Rubén González-Núñez

Effect of Mold Temperature on Morphology and Mechanical Properties of Injection Molded HDPE Structural Foams

In this study, HDPE structural foams are produced by injection molding under different mold temperatures to study the effect of this variable on average cell dimension, cell density, and skin thickness ratio. Samples are also produced by setting independently the temperature of the fixed and moving plate of the mold to detect the sensitivity of foam structure to a temperature gradient in processing. The resulting foams are also characterized in terms of mechanical properties including impact and flexural tests. It has been found that for homogeneous mold temperatures, symmetrical skin thicknesses are obtained, which increase with decreasing mold temperature. On the other hand, by keeping one mold face at a constant temperature and varying the second one, asymmetric skin thicknesses are obtained. The degree of asymmetry is found to increase as the temperature difference between both molds increased. Furthermore, decreasing mold temperature produces a small increase in average cell sizes and reduced cell density. In general, both impact strength and flexural moduli of the structural foams increase with increasing skin thickness. For the particular case of asymmetric foams, the flexural moduli are slightly higher when the load is applied on the thicker skin; while much higher impact strength is obtained when the falling weight strikes the samples on the face having the smaller skin thickness.

Morphology and Mechanical Properties of Foamed Polyethylene-Polypropylene Blends

Blends of high-density polyethylene and polypropylene are foamed by means of extrusion using azodicarbonamide as a chemical blowing agent to study the effect of blending on the morphological and mechanical properties. At 0.5 wt% of blowing agent, optimum foam density is found to be around 417 kg/m3 for each blend composition, but the average cell size ranges between 130 and 301 mm depending on the blend composition. It is believed that the dispersed polymer phase acts as nucleating sites producing foams with smaller cell sizes. Owing to the incompatibility between both the polymers, the best tensile and impact properties are obtained for neat polymers. Simple semi-empirical models are proposed to predict the tensile and impact properties of the foams.

Morphology of Extruded PP/HDPE Foam Blends

A complete three-dimensional morphological analysis is performed to determine the size and deformation of both foam cells and dispersed phase particles in the extrusion foaming of HDPE/PP blends. Each dimension is carefully measured to determine interaction between blending and foaming on the final foam morphology. In this study, blends of 0, 10, 30, 50, 70, 90, and 100% PP in HDPE are foamed using azodicarbonamide (ACA). The effect of using a compatibilizing agent (Kraton D 1102) is also included.

Deformation of drops in extensional viscoelastic flow

The deformation of nylon drops in polyethylene, with and without an interfacial agent, in an extensional flow has been studied. The presence of an interfacial agent reduces the size of the dispersed phase, and the deformation of the drop is reduced. An analysis is given, which accurately predicts the deformation for all values of the capillary number considered. The predicted and observed shapes are, however, only in agreement at low values of capillary number. Possible causes for this discrepancy are discussed.

LDPE/agave fibre composites: Effect of coupling agent and weld line on mechanical and morphological properties

Natural fibre composites based on Agave fibres (Agave tequilana) and low-density polyethylene (LDPE) were produced by injection moulding. Maleic anhydride grafted polyethylene (MAPE) was also added to study the effect of fibre content, coupling agent addition, and weld line presence on mechanical properties. It was found that tensile and flexural moduli increased with fibre concentration while they decreased for impact strength. In all cases, MAPE addition enhanced the effect up to an optimum content between 2 and 5% on a fibre weight basis. The presence of a weld line substantially reduced tensile properties and this effect could be interpreted in terms of molecular entanglement and orientation changes.

Asymmetric microcellular composites: Morphological properties

Asymmetric microcellular composites were prepared by injection molding to study the effects of temperature gradient inside the mold (0 to 60℃) as well as blowing agent (0 to 1%) and natural fibers (0 to 30%) contents. High-density polyethylene, flax fiber, and azodicarbonamide were used as the matrix, reinforcement, and chemical blowing agent, respectively. From the samples produced, a complete morphological characterization was performed. As expected, cell size, cell density, and skin and core thicknesses were affected by blowing agent and natural fiber contents and mold temperatures. It was found that a better microcellular asymmetric structure was obtained with higher fiber and blowing agent contents and higher average mold temperature. From the data obtained, a simple mathematical model was used to fit the relative density of asymmetric foams to include skin, core, and transition zone thicknesses.

Bacterial immobilization by adhesion onto agave-fiber/polymer foamed composites

Adhesion of Pseudomonas putida F1 onto agave-fiber/recycled-polyethylene foamed composites was studied under different controlled conditions. The adhesion process was analyzed in batch experiments controlling factors such as pH, contact time, temperature, initial biomass concentration and ionic strength; and was verified by scanning electron microscopy (SEM). The number of adhered bacteria after the experimental time was determined by difference between concentration of suspended cells in NaCl solution contained in two different Erlenmeyer flasks, one of the flasks with composite pellets and the other one without them. The concentration of cells in each flask was obtained using the serial dilution technique. Experimental data analysis showed that adsorption follows first-order kinetics. And it was further corroborated to be an irreversible process. For the first time, an equation is proposed here to predict the correlation between adhered bacteria and aqueous pH. In addition to the obvious reuse of waste material, these results suggested that agave-fiber/polymer foamed composites could be used as support for bacterial immobilization to be applied, among others in environmental processes such as bioremediation and biofiltration of gases with almost limitless possibilities.

Effect of fiber content and surface treatment on the mechanical properties of natural fiber composites produced by rotomolding

Abstract In this study, natural fibers (agave, coir, and pine) were surface treated with maleated polyethylene (MAPE) with two main objectives: (1) to improve the mechanical properties of natural fiber composites produced by rotational molding and (2) to increase the fiber content in the composite. The rotomolded composites were produced at 0, 10, 20, 30, and 40% wt. of fiber contents (treated or untreated) and characterized in terms of morphology and mechanical properties (hardness, impact, tension, and flexion). The results showed that MAPE surface treatment was more successful for agave and coir than for pine fibers due to their respective chemical composition. In general, surface treatment led to better fiber distribution and a more uniform composite morphology allowing the possibility to use higher fiber contents in rotational molding. At low fiber contents (10 and 20% wt.), the mechanical properties were improved using treated fiber composites (TFC) compared to the neat polymer and untreated fiber composites (UFC). Although the mechanical properties of TFC decreased at high fiber contents (30 and 40% wt.), they were substantially higher (about 160, 400, and 100% for impact, tensile, and flexural properties, respectively) than for UFC.

Ionomer Synthesis by Emulsion Polymerization of Styrene and Sodium Acrylate

The emulsion copolymerization of styrene and sodium acrylate is reported using either a water-soluble initiator (potassium persulfate, or KPS), or an oil-soluble one (2,2-azoisobutyronitrile (AIBN)).Reaction rates are fast with both KPS and AIBN. With KPS, conversions u90% are achieved in 50 min, with AIBN, conversions reach 85% in 100 min. Particle size, measured by quasielectric light scattering (QLS), in- creases with conversion. Particle size in final latices isC 70-80 nm. Copolymer forma- tion is confirmed by infrared (IR) spectroscopy, plasma emission spectroscopy (PES), and scanning electron microscopy (SEM). IR and PES indicate that mainly sodium acrylate reacts at the beginning of the reaction and then styrene is incorporated in the copolymer backbone. The copolymer produced with KPS contains more sodium acrylate than the one made with AIBN. These differences can be explained in terms of the reactivities and partitioning (local concentrations) of the monomers and of the type of initiator used. Thermomechanical analysis (TMA) of the copolymers reveals two transitions: one at C 1007C, which is due to the glass transition temperature (Tg )o f polystyrene blocky segments in the copolymer, and another one at higher temperatures, which is associated to the Tg of segments composed of alternated sodium acrylate and styrene units. The higher-temperature transition shifts to lower values as the reaction proceeds because these segments become richer in styrene. q 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 879-889, 1997

Preparation and characterization of HDPE/PA6 foam blends

In this work, preparation and characterization of HDPE/PA6 foam blends is presented. Using a chemical blowing agent based on azodicarbonamide, foams were produced via extrusion using different concentrations of the dispersed phase PA6. Due to very different thermal and rheological properties of the polymers, good cellular structures could only be obtained by a careful selection of the temperature profile and feeding strategy of the materials. Furthermore, the use of a coupling agent was also found necessary to stabilize the blend before foaming. The final foam morphology was controlled by post-extrusion conditions and the results are discussed in terms of deformation and dimensions of foam cells and dispersed phase particles. The tensile and impact properties of the resulting foams are also reported.