Class 1. The full eq 1 was used to represent all lubricant properties with X3 = molar concentration of C g (cf. Table
11). Class 2. The full eq 1 was used to represent all lubricant properties with X3 = molar concentration of C8 (cf. Table 111). Class 3. The full eq 1 was used to represent all lubricant properties, except -40 OF viscosity, with X3 = molar concentration of (C, Cg). A more limited model had to be used for the -40 O F viscosity representation because only 7 data points were obtained (cf. Table IV where four of the lubricants became solid a t -40 O F ) , and there are 10 coefficients to be estimated in eq 1. The limited model does not include the terms with coefficients B13, B23, B1l,or B 3 3 .
Literature Cited Draper, N. R., Smith, H. "Applied Regression Analysis", Why, New York, N.Y., 1966. Gunderson, R. C., Hart, A. W., "Synthetic Lubricants", pp 388-399, Reinhold, New York, N.Y.. 1962. Niedzielski, E. L., lnd. Eng. Chem., Prod. Res. Dev., 15, 55 (1976).
Received for review June 3 , 1977 Accepted August 16,1977
+
Presented a t the Division of Petroleum Chemistry, 174th National Meeting of the American Chemical Society, Chicago, Ill.August , 1977.
Biodegradable Films from Starch and Ethylene-Acrylic Acid Copolymer Felix H. Otey, Richard P. Westhoff, and Charles R. Russell' Northern RegionalResearch Center, Agricultural Research Service,
U S .Department of Agriculture, Peoria, lllinois 6 1604
Compatible mixtures containing up to 90% starch and ethylene-acrylic acid copolymer were milled or cast from aqueous dispersions into flexible, nonsupported films without the aid of conventional plasticizers. These films were water resistant and appeared to have acceptable physical properties for a variety of packaging and agricultural mulch applications. Cast films, with up to 50% starch and 2 % paraformaldehyde, resisted outdoor weathering for more than 2 months.
Introduction Recently starch has gained a more favorable position as a raw material in film production because of the demands for biodegradable films and because of increasing prices and decreasing availability of conventional film-forming resins. Starch films have a very low oxygen permeability, which makes them attractive for food packaging where flavor retention is important. Starch is potentially useful for making agricultural mulch films because it degrades into harmless products after contact with soil microorganisms. While whole starch has been investigated for many years as a raw material for nonsupported films, these efforts have only recently begun to show industrial promise. Griffin (1973) developed a process that is now used industrially in England for incorporating up to about 10%starch filler into polyethylene (PE) films. A water dispersion of corn starch and a plasticizer, such as glycerol, can be cast into a clear flexible film. However, these films have no industrial potential, as nonsupported films, because they deteriorate in water and become very brittle a t ambient conditions. We found that film quality is greatly improved by adding polyvinyl alcohol (PVA) to the starchglycerol formulation, especially when a water-resistant coating was applied (Otey et al., 1974). Since early 1975,one company has been using combinations of modified starch and PVA to produce water-soluble bags. In the present study, we were interested in eliminating water-soluble plasticizers from starch-based films with the aims of improving their water resistance and preventing them from becoming brittle with age. Special emphasis was given to the possible application of starch films as agricultural mulch
and as packaging. For these applications the films must have good flexibility a t ambient conditions, good water resistance, and when used as a mulch film, they must biodegrade rapidly when plowed into the soil. Various attempts to use waterinsoluble plasticizers were not successful. However, considerable success was achieved by blending starch with an ethylene-acrylic acid copolymer (EAA) both as dry mixtures and as aqueous dispersions. Films made from this system have better water resistance and mechanical properties than any starch films previously observed. Maxwell described the use of EAA-starch (1970) and EAA-PVA (1971) combinations as paper sizes to impart water resistance.
Materials Three forms of EAA obtained from Union Carbide Corp. were evaluated: PCX-300, dry pellets; PCX-140, a mechanically produced latex containing 40% EAA; and PCX-100, a 25%ammonium salt solution of EAA. The EAA, as illustrated below, has a carbon backbone with occasional pendant carboxyl groups that can react with alkali to solubilize the resin.
I
OH
I
OH
If the alkali used to prepare the dispersion is ammonium hydroxide, a water-resistant film is obtained upon drying the Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
305
Table I. Properties of Starch-Ethylene Acrylic Acid Copolymer Films from Aqueous Dispersions Composition, % Stearic Starch EAAa acid 100 86 20 76 30 66 40 56 50 46 60 37 70 29 80 19 90 9 Std. dev. between runs 0 10
0
10 20 30 40 50 60
100 90 80
70 60 50 40
0
4 4 4 4
4 3 1 1 1
0 0 0 0 0 0 0
Runs,b no. 2 5 5 4 5 5 7 5 2 1 1 1
1 1 1 1 I
Mean thickness, mils
Tensile strength, Psi, mean
Starch and PCX-100 3.3 3288 3.1 2395 2.3 2483 2.1 2525 1.9 2528 2.2 3003 2.0 3186 1.5 3933 1.5 5345 1.5 7190 583 Starch and PCX-140 2.6 3336 2.5 3737 1.9 3898 1.6 3969 1.3 4077 1.2 4791 1.2 4993
Elongation, %,
mean 295 181
116 56 44 35 21 10 10
15 18.8
349 319 226 139 87 51 44
Foldingc endurance, no. double folds
Burstd factor
Fungie resistance
No break 5027 2972 1262 531 420 349 194 61 94
21.8 18.4 20.7 22.2 21.7 18.8 20.6 19.9 15.0 17.9
0 0 2
-
-
-
4 4
4 4 4 4 4 0
3 4 4 4 4 4
Dry weight basis of ethylene-acrylic acid copolymer. Number of separate film preparations evaluated for tensile and elongation properties. c Average of 10 specimens from one film preparation. d Average from two film preparations; g/cm2 per g/m2. e ASTM D 1924-63; mold coverage: 0 = none; 2 = 10-30%; 3 = 30-60%; 4 = 60-100%.
dispersion. We used an EAA that contained about 20% reacted acrylic acid. It was produced in limited quantities and was not available for large-scale evaluations. Other dispersible EAA copolymers are available and are being evaluated in starchfilm studies. Commercial grade corn starch was used in this study.
Experimental Section Films from Aqueous Dispersions. In a round-bottomed flask were mixed the desired amounts of air-dried corn starch, PCX-100, enough water to equal 10 times the weight of starch, and 5 to 10% stearic acid based on the dry weight of EAA. The total solids concentrations ranged from 22% for the 10% starch films to 10% for those with 90% starch. The stearic acid was added to reduce the viscosity of the mixture after heating. In some instances, paraformaldehyde was added to the formulation to reduce the rate of biodegradation of the films. The mixture was then heated by rotating the flask in an 80-90 "C water bath for 1h. Finally, the dispersion was cast at a 30-mil wet thickness onto silicone-coated plate glass, preheated to 80-90 "C, and dried in a forced-air oven at 120 "C for 5 min. When the starch level in the dry film was greater than 60%, the cast dispersions had to be dried at 50-60 "C to prevent splitting due to film shrinkage. The resulting dry, clear film was stripped from the plates and equilibrated at 50% relative humidity (RH) prior to testing. Similar films were also prepared from PCX-140 and starch by this same procedure, except that the stearic acid was omitted. The amount of added water was increased to about 30 times the weight of starch to avoid excessive viscosities or coagulation of the latex. The total solids concentrations for these preparations ranged from 15% to 5%, respectively, as the starch level in the films was increased from 10% to 60%. Films from Dry Mixtures. Two techniques were evaluated for producing films from dry mixtures of starch and EAA. In one method, dry starch and PCX-300 were fluxed on a rubber mill at 250-270 "F for 10 min and then pulled from the rolls as a thin sheet or film. 306
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
In the other method, cross-linked starch xanthide and the EAA were coprecipitated from a water dispersion of starch xanthate and PCX-140 latex. In a 2-L beaker was mixed 162 g (dry basis) of starch, 1200 mL of water, and 250 mL of 2 N NaOH. After stirring to produce a uniform gel and cooling to 20 "C, about 6 mL of carbon disulfide was added with stirring. The product was a clear viscous solution containing about 0.07 xanthate group per anhydroglucose unit. In one example, 308 g (30 g dry basis) of this starch xanthate solution was combined with 600 mL of water and 75 g (30 g dry basis) of PCX-140. Then 4.2 g of NaN02, dissolved in 50 mL of water, was added and the solution was stirred for 15 min. Then dilute HzS04 was added to a solution pH of 1.4 which caused starch xanthide and EAA to coprecipitate. The coprecipitate was separated by filtration, washed until neutral with H20 and then three times with methanol, and finally dried in a forced-air oven a t 50 "C. The dried product was then blended on a rubber mill at 250-270 O F for 10 min and pulled from the rolls as a thin sheet or film. The film contained about 50% each of starch xanthide and EAAa. Test Methods. Films were evaluated for the following physical properties: tensile strength and percent elongation using either the Instron or Scott testers; folding endurance, ASTM D 2176-69 using the MIT Tester with 0.5 kg of tension; burst factor, TAPPI Standard T 220 os-71; and tear resistance, ASTM D 1004-66. Fungi resistance was conducted according to ASTM D 1924-70 using a mixed fungus spore suspension containing Aspergillus niger, Penicillium funiculosum,Trichoderma sp., and Pullularia pullulans. Water sensitivity was determined by repeatedly soaking l 1 2 - h . wide strips of the films in water for 24 h followed by air drying the films for 24 h and then repeating this cycle for 13 times. Outdoor exposure studies were performed by laying 8 X 18 in. samples of the films over tilled rows of soil and covering about 4 in. of each end with soil. These studies were conducted from August to December of 1976. If no rain fell during any week, the samples were sprinkled with enough water to equal 'I2 in. of rain.
Table 11. Properties of Films Made by Fluxing Dry Mixtures of Starch and Ethylene-Acrylic Acid Copolymer on a Rubber Mill Film composition, 96 Starch EAAa 0 10
100
20 30 40 60
80 70 60 50 40
0
100
20 40 60
80 60 40
50
90
Film thickness, mils
Tensile strength, psi
Elongation, %
Tear resistance, lb/mil
Coprecipitated Mixtures of Starch Xanthide and PCX-140d 4.6 7471 43 5.6 5026 58 2.6 5463 82 3.2 3603 74 5.8 2873 59 5.0 2558 15 4.1 2272 6 Dry Mixtures of Starch and PCX-300e 3.4 6699 48 4.7 4519 48 5.5 1897 53 5.4 2685 21
0.77 0.66
0.64 0.52 0.46 0.18 0.07 0.61 0.54 0.43 0.45
FungiC resistance 0 0 0
3 4 4 4 0 0- 1 4 4
a Dry weight basis of ethylene-acrylic acid copolymer. ASTM D 624. c See footnote e , Table I. Average data from two separate film preparations. e All data based on one film preparation for each entry.
Results a n d Discussion Cast Films. Properties are listed in Table I for films containing up to 90% starch prepared by casting water dispersions of starch and EAA. Uniform films were obtained from combinations of starch and either the ammonium salt solutions of EAA (PCX-100) or the EAA latex (PCX-140). In the presence of stearic acid, the PCX-100 could be formulated at higher solids concentrations and, in general, appeared to be the more promising of the two dispersions for starch-film production. Tensile strengths and percent elongations were determined after the films aged 24 h at 50% relative humidity, whereas the fold and burst testa were made after the films had aged several days. As the level of starch was increased, the films became less flexible as evidenced by decreasing percentage elongation and fold endurance. Yet, aged films with 90% starch and 9% EAA could be creased without evidence of fracture. Films containing up to 80% starch were judged to have sufficient strength and flexibility for a variety of packaging applications. Tensile and elongation properties of the PCX-100 films containing 10-70% starch were again measured after the films were aged for 125 days at 50% relative humidity. During that time, tensile strengths increased an average of only 20% and percent elongation decreased 30%. Since cast aqueous dispersions of pure starch become very brittle upon drying, we believe that the EAA in these films is retarding extended starch chains from associating to form highly crystalline, brittle regions. Samples of the starch-PCX 100 films, listed in Table I, were repeatedly soaked in water for 24 h, then air dried for 24 h for a total of 13 times. They all became slightly less transparent which indicates some retrogradation of the starch. Those films with 10-40% starch remained very flexible and strong, those with 50-70% starch remained intact but showed an increasing loss of strength and flexibility, while those with 80 and 90% starch were brittle. After the first 48-h cycle of soaking and drying, films containing 10-20% starch shrank about 1%in length, and those with 30-60% starch shrank about 4%.Generally, starch films are destroyed after a few minutes of water soaking; hence, the EAA greatly improved the water resistance of these starch-based films. Duplicate samples of starch-PCX 100 films, listed in Table I, that contained 30,40,50,70, and 90% starch were exposed to outdoor soil contact, with the ends of the film buried in soil, to observe their resistance to sunshine, rain, and soil microorganisms. Those samples with 90% starch were badly torn
within 1-3 days after water soaking, primarily because of embrittlement and shrinkage upon drying. Heavy mold growth occurred within 3-5 days on the buried portion of these samples. Similar deterioration was observed for samples containing 50 and 70% starch, but not until about 5-7 days of exposure. Samples with 30 and 40% starch remained flexible and intact for at least 30 days. Deterioration of the latter films was attributed primarily to microbial attack, as evidenced by heavy mold growth, rather than to embrittlement and shrinkage. Samples formulated with 50 and 70% starch and 2% paraformaldehyde demonstrated much better resistance to microbial attack. Those with 70% starch and 2% paraformaldehyde again became somewhat brittle and had small tears due to shrinkage within 5-7 days, but they continued to provide soil coverage for about 15 days; however, those with 50% starch and 2% paraformaldehyde remained flexible and provided good soil coverage throughout the test period of 70 days. As long as the films remained in good condition, the soil under them was moist even when the surrounding area became dry. Within a few weeks after the exposed surface of the films became severely damaged, the buried portion had degraded into small particles. These preliminary soil exposure tests demonstrated that the starch-EAA films are biodegradable. Furthermore, some control in the rate of biodegradation can be built into the films by varying the amount of starch used and by adding a fungicide such as paraformaldehyde. The simulated field tests suggest that the films could have application as biodegradable agricultural mulch. The preferred life of a mulch film depends upon its application. While some farmers have expressed a need for a mulch that will degrade in 2-3 weeks, others require one that will last 4 months or longer depending upon the climate and types of crops grown. Milled Films. Starch and EAA can also be formed into quality films by dry mixing on a rubber mill or by coprecipitating starch xanthide and the copolymer (PCX-140) from aqueous dispersions and then drying the precipitate and fluxing it on a rubber mill (Table 11). Either method yields films with sufficient strength and flexibility for a variety of applications; however, a lower level of starch must be used to produce quality films than was possible by the casting technique. The coprecipitates containing up to 50% starch yielded clear and flexible films, but when the starch level was higher than 50%, the resulting films were translucent and became brittle with aging. Dry mixing of up to 60% powdered starch and 40% PCX-300 yielded films that remain flexible with age, Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 4, 1977
307
and a higher percent elongation than the corresponding milled films. In addition to processing effects, scanning electron micrographs (SEM) of film surfaces suggest that the starch in the dry-milled samples (Figure l a and l b ) is serving more as a particulate filler, whereas in the cast samples (Figure IC,Id, l e ) i t is a continuous film blended uniformly with the EAA film. The starch in films made by fluxing 209b powdered starch and 8% dry PCX-300 (Figure la) appears as large individual particles held in the EAA matrix. Fluxed coprecipitates, containing 50%starch xanthide and 50% EAA, yielded films (Figure l h ) with a very rough surface, and the starch appears to be dispersed as small particles in the matrix. In contrast, the cast films made with 50% starch and EAA from PCX-100 (Figure ICand Id) or from PCX-140 (Figure le) are fairly smooth with no clearly identifiable particles within the matrix. While the stearic acid, shown blossoming on the surface in Figure IC,is desirable to reduce the formulation viscosity, it was omitted from the film in Figure Id to allow better examination of the starch-PCX 100 film surface. This surfacing characteristic of stearic acid probably explains why the acid is a good mold release agent for cast films.
Figure 1. Surfaces of starch-ethylene acrylic acid copolymer (EAA) films as seen with a scanning electron microscope (SEMI: (a) dry milled film with 20% starch and 80% EAA (b) dry milled film with 50% starch xanthide and 50%EAA from PCX-140; (c) cast film with 50% starch; 4% stearic acid, and 46% EAA from PCX-100; (d) same as (c) without stearic acid; and (e) cast film with 50%starch and 50% EAA from PCX-140. but those with more than 30% starch were nearly opaque. These dry systems appear to have melt properties that would allow them to be extruded into films. However, because of the limited quantity of material available, we were not able to establish the feasibility of using extrusion equipment. Preliminary runs in a Brabender extruder suggest that composites of the EAA, starch, and a small amount of water can he extruded into quality films. Comparison of Cast Films with Milled Films. The properties of films made by casting water dispersions of starch and EAA (Table I) were significantly different from those made by fluxing the dry formulations on a rubher mill (Table 11).The milled samples became weaker with increasing starch levels, while the cast films hecame somewhat stronger with increasing amounts of starch. Higher strengths for milled controls and lower starch-level milled films compared to the corresponding cast films are, in part, attributed to effects caused by the mill rolls, possibly film orientation. Moisture retained by the cast films apparently serves as a plasticizer and causes the lower starch-level films to have less strength 308
Ind. Eng. Chem.. Prod. Res. Dev.. VoI. 16, No. 4, 1977
Conclusions Starch was formulated into water-resistant, flexible films without the aid of conventional plasticizers. The reason that these films remain flexible with aging is not clearly understood. However, it seems possible that starch molecules are expanded and quite flexible when first cast from an aqueous dispersion, but upon drying they contract and various bonding forces cause them to become brittle. The added EAA may he associating with the starch molecules enough to bold them in their expanded, flexible state. As the amount of EAA is decreased, this association can be partially disrupted, especially with age or in the presence of a solvent such as water. However, with about 50%EAA, these starch films remain flexible even after repeated soaking with water. Although conventional plasticizers such as glycerol help starch films to retain some flexibility, they are readily leached out with water. These films appear to have acceptable physical properties for a variety of packaging and agricultural mulch applications, and they are readily attacked by soil microorganisms. This microbial attack can be controlled to some extent by varying the amount of starch and by adding a fungicide such as paraformaldehyde. While casting is an acceptable commercial technique for film production, we believe that additional studies are needed to establish the feasibility of producing films from starch and a commercially available EAA copolymer, using an extrusion technique. Acknowledgment We thank F. L. Baker for preparing the scanning electron micrographs. Mention of firm names or trade products does not imply that they are endorsed or recommended by the US. Department of Agriculture over other firms or similar products not mentioned. Literature Cited Griffin,G. J. L., Am. Chem. Soc.. Div. Org. Coatings Piasf. Chem., Pap., 33 (2). en O L , < O 7 1 >
Maxwell. C. S..Tappi, 53 (8). 1464-1466 (1970). Maxwell, C. S.. Tappi, 54 (4). 568-570 (1971). Otey, F. H., Mark, A. M.. Mehltretler, C. L.. Russell, C. R.. ind Eng. Cham.. Pmd. Res. De”.. 13, 90-92 (1974).
Reeeiued for reuiew June 8,1977 Accepted August 1,1977 Presented at the Symposium “NewConcepts in Coatings and Plastics Chemistry,” Division of Organic Coatings and Plastics Chemistry, American Chemical Society, Chicago, Ill., 1977.
