Effect of the Addition of Natural Rice Bran Oil on the Thermal

sustainability

Article Eﬀect of the Addition of Natural Rice Bran Oil on the Thermal, Mechanical, Morphological and Viscoelastic Properties of Poly(Lactic Acid)

Maria Cristina Righetti 1,* , Patrizia Cinelli 1,2, Norma Mallegni 1, Carlo Andrea Massa 1, Maria Irakli 3 and Andrea Lazzeri 1,2 1 CNR-IPCF, National Research Council—Institute for Chemical and Physical Processes, Via Moruzzi 1, 56124 Pisa, Italy; [email protected] (P.C.); [email protected] (N.M.); [email protected] (C.A.M.); [email protected] (A.L.) 2 Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino 1, 56122 Pisa, Italy 3 Hellenic Agricultural Organization “Demeter”, Institute of Plant Breeding & Genetic Resources, P.O. Box 60411, GR-57001 Thermi-Thessaloniki, Greece; [email protected] * Correspondence: [email protected]

Received: 10 April 2019; Accepted: 10 May 2019; Published: 15 May 2019

Abstract: For the ﬁrst time in this study, the utilization of rice bran oil (RBO) as possible totally eco-friendly plasticizer for poly(lactic acid) (PLA) has been investigated. For comparison, the behavior of soybean oil (SO) has also been analyzed. Both oils are not completely miscible with PLA. However, certain compatibility exists between PLA and (i) RBO and (ii) SO, because demixing is not complete. Although not totally miscible, RBO and SO are able to reduce the viscosity of the PLA+RBO and PLA+SO mixtures, which attests that a small amount of RBO or SO can be successfully added to PLA to improve its processability. Additionally, the mechanical properties of the PLA+RBO and PLA+SO mixtures exhibit trends typical of plasticizer-polymer systems. More interestingly, RBO was found to accelerate the growth of PLA α’-crystals at a low crystallization temperature. This feature is appealing, because the α’-phase presents lower elastic modulus and higher permeability to water vapor in comparison to the α-phase, which grows at high temperatures. Thus, this study demonstrates that the addition of RBO to PLA in small percentages is a useful solution for a faster preparation of PLA materials containing mainly the α’-phase.

Keywords: poly(lactic acid); eco-friendly plasticizer; vegetable oils; crystalline form; morphology

1. Introduction Bio-based and/or biodegradable polymers (bioplastics) [1] represent an important alternative to petroleum-derived polymers, which are commonly non-degradable, although in some cases they can be de-polymerized, and thus made sustainable [2]. For this reason, in recent years, bioplastics have gained increasing attention in both the academia and industry. Among them, poly(lactic acid) (PLA) is the biodegradable polymer most present on the market and widely used for packaging purposes. Many properties of PLA, such as strength and stiffness are comparable to those of traditional petroleum-based polymers, whereas a relatively low toughness limits its use [3]. To improve the ductility of PLA materials, different strategies have been adopted, such as for example polymer blending, copolymerization or plasticization [4]. Plasticizers are low-molecular weight non-volatile substances, which are largely used to improve the processability and flexibility of polymers [5]. The main effects of a plasticizer on the properties of a polymeric material are: (i) Decrease in the glass transition temperature (Tg), (ii) decrease in the melting temperature (Tm), (iii) decrease in the tensile strength, (iv) increase in the elongation at

Sustainability 2019, 11, 2783; doi:10.3390/su11102783 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 2783 2 of 16 break, and (v) reduction in viscosity [6]. These changes also affect the workability of the materials, because the decrease in Tm can allow reducing the temperature of processing, and thus also possible polymer degradation. Ideal plasticizers are miscible or compatible in all proportions with the polymer. Plasticizers reduce the intermolecular forces between the macromolecules, thus increasing the free volume and facilitating the molecular motions [7]. A necessary condition for a good plasticizing action is that the molecular weight of the plasticizer is sufficiently high, so that its mobility is low and possible migration within the polymeric matrix is reduced. To overcome the toxicity of petroleum-based plasticizers, in recent years, plasticizers for PLA produced from natural sources have been widely utilized and investigated [5,7–22]. This class of plasticizers includes low molecular weight citrate esters [8], oligomeric lactic acid [11], and epoxidized fatty acid esters [12]. In addition, modified vegetable oils (epoxidized or maleinized vegetable oils: Soybean, linseed, palm, cottonseed) have been widely utilized as plasticizers for PLA and PLA-based biocomposites [13–22]. These latter studies fall into the recent and comprehensive interest for plant oils, to be used as an alternative resource for the production of polymers and additive for polymers [23–25]. Unmodified vegetable oils are less utilized as plasticizers for PLA in comparison with functionalized vegetable oils, because in general they are less miscible and/or compatible with PLA, although totally eco-friendly [21]. Some studies have however evidenced that also unmodified oils, such as for example cardanol oil and coconut oil, can act as good plasticizers for PLA [26,27]. For the first time in this study, the utilization of rice bran oil (RBO) as a possible plasticizer for poly(lactic acid) (PLA) has been investigated. For comparison, the behavior of soybean oil (SO) has also been analyzed. Rice (Oryza sativa L.) is one of the most important crops worldwide. By-products of rice processing are rice bran and husks, which accounts for 20% of the rough rice [28]. Rice brain, the outer layer of brown rice, serves as a good source of RBO and bioactive compounds like γ-oryzanol, tocopherols, tocotrienols, phytosterols and phenolic compounds, which present an interesting antioxidant activity [29]. The main aim of the present paper is to investigate how the addition of the totally eco-friendly RBO can modify the final thermal, mechanical, morphological and viscoelastic properties of PLA-based materials. This study fits into the widely investigated topic, in general and also by our group [30–34], on the utilization and valorization of agro-food biomass, co-products and by-products, for the production of sustainable polymeric materials, according to the principles of circular economy, in order to favor the production of articles with properties valuable for practical applications, and reduce the cost of the final products.

2. Materials and Methods

2.1. Materials Poly(lactic acid) (PLA) derived from natural resources, was 2003D NatureWorks (Minnetonka, MN, USA), grade for thermoforming and extrusion processes, containing 3% of d-lactic acid units [melt ﬂow index (MFI): 6 g/10 min (210 ◦C, 2.16 kg), nominal average molar mass: 200,000 g/mol]. The refined rice bran oil (RBO) was provided by Hellenic Agricultural Organization “Demeter”, Institute of Plant Breeding and Genetic Resources (Thessaloniki, Greece). The soybean oil (SO) was purchased from Sigma Aldrich (Milan, Italy). The fatty acid composition of the oils, and the concentration of γ-oryzanol, tocotrienols and tocophenols, determined as reported in reference [35], is detailed in Tables1 and2, respectively. Sustainability 2019, 11, 2783 3 of 16

Table 1. Fatty acid composition of rice bran oil (RBO) and soybean oil (SO).

Fatty Acid g/100 g Fat RBO g/100 g Fat SO C12:0 lauric acid 0.2 - C14:0 myristic acid 0.4 0.1 C16:0 palmitic acid 16.9 10.6 C16:1 palmitoleic acid 0.2 0.1 C17:0 eptadecanoic acid - 0.1 C17:1 eptadecenoic acid - 0.1 C18:0 stearic acid 2.3 4.3 C18:1 oleic acid 41.4 23.9 C18:1 vaccenic acid 0.1 - C18:2 linoleic acid 34.9 52.6 C18:3 linolenic acid 1.5 7.1 C20:0 arachidic acid 0.4 0.3 C20:1 eicosenoic acid - 0.2 C22:0 behenic acid 0.2 0.4 C22:1 erucic acid 0.1 - C24:0 lignoceric acid 0.1 0.1 C24:1 tetracosenoic acid 0.1 - total 98.8 99.9 Saturated 20.5 15.9 Monounsaturated 41.9 24.3 Polyunsaturated 36.4 59.7

Table 2. RBO and SO: γ-oryzanol, tocotrienols and tocopherols content.

mg/100 mL RBO mg/100 mL SO γ-oryzanol 81 - δ-tocotrienol 3.14 - γ-tocotrienol 10.26 - β-tocotrienol 0.37 - α-tocotrienol 1.59 - δ-tocopherol 2.40 28.32 γ-tocopherol 30.47 42.63 α-tocopherol 15.65 1.50

2.2. Mixtures Preparation Mixtures of PLA with (i) RBO and (ii) SO with concentrations of 1, 2 and 5 wt.% were prepared by using a MiniLab II HAAKE Rheomex CTW 5 (Waltham, MA, USA), a co-rotating conical twin-screw extruder, which allowsmixing of the components. The mixing was performed at 180 ◦C, with a screw speed of 100 rpm and a recirculating time of 1.5 min. Before processing, PLA had been dried at a temperature of 60 ◦C for at least 24 h. The molten materials were transferred from the mini extruder through a preheated cylinder to a mini injection molder (Thermo Scientific HAAKE MiniJet II) (Waltham, MA, USA), which allows preparing dog-bone tensile bars specimens, to be used for thermal, mechanical, viscoelastic and morphological characterization. The mold temperature was 90 ◦C and the molding time 1 min. The dimensions of the dog-bone tensile bars were: Width in the larger section: 10 mm, width in the narrow section: 4.8 mm, thickness 1.35 mm, length 90 mm. For comparison, pure PLA was processed under the same conditions. After preparation, all the samples were stored in a desiccator to avoid moisture absorption, and analyzed the day after, to reduce physical ageing effects [36,37].

2.3. Mixtures Characterization The thermal stability of RBO, SO and the PLA mixtures with RBO and SO was investigated by the thermogravimetric analysis (TGA) carried out on about 10 mg of sample by using a Perkin Elmer TGA Sustainability 2019, 11, 2783 4 of 16

7 (Waltham, MA, USA), under nitrogen flow (35 mL/min), at a heating speed of 10 K/min from 50 ◦C to 600 ◦C. Differential scanning calorimetry (DSC) measurements were performed with a Perkin Elmer Calorimeter DSC 8500 (Waltham, MA, USA), equipped with an IntraCooler III as refrigerating system. The instrument was calibrated in temperature with high purity standards (indium, naphthalene, cyclohexane) according to the procedure for standard DSC [38]. Energy calibration was performed with 1 indium. Dry nitrogen was used as purge gas at a rate of 30 mL min− . The as prepared PLA samples were analyzed from 50 C to 200 C at the heating rate of 10 K/min, after fast cooling from room − ◦ ◦ temperature. Additional experiments were performed: The as prepared PLA, PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) samples were melted at 180 ◦C for 0.5 min and successively fast cooled to 60 ◦C. After 5 min at 60 ◦C, the samples were heated at 100 K/min to 80 ◦C, and crystallized at this temperature for 60 min. The final heating scans were performed from 50 C to 200 C at the heating − ◦ ◦ rate of 10 K/min, after fast cooling from 80 ◦C. Tensile tests on the PLA mixtures prepared with the injection molder were performed at room temperature, at a crosshead speed of 10 mm/min, by means of an INSTRON 5500 R universal testing machine (Canton MA, USA), equipped with a 10kN load cell and interfaced with a computer running the Testworks 4.0 software (MTS Systems Corporation, Eden Prairie MN, USA). At least five specimens were tested for each sample in according to the ASTM D 638, and the average values reported. Oscillatory shear measurements were performed by means of a Rheometer Anton Paar MCR 302 (Graz, Austria), equipped with parallel plates of 25 mm diameter, under nitrogen flow to minimize oxidation and to maintain dry environment. Frequency sweep experiments were performed at 175 ◦C at a fixed strain (3%), with a gap of 1 mm, in the linear regime, in order to measure the modulus of the complex viscosity |η*| and the storage and loss moduli, G0 and G”, respectively. The angular frequencies were swept from 0.01 to 628 Hz, with five points per decade. The morphology of PLA mixtures was investigated by the scanning electron microscopy (SEM) with an FEG-Quanta 450 ESEM instrument (Waltham, MA, USA). The micrographs of samples fractured with liquid nitrogen and etched with gold were collected. Backscattered electrons generated the images with a resolution provided by the beam deceleration with a landing energy of 2 kV.

3. Results and Discussion The thermal, morphological, mechanical, and viscoelastic properties of the PLA mixtures with RBO and SO were investigated in order to obtain information on possible miscibility/compatibility between the PLA and these natural oils, and to quantify how the addition of RBO and SO modiﬁes the structure of the polymeric material.

3.1. Thermogravimetric Analysis of RBO, SO, PLA, and PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) Mixtures The thermal stability of PLA, RBO, SO and the mixtures PLA+RBO (5 wt.%) and PLA+SO (5 wt.%) was determined by means of the thermogravimetric analysis under nitrogen ﬂow, because the contact of the material with air is reduced in the extruder and moulder. Figure1 shows that the degradation of PLA occurs in a single step, in a narrow temperature range. The initial degradation temperature is located at about 300 ◦C, whereas the maximum degradation rate is centred at about 360 ◦C, in excellent agreement with previous studies [13,26,39]. At temperatures higher than 300 ◦C, the thermal stability of SO appears slightly better than that of RBO and SO, in agreement with literature data [40]. For both oils, the initial main degradation temperature is close to 320 ◦C, whereas the maximum degradation rate is around 410 ◦C. The addition of RBO and SO slightly improves the thermal stability of PLA, as reported also for other mixtures [13,41,42]. This behavior was explained by considering that the dispersion of the oils in the polymer matrix can act as a physical protective additive, which hinders and slows down the release of volatile degradation products, thus delaying the entire degradation process. Sustainability 2019, 11, x FOR PEER REVIEW 5 of 16 Sustainability 2019, 11, x FOR PEER REVIEW 5 of 16 Sustainability 2019, 11, 2783 5 of 16

8 100 8 100 PLA 80 PLA+RBO (5 wt.%) PLA 6 80 PLA+SO PLA+RBO (5 wt.%) (5 wt.%) 6 RBO PLA+SO (5 wt.%) 60 SO RBO SO 60 4 4 Weight % 40

Weight % 40 2 20 2

20 % (%/K) of Weight -Derivative -Derivative of Weight % % (%/K) of Weight -Derivative 0 0 0 100 200 300 400 500 600 0 100 200Temperature 300 (°C) 400 500 600 Temperature (°C)

FigureFigure 1. 1. ThermogravimetricThermogravimetric curves curves of of poly(lactic poly(lactic acid acid)) (PLA), RBO, SO and the mixtures PLAPLA+RBO+RBO (5(5Figure wt.%) wt.%) and1. and Thermogravimetric PLA+SO PLA+SO (5 (5 wt.%) wt.%) atcurves at 10 10 K/min K of/min poly(lactic under under ni nitrogen trogenacid) (PLA), flow. ﬂow. The TheRBO, weight weight SO and % % curvesthe curves mixtures are are also also PLA+RBO shown shown (thin(thin(5 wt.%) solid solid andand and PLA+SOdashed dashed lines). lines). (5 wt.%) at 10 K/min under nitrogen flow. The weight % curves are also shown (thin solid and dashed lines). 3.2.3.2. Morphological Morphological Properties Properties of of PLA PLA and and PLA PLA ++ RBORBO (5 (5 wt.%) wt.%) and and PLA PLA ++ SOSO (5 (5 wt.%) wt.%) Mixtures Mixtures 3.2. Morphological Properties of PLA and PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) Mixtures TheThe morphology morphology of of fracture fracture surfaces surfaces of of PLA PLA and PLA + RBORBO (5 (5 wt.%) wt.%) and and PLA PLA ++ SOSO (5 (5 wt.%) wt.%) fromfrom dog-bone The morphology specimensspecimens of wasfracturewas studiedstudied surfaces by by the the of scanning scanniPLA andng electron electronPLA + microscopy,RBO microscopy, (5 wt.%) in orderinand order PLA to investigateto + investigateSO (5 wt.%) the thedispersionfrom dispersion dog-bone of the of specimensthe oils oils in the in the polymericwas polymeric studied matrix. by matrix. the scanni ng electron microscopy, in order to investigate the FigureFiguredispersion 22 illustratesillustrates of the oils thethe in topologytopology the polymeric ofof thethe matrix. purepure PLA,PLA, whichwhich appearsappears smoothsmooth andand withoutwithout evidentevident voids.voids. FigureThis This is is 2 the theillustrates typical typical surfacthe surface topologye appearance appearance of the pureof of a a fractured PLA, which brittle appears material smooth [43]. [43]. and Conversely, Conversely, without evidentmany many emptyemptyvoids. microvoids microvoidsThis is the are typical are observed observed surfac ine in theappearance the fracture fracture surfof surfacesaaces fractured of the of themixturesbrittle mixtures material PLA+RBO PLA [43].+RBO (5 Conversely, wt.%) (5 wt.%) and PLAmany and +PLA emptySO (5+ SOwt.%). microvoids(5 wt.%). The presence are The observed presence of these in of thenumerous these fracture numerous voids surfaces can voids of be the connected can mixtures be connected to PLA+RBO phase toseparation phase(5 wt.%) separation between and PLA thebetween+ SOPLA (5 wt.%). theand PLA excess The and presence oil, excess which oil,of these whichis removed numerous is removed after voidsafter the can thefracture be fracture connected with with liquidto liquid phase nitrogen. nitrogen. separation A A similarbetween similar morphologymorphologythe PLA and has has excessbeen reported oil, which for several is removed PLA+oils PLA+ oilsafter or or theepoxidized epoxidized fracture oils oilswith mixtures mixtures liquid [12,17,44,45]. [nitrogen.12,17,44,45 A]. similar morphology has been reported for several PLA+oils or epoxidized oils mixtures [12,17,44,45].

PLA PLA+RBO (5 wt.%) PLA+SO (5 wt.%) PLA PLA+RBO (5 wt.%) PLA+SO (5 wt.%) FigureFigure 2. 2. SEMSEM images images of ofthe the PLA PLA and and the thePLA+RBO PLA+RBO (5 wt.%) (5 wt.%) and andPLA+SO PLA +(5SO wt.%) (5 wt.%) mixtures. mixtures. (The scale(TheFigure scalebar 2.shows barSEM shows 30images μm 30). µofm). the PLA and the PLA+RBO (5 wt.%) and PLA+SO (5 wt.%) mixtures. (The scale bar shows 30 μm). 3.3.3.3. Thermal Thermal Properties Properties of of PLA PLA and and PLA PLA ++ RBORBO and and PLA PLA ++ SOSO Mixtures Mixtures 3.3. Thermal Properties of PLA and PLA + RBO and PLA + SO Mixtures The speciﬁc heat capacity (cp) curves of PLA and PLA mixed with RBO and SO with concentrations The specific heat capacity (cp) curves of PLA and PLA mixed with RBO and SO with concentrationsof 1, 2The and 5specific wt.%, of 1, after heat2 and molding capacity 5 wt.%, at after( 90cp)◦ Ccurves molding for 1 min,of atPLA are90 shown°Cand for PLA1 in min, Figures mixed are3 shown andwith4, respectively.inRBO Figures and 3SO and with 4, respectively.concentrationsThe glass transition of 1, 2 and temperature 5 wt.%, after (Tg ),molding which isat located 90 °C for in the1 min, proximity are shown of 60 in◦C, Figures in agreement 3 and 4, withrespectively. literature data [46], is overlapped by an enthalpy recovery peak, due to permanence of the samples at room temperatures for one day [36,37]. The Tg values do not appear to decrease in the presence of RBO and SO. This could mean that both RBO and SO do not act as eﬃcient plasticizers for PLA at a concentration lower than 5 wt.%. Before the melting endotherm, all the curves display an intense cold-crystallization peak located approximately in the interval between 85 ◦C and 130 ◦C. Sustainability 2019, 11, 2783 6 of 16

Sustainability 2019, 11, x FOR PEER REVIEW 6 of 16

At temperatures lower than about 100 ◦C, the crystal α’-form of PLA mainly grows, whereas in the temperature range from 100 ◦C and 120 ◦C, a mixture of α’- and α-forms develops [47]. The two crystal modiﬁcations have a similar chain packing, but the lattice dimensions of α’-crystals are slightly largerSustainability than 2019 the ,α 11counterpart., x FOR PEER REVIEW The α’-phase is indeed(A) characterized by loose chain-packing and6 of 16 PLA+RBO (5 wt.%) (B) conformational disorder, which PLA+RBO leads (2 wt.%) to diﬀerent physical and mechanical properties in comparison PLA+RBO (1 wt.%) with the α-phase [48]. PLA

(A) PLA+RBO (5 wt.%) (B) PLA+RBO (2 wt.%) PLA+RBO (1 wt.%) 4 PLA

3 1.4

Specific heat capacity (J/g K) 2 1.2 4 1 3 1.01.4 -50 0 50 100 150 200 -40 -20 0 20 40

Specific heat capacity (J/g K) 2 Temperature1.2 (°C)

1 Figure 3. (A) Specific heat capacity (cp) of PLA and the PLA1.0 mixed with RBO at the concentrations -50 0 50 100 150 200 indicated, as a function of the temperature. The curves were measured-40 -20 upon 0 heating 20 40at 10 K/min after previous fast cooling to −50 °C; (B) enlargement Temperatureof the cp curves (°C) in the sub Tg region. The ordinate values refer only to the bottom curves. All the other curves are shifted vertically for the sake of Figure 3. (A) Speciﬁc heat capacity (cp) of PLA and the PLA mixed with RBO at the concentrations clearness. indicated,Figure 3. as (A a) functionSpecific ofheat the capacity temperature. (cp) of The PLA curves and the were PLA measured mixed with upon RBO heating at the at 10 concentrations K/min after indicated, as a function of the temperature. The curves were measured upon heating at 10 K/min after previous fast cooling to 50 C; (B) enlargement of the cp curves in the sub Tg region. The ordinate − ◦ valuesprevious refer fast only cooling to the bottom to −50 curves.°C; (B) Allenlargement the other curves of theare cp curves shifted in vertically the sub for Tgthe region. sake ofThe clearness. ordinate values refer only to the bottom curves. All the other curves are shifted vertically for the sake of (B) clearness. PLA+SO (5 wt.%) (A) PLA+SO (2 wt.%) PLA+SO (1 wt.%) PLA

(B) PLA+SO (5 wt.%) (A) PLA+SO (2 wt.%) PLA+SO (1 wt.%) 4 PLA

3 1.4

2 1.2 Specific (J/g heat capacity K) Specific 4 1 3 1.01.4 -50 0 50 100 150 200 -40 -20 0 20 40

2 Temperature1.2 (°C) Specific (J/g heat capacity K) Specific

Figure 4. (A) Speciﬁc1 heat capacity (cp) of PLA and the PLA mixed with SO at the concentrations indicated,Figure 4. as (A a) functionSpecific ofheat the capacity temperature. (cp) of The PLA curves and werethe PLA1.0 measured mixed upon with heatingSO at the at 10concentrations K/min after -50 0 50 100 150 200 -40 -20 0 20 40 previousindicated, fast as cooling a function to of50 theC; te (mperature.B) enlargement The curves of the werecp curves measured in the upon sub T heatingg region. at The10 K/min ordinate after − ◦ valuesprevious refer fast only cooling to the bottom to −50 curves.°C; (B) Allenlargement the other curves ofTemperature the arecp curves shifted (°C) in vertically the sub for Tg theregion. sake ofThe clearness. ordinate values refer only to the bottom curves. All the other curves are shifted vertically for the sake of α α Atclearness.Figure the heating4. (A) Specific rate of heat 10 ◦capacityC/min, ( thecp) of ’-crystalsPLA and the transform PLA mixed into with the SO more at the ordered concentrations-phase by meltingindicated, and almost as a function simultaneous of the temperature. recrystallization, The curves which were cannotmeasured be upon distinguished heating at 10 separately K/min after [ 49]. g As a consequenceTheprevious glass fast transition, thecooling melting totemperature −50 behavior °C; (B) thatenlargement (T ), extends which from ofis thelocated approximately cp curves in the in theproximity 130 sub◦C T tog region. of 165 60◦C °C,The (see in ordinate Figures agreement 3A andwith4A), valuesliterature results refer from data only the [46],to fusionthe is bottom overlapped of both curves. the byα All’- an and the en theotthalpyherα-crystals. curves recovery are Reorganization shifted peak, vert dueically to and permanencefor recrystallization the sake ofof the samplesclearness. at room temperatures for one day [36,37]. The Tg values do not appear to decrease in the presence of RBO and SO. This could mean that both RBO and SO do not act as efficient plasticizers for PLAThe atglass a concentration transition temperature lower than (5T gwt.%.), which Befo isre located the melting in the endotherm,proximity of all 60 the °C, curves in agreement display with literature data [46], is overlapped by an enthalpy recovery peak, due to permanence of the samples at room temperatures for one day [36,37]. The Tg values do not appear to decrease in the presence of RBO and SO. This could mean that both RBO and SO do not act as efficient plasticizers for PLA at a concentration lower than 5 wt.%. Before the melting endotherm, all the curves display Sustainability 2019, 11, x FOR PEER REVIEW 7 of 16 an intense cold-crystallization peak located approximately in the interval between 85 °C and 130 °C. At temperatures lower than about 100 °C, the crystal α'-form of PLA mainly grows, whereas in the temperature range from 100 °C and 120 °C, a mixture of α'- and α-forms develops [47]. The two crystal modifications have a similar chain packing, but the lattice dimensions of α'-crystals are slightly larger than the α counterpart. The α'-phase is indeed characterized by loose chain-packing and conformational disorder, which leads to different physical and mechanical properties in comparison with the α-phase [48]. At the heating rate of 10 °C/min, the α'-crystals transform into the more ordered α-phase by melting and almost simultaneous recrystallization, which cannot be distinguished separately [49]. As a consequence, the melting behavior that extends from approximately 130 °C to 165 °C (see Figures 3(A) and 4(A)), results from the fusion of both the α'- and the α-crystals. Reorganization and recrystallization events overlap the entire fusion process, as generally takes place in semi-crystalline polymers at a relatively low heating rate [50,51]. The enlargements of the cp curves in the sub Tg regions, depicted in Figures 3(B) and 4(B), display an endothermicSustainability event2019, 11 ,between 2783 −30 and 5 °C, which has to be connected with the fusion7 of 16 of the oils. The endothermic process is more pronounced with increasing the oil content, and is appreciable for RBO ≥ 1 wt.%events overlapand SO the ≥ entire 2 wt.%. fusion process,In effects as generally cp curves takes of place pure in semi-crystalline RBO and SO, polymers obtained at a relatively after fast cooling down to –50low heating°C, exhibit rate [50 ,multiple51]. endothermic processes in the temperature range between −50 °C and 10 °C (FigureThe enlargements 5). These peaks of the c parecurves ascribable in the sub toTg regions,the complex depicted fusion in Figures of3 theB and triacylglycerols4B, display (TAGs), an endothermic event between 30 and 5 C, which has to be connected with the fusion of the oils. − ◦ the main Thecomponents endothermic of process the isoils, more which pronounced are presen with increasingt in different the oil content, concentrations and is appreciable in RBO for and SO, as deduced RBOfrom 1Table wt.% and 1. SOThe2 wt.%.shape In of effects thec p peakscurves of is pure multiple, RBO and SO,due obtained to the after many fast cooling crystalline forms ≥ ≥ down to –50 C, exhibit multiple endothermic processes in the temperature range between 50 C and (polymorphism) present◦ in the solid TAGs mixtures [52]. The TAG crystals grown− ◦ upon fast cooling 10 ◦C (Figure5). These peaks are ascribable to the complex fusion of the triacylglycerols (TAGs), the main down to components−50 °C reorganize of the oils, which and are recrystallize present in different upon concentrations heating inat RBO 10 andK/min, SO, as deducedtransforming from from one crystallineTable phase1. The into shape a different of the peaks one is multiple, [53–5 due5]. toThe the measured many crystalline enthalpies forms (polymorphism) of fusion present(Δhm,oil in°) of pure RBO the solid TAGs mixtures [52]. The TAG crystals grown upon fast cooling down to 50 C reorganize and SO are 61 J/g and 58 J/g, respectively. The RBO and SO separate phases− ◦ observed in Figures 3 and recrystallize upon heating at 10 K/min, transforming from one crystalline phase into a different and 4 at low temperatures attest that PLA and the oils are not completely miscible. RBO and SO one [53–55]. The measured enthalpies of fusion (∆hm,oil◦) of pure RBO and SO are 61 J/g and 58 J/g, partially respectively.crystallize The as RBOseparate and SO separatephases phases upon observed cooling, in Figures and3 andthis4 atproves low temperatures that specific attest and strong interactionsthat between PLA and the PLA oils areand not (i completely) RBO and miscible. (ii) SO RBO are and not SO established partially crystallize in the as separate mixtures. phases upon cooling, and this proves that specific and strong interactions between PLA and (i) RBO and (ii) SO are not established in the mixtures.

RBO SO 8

2 Specific heat capacity (J/g K)

-40 -20 0 20 40 Temperature (°C)

Figure 5. Speciﬁc heat capacity (cp) of pure RBO and SO and upon heating at 10 K/min after previous Figure 5. fastSpecific cooling heat to 50 capacityC. (cp) of pure RBO and SO and upon heating at 10 K/min after previous − ◦ fast cooling to −50 °C. An estimation of the amount of the separate oil in the mixtures was performed by comparing the enthalpy of fusion (∆hm) of the oils in the PLA+RBO and PLA+SO mixtures (calculated from An estimationFigures3 and 4of), withthe theamount enthalpy of of fusionthe separate of pure RBO oil and in SOthe (determined mixtures from was Figure performed5). Table3 by comparing the enthalpyshows of the fusion∆hm values, (Δhm derived) of the from oils Figures in 3the and 4PLA+RBO, and the corresponding and PLA+SO∆hm,oil mixturesvalues, obtained (calculated from Figures 3 afterand normalization4), with the to enthalpy the oil concentration. of fusion of The pure comparison RBO and between SO ∆(determinedhm,oil and the enthalpy from Figure of 5). Table fusion of 100% pure oils (∆hm,oil◦) suggests that not all the oil added to the PLA separates, so that 3 shows thea partial Δhm miscibilityvalues, /derivedcompatibility from exists Figures between 3 the and PLA 4, and and (i) RBOthe andcorresponding (ii) SO. With respect Δhm,oil to RBO, values, obtained after normalizationSO appears more to misciblethe oil with concentration. PLA. By considering The the comparison oil amount that between separates, an Δ estimationhm,oil and of the enthalpy of fusion of e100%ﬀective pure oil mixed oils with (Δh PLAm,oil was°) suggests thus achieved, that as not listed all in Tablethe 3oil (last adde column).d to The the data PLA collected separates, in so that a partial miscibility/compatibilityTable3 reveal that the percentage exists of oil between mixed with the PLA PLA increases and with (i) RBO the nominal and ( oilii) amount,SO. With which respect to RBO, suggests that a longer mixing time could favor the incorporation of the oil to PLA. The present cycle time of 1.5 min was however chosen because it was similar to the operating conditions generally used for productive processes. Sustainability 2019, 11, 2783 8 of 16

Table 3. Enthalpy of fusion of the RBO and SO separate phases in the mixtures PLA + RBO and PLA + SO (∆hm and ∆hm,oil) and eﬀective oil mixed with PLA (wt.%).

∆h ∆h Formulation m m,oil Eﬀective Mixed Oil (wt.%) (J/gmixture) (J/goil) PLA + RBO (1 wt.%) 0.20 20.0 0.67 PLA + RBO (2 wt.%) 0.45 22.5 1.25 PLA + RBO (5 wt.%) 1.24 24.8 2.93 PLA + SO (1 wt.%) - - 1.00 PLA + SO (2 wt.%) 0.15 7.5 1.74 PLA + SO (5 wt.%) 0.36 7.2 4.38

Table4 lists the PLA enthalpy of cold crystallization ( ∆hc) and the enthalpy of fusion (∆hm) calculated from the cp curves shown in Figures3 and4 after the construction of a linear baseline from approximately 80 ◦C to 165 ◦C. The ∆hc and ∆hm values collected in Table4 are normalized to the PLA content. From these experimental values, an estimation of the crystalline weight fraction growing during the cold crystallization process (wCc), and disappearing during the melting process (wCm) was thus obtained, dividing ∆hc and ∆hm by the enthalpy of fusion of 100% crystalline PLA phase (∆hm◦) at the crystallization and melting peak temperatures, respectively. As both α’- and α-forms grow during cold crystallization, and disappear during the melting process, average values between the enthalpy of fusion of the α’- and α-forms were utilized [56], i.e., ∆hm◦ = 99 J/g for the cold crystallization centered at about 105 ◦C, ∆hm◦ = 101 J/g for the cold crystallization centered at about 110 ◦C, ∆hm◦ = 103 J/g for the cold crystallization centered at about 114 ◦C, and ∆hm◦ = 119 J/g for the melting process centered approximately at 150 ◦C. The crystalline weight fraction of the as prepared samples (wc) was determined as the diﬀerence (wCm - wCc). The wc values listed in Table4 reveal that the crystallinity degree of the as prepared PLA samples is very low. This can be explained by considering the small times that characterize a typical injection molding and the slow crystallization kinetics of PLA [57]. A slightly higher crystallinity is exhibited by the PLA + SO mixtures, which are characterized by higher eﬀective oil content. This suggests that at the moulding temperature of 90 ◦C, the mobility of the PLA chains is higher in the PLA + SO mixtures, due to the higher miscibility of SO with PLA.

Table 4. Enthalpy of cold crystallization (∆hc), enthalpy of fusion (∆hm), and crystalline weight fraction growing during cold crystallization (wCc) and disappearing during fusion (wCm) for the PLA + RBO and PLA + SO mixtures, after molding at 90 C for 1 min (estimated errors: 1 J/g for ∆hc and ∆hm; ◦ ± 0.02 for w and w , and 0.04 for w ). ± Cc Cm ± C ∆h ∆h c w m w w (J/g) Cc (J/g) Cm C PLA 14.4 0.14 24.3 0.20 0.06 PLA + RBO (1 wt.%) 21.6 0.21 26.8 0.23 0.02 PLA + RBO (2 wt.%) 21.7 0.21 28.7 0.24 0.03 PLA + RBO (5 wt.%) 18.8 0.19 27.3 0.23 0.04 PLA + SO (1 wt.%) 15.7 0.15 26.0 0.22 0.07 PLA + SO (2 wt.%) 11.8 0.12 22.7 0.19 0.07 PLA + SO (5 wt.%) 12.0 0.12 22.6 0.19 0.07

More remarkable is the position and the shape of the cold-crystallization peaks (Figures3 and4). The cold-crystallization peaks of the mixtures are located at slightly lower temperatures with respect to the pure PLA. This means that the mixtures are characterized by a higher chain mobility, which induces crystallization at lower temperatures. The cold-crystallization process of the pure PLA appears as a single peak, as well as that of the PLA + SO mixtures. Conversely, the PLA + RBO mixtures exhibit an asymmetric exothermic peak, which is double and well resolved for the PLA + RBO (1 wt.%) Sustainability 2019, 11, 2783 9 of 16

mixture. It is well known from the literature that in the temperature range 100–120 ◦C both the α’- and α-forms grow in PLA [47]. The peak observed at lower temperatures is ascribable to the growth of α’-crystals, whereas the peak at higher temperatures to the development of the α-phase. Thus, Figure3 suggests that the crystallization of the α’-phase is favored in the PLA + RBO mixtures. In the attempt to confirm this hypothesis, additional experiments were performed, as described in the Materials and Method section. The as prepared PLA, PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) samples, after fusion, were annealed at 60 ◦C for 5 min, to allow the formation of crystal nuclei which accelerate subsequent crystallization at temperatures above Tg [58]. The cp curves of the samples crystallized at 80 ◦C for 1 h, after nucleation for 5 min at 60 ◦C, are displayed in Figure6. For the PLA + RBO (5 wt.%) sample, the crystallization time of 1 h is sufficient to complete crystallization at 80 ◦C, because the relative cp curve shows that cold crystallization during the successive heating run is absent. Conversely, crystallization at 80 ◦C is not complete for the PLA + SO (5 wt.%) sample, which undergoes cold crystallization between 80 and 120 ◦C. Cold crystallization is much more intense for pureSustainability PLA, which 2019, 11 means, x FOR PEER that theREVIEW addition of the oils improves the mobility of the chains and favors9 of 16 PLA crystallization. The enthalpies of fusion, calculated as ∆h = ∆hm - ∆hc and normalized to the PLA content,More are indeedremarkable 18, 29 is and the 35 position J/g for PLA,and the PLA shape+ SO of (5 wt.%)the cold-crystallization and PLA + RBO (5 peaks wt.%), (Figures respectively. 3 and That4). Theα’-form cold-crystallization grows at 80 ◦C is peaks attested of bythe themixtures exotherm are atlocated about 135at slightly◦C, which lower is connected temperatures with thewith reorganizationrespect to the ofpure the PLA.α’-crystals This means into α -crystalsthat the mixtures [59]. This are exotherm characterized is barely by visible a higher in thechaincp curvemobility, of purewhich PLA, induces which crystallization further confirms at thelower slower temperatures. and reduced The formation cold-crystallization of α’-crystals process at 80 ◦C of in the the pure absence PLA ofappears the oils. as These a single experiments peak, as prove well that as that both of RBO the and PLA SO + favor SO mixtures. the growth Conversely, of PLA α’-crystals, the PLA and + thatRBO RBOmixtures is more exhibit efficient an thanasymmetric SO, being exothermic the relative peak,∆h value which higher is double and theand effective well resolved concentration for the lower.PLA + TheRBO promoting (1 wt.%) ofmixture. the α’-crystallization It is well known could from be the ascribed literature to athat nucleant in the effecttemperature exerted range by the 100–120 separate °C liquidboth oilthe microdroplets.α'- and α-forms The grow different in PLA composition [47]. The peak of theobserved unsaponifiable at lower temperatures matter of RBO is andascribable SO, i.e., to tocotrienols,the growth tocophenolsof α'-crystals, and whereas especially the peakγ-oryzanol, at higher which temperatures is present to in the high development percentage inof RBO,the α-phase. could beThus, the cause Figure of 3 the suggests different that nucleating the crystallization action for of the the PLA α'-phaseα’-form. is favored Future in investigations the PLA + RBO on themixtures. PLA crystalline growth in the presence of additional increasing amount of γ-oryzanol could better clarify this issue.

PLA+RBO (5 wt.%) PLA+SO (5 wt.%) PLA

3 Specific heat capacity (J/g K) (J/g capacity heat Specific

-50 0 50 100 150 200 Temperature (°C)

Figure 6. Speciﬁc heat capacity (cp) of pure PLA and PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) as

aFigure function 6. ofSpecific the temperature. heat capacity The (c curvesp) of pure were PLA measured and PLA upon + RBO heating (5 wt.%) at 10 and K/min PLA after + SO nucleation (5 wt.%) as at a 60 C for 5 min, crystallization at 80 C for 1h, and fast cooling to 50 C. function◦ of the temperature. The curves◦ were measured upon heating− ◦ at 10 K/min after nucleation at 3.4. Mechanical60 °C for 5 Properties min, crystallization of PLA and at PLA80 °C+ forRBO 1h, and fast PLA cooling+ SO Mixturesto −50 °C.

TheIn the mechanical attempt to properties confirm this of hypothesis, pure PLA andadditi theonal PLA experiments+ RBO and were PLA performed,+ SO mixtures as described are summarizedin the Materials in Figure and 7Method, which section. displays The that as the prepared elastic modulusPLA, PLA does + RBO not change(5 wt.%) as and a result PLA of+ SO the (5 additionwt.%) samples, of RBO andafter SOfusion, up to were a content annealed of 5 at wt.%, 60 °C whereas for 5 min, the to tensile allow strengththe formation decreases of crystal markedly nuclei andwhich the elongationaccelerate atsubsequent break strongly crystallization increases. at temperatures above Tg [58]. The cp curves of the samples crystallized at 80 °C for 1 h, after nucleation for 5 min at 60 °C, are displayed in Figure 6. For the PLA + RBO (5 wt.%) sample, the crystallization time of 1 h is sufficient to complete crystallization at 80 °C, because the relative cp curve shows that cold crystallization during the successive heating run is absent. Conversely, crystallization at 80 °C is not complete for the PLA + SO (5 wt.%) sample, which undergoes cold crystallization between 80 and 120 °C. Cold crystallization is much more intense for pure PLA, which means that the addition of the oils improves the mobility of the chains and favors PLA crystallization. The enthalpies of fusion, calculated as Δh = Δhm - Δhc and normalized to the PLA content, are indeed 18, 29 and 35 J/g for PLA, PLA + SO (5 wt.%) and PLA + RBO (5 wt.%), respectively. That α'-form grows at 80 °C is attested by the exotherm at about 135 °C, which is connected with the reorganization of the α'-crystals into α-crystals [59]. This exotherm is barely visible in the cp curve of pure PLA, which further confirms the slower and reduced formation of α'- crystals at 80 °C in the absence of the oils. These experiments prove that both RBO and SO favor the growth of PLA α'-crystals, and that RBO is more efficient than SO, being the relative Δh value higher and the effective concentration lower. The promoting of the α'-crystallization could be ascribed to a nucleant effect exerted by the separate liquid oil microdroplets. The different composition of the unsaponifiable matter of RBO and SO, i.e., tocotrienols, tocophenols and especially γ-oryzanol, which is present in high percentage in RBO, could be the cause of the different nucleating action for the PLA Sustainability 2019, 11, x FOR PEER REVIEW 10 of 16

α'-form. Future investigations on the PLA crystalline growth in the presence of additional increasing amount of γ-oryzanol could better clarify this issue.

3.4. Mechanical Properties of PLA and PLA + RBO and PLA + SO Mixtures The mechanical properties of pure PLA and the PLA + RBO and PLA + SO mixtures are summarized in Figure 7, which displays that the elastic modulus does not change as a result of the addition of RBO and SO up to a content of 5 wt.%, whereas the tensile strength decreases markedly and the elongation at break strongly increases. As a general rule, the addition of a plasticizer leads to a progressive decrease in the elastic modulus and tensile strength, whereas an increase in elongation at break is usually detected, due to the increased polymer flexibility. [8,60,61]. Similar trends have been observed for PLA plasticized with epoxidized oils, for which a good plasticizing effect on PLA was proved [13,14,17,19,41,42]. Both the drop in tensile strength and the increase in the elongation at break can be connected to the action exerted by the oil, either mixed or separate. Not only the oil effectively mixed, but also the oil microdroplets, dispersed within the polymeric matrix, can act as a lubricant, by reducing the forces of attraction between the polymer molecules and by increasing the flexibility of the chains. The plastic deformation of PLA comes out promoted, and brittleness reduced. The unchanged elastic modulus can be rationalized by considering that it is calculated in the initial linear stress vs. strain region. For very small deformations, the effect of a low percentage of added oil can be negligible, especially if Sustainabilitythe miscibility2019, 11is ,scarce. 2783 10 of 16

5.0 80 60 PLA PLA PLA PLA+RBO PLA+RBO PLA+RBO 4.5 PLA+SO 70 PLA+SO 50 PLA+SO

4.0 60 40

3.5 50 30

3.0 40 20 Elastic modulus (GPa) Tensile strength (MPa) strength Tensile Elongation at break (%) at break Elongation

2.5 30 10

2.0 20 0 0246 0246 0246 Oil percentage (wt.%)

Figure 7. Elastic modulus, tensile strength and elongation at break of PLA and the PLA + RBO and PLAFigure+ SO 7. Elastic mixtures modulus, as a function tensile of strength the oil percentage. and elongation at break of PLA and the PLA + RBO and PLA + SO mixtures as a function of the oil percentage. As a general rule, the addition of a plasticizer leads to a progressive decrease in the elastic modulus and3.5. tensileViscoelastic strength, Properties whereas of PLA an increase and PLA in + elongationRBO and PLA at break + SO is Mixtures usually detected, due to the increased polymer ﬂexibility. [8,60,61]. Similar trends have been observed for PLA plasticized with epoxidized The study of the viscoelastic properties is crucial to gain a fundamental understanding of the oils, for which a good plasticizing eﬀect on PLA was proved [13,14,17,19,41,42]. processability of a polymeric material. In addition, for a mixture, it provides information on the state Both the drop in tensile strength and the increase in the elongation at break can be connected to the of dispersion of the components. The flow behavior of a polymer melt is generally defined by the action exerted by the oil, either mixed or separate. Not only the oil effectively mixed, but also the oil viscosity, i.e., the ratio of the stress to the deformation rate. For common liquids, the viscosity is microdroplets, dispersed within the polymeric matrix, can act as a lubricant, by reducing the forces of dependent only on the temperature and pressure, and not on the deformation rate and time. The attraction between the polymer molecules and by increasing the flexibility of the chains. The plastic situation is much more complicated for polymeric liquids, because viscosity depends also on the deformation of PLA comes out promoted, and brittleness reduced. The unchanged elastic modulus can be deformation conditions. Polymer melts are in fact viscoelastic materials, because flow is accompanied rationalized by considering that it is calculated in the initial linear stress vs. strain region. For very small also by elastic effects. The melt viscosity of polymers is very sensitive to changes in the deformations, the effect of a low percentage of added oil can be negligible, especially if the miscibility macromolecular chain structure and the addition of plasticizers, which increase the polymer free is scarce.

3.5. Viscoelastic Properties of PLA and PLA + RBO and PLA + SO Mixtures The study of the viscoelastic properties is crucial to gain a fundamental understanding of the processability of a polymeric material. In addition, for a mixture, it provides information on the state of dispersion of the components. The flow behavior of a polymer melt is generally defined by the viscosity, i.e., the ratio of the stress to the deformation rate. For common liquids, the viscosity is dependent only on the temperature and pressure, and not on the deformation rate and time. The situation is much more complicated for polymeric liquids, because viscosity depends also on the deformation conditions. Polymer melts are in fact viscoelastic materials, because flow is accompanied also by elastic effects. The melt viscosity of polymers is very sensitive to changes in the macromolecular chain structure and the addition of plasticizers, which increase the polymer free volume and the polymer chain mobility. Viscoelastic properties are generally measured through dynamic shear experiments, because from oscillatory measurements details on both the elastic and viscous properties can be obtained [62]. Dynamic oscillatory shear measurements of polymeric materials are generally performed by applying a time dependent strain γ(t) = γ sin(ωt), and measuring the resultant shear 0· stress σ(t) = σ [G sin(ωt) + G”cos(ωt)], where G and G” are the storage and loss moduli, respectively 0· 0 0 and ω is the angular frequency. In addition, the modulus of the complex viscosity |η*|can be derived, 2 2 1/2 as it holds that |η*|(ω) = [G0(ω) + G” (ω) ] /ω [62]. Sustainability 2019, 11, 2783 11 of 16

Sustainability 2019, 11, x FOR PEER REVIEW 11 of 16 The bilogarithmic plot of |η*| vs. ω obtained by the parallel plate oscillating rheometer at 175 ◦C for PLA and the PLA/RBO (5 wt.%) and PLA/SO (5 wt.%) mixtures is shown in Figure8. The experiments werevolume performed and the from polymer low to highchain frequencies. mobility. Viscoelastic All the curves properties indicate a are decrease generally in the measured complex viscosity through withdynamic increasing shear the experiments, deformation because frequency. from In general, oscillator PLAy measurements behaves like a pseudo-plastic, details on both non-Newtonian the elastic and fluid,viscous and properties a typical shear can thinningbe obtained fluid, [62]. in which Dynami at highc oscillatory shear rates shear the macromolecules measurements orient of polymeric and the ⋅ ω entanglementsmaterials are numbergenerally decreases performed [63,64 by]. Actuallyapplying also a time in the dependent terminal zone strain at lowγ(t) frequency,= γ0 sin( whicht), and σ σ ⋅ ω ω definesmeasuring the zero-shear the resultant viscosity shear stressη , a small(t) = decrease0 [G'sin( int) viscosity + G"cos( ist observed)], where G' for and pure G" PLA, are the probably storage o ω dueand to loss an moduli, original respectively strongly entangled and is structure the angular not frequency. completely In destroyed addition, before the modulus the beginning of the complex of the viscosity |η*|can be derived, as it holds that |η*|(ω) =[G'(ω6)2 + G" (ω)2]1/2/ω [62]. rheological test. The ηo value for PLA at 175 ◦C is about 5.8 10 mPa s. This value is in the range typical 5 7 of commercial PLA (Mw = 140–160 kg/mol), i.e., 10 –10 mPa s [65].

107 iscosity (mPa s)

106

PLA PLA+RBO (5 wt.%) PLA+SO (5 wt.%) Modulus of theModulus Complex V 0.1 1 10 100 1000 Angular frequency (rad/s)

Figure 8. Modulus of the complex viscosity (|η*|) as a function of the angular frequency (ω ) at 175 ◦C forFigure PLA and8. Modulus the PLA of+RBO the complex (5 wt.%) viscosity and PLA (|+ηSO*|) (5 as wt.%) a function mixtures. of the angular frequency (ω) at 175 °C for PLA and the PLA+RBO (5 wt.%) and PLA+SO (5 wt.%) mixtures. The incorporation of RBO and SO reduces the viscosity, suggesting that both RBO and SO influence the relaxationThe bilogarithmic times of PLA. plot The of | viscosityη*|vs. ω decreasesobtained by at athe higher parallel extent plate in oscillating the presence rheometer of SO, due at to175 the °C higherfor PLA concentration and the PLA/RBO of the oil actually(5 wt.%) mixed and toPLA/SO PLA. Both (5 wt.%) the PLA mixtures/RBO (5 wt.%)is shown and PLAin Figure/SO (5 wt.%)8. The mixturesexperiments exhibit were a strong performed shear from thinning low behaviorto high frequencies. also at low angularAll the curves frequencies. indicate This a decrease behavior in has the tocomplex be ascribed viscosity to the with change increasing in the the shape deformation of the oil frequency. droplets, which In general, elongate PLA in behaves the direction like a pseudo- of the flow,plastic, thus non-Newtonian increasing the surface fluid, and contact a typical with the shear polymer thinning chains fluid, and in the which free volume at high in shear the mixtures. rates the Themacromolecules intermolecular orient forces and between the entanglements the polymer chains number weaken, decreases and [63,64]. the PLA Actually mobility also increases. in the terminal In this wayzone the at oils low act frequency, as lubricants, which reducing defines friction the zero-shear and facilitating viscosity polymer ηo, a small chain decrease mobility [in7]. viscosity The final is effobservedect of the for addition pure PLA, of RBO probably and SO due to PLA to an is thusoriginal similar strongly to that entangled of a classical structure plasticizer not [completely14,66]. destroyedThe storage before and the lossbeginning moduli of ( Gthe0 and rheologicalG”) of PLA test. andThe η theo value PLA +forRBO PLA (5 at wt.%) 175 °C and is about PLA +5.8SO 106 (5mPa wt.%) s. mixturesThis value are is shownin the inrange Figure typical9. Both of commercialG0 and G” of PLA PLA ( presentMw = 140–160 the typical kg/mol), liquid-like i.e., 10 behavior5–107 mPa ofs molten[65]. polymers, with the slope of the G0 and G” curves in the terminal zone equal to 2 and 1, respectively.The incorporation The viscous of nature RBO isand dominant SO reduces at low the frequencies, viscosity, whensuggestingG” > Gthat0, whereas both RBO the elasticand SO propertiesinfluence are the dominant relaxation at times highfrequencies, of PLA. The when viscosityG0 > Gdecreases”. The slopes at a ofhigherG0 and extentG” for in the the PLA presence+ RBO of (5SO, wt.%) due and to the PLA higher+ SO concentration (5 wt.%) mixtures of the are oil slightly actually lower mixed with to PLA. respect Both to PLA,the PLA/RBO as well as (5 the wt.%) values and ofPLA/SOG0 and G(5” wt.%) at all themixtures investigated exhibit frequencies.a strong shear The th lowerinningG behavior0 values ofalso the at mixtures low angular with frequencies. respect to PLAThis can behavior be ascribed has to to be more ascribed mobile to the chains change in the in presencethe shape of of both the oil RBO droplets, and SO. which On the elongate other hand, in the thedirection order of of the theG flow,” curves thus appearsincreasing similar the surface to that co ofntact the viscositywith the polymer (PLA > PLA chains+ RBOand the> PLA free volume+ SO), becausein the mixtures. it represents The the intermolecular viscous part forces of the stress,between i.e., the the polymer part of thechains stress weaken, where and energy the isPLA dissipated. mobility increases.The G0 —InG this” cross way point the shiftsoils act towards as lubricants, higher frequencies reducing friction in the case and of facilitating the mixtures, polymer indicating chain thatmobility the relaxation [7]. The final times effect in the of meltthe addition decreases of inRBO in theand presence SO to PLA of RBOis thus and similar SO. The to that material of a classical starts toplasticizer flow (G” [14,66].> G0) when the stress applied is higher than intermolecular forces. The fact that this Sustainability 2019, 11, 2783 12 of 16 occurs in the mixtures at higher deformation frequency means that the oils cause a weakening of the Sustainability 2019, 11, x FOR PEER REVIEW 12 of 16 intermolecular interactions between the PLA chains, and an increase in the intermolecular spaces, which results in a decrease in the relaxation times.

106

105

104

103

G' PLA 102 G" PLA G' PLA+RBO (5 wt.%) 101 G" PLA+RBO (5 wt.%)

Storage and Loss Moduli (Pa) and Loss Moduli Storage G' PLA+SO (5 wt.%) G" PLA+SO (5 wt.%) 100

0.1 1 10 100 1000 Angular frequency (rad/s)

Figure 9. Storage modulus (G’) and loss modulus (G”) as a function of the angular frequency (ω) at 175Figure◦C for 9. PLA Storage and modulus the PLA + (G'RBO) and (5 wt.%)loss modulus and PLA (G"+ )SO as (5a function wt.%) mixtures. of the angular frequency (ω) at 175 °C for PLA and the PLA + RBO (5 wt.%) and PLA + SO (5 wt.%) mixtures. 4. Conclusions InThe this storage study theand utilization loss moduli of ( naturalG' and G oils,") of in PLA particular and the RBO PLA+RBO and SO, (5 as wt.%) possible and plasticizersPLA + SO (5 forwt.%) PLA mixtures has been are investigated. shown in Figure This 9. is Both the first G' and time G" that of PLA RBO present is taken the into typical consideration liquid-like forbehavior this possibleof molten application. polymers, with the slope of the G' and G" curves in the terminal zone equal to 2 and 1, respectively.Both RBO andThe SOviscous are not nature completely is dominant miscible at with low PLA,frequencies, as they arewhen observed G" > G as', separatewhereas phasesthe elastic at lowproperties temperature. are dominant However, at certain high frequencies, compatibility when exists G' between> G". The PLA slopes and of ( iG)' RBO and G and" for (ii the) SO, PLA because + RBO the(5 demixingwt.%) and is PLA not complete.+ SO (5 wt.%) mixtures are slightly lower with respect to PLA, as well as the values of GAlthough' and G" at not all totally the investigated miscible/compatible, frequencies. RBO The and lower SO G are' values able toof reducethe mixtures the viscosity with respect of the to PLAPLA+ canRBO beand ascribed PLA + toSO more mixtures, mobile which chains proves in the thatpresence the addition of both ofRBO RBO and and SO. SO On to the PLA other reduces hand, thethe intermolecular order of the G forces," curves and appears increases similar the mobility to that ofof thethe PLA viscosity polymeric (PLA chains.> PLA This+ RBO finding > PLA attests + SO), thatbecause a small it amountrepresents of RBO the orviscous SO can part be successfully of the stress, added i.e., tothe PLA part to improveof the stress its processability. where energy is dissipated.In addition, the mechanical properties of the PLA+RBO and PLA+SO mixtures (elastic modulus, tensileThe strength G'—G and" cross elongation point shifts at break)towards are higher different frequencies with respect in the tocase the of pure the mixtures, PLA, with indicating trends typicalthat the of relaxation plasticizer-polymer times in the systems. melt decreases A comparison in in the between presence these of RBO trends and and SO. literature The material data starts for PLAto flow plasticized (G" > G with') when epoxidized the stress vegetable applied is oils higher showed than that inte thermolecular mechanical forces. properties The fact are that on this average occurs comparablein the mixtures [15,19 ].at The higher epoxidized deformation vegetable frequenc oils exhibity means a higher that miscibilitythe oils cause/compatibility a weakening with PLA,of the butintermolecular on the otherhand interactions they are between more expensive the PLA and chains not, totally and an eco-friendly, increase in because the intermolecular they are prepared spaces, bywhich chemical results synthesis. in a decrease This validates in the relaxation the utilization times. of RBO and SO as plasticizers for PLA, although they are not completely miscible with PLA. 4. Conclusions More interestingly, RBO was found to accelerate the growth of the PLA α’-crystals at low crystallizationIn this study temperatures. the utilization This of feature natural is oils, appealing, in particular because RBO the andα’-phase SO, as presentspossible plasticizers lower elastic for modulusPLA has and been higher investigated. permeability This tois the water first vapor time inthat comparison RBO is taken to theintoα consideration-phase, which for grows this atpossible high temperaturesapplication. [ 67,68]. This means that PLA materials processed at different temperatures can exhibit differentBoth properties. RBO and The SO presentare not completely study has demonstrated miscible with thatPLA, the as additionthey are ofobserved completely as separate natural phases RBO toat PLA low in temperature. small percentages However, is a useful certain solution compatibility for a faster exists preparation between PLA of PLA and materials (i) RBO containingand (ii) SO, mainlybecause the theα’-phase, demixing according is not complete. to the principles of circular economy, in order to favor the production of sustainableAlthough polymeric not totally materials miscible/compatible, with properties RBO useful an ford SO practical are able applications. to reduce the viscosity of the PLA + RBO and PLA + SO mixtures, which proves that the addition of RBO and SO to PLA reduces Author Contributions: M.C.R. contributed to funding acquisition and conceptualization of the study, supervised thethe work, intermolecular wrote and reviewed forces, and the paper; increases P.C. the contributed mobility to of funding the PLA acquisition polymeric and chains. reviewed This the finding paper; attests N.M. that a small amount of RBO or SO can be successfully added to PLA to improve its processability. In addition, the mechanical properties of the PLA+RBO and PLA+SO mixtures (elastic modulus, tensile strength and elongation at break) are different with respect to the pure PLA, with trends typical of plasticizer-polymer systems. A comparison between these trends and literature data for Sustainability 2019, 11, 2783 13 of 16 performed the mechanical, thermal and morphological characterization; C.A.M. performed the viscoelastic characterization; M.I. performed the oil chemical characterization and reviewed the paper; A.L. contributed to funding acquisition and reviewed the paper. Funding: This research was funded by the European Commission’s Horizon 2020 Research and Innovation Programme (2014–2020)—Sustainable techno-economic solutions for the agricultural value chain (AgroCycle), under Grant Agreement n. 690142. Conflicts of Interest: The authors declare no conflict of interest.

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