Stabilization of Polylactic Acid and Polyethylene ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Stabilization of Polylactic Acid and Polyethylene with Nutshell Extract: Efficiency Assessment and Economic Evaluation Sarai Agustin-Salazar,† Nohemi Gamez-Meza,† Luis Á ngel Medina-Juárez,† Mario Malinconico,‡ and Pierfrancesco Cerruti*,‡ †

Departamento de Investigaciones Científicas y Tecnológicas de la Universidad de Sonora, Blvd. Luis Encinas y Rosales, C.P. 83000 Hermosillo, Sonora México ‡ Institute for Polymers, Composites and Biomaterials (IPCB-CNR), via Campi Flegrei 34, 80078 Pozzuoli (Na), Italy S Supporting Information *

ABSTRACT: Recovery of functional molecules from byproducts of agro-food industries is an appealing approach to waste reduction and to obtain safe and value-added phytochemicals with antioxidant and antimicrobial properties. However, more information about the operational costs is needed for successful industrial scale-up. In this study, the potential of pecan (Carya illinoinensis) nutshell (NS) as a source of antioxidants was investigated. To this aim, a hydroalcholic NS extract (NSE) was thoroughly characterized, and its effect was investigated on thermo- and photo-oxidative stability of two polymers widely used in food packaging, polylactic acid (PLA) and polyethylene (PE). Twenty-six phenolic constituents, including proanthocyanidins, were identified in NSE, and its effective radical scavenging capacity was assessed. NSE acted as a thermal stabilizer for PLA and PE films, both in an oxygen-depleted environment (i.e., during melt processing) and in the presence of oxygen during polymer service life. NSE showed great potential as a PLA stabilizer, due to the compatibility with the polyester matrix. Under UV-light irradiation, NSE was more effective in protecting PE than PLA, due to combination of peroxy radical scavenging and inhibition of Norrish-type photolytic cleavage. Finally, from the experimental data, an economic evaluation of batch and continuous mode NSE manufacturing was performed, demonstrating that the process is technically and economically viable. Overall, these results emphasize the potential of NSE as a low-cost, safe, and sustainable additive for the stabilization of polymer films in packaging and other applications. KEYWORDS: Polyethylene, Polylactic acid, Nutshell extract, Natural antioxidants, Biodegradable polymers, Phenolic substances, Thermo- and photo-oxidation, Packaging

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INTRODUCTION

even be in competition with food and animal feed production. Therefore, the use of biowaste derivatives as antioxidants source for polymer stabilization in packaging and related technologies is an attracting alternative. Food industry produces a significant volume of waste, with serious disposal issues, despite plant byproducts containing large amounts of phytochemicals analogues of synthetic additives having antioxidant and antimicrobial potential. Several studies have shown that polyphenols present in agroindustrial wastes such as spent coffee grounds, grape pomace, and lignin elicit a powerful stabilizing action on several polymer materials.2,11−13 Nutshell (NS) is a lignocellulose biowaste which contains high amounts of antioxidant phenolic compounds like ellagic, protocatechuic, syringic, and p-hydroxybenzoic acids.14,15 For this reason, several academic efforts are ongoing to convert NS wastes into

The demand for more sustainable materials and technologies in the replacement of synthetic supplies is ever-increasing due to economic and ecological factors. Health and environmental concerns due to the use of synthetic chemicals, such as antioxidants and plasticizers employed in polymer formulations have underscored the need for more eco-friendly functional additives, particularly in food packaging technology.1,2 Unlike natural fibers, which have been used for decades to enhance mechanical properties in polymer composites,3,4 the use of natural additives in polymer formulations has been reported only recently.5,6 In particular, naturally occurring phenolics, such as α-tocopherol, resveratrol, quercetin, as well as natural extracts,2,5 have been incorporated into polymer materials intended for active packaging applications, and their stabilizing effect has been assessed.7−10 In this respect, most papers deal with the use of rather expensive plant extracts or even pure compounds. However, recovery of such compounds from firstgeneration feedstock may be not economically viable or may © 2017 American Chemical Society

Received: December 21, 2016 Revised: March 24, 2017 Published: May 9, 2017 4607

DOI: 10.1021/acssuschemeng.6b03124 ACS Sustainable Chem. Eng. 2017, 5, 4607−4618

Research Article

ACS Sustainable Chemistry & Engineering

Antioxidant Capacity. The antioxidant properties of NSE were evaluated by two assays: 2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS)21 and 2,2-diphenyl-1-picrylhydrazyl (DPPH).22 The reaction was read at 515 nm for DPPH• and 734 nm for ABTS+• in a Cary 100 UV−vis spectrophotometer. The results were expressed as Trolox equivalent (TE) (μmol g−1 of dry sample).14 Thermal Stability. TG was performed under nitrogen and air atmosphere (flow rate 30 mL min−1) using 5−10 mg samples, by means of a PerkinElmer Pyris Diamond TG-DTA. The thermal program used was: 30−100 °C (20 °C min−1), 30 min isotherm, and heating up to 700 °C (10 °C min−1).3 CL measurements were performed on about 3 mg of sample through a Lumipol-3 photoncounting instrument (Polymer Institute of Slovak Academy of Sciences). Nonisothermal measurements were carried out either under air or nitrogen atmosphere. The temperature ramp was from 30 to 250 °C (5 °C min−1). The signal of the photocathode was recorded at a 1 s data collection interval.23 Preparation and Characterization of PE and PLA Films. Film Preparation. NSE (dry powder) was mechanically mixed with the dried polymer powder. Films, neat or containing 1, 2, and 3% w/w NSE (coded PLA0, PLA1NSE, PLA2NSE, and PLA3NSE for PLA films, and PE0, PE1NSE, PE2NSE, and PE3NSE for PE films) were obtained by film casting, using a Collin Teach-Line E20T single-screw extruder equipped with a horizontal flat die, and a Collin CR72T chillroll and calendering unit. The following temperature profile was adopted (from hopper to die): 165, 170, 170, 170, and 170 °C for PLA; 150, 170, 180, 180, and 170 °C for PE.1,24 The film average thickness was 70 ± 10 μm. For comparison, films added with 0.5% w/ w the commercial antioxidant Irganox 1076 were also prepared. Optical Properties of Polymer Films. The total color difference (ΔE*) of the films was determined on an L*a*b* color scale25 with a Chroma Meter (CR-400, Spectra Magic NX Lite model, Konica Minolta Optics). L* corresponds to the lightness (0 = black to 100 = white), a* corresponds to the variation in color from green (−80−0) to red (0−100), and b* corresponds to the variation of color from blue (−100−0) to yellow (0−70).26,27 Thermal Oxidative Stability. Thermal oxidative behavior of polymer films at high temperature was evaluated through nonisothermal TG and CL, performed under nitrogen and air, as reported above for NSE. For PE, CL measurements in isotherm conditions under air at 180 °C were also performed. Photo-Oxidative Aging. Irradiation of dumbbell-shaped film specimens die-cut along machine direction was carried out under dry conditions (RH 0%) at 40 °C in an Angelantoni SU250 forced-air climatic chamber equipped with a low-pressure mercury UV lamp (λ > 250 nm). Aged specimens were collected at different times to follow changes in functional groups and mechanical properties.7 Photo-Oxidative Stability. Transmission FTIR spectra of photooxidized films were acquired by the same equipment reported above, as an average of 32 scans in the range of 4000−400 cm−1 (resolution of 4 cm−1). The rate of polymer oxidation was determined by the increase in the peak area of carbonyl and vinyl functional groups.28 Mechanical properties were measured by an Instron model 5564 dynamometer equipped with a 1 kN load cell in tensile mode at 23 ± 2 °C, 45 ± 5% RH, and 5 mm min−1 clamp separation rate. Prior to the test, the dumbell-shaped specimens were conditioned at 25 °C and 50% RH for 48 h. Economic Analysis. Lab-Scale Manufacturing Cost of NSE. The manufacturing cost of lab-scale production NSE was calculated considering the batch production of 1 kg of NSE by ethanol/water extraction. Since the weight yield of dry extract with respect to nutshell biomass was about 14.5%, 6.92 kg of nutshell was used as the amount of raw material. Operating times and electricity consumption of the process were calculated on the basis of the technical specifications of the following semi-industrial scale equipment: Branson 8510 ultrasonic bath, Flottweg Z3E Decanter, and KD Instruments RE5002 rotary evaporator. The cost of NSE was calculated as the sum of costs of raw material and utilities (Table S2). Equipment purchasing costs and labor costs were not included in the computation. Standard unit costs

value-added products traditionally produced from fossil-fuel resources. The aim of the present paper is to assess the feasibility of using pecan nutshell in polymer stabilization. To this purpose, a phenol-rich nutshell powder extract (NSE) was recovered from pecan biowaste through hydroalcoholic extraction and thoroughly characterized to identify major phenolic and flavonoid components, as well as to evaluate its antioxidant properties and thermal oxidative stability. Then, the influence of NSE content was investigated on thermo- and photo-oxidative durability of the two major conventional and biodegradable polymers used in food packaging, polyethylene (PE) and polylactic acid (PLA), providing an insight on the specific substances involved in the stabilization process. Thermal stability of polymer films was determined using thermogravimetry (TG) and chemiluminescence (CL) emission.1 Furthermore, low-temperature UV−visible light irradiation of polymer films provided information on the mechanism and rate of photodegradation of NSE-doped films under service conditions. Finally, an estimation of the manufacturing cost of the antioxidant extraction from NS was performed to assess its economic feasibility in comparison with traditional oil-based polymer antioxidants.

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EXPERIMENTAL SECTION

Raw Materials. Poly(L-lactide) (PLA, grade 4042D, 94% L-lactic acid) was obtained from NatureWorks LLC. An unstabilized linear low-density polyethylene (PE, DJM1826, melt flow index 2.5 g 10 min−1) was supplied by Versalis. Pecan (Carya illinoinensis) NS was obtained from the Productora de Nuez S.P.R de R.I. (México). NS was ground by a blade mill (0.25 mm average size) and stored in darkness in hermetically sealed bags at −20 °C. The commercial antioxidant Irganox 1076 was used as a reference. All standards, reagents, and solvents employed for the extraction were of analytical grade (SigmaAldrich). Extraction and Characterization of NSE. Extractable Matter. NS biomass (1 g) was extracted16 twice using 10 mL of ethanol/water (6:4 v/v) for 30 min. The reunited extracts (NSE) were filtered with Whatman paper No. 2 (GE Healthcare), and the resulting solution was used to determine phenolic substances (PS), total flavonoids (TF), and antioxidant capacity (AC). Determination of PS. Phenolic compounds were spectrophotometrically determined using the Folin−Ciocalteu method.17 Absorbance was measured at 765 nm after 30 min with a Varian Cary 100 UV−vis spectrophotometer. PS concentration was expressed as mg gallic acid equivalents (GAE) g−1 dry sample.14 TF Assay. Flavonoid substances were determined according to Singleton et al.18 The absorbance was read at 510 nm. The results were expressed as mg catechin equivalents (CE) g−1 dry sample. Spectroscopic Characterization. Infrared analysis (FTIR-ATR) was carried out by a PerkinElmer Spectrum 100 spectrometer, equipped with a Universal ATR diamond crystal accessory. Spectra were recorded as an average of 16 scans in the range of 4000−480 cm−1 (resolution of 4 cm−1).7 Nuclear magnetic resonance spectroscopy (1H NMR) experiments were performed on a Varian VXR200 spectrometer.19 Identification of Major Phenolics in NSE. ESI-MS spectra were collected on a Varian 500-MS Flow Injection Analysis-Electro Spray Ionization-Ion Trap-Mass/Mass Spectrometer. First, 1 mL of sample was injected directly into the mass spectrometer, operated under the following conditions: infusion rate: 10 μL min−1, mobile phase: methanol (HPLC-grade), ionization mode: negative, scan voltage: −17 kV, capillary temperature: 350 °C. Helium was used as auxiliary gas (flow: 0.8 mL min−1). Detection was carried out within a mass range of 100−2000 m/z. MS2 analyses were acquired by automatic fragmentation where the three most intense mass peaks were fragmented.20 4608


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Figure 1. (a) FTIR-ATR spectrum, (b) NMR (DMSO-d6) spectrum, and (c) TG and DTA curves of NSE.

extracts), respectively. As regards flavonoids, the TF content found in the present work was higher than that reported for nutshell from different cultivars of C. illinoinensis using acetone.14 Temperature, solvent systems, particle size, as well as different origins can be responsible for the extraction performance.16 In particular, the present findings suggest that phenol components of NSE possess good water solubility. Chemical characterization of NSE was performed using FTIR and NMR spectroscopy, and detailed identification of major NSE constituents was achieved by ESI-MS spectrometry. Figure 1a shows the FTIR-ATR NSE spectrum. The presence of the main phenol functionalities was confirmed by absorption peaks at about 3200 cm−1 (phenol O−H stretching), 1603 cm−1 (resonance of the aromatic CC), and 1530 cm−1 (in-plane bending of phenyl C−H bonds).7,33 The observed splitting of the latter peak suggested the presence of proanthocyanidin structures.34 This assumption was corroborated by the absence of absorption in the carbonyl range, indicating a comparatively low content of flavone structures. Absorptions at 1442 and 1325 cm−1 were attributed to C−H deformations and in plane O−H bending, while peaks detected from 1200 to 1030 cm−1 were due to C−O stretching and −OH deformation vibrations in secondary alcohols and phenols as well as to C−O−C glycosidic linkage vibrations.35 The 1H NMR spectrum of NSE (Figure 1b) in DMSO-d6 showed significant line broadening typical of the resonance overlap of oligomeric structures. Five distinct bands can be found in the spectral pattern, consistent with the presence of anthocyanin structures.36,37 The resonances at 8.75 and 7.98 ppm can be assigned to aromatic hydroxyl protons, with the proton contribution on C4 of the aglycone moieties and the aromatic protons on C2′ and C6′ of pyrogallol rings. Indeed, addition of a few drops of D2O resulted in an almost complete

of raw materials and utilities were obtained as reported in the Supporting Information. Industrial Process Simulation. The process scale-up and economic simulation were performed using the software SuperPro Designer v9.5 (Intelligen Inc., USA). The scale-up criteria assumed that the extraction yield of the process would be the same as that obtained by the lab-scale operation. The process included three extraction vessels of 6000 L to simulate a batch process in a semicontinuous mode, with an extraction time of 30 min. The process was designed to run 7920 h per year (24 h per day, 330 days per year). All costs for equipment, labor, facility, and utilities required for the operation of each equipment, as well operational conditions, efficiency, mass and energy balances were calculated by the software and adjusted for year 2017.29 The NSE unit production cost (UPC, USD kg−1) was estimated as the ratio between annual operating costs (AOC, USD yr−1) and NSE production rate per year (kg yr−1). AOC included annual labor cost (LC), utilities cost (UC), raw material cost (RC), and facility-dependent cost (FDC). FDC was related to insurance, taxes, factory expenses, maintainance, and depreciation (10% per year) costs. Statistical Analysis. Data were analyzed by one-way analysis of variance using OriginPro 8.5. Significant differences among the means were tested using Tukey’s test (P < 0.05). Experimental data were the means ± standard deviation (SD) of three parallel analyses.

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RESULTS AND DISCUSSION

Chemical Analysis of NSE. NS extraction using ethanol/ water as a solvent system yielded 14.5% w/w dry matter. The PS and TF amounts in NSE were 130.5 ± 5.6 mg GAE g−1, and 80.6 ± 0.8 mg CE g−1, respectively. NSE presented a PS content similar to that of brazilian pecan nutshell extracts14,30 and walnut husk (Juglans regia)31 and much higher than that of grape bagasse.16 de la Rosa et al.14 and Ghasemzadhe et al.32 found lower concentrations of PS and TF in pecan nutshell (using acetone, 80 vol %) and Zingiber of f icinale (in methanolic 4609


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ACS Sustainable Chemistry & Engineering Table 1. Tentative Identification of Phenol Compounds in NS by FIA-ESI-IT-MS/MS tentative identification 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 a

40

(epi)gallocatechin-O-gallate-(epi)gallocatechin-O-gallate (epi)catechin-O-gallate-(epi)gallocatechin-O-gallate (proantocyanidin B)40 (epi)catechin-(epi)-gallocatechin-O-gallate40 galloyl-HHDP-hexoside44 (epi)gallocatechin dimer40 (epi)catechin-(epi)gallocatechin40 (epi)catechin dimer40−42 HHDP-hexosidea,44,46 methyl ellagic acid hexoside46 myricetin 3-O-rhamnoside15 cyanidin 3-O-glucoside42,45 myricetin-O-(O-galloyl)-pentosidea,19 phloretin-hexoside43,44 ellagic acid pentose18,19,44 galloyl-hexoside19,44 methyl ellagic acid18 (epi)gallocatechin18,15,40,41,44 taxifolin15 ellagic acidb,15,44,46 (epi)catechinb,15,16,40−46 trans-resveratrolb,45 gallic acidb,4515,20,41,44−46 coumaric acidb,4520,43,45 protocatechuic acid15,20,43,45 trans-cinannamic acid20 protocatechualdehyde18,15

[M − H]− (m/z)

ESI-MS/MS (m/z) ion fragments

913 897 729 633 609 593 577 481 477 463 449 449 435 433 331 315 305 303 301 289 227 169 163 153 147 137

607 (67), 703 (100), 727 (14), 787 (8) 303 (6), 407 (18), 543 (4), 711 (100) 577 (100) 301 (100) 305 (100), 423 (53),441 (67), 483 (4) 289 (100), 305 (14), 467 (8),473 (16) 289 (38), 407 (3), 425 (44), 451 (45) 275 (92), 301 (100) 300 (26), 315 (51) 301 (30), 315 (80), 316 (100) 301 (9), 315 (26), 316 (100), 317 (81) 316(100), 317(67),269 (18), 287 (33) 273 (100) 300 (33), 301 (100) 169 (39), 170 (19),271 (16), 313 (25) 185 (100),300 (22) 179 (65), 219 (34), 221 (83), 261 (30) 125 (2), 285 (83) 185 (62), 229 (14), 257 (40) 179 (36), 203 (63), 205 (33), 245 (100) 185 (100) 125 (100) 119 (100) 109 (59), 137 (8) 119 (100) 109 (100)

HHDP: hexahydroxydiphenoyl. bCompounds identified by comparison with the corresponding standards.

(HHDP)-hexoside.40,44 The MS2 analyses of anthocyanins containing cyanidin aglycone yielded a characteristic molecular ion at 449 m/z, with MS2 fragment at 287 m/z (loss of hexose moiety), and was identified as cyanidin 3-O-glucoside/ myricetin-O-(O-galloyl)-pentoside.19,42−45 The mass fragments obtained at 289, 301, and 315 m/z identified aglycones as (epi)catechin, 15,16,40−46 quercetin, 15,44−46 and phenolic acids,14,15,40,41,43,46 respectively. The neutral losses of 137, 146, 152, 163, and 169 mass units allowed the identification of phenolic acids.14,15,20,41,43−46 Furthermore, the stilbene transresveratrol was identified at 227 m/z.47 Antioxidant Capacity. According to DPPH and ABTS assays, NSE exhibited a significant antioxidant activity; however, the two methods gave different results, as the AC value of ABTS (2727.1 ± 50.7 μmol TE g−1) was double that measured through DPPH (1386.8 ± 5.7 μmol TE g−1). The difference between the measured radical scavenging activities can be ascribed to the fact that DPPH may show slower kinetics when reacting with the phenolic compounds present in NSE.48NSE presented an AC comparable to that of other pecan nutshells cultivars14,30 and higher than that of grape seeds16 and the strong antioxidant Pycnogenol.49,50 Thermal and Thermal-Oxidative Stability. TG provided information on NSE stability under conditions comparable to those adopted in thermoplastic polymer processing. NSE TG curves under nitrogen and air are shown in Figure 1c. Under nitrogen, NSE showed a slow degradation rate, retaining almost 60% of the initial weight even at 700 °C. The onset temperature value (Tonset, calculated as the intercept of the tangent line to the curve) was 229 °C, while the temperature of maximum decomposition rate (Tmax, calculated as the maximum of the TG derivative curve (DTG), Figure S4)

disappearance of these peaks (Figure S1) due to the deuterium exchange of the phenolic OH protons. The broad multiplet between 7 and 6 ppm was due to hydrogens on aglycone C6 and C8.38 Signals at 5.78 and 4.35 may arise from resonance of protons on the glucose moieties (β-glycosides).36 Despite NMR spectroscopy having the advantage of quantitative detecting a wide range of metabolites, it is less sensitive than other analytical methods, and can suffer from problems with signal overlap.39 Therefore, spectroscopy characterization was complemented with MS/MS analysis, in order to specifically detect the low molecular weight and oligomeric PS present in NSE. Identification of the Main Phenolics. Figure S2a reports the full scan mass spectrum (100−2000 m/z) of NSE. Within the examined mass range, oligomeric proanthocyanidins ranged from dimer to pentamer. The major ions found in the MS spectra were exposed to a second order fragmentation (MS2), leading to the identification of several metabolites (Figure S2b, Table 1). The structure of the main phenolics identified in NSE are reported in Figure S3. In total, 26 NSE constituents were identified.40 C2 and C3 carbons stereochemistry of the monomeric and oligomeric catechins cannot be differentiated in mass spectrometry experiments, which for the purposes of nomenclature will be denominated as (epi)-catechin. Catechin dimers esterified by galloyl units were identified at 729, 897, and 913 m/z. Precursor ions at 577, 593, and 609 m/z, corresponding to catechin dimers, were attributed to proanthocyanidins.40−42 MS2 experiments with the precursor ions at 481, 609, and 633 m/z led to product ions at 305 [M − H − 179]−, 289 [M − H − 152 − 179]−, and 305 [M − H − 179 − 219] m/z, which suggested the presence of (epi)gallocatechin dimers 16,20,43 and hexahydroxydiphenoyl 4610


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ACS Sustainable Chemistry & Engineering Table 2. Color Parameters of the Films and Total Color Change (ΔE)a

Average of three determinations ± standard deviation. Values in the same column (for the same polymer matrix) followed by the same letter indicate that there is no significant difference (p > 0.05).

a

Table 3. Thermal Parameters Calculated from the TG Curves of NSE in Nitrogen and Aira nitrogen Tonset (°C) PLA0 PLAIOX PLA1NSE PLA2NSE PLA3NSE PE0 PEIOX PE1NSE PE2NSE PE3NSE

337.6 342.3 343.5 344.2 339.9 459.6 459.7 459.8 457.6 458.6

± ± ± ± ± ± ± ± ± ±

0.2a 0.2c 0.4d 0.3d 0.2b 0.3c 0.3c 0.2c 0.2a 0.1b

air Tmax (°C) 360.0 363.3 363.0 363.1 361.4 478.2 480.0 479.9 480.3 478.1

± ± ± ± ± ± ± ± ± ±

Tonset (°C)

0.1a 0.3c 0.1c 0.1c 0.1b 0.2a 0.1c 0.3b 0.3b 0.3a

341.5 343.4 340.9 346.3 342.2 361.3 369.5 356.5 365.1 356.4

± ± ± ± ± ± ± ± ± ±

0.3a 0.2c 0.3a 0.2d 0.1b 0.2b 0.2d 0.2a 0.1c 0.2a

Tmax (°C) 364.3 367.2 363.8 365.0 366.0 396.2 405.7 383.1 408.4 390.7

± ± ± ± ± ± ± ± ± ±

0.2b 0.2e 0.1a 0.2c 0.2d 0.2c 0.2d 0.1a 0.1e 0.1b

OOT (°C)

200.0 ± 0.4a 203.0 ± 0.7b 207.0 ± 0.1c 212.1 ± 0.2d

Average of three determinations ± standard deviation. Values in the same column (for the same polymer matrix) followed by the same letter indicate that there is no significant difference (p > 0.05).

a

was 336 °C. In accordance with the observed stability, the differential thermal signal (DTA) of NSE in nitrogen did not show any appreciable peaks. The curve under air showed a similar Tonset (235 °C), but a distinct one-step weight loss was evident (Tmax = 401 °C), which ultimately led to almost complete volatilization of NSE. From the DTA curve, two main exothermal processes could be observed. A first peak at about 265 °C (also present in the DTG curve, Figure S4), related to a first weight loss step, was convoluted with a more intense and broad exotherm with a maximum at about 400 °C, which corresponded to sample oxidation and complete volatilization. The outstanding NSE thermal stability was similar to that observed for predominantly aromatic compounds such as acidinsoluble lignins3 or the antioxidant resveratrol,7 also present in NSE, and significantly higher than that reported for synthetic and natural antioxidants used in polymer formulations.50,51 Therefore, NSE has the potential to increase polymer thermoand photo-oxidative durability,1,7,12,52 as demonstrated in the following sections. Film Characterization. Effect of NSE on Film Color. The color of PLA and PE films containing NSE was determined (Table 2), since color can be very important in applications such as food packaging.53 Undoped films were transparent and

colorless, while NSE-doped samples showed a yellow to red discoloration, related to the tannin contents in NSE.30 The lightness decreased with NSE concentration, from 94 (undoped films) to 79 and 71 for PLA3NSE and PE3NSE, respectively. However, the impact of NSE on PE lightness was lower than that of other natural antioxidants, such as dihydromyricetin (DHM) and quercetin (Q).54 The a* value changed from the lightly negative values to positive values (red color). The b* value intensified its tendency toward yellow direction. Total differences in color (ΔE) for 3% w/w NSE-doped films (19.64 and 33.28 for PLA and PE films, respectively) were lower than those reported for LDPE film with added 2.9% w/w marigold (Tagetes erecta) extract27 and similar to those reported to PLA films with added 1−4% w/w α-tocopherol and resveratrol.8 Thermal and Thermal-Oxidative Stability of PE and PLA Films. TG in nitrogen and air atmospheres provided an insight on the influence of NSE on polymer thermal and thermaloxidative stability. The thermograms of all samples are shown in Figure S5, while Table 3 lists the values calculated for relevant thermal parameters. As regards the thermal behavior under nitrogen, PLA and PE films degraded in one step, with no residual char (Figure S5a, b). PLA0 showed a Tonset of 337 °C, whereas Tmax was 360 °C. 4611


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ACS Sustainable Chemistry & Engineering Scheme 1. Mechanism of Radical Scavenging Activity of Catechins via Hydrogen Transfer

Figure 2. Nonisothermal CL emission of: (a) PLA and (b) PE films under N2; (c) PLA and (d) PE films under air.

All doped films were more stable than PLA, with the highest Tonset and Tmax shown by PLA2NSE. This suggests that NSE unlike other phenolics such as butylated hydroxytoluene (BHT) and resveratrol does not favor thermally induced transesterification reactions which cause PLA degradation.7 Regarding PE films in nitrogen, since the onset of PE thermal degradation was remarkably higher than PLA and NSE, the additive did not modify significantly neither Tonset nor Tmax (Figure S5c, d). TG curves in air showed a second decomposition step at higher temperature due to the polymer combustion. Under oxidizing conditions, the doped PLA films also performed better than the control, due to the peroxide radical scavenging ability of PS contained in NSE. In particular, Tonset of PLA2NSE was about 5 °C higher than that of PLA0. As concerning PE, under air the degradation temperatures were significantly lower compared to those in inert atmosphere, and the curves exhibited several consecutive weight loss steps. In Table 3, the value of the first peak of the TG derivative curve is reported as Tmax, along with the oxidation onset temperature (OOT). Among doped PE films, only Irganox and 2% NSE

improved the oxidative stability, as confirmed by Tonset and Tmax, while 1 and 3% NSE were detrimental to PE stability. However, a detailed view of the early stage of weight loss between 180 and 260 °C (Figure S6) provided a more clear insight to the NSE effect. All curves except PEIOX exhibited a weight increase due to oxygen chemisorption, followed by a rapid weight loss attributed to the rapid decomposition of the unstable peroxidized polymer species.1 From the table, we noted that oxidation of unstabilized PE can initiate at temperatures as low as those used in polymer processing. NSE retarded polymer peroxidation to an extent proportional to additive concentration, due to hydrogen-transfer reactions (Scheme 1). In this regard, Irganox showed the best performance, as it substantially suppressed polymer peroxidation. Since TG is not as sensitive in providing detailed information on the early stages of oxidation, CL emission of samples upon temperature ramping was also studied. CL allows to accurately predict the oxidative stability of polyolefin formulations, as light emission is due to the thermally stimulated radiative 4612


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Figure 3. Evolution of the carbonyl region of FTIR spectra upon UV irradiation for (a) PE0 and (b) PLA0.

Figure 4. Increase of main functional groups in the FTIR spectra of photo-oxidized films: (a) PE carbonyl groups, (b) PE vinyl groups, (c) PLA anhydride groups, and (d) PLA vinyl groups.

that of Irganox, confirming the ability of the extract to withstand the temperatures adopted during polymer processing without evident degradation (Figure 2b). In this regard, it is worth noting that PE3NSE showed almost no CL increase upon heating. In the case of colored films, a reduction in CL intensity could simply be related to quenching due to the simultaneous light emission and absorption by the tinted material. However, isothermal experiments performed on PE at 180 °C under air indicated that the protective effects of NSE could not be ascribed to simple absorption (Figure S7). Indeed, a significant increase in oxidation induction time (OIT) was recorded, particularly for PE2NSE and PE3NSE (PE0 510 s, PE1NSE 700 s, PE2NSE 1100s, and PE3NSE 1250 s). Nonisothermal curves in air showed increased CL intensity compared to those in nitrogen, since polymer oxidation

decomposition of polymer peroxides, and antioxidants reduce the emission intensity according to their stabilizing efficiency.55 CL emission in nitrogen was first studied, to evaluate the peroxide content formed during film processing at high temperature. Nonisothermal CL curves under nitrogen for PLA and PE films doped with NSE and Irganox are shown in Figure 2. CL emission increased with increasing temperatures, due to the gradual thermal decomposition of peroxides formed upon polymer processing. All films containing NSE displayed lower CL intensity compared to the pure matrix, indicating that NSE acted as a processing stabilizer. An effect comparable to that elicited by Irganox was observed for PLA3NSE (Figure 2a). PE showed a level of CL emission comparable to that of PLA, and in this case, NSE attained even lower intensity compared to 4613


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Figure 5. Change in tensile properties of films upon irradiation: (a) PE strain at break, (b) PE stress at break, (c) PLA strain at break, and (d) PLA stress at break.

acids (1715 cm−1).56 For PLA films, the increasing absorption in the region 1860−1830 cm−1 is attributed to the formation of anhydride groups.28 The accumulation of these species occurs prevalently by a radical mechanism initiated by hydrogen abstraction, which leads the formation and decomposition of hydroperoxides.28,57 Mainly carboxylic acids and ester groups are formed in PE (Scheme S1), while anhydride groups are found in oxidized PLA (Scheme S2). For both polymers, these spectral changes were accompanied by the formation of a peak at 1645 cm−1, characteristic of the carbon−carbon double-bond stretching. The evolution of this peak is shown in Figure S8. This finding is related to a different reaction pathway. Indeed, with wavelengths lower than 290 nm, oxidation can be initiated by Norrish I and Norrish II photolytic cleavage reactions involving carbonyl groups present on the polymer backbone.58 Such moieties are already present as ester carbonyls in pristine PLA, while they can be formed in PE (mostly as ketones) during processing or directly from alkyl macroradical oxidation59 (Scheme S2, bottom pathway). The comparison of the evolution rate of these functional groups on irradiation is provided in Figure 4 for all samples. For PE, vinyl peak integration was performed in the 920− 900 cm−1 range, since absorption in this region was more intense (Figure S9). Carbonyl group content (Figure 4a) remained low up to 300 h of irradiation for all PE samples, confirming that most carbonyls are secondary photoproducts of hydroperoxide homolysis. Subsequently, their formation rate increased particularly for PE0 and PE1NSE, due to the autoaccelerating nature of the oxidative reactions of polyolefins. The slope of the carbonyl curve of the films containing 2 and 3% NSE was lower, and that of the PEIOX curve was in

resulted in a greater amount of thermally decomposing peroxides (Figure 2c,d). Moreover, CL intensity of neat PE was about 1 order of magnitude higher than that of PLA. Although a direct relation between CL intensity and oxidizability cannot be established when different polymers are considered, this finding confirmed the higher propensity of PE to undergo oxidation with respect to PLA.23 Under these conditions, the antioxidant effect of NSE on both polymers was also clear. In particular, NSE was more effective when blended to PLA, where the suppression of the CL signal was comparable to that of the commercial stabilizer, irrespective of additive concentration (Figure 2c). For PE, stabilization was significant until 210 °C, whereas higher temperatures led to increased CL emission due to the initiation of polymer oxidation (Figure 2d). Overall, these results highlight the activity of NSE as thermal stabilizer for different polymer matrices, both in an oxygendepleted environment (i.e., during melt processing), and in the presence of oxygen during service life of the polymer. In particular, NSE has a great potential as PLA stabilizer, likely because the additive is more compatible with polar polymers. Photo-Oxidative Stability of PE and PLA Films. To evaluate the potential of NSE as a polymer stabilizer under conditions closer to service life, all films were UV-irradiated at 40 °C and periodically analyzed by FTIR spectroscopy and mechanical tests until embrittlement. The major changes in FTIR spectra of PE and PLA were recorded in the carbonyl region due to photo-oxidation and formation of carbonyls as stable products (Figure 3). For PE, the increase is related to the formation and accumulation in the polymer of γ-lactones (1780 cm−1), esters (1735 cm−1), ketones (1720 cm−1), and associated carboxylic 4614


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Figure 6. Flowchart of the continuous NS extraction process.

PLA films. However, further studies are required to elucidate the photodegradation mechanism of PLA in the presence of NSE. The increase in carbonyl and vinyl groups observed by spectroscopic analysis was correlated to the changes in tensile properties of the UV-irradiated films. Strain and stress at break as well as tensile modulus were measured as a function of the photo-oxidation time. All PE samples displayed strain at break values of about 600% prior to UV irradiation, whereas PLA exhibited poor ductility (about 20% strain at break) (Table S1). Figure 5 shows the changes in strain and stress at break for PE and PLA films subjected to the photo-oxidative treatment. All films exhibited a drop in tensile strain due to molecular weight decrease. For PE0, strain and stress at break declined soon after 100 h (Figure 5a), while a slight increase was detected for the stabilized samples, since thermal annealing favored crystalline phase aggregation.11 Interestingly, all NSEdoped samples retained about 90% deformation at break even after 300 h, under which conditions PE0 degraded significantly (Figure 5a,b). Mechanical tests confirmed that Irganox has no UV stabilizing activity. As far as PLA is concerned, no appreciable differences among the samples were noted in terms of strain changes, and all of them failed after about 30 h irradiation (Figure 5c). It should be underlined that the strain at break values of unaged PLA was about 20%; therefore, even small changes dramatically impacted on polymer ductility. Notably, stress at break was more sensitive to the presence of NSE, and the NSE-doped samples exhibited an improved mechanical resistance over time, which was particularly significant for PLA1NSE (Figure 5d). Manufacturing Costs of NSE. The cost estimation of NSE manufacturing was first performed considering the batch production of 1 kg of extract, which required lab-scale equipment consisting of an extraction tank, a decanting device for separating the solid residue from the extract mixture, and a batch rotary evaporator for recycling the solvent mixture and yielding the extract powder. In the procedure, NS extraction included two 30 min ultrasound-assisted extraction steps. Therefore, a total extraction time of 1 h was considered. A 1:10 (w/v) ratio between nutshell biomass and extracting solvent

between. After 1200 h, PE3NSE attained a carbonyl value about 40% lower than the control. A similar behavior was found for vinyl groups (Figure 4b) formed upon ketone photolysis, and NSE was even more effective. Indeed, the amount of vinyl groups formed in PE3NSE after 1200 h was one-third that of PE. Moreover, 1% NSE yielded the same result of Irganox. These results demonstrate the effectiveness of NSE in protecting PE from photo-oxidation, suggesting that besides a peroxy radical scavenging mechanism NSE is also capable of retarding photolytic cleavage through Norrish type pathways. This outcome may be due to the strong UV absorbing activity ascribed to flavonoid structures such as quercetin, which can be formed through in situ photo-oxidation of catechins present in NSE.60,61 A slightly different picture was observed for PLA. Only PLA1NSE showed a retardation in anhydride accumulation, while all the other samples containing NSE exhibited a stability comparable to that of pure PLA, and Irganox even acted as a prodegradant (Figure 4c). Since TG and CL showed that under thermal oxidative conditions NSE stabilized PLA through radical scavenging, it is reasonable to attribute this apparent discrepancy to the photochemical behavior of NSE in PLA. The curve relative to the vinyl groups accumulation (Figure 4d) clearly showed that compared to PLA0 both NSE and Irganox promoted the formation of carbon−carbon double bonds. Since vinyl unsaturations are produced by Norrish II type scissions, it is likely that the phenolic additive promotes PLA degradation via direct photolysis. This consideration was supported by the kinetics of carboxylic acids (integration in the range between 1700 and 1720 cm−1, Figure S10), which showed a trend comparable to that found for vinyl groups. Such a similarity was expected, as in PLA carboxylic acids are mainly formed upon Norrish II photolysis. In this regard, unsaturations tend to disappear during oxidation by a mechanism involving addition to vinyl double bonds; therefore, all samples except PLA showed an autoretardant behavior. In contrast, carboxylic acids are photostable products; therefore, they showed a constant increase. The behavior reported here recalls that reported by Tsuji et al.,62 who used a photosensitizer, tetramethyl-1,4phenylenediamine (TMPD), to enhance photodegradation of 4615



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CONCLUSIONS Extractable phenolics of pecan NS are mainly constituted of oligomeric anthocyanidins and phenolic acids, endowed with antioxidant activity and high thermal stability. When compounded with PE and PLA, NSE acted as a thermal processing stabilizer, as well as an effective antioxidant. In particular, NSE was very active when blended with PLA, while in PE the antioxidant effect was significant until 210 °C. The influence of NSE on the photochemical behavior of PE and PLA films was studied by low-temperature UV-light irradiation. FTIR analysis demonstrated that NSE-doped PE films were more stable to photo-oxidation than the PLA-based counterparts. PE films doped with 1 and 3% w/w NSE retained about 90% strain at break even after 300 h of irradiation, under which conditions the undoped sample degraded. An estimation of NSE manufacturing costs demonstrated that NSE production on an industrial scale could be competitive with that of traditional oil-based antioxidants. These results are promising in view of using NSE as a safe and sustainable stabilizer in polymeric materials for packaging and other applications.

was considered so that 69.2 L of ethanol/water (6:4 v/v) solution was required for extracting 6.92 kg of nutshell. On the basis of the calculation reported in the Table S2, the manufacturing cost of NSE was 24.69 USD kg−1. A cost breakdown analysis showed that the final cost was mostly affected by raw material expenditure (21.53 USD), mainly due to ethanol cost, followed by electricity cost (3.16 USD); the latter was almost exclusively due to the solvent evaporation step. However, a second extraction cycle using recycled solvent resulted in a remarkable decrease of raw material cost. Indeed, considering a solvent recovery rate of 95%, the overall cost of NSE dropped to 4.64 USD kg−1. As a comparison, the cost of industrial supplies of Irganox 1076 ranges between 5 and 15 USD kg−1, depending on country of origin and purchased quantity.13,63 However, since the assessment of the economic viability on an industrial scale requires also considering capital investment and operating facility and labor costs,29 a comprehensive simulation of a scaled-up continuous process was performed. In the simulation, it was assumed that the extraction yields obtained by industrial-scale units were similar to those obtained on the lab scale. The flowchart of the continuous process (Figure 6) included three extraction tanks, a decanting device for separating the residual biomass from the extract mixture, a thin film evaporator to concentrate the solvent mixture, a condenser to recycle the ethanol solution, and a spray dryer for producing the extract powder. Temperature (80 °C) and pressure (1.013 bar) used in the thin film evaporator yielded a solution with a mass solid content of about 50%. After spray drying, the final solid content of NSE powder was 95%.29 Table S3 lists the cost data fed into the simulation software to calculate the manufacturing cost of NSE. From the simulation, it turned out that a 1:10 w/v feed to solvent ratio allowed to work up a total of 357 000 kg of NS per year, yielding about 35 000 kg of NSE, with a manufacturing cost (UPC) of 35.24 USD kg−1 (see NSE 1:10 Economic Evaluation Report section in Supporting Information). These results are comparable to the results reported by Viera et al., who used a different equation to calculate the cost of manufacturing of crude jussara extracts.29 As already reported,64 the FDC was the most important item, accounting for about 80% of the AOC. LC accounted for about 13%, while UC and RC rates were lower be due to the low costs of feedstock, electricity, and heat exchange agents.30 However, optimization of the extraction process could result in a dramatic change in the cost of manufacturing. Indeed, an increase in the feed to solvent ratio to 1:4 w/v led to an annual NSE throughput of 134 000 kg, with a UPC of 14.40 USD kg−1, which is much closer to the cost range of traditional oil-based antioxidants (see NSE 1:4 Economic Evaluation Report section in Supporting Information). Under these conditions, if the selling price of NSE is 20 USD kg−1, then the payback time is 7.13 years, with a rate of return on investment (TRI) of 14.02%. Therefore, hydroalcoholic NS extraction can be an economically viable method to obtain an efficient polyphenol polymer stabilizer safe for food contact applications. In order to optimize the process, the effect of processing parameters, such as extraction time, temperature, and solvent/biomass ratio should be further addressed. Moreover, it is known that the cost of manufacturing is inversely proportional to the plant capacity, representing an advantage for the investment on a large industrial scale.65

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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.6b03124. MS spectra, chemical structure of NSE constituents, TG curves, mechanism of PLA and PE thermal and photooxidation, rate of functional groups increase, economic evaluation data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +39-081-8675214. ORCID

Pierfrancesco Cerruti: 0000-0003-0822-2866 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge Ch. Claudia Molina-Dominguez (DICTUS), and Ch. Giovanni Dal Poggetto (IPCB) for their support in HPLC-mass spectrometry and NMR analysis, respectively. This research was supported by Viñedos Alta S.A. de C.V. (Mexico), and the Italian Ministry of University and Research (MIUR) in the framework of the PON03PE_00107 BioPoliS project. S. Agustin-Salazar thanks CONACyT for the grant.

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REFERENCES

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Stabilization of Polylactic Acid and Polyethylene ... - ACS Publications

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