Cynara cardunculus Biomass Recovery: An Eco-Sustainable

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

pubs.acs.org/journal/ascecg

Cynara cardunculus Biomass Recovery: An Eco-Sustainable, Nonedible Resource of Vegetable Oil for the Production of Poly(lactic acid) Bioplasticizers Rosa Turco,† Riccardo Tesser,† Maria Elena Cucciolito,† Massimo Fagnano,§ Lucia Ottaiano,§ Salvatore Mallardo,‡ Mario Malinconico,‡ Gabriella Santagata,*,‡ and Martino Di Serio*,†,∥

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 02/05/19. For personal use only.

†

Department of Chemical Sciences, University of Naples Federico II, Complesso Universitario di Monte Sant’Angelo, 80126 Naples, Italy ‡ Institute for Polymers, Composites and Biomaterials, National Council of Research, Via Campi Flegrei 34, 80078 Pozzuoli, Italy § Department of Agricultural Sciences, University of Naples Federico II, Via Università, 80126 Naples, Italy ∥ International Research Organization for Advanced Science and Technology (IROAST), University of Kumamoto, 860-8555 Kumamoto, Japan S Supporting Information *

ABSTRACT: Cardoon seed oil (CO), derived from the nonedible Cynara cardunculus plant, growing in marginal and contaminated soils of Mediterranean regions, was successfully epoxidized (ECO) in a fed-batch modality. The cost-effective and environmentally friendly oils have been used as bioplasticizers of poly(lactic acid) (PLA), to improve the overall properties and broaden its industrial applications as a biodegradable packaging material. Hence, physical blends and films of PLA, containing 3% by weight of CO and ECO, were prepared by melt extrusion and compression molding, and the effect of the both bioplasticizers on structural, thermal, and mechanical properties of the obtained films was investigated. Cardoon oils induced the decreasing of glass transition temperature due to PLA free volume enhancement. This effect was particularly marked in PLA−ECO film. Thermal stability of PLA was meaningfully improved upon addition of the oils, and the mechanical properties made evident the increase of PLA ductility, particularly enhanced in the PLA−ECO system, where the polymeric matrix and the oil showed stronger physical interaction and improved phase compatibility, as also revealed by spectroscopic and morphological analyses. Therefore, the plasticization action exerted by very low concentrations of epoxidized cardoon oil efficiently overcomes PLA drawbacks, thus encouraging the feasibility of its use as a bioplastic for packaging material. KEYWORDS: Biomass recovery, Cardoon epoxidized oil, Poly(lactic acid), Bioplasticizer, Hydrogen bonding, Bioplastics

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Bocquè, et al.5 In addition, the large demand for new “green” plasticizers shifted the focus on natural-based resources, such as vegetable oils, extracted from oleaginous plants and trees. Vegetable oils are eco-sustainable, biodegradable, nontoxic plasticizers that come from easily renewable sources. Their chemical structure, characterized by long and complex fatty acid chains, is suitable to interpose among the intermolecular space of the polymeric chains, thus improving its mechanical elasticity. Epoxidized soybean oil (ESBO) is one of the oldest biobased plasticizers of PVC.4−7 It acts as both a primary and a secondary plasticizer depending on the residual double bond content, expressed by iodine number.8,9

INTRODUCTION The requirement of more ecofriendly materials to replace oilderived industrial products is constantly growing because of the environmental concerns. In particular, some of the most common petroleum-based plasticizers, applied in polymeric formulations to enhance their final properties, such as phthalate, are nowadays debated because of the toxicity issues arising from their aptitude to migration. Thus, the scientific and industrial communities increasingly address the use of more eco-sustainable alternatives in applications that are particularly sensitive to the problem, such as in packaging materials.1−4 To this aim, carboxylic acid esters with linear or branched alcohols of medium chain lengths (C6−C11), such as adipates, benzoates, or alkane-dicarboxylic, represent interesting nontoxic petro-based substitutes widely used in petroleum-based and also biobased polymers, as reported by © XXXX American Chemical Society

Received: October 25, 2018 Revised: January 11, 2019

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DOI: 10.1021/acssuschemeng.8b05519 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

emissions, biodiversity, landscape, and water resources) for cardoon also if compared to other perennial crops such as giant reed. Razza et al.25 found that the highest GHGs emissions and nonrenewable energy resource consumption were due to urea fertilization (70−80%), suggesting that the substitution of this fertilizer with compost or legume green manure could further increase the sustainability of cardoon cultivation in the Mediterranean area. Concerning economic sustainability, different estimates demonstrated that cardoon is more profitable than wheat thanks to lower cultivation costs. In Greece the net profit (without subsides) was from 111 to 456 € ha−1 for cardoon, while it was negative (from −284 to −243 € ha−1) for durum wheat.26 In this area, social impacts were also considered positive since direct and induced additional jobs were increased by cardoon. These results were confirmed also in more recent research in Italy by Toscano et al.27 who demonstrated that the use of cardoon residual press-cake as feed, in the substitution of biomass and grain cultivations mostly devoted to different energetic purposes, was very advantageous and competitive for the farmers of marginal lands, thanks to the low input management required and to the compliance of the pedoclimatic conditions of Mediterranean regions. The economic sustainability of cardoon could be further improved considering its potential to produce roots with high inulin content28 and seeds with high yield of oil extraction, about 25−30%. Finally, for the recovery and use of nonedible oils as natural plasticizers of polymers, the first part of this work concerned the chemical modification, by means of epoxidation, of neat cardoon oil (CO) extracted from Cynara cardunculus seeds, to obtain green, nontoxic, and low-cost plasticizers. It is worth underlining that, so far, no literature data has been investigated on the cardoon plant as a potential renewable source of oil plasticizers. Moreover, the synthesis of the epoxidized cardoon seed oil (ECO) represents a remarkable novelty in the field of biodegradable natural oil-derived plasticizers. In this paper, both CO and ECO were used as plasticizers of poly(lactic acid) (PLA), to preserve the eco-sustainability of the whole investigated system. Poly(lactic acid), a linear aliphatic polyester, is classified as an “environmentally f riendly” material, being one of the most promising alternatives to petroleumderived plastics29,30 because of its biodegradability and highperforming plastic commodities. Suffice it to say that its mechanical properties, such as tensile strength and Young’s modulus, are comparable to those of poly(ethylene terephthalate).12 Moreover, the easy processability and biocompatibility make PLA a suitable biopolymer for a wide range of industrial products, such as packaging materials,31,32 films for agriculture,33 and drug delivery and tissue engineering.34,35 Additionally, because of its high crystallinity, it is a very brittle polymer, showing both low toughness and tensile strain. These drawbacks severely restrict some uses in the engineering sectors.36 Therefore, for an improvement in PLA performance, thus extending its industrial applications, considerable efforts have been made, by blending it with plasticizers such as epoxidized vegetable oils.19,37 Last but not least, it is worth highlighting that the cardoon oil yield was comparable with the oil production found for soybean and sunflower crops, as reported by Ottaiano et al.38

The presence of epoxy groups improves the compatibility of vegetable oils with the polymers and makes the plasticizers more prone to microbial degradation.10 In addition, since they come from renewable and accessible sources, they are relatively inexpensive and competitive.11−13 However, most vegetable oils are edible; therefore, the increase of oil demand from bioenergy or biochemicals could cause an increase of their global prize, thus threatening the food security of the poor.14 For this reason, the first-generation feedstock may be not sustainable at the global scale. Furthermore, the spread of nofood crops in agricultural soils could compete for land with food crops,15 thus reducing food supply and consequently increasing their prices. Also, in this case, the effect of the growth of energy crops on good agricultural land could seriously reduce food-security-inextensive areas worldwide.16,17 With the aim to evaluate the cultivation sustainability of biomass crop as feedstock for biorefineries, regional research has been carried out in the Campania region, southern Italy, for identifying croplands not competitive with the food chain. To this aim, the attention has been mainly focused on marginal lands and polluted soils,18 such as abandoned land previously used for cereal production. In fact, in such hilly areas, the very low production of durum wheat (under 3 t ha−1) pitted by high gross income (400−500 € ha−1) could not balance the cultivation costs.19 In addition, because of the deep soil tillage at the end of August and sowing in November, no vegetation grew in the soil until December. Because of this cropping procedure, the plain soil underwent the copious rain fall period responsible for a significant susceptibility to its erosion; as a consequence, about 200−300 t ha−1 of soil loss occurred, mainly during the period of September and October.20 A valid, eco-sustainable, and cost-effective alternative to protect soil from organic matter (SOM) depletion following the above concerns could be represented by the cultivation of perennial crops, able to cover the soil for a long period, in this way increasing the farm profits and avoiding the leaving of the land.17 The analyses made by Fernando et al.21 demonstrated that all perennial biomass crops cultivated in marginal lands provided benefits, as compared with the traditional crop of wheat, regarding soil properties, erodibility, and pesticiderelated emissions, confirming the results of Torres et al.22 Among the species of perennial crops, cardoon resulted as the best one in regards to benefits for biological and landscape diversity. Schmidt et al.23 concluded that only truly unused land in the Mediterranean region should be used for the cultivation of perennial crops to avoid negative indirect effects, highlighting the impact of irrigation on their environmental performances. These authors considered cardoon as a crop with great applicative potentialities for producing liquid fuels based on the oil derived from the plant seeds. The environmental sustainability of cardoon was also confirmed by several research efforts in the past 30 years. Gominho et al. 24 confirmed that Cynara cardunculus represented a challenging, strong, and productive crop able to grow in dry lands of the Mediterranean regions; its applications ranged from energy to green chemistry or phytochemicals. Moreover, in arid and saline environments, cardoon allowed interesting yields of lingo-cellulosic biomass (10−20 t ha−1) and of oil seeds (0.6−4.3 t ha−1), with a low energy requirement for cultivation (until 2.2 GJ ha−1 year−1). These authors reported lower environmental impacts (GHGs B


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For the acquisition of both a fine distribution of the oil between the polymeric chains and a complete dissolution of the polymer, the oils were previously dissolved in the minimum amount of chloroform at room temperature under mechanical stirring; then, PLA granules were added to the solutions, and the systems were heated at 60 °C for about 1 h, under stirring and with a reflux condenser. Then, the solutions of PLA, PLA−CO, and PLA−ECO were cast in glass crystallizing dishes and kept in an oven at 40 °C under vacuum for 24 h, for removing the solvent. The physical blends thus obtained and the neat polymer were melt blended using a twin counter-rotating screw extruder (HAAKE MINILAB from Thermo Electron Corporation). The thermoprocess was performed at 180 °C and 20 rpm of screw speed. The permanence time inside the extruder was 1.5−2 min. The blending was performed in “flush” mode, and the melt compounded materials were collected in a “spaghetti-like” form. The sheets were prepared by means of a hydraulic hot-press (Carver Laboratory Press Model C) at 180 °C. The compounded material was previously melted by using a low plate pressure of 0.5 bar for 3 min; then, a second step, at a pressure of 0.7 bar for 1 min, allowed a homogeneous distribution of the melt blend between the hot plates. Finally a pressure of 2 bar for 2 min followed by a rapid cooling at 50 °C for 10 min induced the production of homogeneous PLA, PLA− CO, and PLA−ECO films. Film Characterization. Fourier Transform Infrared (FTIR-ATR) Spectroscopy. Fourier transform infrared spectroscopy (FTIR) was performed on samples by using a PerkinElmer Spectrum 100 spectrometer (Waltham, MA). Before testing, the samples were dried in an oven at 60 °C for 24 h. Spectroscopic analyses were performed at room temperature. Spectra were collected as an average of 16 scans in the range 4000−480 cm−1, with a resolution of 4 cm−1. Scanning Electron Microscopy (SEM). Morphological analysis of films was performed by means of a scanning electron microscope (SEM) (Quanta 200 FEG, 338 FEI, Eindhoven, The Netherlands), on cryogenically fractured cross-sections. Prior to the observation, the surfaces were coated with a homogeneous layer (18 ± 0.2 nm) of Au and Pd alloy by means of a sputtering device (MED 020, Bal-Tec AG, Tucson, AZ). SEM micrographs were performed at room temperature, in high vacuum mode and internal water vapor pressure of 66.66 Pa, by using a large-field detector (LFD) and an acceleration voltage of 20 kV. Differential Scanning Calorimetry (DSC). Thermal properties of PLA-based films were studied by differential scanning calorimetry (DSC Q2000, TA Instrument, equipped with a RCS cooling accessory). Samples of 5−8 mg were placed on sealed aluminum pans under a dry nitrogen flow of 50 mL min−1. The samples were equilibrated at 25 °C and heated up to 250 °C by a heating ramp of 10 °C min−1 to erase the previous thermal history. A brief isotherm step of 1 min antedated a nonisothermal crystallization cooling step up to −80 °C at a rate of 10 °C min−1. Finally, a second heating ramp up to 250 °C at 10 °C min−1 was recorded. All the samples were dried under vacuum at room temperature for 24 h before each DSC test. All the experiments were repeated three times to ensure reproducibility, and for each analysis, a fresh specimen was used.44 Mechanical Properties. Tensile tests were performed on dumbbell-shaped film specimens whose dimensions were, respectively, 4 mm in width and 28 mm in length, while the thickness of each film was measured at five random points, and the result was expressed as the average value. The tests were performed by means of a dynamometer model 4301, Instron (Canton, MA) equipped with a 1 kN load cell on six specimens per film, and the results were expressed as the average values. Before testing, the samples were equilibrated in climatic chamber set at 25 °C and 50% RH for 24 h. The measurements were carried out at 23 ± 2 °C and 45 ± 5% RH, at a crosshead rate of 2 mm min−1. Young’s modulus, stress, and strain at break points were calculated.

In line with the above introduction, in the present investigation, neat cardoon oil (CO) and epoxidized cardoon oil (ECO) were used as plasticizers of PLA; hence, PLA/CO and PLA/ECO blends were prepared. The melt blending and compression molding were employed to prepare the films, thus resembling the most cost-effective and industrial thermoprocessing method. The different behavior of both plasticizers was reflected in the final properties of PLA-based films. In particular, the effect of CO and ECO loadings in PLA was investigated by means of structural, morphological, thermal, and mechanical analyses. The improvement of PLA properties, particularly heightened when ECO was used, accounted for the stronger hydrogen bonding occurring between PLA and the epoxidized oil.

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EXPERIMENTAL SECTION

Material and Chemicals. Commercial grade poly(lactic acid) 4042D resin with an MW of ∼390 000 g mol−1, was supplied by NatureWorks LCC, Minnetonka, MN. Chloroform (HPLC grade) was provided by Sigma-Aldrich. Plant Material and Oil Extraction. The oil was obtained from cardoon seeds produced in an open field experiment carried out in the Vesuvius plain [southern Italy, 28 m a.s.l. (meters above sea level)] characterized by semiarid climate and sandy loam soils.38 The weather of the cropping cycle (2015) was characterized by air temperatures increasing from April to August, with minimum temperatures from 6 (December−March) to 20 (June−September) °C and maximum temperatures from 15 (December−March) to 34 (June−September) °C. The mean annual rainfalls were 653 mm, and summer water balance (rainfall − evapotranspiration from May to August) was strongly negative (−689 mm). The soil of experimental plots was sandy loam (clay 14.4%, silt 22.6%, sand 63.0%), neutral (pH 7.3), with a good content of total N (0.2%) and organic matter (2.6%). Cardoon cultivar Trinaseed was sowed on May 2012 with two plant densities: 4 (0.75 × 0.33 m) and 8 (0.75 × 0.17 m) plants per m2. Low-input cropping techniques were used (no irrigation and fertilization in March with 150 kg N ha−1 from ammonium nitrate). The harvest of plants was made at the beginning of August 2015, after the third growth cycle. A wheat thresher was used for collecting cardoon seeds from which oil was extracted with a thermostated mechanical press (regulated at 50 °C) with an operative capacity of 14 kg h−1. Epoxidation Process. The experimental apparatus and procedure were previously reported by Santacesarea et al.39 In particular, the epoxidation of cardoon oil was carried out in a jacketed glass reactor (1 L), equipped with a thermocouple, a reflux condenser, and a mechanical stirrer (750 rpm). The temperature was kept isothermal by using a thermostat with recirculating water. The oil was epoxidized according to the following procedure: the mixture of orthophosphoric acid (1.2 g) and cardoon oil (100 g) was well-stirred and heated at the reaction temperature of about 65 °C. Afterward, a solution of hydrogen peroxide (37 g) and formic acid (5.38 g) was added, and the reaction mixture was stirred and kept at constant temperature for 3 h. Different small samples were withdrawn from the reactor at different times, quenched, and centrifugated at 3000 rpm for 20 min. The organic phase was separated from aqueous phase, and it was treated with a sodium bicarbonate solution (5 wt %) to neutralize all the residual acidity, and then dried with anhydrous magnesium sulfate. Then, the iodine number (I.N.) and oxirane number (O.N.) were determined according with standard methods (Wijs method, ISO3961:2009 and ASTM D1652-97).40,41 Preparation of PLA-Based Films. Before the preparation of physical blends, PLA was dried for 24 h to avoid hydrolytic degradation phenomena. With the aim to avoid any plasticizer migration or essudation from the polymeric matrix and on the basis of the literature data, PLA−CO and PLA−ECO blends were prepared by including the 3% by weight of oils with respect to PLA weight.42,43

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RESULTS AND DISCUSSION Oil Extraction and Epoxidation Process. In Table 1, the average values of total biomass, grain and oil yields from

C


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The FTIR spectra of CO and ECO are reported and discussed in Figure S1a,b of the Supporting Information (SI). Herein, the worthy results have been detailed. First, it was confirmed that epoxidation occurred at the expense of all unsaturated group conversion. By means of infrared spectroscopy, it is possible to evaluate possible interactions between components of polymeric systems. The main absorption regions of PLA and PLA/oil systems were attributed to the vibrations of CH stretching between 3100 and 2800 cm−1, CO stretching in the range 1760−1745 cm−1, CH bending at 1500−1400 cm−1, and  CO stretching in the interval between 1100 and 1000 cm−1.49 Unfortunately, most of the vibration bands of the above functional groups were overlapped, so the corresponding spectra comparisons were not enlightening (see Figure S2). Nevertheless, by means of a spectral subtraction between the PLA/oils and neat PLA spectra, the main peaks could be detected. In Figure 2a,b, the carbonyl region of PLA−CO, PLA, and their spectral subtraction (a), and PLA−ECO, PLA, and their spectral subtraction (b) are reported. From the analysis of Figure 2a, it is possible to observe that the ester carbonyl absorption for both neat and doped PLA was centered at 1758 cm−1; in addition, the sharp profile of the PLA peak was counterposed by a broader vibrational band of PLA−CO, composed of different overlapping peaks. Previous studies showed that, in the case of semicrystalline polymers, the crystalline and the amorphous ester carbonyl group domains are centered at slightly different absorption frequencies. In particular, as shown by Mallardo et al., the carbonyl stretching of the amorphous fraction can be found at higher frequencies, because of the greater intrinsic vibrational energy associated with the enhanced free volume.50,51 In fact, after the spectral subtraction, two absorption peaks could be clearly detected, one at 1781 cm−1 and one at 1740 cm−1. The peak at higher frequency likely referred to the stretching of the PLA amorphous fraction carbonyl group; it results in a shift to higher frequency with respect to the neat PLA, thus indicating a scarce interaction of PLA−CO. On the other hand, the peak at 1740 cm−1 may be attributed to the stretching vibration of the CO ester carbonyl group (see Figure S1a). The presence of both peaks emphasized a phase separation occurring between the polymer and the oil, as subsequently confirmed by morphological, thermal, and mechanical analysis and also as reported by Orue et al.37 In Figure 2b, the spectra of the carbonyl region of PLA− ECO and neat PLA are plotted together with their subtraction. From the analysis of data, it is worthy to note a substantial different behavior from the PLA−CO system. Indeed, the spectra were fairly overlapped, and their spectral difference resulted in a disappearing of the amorphous component of the PLA carbonyl ester group and in a slight shifting of the peak related to the vibrational mode of the PLA crystalline fraction from 1758 to 1755 cm−1; in addition, at 1740 cm−1, a weak shoulder of the ECO ester carbonyl group stretching was visible. The shifting toward lower vibrational frequency of the PLA crystalline carbonyl group suggested its involvement in hydrogen bonding with the polar fraction of epoxidized cardoon oil. In fact, in ECO, in addition to the carbonyl residues, the epoxidized groups also provided an additional polar contribution to the interaction with the polymer as reported by Buong et al.52 The formation of physical

Table 1. Average Values of Total Biomass, Grain and Oil Yields from Cardoon (Cynara cardunculus cv. Trinaseed) at Different Plant Densities plant density (no. m−2)

total biomass (d.w.) (t ha−1)

grain yield (d.w.) (t ha−1)

oil yield (%)

4 8

27.1 ± 3.2 31.6 ± 4.1

2.0 ± 0.2 2.3 ± 0.3

25.6 ± 2.2 24.7 ± 2.3

cardoon (Trinaseed cultivar) at different plant densities are reported. The effect of plant density was not significant for dry weight (d.w.) of total biomass, heads and grains, and for oil yield, thus confirming the plasticity and adaptability of this crop in semiarid environments.45,24 On the average, the cultivar Trinaseed showed a good productivity both of total biomass and of seeds, with values similar to the ones obtained in other field experiments in southern Italy.38,46 According to these results, cardoon is confirmed to be a sustainable alternative for farmers of the Mediterranean area,27 resulting in being able to limit the abandonment of marginal lands. The profile of double bond conversion and selectivity to the oxirane group is reported in Figure 1. It is evident that the

Figure 1. Iodine number (blue curve) and oxirane number (red curve) profiles obtained in cardoon oil epoxidation with performic acid.

initial iodine number (124.5) was reduced up to values (1) after 4 h of reaction with a double bond conversion greater than 99%. This parameter is important for the application of ECO as a primary plasticizer, as reported by Karmalm et al.47 The selectivity was kept almost constant at a value of 70%, with a final oxirane number of 6.0. These results were comparable with those obtained for soybean oil in similar reaction conditions.48 This finding confirms that cardoon oil could be used as a valid substitute of soybean oil in these applications. Last, but not least, since the main drawback related to the use of epoxidized oils as plasticizers is their exudation from the polymeric matrix, due to the presence of residual unsaturations in the oil, a very low values of iodine number (I.N.), associated with the high conversion of double bonds, may ensure the permanence of the epoxidized oil in the plastic. Spectroscopic (FTIR), Morphological (SEM), Thermal (DSC, TGA), and Mechanical Properties of the Films. D


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Figure 2. (a) FTIR spectra of PLA, PLA−CO carbonyl region, and their spectral subtraction. (b) FTIR spectra of PLA, PLA−ECO carbonyl region, and their spectral subtraction.

dark interstitial region, emphasized the phase separation between the polymer and the oil; similar results were found by Ali et al.53 On the other hand, the morphological analysis of the PLA− ECO film fracture surface (Figure 3b) made evident a drastic reduction of both interstitial cavity concentration and their hole dimensions, ranging around 900 nm. In addition, the surface topography changed from a ridge-and-valley structure to a more smoothed and regular one, in which, in addition to some sporadic voids, most of the oil droplets were wellembedded inside the polymeric matrix surface.54 The morphological behavior of the PLA−ECO system underlined a better interfacial adhesion and compatibility between the polymer and the oil with respect to the PLA−CO system; this outcome was likely due to the physical interactions occurring between the polar groups of both polymer and epoxidized oil.43,55 DSC thermograms recorded during the cooling ramp and second heating of PLA-based films are reported in Figure 4 and Figure S4, respectively, while DSC parameters are detailed in Table 2 and in Table S1. The analysis of thermograms during cooling from the melt (Figure 4) made evident the crystallization kinetics of PLA in the plain and doped polymer. Because of the rapid cooling rate, the crystallized fraction of PLA consisted of defective

entanglements will be confirmed also by thermal and mechanical properties. Additionally, from the investigations of the PLA−ECO fingerprint region, it was still possible to detect the vibrational frequencies (Figure S3) of the oxirane ring C−O−C group, thus confirming that the PLA−ECO interaction was mainly of a physical nature. In Figure 3a−c, the micrographs of cryogenically fractured cross-sections of neat PLA (a), PLA−CO (b), and PLA−ECO (c) films are reported.

Figure 3. SEM micrographs of PLA (a), PLA−CO (b), and PLA− ECO (c) cross-sectional surface.

As shown in Figure 3a, neat PLA showed an uneven and rough fracture surface, due to its brittle nature. Nevertheless, its morphology severely changed in the presence of cardoon oil. In particular, from the analysis of PLA−CO micrographs (Figure 3b), some spherical voids could be observed. They were due to the oil droplet evolution likely occurring during the cryogenic film surface delamination. The system PLA−CO showed several discrete microdomains quite homogeneously distributed inside the polymeric matrix. The high concentration of micrometric (2.5−3 μm) and deep interstitial cavities, showing alternating bright nodular domains to the

Figure 4. DSC melt crystallization thermograms of PLA, PLA−CO, and PLA−ECO films. E


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Table 2. Thermal Parameters of PLA, PLA−CO, and PLA−ECO Films Measured by DSC and TGA and Mechanical Properties of the Films sample PLA PLA−CO PLA−ECO ECO CO

Tc,melt (°C) ± 2%

ΔHc,melt (J/g) ± 2%

Tg (°C) ± 1%

ΔHm (J/g) ± 1%

Tm (°C) ± 1%

Tonset10%WL (°C) ± 2%

Tpeak (°C) ± 2%

97.4 98.1 94,5

33.5 30.4 28.4

62.2 59.8 57.8

44.0 47.7 44.3

174 174 173

265 345 310 375 350

325 365 350 425 405

crystals.56,50 The inclusion of cardoon oils gave rise to the development of more homogeneous crystals; indeed, the enhancing of the chain mobility induced a collapse of the previous unstable PLA small crystals, in favor of more regular macromolecular chain organization. In addition, as expected in plasticized systems, the crystal growth from the melt was inhibited, as shown by the decreasing of normalized crystallization enthalpy (ΔHc,melt) values and by the lower crystallization temperatures (Tc,melt). These outcomes, well discernible in the PLA−ECO system, underlined the plasticizing effect of epoxidized oil on the polymeric matrix.52 During the second heating ramp, a slight shifting of the glass transition toward lower temperatures (Tg) could be observed (see Table 2). This outcome was due, as expected, to the plasticizing effect of the oils at the expenses of the amorphous fraction of the polymer. The oil, interposing between the macromolecular chains, induced the free volume increase thus heightening the molecular segment mobility.57 It is worth underlining that the plasticizing effect was particularly boosted in the PLA−ECO system, where the epoxidized oil improved its miscibility with the polymer, as already made evident by morphological analysis. In addition, it deserves mentioning that the plasticization occurred at relatively low content of both oils. Similar results were found by Ferri et al.42,58 During the second heating run, PLA-based systems made evident two cold crystallization phenomena (red circle in Figure S4). A specific description is reported in the SI, while the relative measured parameters are detailed in Table S1. Cold crystallization phenomena, observable after the glass transition temperature, are very common mostly in the case of polyesters, as widely reported in the literature.59,60 The most resounding results concern the cold crystallization temperature decreasing in PLA−CO and PLA−ECO plasticized systems, as a consequence of PLA segmental mobility rising; this effect was particularly pronounced in the PLA− ECO system, thus confirming the good compatibility between PLA and epoxidized oil.61,62 Usually, the cold crystallization provides the development of intense spherulite nucleation responsible for a faster crystallization and smaller spherulite sizes.63 This outcome represents a key point in terms of mechanical performances of the polymer; indeed, the development of small spherulites improves the tensile properties, as discussed in the following.64 TGA thermograms of neat CO, ECO, PLA, and PLA-oilbased films and their first derivative curves (DTG) are reported in Figure S5a,b, respectively. All the curves were normalized with respect to starting sample weights. The thermal parameters analyzed were the onset of temperature degradation, taken as the temperature at which 10% weight loss (WL) of sample was observed (Tonset) and the minimum of DTG curves, corresponding to the maximum degradation rate temperature (Tpeak). All the results are summarized in

Young’s modulus (MPa) ± 5%

stress at break (MPa) ± 10%

strain at break (%) ± 10%

2377 2020 1823

12.6 10.4 11.7

7.6 17.3 31.6

Table 2. For brevity, a detailed description of the results is in the SI. Herein the main results have been described. PLA was thermally stabilized by both oils, as shown by thermograms (Figure S5a,b) and Table 2. In fact, the physical interaction occurring between PLA chains and oils induced a structural reassembling of the polymer creating a protective physical barrier, able both to hinder the permeability of volatile degradation products out from the blend, and to promote a drastic delay of blend thermal degradation.65 In addition, in the PLA−ECO system, the higher plasticization and the increased free volume made the polymer more prone to thermal degradation.64−66 Finally, the presence of one thermal degradation profile of PLA-doped films, as shown in DTG thermograms (Figure S5b), suggested that the oils were wellembedded inside the polymeric matrix, did not migrate, and degraded separately from it. The results of tensile tests are summarized in Table 2. Neat PLA is a brittle polymer, with a high elastic modulus and a very low elongation at break. The addition of both plasticizers to the polymer caused a progressive decrease of the elastic modulus and tensile stress in favor of an enlightened rising of elongation at break, particularly marked in PLA−ECO film. This result is consistent with a more heightened slithering of macromolecular chains under the force action since the homogeneous distribution of the oil molecules among the polymeric matrix. Silverajah et al.65 argued that a good plasticizer should contain both polar and nonpolar structural components. If the plasticizer used is mainly nonpolar (high unsaturation degree), as in the case of CO, the scarce physical interactions occurring with the polar residues of the polymeric matrix result in mechanical strain failings, as resembled by the PLA−CO system. In the epoxidized oil, in addition to the carbonyl group of carboxylic ester functionality, the polar oxirane residues (OOC) account for stronger physical interaction with the polar groups of the polymer. Thus, the presence of 3% by weight of ECO, reducing the intermolecular forces between macromolecular chains by increasing their mobility, enhanced the flexibility and extensibility of the film, as shown by the surge increasing of strain at break and Young’s modulus decreasing. Similar results were also found by Wang et al.66

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CONCLUSIONS In this work, a vegetable oil obtained by seeds of cardoon plant (CO), coming from marginal and abandoned lands, was successfully epoxidized (ECO) and proposed as a novel ecosustainable plasticizer of poly(lactic acid), a thermoplastic biodegradable polyester, suffering high stiffness and brittleness, drawbacks that limit its industrial application. The use of 3% w/w of nonedible CO and ECO as plasticizers of PLA deeply tailored PLA properties. The best F


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ACS Sustainable Chemistry & Engineering performances were made evident in PLA−ECO film, because of the strong physical interactions occurring between the polar groups of both oil and polymeric matrix, as made evident by spectroscopic analysis. The enhanced phase compatibility made evident by morphological analysis was reflected in an improvement of PLA plasticization, as shown by the decreasing of glass transition temperatures and by the enhancement of PLA tensile properties. In particular, a worthy increasing of PLA plastic deformation was highlighted in the PLA−ECO system. TGA analyses made evident a higher thermal stability of PLA upon the addition of both oils. In particular, the enhanced plasticizing effect of epoxidized cardoon oil on the polymeric matrix, associated with the increase of macromolecular chain free volume, rendered the PLA−ECO system more prone to thermal degradation; this effect was made evident by a bringing forward the of thermogravimetric depolymerization process. Finally, the cardoon oil, in particular the epoxidized one, represented novel environmentally friendly and cost-effective plasticizers able to positively contribute to the enhancement of PLA properties, for the broadening of its industrial application as a packaging material.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05519.

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FTIR analyses, cold crystallization phenomena related to the second heating run of DSC thermal analysis, and TGA−DTG thermograms (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Phone: +390818675372. E-mail: [email protected]. *Phone: +39081674414. E-mail: [email protected]. ORCID

Riccardo Tesser: 0000-0001-7002-7194 Gabriella Santagata: 0000-0002-3370-195X Martino Di Serio: 0000-0003-4489-7115 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the “Finanziamento della Ricerca di Ateneo (000023-ALTRI_DR_3450_2016_Ricerca di AteneoCA_BIO)” for the financial support. The authors also thank Maria Cristina Del Barone, for her valid support in morphological analysis. The authors are grateful to Solvay Italy for having provided hydrogen peroxide.

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Cynara cardunculus Biomass Recovery: An Eco-Sustainable

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