Agriculture Reference
In-Depth Information
Sirichaiwat, 1996). This was explained in section 3.3. They found that after the capsule was
coated with unvulcanized NR, the lowest rate of urea released from the capsules was achieved
using the urea coated by unvulcanized rubber compared to the non-vulcanized NR as the
polymer matrix and the complete release of urea from the capsule was roughly 50 days.
Another study, by Hepburn and co-workers (Hepburn, et al., 1989a) studied the experience of
incorporating the urea in unvulcanized NR and vulcanized NR matrix by using the two-roll
mill method. They reported that the rate of urea released from beads made with unvulcanized
NR was faster than that of beads made from vulcanized NR due to the different dispersions
between the urea and the rubber matrix.
4.
B
IODEGRADATION
NR, biopolymers can be degraded by aerobic soil bacteria mainly in the order
Actinomycetales and genus Streptomyces. The initial attack occurs at the C=CH bonds to
produce C=O and CHO groups by radical-generating enzymes (Figure 35). Attack by these
enzymes is often restricted by poor access to the substrate probably because of the
hydrophobicity of the polyisoprene chains. When NR is modified chemically, or by mixing
with a biopolymer, the enzymes have more access to the C=CH bonds so the rate of
degradation is often enhanced. There is very little information yet on the further degradation
pathways to CO
2
and H
2
O but they are likely to involve the normal oxidation metabolic
pathways. Three different enzyme mediator systems, consisting of radical-generating
enzymes and their substrates, acting as radical precursors have been investigated with regard
to the biodegradation of polyisoprene and rubber material. Although these enzymes may have
a different physiological function, these studies have demonstrated that biochemically
generated radicals are capable of attacking C=CH bonds in the polyisoprenoids. After use, the
polymer membrane from the capsule can be degraded in natural soil. In this experiment, water
(50 mL) was poured into the soil every week for 1 month. Each week, the specimen was
carefully taken out, washed with distilled water and dried at 45°C for 2 days before being
weighed. The % weight loss of the residual sample was calculated from the equation (2).
(2)
Where W
e
is the weight of the residual sample after being buried in soil for various times.
Until fairly recently the mechanisms involved in the degradation of natural polyisoprenoids
have been mostly unknown but now there has been considerable progress in our
understanding of microbial rubber degradation. Analyses of the degradation products have
indicated a possible biodegradative pathway for this abundant and technically vital polymer.
Two nonhomologous enzymes involved in this process and their respective genes were
recently identified in species of the genera Streptomyces and Xanthomonas. This should
allow for more detailed molecular and biochemical studies to determine the mechanisms by
which natural and synthetic polyisoprenoids are degraded. These enzymes have been studied
in the mycelium-forming actinomycetes in the genera
Streptomyces actinoplanes
as well as
in
Micromonospora
(Jendrossek et al., 1997; Linos et al., 2000; Linos and Steinbüch et al.,
Search WWH ::
Custom Search