Preparation and Properties of Biodegradable Multilayer Films Based on SoyProtein 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 wereprepared by a simple solvent casting method in order to exploit the advantageous properties of both filmmaterials. 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 comparedwith that of the SPI film. The mechanical properties of the multilayer film were comparable to those oflow-density polyethylene (LDPE) or high-density polyethylene (HDPE) films. The lamination of PLA layerson 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 multilayerfilm [(2.40 ( 0.24) × 10-18 m3‚m/m2‚s‚Pa] decreased more than 26-fold compared with that of the PLAfilm. In addition, the multilayer film had adequate water resistance over short periods. All of these propertyimprovements may be attributed to the strong adhesion between both polymers used, i.e., SPI and PLA.

Introduction

Considerable interest in biopolymer-based films has beenrenewed due to their environmentally friendly nature and theirpotential use in the food and packaging industries.1-5 Biopoly-mers are natural polymers obtained from agricultural productsor animals. Biopolymers produced from various natural re-sources such as starch, cellulose, and protein have beenconsidered attractive alternatives for nonbiodegradable petroleum-based plastics since they are abundant, renewable, inexpensive,environmentally friendly, and biodegradable. Soy protein, inparticular, has tremendous potential to substitute for nonbio-degradable plastics, and their potential use as an alternativeresource to bioplastics in packaging applications has beenextensively studied.6-12 However, there are some limitationsto the application of soy protein based films for packaging dueto their poor mechanical properties and high sensitivity tomoisture.13 Various efforts have been made to overcome theseproblems and to improve the property of soy protein based filmsthrough physical, chemical, or enzymatic treatments. Suchefforts have included treatment with alkali,14 alkylation withsodium alginate or propylene glycol alginate,15,16acylation withacetic and succinic anhydrides,17 aldehyde cross-linking,18,19UVirradiation,20,21 heat curing,22,23 blending with hydrophobicadditives such as neutral lipids, fatty acids, or waxes,24-26 andenzymatic cross-linking.27-29 Recently, nanocomposite technol-ogy, compositing soy protein with layered silicate clay materials,has been tested to improve film properties. For example, Otaigbeand Adams30 obtained better mechanical properties with im-proved water resistance for soy protein composites by blendingwith polyphosphate fillers. Rhim et al.31 also demonstrated that

soy protein isolate (SPI) films composited with organicallymodified montmorillonite or bentonite increased tensile strengthwith improved water vapor permeability. Though previouslyreported methods indicated a significant improvement in filmproperties, the moisture barrier property of soy protein basedfilms has not yet been fully addressed.

Another strategy to overcome the problem is to associate soyprotein with a moisture-resistant polymer, while maintainingthe overall biodegradability of the product. One of the mostpromising polymers for such a purpose is poly(lactide) (PLA).32,33

PLA is synthesized from lactic acid which is derived fromrenewable resources, such as corn or sugar beets,32 is athermoplastic with high strength, high modulus, and goodprocessability, and is completely biodegradable and thereforeperfectly safe for the environment. Generally, associationbetween polymers can be by blending or making multilayerswith component polymers, but blending is a more easy andeffective way to prepare muiltiphase polymeric materials withdesirable properties. However, natural polymers are usuallyhydrophilic in nature and not miscible with synthetic polymersbecause of poor interfacial adhesion between the two phases inthe blends. Hence, it is necessary to use a synthetic polymerwith a reactive group capable of reacting with the naturalpolymer. On the other hand, multilayer films can be preparedwith fewer problems of compatibility than experienced in thepreparation of blend films. A coextrusion technique has beenwidely used in the plastic industry to prepare multiple layerfilms.34 However, research work on biodegradable multilayerfilms based on biopolymers is scarcely found in the literature.Martin et al.35 reported on the preparation of multilayerbiodegradable films based on plasticized wheat starch andvarious biodegradable aliphatic polyesters using flat filmcoextrusion and compression molding techniques. They foundthat the multilayer films prepared by those methods were easilydelaminated depending on the affinity between base film andcap film layers, and their composition. However, they recognized

* To whom correspondence should be addressed. Tel.:+82-61-450-2423. Fax: +82-61-454-1521. E-mail: jwrhim@mokpo.ac.kr.

† Mokpo National University.‡ School of Packaging, Michigan State University.§ Department of Food Science and Human Nutrition, Michigan State

University.

3059Ind. Eng. Chem. Res.2006,45, 3059-3066

10.1021/ie051207+ CCC: $33.50 © 2006 American Chemical SocietyPublished on Web 03/24/2006

that better barrier properties could be obtained with starchproducts layered with moisture-resistant polyesters than withblends.

The main objective of this study was to prepare multilayerfilms with SPI and PLA through the solvent casting method toimprove mechanical and barrier properties of individual films.The properties of the prepared films were characterized throughmeasuring some selected properties including tensile strength(TS), elongation at break (EB), water vapor permeability (WVP),oxygen permeability (OP), water contact properties, and watersolubility (WS) of the films.

Experimental Section

Materials. Soy protein isolate (minimum 90% protein contenton a dry basis, Pro-FAM 646) was obtained from Archer DanielsMidland (Decatur, IL) and poly-L-lactide (PLLA, BiomerL9000) was obtained from Biomer Inc. (Krailling, Germany).The latter had a weight-average molecular weight of 200 kDaand was polymerized mainly (>98%) from L-lactic acid.Analytical grade chloroform and glycerin were purchased fromJ. T. Baker (Mallinckrodt Baker, Inc., Phillipsbury, NJ).

Preparation of Films. SPI films were prepared accordingto the method of Brandenburg et al.14 Five grams of SPI wasdissolved in a constantly stirred mixture of distilled water (100mL) and glycerin (2.5 g). The solution pH was adjusted to 10( 0.1 with 1 M sodium hydroxide solution. The film solutionswere heated for 20 min at 90°C in a constant-temperature waterbath, to denature the soy protein, and then cast onto a leveledTeflon 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 sameamount (100 mL) of film-forming solution per plate. Thecastings were dried at ambient conditions (≈23 °C) for about20 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 chloroformwhile mixing vigorously at room temperature. The dissolvedsolution was poured onto a Teflon-coated glass plate as for SPIfilms and then allowed to dry for about 20 h at room temperature(≈23 °C). The resultant film was peeled intact from the castingsurface.

Multilayer films, composed of a PLA outer layer, a SPImiddle layer, and another PLA outer layer, were prepared withbasically the same method as above using the same amount ofsolid content as control SPI or PLA films to control the filmthickness. First, one outer PLA layer was prepared by castingand drying, then a middle layer of SPI was cast over it, followedby the other PLA outer layer. Film-forming solutions for eachlayer were prepared by dissolving 1.25 g of PLA in 70 mL ofchloroform for the first layer, 2.5 g of SPI and 1.25 g of glycerinin 100 mL of distilled water for the second layer, and 1.25 g ofPLA in 70 mL of chloroform for the third layer. Each layerwas 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), andelongation at break (EB), respectively.

Film Thickness and Conditioning. Film thickness wasmeasured to the nearest 0.01 mm using a hand-held micrometer(Dial Thickness Gauge 7301, Mitutoyo, Japan). Five thicknessmeasurements were taken on each tensile testing specimen alongthe length of the rectangular strip, and the mean value was usedin tensile strength calculation. Similarly, five measurements weretaken on each water vapor permeability specimen, one at the

center and four around the perimeter, and the mean values wereused in calculating water vapor permeability. All film sampleswere preconditioned for at least 48 h in a constant-temperaturehumidity chamber set at 25°C and 50% relative humidity (RH)before testing.

Transparency. Transparency of the films was determinedby 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 atbreak (EB) of each film type sample were determined with anInstron Universal Testing Machine (Model 5565, InstronEngineering 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). Initialgrip separation was set at 50 mm and cross-head speed was setat 50 mm/min. The TS and EB measurements for each type offilm were replicated three times with individually prepared filmsas the replicated experimental unit; each replicate was the meanof seven specimens taken from the same film.

Water Vapor Permeability (WVP). WVP (g‚m/m2‚s‚Pa)was calculated as

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) acrossthe two sides of the film. WVTR was determined gravimetricallyusing a modified ASTM Method E 96-95. In calculating WVP,the effect of the resistance of the stagnant air layer between thefilm underside and the surface of the water in the cup wascorrected using the method of Gennadios et al.37

Water Solubility (WS). WS of each film was determinedas the percentage of film dry matter solubilized after 24 himmersion in distilled water.18 Three randomly selected 2× 2cm samples from each type of film were first dried at 105°Cfor 24 h to determine the weight of the initial dry matter. Anadditional three pieces of weighed film were placed in a 50-mL beaker containing 30 mL of distilled water. Beakers werecovered with Parafilm (American National Can, Greenwich, CT)and stored in an environmental chamber at 25°C for 24 h withoccasional, gentle swirling. Undissolved dry matter was deter-mined by removing the film pieces from the beakers, gentlyrinsing them with distilled water, and then oven drying them(105 °C, 24 h).

Contact Angle of Water. A contact angle analyzer (ModelPhoenix 150, Surface Electro Optics Co. Ltd., Kunpo, Korea)was used to measure the contact angle of water in air on thesurface of SPI and PLA films. A film sample (3× 10 cm) wasglued on a movable sample stage (black Teflon coated steel, 7× 11 cm) and leveled horizontally; then a drop of about 10µLof distilled water was placed on the surface of the film using amicrosyringe. The contact angles on both sides of the drop weremeasured to ensure symmetry and horizontal level. Wettingenergy was calculated using the following relationship:

whereEwet is the wetting energy (mJ/m2), θ is the contact angleof the water drop (deg), andγ is the surface tension of the probeliquid, which is 72.8 mJ/m2 for water.

Dynamic contact angle change was measured by recordingthe contact angle change of a water drop with time within 200s at room temperature (≈23 °C) at 50( 5% RH.

WVP ) [(WVTR)l]/∆p

Ewet ) γ cosθ

3060 Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006

Oxygen Permeability (OP).Oxygen transfer rate (OTR) wasdetermined at 23°C and 0% RH with an Ox-Tran 2/21 (MoconInc., Minneapolis, MN) according to the method of ASTM D3895-95. Aluminum foil masks with an exposure area of 1 cm2

were used to mount a test film sample in a diffusion cell whichwas subsequently purged with a carrier gas of N2 containing2% H2. One side of the sample was then exposed to the permeantgas of O2 (99.95%) at atmospheric pressure. The permeationrate through the specimen was measured until it reached steadystate. OP was determined by normalizing the OTR with respectto the oxygen pressure differential (∆p) and film thickness (l),i.e., dividing the OTR by∆p and multiplying byl. Two replicatemeasurements for each film sample were tested.

Thermal Analysis. Thermal analyses of the SPI, PLA, andmultilayer films were performed on a differential scanningcalorimeter (DSC Q100, TA Instruments, USA) using themethod of Martin and Ave´rous.38 For each film sample, about5 mg was sealed in an aluminum pan and heated from 25 to100 °C at a rate of 10°C/min, held at that temperature for 1min, and then cooled to-100°C with liquid nitrogen (coolingrate of 25°C/min) before a second heating scan to 200°C at a10 °C/min scan rate. A nitrogen flow (60 mL/min) wasmaintained throughout the test. The glass transition temperature(Tg), melting temperature (Tm), and enthalpy of fusion (∆Hf)were determined from the second heating scans. TheTg wastaken at the midpoint of heat capacity changes andTm at thepeak value of the respective endotherms of the second heatingscan.

Thermomechanical properties of the films were tested witha 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 afrequency of 1 Hz and deformation amplitude of 20µm. Thetemperature was programmed to increase from room temperatureto 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 fracturesurfaces of the single and multiple layer films was observed byscanning electron microscopy (SEM) at room temperature. AJEOL (Model JSM-6300F, Tokyo, Japan) SEM with fieldemission gun and accelerating voltage of 10 kV was used tocollect SEM images for the film specimen. A gold coating of afew nanometers in thickness was coated on impact fracturesurfaces. The samples were viewed perpendicular to thefractured surface.

Statistical Analysis. The measurements of TS, EB, WVP,and WS were triplicated with individually prepared films asthe replicated experimental units. Statistics on a completelyrandomized design were determined using the General LinearModels procedure in the SAS program. Duncan’s multiple rangetests 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 withPLA, SPI, and PLA were prepared with the solvent castingmethod. SPI film layers were homogeneously laminated on bothsides with PLA film layers, and the resulting multilayer filmhad good visual appearance. PLA, an inherently polar materialdue to the basic repeat unit of lactic acid, is likely to adhere tothe SPI layer firmly through hydrogen bonding interaction. ThePLA layers were so tightly adhered to the SPI film matrix thatmanual separation of individual layers was not possible.Considering the fact that the delamination of component layers

with multilayer films is mainly due to incompatibility with eachother,35 this indicates that the SPI layer is quite compatible withthe PLA layer.

Apparent properties of the multilayer film along with controlSPI and PLA films are shown in Table 1. Mean thicknesses forcontrol SPI and PLA films were 90.8( 1.0 and 91.4( 1.2µm, respectively. The thickness of the multilayer film did notchange significantly (P > 0.05). Generally, film thickness isdetermined by the solids content of the casting solution;however, it is not unusual for a thicker than expected multilayerfilm to be produced when the component layers are notcompatible with each other. In this study, multilayer films witha fairly consistent thickness were prepared by controlling thesolids 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 importantto 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 solventin the solvent-cast PLA films through thermogravimetricanalysis. 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 areexpected to act as plasticizers and to affect other film properties.

PLA films prepared by the solvent casting methods were astransparent as polystyrene films. Transmittance of PLA and SPIfilms prepared by the solvent casting methods were 95.2( 0.1%and 91.5( 0.4%, respectively. In general, the clarity of a filmis affected by additives, such as plasticizer, colorant, and fillers,and by processing temperature11 as well as compatibilitybetween component layers in multilayer films.40 The transmit-tance of the multilayer film was significantly (P < 0.05) higherthan that of the SPI film and slightly lower than that of thePLA film. Increase in transmittance of SPI films laminated withPLA layers is indirect evidence for compatibility between SPIand PLA film layers. Overall, transparency of the multilayerfilm was good enough for the film to be used as a see-throughpackaging material.

Tensile Properties.Table 2 shows the results for TS andEB for SPI, PLA, and multilayer films. TS and EB of SPI filmswere 3.3( 0.4 MPa and 148.2( 28.0%, respectively. Thesevalues were in good agreement with previously reported valuesfor SPI films.14-23 SPI film is known to have a rather lowmechanical strength with a medium degree of resilience. Thelow mechanical strength of SPI films was one of the reasons

Table 1. Apparent Properties of Soy Protein Isolate (SPI),Multilayer, and Poly(lactide) (PLA) Films a-d

film thickness (µm) DMe (%) T f (%)

SPI 90.8( 1.0b 80.2( 0.3b 91.5( 0.4b

multilayer 90.5( 1.4b 83.7( 0.6c 94.5( 0.2c

PLA 91.4( 1.2b 87.3( 0.1d 95.2( 0.1d

a-d Means of three replicates( standard deviation. Any two means inthe same column followed by the same letter were not significantly different(P > 0.05) by Duncan’s multiple range test.e Dry matter.f Transmittanceof the film determined at 660 nm.

Table 2. Tensile Strength (TS) and Elongation at Break (EB) of SoyProtein Isolate (SPI), Multilayer, and Poly(lactide) (PLA) Filmsa-c

film TS (MPa) EB (%)

SPI 3.3( 0.4b 148.2( 28.0b

multilayer 17.0( 0.3c 176.9( 27.9b,c

PLA 17.2( 0.5c 203.4( 20.8c

a-c Means of three replicates( standard deviation. Any two means inthe same column followed by the same letter were not significantly different(P > 0.05) by Duncan’s multiple range test.

Ind. Eng. Chem. Res., Vol. 45, No. 9, 20063061

preventing their use in food packaging or related applications.This explains why the mechanical strength of SPI film needsto be improved by associating it with stronger biopolymers suchas PLA. TS and EB values of solvent-cast PLA films were 17.2( 0.5 MPa and 203.4( 20.8%, respectively. In general, PLAfilm is known to be strong but brittle, with TS values of 45.6-61.4 MPa and EB values of 3.1-5.8%.32 However, the PLAfilms prepared in the present study showed significantly differentmechanical properties, being less strong and more resilient thanthose previously reported. It is presumed that this discrepancyis mainly caused by the difference in film preparation method.Most previously reported results were obtained with PLA filmsprepared by the extrusion casting method. Rhim et al.36

demonstrated that mechanical properties of solvent-cast PLAfilms were quite different from those of thermocompression PLAfilms (TS 44.0 ( 2.2 MPa; EB 3.0( 0.1%). Generally,plasticizers function by weakening intermolecular forces be-tween adjacent polymer chains, resulting in decreased tensilestrength and increased film flexibility.41 The increase inresiliency and decrease in tensile strength of the solvent-castPLA film is most likely due to a plasticizing effect of the solventretained in the film. Martin and Ave´rous38 also reported thatmechanical properties of PLA film could be improved by addingplasticizers such as glycerol, citrate ester, poly(ethylene glycol),and oligomeric lactic acid. They showed that the elastic modulusof PLA film decreased from 2050( 44 MPa to 744( 22 MPawhile flexibility of the film increased from 9( 2% to 200(24% by adding 20% oligomeric lactic acid as a plasticizer. BothTS and EB of multilayer films were significantly (P < 0.05)increased compared with those of SPI films. The TS value ofmultilayer film was similar to that of control PLA film. Theincrease in TS of multilayer film is mainly due to the effect ofPLA itself.42 The EB value of multilayer film was 176.9(17.0%, which is higher than that of SPI film and lower thanthat of PLA film. Change in flexibility of the multilayer filmmay also be ascribed to the plasticizing effect. It is also well-known that water plasticizes hydrophilic films and improvesfilm extensibility.43 In the multilayer film, both water in theSPI film layer and solvent in the PLA layers seemed to workin tandem as plasticizers to result in increased film extensibility.The mechanical property of TS of the multilayer film iscomparable to the TS values of widely used plastic films suchas low-density polyethylene (LDPE) and high-density polyeth-ylene (HDPE), which are known to be 13 and 26 MPa,respectively.44 Mechanical properties of multilayer films are alsoknown to be affected by the compatibility of each componentlayer.45 For example, Ghorpade et al.42 tested the effect of PLAcoating on wheat gluten film and found that the TS value ofthe gluten film changed from 3.09( 0.26 MPa to 3.83( 0.16MPa by coating with 8% PLA coating solution. Such a slightchange in TS may be due to incompatibility between wheatgluten and PLA. Martin et al.35 prepared multilayer films basedon plasticized wheat starch (PWS) and PLA and measuredmechanical properties of the multilayer films. They found thatTS and EB of PWS films, plasticized with glycerol (glycerol/

starch ratio) 0.14), increased from 17.4 MPa and 2.2% forcontrol PWS films to 26.4 MPa and 2.5% for multilayer filmswith PLA (PLA/PWS/PLA ratio) 14/74/12), and those ofanother PWS film (glycerol/starch ratio) 0.54) changed from2.1 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 secondtypes of multilayer films, respectively. Such a low peel strengthof these multilayer films is an indication that starch is notcompatible with PLA.44 In the present study, both the SPI andPLA layers of the multilayer film were fractured withoutseparation of component layers upon tensile testing, indicatinga strong adhesion between SPI and PLA layers. This resultsuggests that mechanical properties of multilayer films can beimproved by choosing proper polymer layers with high compat-ibility.

Water Vapor Permeability. The WVP values, along withactual 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 isnoteworthy that the WVP value of PLA film is 2 orders ofmagnitude lower than that of SPI film. Lamination with PLAimproved water vapor barrier properties of the SPI filmsdramatically as evidenced by a decrease in WVP value. Thewater vapor barrier of multilayer film improved 40-fold incomparison with that of control SPI films, and was comparableto that of PLA film. The WVP value of the multilayer film iscomparable to those of widely used plastic films.37 The WVPvalues (in kg‚m/m2‚s‚Pa) for various polymeric films aredocumented in the literature45 as follows: for poly(vinylidenechloride), (0.7-2.4)× 10-16; for high-density polyethylene, 2.4× 10-16; for low-density polyethylene, (7.3-9.7)× 10-16; forcast polypropylene, 4.9× 10-16; for ethylene vinyl acetate,(2.4-4.9) × 10-15; for polyester, (1.2-1.5) × 10-15; and forcellulose acetate, (0.5-1.6) × 10-14. Results from the presentstudy indicate that the water vapor barrier of the SPI filmlaminated with PLA layers is comparable to that of celluloseacetate films. Ghorpade et al.42 also found a similar result ofWVP values with PLA-coated wheat gluten films. They reportedthat WVP of wheat gluten films decreased exponentially withincreases in the PLA concentration of the coating solutions.

It was also noted in the current study that the calculated actualRH values at the inner film surface came close to the theoreticalvalue, i.e., 100%, for PLA and multilayer films. This indicatesthat it is not necessary to account for resistance of the stagnantair layer between the film sample and the water surface in thewater vapor transmission rate measuring cups.37 Ghorpade etal.42 also observed an increase in actual RH at the underside ofwheat gluten films coated with PLA. Since the increase in actualRH at the underside of a film sample means an increase in RHgradient across the film layer, expected WVP values for PLA-laminated films would most likely have been even lower if equal

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 (%) WS (%) OP (×10-18 m3‚m/m2‚s‚Pa)

SPI 263.87( 11.02d 72.4( 0.1b NDf 1.99( 0.11b

multilayer 6.66( 0.27c 98.2( 0.1c 45.4( 1.6c 2.40( 0.24c

PLA 4.66( 0.25b 98.7( 0.1d 0.0( 0.0b 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 dueto disintegration after immersion in water.

3062 Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006

RH gradient conditions had been applied across the various filmsamples. The observed increase in water vapor barrier propertiesof SPI films laminated with PLA was attributed to thehydrophobicity of PLA.

Water Solubility (WS). Results for WS of the control SPIand PLA films as well as multilayer films are shown in Table3. WS is a measure of resistance of film against water. BecauseSPI films are hydrophilic and readily dissolved in water, WSof SPI film could not be determined. However, PLA film wasnot soluble at all, indicating its higher degree of hydrophobicity.The WS value of the multilayer film was 45.4( 1.6%, whichwas between those of the SPI and PLA films. The solubility ofthe multilayer film was attributed to the solubilization of theSPI layer between the PLA layers. The film samples kept theirshape even after immersion in water for several hours, but theywere separated into two layers after immersion for 24 h due todissolution of the middle SPI layer. Most of the absorbed waterprobably entered from the cut edges of the film sample due tothe absence of a protective layer of PLA where film was cutfor WS measurement (2× 2 cm size).

However, water resistance of the SPI films was greatlyimproved by laminating with PLA films. High water resistanceof a film is one of the most important properties from a foodpackaging point of view, especially for high water activity foodsor foods contacting high humidity environments during trans-portation and storage. Furthermore, it was observed that themultilayer film was dimensionally stable even when stored underhigher RH conditions, while the SPI film tended to curl orwrinkle. Again, the improvement in water resistance of themultilayer film was attributed to the protective effect of thehydrophobic 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 plasticfilms 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 suchas corn zein, wheat gluten, and whey protein isolate. Generally,films prepared with polymers that can associate throughhydrogen or ionic bonding are excellent oxygen barriers butare susceptible to water vapor.50 Being hydrophilic in nature,most protein films are good oxygen barriers at low-to-intermedi-ate 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 shownin Table 3, the PLA film, being hydrophobic, had a much higherOP value than the other films tested, (52.21( 3.63)× 10-18

m3‚m/m2‚s‚Pa, which is comparable to that of high-densitypolyethylene (HDPE) film (see Table 4) but is 26 times higherthan that of control SPI film. The multilayer film, which was alaminate 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 tothe SPI film layer. Though protein films50,51 as well as other

biopolymer films52,53 are good oxygen barriers at low RHconditions, their OP values increase exponentially with RHincreases above intermediate RH. It is important to recognizethat the multilayer film can maintain its high oxygen barrierproperty even under high humidity conditions because thehydrophobic PLA outer layers keep the inner SPI layer fromcontacting high humidity directly. This is of utmost importancefor applications in packaging areas that have requirements forboth moisture and oxygen barrier properties.

Contact Angle of Water. The contact angle of water is oneof the basic wetting properties of packaging materials, indicatinghydrophilic/hydrophobic properties of the material.54 The wet-ting properties of a material against a water drop can beillustrated by measuring the initial values of contact angle ofwater immediately after deposition of the droplet and byfollowing the kinetics of water absorption.

Results of the initial contact angle of water measurementsfor the films are shown in Table 5. Usually, the more hydrophilica material is, the lower the contact angle value it has.Consequently, a more hydrophilic surface of material results ina higher wetting energy. The contact angles of water on SPIand PLA films were 42.4° ( 1.4° and 63.8° ( 0.9° with wettingenergy values of 51.1( 1.3 mJ/m2 and 32.3( 1.1 mJ/m2,respectively. This supports that PLA film is more hydrophobicthan SPI film and explains the fact that PLA film has lowerWVP with higher water resistance than SPI film.

Figure 1 shows the results for dynamic change of contactangle of water droplet on SPI and PLA films. The contact anglesof a water drop on both the SPI and PLA films decreasedlinearly over a short time period. This result clearly indicatesthe difference in initial contact angle of water on each film andthe difference in the degree of decrease in contact angle withtime.

The linear regression results showed that the linear model fitwell with the dynamic contact angle change data, with highvalues of the coefficient of determination (R2) as follows:

They-intercept value of the linear equation indicates the initialcontact angle of water of the material. The initial contact anglesdetermined in this method were 44.9° and 63.5° for the SPIand PLA films, respectively. This agrees with the results with

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 30°C, 0% RH 1.50-5.20 46wheat gluten 30°C, 0% RH 1.11-2.80 46whey protein isolate 23°C, 50% RH 0.50-8.81 50methylcellulose 30°C, 0% RH 31.02 47hydroxypropyl cellulose 30°C, 0% RH 29.98 47low-density polyethylene 23°C, 50% RH 216.44 48high-density polyethylene 23°C, 50% RH 49.42 48cellophane 23°C, 0/50/95% RH 0.08/1.85/29.17 49ethylene vinyl alcohol 23°C, 0/95% RH 0.01/0.14 48

Table 5. Water Contact Propertiesa of Soy Protein Isolate (SPI) andPoly(lactide) (PLA) Filmsb-d

film θ (deg) Ewet (mJ/m2) k (deg/s)

SPI 42.4( 1.4c 51.1( 1.3d 0.029( 0.001PLA 63.8( 0.9d 32.2( 1.1c 0.021( 0.001

a θ, water contact angle;Ewet, wetting energy;k, rate constant for changein 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 notsignificantly different (P > 0.05) by Duncan’s multiple range test.

y ) -0.029x + 44.879 (R2 ) 0.98) for SPI film

y ) -0.021x + 63.543 (R2 ) 0.99) for PLA film

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direct measurement of the initial contact angle. Since the contactangle of water on the film decreases due to water absorption ina 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 thewater absorption rate by the film,56 and low values of the slope(in absolute values) indicate a stronger hydrophobic characterof the film. As shown in Table 5, the rate constants for contactangle change (k) for the SPI and PLA films were 0.029 and0.021 deg/s, respectively. These results also support that PLAfilm is more hydrophobic than SPI film.

Thermal Properties. Thermal properties of the SPI, PLA,and multilayer films were investigated using DSC, and theresults are shown in Figure 2 and Table 6. The first endothermicpeak (around 5°C) in all the thermograms may be attributed tothe melting of ice. Thermal properties of multilayer filmreflected the properties of both component materials. Thoughnot clearly visible in the thermogram, the glass transition

temperature (Tg) determined using TA Instruments AdvantageSoftware (P/N 925710.001, version 4.0) for the multilayer filmshowed two transition points between theTg values of SPI andPLA films, 117.7 and 50.9°C, respectively. TheTg of PLAfilm was a little bit lower than the reported values (55-60°C),33

and may be due to the plasticizing effect of chloroform solventremaining in the film (about 10%), as shown by Rhim et al.36

The melting temperature peak for control SPI films was notdetected. The melting temperature (Tm) and the apparententhalpy of fusion (∆Hf) of the multilayer film were affectedby 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 themultilayer film was about half that of PLA film. Generally,Tg

is used as one of the most important criteria for the compatibilityof a polymer blend. It is known that, for a compatible polymerblend, usually only oneTg will appear in DSC thermograms atan intermediate temperature compared to that of theTg valuesof the component polymers.55

Thermomechanical Properties.Thermomechanical proper-ties of the SPI, PLA, and multilayer films were investigated byDMA, and the storage modulus and loss tangent curves as afunction of temperature are shown in Figure 3. The storagemodulus 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 beganto drop steadily as the temperature increased and reachedminimum plateau values at about 80°C. Tanδ values of theSPI and multilayer films showed one distinctive peak, whilethose of the PLA film showed a broad band for a peak. Thebroad-band peak of the PLA film is attributed to the solventstill present in the film.36 The glass transition temperature (Tg)values, determined as the temperature at which the tanδ peaked,were 97.3, 76.2, and 57.8°C for the SPI, multilayer, and PLAfilms, respectively. TheTg values determined by DMA curvesagreed fairly well with those determined from DSC thermo-grams, although absolute values were not exactly matched. Itis common for different methods to yield slightly different valuesfor Tg. It is worthwhile to note that the multilayer film indicatedonly one distinctiveTg value ranged between those of the SPIand PLA films as observed in DSC thermograms. This providesindirect evidence for the compatibility between the SPI andPLA.55

Film Microstructure. Figure 4 shows SEM images of theSPI, multilayer, and PLA films. The outer PLA layers are foundbeing tightly bonded to the inner SPI layer to make continuousfilm layers. Usually, adhesives are used between surfaces to bebonded when manufacturing multilayer films. The film surface

Figure 1. Dynamic change in contact angle of water drop on soy proteinisolate (SPI) and poly(lactide) (PLA) films.

Figure 2. Differential scanning calorimetry (DSC) thermographs of soyprotein isolate (SPI), multilayer, and poly(lactide) (PLA) films.

Table 6. Differential Scanning Calorimetry (DSC) MeasurementResultsa of Soy Protein Isolate (SPI), Multilayer, and Poly(lactide)(PLA) Films

film Tg (°C) Tm (°C) ∆Hf (J/g)

SPI 117.7 ndb ndmultilayer 61.2, 112.0 165.7 12.82PLA 50.9 167.3 24.64

a Tg, glass transition temperature;Tm, melting temperature;∆Hf, apparententhalpy of fusion b Not detected.

Figure 3. Storage modulus and tanδ curves of soy protein isolate (SPI),multilayer, and poly(lactide) (PLA) films.

3064 Ind. Eng. Chem. Res., Vol. 45, No. 9, 2006

is completely wet with the adehesive to fill any irregularitiesto obtain a homogeneous bond between each layer. In themultiple layer film in this study, each layer probably workedas an adhesive between layers; i.e., the middle SPI layerfunctioned like water-borne adhesive and the outer PLA layerworked as a solvent-borne adhesive. When the next film solutionwas cast over the previously dried film surface, it flowed intoand filled any irregularities in the film surface, enabling theformation of a close bond between layers on a molecular scale.Structurally, PLA is a linear aliphatic polyester based on lacticacid, and SPI (which contains at least 90% protein on a drybasis) is composed of 20 amino acids, with more than 30% ofits content comprised of the acidic amino acids, i.e., glutamicand aspartic acids. It is postulated that hydrogen bonds betweencarbonyl groups of PLA and amino groups of SPI may increasethe bond strength between the PLA and SPI layers.

This would explain the strong adhesion between each layerof the multilayer film, the consistent film thickness, the goodtransparency, and the desirable mechanical, water, and gasbarrier properties of the multilayer film. It is noted that the SEMimages of multilayer films show void spaces between polymerlayers. However, this may be partly attributed to the tensiletesting these films were subjected to; i.e., SEM images weremade of a cross section of the tensile fracture surface of thefilms that had been laminated with film layers with differentextensibility values.

Conclusion

Multilayer film samples composed of an SPI inner layer andPLA outer layers were prepared by a simple solvent-cast methodwithout addition of any compatibilizer or chemical modificationof film surfaces. The mechanical properties of SPI film wereimproved through lamination with PLA layers, which were thencomparable to those of LDPE or HDPE. The lamination of PLAlayers on SPI film also resulted in desirable gas barrier propertiesof the film with a low WVP of PLA and low OP of SPI. Inaddition, the film had an adequate water resistance over a shortperiod of time. All of these property improvements may beattributed to the compatibility between both polymers used, i.e.,SPI and PLA. The multilayer film, composed of SPI and PLAlayers, being completely biodegradable as well as having highwater vapor and oxygen barriers, has a high potential forsubstituting for presently used barrier polymeric films coatedor laminated with barrier polymers such as poly(vinylidenechloride) (PVDC), ethylene vinyl alcohol (EVOH), nylon, andpolyesters, which are expensive and nonbiodegradable.

Acknowledgment

Support from the Korea Science and Engineering Foundation(KOSEF; R01-2003-000-10389-0) and the School of Packagingand the Department of Food Science and Human Nutrition,Michigan State University, is gratefully acknowledged.

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ReceiVed for reView October 31, 2005ReVised manuscript receiVed February 7, 2006

AcceptedFebruary 28, 2006

IE051207+

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