Cryogenic pre-processing and centrifugal sifting create super-fine rubber powders that spawn new applications and dramatic energy savingsTUCKER, GA—Lehigh Technologies combines a patented cryogenic pre-processing technique and a Kason centrifugal sifter to turn recycled tire granules into the finest rubber powders on the market. The company's fine, 80 to 140 mesh (177 to 104 microns) and super fine powders, 140 mesh (104 microns) and above, are opening new applications in sealants and coatings, and expanding rubber powder consumption in new tires and recaps.
Recycled rubber particles have long been used as fillers and performance enhancers for more costly elastomers. Approximately one billion pounds will ship this year for use in carpet underlay, flooring material, belts, hoses, engine mounts and dampers, wire, cable, paints and adhesives. While the market has been growing at a robust 25% per year, even greater growth is predicted.
"As we make powders finer, the market for them becomes almost unlimited," notes Bill Meckert, Lehigh's vice president of engineering.
A number of manufacturers ship rubber powders 40 mesh (420 microns) and larger, and a few produce up to 100 mesh (149 microns), but no one matches Lehigh's 240 mesh (60 microns) rubber powder. Ultra fine powders in the 200 to 240 mesh (74 to 60 microns) range are essential for applications, such as sealants and automobile undercoatings. These materials are applied with fine orifice nozzles that would clog if larger particles were used.
The smaller the particle size, the more rubber powder can be added to tires and recaps. Finer particles present a larger surface area for bonding and, therefore, lower the risk of delamination. Since recycled rubber powder costs about half the price of virgin material, the higher the powder content, the greater the cost benefits. One pound of recycled rubber added to a tire saves almost 1 gal of crude oil, the amount required to produce an equal amount of virgin rubber.
Producing Fine and Superfine
Lehigh's manufacturing site in Tucker, GA is one of the largest and most advanced rubber powder manufacturing facilities in the world. Here, proprietary technology is combined with commercially-available industrial equipment to produce consistently-sized rubber particles, 40 mesh (420 microns) and smaller, that are sold under Lehigh's Polydyne™ label.
The raw stock is 100% recycled content, granulated rubber obtained from tire processors in sizes ranging from 0.75 in. (19.05 mm) down to 10 mesh (2000 microns). Delivered in bulk bags or truckloads, the material is dumped into a hopper and shaker-screened to classify the granules into sizes between 6 mesh (3360 microns) and 14 mesh (1410 microns). Granules exceeding 6 mesh (3360 microns) are ground down before the sized material is stored in bins.
Upon demand, stock is drawn from the bins and fed by insulated screw conveyor into the liquid-nitrogen-cooled cryogenic chamber where temperature, typically between -100F to -130F (-73C to -90C), is regulated based on the particle size desired. Freezing turns the rubber granules into a glass-like state that fractures easily during grinding in a high-speed attrition mill. After size-reducing, the material passes through a rotary drum magnet to remove steel belting freed during the grinding process.
At this stage, the powder exhibits an oily texture and is electro-statically charged, causing the particles to agglomerate and making them difficult to screen. A flow aid, such as finely-ground talc or fumed silica, is metered in and the mixture screw-fed into a Kason Model MO-CS CENTRI-SIFTER™ centrifugal sifter that blends the two, causing the rubber particles to separate.
Inside the screener's horizontally-oriented cylindrical chamber, rotating helical paddles breakup soft agglomerates and continuously propel the material against a 20 to 30 mesh (840 to 589 microns) woven nylon monofilament screen. The centrifugal force accelerates the on-size particles through the apertures and scalps any fiber from the tire cord. The fibers ball up like cotton and are discharged through the open end of the screen cylinder along with any oversized rubber and residual steel.
"The CENTRI-SIFTER sifter, which we purchased from Cino Equipment Co., Berkeley Heights, NJ, does everything simultaneously, which is why we use this equipment," declares Meckert.
The CENTRI-SIFTER centrifugal sifter's ability to work efficiently within the demanding production environment contributes to the quality of powder processed on the line. Its heavy-duty construction makes it well-suited for continuous inline operations. The unit is compact and operates quietly with vibration-free action. A large access door facilitates cleaning, and screens can be changed in under two minutes.
The Final Step
Following the CENTRI-SIFTER sifter, the powder is discharged into a proprietary tumbler screening process that classifies it. The line produces 1.25 tons/h of powder in sizes ranging from 40 to 240 mesh (60 microns), depending on the application. Currently, products are shipped only within the USA, but at the request of Lehigh's globally-diverse customer base, the company will be shipping internationally.
"Our growing production will ensure a good supply of powder for our customers and enable them to lower their production costs by increasing the amount of recycled material in their products," remarks Meckert.
In the future, Lehigh foresees blending rubber with plastics to increase their durability and flexibility, as well as to improve their UV resistance so they are less susceptible to brittleness and cracking from sunlight. This application, which is still in the embryonic stage, will require even finer rubber powders than are now being produced, and according to Meckert, the company's centrifugal sifter will adapt to the new requirements with a simple change of screens.