Ind. Eng. Chem. Res. 2006, 45, 3059-3066
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Preparation and Properties of Biodegradable Multilayer Films Based on Soy Protein Isolate and Poly(lactide) Jong-Whan Rhim,*,† Kmar A. Mohanty,‡ Sher P. Singh,‡ and Perry K. W. Ng§ Department of Food Engineering, Mokpo National UniVersity, 61 Dorimri, Chungkyemyon, Muangun, Chonnam 534-729, Republic of Korea, School of Packaging, Michigan State UniVersity, East Lansing, Michigan 48824-1223, and Department of Food Science and Human Nutrition, Michigan State UniVersity, East Lansing, Michigan 48824-1224
Multilayer film composed of a soy protein isolate (SPI) inner layer and poly(lactide) (PLA) outer layers were prepared by a simple solvent casting method in order to exploit the advantageous properties of both film materials. Tensile strength and elongation at break of the multilayer film were 17.0 ( 0.3 MPa and 176.9 ( 27.9%, respectively. Especially the tensile strength of the multilayer film increased more than 5-fold compared with that of the SPI film. The mechanical properties of the multilayer film were comparable to those of low-density polyethylene (LDPE) or high-density polyethylene (HDPE) films. The lamination of PLA layers on SPI film also resulted in desirable gas barrier properties of the film with both low water vapor permeability (WVP) of PLA and low oxygen permeability (OP) of SPI. The WVP of the multilayer film [(6.66 ( 0.27) × 10-14 kg‚m/m2‚s‚Pa] decreased 40-fold compared with that of the SPI film, and the OP of the multilayer film [(2.40 ( 0.24) × 10-18 m3‚m/m2‚s‚Pa] decreased more than 26-fold compared with that of the PLA film. In addition, the multilayer film had adequate water resistance over short periods. All of these property improvements may be attributed to the strong adhesion between both polymers used, i.e., SPI and PLA. Introduction Considerable interest in biopolymer-based films has been renewed due to their environmentally friendly nature and their potential use in the food and packaging industries.1-5 Biopolymers are natural polymers obtained from agricultural products or animals. Biopolymers produced from various natural resources such as starch, cellulose, and protein have been considered attractive alternatives for nonbiodegradable petroleumbased plastics since they are abundant, renewable, inexpensive, environmentally friendly, and biodegradable. Soy protein, in particular, has tremendous potential to substitute for nonbiodegradable plastics, and their potential use as an alternative resource to bioplastics in packaging applications has been extensively studied.6-12 However, there are some limitations to the application of soy protein based films for packaging due to their poor mechanical properties and high sensitivity to moisture.13 Various efforts have been made to overcome these problems and to improve the property of soy protein based films through physical, chemical, or enzymatic treatments. Such efforts have included treatment with alkali,14 alkylation with sodium alginate or propylene glycol alginate,15,16 acylation with acetic and succinic anhydrides,17 aldehyde cross-linking,18,19 UV irradiation,20,21 heat curing,22,23 blending with hydrophobic additives such as neutral lipids, fatty acids, or waxes,24-26 and enzymatic cross-linking.27-29 Recently, nanocomposite technology, compositing soy protein with layered silicate clay materials, has been tested to improve film properties. For example, Otaigbe and Adams30 obtained better mechanical properties with improved water resistance for soy protein composites by blending with polyphosphate fillers. Rhim et al.31 also demonstrated that * To whom correspondence should be addressed. Tel.: +82-61-4502423. Fax: +82-61-454-1521. E-mail:
[email protected]. † Mokpo National University. ‡ School of Packaging, Michigan State University. § Department of Food Science and Human Nutrition, Michigan State University.
soy protein isolate (SPI) films composited with organically modified montmorillonite or bentonite increased tensile strength with improved water vapor permeability. Though previously reported methods indicated a significant improvement in film properties, the moisture barrier property of soy protein based films has not yet been fully addressed. Another strategy to overcome the problem is to associate soy protein with a moisture-resistant polymer, while maintaining the overall biodegradability of the product. One of the most promising polymers for such a purpose is poly(lactide) (PLA).32,33 PLA is synthesized from lactic acid which is derived from renewable resources, such as corn or sugar beets,32 is a thermoplastic with high strength, high modulus, and good processability, and is completely biodegradable and therefore perfectly safe for the environment. Generally, association between polymers can be by blending or making multilayers with component polymers, but blending is a more easy and effective way to prepare muiltiphase polymeric materials with desirable properties. However, natural polymers are usually hydrophilic in nature and not miscible with synthetic polymers because of poor interfacial adhesion between the two phases in the blends. Hence, it is necessary to use a synthetic polymer with a reactive group capable of reacting with the natural polymer. On the other hand, multilayer films can be prepared with fewer problems of compatibility than experienced in the preparation of blend films. A coextrusion technique has been widely used in the plastic industry to prepare multiple layer films.34 However, research work on biodegradable multilayer films based on biopolymers is scarcely found in the literature. Martin et al.35 reported on the preparation of multilayer biodegradable films based on plasticized wheat starch and various biodegradable aliphatic polyesters using flat film coextrusion and compression molding techniques. They found that the multilayer films prepared by those methods were easily delaminated depending on the affinity between base film and cap film layers, and their composition. However, they recognized
10.1021/ie051207+ CCC: $33.50 © 2006 American Chemical Society Published on Web 03/24/2006
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that better barrier properties could be obtained with starch products layered with moisture-resistant polyesters than with blends. The main objective of this study was to prepare multilayer films with SPI and PLA through the solvent casting method to improve mechanical and barrier properties of individual films. The properties of the prepared films were characterized through measuring some selected properties including tensile strength (TS), elongation at break (EB), water vapor permeability (WVP), oxygen permeability (OP), water contact properties, and water solubility (WS) of the films. Experimental Section Materials. Soy protein isolate (minimum 90% protein content on a dry basis, Pro-FAM 646) was obtained from Archer Daniels Midland (Decatur, IL) and poly-L-lactide (PLLA, Biomer L9000) was obtained from Biomer Inc. (Krailling, Germany). The latter had a weight-average molecular weight of 200 kDa and was polymerized mainly (>98%) from L-lactic acid. Analytical grade chloroform and glycerin were purchased from J. T. Baker (Mallinckrodt Baker, Inc., Phillipsbury, NJ). Preparation of Films. SPI films were prepared according to the method of Brandenburg et al.14 Five grams of SPI was dissolved in a constantly stirred mixture of distilled water (100 mL) and glycerin (2.5 g). The solution pH was adjusted to 10 ( 0.1 with 1 M sodium hydroxide solution. The film solutions were heated for 20 min at 90 °C in a constant-temperature water bath, to denature the soy protein, and then cast onto a leveled Teflon protective overlay (Cole-Parmer Instrument Co., Chicago, IL) mounted on a glass plate (24 × 30 cm) framed at four sides. Films with uniform thickness were obtained by casting the same amount (100 mL) of film-forming solution per plate. The castings were dried at ambient conditions (≈23 °C) for about 20 h and peeled from the plates. PLA films were prepared using the solvent casting method.36 Five grams of PLA was dissolved in 100 mL of chloroform while mixing vigorously at room temperature. The dissolved solution was poured onto a Teflon-coated glass plate as for SPI films and then allowed to dry for about 20 h at room temperature (≈23 °C). The resultant film was peeled intact from the casting surface. Multilayer films, composed of a PLA outer layer, a SPI middle layer, and another PLA outer layer, were prepared with basically the same method as above using the same amount of solid content as control SPI or PLA films to control the film thickness. First, one outer PLA layer was prepared by casting and drying, then a middle layer of SPI was cast over it, followed by the other PLA outer layer. Film-forming solutions for each layer were prepared by dissolving 1.25 g of PLA in 70 mL of chloroform for the first layer, 2.5 g of SPI and 1.25 g of glycerin in 100 mL of distilled water for the second layer, and 1.25 g of PLA in 70 mL of chloroform for the third layer. Each layer was dried for about 20 h at room temperature (≈23 °C). All of the films were cut into 7 × 7 cm, 2 × 2 cm, and 2.54 × 15 cm pieces for the measurement of water vapor permeability (WVP), water solubility (WS), tensile strength (TS), and elongation at break (EB), respectively. Film Thickness and Conditioning. Film thickness was measured to the nearest 0.01 mm using a hand-held micrometer (Dial Thickness Gauge 7301, Mitutoyo, Japan). Five thickness measurements were taken on each tensile testing specimen along the length of the rectangular strip, and the mean value was used in tensile strength calculation. Similarly, five measurements were taken on each water vapor permeability specimen, one at the
center and four around the perimeter, and the mean values were used in calculating water vapor permeability. All film samples were preconditioned for at least 48 h in a constant-temperature humidity chamber set at 25 °C and 50% relative humidity (RH) before testing. Transparency. Transparency of the films was determined by measuring the percent transmittance at 660 nm using a UV/ visible spectrophotometer (Lamda 25, Perkin-Elmer Instruments, Norwalk, CT). Tensile Properties. Tensile strength (TS) and elongation at break (EB) of each film type sample were determined with an Instron Universal Testing Machine (Model 5565, Instron Engineering Corporation, Canton, MA). Rectangular specimens (2.54 × 15 cm) were cut using a precision double blade cutter (Model LB.02/A, Metrotec, S.A., San Sebastian, Spain). Initial grip separation was set at 50 mm and cross-head speed was set at 50 mm/min. The TS and EB measurements for each type of film were replicated three times with individually prepared films as the replicated experimental unit; each replicate was the mean of seven specimens taken from the same film. Water Vapor Permeability (WVP). WVP (g‚m/m2‚s‚Pa) was calculated as
WVP ) [(WVTR)l]/∆p where WVTR was the measured water vapor transmission rate (g/m2‚s) through a film, l was the mean film thickness (m), and ∆p was the partial water vapor pressure difference (Pa) across the two sides of the film. WVTR was determined gravimetrically using a modified ASTM Method E 96-95. In calculating WVP, the effect of the resistance of the stagnant air layer between the film underside and the surface of the water in the cup was corrected using the method of Gennadios et al.37 Water Solubility (WS). WS of each film was determined as the percentage of film dry matter solubilized after 24 h immersion in distilled water.18 Three randomly selected 2 × 2 cm samples from each type of film were first dried at 105 °C for 24 h to determine the weight of the initial dry matter. An additional three pieces of weighed film were placed in a 50mL beaker containing 30 mL of distilled water. Beakers were covered with Parafilm (American National Can, Greenwich, CT) and stored in an environmental chamber at 25 °C for 24 h with occasional, gentle swirling. Undissolved dry matter was determined by removing the film pieces from the beakers, gently rinsing them with distilled water, and then oven drying them (105 °C, 24 h). Contact Angle of Water. A contact angle analyzer (Model Phoenix 150, Surface Electro Optics Co. Ltd., Kunpo, Korea) was used to measure the contact angle of water in air on the surface of SPI and PLA films. A film sample (3 × 10 cm) was glued on a movable sample stage (black Teflon coated steel, 7 × 11 cm) and leveled horizontally; then a drop of about 10 µL of distilled water was placed on the surface of the film using a microsyringe. The contact angles on both sides of the drop were measured to ensure symmetry and horizontal level. Wetting energy was calculated using the following relationship:
Ewet ) γ cos θ where Ewet is the wetting energy (mJ/m2), θ is the contact angle of the water drop (deg), and γ is the surface tension of the probe liquid, which is 72.8 mJ/m2 for water. Dynamic contact angle change was measured by recording the contact angle change of a water drop with time within 200 s at room temperature (≈23 °C) at 50 ( 5% RH.
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Oxygen Permeability (OP). Oxygen transfer rate (OTR) was determined at 23 °C and 0% RH with an Ox-Tran 2/21 (Mocon Inc., Minneapolis, MN) according to the method of ASTM D 3895-95. Aluminum foil masks with an exposure area of 1 cm2 were used to mount a test film sample in a diffusion cell which was subsequently purged with a carrier gas of N2 containing 2% H2. One side of the sample was then exposed to the permeant gas of O2 (99.95%) at atmospheric pressure. The permeation rate through the specimen was measured until it reached steady state. OP was determined by normalizing the OTR with respect to the oxygen pressure differential (∆p) and film thickness (l), i.e., dividing the OTR by ∆p and multiplying by l. Two replicate measurements for each film sample were tested. Thermal Analysis. Thermal analyses of the SPI, PLA, and multilayer films were performed on a differential scanning calorimeter (DSC Q100, TA Instruments, USA) using the method of Martin and Ave´rous.38 For each film sample, about 5 mg was sealed in an aluminum pan and heated from 25 to 100 °C at a rate of 10 °C/min, held at that temperature for 1 min, and then cooled to -100 °C with liquid nitrogen (cooling rate of 25 °C/min) before a second heating scan to 200 °C at a 10 °C/min scan rate. A nitrogen flow (60 mL/min) was maintained throughout the test. The glass transition temperature (Tg), melting temperature (Tm), and enthalpy of fusion (∆Hf) were determined from the second heating scans. The Tg was taken at the midpoint of heat capacity changes and Tm at the peak value of the respective endotherms of the second heating scan. Thermomechanical properties of the films were tested with a dynamic mechanical analyzer (DMA Q800, TA Instruments, USA) following the procedure of Ogale et al.39 Each film sample (about 6 mm × 40 mm) was tested in the tensile mode at a frequency of 1 Hz and deformation amplitude of 20 µm. The temperature was programmed to increase from room temperature to 100 °C for the control PLA and multilayer films and to 160 °C for the control SPI film at a rate of 2 °C/min. Film Microstructure. The morphology of impact fracture surfaces of the single and multiple layer films was observed by scanning electron microscopy (SEM) at room temperature. A JEOL (Model JSM-6300F, Tokyo, Japan) SEM with field emission gun and accelerating voltage of 10 kV was used to collect SEM images for the film specimen. A gold coating of a few nanometers in thickness was coated on impact fracture surfaces. The samples were viewed perpendicular to the fractured surface. Statistical Analysis. The measurements of TS, EB, WVP, and WS were triplicated with individually prepared films as the replicated experimental units. Statistics on a completely randomized design were determined using the General Linear Models procedure in the SAS program. Duncan’s multiple range tests were conducted to determine the significant differences (P < 0.05) between each type of film. Results and Discussion Apparent Film Properties. Stratified three-layer films with PLA, SPI, and PLA were prepared with the solvent casting method. SPI film layers were homogeneously laminated on both sides with PLA film layers, and the resulting multilayer film had good visual appearance. PLA, an inherently polar material due to the basic repeat unit of lactic acid, is likely to adhere to the SPI layer firmly through hydrogen bonding interaction. The PLA layers were so tightly adhered to the SPI film matrix that manual separation of individual layers was not possible. Considering the fact that the delamination of component layers
Table 1. Apparent Properties of Soy Protein Isolate (SPI), Multilayer, and Poly(lactide) (PLA) Filmsa-d film
thickness (µm)
DMe (%)
T f (%)
SPI multilayer PLA
90.8 ( 90.5 ( 1.4b 91.4 ( 1.2b
80.2 ( 83.7 ( 0.6c 87.3 ( 0.1d
91.5 ( 0.4b 94.5 ( 0.2c 95.2 ( 0.1d
1.0b
0.3b
a-d Means of three replicates ( standard deviation. Any two means in the same column followed by the same letter were not significantly different (P > 0.05) by Duncan’s multiple range test. e Dry matter. f Transmittance of the film determined at 660 nm.
Table 2. Tensile Strength (TS) and Elongation at Break (EB) of Soy Protein Isolate (SPI), Multilayer, and Poly(lactide) (PLA) Filmsa-c film
TS (MPa)
EB (%)
SPI multilayer PLA
3.3 ( 0.4b 17.0 ( 0.3c 17.2 ( 0.5c
148.2 ( 28.0b 176.9 ( 27.9b,c 203.4 ( 20.8c
a-c Means of three replicates ( standard deviation. Any two means in the same column followed by the same letter were not significantly different (P > 0.05) by Duncan’s multiple range test.
with multilayer films is mainly due to incompatibility with each other,35 this indicates that the SPI layer is quite compatible with the PLA layer. Apparent properties of the multilayer film along with control SPI and PLA films are shown in Table 1. Mean thicknesses for control SPI and PLA films were 90.8 ( 1.0 and 91.4 ( 1.2 µm, respectively. The thickness of the multilayer film did not change significantly (P > 0.05). Generally, film thickness is determined by the solids content of the casting solution; however, it is not unusual for a thicker than expected multilayer film to be produced when the component layers are not compatible with each other. In this study, multilayer films with a fairly consistent thickness were prepared by controlling the solids content, indicating a likely compatibility between layers. Dry matter content for control SPI film was 80.2 ( 0.3%, while that of control PLA film was 87.3 ( 0.1%. It is important to recognize that the balance in the matrix of SPI film (19.8%) is water; however, that in the PLA film is the dissolving solvent, i.e., chloroform. Rhim et al.36 showed the presence of solvent in the solvent-cast PLA films through thermogravimetric analysis. Dry matter for the multilayer films was 83.7 ( 0.6%, indicating that the film contained 16.3% moisture and solvent. The residual moisture and solvent (chloroform) in the films are expected to act as plasticizers and to affect other film properties. PLA films prepared by the solvent casting methods were as transparent as polystyrene films. Transmittance of PLA and SPI films prepared by the solvent casting methods were 95.2 ( 0.1% and 91.5 ( 0.4%, respectively. In general, the clarity of a film is affected by additives, such as plasticizer, colorant, and fillers, and by processing temperature11 as well as compatibility between component layers in multilayer films.40 The transmittance of the multilayer film was significantly (P < 0.05) higher than that of the SPI film and slightly lower than that of the PLA film. Increase in transmittance of SPI films laminated with PLA layers is indirect evidence for compatibility between SPI and PLA film layers. Overall, transparency of the multilayer film was good enough for the film to be used as a see-through packaging material. Tensile Properties. Table 2 shows the results for TS and EB for SPI, PLA, and multilayer films. TS and EB of SPI films were 3.3 ( 0.4 MPa and 148.2 ( 28.0%, respectively. These values were in good agreement with previously reported values for SPI films.14-23 SPI film is known to have a rather low mechanical strength with a medium degree of resilience. The low mechanical strength of SPI films was one of the reasons
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Table 3. Water Vapor Permeability (WVP) and Oxygen Gas Permeability (OP) of Soy Protein Isolate (SPI), Multilayer, and Poly(lactide) (PLA) Filmsa-d film
WVP (×10-14 kg‚m/m2‚s‚Pa)
RH inside cupe (%)
SPI multilayer PLA
263.87 ( 6.66 ( 0.27c 4.66 ( 0.25b
72.4 ( 98.2 ( 0.1c 98.7 ( 0.1d
11.02d
0.1b
WS (%) NDf
45.4 ( 1.6c 0.0 ( 0.0b
OP (×10-18 m3‚m/m2‚s‚Pa) 1.99 ( 0.11b 2.40 ( 0.24c 52.21 ( 3.63d
a-d Means of three replicates ( standard deviation. Any two means in the same column followed by the same letter were not significantly different (P > 0.05) by Duncan’s multiple range test. e Actual RH values underneath the film samples covering of the WVP measuring cup. f Could not determined due to disintegration after immersion in water.
preventing their use in food packaging or related applications. This explains why the mechanical strength of SPI film needs to be improved by associating it with stronger biopolymers such as PLA. TS and EB values of solvent-cast PLA films were 17.2 ( 0.5 MPa and 203.4 ( 20.8%, respectively. In general, PLA film is known to be strong but brittle, with TS values of 45.661.4 MPa and EB values of 3.1-5.8%.32 However, the PLA films prepared in the present study showed significantly different mechanical properties, being less strong and more resilient than those previously reported. It is presumed that this discrepancy is mainly caused by the difference in film preparation method. Most previously reported results were obtained with PLA films prepared by the extrusion casting method. Rhim et al.36 demonstrated that mechanical properties of solvent-cast PLA films were quite different from those of thermocompression PLA films (TS 44.0 ( 2.2 MPa; EB 3.0 ( 0.1%). Generally, plasticizers function by weakening intermolecular forces between adjacent polymer chains, resulting in decreased tensile strength and increased film flexibility.41 The increase in resiliency and decrease in tensile strength of the solvent-cast PLA film is most likely due to a plasticizing effect of the solvent retained in the film. Martin and Ave´rous38 also reported that mechanical properties of PLA film could be improved by adding plasticizers such as glycerol, citrate ester, poly(ethylene glycol), and oligomeric lactic acid. They showed that the elastic modulus of PLA film decreased from 2050 ( 44 MPa to 744 ( 22 MPa while flexibility of the film increased from 9 ( 2% to 200 ( 24% by adding 20% oligomeric lactic acid as a plasticizer. Both TS and EB of multilayer films were significantly (P < 0.05) increased compared with those of SPI films. The TS value of multilayer film was similar to that of control PLA film. The increase in TS of multilayer film is mainly due to the effect of PLA itself.42 The EB value of multilayer film was 176.9 ( 17.0%, which is higher than that of SPI film and lower than that of PLA film. Change in flexibility of the multilayer film may also be ascribed to the plasticizing effect. It is also wellknown that water plasticizes hydrophilic films and improves film extensibility.43 In the multilayer film, both water in the SPI film layer and solvent in the PLA layers seemed to work in tandem as plasticizers to result in increased film extensibility. The mechanical property of TS of the multilayer film is comparable to the TS values of widely used plastic films such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE), which are known to be 13 and 26 MPa, respectively.44 Mechanical properties of multilayer films are also known to be affected by the compatibility of each component layer.45 For example, Ghorpade et al.42 tested the effect of PLA coating on wheat gluten film and found that the TS value of the gluten film changed from 3.09 ( 0.26 MPa to 3.83 ( 0.16 MPa by coating with 8% PLA coating solution. Such a slight change in TS may be due to incompatibility between wheat gluten and PLA. Martin et al.35 prepared multilayer films based on plasticized wheat starch (PWS) and PLA and measured mechanical properties of the multilayer films. They found that TS and EB of PWS films, plasticized with glycerol (glycerol/
starch ratio ) 0.14), increased from 17.4 MPa and 2.2% for control PWS films to 26.4 MPa and 2.5% for multilayer films with PLA (PLA/PWS/PLA ratio ) 14/74/12), and those of another PWS film (glycerol/starch ratio ) 0.54) changed from 2.1 MPa and 109.4% for control film to 12.3 MPa and 52.4% for a multilayer film (PLA/PWS/PLA ratio ) 13/75/12), respectively. Peel strengths for these multilayer films were 0.12 ( 0.02 N/mm and 0.05 ( 0.01 N/mm for the first and second types of multilayer films, respectively. Such a low peel strength of these multilayer films is an indication that starch is not compatible with PLA.44 In the present study, both the SPI and PLA layers of the multilayer film were fractured without separation of component layers upon tensile testing, indicating a strong adhesion between SPI and PLA layers. This result suggests that mechanical properties of multilayer films can be improved by choosing proper polymer layers with high compatibility. Water Vapor Permeability. The WVP values, along with actual RH conditions at the undersides of films during testing, of the SPI, PLA, and multilayer films are shown in Table 3. WVP values of the SPI and PLA films were (268.37 ( 11.02) × 10-14 and (4.66 ( 0.25) × 10-14 kg‚m/m2‚s‚Pa, respectively, which is in good agreement with reported values.14,36 It is noteworthy that the WVP value of PLA film is 2 orders of magnitude lower than that of SPI film. Lamination with PLA improved water vapor barrier properties of the SPI films dramatically as evidenced by a decrease in WVP value. The water vapor barrier of multilayer film improved 40-fold in comparison with that of control SPI films, and was comparable to that of PLA film. The WVP value of the multilayer film is comparable to those of widely used plastic films.37 The WVP values (in kg‚m/m2‚s‚Pa) for various polymeric films are documented in the literature45 as follows: for poly(vinylidene chloride), (0.7-2.4) × 10-16; for high-density polyethylene, 2.4 × 10-16; for low-density polyethylene, (7.3-9.7) × 10-16; for cast polypropylene, 4.9 × 10-16; for ethylene vinyl acetate, (2.4-4.9) × 10-15; for polyester, (1.2-1.5) × 10-15; and for cellulose acetate, (0.5-1.6) × 10-14. Results from the present study indicate that the water vapor barrier of the SPI film laminated with PLA layers is comparable to that of cellulose acetate films. Ghorpade et al.42 also found a similar result of WVP values with PLA-coated wheat gluten films. They reported that WVP of wheat gluten films decreased exponentially with increases in the PLA concentration of the coating solutions. It was also noted in the current study that the calculated actual RH values at the inner film surface came close to the theoretical value, i.e., 100%, for PLA and multilayer films. This indicates that it is not necessary to account for resistance of the stagnant air layer between the film sample and the water surface in the water vapor transmission rate measuring cups.37 Ghorpade et al.42 also observed an increase in actual RH at the underside of wheat gluten films coated with PLA. Since the increase in actual RH at the underside of a film sample means an increase in RH gradient across the film layer, expected WVP values for PLAlaminated films would most likely have been even lower if equal
Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006 3063 Table 4. Comparison of Oxygen Gas Permeability (OP) of Biopolymer and Plastic Films film
measuring conditions
OP (×10-18 m3‚m/m2‚s‚Pa)
ref
corn zein wheat gluten whey protein isolate methylcellulose hydroxypropyl cellulose low-density polyethylene high-density polyethylene cellophane ethylene vinyl alcohol
30 °C, 0% RH 30 °C, 0% RH 23 °C, 50% RH 30 °C, 0% RH 30 °C, 0% RH 23 °C, 50% RH 23 °C, 50% RH 23 °C, 0/50/95% RH 23 °C, 0/95% RH
1.50-5.20 1.11-2.80 0.50-8.81 31.02 29.98 216.44 49.42 0.08/1.85/29.17 0.01/0.14
46 46 50 47 47 48 48 49 48
RH gradient conditions had been applied across the various film samples. The observed increase in water vapor barrier properties of SPI films laminated with PLA was attributed to the hydrophobicity of PLA. Water Solubility (WS). Results for WS of the control SPI and PLA films as well as multilayer films are shown in Table 3. WS is a measure of resistance of film against water. Because SPI films are hydrophilic and readily dissolved in water, WS of SPI film could not be determined. However, PLA film was not soluble at all, indicating its higher degree of hydrophobicity. The WS value of the multilayer film was 45.4 ( 1.6%, which was between those of the SPI and PLA films. The solubility of the multilayer film was attributed to the solubilization of the SPI layer between the PLA layers. The film samples kept their shape even after immersion in water for several hours, but they were separated into two layers after immersion for 24 h due to dissolution of the middle SPI layer. Most of the absorbed water probably entered from the cut edges of the film sample due to the absence of a protective layer of PLA where film was cut for WS measurement (2 × 2 cm size). However, water resistance of the SPI films was greatly improved by laminating with PLA films. High water resistance of a film is one of the most important properties from a food packaging point of view, especially for high water activity foods or foods contacting high humidity environments during transportation and storage. Furthermore, it was observed that the multilayer film was dimensionally stable even when stored under higher RH conditions, while the SPI film tended to curl or wrinkle. Again, the improvement in water resistance of the multilayer film was attributed to the protective effect of the hydrophobic PLA layers. Oxygen Gas Permeability (OP). Results of OP for SPI, PLA, and multilayer films are also shown in Table 3. For comparison, reported values of OP for some selected biopolymer and plastic films with their measuring conditions are presented in Table 4. The OP of SPI film was (1.99 ( 0.11) × 10-18 m3‚m/m2‚s‚Pa, which is comparable to those of other biopolymer films such as corn zein, wheat gluten, and whey protein isolate. Generally, films prepared with polymers that can associate through hydrogen or ionic bonding are excellent oxygen barriers but are susceptible to water vapor.50 Being hydrophilic in nature, most protein films are good oxygen barriers at low-to-intermediate RH, but poor water vapor barriers. On the contrary, hydrophobic films such as polyethylene (PE) and polypropylene (PP) are high oxygen but low water vapor barriers. As shown in Table 3, the PLA film, being hydrophobic, had a much higher OP value than the other films tested, (52.21 ( 3.63) × 10-18 m3‚m/m2‚s‚Pa, which is comparable to that of high-density polyethylene (HDPE) film (see Table 4) but is 26 times higher than that of control SPI film. The multilayer film, which was a laminate of SPI and PLA films, had an average OP value of (2.40 ( 0.24) × 10-18 m3‚m/m2‚s‚Pa, close to that of SPI film. The high oxygen barrier of the multilayer film is attributed to the SPI film layer. Though protein films50,51 as well as other
Table 5. Water Contact Propertiesa of Soy Protein Isolate (SPI) and Poly(lactide) (PLA) Filmsb-d film
θ (deg)
Ewet (mJ/m2)
k (deg/s)
SPI PLA
42.4 ( 63.8 ( 0.9d
51.1 ( 1.3d 32.2 ( 1.1c
0.029 ( 0.001 0.021 ( 0.001
1.4c
a θ, water contact angle; E , wetting energy; k, rate constant for change wet in contact angle of water drop. b-d Means of replicates ( standard deviation. Any two means in the same column followed by the same letter were not significantly different (P > 0.05) by Duncan’s multiple range test.
biopolymer films52,53 are good oxygen barriers at low RH conditions, their OP values increase exponentially with RH increases above intermediate RH. It is important to recognize that the multilayer film can maintain its high oxygen barrier property even under high humidity conditions because the hydrophobic PLA outer layers keep the inner SPI layer from contacting high humidity directly. This is of utmost importance for applications in packaging areas that have requirements for both moisture and oxygen barrier properties. Contact Angle of Water. The contact angle of water is one of the basic wetting properties of packaging materials, indicating hydrophilic/hydrophobic properties of the material.54 The wetting properties of a material against a water drop can be illustrated by measuring the initial values of contact angle of water immediately after deposition of the droplet and by following the kinetics of water absorption. Results of the initial contact angle of water measurements for the films are shown in Table 5. Usually, the more hydrophilic a material is, the lower the contact angle value it has. Consequently, a more hydrophilic surface of material results in a higher wetting energy. The contact angles of water on SPI and PLA films were 42.4° ( 1.4° and 63.8° ( 0.9° with wetting energy values of 51.1 ( 1.3 mJ/m2 and 32.3 ( 1.1 mJ/m2, respectively. This supports that PLA film is more hydrophobic than SPI film and explains the fact that PLA film has lower WVP with higher water resistance than SPI film. Figure 1 shows the results for dynamic change of contact angle of water droplet on SPI and PLA films. The contact angles of a water drop on both the SPI and PLA films decreased linearly over a short time period. This result clearly indicates the difference in initial contact angle of water on each film and the difference in the degree of decrease in contact angle with time. The linear regression results showed that the linear model fit well with the dynamic contact angle change data, with high values of the coefficient of determination (R2) as follows:
y ) -0.029x + 44.879 (R2 ) 0.98)
for SPI film
y ) -0.021x + 63.543 (R2 ) 0.99)
for PLA film
The y-intercept value of the linear equation indicates the initial contact angle of water of the material. The initial contact angles determined in this method were 44.9° and 63.5° for the SPI and PLA films, respectively. This agrees with the results with
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Figure 1. Dynamic change in contact angle of water drop on soy protein isolate (SPI) and poly(lactide) (PLA) films.
Figure 2. Differential scanning calorimetry (DSC) thermographs of soy protein isolate (SPI), multilayer, and poly(lactide) (PLA) films. Table 6. Differential Scanning Calorimetry (DSC) Measurement Resultsa of Soy Protein Isolate (SPI), Multilayer, and Poly(lactide) (PLA) Films film
Tg (°C)
Tm (°C)
∆Hf (J/g)
SPI multilayer PLA
117.7 61.2, 112.0 50.9
ndb 165.7 167.3
nd 12.82 24.64
a T , glass transition temperature; T , melting temperature; ∆H , apparent g m f enthalpy of fusion b Not detected.
direct measurement of the initial contact angle. Since the contact angle of water on the film decreases due to water absorption in a short time span, the change in contact angle with time (i.e., the slope of the linear line) can be used as an indication of the water absorption rate by the film,56 and low values of the slope (in absolute values) indicate a stronger hydrophobic character of the film. As shown in Table 5, the rate constants for contact angle change (k) for the SPI and PLA films were 0.029 and 0.021 deg/s, respectively. These results also support that PLA film is more hydrophobic than SPI film. Thermal Properties. Thermal properties of the SPI, PLA, and multilayer films were investigated using DSC, and the results are shown in Figure 2 and Table 6. The first endothermic peak (around 5 °C) in all the thermograms may be attributed to the melting of ice. Thermal properties of multilayer film reflected the properties of both component materials. Though not clearly visible in the thermogram, the glass transition
Figure 3. Storage modulus and tan δ curves of soy protein isolate (SPI), multilayer, and poly(lactide) (PLA) films.
temperature (Tg) determined using TA Instruments Advantage Software (P/N 925710.001, version 4.0) for the multilayer film showed two transition points between the Tg values of SPI and PLA films, 117.7 and 50.9 °C, respectively. The Tg of PLA film was a little bit lower than the reported values (55-60 °C),33 and may be due to the plasticizing effect of chloroform solvent remaining in the film (about 10%), as shown by Rhim et al.36 The melting temperature peak for control SPI films was not detected. The melting temperature (Tm) and the apparent enthalpy of fusion (∆Hf) of the multilayer film were affected by both components of the film: Tm of the multilayer film (165.7 °C) was close to that of PLA film (167.3 °C); ∆Hf of the multilayer film was about half that of PLA film. Generally, Tg is used as one of the most important criteria for the compatibility of a polymer blend. It is known that, for a compatible polymer blend, usually only one Tg will appear in DSC thermograms at an intermediate temperature compared to that of the Tg values of the component polymers.55 Thermomechanical Properties. Thermomechanical properties of the SPI, PLA, and multilayer films were investigated by DMA, and the storage modulus and loss tangent curves as a function of temperature are shown in Figure 3. The storage modulus values had magnitudes of approximately 270, 1240, and 2600 MPa for the SPI, multilayer, and PLA films, respectively, at the starting temperature of 30 °C. They began to drop steadily as the temperature increased and reached minimum plateau values at about 80 °C. Tan δ values of the SPI and multilayer films showed one distinctive peak, while those of the PLA film showed a broad band for a peak. The broad-band peak of the PLA film is attributed to the solvent still present in the film.36 The glass transition temperature (Tg) values, determined as the temperature at which the tan δ peaked, were 97.3, 76.2, and 57.8 °C for the SPI, multilayer, and PLA films, respectively. The Tg values determined by DMA curves agreed fairly well with those determined from DSC thermograms, although absolute values were not exactly matched. It is common for different methods to yield slightly different values for Tg. It is worthwhile to note that the multilayer film indicated only one distinctive Tg value ranged between those of the SPI and PLA films as observed in DSC thermograms. This provides indirect evidence for the compatibility between the SPI and PLA.55 Film Microstructure. Figure 4 shows SEM images of the SPI, multilayer, and PLA films. The outer PLA layers are found being tightly bonded to the inner SPI layer to make continuous film layers. Usually, adhesives are used between surfaces to be bonded when manufacturing multilayer films. The film surface
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Figure 4. Scanning electron micrographs (SEM) of soy protein isolate (SPI), multilayer, and poly(lactide) (PLA) films (magification: 3000×).
is completely wet with the adehesive to fill any irregularities to obtain a homogeneous bond between each layer. In the multiple layer film in this study, each layer probably worked as an adhesive between layers; i.e., the middle SPI layer functioned like water-borne adhesive and the outer PLA layer worked as a solvent-borne adhesive. When the next film solution was cast over the previously dried film surface, it flowed into and filled any irregularities in the film surface, enabling the formation of a close bond between layers on a molecular scale. Structurally, PLA is a linear aliphatic polyester based on lactic acid, and SPI (which contains at least 90% protein on a dry basis) is composed of 20 amino acids, with more than 30% of its content comprised of the acidic amino acids, i.e., glutamic and aspartic acids. It is postulated that hydrogen bonds between carbonyl groups of PLA and amino groups of SPI may increase the bond strength between the PLA and SPI layers. This would explain the strong adhesion between each layer of the multilayer film, the consistent film thickness, the good transparency, and the desirable mechanical, water, and gas barrier properties of the multilayer film. It is noted that the SEM images of multilayer films show void spaces between polymer layers. However, this may be partly attributed to the tensile testing these films were subjected to; i.e., SEM images were made of a cross section of the tensile fracture surface of the films that had been laminated with film layers with different extensibility values. Conclusion Multilayer film samples composed of an SPI inner layer and PLA outer layers were prepared by a simple solvent-cast method without addition of any compatibilizer or chemical modification of film surfaces. The mechanical properties of SPI film were improved through lamination with PLA layers, which were then comparable to those of LDPE or HDPE. The lamination of PLA layers on SPI film also resulted in desirable gas barrier properties of the film with a low WVP of PLA and low OP of SPI. In addition, the film had an adequate water resistance over a short period of time. All of these property improvements may be attributed to the compatibility between both polymers used, i.e., SPI and PLA. The multilayer film, composed of SPI and PLA layers, being completely biodegradable as well as having high water vapor and oxygen barriers, has a high potential for substituting for presently used barrier polymeric films coated or laminated with barrier polymers such as poly(vinylidene chloride) (PVDC), ethylene vinyl alcohol (EVOH), nylon, and polyesters, which are expensive and nonbiodegradable. Acknowledgment Support from the Korea Science and Engineering Foundation (KOSEF; R01-2003-000-10389-0) and the School of Packaging and the Department of Food Science and Human Nutrition, Michigan State University, is gratefully acknowledged.
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ReceiVed for reView October 31, 2005 ReVised manuscript receiVed February 7, 2006 Accepted February 28, 2006 IE051207+
