The available data on permeability, diffusivity, and solubility of water and water vapor through elastomers and plastics have been summarized. In many ways, the last five years have been a relatively dormant period, following the previous fifteen years when most theories and experimental data were generated. From a practical and technical viewpoint, knowledge of permeation, diffusion, and solution behavior is essential for the successful design and use of many products, such as packaging films and other protective coatings. This knowledge had an immediate impact on the development of efficient permselective membranes to satisfy the exacting conditions required for use as media for reverse osmosis desalination, artificial kidney and lung components, and for other precise separations of multicomponent penetrant mixtures. The interdependence of polymer structure and transport behavior—a major factor affecting the ultimate properties of films and membranes—is of increasing importance as our ability to control polymer synthesis and characterize polymer structure has become more precise and predictable. It is to be expected that even more dramatic progress in membrane technology will result from the ever quickening pace of research in related areas of science spurred on by the increased awareness of the present and potential importance of membrane phenomena. There are two serious and pandemic problems which plague researchers in the field of transport of water through elastomers and plastics. One of these is that a variety of techniques are used to measure permeability that cannot be compared to one another. The second is that the composition of the membrane is often not reported precisely in the published data.
Uses are growing for rubbers with varying levels of resistivity. High electrical resistivity is very much essential in wire and cable insulation applications. Low levels of resistivity are needed for electrostatic discharge in phonograph records and many medical, industrial, and military products and for semiconductive cable compounds. The level of resistivity depends upon the number of contacts or near contacts between conductive particles in the rubber matrix. Loading level is obviously a major determinant in addition to physicochemical characteristics of the black. In the latter regard, the highly conductive grades are characterized by small particle size, high structure, high surface porosity, and low volatile content. One would, therefore, seek the reverse of those factors for high-resistivity rubbers. One of the goals of materials research now is to create new materials with physicomechanical properties tailored to a particular application and to understand the physical processes which determine the end properties. In this review, an attempt has been made to discuss the electrical properties of carbon-black-loaded rubber composites, a class of materials which covers the range from insulators to conductors. The carbon-black-loaded rubbers are formed by dispersing carbon black into the rubber. The compounding is done by adding the carbon black to the rubber, mixing at temperatures above Tg and subjecting the mixtures to high shears until a uniform blend is obtained. The carbon-black particles may be as small as 14 nm in diameter or as large as 300 nm, and they may be individually dispersed or agglomerated in micron-sized clusters. Morphology of the rubber has a profound effect on its electrical properties. High electrically resistive rubbers are becoming increasingly important. Their wide array of applications include antistatic products, shielding materials, insulating membranes, resistors, etc. In the vicinity of the crystalline transition region the rubber shows a dramatic resistivity increase which can be utilized for self-regulation processes. Compounds suitable for such various applications differ appreciably in the nature of their components and composition depending on the specific performance required.
Commercial polyurethanes, although convenient, do not provide close sound-speed or density matches with seawater. Several series ofdiblock copolymer polyurethanes were prepared in order to study the compositional parameters that determine the acoustic properties of this class of compounds. Compositional changes that principally affect the hardblock part of the structure were shown to exert little influence on sound speed or density. Conversely, an increase in the size or quantity of the softblock causes large decreases in the properties, allowing production of polyurethanes with very closely matched properties. Correlation equations for sound speed, density, and specific acoustic impedance are given as a function of calculated number-average molecular weights of the polyurethane softblock. The essential independence of the sound speed from the Shore hardness suggests that the hardblock composition controls the hardness property whereas acoustic properties are determined by the softblock parameters. PACS numbers: 43.85.Bh, 62.65. d-k, 43.85.Dj, 43.30.Jx BACKGROUND
Polyurethane prepolymers are a complex mixture of oligomers. The proportion of the various species in this mixture determines the handling properties of the prepolymer as well as the physical properties of the final polyurethane. An analytical method has been developed that gives a clear and sensitive picture of both the reaction kinetics and the concentrations of the oligomeric species in the prepolymer mixture. The analytical method is applied to the polypropylene glycol/tolylene diisocyanate/catalyst system. The expected changes in reaction rates and in the formation of higher oligomers in the prepolymer were observed when catalyst was added at three different polyol molecular weights. An alternative equation for predicting the number average degree of polymerization is developed for the cases where reactant ratios are significantly less than one. An empirical equation is derived that permits expressing the reaction kinetic data in a linear plot. This equation is used to express the results of this work.
Elastomers are commonly used as electrical insulators in marine acoustic devices. A previous paper discussed the very large decrease in electrical resistivity of a Neoprene GRT formulation when subjected to moderately high molding pressures. In the present paper, data will be presented showing similar dependencies on mold pressure for natural, polybutadiene, 39%-acrylonitrile NBR, and chlorobutyl elastomers and the absence of the effect for an EPDM, and a 27%-acrylonitrile NBR elastomer. These data are not easily explained. The previous paper also discussed that increasing the carbon-black loading caused the electrical resistivity to decrease at even lower molding pressures. Similar trends are presented here for the other elastomers whose electrical resistivities are dependent on molding pressure. Data will also be presented that show that the addition of poly-para-dinitrosobenzene (Poly DNB®) to a CR formulation will eliminate the mold pressure vs. electrical resistivity effect.
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