Crumb rubber is the name given to any material derived by reducing scrap tires or other rubber into uniform granules with the inherent reinforcing materials such as steel and fiber removed along with any other type of inert contaminants such as dust, glass, or rock.
Crumb rubber is manufactured from two primary feedstocks: tire buffings, a byproduct of tire retreading and scrap tire rubber. Scrap tire rubber comes from three types of tires: passenger car tires, which represent about 84 percent of units or approximately 65 percent of the total weight of U.S. scrap tires; truck tires, which constitute 15 percent of units, or 20 percent of the total weight of U.S. scrap tires; and off-the-road tires, which account for 1 percent of units, or 15 percent of the total weight of U.S. scrap tires. End product yields for each of these tire types are affected by the tire’ s construction, strength and weight. On average, 10 to 12 pounds of crumb rubber can be derived from one passenger tire.
Overall, a typical scrap tire contains (by weight):
- 70 percent recoverable rubber
- 15 percent steel
- 3 percent fiber
- 12 percent extraneous material (e.g. inert fillers)
Crumb Rubber Manufacturing Technologies
Generally, crumb rubber is produced by reducing scrap tires down to sizes ranging from 3/8” to 40 mesh particles and removing 99 percent or more of the steel and fabric from the scrap tires. There are several processes for manufacturing crumb rubber. Two of the most common are ambient grinding and cryogenic processing. A third technology-the wet grind process-is also in use in the U.S. to produce finer mesh crumb rubber ranging from 40 mesh to 200 mesh.
Ambient grinding can be accomplished in two ways: granulation or cracker mills. In an ambient system, the rubber, tires or other feedstock remain at room temperature as they enter the cracker mill or granulator.
Ambient grinding is conducive to any size particle, including whole tires. It can be accomplished in two ways: granulation or cracker mills. In an ambient system, the rubber, tires or other feedstock remain at room temperature as they enter the cracker mill or granulator.
Ambient grinding is a multi-step processing technology that uses a series of machines (usually three) to separate the rubber, metal, and fabric components of the tire. Whether using granulation equipment or cracker mills, the first processing step typically reduces the original feedstock to small chips. The second machine in the series will grind the chips to separate the rubber from the metal and fabric. Then a finishing mill will grind the material to the required product specification. After each processing step, the material is classified by sifting screens that return oversize pieces to the granulator or mill for further processing. Magnets are used throughout the processing stages to remove wire and other metal contaminants. In the final stage, fabric is removed by air separators.
Rubber particles produced in the granulation process generally have a cut surface shape and rough texture, with similar dimensions on the cut edges. Uses for the crumb rubber or granulate produced in this process include safety and cushioning surfaces for playgrounds, horse arenas and walking/jogging paths.
Cracker mills – primary, secondary or finishing mills – are all very similar and operate on basically the same principle: they use two large rotating rollers with serrations cut in one or both of them. The roll configurations are what make them different. These rollers operate face-to-face in close tolerance at different speeds. Product size is controlled by the clearance between the rollers. Cracker mills are low speed machines operating at about 30-50 RPM. The rubber usually passes through two to three mills to achieve various particle size reductions and further liberate the steel and fiber components.
These mills do not have screens built into the mill and as such the mill itself does not control the final particle. A stand-alone screening system will separate “sized” particles from oversize granules following the mill and re-circulate the oversize products. The crumb rubber particles produced by the cracker mill are typically long and narrow in shape and have a high surface area.
Crumb or ground rubber produced in this process is used in the manufacture of numerous rubber products. These include mats for domestic, commercial, recreational, industrial and agricultural use; rubber wheels for carts and lawnmowers; insulation products; lumber and other construction products.
Cryogenic processing refers to the use of liquid nitrogen or other materials/methods to freeze tire chips or rubber particles prior to size reduction. Most rubber becomes embrittled or “glass-like” at temperatures below -80°C. The use of cryogenic temperatures can be applied at any stage of size reduction of scrap tires. Typically, the size of the feed material is a nominal 2 inch chip or smaller. The material can be cooled in a tunnel style chamber, immersed in a “bath” of liquid nitrogen, or sprayed with liquid nitrogen to reduce the temperature of the rubber or tire chip. The cooled rubber is ground in an impact type reduction unit, usually a hammer mill. This process reduces the rubber to particles ranging from 1/4 inch minus to 30 mesh, with the majority of the particle distribution between 1/4 inch minus and 20 mesh. A typical throughput is 4,000 to 6,000 pounds per hour. Cryogenic grinding avoids heat degradation of the rubber and produces a high yield of product that is free of almost all fiber or steel, which is liberated during the process.
For scrap tire derived rubber, the steel is separated out of the product by the use of magnets. The fiber is removed by aspiration and screening. The resulting material appears shiny, has clean, fractured surfaces and low steel and fiber content. The final product has a range of particle sizes, which can be used as is or further size-reduced. Production of finer (40 to 60 mesh) and very fine crumb rubber (60 minus and smaller mesh) requires a secondary high intensity grinding stage.
Fine Grind – Ambient Method
Micro milling, also called wet grinding, is a processing technology used to manufacture crumb rubber that is 40 mesh and finer.
The wet grind process mixes partially refined crumb rubber particles with water creating a slurry. This slurry is then conveyed through size reduction and classification equipment. When the desired size is achieved, the slurry is conveyed to equipment for removing the majority of the water and then drying. Aside from the use of water, the same basic principles that are used in an ambient process are utilized in a wet grinding process.
The major advantage for a wet grind process is the ability to create fine mesh crumb rubber. While products as coarse as 40 mesh are produced, the majority of the particles are 60 mesh and finer. A percentage of the overall throughput is finer than 200 mesh. Another advantage for a wet grind process is the cleanliness and consistency of the crumb rubber produced. The process literally “washes” the crumb rubber particles. The wet process removes the fine particles of fiber from the crumb rubber making a very clean product.
The wet process also produces a unique morphology in the particles. This has proven effective in the manufacturing of several goods used in automotive applications and in certain molded goods.
CRUMB RUBBER MARKETS
Crumb rubber is used in a growing number of products and applications in a growing number of diverse markets. The following list is intended as a reference to the kind of markets and end-use applications that can or currently use tire-derived crumb rubber. It is in no way comprehensive.
Major markets for crumb rubber:
- Athletic surfaces and fields
- Agrimats and equestrian footing
- Automotive parts and tires
- Landscape, trails and walkways
- Molded and extruded products
- Playground and other safety surfaces
- Rubber modified asphalt and sealants
- Rubber and plastic blends
Crumb Rubber Particle Size Measurement
Different crumb rubber market segments have different crumb rubber size requirements. Within a specific crumb rubber market, each application has its own requirements in terms of particle size and purity.
Although ASTM International, formerly known as the American Society for Testing and Materials (ASTM), has standards for specifying different size ranges of crumb rubber, such as 30 mesh or 80 mesh, it is often necessary for crumb rubber manufacturers to have a more detailed knowledge of the range of particle sizes, not just the maximum size. To accomplish this, some method of particle size determination is necessary.
Sieve analysis is the most common technique for determining particle size measurement. It consists of shaking and tapping a measured quantity of crumb rubber sample through a specified number of test sieves over a specified time. The amount of sample retained on each screen is weighed and results are given as the percentage of sample retained on each screen. The recommended procedure for sieve analysis using the Rotap method is provided in ASTM 5644.
Inch, Millimeter, Mesh
In addition to sieve analysis, crumb rubber manufacturers need to understand the relationship between the inch, millimeter and mesh units of measure. There are some readily available tools commonly used in the crumb rubber processing industry to help crumb manufacturers. One such tool is the “inch to mm to mesh” chart. A sample listing from the chart shows sieve designations for test sieve openings expressed in inches, millimeters and mesh. In the example shown particles that are smaller than .111 inch or 2.80 mm in diameter, will fall through a 7 mesh screen. Processors also use ASTM specification E-11 “Standard Specification for Wire Cloth and Sieves for Testing Purposes” to conduct sieve analysis of crumb rubber. Another very useful chart is the dime chart. This compares a magnified dime to different particle sizes.
Quality is Key
There is no U.S. standard for processing crumb rubber. Nearly every crumb rubber manufacturer has their own unique processing system that they have tweaked and modified to meet customers’ specifications or end-product requirements. As a result, crumb rubber quality varies from operation to operation.
Despite this variability several key requirements for crumb rubber have emerged and are becoming the industry-recognized product characteristics that determine quality. In general, high quality crumb means low fiber content (less than 0.5 percent of total weight), low metal content (less than 0.1 percent) and high consistency. The accepted level of maximum moisture content is about 1 percent by weight. Excess moisture content limits crumb rubber use in many applications, especially molded and extruded products. Excess heat during processing can also degrade the rubber.
In addition, processors have adopted a “clean” standard for crumb rubber. At minimum producing higher quality, cleaner crumb rubber involves pre-qualifying feedstock and introducing and re-introducing crumb rubber to multiple types of equipment including magnets, density separators, and reciprocating screens to remove contaminants and sort the crumb rubber to the required sizes for different markets.
Crumb Rubber Standards
ASTM International, formerly the American Society for Testing and Materials (ASTM) produces the largest voluntary standards development systems in the world. This not-for-profit organization publishes thousands of standards per year used around the world to improve product quality, enhance safety, facilitate market access and trade, and build consumer confidence. Working in an open and transparent process, ASTM producer, user and consumer members participate in developing industry standards, test methods, specifications, guides and practices that support industries and governments worldwide.
There are several ASTM standards that apply to crumb rubber:
ASTM D5603 Standard Classification for Rubber Compounding Materials-Recycled Vulcanizate Particulate, classifies vulcanized particulate rubber according to maximum particle size, size distribution and parent materials including whole tires, tire peels, buffings generated from the tire tread and shoulder, buffings generated from tire tread, shoulder and sidewall and non-tire rubber.
ASTM D5644 Test Methods for Rubber Compounding Materials-Determination of Particle Size Distribution of Recycled Vulcanizate Particulate Rubber, discusses methods of determining particle size and particle size distribution for crumb rubber. The standard refers to the use of a mechanical sieve shaker for determining particle size and particle size distribution for crumb rubber and gives experimental details of running the test. Particle size and particle size distribution of recycled rubber depends on the end use of the material and may affect its handling, processing, and properties. Therefore, methods for analyzing particle size and particle size distribution are important tools for the rubber recycling industry.
CRUMB RUBBER GLOSSARY OF TERMS
The screens or sieves that are mounted in a production piece of equipment to separate the desired size of crumb from the other sizes, and recycle the other for the purpose of further size reduction.
A machine that tears apart scrap tire rubber by passing the material between rotating serrated steel drums, reducing the rubber to various sizes.
A technology for processing materials at very low temperatures, (typically – 80 to –120 ° C). In processing rubber, liquid nitrogen or commercial refrigeration methods are commonly used to embrittle rubber. The embrittled rubber is then processed in a hammer mill or granulator to a desired product size.
A machine that shears apart scrap tire rubber, cutting the rubber with revolving steel plates that pass at close tolerance, reducing the rubber to smaller sizes.
A machine that impacts materials against heavy metal hammers loosely pinned to a shaft, which rotates at a high velocity to reduce scrap tire rubber particle size.
1/36th of a yard, 25.4 millimeters
The opening between the wires of a screen – term commonly used to describe or measure the size of crumb rubber. Crumb rubber is sized by the screen or mesh through which it passes in the production process. The finer the screen/mesh the more openings it will have per linear inch, i.e. 30 mesh means there are 30 holes or openings per linear inch. The greater number of openings, the smaller the material must be to pass through the screen. A minus “-” symbol refers to material that has passed through the screen, i.e. -30. Technically crumb rubber sizes are expressed in two numbers. The second number is preceded by a plus (+) symbol and indicates the size particle that has been retained on the screen.
A machine that further reduces crumb rubber to a very fine particle, at ambient temperatures, using rotating abrasive discs or other abrasives.
1/1000th of a meter, .0394 inch
A large sieve of suitably mounted wire cloth, grate bars, or perforated sheet iron used to separate materials by size.
A process for classifying rubber particles. Gradations are usually expressed in terms of total percent passing or retained. The percent passing indicates the total percent of rubber that will pass each given sieve size. The total percent retained is the opposite of percent passing or the total percent passing each given sieve.
Crumb Mesh Size Range in Each Market Category
|Molded and Extruded Products||4 – 100 mesh|
|Asphalt Modification||16 – 40 mesh|
|Sport Surfacing||1/4″ – 40 mesh|
|Automotive Products||10 – 40 mesh|
|Tires||80 – 100 mesh|
|Rubber and Plastic Blends||10 – 40 mesh|
|Construction||10 – 40 mesh|