A Computer-Based Technique for the Evaluation of Sedimentation

6 A Computer-Based Technique for the Evaluation of Sedimentation Rates/Shelf Life of Pesticide Flowables J. P. GLATZHOFER

Downloaded by CORNELL UNIV on October 7, 2016 | http://pubs.acs.org Publication Date: June 5, 1984 | doi: 10.1021/bk-1984-0254.ch006

SDS Biotech Corporation, Agricultural Chemicals Research and Development, Painesville, OH 44077

In the development and maintenance of a quality pesticide flowable formulation, the rate at which the suspension forms a non-redispersible sediment under various storage conditions is a highly important product characteristic which must be thoroughly evaluated. This paper will describe equipment and methodology which are useful for the fast and accurate determination of the relative stability performance and shelf life of flowable formulations. The methodology described utilizes a MINC computer/data acquisition system interfaced to a Brookfield Digital Viscometer. Viscosity versus sample depth measurements for evaluation samples are obtained by using a Brookfield Heliopath lowering stand and modified "T-bar" viscometer spindles. Data acquired via this system permits accurate and reproducible evaluations of relative formulation stability performance after 0.5-3 months of accelerated 50°C sample storage or after 3-6 months of ambient temperature storage. The formation of non-redispersible sediment in a pesticide flowable formulation is often the primary cause of product failure during inventory/sheIf storage. In order to develop and maintain a quality pesticide flowable formulation, a formulator needs evaluation techniques which will enable him to quickly determine the relative stability performance of a sample in a non-subjective manner. Accurate sample characterization and early prediction of shelf life is also highly desirable. Many methods for flowable sample evaluation, such as sediment 'probing , are subjective and destructive to the integrity of the sample. Often, samples have to be 'aged at elevated temperatures to obtain measurable differences in stability performance within reasonable time constraints. The purpose of this paper is to describe equipment and methodology which can be utilized to measure the relative 1

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0097-6156/84/0254-0077$06.00/0 © 1984 American Chemical Society

Scher; Advances in Pesticide Formulation Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

ADVANCES IN PESTICIDE FORMULATION TECHNOLOGY

78

v i s c o s i t y versus sample depth f o r aqueous flowable suspensions stored i n various sized containers. The u t i l i t y of the i n t e g r a t i o n of a computer i n t o the data a c q u i s i t i o n system w i l l be discussed and the p o t e n t i a l usefulness of the methodology to q u i c k l y evaluate a flowable sample's q u a l i t y , sedimentation r a t e , and i t s expected s h e l f l i f e w i l l a l s o be demonstrated.


The Viseometry

System

The r e l a t i v e v i s c o s i t y versus sample depth o f a p e s t i c i d e flowable sample can be determined by lowering a r o t a t i n g s p i n d l e (with 1 t h i n bar extension) i n t o the sample at a f i x e d rate of descent u n t i l the bottom i s reached. The apparent v i s c o s i t y at any depth i s r e l a t i v e l y p r o p o r t i o n a l to the measured torque on the spindle s h a f t at a given spindle r o t a t i o n speed. The terms " r e l a t i v e v i s c o s i t y " and "apparent v i s c o s i t y " are being used to describe the r e s i s t a n c e encountered by the r o t a t i n g s p i n d l e i n the concentrated suspension. V i s c o s i t y , by s t r i c t d e f i n i t i o n , i s not the property being measured. E v a l u a t i o n of a sample i n t h i s manner can e a s i l y be performed by u s i n g the f o l l o w i n g commercially available equipment : 1.

A B r o o k f i e l d D i g i t a l Viscometer (Models 0.5 RVT) with analog recorder output.

2.

A B r o o k f i e l d H e l i o p a t h Lowering g r e a t e r ) stand support r o d .

3.

Specially modified T-Bar s p i n d l e s without the v e r t i c a l extension t i p below the t h i n , h o r i z o n t a l extension arms. (Available through B r o o k f i e l d Engineering L a b o r a t o r i e s by custom order.)

4.

A computer data a c q u i s i t i o n system or an X-Y recorder capable of p r o c e s s i n g a 0-10 m i l l i v o l t analog s i g n a l .

f

Stand

LVT, 0.25X RVT, and

with

a

17

inch (or

f

The D i g i t a l Viscometer outputs data as values w i t h i n a range o f 0.0-100.0 u n i t s . The magnitude of the readings f o r a given sample i s dependent on the spindle type, s p i n d l e RPM, sample temperature, and model o f viscometer used. The models d i f f e r from one another because of changes i n the spring constant of the spindle torque measurement system. The LVT, 0.25X RVT, and the 0.5X RVT models are s e n s i t i v e enough f o r u s e f u l measurement of sediment i n t y p i c a l aqueous based flowables. T-bar s p i n d l e s are a v a i l a b l e with v a r i o u s h o r i z o n t a l bar lengths f o r i n c r e a s i n g or decreasing viscosity sensitivity. They are designated T-A thru T-F, with T-A (47.5 mm long) being the most s e n s i t i v e and T-F (11 mm) being the least sensitive. The mouth width of the sample c o n t a i n e r w i l l determine the maximum spindle s i z e which can be used i n many


6.

GLATZHOFER

79

Sedimentation Rates I Shelf Life

cases. Figure 1 shows the v a r i o u s system components mounted f o r a c t u a l use. Note the presence of a ' c o l l a r on the H e l i o p a t h Stand support column which 'stops the descent of the s p i n d l e at a set depth (set to be equal to the bottom of the sample c o n t a i n e r ) . 1

1

The f o l l o w i n g viscometer components and operating c o n d i t i o n s are c u r r e n t l y being used i n the determination of r e l a t i v e v i s c o s i t y versus sample depth i n flowable samples:


- Τ-Bar s p i n d l e s (T-A, T-B, T-C, T-D models were modified to eliminate the vertical extension below the h o r i z o n t a l s p i n d l e arm; v e r t i c a l lengths used are 120 mm and 400 mm). - Spindle R o t a t i o n a l Speed = 5 RPM. This speed was chosen because i t minimizes sample disturbance and e c c e n t r i c spindle motion, while still providing acceptable sensitivity. - Rate of Spindle Descent = 1.00 mm per 2.570 seconds (Not a d j u s t a b l e ) = 1 inch per 65.35 seconds - The spindle i s placed as c l o s e to the top of the l i q u i d (without being wetted) p r i o r to the s t a r t of descent, and the h o r i z o n t a l extension arm i s 'centered' i n the middle of the c o n t a i n e r opening. The graphic p o r t r a y a l of a ' t y p i c a l ' v i s c o s i t y versus sample depth e v a l u a t i o n performed on a 16 oz. sample j a r i s depicted i n Figure 2. Important e v a l u a t i o n parameters for the r e s u l t a n t curve are also depicted. The t o t a l time necessary f o r the e v a l u a t i o n of t h i s sample (approx. 90 mm deep) was four minutes. S i m i l a r data has been generated for flowable suspensions i n sample c o n t a i n e r s as t a l l as 15 inches ( i . e . , 5 g a l l o n p a i l ) . This i s accomplished by using 400 mm long s p i n d l e s i n c o n j u n c t i o n with an extension f o r the Heliopath Stand supporting rod to provide the amount of stand t r a v e l necessary to permit the spindle to 'probe' the e n t i r e sample depth. Computer Data A c q u i s i t i o n and Handling

System

Figure 3 i s the schematic of the data a c q u i s i t i o n system which i s c u r r e n t l y being used to generate the v i s c o s i t y versus sample depth 'profiles'. The 0-10 m i l l i v o l t analog output s i g n a l from the viscometer i s p r e a m p l i f i e d to improve r e s o l u t i o n before i t i s converted to a d i g i t a l value v i a the A/D c o n v e r t o r . The e f f e c t s of transient " n o i s e " are minimized using signal averaging techniques. The A/D convertor i s c o n t r o l l e d by a c l o c k which specifies the c o l l e c t i o n of a d i g i t a l v i s c o s i t y value averaged


80



Figure 1.

B r o o k f i e l d D i g i t a l Viscometer with "Modified" T-Bar Spindle Mounted on a Heliopath Stand.


6.

81

Sedimentation Rates/Shelf Life

GLATZHOFER

TOTAL SAMPLE DEPTH TOTAL DEPTH OF PHASE HITH 'SOLIDS'


RELATIVE VISCOSITY

SLOPE

SEDIMENT DEPTH 'STEADY STATE' VISCOSITY

DEPTH OF SUPERNATANT/ „ VISCOSITY OFI I SUPERNATANT]. Figure 2.

SAMPLE DEPTH

" T y p i c a l " Graph of R e l a t i v e V i s c o s i t y v s . Sample Depth with Important E v a l u a t i o n Parameters.

DIGITAL READOUT VISCOMETER ANALOG

PREAMP

A/D CONV. CLOCK

8 - 18 mv

OUTPUT GRAPHICS TERMINAL (FOREGROUND)

4

COMPUTER SAMPLE CONTAINER

DISK DRIVES

HARDCOPY PLOTTER F i g u r e 3.

Schematic System.

Diagram

of

the

Data

Acquisition


82


every 0.514 seconds, which i s e q u i v a l e n t to 0.20 mm of spindle descent through the sample. The d i g i t a l l y converted data i s handled by the computer i n a 'background processing mode which enables the points to be g r a p h i c a l l y d i s p l a y e d immediately as they are c o l l e c t e d and a l s o simultaneously stored on floppy d i s k s f o r f u r t h e r use. A m u l t i - c o l o r e d p l o t t e r i s used f o r hard copy generation of the ' p r o f i l e ' c u r v e s . A variable speed X-Y strip chart recorder c o u l d be s u b s t i t u t e d f o r the whole data a c q u i s i t i o n end of the system. But, t h i s type of system i s not a d v i s a b l e , except f o r method e v a l u a t i o n purposes, because of the d i f f i c u l t y involved i n handling s t r i p c h a r t data.


1

The computer system used f o r t h i s purpose i s a D i g i t a l Equipment Corp. (DEC) 'MINC l a b o r a t o r y data a c q u i s i t i o n system c o n s i s t i n g of the f o l l o w i n g components: - PDP 11/23 Processors with 'MINC' Basic Language (V3.0). - VT 125 Graphics Terminal. - MINC Preamp (4 channel) and A/D Convertor (16 channel) Modules. - DEC RX02 Dual 8-inch Disk D r i v e s . - Hewlett Packard 9872 P l o t t e r (4 c o l o r ) . Any computer system which i s capable of being c o n f i g u r e d i n a s i m i l a r manner to the schematic diagram c o u l d be used f o r t h i s type of data a c q u i s i t i o n . Data I n t e r p r e t a t i o n and P o r t r a y a l Data i n t e r p r e t a t i o n of v i s c o s i t y versus sample depth 'profile' curves u s u a l l y e n t a i l s u s i n g an X-Y p l o t t e r of the data to determine v a r i o u s important parameters which y i e l d information about the sample. Figure 2 portrays the most important curve c h a r a c t e r i s t i c s and slopes which need to be c a l c u l a t e d . The determination of these values i s g r e a t l y f a c i l i t a t e d by the use of the computer. Current programming enables the use of curve smoothing r o u t i n e s and f i r s t d e r i v a t i v e a n a l y s i s to a i d i n the c a l c u l a t i o n of slopes and to " p i c k " p o i n t s of i n t e r e s t . Values of these important parameters can then be cross-compared with other samples or c o n t r o l s and c o r r e l a t e d with p r e v i o u s l y acquired data f o r rankings of r e l a t i v e performance and estimation of s h e l f l i f e . Comparison of samples i s h i g h l y f a c i l i t a t e d by the a b i l i t y to 'overlay' p r o f i l e curves g r a p h i c a l l y and to 'blow up' s p e c i f i c regions of these overlays ( i . e . , supernatant region) f o r more accurate v i s u a l p o r t r a y a l . Figure 4 portrays the ' p r o f i l e s ' of three d i f f e r e n t flowables stored i n 16 oz. j a r s f o r 10 months at 50 C. The s t a b i l i t y performance of the samples can be 'ranked' by comparing the depth of the supernate and the sample depth at which the curve goes ' o f f - s c a l e ' . Note the c h a r a c t e r i s t i c s o f the most 'stable' formulation compared to the l e a s t ' s t a b l e ' formulation.



Figure 4.

mm

Comparison of Three D i f f e r e n t A f t e r 10 Months at 50°C.

SAMPLE D E P T H ,

Flowable


Samples oo

84



Figure 5 i l l u s t r a t e s the formation of the supernatant layer and sediment for a s i n g l e sample versus storage time under ambient conditions. The computer provides the capability to mathematically 'normalize' the curve data to permit direct comparison of samples evaluated with d i f f e r e n t spindle s i z e s or viscometer models. 'Normalization' routines could also c o n c e i v a b l y be developed for a more accurate comparison of samples with different sample depths, different initial v i s c o s i t i e s , and d i f f e r e n t storage times and temperatures. It is o f t e n p o s s i b l e to make v a l i d sample comparisons and s h e l f l i f e p r e d i c t i o n s a f t e r sample storage durations of 0.5-3 months at 50 C and 3-6 months at room temperature using t h i s technique, depending on the q u a l i t y and type of flowable system. This v i s c o s i t y versus sample depth methodology a l s o provides a means to discover 'abnormal' flowable sample c h a r a c t e r i s t i c s , which would be difficult to detect v i a more c o n v e n t i o n a l e v a l u a t i o n methods. E v a l u a t i o n of 'abnormal' c h a r a c t e r i s t i c s would be 'curds or lumps' i n sample, t h i c k l a y e r s midway i n the sample (see Figure 6), and other signs of 'premature' sample failure. Freeze/thaw performance can a l s o be r e a d i l y determined u s i n g t h i s methodology. Figure 7 shows the r e l a t i v e performance of three flowable samples a f t e r exposure to 5 freeze/thaw c y c l e s . Only small amounts of sediment are present i n these samples, but the o v e r a l l sample t h i c k e n i n g i l l u s t r a t e d by two of these samples i s h i g h l y i n d i c a t i v e of poor freeze/thaw performance. A p p l i c a t i o n s f o r U t i l i z a t i o n of This Methodology T h i s methodology has a p p l i c a t i o n and usefulness areas p e r t i n e n t to p e s t i c i d e flowables. 1.

i n the f o l l o w i n g

Formulation Development - Comparison of d i f f e r e n t flowable i n g r e d i e n t s and ingredient concentrations. - Comparison of d i f f e r e n t manufacturing processes. - Freeze/thaw s t u d i e s .

2.

R e l a t i v e Sample Q u a l i t y Comparison - Shelf l i f e determination at various storage - Inventory c o n t r o l of production m a t e r i a l . - F i e l d complaint a n a l y s i s .

temperatures.



10

Figure 5.

20

1/4X RVT VISCOMETER T-R SPINILE

40

50

SAMPLE D E P T H ,

ι

MOST STABLE

60

7*"

Comparison of " P r o f i l e " Curves f o r Flowable Sample After Various Temperature Storage I n t e r v a l s .

30

τ

77T

LERST 6TBBUE

70

the Same Ambient


80

90

00

86


ω CO

ΚΛ


CO

c α •Η Vi

υ u ο ω ο ^ CO >J OO

C *J

α co CD JU ^ CO

υ C

•Η

μ

e & = ι—I

5

φ

o |* α ο

0)

U

60 •H fa



Figure 7.

Comparison of "Profile" D i f f e r e n t Flowable Samples Thaw C y c l e s .

Curves for Three A f t e r Five Freeze/


oo

88

3.

A D V A N C E S IN PESTICIDE F O R M U L A T I O N T E C H N O L O G Y

Flowable System Research 1

- S h e l f l i f e p r e d i c t i o n from ' a c c e l e r a t e d , high temperature storage data. - Comparison with other commercial products. - Sediment r e d i s p e r s i o n / m i x i n g s t u d i e s . - Flowable rheology s t u d i e s , i . e . , s t r u c t u r e breakdown and reconstitution· - Packaging/inventory/transportâtion s t u d i e s .


Evaluations have c u r r e n t l y been made only on aqueous based flowables; however, the methodology should be a p p l i c a b l e to other suspension systems such as p a i n t s , c o a l / o i l s l u r r i e s , e t c . Cone lu s ion This technique/methodology i s h i g h l y u s e f u l for c h a r a c t e r i z i n g a pesticide flowable formulation's s t a b i l i t y performance. The components of the viseometry system are commercially a v a i l a b l e , and the system i s easy to c o n f i g u r e and use. Use of a computer system to acquire, process, and portray the r e s u l t i n g data i s h i g h l y recommended, although a s t r i p c h a r t recorder c o u l d a l s o be used. Computer systems capable of p r o v i d i n g the proper data a c q u i s i t i o n and graphics f a c i l i t i e s are a l s o r e a d i l y a v a i l a b l e and need not be s o l e l y dedicted to t h i s a p p l i c a t i o n . Storage s t a b i l i t y studies and the samples generated by these s t u d i e s consume extensive amounts of time, space, and e f f o r t during e v a l u a t i o n . This methodology provides an o b j e c t i v e , more complete a n a l y s i s of these v a l u a b l e samples and permits d e c i s i o n making determinations of the r e s u l t s of these s t u d i e s to be r e l i a b l y made much e a r l i e r i n the study than i s p o s s i b l e with more c o n v e n t i o n a l methods. Ac kn ow le dg emen t The work of P a t r i c i a L. Csumitta, SDS Biotech Corp., on the preliminary development of the techniques and methodology described i n t h i s paper i s g r a t e f u l l y acknowledged. R E C E I V E D February 9 , 1984


A Computer-Based Technique for the Evaluation of Sedimentation

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