I n d . Eng. Chem. Res. 1987,26, 2366-2372
2366
Schwarz, B. J.; Wilhelm, J. A.; Prausnitz, J. M. Znd. Eng. Chem. Res. 1987,in press. Sebastian, H. M.; Lin,H.-M.; Chao, K.-C. J. Chem. Eng. Data 198Oa, 25,381. Sebastian, H. M.;Nageshwar, G. D.; Lin, H. M.; Chao, K. C. Fluid Phase Equilib. 1980b,4 , 257. Tremper, K. K.; Prausnitz, J. M. J. Chem. Eng. Dat 1976,21,295.
Tsonopoulos, C.; Heidman, J. L.;Hwang, S.-C. Thermodynamics and Transport Properties of Coal Liquids; Wiley: New York, 1986.
Receiued for reuiew December 24, 1986 Reuised manuscript received July 13, 1987 Accepted July 27, 1987
Development of Degradable Slow Release Multinutritional Agricultural Mulch Filmf Shawqui M. Lahalih,* Saed A. Akashah, and Farouk H. Al-Hajjar Petroleum, Petrochemicals & Materials Division, Kuwait Institute for Scientific Research, 13109 Safat, Kuwait
A novel agricultural mulch film is prepared by mixing conventional plant nutrients with a watersoluble polymer such as poly(viny1 alcohol). The release of the nutrients contained in the mulch film is controlled by the addition of nitrification inhibitor along with urea or the addition of water-soluble urea-formaldehyde polycondensate. Further control of the nutrient release is realized by the addition of a water-resistant coating layer such as poly(viny1 acetate) and other additives such as glycol, urea, and starch. The agricultural mulch film is degradable and has balanced properties. Although the mulch film is clear, it prevents weed growth. The mechanical properties, accelerated aging behavior, and dissolution rates for the coated film are tabulated and discussed. Agricultural mulch films are used to cover the soil around crops or other newly planted areas to prevent or retard weed growth and to increase the water retention and temperature of the soil. Currently, polyethylene film used as an agricultural mulch is collected and burned after it has served its purpose. Obviously, this operation increases cultivation casta and air pollution, so it would be desirable to eliminate it. Accordingly, the efforts of research were directed to have mulch films to be photodegradable (Bryzgalov et al., 1984; Newland et al., 1969; Nissan Chemical Industries, 1985; Plastopil, 1984; Reich and Hudgin, 1976) or biodegradable (Agency of Industrial Sciences and Technology, 1981; Clendinning, 1975; Clendinning et al., 1976; Ingram, 1974; Kuraray 1982; Otey and Mark, 1976;Otey et al., 1977) to make their disposal easier. Most of these plastics are photooxidatively decomposed into small pieces or they are consumed by biodegradation in the soil. While some previous attempts produced nonnutritional mulch films with limited success, no plastic mulching films reported to date have been completely satisfactory, in the sense that they provide an adequate balance of the important properties needed for a good mulch film such as good mechanical properties, degradability, the incorporation of slow release multinutritional materials, and the ability to retard weed growth. This paper describes a novel degradable plastic mulch film containing multinutrients, which are slowly released to plants, that is characterized by an adequate balance of these important properties, and it has to be neither removed from the field nor buried at the end of cropping season. This film solves the problem of littering and decreases the cost of both mulch removal and adding fertilizers and nutrients to the soil a t controlled rates.
Experimental Section A urea-formaldehyde solution (1:2.5; 70% total solid) (U-F) used as a slow release nitrogen was prepared as follows. A solution of urea (44.46 g) in water (52 mL) was heated to 90 "C. Then 120 g of 94% paraformaldehyde was
'Publication KISR 2215.
added. The pH of this solution was adjusted to 8.0. During the addition of paraformaldehyde, the mixture was stirred continuously until a clear solution was obtained. The pH of the solution was then lowered to 4.8 by adding sulfuric acid. After the reaction was carried for an additional 30 min, the solution was neutralized by adding a 40% concentrated potassium hydroxide solution and was cooled to 45 "C. Urea (51.4 g) was then added to obtain a formaldehyde-to-urea ratio of 2.5, and the pH was adjusted to 5.0 by adding formic acid the reaction continued at 95 "C for 10-20 min. The solution was then neutralized by adding 40% potassium hydroxide solution after the reaction solution was cooled to room temperature. The solid content of the final solution obtained was 70%. Its final viscosity at 20 "C was 1000-1200 CP (nitrogen % = 17).
Laboratory Preparation of Mulch Film Different mulch films containing slow-release nitrogen or multinutrients (NPK) were prepared by the following procedure. The concentration and the volume of the casting solution were 15% and 10 L, respectively. Water (3400 mL) was added to a heated, stirred suspension of poly(viny1alcohol) (PVA), 375 g (MW 100000 manufactured by Fluka AG Chemische Fabrik, degree of polymerization 2000, and degree of hydrolysis 86-89 % ) in methanol (6375 mL), contained in a refluxed reactor (12-L capacity). The mixture was heated with continuous stirring a t 60-75 "C until a clear solution was obtained and the other components such as dipotassium hydrogen phosphate, triethyl phosphate, starch, urea, and ethylene glycol were added. The mixture was heated with continuous stirring for another 10-30 min. Then the required amount of urea-formaldehyde solution (70% solid) was added, and the resulting solution was cooled to 30-40 O C . The viscosity of the final casting solution ranged between 55 and 91 CPat 20 "C depending on the type of additives included. The solution was poured on a glass sheet (2.5 X 1m) that has a dried coating layer of poly(viny1acetate) equivalent to 0.2 mm thick of MW 160 000 (BDH Chemicals; viscosity of 8.6% w/v solution in benzene at 20 "C is 80-90 cP). The compositions of the various prepared
08SS-5885/87/2626-2366$01.50/00 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2367 Table I. ComDosition of PreDared Mulch Film film 1 2 3 4 5 6 7 8 9
lo*
PVA 30 25 25 25 25 25 25 25 20 25
U-F
KNOq
70 65 40 50 50 55 50 45 50 43
10 10 10 5 16
TEP
10 10 10 5 11
composition, %" KVHPOd
starch
urea
ethylene glycol
10 15 15 15
10 10
10
5 10 20 5
"PVA = poly(viny1alcohol) (MW lOOOOO), KN03 = potassium nitrate, TEP = triethyl phosphate, KzHPOl = dipotassium hydrogen phosphate. Sample 10 contained 1% thiourea.
*
mulch films are recorded in Table I. Coating Solution. Different water-resistant polymeric materials were used as a coating material for the prepared mulch films. These include the following: (a) 2 % , 5 % , 7%, lo%, and 12% poly(viny1acetate) in acetone (PVAC); (b) 5% poly(ethy1ene vinyl acetate) (EVA) (sold by Du Pont under EVa3125); 5% poly(viny1chloride) (PVC) (sold by Singapore Polymer Corporation, under SH 61); 5% poly(ethy1ene ethyl acrylate) (EEA) (sold by Union Carbide under DPDA-6182);and 5% vinyl chloride copolymer (VCC) (sold by BDH Chemicals), a vinyl chloride/vinyl acetate copolymer (Brookfield viscosity of 20 % solution in acetone at 25 O C is approximately 40 CPin toluene). Coating Procedure. The mulch film was coated with a machine designed and constructed at Kuwait Institute for Scientific Research by using dip-coating technique (see Figure 1). The film to be coated is passed through a gap (no. 1)into the fluid tank (no. 2) containing the required concentration of the coating solution. It is then guided through other gaps (nos. 3 and 4) into a chamber heated at 70 "C. After this, the drycoated film was rolled on a stainless steel roller. The drying time was 2 min at 70 "C. The film was weighed before and after coating. The measured percentage of coating was 2.1%, 4.9%, and 10.1% when 2%, 5%, and 10% poly(viny1acetate) solution in acetone (PVAC) was used, respectively. The viscosities of the 2%, 5%, and 10% PVAC solutions were 2.9,9.6, and 49.7 CPat 20 OC, respectively. Characterization of the Coated Nutritional Mulch Film. The mulch films were characterized by physical measurements, mechanical properties, weathering studies, and dissolution rates. Physical Measurements. The thickness and volume of the tested sheets were measured by an HBM strain gauge apparatus (Germany) and by a Beckman air comparison pycnometer (USA), respectively. Mechanical Properties. The tensile strength, tensile yield, and elongation at break were measured on a standard film testing machine, Testometric 220 D, an electronic tensile tester. The cross-head speed of the machine grips was 200 m/min. The test was run at 25 f 1"C on standard dumbbell-shaped disc samples according to ASTM D412. Accelerated Weathering Studies. Accelerated weathering studies were conducted according to ASTM G 53-77 by using an Atlas UVCON weatherometer. The cycle used was 4 h of UV at 60 "C and 4 h of condensation at 40 "C. The test was conducted at this cycle for 2 weeks. The samples (8 cm X 30 cm) exposed in the weatherometer were removed and tested every 2 days. Two samples were taken each time; one was removed during the condensation cycle (wet samples), and the other was removed during the UV cycle (dry samples). Five pieces were cut from each dry and wet sample. The mechanical properties of these
10
U-F = urea-formaldehyde condensate,
Table 11. Stability of Urea-Formaldehyde Condensation Product (60% Solid Content) at 10 "C Storage Temperature and pH 10 viscosity a t viscosity at time, h 20 "C, CP pH time, h 20 "C, CP pH 399 10.22 10.28 722 0 409 393 10.07 72 402 10.26 818 116 410 10.27 912 405 10.15 160 395 10.28 938 407 10.03 402 10.13 10.33 1008 301 403 10.10 1080 400 396 420 10.29 467 408 10.10 1152 410 10.04 605 397 10.33 1200 398 9.93 Table 111. Stability of Urea-Formaldehyde Condensation Products (70% Solid Content) at 10 OC Storage Temperature and pH 10 viscosity at viscosity a t time, h 20 "C,cP pH time, h 20 "C, CP pH 0 1050 10.00 132 2132 7.82 24 1275 8.15 224 3781 7.81 48 1390 8.13 268 5371 7.75 67 1672 7.98 312 80001 7.70 88 1760 7.88 336 gelled
pieces were measured. Then the average values for these five samples were reported. Dissolution Rates. Samples of the prepared mulch film were cut, weighed, and subjected to natural conditions to check for their dissolution rate as follows. The coated mulch films were placed on top of a tank (50 X 50 X 160 cm) filled with sand at 25 f 1OC. Four to eight pieces of each type of mulch film (10 X 50 cm) were placed on the top of the sand in which 2 L of water was added above the film daily. The weight of the samples was taken at zero time (before testing). On a weekly basis, one sample was taken, cleaned, dried, and weighed. The loss in the weight of sample was recorded. NPK analysis of the film was measured by using analytical techniques. Results The prepared urea-formaldehyde condensate containing approximately 70% solid was used in the preparation of mulch film, as a source of slow-release nitrogen. The viscosity and the density of prepared urea-formaldehyde (U-F) at 20 "C were 1050-1200 CPand 1.293-1.312 g/cm3, respectively. Stability studies (Table 11)show that, when 60% solid content of urea-formaldehyde solution was stored at 10 "C up to 50 days, its viscosity and pH were fairly constant. On the other hand, when the 70% U-F solution was stored under the same conditions, its viscosity increased with time and gelled after 14 days. The pH of the solution dropped from 10 to 8.15 after 1 day for 70% concentrated U-F when stored at 20 "C (Table 111). These results show that the condensation process can be slowed
2368 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987
+',
u Figure 1. Coating and drying apparatus for mulch film. Table IV. Stability of Urea-Formaldehyde Condensation Product (70% Solid) at 60 "C original sample + original sample 1% 1-butanol viscosity at viscosity a t time, h 20 "C.C P DH 20 "C. CP uH 1201 0 1246 10.00 10.00 1210 24 8.11 1322 7.98 1220 7.94 1672 7.68 67 7.73 1310 1760 7.46 88 1350 7.78 2132 7.52 132 1360 7.95 3781 7.61 224 1370 1.87 8001 7.45 268 gelled 1340 312 7.95 1322 7.91 450 540 1570 7.65 609 1539 7.74 747 1839 7.65 864 1942 7.48 2021 7.45 960 7.45 1080 2621
down or completely stopped by diluting the solution to 60% solid content and lowering the storage temperature to 10 "C. The stability of the original solution of U-F (70% solid) was studied at 60 "C in the absence and in the presence of 1%1-butanol. The results show that, in the absence of 1-butanol, the solution gelled after 11 days at 60 "C. However, in the presence of 1-butanol, the viscosity
of the solution was in the range 1200-2600 CPafter 45 days of storage at 60 "C. In both cases, the pH of the solution dropped to about 8 after 1 day (Table IV). This may be due to oxidation of the free formaldehyde to formic acid or to the hydroxylated urea (-CO-NHCH,OH) to carboxyl group. From these results, it was found that the addition of 1% 1-butanol to the urea-formaldehyde solution retards the condensation process and will help in storing the solution for a longer time. In addition to the use of urea-formaldehyde solution as a slow-release nitrogen source, the mulch films contained ethylene glycol, urea, or starch or their combination or potassium phosphate salts in addition to triethyl phosphate (TEP). Most of the prepared films are clear, strong, and flexible. Film thickness varied from 0.12 to 0.18 mm, depending on the concentration and viscosity of the casting solution used and on the concentration of the coating materials. Tensile strength and elongation at break were measured on both fresh samples and samples that had been aged in the accelerated weathering apparatus. These studies provided information on the effect of different additives, especially urea, starch, ethylene glycol; triethyl phosphate; and potassium phosphate salts; on the properties of different mulch films. Table V shows the mechanical properties of the different mulch films. Samples 2-5 were coated with three different concentrations of poly(viny1 acetate) (2%, 570, and 10%) to investigate the effect of coating thickness on mechanical properties. Results show that elongation at break decreased and tensile strength increased when the concentration of the coated material was increased from 2% to 10%. When the mulch film contained ethylene glycol, starch, and urea, the elongation at break was increased compared to other samples. Table V also shows that the tensile strength of the different samples coated with 10% poly(viny1acetate) ranged between 63 and 132.6 kg/cm2, and the elongation at break ranged between 73% and 372%) depending on the composition of the film. By comparison, a commercial black polyethylene f i i had a tensile strength of 180 kg/cm2 and an elongation at break of 313%. The experimental results show that the prresence of triethyl phosphate in the composition of the mulch film prevents the formation of microcrystals that would form if a high percentage of potassium nitrate and dipotassium hydrogen phosphate were included in the mulch f i i . The formation of microcrystals leads to an opaque film and sometimes causes embrittlement. Accelerated Weathering Studies Accelerated weathering studies were performed to predict the service life of the film. Since the mechanical
Table V. Mechanical Properties of Prepared Mulch Film" 2% PVAC 5% PVAC 10% PVAC elongation elongation elongation film tensile strength, kg/cm2 a t break, 7~ tensile strength, kg/cm2 a t break, % tensile strength, kg/cm2 at break, % 1 102.0 324 114 2 56.2 396 58.0 234 85.6 324 3 45.6 493 64.1 379 63.0 53.1 79 132.6 4 41.3 411 73 5 41.0 346 58.0 343 64.4 239 318 6 62.4 282.3 84.5 7 70.0 372 270 8 103.0 209 9 76.7 10 93.7 208 aFor 10% poly(viny1 alcohol) (MW 100000): tensile strength (kg/cm2) is 390 and elongation a t break is 291%. For 10% poly(viny1 acetate) (MW 160000): tensile strength (kg/cm2) is 58 and elongation at break is 486%.
Ind. Eng. Chem. Res., Vol. 26,No. 11, 1987 2369 Table VI. Mechanical Properties of Mulch Films Coated with 10% PVAC upon Accelerated Weatbering Test wet cycle dry cycle tensile tensile strength , elongation at strength, elongation a t sample composition, % exposure time, days kg/cm2 break, % kg/cm2 break, % 2 25 PVA; 0 94.1 405 45.6 43.0 44.6 2.4 65 U-F; 2 49.2 31.0 31.1 2.4 10 starch 4 6 48.8 50.0 49.2 2.7 50.4 2.9 8 41.7 31.0 68.1 2.7 10 29.9 22.4 12 15.8 15.2 17.0 2.7 3 25 PVA; 0 54.3 408 321 58.7 274 40 U-F; 1 45.4 204 57.1 3.3 15 starch; 3 29.7 149 47.3 8.6 10 urea; 5 20.4 98.0 88.1 11.6 10 ethylene 7 18.9 glycol 9 35.9 53.0 39.1 2.9 11 25.1 48.5 25.2 2.5 4 25 PVA; 0 63.2 327 2 40.5 264 55.7 2.3 50 U-F; 164 22.2 2.7 15 starch; 4 48.5 85.0 20.5 2.6 10 urea 6 58.9 8 37.9 76.5 51.3 3.2 10 54.2 5.7 69.2 4.6 12 38.8 15.3 45.1 3.1 5 25 PVA, 0 92.1 303 50 U-F; 2 44.8 221.0 46.3 27 15 starch; 4 42.3 113 56.0 12 10 ethylene 6 46.4 125 71.5 2.1 glycol 8 34.3 148 28.4 2.4 10 35.0 80.5 58.4 40.0 12 31.0 57.0 76.7 32.5 "PVAC = poly(viny1 acetate) (MW 160000). PVA = poly(viny1 alcohol) (MW 100000).
Table VII. Mechanical Properties of NPK Mulch Film Coated with Different Coating Materials upon Accelerated Weathering time of accelerated weathering, day mechanical coating material type of cycle properties 0 2 4 6 8 10 12 5 % EEA" dry tensile 85.2 79.2 101.2 brittle elongation 196 18.8 10.4 brittle wet tensile 85.2 52.9 46.9 38.0 17.1 brittle brittle elongation 196 147.0 101.0 57 25.7 brittle brittle 5% EVA" dry tensile 90 49.8 brittle elongation 206.3 126.2 brittle wet tensile 90 46.5 39.6 27.9 17.6 brittle elongation 206.3 126.2 61.1 16.1 8.1 brittle 5% PVAC" dry tensile 62.4 126.4 brittle elongation 282.3 9.1 brittle wet tensile 62.4 78.5 72.2 67.3 32.4 16.2 brittle elongation 282.3 101.6 66.1 49.3 18.2 8.4 brittle 84.5 119.6 172.4 181.4 188.1 brittle 10% PVAC" dry tensile 318.0 126.2 27.6 27.6 7.1 brittle elongation wet tensile 84.5 83.0 103.5 83.6 112.7 95.0 104.4 elongation 318.0 101.0 106.0 87.0 88.0 44.6 17.1 70.0 107.3 116.7 123.1 brittle 10% PVACb dry tensile elongation 372.0 133.3 80.7 50.1 brittle wet tensile 70.0 72.6 70.0 36.5 30.7 15.2 brittle elongation 372.0 105.2 80.2 42.1 18.2 6.0 brittle
14
106.7 14.2
"Mulch film sample 6 containing 25% PVA, 10% KN03, 10% TEP, and 55% U-F. bMulch film sample 7 containing 25% PVA, 50% U-F, 5% K2HP04,10% TEP, and 10% KNO,.
properties of the films are strongly affected by the level of moisture, one set of the prepared films were removed during the condensation cycle (wet sample) and another was removed during the UV cycle (dry sample). Table VI shows the mechanical properties of mulch films (samples 2-5) coated with 10% poly(viny1acetate) upon accelerated weathering test. Results show that the presence of urea and ethylene glycol in the composition of the mulch film improves the properties of the film, which becomes stronger and more flexible. Table VI1 shows the mechanical properties of mulch films 6 and 7 coated with 5%
poly(ethy1ene ethyl acrylate) (EEA), 5% poly(ethy1ene vinyl acetate) (EVA), and 5% or 10% poly(viny1acetate) (PVAC) on accelerated aging. In general, elongation of f i i deteriorates with age, and it becomes brittle although the tensile strength is still high, especially for samples coated with 10% PVAC. The results obtained from the' samples coated with 5% and 10% PVAC show that the deterioration rate can be decreased by increasing the thickness of the coating film. Sample 6 coated with 10% PVAC survives twice as long as the same samples coated with 5 % PVAC. The results also show that if the films
2370 Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 Table VIII. Natural Dissolution Rates of NPK Film Sample 2 (25% PVA, 50% U-F, 5% K2HP04,10% TEP, and 10% KNO& with Different Coating Materials at 25 i 1 'C NPK weight loss, % 5% PVAC 5% EVA 5% EEA 5% vcc type of water time, weeks a b a b a b a b tap water 1 27.1 23.0 22.9 33.2 16.0 33.0 20.0 44.0 2 31.3 33.0 35.5 51.9 22.3 52.0 32.9 60.0 3 43.0 40.0 37.0 64.8 43.6 65.0 42.2 68.0 4 45.9 46.0 69.0 45.7 73.0 45.4 73.0 5 49.7 51.0 44.7 71.5 47.7 78.0 49.2 75.0 6 50.2 55.0 47.0 72.1 50.0 81.1 51.1 76.0 7 50.8 58.0 48.0 72.5 51.4 82.5 52.9 77.5 8 51.1 61.0 49.1 73.0 51.5 83.1 53.1 79.0 9 51.5 64.0 49.5 73.5 54.2 84.5 55.4 80.0 67.0 50.1 74.0 57.2 85.0 56.2 81.5 10 53.0 19.0 29.4 42.5 19.5 32.5 40.5 56.0 brackish water 1 18.0 33.5 55.5 33.8 52.5 41.9 63.0 2 23.2 31.0 67.0 40.0 44.0 63.3 65.0 43.1 3 30.2 39.8 72.0 46.0 71.0 41.1 44.1 69.0 4 34.0 47.0 71.0 76.0 45.4 53.0 47.1 74.0 43.3 5 39.5 57.5 47.5 76.0 49.1 79.0 46.1 72.0 6 41.1 73.0 48.5 77.7 52.3 80.0 46.4 7 42.3 61.0 81.0 46.4 74.0 79.3 52.4 64.0 49.1 8 43.4 82.0 47.2 76.0 66.0 50.2 80.0 53.3 9 46.3 68.0 51.4 81.5 54.5 83.0 48.9 77.0 10 47.1 "The percentage of NPK weight loss calculated from total weight dissolved to the original weight of film. "he percentage of NPK weight loss is calculated from actual measurements of N, P, and K on the remaining film compared with the original amount of NPK in the film.
Table IX. Natural Dissolution Rates of NPK Mulch Film (25% PVA, 50% U-F, 5% K2HP04,10% TEP, and 10% KN03) Coated with Different Concentrations of PVAC at 25 i 1 OC Using Tap Water NPK weight loss, % 2% PVAC 7% PVAC 10% PVAC 12% PVAC time, weeks a b a b a b a b 17.0 35.0 14.0 22.8 42.5 18.9 1 44.8 57.0 55.0 30.9 50.0 30.9 28.5 81.0 32.6 2 48.5 63.0 35.0 55.0 31.3 37.0 84.0 32.9 3 54.7 38.5 58.0 32.0 42.0 36.3 67.0 4 57.3 85.9 42.0 60.0 33.9 44.0 39.7 70.0 5 60.3 87.4 72.0 42.2 61.0 34.5 46.0 40.9 6 61.1 88.5 73.5 89.6 42.3 42.2 62.0 35.0 47.0 7 63.0 75.0 42.3 63.0 36.2 48.0 90.2 47.3 8 64.9 75.5 44.1 64.5 37.8 49.5 90.5 49.1 9 65.1 10 66.3 91.5 50.9 76.0 43.3 65.5 38.1 51.0 The percentage of NPK weight loss calculated from total weight dissolved to the original weight of the film. The percentage of NPK weight loss calculated from actual measurements of N, P, and K on the remaining film compared with the original amount of NPK in the film.
are allowed to dry, deterioration quickly occurs and the films become brittle and break down into small pieces, which accelerates the degradability of these films. Dissolution Rates. Dissolution is an important property that determines the design of the mulch film to be used for a particular crop. Dissolution rates of the prepared mulch film were studied under natural conditions in which the sample was placed on top of soil. The dissolution rate results of sample 2 coated with different coating materials (5% EVA, 5 % EEA, 5% VCC) and different concentrations of poly(viny1acetate) (2%, 5 % , 7%, lo%, and 12%) tested at 25 f 1 "C using tap (total dissolved solid is 0.25-0.38 g/L) and brackish waters (total dissolved solid is 2.8-3 g/L) are reported in Tables VI11 and IX. The samples were placed on top of soil and irrigated daily by pouring 2 L of water on the surface of the tested mulch film. One sample was removed every week, and its weight was recorded. The amounts of N% , P% and K% on the tested film were also measured. The weight loss and N, P, and K analysis were monitored for 10 weeks. The results are reported in Table VIII. Samples coated with poly(viny1acetate) dissolve much slower than those coated with other materials. Table VI11 also shows the total dissolved material from the coated films com-
pared with the dissolved amount of N, P, and K. The dissolution of N, P, and K from the films was obtained from actual NPK analysis of the films at various times. Although this is the best method to check for the dissolution of the nutritional elements, it is a lengthy and costly process. If it is assumed that only fertilizers leach out or dissolve, then the weight loss determination of dissolution rates can be just as accurate as the NPK analysis of the films. This could, however, lead to serious errors if the PVA dissolves in the long run. The effect of the coating thickness of PVAC on the dissolution rate is shown in Table IX. The data show that the dissolution rate can be controlled by the coating layer thickness. Therefore, the release of nutrients to the plants is controlled by the slow-releasecharacteristics of the constituents, in addition to the polymer matrix control and the coating thickness control. This is essential for crops of different seasonal period. For example, a cucumber crop, which has a 3-4month season, would require a film coated with 2-5% PVAC, while a tomato crop, which has a 8-10-month season, would require a film coated with 10-129'0 PVAC. Table X shows the result of natural dissolution rates for mulch films (samples 2-5) coated with different concentrations of poly(viny1 acetate) at 25 "C using tap water.
Ind. Eng. Chem. Res., Vol. 26, No. 11, 1987 2371 Table X. Dissolution Rates of N Mulch Films Coated with Different Thicknesses of PVAC and Placed on Top of Tanks at 25 OC Using Tap Water dissolution rates (%) after composition, % coating material (PVAC % ) 1 week 2 weeks 3 weeks 4 weeks sample 2 17.7 2 25 PVA, 65 U-F, 21.3 41.5 41.0 5 10.6 10 starch 26.7 26.0 26.0 11.0 14.0 24.1 10 25.0 3 25 PVA, 40 U-F, 20.0 34.7 40.0 2 60.5 18.6 51.2 5 15 starch; 10 urea, 27.6 40.6 15.8 10 10 ethylene glycol 25.0 39.0 44.0 4 25 PVA, 50 U-F, 25.1 2 48.9 49.2 50.0 15 starch; 10 urea 5 12.0 26.5 29.5 30.6 10 13.7 14.5 21.3 24.0 2 5 25 PVA, 50 U-F, 31.0 35.5 40.0 41.0 5 25.1 15 starch, 10 ethylene 29.3 29.4 30.0 10 glycol 14.5 22.0 24.0 28.0 ~~
Results indicate that the weight loss of the film coated with 2% PVAC was more than that of film coated with 5% or 10% PVAC-as would be expected. Also, this type of information can be used to design suitable films for any crop duration.
Discussion The idea of developing a nutritional mulch film to be used for agriculture with the slow-release characteristic is novel. Whereas most research has concentrated on developing biodegradable or photodegradable mulch films, the approach discussed in this paper provides a film that degrades due to the leaching of its constituents under the action of water. Most of the film’s constituents are water soluble. In addition to its ability to degrade, the mulch film prepared according to this procedure has good mechanical properties. Good mechanical properties are needed so the mulch film can sustain stresses during the laying-on-the-ground operation by mechanical means and the stresses generated from the wind and thermal gradients. The maximum stresses the film will face are during the mechanized operation of laying the film on top of the soil, and these are met by intially high values of tensile strength and elongation (Table VI). As time progresses during the growing season of the crop, the stresses on the film are reduced. A t this stage, the film provides nutrition to the plants as well as performing the other functions of a mulch film. By the end of the season, the film’s mechanical properties are reduced, and it becomes very brittle, so it breaks down and dissolves very slowly into the ground. It is estimated that 1-2 weeks of accelerated testing is equivalent to a crop season of 6-months to 1-year duration. Although the presence of starch and a nitrogenous source in a poly(viny1 alcohol) matrix tend to provide a f i i that degrades relatively quickly and becomes very brittle, the presence of urea and ethylene glycol extends the degradation (Table VI). This is also true of the dissolution rates (Table X). In the presence of urea and ethylene glycol, the dissolution rate of these mulch films increases appreciably. Although the degradation mechanism of the mulch film described here depends primarily on the fact that the NPK mulch film is prepared from a water-soluble resin, the mechanism also depends on the fact khat the water-soluble nutrients and additives are leached from the film, thereby changing the morphology of the composite film and rendering more susceptible to mechanical failure. The degradation rate due to the presence of nutrients is greater than that of the dissolution of the water-soluble polymer carrier. This morphological change is accomplished by the presence of nutrients in the film, without resorting to the inclusion of starch, antioxidants, or other additives that
~
jeopardize the mechanical properties of the films or make them too expensive. The mechanical properties and the dissolution rate of these mulch films can be controlled by the proper inclusion of additives. This is essential since different crops have different season durations. It is believed that these additives improve the segmental mobility of the various constituents in the matrix of the thermoplastic carrier. In addition to these additives, the water-resistant coating material (i.e., poly(viny1acetate)) can also control both the mechanical properties and the dissolution rate of these films (Table VI). The NPK mulch films of the present work also exhibit the distinct advantage of containing several multifunctional ingredients, such as a slow-release nitrogenous source. The nitrogenous source comprises either first urea in combination with a nitrification inhibitor or second a urea-formaldehyde polycondensate that releases nitrogen at a controlled rate. Third, the urea-formaldehyde condensate has an adhesive property whereby it binds the substrate (i.e., poly(viny1 alcohol) with the various nutrients) to the thin water-resisting coating layer, thus preventing delamination of the composite film when it is exposed to strenuous mechanical stresses. Fourth, the urea-formaldehyde condensate nitrogenous source is added to the water-solubleresin solution in an aqueous form, and hence the resulting film is homogeneous. Another multifunctional additive that may be included in the film preparation is an alcohol such as methanol. The main role of the use of methanol as a solvent is to prevent or retard the reaction of urea-formaldehyde with poly(vinyl alcohol) through the formation of an ether linkage (-CH,OR) (Schwaar, 1976). Also it raises the viscosity of the solution, thereby resulting in a uniform casted mulch film. Since methanol has a lower boiling point than water, the film can dry in a faster time, thus maximizing the production rate and minimizing the production cost. Yet another multifunctional additive that may be used is triethyl phosphate. This acts as a source for phosphorus and, also, prevents the formation of microcrystals that would otherwise form when high percentages of potassium nitrate and dipotassium hydrogen phosphate are included in the mulch film. In the presence of triethyl phosphate, therefore, it is possible to have a mulch film with 80% fertilizers and still have a homogeneous and clear f i i with good balanced properties. Light transmission tests show that only 5 1 0 % of visible light passes through. The reduction in light transmission is enhanced when water is absorbed during irrigation. This reduction results in an increase in light absorption and reflection at the shorter wavelengths, thus reducing the light transmission necessary for weed growth. That is, this clear mulching film prevents weed growth without the
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addition of herbicides and carbon black which provides an economic advantage to the user. On the basis of the above-mentioned results and discussion, the mulch films described here contain an adequate balance of the important properties needed for an agricultural mulch film. These properties include (1)good mechanical properties, (2) the ability to retard weed growth without the incorporation of toxic chemicals such as herbicides, (3) degradability, (4)the incorporation of multinutrirional materials that can be released slowly, (5) harmless to the environments, and (6) clarity. Conclusions Various types of multinutritional, degradable, and slow-release clear agricultural mulch films containing poly(vlny1 alcohol), urea-formaldehyde, Pz05,K20, and other additives were prepared and coated with different water-resistant coating materials. The mechanical properties and the dissolution rates of these films were studied. The mechanical properties of these films are affected by the type of materials involved and the percentage of moisture present. The results also indicate that the presence of starch, ethylene glycol, and urea improves tensile strength, elongation at break, and dissolution rate of these films. The dissolution rate of the mulch film is affected by the type of coating material and its thickness, types of additives present, the temperature, and the type of water used. This information can be used to design
suitable films for any crop duration. It was also found that the prepared mulch films can prevent weed growth despite their clarity. Registry NO.PVA, 9002-89-5; U-F, 9011-05-6; TEP, 78-40-0; PVAC, 9003-20-7; EVA, 24937-78-8; EEA, 9010-86-0; VCC, 9003-22-9; KNO,, 7757-79-1; KZHPO,, 7758-11-4; starch, 9005-25-8; urea, 57-13-6; ethylene glycol, 107-21-1.
Literature Cited Agency of Industrial Sciences and Technology, Japan Kokai Tokyo Koho 81 22 324, 1981. Bryzgalov, V. A.; Sakova, T. M.; Zakharova, E. I. Plast. Mussy 1984, 2, 51-54. Clendinning, R. A. U S . Patent 3 929 937, 1975. Clendinning, R. A,; Potts, J. E.; Niegish, W. D. US. Patent 3 931068, 1976. Ingram, A. R. U S . Patent 3833401, 1974. Kuraray Co. Ltd. Japan Kokai Tokyo Koho JP 92 582, 1982. Newland, G. C.; Graecar, G . R.; Tamblyn, J. W. U S . Patent 3 454 510, 1969. Nissan Chemical Industries Ltd. Tokyo Koho J p 60 18 369, 1985. Otey, F. H.; Mark, A. M. U.S.Patent 3949145, 1976. Otey, F. H.; Westhoff, R. P.; Russel, C. R. Ind. Eng. Chem. Prod. Res. Deu. 1977, 16, 305-308. Plastopil, H. Israeli 1168 641, 1984. Reich, M.; Hudgin, D. E. U.S.Patent 3984940, 1976. Schwaar, R. H. ’Thermosetting Resins”, Report 93, 1976, p 132; Stanford Research Institute.
Received for review January 13, 1987 Revised manuscript received July 10, 1987 Accepted July 27, 1987
Separation of Isomers Using Retrograde Crystallization from Supercritical Fluids Keith P. Johnston,* S t e p h e n E.B a r r y , Nolan K. Read, and T y l e r R. Holcomb Department of Chemical Engineering, University of Texas, Austin, Texas 78712
Solubilities have been measured for a mixture of 2,3- and 2,6-dimethylnaphthalene in supercritical fluid carbon dioxide to identify retrograde regions, which have an inverse solubility versus temperature relationship. A retrograde crystallization process has been designed and tested for the separation of the two isomers at supercritical conditions. A thermodynamic framework has been developed which may be used to locate retrograde regions in order to evaluate the feasibility of retrograde crystallization processes. The complex effect of temperature on solubility at constant pressure has been expressed in terms of two approximately constant derivatives and the volume expansivity of the pure solvent. Supercritical fluids have several desirable properties that make them attractive for certain separation processes; e.g., the product is not contaminated with residual solvent (Paulaitis et al., 1983; Johnson, 1984; McHugh and Krukonis, 1986). A retrograde solubility region may be found near the critical pressure of the solvent where small increases in temperature cause a large decrease in the density. Chimowitz and Pennisi (1986) described recently a novel retrograde deposition process, which is a crystallization process in which the slopes of the solubility versus temperature are opposite for two solids. A crossover pressure was defined as the point where the slope of the solubility versus temperature curve changes sign. For two solids which have different crossover pressures, there exists a “crossover region ’ between the crossover pressures where an increase in the temperature precipitates only the solid with the inverse solubility versus temperature behavior. Therefore, this process has the potential to achieve infinite 088S-588~/87/2626-2372$01.50/0
selectivity in a single stage for the separation of a binary mixture. This phenomena will be explained in greater detail below. Chimowitz and Pennisi (1986) found that the process is quite successful for the separation of benzoic acid from a mixture of benzoic acid and 1,lO-decanediol, perhaps since the crossover region is large for this system. Supercritical fluid solvents have an additional degree of freedom compared with liquids, in that the solubility behavior is strongly pressure dependent as well as temperature dependent. For certain binary mixtures, it is conceivable that a pressure region may be available to perform a retrograde separation even if both components have the same solubility versus temperature slopes in liquid solvents. The objective is to achieve a fundamental understanding of the range of the crossover region for two solids, e.g., isomers, and to design and test an actual retrograde crystallization process for a system with a small crossover 0 1987 American Chemical
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