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To produce
crumb rubber, it is usually necessary to further reduce the size
of the tire shred or chip. This is accomplished by grinding techniques
generally categorized as ambient or cryogenic.
Ambient Process
Ambient grinding can be accomplished in two ways:
granulation and crackermills. Ambient describes the temperature
of the rubber or tire as it is being size reduced. Typically, the
material enters the crackermill or granulator at "ambient"
or room temperature. The temperature of the rubber will rise significantly
during the process due to the friction generated as the material
is being "torn apart." Granulators size reduce the rubber
by means of a cutting and shearing action. Product size is controlled
by a screen within the machine. Screens can be changed to vary end
product size.
Rubber particles produced in the granulation process
generally have a cut surface shape, rough in texture, with similar
dimensions on the cut edges. Uses for the crumb rubber or granulate
produced in this process are covered in the products
and applications section of the crumb rubber area.
Crackermills - 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. Crackermills are low speed machines and the rubber is usually
passed through 2-3 mills to achieve various particle size reductions
and further liberate the steel and fiber components. The crumb rubber
particles produced by the crackermill 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 are discussed
in the products and applications section.
see
typical ambient grinding process
Cryogenic
Process
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°F. 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 is cooled in a tunnel style chamber or immersed in
a "bath" of 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 hammermill. This process reduces
the rubber to particles ranging from 1/4 inch minus to 30 mesh.
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 due
to the clean breaks between the fiber, steel, and rubber.
Other
Processes
In addition to conventional ambient grinding techniques
and the cryogenic process, there are several proprietary wet-grinding
processes in use today in the U.S. for producing fine and super-fine
grades of crumb rubber.
Production of finer crumb rubber (40-60 mesh) and
veryfine (60- mesh) requires a secondary high intensity grinding
stage.
Micromilling, also called wet-grinding, is a patented
grinding process in which tiny rubber particles are further size
reduced by grinding in a liquid medium, usually water. Grinding
is performed between two closely spaced grinding wheels.
Fine mesh crumb or ground rubber can be used in asphalt
rubber, roofing product, coating and sealant products, as an ingredient
in numerous automotive products including
new tires.
see
typical cryogenic grinding system
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