Baked starch foams: starch modifications and additives improve process parameters, structure and properties

Industrial Crops and Products 16 (2002) 69 – 79 www.elsevier.com/locate/indcrop

Baked starch foams: starch modifications and additives improve process parameters, structure and properties R.L. Shogren a,*, J.W. Lawton a, K.F. Tiefenbacher b a

Plant Polymer Research Unit, National Center for Agricultural Utilization Research, US Department of Agriculture, Agricultural Research Ser6ice, 1815 North Uni6ersity Street, Peoria, IL 61604, USA b Franz Haas Machinery of America, Richmond, VA 23231, USA Received 14 September 2001; accepted 29 January 2002

Abstract Single-use packaging articles made of expanded polystyrene (EPS) are currently used to serve and pack a variety of food and non-food products. Recently, there have been efforts to develop and commercialize materials from renewable resources such as starch to replace EPS. Starch based foams are, however, brittle and sensitive to water, and thus require expensive coating steps when exposure to cold or hot liquids is required. In this report, various modified starches and additives were tested in baked foam plate formulations to improve strength and water resistance properties in lieu of coating. Foam plates made from chemically modified starches had shorter baking times, lighter weights and higher elongations at break than unmodified starch. Plates made from genetically modified (waxy) starches and polyvinyl alcohol (PVOH) had elongations to break at low humidities, which were much higher than those made from normal starches and PVOH. Addition of softwood fibers increased starch foam plate strengths at low and high humidities. Addition of monostearyl citrate to starch batter formulations gave the best improvement in water resistance among the compounds tested. Baked foams made from potato amylopectin, PVOH, aspen fiber and monostearyl citrate appeared to have adequate flexibility and water resistance to function as clamshell-type hot sandwich containers. Published by Elsevier Science B.V. Keywords: Starch; Foam; Biodegradable; Packaging

1. Introduction

 Product names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name USDA implies no approval of the product to the exclusion of other that may also be suitable. * Corresponding author. Tel.: +1-309-681-6354; fax: + 1309-681-6691. E-mail address: [email protected] (R.L. Shogren).

Materials derived from agriculture are beginning to emerge as promising substitutes for petroleum-based plastics for a variety of applications including packaging, consumer nondurables, non-wovens, coatings, medical plastics, agricultural plastics, and textile fibers (Narayan, 1994; Mayer and Kaplan, 1994; Chang, 1997; Bastioli, 1998; Petersen et al., 1999). Disposable

0926-6690/02/$ - see front matter. Published by Elsevier Science B.V. PII: S 0 9 2 6 - 6 6 9 0 ( 0 2 ) 0 0 0 1 0 - 9

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products such as plastic tableware and packaging are targeted since they are generally used only once and are difficult to recycle. Petroleum-based plastics such as polystyrene foam may require hundreds of years to degrade and can kill wildlife if ingested. Biobased materials such as starch and cellulose have the advantage of being biodegradable into a useful compost. Plastics and fast food industries also are realizing that they need to begin to identify renewable sources of raw materials as petroleum supplies decline. To promote this approach, the Biobased Products Act of 2000 was enacted to encourage the tripling of the use of biobased materials and energy by 2010. Increasing demand for low priced agricultural commodities such as corn and wheat are also motivating factors. Previous work (Tiefenbacher, 1993) has shown that starch/water batters can be baked in a closed, heated mold and thereby gelatinize and foam the starch into the shape of the mold. Starch alone, however, is rather brittle and water sensitive so further treatments are necessary to attain the strength, flexibility and water resistance needed for applications such as foam plates or clamshells intended for moist food. Earthshell Corporation (Andersen and Hodson, 1998a; Andersen et al., 1999) has added mineral fillers and wood fibers to improve strength and have coated the plates with wax and other materials to improve water resistance. These are beginning to be commercialized as hinged lid food containers for fast-food restaurants. Clamshell hinges with sufficient bending flexibility have been made by spraying the hinge with glycerol or polyvinyl alcohol (PVOH) solution (Andersen and Hodson, 1998b). However, the effects of humidity on the containers were not adequately characterized. Water resistance and foam strength were improved by coating with polyesters (Shogren and Lawton, 1998) or adding PVOH to the batter before baking (Shogren et al., 1998a, 2000). In this study, we have further investigated the possibilities for improving baked foam properties by additions to the starch/water and starch/ PVOH solution batters. The effects of adding modified starches, fiber, and hydrophobic compounds on baking process parameters, strength,

elongation to break and water resistance were determined. Morphologies of the foams were assessed by scanning electron microcopy.

2. Experimental

2.1. Materials Potato amylopectin was provided by Lyckeby Sta¨ rkelsen, Kristianstad, Sweden. Waxy cornstarch was Amioca from National Starch and Chemical, Bridgewater, NJ. Normal cornstarch was Buffalo 3401 from CPC International, Englewood Cliffs, NJ. High (50%) amylose cornstarch and high (50%) amylose cornstarch acetate were Amaizo 5 and Amaizo Crisp Tex, respectively, from Cerestar, Hammond, IN. Hydroxyethyl normal cornstarch (M.S.0.06) was Ethylex 2095 from A.E. Staley, Decatur, IL. Starches contained 10–18% moisture. PVOH was 98% hydrolyzed with average molecular weights of 85–146 000 (Airvol 325) and 124–186 000 (Airvol 350) from Air Products, Inc., Allentown, PA. Aspen fiber was from Super Wood Corp., Phillipps, WI. The fibers had an average length of 3.5 mm with a range of 1–11 mm. Guar gum and magnesium stearate were from Sigma Chemical, St. Louis, MO. Monostearyl citrate was from Morflex Inc., Greensboro, NC. Ethanedial oxalaldehyde resin (Curez 5550), glyoxal resin (Curez 5650) and methylated melamine resin (Melrez 5480) were from Hopton Technologies Inc., Rome, GA. Paraffin wax, soybean oil, polyethylene glycol stearate, silicon oil, rosin, gum elemi, shellac, stearic acid, citric acid, butanetetracarboxylic acid and octenyl succinic anhydride were from Aldrich, Milwaukee, WI. Epoxidized soybean oil (Paraplex G-62) was from C.P. Hall, Bedford Park, IL.

2.2. Methods 2.2.1. Foam tray preparation Starch, aspen fiber and magnesium stearate were first mixed using a Kitchen Aid mixer with a wire wisk attachment. Magnesium stearate acts as a mold release agent, preventing sticking of starch

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to the mold. For batters containing no PVOH, guar gum (1% by weight of starch) was also added to prevent settling of the starch. PVOH solution (10% by weight, heated to 90 °C for 1 h) and/or water and other liquid reagents were then added so as to give the desired level of PVOH and a total solid content of 33%. Other compounds (see Section 2.1) were added to the batter at 1 –2% levels based on starch weights. Acetic acid was added to batters containing the Curez or Melrex resins to decrease the pH to nearly 5. Mixing was continued for 20– 30 min. Starch foam trays were prepared using a laboratory model-baking machine (model LB TRO) supplied by Franz Haas Machinery of America, Richmond, VA. This equipment consists of two heated steel molds, the top of which can be hydraulically lowered to mate with the bottom half for a set amount of time. Dimensions of the mold were 217 mm long, 134 mm wide, 19 mm deep and 3 mm (plate separation). Baking temperatures were set at 200– 205 °C. Actual temperatures at the mold surface were about 10 °C lower as measured using a Temp-Sure Digital Pyrometer TS200 (D-M-E Co., Madison Heights, MI). Baking times were the minimum required to avoid a soft or bubbled tray and varied from about 80– 140 s.

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stron model 4201 Universal Testing Machine (Canton, MA) equipped with a cylindrical probe (35 mm diameter) and a cylindrical base (80 mm inside diameter). The probe was lowered onto the tray until a load of 0.5 N was reached and then lowered at 30 mm/min. Parameters calculated were the maximum force (Fm) and deformation to Fm (Lm). Five trays were normally tested. Tests of statistical significance between values of tray weight, Fm, Lm and water absorbance were done using t-tests on SIGMA-STAT (SPSS Inc., Chicago, IL). Water resistance tests were performed by first weighing a tray equilibrated to 50% r.h., adding 100 ml of distilled water at 23 °C, waiting 25 min, pouring off the water and reweighing the tray. Two replicate tests were normally performed.

2.2.3. Scanning electron microscopy Tray samples were mounted on aluminum stubs with graphite filled tape and vacuum coated with gold/palladium. Specimens were then examined with a JSM 6400V scanning electron microscope (JEOL Ltd, Tokyo, Japan).

3. Results and discussion

2.2.2. Testing of trays Trays were equilibrated at 5, 20, 50, 85 and 95% relative humidity (r.h.) at 23 °C for 7 days prior to mechanical testing. An Electro-tech Systems environmental chamber (Model 518, Glenside, PA) was used for 5% r.h. The 20 and 95% r.h. environments were achieved by placing saturated solutions of sodium acetate and disodium hydrogen phosphate, respectively, in large glove boxes. For 85% r.h., a Hotpack (Philadelphia, PA) constant humidity oven was used. Equilibration at 50% r.h. and testing was carried out in a special room maintained at that humidity. Humidities were checked in each environment using a Vaisala humidity meter (HMI 31, Finland), which was calibrated with saturated LiCl and NaCl. Trays conditioned at humidities other than 50% were placed in Ziplock™ polyethylene bags and then were removed one by one for testing. Mechanical testing was performed using an In-

The effects of chemical modification on process parameters and foam plate properties are shown in Tables 1 and 2. Plates made with unmodified cornstarch have a smooth surface, but will occasionally have cracks. No cracks were seen in plates made with hydroxyethyl (HE) starch. Also, batter volume and baking times required for the latter were less than unmodified cornstarch. Hydroxyethylation causes the starch to gelatinize more quickly (at a lower temperature), and thus the starch paste can then expand into a foam and dry more rapidly. There were no cracks in the modified starch foams probably because chemically modified starch granules will usually dissolve more completely in water and thus form a stronger and more continuous film. Strength (Fm) values for the HE starch foam plates, as shown in Table 2, were lower than the unmodified starch plates due

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to the lower weight of the former. On a unit weight basis, however, the HE starch plates were slightly stronger. The Lm or flexibility of HE starch plates was higher at 50% humidity, likely due to their lower density. The trend of increasing flexibility with lower density has been observed previously where the density of starch plates decreased with decreasing amylose content (Shogren et al., 1998b) or starch batter solids level (Lawton et al., 1999). Scanning electron micrographs of cross-sections of foam plates made from native and hydroxyethyl starches are presented in Fig. 1. The modified starch foams have a more expanded, lower density structure than the unmodified ones particularly at the top and bottom of the plate due to the more rapid gelatinization of the modified starches. The cell walls of the modified

starch foams were thinner, and hence could probably flex more easily thus giving a larger Lm. Foam plates made from high (50%) amylose cornstarch were irregular in shape (see Fig. 1) and very heavy because these starches have a high melting temperature, and thus do not give a very elastic paste, which can hold a steam bubble during expansion. Plates made from acetylated high amylose starch had regular shapes and smooth surfaces, baked in much shorter times and had lighter weights than plates made from unmodified high amylose starch. Batter solids levels needed to be at least 36% to make a well-formed tray. Attempts were made to make foam plates using hydroxypropyl high (70%) amylose starch (HP Hylon VII from National Starch), but the plates were not well formed. Visually, the paste viscosity and elasticity of this starch were too low

Table 1 Baking parameters and tray properties for modified cornstarches Starch

Solids (%)

Batter volume (ml) Baking time (s)

Tray weight (g)a

Appearance

Corn HE cornb High Am. cornc High Am. corn acetate

33 33 33 36

55 35 80 60

15 a 10.7 b 27 c 20 d

Smooth surface, some cracks Smooth surface Irregular surface Smooth surface

115 80 180 95

a Measured after equilibration at 50% r.h.; values within columns followed by a different letter are statistically different (95% confidence). b Hydroxyethyl cornstarch. c High (50%) amylose cornstarch.

Table 2 Mechanical properties of modified cornstarch foam plates Starch

Corn HE cornc High Am. cornd High Am. corn acetate

Fm (N)a at r.h. (%)

Lm (mm)b at r.h. (%)

20

50

80

20

50

42 aA 34 bA

72 aB 59 aB 160 b 123 bB

24 aC 16 bC

2.4 aA 2.7 bA

45 cC

2.7 bA

4.0 6.9 5.4 5.9

62 cA

80 aB bB c bcB

5.0 aB 3.4 bC 4.6 aB

a Force at break measured at indicated relative humidities (r.h.); values within columns followed by a different lower case letter are statistically different (95% confidence); values within rows followed by a different upper case letter are statistically different (95% confidence). b Elongation at break measured at indicated relative humidities (r.h.). c Hydroxyethyl cornstarch. d High (50%) amylose cornstarch.

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Fig. 1. Scanning electron micrographs of cross-sections of baked starch foam plates containing normal cornstarch (A), hydroxyethyl normal cornstarch (B), high (50%) amylose cornstarch (C), and acetylated high (50%) amylose cornstarch (D).

to form an expanded foam using the baking process. The effects of addition of fiber, PVOH and different hydrophobic agents on the properties of cornstarch foam plates are shown in Tables 3 and 4. Addition of 5–10% aspen fiber clearly gives higher strength foams, particularly at low and high humidities (Table 4). As shown in Fig. 2, the fibers adhere well to the starch matrix, and thus act as a reinforcement. This is critical at low humidities when starch becomes very brittle and fibers can ‘bridge’ between cracks in the starch. At high humidities, the amorphous starch foam started to become soft, and hence a fibrous network provides additional strength. Baking times increased slightly

with increasing fiber content probably because this loose fibrous network increased viscosity and resistance to expansion. Addition of PVOH to cornstarch foams gave some improvement in strength. Further addition of fiber to starch/PVOH foams yielded a plate with high and fairly constant strength versus humidity without an increase in tray weight. Flexibility of cornstarch foam plates was increased slightly by adding PVOH and changed little by fiber addition. Improvement in the mechanical properties of gelatinized starch on fiber addition has been noted by others in extruded films as well as baked foams (Andersen and Hodson, 1998a; Bergthaller et al., 1999; Dufresne and Cavaille, 1999; Curvelo et al., 2001; Averous et al., 2001).

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In the effort to improve water resistance, many different hydrophobic materials and cross-linking agents were added to the batter formulations shown in Table 3 at levels of approximately 1– 2%. Addition of paraffin waxes, vegetable oils, epoxidized oils, PEG stearates, silicon oils, rosin, gum elemi, shellac, stearic acid, citric acid, butanetetracarboxylic acid, octenyl succinic anhydride, ethandial resin, glyoxal resin and melamine resin gave no improvement in water resistance. Only monostearyl citrate (MSC) gave significant improvement in water resistance over the control.

For example, water absorption decreased from 11 to 4.7 g after addition of 2% MSC to a cornstarch/20% PVOH/10% fiber foam plate formulation (Table 3). Foam plates containing MSC tended to have a shiny surface, suggesting that it migrated to the surface and formed a water repellent layer there. Some reaction with starch may also be possible via dehydration of adjacent carboxyl groups to the anhydride followed by esterification by starch hydroxyls. MSC also acted as a very effective mold release agent, leaving mold surfaces as clean or cleaner than magnesium

Table 3 Effects of additives on baking parameters, weights and water absorption of normal cornstarch foam plates PVOH (%)

Fiber (%)

Additive (%)

Solids (%)

Batter volume (ml)

Baking time (s)

Tray weight (g)a

Weight gain after water addition (g)a

0 0 0 20 20d 20d

0 5 10 0 10 10

MgStb, 2 MgSt, 2 MgSt, 2 MgSt, 2 MgSt, 2 MSCe, 2

33 33 33 33 32 32

55 55 60 55 60 60

115 120 130 115 120 120

14.0 15.5 17.8 13.7 13.4 15.0

12.7 a – 11.3 b 10.4c 11.0c 4.75c

a b c a a b

a

Measured at 50% r.h.; values within columns followed by a different lower case letter are statistically different (95% confidence). Magnesium stearate. c Single measurement. d Airvol 350. e Monostearyl citrate. b

Table 4 Effects of additives on mechanical properties of normal cornstarch foam plates PVOH (%)

Fiber (%)

Additive (%)

Fm (N)a at r.h. (%) 20

0 0 0 20d 20 20

0 5 10 0 10 10

MgStc, 2 MgSt, 2 MgSt, 2 MgSt, 2 MgSt, 2 MSCe, 2

42 63 73 65 91 80

aA bA bcA bcA cA cA

Lm (mm)b at r.h. (%)

50

80

20

72 aB 74 aB 85 bA 106 bB 90 bA 102 bAB

24 aC 62 bA 70 bA 40 cC 106 dA 135 eB

2.4 2.8 2.8 2.6 2.9 2.5

50 aA bA abA abA abA aA

4.0 4.8 3.9 5.2 4.0 3.8

80 aB abB abAB bB abA abB

5.0 6.0 5.5 7.0 8.6 7.8

aB aB aB bC cB bC

a Force at break measured at indicated relative humidities (r.h.); values within columns followed by a different lower case letter are statistically different (95% confidence); values within rows followed by a different upper case letter are statistically different (95% confidence). b Elongation at break measured at indicated relative humidities (r.h.). c Magnesium stearate. d Airvol 350. e Monostearyl citrate.

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Fig. 2. Scanning electron micrograph of a cross-section of baked starch foam plate containing normal cornstarch with 10% wood fiber and 2% monostearyl citrate.

stearate. Addition of the aldehyde based resins, commonly used as water-resistance additives in the paper industry, caused severe browning and loss of strength. The oils and waxes caused an ‘overlubrication’ to produce an irregular, malformed tray. Addition of Ca and Zr salts was previously found to improve starch foam water resistance (via starch cross-linking) but plate weight and baking time also increased significantly (Shogren et al., 1998a). The effects of addition of PVOH and fiber to formulations containing waxy (amylopectin) starches are listed in Tables 5 and 6. As documented previously (Shogren et al., 1998b), trays made with waxy starches bake quickly and are lighter in weight due to their very rapid gelatinization. They are, however, not very strong or flexible (relatively low values for Fm and Lm). After addition of PVOH, however, plates made from waxy starches (especially potato) become very flexible and have acceptable strength even at low humidities. For example, Lm for waxy potato/ PVOH trays is 7.3 mm at 20% humidity versus only 3.3 mm for waxy potato starch alone (Table

6). Additions of fiber and MSC help to increase strengths at higher humidities and water resistance, respectively. Fibers are apparently well dispersed in the starch/PVOH matrix, as shown in the scanning electron micrograph in Fig. 3A. Adherence of fibers to the starch/PVOH matrix appears strong (Fig. 3B). The reason why Lm increases so much more on addition of PVOH for waxy starch than normal cornstarch is unknown, but may reflect differing compatibilities of the polymers involved. Previous work has shown that amylopectin is less compatible with poly(ethylene-co-vinyl alcohol) than amylose (Simmons and Thomas, 1995, 1998). Removal of starch from starch/PVOH 80/20 foam plates by microbial enzymes leaves a continuous, hollow framework of PVOH (Shogren et al., 1998a). Thus, the higher flexibility of waxy starch/ PVOH foams may result from a morphology in which the stronger PVOH occupies the continuous phase and swollen starch granules serve as a filler. The thin cell walls in these foams are probably also an important factor.

m.b m. p.e p. p.

0 20d 0 20 20

PVOH (%)

0 0 0 0 10

Fiber (%)

MgStc, 2 MgSt, 2 MgSt, 2 MgSt, 2 MSCf, 2

Additive (%)

33 33 33 33 31

Solids (%)

27 27 25 30 37

Batter volume (ml) 75 80 80 85 95

Baking time (s)

7.4 8.5 8.4 7.8 9.3

a b b a c

Tray weight (g)a

– 7.2 a 17 b 9.4 c 5.6 d

Weight gain after water addition (g)a

b

Measured at 50% r.h.; values within columns followed by a different lower case letter are statistically different (95% confidence). Waxy maize starch. c Magnesium stearate. d Airvol 350. e Waxy potato starch. f Monostearyl citrate.

a

Waxy Waxy Waxy Waxy Waxy

Starch

Table 5 Effects of additives and starch type on baking parameters, weights and water absorption of starch foam plates

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m.c m. p.f p. p.

0 20e 0 20 20

PVOH (%)

0 0 0 0 10

Fiber (%)

MgStd, 2 MgSt, 2 MgSt, 2 MgSt, 2 MSCg, 2

Additive (%)

13 aA 30 aA

14 35 22 54 81

aA bA cB dB eA

26 66 42 54 81

aB bB cC dB bA

10 17 15 27 58

80 aC bC bA cC dB

95

3 aD 6 bD

2.0 aA 5.4 bA

2.4 4.0 3.3 7.3 5.9

20 aA bA bB cB cA

5.2 aB 6.7 bB 7.1 bC 10 cB 6.2 abA

50

5.5 aB 5.3 aC 5.6 aD 11 bB 8.7 bB

80

5

50

5

20

Lm (mm)b at relative humidity (%)

Fm (N)a at relative humidity (%)

\20 \20

95

a Force at break measured at indicated relative humidities; values within columns followed by a different lower case letter are statistically different (95% confidence); values within rows followed by a different upper case letter are statistically different (95% confidence). b Elongation at break measured at indicated relative humidities. c Waxy maize starch. d Magnesium stearate. e Airvol 350. f Waxy potato starch. g Monostearyl citrate.

Waxy Waxy Waxy Waxy Waxy

Starch

Table 6 Effects of additives and starch type on mechanical properties of starch foam plates

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This flexibility is critical for applications such as a flexible hinge region of a clamshell container that must flex without breaking. As a test of the suitability of the above formulations for use as sandwich containers, a hamburger and bun was placed between two foam plates consisting of waxy potato starch/PVOH/fiber/MSC 70/20/10/2 and microwaved for 30 s. The internal temperature of the hamburger reached 75 °C. After 10 min, the bottom plate was slightly soft under the hamburger, but as a whole, the container was still rigid and did not deform significantly. The weights of the plates increased from 19.4 to 22.2 g after the hamburger was removed.

In conclusion, the flexibility of starch foam plates can be improved and baking times decreased by incorporating chemically or genetically modified starches into the foam formulations. Strength, especially at low and high humidities can be improved by addition of fibers and water resistance is improved by adding monostearyl citrate. Although the plates we prepared are not adequate for applications such as hot drink containers, the range of uses are extended to applications where moist food contact occurs for short periods. The eventual goal is to have as short baking times as possible and no secondary coating steps to reduce process costs.

Acknowledgements The technical assistance of Elizabeth Krietemeyer for the baking work and Dr Arthur Thompson for the scanning electron micrographs are gratefully acknowledged. This work was conducted under Cooperative Research and Development Agreement (CRADA) No. 58-3K95-M-228 with Franz Haas Machinery of America.

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Fig. 3. Scanning electron micrographs of baked starch foam plates containing waxy potato starch with 20% PVOH, 10% wood fiber and 2% monostearyl citrate: surface (A), and cross-section (B).

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