The Use of Zinc Oxide Nanoparticles in Eva to Obtain Food Packing Films

The increasing demand for new packages with increased shelf life properties has stimulated the increase of research in the active packaging sector. The use of antimicrobial agents requires an in-depth study of their properties to avoid loss of efficiency of the polymer processing. In this context, the objective of this work was to evaluate the preparation of an 18% ethylene vinyl acetate copolymer (EVA) nanocomposite and zinc oxide (ZnO) as microbicidal nanoparticle, prepared in a monosulfon extruder. The nanoparticle was modified with octadecylamine and EVA 18 nanocomposite films were prepared and compared to the systems containing modified nanoparticle. These new materials were characterized by thermogravimetric analysis (TGA), Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), Dynamic Mechanical Analysis (DMA), Time Domain Nuclear Magnetic Resonance (NMR) to investigate the effect of zinc oxide nanoparticles on thermal properties, EVA crystallinity and antimicrobial effect. The TGA showed a tendency of increase of the thermal stability in different proportions of ZnO. DSC results did not show significant changes in thermal parameters. The XRD analysis showed an increase in the degree of crystallinity of the nanocomposites in relation to the EVA matrix and change in the crystallinity with the increase of ZnO percentages. DMA analysis indicates change in structural organization through the variation of storage modulus, loss, and tan delta. Time domain NMR data corroborate with XRD data through the change in molecular mobility.

with a growing demand for packaging materials that are stronger, lighter and have certain functional properties, food needs adequate packaging to maintain quality and freshness during transportation and storage, as well as to prolong the shelf life by controlling humidity, gases and certain volatile components, such as flavorings [1].
Traditionally, the food packaging industry develops materials for preserving and protecting food from environmental factors starting from production site to the point of consumption. These traditional systems are reaching their limits regarding extending the shelf life of packaged foods. To provide this shelf life extension and improve quality and integrity of packaged foods, innovative concepts of active packaging are being developed [2] [3].
The purpose of the active packaging is to extend shelf life by applying various strategies, such as: oxygen removal, moisture control and addition of other active materials in its composition for food protection. These developments in active packaging have led to advances in many industries, including late oxidation in sport supplements, controlled rate of respiration in vegetables and antimicrobial action [4].
Antimicrobial packaging is a form of active packaging in which the microbicidal agent acts to reduce, inhibit or retard the growth of microorganisms present in the packaged food or in the packaging material itself. Incorporation of antimicrobial agents can be done via direct coating in packaging or processed along with polymer matrix [5] [6].
To develop antimicrobial packaging films, microbicidal nanoparticles are incorporated into the polymer matrix through two possible methods: solution or fusion. Zinc oxide (ZnO) is an inorganic compound widely used in everyday applications and is a material generally recognized as safe by the Food and Drug Administration (21CFR182.8991) and has already shown antimicrobial activity against foodborne pathogens [6]- [28].
Ethylene-co-vinyl acetate (EVA) is a copolymer with wide industrial applications due to its stable physicochemical properties, which make it distinct from other copolymers. This copolymer has low gas permeability and, therefore, it is used in the packaging industry in the form of film [29] [30].
Due to the increasing demand for research in the field of bactericidal and fungicidal for packaging, this work aims the detailed study of polymer nanocomposites containing antimicrobial agents, as is the case of zinc oxide and its well-known antimicrobial property. The proposal of the present work addresses the method of processing, which includes compatibility with the polymer matrix, types of pathogens affected and mechanical properties of the new packaging.

Zinc Oxide Modification
Experimental procedure started by weighting 0.8 mg of stearic acid along with 200 ml butanol, then sonicating for 5 minutes at max power. Zinc oxide was added to butanol solution of stearic acid, and then magnetic stirring was applied for 24 h.
Subsequently, the solution was split and poured into 5 falcon tubes and centrifuged at 3500 rpm for 5 minutes. The acid supernatant was removed, and ethyl acetate was added to complete the tubes. All tubes were sonicated for 20 minutes; then the tubes were centrifuged under the same conditions 3 times. The ethyl acetate was withdrawn from the tubes and solid was dried for 24 h.
The oxide was transferred to an erlenmeyer flask containing 100 mL ethanol, then the solution was added to another solution containing 100 mL of ethanol and 1.2 g of octadecylamine. The erlenmeyer was kept under stirring for 1 h, then the solution was centrifuged under the same previously mentioned conditions and there were 5 washes with ethyl acetate, then the solution was withdrawn and divided into two falcon tubes again. After the last wash, the ethyl acetate was withdrawn and the modified ZnO dried in the falcon tube for at least 24 hours.
After the modification of the zinc oxide, three methods of characterization were used: Thermogravimetric analysis (TGA), X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR)

Films Fabrication
The polymer matrix used for antimicrobial packaging formulation was the ethylene-vinyl acetate copolymer with 18% acetate (EVA 18), purchased as pellet.
The EVA18 copolymer was frozen in Ultrafreezer at −90˚C for grinding to avoid possible melting of the copolymer with the heat generated by the mill.
The frozen EVA18 pellets were milled using a knife mill. This step guarantees the increase of the contact surface of EVA, which facilitates the homogeneous diffusion of the heat of the extruder during melting and allowing greater contact with the nanoparticle.
The obtained EVA18 granules were oven dried at 40˚C for 24 hours for complete removal of moisture from the material. This drying was important to avoid bubbles during extrusion, generating a more homogeneous material during processing.
After drying, the material was processed in a single-screw extruder from AX Rotation speed of screw was modified following the variation of zinc oxide amount incorporated in the nanocomposites. Increasing the amount of oxide increased the viscosity of the material, increasing its resistance to flow; therefore, the rpm of the thread was increased during extrusion. 60, 70, 80, 90 and 90 rpm were used for the films containing 0%; 0.25%; 0.5%; 0.75% and 1% ZnO, respectively. The sample was cooled and solidified on the roll, which permitted to obtain the film form.

Characterization
The obtained films were characterized by the techniques of: Fourier Transform

Results and Discussion
The EVA/ZnO systems were obtained in proportions of 0.25%; 0.5%, 0.75% and

Fourier Transform Infrared Spectroscopy
Zinc oxide was analyzed by FTIR to confirm the success of its modification process.    [33].
The chemical modification was confirmed by the presence of the octadecylamine peaks relative to the -CH 2 bond at wavelength 2918, 2848 and 720 cm −1 , and the band at 1466 cm −1 related to the stretching of the C-N amide bond [34].
FTIR analysis of the obtained EVA/ZnO systems showed no change in the characteristic peaks of the EVA copolymer after zinc incorporation (Figure 3), indicating only a possible physical interaction between the oxide and the copo- Through the carbonyl band it can be verified degradation reactions of polymer matrix throughout processing [37]. As there is no new carbonyl band pertaining to EVA degradation, increase of intensity and nor displacement of the EVA acetate band at 1744 cm −1 , the analysis indicates absence of copolymer degradation during processing.
Regarding to pure EVA, the analysis of the modified nanoparticle-containing systems ( Figure 4) also showed no change in the peak intensities for the carbonyl, indicating that the nanoparticle modification did not accelerate the degradation process and indicates the formation only of a physical interaction in the systems.    EVA [38]. Vinyl acetate incorporation controls crystallinity degree, which is corresponding to the long ethylene segment [39] [40] [41] [42]. Thus, the broad peak of EVA is due to the 18% vinyl acetate content present in this copolymer.

X-Rays Diffraction
The addition of zinc oxide in the EVA matrix intensifies the crystalline peak of the EVA/ZnO systems indicating that the oxide favors the organization of the system. A significant change is observed in the amorphous phase, which may indicate that the zinc oxide induces crystalline packaging of polymer chains that consequently decreases the amorphous region.
The variation of the nanoparticles content in the EVA/ZnO systems evidenced increased crystallization of the copolymer, as shown in Figure 5, indicating that there was greater structural organization of EVA along oxide incorporation.
Moreover, oxide contents above 0.5% induced a decrease of composites organization, with the appearance of peaks related to zinc oxide. In the system containing 1% of ZnO it is observed a similar behavior to the pure polymer.
When evaluating the organic modifier 0.25% ZnO composite (Figure 6), it is was verified the base narrowing indicating a decrease of the amorphous area with the presence of the modifier. This may be due to the better interaction of matrix-modified zinc in the amorphous area, limiting the movement of the chains and increasing the structural organization of the polymer.
In the 0.5% oxide system (Figure 7), there was an inversion behavior with respect to the 0.

Thermogravimetric Analysis (TGA)
EVA presents two thermal degradation stages as shown in Figure 8. The first one starts at about 300˚C with a peak at 328˚C and refers to the loss of acetic acid. The second degradation stage starts at 420˚C with a peak at 447˚C, which refers to the degradation of the olefin meres (C-C and C-H bonds) [38].
However, by observing the TGA profiles of Figure    for increased thermal stability. This increase in thermal stability can be attributed to the high thermal stability of ZnO networks and the existence of interactions between nanoparticles and polymer matrix [43].
Systems containing modified ZnO (Figure 9) showed the same behavior of unmodified ZnO systems, indicating that the modification does not alter the degradability of the material and the intermolecular interactions is only physical. Figure 10 presents the system's first heating curve evidencing the thermal history

Dynamic-Mechanical Analysis (DMA)
The dynamic-mechanical analysis allows the separation of the elastic and viscous contribution in viscoelastic materials, as a function of both temperature and time.
Regarding the pure polymer, the storage modulus decreased with the insertion      The increase in ZnO ratios to 0.75% and 1% further intensifies the loss modulus diminish, indicating a decrease in free volume by ZnO dispersion in the matrix.
Regarding the pure copolymer, the systems containing modified nanoparticle presented variation of the loss modulus, where the EVA/ZnOmod system containing 0.25% ( Figure 16) showed even greater decrease of the loss modulus.
This suggests an increase in the interactions between polymer and nanoparticle, making even more difficult to dissipate energy in the internal medium of the polymer due to decrease of the free volume.
The system containing 0.5% modified nanoparticle also showed inverted behavior comparing to the 0.25% EVA/ZnOmod system (Figure 17     The intensity of the tan delta also allows evaluating the relationship between the storage and loss modules, indicating the variation of the damping as a function of the presence of the nanoparticle. It is possible to verify that the system containing 0.5% of modified nanoparticle presents higher damping than the other systems, which agrees with the increased contribution of the amorphous

Time Domain Nuclear Magnetic Resonance
The samples were evaluated focusing the domain relaxation curves and the be-  Table 2.
Through the percentages of the mobile and rigid phases in EVA/ZnO systems, it is possible to verify if the system stiffness increases. With 0.25% nanoparticle, the increase in stiffness confirms the decrease in the amorphous phase indicated   by XRD. In 0.5% there is a slight decrease in the rigid fraction, corroborating the XRD which showed an increase in the amorphous halo contribution. At higher nanoparticle percentages, such as 0.75% and 1%, the increase in hard T 1 still occurs. Increasing the amount of oxide in the systems decreased the contribution of mobile T 1 , indicating the existence of ZnO in the amorphous area of the polymer.
The spin-lattice relaxation data of the systems containing modified nanoparticle, presented differences in relation to the EVA/ZnO systems with 0.25% and 0.5%. In both systems, the nanoparticle modification led to T 1.1 H increase, elucidating decrease of the molecular mobility of the polymer along with the increase of the polymer-nanoparticle interaction and indicating greater dispersion of the oxide in the amorphous area of the material [47] [48] [49], where there is no variation of the percentage of T 1,2 H with nanoparticle modification. These data corroborate the results of DMA, where the analysis indicated that the modification induced an increase in the polymer-nanoparticle interactions. The DRX also rectifies this behavior, where the 0.25% modification possibly caused interference in more amorphous regions.
Finally, for the concentration of nanoparticle in the systems, it is possible to verify that increasing zinc oxide proportion in nanocomposites affected the structural organization of the system; the 0.5% ZnO proportion was observed to be a borderline for nanoparticle incorporation, with changes in the structural characteristics of the system. The modification indicated higher nanoparticle-polymer interaction, with higher contribution of the amorphous halo in the crystallinity of the polymer matrix, change of the system mobility and variation of the dynamic mechanical characteristics.

Conclusion
From the results obtained, it was possible to infer that the presence of zinc oxide in the EVA matrix can alter the structural organization of the polymer, resulting in different properties when compared to the pure polymer. The concentration of 0.25% turned out to perform better among EVA/ZnO systems, where the amount of nanoparticle inserted in the matrix was enough to change the crystallinity and mechanical properties of the material, with subtle increase of thermal stability.