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Mar 23, 2015 - Development of Films of Novel Polypropylene based Nanomaterials for Food Packaging Application. By Z. Ayhan,1* S. Cimmino,2 O. Esturk,3 ...

PACKAGING TECHNOLOGY AND SCIENCE Packag. Technol. Sci. 2015; 28: 589–602 Published online 23 March 2015 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/pts.2126

Development of Films of Novel Polypropylene based Nanomaterials for Food Packaging Application By Z. Ayhan,1* S. Cimmino,2 O. Esturk,3 D. Duraccio,2 M. Pezzuto2 and C. Silvestre2

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1 Faculty of Engineering, Department of Food Engineering, Sakarya University, Sakarya, Turkey Istituto per i Polimeri, Compositi e Biomateriali (IPCB)-CNR, Via Campi Flegrei 34. Comprensorio Olivetti, 80078, Pozzuoli (Naples), Italy 3 Faculty of Agriculture, Department of Food Engineering, Mustafa Kemal University, Hatay, Turkey

In order to design new antimicrobial nanocomposites with properties for food packaging application, films of polypropylene random copolymer (PPR), PPR/Poly-β-pinene (PβP), PPR/clay and PPR/PβP/clay were prepared by melt extrusion. Structural, morphological, mechanical, barrier, antimicrobial properties and thermal stability of the films were determined. PPR and PβP always form a homogeneous system in the amorphous phase; in the binary PPR/clay system, PPR molecules intercalate the clay galleries; in the ternary PPR/PβP/clay system, the miscibility between PPR and PβP prevents the intercalation of the PPR macromolecules into the clay galleries. The addition of clay and PβP increased the thermal stability and the tensile mechanical properties of PPR and reduced the oxygen transmission rate and the water vapor transmission rate compared with plain PPR. Films of nanomaterials containing PβP provided a reduction of the test microorganisms (Escherichia coli 25922) of 24% comparing to the control (PPR/clay film). Copyright © 2015 John Wiley & Sons, Ltd.

Received 31 July 2014; Revised 8 January 2015; Accepted 11 February 2015 KEY WORDS: polypropylene; poly-β-pinene; antimicrobial nanocomposite; permeability; food packaging

INTRODUCTION Application of nanotechnology in food packaging is considered highly promising because nanotechnology can improve packaging performances such as gas, moisture, UV and volatile barriers, mechanical strength, heat resistance and flame retardancy, oxidation stability and biodegradability of conventional polymeric material.1–3 These improvements increased the potential use of these materials for food packaging applications of several categories such as processed meats, confectionery, cereal, boil-in-the-bag foods, fruit juices and dairy products, beer and carbonated drink bottles.4 Many studies have focused on preparation, material properties and characterization of nanocomposites. In the area of nanocomposites, layered silicate nanocomposites have attracted great interest, both in industry and academia because they often exhibit remarkable improvement in properties when compared with virgin polymer or conventional micro and macrocomposites. The properties of the nanocomposites depend not only on the kind of constituents and composition but mainly on the degree of dispersion of nanoscale particles inside the matrix. When the nanoparticles are

* Correspondence to: Zehra Ayhan, Sakarya University Faculty of Engineering, Department of Food Engineering, Serdivan, Sakarya, Turkey. E-mail: [email protected] Partially (Only OTR and WVTR data) presented at International Conference on Food, Engineering and Biotechnology IV (ICFEB 2013) on May 19–20, 2013 in Copenhagen, Denmark Copyright © 2015 John Wiley & Sons, Ltd.

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well dispersed in the matrix, their large interfacial area and nanoscopic dimension of the particles lead to the formation of hybrid structures which fundamentally differentiates polymer nanocomposites from traditional filled plastic.5 Scientific studies showed that clays used in the polymer matrix provided marked an increase in the gas and water vapour barrier properties of materials. There are many studies presenting the effectiveness of clays in reducing oxygen and water vapour permeabilities of different polymers.5–13 Nanomaterials having substances such as antimicrobials are promising applications of active packaging. Silver, gold and zinc oxide nanoparticles having antimicrobial function are extensively studied, and silver was already commercialized in some applications.14–16 Silver with high-temperature stability and low volatility is known to be a very effective antimicrobial at the nanoscale, being effective against 150 different bacteria.17–20 For example, nanocomposites with low silver content showed a better increased efficacy against Escherichia coli than microcomposites with a much higher silver content.21 Antifungal active paper incorporating cinnamon oil with solid wax paraffin using nanotechnology as an active coating was proved to be effective packaging material for bakery products.22 Food packaging with antimicrobial properties could be useful for products susceptible to surface spoilage such as cheese, bakery goods and sliced meat products, which are vacuum packed or film wrapped with most of the product surface in contact with the packaging.23 As an alternative to the antimicrobial nanoparticles reported earlier, the paper proposes the use of a natural substance, in particular, terpenic resin, derived from pine tree. The terpene resins are nontoxic, nonphytotoxic, nonsensitizing to the skin and inert to diluted acids, alkalis and salts, and could constitute valuable ecosustainable candidates as antimicrobial packaging agents. Based on their properties, they can be approved for food contact application.24,25 Terpene resins are low molecular weight hydrocarbon polymers prepared by cationic polymerization of terpene monomers that are widely available from pine trees. They have been used for many years in commercial applications such as adhesives, coatings and elastomeric sealants. Among them, poly-β-pinene (PβP) is well known to be an excellent tackifier of numerous elastomers such as natural rubber, polyisoprene and styrene/butadiene rubber because of its chemical and physical characteristics. Numerous other uses are described in sheet carbon copying paper, paints, varnishes, plasticizers and so on.24–26 Systems formed by isotactic polypropylene (iPP) and natural terpenic resins (polylimonene and poly-α-pinene) have been found to produce blends suitable for the production of ecosustainable films for food packaging with antibacterical properties and reduced permeability to oxygen.24,25 In the family of polypropylenes, polypropylene random copolymers (PPR) constitute an important class of plastic materials that are particularly suitable for the packaging industry to be transformed into films and containers with a satisfying balance of clarity, flexibility and mechanical strength. Recent market trends are in fact towards products with increasingly lower heat-sealing initiation temperatures, which allow faster and more economic processing.27 This property belongs to the copolymers and is mainly related to the melting temperature (Tm), which decreases with the insertion of the comonomer along the polymer chain with respect to plain iPP. The introduction of the second comonomer along with propylene segments, in fact, promotes the formation of shorter stereo regular blocks, which melt at a lower temperature. The physical properties of these copolymers depend on the degree of crystallinity, structure and overall morphology, as well as on crystallization/processing conditions.27 Based on the aforementioned state of the art, we plan to investigate the conditions to develop innovative food packaging materials by synergistically exploiting the following: (a) the potentiality of clay to increase matrix properties, mainly barrier; (b) the antibacterial properties of PβP; (c) the properties of polypropylene random copolymers (PPR), mainly transparency, brilliance, low specific weight, chemical inertness and good processability, that make these copolymers suitable for the packaging industry to be transformed into films and containers. The specific objectives of this study are therefore to prepare films of PPR/PβP, PPR/nanoclay and PPR/PβP/nanoclay composites and to determine their mechanical, thermal, barrier and antimicrobial properties in order to propose innovative ecosustainable nanocomposites with antimicrobial activity for food packaging applications. Copyright © 2015 John Wiley & Sons, Ltd.

Packag. Technol. Sci. 2015; 28: 589–602 DOI: 10.1002/pts

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MATERIALS AND METHODS Materials The following commercial materials were used in this work: 1. Polypropylene random copolymer [PPR 3221® of Total Petrochemicals (Houston, TX, USA) with a melt-flow index of 1.8g/10 min (230°C, 2.16 kg]; 2. Clay (Dellite 67G from Laviosa (Livorno, Italy)–modified montmorillonite with high content of quaternary ammonium salt (dimethyl dehydrogenated tallow ammonium salt); 3. Poly-β-pinene (Piccolyte® S115 from Hercules Inc. (Wilmington, DE, USA) thermoplastic hydrocarbon resin with low molecular weight (Mw = 2580 g/mol, Mn = 950, Mz 5100) and Tg = 61°C (by Differential Scanning Calorimetry (DSC)), obtained from high-purity β-pinene isolated from turpentine). Sample preparation The following materials are produced and tested in this study as follows: PPR as control; PPR/clay 99/1 (wt/wt), PPR/PβP 95/5 and the ternary system PPR/ PβP/clay 94/5/1. All the samples were processed through a 25 mm twin-screw co-rotating extruder (L/D = 24) (Collin Teach-Line Twin screw kneader ZK25T/SCD15, Dr. Collin GMBH, Ebersberg, Germany). The temperature of the extruder was set at 160°C at the hopper and 180°C in all other parts of the extruder to the die, and the screw speed was set at 30 rpm. The extruded materials were converted into pellets by using pelletizer (Collin Teach-Line Pelletizer 171 T). The pellets were processed again by single screw extruder (Collin Teach-Line Extruder E20T/SCR15) and converted to films by a Collin Teach-Line Chill Roll CR72T. The temperature setting of the single-screw extruder from the hopper to the die was 160/180/190/185/180°C, and the screw speed was 60 rpm. The film thickness was about 90 μm. For dynamic mechanical thermal analysis tests, the samples were obtained by compression moulding. The pellets were inserted inside a mould with a parallelepiped-shape of 65 × 100 mm and suitable to give a sample with a thickness of 1.0 mm. The mould was placed between the plates of a press preheated at 210°C for 5 min without applied pressure to allow complete melting of the polymer material. After this period, a pressure of 100 bar was applied for 5 min. Then, the plates of the press containing coils for fluids were cooled to room temperature in about 3 min. Finally, the pressure was released and the mould removed from the plates. Diffusivity of PβP Standard solution for PβP was prepared by dissolving 200 mg in 100 ml of petroleum ether to give a stock solution concentration of 2 mg/ml. Standard working solutions were prepared at five concentration levels in the range of 0.125–2 mg/ml by dilution of appropriate aliquots of the stock solution to 25 ml with petroleum ether. The absorbance of PβP solutions was measured at 342 nm with an ultraviolet–visible spectrophotometer (UV-160A, Shimadzu, Kyoto, Japan), and a calibration curve was constructed. The diffusion of PβP from PPR/PβP/clay film was determined by immersing film squares (2 × 2 cm) into a glass bottle containing 100 ml of petroleum ether. The bottle was placed into a controlled temperature room at 25.0 ± 0.5°C and was well stirred with a magnetic stirring bar to minimize boundarylayer formation around the film. Films were removed from the bottle at various time intervals, and the absorbance of the solution was measured at 342 nm with a ultraviolet–visible spectrophotometer. The amount of PβP present in the solution was computed from the calibration curve. The sliced salami (2.5 mm thickness) was packaged with PPR/PβP/clay film under three different atmospheres (air, 50% CO2 and 50% N2, and vacuum) and stored at 4°C for 90 days, to determine the diffusion of PβP from the packaging material contacted with meat products. Three pieces (2 × 2 cm) of film squares were taken for each application at day 5, 10, 20, 30, 40 and 90 and immersed into a glass bottle containing 100 ml of petroleum ether for 10 min. The absorbance of the extract was measured, and the amount of PβP present in the solution was computed from the calibration curve. Copyright © 2015 John Wiley & Sons, Ltd.

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Structural and morphological analyses Wide-angle X-ray diffraction. Wide-angle X-ray diffraction measurements were conducted using a Philips XPW diffractometer (Philips Analytical, Almelo, The Netherlands) with Cu Ka radiation (1.542 Å) filtered by nickel. The scanning rate was 0.02° (Δ2θ)/2 s(Δt), and the scanning angle was from 2 to 45°. The diffraction profiles were collected on milled films obtained with a mill Retsh ZM1 (Retsch, Haan, Germany) (grid with hole diameters of 1 mm) in liquid nitrogen, in order to randomize the orientation of the crystals while preventing melting/recrystallization. The crystallinity xc was evaluated from the X-ray powder diffraction profiles by the ratio between the crystalline diffraction area (Ac) and the total area of the diffraction profile (At), xc Wide Angle X-Ray Scattering (WAXS) = A(C)/A(t). The crystalline diffraction area was obtained from the total area of the diffraction profile by subtracting the amorphous halo. The profile of amorphous phase was approximated by one of an atactic polypropylene and indicated in Figure 3 by a dashed curve. Transmission and scanning electron microscopic analyses (TEM and SEM). The surface analysis was performed with a SEM (FEI Quanta 200 SEM, Eindhoven, The Netherlands) on the cryogenically fractured surface of the film. The fractured surface was obtained by bathing the film for 5 min in the liquid nitrogen, and then it was removed and immediately broken. Before the observation, the fractured surface was coated with an Au/Pd alloy using an E5 150 SEM coating unit. The TEM analysis was performed at 120 kV with a FEI Tecnai G2 Spirit twin microscope (Hillsboro, OR, USA) on ultramicotomed sections of 80 nm. Thermal analyses Thermogravimetric analysis (TGA). Thermogravimetric analysis was carried out at 20°C/min heating rates from 30°C to 700°C in air (flow rate 200 ml/min) on produced film samples (approximately 5 mg) by means of a Perkin Elmer Pyris Diamond TGDTA (Perkin Elmer, Waltham, MA, USA). Dynamic mechanical thermal analysis (DMTA). Dynamic mechanical data were collected at 1 Hz and at heating rate 3°C/min from 80 to 110°C under nitrogen atmosphere with a Pyris Diamond DMA configured for automatic data acquisition. The experiments were performed in tensile mode. Parallelepiped-shaped specimens with a length of 50 mm, width of 10 mm and thickness of 1 mm were cut from the compression-moulded samples. Mechanical analysis Stress–strain curves were obtained using an Instron machine (Model 4505, Norwood, Ma, USA) at room temperature (25°C) and at a crosshead speed of 5 mm/min. Dumbbell-shaped specimens were prepared on the basis of ASTM D638 for the experiments in both machine direction (MD) and transverse direction (TD). Ten tests were performed for each composition. Barrier properties (OTR and WVTR) OTR (ASTM D3985) and WVTR (ASTM E96) were determined by using a Multiperm ExtraSolution (Pisa, Italy). The film was inserted between two chambers: for water vapour measurement, a nitrogen flux with water vapour enters in the lower one and a nitrogen flux enters in the upper one. The water vapour diffusion through the film is measured by a zirconium oxide sensor. For OTR measurement, oxygen is used instead of water-vapour flux. The area of the film involved is 50 cm2. OTR was performed at 23°C and 0% RH. The WVTR was performed at 38°C and 90% RH. Three measurements were performed for each material. Antimicrobial activity Antimicrobial activity of samples containing PβP was determined under dynamic contact condition according to the ASTM E-2149-10 standard method for quantitative evaluation by using E. coli (ATCC 25922)28 as test microorganisms. Film samples of 10.16 cm2 (4 in2) was cut in sterile conditions. Clay containing nanocomposite film was used as control. Each film sample was placed in a screw-cap bottle Copyright © 2015 John Wiley & Sons, Ltd.

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containing 50.0 ± 0.5 ml of buffer solution (0.25 M KH2PO4) with wetting agent surfactant (DC Q2-5011) and E. coli at a final concentration of 1.5–3.0 × 105 CFU/ml. The flasks were shaken for 1 h at 180 rpm. The bacterial concentration of the solution has been evaluated by performing serial dilution and standard plate-count techniques. Three experiments have been carried out for each composition. The number of colonies in a Petri dish after incubation at 35°C in 24 h was determined by using selective-differential media and Gram staining, and the results are presented in colony-forming unit per millilitre (CFU/ml) of buffer solution in the flask. The results are presented in both percent reduction when measuring CFU/ml and log10 bacterial reduction when calculating mean log10 density of bacteria as follows:  Reduction; % CFU=ml1 ¼ ½ðB  AÞ=B100 Log10 reduction ¼ Log10 ðBÞ  Log10 ðAÞ where A = CFU/ml for the flask containing materials (PβP containing nanomaterial or PβP containing material); B = CFU/ml for the flask containing control material (material containing only clay or only inoculums (no film)). Data analysis Data were subjected to analysis of variance and Duncan’s multiple range test (P > 0.95) by using SAS version 8.02 (Statistical Analysis System (SAS) Institute, Cary, USA).

RESULTS AND DISCUSSION Diffusivity of PβP PβP release values from immersed film in petroleum ether are given in Figure 1. Initial PβP content of PPR/PβP/nanoclay films was 5% (w/w). PβP concentration in the solution was 0.2 mg/g after 5 min immersing time and stayed almost stable during a 150 min period (Figure 1A). The amount of PβP increased to 0.4 mg/g after 12 h and stayed around this concentration level with prolonged immersing time (Figure 1B). Even though petroleum ether was the best solvent for PβP among the tested ones, only a limited amount of PβP released from the film samples. In addition to that, PβP concentration did not show a linear increase with time. No significant difference was observed between the absorbance readings of PPR/PβP/clay films samples contacted with sliced salami and PPR/PβP/clay films squares immersed in petroleum ether for 10 min (P > 0.05, data not shown). Therefore, it was concluded that PβP does not diffuse from the material. Further evidence that PβP does not diffuse from

Figure 1. Diffusivity of poly-β-pinene (PβP) for 150 min (A) and 14 400 min (B) immersing times. Copyright © 2015 John Wiley & Sons, Ltd.

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the material comes from the results of the application of the agar diffusion method for the evaluation of the microbial activity, where no zones on agar were detected. However, PPR/PβP/clay films showed antimicrobial activity, when dynamic contact conditions method of immobilized agents was used. Structural and morphological properties Wide-angle X-ray diffraction. Figures 2 and 3 show the WAXS intensity profiles of the four samples under investigation. In particular, in Figure 2, the profiles at low value of 2θ are reported together with the profile of the pure clay in order to investigate the possible intercalation/exfoliation of the clay layers by PPR and/or PβP. In Figure 3, the profiles of the sample at high value of 2θ are reported in order to investigate the crystalline structure of the PPR and crystallinity content of the samples. From the analysis of the profiles, it emerges as follows: 1. The clay profile presents three peaks at 2θ = 2.6° (d = 3.4 nm), 4.7° (d = 1.9 nm) and 7.2° (d = 1.2 nm) indicating a multilayer structure. The addition of clay to the PPR shifted all the characteristic peaks of the clay to lower 2θ values (namely 2θ = 2.2°, 2.7° and 5.4°), probably suggesting an occurrence of intercalation/partial intercalation of the PPR molecules into the clay galleries. This could indicate that the clay galleries slightly open to accommodate some molecules of PPR and an intercalated structure could develop. In the case of the ternary system (PPR/PβP/clay), three reflections are present. It is clear that these reflections are set at angles very close to those observed for the pure clay, possibly indicating that the presence of PβP does not allow the PPR molecules to enter into the clay galleries and increase the interlayer distances. 2. At high 2θ values several features can be observed in the X-ray profiles. For all the samples, the presence of the intensity of 130 reflections at 2θ between 18 and 19° indicates that the crystals are mainly organized according to the monoclinic α form. For the pure PPR, an amount of crystallinity of 37% is calculated by the profiles using the methodology reported in the experimental part. Adding PβP to PPR crystalline peaks of lower intensity are present, indicating a lower amount of crystallinity (32%) of this sample compared with the plain PPR. Adding clay to PPR, an increase in the peak intensity of the α form crystalline reflections is observed, suggesting that the clay probably acts as a nucleating agent for this phase and causes an increase in the crystallinity (40%). Finally for the ternary blends, a clear α form pattern with high-intensity peaks is found indicating that for this system, the clay has a very high nucleating effect, and the crystallinity reaches a very high value, 52%. The increase of the crystallinity with the clay confirms the

Figure 2. Wide-angle x-ray diffraction (WAXD) profiles at low 2θ of the polypropylene random copolymer (PPR) based samples and clay powder. Poly-β-pinene, PβP. Copyright © 2015 John Wiley & Sons, Ltd.

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Figure 3. Wide-angle x-ray diffraction (WAXD) profiles of the polypropylene random copolymer (PPR) based samples. hypothesis of the nucleating effect of the nanoparticles on the PPR crystals as reported in literature.29,30 This effect favours the crystallization process and the formation of crystals. SEM and TEM. Scanning electron and transmission electron micrographs of the samples are reported in Figures 4A–D and 5A–B, respectively. For the PPR and the binary PPR/PβP system, homogeneous surfaces are observed with no evidence of segregated domains, supporting the hypothesis of miscibility in the amorphous phase between PPR and PβP. For both blends containing clay nanoparticles, a good dispersion of the clay particles is observed. From the TEM micrographs (Figure 5A–B) it is evident that in the binary PPR/clay system with respect to the ternary system, the average dimension of the clay aggregate is smaller, and that in these aggregates, the distance between layers slightly increase, in agreement with the X-ray results. Also in the ternary system, no domains of the PβP are detectable, indicating that the miscibility in the amorphous phase between PPR and PβP is not influenced by the addition of the clay nanoparticles. Thermal properties TGA. TGA curves recorded in air at 20°C/min are presented in Figure 6. The curves indicate that the thermal decomposition for all samples is a one-step process, and the addition of clay and/or PβP enhanced the thermal stability of PPR. The TGA curves of PPR/PβP, PPR/clay and PPR/PβP/clay composites are shifted at higher temperatures as compared with neat PPR. The temperatures of the inflection point (Tmax) of PPR/PβP, PPR/clay and PPR/PβP/clay samples are almost similar but higher of about 40° than that of the plain PPR (Table 1). Several studies on polymer/clay nanocomposites Copyright © 2015 John Wiley & Sons, Ltd.

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Figure 4. Scanning eletron micrographs of cryogenically fractured surfaces of films of polypropylene random copolymer (PPR) (A), PPR/poly-β-pinene (PβP) (B), PPR/clay (C) and PPR/PβP/clay (D).

Figure 5. Transmission electron micrographs of thin sections of polypropylene random copolymer (PPR)/clay (A) and PPR/poly-β-pinene (PβP)/clay (B). have reported similar results in the thermal properties. Incorporation of nanoclay resulted in a significant improvement on thermal stability of polystyrene,31 polyvinyl alcohol,32 PP–EP/EVA33 and PP.34 The increase of the thermal stability for PPR/clay and PPR/PβP/clay samples could be attributed to the clay particles that act as barrier to the diffusion of gases, originated by the thermal degradation process, from the bulk to the surface of the film, contributing to slow the degradation process of the PPR.35 Copyright © 2015 John Wiley & Sons, Ltd.

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Figure 6. Thermogravimetric analysis (TGA) in air for polypropylene random copolymer (PPR), PPR/ clay, PPR/poly-β-pinene (PβP) and PPR/PβP/clay films.

Table 1. Temperatures at inflection point (Tmax) calculated from TGA for films. Sample PPR PPR/clay PPR/PβP PPR/PβP/clay

Tmax (°C) 358 400 405 396

The increase of the thermal stability for PPR/PβP could be probably due to the double bond present in each unit of the PβP formula. The double bond can neutralize some radicals of PPR macromolecules generated during the heating. This is just a hypothesis and needs further investigation. Dynamic mechanical thermal analysis. The glass transition temperatures of the films were obtained from tanδ curves (Figure 7). Before analysing the results, it has to be recalled that semicrystalline polymers, such as PP, PE, PVF, PVDF and P4MP1, present two glass transitions named as Tg (L) and Tg (U) (where L and U stand for ‘lower’ and ‘upper’)36–38 and later as MAF (mobile amorphous phase) and RAF (rigid amorphous phase), respectively.39 Tg (L) is the main transition, and it is generated by the bulk-amorphous phase; Tg (U), secondary transition, arises from the relaxation of amorphous materials bordering the crystals, which are macromolecules or a section of them interconnected to the crystalline phase, such as cilia, loose loops and tie molecules. Tanδ curve of PPR in Figure 7 shows two clear and distinct peaks: a first peak with a maximum of about 6°C and a second peak with a maximum of about 71°C. The first peak is due to the main glass transition, Tg(L), and the second peak to the secondary transition, Tg(U). The addition of 1% in weight of the clay does not have any influence on the two transitions of the amorphous phase. No influence on Tg by clay particles was reported for polypropylene composites by Rahman et al.40 and Modesti et al.41 The addition of the PβP to the plain PPR and PPR/clay composite had significant influence on the two transitions. The Tg(L) peak shifts at higher T on the tanδ curves of the two films containing PβP estimated values of 11°C and 12°C are reported, where the curves present a beginning or a sign of a plateau. PβP has also a significant effect on Tg(U) that decreases from 71°C to 61°C and 64°C for the PPR/PβP and PPR/PβP/clay formulations, respectively. The increases of Tg(L) and the decreases of Tg(U) are due to the Tg of PβP, which is higher than Tg(L) and lower than the Tg(U). The Copyright © 2015 John Wiley & Sons, Ltd.

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Figure 7. Tanδ of polypropylene random copolymer (PPR), PPR/clay, PPR/poly-β-pinene (PβP) and PPR/PβP/clay. increase of Tg(L) and the decrease of Tg(U) for the films with PβP clearly indicate that PβP is compatible with the amorphous phase of PPR. This result is in agreement with the results obtained by Cimmino and Silvestre et al.25 on similar systems, in particular, blends of iPP with natural terpene resins.

Mechanical properties Stress–strain curves for all the samples in machine direction (MD) and transversal direction (TD) are reported in Figure 8A–B. Young modulus (E), stress (σ y) and strain (εy) at yield point, and stress (σ b) and strain (εb) at break point parameters in MD and TD are reported in Tables 2 and 3, respectively. The results indicated that the properties are depending on the test direction and on the composition. All the samples have a ductile behaviour. In MD, the Young modulus (E) of the film PPR/PβP/clay presented a value higher than that of plain PPR, whereas the film containing only PβP had the lowest value. In TD, addition of clay and/or PβP significantly increased the Young modulus. It is worth to underline that in both of the test directions, the addition of both clay and PβP is always more effective on increasing the Young modulus comparing with the addition of clay and/or PβP by itself.

Figure 8. A) Stress–strain curves of blown films of polypropylene random copolymer (PPR), PPR/ clay, PPR/poly-β-pinene (PβP) and PPR/PβP/clay in machine direction (MD). B) Stress–strain curves of blown films of PPR, PPR/clay, PPR/PβP and PPR/PβP/clay in transverse direction (TD). Copyright © 2015 John Wiley & Sons, Ltd.

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Table 2. Mechanical properties of the films in machine direction (MD). Sample PPR PPR/clay PPR/PβP PPR/PβP/clay

E (MPa)

ɛy (%)

σ y (MPa)

ɛb (%)

σ b (MPa)

638 ± 63.3c 692 ± 57.9b 573 ± 36.1d 930 ± 76.9a

9.5 ± 0.8a 9.6 ± 1.7a 9.9 ± 0.7a 10.0 ± 0.3a

14.4 ± 0.9c 15.4 ± 1.2b 12.3 ± 0.6d 19.2 ± 0.6a

630 ± 48.4ab 637 ± 41.2a 594 ± 59.2b 622 ± 54.4ab

48.7 ± 5.0a 48.4 ± 3.7a 47.5 ± 4.5a 49.4 ± 2.5a

For each column, mean values with similar letters are not significantly different at P ≤ 0.05 among the materials.

Table 3. Mechanical properties of the films in transversal direction (TD). Sample PPR PPR/clay PPR/PβP PPR/PβP/clay

E (MPa)

ɛy (%)

σy (MPa)

ɛb (%)

σb (MPa)

632 ± 35.8d 888 ± 91.4b 792 ± 37.5c 1020 ± 66.5a

4.8 ± 1.0c 5.2 ± 1.0c 7.5 ± 1.3a 6.2 ± 0.9b

10.7 ± 1.0c 15.3 ± 1.3b 14.6 ± 0.6b 18.2 ± 1.2a

739 ± 80.6ab 684 ± 51.1b 740 ± 42.8ab 756 ± 28.2a

24.3 ± 1.5c 23.6 ± 2.0b 28.9 ± 2.5a 26.7 ± 1.5b

For each column, mean values with similar letters are not significantly different at P ≤ 0.05 among materials

In MD, there was no significant difference in strain at yield (εy) of the films, whereas in TD, an increase in εy by addition of PβP and clay/PβP comparing to neat PPR was observed. In both directions, strain at break (εb) does not change significantly for all the samples, whereas PPR/clay and PPR/PβP/ clay films have higher yield stress (σ y) comparing to neat PPR. There is no statistically significant difference for the films in terms of stress at break (σ b) for a given test direction. The higher strength of the film in the MD direction is probably due to a particular orientation of the PPR macromolecules during sample preparation. In summary, the mechanical properties of the binary and ternary system containing clay in both of the test directions improved E and σ y comparing to neat PPR. This mechanical behaviour makes these samples suitable for potential use in the field of food packaging from a mechanical point of view. Several studies on PP/clay nanocomposites as film or as moulded forms have reported similar increases in the mechanical properties.42–44 Liu and Wu42 reported 27% increase in the tensile strength and 42% increase in the tensile modulus with 7 wt% clay concentration for PP/clay nanocomposite samples prepared via grafting and melt compounding. Joshi and Viswanathan43 reported 30% improvement in the tensile strength and 70% improvement in the tensile modulus at loadings of 0.25–0.5 wt% clay concentration for PP/nanoclay composite filaments produced by melt spinning. Barrier properties (OTR and WVTR) OTR and WVTR of the produced films are shown in Table 4. The addition of clay or PβP to PPR reduced the OTR of about 10% comparing to neat PPR. The OTR was further reduced in the case of ternary system (24% compared with plain PPR). The addition of clay or clay/PβP also significantly reduced WVTR by 24% and 31%, respectively, in comparison to plain PPR. The decrease of OTR and WVTR of the PPR films filled with clay can be attributed to the tortuous path for oxygen and water molecules to diffuse through the Table 4. OTR and WVTR of the produced films. Sample PPR PPR/clay PPR/PβP PPR/PβP/clay

Thickness (micron)

OTR (ml/m2/day)

WVTR (g/m2/day)

93.2 ± 4.5a 88.6 ± 3.2bc 86.7 ± 4.8c 92.5 ± 5.9ab

1410 ± 59.0a 1280 ± 1.5b 1260 ± 0.7b 1060 ± 35.4c

1.88 ± 0.06a 1.43 ± 0.02c 1.59 ± 0.03b 1.30 ± 0.09d

For each column, mean values with similar letters are not significantly different at P ≤ 0.05 among materials Copyright © 2015 John Wiley & Sons, Ltd.

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film created by the presence of clay layers in the PPR matrix in agreement with several results reported in literature.21,45–47 In the case of PPR/PβP, the decrease is probably due to higher values of the main Tg of the amorphous phase (Tg(L)) due to the compatibility in the amorphous of PPR and PβP. At the temperatures of the two tests (23°C for OTR and 38°C for WVTR), the amorphous phase of PPR/PβP system has less free volume available for the molecules of O2 and H2O to permeate through the film, compared with the same amorphous phase of plain PPR. In the ternary system, the decrease of OTR and WVTR is probably due to the presence of clay particles acting as barrier (as claimed for the PPR/clay system), to the increase of the Tg (as claimed for the PPR/PβP system) and also to the higher crystallinity content; these three effects together would contribute to further increase in the barrier properties of the PPR/PβP/clay film. Antimicrobial activity The data showed that the material containing PβP/clay had an antibacterial effect. Antimicrobial properties of nanomaterial containing PβP reduced the test microorganisms to 1.33 log CFU/ml (or 24.3% reduction) comparing to the control. Material only containing PβP also showed similar results with bacterial reduction of 1.35 log CFU/ml (or 25.5% reduction). Diffusion study showed that PβP is not diffusing from the material, indicating that it is an immobilized antimicrobial agent. In this case, there would be direct contact of the material containing PβP needed for antibacterial effect for food. In the current study, the antimicrobial efficiency of nanocomposite films was also tested by using agar-diffusion method, but the antimicrobial efficacy of nanocomposite films were quantitatively determined under dynamic contact conditions using ASTM method. Although in the literature, there are a few reports on the antimicrobial activity of β-pinene as monoterpene; to the best of our knowledge, this study is the first report showing the antimicrobial activity of PβP as polyterpene containing films. β-pinene MIC values were found between 2.5 and 40 μl/ml on potential infectious endocarditis causing gram-positive bacteria Staphylococcus strains.48 Antimicrobial activity of the positive enantiomers of α- and β-pinene were shown against Candida albicans, Cryptococcus neoformans, Rhizopus oryzae and Methicillin-resistant Staphylococcus aureus .49 Uribe et al.50 reported the antimicrobial effect of β-pinene, which has been attributed to alterations produced at the level of the membranes. Along with β-pinene, citral and linalool have been shown to be active against Saccharomyces cereviciae, inoculating orange-based soft drink.51

CONCLUSIONS The results reported in this paper can bring to the following conclusions: 1. The increase in the main glass transition Tg(L) and the decrease in the Tg(U) of PPR/PβP/clay and PPR/PβP systems, compared with the transitions of plain PPR and PPR/clay, indicate that PβP and PPR are miscible/compatible in the amorphous phase. This is supported also by the homogeneity observed on the fracture surfaces. 2. The X-ray profiles and the morphological analysis for the binary system PPR/clay, suggest a possible occurrence of intercalation of the PPR molecules into the clay galleries. In the case of the ternary system, the presence of PβP, molecularly mixed with the PPR, does not allow the PPR molecules to enter the clay galleries supporting the hypothesis that in the ternary system the PPR/PβP miscibility prevents the intercalation phenomenon. 3. The clay acts as a nucleating agent for the crystallization of the monoclinic α form of the PPR and causes an increase in crystallinity. This effect is very marked in the case of the ternary system, where no intercalation phenomenon is present. 4. Addition of PβP and clay improves gas barrier and thermal stability comparing to neat PPR and binary system. Several effects contribute to this behaviour: the increase of Tg due to compatibility between PPR and PβP, the barrier labyrinth created by the clay nanoparticles to the diffusion, as well as the increase of crystallinity, just in the case of the ternary system. Copyright © 2015 John Wiley & Sons, Ltd.

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NOVEL POLYPROPYLENE BASED NANOMATERIAL FOR FOOD PACKAGING

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5. PβP has antibacterial effect on the target bacteria and direct contact of the material containing PβP needed for antibacterial effect. In a forthcoming paper, the effectiveness of the materials with clay and both clay and PβP on sliced salami and pastrami will be reported. Preliminary results seem to show that the shelf lives of salami and of pastrami packed under vacuum with films of three-component nanomaterial (PPR/PβP/clay) are quite comparable with those obtained with commercial multilayer films.

ACKNOWLEDGEMENTS This work was supported by TUBİTAK-COST project (111O333) and COST ACTION FA0904.

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