Research on the rheological properties of pesticide suspension concentrate.

Abstract

This study reports research on pesticide suspension rheology and a new rheological parameter, the relative value of approach, which has great advantage for judging the physical stability of a pesticide suspension concentrate. Experiments showed that the system can form stable dispersions when the value of the relative value of approach (S(r)) is less than 0.1.

Full text

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Tan et al. / J Zhejiang Univ SCI 2004 5(12):1604-1607
1604




Research on the rheological properties of
pesticide suspension concentrate

TAN Cheng-xia (谭成侠)†1,2, SHEN De-long (沈德隆)1, WENG Jian-quan (翁建全)1
CHEN Qing-wu (陈庆悟)1, LIU Hui-jun (刘会君)1, YUAN Qi-liang (袁其亮)1
(1College of Chemical Engineering and Materials Science, Zhejiang University of Technology, Hangzhou 310014, China)
(2College of Agriculture and Biotechnology, Zhejiang University, Hangzhou 310029, China)
†E-mail: tanchengxia@zjut.edu.cn
Received Oct. 8, 2003; revision accepted Dec. 22, 2003

Abstract: This study reports research on pesticide suspension rheology and a new rheological parameter, the relative value
of approach, which has great advantage for judging the physical stability of a pesticide suspension concentrate. Experiments
showed that the system can form stable dispersions when the value of the relative value of approach (Sr) is less than 0.1.

Key words: Suspension concentrate, Rheology, Stability, Relative value of approach
doi:10.1631/jzus.2004.1604 Document code: A CLC number: TQ450.1; O373


INTRODUCTION

Pesticides formulated as suspension concen-
trate are better than many other pesticide prepara-
tions as they have advantages in terms of biological
efficacy, safety, and economic viability (Kanel-
lopulos, 1974; Mulqueen et al., 1990; Seaman,
1990; Wigger and Guckel, 1989; Zhang, 2002).
Consequently this type of formulation has become
widely accepted by agricultural users. However, the
difficulty in controlling the physical stability of the
S.C. is one of the major limitations to its devel-
opment (Brian, 1980; Cheng, 1980; John, 1987;
Makarov et al., 1988; Luckham, 1989; Guido and
Tadros, 2000).
Rheology is a science which deals with mate-
rial flow and deformation. The rheological proper-
ties of an S.C. pesticide formulation can be either
Newtonian or non-Newtonian; both aqueous or non
aqueous dispersions can be prepared (Tadros, 1980;
Chen et al., 2002).
One of the purposes of the current study was to
gain insight into the stability of the S.C. A method
for judging the physical stability of S.C. is very
important in preparation of suitable formulation for
pesticide S.C. (Winzeler et al., 1980; Woldemar et
al., 1984).


THEORY OF PLASTIC FLUID

The suspension concentrates studied in this
work were mainly plastic fluids; the characteristics
of plastic fluids will be discussed initially (Fig.1).
The plastic property of a dispersing system is
thought to be produced by a three dimensional
network structure of particles that comprise the S.C.
The network structure must be destroyed for the
system to flow. In a plastic material, the system
starts to flow once the stress exceeds a fixed value.
The more the shear stress increases, the more the
structure is destroyed, and the viscosity will decline
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Tan et al. / J Zhejiang Univ SCI 2004 5(12):1604-1607
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simultaneously.
Winzeler thought that the system has three
yield values that are shown in Fig.1. τ0 is static
shearing force which Winzeler (1980) called it the
value of plastic deformation. τB is called motive
shearing force (or Bingham value) which is the
value of the part of line extending to the ordinate.
τM is the shear stress of the system starting to flow
and corresponds to the shear stress of a single par-
ticle appearing in the flowing system when the
particle structure of the whole system is destroyed.
Here τ0 is obtained from the Epprecht equation:

1 2
D
2
−
1
0
2 2
τ
1 1
τ
2222
1 221 2 2
τ
τ
1 1
τ
τ
2 1
τ τ
2 1 1 2
D
ττ
2 2 1 1
2
2(
()
)
[2( )][2(
2(
)] ()
)
DD
D
DDDDD
DD
ττ
τ
ττ
−
=
−−−−
−
−
(1)

For non-Bingham fluid, the equation is:

τ=k0Dn+τ0 (2)


RHEOLOGICAL MODEL

Creation of rheological model
In a Bingham fluid, the fluid starts to flow
after the shear stress exceeds the yield stress τB.
This phenomenon can be explained as follows: in a
three-dimensional structure, the fluid at rest can
resist a fixed shear stress and does not flow until the
stress exceeds the Bingham yield value. The rela-
tive movement of particles in the interior of the
system does not occur until the stress exceeds the
value. If the weight of particles is not sufficiently
large, the Brownian motion between the particles
cannot reach the yield value. Thus, the system can
only keep stability in a fixed scope.
The structure of a non-Bingham fluid is not as
good as that of a Bingham fluid; because its Casson
yield value is less than the Bingham yield value of
Bingham fluid. Therefore, the particles cannot be
arranged well when the system is at rest.
We can draw the conclusion that the more a
system approaches the ideal state, the more stable it
is. The parameter Sr is applied to judge the distinc-
tion between non-Bingham and Bingham fluid. The
smaller Sr is, the more stable is the system. The
equation of Sr is as follows:

/SS S
= ∆

The curves of CE and BE overlap from the
point ‘E’. ∆S is the area of BCE, which means the
distinction between a non-Bingham and a Bingham
fluid and its unit is Pa/s; SB is the area of quadri-
lateral BOAE, and its unit is also Pa/s (Fig.2).













Verification of rheological model
The normal method of evaluating pesticide
S.C. physical stability is to measure the suspending
ratio after it is stored for two weeks at (54±2) °C.
The higher the suspending ratio, the more stable is
the S.C. When the suspending ratio is less than 90%,
the S.C. will be unstable.
To investigate the S.C. rheology, we devel-
rB
(3)
Fig.1 Rheological curve of plastic fluid
0.00 2.00 4.00 6.00
Shear rate (1/s)
1.20
1.00
0.80
0.60
0.40
0.20
Shear stress (Pa)
τ0
τB
τM
Fig.2 Legend of non-Bingham fluid and Bingham fluid
1.20
1.00
0.80
0.60
0.40
0.20
0.00 2.00 4.00 6.00
Shear rate (1/s)
Shear stress (Pa)
A
E
B
C

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Tan et al. / J Zhejiang Univ SCI 2004 5(12):1604-1607
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oped 30% thiram-triadimefon S.C. After studying
the formulation, we found that dispersants, viscos-
ity regulators and wettable agents were the major
factors influencing the S.C. stability. First, the 10
samples (W-01∼W-10) were obtained by orthogo-
nal experiments. Then, we measured their values of
Sr and suspending ratios (Table 1).
As can be seen from Table 1, the S.C. was
stable only when Sr was less than 0.1. The more the
value of Sr approaches zero, the more stable S.C. is.
On the other hand, the higher the value of Sr, the
bigger is the deviation from Bingham fluid behav-
ior, and the more unstable is the S.C.


APPLICATION OF THE
MODEL

The rheological model was applied to measure
the physical stability of 40% chlorothalonil S.C.
supplied by Suli Fine Chemistry Co., Ltd. (Jiangyin,
China).

Measurement of viscosity
The viscosity values of 40% chlorothalonil
S.C. were measured at (30±1) °C by using NDJ-79
Rotary Viscometer (Table 2).

Determining the curve type
Table 2 data were used to construct the curve
of 40% chlorothalonil S.C. (Fig.3). Fig.3 shows that
the system is a plastic fluid. Eq.(2) was used to
obtain values of k0, n and τ0 as follows:

k D
n
YBX A
=+









τ (Pa)
0.4398

RHEOLOGICAL
00
00
log()log( )log( )
n
Dk
ττ
ττ
−=
−
⇒
=+⇒

where, B=n; A=log(k0); Y=log(τ−τ0); X=log(D). In
this way, we have changed the curve to a line. The
theory of curve fitting was used to get the resulting
equation corresponding to the equation (Y=BX+A)
as:

1
i
x
xx
 

∑∑

and we can get the values of A and B. Changing X
and Y of the equation to τ and D will yield the
equation of non-Bingham fluid (Fig.4) as

τ=0.52D0.43+0.16































0.75140.84820.9085
2
i
i
iii
y
A
B
x y










 =
 
∑∑∑
∑

Table 1 Sr and suspending ratio of W-01∼ ∼W-10
W-02 W-03W-04
0.146 0.0820.147
88.7 91.284.5
Exp.
Sr
W-01
0.079
92.4
W-05
0.093
90.3
W-06
0.064
95.0
W-07
0.187
82.1
W-08
0.070
93.5
W-09
0.024
97.2
W-10
0.020
98.3 Suspending ratio (%)

Table 2 Shear stress and shear rate of 40% chlorothalonil S.C.
D (1/s)
η (cp)
0.6283
700
1.2566
472
0.5969
1.8849
328
0.6182
2.5132
299
3.1416
270
3.7698
241
5.0264
215.5
1.0832
6.2832
190
1.1938
Fig.3 The flow curve of the system
0.00 2.00 4.00 6.00
Shear rate (1/s)
1.20
1.00
0.80
0.60
0.40
0.20
Shear stress (Pa)
+
+
+
+
+
+
+
+
Fig.4 Curve of non-Bingham fluid
0.00 2.00 4.00 6.00
Shear rate (1/s)
1.20
1.00
0.80
0.60
0.40
0.20
Shear stress (Pa)
B
C
A
E

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Tan et al. / J Zhejiang Univ SCI 2004 5(12):1604-1607
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Estimation of DE and τ τE
According to the above theory of plastic fluid,
the system starts to flow once the stress exceeds a
fixed value. When log(η1−η2) ≤10−9, we can get

DE=3.90107,

Solving the equation of Bingham fluid
We constructed the curve of non-Bingham
fluid using Graf4win and got the relevant equation
by line regress. The equation of Bingham fluid is

τ=0.74+0.09D

Getting the value of the relative value of ap-
proach (Sr)
From Fig.4 and the equations of non-Bingham
fluid, and Bingham fluid, we can obtain the value of
the area of BCE as

3.90107
[(0.740.09 ) S
∆ =+
∫
τE=1.19496
0.43
0
(0.520.16)]d
0.223,
xxx
−+
=


the area of quadrilateral AEBO as

3.90107
S
=
∫

and the value of relative approach (Sr) as

Sr=∆S/SB=0.08.

The thiram-triadimefon S.C. system can form
stable dispersion when the relative value of ap-
proach (Sr) is less than 0.1. The system should be
stable when Sr of 40% chlorothalonil S.C. is 0.08.
This experiment showed that the rheological pa-
rameter can be used to evaluate and foresee the
physical stability of pesticide S.C.


CONCLUSION

The authors’ S.C. rheology research is pre-
sented and a new rheological model, relative value
of approach, is advanced to judge the physical sta-
bility of a pesticide S.C. by a quantitative method.
The authors learned from the experiments that the
system can form stable S.C. when the relative value
of approach is less than 0.1. The experiments also
B
0
(0.740.09 )d2.778,x x
+=

showed that the results determined by the method
of relative value of approach to evaluate and fore-
cast the physical stability of S.C. is the same as that
of the present storage method for two weeks at
(54±2) °C. The former who needs only one day to
judge the physical stability of S.C. can replace the
latter. What is more, this method can evaluate and
forecast the physical stability of pesticide S.C.,
which will benefit development of pesticide S.C.

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