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Physicochemical Characteristics and Slow Release Performances of Chlorpyrifos Encapsuled by Poly(butyl acrylate-costyrene) with the Cross-linker Ethyleneglycol Dimethacrylate Yu Wang, Zideng Gao, Yang Li, Sainan Zhang, Xueqin Ren, and Shuwen HU J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b01378 • Publication Date (Web): 06 May 2015 Downloaded from http://pubs.acs.org on May 12, 2015
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Journal of Agricultural and Food Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Journal of Agricultural and Food Chemistry
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Physicochemical Characteristics and Slow release Performances of
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Chlorpyrifos Encapsulated by Poly(butyl acrylate-co-styrene) with the
3
Cross-linker Ethyleneglycol Dimethacrylate
4
Yu Wang, Zideng Gao, Yang Li, Sainan Zhang, Xueqin Ren*, Shuwen Hu*
5
(College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, China)
6 7
Abstract
8
Chlorpyrifos’ application and delivery to the target substrate needs to be
9
controlled to improve its use. Herein, poly(butyl acrylate-co-styrene) (poly(BA/St))
10
and poly(BA/St/ethylene glycol dimethacrylate (EGDMA)) microcapsules loaded
11
with chlorpyrifos as a slow release formulation were prepared by emulsion
12
polymerization. The effects of structural characteristics on the chlorpyrifos
13
microcapsules particle size, entrapment rate (ER), pesticide loading (PL), and release
14
behaviors in ethyl alcohol were investigated. FT-IR and TGA analysis confirmed the
15
successful entrapment of chlorpyrifos. The ER and PL varied with the BA/St monomer ratio,
16
chlorpyrifos/monomer core-to-shell ratio and EGDMA cross-linker content with consequence that
17
suitable PL was estimated to be smaller than 3.09% and the highest ER observed as 96.74%. The
18
microcapsules particle size (88.36–101.8 nm) remained mostly constant. The extent of
19
sustainable release decreased with increasing contents of BA, St, or chlorpyrifos in oil
20
phase. Specifically, adequate degree of cross-linking with EGMDA (0.5–2.5%)
21
increased the extent of sustainable release considerably. However, higher levels of
22
cross-linking with EGDMA (5–10%) reduced the extent of sustainable release. 1
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Chlorpyrifos release from specific microcapsules (monomer ratio 1:2 with 0.5% EGDMA or 5 g
24
chlopyrifos ) tended to be a diffusion-controlled process, while others the kinetics probably
25
indicated the initial rupture release.
26
Keywords
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Microcapsule, slow release, chlorpyrifos, poly(butyl acrylate-co-styrene),
28
ethyleneglycol dimethacrylate, emulsion polymerization.
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2
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Introduction
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Chlorpyrifos (O,O-diethyl O-(3,5,6,-trichloro-2-pyridyl phosphorothioate)) is a
32
broad-spectrum, chlorinated organophosphate insecticide, acaricide and nematicide 1.
33
It is used worldwide as a moderately toxic organophosphate pesticide against diverse
34
chewing and sucking mouthparts pests on rice, wheat, cotton, fruit, vegetables, and
35
tea trees through the triple effects of stomach action, contact poison, and fumigation.
36
Because of its high volatilization and widespread use, chlorpyrifos represents one of
37
the most significant sources of organophosphate exposure during application,
38
resulting in adverse effects such as pollution of surface/underground water and
39
biological systems. Additionally, its high toxicity to humans can result in severe
40
illnesses, such as prostrate cancer 2, and respiratory problems 3. In China, the high
41
volatilization and easy spreading of chlorpyrifos in the soil and water lead to low
42
utilization efficiencies. As a result, chlorpyrifos needs to be repeatedly applied for
43
effective performance, and that consequently increases the level of adverse health
44
risks to both the environment and humans. Thus, control over the application of
45
chlorpyrifos in the agricultural sector is necessary.
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Microcapsule-controlled release technology has been widely used for the
47
delivery of target molecules over the past decades. Microencapsulation offers
48
advantages such as target molecule protection, controlled and triggered release, and
49
the development of new product features 4. It has been recognized as a new
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technology and promising solution in the fields of pharmaceuticals, biotechnology,
51
pesticides, environmental engineering, cosmetics, coatings, and food chemistry 5. 3
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Additionally, microcapsule technology has bright prospects in the agriculture sector to
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prepare controlled release formulation systems for pesticides. It is an effective
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approach to efficiently increase the performance level of pesticides, and reduce
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human labor and hazard risks to the environment and humans.
56
In recent years, many papers have reported the preparation of pesticide-based
57
microcapsules. Numerous methods were investigated toward the encapsulation of
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pesticides namely, interfacial polymerization
59
condensation
60
successfully achieved the preparation of wall-type material membranes and
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chlorpyrifos-encapsulating matrices. The materials included synthetic polymers (e.g.,
62
polyamide and polyurethane) prepared by interfacial polymerization and natural
63
polymeric membranes (e.g., alginate, chitosan, starch, and cellulose) prepared by
64
condensation methods. However, studies typically focused on the properties of the
65
encapsulating units only rather than on the release profiles of the encapsulated target
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pesticide 16-18.
67
11
and solvent evaporation
12-15
6-9
, in situ polymerization
10
, and
methods. Some of these methods
It has been reported that some wall-type materials such as urea formaldehyde, 19
68
prepared by in situ polymerization
, afford short sustainable release profiles,
69
whereas other materials, such as polylactic acid 20, 21 and mesoporous silica 22, afford
70
longer sustainable release profiles. In contrast, emulsion polymerization affords the
71
synthesis of microcapsules with a relatively wide range of sustainable release profiles
72
for the delivery of active ingredients by varying the contents and ratios of typical
73
monomers including butadiene, styrene, acrylonitrile, acrylate ester, and methacrylate 4
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ester
. Furthermore, the preparation process of emulsion polymerization for
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chlorpyrifos won’t involve any organic solvent, such as toluene, xylene and
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cyclohexanone etc., which might be considered more environmentally friendly. And
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the chlorpyrifos-loaded microcapsules emulsion can be directly sprayed onto plants
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following synthesis, thereby not requiring the dispersion of the pesticide
79
microparticles in water before application. The chlorpyrifos-loaded microcapsules
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prepared by emulsion polymerization have been discussed in several patents, and in
81
several research papers
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characteristics of wall materials on the sustainable release of chlorpyrifos
83
encapsulated in microcapsules are rare.
27-30
. However, reports on the effects of the structural
84
In the present study, butyl acrylate (BA), styrene (St), and cross-linker ethylene
85
glycol dimethacrylate (EGDMA) were used to synthesize microcapsules by emulsion
86
polymerization. Two series of chlorpyrifos-loaded polyacrylate microencapsulating
87
systems were prepared i.e., microcapsules of chlorpyrifos-loaded poly(BA/St) and
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chlorpyrifos-loaded poly(BA/St/EGDMA) (poly(BA/St) cross-linked with EGDMA).
89
The glass transition temperature (Tg) of poly(butyl acrylate) is low, and hence
90
features an elastomeric state (soft and sticky) at room temperature, whereas
91
polystyrene features a glassy state (hard and brittle) because of its higher Tg. Hence, it
92
is expected that copolymerization of BA and St whose Tg can be easily designed by
93
Fox equation may generate microcapsules with the required hardness and viscosity.
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Furthermore, the addition of cross-linker EGDMA is expected to afford the synthesis
95
of a denser microcapsule wall material or better connections among the microcapsule 5
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particles. Thus, copolymerization between BA and St or between BA/St and EGDMA
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would afford microcapsules with fine characteristics (hardness, viscosity, and dense)
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to achieve long sustainable release properties as desired. More particularly, the
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preparation conditions of the chlorpyrifos microcapsules i.e., the effects of monomer
100
ratio (BA/St), core-to-shell ratio, and content of cross-linker EGDMA in oil phase on
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the characteristics of the microcapsules (particle size, entrapment rate, and pesticide
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loading), especially, the sustained release performance were investigated.
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Materials and methods
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Materials
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BA (chemical pure, National Medicine Group Chemical Reagent Co., Ltd.), St
106
(chemical pure, National Medicine Group Chemical Reagent Co., Ltd.), nonylphenol
107
polyoxyethylene (OP-10), sodium dodecyl sulfate (SDS), ammonium persulfate (APS;
108
analytical pure, National Medicine Group Chemical Reagent Co., Ltd.), sodium
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bisulfite (SBS; analytical pure, Beijing Chemical Reagent Co.), chlorpyrifos (Jiangsu
110
YiJin Agrochemical Co., Ltd.), and EGDMA (99%, J&K Beijing Science and
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Technology Co., Ltd.) were used in this study.
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Preparation of chlorpyrifos microcapsules
113
The
chlorpyrifos-loaded
poly(BA/St)
and
chlorpyrifos-loaded
114
poly(BA/St/EGDMA) microcapsules were synthesized by emulsion polymerization in
115
a 250-mL four-neck flask equipped with a mechanical stirrer, a heating mantle with a
116
digital thermometer, and a condenser. In a typical synthesis, exact amounts of
117
chlorpyrifos were dissolved in a mixture of BA, St, and EGDMA to form the oil 6
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phase. Then, the oil phase was poured into distilled water (30 g) containing OP-10
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(0.15 g) and SDS (0.4 g) and emulsified using a homomixer at 1500 rpm for 10 min to
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form the pre-emulsion. Another solution that was prepared by dissolving OP-10 (0.1 g)
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and SDS (0.2 g) into distilled water (25 g) was added to the four-neck flask with
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mechanical stirring at 300 rpm. The initiator solution was prepared by dissolving APS
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(0.25 g) and SBS (0.25 g) in distilled water (15 g). One-third of the prepared
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pre-emulsion was added to a four-neck flask under nitrogen gas, and maintained at
125
80 °C. Then, one-third of the initiator was introduced to initiate polymerization. The
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remainder of the pre-emulsion and initiator solutions was slowly dropped into the
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reaction mixture over a period of 1 h. The reaction was continued further for 3 h
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under nitrogen gas. The resulting emulsion was collected after cooling to room
129
temperature for further characterization. The formulations employed for the synthesis
130
of the chlorpyrifos microcapsules are listed in Table 1.
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Effect of different formulations on the characteristics of microcapsules
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The individual effects of different factors on particle size, entrapment rate,
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pesticide loading, and slow release behaviors were examined. More specifically, the
134
effects of the monomer ratio, core-to-shell ratio, and EGDMA cross-linker content in
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the oil phase on the characteristics of the microcapsules were studied. The BA/St
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monomer ratios (w/w) in the oil phase were 1:5, 1:2, 1:1, 2:1, and 5:1 in the oil phase.
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The chlorpyrifos/monomer core-to-shell ratios (w/w) in the oil phase were 1:30, 5:26,
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10:21, and 15:16. The contents of cross-linker EGDMA in the oil phase (relative to
139
EGDMA/monomer ratio (mol %/mol %)) were 0, 0.5, 1.0, 2.5, 5, and 10%. 7
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Characterization
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Entrapment rate and pesticide loading
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The two parameters, entrapment rate (ER, %) and pesticide loading (PL, %),
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were used to characterize the efficiency of the encapsulation process. ER is defined as
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the ratio between the weight of encapsulated chlorpyrifos and the theoretical weight
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of chlorpyrifos during the preparation process,
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ER % =
147
PL is defined as the weight of encapsulated chlorpyrifos divided by the weight of
148
× 100 %.
the corresponding chlorpyrifos-containing microcapsules,
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PL % =
150
Prior to measuring ER and PL of the pesticide microcapsules, the emulsion was
151
demulsified. Typically, the emulsion (2 mL) was added to a 1:1 (w/w) mixture of
152
ethanol and 2% CaCl2 aqueous solution. The demulsified pesticide microcapsule
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solution
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chlorpyrifos-containing microcapsules. Then, the microcapsules were washed with 50
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wt.% ethanol/water solution once and distilled water twice to remove any
156
non-encapsulated chlorpyrifos, followed by drying at 45 °C for 24 h.
was
filtered
using
a
mutche
× 100 %.
filter
to
obtain
aggregates
of
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The microcapsule aggregates (~0.3 g) were introduced into 50-mL centrifuge
158
tubes containing n-hexane (20 mL). The encapsulated chlorpyrifos was extracted into
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n-hexane upon stirring at 150 rpm for 4 h at 25 °C. The concentration of chlorpyrifos
160
in
161
spectrophotometer (UNICO Shanghai Instrument Co., Ltd., China) at 292 nm and the
n-hexane
was
analyzed
on
a
WTF
UV-2102PC
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weight of encapsulated chlorpyrifos was calculated.
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Particle size measurement
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The mean diameter of the microcapsule particles was calculated using a
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HORIBA LA-950 laser particle analyzer (HORIBA Co., Ltd., Japan) at the Beijing
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Centre for Physical & Chemical Analysis (Beijing, China)
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Fourier transform infrared (FTIR) spectroscopy
168
For sample analysis, the microcapsules obtained from the demulsified solution
169
described above were dried at 40 °C overnight. All the FTIR measurements were
170
conducted on a Nicolet NEXUS-470 spectrometer (Madison, WI, USA).
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Thermo-gravimetric analysis (TGA)
172
Thermo-gravimetric analysis for the chlorpyrifos microcapsules was conducted
173
by using a TGA/DSC simultaneous thermal analyzer (Mettler Toledo, Switzerland) at
174
a heating rate of 10 °C/min under a nitrogen atmosphere from 25 °C to 500 °C.
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Transmission electron microscopy (TEM)
176
Prior to TEM analysis, the emulsion was diluted 600-fold, followed by negative
177
staining. TEM photographs were taken on a JEM-1230 transmission electron
178
microscope (JEOL Japan Electronics Co., Ltd., Japan) operating at an accelerating
179
voltage of 80 kV at the required magnification (80,000–200,000×) at room
180
temperature.
181
Slow release behaviors of chlorpyrifos microcapsules
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For the chlorpyrifos release experiments, a chlorpyrifos microcapsule emulsion
183
(5.0 g) was injected into a dialysis bag with molecular weights of 8,000–12,000. The 9
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dialysis bag was placed in a conical flask (250 mL) containing absolute ethyl alcohol
185
(100 mL) as the release medium under static conditions at 25 °C. The amounts of
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chlorpyrifos released at different time intervals were measured by recording the
187
absorbance of the release medium at 291 nm on an ultraviolet–visible
188
spectrophotometer (UNICO Shanghai Instrument Co., Ltd., China).
189
Results and Discussion
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FTIR spectroscopy analysis of chlorpyrifos microcapsules
191
To assess the successful encapsulation of chlorpyrifos, the FTIR spectra of the
192
chlorpyrifos, poly(BA/St) microcapsules, and chlorpyrifos-loaded poly(BA/St)
193
microcapsules were recorded (Figure 1). The spectrum of chlorpyrifos (Figure 1a)
194
featured stretching vibration peaks corresponding to pyridine ring C=N, aromatic
195
P–O–C, aliphatic P–O–C, and P=S at 1549.64, 1169.59, 852.84, and 1016.97 cm−1,
196
respectively. These characteristic peaks are typical of chlorpyrifos. The poly(BA/St)
197
microcapsules (Figure 1b) were characterized by the absorption peak at 1727.69 cm−1
198
(corresponding to C=O), which proved the existence of BA/St copolymer. Moreover,
199
the characteristic absorption peak of C=C belonging to monomer BA and St at 1637 cm−1 was
200
absent, which proved the successful preparation of the poly(BA/St) wall material. The
201
characteristic peaks observed in both Figure 1a and Figure 1b were present in the
202
spectrum of the chlorpyrifos-loaded poly(BA/St) microcapsules (Figure 1c), thereby
203
confirming the successful entrapment of chlorpyrifos by poly(BA/St) wall material.
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TG analysis of chlorpyrifos microcapsules
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Figure 2 demonstrated TGA thermogram of prepared poly(BA/St) chlorpyrifos 10
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microcapsules (Figure 2. c) comparing with pure chlorpyrifos (Figure 2. a), blank
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poly(BA/St) microcapsule (Figure 2. b) and physical mixture of chlorpyrifos and
208
blank poly(BA/St) microcapsule (Figure 2. d). TG curves presented the weight loss of
209
pure chlorpyrifos occurred at 143 °C while that of blank poly(BA/St) microcapsule
210
occurred at 360 °C. TG curves of poly(BA/St) chlorpyrifos microcapsules showed
211
two stage degradation with an initial weight loss from 234 °C to 301 °C and a second
212
from 360 °C to 425 °C. Comparing with TG curves of pure chlorpyrifos and blank
213
poly(BA/St) microcapsule, the first stage might result from the gasification of
214
chlorpyrifos and the second one was attributed to the decomposition of poly(BA/St).
215
It also proved that the chlorpyrifos was encapsulated by poly(BA/St) microcapsule
216
successfully. Additionally, TG curves of poly(BA/St) chlorpyrifos microcapsules had
217
the same degradation feature as the TG curves of physical mixture of chlorpyrifos and
218
blank poly(BA/St) microcapsule, might imply the physical combination between
219
chlorpyrifos and poly(BA/St) walls material.
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Particle size of chlorpyrifos microcapsules
221
In order to investigate the diameter feature of microcapsule pesticide sample
222
prepared in this study, particle size distribution analysis was conducted. The particle
223
size distribution of M3 was demonstrated in Figure 3. The diameter presented an
224
approximately normally distribution and the particle size mostly distributed at about
225
90 nm with the mean diameter of 90.21 nm. For other samples, similar distribution
226
was observed. It revealed a narrow and concentrated distribution of the particle size of
227
each sample. As to the mean diameter, presented on Table 1, mean diameters of 11
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sample P1-P3 were slightly bigger than others, which probably indicated the influence
229
of chlorpyrifos content on particle mean diameter. When only considering the
230
monomer ratio (M1-M5), M2 is slightly bigger than others’ formulation (M1, M3, M4
231
and M5) where no significant difference was observed. Similarly, the mean diameters
232
of E1-E5 were almost the same (about 90nm), which indicated that the change of
233
EGDMA content had little effect on particle size. These results demonstrated that the
234
content of the monomer, pesticide, and EGDMA minimally influenced the particle
235
size. Consequently, it could be inferred that the release performance of the
236
microcapsules would not be influenced by the diameter of the microcapsules that
237
remained mostly unchanged across the different synthesis conditions studied herein.
238
Effect of monomer ratio on characteristics of chlorpyrifos microcapsules
239
As observed in Table 2 and Figure 4, when the BA/St monomer ratio changed
240
from 1:5 to 1:1, ER and PL increased from 81.28 to 89.31% and 2.62 to 2.88%,
241
respectively. In contrast, when the BA/St monomer ratio changed from 1:1 to 5:1, ER
242
and PL decreased from 89.31 to 73.62% and 2.88 to 2.37%, respectively. As deduced,
243
excess amounts of BA and St were detrimental to the encapsulation of chlorpyrifos as
244
indicated by the corresponding lower measured ER and PL values when compared
245
with those of other formulations comprising lower amounts of BA and St. At
246
excessive amounts of St (M1), the resulting poly(BA/St) became hard and brittle
247
because of its higher Tg over 55 °C. Hence, the polymer was prone to rupture during
248
the encapsulation of chlorpyrifos. Similarly, when the content of BA in the
249
formulation was high (M4, M5), poly(BA/St) featured soft and sticky characteristics 12
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with the lower Tg of about -21 and -39 °C, and hence was not adequately
251
mechanically stable for the effective entrapment of the pesticide. Therefore, the ER
252
and PL values of the microcapsules were satisfactory at moderate monomer ratios of
253
1:2 and 1:1 owing to the desirable soft and hard characteristics of poly(BA/St). The
254
monomer ratio of 1:2 was studied further in the subsequent experiments.
255
Effect of core-to-shell ratio on characteristics of chlorpyrifos microcapsules
256
The influence of different chlorpyrifos/monomer BA and St core-to-shell ratios
257
on ER and PL was investigated (Table 3, Figure 5). The weight of the oil phase was
258
kept constant and the total weight of chlorpyrifos and monomer BA and St was 31 g.
259
Only the weight of chlorpyrifos was varied from 1 to 15 g to achieve
260
chlorpyrifos/monomer BA and St weight ratios of 1:30, 5:26, 10:21, and 15:16. As
261
observed in Figure 5, PL increased from 2.74 to 29.11% when the loading content of
262
pesticide increased owing to the increased content of pesticides in the oil phase. In
263
contrast, ER decreased from 85.14 to 60.15% when the loading content of
264
chlorpyrifos increased from 1 to 15 g. These results showed that moderate dosages of
265
monomer in the chlorpyrifos emulsion preparation process afforded desirable ER and
266
PL values, whereas similar weight levels of chlorpyrifos and monomer did not result
267
in desirable ER values. However, higher PL values were obtained. This phenomenon
268
was attributed to ineffective chlorpyrifos entrapment owing to the inadequately dense
269
polymer obtained in the presence of relatively low monomer dosages. Optimum
270
weight ratios of the pesticide and monomer were determined at 1:30–5:26 for the
271
effective encapsulation of chlorpyrifos into poly(BA/St) microcapsules by emulsion 13
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272 273 274
polymerization. Effect of content of cross-linker EGDMA on characteristics of chlorpyrifos microcapsules
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EGDMA, a widely employed cross-linker for emulsion polymerization, will
276
form a dense network within the microcapsule wall material or generate links among
277
the microcapsule particles upon copolymerization with BA and St. In the present
278
study, the effect of EGDMA on the characteristics of the microcapsules was
279
investigated by varying its contents at 0, 0.5, 1.0, 2.5, 5, and 10% (relative to the
280
EGDMA/monomer ratio, mol/mol). The results are shown in Table 4. As observed in
281
Figure 6, ER and PL both reached a maximum when an EGDMA concentration of 0.5%
282
was used, and then decreased with increasing concentrations of cross-linker EGDMA.
283
The observed increase in ER and PL may be attributed to the denser structure of
284
poly(BA/St/EGDMA) relative to that of poly(BA/St) owing to the higher degree of
285
cross-linking. In contrast, the considerable decrease in ER and PL with further
286
increases in the amount of EGDMA may be attributed to the excessively high degrees
287
of cross-linking owing to the higher content of cross-linker. Higher degrees of
288
cross-linking would result in tighter connections among the polymer chains, thereby
289
inhibiting effective encapsulation of the pesticide.
290
Pesticide release study
291
An important aim of the present study was to study the release performance of
292
chlorpyrifos from the poly(BA/St) and poly(BA/St/EGDMA) microcapsules. First, the
293
effect of the monomer ratio (BA/St) used in emulsion polymerization on the release 14
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profile is discussed. The pesticide microcapsule emulsion was introduced into a
295
dialysis bag and the release of the pesticide from the dialysis bag was monitored over
296
~170 h. The results are shown in Figure 7. As observed, the release of chlorpyrifos
297
from the poly(BA/St) microcapsules M1 and M5 prepared with monomer ratios of 1:5
298
and 5:1 was fast. A sustainable pesticide release over ~72 h was observed. A slower
299
release profile was obtained for the chlorpyrifos-loaded poly(BA/St) microcapsules
300
M4 prepared with a monomer ratio of 2:1. The release profile of the microcapsules
301
M2 prepared with a monomer ratio of 1:2 was satisfactory: a lower chlorpyrifos
302
release was observed at the early stages of the release studies that lasted for 168 h.
303
The maximum sustainable release content of the pesticide was 63.9%. These results
304
may be explained by the different chlorpyrifos microcapsule characteristics as a result
305
of the different monomer ratios used in the formulations as observed in Table 2. The
306
ER and PL values of the chlorpyrifos-loaded microcapsules M1 and M5 were
307
relatively lower, which not only led to poor chlorpyrifos encapsulation, but also to a
308
high content of chlorpyrifos retained on the microcapsule surface or in the water
309
phase. Thus, the fast release profile was caused by the fast dissolution of chlorpyrifos
310
in the external phase at the early stages of the release process and prompt release of
311
the pesticide from the mechanically weak chlorpyrifos-loaded poly(BA/St)
312
microcapsules. Furthermore, the Tg of M1 was much higher than the release
313
temperature of 25 °C while the Tg of M5 was much lower than 25 °C. A relatively
314
large difference between Tg and environmental temperature might lead to a relatively
315
complete polymer state transitions to elastomeric state for M5 and glassy state for M1, 15
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which seemed to be conducive to the chlorpyrifos release in the present study. The
317
rapid early release observed for the microcapsules M4 was due to the same
318
phenomena occurring in microcapsules M1 and M5 as discussed previously. However,
319
the slower release profile was due to the stronger mechanical property of the
320
microcapsules. Poly(BA/St) microcapsules M2 and M3 had mechanical properties
321
suited for entrapping the pesticide as well as obtaining high ER and PL values. Tg of
322
M2 and M3 were close to the release temperature, which might let the microcapsule
323
had characteristics of both elastomeric state and glassy state. It seemed to be very
324
suitable for slow release of chlorpyrifos.Therefore, both the early release and
325
sustainable release (>168 h) profiles of these particular pesticide microcapsules were
326
satisfactory. More importantly, our results indicated that the poly(BA/St)
327
microcapsules prepared at a monomer ratio of 1:2 (M2) were the best candidate for
328
achieving sustainable release of chlorpyrifos.
329
Secondly, the effect of varying the EGDMA cross-linker content in the
330
microcapsule formulation on the release profile of chlorpyrifos was investigated. The
331
content was varied from 0 to 10%, while the BA/St monomer ratio was fixed at 1:2.
332
The results are depicted in Figure 8. As observed, the sustainable release of
333
chlorpyrifos loaded in the microcapsules M2-E3 prepared with increasing EGDMA
334
contents from 0 to 2.5% increased. Further increases in the EGDMA content above
335
2.5% led to shorter sustainable releases (E4-E5). The reduction in the sustainable
336
release behavior may be due to the dense poly(BA/St/EGDMA) network that prevents
337
effective release of chlorpyrifos from the microcapsule. Moreover, more EGDMA 16
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338
reacted with BA and St, resulting in tighter and denser polymer chains, consequently
339
leading to the decrease in sustainable release. In contrast, increasing the content of
340
EGDMA from 2.5 to 10% resulted in considerably fast release profiles of chlorpyrifos.
341
The fast release profiles may be due to the low mechanical stability of the
342
microcapsules in ethyl alcohol release medium owing to the greater extent of
343
cross-linking by EGDMA. Besides, the relatively lower ER and PL values resulted in
344
a higher level of pesticide retainment on the microcapsule surface or in the water
345
phase that may account for the rapid initial release rate.
346
To
further
understand
the
release
profiles
of
the
pesticide-loaded
347
EGDMA-cross-linked microcapsules, TEM analysis was conducted on the
348
pesticide-loaded microcapsules prepared with different cross-linker EGDMA contents.
349
The morphology and dispersion of the microcapsules can be observed in Figure 9.
350
The dark circles of microcapsule from TEM might also represent the relative thin
351
shell polymer forming by the slowly dropped process of monomer, which indicated a
352
core-shell structure of these microcapsules. Figure 9a shows the homogeneous
353
dispersion of the chlorpyrifos-loaded poly(BA/St) microcapsules prepared in the
354
absence of cross-linker EGDMA. In contrast, connections among the pesticide-loaded
355
microcapsules prepared in the presence of 5% EGDMA were obvious (Figure 9b).
356
The external connections among the microcapsules that resulted in denser structures
357
led to slower release of chlorpyrifos when compared with that observed for
358
microcapsules prepared without EGDMA. Unfortunately, excessive connections led to
359
the aggregation of the microcapsules that were prepared in the presence of 10% 17
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360
EGDMA (Figure 9c). Furthermore, the inhomogeneous microcapsule structure may
361
be prone to rupture during the microcapsule synthesis and pesticide release processes.
362
Therefore, poly(BA/St/EGDMA) microcapsules prepared in the presence of 5–10%
363
EGDMA featured fast release profiles, whereas those prepared in the presence of
364
0.5–2.5% featured slower release characteristics.
365
Figure 10 shows that the length of sustainable release of chlorpyrifos decreased
366
significantly with increasing loading contents of chlorpyrifos (1–10 g). Accordingly,
367
changes in the loading content of chlorpyrifos altered the chlorpyrifos/monomer BA
368
and St core-to-shell ratio. More specifically, increasing the dosage of pesticide led to
369
decreasing dosages of the monomer at a given combined pesticide and monomer
370
content. The polymer matrix prepared with a lower amount of monomer was not
371
dense enough for chlorpyrifos entrapment. Based on the results discussed previously,
372
the lower ER of the microcapsules P1 and P2 loaded with 5 and 10 g pesticide is
373
another reason for the rapid release profile observed. These results indicated that a
374
core-to-shell ratio of 1:30 was optimum to achieving good release profiles, whereas
375
microcapsules loaded with higher contents of chlorpyrifos (5–10 g) featured faster
376
release profiles.
377
Chlorpyrifos Release Kinetics
378
All of the slow release profile curves were composed of the gradual release curve
379
followed by constant release curve that implied the fully release of chlorpyrifos. In
380
order to investigated the release mechanism of chlorpyrifos-loaded microcapsule in
18
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the present study, data of the gradual release curve from chlorpyrifos release
382
experiments were fitted to the following equation 31: !
= #$ %
"
383 384 385
!
is the amount of chlorpyrifos released at time t,
"
is the total amount of
chlorprifos in microcapsule, k is a release constant and n is a diffusional exponent. As descripted in previous study
20, 32, 33
, the diffusion exponent n was equal to
386
0.43 for a Fickian diffusion system, when the diffusion of chlorpyrifos was controlled
387
by its concentration difference, and diffusional exponent n was close to 0.85 for a
388
degradation release system, when the release of chlorpyrifos was controlled by the
389
degradation of microcapsules.
390 391
Slow release data were fitted with nonlinear regression. The release constant, diffusional exponent and the correlation coefficient R were presented in Table 5.
392
The release exponent n close to 0.43 of samples M2, M3, E3 and P1 with a high
393
R values that indicated good fitting of exponential equations, revealed the release
394
mechanism of these microcapsule to be controlled by diffusion. When the release
395
exponent n values were much lower than 0.43, there might be some other factors that
396
influenced the chlorpyrifos release system and the rapid initial release profile of these
397
sample probably implied the rupture of microcapsule in the early release stage. All n
398
values were below 0.43, proving that degradation of microcapusle was not the
399
dominant factor of the release.
400
As to the effects of monomer ratio on the microcapsule, release profiles of M2
401
and M3 with BA/St ratios of 1:2 and 1:1 fitted well with the equation while profiles of 19
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402
microcapsules of other BA/St monomer ratio did not, which indicated the same
403
conclusion that the suitable monomer ratio for chlorpyrifos slow release were 1:2 and
404
1:1. The n value of E3 with 2.5% EGMDA was 0.428, which represent a good
405
diffusion system of chlorpyrifos. As a result, E3 with 2.5% EGMDA was the finest
406
slow release formulation for controlled chlorpyrifos. For the effects of different
407
core-to-shell ratios, results of slow release kinetics showed that M2 and P1 with
408
core-to-shell ratios of 1:30 and 5:26 basically matched the diffusion system while
409
ratio of 10:21 might lead to an initial rupture of microcapsule. It might be related with
410
the lower ER as a consequence of the higher PL.
411
A series of poly(BA/St) and poly(BA/St/EGDMA) microcapsules loaded with
412
various amounts of chlorpyrifos were prepared by emulsion polymerization. All
413
prepared microcapsules featured particles sizes of ~90 nm with a little difference
414
among each samples. The microcapsules show promise as a carrier for the slow
415
release of pesticide chlorpyrifos. FT-IR and TG analysis of the microcapsules
416
confirmed the successful entrapment of chlorpyrifos in the poly(BA/St) wall material;.
417
The microcapsule ER varied from 60.15 to 96.74%. The highest ER (96.74%) was
418
observed for microcapsules prepared with a BA/St monomer ratio of 1:2 in the
419
presence of moderate EGDMA cross-linker content of 0.5%. In contrast, lower ERs of
420
60.15–67.97% were observed for microcapsules prepared with higher contents of
421
chlorpyrifos (5-10 g). Variations in ER were consistent with variations in PL except
422
for the series of microcapsules prepared with different core-to-shell ratios. In this
423
series, maximum PL (29.11%) resulted in the lowest ER (60.15%). 20
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The release performance of chlorpyrifos from the microcapsules was
425
characterized by the medium dialysis method. Microcapsules prepared with a
426
monomer ratio of 1:2 whose Tg was close to the release temperature displayed
427
optimum sustainable release of chlorpyrifos. In contrast, microcapsules prepared with
428
monomer ratios of 1:5 and 5:1 displayed significantly high pesticide release at the
429
early stages of the release process. Microcapsules featuring adequate degrees of
430
cross-linking by EGDMA (0.5–2.5% EGDMA) displayed prolonged sustainable
431
release profiles, whereas those prepared with higher EGDMA contents of 5–10%
432
displayed enhanced chlorpyrifos release. Additionally, microcapsules prepared with a
433
chlorpyrifos/monomer BA and St core-to-shell ratio of 1:30 displayed optimum
434
release profiles, whereas those prepared at other ratios displayed faster release profiles.
435
Chlorpyrifos release was characterized by nonlinear regression analysis. The diffusion
436
release from microcapsules was found due to diffusional exponent n closing to 0.43,
437
while for samples that had n values much lower than 0.43, their faster release might
438
be caused by the initial rupture of microcapsule.
439
The current study investigated the influences of monomer ratio, core-to-shell
440
ratio, and degree of cross-linking on the characteristics and sustainable release
441
behaviors of chlorpyrifos entrapped in poly(BA/St) and poly(BA/St/EGDMA)
442
microcapsules via emulsion polymerization. Moreover, the findings provide a
443
reference for the release profile studies of polyacrylate chlorpyrifos microcapsules.
444
References:
445
(1) Bending, G. D.; Lincoln, S. D.; Sorensen, S. R.; Morgan, J. A. W.; Aamand, J.; Walker, A. In-field 21
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Journal of Agricultural and Food Chemistry
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spatial variability in the degradation of the phenyl-urea herbicide isoproturon is the result of
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interactions between degradative Sphingomonas spp. and soil pH. Appl. Environ. Microb. 2003, 69,
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827-834.
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(2) Fleming, L. E.; Bean, J. A.; Rudolph, M.; Hamilton, K. Cancer incidence in a cohort of licensed
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pesticide applicators in Florida. J. Occup. Environ. Med. 1999, 41, 279-288.
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(3) Salameh, P. R.; Baldi, I.; Brochard, P.; Raherison, C.; Abi Saleh, B.; Salamon, R. Respiratory
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symptoms in children and exposure to pesticides. Eur. Respir. J. 2003, 22, 507-512.
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(4) Hack, B.; Egger, H.; Uhlemann, J.; Henriet, M.; Wirth, W.; Vermeer, A. W. P.; Duff, D. G.
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Advanced agrochemical formulations through encapsulation strategies? Chem. Ing. Tech. 2012, 84,
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(5) Shim, T. S.; Kim, S. H.; Yang, S. M. Elaborate design strategies toward novel microcarriers for
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(6) Hong K.; Park S. Preparation of polyurethane microcapsules with different soft segments and their
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characteristics. React. Funct. Polym. 1999, 42, 193-200.
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(7) Hashemi S. A.; Zandi M. Encapsulation process in synthesizing polyurea microcapsules containing
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pesticide. Iran. Polym. J. 2001, 10, 265-270.
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(8) Hirech, K.; Payan, S.; Carnelle, G.; Brujes, L.; Legrand, J. Microencapsulation of an insecticide by
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interfacial polymerisation. Powder Technol. 2003, 130, 324-330.
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(9) Mihou, A. P.; Michaelakis, A.; Krokos, F. D.; Mazomenos, B. E.; Couladouros, E. A. Prolonged
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slow release of (Z)-11-hexadecenyl acetate employing polyurea microcapsules. J. Appl. Entomol. 2007,
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polymer. Amer. J. Appl. Sci. 2010, 7, 39-45.
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(11) Mayya K. S.; Bhattacharyya A.; Argillier, J. F. Micro-encapsulation by complex coacervation:
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influence of surfactant. Polym. Int. 2003, 52, 644-647.
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(12) Dailey O. D.; Dowler C. C. Polymeric microcapsules of cyanazine: preparation and evaluation of
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efficacy. J. Agric. Food Chem. 1998, 46, 3823-3827.
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(13) P Rez-Mart Nez, J. I.; Morillo, E.; Maqueda, C.; Gin S, J. M. Ethyl cellulose polymer
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microspheres for controlled release of norfluazon. Pest Manag. Sci. 2001,57, 688-694.
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(14) El Bahri Z.; Taverdet J. L. Elaboration and characterisation of microparticles loaded by pesticide
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model. Powder Technol. 2007, 172, 30-40.
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(15) Adak, T.; Kumar, J.; Shakil, N. A.; Walia, S. Development of controlled release formulations of
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imidacloprid employing novel nano-ranged amphiphilic polymers. J. Environ. Sci. Heal. B. 2012, 47,
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217-225.
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(16) Zhang P.; Zhang Q.; Jiao, Q. Synthesis and characterization of microcapsules with chlorpyrifos
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cores and polyurea walls. Chem. Res. Chinese U. 2006, 22, 379-382.
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(17) Zhu, L.; Wang, Z.; Zhang, S.; Long, X. Fast microencapsulation of chlorpyrifos and bioassay. J.
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Pestic. Sci. 2010, 35, 339-343.
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(18) Zhang, J.; Li, M.; Fan, T.; Xu, Q.; Wu, Y.; Chen, C.; Huang, Q. Construction of novel amphiphilic
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chitosan copolymer nanoparticles for chlorpyrifos delivery. J. Polym. Res. 2013, 20, 107.
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(19) Kumbar, S. G.; Kulkarni, A. R.; Dave, A. M.; Aminabhavi, T. M. Encapsulation efficiency and
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release kinetics of solid and liquid pesticides through urea formaldehyde crosslinked starch, guar gum,
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and starch +guar gum matrices. J. Appl. Polym. Sci. 2001, 82, 2863-2866.
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(20) Stloukal, P.; Kucharczyk, P.; Sedlarik, V.; Bazant, P.; Koutny, M. Low molecular weight 23
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poly(lactic acid) microparticles for controlled release of the herbicide metazachlor: preparation,
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morphology, and release kinetics. J. Agric. Food Chem. 2012, 60, 4111-4119.
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(21) Zhang, S. F.; Chen, P. H.; Zhang, F.; Yang, Y. F.; Liu, D. K.; Wu, G. Preparation and
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physicochemical characteristics of polylactide microspheres of emamectin benzoate by modified
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solvent evaporation/extraction method. J. Agric. Food Chem. 2013, 61, 12219-12225.
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(22) Zhang, W.; He, S.; Liu, Y.; Geng, Q.; Ding, G.; Guo, M.; Deng, Y.; Zhu, J.; Li, J.; Cao, Y.
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Preparation
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silica-alginate-elements as controlled release carrier materials. ACS Appl. Mater. Inter. 2014, 6,
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11783-11790.
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(23) Liu, D.; Ichikawa, H.; Cui, F.; Fukumori, Y. Short-term delayed-release microcapsules
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spraycoated with acrylic terpolymers. Int. J. Pharm. 2006, 307, 300-307.
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(24) Chen, C.; Chen, Z.; Zeng, X.; Fang, X.; Zhang, Z. Fabrication and characterization of
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nanocapsules containing n-dodecanol
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redox
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(25) Swamy, B. Y.; Prasad, C. V.; Rao, K. C.; Subha, M. C. S. Preparation and characterization of poly
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(hydroxyl ethyl methyl acrylate-co-acrylic acid) microspheres for drug delivery application. Int. J.
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Polym. Mater. 2013, 62, 700-705.
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(26) Wu, H.; Xu, Y.; Liu, G.; Ling, J.; Dash, B.; Ruan, J.; Zhang, C. Emulsion cross-linked
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chitosan/nanohydroxyapatite microspheres for controlled release of alendronate. J. Mater. Sci.-Mater.
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M. 2014, 25, 2649-2658.
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(27) Casana, G. V.; Gimeno, S. M.; Gimeno, S. B. Agrochemical formulations containing
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microcapsules. U.S. Patent 08263530, 2012.
and
characterization
of
novel
by
functionalized
miniemulsion
prochloraz
polymerization
microcapsules
using
using
interfacial
initiation. Colloid Polym. Sci. 2012, 290, 307-314.
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512
(28) Casana, G. V.; Gimeno, S. M.; Gimeno, S. B. Composition for delivering e.g. pyrethroids
513
comprises microcapsules that each encloses microencapsulated material, where wall of microcapsule is
514
formed by polymerization of aliphatic/aromatic isocyanate, and acetylene carbamide derivative. U.S.
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Patent 2012245027-A1, 2012.
516
(29) Murakami M.; Ogawa M.; Fujinoto, I.; Ohtsubo, T. New pesticidal compositions comprise
517
microcapsules microencapsulating organophosphorus compound e.g. chlorpyrifos, used to control
518
wood-injuring insects, termites and nuisance injurious insects and to produce insect-proof wood. U.S.
519
Patent 5929053-A, 1999.
520
(30) Yang C.; Pan I. Controlled release pesticidal microcapsule prodn.|by mixing vegetable oil,
521
pesticide and aq.urea-formaldehyde prepolymer, acidifying and cross-linking, giving high
522
encapsulation rate. U.S. Patent 5576008-A, 1996.
523
(31) Korsmeyer R. W.; Gurny R., Doelker, E.; Buri, P.; Peppas, N. A. Mechanisms of solute release
524
from porous hydrophilic polymers. Int. J. Pharm. 1983, 15, 25-35.
525
(32) Zuleger S.; Lippold B. C. Polymer particle erosion controlling drug release. I. Factors influencing
526
drug release and characterization of the release mechanism. Int. J. Pharm. 2001, 217, 139-152.
527
(33) Asrar, J.; Ding, Y.; La Monica, R. E.; Ness, L. C. Controlled release of tebuconazole from a
528
polymer matrix microparticle: release kinetics and length of efficacy. J. Agric. Food Chem. 2004, 52,
529
4814-4820.
530
25
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531 532
Figure 1. FTIR spectra of (a) chlorpyrifos, (b) poly(BA/St) (1:1, w./w.) microcapsules, and (c)
533
chlorpyrifos-loaded poly(BA/St) (1:1, w./w.) microcapsules.
534 535
Figure 2. TGA thermogram: (a) chlorpyrifos; (b) blank poly(BA/St) microcapsule; (c) poly(BA/St)
536
microcapsule of chlorpyrifos; (d) physical mixture of chlorpyrifos and blank poly(BA/St)
537
microspheres.
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538 539
Figure 3. Particle size distribution of chlorpyrifos poly(BA/St) microcapsule with the monomer ratio
540
1:1 (BA/St w./w.)
541 542
Figure 4. Effect of BA/St monomer ratio in the oil phase on the ER and PL of the microcapsules.
27
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543 544
Figure 5. Effect of additive amount of pesticide in the oil phase on the ER and PL of the microcapsules.
545 546
Figure 6. Effect of additive amount of EGDMA in the oil phase on the ER and PL of the microcapsules.
28
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547 548
Figure 7. Profiles of chlorpyrifos release from formulations prepared using different ratios of BA/St
549
monomer.
550 551
Figure 8. Profiles of chlorpyrifos release from formulations prepared using different contents of
552
EGDMA (The BA/St monomer ratio was 1:2).
29
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553 554
Figure 9. TEM images of the different pesticide microcapsules prepared with (a) 0, (b) 2.5, and (c) 10%
555
EGDMA.
556 557
Figure 10. Profiles of chlorpyrifos release from microcapsules prepared with different loading contents
558
of chlorpyrifos (1–10 g).
559
Table 1. Particle size of the chlorpyrifos microcapsules prepared under different conditions.
Oil phase Mean Diameter Sample
Monomer
Chlorpyrifos
EGDMA/ monomer (nm)
(BA/St w./w.)
(g)
(mol./mol.)
M1
1:5
1.0
0
88.80±1.47 ef
M2
1:2
1.0
0
95.45±1.96 c
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M3
1:1
1.0
0
90.21±1.93 e
M4
2:1
1.0
0
87.40±1.75 ef
M5
5:1
1.0
0
89.21±0.42 ef
E1
1:2
1.0
0.5%
92.82±1.68 d
E2
1:2
1.0
1.0%
88.59±1.71 ef
E3
1:2
1.0
2.5%
89.82±1.66 ef
E4
1:2
1.0
5.0%
88.36±0.89 ef
E5
1:2
1.0
10%
90.66±0.57 de
P1
1:2
5.0
0
97.96±1.10 b
P2
1:2
10.0
0
101.8±1.21 a
P3
1:2
15.0
0
97.17±1.71 bc
560
Table 2. Characteristics of the pesticide microcapsules prepared under different BA/St monomer ratio
561
conditions.
Monomer
Designed
(BA/St
Tg
Entrapment Rate
Pesticide Loading
w./w.)
(°C)
(%)
(%)
M1
1:5
55.13
81.28±2.02 b
2.622±0.07 b
M2
1:2
25.02
85.14±2.80 b
2.747±0.09 b
M3
1:1
-0.023
89.31±2.21 a
2.881±0.07 a
M4
2:1
-21.18
76.37±1.37 c
2.464±0.04 c
Sample
Characteristic Parameter
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M5
5:1
-39.31
73.62±2.34 c
Page 32 of 34
2.375±0.08 c
562
The designed Tg (glass transition temperature) was calculated by the Fox equation.
563
Table 3. Characteristics of the pesticide microcapsules prepared under different content of chlorpyrifos.
Characteristic Parameter
Chlorpyrifos Core-to-shell Sample (g)
Ratio (w./w.) Entrapment Rate (%) Pesticide Loading (%)
M2
1.0
1/30
85.14±2.02 a
2.747±0.09 d
P1
5.0
5/26
78.18±1.51 b
12.61±0.24 c
P2
10.0
10/21
67.97±2.87 c
21.93±0.93 b
P3
15.0
15/16
60.15±2.73 d
29.11±1.32 a
564
Table 4. Characteristics of the pesticide microcapsules prepared under different content of crosslinker
565
EGDMA.
Characteristic Parameter
EGDMA/monomer Sample
566
(mol./mol.)
Entrapment Rate (%)
Pesticide Loading (%)
M2
0
85.14±2.02 c
2.747±0.09 c
E1
0.5%
96.74±0.47 a
3.094±0.01 a
E2
1.0%
93.05±1.69 ab
2.951±0.05 b
E3
2.5%
90.02±1.89 b
2.783±0.06 c
E4
5.0%
86.11±3.24 c
2.557±0.10 d
E5
10%
85.78±1.45 c
2.359±0.04 e
Table 5. Parameters characterizing fitting of the model equation on chlorpyrifos gradual release data
Sample
k
n
R
M1
0.433
0.180
0.980
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M2
0.132
0.317
0.998
M3
0.120
0.398
0.993
M4
0.161
0.383
0.976
M5
0.234
0.316
0.985
E1
0.167
0.274
0.992
E2
0.126
0.288
0.998
E3
0.067
0.428
0.996
E4
0.187
0.327
0.964
E5
0.337
0.232
0.991
P1
0.086
0.431
0.997
P2
0.230
0.237
0.990
567
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568
TOC Graphic
569
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