Ind. Eng. Chem. Res. 1994,33, 1821-1827 1821

Biodegradable Plastic Made from Soybean Products. 1. Effect of Preparation and Processing on Mechanical Properties and Water Absorption?

Inke Paetau, Chin-Zue Chen,t and Jay-lin Jane' Department of Food Science and Human Nutrition and Center for Crops Utilization Research, Iowa State University, Ames, Iowa 50011

Preparation and processing conditions for making biodegradable plastics from soy isolate and soy concentrate were explored. Soy isolate and concentrate, as well as acid-treated soy isolate and soy concentrate, were compression molded at various moisture levels and molding temperatures. Hydrochloric acid, sulfuric acid, acetic acid, and propionic acid were examined for their suitability for treating soy protein with regard to final properties. The molded specimens were tested and calculated for their tensile and yield strength, percent elongation, Young's modulus, and water absorption. The pH of the molding material was crucial with respe,ct to the water absorption of the plastic, with an optimum pH around the isoelectric point of the protein (pH 4.5). The plastics obtained were rigid and brittle with tensile strength values from 10 to 40 MPa, 9e ld strength values from 1.0 to 5.9 MPa, elongation values from 1.3% to 4.8%, and water absorption values from 30% to 167% weight after 26-h submersion in distilled water a t 25 "C. Plastic specimens made from soy concentrate displayed similar tensile strength but greater water absorption compared with plastics made from soy isolate.

Introduction

Although useful and desirable for many purposes, the indestructibility of petroleum-based plastic is a growing concern because of its accumulation in the environment. The development of biodegradable plastics, which degrade in the environment by means of humidity and the action of microorganisms, is needed as one alternative to help solve solid-waste-related environmental problems.

A great deal of research on soybean plastics was conducted in the 1930s and 1940s. Petroleum was expensive, whereas soybeans were abundant. At that time, soybean products were incorporated in phenolic resins mainly as a filler or extender to decrease the cost of the plastic (Brother and McKinney, 1939; McKinney et al., 1943). Decreasing petroleum prices and better performing, petroleum-based plastics dominated the market after World War 11.

To improve biodegradation rates, several products have been studied that use starch or other biodegradable materials as fders in petroleum-based plastics (Evangelista et al., 1991; Fanta et al., 1992; Griffin, 1974; Lim et al., 1992; Otey and Westhoff, 1979; Westhoff et al., 1974). The amount of inherently biodegradable polymers, such as poly(hydroxybutyrate), poly(1actic acid), or starch-based plastics, is close to 1.5 billion kg/year; these polymers are 100 % biodegradable in soil and are water-soluble (Lindsay, 1992). Other biodegradable plastics made from starch and other polymers, such as poly(viny1 alcohol) or proteins, are suitable €or manufacturing extrudedor molded articles (Jane et al., 1993; Lay et al., 1992; Nakatsuka et al., 1978; Otey et al., 1987). A starch-protein thermoplastic com- position with fairly good water resistance was reported (Lim and Jane, 1993).

Plant protein is a readily available, renewable, and biodegradable polymer. Among plant protein sources, soy

* Corresponding author. 'Journal Paper No. 5-15348 of the Iowa Agriculture and Home

$Current address: Department of Engineering Technology, Economics Experiment Station, Ames, IA. Project No. 2863.

Austin Peay State University, Clarksville, TN 37047.

0888-5885/94/2633-1821$04.50/0

protein is relatively low cost with vast available supplies. Commercially available soy protein products include soy isolate, soy concentrate, and soy flour. Soy isolate, prepared by precipitation at pH 4.5, consists of more than 90% protein (Kinsella, 1979; Salt et al., 1982; Wolf, 1970), whereas soy concentrate, prepared by eluting soluble components from defatted soy flour, contains more than 70% protein and about 18% carbohydrate. Soy flour contains about 56 % protein and about 34% carbohydrate (Kinsella, 1979). Approximately 90% of the proteins in soybeans are dehydrated storage proteins. Major com- ponents in the storage proteins are conglycinin (75) (35% ) and glycinin (11s) (52%). Both 75 and 11s proteins are of quaternarystructures. The 75 protein has nine subunits with an average molecular weight of 20 OOO Da, whereas the 11s has three acidic and three basic subunits of ca. 35 000 and 20 OOO Da, respectively (Catsimpoolas et al., 1971).

In this study we explored the suitability of soy protein products, such as soy isolate and concentrate, for manu- facturing molded specimens. Effects of preparation and processing conditions on the mechanical properties and water absorption of the molded plastics were examined.

Experimental Section Materials and Met hods Materials. Soy isolate (PRO-Fam) and soy concentrate

(ARCON S) were provided by Archer Daniels Midland (Decatur, IL). The reported protein contents for PRO- Fam and ARCON S were 90 % -91 % (dry basis) and 70 % - 71 % (dry basis), respectively. Both protein products were prepared by acid and neutralized before drying. Acetic acid, hydrochloric acid (HCl), propionic acid, and sulfuric acid were reagent grade and were used without further treatment (Fisher Scientific, Pittsburgh, PA).

Preparation of Acid-Treated Soy Protein. Soy isolate or concentrate was mixed with distilled water at a ratio of 1:6. The slurry was continuously stirred (Jumbo stirrer, Fisher Scientific, Pittsburgh, PA), and pH was monitored with a pH meter electrode placed in the slurry. Concentrated acetic acid, propionic acid, 1 N HC1, or 10% sulfuric acid solution was added by drops to the aqueous

0 1994 American Chemical Society

1822 Ind. Eng. Chem. Res., Vol. 33, No. 7,1994

soy protein slurry until the pH reached 4.5, the isoelectric point of soy protein. It was then centrifuged (Sorvall Superspeed RC2-B, 454lg, 10 min) to remove excess water. The remaining material was dried in a convection oven at about 50 "C until it reached 110% moisture, at which point it was ground (cyclone sample mill, UDY Corpora- tion, Fort Collins, CO). The ground material was sieved (no. 40, 0.42-mm opening), and the material's moisture content was analyzed prior to molding (Sartorius moisture analyzer (MA 30), Goettingen, FRG). To adjust the moisture content, the material was either subjected to additional drying in the oven or placed in a desiccator over water (95 5% relative humidity) to increase the moisture content. If large amounts of moisture were added, distilled water was sprayed onto the material. After sieving, the moisture was allowed to equilibrate overnight before the moisture content was determined.

Specimen Preparation. Type I specimens (ASTM standard D638-86, dumbbell shaped, overall length of 165 mm) were molded from 13.0 g of soy isolate or concentrate on a Wabash compression-molding machine. Soy isolate with 11.7 % moisture was molded at temperatures ranging from 80 to 160 "C. In another experiment soy isolate with different moisture contents, ranging from 7.1 % to 16.9 % , was molded at 125 "C. Soy concentrate with 11.2% moisture was molded at temperatures ranging from 100 to 160 "C. Soy concentrate with moisture levels ranging from 5 % to 15% was molded at 125 "C. The mold temperature during material filling was 150 "C. Molding was performed at 20.7 MPa for 6 min at different temperatures. After molding, the mold and specimen were cooled to 170 "C before the specimen was removed and allowed to continue cooling. Flash was carefully removed by sanding the edges of the specimen with grade 80 and 120 abrasive sandpaper.

Mechanical Property Tests. Tensile strength, per- cent elongation at peak, and yield strength at peak were measured by using an Instron Model 4502 testing system (Canton, MA), and Young's modulus was calculated. The test was performed according to the Standard Test Method for Tensile Properties of Plastics (ASTM D638-86). Specimens were preconditioned at 50 % relative humidity for 40 f 2 h. Five or more specimens for each treatment were tested at 5 mmtmin crosshead speed.

Water Absorption. The test applied was a modifica- tion of the ASTM standard test method D570-81. The tensile testing specimens also were used for water absorp- tion testing. Three specimens for each treatment were tested. The specimens, having the same surface-areal volume ratio, were conditioned by drying them in an oven (24 ha t 50 f 3 "C), cooled in a desiccator, and immediately weighed. The conditioned specimens were submerged in distilled water at 25 OC. After 2 h the specimens were removed, surface water dried with a paper towel, im- mediately weighed, and resubmerged. They were weighed again after another 24 h following the same procedure. Water absorption was calculated as a percentage of initial weight. The specimens were checked for soluble matter lost during immersion by weighing them following recon- ditioning (same time and temperature used in the original drying period). If any loss of water-soluble matter occurred, it was taken into consideration in the water absorption calculation.

Results and Discussion Soy Isolate. A. Effect of Moisture Level in the

Molding Material. Soy isolate with four different moisture contents, 7.1 9% , 10.0 % , 12.5 % , and 16.9 7% , was

I I I I- 4 1800

1 \'@I 1000 -i Young's Modulus v Yield Strength v Elongation 0 Tenslle Strength 2.

0 10 15 Moisture Content (%)

Figure 1. Effect of moisture content in the soy isolate molding material on the mechanical properties of the molded specimens, molded at 125 "C.

s 100 ; 1 4

0 10 15

Figure 2. Effect of moisture content in the soy isolata molding material on water absorption behavior of the molded Specimens, molded at 125 OC.

Moirture Content (%)

molded into plastic specimens at 125 "C. At this tem- perature, complete molding of the specimens took place and the tensile strength showed a maximum of 36 MPa at a moisture level of 12.5% (Figure 1). The percent elongation increased as water content increased in the molding material. At 16.9% moisture, the elongation of the plastic was 4.8%. Water acted as a plasticizer in the material. The extensibility of the plastic increased as the moisture content increased. This also was expressed in Young's modulus, which steadily declined as the moisture content increased. The yield strength showed a maximum of 4.9 MPa at a moisture level of 10.0%. The ability of the plastic to absorb water decreased as the moisture content in the molding material increased. The molded plastic specimens prepared with 7.1 % and 16.9% moisture in the molding material absorbed 133% and 98% water, respectively, after 26-h submersion (Figure 2).

It is plausible that at a low moisture content the material has a more porous matrix that allows water to readily penetrate. Protein gelation is possibly more complete and molecules are arranged more orderly at higher moisture contents. This results in a three-dimensional molecular network which makes it more difficult for water to enter.

Ind. Eng. Chem. Rea., Vol. 33, No. 7,1994 1829

b =-

Figures. Seanningelectronmicrographs (scale bar represents 30rrm)ofmoldedsoyisolateapecimenswith 16.9% moisturemntentinmold~ material (A), after 24-h submersion in distilled water (B). with 7.1% moisture (C), and after 24-h submersion in distilled water (D).

Scanning electron micrographs (Figure 3) revealed that specimens made with higher moisture contents displayed smoother surfaces. After submerging in distilled water for 24 h, these specimens also displayed many voids. The voids, resulting from blistering during molding, decreased the tensile strength of the specimens. B. Effect of MoldingTemperature. Soy isolatewith

11.7% moisture content was molded into specimens at five different temperatures. Starting at 80 O C , the temperature was raised in 20 O C increments to 160 OC.

of 140 OC. The elongation of the plastic increased up to 140 OC to a value of 4.6 % . Plastics molded at 140 and 160 OC were the strongest with a yield strength of 4.9 MPa.

These results may be attributed to the fact that high molding temperaturea increase the mobility ofthe polymer chains, which in turn enhance the flow properties of the material and improve the alignment and interaction of the chains (McCrum et al., 1988). At all the processing temperatures, Young's modulus displayed values in a narrow range of 1340-1400 MPa. They increased slightly

The~strength of the molded specimens increased as the temperature increased; the specimens molded at high temperature became stiffer and less extensible (Figure 4). Temperatures above 160 "C led to thermal degradation of the specimen during molding. The tensile strength showed a maximum of 39 MPa at a molding temperature

as the molding temperature increased but were greatly affected by the moisture content of the molding material (Figure 1). Water absorption of the molded specimens alsodecreased as temperature increased (Figure 5). High- temperature processing probably contributes to a closer packing of polymer chains and is likely to exclude water.

1824 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994

- 50 +/-, 2000 - - E 45 1 Young's Mod. c v Elongation

40 - c 35 -

30 - Gi c;n B t 2 5 - E =

c 2 1 5 - !!m G o 1 0 -

0 rn

= p o - 4

- 1800 z 1 -

0 80 100 120 140 160 Molding Temperature ('C)

Figure 4. Effect of molding temperature on mechanical properties of the molded specimens made from soy isolate at a moisture level of 11.7%.

180 c i\

\- 80 t\i 1 looj 0 L,l I I I I

0 80 100 120 140 160 Molding Temperature ("C)

Figure 5. Effect of molding temperature on water absorption behavior of the moldedspecimens made from soy isolate at a moisture level of 11.7%. C. Acid Treatments at the Isoelectric pH (4.5) by

Different Acids. The effect of molding temperature on soy isolate at pH 4.5 by HC1 is shown in Figure 6. With a moisture content of 11.3%, the highest tensile strength was reached at 160 "C (22 MPa). The strength of the plastic specimens increased as the temperature increased but did not reach the values obtained from the soy isolate that had not been adjusted to pH 4.5. Acid treatments at pH 4.5 decrease the net charge of the protein to zero leading to minimum water absorption. Nash et al. (1971) reported a rapid insolubilization of part of soybean globulins at pH 4.5. Acid treatments decreased the water absorption of the plastic specimens. Acid-treated speci- mens molded at 140 "C absorbed 30% water after 26-h submersion (Figure 7), which is a reduction in water absorption by almost 70% compared to the soy isolate without acid treatment.

Decreasing the moisture content of the molding material to8.4% led to stronger specimens. Molded at 160 "C, this material resulted in a plastic with a tensile strength of 35 MPa, elongation of 3.7 5% , yield strength of 4.9 MPa, and Young's modulus of 1550 MPa. The specimens absorbed 32% water after 26 h. At these processing conditions, the strength of the specimens was comparable to the strength of those made from soy isolate without acid treatment.

I H Youna's Mod. T

V Elondotion 0 Tensile Str. v Yield Str.

1 1300 1200

n

0 80 100 120 140 160 Molding Temperature ("C)

Figure 6. Effect of molding temperature on mechanical properties of the molded specimens made from HC1-treated soy isolate at a moisture level of 11.3%.

n 6 40

R 'y. v

C 0 - - 35 ' t s (D

L

{ 30 %

Y I I I I

0 80 100 120 140 180 Molding Temperature (" C)

0

Figure 7. Effect of molding temperature on water absorption behavior of the molded specimens made from HCEtreated soy isolate at a moisture level of 11.3%.

Several acids were used to examine their effects on the final properties of the plastic specimens. Soy isolate treated by HC1, propionic acid, or acetic acid did not show any significant detrimental effect on the tensile and yield strength of the specimens (Figure 8a). The elongation, however, was diminished when HCl or acetic acid was used. Sulfuric acid decreased both the strength and elongation of the plastic specimens. All acids decreased the water absorption of the plastics to about 30% after 26-h submersion (Figure 8b). Overall, the resultsfor mechanical properties and water absorption were similar for the four different acids tested.

Soy Concentrate. Soy concentrate molded at 125 O C

with different moisture contents showed a maximum tensile strength of 32 MPa and yield strength of 4.9 MPa at 7.4% moisture (Table 1). The percent elongation increased as the moisture content in the molding material increased (Table 1). Molded soy concentrate showed a very high Young's modulus which indicated that the material was quite rigid. At 5.0% moisture, Young's modulus was 1950 MPa and decreased as the moisture content in the molding material increased (Table 1). By increasing the molding temperature, it was possible to obtain stronger plastic specimens (Table 2). The maxi-

Ind. Eng. Chem. Res., Vol. 33, No. 7,1994 1826

- 5 80

J N

v

s 60 .- U e 0

9 40 a L

U

20 B

n -

a

-

-

-

-

0 m c 35

30

5 25

- W

A n

I v l

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C 5 15 !!m I! 10

a .I! m 5

I - 0 0

c

.= > c u Q C

- 1 2 3 4 5

b

401 20

0 1 2 3 4 5

Figure 8. (a) Mechanical properties of soy isolate molded specimens affected by various acid treatments (1, no acid; 2, HCI; 3, propionic acid; 4, sulfuric acid; 5, acetic acid; 8% moisture in molding material). (b) Water absorption of soy isolate molded specimens affected by various acid treatments (1, no acid; 2, HCI; 3, propionic acid; 4, sulfuric acid; 5, acetic acid; 8% moisture in molding material).

mum tensile strength for molded soy concentrate a t a moisturelevel of 11.2% was reachedat 160 "C and showed a value of 40 MPa. Yield strength reached ita maximum at 140 O C with 5.9 MPa. Specimens molded at 160 "C

1 2 3 b

100

- 1 2 3

Figure 9. (a) Mechanical properties of soy concentrate molded specimens affected by various acid treatments (1, no acid; 2, HCI; 3, propionic acid; 9% moisture in molding material; molded at 160 OC). (b) Water absorption of soy concentrate molded specimens affected by various acid treatments (1, no acid; 2, HCI; 3, propionic acid; 9% moisture in molding material, molded at 160 OC).

were the strongest and showed the lowest water absorption (89% in 26 h).

Plastic molded from soy concentrate which had been treated with HC1 or propionic acid resulted in stronger

Table 1. Effect of Moisture Level on Mechanical Properties and Water Absorption of Plastic Specimens Made from Soy Concentrate Molded at 125 'C

moisture (% ) tensile' elongation' yield strength' Young'sb modulus watef absorpn

5.0 22 (1) 1.8 (0.1) 4.0 (0.1) 1950 (40) 133 (4) 7.4 32 (3) 2.6 (0.3) 5.0 (0.5) 1870 (40) 128 (3) 10.0 29 (7) 2.9 (0.8) 4.0 (0.7) 1500 (70) 101 (4) 15.1 27 (4) 3.8 (0.4) 3.7 (0.4) ~OOo (80) 104 (1)

in molding compd. strength (MPa) ( % I (MPa) (MPa) (%, 26 h)

0 Measured by using an Inatron Model 4502 testing system, average of a t least five measurements; ( ) standard deviation. Average of five calculated values. Average of three measurements.

Table 2. Effect of Molding Temperature on Mechanical Properties and Water Absorption of Plastic Specimens Made from Concentrate a t a Moisture Level of 11.2%

molding tensile elongation' yield strength' Young'sb modulus wateF absorpn

100 29 (3) 3.1 (0.3) 4.5 (0.6) 1630 (50) 137 (4) 120 36 (5) 3.7 (0.3) 5.3 (0.8) 1600 (60) 119 (3) 140 40 (6) 4.0 (0.5) 5.7 (0.8) 1630 (30) 98 (2) 160 41 (2) 3.9 (0.2) 5.4 (0.2) 1600 (100) 89 (2)

temp ( O C ) strength' (MPa) (%) (MPa) (MPa) (%, 26 h)

a Measured by using an Instron Model 4502 testing system, average of a t least five measurements; ( ) standard deviation. b Average of five calculated values. c Average of three measurements.

1826 Ind. Eng. Chem. Res., Vol. 33, No. 7,1994

Figure 10. SEM micrographs of the fracture surface of soy isolate plastic; d e bars represent 200 rm.

material. HCl-treated soy concentrate produced less strong specimens with 34MPa tensile strength and 4.9- MPa yield strength when compared with propionic acid- treated soy concentrate plastic which showed 40-MPa tensile strength and 5.9-MPa yield strength (Figure 9a). Adjustment of the pH to 4.5 diminished the water absorption of the molded specimens to about 50 % (Figure 9b).

Scanning electron micrograph (Figure 10) illustrates a typical brittle fracture of soy isolate plastic. Brittle fracture surfaces show a smooth surface center region where the crack initiated and grew slowly, surrounded hy a rough region where the crack rapidly propagated. The original cracklike defecta may be caused by voids or impurities in the material.

Conclusions

Soy isolate and soy concentrate were compression moldedintorigidand brittle plasticspecimenswithtensile strength comparable to polystyrene. By acid treatment at the isoelectric point of soy protein, the 26-h water absorption of the specimens was reduced to 32%. The treatment did not detrimentally affect the mechanical properties. Molding the material at 140 "C resulted in

specimens with higher tensile strength compared with thosemoldedat lowertemperatures. Themoistwecontent of the molding material was crucial for the rigidity and extensibility of the specimens. Moisture contents above 10% resulted in more extensiblespecimenswith decreased tensile strength.

Ingeneral, plastiesmadefrom soy concentrateandacid- treated soy concentrate displayed tensile strength similar tothose made from soy isolate and acid-treated soy isolate. The water absorptionof moldedsoy concentrate specimens wasgreater than that ofmoldedsoy isolate. The difference in the water absorption between the molded concentrate and isolate is attributed to the carbohydrate content in the soy concentrate.

Acknowledgment The authora thank Dr. C. Schilling for reviewing the

manuscript and for suggestions, D. Burden for help in preparing the manuscript, and the Iowa Soybean Promo- tion Board for a grant support of this research.

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Received for review August 12, 1993 Revised manuscript received February 2, 1994

Accepted April 26, 1994.

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