## 1 Introduction
Due to their natural source, biodegradable starch films present poor mechanical and barrier properties, such as low tensile strength and elongation at break, and high water vapor and oxygen permeability [10]. Incorporation of secondary plasticizers, such as glycerol, is necessary to produce long-lasting films but, on the other hand, negatively affects water barrier properties and tensile strength. However, many ways of improving the mechanical and barrier properties of starch biodegradable films have been proposed in the literature. The most promising solution is the incorporation of nanoclay particles acting as nanofillers on the matrix that form stacked layers. Nanoclays can be classified as smectites, chlorites, kaolinites, illites and halloysites [4]. Negative charges, generated by isomorphic substitution within the interlayer spaces, are neutralized by alkali or alkaline earth cations. Apart from their contribution to improving the mechanical and barrier properties of food packaging materials, nanoclays have been shown to reduce the migration of chemicals from food packaging to food [11]. The most established nanoclay addition technology is organic surface modification, which enhances the intercalation of biopolymers within nanoclay platelets by increasing the interlayer spacing between them [12]. Current trends include the preparation of nanoclay hybrids with carbon nanomaterials and several metal or metal oxide nanoparticles, which increases surface area, heat resilience, loading and strength, and adds additional functionalities [12], and emulsion techniques such nano-emulsion and the Pickering emulsion, which contribute to the homogenous dispersibility of nanoclays in the food packaging matrix [13]. The migration/release of nanoclays from the food packaging material, during the manufacture, use, disposal or weathering because of environmental conditions, is affected by temperature, the polymer/nanoclay interaction, mechanical impact and direct contact with food, and requires further investigation [13]. Finally, the potential toxicity of nanoclays is a research subject that has not been extensively studied due to the fact that the different toxicity profiles of nanoclays and modified nanoclays require a case-by-case evaluation [13]. Among nanoclay particles, montmorillonite (MMT) is the one that is used more frequently due to its low cost, large surface area and high cation exchange capacity [14]. In order to form nanocomposites of MMT-starch, and thus obtain the improved tensile and barrier properties, MMT needs to be intercalated through amylose chains, and this is facilitated by melt or solution intercalation at temperatures above 85 \({}^{\circ}\)C [15]. The international protocol of food packaging mechanical testing (ASTM D882), which is used by most packaging industries, subjects the film to tensile stress until disruption at a specific speed and, at the same time, the force needed to strain the film specimen, the distance and the time of the test are monitored. The tensile strength, elongation at break and Young's modulus can be calculated from the stress-strain plot of the test, among others. Notwithstanding the fact that, with this test, the industry obtains an objective evaluation of the film tensile properties; in reality, the film is stressed in a very different way, meaning that the elongation ranges are substantially lower than the disruptive elongation ranges of the aforementioned test. Consumers reuse packages repeatedly, with different loads for different times and with repeating fatigue cycles [16], thus submitting the package to additive stress in lower elongations compared to the disruptive test and, consequently, this additive stress on the package will lead, after some time, to disruption. It is rational, then, that the food packaging industry, which uses biodegradable starch films, needs to be able to evaluate the stress behavior of these products at a range of stress levels, stress durations and cycles of stress.