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

This is the second of a two-part series. The first installment appeared in the November issue.

Novel concepts

If it is accepted that current economic and technical approaches do not meet the environmental, social and legislative pressures being brought to bear on the industry, then what alternatives are there? Maybe it is time to challenge fixed beliefs and open up new directions for the rubber industry? Here, a number of concepts are briefly outlined, more to initiate discussion than to present a complete solution.

Rubber as a recyclable material

If rubber can be considered to be a material that could be recycled, then a totally different economic model could be envisaged, along the lines of models applied to the glass and steel industries. This would require recycling technologies that retain the highest level of the rubber properties for the lowest additional energy/resource investment during processing.

In ascertaining criteria for this technology, use can be made of the cost analysis data from the existing industry to identify rough performance boundaries. The following assumptions can be made for end of life products to be recycled into new products:

* Useful product: The recycling process should provide a material stream that is competitive in price to that of raw materials. (Although the raw materials will contain fillers, curing agent residues and other expensive processing additives, it is assumed here that these will exert a low order effect on the overall material price). This means that the price commanded by the recycled material must be in the region of 750 Euro/mt for natural rubber products and typically 1,400 Euro/mt for synthetic products such as SBR, based on current raw material prices.

Technical performance (mechanical and chemical) for critical parameters of the recycled materials would need to be within product tolerance, typically within 10%, although letting down materials by combining raw and recycled stock may reduce this requirement. In most cases, reformulation of the compound to take recycled material properties into account is not favored.

Further, the recycled material stream would have to meet the typical industry supply criteria such as delivery guarantees, quality specifications and delivery format. Recycled materials for higher specification may also need to undergo a more rigorous sorting before recycling to ensure compatibility of formulation as most manufacturers do not want to have to constantly reformulate to meet the deficiencies of the feedstock. For products such as tires and seals, failure to perform can be a life threatening issue.

* Resources (energy and materials): Processing costs should be of the same order as those for raw material feedstocks. (It can be argued that recycled materials could be regarded as compounded stocks and therefore capable of being introduced at a later stage in the manufacturing process with associated energy savings. As this introduces another variable which will be a function of the process into which the material is to be introduced, it is not included in this rough order-of-magnitude estimate). This means that typical processing for NR based products would be of the order of 16 GJ/mt (approximately 4.5 liters of oil equivalent), while SBR would be of the order of 160 GJ/mt (approximately 22.5 liters of oil equivalent). (These figures are based on old calculations and require revision to reflect advances in manufacturing process efficiencies). It should be noted that this is an integral energy balance that includes recovery, logistics and processing.

* Emissions and waste by-products: In order to maintain the environmental balance, emissions and waste by-products from the recycling processes should be less than the emissions and waste by-products generated by the initial material manufacturing processes.

For in-house recycling, the following assumptions can be made:

* Useful product: As this recycled material is already in-house, market price is less of an issue as long as the material is available in a form suitable for reprocessing, although there are clear economic arguments to be made for focusing on materials that have a high initial cost or high energy/resource demands. The windfall from reduced raw material and waste disposal costs should be included in the economic analysis. For operations where an external operation deals with the waste and returns the recycled materials, similar price constraints apply as for end-of-life material streams.

Technical performance requirements remain the same as for general recycling, although given the detailed knowledge of material history, a closer match to the original material may be expected. Given the typical in-house fall-off rates, in-house recycled materials need only comprise 5 to 15% of the final product composition.

As the material will have been subject to in-house inspection and controls, the history of the scrap is well known.

* Resources (energy and materials): Processing costs should be of the same order as raw material feedstocks described above, although, depending on the stage within the process generating the waste, initial processes may be skipped when incorporating the recycled materials.

Recycling technologies

The lack of an economical recycling technology that can unlock the true economic value of waste rubber by providing a recycled material with properties approaching those of virgin feedstock is the major barrier to the implementation of models based on rubber as a recyclable material. The ideal form of recycling is through devulcanization in which the vulcanization process is reversed to provide a soluble material for re-use. In simplistic terms, a technology that can break existing crosslinks while causing minimum damage to the polymer backbone and result in a material which can readily take part in further crosslinking processes is all that is required.

Five categories of recycling technology are identified here, namely:

* Downcycling;

* high energy breakdown;

* chemical modification;

* biological breakdown; and

* mechanochemical recycling.

These can be roughly positioned by examining their consumption of resources in terms of energy, materials and chemicals, plant/process and the emissions and waste that result from the recycling process. If this is viewed against the ability to retain the properties of the original material, then a simple classification can be made, as illustrated in figure 7. An ideal technology would tend towards a low resource consumption and high property retention.

[FIGURE 7 OMITTED]

These categories represent a very rough bundling of technologies.

Downcycling

The term "downcycling" is sometimes used when describing processes in which the properties of the recycled material are significantly reduced when compared to the original material or compound. For rubber, this often indicates either a significant change in mechanical properties as a result of backbone breakage or a change in chemical properties as a result of chemical additives.

When downcycled, typically by mechanical crumbing, rubber is often used as a filler in another material which is sold either as a base material or incorporated into a suitable product or application. It is often found that the finer the crumb, the easier it is to incorporate into fresh recipes. Typically only the bulk properties of the crumb are used, although sometimes additional performance characteristics are achieved through chemical modification. The economics indicate that the new materials, products and applications must command prices that are sufficiently higher than the base crumb. These materials often require new markets and new products (and applications) which classically represents the most difficult of marketing challenges requiring time and resource to develop.

A number of applications have already been identified for rubber crumb, such as bulking materials in asphalt road surfaces and other civil engineering applications, although there are limits to the volume that can be included. A number of products has also been identified for materials with reduced performance, such as rubber playground surfaces, car mudflaps, mats and carpet backing material. The limited uptake of these materials reflects the effort required in developing these new markets, products and applications.

From the point of view of chemistry, the vulcanization process links up the raw rubber chains into a gigantic single molecule. "Devulcanizing" is the process of unlinking all these chains again so that they can again behave virtually as the original raw rubber and mixtures with carbon black as virtually that of the compounded stock from which rubber products are made.

Chemical devulcanization has been an aim all the way back to Charles Goodyear's discovery of vulcanization in 1839. Full chemical devulcanization has yet to be achieved, mainly due to the complexity of the rubber chemistry coupled with the large range of material formulations and the need for a resultant material which cannot contain chemicals that upset the curing reaction when added to a compound stock. Academically, some thermal chemical reactions can so convert a vulcanizate. Warner (ref. 17) has summarized such chemicals, which have been found to be chemical probes capable of splitting various sulfur bonds. Unfortunately, none has led to a commercial process yet and no chemists have foreseen one.

An alternative approach is to reduce the rubber material to a small particle and then to chemically reactivate the surface to enable it to take part in further curing reactions. This can be done by either chemically treating the surface of the crumb or by coating it in a latex film. Both these approaches have met with limited success to date.

A new phenomenon of an alcohol devulcanization was announced earlier this year by Goodyear, but it currently seems to be just a chemical curiosity.

High-energy breakdown

An alternative approach to devulcanization is through the use of high-energy fields to excite the material to a point of crosslink rupture. Typical excitation sources are in the ultrasonic and microwave spectrum. Larger scale plants are in operation, although resource consumption (high energy inputs) may be a limiting factor.

Through pyrolysis, rubber can be thermally degraded in the absence of oxygen to form component materials such as gas, oil and carbon which can be sold as base materials or energy sources. The environmental balance of pyrolysis plants is based on the premise that the only way to unlock the resources within waste rubber is to return it to component materials. As such it represents a high resource consumption.

Coalite has recently announced in the U.K. that a useful fuel/oil can be obtained from tires that have been "processed" for nine hours at 600 [degrees] C in their specially designed ovens.

Biological breakdown

For sulfur crosslinked materials, research indicates that microbes with sulfur affinity may be introduced to rubber waste streams to break down the crosslinks. Such technology is still a long way away.

Mechanochemical solubilizing

The basis of mechanochemistry is the change in the chemistry of a material when mechanical energy is applied. Initially, this was applied to increase the understanding of the effects of mastication on uncured material stocks. Watson and co-workers in the 1950s showed (ref. 6) that the softening of rubber by cold mastication was due to the rubber chains being so extended in their central sections that a main-chain bond there was ruptured. The ruptured ends were free radicals. Normally, these free ends were terminated by combining with oxygen. They could be arranged to react otherwise, combining in pairs in absence of oxygen, reacting with an added small-molecule radical acceptor, adding to the surface of a reinforcing filler and initiating free radical chain reactions.

Based on this principle, Watson in the 1990s conducted a series of laboratory scale experiments (2 g sample size) using a novel mixer to show that a rubber network, sufficiently extended, would rupture preferentially at crosslinks. These were likely to be regions of stress concentration and the sulfur bonds at the crosslinks were of less bond strength than the carbon-carbon bonds of the chain segments between crosslinks. It was further expected that bonds within the crosslink, rather than adjoining bonds in the chain segments, would split (figure 8). This work has been patented (PCT/ GB96/00956) and the mixer termed a high shear mixer (HSM).

[FIGURE 8 OMITTED]

This early work has been extended to include a wide range of viscoelastic polymers, including NR, SBR, BR, EPR, EPDM, CR and NBR, and an initial 1,000 times scale-up to a 2 kg batch size in 1998.

While work on SBR and NR tire compounds reported in 1998 indicated the potential of the technology, recent work has revealed very promising results. For example, recycled SBR tire compound which has been let down and cured, with more than 80% recycled material in the sample, has yielded UTS values in excess of 16 Mpa and which are within 80% of the original material values, with EB above 350%. Already providing a useful material for a number of applications, further optimization of the process may result in a technology that is capable of fulfilling the recycling role using mixing energies, which are of the same order of magnitude as established mixing technologies already in the industry.

A European funded, two year Craft project, was started recently in which nine partners in Germany, the Netherlands, Portugal and the U.K. will be actively assessing this technology, looking to both scale it up and apply it to existing and new products and processes.

Discussion

Historical note

The authoritative "History of the Rubber Industry," published by Hoffman in 1952 with contributions from prominent members of the rubber industry, states that, before World War II, of the total of two million tons of rubber goods manufactured annually, some 360,000 metric tons consisted of reclaimed rubber (some 18%). While the reclaimed rubber gave tensile strengths of only around 5 Mpa, compared with 20 Mpa for new materials, it still constituted a usable material suitable for a number of products produced by the current industry.

Rubber as a "recycling industry"

Returning to the simplified life-cycle model presented earlier, the following modifications could be considered:

* The elimination of the waste stream to landfill/stockpiling which will result in an additional 20% to 30% of waste materials to be processed;

* the inclusion of in-house recycling streams that will return much of the 5% to 15% in house scrap into product; and

* the strengthening of the recycling stream to enable 20% to 25% of materials to be returned to the supply chain. These would typically be "high value" materials based on economic and environmental criteria.

These changes are reflected in figure 9.

[FIGURE 9 OMITTED]

While it is too early to predict the impact of changes such as these on the rubber industry, the following issues are clear:

* While considerable pressure is being brought to bear on the industry to move towards a recycling model, care must be taken to ensure that any new economic model ensures a better future for the entire industry, including, in particular, the natural rubber producers if the diversity of materials is to be maintained.

* The premise that rubber waste is of no or low value and therefore an environmental burden needs to be changed. Bodies currently coordinating the mass destruction of waste rubber need to adopt a new premise that reflects the true economic and environmental value of waste rubber, and set economic and technical targets accordingly.

* Finally, environmentally friendly recycling technologies are being developed to unlock the materials and energy in rubber waste and end of life products, including scrap tires. These must be encouraged and judged using clearly defined technical and economic criteria.

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.

(6.) The chemistry and physics of rubber-like substances, L. Bateman ed., Maclaren Press, 1963.

(7.) Tire recycling, European Tire Recycling Association, Paris, France (http://www.etra-www.org).

(8.) Finnish Tire Recycling Ltd. (http://rengaskierratys.com).

(9.) Dismantling and recycling strategies and processes, Georgia Tech Research Institute. (http://mime 1.marc.gatech.edu/Courseware/auto2/D& R.html).

(10.) Exchange Floor - World Chemical Exchange, Chem-Connect Inc. (http://www.chemconnect.com).

(11.) Spot prices of crude oil motor gasoline and heating oils, 1999 to present, National Energy Information Center, U.S. (http://www.eia.doe.gov).

(12.) European Union (EU), Coking coal import costs from non-EU countries, National Energy Information Center, U.S. (http://www.eia.doe.gov).

(13.) Power from scrap tires at Wolverhampton, Caddet Renewable Energy, Caddet Renewables Technical Brochure, June '94.

(14.) Rubber Technology - 3rd Edition, Maurice Morton (ed.) Van Nostrand Reinhold, New York, 1987, ISBN 0-442 -26422-4.

(15.) Rubber Technology Handbook (Kautschuk-Technologie), Werner Hoffman, (translated Bauer, Meinecke), Hanser/Gardner Publications, 1996, ISBN 1-56990-145-7.

(16.) Handbook of polymer testing: Physical methods, Roger Brown (ed.), Marcel Dekker, 1999, ISBN 0-8247-0171-2.

(17.) W.C. Warner, Rubber Chemistry and Technology 1994, 67, 559.

(18.) Principles of Polymer Chemistry, Paul J. Flory, Cornell University Press, 1953.

(19.) Properties of Polymers, D.W. van Krevelen, Elsevier, 1990, ISBN 0-444-88160-3.

(20.) Natural Rubber Science and Technology, A.D. Roberts (ed.), Oxford University Press, 1988 ISBN 0-19-855225-4.

(21.) Rubber Technology and Manufacture (2nd Edition), C.M. Blow, C. Hepburn (ed.), Butterworth Scientific, 1982, ISBN 0-408-00587-4.

(22.) Rubber Chemistry, J.A. Brydson, Applied Science Publishers, Ltd, 1978, ISBN 0-85334-779-4.

(23.) Rubber Compounding (2nd Edition), F.W. Barlow, Marcel Dekker Inc, 1993, ISBN 0-8247-8968-7.

(24.) Science and Technology of Rubber, F.E. Eirich, Academic Press Inc., 1978, ISBN 0-12-234360-3.

(25.) The Physics of Rubber Elasticity, (3rd Edition), L.R.G. Treloar, Clarendon Press, 1975, ISBN 0-19-851355-0.

(26.) Rubber Processing, J.L. White, Carl Hanser Verlag, 1995, ISBN 1-56990-165-1.

(27.) Polymer Stress Reactions (Volumes 1 and 2), A. Casale, R.S. Porter, Academic Press, 1978.

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.