Starch-Based Film for Degradable Agricultural Mulch Felix H. Otey, Arthur M. Mark, Charles L. Mehltretter, and Charles R . Russell* Northern Regional Research Laboratory, Peoria, lliinois 61604
Several starch-based films were prepared and evaluated as degradable agricultural mulch. The films were made by casting a water dispersion of starch, poly(viny1 alcohol), glycerol, surfactant, and formaldehyde onto glass plates, oven drying, and coating the films with poly(viny1 chloride) or a copolymer of vinylidene chloride and acrylonitrile. The films have 2000-3000 psi tensile strength, 10-1 50% elongation, and resistance to artifical weathering of 40-120 hr. The range of properties is controlled by the ratios of coating, plasticizer, and cross-linking agent.
Applying plastic film for agricultural mulching has increased rapidly in the past few years. At present about 30 million lb/year of plastic mulches is used in the United States, and some predictions are that by 1975 the market will have doubled (Chemical Week, 1972). Vegetables, tomatoes, and strawberries are among the more common crops on which plastic mulch is used. Principal benefits of mulching are to provide weed control, warm the soil for early crop production, control soil moisture, and reduce nutrient leaching. The monetary advantage of mulching certain crops was illustrated in a study by Albregts and Howard (1973). They observed increases in yield due to full mulching over no mulching of 447 bu/acre for peppers and 136 bu/acre for okra. Assuming an average selling price of these two vegetables of $4/bu, the additional return from full mulching of peppers was $1788/acre and of okra, $544/acre. These yield increases alone pay for the mulching several times over. In addition the chances for crop production are greatly improved during unfavorable weather conditions, such as excessive rainfall or subnormal temperatures. Generally mulch is applied after the soil is prepared and planting is done through holes punched in the plastic with a planter. Polyethylene film is the most common plastic mulch; however, it must be removed from the field and buried or burned a t the end of each fruiting season since it does not decompose in the soil. The application of degradable films would eliminate the estimated $40-$70 cost/acre required to remove it as a mulch and would reduce deleterious environmental effects caused by burning or burial of nondegradable plastics. Paper coated on one or both sides with a thin layer of polyethylene is being extensively investigated as an agricultural mulch because it disintegrates when plowed into the soil (Albregts and Howard, 1972). Also, a new film based on butene-1 and sold under the trade name of "Ecolan" shows potential as a degradable plastic mulch. Reportedly the product is highly susceptible to degradation by sunlight (Chemical Week, 1971). Some new starch-based films have been prepared that may have application as degradable plastic mulch. Although whole starch has been investigated for many years as a potential raw material for nonsupported films, it has never been Buccessful because its films are brittle and are greatly affected by moisture (Lloyd and Kirst, 1963). The linear amylose fraction does produce quality films (Sweeting, 1968). Large amounts of compatible plasticizers, such as glycerol or ethylene glycol, are effective softening agents for whole starch, but the resulting films are too soft and tacky a t high humidity and have virtually no wet strength. These problems were ameliorated, for the specif90
Ind. Eng. Chem., Prod. Res. Develop.,Vol. 13, No. 1 , 1974
ic objective of this study, by adding some poly(viny1 alcohol) (PVA, Du Pont Elvanol 72-60, 99-10070 hydrolyzed), cross-linking with formaldehyde, and coating the film with a thin layer of either poly(viny1 chloride) (PVC, Monsanto Opalon) or a vinylidene chloride-acrylonitrile copolymer (Saran, Dow Chemical experimental resin XD-2364.2). Any of a great variety of water-resistant coatings could be selected, including those which are light sensitive or biodegradable.
Experimental Section Starch-PVA Film Preparation. In a three-necked, 1-1. flask, equipped with a glass-sleeve Teflon stirrer and steam bath, were mixed 32 g (dry basis) of air-dried pearl corn starch, 8.0 g of PVA, 12 g of glycerol, 360 ml of water containing 0.4 g of Tween 80 surfactant (Atlas Chemical Industries poly(oxyethy1ene) sorbitan monooleate), and 2 ml 40% formaldehyde solution. The mixture was stirred slowly and full steam heat was applied so that the temperature was above 90" as quickly as possible. The mixture becomes a thick paste in about 2 min. Heating and stirring were continued at 95-98' for at least 45 min. Then 0.5 g of NH4C1 catalyst dissolved in 5 ml of water was added, and the mixture was stirred for a t least 15 min a t 95-98". The hot bubble-free mixture was cast a t a 30-mil wet thickness onto a silicone-coated plate glass preheated to 80-90". Then the material was dried to a clear film in a forced-air oven a t 130" for 5 min. Immediately the film was removed from the hot plate and equilibrated at 50% relative humidity. TDI-Castor Oil Prepolymer. In a dry 250-cm3 roundbottomed flask, equipped with thermometer, nitrogen inlet, magnetic stirrer, condenser, and heating mantle, were mixed 33.6 g (0.1 equiv) of castor oil, 13.5 g of toluene diisocyanate (TDI, 0.15 equiv), and 20 ml of toluene. The mixture was stirred and swept with nitrogen a t 70" for 40 min and cooled to room temperature. The product was a clear viscous liquid with a few weeks' shelf life. Coating Solution. Fifteen grams of PVC and 10 g of dioctyl phthalate (DOP) were dissolved in 175 ml of methyl ethyl ketone (MEK) with steam heat, and then the mixture was cooled to room temperature. Three grams of the preparef TDI-castor oil prepolymer solution and 0.2 g of dibutyltin dilaurate were added to the MEK solution. The starch-PVA film was pulled through this coating solution and then between two steel rods spaced about Y I ~ in. apart. Alternatively, 15 g of Saran could be dissolved in 175 ml of MEK, 3 g of the TDI-prepolymer solution added, and the film coated ih the same manner as described. These amounts of PVC-DOP or Saran yield a film with about 8-10% coating by weight. For a 15-20%
coating the film was coated, air-dried, and again coated with the same solution.
Table I. Effects of Formaldehyde and of Glycerol on Starch-PVAa Films with No Coating (Formulation: 8 g of Cornstarch, 2 g of PVA Plus Designated Additives)
Results and Discussion
Additives
Generally the addition of cross-linking agents (such as formaldehyde), plasticizers (such as glycerol), and PVA causes starch film to be more flexible and to have a higher stretch. A surfactant, for example Tween 80, was added to the formulation to lower the surface tension and reduce gas entrapment. Otherwise the mixture must be deaerated by alternate application and release of a vacuum while keeping the mixture above the gelation temperature. Extended heating of the mixture for at least 45 min at steam-bath temperature is also essential if films are to be free of bubbles. Longer heating of 2-3 hr does not adversely affect the films. After 1 hr of heating, including 15 min after the NH4C1 addition, the mixture has a viscosity of 1600 CPa t 70". Experiments were designed to produce the films from low-cost raw materials and by a process that is adaptable to industrial band casting. Here the starch-PVA suspension would be cast onto a steel belt, passed through a drying oven, stripped from the belt, and passed through a coating solution. Although the films are clear and transparent, their appearance can be improved by using watersoluble grades of PVA and modified starches, such as hydroxyethylated starch. These modifications allow the starch-PVA dispersion to be filtered before casting to remove trace amounts of undispersed particles. However, our preliminary findings indicate that the additional cost is not justified for mulch applications. Table I lists some properties of uncoated starch-PVA films tested within 48 hr after preparation. Each value in the table represents an average of 50 specimens taken from five film preparations. The films show increased per cent elongation with addition of CH20 and NH4Cl. We find that the higher (22%) glycerol level provides better films although they tend to block if not coated. After these films have been held even at 50% relative humidity, their tensile strength increases and the percentage of elongation decreases. Further studies are under way to determine the cause and prevention of these changes in physical properties. All films tested show no permeability to oxygen by the procedure of Major (1963) which suggests that the film would help retain soil fumigants applied just prior to the mulch.
!z
!z
Film thickness, mils
0 0 0.5 0.5 0.5 0.5 0.5 0 0 0.5 0.5 0.5 0.5
2 2 2 2 2 2 2 3 3 3 3 3 3
1.3 1.3 1.3 1.5 1.2 1.3 1.3 1.5 1.5 1.4 1.5 1.4 1.4
CHzO, Glyml of 40% NHaC1, cerol, soln 0 0.5 0 0.5 1.o 2.0 4 .O 0 0.5. 0 0.5
1.o 2 .o
Tensile
strength,b Elongapsi tion, % 3160 3100 1960 2000 1760 2110 2250 2040 2010 1610 1290 1360 1580
20 26 54 68 77 110 88 100 110 88 140 150 130
PVA = poly(viny1 alcohol). b Tested within 24 hr after film preparation. Q
Starch-PVA films remain insoluble after water soaking for 16 hr; however, their wet strength is low. Furthermore, they are greatly affected by humidity changes. Numerous additives and cross-linking agents were evaluated to overcome these problems but none proved adequate. Hence, we resorted to water-resistant coatings. PVC and Saran are promising coatings because they provide water resistance for a time, then become brittle, and allow deterioration of the film-a desirable characteristic for degradable mulch film. Table I1 lists properties of some starch-PVA films coated with PVC or Saran and equilibrated for about 2 weeks before testing. TDI-castor oil prepolymer was mixed with the coating solution to bond the coating to the starchPVA film. Otherwise, the coating delaminates during water soaking. Since standard coating equipment was not available, the data in Table I1 should not be used to interpret differences between PVC and Saran coatings. The most significant difference in the properties of coated film us. noncoated film is the improved wet strength. Without coating, the films in Table I1 have wet strengths that are almost too low to measure, but with coating they show wet strengths of 200-500 psi after 16 hr of water soaking. The effect of CHZO-NHICl and increasing the amounts of
Table 11. Properties of Coated Starch-PVA Filmsa Item Compn, % Starch PVA Glycerol CHzO NH&I Tween 80 Thickness, mils Tensile strength, psi 50% re1 humidity 10% re1 humidity Wet, after 16-hr HzO soak Elongation, % 50% re1 humidity Wet, after 16-hr H 2 0 soak M I T fold (50% re1 humidity,
Coated with 7 . 5 % Saran soln
Coated with 7 . 5 % PVCb-5% DOP soln
No coating
64 16 20 0 0 0.8 1.6
64 16 16 1.6 2.1 0.8 1.8
61 15 19 1.5 2.1 0.8 1.9
59 15 22 1.5 2.0 0.8 2.1
64 16 20 0 0 0.8 1.9
64 16 16 1.6 2.1 0.8 1.8
61 15 19 1.5 2.1 0.8 1.9
59 15 22 1.5 2.0 0.8 2.0
62 15 23 0 0 0
58 15 22 1.5 3.7
3010
2520
2460
2500
2750
3060
2250
380
2690 2960 210
2610
390
2450 2510 260
180
550
450
490
10 30 51
10 120 237
75 100
55 80
10 120 39
13 210 18
81 230 50
37 200 52
11
52
11
14
13
15
14
8
15
15
0
500 lb)
Mullen burst, psi a
Tested 11-14 days after film preparation.
b
PVC
=
poly(viny1 chloride); DOP
=
dioctyl phthalate.
I n d . Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1 , 1974
91
Table 111. Weather-Ometer Data on Coated Starch-PVA Film Coating
Film compn," % Wt
TYPeC
%
PVC-DOP PVC-DOP Saran Saran
8 18 5 8
GlyStarch cero1 54 63 54 54
25 19 25 25
PVA 16 13 16
16
Weather-
Ometer time,b hr 60 120 40 80
Each film also contained CH20, "&I, and Tween 80. Weather-Ometer, Atlas Electric Devices Twin Arc Model DMC-HR; operated on a cycle of 102 min of light only and then 18 min of light and water spray; black panel temperature 63"C; hours represent time when significant deterioration was observed. c Coating solutions were 7.5 % PVC-5% DOP in methyl ethyl ketone (MEK) and 7.5% Saran in MEK; higher weight per cent values were obtained by double coating. glycerol are evidenced by improved stretch. The MIT fold and Mullen burst for the films are quite low; however, they are believed acceptable for mulch applications. Where strength and related properties are important, the ratio of PVA to starch should be increased. For our study, we used the minimum amount of BVA to reduce the total cost and to improve the potential biodegradability of the films. The amount of coating-5-10% by weight-allowed the films to withstand Weather-Ometer conditions for 40-60 hr. Based on the weight difference between coated and noncoated films, the data in Table I11 show that films with 5-10% coating will remain in good condition after 40 hr in the Weather-Ometer. Films with 15-20% coating
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Ind. Eng. Chem., Prod. Res. Develop., Vol. 13, No. 1, 1974
were resistant to the same conditions for about 100-120 hr. These data suggest that the life of films when exposed to rain and summer sun can be controlled by the amount of coating applied. We are not aware of any direct correlation between Weather-Ometer data and field testing of mulch film. However, as a general average for some materials about 300 hr in the Weather-Ometer is equivalent to 1 year's exposure in the central portion of the North Temperate Zone (Atlas, 1960). The preferred mulch life depends upon the individual crop. For tomatoes we estimate that the film should remain intact for 3-4 months; for lettuce, radishes, and other quickly maturing crops with a one-time harvest, the film needs to last only a few weeks. In summary, techniques were developed for preparing starch-derived films that have potential application as agricultural mulch. Coatings provide resistance to water and adverse effects of humidity changes during storage. Decomposition of the coatings during weathering will expose the starch moiety to soil microorganisms for further decomposition. Literature Cited Albregts, E. E., Howard, C. M., HortScience, 7 (6),568 (1972). Albregts, E. E., Howard, C. M., HortScience, 8 (l),36 (1973). Atlas Electric Devices Co., Chicago, Ill. "Atlas Weather-Ometers Bulletin," 1960. 45,49(1971) Chem. Week, 109 (23), Chem. Week, 110 (7),44 (1972). Lloyd. N. E., Kirst, L. C., Cereal Chem., 40, 154 (1963). Major, C J., Mod. Packag., 36 (6),119 (1963). Sweeting, 0.J., "The Science and Technology of Polymer Films," Vol. I, p 176,Interscience, New York, N . Y.. 1968.
Received for review August 30, 1973 Accepted September 26,1973 Mention of firm names or commercial products does not constitute an endorsement by the U. S. Department of Agriculture.
