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Continued interests in green credentials of natural rubber

19 Sep 2012 - {{hitsCtrl.values.hits}}

Rubber ( Hevea brasiliensis) has retained many features from it’s Amazonion origins as an environmentally-friendly forest tree, although it has been cultivated as a plantation crop for latex with wood as a side-product and now for carbon credits.

One key factor in the relationship between any activity and the environment is that it is impossible to consider any individual activity without reference to the overall impact. In the case of the rubber industry, it is helpful to break down the activities which impinge upon the environment ( Table 1) into those associated with (1) the production of the raw material, (2) the transformation of the raw material into finished products (3) the use of such products in service and (4) the final recycling or disposal of the products. Many studies relating to the last-named, such as investigations of the scrap tyre problem, fail to recognise the importance of the other elements which may either amplify or mitigate the problem. It is unavoidable that many authors tend to base their analysis upon natural rubber, frequently in comparison with synthetic rubber, but many of the factors ( such as factory emissions, product service and ultimate disposal) apply to all elastomers.

In most discussions, the environment resources are divided into renewable and non-renewable categories. The former includes most natural products. The latter includes most mineral resources, although many of these are re-cycle able, and fossil fuels. Some tend to consider their large hydroelectric capacity as a green resource. Fossil fuels are not only non-renewable, but their combustion contributes to increases in global carbon dioxide levels and a possible green house effect which may even lead to higher ocean levels and the loss of global land mass.

Industrial material
Natural rubber is an unusual industrial material as it is renewable resource. As such natural rubber enjoys very considerable environmental benefits, and these have tended to be understated in most discussions. In broad energy input terms, natural rubber enjoys a very considerable advantage over synthetic elastomers, whose energy inputs is in the region of 210 – 275 GJ / tonne, as against 30 – 35 GJ / tonne in NR production. It is probable that the synthetic rubber industry has now reduced its energy inputs for processing and that the use of yield stimulation etc. may marginally increase the energy consumed in natural rubber production besides increased fuel costs. Nevertheless, the natural rubber production data assumes long-distance transportation for the raw rubber from the producing countries to the major consumers.

Rubber wood
The use of rubber wood is growing rapidly and Hevea is even being grown primarily as a source of timber, with rubber being produced as a by-product. Rubber wood is used in furniture, flooring , building components, chipboard, etc and enjoys a growing market. Obviously, the timber so-produced is an eco-friendly material and it is highly pertinent to note that some of the companies involved are subjecting rubber wood production to environmental audits. It has been estimated that the energy input for wood as a raw material is about 6 GJ / tonne as compared with 38 GJ / tonne for steel and around 100 GJ / tonne for most thermoplastics.

CO2 sequestration
The most understated aspect of Hevea cultivation is that of a sink for the carbon dioxide which is produced by animals ( including man), the natural combustion of plant tissue, and especially through the burning of fossil fuels. Photosynthesis enables the carbon dioxide to be converted into life-sustaining oxygen whilst fixing the carbon as biomass. Hevea’s effectiveness in this respect is probably at least equal to that of virgin forest and may even exceed it. Tropical forests, which cover 20 percent of the earth’s surface, account for at least 25 percent of global terrestrial carbon fixation, and it is becoming increasingly recognised that the forest makes a major contribution to global ecology.

Hevea rubber compares well with virgin jungle in terms of biomass, especially once the trees reach maturity. Physiological studies have shown that Hevea is more effective than teak grown in plantation conditions in taking up carbon dioxide. This is probably due to the extra energy required to produce the latex inside the tree: thus, in contrast to a synthetic rubber plant, which consumes energy and produces carbon dioxide to convert pure energy ( crude oil) into elastomers, the natural rubber plant converts carbon dioxide into an elastomer. The biomass production potential of a plant species is related to its photosynthetic capacity per unit leaf area and the total leaf area produce per plant.

In full sunlight the photosynthetic rate of a mature rubber leaf is around 11 µmol/m2/s1 as compared with 5 – 13 µmol/m2/s1 in other tree species. The leaf area produced by a mature rubber tree is quite substantial : the leaf area index of a mature rubber plantation can be as high as 6 or 7. Because of the high photosynthetic rate and leaf area index, the biomass production per unit land area within a given time is very high in Hevea. With a planting density of 500 trees per hectare the canopy closes in less than five years.

Agronomic efficiency
Natural rubber does not impoverish the land upon which it is grown. Fertiliser inputs are very low and the surrounding soil appears to be enriched by the abundant leaf fall. Furthermore, biodiversity remains remarkably high in rubber plantations in marked contrast to most forms of monoculture. Excellent agronomic techniques assist in the conservation of the environment within rubber plantations. Measures include terracing, slit pitting, bunding and mulching and the growth of leguminous cover plants between the rows to assist with nitrogen fixation. Biomass burning is now discouraged during replanting. Moreover, it is possible to grow a wide variety of crops during the tree’s immature period, further enhancing its environmental credentials.

Energy inputs
It is possible to produce dry rubber with remarkably low energy inputs especially if maximum use is made of human and solar energy. It is possible to produce air –dried sheet solely by the exploitation of these two forms of energy. Most dry rubber and latex concentrate production does exploit modest inputs of electricity (which in many producing countries is green power from hydro generators) and other forms for drying. Obviously energy is also required to convert dry rubber into a form where it can be shaped and vulcanized.

Unfortunately, primary processing of natural rubber can lead to significant environmental pollution, especially of water courses and through localized unpleasant odours. Considerable progress has been made in reducing water-borne pollution. especially in India, Malaysia and Sri Lanka. But, in most countries, a considerable problem still remains. This endangers many other activities such as the use if water for agriculture for industrial use and for fish cultivation

In-service segment
There are both positive and negative environmental factors in the in-service segment of an elastrometric product life cycle. The positive factors include a reduction in environmental noise, although tyre noise is a major contributor to environmental disturbance from roads, especially where vehicles travel at high speed. A clear positive contribution to noise and vibration control is to be found in the application of elastrometric mountings and bearings.

The negative factor for it is essentially that the main outlet of rubber is associated with the automotive industry. The road transport industry accounts for disproportionate uptake of the world’s natural resources. In the USA, is has been reported that approximately 25 percent of crude oil is consumed in personal transportation. This industry is a major contributor to global increases in carbon dioxide emissions and endangers health, especially that of children, through asthma and other dangers. It must be stressed that these effects are not directly associated with the use of rubber, but that the system which induces them is inherently dependent upon rubber for its tyres, its engine mounts, its weatherstrip and so on. In the product life cycle, it is seen that the energy required to manufacture or dispose of a passenger car tyre is trivial in proportion to that associated with its use in service.

Another, lesser, negative factor is that rubber components, especially tyres, tend to wear in service. This leads to the emission of particulate materials into the

environment. There have been some dubious suggestions that such particulate material may contain latex proteins and that this could produce allergic responses in susceptible individuals. Presumably, some of the airborne fragments could combine with the exhaust gases emitted by vehicles to contribute to the unpleasant urban smog which is found where the pollution is trapped either within an inversion layer and / or within a confined environment. There has also been some suggestion that tyre particles could lead to waterborne pollution in rivers and lakes.

However, it must be remembered that the carbon dioxide produced is not a problem for the natural rubber element of the tyres as this will be recycled by the rubber trees that produced it in the same way that it is possible to grow biomass as source of fuel Pyrolysis is interesting as there is no air pollution problem and the products other than heat, are in the form of gases ( which can be burned), solids include a form of carbon black, which if the input consists predominantly of natural rubber, can be claimed to be ‘green”.

(The writer can be contacted at [email protected])

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Continued interests in green credentials of natural rubber

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