Agriculture Reference
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The decomposition of the cured ENR samples, immersed in 9% w/w GA solution for 10,
20 or 30 days buried in soil was investigated and the percentage of weight loss as a function
of buried time was determined as presented in Figure 36 (a), (b) and (c), respectively. The
results showed that the weight loss (%) of the cured ENR dramatically decreased with an
increase of burial time. The rate of biodegradability of cured ENR for 10 days was faster than
that of ENR cured for 20 and 30 days (Figure 36). It was explained that the higher cross-
linked density of the cured ENR in the latter cases impeded the penetration of bacteria into
the sample. It is well known that high molecular weight NR (~10 6 ) cis 1,4- polyisoprene is
responsible for the more difficult biodegradation compared to St (Riyajan et al., 2012a). NR
can be slowly degraded in nature by specific microorganisms i.e., 31 Streptomyces strains, 5
Micromonospora strains, 3 Actinoplanes strains, 2 Nocardia strains, 1 Dactylosporangium
strain, and some fungi (Bhatt et al., 2008). The total aerobic activity began by splitting the
polymer chains at the C=CH bonds followed by the metabolism of the depolymerization
products with mixed microbial populations leading to their eventual mineralization (Bhatt et
al., 2008). Certainly, the biodegradability of rubber by microorganisms decreases after curing
(Jendrossek et al., 1997). As a result of the ester linkage between ENR and GA, ENR cured
with GA would be easier to decompose by microorganisms and hydrolysis reactions in the
soil compared with those cured with sulfur. The natural soil environment contains fungi,
bacteria and moisture (Jacob et al., 2004). The growth of many fungi can also cause small-
scale swelling and bursting, as the fungi penetrate the modified NR. In addition, the moisture
in natural soil and the added water at weekly intervals for 3 months allowed the degradation
process to penetrate into the samples. Three physical forces: physical, chemical, and
biological modify the polymer surfaces and create new surfaces for reacting with chemical
and biochemical agents, a critical phenomenon for the degradation of solid polymers (Kamal
and Huang, 1992). Depolymerization of many biopolymers such as starch, cellulose, and
hemicelluloses (Kamal and Huang, 1992) can be initiated by the many different
microorganisms in the soil, which can produce many different enzymes (Alexander, 1977).
The cellulose fibers and St in the NR composites are therefore more easily degraded than NR
and are the first to be hydrolyzed before the microorganisms can utilize the NR as a nutrient
source (Abrahama et al., 2012).
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