Reclaim rubber usage and trends.
In the early days of the rubber industry, natural rubber was the only polymer available and it was very expensive and in short supply. This, along with the tact that natural rubber scrap was easily reclaimed and retained excellent properties, soon led to the development of a thriving rubber reclaiming industry. Most major rubber companies even established their own reclaiming divisions. These divisions, along with major independents, grew and prospered for most of the next century.
Natural rubber shortages encountered during World War I made the industry aware of the risk potential of being dependent on the raw material sources from politically unstable areas of the world. This, along with the rapid growth of the automobile as the primary mode of transportation, prompted research toward developing alternative natural and synthetic rubbers and improved reclaiming processes.
World War II marked the beginning of several events that combined to virtually destroy the rubber reclaiming industry and bring us to the present situation. Today there is no tire reclaim produced in the U.S. and only limited quantities of specialty reclaims such as butyl, natural rubber and silicones.
Events impacting the reclaiming industry
A summary of events impacting the rubber reclaiming industry are:
* Development of synthetic rubber and its replacement of natural rubber.
* Major tire recalls made tire producers and compounders extremely cautious.
* The Clean Air and Water Act forced established reclaimers to clean up or shut down.
* Development of the steel belted tire made scrap tire processing more difficult and produced a tire that lasted longer.
* Radial tire acceptance in the marketplace increased life expectancy of the tire and made compound cost savings less significant when spread over the expected life of the tire.
* Low priced crude oil made substitutable virgin rubber and carbon black more cost competitive with reclaim.
Before discussing these events in greater detail, let's first attempt to distinguish between the two terms commonly used as pertaining to the reuse of scrap rubber, recycling and reclaiming.
Although the two terms are now often used intechangeably, it was only in recent years that the word recycling began to appear in dictionaries. It is ironic that the recycling move came at the time when the rubber reclaiming industry, that had been in existence for over a hundred years, began a sharp decline.
Many now use a broad definition of recycling that may encompass any method from reclaiming the rubber for use directly back into a new tire compound, to burning, to building fishing reefs or swings. This article will limit the discussion to areas where the scrap rubber is processed into "reclaim" for use back into similar products where it imparts its resilient elastic properties. Certainly burning would not fit this definition.
Most vulcanized elastomers can be mechanically ground (for recycling) to a fine particle size with none of the compounding ingredients contained in the scrap being changed or removed. The size and morphology of the crumb rubber particles and the type scrap from which it is produced is normally dictated by the properties expected of the finished product.
Recycled ground rubber is being used today in a wide range of products and applications ranging from rubber compounding to brake linings to asphalt rubber for roads.
In reclaiming of scrap vulcanized rubber, the scrap is normally ground and is then treated with the application of heat, chemical peptizers and is then intensely worked mechanically to partially devulcanize (depolymerize) the rubber component. This partially devulcanized product is commonly called reclaimed rubber and can be compounded, processed and revulcanized much like virgin rubber. The availability and processing characteristics of suitable scrap sources has dictated the types of reclaim produced.
Types of reclaim
Historically, scrap tires, tire parts and tire buffings have been the primary source of raw scrap rubber available for reclaiming. Tire reclaim was therefore by far the largest volume reclaim produced. New tires were also the largest market for reclaimed rubber. However, for the reasons listed above, this market for reclaimed rubber has virtually disappeared.
Various types of natural rubber scrap have been available to the reclaimers through the years and substantial volume of both black and light colored natural reclaim have been produced. Black and red natural innertubes were a good source for scrap and were relatively easy to reclaim. The reclaimed rubber produced from natural rubber scraps is generally very tacky and was (and is) used in tapes, adhesives and other rubber compounds where increased tack and good green strength are desired. Transfer pads from molding operations are also an excellent source for good quality scrap.
With the recent spread of AIDS and growing use of latex gloves and condoms. substantial quantities of factory reject and outdated product scrap is available for reclaiming.
Like other synthetic rubbers produced in the U.S., butyl rubber production and use was spurred by shortages that developed before and during WWII. Once produced on a commercial scale, it became the polymer of choice for those applications where air retention or good aging characteristics were needed.
Factory reject and road worn innertube scrap is generated in substantial quantities, but collection of road worn scrap is difficult since tubes are generally scrapped in relatively small quantities at any one location. Butyl pad scrap from transfer molding operations also offers a good potential source of butyl scrap.
When reclaimed, butyl rubber retains much of its original properties and can be blended with virgin rubber to produce new innertubes, innerliners for tires and various types of tape. All these applications are very high tech products and the reclaim must meet stringent quality specifications for acceptance. There continues to be an excellent market for butyl reclaim.
This is a very expensive polymer and many who generate any significant quantity have their scrap custom reclaimed for reuse in their own compounds. Volumes are very small compared to other types discussed above.
Other scraps such as EPDM can be reclaimed but, generally, at much higher cost and do not otter the cost savings normally expected. These types can be and are being ground to a fine powder for recycling back into various products (ref. 1)
Through the years the reclaiming industry has been very innovative in adapting or developing processes to reclaim the various types of scrap rubber that were available. Then, as now, markets and market value of the reclaim rather than technical constraints have dictated whether a given type reclaim would be produced.
Many different reclaiming processes have been used through the years, depending on scrap characteristics and economics. Time does not allow a review of all, but an excellent review can be found in "Reclaiming rubber and other polymers," by J.A. Szilard.
The processes used currently differ from those used earlier only in the devulcanization stage. Ground rubber scrap is in most cases the teed stock for the devulcanization step. The pan process, digestor process (either wet or dry) and mechanical or reclaimator process are the current common processes used for devulcanization.
The ground rubber may be mixed with various reclaiming oils before being added to bins or boats. These bins are inserted into an autoclave then subjected to steam pressure for a period of time. The pan process was widely used in the early stages of the reclaim industry before the advent of the wet digestor. Its use is now limited to specialty types such as light colored natural rubber reclaim.
The digestor process uses a steam vessel equipped with a paddle agitator for continually stirring the crumb rubber while steam is being applied. The wet process may use caustic and water mixed with the rubber crumb while the dry process uses steam only. Various reclaiming oils may or may not be added to the mixture in the vessel. The dry digestor has the advantage of less pollution being generated and was adopted after the Clean Air and Water Act was enacted.
A mechanical or reclaimator process has been used for the continuous reclaiming of whole tire scrap. Fine rubber crumb (30 mesh) mixed with various reclaiming oils is subjected to high temperatures with intense mechanical working in a modified extruder for the devulcanization of the rubber scrap. Depending on the specification of the finished products, fillers may be added to the devulcanized product before further processing. The devulcanized rubber from each process is then strained and refined as dictated by the specification of the finished product before being powdered, baled, sheeted or extruded into the finished form.
Advantages of reclaim
Although reclaim rubber may offer a lower polymer cost, the most outstanding contribution may be as a processing aid. Some advantages of using reclaim rubber are:
* A shorter breakdown and mix time;
* Low power consumption during breakdown and mixing;
* Low mixing, calendering and extrusion temperatures;
* Fast uniform calendering and extrusion;
* Improved building tack;
* Improved green strength and firming of uncured stocks;
* Reduced swelling and shrinkage during extrusion:
Restraints in reclaimed rubber use
One would think with all these advantages and the current trends toward recycling, that the reclaiming industry would be booming. However, there are technical restraints in the devulcanization of rubber polymers, and vulcanization is, in fact, not truly reversible. The partial devulcanization or depolymerization of scrap rubber will result in a degradation of physical properties. This degradation may range from 30% to 70 o of the virgin state. This limits substitution levels in high-tech applications such as tires, but can provide the compounder of less stringent products with an excellent low cost polymer that can be used as the prime polymer or at very high substitution levels.
When natural rubber was the primary polymer used in tire compounding. the reclaim rubber was inherently a better product since it retained a higher of original properties and still gave much improved processing properties. Tire reclaim used in a typical tire sidewall compound would make up over 50% of the total rubber. As various new synthetic elastomers began to replace natural rubber, the reclaiming industry was able to develop a process as noted previously to reclaim the synthetic polymers. However, the reclaimed product was normally not as good, and substitution levels began to decline.
The introduction of the oil extended synthetic polymers in the manufacture of tires was another significant event in the history of the rubber reclaiming industry. Even with these problems the industry overcame most of the obstacles associated with the synthetic polymers and it was widely accepted in the manufacture of tires and other rubber products. As recently as 1970, the four major tire manufacturers continued to operate reclaiming plants and there were major independent companies still producing reclaim.
As auto and tire design technology evolved, problems were encountered that resulted in major tire recalls. D.O.T. became involved in some of these cases and the public, along with tire company management, soon became aware of and recognized the potential liability of producing anything but the best tire possible. Tire compounders had always (and for good reason) been a conservative lot since there was inherently a long lag between instituting change and knowing the real result.
These recalls triggered a general review of not only tire design, but compounding as well. With the natural stigma of reclaimed rubber being produced from scrap and its not having equal physical properties obtainable with virgin rubber compounds, a decision was made by several tire manufactures to phase reclaim out of their compounds even though the compound most likely had nothing to do with tire failures.
Passage of the Clean Air and Water Act and the introduction of steel belted tires were the next significant events in the decline of the rubber reclaiming industry. Technology was available to make engineering and process changes to comply with regulations and to remove the steel wire. However, a switch to the dry dynamic digestor or mechanical reclaimator process and installation of magnetic separation equipment required a commitment of large capital investment. With a declining trend in reclaim use clearly evident, all the major tire companies chose to close their reclaiming plants and accelerate the shift to virgin rubber. The fact that they all owned their own virgin synthetic rubber plants, none of which were operating at capacity. Likely influenced their decision. Several independents, spurred by the prospect of the majors outsourcing their tire reclaiming requirements, did invest the necessary capital to reach compliance and continued operating. However, as tire companies continued to remove reclaim from their compounds and with continuing tightening of the screws by government, the remaining independents found it more and more difficult to remain profitable with demand for their product falling below the break even level.
With an executive decision having already been made to substantially remove all reclaim from top of the line tires, and with continuing low crude oil prices making petroleum derived virgin synthetic rubber, processing oils and carbon black price competitive for non-tire applications such as semi-pneumatic tires, the rapid shift of the market to radial tires delivered the death blow to the rubber reclaiming industry as it had existed.
When radial tires were first introduced it was found that reclaim in carcass stocks increased building tack and improved tire building efficiency. However, with the edict to remove reclaim due to its lower physical properties, compounders soon developed compounding techniques that no longer required reclaim as a processing building aid.
Those remaining in the reclaim business had been able to make marked improvements in the reclaim quality by developing inner grinding, improving wire and fiber removal techniques, and had even improved tensile properties significantly by blending high quality natural rubber peel scrap into the product mix. However, even this improved product gained very little acceptance.
At this point, U.S. Rubber Reclaiming recognized that a total new level of quality would be required for acceptance in radial tire compounding. We developed a new process that elevated the quality level far beyond what had been available. Details of the process were reported by LaGrone and Lynch (ref. 1).
One of the main problems that causes a loss of physical properties when using a normal reclaiming process is achieving a homogeneous product without depolymerizing the rubber beyond the desired level. The old pan or wet digestor processes used a coarse crumb (approximately 10 mesh) and often put what we referred to as "guts, feathers and all" into the pot. Obviously, quality would be low, as one had to overcook the exposed surface of the rubber crumb in order to get any level of softening of the core. These "tailings" were then refined or strained out to the degree possible, but the extensive working and the remaining small tailings resulted in lower physical properties.
Finer crumb rubber (30 mesh) was later used with improved results, but this improved product, due to the same problems as described above, was still not acceptable for radial tires.
The new process described by LaGrone and Lynch used a wet grinding process to achieve a crumb fineness of approximately 200 mesh. When this product, which had a much higher surface to mass ratio, was reclaimed, no chemicals were required and only minimal heating and mechanical processing was needed. The end result was a very homogeneous reclaimed lubber with improved physical properties that from a technical view was acceptable for use in radial tires.
Unfortunately the production plant for producing this product came on line at the time when oil prices dropped to an all time post-OPEC low. This caused several companies that had approved the new product (and who had placed orders) to reconsider. They then chose to use the margin product from their own synthetic plants rather than adopt the new reclaim.
At this point, one would probably agree that this article could have been entitled "The rise and tall of the rubber reclaiming industry," and that is essentially the situation.
The recent awareness of a scrap tire problem and all the recycling activity has had no positive impact on the reclaiming industry, and is not likely to have. Those solutions that are being called reclaiming by many in the industry amount to nothing more than garbage disposal. Government won,t let us bury the scrap tire garbage, so they are paying for shredders to cut them up so that they can be burned. After 110 years with sustained research effort directed toward finding profitable products to be produced from scrap rubber we are not optimistic that substantial new uses will be found. Old ones are likely to be rediscovered, some small areas will grab fad publicity but will have little impact on actual reclaiming or recycling of waste rubber.
For the rubber reclaiming industry to have new life and experience a new life cycle, the product must be accepted and used back into tires and other rubber compounds. We feel that the technology is available to produce a suitable product for use back into most tire and many other rubber compounds. However, the products must be given a fair technical and economic evaluation. Simply saying "waste rubber will not be used in our product" has essentially destroyed the reclaiming industry and it will not see any hope for revival if this attitude does not change.
We would speculate that more tires have to date been produced that did contain reclaim than did not. Granted, some people may plan to drive 120 mph and if so would need the best possible tire made. However, neither speed limits nor auto weights are appreciably different than they were 20-30 years ago, and most people do not drive over 70 mph.
Government mandates for use of recycled rubber has not been effective. The ISTEA bill that dictated use of recycled rubber in roads has come under fire from all sides, including inside government. We do not expect that it would be any different if regulations were to be passed dictating scrap rubber use back into tires or other rubber products. What could help revive the industry would be a more realistic approach in compounding and assessing requirements for acceptable value oriented performance targeted products.
There has simply been too much scare tactic advertising to sell high priced tires that are not needed and do nothing for the consumer other than cost more. We were recently asked to look at performance tires that were sold to a lady who has never been known to drive over 50 mph. She thought she was getting the best and paid for it. She did not understand why the tread had all worn off after only 12,000 miles. I explained that the tire had a very thin tread so that it could safely run 100 mph. Did she get real value and quality?
We all know that all parts of the tire and most other rubber products are a compromise of properties. Also, that quality is an inexact term - the lady referred to above certainly did not think she got quality.
Without a reexamination of what is quality and what the customer needs, there is little chance for a revival of the rubber reclaiming industry. It will be limited to specialty reclaims such as butyl, natural rubber and very high cost polymers like silicones, where it competes on price and where it imparts special desired properties.
[1.] Jerry Lynch and B.D. LaGrone, "Ultrafine crumb rubber," 1986.
|Printer friendly Cite/link Email Feedback|
|Date:||May 1, 1994|
|Previous Article:||Dynamic properties of rubber.|
|Next Article:||Curing fumes - a threat to the environment?|