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High-value engineering materials from scrap rubber.

Scrap tires are a significant problem. As a post-consumer waste, tires have few uses; most are landfilled. Whole tires in landfills are known to "float" to the surface over time and disrupt the layered structure of modem landfills. Unused tires that are not landfilled are collected in tire piles. These frequently catch fire. Tire fires are difficult to extinguish, and they create considerable air, land and water pollution. Scrap tire piles are ideal breeding grounds for mosquitoes and rats. Outbreaks of encephalitis in the U.S. have been traced to scrap tire piles. The non-reuse of over 7 billion lbs./year of scrap tires constitutes a major lost resource.

Scrap tires also represent a great opportunity. These highly-engineered automotive components are composed of very durable, high performance materials that can be reused in high-value applications. Furthermore, scrap tires are already source separated from other waste materials, and collected in centralized locations. All that is needed is technology for enabling the rubber from scrap tires to be reformed into end products having sufficient physical properties to make its use economically viable.

The highest value for reusing scrap tires is in applications where the rubber is reused as an engineering material, as opposed to a fuel. Rubber and other elastomeric materials have prices ranging from $0.50-$5.00/lb. In contrast, scrap tire rubber used as a fuel is competing with coal, which costs approximately $0.03/lb.

The reuse of scrap tire rubber has been a challenge in the past. Because the rubber is a crosslinked polymer (thermoset), it cannot be simply melted down and molded into new products. Heating rubber above a critical temperature simply results in decomposition of the rubber. Combining ground-up scrap rubber with uncured rubber or other moldable polymer materials followed by molding/curing has not been particularly successful thus far. This is because of poor bonding between the rubber particles and moldable resins. This results in end-products with inferior physical properties. Thus, these end-products generally find applications with lower performance requirements. Materials that are used in low-end applications generally are low priced. When rubber is used in such applications, it generally is priced in the range of $0.10-$0.35/lb. In these applications the rubber is competing with other inexpensive inert, fillers, such as sand and calcium carbonate.

Recycle approach

Surface-modification is a unique technology for recycling scrap tire rubber into high value applications. This approach consists of modifying the exterior surface of scrap tire rubber that has been finely ground, and liberated from the metal and fabric. The rubber particles are surface-modified in order to facilitate combination with other types of polymers. In comparison to untreated rubber, particles that have been surface-modified are more compatible with, and bond more tenaciously to, other types of polymers. When surface-modified rubber particles are combined with other continuous phase polymers, novel "composites" are formed. This concept is illustrated in figure 1, which also lists amenable continuous phase polymers. It should be noted that different surface-modifications will be required for different classes of these amenable polymers. These "composite" materials made with treated rubber particles frequently have physical properties that enable their use in high-performance, high-value applications.

The first surface-modification that we have developed employs a proprietary reactive gas atmosphere to oxidize the surface. The treatment causes the formation of polar functional groups such as carboxylate and hydroxyl. The presence of these moieties causes the normally hydrophobic rubber particles to become very hydrophilic; they are readily wetted by water. These polar groups enhance the dispersibility and bonding characteristics of rubber in other polymer materials.

The effectiveness of surface-modification in enhancing the bonding between rubber and other material is demonstrated by comparing the bond strength of strips of rubber with polyurethane cast onto them. In T-peel tests, it was found that where polyurethane was cast onto non-treated strips of rubber, the bond strength was 3 lb./inch. In analogous tests with strips of surface-treated rubber, the bond strength exceeded 150 lb./inch; the rubber tore before the adhesive bond failed.

Because the modification involves a chemical change to the backbone, the treatment is permanent.

It is important to note that this treatment process is nonpolluting. In addition, toxicological testing has indicated that the treated rubber particles are nontoxic.

Most of our work to date has involved combining the treated rubber particles with high cost materials such as polyurethane, polysulfide and epoxy. This is because the commercial incentive for using a lower cost filler is greatest in these systems. Furthermore, it has been more convenient to perform development work on thermoset systems.

The physical properties of composites formed from the combination of treated particles with other polymers may be superior, similar or inferior to those of the unfilled matrix polymer. In part, what constitutes superior physical properties depends on the specific end use application being considered. The impact of cost reduction derived from the use of surface-modified rubber must also be factored in.

Most materials are evaluated on the basis of performance/cost. Cost reduction, in combination with having adequate properties for specific applications, often enables these composites to displace other materials for specific applications.

Economics

The cost of materials is extremely important in determining their commercial success. The current list price for surface-modified rubber particles is as low as $0.50/lb. These prices will fall to as low as $0.30/lb. from production economy of scale as commercial usage grows. There is generally a significant raw material cost reduction with use of treated rubber in engineering resins such as polyurethanes, epoxies and polysulfides, where resin prices are in the range of $1.25-$4.00/lb. For example, a molded part made in 25% treated rubber costing $0.65/lb. and 75% polyurethane costing $3.50/lb. will have raw material costs 20% lower than if it were molded in pure polyurethane.

Current public opinion and governmental regulations favoring products with recycle content provide an economic benefit to composites made with surface-modified scrap rubber particles. End products with sufficient recycle content are increasingly appreciating premium pricing.

Composite properties

The composite materials formed when treated rubber particles are combined with polyurethanes often have physical properties similar to or even superior to the unfilled polyurethane. For example, a composite consisting of 15% treated rubber particles/85% polyurethane has physical properties very similar to those of the pure polyurediane (table 1). Dynamic mechanical analysis (DMA) curves for composites consisting of 20% treated rubber particles/polyurethane are nearly identical to those of the unfilled polyurethane (ref. 1). Materials made with as much as 50% treated rubber/50% polyurethane have been shown to have engineering properties very similar to those of the unfilled polyurethane.
Table 1 - property comparison
Property Unfilled 15% Treated
 PU(*) rubber/85% PU(*)
Tensilee strength (psi) 4100 3500
% Elongation 278 275
Tear resistance (Die C) 593 522
Tear resistance (Trouser) 113 104
% Rebound 49 48
Hardness (Shore D) 50 50
(*) Polyurethane is Airthane PET 95A cured with Ethacure 300


When the properties obtained for composites made with treated rubber particles, such as with polyurethanes, are similar to those of the unfilled polymer, treated rubber can be used in place of a percentage of these other polymers on a pound for pound basis.

Property enhancements

One example of a material property of a composite made with treated rubber particles having superior physical properties is the wet coefficient of friction in polyurethanes. It is known that polyurethane tires and shoe soles become very "slippery" on wet surfaces. It has been demonstrated that inclusion of treated rubber in a polyurethane formulation can significantly increase the coefficient of friction (table 2).
Table 2 - wet coefficient of friction(*)
 Static (lb.) Dynamic (lb.)
Unfilled PU(**) 0.57 0.55
20% treated rubber
particles/PU(**) 0.58 0.69
(*) Tested via ASTM D1894-90 (modified)
(**) Polyurethane is Uniroyal Adiprene LF95 cured with Ethacure 300


Other examples of enhanced properties from composites made with surface modified rubber particles include better tear resistance of polysulfides and polyurethanes, increased abrasion resistance for polysulfides, better tensile strength for polysulfides and "low-end" polyurethanes, greater flexibility and impact resistance for polyesters, and better substrate adhesion for epoxy coatings.

Applications for treated rubber composites

The real indication of the value of this surface-modified rubber approach for rubber recycle is its success in gaining commercial usage. The market response thus far has been very strong, and numerous specific applications for surface-modified rubber particles have been developed and qualified. It is obvious that this is just the beginning and that a large number of additional uses will be developed in the future. These examples merely demonstrate the technical and commercial feasibility of using treated rubber particles.

Our modus operandi has been to support the development activities of molders in the marketplace, as opposed to performing applications development ourselves. This is because only molders and their customers really understand all of the performance requirements for specific applications. Hence, the examples summarized below have much credibility. It should also be noted that in many of these cases customers have not revealed to us all of the benefits they observe nor all of the techniques and formulations that they employed. In some cases we are under confidentiality agreements to maintain certain aspects of their applications secret.

Polyurethane foam product

A polyurethane flexible foam product using treated rubber particles has been developed. The initial incentive for using treated rubber was cost reduction. As development proceeded, it was discovered that incorporation of 20% treated rubber created a superior foam in terms of resiliency and uniformity. It is important to note that treated rubber particles, which are far larger than the cell dimensions, work very well. The company that developed this product termed the new foam a rubber reinforced foam. The company reported that they had absolutely no problems using the rubber particles in their existing plant equipment and molding process.

The development of this application is noteworthy in that it is in one of the largest current markets for polyurethanes. This presages a large use of treated rubber particles in polyurethane for property enhancements as well as for cost reduction.

Solvent-impermeable membranes and coatings

Surface-modified rubber particles are the basis of a new type of solvent-impermeable membrane/coating from Superior Environmental Products, Inc., Dallas, TX. This material was developed for the secondary containment spill dikes under gasoline/oil/chemical storage tanks. The need FOT such membranes is a result of recent Clean Water legislation. This specific membrane/coating is composed of 40% treated rubber in a liquid polysulfide. This rubber/polysulfide structure has been shown to have outstanding barrier properties for the containment of a broad range of chemicals, including organic chemicals and acids (table 3). This material is applied by spraying or rolling in place. This means that there are no scams to leak, and that it is easy to form seals around tank legs and pipes. Since the system is cured catalytically, the cure rate can be tailored to each application and can be compensated for a broad temperature range.
Table 3 - amenable materials for containment
Acetic acid 85% Phosphoric acid
Toluene 10% Nitric acid
Mineral spirits 50% Sulfuric acid
Crude oil 38% Hydrochloric acid
Butyl acetate 5% Lactic acid
Ethanol Bleach
MlBK Gasoline
Diisopropyl amine


The rubber particles are important in this application because it increases the toughness of the membrane to the point that it can be walked on and can have light vehicles driven on it. In addition, the use of treated rubber significantly reduces the raw material costs. Rubber particles that have not been surface-treated cannot be used in this application because of the insufficient dispersion in the polysulfide, and there is less of a toughening effect.

This application is noteworthy in that one environmental problem, scrap tires, has been used to solve another environmental problem, soil and water pollution from leaking tanks.

Roofing material

Superior Environmental Products, Inc. has sold the same treated rubber/polysulfide coating system, described above, as a premium roofing material.

Solid cast PU industrial wheels and forklift tires

Two firms have developed treated rubber/polyurethane formulations for molding a variety of industrial tires, casters and wheels. Prototypes have been made and tested with as much as 46.5% treated rubber particles. In general, these molding tests and the subsequent performance testing of the wheels were very successful. Several of these wheels are still being evaluated in field tests at customers' sites.

The primary incentive for these development efforts on wheels was to reduce raw material costs. However, there was at least one performance benefit bonus observed - better wet traction.

In one case, a 6" diameter, 2" wide caster wheel was molded in a composite consisting of 46.5% treated rubber/53.5% polyurethane. The polyurethane used is the same polyurethane normally used to make these wheels. The sample wheels were tested by Faultless Caster Corp. They passed the compression set test and the impact test, and they just barely failed the endurance test. The sample failed after 9,784 cycles, with passing being 10,000 cycles. The technician who performed the test reported that the composite wheel was deforming more under the 800 lb. load than do wheels that pass. This indicates that the early failure was a result of heat buildup from this flexing. Incorporating a high percentage of rubber that is considerably softer than the polyurethane used in this application would be expected to give a composite with greater overall flexibility. Clearly a slight reformulation with a harder polyurethane would produce a wheel that would pass this test.

The significance of these results is the demonstration of the technical feasibility of these hybrid rubber/PU wheels. Historically, polyurethane and rubber have competed with each other for many industrial wheel applications. Often wheels made in rubber and in polyurethane have similar prices. This is because the lower raw material costs for rubber are offset by higher molding costs. Treated rubber offers the best of both worlds; a combination of lower material costs of a treated rubber/polyurethane formulation and the lower molding costs of the urethane casting process.

More field testing of these industrial wheels and tires in a broad range of applications is required before it is determined how much of the industrial wheel market can be served by this hybrid combination. Nevertheless, these initial efforts are very encouraging.

Shoe soles

A proprietary shoe sole formulation using surface-modified rubber particles has been developed and has been approved for commercial usage.

Extruded polyester panels

A firm has developed a new line of extruded thermoset polyester panels for use as a construction material. The treated rubber particles enhance the flexibility of the panels and increase their impact resistance.

Polyurethane industrial enclosures

A family of proprietary commercial enclosures has been developed. These products, molded into 40% treated rubber particles/60% polyurethane, will replace various enclosures currently made in metal, wood and concrete. The treated rubber is a critical component in this application. This is because it improves the overall physical properties, and because it reduces the raw material costs as compared to the use of unfilled polyurethane or polyurethane filled with other fillers. Nontreated rubber particles have insufficient dispersibility in this system, and do not form an acceptable product.

PU RIM automotive components

A leading supplier of PU RIM resins is developing formulations that use treated rubber particles. The initial incentive for using treated rubber is cost reduction. Their results thus far are promising. Unlike other fillers commonly used in PU RIM (inorganic materials), the treated rubber particles do not reduce the elasticity of the end product. Thus, critical properties such as impact resistance and hardness are not compromised.

Roof sealant

Another application developed for treated rubber particles is a roof sealant. For this particular product, the formulator needed to develop a system that:

* is applied as a viscous liquid;

* cures within one minute;

* forms an elastomer with specific tensile strength, tear resistance and percent elongation; and

* is within cost constraints. The system developed is a liquid polysulfide with 40% treated rubber.

Molded epoxy components

A laboratory study on treated rubber particles in epoxy formulations was conducted at Lehigh University This work showed that the fracture toughness of epoxy resins is enhanced when treated rubber is used along with conventional carboxyl terminated nitrile rubber additives. These results suggest a potential use for treated rubber in epoxy resins used in compression molded composites, linings and coatings applications.

Cast polyurethane rollers

A manufacturer of cast polyurethane rollers is using treated rubber particles in a commercial formulation in order to increase the coefficient of friction and to reduce slippage. Rubber Millers makes a roller for a piece of equipment that moves paper. This particular roller is in contact with the paper for only about 15 degrees, which resulted in slippage and premature failure. Rubber Miller found that by incorporating 15% treated rubber particles in their formulation, the slippage was eliminated and the roller gaves much greater longevity. Thus, Rubber Millers is making a superior product while appreciating lower raw material costs.

Microcellular polyurethane tires

Three firms have developed formulations for microcellular PU tires in formulations using treated rubber. The types of tires made were for wheel chairs, bicycles and hand trucks. These developmental efforts employed loading levels of treated rubber particles as high as 30% by weight. There were few problems encountered in molding the prototype test tires. Laboratory and field testing of the various wheels revealed that the performance of the specimens made in treated rubber/polyurethane composite materials were nearly identical to those of tires made of unfilled polyurethane. Once again, a noteworthy difference was that inclusion of treated rubber improved the wet traction of the wheels - a big plus. The wheel chair tire is in final stages of testing. It is anticipated that the tire will be commercialized later this year.

DOE support

It is gratefully acknowledged that the development of this technology has been partly funded by the Office of Industrial Technologies, U.S. Department of Energy.

The development and commercialization of this technology hold promise to save a significant amount of energy on a national scale. It takes between 40,000 and 100,000 BTUs to manufacture a pound of virgin polymer, such as polyurethane precursors. It only requires about 2,000 BTUs to manufacture a pound of surface-modified rubber particles. Hence, for each pound of virgin polymer substituted by a pound of treated rubber, significant quantities of energy are saved. If all of the scrap tires generated in the U.S. were processed via this technology, it would conserve about 0.3% of the annual U.S. consumption of energy.

Conclusion

The surface-modification approach for rubber recycle holds much promise. It is reasonable to expect that this method can also be used to reuse industrial scrap rubber. The technology is economically viable, even in the absence of user taxes or tipping fees. Reusing scrap rubber as an engineering material will conserve nonrenewable resources and energy. Initial success in developing applications for surface-modified rubber particles indicates that this technology will eventually have a significant impact because of wide spread commercial implementation.

In fact, this technology represents a new dimension in materials engineering, not just a way to recycle rubber. It facilitates reduction in raw material costs and, in some cases, provides better physical properties. This will lead to material substitutions by altering the performance/price ratio of some continuous phase polymers and make them more competitive with other materials.

Ackowledgements

"Application of CRM in asphaltic materials" is based on a paper given at the April, 1994 Rubber Division meeting. "High-value engineering materials from scrap rubber" is based on a paper given at the April, 1994 Rubber Division meeting.

"Reclaimed tire rubber in TPE compounds" is based on a paper given at the October, 1994 Rubber Division meeting. Quantitative determination of PCDD/Fs during combustion of chlorobutyl-lined tires" is based on a paper given at the October, 1994 Rubber Division meeting.
COPYRIGHT 1995 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1995, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Bauman, Bernard D.
Publication:Rubber World
Date:May 1, 1995
Words:3311
Previous Article:Application of CRM in asphaltic materials.
Next Article:Reclaimed tire rubber in TPE compounds.
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