Cryogenic processing and recycling.
Economically attractive; performance advantageous; environmentally beneficial; state-of-the-art technology; cryogenic recycled rubber meets each of these criteria.
Corporate executives, technical directors, purchasing agents and compounders continuosly search for a material or process that works better, saves money and enhances their corporate and their personal image. Pressured by their own Boards of Directors and stockholders, competitive forces, economic and political considerations, and the federal and state governments, rubber companies are finding it increasingly politic to do the "right" thing.
Of course the right thing does vary from time to time, i.e., often it is necessary for a company to assert technical superiority or maintain technical parity, at other times it is required that a company minimize its cost and conserve previous dollars, other times it is appropriate to accede to governmental and legislative regulations, and, as always, it is important to reduce product and process variation to an absolute minimum following current dictates of SPC and Pareto analysis in accordance with good manufacturing practices.
The ability of cryogenically recycled materials to meet each or all of these varying emphases accounts for the rapid growth of the utilization of these materials in the rubber industry over the past 10 years. As a practical barometer of materials trends, 10 years ago the Rubber Division of the American Chemical Society's biannual exposition had only one booth dedicated to cryogenic recycling. At the recent mini-exhibition of the Rubber Division ACS in Washington, DC in October there were no less than five booths representing cryogenic and ambient grinding companies. Currently, there are approximately 12 cryogenic-licensed facilities in North America.
Several market factors have contributed to the rise in popularity for the use of cryogenically recycled materials in the rubber industry. The consolidation within the industry attributable to merger, acquisition and joint venture has effected globalization of the industry which has in turn lead to stronger but fewer players competing for their share of the market. At the same time advances in process technology, statistical process control and demands on suppliers by their OE customers have all driven the search for advantages in processes or materials which will allow individual rubber companies to effectively compete, or in some cases simply survive. Complicating matters even more is the environmental movement concerning right-to-know, the scrap tire disposal problem and U.S. foreign trade policy, all of which have had substantial impacts on the global rubber industry.
These pressures within and outside the industry have lead to increased usage of cryogenically ground recycled rubber. There is a wealth of technical information indicating the economic advantages associated with using 5 PHR to 10 PHR in various passenger and truck tire compounds (ref. 1). The economic benefit for a modest usage (5%) in passenger tires and in truck tires has been estimated at approximately $.10 and $.54 per tire respectively. At the 10% usage level the economic benefit correspondingly doubles (ref. 2). See cost savings per tire using cryogenically ground rubber (ref.2)
Table 1 - cost savings per tire using cryogenically
ground rubber (ref.2)
Percent Passenger Truck tires tires 5% $.0980 $.5424 10% $.1861 $1.031
Based on oil at $20.00/barrel
The unique nature of the surface morphology of the cryoground rubber particle facilitates venting of trapped air in unvulcanized rubber laminate products, particularly tires, thus reducing propensity for cure blistering. Significant reductions in halobuty1 inner liner blistering have been reported.
When cryoground buty1 is incorporated into the tire inner liner, reinforcement of the cryogenically ground particle in the rubber matrix is a function primarily of particle size. Surface oxidation of the cryoground particle is of little concern because of its inherent low surface area (< .2 [meter.sup.2/] gram), thus distinguishing itself from ultra-fine, high surface area ambient ground fillers. Similarly, the cryoground rubber particle has a narrow particle size distribution within a given mesh size compared to ambient or wet ground particles, analogous to a solution polymer compared to its emulsion counterpart. This narrower particle size distribution makes a given mesh size more predictable in performance allowing less variation within a given application.
From coast-to-coast and also in the heartland, landfill costs and tipping fees have risen dramatically in the last three years and are expected to continue to rise. A number of landfills have closed their doors to tires and scrap rubber already making re-utilization of rubber scrap by manufacturers a viable economic alternative.
Concern over nitrosoamines, heavy metal contamination, and concern about effects on the water table of deteriorating rubber products further drives the incentive toward recycling of these materials. Use of cryogenically recycled materials reduces chemical releases by virtue of substitution of vulcanized for unvulcanized materials (ref. 3).
There has been substantial adverse publicity regarding tire fires that have taken place in outdoor scrap tire storage prompting the National Tire Protection Association to update its standards regarding storage of rubber tires (ref. 4).
From an environmental standpoint, cryogenic recycling makes solid economic sense and is in the best interest of both manufacturer and consumer.
Regarding state-of-the-art technology the cryogenic process represents the only technology for size reduction utilizing minimum enthalpy and entropy effects thus leaving the size-reduced particle in exactly the same state as it was prior to size reduction. Surface morphology is fundamentally a geometric (cubic/rhombic) surface readily evident on examination with an SEM (see figure 1). Ambient grinding technologies, wet and dry, impart surface abrasion/cutting with corresponding heat build-up and relatively high surface area which increase the propensity for oxidation. The cryogenic process uses liquid nitrogen to cool the feedstock particles to approximately -325 [degrees] F, far, far below first order transition temperatures for polymeric materials. The ultra-cooled material is fed directly into a high technology hammermill/screen which then feeds through a closed loop system to a multi-state screener which is also non-adiabatically precooled with low temperature nitrogen gas.
The product is thus manufactured without exposure to oxygen during the important grinding process as well as the partitioning process thus leaving the possibility of surface oxidation nonexistent. For all the above reasons: economy, performance, environmental benefit and state-of-the-art technology, the cryogenic process provides material ideally suited for both tire and non-tire areas. In the non-tire areas, the largest market segment applications are in asphalt, athletic surfaces in combination with urethane or latex binders, specialty molded rubber goods or coatings applications and color dispersions. The cryogenic process is uniquely suited for processing unvulcanized rubber scrap or virgin materials. These materials cannot be processed by conventional means without great difficulty, i.e., the heat generation associated with ambient processing precludes that technology from being used on uncured or "sticky" materials. Both cryogenically recycled scrap and virgin materials find a significant market in the brake industry and in the coatings industry primarily as a means for cost reduction, process enhancement and performance improvement.
The recycling effort, in all its multiple phases, will continue to have significant impact on the rubber industry. As opportunities for disposal of scrap materials diminish, similar opportunities arise or are treated by necessity for alternative uses of recycled rubber materials. Advances in state-of-the-art partitioning and separation technology will allow particle size reduction costs to trend downward over the foreseeable future. The liquified gas industry is less susceptible to price increases associated with petroleum dependent materials and technologies, thus increases in processing costs in the cryogenic recycling industry are forecast to increase at a rate lower than the general increase in the majority of petroleum-derived materials used in rubber, thus encouraging the utilization of this technology for continued cost reduction and performance improvement within the rubber industry.
"Cryogenics advances ground rubber technology," Eckart, R., Modern Tire Dealer June, 1980. "Tires as a consumer and source of recycled materials," Smith, Fernley G. and Klingensmith, William H., Paper #15, presented at the Rubber Division, American Chemical Society meeting Washington DC, October 1990. "Estimating chemical releases from rubber production and compounding," U.S. EPA office of Pesticides and Toxic Substances, publication EPA 560/4-88-004q, March 1988. NFPA Publication 231D "Standard for storage of rubber tires," 1989 Edition, Copyright 1989, NFPA all rights reserved.
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|Title Annotation:||rubber recycling|
|Author:||Leyden, Jerry J.|
|Article Type:||Cover Story|
|Date:||Mar 1, 1991|
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