curcumin melt extruded composites with enhanced water vapor barrier and antioxidant properties for active food packaging

Polymer 175 (2019) 137–145

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Low-density polyethylene/curcumin melt extruded composites with enhanced water vapor barrier and antioxidant properties for active food packaging

T

Jasim Ziaa,b, Uttam C. Paula, José Alejandro Heredia-Guerreroa, Athanassia Athanassioua, Despina Fragoulia,∗ a b

Smart Materials, Istituto Italiano di Tecnologia, Via Morego 30, Genova, 16163, Italy Dipartimento di Informatica Bioingegneria, Robotica e Ingegneria dei Sistemi (DIBRIS), Università degli studi di Genova, Via All' Opera Pia 13, Genova, 16145, Italy

H I GH L IG H T S

antioxidant food packaging films from low-density polyethylene (LDPE) and curcumin. • Highly water vapor barrier properties. • Excellent thermal stability without altering thermal processability. • High • Fabrication through industrially applicable methods.

A R T I C LE I N FO

A B S T R A C T

Keywords: Active packaging Biocomposites Melt mixing Barrier properties

In this study, active biocomposite films of low-density polyethylene (LDPE) containing 5% wt. of curcumin, a natural chemical produced by some plants, are developed by melt extrusion and hot pressing. The detailed study of the physicochemical properties of the formed films demonstrates that the curcumin molecules, homogeneously dispersed in the whole volume of the LDPE matrix, cause a notable increase in the thermal stability of the LDPE polymer, without altering its thermal processability. At the same time the LDPE's water vapor barrier properties are significantly improved, while an effective antioxidant scavenging activity is observed against the 2,2-diphenyl-1-picrylhydrazyl free radicals (DPPH•). These results indicate that the developed active films based on the combination of LDPE and curcumin, following industrially applied preparation methods, are ideal candidates for active food packaging applications.

1. Introduction In the past few years, the development of active food packaging materials, able to enhance the shelf life of the products while maintaining their nutritional quality, is continuously gaining a great research interest in materials engineering. This can be mainly achieved by developing plastic packaging systems able to decelerate the oxidation of the food, the growth of microorganisms and the migration of the contaminants [1,2], succeeded by introducing in the polymers antioxidant and antimicrobial releasing components, and fillers with enhanced barrier properties against vapors, oxygen, etc. [2,3]. There are diverse studies where synthetic antioxidant compounds have been combined with polymers for packaging applications [4,5], however the potential toxicity derived from their migration into food

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products is making their application questionable, and strict statutory controls are required before reaching the consumers [6]. Similarly, to enhance the barrier properties of the packaging polymeric materials, synthetic additives, such as clays, silicate nanomaterials etc., have been employed [7–9]. However, the migration towards food and the effects to the health are still under consideration and therefore the, still not defined, acceptable limits for migration of these substances into food may restrict their large scale application in the food packaging field [9–11]. Therefore, due to the adverse effects of the synthetic additives to the human health and environment, there is an increasing trend in the use of molecules and particles of natural origin [1,2,6,12–15]. Some of them can significantly increase the antioxidant activity of the films [16,17] but at the same time they do not contribute sufficiently to the enhancement of the water vapor barrier properties.

Corresponding author. E-mail address: [email protected] (D. Fragouli).

https://doi.org/10.1016/j.polymer.2019.05.012 Received 25 January 2019; Received in revised form 3 May 2019; Accepted 5 May 2019 Available online 06 May 2019 0032-3861/ © 2019 Elsevier Ltd. All rights reserved.

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However, the utilization of phenolic compounds, like tea extracts [2,18] and curcumin [19] may offer the possibility to improve simultaneously both properties if combined with the right polymers, making possible the development of packaging materials with enhanced properties. In particular, curcumin is widely used in the food industry and pharmaceuticals due to its excellent antiviral, anticancer, antimicrobial, anti-inflammatory and antioxidant activities [20–26], which combined to its ability to change color upon pH modifications [27,28] have led to the development of curcumin based smart packaging systems. Most of the studies on the utilization of the curcumin and other natural additives in food packaging, are focused on their combination with natural polymers e.g. gelatin, k-carrageenan, chitosan, corn zein and tara gum [19,27–30] as host matrices, in order to achieve functional biocomposites utilized either as is or as coatings and components of multilayered films [31–33]. However, most of the fabrication methodologies proposed, are not directly correlated with the classical polymer technological processes for large scale industrial production, such as the melt processes, (e.g. melt extrusion, injection, etc.), making difficult to envision their use in a large scale commercial production compromising, thus, their direct applicability in packaging systems. On the other hand, polyethylene (PE), the first polyolefin used in food packaging since the 1940s [34,35], is widely used in packaging due to its easy feedstock availability and processability, versatility, low cost and recyclability. Recently, due to the development of ethylene feedstocks from renewable resources, like the bio-ethanol produced by fermentation processes of biomass, PEs are developed from renewable natural resources (BIO-PE) with identical properties as the petroleum derived PEs [36] offering thus a sustainability advantage relative to other thermoplastic resins. Among PEs, LDPE is a widely used polymer in food packaging, due to its favorable sealing, stiffness, moisture barrier, and transparency properties, and it meets the requirements of thermal stability for the melt-flow processes applied in most manufacturing applications [37–39]. However, to the best of our knowledge, studies on the development of active composite packaging systems based on LDPE, following melt flow processes are pretty limited [40,41] while, in most of the cases, the active components are combined with the polymer in the form of multilayers to achieve antioxidant and antibacterial activity [42–45]. Furthermore, in the few studies dealing with the combination of PEs with natural fillers, the focus is on the combination with small quantities of natural antioxidants, in order to prevent the polymer's oxidation due to elevated temperatures applied during conventional melt processing operations without further modification of the overall properties of the polymer [12,46]. Herein, is presented the development of active food packaging composite films based on the combination of LDPE with curcumin following a conventional fabrication method. In particular, we adopt a hot melting process: melt extrusion, in order to mix effectively the two components, followed by compression molding to obtain the active food packaging films. The results of the present study reveal that the addition of curcumin increases the thermal stability of the LDPE biocomposite films, without changing their melting behavior, and thus, without compromising their thermo-formability. In addition, the presence of curcumin enhances the tensile modulus of the active films and improves significantly their water vapor barrier performance. On the top, the active biocomposite films possess effective antioxidant scavenging activity against the DPPH• free radicals. Therefore, following an easily scalable fabrication method already well established for bulk stage production, LDPE active biocomposites are formed, ready to be applied in smart food packaging applications.

Fig. 1. Chemical structure of curcumin.

(Turmeric powder) (Fig. 1), ethanol with 96% purity and 2,2-diphenyl1-picrylhydrazyl (DPPH) were purchased from Sigma Aldrich and were used without further purification. The curcumin particles have sizes ranging between few μm and tens of μm as presented in the SEM image of Fig. S1 of the Supporting Information. 2.2. Fabrication of the films The extrusion of the pure LDPE and LDPE containing 1, 2, 3, 5 and 7% wt. of curcumin (Cur/LDPE) was carried out through a three heating zone single-screw Rheoscam extruder (Scamex-France) with a screw of 20 mm diameter, and length to diameter ratio (L/D) of 20. The screw was attached to a die of 2 mm diameter at the extrusion point, and its rotation speed was maintained at 50 rpm throughout the extrusion process. During the process, the temperature of the three heating zones was set at 145 °C, while the entire length of the heating zones is 11.5 cm. The extruded filaments were subsequently pelletized, and the formed pellets (dimensions 3.9 mm × 1.7 mm) were used for the fabrication of the films. Following this process, pure LDPE and Cur/ LDPE composite films were produced by compression molding (Carver Inc., Model 3850, USA) at 145 °C for 40 min with a 10 MPa applied pressure. The dimensions of the final films were 9.8 cm × 9.8 cm and their thickness 450 μm. 2.3. Characterization 2.3.1. Morphological characterization To study the surface and cryofractured cross sectional morphologies of the pure LDPE and Cur/LDPE filaments and films, scanning electron microscopy imaging was performed (SEM, JEOL JSM-6490AL) after coating the specimens with 10 nm thin gold layer using a Cressington 208HR high-resolution sputter coater (Cressington Scientific Instrument Ltd., UK). 2.3.2. Chemical characterization The chemical composition of curcumin and LDPE and the potential intermolecular interactions between the different components were characterized by Fourier Transform Infrared Spectroscopy (FTIR) analysis. Infrared spectra were obtained with a single-reflection attenuated total reflection (ATR) accessory (MIRacle ATR, PIKE Technologies) coupled to a Fourier Transform Infrared (FTIR) spectrometer (Vertex 70v FT-IR, Bruker). All spectra were recorded in the range from 3800 to 600 cm−1 with a resolution of 4 cm−1, accumulating 128 scans. 2.3.3. Thermal and mechanical characterization The thermal stability of the pure curcumin powder, pure LDPE and Cur/LDPE films was investigated by means of thermo-gravimetric analysis (TGA) using a TGA Q500 system (TA Instruments USA). The measurements were carried out on 14 mg samples weighed in aluminum pans and heated from 30 °C to 800 °C with a heating rate of 10 °C/min under air of a constant flow rate of 50 ml/min. The thermal behavior of the fabricated films was studied through differential scanning calorimetry (Diamond-DSC, PerkinElmer, USA). The 20 ± 1 mg of pure LDPE and Cur/LDPE films were weighed accurately in sealed aluminum pans and heated from 30 °C to 220 °C with

2. Experimental 2.1. Materials Low-density polyethylene (LDPE), curcumin from Curcuma longa 138

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2.3.6. Antioxidant activity test 2.3.6.1. Release kinetics. For the release kinetics, samples of 60 mg weight were cut with a dye cutting press and placed in ethanol solution such that the sample concentration in the solution to be 1% (w/v). The evolution of the absorbance of the extracted curcumin in the solution was recorded after defined time intervals up to 24 h using pure ethanol for the baseline correction. Following this process, 10 different samples were tested, and the average absorbance value of the characteristic peak of the curcumin (428 nm), was calculated for each time interval. In order to evaluate the concentration of the curcumin in solution in each case, the absorption spectra of five different curcumin solutions with known concentrations ranging between 0.001 and 0.005 mg/ml were recorded. The intensity of the absorption peak at 428 nm was then plotted and the equation which gives the dependence of the absorption intensity to the curcumin concentration was defined by a linear fit (Fig. S2, Supporting Information). According to the linear fitting, the concentration of the curcumin in unknown solutions was calculated by the equation

a rate of 20 °C/min under a dry nitrogen purge (flow rate 20 ml/min). After keeping the samples at 220 °C for 1 min, they were then cooled down from 220 °C to 30 °C with a rate of 20 °C/min. After 1 min at 30 °C they were subsequently subjected to a second heating cycle with the same parameters. An empty pan was used as the reference cell. The instrument was calibrated using indium before testing the samples. The first heating cycle was performed in order to erase any aging event occurring during samples storage, and the second one was conducted to determine the melting temperature (Tm) while the first cooling cycle was carried out to determine the crystallization temperatures (Tc). The mechanical properties of the pure LDPE and Cur/LDPE composite films were determined using an Instron (3365 Instron, USA) dual column tabletop universal testing system, with a load cell of 5 kN at a crosshead speed of 5 mm/min, following the ASTM D882-12 standard method. The specimens were cut into dog-bone shape with dimension of 35 mm × 4 mm using a mechanical cutter. The elastic modulus in MPa and elongation at break in % were automatically calculated by the Instron software. In each case, 5 different samples were tested from different films and the average values are reported.

Iabs = 181.25 × C

where Iabs is the intensity of the absorbance peak at 428 nm and C is the curcumin concentration.

2.3.4. Water vapor permeability and wetting behavior The water vapor permeability (WVP) of the pure LDPE and Cur/ LDPE films was determined gravimetrically according to the ASTM E96/E96 M standard method [47,48]. The tests were carried out at ambient temperature under 100% relative humidity gradient (ΔRH%). 400 μl of distilled water (which provide 100% RH inside the permeation cell) was placed in each of the permeation cells with a 10 mm inner depth and an inner diameter of 7 mm. The test films were cut in circular form using a dye cutting press, and then were mounted on the top of the permeation cells. The permeation cells were placed in a desiccator, which contained anhydrous silica gel as a desiccant in order to maintain 0% RH [49]. The water transferred through the test films and absorbed by the desiccant was determined from the weight loss of the permeation cell after every hour for an 8 h period using an electronic weighing balance (0.0001 g accuracy). The weight loss of the permeation cells was plotted as a function of time. Then, the water vapor transmission rate (WVTR) was determined from the slope of each line obtained from the linear regression, as shown by the following expression.

WVTR =

Slope Area of the film

2.3.6.2. Radical scavenging activity. To investigate the antioxidant activity of Cur/LDPE films, the curcumin extract from different vials containing 1% (w/v) Cur/LDPE films was collected after time intervals of 1 h for a period of 24 h. For comparison reasons, the antioxidant activity of pure curcumin powder was also evaluated by preparing the same concentrations of curcumin powder in ethanol (measured through the process described above (Equation (3))). The antioxidant activity of the Cur/LDPE films was determined by using a slightly modified standard DPPH• free radical scavenging method [50,51]. In particular, 2 ml of the curcumin extract solution was mixed with 2 ml of 50 μM solution of DPPH• in ethanol and kept in the dark. After 30 min, the absorbance of this solution was measured. The absorbance of a second solution was measured by mixing 2 ml of curcumin extract solution with 2 ml of ethanol. Finally, the spectrum of the absorbance was obtained by mixing 2 ml of 50 μM solution of DPPH• in ethanol with 2 ml of ethanol. The percentage of DPPH• scavenging activity was calculated according to the following equation.

(1)

Radical scavenging activity (%) = ⎡1 − ⎣

The WVP of the films was calculated as follows [48,49].

WVP =

WVTR × L × 100 Ps × ΔRH

(3)

A1 − A2 ⎤ × 100 A3 ⎦

(4)

where A1 is the absorbance value at 517 nm of the solution containing the curcumin extract and the DPPH• radical, A2 is the absorbance at 517 nm of the curcumin extract in ethanol and A3 refers to the absorbance of DPPH• control solution at the same wavelength. All the results are reported as an average of three repetitions of different samples.

(2)

where, L (m) is the film thickness, ΔRH (%) is the relative humidity gradient and Ps (Pa) is the saturation water vapor pressure at 25 °C. The results are reported as a mean of 10 repetitions from different films with their standard deviation. The wettability of the fabricated films was studied using an optical contact angle measurement instrument (Data Physics OCAH 200, Germany). A 5 μl distilled water droplet was deposited on the surface of the films. Three films of each type were used and for each film ten measurements were recorded after the drop deposition at different areas of the films’ surface. The average values were then calculated and reported.

3. Results and discussion 3.1. Morphology Fig. 2 demonstrates the consecutive transformations of the materials, in order to reach the formation of the Cur/LDPE films. It should be mentioned that Cur/LDPE films with different curcumin concentrations are fabricated and characterized, and we will focus on the 5% Cur/ LDPE film, since, as will be proved below, this combination results in films with good combined properties for packaging applications. In all cases, the two components are first coextruded into filaments (Fig. 2a). The extruded filaments are, then, transformed into pellets, which are subsequently hot pressed in order to form the films with the typical orange color attributed to the curcumin filler (Fig. 2b). The composites fabricated with less curcumin content have lighter orange color, which

2.3.5. UV–visible spectroscopy UV–visible (UV–Vis) spectroscopy was used to determine the absorption of solutions for the release kinetics of the curcumin from the Cur/LDPE films, and their antioxidant activity. Each solution was kept in a quartz cuvette (3.5 ml capacity) that was placed in the sample holder of Varian CARY UV–Vis spectrophotometer (CARY 6000i, USA). Spectra were recorded in the wavelength range of 200–800 nm. 139

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Fig. 2. Photographs of (a) extruded 5% Cur/LDPE filament, and pellets, (b) 5% Cur/LDPE film (c) pure LDPE film.

the typical bands associated with the PE in a structure with a low degree of crystallinity: CH3 stretching mode at 2957 cm−1, asymmetric and symmetric CH2 stretching at 2914 and 2849 cm−1, respectively, symmetric CH3 bending at 1377 cm−1, CH2 bending, wagging and rocking modes at 1470 cm−1, 1302 cm−1, and 718 cm−1 respectively [38,57–60]. The 5% Cur/LDPE composite presented bands representative for both components, with no shifts or changes in the relative intensity of the curcumin bands, indicating no chemical secondary interactions with the matrix. However, some subtle modifications of the main LDPE bands (i.e., CH2 stretching, bending, and rocking modes) are detected. To highlight these differences the subtraction of the LDPE spectrum form the 5% Cur/LDPE one is performed and the results are presented in Fig. 4b,c,d. After subtraction, in the region associated with the asymmetric and symmetric CH2 stretching modes, positive absorptions were observed at higher wavenumbers than the maxima of νa(CH2) and νs(CH2) of the LDPE spectrum, indicating a blue shift. A similar behavior was noticed for the CH2 rocking mode. On the contrary, the CH2 bending mode presented positive absorptions at lower wavenumbers, that is, a red shift. This type of slight shifts has been previously reported for LDPE as a function of the temperature and are ascribed to a bulk contraction and subsequent increase of van der Waal forces between polymer chains [61]. In this sense, the presence of curcumin clusters in the composite and the high pressure during extrusion molding can effectively induce a polymer shrinkage, decreasing the distances between LDPE chains.

becomes darker and more intense with the increase of the curcumin concentration in the LDPE. The white pure LDPE films (Fig. 2c) are formed following the same process (without adding the curcumin). Both types of films have a smooth, flat surface of similar roughness, as shown in the AFM surface characterization study presented in Fig. S3 of the Supporting Information. Due to their thickness, all films are opaque. Nonetheless, thinner LDPE, as also the orange colored Cur/LDPE films are transparent, and therefore can be easily used in packaging applications, where transparency is needed. In the SEM images presented in Fig. 3, it is evident that the LDPE filaments and films have a smooth, compact, and nonporous morphology. By introducing the curcumin powder filler (5% wt.), the filaments' and films’ top and cross section surface becomes slightly less smooth, but no big particle agglomerates, air gaps, cracks, or detachment zones, are evident indicating a good dispersion of the curcumin powder in the polymer matrix. 3.2. Chemical characterization Curcumin, LDPE, and the blend of both components were chemically characterized by ATR-FTIR spectroscopy (Fig. 4). As shown, in the full-range infrared spectra of the curcumin (Fig. 4a) the main assignments are the OH stretching at 3507 cm−1, different C–H stretching modes in the region between 3020 and 2940 cm−1, conjugated C=O stretching at 1627 cm−1, aromatic C=C stretching at 1602 cm−1, C=O stretching at 1506 cm−1, olefinic C–H bending at 1427 cm−1, C–O stretching of the enol tautomer at 1273 cm−1, C–O–C stretching at 1026 cm−1, and the C–H out of plane bending of the aromatic rings at 855 cm−1 [28,52–56]. On the other hand, the polymer matrix showed

3.3. Thermal and mechanical properties The thermal stability of the pure curcumin, pure LDPE and 5% Cur/ LDPE films were evaluated by TGA analysis under air atmosphere (Fig. 5). As evident in the spectra, the decomposition of curcumin takes place in two different steps [62]. After the thermal elimination of the adsorbed moisture at lower temperatures, the first degradation step occurs between 201 °C and 337 °C with a maximum rate of weight loss (Tmax) at 290 °C attributed to the decomposition of the substituent groups of curcumin [63]. The second degradation step takes place at higher temperatures, between 337 °C and 587 °C with a Tmax at 535 °C due to the decomposition of the two benzene rings [63]. The degradation of pure LDPE takes place in a single step at the temperature range from 237 °C to 461 °C (Tmax: 373 °C) due to the random polymer chain scission and branching [64]. In the case of 5% Cur/LDPE composite, two degradation steps occur during the heating ramp. The first step takes place at a very slow rate, between 224 °C and 387 °C (Tmax 345 °C) with a very short weight loss of about 4.3%, while the second degradation step takes place in between 387 °C and 459 °C with a faster degradation rate and a Tmax of 390 °C. Compared to the pure LDPE, the Cur/LDPE composite films present higher thermal stability with a degradation temperature shifted by 17 °C (from 373 °C to 390 °C), attributed to the presence of curcumin, which is able to capture the generated radicals of the polymer during the thermal oxidation decelerating the degradation process [46,65]. The possible existence of thermal phase transitions of the Cur/LDPE

Fig. 3. Surface and cryofractured cross sectional morphologies of (a) pure LDPE filament, (b) 5% Cur/LDPE filament, (c) pure LDPE film and (d) 5% Cur/LDPE film. 140

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Fig. 4. (a) ATR-FTIR spectra of pure curcumin powder, pure LDPE, and the 5% Cur/LDPE composite. Main assignments of curcumin and LDPE are included. (b,c,d) CH2 stretching (3000-2750 cm−1), bending (1500-1400 cm−1), and rocking (740-700 cm−1) modes, respectively, of pure LDPE and 5% Cur/LDPE composite. The corresponding subtraction spectra are included and the regions of interest are highlighted by filling the peaks in a light blue color. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 5. (a) TGA thermograms and (b) derivative thermogravimetric curves of curcumin, pure LDPE and 5% Cur/LDPE films. 141

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vapor transmission rate (WVTR) and water vapor permeability (WVP) of the Cur/LDPE films decrease with the increase of the curcumin content reaching 73% lower values for the 7% Cur/LDPE compared to those of the pure LDPE film, indicating that the presence of curcumin improves the water vapor barrier performance. Concerning the 5% Cur/ LDPE composite, a reduction of 51.5% in both parameters is observed. The reduction of the WVTR and WVP values can be attributed to the hydrophobic nature of the curcumin which is homogeneously dispersed in the polymer matrix and acts as a physical barrier in the path of water vapors [68]. In fact, as demonstrated to Fig. 7b and c the hydrophobicity of the composite film is enhanced with water contact angle (WCA) of the 5% Cur/LDPE films about 10° higher (110.8° ± 1.6°) with respect to that of the pure LDPE (101.2° ± 1.1°). Furthermore, this can be also attributed to the fact that the curcumin particles in the composite film cause the decrease in the distance between the polymeric chains as revealed by the FTIR study, hindering the diffusion of the water molecules through the composite films. 3.5. Antioxidant activity Although the introduction of curcumin in the LDPE films slightly decreases the oxygen barrier properties of the LDPE (see Fig. S6 and Table S4 of Supporting Information) the Cur/LDPE films present good antioxidant activity necessary for an efficient packaging system. Indeed, in active packaging applications, the necessity to use polymeric films with high antioxidant activity is of high importance, since it can dramatically decrease the free radical oxidation of the lipid components of the food and subsequently increase of food shelf life and quality [2,42,51]. In order to evaluate the antioxidant capability of the Cur/ LDPE film the DPPH• standard assay was adopted [2,18,67] as a representative method to measure the antioxidant activity towards lipid oxidation [67].

Fig. 6. Normalized DSC thermograms, second heating cycle and first cooling cycle for pure LDPE and 5% Cur/LDPE films.

films was investigated by DSC analysis, where the crystallization temperature (Tc) and melting temperature (Tm) were evaluated. The normalized thermograms for pure LDPE and 5% Cur/LDPE are presented in Fig. 6, and for the pure LDPE the Tc and Tm are 92 °C and 113 °C respectively. The presence of the curcumin in the 5% Cur/LDPE polymer blend does not cause any significant alteration to the melting or crystallization peaks and therefore causes minimal changes to the thermal properties of the polymer matrix indicating that the polymer chain mobility is preserved. Therefore, the Cur/LDPE films can be easily processed thermally, without altering the so far stabilized melt processing methods adopted in industrial scale for the LDPE polymers. Complete details of TGA and DSC results are shown in Table S1, Supporting Information. The effect of the presence of curcumin on the properties of the LDPE is further investigated by characterizing the mechanical properties of the composite systems. As shown in Fig. S4 and Table S2 of the Supporting Information, the increase of the amount of curcumin in the LDPE polymer causes the enhancement of the elastic modulus, and the decrease of the elongation at break, reaching 47% and 68% respectively for the 7% Cur/LDPE film. Concerning the 5% Cur/LDPE films, the elastic modulus is 34% higher compared to the pure LDPE (361 ± 9 MPa and 269 ± 8 MPa respectively), while the elongation at break is 57% lower (75% ± 9% compared to the 175% ± 13% of the pure polymer film). The increase in the elastic modulus with the enhancement of the curcumin content, is attributed to the presence of the uniformly dispersed curcumin filler in the polymer matrix, which, as also proved by the FTIR analysis previously presented, induces the bulk contraction by causing the increase of the van der Waals forces between the polymer chains and therefore by reinforcing significantly the material. The decrease in the elongation at break is caused by the increase in the stiffness of the composite due to the close packing of the polymeric chains after curcumin inclusion [66].

3.5.1. Release kinetics First, the release kinetics of the curcumin from the composite films in ethanol was studied by UV–vis spectroscopy, as shown in Fig. 8a and b and Fig. S7 of Supporting Information. By increasing the contact time with the solvent, the amount of curcumin released from the Cur/LDPE films increases, as the intensity increase of the characteristic absorbance peak at 428 nm indicates (Fig. 8a). In the first six hours the release rate is high, possibly due to the release of the curcumin present on the superficial layers of the film, and the total amount of 2.518 μg/ml (5% Cur/LDPE film) is present in the solution, as calculated by equation (3). Subsequently the curcumin release slows down reaching 3.392 μg/ ml after 24 h of contact of the film with the ethanol solution. The continuous and controlled release of the curcumin from the films over time, will help to prevent the oxidation of the food by maintaining the critical concentration of the antioxidant at the surface that is essential for inhibiting the oxidative change during time [69,70]. 3.5.2. Radical scavenging activity The DPPH• free radical scavenging activity of the Cur/LDPE films was evaluated by monitoring the decrease in the absorbance peak of the DPPH• at 517 nm when in contact with the curcumin extract [71]. As shown in Fig. 9 and Fig. S8 of the Supporting Information, the antioxidant activity of Cur/LDPE films increases with the concentration of curcumin in the composite and with time, since, in both cases, the concentration of the curcumin released in the solution increases. Therefore the increasing amount of hydrogen radicals or electrons in the solution stabilizes more and more DPPH• free radicals forming DPPH-H compounds, as also observed macroscopically with a clear color change in the solution from violet to yellow [71–73]. The antioxidant activity of 5% Cur/LDPE films increases from 24.2% to 44.5% for an observed period from 1 to 24 h (at a minor release of 3.392 μg/ml of curcumin after 24 h) owing to the free radical scavenging activity of the curcumin molecules through hydrogen donation [71]. Similar

3.4. Water vapor permeability and wetting behavior LDPE is a polymer widely used in food packaging also due to its favorable water vapor barrier properties. Packaging films with enhanced water vapor barrier properties contribute significantly to the maintenance/extension of the shelf life of the food products, because they prevent the transfer of water molecules from the internal or external environment through the polymer package wall [2,67]. As shown in Fig. 7a, Fig. S5 and Table S3 of the Supporting Information, the water 142

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Fig. 7. (a) Water vapor transmission rate (WVTR) and water vapor permeability (WVP) for pure LDPE and 5% Cur/LDPE films. WCA of (b) pure LDPE and (c) 5% Cur/LDPE film.

behavior is observed also for the 7% Cur/LDPE films, which reach 55.2% after 24 h, while films with lower curcumin concentration are not so efficient. These results show that the higher the curcumin concentration in the LDPE polymer the better the antioxidant activity as well as the water vapor barrier properties. However, the 7% Cur/LDPE films are rigid and have significantly low elongation at brake, making their application in food packaging quite difficult. Therefore, we can conclude that the 5% Cur/LDPE films possess excellent properties, and their slow scavenging process can find applications in cases where the prolonged and sustained antioxidant effect is needed, as in the active food packaging conditions.

4. Conclusions In conclusion, it is proved that the combination of curcumin with LDPE in the form of films, following industrially applied methods such as melt extrusion and hot pressing results in active composite homogenous films. The resulting Cur/LDPE films present enhanced stability against the thermal degradation without altering the thermal processability of the polymer. The incorporation of curcumin in the LDPE

Fig. 9. DPPH• free radical scavenging activity for 5% Cur/LDPE film.

Fig. 8. Release kinetics of 5% Cur/LDPE film (a) absorbance profile and (b) released curcumin concentration as calculated by equation (3). 143

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matrix significantly improved the water vapor barrier performance of the polymer, due to the enhanced hydrophobicity of the composite and the more compact packing of the polymer chains. The active films showed continuous and steady release of the active compound to the surrounding environment with sufficient antioxidant activity towards the lipid oxidation. Considering that LDPE is a widely used polymer in food packaging, and its use will continue to grow due to its excellent properties among which the recyclability, but also due to the possibility to produce it using renewable resources, the present biocomposites developed following a widely applied industrial method are feasible materials in the active food packaging field.

[17]

[18]

[19]

[20]

Acknowledgements [21]

Authors acknowledge L. Marini for the DSC and TGA studies, Dr. M. Salerno for the AFM analysis, and Dr. M. Zahid, Dr. M. Tamoor Masood and Dr. A. Sentahamizhan (Istituto Italiano di Tecnologia, Genova) for their kind support and useful discussions throughout this research project.

[22]

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Appendix A. Supplementary data

[24]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.polymer.2019.05.012.

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