Agriculture Reference
In-Depth Information
Torres
2007
). In the middle of the last decade, research started being widespread
focusing particularly on the controlled release of pesticides in both theoretical and
experimental directions (Yan et al.
2013
).
Encapsulation of pesticides by natural polymers (starch, for instance) has
received much attention recently (El Bahri and Taverdet
2005
; Chen et al.
2008
;
Jerobin et al.
2012
). The use of starch is being encouraged because of its nontoxic
characteristics, biodegradability, easy handling and processing, and low cost. When
using these systems, the release is primarily controlled by diffusion processes: the
starch granules applied to the soil absorb water and swell, and the encapsulated
compound diffuses out of the starch matrix (Wienhold and Gish
1994
; Giroto
et al.
2014
). In this type of release process, the release itself is difficult to be
controlled, since the diffusion of the active compound is influenced only by the
matrix properties and environmental conditions. Biopolymers produced using
starch have attractive features, however, tend to present some limitations in certain
aspects, such as weak mechanical resistance and hydrophilic degradation. In this
last case, the water molecules can attack the hydrogen bonds of the starch structure,
weakening its bonding strength, thus diminishing its functional properties that are
essential for use in agriculture (Matsuda et al.
2013
). Another important aspect is
the affinity between the polymer matrix and the compound to be encapsulated, as
the case of many pesticides and starch. The hydrophilic character of starch and the
low water solubility of pesticides result in a poor chemical affinity between two
components, making the pesticide encapsulation in starch matrix an unsuitable
strategy.
In order to minimize or even nullify these drawbacks, modifications of starch are
reported. The preparation of starch-clay composites has been widely explored
(Cyras et al.
2008
; Chivrac et al.
2010
; Aouada et al.
2011
; Ojijo and Ray
2013
).
The clay can be included into the matrix by intercalation as well as exfoliation
processes. If the clay presents an attraction to the polymer matrix, it binds to
hydrophobic sites of the starch molecule chains, reducing its hydrophilic character
and leading to a more compatibility with the herbicide. In this way, aluminosilicates
(clays) have become one of the most commonly reported materials in the prepara-
tion of starch-based biodegradable composites. The improvement on properties is
proportional to the dispersion level of the clays within the polymer matrices. A
homogeneous dispersion of clay can provoke an increase on mechanical properties
and generate a physical barrier effect against the release of compounds or even to
water absorption. Thus, the water molecules need to follow through a more tortuous
path in the polymer-clay composite, reducing the diffusion rates of compounds
within and outside the matrix, and thus to high barrier levels.
C´spedes et al. (
2013
) observed that the modification of alginate with kaolin and
bentonite reduced the release rate of pesticides from the composites. Clays acted as
adsorbents, slowing the release time of pesticides and reducing the amount of active
compounds available for leaching or volatilization. Figure
11.6
shows that 89.78 %
of technical grade product is dissolved in less than 6 days, whereas it takes at least
108 days to release 93.43 % of active compound from the alginate-based
