LDPE (low-density polyethylene) foams were prepared using the improved compression moulding technique (ICM) with relative densities ranging from 0.3 to 0.7 and with different levels of chemical blowing agents (from 1% to 20%). The density gradients, cellular structure and thermal conductivity of the foams were characterized. The density and amount of CBA used were found to have a significant effect on the cellular structure both at the mesoscale (density gradients) and at the microscale (different cell sizes and cell densities). In addition, the thermal conductivity of the samples is very sensitive to the local structure where the heat flux is located. The technique used to measure this property, the Transient Plane Source method (TPS), makes it possible to detect the presence of density gradients. A simple method for determining these gradients based on thermal conductivity data was developed.

The understanding of the interaction between graphene with chitinous polymers remains a challenge. Herein, we propose the extraction of partially deacetylated chitin from Litopeneaus Vannamei shrimp’s waste (CH30) to produce a nanocomposite film with graphene and compared it with a high deacetylated chitosan (CH75). Structural characterization was carried out by X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), Raman, Scanning Electron Microscopy (SEM), and Thermogravimetric Analysis (TGA-DTG). Our findings reveal the successful functionalization of graphene with chitin to form a nanocomposite film. These findings shed light on the interaction between graphene and chitinous polymers in regards to give an additional value to shrimp shell waste.

Nanofiller reinforcing agents can significantly improve the strength and modulus of polymer foams. But these improvements are often accompanied by changes in foam density (or equivalently the expansion ratio or void volume). The efficacy of nanofillers as reinforcing agents can only be judged once the density differences are accounted for. We review the literature and show that representing the data on Ashby charts of modulus against foam density is an effective way of evaluating whether nanofillers have a significant reinforcing effect or not. The literature suggests that strength and improvements due to nanofiller – after accounting for foam density changes – are typically modest for thermoplastic foams. However, major improvements are possible for reactively generated foams, especially flexible polyurethane foams.

Polymers such as poly(lactic acid) (PLA), which have poor melt strength, are difficult to foam due to severe cell coalescence during foaming. We show that addition of a few percent of polytetrafluoroethylene (PTFE) particles can stabilize PLA foams against bubble coalescence and collapse. The particles and a chemical blowing agent, were dispersed into the PLA by extrusion, and then foamed by heating. The PTFE‐containing foams remained stable even when the foams were held under molten conditions for extended periods. Foam stability is attributed to an interfacial mechanism: due to their low surface energy, the PTFE particles adsorb on the inner surface of the foam bubbles at a high surface coverage, and endow the bubbles with an interfacial “shell” that prevents coalescence. This mechanism resembles the particle‐stabilization of Pickering emulsions in oil/water systems. Particle adsorption at the interface is a necessary condition for using this approach, and hence this approach is most likely to be successful if the particles have a low surface energy and the polymer has a high surface tension. The approach of using interfacially adsorbed particles can be broadly generalized, and offers the opportunity of foaming various polymers with low melt strength, or for expanding the processing window within which foaming can be conducted. POLYM. ENG. SCI., 56:9–17, 2016. © 2015 Society of Plastics Engineers

Existe una cantidad significativa de materiales porosos en la naturaleza, como es el caso de la madera, huesos, tallos y hojas. Las celdas o poros en estos materiales poseen multiples funciones, como aligerar el peso de la estructura, permitir que exista permeabilidad a los fluidos, lograr aislamiento termico, mejorar en masticabilidad, etc. Un ejemplo muy interesante es el hueso. Superficialmente los huesos tienen un aspecto solido, pero al explorarlos detenidamente la mayoria de los huesos presentan una estructura mas elaborada compuesta de una corteza mas densa y compacta, envolviendo un nucleo poroso de celdas o tejido esponjoso. Los materiales sinteticos celulares, tanto polimericos como metalicos, se utilizan principalmente como materiales funcionales para aislamientos termico, aislamiento acustico, absorcion de impactos y vibraciones, etc. No obstante, la presente tesis se centra en materiales sinteticos celulares cuyo uso seria principalmente estructural. Esta funcion estructural para los materiales celulares, muy comun en la naturaleza, no es tan habitual en nuestros disenos. De hecho estos disenos usuales utilizan materiales macizos con fines estructurales, a pesar de que hay una clara tendencia a cambiar este concepto clasico. El objetivo principal de esta investigacion es mejorar la rigidez y la resistencia de dos clases de materiales celulares. El primero es cobre poroso tipo lotus (como ejemplo de un metal). Para estos materiales el estudio se centro en mejorar la microestructura y propiedades de la matriz metalica, manteniendo al mismo tiempo la estructura celular. El segundo es un material celular, basado en polietileno de baja densidad (como ejemplo de un termoplastico). En este caso particular el principal proposito es producir un material sin reticulacion con una estructura celular mejorada mediante la reduccion del tamano de celda y una mayor homogeneidad en na distribucion de tamanos celulares. Para poder obtener estos materiales se ha desarrollado un nuevo proceso de espumado. El presente trabajo se ha desarrollado en dos localizaciones distintas: El metal poroso estructural se desarrollo y caracterizo en el Nakajima Laboratory perteneciente al ISIR (Institute for Scientific and Industrial Research), de la Universidad de Osaka, en Japon, bajo la supervision del profesor H. Nakajima. El polimero celular basado en el polietileno se desarrollo y caracterizo en el CellMat Laboratory en el Departamento de Fisica de la Materia Condensada de la Universidad de Valladolid, en Espana, bajo la supervision del profesor M.A. Rodriguez-Perez. La tesis se ha dividido en 9 capitulos conteniendo la siguiente informacion: El Capitulo 2 introducen varios conceptos necesarios para entender el trabajo. Algunos de ellos tratan sobre sobre las propiedades mecanicas de los metales, los polimeros y sus caracteristicas estructurales. Ademas se introducen los conceptos basicos asociados a las propiedades mecanicas de los materiales celulares y como pueden utilizarse para aligerar el peso total de una determinada estructura manteniendo el comportamiento mecanico. Los Capitulos 3 a 5 describen la investigacion llevada a cabo sobre metales porosos tipo lotus (metales lotus). La introduccion (capitulo 3) muestra entre otros aspectos como los metales porosos tipo lotus consiguen una dependencia lineal entre las propiedades mecanicas y la porosidad[11]. Los metales para uso estructural se fabrican mediante un proceso de forjado. El forjado puede producir una pieza mas resistente que una de la misma forma conseguida mediante relleno de un molde con metal fundido. Para conseguir su estructura porosa, los metales porosos tipo lotus fueron fabricado mediante fundicion del metal, de modo que la matriz metalica del material resultante es mas ductil que la que se consigue en los metales forjados. Esta investigacion se centro en mejorar las propiedades mecanicas de la matriz metalica para que se asemejen a aquellas de los metales forjados, sin deteriorar la estructura celular del cobre poroso tipo lotus.??El Capitulo 4 es un intento de hacer una mejora parcial de la estructura metalica mediante un proceso de tratamiento en la superficie de las piezas basada en la aplicacion de deformacion mediante un cepillo de alambre (wire-brush technique). El Capitulo 5 introduce una tecnica disenada para la forja de metales, ECAE (Equal Channel Angular Extrusion), la cual fue modificada para poder aplicarla a metales porosos sin alterar la estructura porosa. Los Capitulos 6, 7 y 8 describen la investigacion llevada a cabo en espumas polimericas en CellMat. Las espumas polimericas convencionales no mantienen la relacion lineal entre propiedades mecanicas y porosidad. Nam Suh [12] propuso la idea de desarrollar plasticos microcelulares los cuales mantendrian las propiedades mecanicas mas proximas a las de los polimeros solidos. Habitualmente estas espumas polimericas microcelulares han sido producidas con polimeros amorfos [13]. En nuestro caso el objetivo de la investigacion era obtener una espuma microcelular a partir de un polimero semicristalino como el polietileno, siguiendo la idea principal de mejorar sus propiedades mecanicas. El Capitulo 6 es una introduccion a los procesos industriales utilizados en la actualidad para producir espumas polimericas; y espumas microcelulares tras las cuales entramos en profundidad en el nuevo proceso de moldeado desarrollado para producir las espumas de esta tesis. Esta tecnica permite producir espumas estructurales no reticuladas con una densidad baja. El Capitulo 7 se centra en el analisis de las propiedades mecanicas de las espumas resultantes. El Capitulo 8 estudia en profundidad la estructura celular resultante, la conductividad termica y los efectos globales de usar mas cantidad de agente espumante. El Capitulo 9 hace un resumen de las conclusiones obtenidas durante esta tesis.

Lotus-type porous copper with directional cylindrical pores was fabricated by unidirectional solidification in a pressurized hydrogen atmosphere. Improvements in microstructure and mechanical properties of the lotus-type porous copper by equal-channel angular extrusion (ECAE) were investigated using a die with a channel angle of 150°. The porosity decreases with increasing pass number, and decreases from 46% to 30% by four passes. It means that pore closure in the ECAE process is not significant. Both the specific compressive yield strength σ0.2%* and the Vickers hardness HV of the lotus copper increase with increasing pass number of ECAE. The ECAE processed porous copper shows improved σ0.2%* and HV comparable to those of the extruded nonporous copper. It is suggested that ECAE is a promising method to strengthen porous metals without significant pore closure.

Lotus-type porous copper with long cylindrical pores aligned in one direction was processed by the Equal-Channel Angular Extrusion (ECAE) with a die with a channel angle of 150 degrees. The effects of the pass route and the pass number on the compressive yield strength and the Vickers hardness were investigated. Three kinds of pass routes were adopted; route A (no rotational change of sample rod between passes), route B-C (axial rotation of 90 degrees of the sample rod between passes), and route C (axial rotation of 180 degrees of the sample rod between passes). Although the porosity slightly decreased after every pass, the porous structure remained after the process. The Vickers hardness and the compressive yield strength of lotus copper increased with an increase in the pass number till 4 passes. The maximum specific yield strength was the same level as that of non-porous copper processed by the ECAE by accumulation of strain. The specific yield strength decreased due to the Bauschinger effect via route C on the 5th pass. The cracking occurred in the sample rod via route A and B-C on the 5th pass. Inhomogeneous hardness distribution was found in lotus copper processed by the ECAE. The Vickers hardness in the outer region was higher than that in the central region.

This paper presents the mechanical properties of a collection of high density Polyethylene based foams. The produced materials are characterised by a reduction in density up to 60%, an excellent surface quality, cell sizes in the microcellular range (around 50 microns) and a multi-structured cellular structure (cranial structure) with dense skin and foamed core. The mechanical properties of these materials showed linear relationships between Young's modulus and density for densities above 0.7 g/cm(3). In addition, variations in the cell size did not influence the elastic properties.

The surface of lotus-type porous copper plates that had cylindrical open pores in the thickness direction (porosity 50.4%, average pore diameter 144.4 μm) were processed by wire-brushing. Open pores on the surface of the lotus copper are closed by a newly formed nonporous thin layer. Electron backscatter diffraction patterns of the processed plate cross section show that the deformed surface consists of ultra-fine grains and that a nonporous layer was formed on the deformation of the surface layer. The Vickers hardness of the wire-brushed lotus copper is higher than that of the wire-brushed non-porous copper. The Vickers hardness increases with the increase in the rate of revolution of the wire–brush due to grain refinement. The increment of the ultimate tensile strength of lotus copper by wire-brushing is larger than of non-porous copper. The increment of ultimate tensile strength of the lotus copper reaches maximum when the newly deformed layer closes all the pores on the surface. These results show that wire-brushing is an effective process for the improvement of mechanical properties for lotus metals.

Deformation behavior of lotus-type porous copper with long cylindrical pores aligned in one direction through equal-channel angular extrusion (ECAE) process was investigated using a die with channel angle of 150º. Although the density slightly increased after every pass, the porous structure remains in the process. The Vickers hardness and the compressive yield strength of lotus copper increased through the ECAE process. The compressive yield strength after 3 passes increased up to 10 times larger than that before processing. The deformation of lotus copper takes place by buckling and the shearing of the cell walls. The increase in hardness is considered to be caused by work hardening.

This article presents the compressive mechanical response at low strains for a collection of polyethylene foams with high densities and cell sizes in the microcellular range. The materials under study had a relative density between 0.27 and 0.92, a homogeneous and multi-structured cellular structure with a dense skin and a foamed core. The Young's modulus and collapse stress were reduced when density did, the modulus following a linear trend and the collapse stress a quadratic tendency. For relative densities higher than 0.7, the materials showed Young's modulus slightly above the limit given by a potential law with exponent equal to one. In addition, it has been proved that variations in the cell size did not influence the elastic properties. The advantages of using these materials for flat structural panels have been analyzed. A reduction of the weight of flat panels loaded in bending of up to 35% can be reached by using these foams in spite of the solid sheet from which the foam was produced.

This paper presents the production method and the compressive mechanical response at low strains for a collection of polyethylene foams with high densities and cell sizes in the microcellular range. The materials were produced using an improved compression moulding technique that allows and independent control of density and cell size. The materials had a relative density between 0.27 and 0.92, an homogeneous and multi-structured cellular structure with dense skin and foamed core and cell sizes in the range 30 to 100 microns. The Young's modulus decreased with density. For relative densities higher than 0.7, the reduced Young's modulus of the foams was higher than that of the solid. In addition, it has been proved that variations in the cell size at constant density did not influence the Young's modulus. The advantages of using these materials for the production of plastic pipes have been analysed. In comparison with a solid pipe a reduction of the weight of foamed pipes loaded in compression of up to 40% can be reached.

The coexistence curves of the liquid-liquid equilibria (LLE) for systems of dimethylformamide (DMF) with hexane, heptane. octane, or nonane and of dimethylacetamide (DMA) with heptane have been determined visually. All the curves show an upper critical solution temperature (UCST) and have a rather horizontal top. The measured LLE curves for DMF mixtures show that their symmetry depends on the size of the alkane. For a given alkane, the UCST is higher for systems with DMF. This reveals that dipole-dipole interactions between amide molecules are stronger in such solutions. The DISQUAC model represents fairly well the LLE curves.