jrups
Editor’s Choice : Mechanism of Micro-Crack Propagation in Semicrystalline Polymers
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Abstract: The development and propagation of cracks is the principle reason for premature mechanical failure of polymeric materials. The well known and widely accepted fracture theories, namely the Griffith fracture theory and the Irwin model, both assume that fracture takes place through the presence of preexisting cracks in the polymer. These minor preexisting cracks, or micro-cracks, are practically present in most polymeric samples. The Griffith approach assumes that for any particular material, the fracture stress is controlled by the size of the flaws present in the structure. The control and minimization of micro-crack size during polymer processing requires an understanding of the inherent micro-crack propagation mechanism. The present research reveals a mechanism of internal stress-induced micro-crack propagation in semicrystalline polymers and describes the effect of the intricate crystalline morphological interactions on the extent and direction of intra-spherulite and inter-spherulite micro-crack propagation. In conclusion, a method for minimizing inter-spherulite micro-crack propagation is presented in this article. Keywords: Crystalline morphology, fracture, micro-cracks, internal stress, high -density polyethylene (HDPE). Download Full Article |
Editor’s Choice : Modification of Nanocrystalline Cellulose for Bioactive Loaded Films
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Abstract: Despite the use of petrochemical derived packaging, many problems such as browning and food spoilage still happen in food after harvesting. There is an increasing consumers concern for food shelf life to be extended as much as possible along with a big interest in green and bioactive materials, that could be used in direct contact with aliments. In order to reach public demand, biopolymers coming from natural sources such as plants or animals have been used to replace synthetic materials. Even though natural polymers are biodegradable, they do not reach regulations required with respect to mechanical properties in commercial applications. However, the mechanical properties can be improved when reinforced with nanoparticles. Several reinforcing nanoparticulessuch as clays, silica or silver have been used for industrial applications, but cellulose nanocrystals (CNCs) are a better choice for food industry due to their biodegradable and biocompatible nature as well as their outstanding potential in improving mechanical and barrier properties of nanocomposites. CNCs consist of anhydroglucopyranose units (AGU) linked together and several functional hydroxyl groups found on its surface. Modifications of the CNC surface chemistry can give to cellulose new functionalities that open the way to the development of new bioactive reinforcement in food packaging. The present review will be focused on covalent and non covalent modifications that can be achieved on surface CNC with the aim of adding functionalities to be applied for food industry. Keywords: Food packaging, cellulose nanocrystals, CNC, acetylation, polymer grafting, TEMPO oxidation, layer-by-layer, cationic surfactants, radiation-induced polymer grafting. Download Full Article |
Editor’s Choice : Study on Flexural Strength and Flexural Failure Modes of Carbon Fiber/Epoxy Resin Composites
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Abstract: The flexural failure modes and flexural strength of unidirectional carbon fiber/epoxy (CF/epoxy) composites was theoretically analyzed and calculated. The hypothesis that the maximal flexural strength of unidirectional CF/epoxy composites occurred in outer sheet layer was brought forward. Load-displacement curve of unidirectional CF/epoxy composites also demonstrates that it is correct, which is consistent with the assume. Unidirectional CF/epoxy composites were fabricated with winding and compression molding, and three different kinds of fiber packing modes were proposed and the effects of these modes on flexural prosperities of composites had also been analyzed respectively. Three failure modes, namely fracture of fiber, fracture of epoxy resin and interfacial delamination between fiber and matrix, were analyzed by scanning electron microscopy (SEM). The load-displacement curve of unidirectional CF/epoxy composites indicates the plastic deformation is increased with increasing phenolic resin. Keywords: Bending strength, carbon fiber, packing modes, failure mode, composite.Download Full Article |
Editor’s Choice : Structurally Functionalized Polyurethane Foam for Elimination of Lead Ions from Drinking Water
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Abstract: Polyurethane foams functionalized with Sulfonic acid groups have been found to be strong cation exchangers. This novel property of the foam was used to exchange lead (Pb2+) ions from aqueous solutions. Polyurethane foam synthesis is based on addition polymerization of the highly reactive isocyanate (-NCO) groups of an isocyanate with the hydroxyl (–OH) groups of a polyol to form the urethane species. Toluene-2,4-2,6- diisocyanate was reacted with Polypropylene glycol 1200 in 2:1 molar ratio to form a linear pre-polymer. The linear pre-polymer was further polymerized using a chain extender, N, N-bis (2-hydorxyethyl)-2-aminoethane-sulfonic acid (BES). BES also acts as a functional group to exchange Pb2+ ions. A set of experiments were designed to study various process parameters. The functionalized polyurethane foam was characterized by Fourier transform infrared spectroscopy, gel permeation chromatography, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The Pb2+ ion exchange capacity was determined using an Inductively Coupled Plasma Mass Spectrometer. The maximum Pb2+ ion exchange capacity of the foam was found to be 47 parts per billion per gram (ppb/g) from a 100 parts per billion (ppb) Pb2+ solution over a period of 60 minutes. A multistage batch filtration process increased the Pb2+ ion removal to 50-54 ppb/3g of foam over a period of 90 minutes. Keywords: Adsorption, Functionalized, Ion exchange, Polyurethane foams, Sulfonic.Download Full Article |
Editor's Choice : Optimization of Process Parameters for Generation of Nanocellular Polymer Foams
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Abstract: High melt strength polypropylene nanocomposites, PPNC/Cloisite 20A (clay) with exfoliated and intercalated morphologies were prepared and subsequently foamed in a batch setup under different foaming conditions. The foaming parameters were varied to relate the foam cell structure to these parameters and determine the efficiency of clay in producing fine cell foams. A Box Benkhen design approach was used initially to determine the effect of processing parameters on foam cell morphology and also to perform optimization studies. The optimization process helped in identifying the range of operating conditions needed to minimize foam cell sizes. Saturation pressure and temperature and foaming time and temperature are the four processing variables used in these studies. Nanocellular foam cells were effectively generated for the first time in Polypropylene nanocomposites. Keywords: Nanocellular, DOE, optimization, batch foaming.Download Full Article |