*3.1. 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.

**Figure 1.** Thermostability curve of ZnO and ZnOc 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].

**Figure 2.** X-ray diffraction pattern of ZnO and ZnOc particles.

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 [35–38]:


' 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´<sup>1</sup> (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.

**Figure 3.** FT-IR spectrum of ZnO and ZnOc powder.

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, Δ, (Δ = *ν*as(COO–) ´ *ν*s(COO–)), the mode of the carboxylate binding with ZnO can be determined [36–38]. 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´<sup>1</sup> = 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´<sup>1</sup> [35], it can be concluded that the coordination is monodentate.

The band at 454 cm´<sup>1</sup> 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.

**Figure 4.** SEM micrographs of (**a**,**c**) ZnO and (**b**,**d**) ZnOc powders. (a,b) 40000ˆ; (c,d) 20000ˆ.

### *3.2. Analysis of the iPP/ZnOc Composites*

#### 3.2.1. 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˝.

**Figure 5.** WAXD spectrum of iPP and iPP/ZnOc composites.

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].

**Figure 6.** UV-Vis spectrum of iPP and iPP/ZnOc samples.

Figure 7 shows SEM micrographs of the fractured surface of iPP, iPP/2%ZnOc, iPP/5%ZnOc. It is possible to note that the particles are fairly distributed within the polymer matrix. Only a few aggregates with a size of about 5 μm can be observed.

**Figure 7.** Micrographs of iPP and iPP/ZnOc pellets fractured in liquid nitrogen at magnification (**a**) 2000ˆ and (**b**) 5000ˆ.

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.

#### 3.2.2. Thermostability

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.** Thermostability curve of iPP and iPP/ZnOc composites.

**Table 2.** Thermal degradation temperature at which the degradation rate is maximum (*T*max).


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.

#### 3.2.3. Mechanical and Impact Properties

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*)).

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.

**Figure 9.** Stress–strain curves of iPP and iPP/ZnOc films.

**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.

**Table 4.** Impact tests values for iPP and iPP/ZnOc composites.


#### 3.2.4. 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 ~2 ˆ 106. 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 ˆ 10<sup>5</sup> for iPP/2%ZnOc and 1.7 ˆ 105 for iPP/5%ZnOc. After 48 h, the bacterial concentration was significantly decreased for the iPP/5%ZnOc sample (2 ˆ 103 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.

**Figure 10.** Effect of time and filler content on the antibacterial activity of iPP and iPP/2%–5% ZnOc composites.



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:


**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]).


\* PP(9k)gMA(4.8) *M*w = 9100 MA (wt %) = 4.8; \*\* PP(65k)gMA(1.4) *M*w = 65,000 MA (wt %) = 1.4; \*\*\***/**PP(95k)gMA(0.5) *M*w = 95,000 MA (wt %) = 0.5.

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].

#### **4. 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.

**Acknowledgments:** The research described herein was partially supported by the European Community's Seventh Framework Programme (ERA-Net Susfood-CEREAL "Improved and resource efficiency throughout the post-harvest chain of fresh-cut fruits and vegetable"), Italian Health Ministry (Progetti Ricerca Corrente/2013 "Impiego di nanopackaging innovativo nella filiera della carne: valutazione dell'efficacia antibatterica e della sicurezza d'uso"), and Italian Ministry of Foreign Affair (Bilateral Project Italy/Quebec 2014–2016 "Sviluppo di nanomateriali ecosostenibili per l'Imballaggio alimentare adatti alla sterilizzazione per radiazione"). Jannette Dexpert-Ghys and Marc Verelst from The Centre d'Élaboration de Matériaux et d'Etudes Structurales (CEMES-CNRS), Toulouse-France, are kindly thanked for supplying ZnO particles obtained by spray-pyrolysis. Ida Romano of the Istituto di Chimica Biomolecolare (CNR), Pozzuoli (NA) Italy is kindly thanked for performing the antimicrobial tests.

**Author Contributions:** Clara Silvestre and Sossio Cimmino supervised the research program. Clara Silvestre, Sossio Cimmino and Donatella Duraccio designed the setup. Antonella Marra, Valentina Strongone and Donatella Duraccio performed the experiments. All authors contributed to the analysis of the presented experiments and correlation of the different means of investigations. Antonella Marra and Clara Silvestre wrote the initial draft. Clara Silvestre and Sossio Cimmino coordinated the revisions of the draft in the final form.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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