Controlled Release of Herbicide from an Unmodified Starch Matrix

Chapter 14

Controlled Release of Herbicide from an Unmodified Starch Matrix R. E . Wing , M . E . Carr , W. M . Doane , and M . M . Schreiber Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: March 5, 1993 | doi: 10.1021/bk-1993-0520.ch014

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Plant Polymer Research, National Center for Agricultural Utilization Research, Agricultural Research Service, U.S. Department of Agriculture, Peoria, IL 61604 Insect and Weed Control Research, Department of Botany and Plant Pathology, Agricultural Research Service, U.S. Department of Agriculture, Purdue University, West Lafayette, IN 47907 1

2

Unmodified starch is an effective matrix for encapsulating solid and/or liquid active agents such as herbicides. Gelatinization of starch in the presence of water and herbicide via continuous twin-screw extrusion processing followed by particularization to desirable mesh sizes yields slow release herbicide products. Rate of release can be controlled by varying extruder conditions, particle size and the addition of other chemical additives to the starch matrix. Data w i l l be presented to show efficacy of weed control and reduction of ground water contamination by various herbicide formulations. Studies were presented earlier emphasizing improving worker safety in handling herbicides and providing a release profile for s o i l incorporated herbicides such that the release rate of active ingredient was sufficient to control the target weed but slow enough to avoid phytotoxicity to the crop. Encapsulation of herbicides was achieved by dispersing the herbicides into an aqueous dispersion of gelatinized starch and then crosslinking the starch by (a) xanthide (1); (b) calcium chloride (2); (c) borate (3); or (d) calcium-borate (4) methods. Several researchers have shown that starch encapsulation: a) significantly decreases percutaneous permeability of parathion as compared with clay formulated samples (5); b) extends the duration of weed control by extending the period of release (6-7); c) obviates the need for s o i l incorporation of certain herbicides (8); and d) reduces amounts of herbicide required (9). Starch exhibits many properties required of a polymer to function as an encapsulation matrix. Broader acceptance of starch This chapter not subject to U.S. copyright Published 1993 American Chemical Society

In Polymeric Delivery Systems; El-Nokaly, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

POLYMERIC DELIVERY SYSTEMS

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214

as a n e n c a p s u l a t i o n m a t r i x depends o n i m p r o v i n g t h e economics o f encapsulation and/or e l i m i n a t i n g the use o f c h e m i c a l s needed t o form t h e c r o s s l i n k e d m a t r i x . Recent a r t i c l e s (20-23) and a patent (24) showed herbicides are effectively encapsulated in s t a r c h without the use o f c r o s s l i n k i n g agents. Cornstarch i s f i r s t solubilized o r h i g h l y d i s p e r s e d by steam i n j e c t i o n c o o k i n g a n d t h e n c r o s s l i n k e d through the n a t u r a l process o f rétrogradation a f t e r the herbicide is added. Rétrogradation i s the formation o f aggregates resulting from h y d r o g e n b o n d i n g between h y d r o x y l g r o u p s o f a d j a c e n t starch chains. Factors such as rate of c o o l i n g , p H , amy l o s e content, moisture content, and d i s p e r s i o n temperature affect rétrogradation of solubilized starch. This property i s b e n e f i c i a l for enhancing encapsulation and c o n t r o l l i n g t h e r e l e a s e r a t e o f pesticides from t h e formed starch matrix. Mixtures of various starches (waxy, p e a r l , and h i g h amy l o s e ) c a n be s e l e c t e d t o c o n t r o l the amy l o s e c o n t e n t and t h u s t h e d e g r e e o f r é t r o g r a d a t i o n . Several researchers (25-26) have reported excellent results in c o n t r o l l i n g weeds and r e d u c i n g p o t e n t i a l groundwater c o n t a m i n a t i o n w i t h p r o d u c t s p r e p a r e d by t h i s e n c a p s u l a t i o n t e c h n i q u e (17-22). More r e c e n t l y , we p r o d u c e d starch-encapsulated products i n a continuous, efficient, and e f f e c t i v e p r o c e s s b y u s i n g a t w i n - s c r e w extruder to gelatinize s t a r c h and incorporate the herbicide (23-25). A preblend of s t a r c h and a t r a z i n e was f e d i n t o t h e twin-screw extruder while w a t e r was injected to gelatinize the starch and a l l o w t h e a t r a z i n e to enter the matrix. Other herbicides ( a l a c h l o r o r m e t o l a c h l o r ) were m e t e r e d i n t o o t h e r s t a r c h formulations as liquids. The e x t r u d a t e s were d r i e d and g r o u n d t o provide p r o d u c t s w i t h 94-99% a t r a z i n e , 72-95% a l a c h l o r , a n d 87-92% metolachlor encapsulated. We now r e p o r t a scale-up of the extrusion process t o produce thousand-kilogram q u a n t i t i e s o f these starch encapsulated herbicides. Preliminary data of their evaluation i n greenhouse b i o a s s a y and s o i l columns a r e p r e s e n t e d . Preliminary r e s u l t s of s t a r c h - c l a y blends with metolachlor are a l s o discussed. EXPERIMENTAL METHODS Chemicals. S t a r c h from CPC I n t e r n a t i o n a l , Englewood C l i f f s , N J a n d A g s o r b 30/60 LVM-GA c l a y , Oil Dri, Chicago, I L was u s e d . Technical grade atrazine [2-c±aoro-4-ethyl-amino-6-isopropylamino-s-triazine (97%)], t e c h n i c a l grade, metolachlor [2-chloro2 ' - e t h y l - 6 ' - m e t h y l - N - ( l - m e t h y l - 2 - m e t h o x y e t h y l ) a c e t a n i l i d e (95%)] and D u a l 8E (86% m e t o l a c h l o r ) were s u p p l i e d b y C i b a G e i g y C o r p . , Greensboro, NC. T e c h n i c a l grade a l a c h l o r [2-chloro-2 ,6'-diethylN-(methoxy- m e t h y l ) - a c e t a n i l i d e (94.3%] was s u p p l i e d by Monsanto C o . , S t . L o u i s , MO. A l a c h l o r was m e l t e d a t 50°C b e f o r e u s e . /

E x t r u s i o n and B i c a p s u l a t i o n . The extruder u s e d was a ZSK 57 corotating, fully intermeshing twin-screw extruder (Werner and Pfleiderer). The b a r r e l l e n g t h / s c r e w d i a m e t e r (L/D) r a t i o was 3 0 . S t a r c h o r a s t a r c h - c l a y b l e n d was f e d i n t o b a r r e l s e c t i o n (BS) 1 a t the rate of 150 l b / h r (135 l b / h r d r y b a s i s ) . A t r a z i n e as a s o l i d was also fed i n t o B S - 1 a t t h e 10% a d d i t i o n l e v e l b a s e d on s t a r c h , while i n other formulations D u a l o r m e l t e d a l a c h l o r were m e t e r e d into BS-3. B a r r e l t e m p e r a t u r e s were 25° C a t B S - 1 , 75° C a t


14. WING ET AL.

Release of Herbicide from Unmodified Starch Matrix

215


BS-2 t o B S - 4 , a n d 95° C a t B S - 5 t o B S - 1 0 . T h e s c r e w a t 200 rpm was composed o f a l t e r n a t i n g c o n v e y i n g a n d k n e a d i n g b l o c k e l e m e n t s in t h e s t a r c h g e l a t i n i z a t i o n zones. I h e d i e h e a d a s s e m b l y was e q u i p p e d w i t h a d i e h a v i n g t w e n t y 5-mm d i a m e t e r h o l e s . Production rate was 100 k g / h r p r o d u c t w i t h 30% m o i s t u r e . P r o d u c t s were c u t with a die-face c u t t e r a t 70% s o l i d s a t t h e e x t r u d e r e x i t . Ihe extrudate was d r i e d a n d t h e n g r o u n d t o t h e d e s i r e d p a r t i c l e s i z e s in a Bauer d i s c m i l l . T h e m i l l e d samples (2 t o 25%) m o i s t u r e were s i e v e d t o o b t a i n 1 4 - 2 0 a n d 20-40 mesh p r o d u c t s . A ZSK 30 e x t r u d e r was u s e d f o r t h e s t a r c h - c l a y b l e n d s . Percent herbicide encapsulated. The amount o f herbicide entrapped i n t h e s i e v e d p r o d u c t s was d e t e r m i n e d from samples (100 g) washed t h r e e times with chloroform (300 m l t o t a l ) t o remove absorbed h e r b i c i d e . P r o d u c t s were a n a l y z e d f o r percent active ingredient (a.i.) v i a nitrogen analysis for atrazine and by chlorine analysis f o r m e t o l a c h l o r and a l a c h l o r as previously d e s c r i b e d (23-25). Swellability. Samples (0.20 g , 1 4 - 2 0 a n d 2 0 - 4 0 mesh) were p l a c e d in a 10-ml graduated c y l i n d e r w i t h water ( 4 . 0 ml) a t 30°C a n d gently s t i r r e d several times during t h e f i r s t three hours t o prevent c l u m p i n g . A f t e r 24 h , t h e h e i g h t o f t h e s w o l l e n p r o d u c t i n t h e c y l i n d e r was u s e d t o c a l c u l a t e t h e p e r c e n t i n c r e a s e i n v o l u m e . Soil column l e a c h i n g s t u d i e s . Dry-screened (1/2 cm hardware cloth) Miami s i l t loam t o p s o i l was p a c k e d i n t o 7 . 5 cm d i a . aluminum t u b e s . E a c h t u b e h a d a 2 . 5 - c m w i d e s l o t , 40-cm l o n g down one s i d e . This s l o t was c o v e r e d a n d s e a l e d w i t h f l o r a l c l a y t o produce a water-tight seal. Ihe dry s o i l was added i n t o t h e columns i n portions. A f t e r e a c h p o r t i o n t h e column was d r o p p e d 4 times o n t o a r u b b e r s t o p p e r from a h e i g h t o f 5 cm. T h i s p r o c e d u r e was repeated until the s o i l reached 2 cm above t h e t o p o f t h e slot. T h e columns were t h e n s a t u r a t e d w i t h - 7 5 0 m l w a t e r a n d allowed t o d r a i n . S t a r c h - e n c a p s u l a t e d p r o d u c t s (20-40 mesh) a n d commercial e m u l s i f i a b l e concentrate (EC) samples were a p p l i e d t o t h e t o p o f t h e columns a t t h e r a t e o f 3 . 3 6 k g / h a . Sand (1 cm) was placed over t h e samples a n d 375 m l w a t e r ( e q u i v a l e n t t o 7 . 5 cm rainfall) was p a s s e d t h r o u g h t h e column a t a r a t e o f 4 m l / m i n . A 1-cm h e a d o f w a t e r was a l w a y s m a i n t a i n e d d u r i n g t h e l e a c h i n g p e r i o d t o prevent channeling. A f t e r l e a c h i n g , t h e column was p l a c e d o n its side and t h e s l o t was opened. Foxtail s e e d s (-200) f o r alachlor a n d m e t o l a c h l o r and b e n t g r a s s f o r a t r a z i n e were sown o n the exposed s o i l and a 0 . 5 - c m l a y e r o f s a n d was added t o c o v e r t h e seeds. The s a n d was k e p t s l i g h t l y m o i s t d u r i n g t h e 1 4 - d a y g r o w i n g period. The d i s t a n c e from t h e t o p o f t h e s l o t t o where g r o w t h s t a r t e d determined t h e depth o f l e a c h i n g . Herbicide release studies i n t o water. Samples (14-20 mesh) o f encapsulated a t r a z i n e (25 mg), a l a c h l o r (100 mg), o r D u a l (100 mg) w i t h water (100 ml) were p l a c e d i n 125-ml E r l e n m e y e r f l a s k s . The m i x t u r e s were a g i t a t e d i n an o r b i t a l shaker (Lab-Line, Melrose Park, IL) a t 250 shakes p e r m i n a n d sampled a t 1, 2 , 3 , 24 a n d 48 h. T h i s w a t e r volume i s i n l a r g e excess o f t h a t r e q u i r e d t o dissolve a l l of the herbicides ( a t r a z i n e s o l u b i l i t y i s 33 mg/1



216

water a t 27°C, alachlor solubility is 240 mg/1 a t 23°C, and metolachlor solubility is 530 mg/1 at 20° C Herbicide Handbook. Herbicide concentrations i n t h e w a t e r p h a s e were determined spectrophotom e t r i c a l l y (Beckman DU-50, I r v i n e , CA) a t their u l t r a v i o l e t a b s o r p t i o n maxima (262-264 nm). C o r r e c t i o n s were made a t t h e s e w a v e l e n g t h s u s i n g s t a r c h c o n t r o l s w i t h o u t h e r b i c i d e .


KESUUPS AND DISCUSSION Large quantities (750 kg) of s t a r c h encapsulated atrazine, alachlor, and m e t o l a c h l o r were p r e p a r e d on a ZSK 57 e x t r u d e r . T a b l e I shows t h a t t h e s e h e r b i c i d e s were e n c a p s u l a t e d e f f e c t i v e l y

Table I.

Scale-up Encapsulation of Atrazine, and M e t o l a c h l o r

Alachlor,

3

14-20 Mesh 20-40 Mesh % t o t a l % encap- % t o t a l % encap- % s w e l l active sulatactive sulatability agent iorr agent ion* in water

A c t i v e % Water D r y i n g agent at method grinding

5

0

atrazine alachlor alachlor alachlor alachlor metolachlor metolachlor metolachlor metolachlor

4 16.5 7.8 2.7 2.3 25 18 10 7

air air air oven oven air air oven air

a

11.3 9.6 9.4 10.0 9.7 9.2 9.2 9.0 8.7

94 80 93 90 95 50 41 70 77

11. 1 9. 6 9. 5 9. 7 9. 8 9. 4 9. 2 9. 0 8. 9

95 66 91 90 89 34 29 55 62

340 220 180 180 220 200 300 180 180

f* S t a r c h a t 70% c o n c e n t r a t i o n , ~ 10% h e r b i c i d e . Herbicide remaining a f t e r e x t r a c t i n g surface m a t e r i a l . ° Sample (0.2 g) i n w a t e r (4 m l ) . D r i e d a t 50°C, 24 h r . d

in the scale-up at a production rate of 70 k g / h . The swellabilities listed i n Table I c o r r e l a t e w e l l o n how f a s t t h e herbicides will be r e l e a s e d . P o r t i o n s o f t h e p r o d u c t s were g r o u n d at different moisture contents a f t e r e i t h e r a i r o r oven d r y i n g . Drying the samples prior to grinding seemed t o improve the encapsulation process. Therefore it may be important during continuous, commercial s c a l e p r o c e s s i n g t h a t t h e products a r e d r i e d t o a b o u t 10% m o i s t u r e i f g r i n d i n g i s n e c e s s a r y . Metolachlor was encapsulated i n various starch-clay blends using a ZSK 30 e x t r u d e r . Table I I shows a s l i g h t d e c r e a s e i n encapsulation efficiency as the percent c l a y increases. However, t h e product p r o c e s s i n g improves, s u r f a c e q u a l i t y improves, d e n s i t y i n c r e a s e s , and t h e c o s t d e c r e a s e s a s t h e amount o f c l a y i n c r e a s e s .


14. WING ET AL.


Table H .

Encapsulation o f Metolachlor i n Starch-Clay Blends*

Percent Clay

14-20 Mesh % total % active encapsuagent lation 1 3

% swellability in water

2 0 - 4 0 Mesh % total % active encapsuagent lation

% swellability in water

1 3

c


217

0 20 50 60 80

10.8 10.6 11.1 9.9 9.7

79 71 72 68 67

0

240 240 200 160 100

10.0 10.1 11.2 9.8 11.0

81 61 63 55 51

280 280 200 200 100

S t a r c h - c l a y b l e n d a t 65% c o n c e n t r a t i o n , -10% m e t o l a c h l o r . Metolachlor remaining a f t e r e x t r a c t i n g surface m a t e r i a l . Sample (0.2g) i n w a t e r (4 m l ) .

In Table III the release data in a laboratory assay are reported. The d a t a shows c o n t r o l l e d r e l e a s e i n a v e r y h a r s h t e s t . As t h e water s o l u b i l i t y o f t h e a c t i v e agent i n c r e a s e s , t h e r a t e o f release increases. Initial r a t e o f m e t o l a c h l o r r e l e a s e from t h e s t a r c h - c l a y m a t r i x i n c r e a s e s s l i g h t l y up t o a 50% c l a y l e v e l .

Table m .

Rate o f Release o f B x a p s u l a t e d into Water

Herbicides

3

Product encapsulated

1 hr

atrazine alachlor metolachlor 0% C l a y 20% C l a y 50% C l a y 60% C l a y 80% C l a y

9 31 37 29 32 40 40 27

14-20 mesh unwashed samples s w i r l e d a t 250 rpm.

2 hr

17 46 49 39 42 51 48 40

% Released 3 hr 24 h r

22 56 61 50 55 58 58 51

(25 o r 100 mg)

56 80 91 80 91 93 93 100

48hr

67 91 97 85 98 100 100

i n 100 m l o f w a t e r

The s t a r c h e n c a p s u l a t e d herbicides were compared w i t h the commercial EC f o r m u l a t i o n s in soil column t e s t s using 7 . 5 cm simulated r a i n f a l l . The r e s u l t s i n T a b l e I V show a s i g n i f i c a n t l y r e d u c e d movement o f a c t i v e a g e n t from e n c a p s u l a t e d p r o d u c t s . All t h e e n c a p s u l a t e d samples were unwashed s o t h e s u r f a c e h e r b i c i d e was s t i l l present.


218


T a b l e I V . S o i l Column L e a c h i n g o f H e r b i c i d e s f r o m S t a r c h and Ocflmercial EC F o r m i l a t i o n s 3


Herbicide

formulation

D e p t h , cm

atrazine starch EC alachlor starch EC metolachlor starch EC α

S t a r c h (20-40 mesh) a n d EC f o r m u l a t i o n s 7 . 5 cm r a i n f a l l .

6 33 5 24 5 27 a t 3.36 kg/ha and

OCNCLUSICNS Using a twin-screw extruder f o r encapsulating herbicides i n s t a r c h and s t a r c h - c l a y m a t r i c i e s i s e f f i c i e n t , e f f e c t i v e and continuous. Laboratory evaluation o f t h e products shows them t o b e s l o w release, effective i n c o n t r o l l i n g weeds, and a b l e t o reduce l e a c h i n g under c o n t r o l l e d c o n d i t i o n s . * T h e m e n t i o n o f f i r m names o r t r a d e p r o d u c t s d o e s n o t i m p l y t h a t they a r e endorsed o r recommended b y t h e U . S . Department o f A g r i c u l t u r e over other firms o r s i m i l a r products n o t mentioned. LITERATURE

CITED

1. Wing, R. E . ; Otey, F. H. J. Polym. Sci., Polym. Lett. Ed.; 1983; 21(1):121-140. 2. Shasha, B. S.; Trimnell, D.; Otey, F. H. J. Polym. Sci., Polym. Chem. Ed., 1981; 19(11):1891-1899. 3. Trimnell, D.; Shasha, B. S.; Wing, R. E . ; Otey, F. H. J . Appl. Polym. Sci., 1982; 27(11):3919-3928. 4. Wing, R. E . ; Maiti, S.; Doane, W. M. J. Contr. Rel., 1987; 5(7):79-89. 5. Reily, R. T. J. Ag. Food Chem., 1983; 31 (2):202-206. 6. Schreiber, M. M.; Shasha, B. S.; Ross, Μ. Α.; Orwick, P. L . ; Edgecomb, Jr., D. W. Weed Sci., 1978; 26(6):679-686. 7. Devisetty, Β. N.; McCormick, C. L.; Shasha, B. S. Proc. 7th Inter. Sym. Contr. Rel. Bioact. Mat., 1980; 187-188. 8. White, M. D.; Schreiber, M. M. Weed Sci., 1984; 32(4): 387-392. 9. coffman, C. B.; Genter, W. A. Ind. J. Ag. Sci., 1984; 54(2):117-122. 10. Wing, R. E . ; Maiti, S.; Doane, W. M. Starch, 1987; 39(12):422-425. 11. Wing, R. E . ; Maiti, S.; Doane, W. M. J. Contr. Rel., 1988; 7(1):33-37.


14.

12. 13. 14. 15. 16. Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: March 5, 1993 | doi: 10.1021/bk-1993-0520.ch014

17. 18. 19. 20. 21. 22. 23. 24. 25.

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Wing, R. E. Proc. Corn Util. Conf., 2nd., 1988; 1-17. Wing, R. E. Proc. 16th Inter. Symp. Contr. Rel. Bioact. Mat., 1989; 430-431. Doane, W. M.; Maiti, S.; Wing, R. E. U. S. Patent 4,911,952 1990. Schreiber, M. M.; Wing, R. E . ; Shasha, B. S.; White, M. D. Proc. 15th Inter. Symp. Contr. Rel. Bioact. Mat., 1988; 223-224. Schreiber, M. M.; White, M. D.; Wing, R. E . ; Trimnell, D.; Shasha, B. S. J. Contr. Rel., 1988; 7(3):237-242. Schoppet, M. J.; Gish, T. J.; Helling, C. S. Proc. Am. Soc. Ag. Eng., 1989; 1-13. Mills, M. S.; Thurman, Ε. M.; Wing, R. E . ; Barnes, P. L . Trans. Am. Geophy. Un., EOS, 1990; 71(43):1331. Mills M. S.; Thurman, Ε. M. Environ. Sci. Technol. (In Press). Gish, T. J.; Shoppet, M. J.; Helling, C. S.; Shirmohammadi, Α . ; Schreiber, M. M.; Wing, R. E. Trans. Am. Soc. Agric. Engin., 1991; 34(4):1738-1744. Gish, T. J.; Schoppet, M. J.; Weinhold, B. J.; Helling, C. S.; Wing, R. E.; Schreiber, M. M. Weed Sci. (In Press). Fleming, G. F . ; Simmons, F. W.; Wax, L . M.; Wing, R. E . ; Carr, M.E. Weed. Sci. (In Press). Carr, M. E . ; Wing, R. E . ; Doane, W. M. Cereal Chem., 1991; 68(3):262-266. Wing, R. E . ; Carr, M. E . ; Trimnell, D.; Doane, W. M. J . Contr. Rel., 1991; 16(30):267-278. Trimnell, D.; Wing, R. E . ; Carr, M. E . ; Doane, W. M. Starch, 1991; 43(4):146-151.

R E C E I V E D October 1, 1992


Controlled Release of Herbicide from an Unmodified Starch Matrix

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