ABSTRACT:The objective of this article was to review the use of carbon black (CB) as a conductive filler in polymers and polymer blends. Important properties of CB related to its use in conducting polymers are discussed. The effects of polymer structure, molecular weight, surface tension, and processing conditions on electrical resistivity and physical properties of composites are discussed. Several percolation models applicable to CB/polymer blends are reviewed. Emphasis is placed on recent trends using polymer blends as the matrix to obtain conducting composites at a lower CB loading. A criterion for the distribution of CB in polymer blends is discussed.
The objective of this article is to review the use of conductive plastics in the shielding of electromagnetic interference (EMI). The theory of EM1 shielding and evaluation methods of EM1 materials are discussed. Different approaches in achieving high conductivity of plastics are reviewed. Emphasis is placed on filled plastics which are a promising way for shielding and do not require a secondary process. Literature on properties and processing of several common conducting plastics is reviewed. Design and processing requirements of such filled plastics-are also given. 01995 John Wiley & Sons, Gc.
In this paper, the approach of biodegradation to reduce the environmental impact of plustic wastes is reviewed. The mechanism of biodegradation and the processing and applications of these materials are also discussed in relation to the structural characters of their molecules. The synthetic biodegradable materials such as starch based biodegradable materials are compared with naturdy occumhg biodegmdable materials such as poly(hydr0xy butyrate) (PHB) in terms of their applications, processing difJic u b , and degradation mechanism. The future trends of these materials are anticipated.
Epoxy resin is a material commonly used for the encapsulation of light-emitting diodes (LED). Three epoxy resins were chosen in this study: D.E.R.-331 was a bisphenol A type epoxy resin, Eporite-5630 was a bisphenol A type epoxy resin modified with a UV stabilizer, and ERL-4221 was a cyclo-aliphatic epoxy resin. The curing agent was methyl hexahydrophthalic anhydride (MHHPA), and the catalyst was tetra-butyl ammonium bromide (TBAB). Thermal, environmental, and optical properties of the cured epoxy resins were studied. It was found that cured ERL-4221 had better UV resistance, but other properties of ERL-4221 were lower because of the internal stress generated during the curing process. D.E.R.-331 showed better thermal stability, but the aromatic groups in the main chains caused sensitivity to photodegradation. Eporite-5630 had the advantages of D.E.R.-331 and better UV resistance than D.E.R.-331 because of the addition of UV stabilizer. Among the three materials, Eporite-5630 was the best choice of the balance properties for LED applications. This finding may assist in the selection of epoxy resins used in LED technology. C
ABSTRACT:The shear and extensional viscosities of a polypropylene resin were studied using a capillary rheometer and capillary dies of 1-mm diameter and length of 10, 20, and 30 mm. Melt temperatures at 190, 205, and 220°C and shear rates between 100 and 5000 s Ϫ1 were used. At the highest shear rate a visible melt fracture was observed. An equation relating the pressure drop and die length was derived with consideration of pressure effects on melt viscosities and the end effect. After the correction for pressure effects the true wall shear stress and end effect at zero pressure were calculated. The end effect showed a critical stress of melt fracture around 10 5 Pa, and increased rapidly when shear stress increased above the critical stress. From shear stress the shear viscosity was calculated, and a power law behavior was observed. Extensional viscosity was calculated from the end effect and showed a decreasing trend when strain rate increased. After time-temperature superposition shift shear viscosity data correlated well, but an upward trend was observed in extensional viscosity when melt fracture occurred. Die swell ratio at different temperatures can be plotted as a function of wall shear stress and was higher for shorter dies.
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