284
Ind. Eng. Chem. Prod. Res. Dev. 1904, 23,284-287
Starch-Based Films. Preliminary Diffusion Evaluation Fellx H. Otey' and Rlchard P. Westhoff Northern Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture. Peoria, Illinois 6 1604
Highly transparent films were prepared from compositions of up to 60% starch and various levels of poly(ethylene-co-acrylic acid) and strong alkali. Preliminary diffusion studies revealed that the films have promising semipermeable characteristics. Observed permeabilities of six solutes were determined for four separately prepared films by use of a rotating dialysis cell. After 1 h exposure of 1.5 % solute concentration (w/w), urea diffused through one film 7.6 times faster than glucose and 32 times faster than sucrose. Increasing starch levels or incorporating water-soluble compounds into the films significantly increased diffusion rates.
Introduction During a study designed to develop biodegradable plastic film from starch, we found that certain formulations yield semipermeable films that may have application in dialyzers. This paper describes the preparation and preliminary evaluation of these films. Although cellulose-based films have achieved prominence as semipermeable membranes, starch-based films have not been available that would withstand prolonged exposure to water. Starch and cellulose are high polymers composed of D-glucopyranosyl units. They differ in weight and in the manner in which the glucopyranosyl units are joined together. Cellulose is a linear polysaccharide consisting of 6000 to 8000 1,4-linked fl-D-glUCOpYranOSylunits. This p- 1,Clinkage permits molecular alignments yielding crystals or microfibrils with the potential for strong hydrogen bond interactions to produce good film-forming capabilities and high resistance to gelatinization in water. In contrast, most common starches contain 17-27% linear polysaccharide (amylose) consisting of 400 to 1000 1,4linked a-D-glucopyranosylunits, and the remaining composition is a branched molecule (amylopectin) having 10000 to 40 000 1,4- and 1,6-linked a-D-glucopyranosyl linkage. Because of the a-l,6linkage, the amylose molecules assume a spiral or helical shape having six glucose units per spiral. Starch readily disperses in hot water to form starch pastes possessing unique viscosity characteristics and film-formingbehavior. However, such films are very brittle upon drying and are very sensitive to water. Recently, we reported the first technique for formulation and extrusion blowing of films with up to 60% corn starch, and they can withstand prolonged exposure to water (Otey et al., 1980). Extrusion blowing is the most common and economical method for producing film, in which a tubular extrudate is expanded and shaped by internal air pressure to form a continuousbubble several diameters greater than the die opening. The successful preparation of blown starch-based films was achieved by combining starch, poly(ethy1ene-co-acrylicacid) (EM),ammonium hydroxide, and water, and then heating with stirring until the starch was gelatinized and a uniform mixture was obtained. Films made from this formulation had no observed permeability characteristics. When a strong alkali, such as sodium hydroxide, was used in place of ammonia, we found the films to be much more transparent and quite permeable to small molecules. Permeability characteristics of the films were measured with a rotating dialysis cell prepared as described by Regan et al. (1968) and Wendt et al. (1971).
Experimental Section Film Preparation. Extrusion blown films were prepared with various levels of starch as reported earlier, but sodium hydroxide was used in place of ammonia (Otey et al., 1980) (Table I). A mixture of air-dried corn starch (11% moisture) and enough water to equal the total solids in the final composition and dissolved sodium hydroxide were blended for 2-5 min at 95 to 100 "C in a steam-heated Readco mixer (type: 1-qt lab made by Read Standard Div., Capitol Products Corp.) to gelatinize the starch. EAA pellets (type: 2375.33 manufactured by Dow Chemical Co.) were added while heating and mixing were continued. Other materials such as polyvinyl alcohol and plasticizers such as glycerol and various sugars can be blended into the dough. After the mixture was stirred and heated for a total of about 45 min, the resulting doughlike product was extrusion processed with an extrusion head attached to a Brabender Plasti-Corder (type: PL-V300, manufactured by C. W. Brabender Instruments, Inc.). The dough was first extruded at a barrel temperature of 100-105 OC through a die having 24 holes of 1/32 in. diameter until the moisture content of the extrudate was about 5 to 10%. The extrudate was then blown into a film by passing it through the same extruder, but the die was replaced with a heated 1/2-in.blown film die. The barrel and die temperature ranged from 105 to 110 "C. Both wet and dry tensile measurements were made on the films with an Instron. Twenty 0.635 X 10 cm strips were cut from each film. Ten were used for dry tensile measurements; the other 10 were soaked for 4 h in water, lightly blotted with paper towels, and their length and weight changes and tensile properties were measured while wet. Solute Analyses. Diffusion rates of solutes through the membranes were followed by colorimetric analyses for individual runs of urea (Potts, 1963) and sugars (Hodge et al., 1962). Sodium chloride concentrations were measured by atomic absorption analysis of sodium on a Varian Techtron AA120 spectrophotometer. T h e Rotating Dialysis Cell. Cells were made in our laboratory from Plexiglas, as shown disassembled in Figure 1. Each half of a cell was prepared by cutting a round 9.4 cm diameter hole from a 0.6 cm thick Plexiglas acrylic disk (13 cm diameter); this disk was then laminated to the same diameter Plexiglas disk, which was 1.3 cm thick. A Plexiglas rod (3.5 cm diameter and 3 cm long), glued to the outside center of each cell half, was used for attaching the motor chuck. Two filling holes drilled 180" apart on the edge of each compartment were provided with
This article not subject to U.S. Copyright. Published 1984 by the American Chemical Society
Ind. Eng. Chem. Rod. Res. Dev.. Vol. 23. No. 2. 1984 285
Table 1. Film Formulations and Roperties ~
film no.
10 11 60 62d
formulationo starch, EAA, NaOH, 90
90
60 40 40 40
40 60 60 52
tensile strength, ~
pph
1.5 5 5 5
kglcmab dry wet
elongation, % b dry wet
255 238 212 198
7.6 50.4 36.9 35.1
24.4 44.9 41.3 44.5
27.5 48.3 57.1 45.0
effect of water soakingb wt' length increase, increase, wte 5% 12.4 9.5 7.5 8.4
%
loss, 96
58.9 40.4 54.9 33.8
7.5 1.8 1.8 8.7
Based on dry weight exclusive of water. NaOH given in parts per 100 parts formulation. Dry measurements made after ambient aging, 10 days for no. 10 and 1 year for the other 3. Wet measurements made after soaking in water for 4 h. All values are average of 10 specimens (0.64x 10 em). Wet strengths based on dry crosa-sectional areas; all samples broke at jaws. Weight increase determined on wet, blotted film. Weight loss determined after films had air dried for 24 h. Contained 2% glycerol and 6% glycol glucoside.
Figure 2. The rotating dialysis cell, assembled Figure
1. The rotating dialysis cell. disassembled.
threaded plugs fitted with O-rings. Four-inch O-rings were fitted into grooves machined in the face of each compartment to seal the membranes clamped between the two halves. The maximum available volume in each compartment was 46 em3,and the area of exposed film, measured acrw contact with the large O-ring (100mm diameter), was 78.5 cm2. Wendt et al. (1971)found that the entire measured area of the membrane should be used to calculate observed penneabilities (Paeven though the cell is only partly fad. Apparently, the diffusion process continues due to a thin film of solution being carried over the arc above the solution during rotation of the cell. The thickness of air-dried film specimens measured a t 9 locations was reported as average, minimum, and maximum thicknesses. The film was soaked in water for several hours and then clamped while wet between the two cell halves. The cell then was attached to a motor chuck, and the complete assembly was clamped in a horizontal position (as shown in Figure 2). A weighed syringe was used to introduce about 34 cm3 of water into one side (B) and the same volume of solution into the other side (A). The filling holes were immediately plugged and the motor was started. The time that filling of compartment A commenced was assigned t , , and the time rotation began was tl. This usually required 15 s. Cells were rotated a t 73.5 rpm. After a given time, t3.the motor was stopped; for two of the films, the cells were dumped to terminate the dialyses. For the other two films, 2.5 cm3 of solution was removed from each compartment for analysis. The time when withdrawal was complete was designated t,. Diffusion time was calculated from the equation (1) t = (t3+ t J / 2 - (t2 + t,)/2 However, for most solutes tested the diffusion raten were
slow enough that averaging filling and withdrawal times waa not essential for data accuracy. Po was calculated from the equation
Po= V In (ACo/AC)/2At
(2)
where AC,/AC is the ratio of the concentration differences initially and at time t , A is the membrane area (78.5cm2) through which transport took place, and Vis the volume of liquids (about 34 cm3) in each cell (A = €0. For those runs on films 10 and 11 (Table I), where we chose to sample the solutions periodically to determine effect of time on dialysis, as soon as the 2.5-cm3 sample was withdrawn, rotation was again started for subsequent values on the run. Usually, three or four samples were analyzed from each of these runs a t various times. Only side B was analyzed, and the amount found in side B was subtracted from original solution concentration to determine concentration of solution side at any time. This method gave more reproducible results, because side A was 80 much more concentrated than B that large dilutions for small samples of A gave poor material balance. Results and Discussion Film Preparation and Properties. Film composition and properties are listed in Table I. As the level of starch was increased, extrusion blowing became increasingly difficult. For the available equipment, the maximum starch level was about 60% and the preferred level was less than 50%. All of the films expanded and imbibed considerable water upon soaking,however, the expanded f h s appeared to remain stable in the dialysis cells with water for several weeks with no apparent distortion or change in appearance. Weight losses during water soaking indicate that the sodium hydroxide, in excess of that needed to neutralize the
286
Ind. Eng. Chem. Prod. Res. Dev., Vol. 23, No. 2, 1984
Table 11. Permeability Data for Several Solutes ~
urea
NaCl
glucose
fructose
sucrose
raffinose
15.17
15.09
concn, mg/cm3 15.09
15.03
15.06
15.01
Film No. 10 60% Starch Thickness, Mil: Max, 3.4; Min, 1.8; Av, 2.5 Po,cmls x lo6 32.8 31.6 6.27 10.0 2.9 no. of experiments 4 4 3 3 2 std dev,P, x l o 6 0.37 1.7 0.42 1.1 1.1 slope eq 3a x IO4 83.8 53.1 10.7 10.9 5.18 slope eq 4b x i o 4 82.9 49.6 9.9 15.8 4.62 %, 1st h, calcdC 30.3 21.5 5.3 7.9 2.5
P o , cm/s x IO6 no. of experiments std dev, Po x lo6 slope eq 3= x IO4 slope eq 46 x i o 4 %, 1st h, calcdC
Film No. 11 40% Starch Thickness, Mil: Max, 2.0;Min, 1.8; Av, 1.9 12.0 7.15 1.46 1.05 0.341 4 3 4 4 3 0.031 0.367 0.067 0.094 0.006 21.1 11.9 2.33 1.63 0.54 18.9 11.2 2.30 1.65 0.54 10.3 6.18 1.36 0.93 0.32
P o , cm/s x l o 6
Film No. 60 40% Starch Thickness; Mil: Max, 3.2; Min, 2.4; Av, 2.7 11.3 2.5 0.96
2.5 3 0.25 3.61 3.91 2.0 0.078 3 0.001 0.12 0.12
__
Film No. 62 40% Starch; 6% Glycol Glucoside; 2% Glycerol Thickness, Mil: Max, 3.0; Min, 2.0; Av, 2.3 P o , cm/s x lo6 21.6 4.1 2.0 1.1 a Slope of In AC,/AC vs. T / V fitted by eq 3. Slope of lines in Figure 3 fitted by eq 4. diffuse in 1 h, calcd from P o . Slope of lines in Figure 4 fitted by eq 4.
% of initial solute that would
200
'
80
-
I
/
/ I
i
I
/
i
C
x x
:I
