Conductive rubber-based composites were derived from EVA, EPDM, and 50/50 EVA/EPDM blend filled with carbon black and short carbon fiber (SCR The electromagnetic interference (EMI) shielding effectiveness and return loss were studied. The measurements of shielding effectiveness (SE) of the composites were carried out in two different frequency ranges: 100-2,000 MHz and 8-12 GHz (X-band). It has been observed that the shielding effectiveness of the composites is frequency dependent and increases with increasing frequency. The increasing of filler loading also enhances the SE of the composites. It is also found that 50/50 EVA/EPDM blend systems have higher shielding effectiveness than pure EVA and EPDM SCF-filled composites. The correlation between shielding effectiveness and bulk conductivity of various composites is also discussed. (C) 2001 John Wiley & Sons, Inc.
Using the biaxially oriented film process, polypropylene (PP)/ethylene-vinyl alcohol copolymer (EVOH) blends with an improved barrier property could be obtained by generating a laminar structure of the dispersed phase in the matrix phase. This laminar morphology, induced by biaxial orientation, was found to result in a significant increase in the oxygen barrier property of PP/EVOH (85/15) blends by about 10 times relative to the pure PP. In this study, compatibility in the PP/EVOH blend system was evaluated by investigating the influence of compatibilizer on the rheological, morphological, and mechanical properties of the blends. In addition, the effects of compatibilizer content, draw ratio, and draw temperature on the oxygen permeability and morphology of biaxially drawn blend films were also studied. It was revealed that an optimum amount of compatibilizer, maleic anhydride grafted PP, should be used to improve the barrier property of the PP/EVOH blends with a well-developed laminar structure. The draw ratio and draw temperature had a significant influence on the permeability of the blends. The blend films exhibited a more pronounced laminar structure when the blends were stretched biaxially under processing conditions of higher draw ratio and draw temperature, resulting in higher barrier properties. (C) 2001 John Wiley & Sons, Inc.
Computer simulation packages have had success in predicting filling behavior in extremely complicated geometries. However, most of the current numerical implementation is based on a hybrid finite-element/finite-difference solution with the middle-plane mode. This kind of model leads to some inconvenience in applications. Instead of using the middle-plane as the datum plane, the surfaces of the cavity are used as datum planes in this article. Additional boundary conditions in the gapwise direction are employed to keep the flows in the surfaces at the same section coordinative. Comparisons with experimental results obtained with an instrumented test mold and C-Mold results show good agreement in terms of pressure traces and melt-front advancement. It is demonstrated that the present formulation is well suited to handle cavities generated directly by current CAD systems. (C) 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 125-131, 2001.
Superabsorbent copolymers (SAPs) based on monomers acrylamide (AM), sodium acrylate (SA), and potassium acrylate (KA) were prepared by aqueous solution polymerization using ammonium persulfate (APS) as an initiator and N,N-methylene-bis-acrylamide (MBA) as a crosslinking agent. The copolymers were synthesized by varying, the monomer concentration, MBA concentration, and initiator concentration The experimental results show the SAPs have better absorbency both in water and NaCl solutions. The copolymers are characterized by LR spectroscopy. The water retention in the soil is enhanced using the SAPs. The SAPs are tested for the growth of cosmos bipinnata plants and the results are presented. SAPs can be used as water management materials for agricultural and horticultural purpose in desert and drought prone areas. (C) 2001 John Wiley & Sons, Inc. Adv Polym Techn 20:146-154, 2001.
Rotational molding of polyethylene foams has increasingly become an important process in industry because of its resultant thicker walls, low sound transfer, high stiffness, and good thermal insulation. This report assesses the rotomoldability of two-layer polyethylene foamed parts. The polymeric material used in this study was linear low-density polyethylene and the foaming material was an endothemic chemical blowing agent. Two different molding methods, by powder and by pellet, were used to mold the multilayer foamed parts. Rotational molding experiments were carried out in a laboratory scale uniaxial machine, capable of measuring internal mold temperature in the cycle. Characterization of molded part properties was performed after molding. Optical microscopy was also employed to determine the bubble distribution in foamed parts. The final goal of this study was to investigate how the blowing agent and processing conditions can influence the process of rotational molding and the final product quality. It was found that the rotational molding of two-layer polyethylene foams produced parts of better impact properties, as well as fine outside surfaces. In addition, rotational molding of foamed parts by pellets saves the cost of powder grinding, but is counteracted by uneven inner surfaces. (C) 2001 John Wiley & Sons, inc. Adv Polym Techn 20: 108-115, 2001.
It is well known that blade geometry significantly affects the performance and efficiency of partially filled internal mixers. Intermesh type blades with very long tips are regarded as effective alternatives to almond-shaped blades used in more traditional tangential rotor mixers. However, using long tip blades results in the formation of long and very narrow sections between the chamber wall and the blade in these mixers. These narrow sections may act as bottlenecks, disrupting flow and mixing of highly viscous compounds in the mixing of elastomers with fillers. Therefore, to avoid costly and time-consuming trial-and-error procedures, the design of long tip blades requires careful quantitative evaluation of their performance. In this article, a computer model, based on the finite element method for the simulation of mixing in twin-blade intermesh type mixers, is presented. This model represents transient nonisothermal, non-Newtonian flow and mixing of a polymer-filler compound in a free surface regime consisting of incompressible and compressible parts. Mixing dependent variations of compound viscosity are also taken into account. To incorporate all of the described realistic aspects of the mixing process in the geometrically complex domain of intermesh type mixers, the developed model is based on a novel and robust solution algorithm. (C) 2001 John Wiley & Sons, Inc. Adv Polym Techn 20: 132-145, 2001.
Distributive mixing was experimentally measured during polymer melt blending along the length of a co-rotating twin-screw extruder. A mixing limited interfacial reaction between two reactive polymer tracers was employed to gain direct evidence of the generation of interfacial area. Model reactions were performed to validate this novel experimental method. In particular, the conversion was a direct indicator of the interfacial area available for the reaction. Specially designed sampling ports were used to obtain polymer samples along the length of the extruder during its continuous operation. The mixing capabilities of conveying sections and kneading blocks were compared over a wide range of operating conditions. In conveying sections, distributive mixing was controlled by the fully filled fraction. The mixing in kneading blocks depended on the combination of the operating conditions and the stagger angle. (C) 2001 John Wiley & Sons, Inc.
Injection molding is one of the most important methods for the manufacture of plastic products; however, there are several unresolved problems that confound the overall success of this technique. Sinkmarks occurring on the surface of molded parts caused by inappropriate mold design and processing conditions is one problem. In this report, an L'18 orthogonal array design based on the Taguchi method was conducted to minimize the sinkmarks of injection-molded thermoplastic parts. The polymeric materials used were general-purpose polystyrene and low-density polyethylene. A plate cavity with various ribs was used for molding. Experiments were carried out on an 80-ton reciprocating injection-molding machine. After molding, the sinkmarks on the surface of molded parts were characterized by a profile meter. For the factors selected in the main experiments, the corner geometry and the width of the rib were found to be the principal factors affecting sinkmark formation in injection-molded thermoplastics. A rib of an undercut geometry and a small width produces parts with the least sinkmark. Experimental investigation of an injection-molding problem can help illuminate the formation mechanism of sinkmarks so that steps can be taken to optimize the surface quality of molded parts. (C) 2001 John Wiley & Sons, Inc.