MICROCOMPOSITES BASED ON POLYPROPYLENE FOR ACTIVE FOOD PACKAGING

Zinc oxide (ZnO) is an important electronic and photonic component for UV light-emitters, varistors, gas sensors, acoustic wave devices, etc.1 Moreover, ZnO exhibits antibacterial activity against Gram-positive and Gram-negative bacteria. It has been reported that 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. Currently there was a great deal of interest in this antimicrobial property of ZnO for food packaging application, as a viable solution for stopping infectious diseases. For the aforementioned reasons and also considering their low cost, ZnO particles (of sub-micro and nano- dimensions) are ideal fillers for polymer composites. Thus, ZnO particles have been incorporated into a number of different polymers, including low density polyethylene (LDPE), isotactic polypropylene (iPP),5 polyvinyl alcohol (PVA),6 etc., for antimicrobial purposes, for increasing dielectric and conductivity properties, mechanical and barrier properties and for reduction of polymer photo-degradation. The aim of this work is the preparation and characterization of microcomposites based on polypropylene for application as "active packaging" in the field of "food". Zinc oxide, modified with stearic acid (ZnOm), were mixed to isotactic polypropylene (iPP) using a twin screw extruder for verifying the effect they have on the structure, morphology, thermal properties, mechanical properties, UV-visible absorption and antibacterial properties of the polymer. The ZnO particles were obtained by spray pyrolysis, the function of stearic acid is to improve the compatibility between the inorganic phase (ZnO) and the organic (iPP) one. Composites were prepared in 3 different formulations: 1,2,5% by weight of ZnOm. Morphological analysis revealed that the extrusion process allows to obtain a good dispersion of the particles, but it also showed the presence of sporadic agglomerations, which number depends on the composition. The addition to ZnOm determines an improvement of iPP thermal stability and has a screen effect to the UV radiation. The iPP/ZnOm systems with 2 and 5% in weight of ZnOm exhibit lower stress and elongation at break values than those of iPP, but retain a ductile behavior. Finally, all systems iPP/ZnOm have significant antibacterial activity against Escherichia-coli, this activity depends on the percentage of ZnOm and on the contact time between the bacterial solution and films.