Cross-linked bond formation and degradation of polymer main chains occur simultaneously during polymer irradiation. The radiation crosslinking G-value of conventional rubber is called crosslinking efficiency. The G value is an important indicator of crosslinking of rubber radiation. Table 3.2 shows the G values of ordinary rubber. The vast majority of alkene rubbers are crosslinkable. However, many factors, including the chemical structures of the polymer main chains, the morphology of the polymer, the irradiation conditions and properties, and the amount of additional ingredients and sensitizers in the rubber, can significantly influence the formation of rubber crosslinking bonds. In recent decades, many methods of recycling rubber waste have been developed.15, 16 These methods generally fall into two categories. The first is on the physical plane. The material is mechanically ground into smaller pieces, so that the chemical bonds are barely broken.
The end result is comminution at varying degrees of finesse. Another category of methods attempts to mechanically break the three-dimensional network using various forms of energy. These include mechanical and thermal treatment, chemical and biological treatment, microwave and ultrasonic waves. Such treatments convert the three-dimensional, insoluble and non-meltable thermosetting into a soft, sticky, retreatable and repulsible elastomer that simulates the properties of the new rubber. The recovery and recycling of vulcanized NRs from used products and production waste saves valuable oil resources and solves the problem of rubber waste disposal. Accordingly, this chapter describes the state of the art in NR vulcanizate recycling. These include recovery, landfilling, mechanical grinding, spraying, mechanochemical, combustion and pyrolytic, chemical, microwave, biotechnological and ultrasonic techniques for NR rubber recycling. It also describes some efforts to recycle synthetic isoprene (IR) rubber using different methods, as the chemical structure of IR is similar to that of NR. The vulcanization of natural rubber requires heat, but is not an essential factor in all processes. For example, silicone vulcanization takes place at room temperature.
Some processes use radiation instead of heat. In most cases, general purpose raw rubber (natural rubber, butadiene or styrene butadiene) is vulcanized by heating it with elemental sulfur to 140°-160°C (sulfur vulcanization). The intermolecular bonds formed consist of one or more sulfur atoms. If 0.5-5% sulfur is added to the raw rubber, a soft vulcanizate is formed (for inner tubes and tire carcasses, balls, inner tubes, etc.). The addition of 30-50% sulfur leads to the formation of a hard and inelastic substance, ebonite. Sulfur vulcanization can be accelerated by adding small amounts of organic compounds – called vulcanization accelerators such as kaptaks or thiuram. These substances are fully active only in the presence of metal oxides (most often zinc oxide), which are activators. In industry, sulfur vulcanization is carried out by heating the articles to be vulcanized in molds under high pressure or in the form of unformed articles (in “free” form) in boilers, autoclaves, individual vulcanizers or continuous vulcanization devices.
In these devices, heating is carried out by steam, air, superheated water, electricity or high-frequency current. The molds are usually placed between the heated plates of a hydraulic press. The vulcanization of sulfur was discovered by C. Goodyear (USA, 1839) and T. Hancock (Great Britain, 1843). To vulcanize raw rubber for special applications, organic peroxides (such as benzoyl peroxide), synthetic resins (e.g. phenol-formaldehyde) as well as nitro and diazo compounds are used. The vulcanization conditions are the same as for sulfur vulcanization. Simply put, vulcanization accelerators accelerate the cleavage of the sulfur cycle and the formation of thiyl and polysulfenyl residues. Accelerators react in the form of their more active zinc salts due to the almost ubiquitous presence of zinc oxide in sulfur vulcanized compounds.
The choice of accelerator influences the safety of combustion (premature vulcanization), the curing rate and the length and number of crosslinks formed. These properties, which are generally related to the rate at which the accelerator is converted to its highly active saline form, are compared in Table 1. Vulcanization is a process of chemical crosslinking of rubber molecules with organic/inorganic matter through the action of heat and pressure. Chemically cross-linked rubber is called vulcanizate. The introduction of cross-linking into the rubber matrix may be relatively small, but it is sufficient to prevent unlimited flow of whole molecules beyond neighboring molecules. The low concentration of cross-linking means that the vast majority of the segments that make up long-chain molecules can move freely due to kinetic energy. An unvulcanized rubber dissolves completely in its solvent. On the other hand, a vulcanized rubber only swells.
Chemical cross-linking prevents complete dissolution. Rubber vulcanized in this sense is a solid and retains its shape and dimensions. According to the theory of rubber elasticity (Flory, 1953), the shrinkage force to resist deformation is proportional to the number of grating-bearing polymer chains per unit volume elastomer. A polymer carrier chain is a linear polymer molecule segment between network connections. An increase in the number of joints or networks leads to an increase in the number of support chains. In a high, unvulcanized linear polymer (above its melting point), only tangles of molecular chains form compounds. In general, the most widely used technique in curing various industrial applications is the accelerated sulfur curing method, as it offers better physical properties, provides a considerable rapid crosslinking rate, and is able to provide the delayed actions required for processing, forming, and molding prior to the formation of the vulcanized rubber network.39 Depending on sulfur content and ratio of the sulfur accelerator Sulfur vulcanization systems are classified as conventional, semi-efficient (semi-EV) and efficient (EV).37 The introduction of radiation vulcanization, the effect of radiation on rubber, and the advantages of radiation vulcanization over chemical vulcanization are described below. In addition, there is a summary of the application and development of EB irradiation in the rubber and tire industry at home and abroad.
Raw rubber is a tangle of high molecular weight hydrocarbon chains. As a result, the rubber flows when stationary and does not retain its shape.