Toward Sustainable PLA-Based Multilayer Complexes with Improved

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Toward Sustainable PLA-Based Multilayer Complexes with Improved Barrier Properties Jeancarlo R. Rocca-Smith,† Roberta Pasquarelli,† Aureĺ ie Lagorce-Tachon,† Jeŕ ôme Rousseau,‡ Steṕ hane Fontaine,‡ Veŕ onique Aguie-́ Beǵ hin,§ Fred́ eŕ ic Debeaufort,†,∥ and Thomas Karbowiak*,† †

Univ. Bourgogne Franche-Comté, Agrosup Dijon, UMR PAM A 02.102, 1 Esplanade Erasme, 21000 Dijon, France Univ. Bourgogne Franche-Comté, DRIVE EA1859, 58000 Nevers, France § FARE Laboratory, INRA, Université de Reims Champagne - Ardenne, 51100 Reims, France ∥ IUT Dijon - Auxerre, Département BioEngineering, 21078 Dijon, France ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF EDINBURGH on 01/29/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: Poly(lactic acid) or PLA is currently considered as one of the most promising substitutes of conventional plastics, with low environmental impact, especially for food packaging applications. Nevertheless, some drawbacks, such as high permeability to oxygen, are still limiting its industrial applications. The objective of this study was to highly increase the oxygen barrier performance of PLA without compromising its sustainable nature and following the principles of circular economy perspective. Coproducts coming from mill industries, such as wheat gluten proteins (WG), were used to produce PLA-WG-PLA multilayer complexes with improved barrier performance. Different technologies of industrial interest were considered: high-pressure homogenization of WG film forming dispersions, corona treatment of industrial PLA films, wet casting and spin coating for tailoring the WG coating thickness, and hot-pressing for shaping the multilayers. The impact of all these strategies on the properties (surface and bulk) and performances (barrier and adhesion) were investigated on the single constituent layers as well as on the final laminate. The most efficient complex increased more than 20 times (or 2000%) the barrier properties to oxygen and ∼20% the barrier properties to water vapor, considering application conditions (50% relative humidity and 25 °C). The low thickness (∼60 μm) of this complex also matched the requirement for flexible packaging applications. High-pressure homogenization, WG coating thickness, and hot-pressing positively and highly impacted the final properties of the multilayer, while the contribution of corona treatment was limited. This study unambiguously evidenced the potential of PLA-WG-PLA complexes as a valid sustainable substitute for high performing conventional plastics, and it could open an unexplored PLA market opportunity. In addition, it could motivate further investigations on PLA-based laminates for industrial interest, using other biopolymers from agroindustrial waste or byproducts. KEYWORDS: Biobased and biodegradable polymers, Poly(lactic acid) PLA, Wheat gluten, Hot-pressing, High-pressure homogenization, Spin coating, Corona treatment, Surface modification

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INTRODUCTION Poly(lactic acid) or PLA is a biobased and biodegradable polyester, which is currently considered as one of the most promising alternatives of conventional plastics, especially for food packaging applications.1,2 Notwithstanding its apparent success, PLA shows some drawbacks to be faced in order to make it a more competitive material. Its low gas barrier properties compared to high performing conventional plastics,3−5 its water reactivity (hydrolysis),6 the undesirable crinkling noise that PLA makes when it is handled,7 its low thermal stability,4 and its high price still restrict its use in the market. The efforts accomplished by research for tailoring the gas barrier properties of PLA are numerous and can be classified in two main strategies.8 On the one hand, the most adopted approach consists in dispersing into the PLA continuous matrix © XXXX American Chemical Society

one (or more) additional phase (e.g., polymers, biopolymers, inorganic materials, fibers, and nanofillers) for complemental barrier performance, thus producing blends,9−11 composites,9 nanocomposites,12−15 or biocomposites.16,17 Although rather easy to produce, this generates materials with intermediate barrier performances due to problems of compatibility or dispersion, and the permeant molecules are not forced to diffuse through the dispersed phase. On the other hand, the second approach aims at modifying the PLA surface for producing multilayer complexes, by using different techniques, such as wet casting,18−23 electrospinning,24 hot-pressing,2,25,26 or coextrusion.27 Received: August 16, 2018 Revised: December 9, 2018

A

DOI: 10.1021/acssuschemeng.8b04064 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

subjected to high-pressure homogenization (WG-high pressure). Briefly, 2.5 g of glycerol (≥99.5%, Sigma-Aldrich, St Louis, MO, U.S.A.), 10 g of gluten from wheat (protein purity 85%, Sigma-Aldrich), 40 g of deionized water, 50 g of ethanol (96% v/v, Sigma-Aldrich) and 0.7 mL of hydrochloric acid (4 M, Sigma-Aldrich) were stirred at 25 °C for 30 min. The film-forming dispersions (pH = 4) were thermally treated for 15 min at 70 °C under magnetic stirring. The dispersion was then submitted to high-pressure homogenization for 7 cycles at 100 MPa (LM10 Microfluidizer, Microfluidics, Westwood, MA, U.S.A.) in a multislotted Z-type chamber. The selected conditions were chosen based on a recent study involving chitosan and lignin composites.38 The high-pressure circuit was covered with ice to prevent an excessive temperature increase of the dispersion. After high-pressure homogenization, 10, 15, 20, or 25 mL of the film-forming dispersion was poured in polystyrene Petri dishes (diameter = 14 cm) or casted on Poly(methyl methacrylate) (PMMA) supports using an automatic film applicator (Model: 1137, Sheen Instruments, Surrey, U.K.). The different volumes and deposition techniques ensured to cover a thickness range from 20 to 150 μm. Films were peeled after drying at 25 °C and 40% RH for ∼15 h. The films were then equilibrated at around 50% RH, using microclimate chambers at 25 °C containing magnesium nitrate saturated salt solution (53% RH). WG films without high-pressure homogenization step were produced for the purpose of comparison (WG-no high pressure). The surface in contact with air during drying was named “air side”, and the surface in contact with the support during drying was named “support side”. In addition, the particle size distribution in the WG film-forming dispersion was determined by laser light scattering (Mastersizer 3000, Malvern, Malvern, U.K.). The Mie theory39 was applied considering particles as spheres for determining the mean volume diameter (d4,3), and using a refractive index of 1.35 and 1.45 for the dispersant (water− ethanol, 4:5 in weigh) and wheat gluten, respectively. The particle size distribution was measured in triplicate (from three different film forming suspensions), and each sample was scanned three times. Production of PLA-WG Laminates. Bilayer Films. The WG filmforming dispersion subjected to high-pressure homogenization was deposited at around 60 °C on the surface of the PLA films by wet casting or by spin coating. The WG coating was always in contact with CT-treated side or with NCT-A side of PLA. The corresponding bilayer complexes were referred to as “CT + WG” or “NCT + WG”. WG coating layers with a thickness of approximatively 1, 2.5, and 5 μm were deposited by spin coating on PLA films (surface area = 5.5 × 5.5 cm2) by using 700 μL of the film-forming dispersion and two successive dilutions (1:2 and 1:4) in ethanol−water solution (1:1) respectively. The spin-coater device (Model: P6700, Specialty Coating Systems, Inc., Indianapolis, IN, U.S.A.) was used following a spin acceleration program of 500 rpm for 10 s, 1500 rpm for 20 s, and 3000 rpm for 20 s, with a deceleration time of 5 s. WG coating layers with 20 and 60 μm thicknesses were prepared by wet casting of 12 and 36 mL film-forming dispersion on PLA films (surface area of about 20 × 30 cm2) using an automatic film applicator (Model: 1137, Sheen instruments, Surrey, U.K.), respectively. Trilayer Films. The laminates were produced with a hot-press (LabPro 600, Fontijne Presses, Vlaardingen, Netherlands) using preformed single films (CT, NCT, and WG-high pressure) or preformed bilayers obtained by wet casting or spin coating (CT + WG, NCT + WG), as previously described. They were prepared on 15 × 15 cm2 squares, except for spin-coated films (5.5 × 5.5 cm2). Both surfaces of WG film were always in direct contact with CT-treated side or NCT-A side of PLA. The system was heated to 130 °C at 10 °C·min−1 and hot pressed at 10 MPa for 10 min. It was then cooled down to 25 °C at 10 °C·min−1. Pressure was released when the temperature of the sample was below 30 °C. The trilayer complexes were then stored at 25 °C and 50% RH. The samples made from preformed single films were named CT/WG/CT and NCT/WG/NCT, while the trilayers prepared from bilayers (previously made by coating or spin coating) were referred to as CT+WG/CT and NCT+WG/NCT.

This latter strategy provides the most efficient structure as continuous blocking layers (or laminates) to permeant molecules.28 Proteins and polysaccharides can be considered as good candidates for developing sustainable multilayer complexes with PLA. They are intrinsically biodegradable and thus do not compromise the sustainable nature of PLA, as inorganic compounds (e.g., nafion,29 graphene,2 montmorillonite,13,29 aluminum oxide,30,31 or carbon32) or poly(vinyl alcohol) and poly(vinylidene chloride) coatings do. In addition, although proteins and polysaccharides lead to hydrophilic films, having lower barrier performances to water vapor with much higher water sensitivity than PLA, their barrier properties to gases are much higher than PLA, when exposed to dry environments. This complementarity suggests that a laminate structure PLA-proteinPLA or PLA-polysaccharide-PLA would not only protect the inner layer from water molecules exposition, but it would also increase the barrier performances of PLA. Some advances have been recently accomplished in this direction using different biopolymers such as cellulose,18,22 chitosan,20 whey proteins,21 gelatin,23,26 wheat gluten,25 starch,27 soy proteins,33 and zein.34 Although these results are encouraging, with an increased oxygen barrier efficiency from 15% to 10 000%, further effort has to be made, considering the PLA production at a large scale, with standardized procedures and technologies as well as the single strategies for producing the additional layers. This study proposes wheat gluten protein (WG) as the complementary polymer to PLA, being also biobased and biodegradable. WG is able to form films, usually opaque, with good mechanical properties and low permeability to gases, comparable to high barrier plastics in dry environments (e.g., PVC poly(vinyl chloride), PA poly(amide), EVA ethylene-vinyl alcohol).35,36 In addition, WG is a coproduct of the mill industry, and its use in PLA based films can be a useful strategy for valorising this protein from a circular bioeconomy perspective. The main objective was to develop multilayer complexes PLA-WG-PLA able to highly increase the barrier performance of PLA to O2. In order to assess the feasibility, the PLA films used in this study were produced at industrial level. Different technologies of industrial interest were studied, such as high-pressure homogenization of wheat gluten film forming dispersion, corona treatment of PLA films and hot-pressing, as well as the impact of different variables commonly involved in multilayer shaping processes, like wet casting, spin coating, and coating thickness.

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MATERIALS AND METHODS

Production of single layers. Production of Support Layers (PLA Films). Corona Treated (CT) and Non-Corona Treated (NCT) PLA films available in the market for food packaging applications (thickness = 20 μm, Nativia NTSS, Taghleef Industries, San Giorgio di Nogaro, Udine, Italy) were produced as support layers. Both films were subjected to biaxial orientation and annealing processes during production. The drawn down ratio and the temperature of the orientation were 2−3 and 50−65 °C in the Machine Direction (MD) of processing line, while 4−5 and 70−85 °C in Transversal Direction (TD), respectively. Only one surface of films was treated by corona (30 W·m−2·min−1) and referred to as CT-treated in the present paper, the other surface being referred to as CT-untreated. In the case of NCT films the surfaces were named NCT-A and NCT-B. To reduce the effect of aging on PLA film properties, the samples were stored at −30 °C and 0% Relative Humidity (RH) until their final use and characterization. Production of Coating Layers (Wheat Gluten Films). Wheat gluten films (WG) were used as coating layers. They were produced using the solvent casting method adapted from Rocca-Smith et al. (2016),37 and B

DOI: 10.1021/acssuschemeng.8b04064 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Thermal Transitions. The thermal transitions related to amorphous and to crystalline phases of PLA films were analyzed by Differential Scanning Calorimetry (DSC) using a Q20 calorimeter (TA Instruments, New Castle, DE, U.S.A.). At least four samples (∼5 mg) were analyzed for each tested condition. They were submitted to two different heating programs at 10 °C·min−1 under N2 atmosphere (flow rate 25 mL·min−1). The first one (double heating from −10 °C to 100 °C) to focus on the amorphous phase of PLA: glass transition temperature (Tg) and variation of specific heat (ΔCp).The second one (double heating from −10 to 190 °C) to focus on the crystalline phase of PLA: temperature of crystal melting (Tm) and enthalpy of crystal melting (ΔHm). Calculations were performed following the same methodology as reported in a previous work6 for determining the crystallinity percentage (Xc), the percentage of Mobile Amorphous Phase (XMAP), and the percentage of the Rigid Amorphous Fraction (XRAF) in the PLA films. Transport Properties of Films to Gases and Water Vapor. The permeance of films to oxygen (PO2) was determined using a manometric method (GDP-C permeameter, Brugger Feinmechanik GmbH, Munich, Germany) under dry (0% RH) and wet conditions (50 and 84% RH) at 25 °C. Film samples were previously equilibrated at the same RH of the test before analysis. In dry conditions, the permeation system was previously outgassed under primary vacuum. At time zero, one side of the film was flushed with the gas (at a flow rate of ∼100 cm3·min−1) and the pressure increase was recorded over time on the other side. In wet conditions, only one side of the permeation cell was outgassed, while the other side was flushed by the humidified gas at the desired RH. The permeance (expressed in mol·m−2·s−1·Pa−1) was determined from the steady state according to eq 2.

Figure S1 (Supporting Information, SI) summarizes the sampling design, with the processing conditions and all samples considered in this study. Characterization of Surface Properties of Films. Surface Free Energy. The surface free energy of films (γs), and its polar (γps ) and dispersive (γds ) components, were determined by goniometry and using the Owens-Wendt method.40 Three liquids were used: water, ethylene glycol, and diiodomethane. The contact angles (θ) were measured in quintuplicate using the sessile drop method with a goniometer (Drop Shape Analyzer 30, Krüss GmbH, Hamburg, Germany). Both surfaces of each film were analyzed at 25 °C and 50% RH. Work of Adhesion. The work of adhesion (Wa) at the interface between layers in the PLA-WG-PLA laminates was calculated using the Dupré expression (eq 1):41 Wa = 2(γ1dγ2d)0.5 + 2(γ1pγ2p)0.5

(1)

where γd1, γp1, γd2, and γp2 are the dispersive and the polar contributions of the surface free energy of the layers 1 (PLA) and 2 (WG), respectively. T-Peel Test. The T-peel strength of PLA-WG-PLA laminates was measured using a texture analyzer (TA HD plus, Texture Technologies, Hamilton, MA, U.S.A.) according to the D 1876-01 ASTM (American Society for Testing and Materials) standard method.42 The load cell was 5 kg, the crosshead speed was 100 mm·min−1 and the trigger force was 0.1 g. At least seven rectangular specimens (15 × 2.5 cm2) with unbounded ends (2.5 × 2.5 cm2) were tested for each condition. The test was performed in the transverse direction (TD) of films. Before analysis, the samples were stored at 25 °C and 50% RH. Structural Properties of Films. Thickness. The thickness of films was measured in at least 5 different positions using a micrometer (Precision 1 μm, Coolant Proof micrometer IP 65, Mitutoyo, Aurora, IL, U.S.A.). The WG coating thickness of complexes produced by spin coating was estimated from at least two cross-section SEM images. Two-Photon Microscopy. Two-photon microscopy was performed to investigate the internal structure of wheat gluten films, using the autofluorescence of wheat gluten proteins. Images were collected using a Nikon A1-MP scanning microscope equipped with a Plan Apo IR × 60 objective (NA: 1.27, Water Immersion, Nikon, Tokyo, Japan) at a scanning speed of 1 frame per second. An InfraRed laser (Chameleon, Coherent, Santa Clara, CA, U.S.A.) was used to provide a 820 nm excitation. Fluorescence emission was collected on two detection channels: FF01-492/SP-25 (400−492 nm), FF03-525/50-25 (500− 550 nm). The samples were previously casted on a coverslip and stored at 100% RH for 4 days before observation in order to achieve better resolution. Cross-Section Microstructure by Scanning Electron Microscopy (SEM). The microstructure of the cross-section of PLA-WG-PLA laminates were assessed by SEM (Scanning Electron Microscopy) analysis. Images were collected using a JSM-7600F scanning electron microscope (JEOL USA Inc., Peabody, MA, U.S.A.) with 1 kV accelerating voltage, 9 × 10−6 Pa vacuum and lower detector (LEI) as secondary electron detector. Molecular Weight Distribution. The molecular weight distribution of PLA films were analyzed by Size-Exclusion Chromatography (SEC), using a 1260 Infinity liquid chromatography system (Agilent Technologies, Santa Clara, CA, U.S.A.) equipped of a refractive index detector. The device was composed of 2 Polypore Size Exclusion columns (Agilent Technologies) connected in series. PLA samples (approximately 50 mg) were dissolved in 1 mL of tetrahydrofuran (THF, Carlo Erba reagents, Val de Reuil, France). These samples were filtrated using a PTFE (poly(tetrafluoroethylene)) syringe filter (0.2 μm) before injection (10 μL). THF was used as mobile phase with a constant flow rate of 1 mL·min−1, at 45 °C. Polystyrene standards ranging from 1.28 to 1820 kDa (Advancing Polymer Solutions, Agilent Technologies) were used for calibration. The number-average molecular weight ( M n ), the weight-average molecular weight ( M w ) and the polydispersity index (PDI) were calculated from the experimental molecular weight distribution curve using Agilent GPC/ SEC software (version 1.2, Agilent Technologies).

P=

Δn AΔt(p1 − p2 )

(2)

where P is the permeance of oxygen, Δn is the variation of moles associated with the mass transfer (mol), A is the film surface area (m2), Δt is the time (s), p1 − p2 is the pressure difference between both sides of the film (Pa). Analyses were carried out in triplicate. The permeance of films to water vapor (PH2O) was measured gravimetrically at 25 °C using a modified ASTM E96−80 standard method.43 Three different relative humidity differentials were tested (0−30% RH, 30−75% RH, and 30−100% RH), using silica gel (SigmaAldrich), NaCl (Sigma-Aldrich) saturated solution or deionized water for maintaining around 0, 75, or 100% RH, respectively in the permeability cells, and placing them into a climatic chamber maintained at 30% RH and 25 °C. The permeance to water vapor was determined from the steady state according to eq 2. Analyses were carried out in quadruplicate. Statistical Analysis. Data were analyzed with Student t test and, when required (groups >2), with one−way analysis of variance (ANOVA) and with Tukey-Kramer multiple comparison test, using GraphPad Prism software (version 5.01, GraphPad Software Inc., La Jolla, CA, U.S.A.). The significance level of all statistical tests was fixed at 0.05.

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RESULTS AND DISCUSSION Influence of High-Pressure Homogenization (HPH) on the Wheat Gluten (WG) Films. HPH is an innovative strategy successfully used for increasing the solubility and dispersibility of particle aggregates, active compounds or nonmiscible materials in film forming solutions of biopolymers.38,44,45 Even if HPH is an energy and time-consuming step, it could considerably influence the functional properties of the resulting WG film by increasing the dispersibility of wheat gluten. Particle Size Distribution of WG Film Forming Dispersion. The particle size distribution in volume of the WG film forming dispersions prior to high-pressure homogenization followed a unimodal distribution (Figure 1). It was centered on a large diameter (∼270 μm). After being homogenized, the distribution C

DOI: 10.1021/acssuschemeng.8b04064 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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films (Figure S3, SI). The reduced thickness of the WG-high pressure films indicated a denser structure of the polymer network. This meant that the interactions between wheat gluten proteins were favored by HPH. Barrier Properties of WG Films to Water Vapor and Oxygen. It is important to report that it was not possible to carry out oxygen transfer experiments with WG-no high pressure films (at 0, 50 and 85% RH, 25 °C) due to their poor structure that makes films too brittle to determine oxygen permeability. Table 1 reports the water vapor permeance (PH2O) of films at three water vapor pressure differentials, while Table S1 (SI) reports water vapor transfer rate (WVTR). Whatever the RH differentials, HPH improved their water barrier properties. The most probable explanation of this behavior comes again from the different structure and densification of the polymer matrix. It is noteworthy that the PH2O of WG-high pressure films, although the films were thinner, was always lower than that of untreated films. From the lowest to the highest RH differential, PH2O increased by approximately 15 times for both wheat gluten films. This was attributed to the well-known plasticization effect by water molecules.46,47 Wheat gluten films are characterized by a high density of polar groups (e.g., hydroxyl, amino, carboxyl, and sulfhydryl) from hydrophilic amino acids or from the plasticizer (glycerol), which are able to interact with water and can induce subsequent plasticization and swelling. This reduced the intermolecular interactions, increased the internal mobility, and thus favored the mass transfer of molecules, such as water or gas. The impact of water plasticization on the barrier properties was further evidenced on oxygen transfer. Indeed, the oxygen permeance (PO2) increased from 15 (10−15 mol·m−2·s−1·Pa−1) to 208 and 498 (10−15 mol·m−2·s−1·Pa−1) when the RH of the surrounding environment increased from 0% to 50% and 84%, respectively. It is very interesting to notice the good barrier properties to O2 of WG-high pressure in comparison to conventional plastics. The O2 permeability of WG-high pressure films was 0.8, 12.1, and 28.9 × 10−18 mol·m−1·s−1·Pa−1 at 0%, 50%, and 84% RH, respectively. Guilbert et al. (2002)36 reported O2 permeability (0% RH, 23 °C) of 0.16, 11.9, 11.9, and 16.0 × 10−18 mol·m−1·s−1·Pa−1 for EVOH (ethylene-vinyl alcohol),

Figure 1. Change in particle size distribution of WG film forming dispersion after being submitted to high-pressure homogenization (100 MPa, 7 cycles). WG = wheat gluten.

shifted to lower values of at least 1 order of magnitude, but also turned to a multimodal distribution. The wheat gluten particles were thus sensitive to high-pressure and two overlapping populations were clearly observed, one centered on diameters of approximately 15 μm and the other one centered on approximately 3 μm. The corresponding d4,3 value decreased from 314.0 to 8.3 μm after homogenization. The HPH of the film forming solution was thus applied in order to help in the dispersion of wheat gluten and to prevent the formation of aggregates in the film. Macroscopic and Internal Structure of Films by Two Photon Microscopy. The HPH treatment also strongly influenced the appearance of WG films (Figure S2, Supporting Information, SI). Nearly transparent WG films were obtained, with smooth and homogeneous surface and without aggregates. The distribution of the wheat gluten protein aggregates within the films was analyzed with two-photon microscopy, based on their fluorescence properties to generate 3D images (Figure 2). Films without HPH treatment lead to protein aggregates of irregular shape, randomly distributed in the film. In contrast, HPH treatment was an efficient strategy for better dispersing wheat gluten, as suggested by the continuous light blue color in the image background, attributed to the better distributed protein network. Difference in the polymer structure was also evidenced by the thickness study of WG-high pressure and WG-no high pressure

Figure 2. 3D reconstruction of the internal structure of WG films using two-photon fluorescence microscopy. WG films were produced from film forming dispersions subjected (WG-high pressure) or not (WG-no high pressure) to high-pressure treatment (100 MPa, 7 cycles). WG = wheat gluten. D

DOI: 10.1021/acssuschemeng.8b04064 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Table 1. Water Vapor Permeance (PH2O) and Oxygen Permeance (PO2) of Films as a Function of High-Pressure Homogenization, Corona Treatment and Hot-Pressing Treatment, at Three Relative Humidity (RH) Differentials at 25 °C1 PH2O (10−9 mol·m−2·s−1·Pa−1)2 film

process high-pressure homogenization (WG films)

WG-no high pressure WG-high pressure

thickness (μm)

PO2 (10−15 mol·m−2·s−1·Pa−1)2

0−30% RH

30−75% RH

30−100% RH

20 ± 3

183 ± 2

276 ± 43

58.0 ± 8.0b

16 ± 2b

159 ± 5b

253 ± 21a

108.0 ± 7.0

a

a

a

a

0% RH §

50% RH §

3

3

15 ± 3

208 ± 19

85% RH §3 498 ± 16

corona treatment (PLA films)

NCT CT

20.0 ± 0.1a 20.0 ± 0.1a

30 ± 1a 31 ± 1a

34 ± 1a 32 ± 2a

41 ± 1a 41 ± 1a

8360 ± 1240a 5200 ± 370b

ND4 ND4

ND4 ND4

hot-pressing (PLA films)

NCT-hot pressing CT-hot pressing

20.0 ± 0.1a 20.0 ± 0.1a

26 ± 1a 24 ± 1b

27 ± 1a 29 ± 1b

32 ± 1a 25 ± 1b

3810 ± 60a 3460 ± 2b

ND4 ND4

ND4 ND4

1 WG = wheat gluten, PLA = poly(lactic acid), WG-no high pressure = WG films obtained without high-pressure homogenization, WG-high pressure = WG films obtained with high-pressure homogenization, NCT = non-corona treated PLA films, CT= corona treated PLA films, NCT-hot pressing = non-corona treated PLA films subjected to hot-pressing, CT-hot pressing = corona treated PLA films subjected to hot-pressing. 2Values are reported as mean ± standard deviation. Significant differences (p value < 0.05) are indicated with different letters in the same column for each process condition. 3§, Analysis not possible. 4ND, Not determined.

Table 2. Surface Properties of Both Sides of Wheat Gluten Films Obtained with High-Pressure Homogenization (WG-High Pressure) and PLA Films Treated (CT) or Not (NCT) by Corona1 contact angle (deg)2

surface free energy (mN·m−1)

film surface

θwater

high-pressure homogenization (WG films)

WG-high pressure-air WG-high pressure-support

91.0 ± 5.2 76.4 ± 1.6b

1.5 6.1

34.3 35.1

35.8 41.2

corona treatment (PLA films)

NCT-A CT-treated

77.7 ± 1.6a 65.8 ± 1.2b

4.8 10.6

32.9 31.3

37.7 41.9

process

γps

γds

γs

a

1

Water contact angle (θ) and surface free energy (γs) with polar (γps ) and dispersive contributions (γds ). WG = wheat gluten, PLA = poly(lactic acid), WG-high pressure-air = air surface of wheat gluten films obtained with high-pressure homogenization, WG-high pressure-support = support surface of wheat gluten films obtained with high-pressure homogenization, NCT-A = A surface of non-corona treated PLA films, CT-treated = treated surface of corona treated PLA films. 2Values are reported as mean ± standard deviation. Significant differences (p value