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Pulverization of Cured Rubber
The S3P technology can be applied to the plverizing of vulcanized rubber into a fine powder. The pulverized used-tire rubber is then used successfully as high-grade filler or additive that is found in a variety of recycled products:
Floor mats for farms and horse stables
Cast with TPU for tracks on sports arenas and tennis courts
Noise-dampening material
Mixed with asphalt for road construction
Fine rubber powder used as an abrasion-resistant additive in new tires and rubber products. The typical particle size for rubber powder is less than 140 meshes (100 microns).
FIG.1 shows a Bernstorff production line that converts used tires into rubber powder. The first step is a crushing mill where the robberies separated from steel cords, and then the rubber pieces of 10 to 20 mm can be fed to the pulverizer. The process of rubber pulverization is different from the S3P process with thermoplastic materials because high shear stress does not lead to melting of the rubber material. Experience has shown, however, that the process temperature should not exceed 180℃ in the pulverization sections. Above that temperature, already ground particles become sticky and tend to reagglomerate. At the same time, degradation of the rubber will set in, causing volatiles to escape. A further rise in temperature can lead to smoldering fires in the powder in the presence of oxygen due to the high surface area of the rubber powder.
For economic operation of the rubber pulverizer, the average particle size of the rubber should be less than 0.5mm. A rate of at least 1000 lb/hr is expected on a pulverizer of the size PT-90A (90-mm diameter). Experiments during the developmental stage of the pulverizer have resulted in 400 to 600 lb/hr rates for the target powder grade. Higher rates could be accomplished at a higher rotor speed but high product temperatures or uneven particle size would restrict the process and prevent it from meeting the performance targets. Also, the fraction of smaller particles decreased at higher speeds.
FIG.2 shows the machine layout of a three-stage pulverizer where the mechanical crushing of the rubber stock is carried out inside the barrels. in order to assist the cooling process, an” outboard" cooling device is a attached to the barrels; this device continuously conveys, cools, and returns to the extruder a liquid refrigerant. The powder can leave the barrels through bottom openings and enter a spiral-type cooling conveyor that elevates the powder for a gravity-fed re-entry into the next barrel section through a top opening. The powder temperature at the exit is 170℃ , and at the re-entry, it is around 40℃, it was found to be useful to screen out the fine particles of the mix before the re-entry, which improved the efficiency of the unit, the barrels and screws could be set up effectively by utilizing the existing building blocks of the twin-screw system. The pulverizer produces rubber powder at a rate of about 1000 lb/hr. the typical specific energy input is 0.4 kWhr /kg, the achieved cooling power from barrels and screws is approximately 0.1 kWhr /kg.
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