Development of Antibacterial Composite Films Based on Isotactic Polypropylene and Coated ZnO Particles for Active Food Packaging

This study was aimed at developing new films based on isotactic polypropylene (iPP) for food packaging applications using zinc oxide (ZnO) with submicron dimension particles obtained by spray pyrolysis. To improve compatibility with iPP, the ZnO particles were coated with stearic acid (ZnOc). Composites based on iPP with 2 wt % and 5 wt % of ZnOc were prepared in a twin-screw extruder and then filmed by a calender. The effect of ZnOc on the properties of iPP were assessed and compared with those obtained in previous study on iPP/ZnO and iPP/iPPgMA/ZnO. For all composites, a homogeneous distribution and dispersion of ZnOc was obtained indicating that the coating with stearic acid of the ZnO particles reduces the surface polarity mismatch between iPP and ZnO. The iPP/ZnOc composite films have relevant antibacterial properties with respect to E. coli, higher thermal stability and improved mechanical and impact properties than the pure polymer and the composites iPP/ZnO and iPP/iPP-g-MA/ZnO. This study demonstrated that iPP/ZnOc films are suitable materials for potential application in the active packaging field.


Introduction
It is extensively reported that the nano/microparticles of zinc oxide (ZnO) exhibit antibacterial activity against gram-positive and gram-negative bacteria. This activity does not require the presence of UV light (unlike TiO 2 ), being stimulated by visible light, and it is inversely dependent on particle size [1][2][3][4][5]. The mechanisms responsible for the antibacterial activity are not fully understood. Distinctive mechanisms that have been put forward in the literature are listed as the following: direct contact of ZnO with cell walls, resulting in destructing bacterial cell integrity [6][7][8], liberation of antimicrobial ions, mainly Zn 2+ ions [8], and reactive oxygen species formation [9][10][11].

Composite Preparation
The powders of ZnOc were mixed with iPP at two different compositions: 2% and 5% (see Table 1), in a twin screw extruder Collin ZK 25 (D = 25 mm and L/D = 24). The temperature setting of the extruder from the hopper to the die was: 180/195/195/190/180˝C. The screw speed of the dispenser was 20 rpm while the speed of the extruder screws was 25 rpm.
Films of iPP and iPP/ZnOc were obtained by compression molding in a press at 210˝C and 100 bars. The films had a thickness of about 110-120 µm.

Characterization
The following technologies were used to determine the properties of the films:

1) FT-IR Spectroscopy
Infrared spectra of the compression molded films were recorded with a PerkinElmer FT-IR spectrometer, model Paragon 500 equipment (PerkinElmer, Boston, MA, USA). The IR spectra were recorded in the range 4000-800 cm´1 with 4 cm´1 resolution and 20 scans.

2) Wide-Angle X-ray Diffraction
Wide-angle X-ray diffraction (WAXD) measurements were conducted using a Philips XPW diffractometer (Philips, Almelo, The Netherlands) with CuKa radiation (1.542 Å) filtered by nickel. The scanning rate was 0.02˝/s and the scanning angle was from 5˝to 45˝. The ratio of the area under the crystalline peaks and the total area multiplied by 100 was taken as the crystalline percentage degree.

3) Thermogravimetric Analysis
The thermal stability of the blends was examined by thermogravimetric analysis (TGA), using a PerkinElmer-Pyris Diamond apparatus (PerkinElmer, Boston, MA, USA) with a heating rate of 10˝C/min in air. Two measurements were performed for each sample.

4) Scanning Electron Microscopy
The surface analysis was performed using SEM, Fei Quanta 200 FEG (Fei, Hillsboro, OR, USA), on particles of the powders and on cryogenically fractured surfaces of composites. Before the observation, samples were coated with an Au/Pd alloy using an E5 150 SEM coating unit.

5) Tensile Tests
Dumbbell-shaped specimens were cut from the compression molded films and used for the tensile measurements. Stress-strain curves were obtained using an Instron machine, Model 4505 (Instron, Torino, Italy) at room temperature (25˝C) at a crosshead speed of 5 mm/min. Ten tests were performed for each composition.

6) Charpy Impact Test
The impact test allows for determining the degree of toughness of a polymer. Charpy impact tests were performed by using a pendulum CEAST (CEAST, Torino, Italy) with appropriate software for processing the data. The tests were carried out at room temperature on slabs obtained by compression moulding. Rectangular samples, with width of 3 mm, thickness of about 4 mm and length of 5 cm were used. The samples were cut from slabs obtained by using the same conditions adopted for the preparation of films.

7) Analysis UV-Visible Spectrometric
The UV-Visible spectrometry is useful for evaluating the ability of a material to minimize radiation potentially dangerous to the packaged food. The instrument used is a spectrometer Shimadzu UV-2101PC (Shimadzu, Columbia, MD, USA). UV-Vis spectra were recorded in transmission in the range 200-850 nm.

8) Antibacterial Test
The antimicrobial activity of the iPP/ZnOc composites was evaluated using E. coli DSM 498T (DSMZ, Braunschweig, Germany) as test microorganisms. The evaluation was performed using the ASTM Standard Test Method E 2149-10 [34]. The preparation of the bacterial inoculum required to grow a fresh 18-h shake culture of E. coli DSM 498T in a sterile nutrient broth (LB composition for 1 L: 10 g of triptone, 5 g of yeast extract and 10 g of sodium chloride) The colonies were maintained according to good microbiological practice and examined for purity by creating a streak plate. The bacterial inoculum was diluted using a sterile buffer solution (composition for one litre: 0.150 g of potassium chloride, 2.25 g of sodium chloride, 0.05 g of sodium bicarbonate, 0.12 g of CaCl 2¨6 H 2 O and pH = 7) until the solution reached an absorbance of 0.3˘0.01 at 600 nm, as measured spectrophotometrically. This solution, which had a concentration of 1.5ˆ10 8 -3.0ˆ10 8 colony forming units/mL (CFUs/mL), was diluted with the buffer solution to obtain a final concentration of 1.5ˆ10 6 -3ˆ10 6 CFUs/mL, that was the working bacterial dilution.
The experiments were performed in 50 mL sterilized flasks. One gram of the film was maintained in contact with 10 mL of the working bacterial dilution. After 2 min, 100 mL of the working bacterial dilution was transferred to a test tube, which was followed by serial dilution and plating out on Petri dishes (10 mmˆ90 mm) in which the culture media was previously poured. The Petri dishes were incubated at 35˝C for 24 h. These dishes represented the T 0 contact time. The flasks were then placed on a wrist-action shaker for 1 h, 24 h, 48 h, 5 days and 10 days. The bacterial concentration in the solutions at these time points was evaluated by again performing serial dilutions and standard plate counting techniques. Three experiments were performed for each composition. The number of colonies in the Petri dish after incubation was converted into the number of colonies that form a unit per millilitre (CFUs/m) of buffer solution in the flask. The percentage reduction (R%) was calculated using the following formula: where A = CFUs/mL for the flask containing the sample after the specific contact time and B = CFUs/mL at T 0 .

Analysis of ZnO and ZnOc Particles
Before blending the ZnOc particles with iPP, an investigation of properties of the ZnO and ZnOc particles has been performed, to assess the amount of stearic acid present on the ZnO particles and the influence of the coating process on the structure, morphology, and thermal stability of the zinc oxide particles. The content of stearic acid present on the surface of the coated particles was evaluated through thermogravimetric analysis. Figure 1 reports the thermogravimetric curves of ZnO and ZnOc particles recorded during the heating rate of 20˝C/min in air from room temperature to 700˝C.
In the entire T range, for ZnO particles, no weight loss was observed. In the case of ZnOc, after the thermal treatment, a weight reduction of about 9% is found. Considering that ZnO does not undergo degradation, it can be concluded that the weight reduction percentage observed for ZnOc corresponds probably to the percentage of stearic acid present on the surface of the particles. To study the influence of the stearic acid on the crystalline structure of the ZnO particles, the spectra of X-ray diffraction at high angles were recorded. As shown in Figure 2, the spectrum of the particles of ZnOc presents the same peaks as those of ZnO, suggesting that the coating does not alter the crystalline structure of the particles (zincite) [35]. In order to recognize the functional groups present, the interaction between the stearic acid and the ZnO particles and to obtain information on the shape of the particles, FTIR analysis was been performed. Figure 3 reports the FTIR spectrum of ZnOc and ZnO. For the coated sample, different bands can be observed, in particular according to literature [35][36][37][38]:  at 2916 and 2848 cm −1 : these vibration bands can be assigned to the "stretching" of the symmetric and asymmetric aliphatic group CH2;  at 1460 cm −1 : this band is assigned to the vibration of "bending" of aliphatic groups CH2 and CH3 of stearic acid;  at 1540 and 1384 cm −1 : these bands are assigned to the asymmetric and symmetric vibrations of the carboxylate group of the stearic acid;  To study the influence of the stearic acid on the crystalline structure of the ZnO particles, the spectra of X-ray diffraction at high angles were recorded. As shown in Figure 2, the spectrum of the particles of ZnOc presents the same peaks as those of ZnO, suggesting that the coating does not alter the crystalline structure of the particles (zincite) [35]. To study the influence of the stearic acid on the crystalline structure of the ZnO particles, the spectra of X-ray diffraction at high angles were recorded. As shown in Figure 2, the spectrum of the particles of ZnOc presents the same peaks as those of ZnO, suggesting that the coating does not alter the crystalline structure of the particles (zincite) [35]. In order to recognize the functional groups present, the interaction between the stearic acid and the ZnO particles and to obtain information on the shape of the particles, FTIR analysis was been performed. Figure 3 reports the FTIR spectrum of ZnOc and ZnO. For the coated sample, different bands can be observed, in particular according to literature [35][36][37][38]:  at 2916 and 2848 cm −1 : these vibration bands can be assigned to the "stretching" of the symmetric and asymmetric aliphatic group CH2;  at 1460 cm −1 : this band is assigned to the vibration of "bending" of aliphatic groups CH2 and CH3 of stearic acid;  at 1540 and 1384 cm −1 : these bands are assigned to the asymmetric and symmetric vibrations of the carboxylate group of the stearic acid; In order to recognize the functional groups present, the interaction between the stearic acid and the ZnO particles and to obtain information on the shape of the particles, FTIR analysis was been performed. Figure 3 reports the FTIR spectrum of ZnOc and ZnO. For the coated sample, different bands can be observed, in particular according to literature [35][36][37][38]: ‚ at 2916 and 2848 cm´1: these vibration bands can be assigned to the "stretching" of the symmetric and asymmetric aliphatic group CH 2 ; ‚ at 1460 cm´1: this band is assigned to the vibration of "bending" of aliphatic groups CH 2 and CH 3 of stearic acid; ‚ at 1540 and 1384 cm´1: these bands are assigned to the asymmetric and symmetric vibrations of the carboxylate group of the stearic acid; ‚ at around 454 cm´1: These bands give information about the shape of the particles. It is interesting to go deeper into the bands at points 3 and 4.
In particular, the bands at 1540 and 1384 cm´1 (region of absorption of the carbonyl C=O) can be related to the coordination behavior of the carboxylate group when it forms complexes with metals. These bands can give important information on the nature of the link between ZnO and the carboxylate group (COO-) of stearic acid.  at around 454 cm −1 : These bands give information about the shape of the particles. It is interesting to go deeper into the bands at points 3 and 4.
In particular, the bands at 1540 and 1384 cm −1 (region of absorption of the carbonyl C=O) can be related to the coordination behavior of the carboxylate group when it forms complexes with metals. These bands can give important information on the nature of the link between ZnO and the carboxylate group (COO-) of stearic acid. According to literature, the carboxylate group has versatile coordination behavior, when it forms coordination complexes with metals. It can be ionic, monodentate, bidentate chelating or bridging. Measuring the frequency of asymmetric, (νas(COO-)) and symmetric bands (νas(COO-) and νs(COO-)) and the magnitude of their separation, Δ, (Δ = νas(COO-) − νs(COO-)), the mode of the carboxylate binding with ZnO can be determined [36][37][38]. Generally, depending on the value of Δ, the following order is proposed for the coordination of carboxylates of divalent metals: Δ(chelating) < Δ(bridge) < Δ(ionic) < Δ(monodentate).
Finally, the assignment of the type of link is done comparing the Δ(experimental) with that of the corresponding sodium salt (Δ(sodium salt)) with the following rules: From the figure, Δ(experimental) = (1540 − 1384) cm −1 = 156 cm −1 . According to the criterion above, and taking into account that from literature data, the sodium stearate as Δ equal to 138 cm −1 [35], it can be concluded that the coordination is monodentate.
The band at 454 cm −1 can give information about the shape of the particles. Using the theory of dielectric media [34,36], the single band in ZnOc indicates particles with a spherical shape. It is interesting to make a comparison with the spectrum of the uncoated particles where the two bands indicate the presence of a structure with a mainly prismatic shape.
The morphology of the ZnO and ZnOc particles was studied using scanning electron microscopy (SEM). Figure 4 shows SEM micrographs of ZnO and ZnOc respectively. From the micrographs, it is clear that ZnO particles are characterized by a hexagonal crystal structure, as already emerged from FTIR analysis, with a smooth surface. Moreover, the ZnO particles seem to have a strong tendency to form agglomerates. Contrary ZnOc particles, more homogeneously dispersed in the matrix, have a spherical shape. Comparing the dimensions of the kinds of particles, the size of the ZnO particles ranges between 250 to 500 nm while that of the particles of ZnOc varies between 1 to 1.2 µ m.  According to literature, the carboxylate group has versatile coordination behavior, when it forms coordination complexes with metals. It can be ionic, monodentate, bidentate chelating or bridging. Measuring the frequency of asymmetric, (ν as (COO-)) and symmetric bands (ν as (COO-) and ν s (COO-)) and the magnitude of their separation, ∆, (∆ = ν as (COO-)´ν s (COO-)), the mode of the carboxylate binding with ZnO can be determined [36][37][38]. Generally, depending on the value of ∆, the following order is proposed for the coordination of carboxylates of divalent metals: ∆(chelating) < ∆(bridge) < ∆(ionic) < ∆(monodentate).
Finally, the assignment of the type of link is done comparing the ∆ (experimental) with that of the corresponding sodium salt (∆ (sodium salt) ) with the following rules: From the figure, ∆ (experimental) = (1540´1384) cm´1 = 156 cm´1. According to the criterion above, and taking into account that from literature data, the sodium stearate as ∆ equal to 138 cm´1 [35], it can be concluded that the coordination is monodentate.
The band at 454 cm´1 can give information about the shape of the particles. Using the theory of dielectric media [34,36], the single band in ZnOc indicates particles with a spherical shape. It is interesting to make a comparison with the spectrum of the uncoated particles where the two bands indicate the presence of a structure with a mainly prismatic shape.
The morphology of the ZnO and ZnOc particles was studied using scanning electron microscopy (SEM). Figure 4 shows SEM micrographs of ZnO and ZnOc respectively. From the micrographs, it is clear that ZnO particles are characterized by a hexagonal crystal structure, as already emerged from FTIR analysis, with a smooth surface. Moreover, the ZnO particles seem to have a strong tendency to form agglomerates. Contrary ZnOc particles, more homogeneously dispersed in the matrix, have a spherical shape. Comparing the dimensions of the kinds of particles, the size of the ZnO particles ranges between 250 to 500 nm while that of the particles of ZnOc varies between 1 to 1.2 µm.

Structure and Morphology
The WAXD patterns of iPP and iPP/ZnOc composites are reported in Figure 5. All samples show the presence of the peak at 2θ = 18°-19° characteristic of α form of iPP [39,40]. The sample iPP/5%ZnOc is characterized also by a small percentage of the form β, highlighted by the presence of the peak at 2θ = 16°.

Structure and Morphology
The WAXD patterns of iPP and iPP/ZnOc composites are reported in Figure 5. All samples show the presence of the peak at 2θ = 18˝-19˝characteristic of α form of iPP [39,40]. The sample iPP/5%ZnOc is characterized also by a small percentage of the form β, highlighted by the presence of the peak at 2θ = 16˝.

Structure and Morphology
The WAXD patterns of iPP and iPP/ZnOc composites are reported in Figure 5. All samples show the presence of the peak at 2θ = 18°-19° characteristic of α form of iPP [39,40]. The sample iPP/5%ZnOc is characterized also by a small percentage of the form β, highlighted by the presence of the peak at 2θ = 16°.  UV-Vis spectra are reported in Figure 6. For all samples, an absorption band at 280 nm is observed, probably due to the presence of stabilizer added to commercial iPP. For the samples containing ZnOc, an absorption band is also observed in the region around 385 nm, as indicated by arrows. This band is due to the inherent capacity of the ZnO particles to absorb the UV light [29,30,35,36].
UV-Vis spectra are reported in Figure 6. For all samples, an absorption band at 280 nm is observed, probably due to the presence of stabilizer added to commercial iPP. For the samples containing ZnOc, an absorption band is also observed in the region around 385 nm, as indicated by arrows. This band is due to the inherent capacity of the ZnO particles to absorb the UV light [29,30,35,36].  Comparing the results with those obtained for the system iPP/ZnO and iPP/PPgMA/ZnO [24] at a given composition, it seems that the coating of the ZnO with stearic acid favors a better dispersion and distribution of the particles in the iPP matrix and prevents the formation of agglomerates.   UV-Vis spectra are reported in Figure 6. For all samples, an absorption band at 280 nm is observed, probably due to the presence of stabilizer added to commercial iPP. For the samples containing ZnOc, an absorption band is also observed in the region around 385 nm, as indicated by arrows. This band is due to the inherent capacity of the ZnO particles to absorb the UV light [29,30,35,36].  Comparing the results with those obtained for the system iPP/ZnO and iPP/PPgMA/ZnO [24] at a given composition, it seems that the coating of the ZnO with stearic acid favors a better dispersion and distribution of the particles in the iPP matrix and prevents the formation of agglomerates. Comparing the results with those obtained for the system iPP/ZnO and iPP/PPgMA/ZnO [24] at a given composition, it seems that the coating of the ZnO with stearic acid favors a better dispersion and distribution of the particles in the iPP matrix and prevents the formation of agglomerates. Figure 8 shows the % weight loss of the samples as function of temperature for iPP and iPP/ZnOc samples, whereas Table 2 reports the values of the temperature at the inflection point of the curve of Figure 8 detected at the maximum of the peak of the first derivative and which corresponds to the maximum rate of the degradation of the sample.  Figure 8 shows the % weight loss of the samples as function of temperature for iPP and iPP/ZnOc samples, whereas Table 2 reports the values of the temperature at the inflection point of the curve of Figure 8 detected at the maximum of the peak of the first derivative and which corresponds to the maximum rate of the degradation of the sample.

Sample
Tmax (°C) iPP 335 iPP/2%ZnOc 375 iPP/5%ZnOc 381 For the two composites, a delay in the temperature of starting degradation, compared to iPP, and a consistent increase of the Tmax are observed. Taking into account that the presence of uncoated ZnO at the same composition did not have consistent influence on the thermostability of iPP, as reported in the paper at reference [22], the increase in thermal stability should be attributed to the stearic acid that coats the ZnO particles. As it was reported in a previous section (see Figure 1), the degradation of the stearic acid starts before the degradation of iPP. The degradation products of the stearic acid probably act as a barrier for the degradation of the matrix also slowing the diffusion of the degradation products of iPP in the sample causing an increase of the thermal stability of the iPP/ZnOc composites. Figure 9 shows the stress-strain curves of iPP and iPP/ZnOc composites, whereas Table 3 reports the values of mechanical parameters, (Young modulus (E), stress and strain at the yield point (y, y), and at break (b, b)).

Mechanical and Impact Properties
It can be seen that all the samples has the typical behavior of a semi-crystalline polyolefin, with the phenomenon of yield strength, cold drawing, fiber elongation and final break of the fibers.
From the values shown in the Table 3, it can be observed that: (1) the two composite films have similar values of Young modulus and strain at the yield point but higher than those of plain iPP; (2) the elongation at yield and the stress at break point can be considered similar for the three samples (the differences are inside the experimental error), whereas the elongation at break decreases with the addition of ZnOc. Comparing these results with those reported in reference [22], where ZnO not coated was used, it can be observed that the composites with ZnOc present improved mechanical properties.  For the two composites, a delay in the temperature of starting degradation, compared to iPP, and a consistent increase of the Tmax are observed. Taking into account that the presence of uncoated ZnO at the same composition did not have consistent influence on the thermostability of iPP, as reported in the paper at reference [22], the increase in thermal stability should be attributed to the stearic acid that coats the ZnO particles. As it was reported in a previous section (see Figure 1), the degradation of the stearic acid starts before the degradation of iPP. The degradation products of the stearic acid probably act as a barrier for the degradation of the matrix also slowing the diffusion of the degradation products of iPP in the sample causing an increase of the thermal stability of the iPP/ZnOc composites. Figure 9 shows the stress-strain curves of iPP and iPP/ZnOc composites, whereas Table 3 reports the values of mechanical parameters, (Young modulus (E), stress and strain at the yield point (σ y , ε y ), and at break (σ b , ε b )).

Mechanical and Impact Properties
It can be seen that all the samples has the typical behavior of a semi-crystalline polyolefin, with the phenomenon of yield strength, cold drawing, fiber elongation and final break of the fibers.
From the values shown in the Table 3, it can be observed that: (1) the two composite films have similar values of Young modulus and strain at the yield point but higher than those of plain iPP; (2) the elongation at yield and the stress at break point can be considered similar for the three samples (the differences are inside the experimental error), whereas the elongation at break decreases with the addition of ZnOc. Comparing these results with those reported in reference [22], where ZnO not coated was used, it can be observed that the composites with ZnOc present improved mechanical properties.   Table 4 shows the values of the impact test, in particular the values of the force (F) that the pendulum lost on impact with the sample, the energy (U) absorbed by the samples at the break and the toughness (T). The results demonstrate that the presence of ZnOc increases the toughness; in fact, the toughness of iPP/5%ZnOc sample is 26% higher than that of iPP. In Figure 10, the antimicrobial effect against E. coli is presented as a function of time for the different composites. Without ZnO particles, the reference concentration of the micro-organism is measured to be ~2 × 10 6 . After 1 h, no change in the concentration was observed for all samples. By increasing the time, a decrease in the E. coli concentration is observed for the composites. The effect is more evident for the iPP/5%ZnOc composite. Significant variations in concentration are observed increasing the contact time and ZnOc content. After 24 h, the concentration of E. coli decreases to 8.8 × 10 5 for iPP/2%ZnOc and 1.7 × 10 5 for iPP/5%ZnOc. After 48 h, the bacterial concentration was significantly decreased for the iPP/5%ZnOc sample (2 × 10 3 CFU/mL). The sample iPP/2%ZnOc reaches similar values after five days.
The values of percentage reduction (%R) of E. coli for all samples at different contact times are reported in Table 5. Neat iPP exhibits no bactericidal activity, and R was observed to be zero up to day 10. The iPP/5%ZnOc composite exhibited maximum reduction, 99.9%, after 48 h.  Table 3. Stress-strain parameters of iPP and iPP/ZnOc composites. Table 4 shows the values of the impact test, in particular the values of the force (F) that the pendulum lost on impact with the sample, the energy (U) absorbed by the samples at the break and the toughness (T). The results demonstrate that the presence of ZnOc increases the toughness; in fact, the toughness of iPP/5%ZnOc sample is 26% higher than that of iPP.

Antibacterial Properties
In Figure 10, the antimicrobial effect against E. coli is presented as a function of time for the different composites. Without ZnO particles, the reference concentration of the micro-organism is measured to be~2ˆ10 6 . After 1 h, no change in the concentration was observed for all samples. By increasing the time, a decrease in the E. coli concentration is observed for the composites. The effect is more evident for the iPP/5%ZnOc composite. Significant variations in concentration are observed increasing the contact time and ZnOc content. After 24 h, the concentration of E. coli decreases to 8.8ˆ10 5 for iPP/2%ZnOc and 1.7ˆ10 5 for iPP/5%ZnOc. After 48 h, the bacterial concentration was significantly decreased for the iPP/5%ZnOc sample (2ˆ10 3 CFU/mL). The sample iPP/2%ZnOc reaches similar values after five days.
The values of percentage reduction (%R) of E. coli for all samples at different contact times are reported in Table 5. Neat iPP exhibits no bactericidal activity, and R was observed to be zero up to day 10. The iPP/5%ZnOc composite exhibited maximum reduction, 99.9%, after 48 h.   The results obtained from the analysis of antibacterial properties allow us to conclude that the particles of ZnO with stearic acid have relevant antibacterial property against E. coli, similar to that of ZnO and that the coating of the particles does not have a negative influence as the coating of the particles with PPgMA [24].  In Table 6 a comparison of the bacterial activity of the three systems, iPP/ZnO, iPP/ZnOc and iPP/PPgMA/ZnO, at the same ZnO content (2%) is reported. From this table, it is clearly confirmed that:

‚
in the iPP/ZnOc system the ZnOc particles maintain their antibacterial properties against E. coli, with respect to the uncoated particles; ‚ in the system iPP/PPgMA/ZnO, the ZnO particles that are linked to the maleic anhydride groups of PPgMA [24], do not display similar antibacterial activity at least up to 48 h. Probably, the PP chains of the PPgMA, due to the link between MA and ZnO, cover the ZnO particles and hinder the antibacterial activity. Table 6. Percent reduction of E. coli at 48 h and 5 days of contact for iPP and different films at 2% ZnO, (adapted from Table 5 of this paper and references [22,24]). The results obtained from the analysis of antibacterial properties allow us to conclude that the particles of ZnO with stearic acid have relevant antibacterial property against E. coli, similar to that of ZnO and that the coating of the particles does not have a negative influence as the coating of the particles with PPgMA [24].

Conclusions
This work had as its final objective the preparation of the film based on the isotactic polypropylene matrix intended for packaging food, with improved properties by the addition of ZnO particles coated with stearic acid (ZnOc). The latter has the function of compatibilization between the inorganic metal oxide particles phase and the organic matrix of the isotactic polypropylene. The samples were prepared in a twin-screw extruder and then filmed by a compression molding.
It was observed that the stearic acid coating on the ZnO particles reduces the surface polarity mismatch between iPP and ZnO and allows the formation of a composite with fair distribution of particles.
The principal achievement of the novel composites is the strong antibacterial activity against E. coli: the bacterial concentration decreases with increasing concentration of ZnOc and the contact time between the film and the bacterial solution. After 48 h, the bacterial reduction was significantly decreased for the sample containing 5% of ZnOc (R = 99.99%); for the sample iPP/2% ZnOc, it reaches these values after five days.
Moreover, the iPP/ZnOc composites present improvement of the thermal stability, tensile parameters (Young modulus and stress at the yield point) and impact properties with respect to neat iPP, iPP/ZnO, and iPP/PPgMA/ZnO at least for samples containing 2% ZnO [22,24]. It also has to be underlined that the films containing ZnOc show an absorption in the region around 385 nm, confirming that the ZnOc particles also have a shielding effect to UV radiation as those of ZnO as reported in the literature.
On the base of the results obtained, it can be stated that the methodology proposed (using of novel ZnO particles obtained by spray pyrolysis; coating the ZnO with stearic acid and optimization of the processing conditions) is innovative, because no literature is available (at our knowledge) on the properties of such an iPP/ZnOc system prepared directly by melt mixing. Moreover, it is also very efficient in preventing formation of agglomerated domains and in providing a system with improved properties.
In conclusion, the composites iPP/5% ZnOc films have relevant antibacterial property against E. coli, higher thermal stability and improved mechanical and impact properties than the pure iPP film so that they are suitable for application in the food industry as active packaging films.