**1. 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 TiO2), being stimulated by visible light, and it is inversely dependent on particle size [1–5]. 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 [6–8], liberation of antimicrobial ions, mainly Zn2+ ions [8], and reactive oxygen species formation [9–11].

There has been a great deal of interest in the antimicrobial property of ZnO for food packaging applications, as a viable solution for stopping infectious diseases [12–14]. Due also to low cost, ZnO particles (with sub-micro and nano-dimensions) are therefore ideal fillers for polymers to be applied in the field of active food packaging. Thus, ZnO particles have been incorporated into a number of different polymers used in food packaging [15,16], such as low-density polyethylene (LDPE) [17,18], isotactic polypropylene (iPP) [19–24], polyamide (PA) [18,25], and polylactic acid (PLA) [26].

Recently, the influence of ZnO particles, obtained by spray pyrolysis with submicron dimensions [22,27,28], on the structure, morphology, thermal stability, photo stability, and mechanical and antibacterial properties of (iPP)/ZnO composites was investigated [22,24]. The addition of ZnO particles imparts improvements on the photodegradation resistance of iPP to ultraviolet irradiation and the composites exhibit significant antibacterial activity against *Escherichia coli*. This activity is dependent on exposure time and composition.

On the other hand, it was noticed that due to the surface polarity mismatch between iPP and ZnO, agglomeration phenomena of the ZnO particles occur and that these phenomena cause a decrease in the mechanical and other functional properties of iPP/ZnO composites with respect to plain iPP [22–24].

The main problem to be solved in adding ZnO nano/microparticles to an iPP matrix seems therefore related to the formation of agglomerated domains that occur because of the strong intermolecular interactions among the ZnO particles in combination with their high surface area. This prevents transfer of their superior properties to the composite. Good dispersion has been reported for some polar polymers [18,25], but ZnO dispersion in non-polar polymers, such as iPP, during melt processing remains a challenge.

A largely proposed strategy, to improve dispersion consists in adding a compatibiliser, containing groups suitable for interaction with the two components [29–33]. Following this strategy in a previous paper, polypropylene grafted with maleic anhydride (PPgMA) [24] was selected as the most promising candidate as a compatibiliser between iPP and ZnO. In particular, the influence of three PPgMA (with different MW and MA% content) added to iPP/ZnO 98/2 wt % on the structure, morphology, mechanical, thermal, barrier properties and antibacterial activity against *E. coli* was investigated with the aim to verify if the compatibiliser PPgMA could be beneficial in order to increase the dispersion of ZnO in an iPP matrix in order to have films with improved properties. It was found that the presence of this compatilizer improves the dispersion of the particles in the matrix, but, at the same time, does not cause any enhancement in the barrier and mechanical properties and indeed reduces the antibacterial activity with respect to iPP/ZnO. An important aspect found in this study is that the more the ZnO are well embedded in polymer material, the more the antibacterial activity decreases, probably because the surface of the particle available for contact with the solution decreases.

An alternative methodology to improve the dispersion consists of modifying the particles' surfaces with groups suitable for interaction with the matrix.

The main purpose of this paper is to develop new films based on iPP for applications in the food industry as active packaging using coated ZnO particles to improve compatibility between the organic phase and inorganic one. In particular, the surface of the ZnO particles, obtained by spray pyrolysis, was coated with stearic acid (ZnOc). Objective of the paper is also to assess the influence of the coating process on the structure, morphology, and thermal stability of the zinc oxide particles.

#### **2. Experimental**

#### *2.1. Materials and Sample Preparation*

The materials used in this work are: (1) isotactic polypropylene (iPP, Moplen X30S), in pellets, kindly supplied by Basell (Ferrara, Italy), with melt flow index = 9 dg¨min´<sup>1</sup> (2.16 kg, 230 ˝C), *M*<sup>w</sup> = 3.5 ˆ 105 and *M*<sup>n</sup> = 4.7 ˆ 104; (2) zinc oxide coated with stearic acid (ZnOc) (white powder). The ZnO particles were synthesized using a preindustrial spray scale pyrolysis platform at the Pylote in Toulouse-France and then coated with stearic acid. This technique [22,27,28] provides many advantages compared to other techniques of preparation: the simplicity of the process, high purity of the powders obtained, more uniform chemical composition, narrow size distribution, better regularity in shape and the ability to synthesize multicomponent materials. The coating of the ZnO particles was performed by preparing a solution of stearic acid and ZnO (1:10) in isopropanol under stirring for 12 h. The powder was recovered by centrifuge and dried in an oven at a temperature of 70 ˝C.
