Controlled Release Pesticides

14 The Effect of Some Variables on the Controlled Release of Chemicals from Polymeric Membranes AGIS F. KYDONIEUS

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Hercon Products Group, Herculite Protective Fabrics Corp., 1107 Broadway, New York, N.Y. 10010

Controlled release technology is the adaptation of permeation processes to the dispensing of toxic, volatile, unstable and other chemicals. Nature, Naivete and ingenuity have provided a number of well known examples of permeation controlled processes. Some well known processes which depend on permeation are: respiration, osmosis, and the "bloom" or "patina" on grapes and other fruit. Inspection of these examples reveal that gases, liquids, and solids may be observed to pass through various membrane materials. The manner in which permeation takes place through the HERCON laminated polymeric membrane system and the factors affecting such permeation will be discussed. ®

1. The HERCON Dispensing System A schematic cross-section of a typical HERCON laminated membrane structure is shown in Figure 1. The specially formulated inner layer, which behaves as the reservoir, contains dissolved insecticide or other active agent which then migrates continually, due to imbalance of chemical potential, through one or more initially inert outer layers to the surface, rendering it biologically or physico-chemically active. At the surface, the insecticide is removed by volatilization, thermal or ultraviolet degradation, alkaline or acid hydrolysis, or mechanically by humans, insects, rainfall, wind or other agents. The construction and composition o£ the laminated insecticidal membranes vary , o£ course, with the active agent used, release rate and effective life span desired. However, materials containing from 0.5 to 40%, by weight, active agent have been successfuEy prepared and have been shown by laboratory and field tests to be efficacious in a number o£ applications (1, 2).

152 In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

14.

KYDONiEus

Effects of Some Variables

153

on Release

2. The Mathematics of Transport The class of membranes used with the H E R C O N technology is the nonporous, homogeneous polymeric films. These membranes are usually referred to as solution-diffusion membranes. Silicone rubber, polyethylene, polyvinylchloride and nylon films are typical examples. The penetrant is able t o pass through the membrane material in the absence of pores or holes by a process of absorption, solution, diffusion down a gradient of t her mo-dynamic activity and desorption. The process of permeation thus is divisible into a number of indepen dent processes governed primarily by Henry s law and Fick s law (3, 4). Transport of active chemical from the reservoir through the barrier membrane is governed by Fick s f i r s t law:


1

f

1

j = ,p

d C

m dx

[1]

where J is the flux in g/cm^-sec, C is the concentration of permeant in the membrane in g/cm^, d C / d x is the gradient in concentration, and D is the diffusion coefficient of the permeant in the membrane in cmVsec. M

m

(a) The Steady State. As shown in Figure 2, the concentration just inside the membrane surface can be related t o the concentration in the reservoir C by the expressions: Cm (0)

=

K C /Q) at the upstream surface (x = Q) [2]

C (l)

= KC (j) at the downstream surface (x =%)

m

here, Κ is a distribution coefficient and is analogous to the familiar liquid-liquid partition coefficient. In Figure 2, for purposes of illus tration, i t has been assumed that the distribution is less than unity for barrier membrane I, and more than unity for membrane II. Throughout the following, we will assume diffusion coefficients and distribution coefficients to be constant. This is a safe assumption for most polymer-permeant systems. Thus, in the steady state, Equation [1] can be integrated to give: C

C

J = D m(0) - m(l) = D 1

Δ

C

M

[3]

1

where 1 is the thickness of the membrane. Since the concentration within the membrane is usually not known, Equation [3] is frequently

In Controlled Release Pesticides; Scher, H.; ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

154

CONTROLLED RELEASE

PESTICIDES

CONTROLLED AMOUNTS OF PESTICIDE . MOVE FROM RESERVOIR LAYER TO SUSTAIN ACTIVE SURFACE

φ ACTIVE SURFACE '


φ PROTECTIVE PLASTIC LAYER φ PESTICIDE RESERVOIR LAYER ' φ PROTECTIVE PLASTIC BARRIER Figure 1.

Schematic of Hereon laminated controlled release structure

T H R E E - LAYER

LAMINATE

FLUID

FLUID

DILUTE SOLUTION SIDE

THICKNESS

Figure 2.

Schematic of the concentration gradient across a three-layer laminate


14.

KYDONiEus


155

on Release

written:


J = f^t dt

= DKAC

[4]

1

dMt where M t is the mass of agent released,—gftr is the steady state release rate at time t and A C is the difference in concentration (C(0) - C(i)) between the reservoir concentration and the fluid concentration adjacent t o the barrier membrane. It is significant t o note that the rate of release is proportional to diffusivity (a kinetic constant ) and to distribution coefficient (a thermodynamic constant). Equation [4] can be integrated between the limits: M = 0 t = Mt = M t t = t [5] 0

t

to give:

= ADK ÀCf. 1

When the distribution coefficient between the reservoir layer and the barrier membrane is much smaller than unity, as is the case of membrane I in Figure 2, the system has excellent release kinetics and the release rate can be maintained constant f o r extended periods of time (pseudo-zero order delivery). Equation [5] is then governing the process and a straight line is obtained when the mass of agent released (Mt) is plotted against time (t). This is shown in Figure 3, with a polyvinyl chloride-polyester system. (b) The Unsteady State. When the distribution coefficient between the reservoir layer and the barrier membrane i s approximately unity, or larger than unity, as i s the case with membrane II of Figure 2, the HERCON system will approximate the "dissolved system", i.e., the reservoir - barrier membrane system forms a single homogeneous polymeric f i l m . The concentration in the reservoir will not remain constant but will f a l l continuously with time. The system remains continuously under unsteady state conditions and the mass of agent released varies as a function of time (first order delivery). The transport equations have been described by several investigators (5,6, 7). The two useful equations are the early time approximation, which holds over the initial portion of the curve: Mt

= 4/Dt \

1 / 2

n

A l t

/

0.6


[6]

CONTROLLED RELEASE

PESTICIDES

2 -, 10 m i l P V C 8 mil MYLAR

RESERVOIR BARRIER MEMBRANE Moo Κ

«

4.24 m g / i n

2

1

-4

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=

Τ 20

T 40

T "

T

60

80

~ l

100

120

TIME IN DAYS Figure 3.

Release of fly repellent MGK R-874—reservoir-type system

4-1

3 H

RESERVOIR BARRIER MEMBRANE Moo Κ

0 Figure 4.

20 20

"T

40 6o" 60 40 TIME IN DAYS

=4.23 =

10 m i l P V C 10 m i l P V C

mg/irr

1

80 80

Τ"

lOO 100

Release of fly repellent MGK R-874—dissolved-type system


1

120

14. KYDONiEus


on Release

157

and t h e l a t e t i m e approximation, which holds over the f i n a l p o r t i o n o f the release curve, exp (-TT*Pt)

M£=l-J

0 . 4 / L . O

[7]

As i t can be seen f r o m equation [ 6 ] , a p l o t o f mass o f agent released versus t i m e w i l l give a parabolic curve. T h i s i s the case f o r the P V C - P V C s y s t e m shown i n Figure 4. When t h e same data are p l o t t e d versus (time)-*/ , a l i n e a r curve i s obtained, (Figure 5), i n accordance w i t h Equation [6].


2

3. F a c t o r s A f f e c t i n g the Release Looking a t equations [5], [6], and [7], i t becomes apparent t h a t t h e release o f a c t i v e ingredients f r o m H E R C O N laminated membrane s t r u c t u r e s i s controlled by molecular and s t r u c t u r a l f a c t o r s . F o r a given combination o f polymer s t r u c t u r e and a c t i v e agent where energy t o f r e e r o t a t i o n s , f r e e volume, and i n t e r m o l e c u l a r a t t r a c t i o n s a r e constant, a t l e a s t t w o p a r a m e t e r s are available t o regulate t h e r a t e o f t r a n s f e r : r e s e r v o i r concentration and membrane t h i c k n e s s . D i f f u s i v i t y D and reservoir/membrane d i s t r i b u t i o n c o e f f i c i e n t Κ are also d i r e c t l y p r o p o r t i o n a l t o the permeation r a t e . In p o l y m e r s , d i f f u s i v i t y i s s t r o n g l y s e n s i t i v e t o the molecular weight o f t h e d i f f u s a n t and t o t h e s t i f f n e s s o f the backbone o f the polymeric mem brane. Simply speaking, the d i f f u s a n t molecule w i l l have t o r e o r i e n t s e v e r a l segments o f polymer chain t o allow i t s passage f r o m s i t e t o s i t e . T h e higher t h e molecular weight, t h e more the segments t h a t need t o be reoriented f o r passage t o be possible; and t h e s t i f f e r the polymer (glassy and high c r y s t a l l i n i t y ) , the more d i f f i c u l t f o r i t s segments t o undergo large r e o r i e n t a t i o n s . T h e r e f o r e , variables t h a t could a f f e c t the s t i f f n e s s o f polymer membranes such as c o - d i f f u s ants t h a t would s o f t e n , p l a s t i c i z e o r p a r t i a l l y dissolve the membrane would have an e f f e c t on d i f f u s i v i t y and p e r m e a t i o n r a t e . The reservoir/membrane d i s t r i b u t i o n c o e f f i c i e n t Κ can be e s t i m a t e d f r o m the s o l u b i l i t y p a r a m e t e r o f the d i f f u s a n t . S o l u b i l i t y p a r a m e t e r s can be calculated using Hilderbrand s s o l u b i l i t y theory. When t h e s o l u b i l i t y p a r a m e t e r f o r t h e d i f f u s a n t and polymer mem brane i s t h e same, t h e polymer w i l l be soluble i n t h e d i f f u s a n t . T h e s o l u b i l i t y p a r a m e t e r s and d i s s o l u t i o n a r e s t r o n g l y a f f e c t e d by molecular weight and the chemical f u n c t i o n a l i t y o f the molecule, i . e . , hydrogen bonding and p o l a r i t y . L i k e dissolves l i k e i s s t i l l a good r u l e o f thumb. I would l i k e t o t a k e a f e w minutes now t o discuss the e f f e c t f


158

CONTROLLED

R E L E A S E PESTICIDES


C

0

1

2 1 t

Figure 5,

3

4

5

/ 2 ( DAYS1/2 )

Release of fly repellent MGK R-874—dissolved-type

system


14.

KYDONiEus


on

Release

159


o f : 1) r e s e r v o i r c o n c e n t r a t i o n , 2) m e m b r a n e t h i c k n e s s , 3) p o l y m e r s t i f f n e s s , 4) c o - d i f f u s a n t s , 5) m o l e c u l a r w e i g h t o f d i f f u s a n t a n d 6) c h e m i c a l f u n c t i o n a l i t y o n t h e t r a n s p o r t o f a c t i v e i n g r e d i e n t s through H E R C ON laminated membranes. F o r purposes o f i l l u s t r a t i o n and s i m p l i c i t y i n a l l examples t h a t f o l l o w , t h e r e s e r v o i r i s made o f flexible polyvinylchloride. R e s e r v o i r C o n c e n t r a t i o n . In F i g u r e s 6 and 7, t h e r e l e a s e o f t h e p e s t i c i d e chlordane and t h e i n s e c t r e p e l l e n t D E E T a r e i l l u s t r a t e d . In b o t h c a s e s , z e r o o r d e r release r a t e s were obtained. A c l o s e r look i n d i c a t e s t h a t doubling t h e c o n c e n t r a t i o n i n t h e r e s e r v o i r does not double t h e m a s s o f agent r e l e a s e d . T h i s d e v i a t i o n i s m o r e pronounced a t higher concentrations, presumably because t h e i n t e r - m o l e c u l a r a t t r a c t i o n of the diffusant molecules increase exponentially w i t h concentration. W i t h the exception o f these minor differences, the data in g e n e r a l t e r m s f o E o w e q u a t i o n s [5] a n d [6]. M e m b r a n e T h i c k n e s s . B o t h e q u a t i o n s [5] a n d [6] i n d i c a t e t h a t t h e mass o f agent released should be i n v e r s e l y p r o p o r t i o n a l t o t h e t h i c k n e s s o f t h e membrane. T h i s i s shown t o be t h e case w i t h t h e r e p e E e n t D E E T and t h e p h e r o m o n e s dodecenyl a c e t a t e and H e x a d e c y l A c e t a t e ( F i g u r e s 8 and 9 ) . Polymer Stiffness. The distribution coefficient Κ for several p o l y m e r m e m b r a n e s o f d i f f e r e n t b a c k b o n e s t i f f n e s s and P V C w a s studied, by adhering large r e s e r v o i r layers o f P V C t o said membranes. T h e t r a n s p o r t o f a c t i v e agent f r o m t h e r e s e r v o i r t o t h e membrane was s t u d i e d b y s e p a r a t i n g t h e l a y e r s and d e t e r m i n i n g t h e a m o u n t o f a c t i v e agent b y c h e m i c a l a n a l y s i s . T h i s i s shown i n T a b l e A f o r t h e active agents: Captan-an a n t i b a c t e r i a l agent, Malathion-an i n s e c t i c i d e , and Z i n e b - a n a g r i c u l t u r a l f u n g i c i d e . F o r a l l t h r e e a g e n t s , t h e amount transported into the membranes becomes progressively s m a E e r , g o i n g f r o m P V C t o r i g i d v i n y l , p o l y p r o p y l e n e , n y l o n and m y l a r , which also corresponds t o i n c r e a s e i n backbone s t i f f n e s s . D i s t r i b u t i o n c o e f f i c i e n t s w e r e n o t c a l c u l a t e d b e c a u s e 20 w e e k s a f t e r t h e e x p e r i m e n t s w e r e i n i t i a t e d , i t was not c e r t a i n t h a t e q u i l i b r i u m had been reached i n m o s t c a s e s . T h e amount o f a c t i v e c h e m i c a l t r a n s p o r t e d w i t h t i m e f o r a few cases is shown i n Table B , In a second e x p e r i m e n t , t h e d i f f u s i o n i n t o f i l m s o f d i f f e r e n t backbone s t i f f n e s s was m o n i t o r e d b y t h e p r o p e r t i e s i m p a r t e d t o said f i l m s . T h e a n t i b a c t e r i a l a g e n t , C a p t a n , t h e g e r m i c i d e , v i n a z e n e , and t h e a n t i s t a t i c agent, Ethoquad, were used. The r e s u l t s , shown i n Table C , indicate t h a t the effectiveness of the f i l m s was, i n general,


160


CONTROLLED RELEASE

1/2 t

1/2 DAYS

Figure 8. Effect of film thickness on release of repellent

DEET


PESTICIDES

Effects

of Some Variables

on

Release


KYDONiEus


161

162

C O N T R O L L E D R E L E A S E PESTICIDES

best for flexible PVC, followed by rigid PVC, acrylic, polyproplene, nylon and polyester. This order is the same as that of the dats shown in Table A , with effectiveness reduced as the backbone stiffness was increased. Table A


Distribution of Active Agents between Flexible PVC and Polymer Increasing Backbone Stiffness ACTIVE AGENT TRANSPORTED (ppm) Flex PVC Rigid PVC Polyprop Nylon Polyester 250 109 36 3 0 12000 9700 498 23 8 1600 568 62 63 9 1

Captan Malathion Zineb

* A l l Polymer films were 5 mils thick. The t o t a l amount of Captan, Malathion and Zineb in the system were 500, 24000 and 4000 ppm, respectively. Readings were taken 20 weeks after the initiation of Experiment.

Table Β Active Agent transported as a Function of Time (ppm)

Malathion Rigid PVC Nylon Polyprop Captan Rigid PVC Polyprop Zineb Rigid PVC Nylon

2 weeks

7 weeks

20 weeks

6300 6 387

9000 12 334

9700 23 498

29 1

55 26

109 36

303 67

619 60

568 62

Co-diffusants. Chemical agents that are capable in altering the structure (e.g., stiffness) of a polymer would have a pronounced effect on the diffusion of active chemicals that are aEowed t o


14.

KYDONiEus


on

Release

163

c o - d i f f u s e . T h e s e " c a r r i e r " m a t e r i a l s m u s t have t h e a b i l i t y t o s w e l l , s o f t e n a n d / o r d i s s o l v e t h e p o l y m e r m a t r i x . A good e x a m p l e i s t h e i m p a r t m e n t o f a n t i s t a t i c p r o p e r t i e s t o n y l o n and p o l y e s t e r c a r p e t s b y u s i n g t h e c o - d i f f u s a n t s phenol and ethylene g l y c o l p h e n y l e t h e r , r e s p e c t i v e l y (8). T a b l e D s h o w s t h a t 3 f o l d and 100 f o l d i m p r o v e m e n t s i n a n t i s t a t i c r e s i s t i v i t y c a n b e o b t a i n e d f o r n y l o n and p o l y e s t e r respectively b y using the co-diffusants mentioned above.


Table

C.

P r o p e r t y Improvement by Diffusion o f A c t i v e Chemicals Flex PVC Vinazene [germicide] Captan [antibacterial] Ethoquad [antistatic]

7.0

Rigid P V C A c r y l i c Polyprop Nylon Polyester 12

1

99.9+

2

99.9+

10000

3

4200

88.2

2100

15

0

0

97.8

42.3

48.2

1100

420

73

^ Zone o f Inhibition i n m m R e d u c t i o n o f b a c t e r i a o v e r u n t r e a t e d c o n t r o l (NYS-63) 3 Reduction i n Surface R e s i s t i v i t y (Ohmsj over untreated c o n t r o l 2

Molecular Weight. T h e m o l e c u l a r weight o f t h e d i f f u s a n t is very i m p o r t a n t because i t i s d i r e c t l y related t o t h e d i f f u s i v i t y . T o i n v e s t i g a t e t h e e f f e c t o f molecular weight on t h e t r a n s p o r t t h r o u g h p o l y m e r m e m b r a n e s , f i v e (5) i n s e c t p h e r o m o n e s w e r e c h o s e n . A l t h o u g h not e x a c t l y o f t h e same homologous s e r i e s , t h e y w e r e a l l a c e t a t e s r a n g i n g f r o m 12 c a r b o n a t o m s t o 20 c a r b o n a t o m s . T h e release through a 2 m i l flexible polyvinylchloride membrane i s shown i n F i g u r e 10. T h e s t r o n g i n f l u e n c e o f m o l e c u l a r w e i g h t i s a p p a r e n t f r o m t h i s graph, w i t h sharp decreases i n release r a t e as t h e m o l e c u l a r w e i g h t i n c r e a s e d f r o m 198 t o 310 i n s t e p s o f 28 u n i t s . C h e m i c a l F u n c t i o n a l i t y . L i k e dissolves l i k e i s equally a p p l i c able i n t h e polymer a r e a . D i s s o l u t i o n o f the p o l y m e r m a t r i x by t h e d i f f u s i n g molecules i s i m p o r t a n t i n t h e t r a n s p o r t process because i t increases t h e value o f the d i s t r i b u t i o n c o e f f i c i e n t . T o s t u d y t h i s v a r i a b l e , t w o s e t s o f p h e r o m o n e s w i t h 16 and 20 c a r b o n a t o m s , respectively, were investigated. The results are depicted i n Figures U and 12 a n d s h o w t h a t f u n c t i o n a l i t y h a s a s u b s t a n t i a l e f f e c t o n t h e


CONTROLLED RELEASE

PESTICIDES

DODECENYL ACETATE 1 4 C ' s


TETRADECENYL ^ACETATE 16C's

ΔΓΡΤΓΤ 18Πο ACETATE 1 8 C s Ρ

2

4

t1/z Figure 10.

OCTADECENYL ACETATE 20C»s

€Υ

(DAYS 1 / 2 )

Effect of molecular weight on mass of agent released

0

2

4

t Figure 11.

V z

6

DAYS

8

72

Effect of functionality on release rate


14.

KYDONiEus


on Release

165

Table D.


The E f f e c t o f C o - D i f f u s a n t s on t h e A n t i s t a t i c P r o p e r t i e s o f C a r p e t s CARPET FIBER

Antistatic Agent

CoA c t i v e Agent Diffusant Concentration

NYLON (Bigelow)

A d v a s t a t 50

Phenol

14.7

Volume Resistivity (ohms) 7.0x108

A d v a s t a t 50

None

14.8

2.2 χ 10

None

None

None

l.lxl0

17.6

7.0 x l O

8

1 0

POLYESTER (DuPont)

Dowanol E P h

A d v a s t a t 50

9

U

A d v a s t a t 50

None

16.9

8.0 x l O

None

None

None

l.lxl0

U

Reservoir Polymer Matrix: VULCANOL 5023

Ο

1

2 t

Figure 12.

%

3

4

5

(DAYS1/2)

Effect of functionality on release rate


6

166

C O N T R O L L E D R E L E A S E PESTICIDES

Table Ε


Some Commercial Products Using the HERC ON technology Product STAPH-CHEK®

Membrane I 4 mil PVC

INSECT APE®

2 m i l mylar

Membrane II Function Antibacterial 4 mil PVC hospital fabric Roach control 5 m i l PVC

LURE-N-KILL™ FLYTAPE Attractant Pesticide

2 m i l mylar 2 m i l PVC

8 m i l PVC 2 m i l PVC

SCENTSTRIP™I SCENT S T R I P ™ Π

2 m i l mylar cloth

5 m i l PVC cloth

A i r freshener (consumer)

SCENTCOIL™

vinyl

vinyl

A i r freshener (industrial)

2 m i l vinyl

2 m i l vinyl

Houseflycontrol

TM LURETAPE Disparlure Gossyplure Orfralure Multilure Grandlure

m

monitoring gypsy moth monitoring pink 5 m i l vinyl 5 m i l vinyl bollworm 6 mil acrylic 6 m i l acrylic Control of oriental f r u i t moth Control of elm mylar mylar bark beetle 16 m i l rigid 16 m i l rigid Monitoring of boE weevil PVC PVC


14. KYDONiEus


on Release

167

release rate. As a matter o£ fact, octadecane had a much faster release rate than hexadecyl acetate as well as a l l of the 16 carbon atom pheromones shown in Figure E.


4. Flexibility of the HERC ON Process. It has been shown that a large variety of factors affect the release through polymer films. The HERCON process with i t s flex ibility in controlling release by varying membrane polymer thickness, membrane polymer matrix, reservoir polymer matrix and diffusant concentration as well as co-diffusants is capable in controlling the release of chemical agents t o produce consumer and industrial products with improved properties, inexpensively. Table Ε describes some of the commercial products that use the HERCON technology. Thickness and type of films are given for each product. The reservoir layer which is perhaps the most important aspect of the technology varies from product to product and i t is of proprietary nature. Our company is looking forward to the continuous expansion of the market f o r controlled release products and we are aggressively exploring new product applications using the technology mentioned herein.

LITERATURE CITED (1) Kydonieus et al., "Marketing and Economic Considerations for HERCON Consumer and Industrial Controlled Release Products", ACS Proceedings-Chemical Marketing and Economics Division-1976, pp. 140-165. (2) Kydonieus et al., "Controlled Release of Pheromones through Multi-layered Polymeric Dispensers", ACS Symposium Series 33, pps. 283-294, 1976. (3) Crank, J. and G.S. Park, ed. "Diffusion in Polymers" Academic Press, N.Y. (4) Richards, R.W., "The Perme ability of Polymers to Gases, Vapours and Liquids; ERDE (Ministry of Defense) Tech. Report No. 135, March 1973, NTIS AD-767 627. (5) Baker, R.W. and H.K. Lonsdale, "Controlled Release of Biologically Active Agents, A.C. Tanquary and R.E. Lacey (Eds.), Plenum Press, N.Y., 1974. (6) Crank, J., "The Mathematics of Diffusion", Oxford University Press, London, 1956. (7) Barrer, R.M., Diffusion in Polymers", Academic Press, London, 1968. (8) Kydonieus, et al., U.S. Patent 3,961,117 (1976).


Controlled Release Pesticides

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