New solid-state shear extrusion pulverization process for used tire rubber recovery.

by Dr. Klementina Khait, Northwestern University

Due to the large quantity of scrap tires, with about 240 million generated per year and the fact that they are mostly discarded, tires would probably remain the main source of ground rubber. It is projected (ref. 1) that by 1997 the annual consumption of scrap tires will reach 330 million. Used tires continued to be utilized in asphalt, civil engineering applications, as a supplemental fuel in cement kilns, in pulp/paper mills and in tire-to-energy facilities.

One of the innovative uses of tire rubber involves development of compounds with recycled post-consumer plastics. However, since tire rubber is a thermosetting material, it cannot be reprocessed like thermoplastics. It is possible to mix thermosets and thermoplastics to produce some products, such as docks, playgrounds and highway construction materials. Used tires are difficult to recycle because they are reinforced with a steel wire and textile fiber. The most common recycling technology today involves separation of steel and fiber followed by grinding rubber to different size particulates for further reuse as a filler in a variety of rubber products or as an ingredient in asphalt mixes. There are several new emerging technologies which include grinding tire rubber to finer particle size, surface treatments and devulcanized processes which are leading to the increase of incorporation of recycled tire rubber into the manufacturing of rubber products.

As previously reported (refs. 2 and 3) the new Solid-State Shear Extrusion (SSSE) pulverization process can convert tire rubber and/or plastics into controlled particle size powder in a once-through, continuous process using a modified co-rotating twin-screw extruder by Berstorff Maschinenbau, GmbH, Germany. The SSSE process utilizes a novel approach in producing powders by subjecting the polymeric feedstock (in the case of tire recycling the feedstock is shredded and coarsely ground 1/4-inch tire rubber) to the simultaneous action of high shear and pressure in the extruder combined with extensive cooling. This new mechano-chemical process causes bond rupture, which leads to partial devulcanization of rubber. Due to a special, complex design of the screw elements and high shearing forces the resultant powder has large surface area and "cauliflower-like" morphology, which is advantageous for further uses of the powder in rubber processing. This article describes further development of tire rubber compounds with post-consumer polyolefins via the SSSE technology for a variety of rubber products including floor mats of different hardness.

Experimental and discussion

A large quantity (2,000 pounds) of coarsely ground 1/4-inch tread rubber was purchased from former Baker Rubber, Inc., South Bend, IN, and shipped to Berstorff Maschinenbau GmbH, Hannover, Germany for pulverization using their first commercial ZE-90 (diameter of 90 mm) co-rotating twin-screw extruder as a part of a complete tire recycling line. This machine is a scale-up version of the prototype ZE-40 (diameter of 40 mm) extruder which was used in our initial work to demonstrate the new SSSE technology to recover used tire rubber (ref. 2).

Berstarff's new tire recycling line is designed as a three step process. The first step consists of a tire cutter that separates the bead and cuts both truck or car tires to pieces approximately 85 x 50 [mm.sup.2] each. The second step is using a crushing calender which further reduces tire pieces to chips of about 6 mm while steel wire and textile cord are separated and recovered. The third (final) step is using a modified, pulverizing extruder with a patented cooling system. The resultant powder is classified by a screener and packaged.

Tread tire rubber purchased from Baker Rubber, Inc. and pulverized in Germany was labeled APTR (American pulverized tread rubber) to distinguish it from PTR (pulverized tread rubber) used in earlier work (refs. 2 and 3). The SSSE process utilizes a novel approach in producing powders by subjecting rubber and/or plastic feedstock to the very high shear and pressure being generated in the modified co-rotating twin-screw extruder combined with extensive cooling to remove frictional heat. By varying screw configuration and processing parameters, such as feed rate and screw rpm, it is possible to convert coarsely shredded pieces of polymeric feedstock into controlled particle size powder of unusual morphology. In addition, this new mechano-chemical process causes chemical bonds to rupture, which leads to partial devulcanization of thermoses rubber and in the case of pulverization of mixed, traditionally incompatible plastics, a creation of in-situ compatibilizing agents, such as block and graft copolymers. The exact mechanism of the SSSE process is not yet fully understood and requires further investigation.

Sieve-screening analysis of APTR was performed to determine particle size and particle-size distribution of pulverized rubber, as very important characteristics of the powdered product. Baker Rubber's sieve analysis test method BTM-5-01 was used. The sieving was carried out by using ten-minute cycle time and 4 g of talc mixed with 100 g of pulverized tire rubber sample with two rubber balls on the top screen. Talc was used to break agglomerates. A mechanically operated Ro-Tap Sieve shaker, U.S. standard sieves and rubber balls with l-1/2inch diameter were used. Sieve analysis results are presented in table 1. As can be seen from the data, the majority of powder (38.2% by weight) was of 60 mesh followed by 80 mesh (20.5% by weight). Some of the powder was of 140 mesh (11.8% by weight) and small amounts of even finer powder as well as coarser than 60 mesh was measured.

Table 1-sieve analysis for American pulverized Tread (APTR)

Mesh   Micron   Weight   Percent
                         retained
  35      500      7.8        7.5
  60      250     38.2       36.7
  80      178     20.5       19.7
 100      150      6.6        6.3
 140      104     11.8       11.3
 200       74      8.8        8.5
 230       63        2        1.9
 325       45      8.2        7.9

The appearance and uniformity of tire powder was studied by scanning electron microscopy (SEM) at 40 and 250 magnification using a Hitachi S-570 microscope. Micrographs of APTR tire rubber pulverized in Germany with the ZE-9O twin-screw extruder are shown in figure 1. After the pulverization, most particles had a large surface area and were about 100 microns in diameter. The open, "cauliflower-like" morphology is advantageous for further use of rubber powder, especially for mixing with plastics, virgin rubber and other ingredients. For comparison, the sample of ground tire rubber obtained from Midwest Elastomers, Wapakoneta, OH was explained by SEM at the same magnifications. As could be seen from micrographs (figure 2), the rubber particles are flat, with an angular surface which is expected from conventional grinding. Cryogenically ground rubber has a small surface area and requires longer mixing time when blended with other ingredients to make rubber compounds. It was found that compression molding of the APTR even without binder at the temperature of 220 [degrees] C and 6,000 psi pressure gives flexible rubber sheets with moderate elongation.

[Figures 1-2 ILLUSTRATIONS OMITTED]

Heavy duty mats using prototype molds have been made at Pace Mat, Inc., and Pittenger and Cook Engineering, Inc., both of Genoa, IL. Rubber/plastic compounds utilizing 60% by weight of pulverized tire rubber have been compression molded into heavy mats measuring 18 x 18 inches which used eight pounds of the compound. The mats were textured on both sides; one side had large knobs which were sharp and firm, indicating good flow properties of the developmental compounds. Several mats with textured surface measuring 10 x 10 inches have been compression molded at the Polymer Reclamation Center. Rigid and semi-rigid mats have been made by varying tire powder/recycled plastic/ binder ratio. These mats had better surface appearance compared with mats utilizing coarser tire rubber produced by conventional grinding.

Summary

Product development using scrap tire rubber recovered by a novel, SSSE pulverization technology is continued. Large quantity (2,000 pounds) of used American tread rubber has been pulverized by Berstorff with a commercial ZE-90 co-rotating, twin-screw extruder. The resultant powder had open, "cauliflower" morphology as determined by a SEM. This morphology is advantageous for further uses of the tire powder in a variety of rubber products.

It has been shown that tire powder can be co-pulverized with recycled polyolefins, such as HDPE, LDPE or LLDPE. Both tensile and tear strength of APTR/polyolefins mixes made by co-pulverization of tire rubber and post-consumer plastics were higher than those made by mixing of separately made powders. Current effort is focused on a development of value-added products containing both recycled tire rubber and post-consumer plastics using a new ZE-25 laboratory scale co-rotating twin-screw extruder built by Berstorff for SSSE process development and currently operational at the Polymer Reclamation Center at BIRL.

References

[1.] Kearney, A.T., Scrap tire use/disposal study, Scrap Tire Management Council, October 1992.

[2.] Khait, K., "New used tire recovery process for value-added products, " Rubber Division Meeting, ACS, Chicago, IL, April 1994.

[3.] Khait, K, "Application development for used tire rubber recovered by a novel solid-state shear extrusion pulverization process," Rubber Division Meeting, ACS, Cleveland, OH October 1995.