Valuable products from waste tires.

Waste tires have brought significant problems (refs. 1-3) with the increasing number of automobiles. Therefore, many researchers have studied this field. Three methods of waste tires treatment, including combustion, underground burial and recycling, have been developed. The most prevalent method is recycling (ref. 4). However, one of the obstacles of recycling is an economic problem. Among the developed variety of methods, the most potentially attractive is utilization of powder rubber. Commercially, one of the most popular applications of the powder utilization (refs. 3 and 5) is in golf ranges, industrial flooring and pathways. The advantage of powder utilization is that it is easy to apply with simple equipment. Although much work (refs. 3 and 5-8) has been done, problems still remain. The difficulties in recycling of waste tires are that the tire scrap is a crosslinked polymer, which is hard to melt and to process. In recent years, there have been significant technical innovations in the tire recycling field, such as chemical de-vulcanization (refs. 9 and 10), surface treatments and new binders. These developments are poised to re-establish high recycled rubber contents in technical rubber articles. Therefore, increasing concerns regarding the wise use of waste tire products have led to a number of investigations on the utilization of waste tires.

Generally, the methods of obtaining powder from waste tires are cryogenic grinding (ref. 11) and ambient grinding. In cryogenic grinding, shredded tires are frozen with liquid nitrogen. The cryogenic ground rubber has a smaller particle size and higher surface area compared to an ambient ground rubber. But the cryogenic grinding is a more expensive process than the ambient grinding.

Therefore, we studied how to make high valued products by using a cheaper rubber powder. First, we designed a grinding machine. Secondly, we tried to make a good, economic rubber block and an isolator for vibration by using ground rubber powder.

Experimental

Materials

A larger size of rubber powder (300~1,200 [micro]m) was used to make the fine powder, GRT, in this study. GRT has the following composition: Rubber hydrocarbon, 59.8%; carbon black, 27.8%; ash, 8.3%; and acetone extractable volatiles, 4.1%.

Grinding machine

The self-designed grinding machine (SDGM) used in this study has dimensions of 427 cm x 150 cmx 325 cm. The rotating speed of the grinding machine is 11,000 rpm.

Preparation of powder rubber

The GRT (300~1,200 [micro]m before grinding; Sa) was produced using the SDGM. The advantage of this technique is the capability of obtaining a fine elastic rubber powder economically. The powder rubber (after grinding--Sb1) was found to be polydispersed particles. It was then sieved into three different particle sizes, ranging from 300~600 [micro]m (Sb2), 150~300 [micro]m (Sb3) and 100~150 [micro]m. Various particle distributions are presented in figure 1.

[FIGURE 1 OMITTED]

Preparation of specimen

The ground powder (Sb1, Sb2, Sb3) was mixed with some chemicals, reacted in a stirrer, and the mixture was compressed by a hot press. A 50-ton hydraulic press equipped with a hot plate was used to produce a rubber sheet at a temperature of 150[degrees]C, and was maintained at this temperature for curing. The curing time was 15 minutes for the crumb rubber compound. The last step of sample preparation was cutting.

After storing the rubber sample for one day, a specimen with the dumbbell 3 shape was cut for mechanical testing.

Methods of measurement

The measurement of the tensile properties was carried out using an automated materials testing system for tensile strength, elongation and tear strength in accordance with KSM (Korea Standard Method) 6518. The instrument was operated at 500 mm/min, head speed and with a 10 KN load cell. The hardness measurement was carried out using a spring type A durometer; the value was determined by the average of four experimental results.

The morphology and elemental analysis were determined from gold-coated samples using a scanning electron microscope equipped with an energy-dispersive x-ray spectrometer operated at 5 KV.

To measure the surface analysis of the sample, the XPS spectra were obtained from x-ray photoelectron spectroscopy with a VG MT Clam 2 system and Mg ka photon source. An electron take-off angle of 37[degrees] was used. Binding energy of the hydrocarbon component peak at 285 eV was used as a reference.

Swelling experiment

A 0.1 to 0.3 gram sample from each of the cured tensile pieces was placed in n-heptane for 48 hours to swell. The swollen pieces were quickly removed from n-heptane, excess n-heptane blotted off and weighed in a stoppered preweighed vial. The sample was de-swelled in a 60[degrees]C vacuum oven for about three hours and weighed. We calculated the number of crosslinking chains per unit volume to find the crosslink density by using weight differences from equation 1.

(1) [[upsilon].sub.e] = -[ln(1 - [V.sub.R]) + [V.sub.R] + [[chi].sub.1][V.sub.R.sup.2]]/[V.sub.1]([V.sub.R.sup.1/3] - [V.sub.R]/2)

whereas, [upsilon]e: Effective number of chains in a real network per unit volume (mole/[m.sup.3]);

[V.sub.1]: Molecular volume of solvent ([m.sup.3]/mole);

[V.sub.R]: Volume fraction of rubber in swollen state; and

[[chi].sub.1]: Parameter expressing the first neighbor interaction free energy.

Results and discussion

A representative energy dispersive x-ray spectroscopy (EDS) spectrum of Sa (before grind sample) is shown in figure 2. The relative percentages of Si, S, Ca and Zn elements were 12.2, 38.5, 11.5 and 37.8 wt %, respectively. The presence of a trace of metal in GRT has been reported earlier (ref. 12). The GRT used for this study contained natural rubber (NR), styrene-butadiene rubber (SBR) and butadiene rubber (BR). Therefore, the sulfur content strongly affects the mechanical properties of the re-vulcanized rubber. In order to analyze this effect in detail, we have tried a recipe with low sulfur ratio.

[FIGURE 2 OMITTED]

Figure 3 shows the stress-strain curves of the re-vulcanized rubber sheets, which were obtained with different particle sizes. The higher modulus compounds were obtained from the smaller particle sizes. It is further shown that the result is dependent on the particle size diameter. In the case of fine particles, the contact surface area is increased more than that with bigger particles. This can lead to enhanced bonding energy between each particle in the network.

[FIGURE 3 OMITTED]

Figure 4 shows the particle size and sharpness of the surface by SEM. The SEM photographs of Sb1, Sb2 and Sb3 clearly indicate more regular shapes with smooth surface than the Sa sample.

[FIGURE 4 OMITTED]

It is also important that the tendency for agglomeration of rubber powder increases with decreasing particle size. The size of Sa was also slightly larger and with broader size distribution than that of Sb1 from SDGM grinding. Figure 4 shows the surface with smoother faces could be due to the effect of high-speed rotation of the grinding machine. The particles of Sa were crushed into small pieces and most of them lost their sharpness. In this aspect, the grinding technique may be an important factor in increasing physical properties of rubber powder.

Table 1 shows the physical properties of re-vulcanized rubber after curing; where there is a significant increase with decreasing particle size. Sb1 has a broader particle size distribution than Sb2 and Sb3, which may also relate to crosslink density of re-vulcanized samples. However, the specific gravity of all samples showed similar values. Figure 5 shows the possible model of Sb2 and Sb3 particles contained in Sb1 re-vulcanized sheet after vulcanization.

[FIGURE 5 OMITTED]

Tensile strength of the rubber compounds is shown in table 1. These results have similar trends for elongation at break of the rubber compounds. From these experimental results, the mechanical properties (tensile strength, elongation at break and tear strength) of Sb1 were increased compared to Sb2, Sb3 and Sa.

Conclusions

The desired particle size of the waste tire powder was obtained using a new technology of a self-designed grinding machine (SDGM). The smaller particle size showed higher modulus. This study was focused on the development of technology for the prospective high value recycled product from waste tires and from an economic point of view. It is our belief that this technique can contribute to the development of the recycling industry.

Table 1--physical properties of re-vulcanized, rubber after curing

Physical                                       Samples
Properties                             Sa                  Sb1

Tensile strength,                     45.2                52.3
  kgf/[cm.sup.2]
Elongation, %                          159                 169
Tear strength, kgf/[cm.sup.2]         15.6                16.4
Hardness, durometer A                 71.5                74.5
Specific gravity                      1.13                1.15
Crosslink density,              1.613x[10.sup.18]   1.812x[10.sup.18]
  crosslink/[cm.sup.3]

Physical                                       Samples
Properties                             Sb2                 Sb3

Tensile strength,                     46.7                50.1
  kgf/[cm.sup.2]
Elongation, %                          146                 152
Tear strength, kgf/[cm.sup.2]         14.5                16.4
Hardness, durometer A                 74.2                75.3
Specific gravity                      1.15                1.14
Crosslink density,              1.777x[10.sup.18]   1.798x[10.sup.18]
  crosslink/[cm.sup.3]

References

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