially gloves, is that in relation to their restricted bulk they contain low amounts of non-rubber ingredients (curatives and antioxidants). The majority of gloves are used on a disposable basis for a very brief time: frequently for a specific medical treatment to one patient. In the case of medical gloves it would probably be possible to apply recycling processes of the DeLink type (5) to return the rubber for further use. The main problem would to ensure that pathogens present from their medical use were eliminated: the energy involved in such cleaning might eliminate the environmental gain. Furthermore, it would be probably be difficult to find markets for such recycled material. In any event, combustion (the normal disposal method) is an environmentally acceptable method of disposal, although such combustion may take place in association with materials which are environmentally destructive, such as polyvinyl chloride, causing the overall destruction policy to be subject to criticism. For some specific glove applications it has been necessary to lessen the hazard from the combustion products. Thus there has been some interest, Pendle (6), in radiation cured prevulcanized latex to remove the risk of corrosion caused by sulphur fumes from incinerated gloves.

The large scale release of balloons, mainly as part of promotional activities, has led to criticism. It has been argued that once the balloons return to the ground they disfigure the landscape, they endanger wildlife and may introduce alien chemicals to the soil. It is worth stressing that most of this criticism has arisen in the USA, where the large scale balloon releases are popular. Most thin latex articles are capable of biodegradation: most promotional balloons will last as long as fallen leaves once they have fallen to earth. Nevertheless, there has been sufficient pressure for recommendations to be drawn up to lessen the life of balloons (7).

Scrap tyre problem

The bulk of natural rubber (in excess of 70% in most West European countries and North America) goes into tyres, although significant amounts are used in engineering components, belting, hose and footwear. In many of the engineering components the rubber is used in association with metal and the steel is liable to be more valuable than the rubber on a scrap basis. Scrap belting and hose (provided that it does not incorporate chlorine-containing elastomers) can be handled with tyres. Most discarded footwear is included with household waste, but it should be noted that the classification and separation of household wastes is increasing.

The Royal Commission on Environmental Pollution (1) noted that used tyres represent a major waste disposal problem. Of the 40 million scrap tyres requiring disposal each year in the United Kingdom, two thirds have been land filled or dumped illegally. Tyre dumps may catch fire and are then extremely difficult to extinguish. It has been estimated by Allen (8) that two million tonnes of used tyres are released each year within the European Union, and that this will grow to 2.5m by the year 2000. Currently 23% of these are retreaded; 30% are recycled or used in other ways and the remainder go for landfill. Landfill costs have increased because of the shortage of sites and because tyres are undesirable in landfill where they cause lasting problems. Degradation is very slow, and the soil and subsoil remain unstable. Tyres are not biodegradable, although some decomposition will take place after 85-100 years. This is surprising in view of the widely reported (9) biodegradation of seals in underground pipelines. They never completely fill with earth and will not compact in landfill (eg by driving over to compress) and so cause general instability. Discarded tyres rise to the top and prevent air and water from circulating properly.