Recycling, production and use of reprocessed rubbers.

Recycling, production and use of reprocessed rubbers

Reprocessed rubber involves taking used rubber products, like tires, trim, scrap or flash, and processing them in a manner that produces a form of rubber filler or ingredients that can be reincorporated into virgin rubber compounds.

There are two general methods for producing reprocessed rubbers. These are:

* a severing of crosslinks by chemical or steam digestion to produce a product known as reclaim and

* a grinding of rubber compounds by ambient grinding, cryogenic grinding or solution grinding in water.

The purpose of this article is to provide a review of the various methods used to produce recycled rubber and to compare their characteristics and review where and why they are used.

Current usage

All methods of recycling mentioned previously are in use in the rubber industry today. Chemical digestion was extensively used for many years with the products being used extensively in tire compounding. However, the advent of radial tires requiring high green strength for processing reduced or eliminated reclaim from tires at once. It is estimated that during and after World War II reclaim rubber usage reached a volume of 600 to 650 million pounds per year. This has fallen drastically to a current usage of 90-100 million pounds of reclaim with about half of this used by tires. The major single, biggest component is reclaimed butyl. It represents perhaps 40% of all chemical reclaim used.

There are three methods of grinding rubber in the U.S. These are ambient grinding, cryogenic grinding and solution or wet grinding. It is estimated that between 110 to 130 million pounds of rubber are ground in the U.S using these methods.

About 70-75% of this is reincorporated into rubber compounds with the remainder being added to other products like asphalt (figure 1).

Review of various types of recycled rubber

Reclaim rubber The process of reclaiming rubber by chemical digestion has been in use for over 100 years. Early processes involved the treatment of the rubber cord mix with acid. Acids attack cotton, rayon and nylon. The acid treatment was used to remove the reinforcing components. The rubber was ground and treated. It was then washed and the rubber steam devulcanized and formed to shape where it could then be reincorporated into virgin rubber compounds. This process was a multi-step process.

An improvement on this technique was the use of alkali digestion. The rubber fiber mixture was ground and then mixed with a dilute sodium hydroxide system. This mixture was reacted under high pressure steam for about a day. The process of removing fiber, sulfur and supposedly devulcanization, all occurred in one step. This was a major improvement over previous methods.

Later developments involved the use of chemical reagents, e.g. mercaptans that reacted with the carbon-carbon bonds. This allowed the control of the devulcanization process. In addition, the use of refining mills provided a method of producing the reclaim more homogeneously and uniformly. The chemical digestion method remains in use today with minor modifications and improvements.

The major benefit of using reclaim was it lowered cost compared to virgin rubbers. Reclaim typically sells for 20-30% of the cost of its none reclaimed counterparts. In addition to cost reduction, reclaimed rubber also imparts some very desirable improvements in processing. Reclaim has much lower nerve than virgin polymers. As a result, compounds containing reclaim have much lower die swell. This produces uniform die swell and extrusion rates. It also increases calendar rates and in general, improves flow and mold filling.

One of the shortcomings of reclaim is that it lowers the green strength and tensile strength of the compounds in which it is used. Reclaim is in fact a mixture of rubber, black, oil, zinc oxide, stearic acid and other compounding ingredients used in the original compounds. The levels of reclaim used are generally 15 to 45 phr, about half of which is rubber and the other half compounding ingredients. In some low performance requirement rubber products like mats, much higher levels can be used (refs. 1-5).

Steam digestion of silicone rubber produces a useful silicone reclaim. The product is used to reduce the cost of silicone compounds. It has been widely used by silicone automotive ignition systems. The resulting compounds have excellent aging characteristics and still possess outstanding electrical properties (ref. 6).

During and after World War II, reclaim usage expanded radically. This was due to a shortage of rubber during the war when reclaim was used to extend compounds as much as possible. This was followed after the war by the rapid expansion of the automotive and tire industry. Almost all of these tires were of bias design. Reclaim was used in many tire applications. It was especially useful in carcass and innerliner compounds. It was also used in sidewalls, chafers and rubber used in bead components. The introduction of radial tires changed the property needs for compounds used in tire manufacturing. The carcass and black sidewall had to have higher green strength for tire building. In addition, high tensile strength and compound thickness was needed in black sidewalls and carcasses while the tires were in service. This was accomplished by increasing the levels of natural rubber used and either eliminating or greatly reducing the volume of reclaim.

Butyl reclaim made from tubes is still used in the innerliner of both bias and radial compounds while some reclaim is still used in low performance bias tires.

There is still a fairly large volume of some high quality rubbers like natural rubber and butyl being reclaimed. These are used in some solid tires and other mechanical goods where it is used to extend the compound without having too much adverse effect on the product. Tables 1 and 2 show some typical recipes and properties of compounds containing reclaim. One is a bias tire carcass and the other an automotive molded good (ref. 7).

Devulcanization Most recycling techniques involve either the mechanical severing of physical bonds of rubber articles or cleaving the carbon-carbon bonds by chemical agents producing reclaim rubber. In both instances, little effort has been exerted on the surface bonds.

The concept of devulcanization or the severing of sulfur bonds has been explored to a limited degree. Work to-date has been largely by investigating this as an analytical probe for studying crosslink characteristics. There are no known commercial processes that specifically are directed at devulcanization where the polymer sulfur chains would be left largely intact while the three dimensional crosslink network is reversed. The concept of devulcanization, if developed, may offer a means conceptually reversing the vulcanization process, producing a rubber product with many of the compound's original properties remaining intact.

This contrasts with chemical reclaim where carbon-carbon bonds are believed severed resulting in much shorter chains in the polymer network. The result is a loss in physical strength and toughness. Devulcanization could offer an alternative route to larger quantities of used and scrap rubber articles (ref. 11).

Ground rubber The first step in the manufacturing of reclaim is grinding of the rubber part to be reclaimed. The rubber or rubber fiber mixture is ground to increase surface. This is needed to increase the rate of the chemical reaction in reclaiming and also to produce a more uniform product. As mechanical grinding became more effective, it was found that small particle size vulcanized ground rubber could be added to the rubber compound to reduce cost. Retreaders had for many years been adding rubber dust obtained from carcass preparation prior to retreading (ref. 18). However, the amount of dust available was limited. Ground rubber used in compounding varies from 10 mesh to 200 mesh. Figure 2 shows particle sizes for comparison (ref. 21).

Ambient grind Using effective types of mechanical grinding techniques, like serrated grinders, recyclers grind various types of scrap and used rubber articles. Articles with steel and fiber have the reinforcing materials removed by various mechanical methods. The rubber is then ground to a particle size from 10 to 30 mesh.

Ambient ground rubber is widely used in tires and mechanical goods. It generally is used at levels of 5 to 20 phr. The larger the particles (lower mesh size) the rougher processing compounds will be that contain ambient (room temperature ground). The name "ambient grind" is somewhat of a misnomer since the grinding, in fact, generates some heat during processing. With high modulus or high durometer compounds the amount of heat generated can be high resulting in a heating of the rubber and perhaps some degradation. In addition, it is theorized that ambient grinding produces an irregular shaped particle with many small hair-like appendages that attach to the virgin rubber matrix producing an intimately bonded mixture. The cost of ambient grinds varies from $.11 to $.15/lb.

Cryogenically ground rubber In the mid 1960s, the technique of grinding scrap rubber and trim in liquid nitrogen was developed. The process involves placing small pieces (1"x 1" x 1/2") into liquid nitrogen and grinding into a fine powder. The particle size varies from 30 mesh to 100 mesh for most products. The particle size is controlled by the time spent in the liquid nitrogen and by screens placed in the grinding chamber controlling the exodus of the rubber (refs. 8-10). Figure 3 shows the relative particle of four sizes of different cryogenically ground rubber: 40, 60, 80 and 100 mesh (ref. 14).

Uses of cryogenically ground rubber Cryogenically ground rubber is used in tires, hose, belts and mechanical goods, wire and cable, and numerous applications. It is especially useful in producing a product for tire innerliners. The particle size chosen is controlled by cost and the fineness needed to produce the desired processing. The finer the particle size, the smoother the calendered sheets and the finer an edge that can be produced on extrusions. Generally, the cost increases as the particle size decreases. The cost of 40 mesh is usually in the mid to high twenty cents per pound area, while small particle sizes like 80 to 100 mesh cost $.30 to $.40/pound (refs. 12-17).

Processing and mixing of cryogenically ground rubber

The following is a processing and mixing guide for using cryogenically ground rubber (ref. 13). Recommendations, though developed specifically for cryogenically ground rubber, apply to the use of all ground rubbers of varying particle sizes.

Processing It has been found that certain particle sizes are more suitable in specific applications. The following are some recommendations for optimizing the benefits of cryogenically ground rubber (CGR).

Extrusion - On feathered edge extrusions, the 80 to 100 mesh CGR is needed to avoid fracturing and rough edges. On thick section extrusions, the 50 to 60 mesh CGR can be used depending on final product surface smoothness requirements. Normally the percent of CGR is lower, approximately 5% for fine edged configurations.

Calendering - For optimum surface smoothness of products that are .060" or less in thickness, the compound requires 80 to 100 mesh CGR. Where smoothness is not critical, 30 to 60 mesh has been used in all gauges of calendered sheets with satisfactory results. Levels of 5% to 10% are common.

Molding - CGR in all mesh sizes aides in removing trapped air during molding. The cured rubber particles provide a path for the air to escape by bleeding air from the part. Once cured, the compound behaves like a composite with the impermeability being determined by composition of the polymer blend.

Mold flow - It has been observed that CGR generally improves mold flow.

Shrinkage is usually less for compounds containing CGR. The shrinkage reduction is proportional to the amount of CGR in the compound. If CGR is used in concentrations sufficient to reduce shrinkage (5-10%), less mold hang up occurs with intricate parts or undercut parts. The data in table 3 show the properties of a 60 mesh cryogenically ground rubber in a tire inner liner.

Wet or solution process grinding

The Gould Co. and GenStar both developed an ambient grinding process that reduces the particle size of rubber by grinding in a liquid medium. The process involves putting a coarse ground rubber crumb (10-20 mesh) into a liquid medium, usually water, and grinding between two closely spaced grinding wheels. The particle size is controlled by the time spent in the grinding process. By placing a screen of the desired size at an exit port, the ultimate particle size can be controlled (refs. 19-21). Particle sizes as small as 400 to 500 mesh (20 microns) have been reported, while the commercial sizes are commonly reported in the 80 to 200 mesh size. A chart of particle size frequency from some early development work is shown in figure 4 (ref. 19).

Wet ground rubber is used in many of the same applications as ambient and cryogenically ground rubber. This includes tires, retreads, solid tires, and molded and extruded mechanical goods. Wet ground rubber sells for $.22 to $.25/lb. and is available in sizes from 60 to 200 mesh on a commercial basis. It is usually used at levels of 5 to 20 phr. the advantage of the fine particle wet ground rubber is that it allows good processing, producing relatively smooth extrudates and calendered sheets. Tables 4 and 5 show how wet or solution ground rubber affects rubber compound properties in a radial passenger tread and sidewall (ref. 21).

Summary

With the serious tire disposal problem that exists in the U.S., many alternatives have been examined to recycle used tires and scrap rubber. This includes grinding and reclaiming of tires for reincorporation back into virgin compounds (refs. 22-24).

Reclaim usage has been declining in tires due to the increasing use of radial tires, both passenger and truck tires. The decline has probably stabilized and reclaim usage will likely remain constant. Reclaim rubber is a very useful material for reducing cost while having positive benefits on processing, flow, knit, nerve, die swell and bloom. The entire concept of reclaim needs to by investigated for other overall potential as a way of scrap disposal.

The use of the different types of ground rubber has been growing steadily. Ground rubber has replaced reclaim in many tire applications including treads, carcasses, innerliners, sub-treads and bead components. It is also used in mechanical goods, footwear, solid tires, mats and retreads.

In cryogenically and wet ground rubbers, smaller particle size allows recycled rubber to be used at moderately high levels and still allow compounds to be processable. The use of these products and ambient ground rubbers will continue to grow. Table 6 summarizes amounts and types of various recycled rubbers used in the U.S. and lists manufacturers. [Figures 1 to 4 Omitted] [Tabular Data 1 to 6 Omitted]

References and bibliography

[1]"Reclaim rubber" from chapter of R.T. Vanderbilt Handbook by Ball, J.M. Published 1958. [2]"Reclaim rubber" from chapter in Introduction to Rubber Technology, written by Hall, J.M. Published by Reinhold Publishing Corp., 1963. [3]"Reclaim rubber" from chapter in Rubber Technology, written by Brothers, J. Published by Robert E. Krieger Publishing Co., 1973. [4]"Reclaim rubber" from chapter in R.T. Vanderbilt Handbook by Smith, F. Published in 1978. [5]"Reclaimed rubber" from chapter in Rubber Technology 3rd Ed., written by Schaeffer, R. and Isringhaus, R.A. Published by Van Nostrand Reinhold in 1987. [6]"A profile of silicone reclaim," Bowers, B., Barber, D. and Allinger, R. ACS Rubber Division meeting, October, 1986. [7]Manual of Reclaimed Rubber, edited by Ball, J.M. Published by Rubber Reclaimers Association, Inc., 1956. [8]"The Tire Cycle solution: Minnesota's answer to the scrap tire problem." Stark, J. III. Tire Technology Conference, Clemson University, Octover 28-29, 1987. [9]"Ambient ground rubber crumb," Winters, R., ACS Rubber Division meeting, October, 1986. [10]"Reprocessing uncompounded elastomers," Eggleton, R. and Fesus, E., ACS Rubber Division meeting, October 10, 1986. [11]"Microwave devulcanization of rubber," U.S. Patent 4104205, issued to Novotny, D., Marsh, R., Masters, F. and Tally, P. to Goodyear Tire and Rubber Co., No. 4, 104, 205, January 6, 1976. [12]"Tires as a source and consumer for recycled rubber," by Smith, F. Klingensmith, W., at Rubber Division meeting, October 1990. [13]"Fine particle rubber technology: Properties and processing," published by Midwest Elastomers, 1990. [14]"Fine particle rubber technology: Butyl rubber," published by Midwest Elastomers, 1990. [15]"Fine particle rubber technology: EPDM," published by Midwest Elastomers, 1990. [16]"Cryogenically ground rubber," Smith, F., paper presented at ACS Rubber Division meeting, October 1986. [17]"Cryogenics advances ground rubber technology," by Eckart, D., Modern Tire Dealer, June 1980. [18]"Controlled particle size buffing dust," by Eckart, D., Modern Tire Dealer, April 1980. [19]"Ultrafine recycled rubber," Swor, R., Jensen, L. and Budzol, M., Gould Incorporated, ACS Rubbe Division meeting, May 1980. [20]"Ultrafine crumb rubber," Lynch, Jerry and LaGrone, B., ACS Rubber Division meeting, October 1986. [21]Rouse Rubber Industries, Inc., technical brochure, distributed at ACS Rubber Division meeting, Fall 1990. [22]"Scrap tire disposal procedures," Rubber Chem. and Technology, Volume 51, No. 3, July-August, 1978. [23]"The shape and size of the scrap tire problem and some potential solutions," Snyder R., Uniroyal-Goodrich Tire, Clemson University Tire Technology Conference, October 29, 1986. [24]"Grafting of waste rubber," Adam, Osredkar, Sebemik, Veksli and Ranogajee, pp. 660-668, Rubber Chemistry and Technology, Volume 63, No. 5, November-December 1990.