ABSTRACT
Natural rubber has been modified in many different ways since as early as 1801, although the frrst commercial form was not manufactured until about 1915. The term modified rubber can refer to any degree of chemical modification from a very small mole percent for the purposes of introducing bound antioxidant functions, crosslinking, bonding, etc., without introducing any changes to the basic physical properties, through to the reaction of a significant number of the repeat units, say 20-50 mol%, which does result in changes in the physical properties of the rubber. Higher levels of modification tend to alter the nature of the polymer from a rubber to a more plastic-like or resinous material. Because natural rubber (NR) has a fixed cis-polyisoprene structure and cannot have its polymerization process tailored like that of the synthetic rubber industry to provide suitable pendent groups, many of the frrst type of modified rubbers have been prepared with a variety of groups for a whole cross section of purposes, for example, the addition of thiols and related compounds to improve low-temperature properties and for crosslinking (Cunneen and Shipley, 1959; Cunneen et al., 1960), epoxidation for the reaction of the oxirane ring (Colclough et al., 1961; Colclough, 1962), addition of maleic anhydride and maleimides (Pinazzi et al., 1960, 1962), the "ene" addition reaction (Cain et al., 1968; Baker et al., 1970), and many others since. The number of these types of modified NR would be sufficient to fill this handbook by themselves, and therefore this chapter has been restricted to the second and third types of modified natural rubber mentioned above. Even so, taking into account those that have not been commercially successful, the number of forms of modified natural rubber examined has still been quite large.
