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Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 585-589
below 850 OC. An increase in porosity above that level necessitates more open initial structures created either by adding spacing agents or by changing the method of preparation to loose sintering. In all cases as would be expected there is a fall in strength with an increase in porosity, and the porosity/ strength characteristics achieved by the three methods are summarized in Figure 7, which shows that reasonable levels of strength can be achieved at porosities above 85%. At higher porosities Obtained with the more 'pen structures there has been a shift in mean pore size from 10 toward 15 pm, making the secondary pore size region less pronounced (Figures 4-6).
Acknowledgment I wish to thank Inco for permission to publish this paper. Registry No. Oxamide, 471-46-5; nickel, 7440-02-0. Literature Cited Falk, S. U.; Salklnd, A. J. Alkallne Storage Batteries; Wiley International: New York, 1969; Chapter 2, p 111. International Nickel Co. Inc., Product Data Sheet A1155A INCO Nickel Powder Type 255, 1981a, New York. International Nickel Co. Inc., Product Data Sheet A1234A INCO Nickel Powder Type 257, 1981b, New York. Tracey, V, A, Ind, Eng, Chem. prod, Res, De", 1982, 21,626,
Received for review January 14, 1985 Accepted April 25,1986
Controlled-Release Polymeric Herbicide Formulations with Pendent 2,4-Dichlorophenoxyacetic Acid Shukla Bhattacharya, Shyamal K. Sanyal, and Ram N. Mukherjea" Process Engineering and Technology Laboratory, Chemical Engineering Department, Jadavpur University, Calcutta 700 032, India
Controlled-release pesticide formulations in which the pesticide is covalently bound to a polymer, either as a pendent group or as a part of the polymer backbone through a hydrolyticallylabile bond, are gaining increasing importance. As part of an ongoing project on slow-release formulations of 2,4dichlorophenoxyaceticacid (2,4-D), the herbicide was initially converted to a polymerizable derivative, a diolamide of 2,4-D. This diolamide was copolymerized with a number of dicarboxylic acid/anhydrides to give polymeric herbicide formulations in which the backbones are completely biodegradable by hydrolysis. Cross-linking reaction of these polymers with vinyl and acrylic monomers gave partially degradable polymeric herbicides. Release characteristics of these formulations have been studied in neutral, alkaline, and acidic media.
Polymeric controlled-release (CR) pesticide formulations have been introduced in response to growing concern for ecological problems associated with increased use of plant protection chemicals required for intensive agricultural practices. The toxicant is delivered from such formulations at a controlled rate, increasing the period of effectiveness of the biologically active component and obviating the need for overdose. There are basically two approaches for preparing such controlled-release pesticides. The active agent may be covalently bound on a preformed synthetic or natural polymer macromolecule (Neogi, 1970; Allan et al., 1977; Bhattacharya et al., 1985). Another approach is to polymerize a herbicide containing monomer. The second method has the advantage that it gives a much higher weight percentage of herbicide, and the activity of the polymeric herbicide can be varied by introducing comonomers into polymer or by changing the molecular weight. The work on control-activity polymers with pendently bound herbicides has been extensively reviewed by McCormick et al. (1982/1983). Herbicide monomers so far prepared are acryl and vinyl monomers containing hydrolyzable herbicide moieties (Harris et al., 1976). These monomers provide polymeric pesticide systems in which the polymer backbone is stable to hydrolytic degradation. A number of these controlled-release systems, however, failed to show biological activity in bioassay studies, implying no hydrolytic release of the pesticides. The release pattern becomes satisfactory
* Author to whom correspondence should be addressed. 0196-4321/86/1225-0585$01.50/0
when these are copolymerized with hydrophilic comonomers (Harris and Arah, 1980). Only limited studies have been done on the synthesis of biodegradable polymeric pesticides. Desmareta and Bogaerts (Schacht et al., 1978) acetalized diethyl tartarate with 2,4-dichlorobenzaldehyde; the acetal obtained was then polymerized with diamine and diols to give a polymeric herbicide having a biodegradable backbone. These controlled-release systems showed slow release of the aldehyde only in acidic homogeneous hydrolysis medium (dioxane/water, volume ratio = 2.9/1). The present study has been undertaken to synthesize a diolamide of the herbicide 2,4-dichlorophenoxyaceticacid (2,4-D), which on copolymerization with a number of dicarboxylic acids would give 2,4-D-releasing polymer systems. The structures of these polymers have been varied over a wide range by using a number of dicarboxylic acids and cross-linking agents. The labile ester functions in the backbone would provide polymeric herbicides that are completely biodegradable by hydrolysis. Cross-linkingwith vinyl or acrylic monomers would provide partially degradable polymeric herbicides. Release characteristics of these polymeric herbicides have been studied in neutral, alkaline, and acidic media. Materials and Methods 2,4-Dichlorophenoxyaceticacid was purified by crystallization from benzene. Diethanolamine (BDH) was purified by distillation under reduced pressure. Succinic acid (Sarabhai M. Chemicals), maleic anhydride (Loba0 1986 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
Chemie), and phthalic anhydride (BDH) were crystallized and dried before use; styrene and acrylic acid were purified prior to use. Preparation of Diolamide of 2,l-Dichlorophenoxyacetic Acid. A mixture of 2,4-D and diethanolamine was taken in a three-necked flask fitted with a stirrer and a nitrogen inlet and outlet. A little excess over equimolecular proportion of diethanolamine was used. The mixture was heated in an oil bath a t 135-140 "C under nitrogen atmosphere with constant stirring in the presence of a small amount of sodium salt of 2,4-D. After 6 h the mixture was cooled and poured into water. The solution was alkaline. On keeping, a white solid separated and was filtered, and the diolamide obtained was crystallizedfrom benzene. The filtrate was extracted with chloroform to isolate the dissolved diolamide. The chloroform layer was washed with water and dried over sodium sulfate. On evaporation of chloroform, a reddish-brown syrupy mass was obtained which slowly became crystalline and was recrystallized from benzene. The total yield varied from 42 to 50%; mp 95-98 O C . Polymerization. Synthesis of Succinic Acid Ester. The mixture of diacid and diolamide (mole ratio = 1.01) was taken in toluene and refluxed for 12 h in the presence of a trace amount of concentrated sulfuric acid. The water liberated was removed by azeotropic distillation. The tacky mass obtained was solidified on cooling. The solid mass was extracted several times with hot acetone to remove unreacted materials. The product was insoluble in common organic solvents and was characterized by IR. Synthesis of Phthalic Anhydride Ester. A mixture of phthalic anhydride and diolamide (mole ratio = 1.01) was heated slowly to 180-190 "C under stirring and kept at that temperature for 2 h when a greenish-yellow mass resulted, which set to a glassy mass on cooling. It was soluble in chloroform and purified by repeated precipitation from chloroform by methanol. The pure, dry, pale yellow solid was characterized by IR. Synthesis of Maleic Anhydride Ester. A threenecked flask was charged with diolamide (1mol), which was heated to 92-95 O C under a slow stream of nitrogen. Then maleic anhydride (1.01mol) was added with stirring. The temperature was raised to 150 "C over 1 h and then to 190 "C over 4 h and was maintained at this temperature for 1h. The temperature was then lowered to 170 "C and maintained a t this temperature for 1 h. An orange glassy mass was obtained, which was purified by repeated precipitation from chloroform by methanol. On drying, a yellow solid was obtained and characterized by IR. Cross-Linking with Styrene. To 20 g of molten maleic ester of diolamide was added 7.5 g of styrene. Polymerization of the mixture was effected by adding benzoyl peroxide (2% by weight of the mixture). A tough, hard, pale yellow solid was formed within 2 h at 60 "C. The solid was extracted several times with xylene and chloroform to remove impurities. Cross-Linking with Acrylic Acid. To a solution of 20 g of maleic ester of diolamide and 8 g of acrylic acid in 2-butanone (4 mL/g of monomers) was added 0.05% AIBN; the solution was mixed thoroughly and slowly heated under nitrogen to 75 "C. After heating at 75 "C for 3 h, a solid separated from the solution, which was filtered and washed several times with 2-butanone and dried. Analytical Methods. Active-agent content of the formulations was determined by the method of chlorine estimation. IR analyses were performed on a Perkin-Elmer 298 spectrometer. NMR spectra were recorded on a Varian
TGA spectrometer. Molecular weight was determined by vapor pressure osmometric method. Hydrolysis Study. Release study was carried out for each formulation by taking weighed amounts of the herbicide samples, containing 25 mg of releasable 2,4-D, in four conical flasks to which 100 mL of distilled water (pH 6.7) was added. The flasks were shaken mechanically in a water bath at 25 "C for different periods of time. After a week, a definite amount of liquid was withdrawn from the flask marked for the first week. The released 2,4-D, in water, was extracted with ether. From the extract, ether was removed, and the concentration of 2,4-D was determined spectroscopically at 565 nm by complexing with chromotropic acid in the presence of concentrated sulfuric acid (Marquardt and Luce, 1951). Similar procedures were followed for the second, third, and fourth weeks for each of the five formulations. A similar hydrolysis study was carried out in acidic (pH 4) and alkaline (pH 10) media for each of the formulations for the same length of time. Laboratory Bioassay. Petri dishes of 5-cm radius containing soil of 2-cm thickness were used for bioassay experiments. The soil was treated with the herbicide formulations at a dose (Goutam, 1982/1983) of 80% active principle/(700 L of water) ha (10000 X 10000 cm2). Ten sunflower seeds were placed on the soil of each Petri dish a t the same time. Controls containing no herbicide and free 2,4-D were similarly prepared. After 24 days, germinated root structures were compared with those of the controls. Results and Discussions Monomer Synthesis. The polymeric generator of 2,4-D has been prepared following the reaction scheme shown in Figure 1. The synthetic route chosen for these polymers involved preparation of the monomer containing the herbicide by reaction of the herbicide with diethanolamine, which yielded a bifunctional amide. This bifunctional monomer can itself release 2,4-D on hydrolysis and can also undergo further condensation reactions because of the free hydroxyl groups, giving polymeric 2,4-D compounds. The reaction of the acid chloride of 2,4-D with diethanolamine in the presence of a tertiary amine did not afford the corresponding amide in good yield due to the formation of a number of byproducts such as diesters, ester amide, and monoesters, as was evident from the IR study of the crude product. When an attempt was made to prepare the amide by directly heating the mixture of acid and diolamine, the amide was formed exclusively without formation of any byproducts, but the yield was poor, about 20-30%. The yield of the amide improved considerably to 40-50% when the reaction was carried out in the presence of a small amount of sodium salt of 2,4-D. The addition of sodium salt of 2,4-D might have prevented the back-dissociation of the diolamine salt of 2,4-D through which the reaction of amide formation occurred. The structure of the monomer produced has been confirmed by the appearance of the absorption peaks at 1655 cm-l for the amide group and at 3400 cm-' for the hydroxyl groups in the IR spectra. Absorptions at 1075 cm-' and at 1485,1580 cm-' are due to the presence of ether linkage and aromatic moiety, respectively. Further proof of the structure of the monomer was obtained from the NMR spectra: three aromatic protons appearing in the region 6 6.6-7, the hydroxyl protons appearing at 6 4.7, and the -0-CH2- and N-CH2- protons appearing in the region 6 3.4 and 2.8. Physical characteristics of the five polymeric formulations prepared are listed in Table I.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986 587
Table I. Physical Characteristics of the Herbicide Polymer sample description of the herbicide polymer herbicide % HP-1 Dolvester of succinic acid and diolamide 40 50 pofyester of maleic anhydride and diolamide HP-2 40 polyester of phthalic anhydride and diolamide HP-3 35 cross-linked product of HP-2 and acrylic acid HP-4 35 HP-5 cross-linked product of HP-2 and styrene 1 .Preparation of
solubility insoluble chloroform chloroform insoluble insoluble
0 0
A 45
2
Reactions of
3000 6000
IR, cm-' 1640, 1735,1480 1650, 1735, 1450-1470 1635, 1720, 1595, 1475
60 r
Diolamide
2.4-0
mol w t
-
A
HP-1 HP-2 HP-3 HP-4 HP-5
CI Diolamide
Diethanolamine
-
Diolamide w i t h Dicarboxylic acidlanhydride
a) P - C - N (CH2CH20Hh 0
HOOC-(CH2)2-COOH
+
P
(I I
Figure 2. Cumulative concentration of 2,4-D released with time (neutral medium). c ) P-c-N(CH2CH20H12 0
+ oc,o,~,,
. .
p(III)
1
Cross l i n k i n g Reactions of(II1 With Acrylic acid 11+ CH2 zCH-COOH
-
Cross-linked product (IVI
With Styrene CH:CH2
11
+
6 -
C r o s s - linked product (VI
Figure 1. Reaction scheme for the preparation of polymeric herbicides.
Release Study. In all these polymeric herbicides, the active material 2,4-D is firmly attached to the polyester backbone by amide linkage. In the presence of environmental reactants, such as water, air, sunlight, or microorganisms, this specific amide linkage is broken and the active material comes out of the polymeric matrix by diffusion. The properties of the macromolecule and the medium surrounding it play an important role in determining the release rate of the toxicant for laboratory hydrolysis study. The rate of hydrolysis depends on the strength and chemical nature of the polymer-pesticide bond: the more susceptible the bond to hydrolysis, the faster will be the rate of release of the herbicide. An amide linkage is more susceptible to hydrolysis than an ester bond, hence the polymer backbone can be expected to be more stable compared to polymer-pesticide linkage, until and unless the slow degradation of ester polymer in water is accelerated due to some other factors. A cross-linked polymer is expected to show a slower release rate than an uncross-linked or a linear one, the latter being more sus-
ceptible to hydrolysis. A stereoregular or crystalline polymer is also expected to be less susceptible to hydrolytic attack. The release profile of these polymer-herbicides may appear to be further complicated due to the presence of the labile ester bond along the backbone, which may undergo depolymerization by the simultaneous attack of water and microbes on it. The hydrolytic release patterns of the five polymeric formulations containing pendent 2,4-D have been studied. HP-1, HP-2, and HP-3 are essentially linear polymers, while HP-4 and HP-5 are cross-linked solids, which would require hydrolysis of the amide bond and diffusion of the active agent through the matrix, as well as degradation of the ester linkage in the polymer backbone for the release of 2,4-D. Plots of percent release of 2,4-D vs. time (Figure 2) indicate a rapid initial release of the herbicide from the linear polymers HP-1 and HP-2, which was later followed by a more gradual release rate. The lower rate of release of 2,4-D from HP-2 compared to that in the case of HP-1 may be due to the presence of the double bond, imparting rigidity to the backbone by restricting free rotation around carbon-carbon double bonds. Moreover, the olefinic bonds are cis in nature, which may be expected to offer greater steric hindrance. The formulation HP-3, though a linear polymer, showed a poor release rate that may be attributed to the presence of hydrophobic groups such as aromatic nucleus and ester linkage. This strong hydrophobicity combined with the rigid stereoregular backbone structure imparted protection against hydrolysis. HP-5 had a much lower release rate compared to HP-2. After swelling, its release rate was slightly improved. The rate of release of HP-4, though a cross-linked formulation, was greater than that of HP-2 due to the presence of the hydrophilic carboxyl group. The presence of the hydrophilic group helped in swelling of the polymer matrix so that hydrolysis and diffusion would occur at a faster rate. This is evident from the fact that the concentration of 2,4-D released from HP-4
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 4, 1986
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0 0
f
A A 45
0
HP-I
HP-2 HP-3 HP-4 HP-5
Dialomide
' 7 HP-4
I 1
1 - 1
1
1
1
1
1
,
I
20 40 60 80 100 P e r c e n t germination inhibition
Figure 5. Percent germination inhibition. Each bar length represents the percent germination inhibition by CR herbicides of sunflower seeds for 24 days. Figure 3. Cumulative concentration of 2,4-D released with time (acid medium). 75
0
HP-I
0
HP-2 HP-3 HP-4
A A
Figure 6. Catalytic effect of the carboxyl group on the hydrolysis of the polymer backbone.
Figure 4. Cumulative concentration of 2,4-D released with time (alkaline medium).
exceeded that from HP-1 at a given time. The slight increase in the release rate of 2,4-D from all five formulations in acidic pH medium (Figure 3) may be due to the catalytic effect of the medium on the hydrolysis of the amide linkage, but this does not have much influence on the ester backbone. The hydrolyses of amide and ester linkages are both catalyzed by alkali, which is evident from the increased release rate of all the formulations except HP-3 (Figure 4). HP-4 in alkaline medium released more than 70% of 2,4-D in 32 days, which may be due to depolymerization in the presence of pendent carboxylate anion. The carboxylate anions are suitably placed to autoaccelerate the hydrolysis of the backbone ester linkage by the well-known neighboring group participation process leading to rapid cleavage of the backbone according to the reaction scheme
shown in Figure 6. It is an established fact that neighboring group participation enhances the reaction rate many fold, and the formation of the stable six-membered cyclic intermediate drives the reaction almost exclusively to that direction. The reason for the slow rate of release from HP-3 even in alkaline medium is not apparent. The alkaline medium might have failed to overcome the protection against hydrolysis offered by the rigid aromatic backbone. A preliminary evaluation of the biological effectiveness of the prepared formulations has been obtained from the results of laboratory bioassay experiments shown in Figure 5. I t is found that while about 60% germination is inhibited in 24 days by free 2,4-D, about 90% inhibition occurred with controlled-release formulation HP-2. HP-1 and HP-4 showed about 70%-75% inhibition. HP-5 and HP-3, showing similar germination inhibition characteristics to free 2,4-D, had also shown a slow rate of release in hydrolysis experiments (Figure 2). It is important for any controlled-release herbicide formulations not only to attain the minimum effective level but also to subsequently maintain that level by additional hydrolysis. HP-1, HP-2, and HP-4 showed improved effectiveness by maintaining
Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 589-595
the effective herbicide level by slow continuous hydrolysis. Hence the results of hydrolytic release study coupled with limited laboratory bioassay can provide useful information on the expected performance of such controlled-release formulations under actual soil conditions (Allan et al., 1977). The herbicide response of these polymeric formulations would depend to a large extent on the soil properties and the availability of water. 2,4-D is formulated as esters of low molecular weight alcohols, which are known to hydrolyze upon absorption by the plant, and the herbicide moves as a free acid in the plant. It has been found that within 0.5 h, after foliage application, as much as 75% of the 2,4-D esters are hydrolyzed to free acids. With herbicide polymer, the herbicjde response would be due chiefly to free 2,4-D, its soluble salts, and soluble diolamide, produced by slow hydrolytic and microbial degradation of the polymel-herbicide bond and polyester backbone of the herbicide polymer absorbed on the soil. Alkaline soils would degrade the polymer by saponification of the ester backbone, releasing both soluble salts of 2,4-D and diolamide that will be transferred to the root, whereas acid soil would only accelerate the cleavage of the pesticidepolymer bond, which will be transferred as free acid by diffusion. Consequently, these polymeric herbicide formulations will retain their herbicidal effectiveness for
589
longer periods of time in both neutral and acid soils. Registry No. I, SRU, 104778-80-5;I, copolymers, 104778-83-8; 11, SRU, 104778-82-7; 11, copolymer, 104778-85-0; 111, SRU, 104778-81-6;111, copolymer, 104778-84-9; 2,4-D, 94-75-7; 2,4-D diolamide, 19335-99-0; NH(CH2CH20H)2,111-42-2; H02C(CH2)&02H, 110-15-6;phthalic anhydride, 85-44-9; maleic anhydride, 108-31-6.
Literature Cited Alian, 0. G.; Beer, J. W.; Cousin, M. J. In Controlled Release Pestlcides; Scher, H. E. Ed.; ACS Symposium Series 53; American Chemical Soclety: Washington, DC, 1977; p 94. Bhattacharya, S.; Sanyal, S. K.; MukherJea, R. N. Ind. Eng. Chem. Prod. Res. Dev. 1985, 24, 274. Goutam, 6 . American Spring and Pressing Works, ASPEE Foundation, Bom bay, 9th Annual Report 1982-1983. Harris, F. W.; Arah. C. 0. In Proceedings of the 7th InternationalSymposlirm on Controlled Release of Bbactlve hfateflals; Lewis, D. H., Ed.;The Controlled Release Soclety: Dayton, OH, 1980; p 172. Harris, F. W.; Aulabaugh, A. E.; Case, R. D.; Dykes, M. K.; Feld, W. A. I n Controlled Release Polymeric Formulations; Paul, D. R., Harris, F. W., Eds.; ACS symposium Series 33; American Chemical Society: Washlngton, DC, 1976; p 222. Marquardt, R. P.; Luce, E. N. Anal. Chem. 1851, 2 3 , 1484. McCormick, C. L.; Anderson, K. W.; Hutchinson, B. H. J . hfacromol. Scl., Rev. hfacromd. Chem. Phys. 1882f1989, C22, 58. Neogl, A. N. Ph.D. Dlsse~tatlon,University of Washington. Seattle,WA, 1970. Schacht, E.;Desmarets, G.; Bogaert, Y. Mekromol. Chem. 1978, 179. 837.
Received for review October 11, 1985 Revised manuscript received April 14, 1986 Accepted June 9,1986
Reaction Rates for Gas-Phase Hydrogen Fluoride Saccharification of Wood Gregory L. Rorrer" and Martin C. Hawley Depattment of Chemical Engineering, Mkhlgan State Unlvers&, East Lansing, Mlchlgan 48824
Derek T. A. Lamport MSU-DOE Plant Research Lahratory, Mlchlgan State UnIvers&, East Lansing, Michlgan 48824
The intrinsic reaction rates for anhydrous vapor-phase HF saccharification of cellulosic materials in Bigtooth aspen were determined in order to characterize and assess this process for the production of fermentable sugars. Maximum glucose yields of 3.0 mmol/g of wood were recovered from wood chips reacted for 2 min with a pure HF flowstream at 30 O C and 1.0 atm. The intrinsic glucose yield vs. time profile was sigmoidal. The glucose production rate decreased nonlineatiy with decreaslng HF partial pressure from 1.O to 0.2 atm at 30 O C . Surprisingly, the glucose production rate decreased with increasing reaction temperature from 28 to 108 O C at an HF partial pressure of 1.O atm. I t is proposed that physlcal adsorption of HF onto the liinocelubslc matrix and hydrogen-bond breaking of cellulose microfibrils are prerequisites to the cellulose-cracking reaction and therefore may strongly influence the global rate of glucose production.
Introduction The conversion of lignocellulosic materials to simple sugars by acid hydrolysis is a well-established biomass saccharification technology. The acid hydrolysis of cellulosic materials in wood produces sugars that can be readily fermented to ethanol or other useful chemicals. However, the saccharification of wood by conventional sulfuric and hydrochloric acid hydrolysis processes suffers from a va0196-4321f86/1225-0589$01.50/0
riety of drawbacks, including low sugar yields, high acid consumption, long reaction times, and the need for high temperatures and pressures to drive the cellulose hydrolysis (Hawley et al., 1983). The use of anhydrous hydrogen fluoride (HF) to convert cellulosic materials in wood to simple sugars overcomes the classical disadvantages plaguing other conventional acidhydrolysis technologies. The "HF saccharification" of 0 1986 American Chemical Society
