**1. Introduction**

Flexible films (<100 µm), such as low-density polyethylene and polyvinyl chloride, are widely applied in the food industry [1]. The handling of the flexible films in the packaging and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

transportation of food and food products over long distances is an issue bordering the agricultural products processing engineers, due to the dynamic loading experienced by the materials in the process [2]. The viscoelastic behaviour often exhibited by this material has great influence on its degree of wear and tear, especially during high speed transportation of packaged foods. The low mechanical impendence of the flexible films has rendered measurement of the rheological behaviour difficult to achieve using the universal tensile testing equipment but can be measured using nanoindentation analysis [3, 4]. The indirect measurement of the area of contact between the indenter and flexible film under investigation is called nanoindentation. The method provides an avenue of touching the flexible film, whose rheological properties are unknown, with another material of known property. The penetration depth of the indenter, which allowed for proper measurement of the penetration rate in the flexible film, is usually measured in nanometres, thus considering the process as non-destructive because of its relatively smaller area of contact. The accurate measurement of the rheological properties has been made possible using the nanoindentation technique. It is easier to measure the rheological properties of flexible films by this process due probably to the high response of the material to the depth sensing device. The knowledge of the response of flexible films to this device can be used to study their behaviour with respect to deformation or wear [5].

The rheological behaviour of flexible films, under static and dynamic loading, has been reported by some researchers [6–8]. Jian et al. [6] studied the load-displacement behaviour of Cu2O thin films by nanoindentation. The authors observed that the regions of loading and unloading appeared distinct on the hysteresis loop of the films. Also, Syed et al. [7] studied the nanoindentation behaviour of ultra-thin polymeric films and use the finite element modelling to characterise their mechanical properties. The authors observed that the Young's modulus and hardness of the flexible film increased with the indentation depth and the projected area of the deformed region. The nano-rheological properties of flexible films including the hardness and elastic modulus have been studied by Chateauminoisa and Briscoeb [8]. The authors observed a progressive wearing of the flexible film due to compaction brought about by increasing load at the contact area. The intensity of the wearing process was interpreted by considering the evolving load carrying capacity of the contact, which was characterised by progressive redistribution of the contact pressure within the flexible film. However, the behaviour of the flexible films, as reported, does not account for the viscoelasticity, elastic and plastic works of the materials. It was also difficult to determine the plasticity index and creep of the materials because of their inconsistent strain rate sensitivity under dynamic loading. The hardness and Young's modulus of the materials were practically inadequate to withstand the dynamic vibrational loading usually experienced on rough roads. Many of the flexible films therefore are limited in their application for high speed transportation, where higher hardness, Young's modulus and elastic behaviours of the materials are required for food packaging. Hence, there is the need for a suitable food packaging material with the required characteristics for potential application in high speed transportation of packaged foods.

The addition of zinc nanoparticles to renewable resource like starch provides an alternative flexible film with improved rheological properties. The new material offers opportunity to package food under dynamic loading at high speed due to their improved rheological behaviour. The improvement of these properties may be explained by the fundamental length scales of the nanoparticles, whose uniform dispersion with the starch results in ultra-large interfacial area between the constituents. The interface between the organic and inorganic materials alters the mobility of the molecules and thus the rheological behaviour of the nanocomposite material. Thus, the objective of this research was to determine the nanorheological behaviour of cassava starch-zinc nanocomposite film, such as hardness, Young's modulus, creep, elastic and plastic works, under dynamic loading for high speed transportation of packaged food.
