Natural Rubber Biosynthesis and Physic-Chemical Studies on Plant Derived Latex

Abstract

1.1 Natural rubber is an indispensable biopolymer Natural rubber is a biopolymer of high economic importance with incomparable performance properties such as high elasticity, resilience and efficient heat dispersion (van Beilen and Poirier, 2007a). This high molecular mass polymer is formed from isopentenyl diphosphate units (IPP) which are linked in cis-configuration building poly(cis-1,4-isoprene) (Nor and Ebdon, 1998; Wititsuwaannakul et al., 2003; Bushman et al., 2006). 300 to 70,000 isoprene molecules are coupled to form an irregular structure that cannot crystallize under normal conditions mediating the amorphous, rubbery texture (Nor and Ebdon, 1998; Kang et al., 2000a). Upon harvest from plants and processing, along with the biopolymer itself also non-rubber compounds are co-extracted and remain in the natural rubber product (Nor and Ebdon, 1998). Therefore, the final product consists of about 94% poly(cis-1,4-isoprene) and 6% non-rubber contents such as proteins and fatty acids (Sakdapipanich, 2007). These contaminations are thought to contribute to the extraordinary characteristics of natural rubber (Nor and Ebdon, 1998). The plant-derived commodity is required for the production of more than 40,000 consumer products including tires, footwear and medical devices (Davis, 1997; Mooibroek and Cornish, 2000; Hagel et al., 2008). After harvest natural rubber is either kept in solution through addition of solvents and stabilizers or it is coagulated and dried. Solubilised natural rubber is used for products such as gloves or condoms but most of the harvested natural rubber is processed as so called bulk rubber in solid sheets or granules (van Beilen and Poirier, 2007b). Currently, the sole crop exploited for commercial production of high quality natural rubber in viable quantities is Hevea brasiliensis Muell. Arg. (Figure 1A). H. brasiliensis – a tree indigenous to the Amazon Basin – is the most recently domesticated plant among major crops. In their natural environment trees grow widely distributed within the forest, but for large-scale rubber production H. brasiliensis trees are planted in monoculture (Davis, 1997). The main rubber-producing countries presently are Thailand, Indonesia, Malaysia, India and the People’s Republic of China, which together accounted for 89% or 9.33 million tons of the global rubber production in 2005 (Figure 2B) (Hayashi, 2009). With a yield potential of more than 2500 kg ha-1 year-1 H. brasiliensis is a valuable crop in tropical and subtropical countries where its cultivation is possible (Cornish, 2001a; Hayashi, 2009).