Polymers comprise the main fraction among the materials concerning radiation processing. Rubber and tire are also a big part of polymer industry. In rubber and tire industries, radiation practices are focused on vulcanization(cross-linking), pre-vulcanization, and devulcanization (for recycling rubber based wastes).
.3APPLICATION OF RADIATION VULCANIZATION IN RUBBER AND TIRES
The application of radiation for the vulcanization and modification of rubber is a prospective technology in polymer processing. This research began in the middle of the 20th century, and many world-famous tire manufacturers had applied radiation vulcanization technology in their plants by the end of the 2oth century. At present, more than 93% of radial tire made in Japan are treated by low-energy accelerators for prevulcanization. In recent years, EB radiation vulcanization has become gradually recognized and accepted in the rubber and tire industry in China. Some tire manufacturer have already established production lines with the adoption of radiation vulcanization.
The following is the introduction of the radiation vulcanization, the effect of radiation on the rubber, and the advantages of radiation vulcanization compared with chemical vulcanization. In addition, there is a summary of the application and development of EB irradiation in the rubber and tire industry at home and abroad. Modified offline irradiation equipment for the prevulcanization process and application examples of this technique is also described.
EB radiation prevulcanization has many advantages, including reducing the cost and improving the performance of tires. This technology is certainly important for the manufacture of high-performance radial tires. It has an enormous potential market in the future.
3.3.1 Radiation vulcanization of rubber
126.96.36.199Radiation vulcanization and prevulcanization of rubber
Free radicals are formed when rubber macromolecules are subjected to high energy rays such as EB and X-ray at normal temperatures and pressures, causing cross-linking of main chains to form three-dimensional networks, and resulting in the same level of vulcanization compared with chemical vulcanization. The process is defined as radiation vulcanization. Radiation prevulcanization means a certain degree of weak cross-linking between main macromolecular chains by adjusting the irradiation parameters.
188.8.131.52 Radiation effects on rubber
The formation of cross-linked bonds and the degradation of polymer main chains happen simultaneously during the course of polymer irradiation. The radiation cross-linking G value of common rubber is called the cross-linking yield. The G value is an important index of rubber radiation cross-linking. Table 3.2 displays the G values of common rubber. The vast majority of alkene rubbers are cross-linkable. However, many factors, including the chemical structures of polymer main chains, polymer morphology, irradiation conditions and characteristics and the amount of accessory ingredients and sensitizers in rubber can significantly affect the formation of rubber cross-linking bonds.
Table 3.2 Radiation cross-linking G values of common rubber
Butadiene styrene rubber
Ethylenepro pylene rubber
184.108.40.206 Advantages of radiation vulcanization
After radiation vulcanization, the tearing, aging and ozone resistance properties of rubber are improved. By radiation vulcanization, full vulcanization can be achieved rapidly at normal temperature and pressure with many advantages, including speed, flexible production, energy saving and low pollution. Once the setup parameters of radiation vulcanization are established, the EB can be evenly distributed throughout the sample, leading to a uniform crosslinking density and a single network structure. Radiation vulcanization can overcome the drawbacks of chemical vulcanization, including pollution, energy consumption, time consumption, and nonuniform vulcanization.
220.127.116.11 Radiation vulcanization of common rubber in tire industry
A variety of rubber are used in the tire industry include natural rubber(NR), isoprene rubber(IR), butadien styrene rubber(SBR), butadiene rubber(BR), ethylene-propylene diene terpolymer(EPDM), butyl rubber(IIR), chlorinated IIR(CIIR), and brominated IIR(BIIR). Radiation vulcanization of these types of rubbers is introduced briefly in the following sections.
3.3．1.4.1 Natural rubber and isoprene rubber
NR is widely used in all components of tires, with the largest consumption share in the tire industry , and it can be prevulcanized by EB and y-ray. Fig. 3.15 shows that the green strength of radiation-prevulcanized NR increases with absorbed dose. Figs. 3.16 and 3.17 indicate that the properties of NR do not change significantly before and after radiation prevulcanizaing.
Table 3.3 shows the properties of radiation vulcanization and sulfur vulcanization for IR compound made according to the method of ASTM D3184（1995）. The Young’s modulus and tan δ of both vulcanizates are similar, as shown in Figs. 3.18 and 3.19, while the tensile strength, elongation at break, and tensile strength at 100% of radiation vulcanizate are higher than the performance of sulfur vulcanizate. This indicates that radiation vulcanization can obtain vulcanizate with good properties.
18.104.22.168.2 Styrene-butadiene rubber and butadiene rubber
SBR is a copolymer with styrene and cis-1, 4 butadiene, and is widely used in the tread of passenger car tires and carcass compounds. It has the largest consumption share of all synthetic rubbers. Cis-1,4 polybutadiene is the most important blended rubber in the tread formula, polymerized by cis-1, 4 butadiene. The wear Resistance of BR is also the best of the common rubbers. SBR and BR can be cross-linked by irradiation, and the degradation rate for both is close to zero when they are irradiated in vacuum. The mechanical properties of SBR compound measured according to ASTM D3185(2006) for rubbers vulcanized by irradiation and sulfur are shown in Table 3.4. The tensile strength, elongation at break, tensile strength at 100%, and hardness of radiation vulcanizate are superior to those of sulfur vulcanizate, and the dynamic properties of both are similar.
22.214.171.124.3 Ethylene-propylene diene terpolymer
EPDM with unsaturated bond is produced by the terpolymerization of ethylene, propylene, and a small amount of alkadiene. This is applied in tire sidewalls and can be cross-linked by irradiation. As shown in Fig.3.20, the tensile strength of EPDM increases above that level, the tensile strength decrease to a larger extent. The elongation at break decreases with the increase of absorbed dose. Radiation cross-linking leads to an increase in the rigidity, and hence an increase in the tensile strength at 100% and Shore A hardness, as shown in Fig.3.21.
126.96.36.199.4 Butyl rubber, brominated butyle rubber, and chlorinated butyl rubber
IIR, BIIR, and CIIR are widely used in the inner liners of tires due to their excellent pressure tightness. The main chain of IIR is isobutene. IIR is a radiation-degradable rubber, and cannot be vulcanized by irradiation. The properties of BIIR and CIIR are similar to those of IIR, BIIR and CIIR rubbers and can be vulcanized by irradiation at a quite low absorbed dose, and they are degraded by irradiation with a higher absorbed dose. The mechanical properties of CIIR can be improved markedly when it is vulcanized by irradiation at 30 k Gy with the addition of polyfunctional monomers.
（1）Radiation Technology for Advanced Materials:From Basic to Modern Applications Guozhong Wu， Maolin Zhai， Mozhen Wang