Blown Films for Chilled and Frozen Food Packaging Applications

polymers

Article Evaluation of the Suitability of Poly(Lactide)/Poly(Butylene-Adipate-co-Terephthalate) Blown Films for Chilled and Frozen Food Packaging Applications

Arianna Pietrosanto, Paola Scarfato * , Luciano Di Maio , Maria Rossella Nobile and Loredana Incarnato Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy; [email protected] (A.P.); [email protected] (L.D.M.); [email protected] (M.R.N.); [email protected] (L.I.) * Correspondence: [email protected]

Received: 20 December 2019; Accepted: 12 March 2020; Published: 3 April 2020

Abstract: The use of biopolymers can reduce the environmental impact generated by plastic materials. Among biopolymers, blends made of poly(lactide) (PLA) and poly(butylene-adipate-co-terephthalate) (PBAT) prove to have adequate performances for food packaging applications. Therefore, the present work deals with the production and the characterization of blown films based on PLA and PBAT blends in a wide range of compositions, in order to evaluate their suitability as chilled and frozen food packaging materials, thus extending their range of applications. The blends were fully characterized: they showed the typical two-phase structure, with a morphology varying from fibrillar to globular in accordance with their viscosity ratio. The increase of PBAT content in the blends led to a decrease of the barrier properties to oxygen and water vapor, and to an increase of the toughness of the films. The mechanical properties of the most ductile blends were also evaluated at 4 C and 25 C. The ◦ − ◦ decrease in temperature caused an increase of the stiffness and a decrease of the ductility of the films to a different extent, depending upon the blend composition. The blend with 40% of PLA revealed to be a good candidate for chilled food packaging applications, while the blend with a PLA content of 20% revealed to be the best composition as frozen food packaging material.

Keywords: biopolymers; PLA/PBAT; blown ﬁlms; food packaging; ﬁlm for frozen food

1. Introduction In recent years, the growing interest of the population towards environmental issues, and the increasingly stringent directives that various countries are adopting to fight against environmental pollution deriving from plastic materials, have directed research towards the development of ecofriendly materials that can substitute the conventional plastic ones [1]. The packaging sector is the one that uses the highest amount of plastic [2], and thus, the use of so-called “biopolymers” in the food packaging field could be a strategy to reduce the environmental impact generated by the traditional plastics materials [3]. Moreover, since a considerable amount of foods requires sub-ambient storage temperatures (e.g., chilled and frozen food), the retention of satisfactory performance properties at low temperatures has a key role to extending the range of applications of a material in the food packaging field. Different examples can be found in literature on the effect of sub-ambient temperature upon the mechanical and barrier response of conventional polyolefin-based systems [4–7], since, currently, these are the most employed materials for low temperature packaging applications.

Polymers 2020, 12, 804; doi:10.3390/polym12040804 www.mdpi.com/journal/polymers Polymers 2020, 12, 804 2 of 18

As a general trend, in terms of mechanical response, the temperature lowering gives a decrease in the material ductility and shock absorption power; instead, in terms of barrier properties, it gives a decrease of permeability to gases and a change in perm-selectivity. To our best knowledge, no literature data are available on the effect of sub-ambient temperatures on the functional performances of biopolymer-based packaging systems, and their applications for refrigerated food storage are still very limited. Among the biopolymers, PLA is one of the most promising and widespread ones that can replace petrochemical plastics [8–10]. PLA is a compostable and bio-based polymer, it is a linear aliphatic polyester, and its properties can be turned varying the relative content of D- and L-enantiomers [11]. It can be easily processed with conventional process such as extrusion, spinning, injection and compression molding [12–15]. However, its high brittleness limits its use, thus different strategies are tested in order to improve its toughness [16–19]. Among them, blending of PLA with PBAT has shown promising results. PBAT is a compostable aliphatic aromatic co-polyester that consists of two types of comonomer, a rigid butylene terephthalate segment made of 1.4 butanediol and terephthalic acid monomers, and a flexible butylene adipate section made of 1.4 butanediol and adipic acid monomers [20]. This polymer is flexible and tough, and it has complementary properties to PLA [21]. Several researchers have blended PLA with PBAT, through several techniques, like single-screw extrusion, twin-screw extrusion and solvent casting [20,22–26]. Results showed that, even if these polymers have very close solubility values [27], they are not thermodynamically miscible. Li et al. [28] studied the morphology of this system in a wide range of compositions. They showed that for a PBAT content lower than 20 wt%, the blend surface appeared uniform, characterized by the presence of featured small droplets suspended in the continuous phase. When the PBAT content increased between 20 and 50 wt%, the system became heterogeneous, with a co-continuous phase structure formed at 50 wt%. Furthermore, when the PBAT content exceeded 70 wt%, the morphology turned into droplets. In addition, Deng at al. [29] investigated the morphology of these systems, and they found a fibrillar morphology with a co-continuous phase structure when the PBAT content was between 20 wt% and 40 wt%, and PLA dispersed particles in the PBAT matrix when the content of the latter polymer was between 60 wt% and 80 wt%. However, even if these polymers are not miscible, their blending allows us to obtain several advantages. Hongdilokkul et al. [30] reported that the addition of 20 wt% of PBAT into the PLA matrix considerably improved the processability for film blowing by increasing the melt strength of the system. Moreover, the presence of PBAT influenced the crystallization behavior of PLA; in fact, it is reported that PBAT can act as a nucleating agent for PLA [30,31], increasing its crystallization rate. Furthermore, several works deal with the mechanical properties of this system. Wang et al. [32] have melt blended PLA and PBAT, and showed that the brittleness and ductility of PLA could be improved by adding 10% PBAT, which increased the elongation at break from 7.71% to 357.8%, and decreased the tensile strength from 65.21 MPa to 47.52 MPa. Li et al. [33] have analyzed a wide range of the composition of PLA/PBAT blends, showing an increase of the elongation at break with the PBAT content in the blend, and a reduction of the elastic modulus that, in the blend with the 20% of PBAT, was decreased of more than 30% compared to the pure PLA. Farsetti et al. [34] made an impact analysis, and they reported an increase of the GIC (critical release rate of strain energy) value at compositions ranging from neat PLA down to PLA mass fraction equal to 0.8, due to a toughening effect of PBAT. In general, the addition of PBAT lead to an increase of the ductility, but to a decrease of the elastic modulus in the tensile strength of the blends. Moreover, interesting results of these blends are reported also for the food packaging field. Tabasi et al. [35] reported that the sealability properties of PLA/PBAT blends can be tailored by varying the blend composition and PBAT crystallinity degree. Higher content of PLA in the blend and the lower crystallinity of PBAT provide lower hot-tack initiation temperature. Polymers 2020, 12, 804 3 of 18

Li et al. [33] investigated the oxygen permeability of these blends in a wide range of compositions, and with the addition of the 0.15 wt% of a compatibilizer, they found that the oxygen permeability of PBAT was two times higher than PLA, and the permeability of the blends increased as the content of PBAT in the blend increased. Wang et al. [23] reported that the presence of PBAT can considerably improve the UV-screening properties compared to neat PLA and PLA/PBAT films, which proved to be effective in preventing the greening of fresh potatoes. Moreover, the high-water vapor permeability of these films can be useful to prevent fogging on the surface of packaging film, as revealed for fresh, green onions. However, there is a lack of literature on the effect of the temperature on the mechanical properties of these systems. Only Gigante et al. [36] studied the effect of temperature on the fracture behavior of different PLA/PBAT blends, in which PLA was the matrix phase. By the estimation of the ductile-to-brittle transition temperature (DBTT), they highlighted that the ductile behavior for these blends appeared at temperatures higher than the PBAT glass transition temperature, and particularly, for the blends with a PBAT content between 15 wt% and 25 wt%, this happened around 23 C. − ◦ Moreover, the decrease of the ductility is a problem particularly relevant for materials with low thickness, so for the flexible packaging. Thus, the knowledge of the mechanical behavior at low temperatures is essential information to evaluate the suitability for packaging foods requiring low storage temperatures. Therefore, in this work, PLA/PBAT blown films in a wide range of compositions were developed and fully characterized, and the mechanical properties of the most ductile films were evaluated at three different temperatures (25 C, 4 C and 25 C), in order to extend the range of application of ◦ ◦ − ◦ these blends for chilled and/or frozen food flexible packaging.

2. Experimental

2.1. Materials PLA 4032D, supplied by NatureWorksTM (Minnetonka, MN, USA), has a content of D-isomer 3 equal to 1.5 wt%, a speciﬁc gravity of 1.24 g/cm and a melting temperature between 155–170 ◦C. Ecoworld PBAT 009 was manufactured by Jin Hui Zhaolong (Lüliang, China); it is composed by the 29% of adipic acid, 26% of terephthalic acid and 45% of 1,4-butanediol, it has a density of 1.26 g/cm3 and a melting temperature around 110–120 ◦C. Both materials comply with USA Food and Drug Administration (FDA) and European Union (EU) regulations for direct food contact. They also conform the standards of compostability under controlled composting conditions, i.e., standards EN13432 and ASTM D 6400.

2.2. Preparation of the Films

PLA and PBAT pellets were dried under vacuum at 70 ◦C for 20 h prior to processing. PLA and PBAT at different PLA/PBAT concentrations (100/0, 80/20, 60/40, 40/60 20/80 0/100 by weight) were mixed in a Collin ZK25 co-rotating twin extruder (D = 25 mm, L/D = 42), with a screw speed equal to 100 rpm, a temperature profile ranging from 140 ◦C to 190 ◦C, and a mass flow ranging from 50 to 54 g/min. Then, the materials were cooled in a water bath. After, pure materials and each blend were dried under vacuum at 70 ◦C for 20 h before processing. The blown films were prepared in a multilayer plant, using two single screw extruders GIMAC (D = 12 mm, L/D = 24), with a screw speed equal to 50 rpm and a take up speed of 1 m/min. The temperature profile ranged from 190 ◦C to 180 ◦C for pure PLA, from 180 ◦C to 135 ◦C for the blends and from 160 ◦C to 125 ◦C for pure PBAT. Films were produced with a blow-up ratio (BUR) and a take-up ratio (TUR) equal to 2.5 and with constant thickness of 75 5 µm. ± Polymers 2020, 12, 804 4 of 18

2.3. Rheological Characterization The rheological properties in the oscillatory mode of pellets of PLA, PBAT and their blends were evaluated using an oscillatory shear strain-controlled rheometer, ARES. Samples were dried under vacuum at 70 ◦C for 16 h prior to testing. Tests were performed with a parallel-plate geometry (d = 25 mm) with a gap of 1 mm at 180 ◦C under a nitrogen atmosphere, in order to minimize thermo-oxidative degradation. A strain sweep test was initially conducted to guarantee the linear viscoelastic regime for each formulation. Frequency sweep tests were conducted in triplicate, with a standard deviation less than 2%.

2.4. Differential Scanning Calorimetry (DSC) Thermal analysis on the pellets of pure materials and their blends and on the produced films was performed using a Differential Scanning Calorimeter (DSC mod. 822, Mettler Toledo, Columbus, OH, USA) under a nitrogen gas flow (100 mL/min), in order to minimize thermo-oxidative degradation phenomena. The temperature programming consisted of three steps: The samples were heated from 70 to 200 C with a speed of 10 C/min, and held at 200 C for 5 min. They were then cooled at − ◦ ◦ ◦ 70 at 10 C/min, and reheated to 200 C at 10 C/min. The crystallinity degrees, Xc, were calculated − ◦ ◦ ◦ according to the following formula:

∆Hm ∆Hcc Xc = − 100 (1) ∆Hm0 ϕi × × where ∆Hm and ∆Hcc (J/g) are the respective heat of melting and heat of cold crystallization of PBAT and PLA, the ∆Hm0 is equal to 93.6 J/g for PLA [37] and 114 for PBAT [38], and ϕi is the relative weight fraction of PLA or PBAT in the blends.

2.5. Fourier Transformation Infrared Spectroscopy (FT-IR) Infrared spectra of the ﬁlm of pure materials and their blends were obtained with a Thermo Nicolet NEXUS 600 spectrometer in attenuated total reﬂectance (ATR) mode, with a diamond crystal collecting 1 64 scans. Each spectrum was obtained within the range of 4000–400 cm− with the wavelength 1 resolution of 4 cm− .

2.6. Morphological Characterization Film samples were cryo-fractured and then coated with a thin gold layer (Agar Auto Sputter Coater mod. 108 A, Stansted, UK) at 30 mA for 160 s to improve their conductivity. Afterwards, their cross sections parallel to the transversal direction (TD) were scanned by ﬁeld emission scanning electron microscope (FESEM) (LEO 1525 model, Carl Zeiss SMT AG, Oberkochen, Germany). Length evaluation was performed through the SigmaScan Pro™ Software.

2.7. Oxygen Transmission Rate (OTR) OTR measurements were carried out through a permeabilimeter (GDP—C 165 of Brugger), with a manometric operation, connected to a thermo-controlled bath (ThermoHaake). Before testing, the evacuation of the upper and the bottom half-cell was performed to drive away humidity and residual gases. The test temperature was set at 23 ◦C, and the oxygen flow to 80 mL/min, according to ISO 2 15105-1. The area of the tested films was 9 cm . The values of the permeability coefficients (P O2) were obtained by multiplying the measured value of OTR by the respective thicknesses (mm) of the films.

2.8. Water Vapor Transmission Rate (WVTR) Water vapor permeability tests were performed through a Water Vapor Permeation Analyzer (Model 7002—Systech Illinois, Princeton, NJ, USA), which provides a modular system for the determination of water vapor permeability using a sensor based upon P2O5. Tests were carried Polymers 2020, 12, 804 5 of 18

out according to ASTM F 1249-90 standard (with the only exception of sensor technology) at 23 ◦C and 50% of relative humidity. The area of the tested films was 5 cm2. The values of the permeability coefficients (P H2O) were obtained by multiplying the measured value of WVTR by the respectively thickness (mm) of the films.

2.9. Mechanical Properties Tensile testing of blown ﬁlms was performed on a SANS dynamometer (Sans Testing Machine Co. Ltd., Shenzhen, China) equipped with a 100 N load cell. The rectangular shape specimens (width = 12.7 mm and length = 30 mm) were extended at a crosshead speed set according to ASTM D822 Polymers 2020, 12, x FOR PEER REVIEW 5 of 18 standard. Mechanical properties were evaluated in the machine direction (MD). Tests= 12.7 were mm carried and length out at= 30 ambient mm) were temperature extended at (25 a ◦crossheadC) and at speed controlled set according environment to ASTM temperatures D822 (4 C andstandard.25 MechanicalC), using properties an Environmental were evaluated chamberin the machine connected direction to (MD). a WK650 High Precision ◦ − ◦ TemperatureTest controllers were carried and to out a Liquidat ambient nitrogen temperature container. (25 °C) All and of the at controlled data are the environment average of at least seventemperatures measurements. (4 °C and −25 °C), using an Environmental chamber connected to a WK650 High Precision Temperature controller and to a Liquid nitrogen container. All of the data are the average 3. Resultsof at andleast Discussionseven measurements.

3.1. Rheological3. Results Characterization and Discussion of the Pellet Rheological3.1. Rheological oscillatory Characterization analysis of the was Pellet carried out to investigate the morphology and processability of the blends.Rheological First, a oscillatory dynamic strain analysis sweep was testcarried was out performed to investigate for both the the morphology pure materials and at a frequencyprocessability of 10 rad /s, of with the blends. the aim First, to determine a dynamic the strain limit sweep of linear test was viscoelasticity performed for that both was the found pure to be greatermaterials than 5% at of a deformation.frequency of 10 Thus,rad/s, with all of the the aim rheological to determine tests the werelimit of performed linear viscoelasticity at a strain that equal to 5%. Dynamicwas foun timed to sweepbe greater tests than were 5% thenof deformation. performed Thus, to investigate all of the rheological the thermal tests stability were performed during theat ﬂow in an inerta strain atmosphere equal to 5%. at the Dynamic rate of time 1 rad sweep/s. These tests were tests then revealed performed a percentage to investigate of complex the thermal viscosity stability during the flow in an inert atmosphere at the rate of 1 rad/s. These tests revealed a percentage reduction in a time of 7 min (duration of a frequency sweep test) of less than 1% for both materials. of complex viscosity reduction in a time of 7 min (duration of a frequency sweep test) of less than 1% Frequencyfor both sweep materials. tests were Frequency then performedsweep tests were with then a frequency performed ranging with a frequency from 0.1 ranging to 100 radfrom/s. 0.1 The to trend of the complex100 rad/s. viscosityThe trend versusof the complex the frequency viscosity isversus reported the frequency in the Figure is reported1. in the Figure 1.

10000

PLA PLA/PBAT 80/20 PLA/PBAT 60/40

1000 PLA/PBAT 40/60 [Pa·s]

· · PLA/PBAT 20/80 G' PBAT

100 0.1 1 10 100 ω [rad/s]

FigureFigure 1. Complex 1. Complex viscosity viscosity of the extruded extruded pellets. pellets.

ResultsResults show show that boththat both the the pure pure materials materials exhibit exhibit shear shear thinning thinning behavior, behavior, and anda low a-frequency low-frequency NewtonianNewtonian plateau. plateau. Furthermore, Furthermore, it is it evident is evident that that the the complex complex viscosity viscosity ofof the PLA is is higher higher than than that of PBATthat for of all PBAT of the for frequencyall of the frequency tested, particularlytested, particularly at the at viscosity the viscosity ratio ratio of of 100 100 rad rad/s,/s, when when PBAT is the is the dispersed phase, and is equal to 0.25, and when PBAT is the matrix phase, and is equal to 4. It dispersed phase, and is equal to 0.25, and when PBAT is the matrix phase, and is equal to 4. It is well is well known that the viscosity ratio has a great influence on the morphology of the blends [39]. In knownfact, that for the a viscosityviscosity ratio ratio lower has athan great one, influence the application on the of morphology high shear determines of the blends the formation [39]. In fact, of for a viscositythe ratio fibrillar lower morphology than one, of the the application dispersed phase. of high Instead, shear for determines viscosity ratio the higherformation than of one, the the fibrillar morphologydispersed of the phase dispersed has a moderate phase. Instead, deformation for viscosity and keeps ratio its spherical higher than shape one, [40]. the So, dispersed a fibrillar phase has a moderatemorphology deformation is expected when and keeps PLA is itsthe spherical matrix phase shape, and [ 40a globular]. So, a morphology fibrillar morphology is expected iswhen expected when PLAPBAT is is the the matrix matrix phase,phase. and a globular morphology is expected when PBAT is the matrix phase. Moreover, the complex viscosity graph shows that the blends had a shear thinning behavior more accentuated than the pure materials, and the increase of PLA content led to an increase of their complex viscosity. Particularly, the PLA/PBAT 80/20 blend not only exhibited a higher viscosity, but, at low frequencies, that viscosity was higher than both PLA and PBAT, suggesting higher interactions of the two polymers at this composition [28].

Polymers 2020, 12, 804 6 of 18

Moreover, the complex viscosity graph shows that the blends had a shear thinning behavior more accentuated than the pure materials, and the increase of PLA content led to an increase of their complex viscosity. Particularly, the PLA/PBAT 80/20 blend not only exhibited a higher viscosity, but, at low frequencies, that viscosity was higher than both PLA and PBAT, suggesting higher interactions of the two polymers at this composition [28].

FigurePolymers 202 20shows, 12, x FOR the PEER trend REVIEW of the storage modulus (G’) in function of the frequency. The6 of storage18 modulus of the blends, for high frequencies values, increased as the content of PLA in the blend increased,Figure and instead,2 shows atthe low trend frequencies, of the storage it ismodulus higher (G’) for allin function the blends of the than frequency. the pure The materials. storage Formodulus PLA /ofPBAT the blends, 80/20 andfor high 20/80, frequencies G’ shows values, a typical increased shoulder as the that content has of been PLA reported in the blend for many increased, and instead, at low frequencies, it is higher for all the blends than the pure materials. immiscible blends [28]. This behavior of G’ is due to the “additional” elastic response originating For PLA/PBAT 80/20 and 20/80, G’ shows a typical shoulder that has been reported for many from the surface tension of the dispersed phase in the continuous matrix. Moreover, a large G’ plateau immiscible blends [28]. This behavior of G’ is due to the “additional” elastic response originating at lowfrom frequencies the surface istension seen of for the the dispersed blends phase PLA in/PBAT the continuous 60/40 and matrix. 40/60, Moreover, reﬂecting a large a sort G’ plateau of solid-like behavior.at low This frequencies behavior is hasseen been for the reported blends inPLA/PBAT many investigations 60/40 and 40/60 [41, ]reflecting for immiscible a sort of blends solid-like that form co-continuousbehavior. This structures, behavior and has thisbeen is reported due to extrain many stresses investigations associated [41] with for immiscible the shape blends relaxation that of the inter-connectiveform co-continuous structures structures [42]. In, and fact, this as is reported due to extra by Omonovstresses associated et al. [43 ],with when the ashape continuous relaxation network structureof the is inter formed,-connective the relaxation structures [42]. times In shiftfact, as to reported very low by frequency Omonov et values al. [43], that when cannot a continuous be observed experimentally,network structure thus they is formed, used thisthe relaxation criterion totimes set shift the composition to very low freq limitsuency of values a co-continuous that cannot structure.be observed experimentally, thus they used this criterion to set the composition limits of a co-continuous Therefore, the 40 wt% and the 60 wt% of PBAT can be the beginning and the ending points of the structure. Therefore, the 40 wt% and the 60 wt% of PBAT can be the beginning and the ending points co-continuous structure, which are in accordance with the values found in literature that vary from of the co-continuous structure, which are in accordance with the values found in literature that vary 20 wt%from to 20 60 wt wt%% to[ 6028 ,wt29%,44 [28,29,44].].

1000000

100000

10000 PLA 1000 PLA PBAT 80 20 PLA PBAT 60 40 100

PLA PBAT 40 60 G' [Pa] G' 10 PLA PBAT 20 80 PBAT 1

0.1 0.1 1 10 100 [rad/s] ω FigureFigure 2.2. G’ of the the extruded extruded pellets. pellets.

3.2. Thermal3.2. Thermal Analysis Analysis (DSC) (DSC) of of the the Films Films All theAll ﬁlmsthe films were were subjected subjected to to the the samesame thermal cycle cycle described described in the in theexperimental experimental part. The part. The mainmain thermal thermal parameters parameters related related to to the the ﬁrstfirst heatingheating are are shown shown in inthe the Table Table 1. 1.

Table 1. Differential Scanning Calorimetry (DSC) results of the 1st Heating scan of the films. Table 1. Diﬀerential Scanning Calorimetry (DSC) results of the 1st Heating scan of the ﬁlms. I Heating I HeatingTm1 Tm2 ΔHm Tm ΔHm Xc Xc Tg PBAT Tg PLA Tcc ΔHcc T T T PBAT TPBAT ∆PBATH PLAT PLA∆H PBAT XPLA X g g[°C] T [°C] ∆[°C]H [J/g] m1 m2 m m m c c PBAT PLA cc cc PBAT[°C] PBAT[°C] PBAT[J/g] [°C]PLA [J/g]PLA [%] PBAT[%] PLA PLA / [◦C]63.5 [J97.6/g] 35.3 / / / 170.3 36.6 / 1.5 [◦C] [◦C] [◦C] [◦C] [J/g] [◦C] [J/g] [%] [%] PLA PBAT 80 20 −33.1 62.1 97.2 21.7 / / / 168.6 27.9 / 8.3 PLAPLA PBAT 60 /40 63.5−33.9 97.661.3 35.396.0 14.9 ///47.5 118.0 0.8 169.2170.3 22.8 36.6 1.7 14.1/ 1.5 PLA PBATPLA 80 20PBAT 4033.1 60 62.1−34.1 97.261.5 21.796.4 8.0 ///48.2 111.8 2.3 168.8168.6 15.3 27.9 3.3 19.4/ 8.3 − PLA PBATPLA 60 40PBAT 2033.9 80 61.3−35.1 96.063.2 14.995.5 1.7 47.5 48.0 118.0112.6 0.87.9 167.2 169.2 6.0 22.8 8.7 1.722.7 14.1 − PLA PBAT 40 60PBAT 34.1 61.5−35.4 96.4/ 8.0/ / 48.248.8 111.8110.6 2.317.2 168.8/ / 15.315.1 3.3/ 19.4 − PLA PBAT 20 80 35.1 63.2 95.5 1.7 48.0 112.6 7.9 167.2 6.0 8.7 22.7 − PBAT 35.4 /// 48.8 110.6 17.2 // 15.1 / The thermograms− of the films of the 1st heating scan are reported in Figure 3. Thermal properties have a great influence on the mechanical behavior of the films. Particularly, the glass transition temperature (Tg) represents the transition point between the glassy and brittle behavior and the ductile behavior [45]. The glass transition temperature of PLA and PBAT are at 63 °C and −34 °C, respectively. This means that PLA has a brittle behavior at ambient temperatures (around 25 °C), and also for lower temperatures, while PBAT keeps its ductile behavior for

Polymers 2020, 12, 804 7 of 18

The thermograms of the films of the 1st heating scan are reported in Figure3. Thermal properties have a great influence on the mechanical behavior of the films. Particularly, the glass transition temperature (Tg) represents the transition point between the glassy and brittle behavior and the ductile behavior [45]. The glass transition temperature of PLA and PBAT are at 63 ◦C and 34 C, respectively. This means that PLA has a brittle behavior at ambient temperatures (around − ◦ 25 ◦C), and also for lower temperatures, while PBAT keeps its ductile behavior for temperatures above 34 C. Moreover, the thermograms of the first heating of these materials show the presence of one − ◦Polymers 2020, 12, x FOR PEER REVIEW 7 of 18 endothermic peak for PLA at 171 ◦C, and two endothermic peaks for PBAT, at the temperatures of 36 ◦Ctemperatures and 107 ◦C. above −34 °C. Moreover, the thermograms of the first heating of these materials show Thethe presence first melting of one peak, endothermic not very peak intense, for PLA can beat 171 attributed °C, and totwo the en meltingdothermic of thepeaks crystalline for PBAT, phase at of the butylene-adipatethe temperatures of (BA) 36 ° fractionC and 107 [46 °C.], while the second melting peak, more intense and widened, is The first melting peak, not very intense, can be attributed to the melting of the crystalline phase between 70 C and 140 C. All the blends showed two different glass transition temperatures, and this of the butylene◦ -adipate◦ (BA) fraction [46], while the second melting peak, more intense and widened, is a furtheris between confirmation 70 °C and 140 that °C. PLA All the and blends PBAT showed are not two thermodynamically different glass transition miscible temperatures [47]. The, presenceand of PBATthis ledis a to further a decrease confirmation of the cold that PLA crystallization and PBAT temperatureare not thermodynamically (Tcc) of PLA, miscible indicating [47]. that The PBAT increasespresence the of crystallization PBAT led to a ratedecrease of PLA, of the as cold previously crystallization reported temperature [33,48]. (T Moreover,cc) of PLA, asindicating reported by Dengthat etal. PBAT [29], increases the crystallinity the crystallization degree (X c rate) of of PLA PLA, increases as previously with the reported increase [33,48]. of the Moreover, PBAT content as in the blend.reported This by behaviorDeng et al. could [29], bethe due crystallinity to the greater degree mobility (Xc) of PLA of the increases PLA chains with the in theincrease presence of the of the PBAT.PBAT On thecontent other in hand, the blend. the opposite This behavior behavior could is be shown due to for the the greater degree mobility of crystallinity of the PLA of chains PBAT, in which decreasesthe presence as the PLA of the content PBAT. in On the the blend other increases. hand, the Since opposite PLA behavior has a lower is shown chain for flexibility the degree and of higher viscositycrystallinity than PBAT, of PBAT, it restricts which thedecreases mobility as the of thePLA PBAT content chains, in the and blend therefore increases. reduces Since PLA its crystallinity has a lower chain flexibility and higher viscosity than PBAT, it restricts the mobility of the PBAT chains, degree [20]. and therefore reduces its crystallinity degree [20].

PLA

PLA PBAT 80 20

PLA PBAT 60 40

PLA PBAT 40 60 Heat Flow [J/g] Flow Heat PLA PBAT 20 80

PBAT

-60 -20 20 60 100 140 180 T [°C]

FigureFigure 3. 3.DSC DSC thermograms thermograms of the the 1 1stst heating heating scan scan of ofthe the films. ﬁlms.

3.3. FT-IR3.3. FT-IR FTIRFTIR spectroscopy spectroscopy investigations investigations were were performedperformed on on the the films ﬁlms in inorder order to obtain to obtain information information on on blendsblends morphology, morphology and, and the the results results are are reported reported inin thethe Figure 4.4. According to Al-Itry et al. [49], the following main characteristic peaks for PLA can be observed: According to Al-Itry et al. [49], the following main characteristic peaks for PLA can be observed: The symmetric stretching vibration of the axial CH groups in saturated hydrocarbons at 2800–3000 1 The symmetriccm−1, the intense stretching peak vibrationoriginating of from the axialthe C=O CH stretching groups in v saturatedibrations located hydrocarbons at around at 1747 2800–3000 cm−1, acm − , 1 the intenseweaker peak band originatingat 1250–1050 from cm−1 theas a C result=O stretching of the C–O vibrations carboxyl groups located, and at aroundtwo weak 1747 peaks cm −at ,866.5 a weaker 1 1 bandcm at− 1250–10501 and 75,464 cm cm−−1 asas aa resultresult of the C–OC–OCC carboxyl bond stretching groups, andand twoCO bending, weak peaks respectively at 866.5.cm The− and 1 75,464functional cm− as groupsa result of of PBAT the C–OCC can be bond described stretching as: Peaks and at CO around bending, 3000 respectively. cm−1 representing The functional C–H −1 1 groupsstretching of PBAT in aliphatic can be and described aromatic as: portions; Peaks at at around around 1710 3000 cm cm representing− representing carbonyl C–H groups stretching (C=O) in in the ester linkage; at 1267 cm−1 representing C–O in1 the ester linkage; and a sharp peak at 720 cm−1 aliphatic and aromatic portions; at around 1710 cm− representing carbonyl groups (C=O) in the ester representing four1 or more adjacent methylene (–CH2–) groups. Bending peaks of the1 benzene linkage; at 1267 cm− representing C–O in the ester linkage; and a sharp peak at 720 cm− representing substitutes are located at wave numbers between 700 and 900 cm−1. In the blends, some peak shifts four or more adjacent methylene (–CH2–) groups. Bending peaks of the benzene substitutes are located were detected: The peak of the PLA carbonyl group shifts as the PBAT content increased from 1747 at wave numbers between 700 and 900 cm 1. In the blends, some peak shifts were detected: The peak cm−1 to 1753 cm−1 in the PLA/PBAT 60/40− blend; the peak of the carbonyl group of PBAT moved as the PLA increases from 1710 cm−1 to 1714 cm−1 in the PLA/PBAT 60/40 blend; the peak at 866.5 cm−1 of PLA C-OCC moved progressively to values between 870 and 872 cm−1 as the content of PBAT in the blend increased, and the peak relative to the CO bending moved up to 750 cm−1 in the blend PLA/PBAT 20/80. These shifts indicate that these polymers, even though they are immiscible, have some interactions, and a certain degree of compatibility.

Polymers 2020, 12, 804 8 of 18

1 1 of the PLA carbonyl group shifts as the PBAT content increased from 1747 cm− to 1753 cm− in the PLA/PBAT 60/40 blend; the peak of the carbonyl group of PBAT moved as the PLA increases from 1 1 1 1710 cm− to 1714 cm− in the PLA/PBAT 60/40 blend; the peak at 866.5 cm− of PLA C-OCC moved 1 progressivelyPolymers 2020, 1 to2, x values FOR P EER between R EVIEW 870 and 872 cm− as the content of PBAT in the blend increased, and8 of 18 1 the peak relative to the CO bending moved up to 750 cm− in the blend PLA/PBAT 20/80. These shifts indicatePLA/PBAT that these20/80. polymers, These shifts even indicate though that they these are immiscible, polymers, even have though some interactions, they are immiscible, and a certain have degreesome ofinteractions compatibility., and a certain degree of compatibility.

FigureFigur 4.e 4.Fourier Fourier transform transform infrared infrared spectroscopy spectroscopy (FTIR) (FTIR spectra) spectra of of the the films: films: (a ()a in) in the the full full x-axis x-axis scale scale (b()b zoom) zoom of of the the carbonyl carbonyl bonds bonds (c )(c zoom) zoom of of the the C–OCC C–OCC and and CO CO bonds. bonds. 3.4. Morphological Characterization 3.4. Morphological Characterization Figure5 gives the FESEM images of the fracture surface of the films for the neat polymers and Figure 5 gives the FESEM images of the fracture surface of the films for the neat polymers and their blends. PLA and PBAT show homogeneous fracture surfaces. On the contrary, all the blends their blends. PLA and PBAT show homogeneous fracture surfaces. On the contrary, all the blends exhibit a typical two-phase structure characterized by the presence of voids and inclusions of variable exhibit a typical two-phase structure characterized by the presence of voids and inclusions of variable shape and dimension. Different morphologies of the dispersed phase can be observed for these blends. shape and dimension. Different morphologies of the dispersed phase can be observed for these When PBAT was the dispersed phase, the blends showed an elongated fibrous morphology, yet instead blends. When PBAT was the dispersed phase, the blends showed an elongated fibrous morphology, when PBAT became the matrix phase, the morphology turned into globular, in accordance with their yet instead when PBAT became the matrix phase, the morphology turned into globular, in accordance viscosity ratios. Particularly, the blend with the 80% of PLA showed an elongated dispersed phase with their viscosity ratios. Particularly, the blend with the 80% of PLA showed an elongated with thin fibrils that confirms the higher interactions of the two polymers in this composition supposed dispersed phase with thin fibrils that confirms the higher interactions of the two polymers in this in the rheological analysis. Moreover, the blends with a content of the dispersed phase equal to 40% composition supposed in the rheological analysis. Moreover, the blends with a content of the showed bigger inclusions than the other blends. dispersed phase equal to 40% showed bigger inclusions than the other blends. In fact, higher concentrations of the dispersed phase can lead to higher coalescence, and so to In fact, higher concentrations of the dispersed phase can lead to higher coalescence, and so to bigger dimensions of the aggregates [44]. However, discrete domains that are not part of a network bigger dimensions of the aggregates [44]. However, discrete domains that are not part of a network structure can be observed for both the blends with a dispersed phase content of 40 wt%, suggesting that the “fully co-continuous structure” could be formed in a composition between this range.

Polymers 2020, 12, 804 9 of 18 structure can be observed for both the blends with a dispersed phase content of 40 wt%, suggesting thatPolymers the 20 “fully20, 12, co-continuousx FOR P EER R EVIEW structure” could be formed in a composition between this range. 9 of 18

Figure 5.5. FieldField emissionemission scanning scanning electron electron microscope microscope (FESEM) (FESEM) images images of: (a )of: neat (a) poly(lactide) neat poly(lactide) (PLA), ((PLA)b) neat, (b poly(butylene-adipate-co-terephthalate)) neat poly(butylene-adipate-co-terephthalate) (PBAT), (PBAT) (c) PLA, ( PBATc) PLA 80 PBAT/20, (d 80/20,) PLA PBAT(d) PLA 60 / PBAT40 (e) PLA60/40 PBAT (e) PLA 40 /PBAT60, (f) PLA40/60, PBAT (f) PLA 20/ 80.PBAT 20/80.

3.5. Barrier Properties

Oxygen and moisture barrier propertiesproperties are among the most important issues toto be considered in materialsmaterials intendedintended to be used in foodfood packaging,packaging, since their presence in somesome cases maymay lead to detrimentaldetrimental changes changes in in quality quality that that result result in adecrease in a decrease of the food of the shelf-life. food shelf In Table-life.2 the In permeabilityTable 2 the coepermeabilityﬃcient to oxygencoefficient and to water oxygen vapor and are water reported. vapor are reported.

Polymers 2020, 12, 804 10 of 18

Table 2. Oxygen and water vapor permeability coeﬃcients of the ﬁlms.

3 2 2 PO2 [cm mm/m d bar] P H2O [g mm/(d m )] · · · · · PLA 33.4 1.3 PLA PBAT 80 20 40.9 1.4 PLA PBAT 60 40 50.3 2.8 PLA PBAT 40 60 61.9 2.9 PLA PBAT 20 80 71.8 3.0 PBAT 84.0 3.1

From the reported table it is clear that PBAT showed significant lower barrier properties to both oxygen and water vapor than neat PLA; in fact, from the above-reported results, it is clear that adding PBAT to PLA results in an increase of the coefficient of permeability, as reported in literature [23,33]. This could be related to the different glass transition temperatures of these polymers. PLA at 23 ◦C is in the glassy state, which means a lower fraction of free volume between the polymer chains, and a reduced mobility compared to PBAT that has a Tg under 23 ◦C. These factors greatly influence the permeability of gas molecules, whose permeability is facilitated with high chains mobility and free volume fractions. Moreover, for polymer blends, it is useful to predict the macroscopic properties, thus different models were applied in order to find the best fitting for the oxygen permeability of PLA/PBAT blends. In the Maxwell equations, the two different polymers are replaced with two bars consisting of a rubbery and a glassy phase. In the parallel model, the two bars are connected in parallel, and in the series model they are connected in series. These models represent the highest and lowest limits for the permeability coefficient of a polymeric blend. The Maxwell series and parallel model [49,50] are respectively:

P = P P /(ϕ P + ϕ P ) b 1 · 2 1 · 2 2 · 1 P = ϕ P + ϕ P b 1 · 1 2 · 2 where Pb is the permeability of the blend, P1, and P2 are the permeabilities of the respective phases, with ϕ1 and ϕ2, being the corresponding volume fractions. Robeson’s equation describes the eﬀect of a spherical ﬁller on the overall composite permeability [51]. The equation is reported below:

Pb = Pm [Pd + 2Pm 2 ϕd (Pm Pd)]/[Pd + 2Pm + ϕd (Pm Pd)] · − · · − · − where Pb is the permeability of the blend, Pm is the permeability of the matrix, Pd is the permeability of the disperse phase, and ϕd is the volume fraction of dispersed phase. Figure6 shows that the experimental values of oxygen permeability are between the Maxwell series and the parallel equations. The curve that ﬁts best the experimental results is the Robeson equation, particularly for the PLA/PBAT 80/20 and 20/80 samples, likely due to dispersed morphology of these compositions. Thus, the Robeson equation could be applied for the prediction of the oxygen permeability coeﬃcient of the PLA/PBAT blends. Polymers 2020, 12, 804 11 of 18 Polymers 2020, 12, x FOR PEER REVIEW 11 of 18 Polymers 2020, 12, x FOR P EER R EVIEW 11 of 18

90 90 80 80 Experimental Values

70 Experimental Values *d*bar]

2 70

*d*bar] Maxwell Series 2 60 Maxwell Series 60 Maxwell Parallel

*mm/m Maxwell Parallel 3

*mm/m 50

3 Robeoson Equation

[cm 50 Robeoson Equation 2 2 [cm

2 40

P O P 40 P O P 30 30 0 20 40 60 80 100 0 20 40PBAT %60 80 100 PBAT %

Figure 6. Oxygen Permeability in function of the blends’ composition. Figure 6.6. OxygenOxygen PermeabilityPermeability inin functionfunction ofof thethe blends’blends’ composition.composition. 3.6. Mechanical Properties 3.6.3.6. Mechanical PropertiesProperties Results of the tensile tests performed at ambient temperature are reported in Figure 7 and in Results of the tensile tests performed at ambient temperature are reported in Figure7 and in TableResults 3. of the tensile tests performed at ambient temperature are reported in Figure 7 and in TableTable3 .3.

50 T = + 25°C PBAT PLA/PBAT 20/80 40 PLA/PBAT 40/60 PLA/PBAT 60/40

PLA/PBAT 80/20 )

a 30 PLA

(

s 20

t S

0 0,0 0,1 0,2 2 3 4 5 6 Strain (mm/mm)

Figure 7. Tensile stress–strain curves, obtained at 25 ◦C, for all the films. Figure 7. Tensile stress–strain curves, obtained at 25 °C , for all the films. Figure 7. Tensile stress–strain curves, obtained at 25 °C, for all the films. Table 3. Tensile properties of the films at 25 ◦C. Table 3. Tensile properties of the films at 25 °C. MechanicalTable 3. Tensile Properties properties at Ambient of the films Temperature at 25 °C. Mechanical Properties at Ambientε Temperatureσ BlendMechanical E Properties (MPa) at Ambientb (%) Temperature y (MPa) Blend E (MPa) εb (%) σy (MPa) PLA 2411.0 164.3 6.7 1.2 45.3 5.4 Blend ±E (MPa) εb± (%) σy (MPa)± PLA PBAT 80 20PLA 1432.12411.0119.7 ± 164.3 15.46.73.8 ± 1.2 45.3 39.1 ± 5.42.5 PLA 2411.0± ± 164.3 6.7± ± 1.2 45.3 ± 5.4± PLA PBATPLA 60 PBAT 40 80 819.620 1432.173.1 ± 119.7 183.015.420.1 ± 3.8 39.1 22.1 ± 2.52.3 ± ± ± PLA PBATPLAPLA 40 PBAT PBAT 60 80 60 20 288.040 1432.1819.6±27.4 ± 119.773.1 220.8183.015.412.9 ± ± 3.8 20.1 39.122.1 9.7± ± 2.5 2.31.2 ± ± ± PLA PBATPLA 20 PBAT 80 60 40 110.9 819.6±6.6 73.1 531.9183.043.2 ± 20.1 22.1 6.1± 2.30.7 PLA PBAT 40 60 ±288.0 ± 27.4 220.8± ± 12.9 9.7 ± 1.2± PBATPLA PBAT 40 60 69.1 288.05.0 ± 27.4 581.1220.8119.9 ± 12.9 9.7 ± 5.1 1.2 0.8 PLA PBAT 20 80 ±110.9 ± 6.6 531.9± ± 43.2 6.1 ± 0.7± PLA PBATPBAT 20 80 110.969.1 ±± 6.65.0 531.9581.1 ±± 43.2119.9 6.15.1 ± ± 0.7 0.8 From Figure7, it is evidentPBAT that PLA and69.1 PBAT± 5.0 have581.1 an opposite ± 119.9 behavior:5.1 ± 0.8 the first is rigid and brittle,From and theFigure second 7, it isis flexibleevident andthat tough.PLA and Moreover, PBAT have the blendan opposite films exhibit behavior: behavior the first intermediate is rigid and From Figure 7, it is evident that PLA and PBAT have an opposite behavior: the first is rigid and brittle, and the second is flexible and tough. Moreover, the blend films exhibit behavior intermediate brittle, and the second is flexible and tough. Moreover, the blend films exhibit behavior intermediate

Polymers 2020, 12, 804 12 of 18 Polymers 2020, 12, x FOR P EER R EVIEW 12 of 18 between that of the neat components, showingshowing a progressive changechange fromfrom fragilityfragility toto ductilityductility asas thethe PBAT contentcontent grows.grows. In particular, withwith respect toto the neat PLA ﬁlm,film, the blend at the highest PBAT loading has a decrease ofof thethe elastic modulusmodulus (E)(E) andand yieldyield stress ((σy)σy) ofof an order of magnitude, andand an increase of the elongation atat breakbreak ((εb)εb) ofof twotwo ordersorders ofof magnitude.magnitude. Similar trendstrends inin the mechanical behavior of PLA/PBAT PLA/PBAT blends are reported in the literature also by others [[32,33].32,33]. On the contrary, contrary, Deng et al. [29], [29], reporting on a series series of melt melt-blended-blended PLA/PBAT, PLA/PBAT, found aa drop ofof thethe elongationelongation atat breakbreak forfor aa PBATPBAT massmass content content between between 50% 50% and and 60%. 60%. Moreover, asas for for the the oxygen oxygen permeability, permeability, it canit can be be useful useful to predictto predict the the mechanical mechanical properties properties of a polymericof a polymeric blend. Particularly,blend. Particularly, the elastic the modulus elastic is modulus strongly dependentis strongly on thedependent blend morphology on the blend and composition,morphology and couldcomposition be useful, and to assesscould thebe useful miscibility to assess in polymers the miscibility blends [in52 polymers]. The Parallel blends and [52]. the SeriesThe Parallel Model and [53 ]the represent Series Model the highest [53] represent and the lowestthe highest bound and of the the lowest modulus bound of a of polymer the modulus system, of respectively:a polymer system, respectively: Eb = E1ϕ1 + E2 ϕ2 (2) Eb = E1φ1 + E2 φ2 (2) 1/Eb = 1/(E1ϕ1 + E2 ϕ2) (3) 1/Eb = 1/(E1φ1 + E2 φ2) (3) Another model is the Davies Equation [54], which describes the elastic modulus behavior for macroscopicallyAnother model homogeneous is the Davies and Equation isotropic blends,[54], which thus describes it is valid the for co-continuouselastic modulus systems: behavior for macroscopically homogeneous and isotropic blends, thus it is valid for co-continuous systems: 1/5 1/5 1/5 Eb = E1 ϕ1 + E2 ϕ2 (4) Eb1/5 = E11/5φ1 + E21/5φ2 (4) The experimental result of the modulus and the theoretical predictionspredictions ofof the above-reportedabove-reported equations areare givengiven inin FigureFigure8 8..

2500

2000 Experimental Values

1500 Parallel Model

Series Model 1000 Davies Model

ElasticModulus [MPa] 500

0 0 20 40 60 80 100

PBAT %

Figure 8. Elastic modulus in function of the blends’blends’ composition.composition.

From Figure8 8,, itit cancan be be seen seen that that the the experimental experimental values values ofof thethe elasticelastic modulus modulus are are between between the highest and and lowest lowest limits limits of of the the parallel parallel and and series series models. models. However, However, the the model model that that fits ﬁtsbest best the theexperimental experimental values values is the is theDavies Davies ones. ones. Particularly, Particularly, at at40% 40%,, there there is isa achange change in in slope slope of of thethe experimentalexperimental values’values’ curve curve that that intersects intersect thes the Davies Davies Equation Equation in ain composition a composition between between 40% 40% and 60%and of60% PBAT, of PBAT, as a further as a further conﬁrmation confirmation of the of possible the possible formation formation of a co-continuous of a co-continuous phase phase in this in range. this Moreover,range. Moreover, the elastic the moduluselastic modulus of the blends of the withblends a PBAT with a content PBAT content of 20% is of the 20% closest is the to closest the parallel to the model,parallel a model, sign of a the sign highest of the level highest of interactions level of interactions of the two of polymers the two polymers in this composition, in this composition, as revealed as byrevealed the rheological by the rheological and morphological and morphological analysis. analysis. Few works in literature deal with the application of literature models to the mechanical properties of PLA/PBAT blends. Deng et al. [29] also found a similar trend with the experimental

Polymers 2020, 12, x FOR PEER REVIEW 13 of 18 Polymers 2020, 12, 804 13 of 18 values of the elastic modulus comprised between the series and the parallel model, while Gigante et Polymers 2020, 12, x FOR PEER REVIEW 13 of 18 al. [36] found that the experimental values, up to a PBAT content of 25%, followed the Parallel model. FewFrom works the inanalysis literature of dealthe mechanical with the application properties of literatureat ambient models temperature, to the mechanical it is evident properties that the values of the elastic modulus comprised between the series and the parallel model, while Gigante et ofmost PLA /ductilePBAT blends. blends Dengare those et al. in [ 29which] also PBAT found is a the similar matrix trend phase with (i.e. the, PLA/PBAT experimental 40/60, values PLA/PBAT of the al. [36] found that the experimental values, up to a PBAT content of 25%, followed the Parallel model. elastic20/80 modulus and PBAT). comprised Therefore, between the mechanical the series and behavior the parallel of these model, blends while was Gigante determined et al. [also36] foundat 4 °C, From the analysis of the mechanical properties at ambient temperature, it is evident that the thatwhich the experimental is the typical values, fridge up temperature, to a PBAT content and at of−2 25%,5 °C, followed which is the the Parallel typical model. temperature of the most ductile blends are those in which PBAT is the matrix phase (i.e., PLA/PBAT 40/60, PLA/PBAT industrialFrom the freezer, analysis in oforder themechanical to test their properties suitability at as ambient chilled temperature, and frozen itfood is evident packaging that thematerials. most 20/80 and PBAT). Therefore, the mechanical behavior of these blends was determined also at 4 °C, ductileFigures blends 9 and are 10 those show in the which stress PBAT–strain is the cur matrixves obtained phase (i.e., at PLA4 °C/ PBATand −25 40 /60,°C, PLArespectively,/PBAT 20 /80whereas and which is the typical fridge temperature, and at −25 °C, which is the typical temperature of the PBAT).Figures Therefore, 11–13 compare the mechanical respectively behavior the elastic of these modulus, blends was the determinedstress at yield also, and at 4 ◦theC, whichelongation is the at industrial freezer, in order to test their suitability as chilled and frozen food packaging materials. typicalbreak fridge for the temperature, PLA/PBAT 40/60, and at PLA/PBAT25 ◦C, which 20/80 is theand typical PBAT temperaturefilms at the three of the test industrial temperatures freezer,. in Figures 9 and 10 show the stress−–strain curves obtained at 4 °C and −25 °C, respectively, whereas order to test their suitability as chilled and frozen food packaging materials. Figures9 and 10 show Figures 11–13 compare respectively the elastic modulus, the stress at yield, and the elongation at the stress–strain curves obtained at 4 ◦C and 25 ◦C, respectively, whereas Figures 11–13 compare break for the PLA/PBAT 40/60, PLA/PBAT 20/80− and PBAT films at the three test temperatures. respectively the elastic modulus, the stress at yield, and the elongation at break for the PLA/PBAT 30 40/60, PLA/PBAT 20/80 and PBAT ﬁlms at the three test temperatures. PBAT T = + 4°C PLA/PBAT 20/80 30 PLA/PBAT 40/60 T = + 4°C PBAT

20 PLA/PBAT 20/80 )

a PLA/PBAT 40/60

(

)

r t

M 10

(

e r

t 10 S

0 0 1 2 3 4 5 6 Strain (mm/mm) 0 0 1 2 3 4 5 6 Figure 9. Stress–strain curves at + 4 °C Sfortr aPBAT,in (m PLA/PBATm/mm) 20/80 and PLA/PBAT 40/60 films.

Figure 9. Stress–strain curves at + 4 ◦C for PBAT, PLA/PBAT 20/80 and PLA/PBAT 40/60 ﬁlms. Figure 9. Stress–strain curves at + 4 °C for PBAT, PLA/PBAT 20/80 and PLA/PBAT 40/60 films. 50 T = - 25°C PBAT 50 PLA/PBAT 20/80 40 PLA/PBAT 40/60 T = - 25°C PBAT PLA/PBAT 20/80

) 40

a 30 PLA/PBAT 40/60

(

) s

a 30

s 20

(

S s

s 20

e 10

t S

100 0 1 2 3 4

0 Strain (mm/mm) 0 1 2 3 4 Figure 10. Stress–strain curves at 25 ◦C for PBAT, PLA/PBAT 20/80 and PLA/PBAT 40/60 ﬁlms. Figure 10. Stress–strain curves at− −25 °CS tforra PBAT,in (m mPLA/PBAT/mm) 20/80 and PLA/PBAT 40/60 films.

Figure 10. Stress–strain curves at −25 °C for PBAT, PLA/PBAT 20/80 and PLA/PBAT 40/60 films.

Polymers 2020, 12, 804 14 of 18 Polymers 2020, 12, x FOR PEER REVIEW 14 of 18 Polymers 2020, 12, x FOR PEER REVIEW 14 of 18

600 600 25°C 25°C

373.5 4°C 373.5 4°C 400 400 288.00 308.8 -25°C

288.00 308.8 -25°C

E (MPa) E E (MPa) E E (MPa) E 238.6 238.6 186.75 200 139.6 186.75 200 139.6 110.97 96.83 110.97 96.83 69.19 69.19

0 0 PLA PBAT 40/60 PLA PBAT 20/80 PBAT PLA PBAT 40/60 PLA PBAT 20/80 PBAT

Figure 11. Comparison of elastic modulus at different temperatures. Figure 11. ComparisonComparison of of elastic elastic modulus modulus at different diﬀerent temperatures.

60 60 25°C 25°C

4°C 4°C 40 40 -25°C 28.24 -25°C 28.24 28.24 24.75 24.75 22.63

y (MPa) y 22.63

y (MPa) y σ

y (MPa) y 18.36 σ σ 20 18.36 20 13.35 9.78 13.35 12.05 9.78 12.05 6.13 5.06 6.13 5.06 0 0 PLA PBAT 40/60 PLA PBAT 20/80 PBAT PLA PBAT 40/60 PLA PBAT 20/80 PBAT

Figure 12. ComparisonComparison of of yield yield at at stress stress at different di erent temperatures. Figure 12. Comparison of yield at stress at differentﬀ temperatures.

800 800 581.09 25°C 581.09 25°C 579.97 600 4°C 531.94 519.63 579.97 600 4°C 531.94 519.63

-25°C 392.28

(%) (%)

(%) 400 b

ε 400

b 400

ε ε 220.86 220.86 143.3 200 143.3 200 143.3 67.18 67.18 20.18 20.18 0 0 PLA PBAT 40/60 PLA PBAT 20/80 PBAT PLA PBAT 40/60 PLA PBAT 20/80 PBAT

Figure 13. Comparison of the elongation at break at different temperatures. Figure 13. ComparisonComparison of of the the elongation elongation at at break break at different diﬀerent temperatures.

Polymers 2020, 12, 804 15 of 18

From these graphs it is clear that the decrease in temperature caused, for all the tested blends, an increase in both elastic modulus and yield stress, and a decrease in the elongation at break. These results are expected, because the decrease in temperature reduces the polymer chain flexibility, and increases the stiffness of the material. However, this variation had a different extent in the function of the blend’s composition. In fact, the increase of the elastic modulus was more accentuated as the content of PBAT in the blend increased, varying from the 30% for PLA/PBAT 40/60 to 170% for pure PBAT (at 25 C). This can be explained by the fact that, since the glass transition temperature of PBAT − ◦ is close to 25 C, the Elastic modulus has a higher dependence upon the temperature compared to − ◦ PLA, whose elastic modulus in this temperature range ( 25 C to 25 C) is almost constant. As regards − ◦ ◦ the ductility, the effect of the composition was the opposite compared to the elastic modulus. In fact, the decrease in the elongation at break was more evident as the content of PLA increased, ranging from the 90% PLA/PBAT 40/60 to 30% for pure PBAT (at 25 C). Particularly, the blend with a PLA content − ◦ equal to 40%, at 25 C turned its failure mode from ductile to brittle. This could be due the fact that, − ◦ at all of the tested temperatures, PLA is in the glassy state, and instead PBAT is the rubbery state, so its ductility is less sensitive than PLA to the temperature decrease. In conclusion, both PLA/PBAT 40/60 and 20/80 could be good candidates for chilled food applications, showing at 4 ◦C a good compromise between stiffness and ductility. At 25 C, the failure mode of PLA/PBAT 60/40 changed from ductile − ◦ at ambient temperatures to brittle, while the PLA/PBAT 20/80 kept its ductile failure mode, revealing that it is the best candidate for frozen food applications.

4. Conclusions In this study, blown films of PLA and PBAT in a wide range of compositions (100/0, 80/20, 60/40, 40/60 20/80 0/100 by weight) were produced using a lab-scale film blowing plant, and were characterized in terms of microstructure and functional properties in order to determine their suitability as food packaging materials for room and low temperature (chilling and freezing) storage conditions. The two polymers, although immiscible in all the compositions tested, proved to have some interactions, as inferred by infrared spectroscopy. This causes that the microstructure of the blend films depends strongly on their composition. In particular, the presence of PBAT raises the crystallinity degree of PLA (from 8% of pure PLA, to 22% of the PLA/PBAT 20/80 blend), while the presence of PLA had the opposite effect on PBAT (from 15% of pure PBAT to 2% of the PLA/PBAT 80/20 blend). In terms of performances, the barrier properties to oxygen and water vapor, while it decreased with the increase of PBAT content according to the Robeson equation, remained in the same order of magnitude for all the compositions; P O varied from 33.4 to 84 (cm3 mm)/(m2 d bar) and P H O from 1.3 to 2 · · · 2 3.1 (g mm)/(d m2). Moreover, the ductility increases and the stiffness decreases, according to the · · Davies model, as the PBAT content in the blends becomes progressively higher, at all test temperatures. However, at fixed blend composition, the ductility decreased, and the stiffness increased the lower is the temperature. On this basis, among the considered blown films, the following two blends were selected as best candidates as packaging materials for food requiring low storage temperatures, thanks to a balance of mechanical properties in the typical range of conventional polymer systems used for this kind of applications. In particular, the blend with a PBAT content of 60% proved to be adequate for chilled foods: in fact, at 4 ◦C (which is the typical fridge temperature) it had an elastic modulus equal to 308 MPa, and elongation at break of 67%. Instead, the blend with a PBAT content of 80% was adequate for frozen foods: in fact, at 25 C (which is the temperature of an industrial freezer) it had an elastic − ◦ modulus of 238 MPa and elongation at break equal to 143%.

Author Contributions: Conceptualization, L.I.; Data curation, A.P. and L.D.M.; Formal analysis, L.D.M. and M.R.N.; Funding acquisition, L.I.; Investigation, A.P. and M.R.N.; Supervision, P.S. and L.I.; Writing—original draft, A.P.; Writing—review and editing, P.S. and L.I. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Polymers 2020, 12, 804 16 of 18

Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

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