The development of packaging materials with new functionalities and lower environmental impact is now an urgent need of our society. On one hand, the shelf-life extension of packaged products can be an answer to the exponential increase of worldwide demand for food. On the other hand, uncertainty of crude oil prices and reserves has imposed the necessity to find raw materials to replace oil-derived polymers. Additionally, consumers' awareness toward environmental issues increasingly pushes industries to look with renewed interest to "green" solutions. In response to these issues, numerous polymers have been exploited to develop biodegradable food packaging materials. Although the use of biopolymers has been limited due to their poor mechanical and barrier properties, these can be enhanced by adding reinforcing nanosized components to form nanocomposites. Cellulose is probably the most used and well-known renewable and sustainable raw material. The mechanical properties, reinforcing capabilities, abundance, low density, and biodegradability of nanosized cellulose make it an ideal candidate for polymer nanocomposites processing. Here we review the potential applications of cellulose based nanocomposites in food packaging materials, highlighting the several types of biopolymers with nanocellulose fillers that have been used to form bio-nanocomposite materials. The trends in nanocellulose packaging applications are also addressed.
Defects were introduced in a hexagonal liquid foam prepared by a novel technique and their coarsening was observed. Both isolated defect clusters, with and without dislocation character, and grain boundaries were produced. As the defects coarsen, a gradient of cell size develops along the direction of growth. Differences in the growth characteristics of isolated defects were found, depending on whether the average size of the cells in the defect clusters is larger (type I) or smaller (type II) than the size of the surrounding honeycomb cells. Clusters of type I show an approximately linear increase of diameter with time, while clusters of type II have an initial contraction. The evolution to the scaling regime of a honeycomb containing a distribution of type I clusters is discussed.
Composite sandwich materials are very common in structural uses for a wide range of applications in the aerospace and automotive industry that require low weight, high bending strength, and high energy absorption. In general, the core of the sandwich structures has a two-dimensional cellular structure, with a regular honeycomb geometry. While with standard manufacturing processes the geometric structures are limited, the emergence of additive manufacturing provides alternatives to conventional designs. The aim of this work is to analyze and evaluate the effect of the core geometry on the flexural properties of the structure. For that purpose, three different cellular configurations were considered, namely regular honeycombs, lotus, and hexagonal honeycombs with Plateau borders. Four relative densities, with average values of 0.1, 0.25, 0.44, and 0.62, for each configuration, were studied. The flexural properties of cellular structures were evaluated with three-point bending tests, both numerically and experimentally. A modeling approach of the tests in the three configurations was performed, for two materials, polylactic acid and pure aluminum, by means of finite element simulations. Fused deposition modeling was used to obtain polylactic acid samples for the aforementioned configurations, which were experimentally tested to evaluate the mechanical response and the failure behavior of the cores. Results differ with the geometry arrangement and showed a strong dependency with the relative density of the structures in the flexural response in what concerns strength, stiffness, and energy absorbed. The arrangements studied present properties, which make them competitive with the traditional core structures for the same density. A promising agreement between experimental and simulation results was obtained.
Sandwich structures are frequently used in automotive, aerospace and marine industries, as they provide adequate functional properties. The two-dimensional regular hexagonal cell shape, i.e. honeycomb is the most used core structure in sandwich panels. Recently, a new type of cellular structures composed of lattice struts has been proposed, as they combine high stiffness, strength and energy absorption with low weight. The main purpose of this research is to investigate the effect of the lattice topology on the flexural behaviour of sandwich panels. Five lattice geometries inspired in crystalline structures were designed, namely, body-centred parallelepiped, body-centred parallelepiped with struts in z-axis, body- and face-centred parallelepiped with struts in z-axis, face-centred parallelepiped with struts in z-axis and parallelepiped simple. The relative density of all the lattices was kept constant as 0.3. Both numerical and experimental approaches were used to evaluate the flexural properties and failure behaviour of the sandwich structures under three-point bending tests. The numerical analysis was undertaken with the finite element software NX Nastran. Taking advantage of additive manufacturing technologies, material extrusion was used to produce polylactic acid samples with the configurations aforementioned. The sandwich panels are composed by a single layer formed by the lattice core and two thin plates, at the bottom and top. The three parts of the panel were manufactured all together. The simulation results indicate that, among the lattices studied, topologies body-centred parallelepiped with struts in z-axis and body- and face-centred parallelepiped with struts in z-axis exhibit higher strength, while body- and face-centred parallelepiped with struts in z-axis shows higher stiffness and higher energy absorption, attaining values that do not differ much from the ones obtained with a two-dimensional hexagonal cellular structure, with the same relative density. As a consequence, some of the geometries studied may have the potential to be considered as alternatives to conventional structures in the design of sandwich structures.
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