Nanocomposite Foams with Balanced Mechanical Properties and Energy Return from EVA and CNT for the Midsole of Sports Footwear Application

Polymer foam that provides good support with high energy return (low energy loss) is desirable for sport footwear to improve running performance. Ethylene-vinyl acetate copolymer (EVA) foam is commonly used in the midsole of running shoes. However, EVA foam exhibits low mechanical properties. Conventional mineral fillers are usually employed to improve EVA's mechanical performance, but the energy return is sacrificed. Here, we produced nanocomposite foams from EVA and multi-walled carbon nanotubes (CNT) using a chemical foaming process. Two kinds of CNT derived from the upcycling of commodity plastics were prepared through a catalytic chemical vapor deposition process and used as reinforcing and nucleating agents. Our results show that EVA foam incorporated with oxygenated CNT (O-CNT) demonstrated a more pronounced improvement of physical, mechanical, and dynamic impact response properties than acid-purified CNT (A-CNT). When CNT with weight percentage as low as 0.5 wt% was added to the nanocomposites, the physical properties, abrasion resistance, compressive strength, dynamic stiffness, and rebound performance of the EVA foams were improved significantly. Unlike the conventional EVA formulation filled with talc mineral fillers, the incorporation of CNT does not compromise the energy return of the EVA foam. From the long-cycle dynamic fatigue test, the CNT/EVA foam displays greater properties retention as compared to the talc/EVA foam. This work demonstrates a good balanced of mechanical-energy return properties of EVA nanocomposite foam with very low CNT content, which presents promising opportunities for lightweight-high rebound midsoles for running shoes.

A study on fabrication of nanocomposite polyethylene foam through extrusion foaming procedure

Foaming a polymer not only turns it into a lightweight material but also gives some special properties to it. However, the most important issue is controlling the foaming process to achieve a desirable structure with high cell density and low relative density. In the present study, the extrusion foaming process of polyethylene was studied through stepwise amendments. An innovative extrusion system was designed and implemented to produce extrusion foams under different material and process conditions using N2 as blowing agent. In the first step, the final cooling condition was investigated. The air-cooling condition led to a higher cell density/lower cell size compared to the water-cooling condition although a higher relative density was obtained. In the second step, the effects of the addition of talc and the synergetic effect of talc/nanoclay at different contents were investigated in detail. The hybrid of talc/nanoclay had a noticeably improving effect on the cellular structure. In the third step, the effects of processing parameters including the die temperature and screw speed were studied on the foam properties. Finally, up to 49.4% decrease in the relative density of samples was observed, also cell densities up to 2.5 × 104 cell/cm3 and cell sizes as small as 280 µm were achieved.

Polyolefinic nanocomposite foams: Review of microstructure-property relationships, applications, and processing considerations

In this review, we survey the state of the art on polymeric foams incorporating nano-scale fillers. Particular focus of the review is on foams from polyolefinic nanocomposite formulations incorporating a wide variety of fillers. The nano-scale additives can influence the foam structure and properties in two ways: Firstly, they can act as composite reinforcement to enhance the mechanical properties and functionality of the matrix polymer; and secondly, they can act as foaming-processing aids through modification of the rheological, thermal and crystallization properties of the matrix as well as serving as heterogeneous nucleation sites. Through a combination of these influences, and using advanced processing techniques it is possible to achieve nanocomposite foams that have higher cell density, and more uniform cell size or controlled cell-size distribution. Such controlled foam morphologies, in turn, can yield better specific mechanical properties resulting in more effective light-weighting solutions. Further, the nano-scale additives can impart additional desired functionality resulting in multi-functional foams. In this article, we provide an overview of the mechanical, thermal and a few other relevant functional properties – such as piezoelectric sensitivity, acoustics, and filtration efficiency – of foams prepared using nanocomposite formulations, along with the processing considerations for achieving high quality foams using such materials.

Increasing cell density/decreasing cell size to produce microcellular and nanocellular thermoplastic foams: A review

Nowadays, polymeric foams have attracted particular attention in scientific and industrial societies due to their unique properties, such as high strength to weight ratio, excellent thermal and sound insulation, and low cost. Researchers have shown that the extraordinary properties of polymeric foams such as superior thermal insulation, can be achieved by increasing the cell density/decreasing the cell size. In this regard, firstly, the most important foaming processes, i.e. batch, extrusion, and injection molding are studied in the present research. Then, cell nucleation stage as the most crucial phenomenon for achieving high cell density/small cell size is investigated in detail. In the next step, the most important researches in the field of polymeric foams are introduced in which the largest cell densities/smallest cell sizes have been achieved. The investigations show that the most remarkable results (highest cell densities/smallest cell sizes) belong to the batch process. Also, the use of nucleating agents, increasing the solubility of blowing agent into the polymer, and the use of nanoparticles are the most efficient solutions to achieve microcellular and nanocellular structures.

Rheological, Mechanical and Morphological Characterization of Fillers in the Nautical Field: The Role of Dispersing Agents on Composite Materials

Coatings have a fundamental role in covering the external surface of yachts by acting both as protective and aesthetic layers. In particular, fillers represent the essential layer from the point of view of mechanical properties and consist of a polymeric matrix, different extenders and additives, and dispersing agents, with the latter having the role to provide good extender-matrix compatibility. In the present work, the effects of dispersing agents with an ionic or steric action on the interactions between hollow glass microspheres and an epoxy-polyamide resin are evaluated. Un-crosslinked filler materials are studied via rheological tests, whereas the mechanical and morphological properties of the crosslinked samples are assessed. The results clearly indicate that steric dispersing agents provide a much greater compatibility effect compared to ionic ones, owing to their steric hindrance capability, thus leading to better-performing filler materials with a less-marked Payne effect, which is here proved to be an efficient tool to provide information concerning the extent of component interactions in nautical fillers. To the best of our knowledge, this work represents the first attempt to deeply understand the role of dispersing agents, which are until now empirically used in the preparation of fillers.

Tuning of polyurethane foam mechanical and thermal properties using ball-milled cellulose

Cystalline-Cc and ultra-milled Amorphous-Ca cellulose were used as reactive filler to tune the performances of composite polyurethane-cellulose-foams, PUC. The effect of Cc and Ca on chemo-physical and mechanical properties of PUC was analysed through FTIR, morphological analysis, thermal conductivity and compression measurements. FTIR results show that, both Cc and Ca react with isocyanate through the OH functional groups contributing to the formation of a tough cellulose-polyurethane network. Morphological observations show that the addition of both Cc and Ca induces a decrease of average cell-size compared to the pristine-PU, thus confirming that they act as nucleating agent. In addition, the better dispersion of the Ca in the polyol, with respect to Cc induces, a finer cell leading to a reduction of the thermal conductivity around 33 % (for the composite loaded with 20 %wt-Ca) with respect to pristine-PU. Finally, the addition of Ca highly reactive modifies the mechanical behaviour from rigid-brittle to semi-rigid.

Microcellular foaming by using subcritical CO2 of crosslinked and non-crosslinked LDPE/clay nanocomposites

The semicrystalline character of low density polyethylene adds severe difficulties to its foamability by a batch process in which the gas is dissolved into the polymer matrix under subcritical conditions. To improve the low density polyethylene foamability, two strategies have been used: the addition of nanoclays and a partial crosslinking of the polymer matrix. On the one hand, the use of nanoparticles is suggested because they act as heterogeneous nucleating sites reducing the cell size and increasing the cell density. On the other hand, crosslinking is also adopted as a solution because both the crystallinity (and hence, the gas solubility and diffusivity) and the extensional rheological properties of the polymer matrix are highly influenced by the crosslinking degree achieved. Results indicate that despite the fact that the presence of nanoclays deteriorates the rheological behaviour of the nanocomposites and, hence, the later foaming behaviour, the use of partially crosslinked polymer matrices allows achieving high expansion ratios (around 7.5) as well as enhanced cellular structures with cell sizes of approximately 15 µm.

Improving the extensional rheological properties and foamability of high-density polyethylene by means of chemical crosslinking

Obtaining high-density polyethylene-based microcellular foams is a topic of interest due to the synergistic properties that can be obtained by the fact of achieving a microcellular structure using a polymer with a high number of interesting properties. However, due to the high crystallinity of this polymer, the production of low-density microcellular foams, by a physical foaming process, is not a simple task. In this work, the proposed solution to produce these materials is based on using crosslinked high-density polyethylenes. By crosslinking the polymer matrix, it is possible to increase the amount of gas available for foaming and also to improve the extensional rheological properties. In addition, the foaming time and the foaming temperature have also been modified with the aim of analyzing and understanding the mechanisms taking place during the foaming process to finally obtain cellular materials with low densities and improved cellular structures. The results indicate that cellular materials with relative densities of 0.37 and with cell sizes of approximately 2 µm can be produced from crosslinked high-density polyethylene using the appropriate crosslinking degree and foaming parameters.

Low Density Non-crosslinked Closed/Open Cell Polypropylene Foams with High Mechanical Properties

Low density polypropylene based foams with different cellular structures have been produced by the improved compression molding route using a high melt strength polypropylene as polymer matrix. In addition, different types of nanoparticles have been introduced in the formulation (multi-wall carbon nanotubes, organomodified nanoclays and natural nanoclays) to modify the structure and properties. The results have showed a clear correlation between the open cell content of the foams and the mechanical properties in compression. In the unfilled polypropylene high specific mechanical properties are only achievable with low values of open cell content. In comparison, for an equal value of the interconnectivity between cells, the samples containing nanoclays present much higher specific properties. This result is attributed to the reinforcement of these nanoparticles in the solid matrix, due to an improved exfoliation during the foaming process and the presence of a bimodal cellular structure. The produced foams have interesting properties with stiffness similar to those of commercial polymer foams used for the core of sandwich panels.

The simultaneous effect of nucleating and blowing agents on the cellular structure of polypropylene foamed via the extrusion process

The current work presents the preparation of polypropylene (PP) foams by the extrusion process, focusing on the influence of the foaming agent and nucleating agent on the microstructure of the foams. Sodium bicarbonate alone and also its mixture with citric acid were used as the chemical blowing agents. Expanded graphite nanoparticle and talc were also used as the nucleating agents. Great differences were found in terms of the foam structure depending on the type of nucleating and blowing agents. Using expanded graphite nucleating agent instead of talc resulted in foams with higher cell densities and more uniform cellular structures. Moreover, the foams including the mixed blowing agents exhibited higher cell densities and upper expansion ratio.

Microcellular injection molding of polypropylene and glass fiber composites with supercritical nitrogen

Microcellular injection molding of polypropylene and glass fiber composites (PP-1684/GF-950) was performed using supercritical nitrogen as the physical blowing agent. Based on design of experiment matrices, the influences of glass fiber content and operating conditions on cell structure, glass fiber orientation and mechanical properties of molded samples were studied systematically. The results showed the cell morphology and glass fiber orientation of foaming parts were definitely influenced by the cooling and shear effects. The mechanical properties of foamed polypropylene–glass fiber composites could be effectively enhanced by improving the cell morphology, dispersion state and orientation of the glass fiber at optimal weight percentage [Formula: see text]. And the optimal conditions for injection molding were obtained by analyzing the signal-to-noise ratio analysis of the mechanical properties of the molded samples, which were a shot size of 36 mm, a supercritical N2 weight percentage of 0.4%, an injection speed of 60%, a melt temperature of 190℃ and a mold temperature of 70℃. The molded specimens of polypropylene–glass fiber composites, produced under those optimal conditions, exhibited very uniform fiber dispersion and microcellular structures with an average cell size less than 30 µm. And the mechanical properties normalized by weight ratio of the microcellular samples were increased significantly, especially the impact strength.

Twin screw extrusion with Expancel foaming agent

Foam extrusion of polypropylene with a foaming agent, Expancel 950 MB 80 and 950 MB 120, was performed. The process was performed using a co-rotating twin screw extruder, with an 18 mm screw diameter and 24 L/D ratio. Dependences of polymer mass flow rate, extrudate foam rate, and polymer pressure on screw speed and foaming agent amount were determined and relevant conclusions were drawn.