Novel concepts in environmentally friendly recycling. (Tech Service).

This is the first of a two-part series. The conclusion will appear in the December issue.

Rubber is not regarded as an environmentally friendly material. Once valuable resources (raw materials and energy) have been invested in a rubber product, they are effectively locked-in by the vulcanization process. The inability of current technologies to provide environmentally friendly and economical processes to effectively unlock these invested resources has resulted in waste rubber being viewed as a costly problem rather than as a valued asset.

Tires account for nearly half of the robber consumed. An estimated 270 million tires reach their end of life in the U.S. each year; over a billion throughout the world (ref. 1). Each tire constitutes a particularly difficult problem environmentally; its robber does not break down to be recyclable as, for example, do glass, paper and metals; its shape harbors insects; its volume encloses much air; and it is dangerously combustible.

Ideally, if vulcanized rubber could be returned to an unvulcanized state, as is the case for other recyclable materials, rubber in products including tires could be recycled into new products of the same value. This means that high quality or high performance products would require high specification recycled materials.

Current approaches to waste rubber may have the potential to deal effectively with the volume of waste, but are not effective in recovering the investment made in the rubber material or products. A considerable further investment in energy and resources is often required that only serves to reduce the economic and environmental return. These approaches focus on rubber waste, "the problem," rather than "the asset," and typically yield low-specification materials.

Legislation and social pressures, driven by environmental concerns and practical issues such as landfills reaching capacity, are forcing the robber industry to address the problem of waste materials more acutely. Without a technical solution and a sustainable economic model, these pressures will only serve to increase industry costs and further reduce margins throughout the supply chain.

The challenge is twofold - first there is a technical issue as technologies must be found that enable tree recycling to take place, and second, an economic issue as new models need to be developed for the robber industry to include significant material recycling streams.

This article looks at some of the underlying issues and proposes a different approach to the economics while positioning old, new and emerging recycling technologies in the light of the technical challenge.

The problem

Background

Since Goodyear's discovery, that by subjecting a combination of sulfur, carbon and robber gum to heat and pressure a strong durable engineering material can be produced, the global application of vulcanized robber had grown to a yearly production level of 17.02 million metric tons by 1999 (ref. 2).

Approximately 40% of this, some 6.84 mmt, was natural rubber, the remaining 10.18 mmt consisted mainly of oil-based synthetic robbers. In the same year, some one billion tires were manufactured requiring approximately 10 mmt of robber material, including most of the 6.84 mmt of natural robber produced.

Once vulcanized robber has reached the end of its working life it must be recycled or disposed. For tires, in 1998 it was estimated that between 750 million and one billion tires were scrapped globally, weighing some 6 to 10 mmt. Approximately 550 million tires, weighing about 5.7 mmt, were recorded as being scrapped in Europe and the U.S. alone. Although most of the published figures report scrap tires, it may also be assumed that other rubber products are reaching their end-of life and that the total rubber waste stream may therefore tend towards the production totals of 16 to 17 mmt/per annum.

Recycling rubber is not simply a case of melting the material down and reusing it, as can be done with materials such as aluminum and glass. Reversing the vulcanization process is often likened to unbaking a cake and then reusing the eggs. Recovery of useful, uncontaminated materials is even more complex for multi-material laminated products such as modern tires. Because of these difficulties, scrapped rubber, and in particular scrapped tires, have traditionally been landfilled or stockpiled. The estimated quantity of known stockpiled and landfilled tires in the U.S. and Europe is in excess of 21 mmt, with a further 2 to 3 mmt being added yearly.

The pressure to change

There is an increasing pressure to address the problems associated with rubber waste streams. Concerns such as the long-term environmental impact of waste rubber in regard to pollution, environmental loading and energy consumption can be added to a number of practical issues such as the landfill sites reaching their capacity.

Landfilling and stockpiling tires are not regarded as being environmentally sustainable solutions. The U.K. Environment Agency has reported a number of problems associated with the landfilling of tires. If disposed of in large volumes, tires in landfill sites can lead to fires. They also tend to rise to the surface, affecting long-term settlement and possibly causing problems for future land use and reclamation. Tires buried in landfill sites are a fire hazard and ignition can cause serious air pollution as well as the pollution of underground water supplies. A fire that started in 1989 in a dump with 10 million tires in Powys in Wales is still burning. An Environment Agency report (ref. 4) also indicates that the effects of the long term leaching of organic chemicals are not known.

Legislative pressure

As a direct response to this problem, a number of countries are legislating against the landfilling of rubber materials. For example, the European Landfill Directive will ban the burying of whole tires in landfill sites by 2003 and shredded rubber by 2006. A number of landfill site, operators have already responded by dramatically increasing the disposal cost of tires or refusing to accept tires or rubber waste.

Increasing use is being made of legislation to force industry to tackle the long-term problems of pollution. In 1993, the European Commission set pan-European targets for the year 2000 of 65% recovery (including energy recovery) and 25% retreading, with 10% undefined. Although these targets have yet to be achieved, they do form the basis of the EC Landfill Directive.

[FIGURE 1 OMITTED]

Future trends

The increase in environmental concern, legislation and financial penalties will continue to act as primary drivers in this field. Economic alternative approaches are therefore urgently required if the rubber industry is to meet this challenge effectively.

Resistance to change

Resistance to changing the way that we treat waste rubber has a number of origins, which include:

* The lack of technologies capable of economically unlocking the investment in rubber materials and returning them into valuable feedstock;

* the market perception that products containing high levels of recycled materials are of lower quality or value than those made from virgin materials; and

* the low prices of raw materials.

The economics are changing as rising material prices, particularly oil-based materials, force up manufacturing costs. In addition, processing costs, which are sensitive to both energy and taxes, are increasing, waste disposal opportunities are decreasing and waste disposal costs are increasing.

It will be only a matter of time before the rubber industry will be forced to deal with the issue of rubber waste without the traditional response of landfills and stockpiles.

Current approaches

The generally accepted hierarchy of measures for the management of waste materials (ref. 9) as represented in figure 2 shows that the most favorable measures ensure the maximum usage of the resources already invested in the waste product, while the least favorable require considerable further investment of resource for very little, or no return.

[FIGURE 2 OMITTED]

When positioning waste rubber products on this hierarchy, the following observations can be made:

* Disposal, which mainly consists of landfill/stockpile and incineration, currently handles a significant portion of the waste stream, although this is set to change as legislation comes into force.

* Energy recovery is particularly favored in the U.S., handling more than 60% of the tire waste stream. It is noted that although some countries, such as Finland, are taking steps to legislate against rubber incineration as a protection against global warming, this is currently the main technology applied to rubber waste.

* Recycling, while currently accounting for 5% to 15% of the waste stream, is important to the thrust of this article and is therefore discussed in more detail in the following section.

* Reuse ranges from the extension of product life (notably the use of part worn tires for less demanding operating conditions and retreading) through to the use of the product in new application areas, such as in the construction industry.

Although recent efforts in Europe and the U.S. have focused on managing the disposal of scrapped tires, a difference in the current implementation can be observed. The reliance on energy recovery in the U.S. is much higher than in Europe, although the Scrap Tire Management Council in the U.S. has reported a 20% decrease in its usage over the last two years, partly as a result of furnace efficiency requirements.

Although Europe is set to follow suit, concerns associated with the environmental impact of the incineration process, particularly in cement kilns, have been expressed. These concerns range from the emissions and by-products generated through to the environmental impact of burning synthetic rubbers, which represent high hydrocarbon investments. During the oil crises in the 1970s it was estimated that it took 3.5 metric tons of oil to produce one metric ton of SBR. While current manufacturing efficiencies have been improved, a high level of hydrocarbons is still involved, up to 10 times that of natural rubbers. Circa 30-40% carbon black filler content further increases the hydrocarbon value.

It has also been observed that the continuing increases in targets embedded within environmental control legislation tend to reduce the economic feasibility of the total process of incineration.

Given the public concern generated by the topic of global warming, it may be prudent to evaluate current and future rubber recycling strategies in terms of the total energy balance throughout their lifecycle in order to determine their "environmental friendliness."

The gap

Examination of the current economic model and technologies employed when dealing with waste rubber reveals a gap between the current approaches and the present and future requirements.

Economics of the big picture

Waste rubber is regarded as a negative (cost) factor in current economic models, which are often based on the premise that it is of low economic value and that any approaches employed are to be optimized for bulk efficiency.

This pragmatic approach invariably leads to the conclusion that energy recovery is the most viable bulk approach for handling waste rubber. Waste rubber compares favorably with coal and oil in terms of calorific value and can therefore be seen to be a cheap alternative to these fuels. Further arguments have been developed to show that recovering energy from this waste, with careful control of the incineration process, is a fossil fuel alternative. However, this approach is only really of value in the absence of better alternatives that can make use of more of the tire properties.

A life cycle view

The adoption of a life cycle analysis (LCA) approach identifies a number of issues related to the true value of the waste stream. The investment of resources, which include raw materials, energy, process and logistics investments, are identified for each process step up to and including the recycling of the material. Using this approach, a judgement can be made as to the true economic value of the waste material and the environmental impact of a recycling approach.

Material value

As previously indicated, reuse is the most favorable option, although it is not always possible as product wear and aging can degrade a product such as a tire. Parts of the product subject to high mechanical wear can be replaced at a fraction of the cost of manufacture for the total product as indicated by the example of tire retreading.

This is borne out by inspection of the market value of various waste rubber streams as shown in figure 3. Here the market spot prices for various tire waste streams are expressed as a percentage of an average market price for a new passenger car tire. It can be seen that the prices achievable for part worn or retreaded tires are considerably higher than for other waste streams, although these are usually less than 30% of the original new price of the tire. It can also be noted that the price commanded by TDF averages less than 1% of the new tire value. (As a simple verification, while fossil fuels remain at prices around 40 to 50 Euro/mt, assuming a similar calorific value and approximately 100 passenger car tires/mt, the competitive price of TDF would be equal to or less than 0.4 to 0.5 Euro per tire. New car tire prices typically range from 50 to 200 Euro.)

[FIGURE 3 OMITTED]

Crumb materials command similarly low prices, often less than half those obtained from TDF, although these are often incorporated into new products which do command significantly higher prices. (As a reference point, the negative value often commanded by the scrapped tire is also indicated.)

While the instantaneous position of each of these points on the graph is open to fluctuation, the gap between the prices commanded for the new tire and those commanded for the materials in waste tires is all but total, except for retreads - evidence of a non recycling industry.

Process cost

The gap becomes even clearer when the cost of operating these processes is considered. Although no detailed information is available, published measures of the oil equivalents for key processes can be used as an indicator, expressed as a percentage of figures published for new tire manufacture (figure 4). The relative energy costs include relevant materials, processing and logistics energy costs. While these figures must be treated with caution, as they have been collated from a number of sources and therefore still require verification, they do seem to indicate a worrying trend from an environmental viewpoint, namely that waste streams which command less than 1% of the price of the original product require an additional energy input of the same order of magnitude or greater than the original product. This would indicate a case for subsidy rather than profit.

This result would seem to indicate that the current economic model will only be sustainable if:

* The investment in the initial rubber product continues to be written off once the product enters the waste stream;

* the cost of the raw materials, particularly synthetic polymers, chemicals and fillers, isignored;

* subsidies through taxes, levies and fees are used to close the gap between process costs and market price and between new product and recycled materials.

The factory economics

The general trend identified at the macro level can also be seen at the factory scale where it also becomes apparent that the non-tire and factory waste streams need to be taken into account.

For typical in-factory processes, a number of mixing stages can be identified, together with a processing and a curing stage, as shown in figure 5. While the actual technologies used are not important for this discussion, it is important to note that each stage represents an investment in the product in terms of raw materials, energy and process. Factory waste, which typically ranges between 5% and 15% of raw material usage, can be generated at any of the processing stages and typically increases in value as the product nears completion. For example, processes where parts are stamped or cut from strip or sheet usually result in volumes of webbing which form a high value scrap in terms of manufacturer investment.

[FIGURE 5 OMITTED]

Although this material has a known history, composition and formulation, it is usually treated as a waste and rejected from the factory, often at an additional collection cost. For high value materials this can result in significant production costs. Very little of the investment is recovered from this type of factory waste, with the costs being passed on to the customer and raw material supplier. Increases in waste levies and taxes are likely to directly affect this waste stream, although these costs too may be passed on. In general, it can be seen that both at a macro and at a micro level the current approaches are not economically favorable and that this gap will widen as external pressure increases.

Figure 4 - relative energy cost of tire waste streams

                     SBR     NR

Scrap tires          -1%     -1%
Crumb No. 4        -141%    0.1%
Crumb No. 3        -172%    0.1%
Crumb No. 1        -184%    0.2%
Crumb No. 2        -191%    0.3%
Chips/TDF          -128%    0.5%
Part worn tires     -10%     17%
Retreading          -34%     25%

Energy cost as % of new tire energy cost

Note: Table made from bar graph.

References

(1.) Scrap Tire Facts & Figures, Rubber Manufacturers Association, Scrap Tire Management Council, U.S. (http://www. rma.org/scrapfctn.html).

(2.) IRSG preliminary statistics: (http://www.rubberstudy. com/STATS.htm).

(3.) Rubber futures index (Japan, Asia, Europe) Reuters (www.commods.reuters.com).

(4.) Report on issues arising from transport and waste management - tires, The Environment Agency, U.K., 1998.

(5.) Life cycle cost analysis of asphalt-rubber paving materials, R. Gary Hicks and Jon A. Epps, Rubber Pavements Association of America.

Dr. Bill Watson and D.A. Brown are the principles in Watson Brown HSM. Watson began his career with the British Rubber Producers Research Association and founded what is now Rapra Technology. David Brown began his career with Phillips Electronics in 1983 and helped set up TecExec in England in 1995. This article is based on a paper given at the International Rubber Forum, September 2000, in Antwerp, Belgium.