Selecting an Elastomer

When the need arises to specify an elastomer for a specific application, physical, mechanical, and chemical resistance properties must all be taken 

FluorosiUcones Ethylene acrylate Ethylene vinyl acetate Chlorinated polyethylene Good Resistance Polychloroprene Fair Resistance Nitrile 

Poor Resistance Buna S boprene Natural rubber 

Abrasion resistance Electrical properties Compression set resistance Tear resistance Tensile strength Adhesion to metals Adhesion to fabrics Rebound, cold and hot Resistance to heat aging and flame 

It should be remembered that these properties may be altered by compounding, but improvement of one property may result in an adverse effect on another. Because of this, it is best to provide a competent manufacturer with complete specifications and let that manufacturer provide an appropriate elastomer. 

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Several factors must be taken into account when selecting an elastomer for a lining application. First and foremost is the compatibility of the elastomer with the medium at the temperature and concentration to which it will be exposed. It should also be remembered that each of the elastomeric materials may be formulated to improve certain of its properties. However, an improvement in one property may have an adverse affect on another property, such as corrosion resistance. Because of the ability to change formulation of many of these elastomers, the wisest policy is to permit a competent manufacturer to make the selection of the elastomer to satisfy the application. 

When selecting an elastomer there are many properties that must be taken into account besides its resistance to atmospheric corrosion. One factor that must be considered is the ability of the elastomers to be compounded. By blending various additives, specific elastomer properties may be modified. Invariably while compounding improves one specific property, it is at the expense of another property. For this reason it is important that the manufacturer be consulted and made aware of the application and the specific specifications to be met. 

Selecting an elastomer for an application requires consideration (like for plastics and foams) of many factors, including the mechanical and physical service requirements, the product s life cycle, the material s processability, and its cost (see Figs. 6-24 and 6-25 and Tables 6-12 and 6-13). A wide range of properties is available, based on the many different compounds that can be produced. 

Vulcani2ation is a chemical process for improving an elastomer compound s performance. However, in most cases not all of the desired properties reach their optimum levels simultaneously. One of the mbber compounder s key responsibiHties is to achieve a balance of the most important property requirements by the proper selection of cure system (chemical) and time—temperature cure cycle (physical). 

Recently, a new concept in the preparation of TPVs has been introduced, based on the reaction-induced phase separation (RIPS) of miscible blends of a semicrystalline thermoplastic in combination with an elastomer, with the potential for obtaining submicrometer rubber dispersions. This RIPS can be applied to a variety of miscible blends, in which the elastomer precursor phase was selectively crosslinked to induce phase separation. Plausible schematic representation of the morphological evolution of dynamic vulcanization of immiscible and miscible blends is shown in Fig. 9. For immiscible blends, dynamic vulcanization leads to a decrease in the size... 

The addition of an elastomer (typically a high butadiene content SBS) to SBC will serve to enhance further the elastomeric properties of the SBC. One key feature mentioned previously was improvement in hinge life properties. SBS copolymers can also be added to thermoformed sheet in blends of SBC and crystal polystyrene. The SBS does cause some loss of clarity, but gives more impact resistance to the sheet. Selection of the proper SBS can result in minimal loss of clarity, typically at 3-10% loadings. Styrene-isoprene copolymers (SIS) have also been tested with SBC and can give similar results in impact property improvement. 

Elastomer compounds can be plasticized by addition of organic compounds. Elastomer compounds are inherently flexible and selection of a base polymer on the basis of molecular weight characteristics, chemical composition, and degree of crystallinity serves as the basis for the properties of the compound from which an elastomer is made. Oils are the most common plasticizer for elastomers. Oils of paraffinic structure or aromatic structure can be used with elastomers in which they are compatible. Paraffin wax would also be included in this category. Other plasticizers include phthalic acid esters and adipic acid esters. Fatty acids can be used as plasticizers but these contribute to an increase in surface tack of elastomer compounds. Examples include stearic and palmitic acid. Plasticizer addition has the added benefit of aiding with incorporation of inorganic materials. 

As indicated in Fig. 23.1, a sample consists of a rigid glass indenter and an elastomer substrate of crosslinked poly(dimethyl siloxane) (PDMS), which are both coated with the polymer layers of interest. These layers include a semicrystalline layer of poly(ethylene oxide) (PEO) sandwiched between glassy polymer layers of poly(tetramethyl bisphenol A polycarbonate) (TMPC). These polymers will be described in more detail within this section along with the steps that were taken to select these polymers for the study of a glassy/semicrystalline interface. 

The Polymer Data Handbook offers, in a standardized and readily accessible tabular format, concise information on the syntheses, structures, properties, and applications of the most important polymeric materials. Those included are currently in industrial use or they are under study for potential new applications in industry and in academic laboratories. Considerable thought was given to the criteria for selecting the polymers included in this volume. The first criterion was current commercial importance—the use of the polymer in conunercial materials—for example, as a thermoplastic, a thermoset, or an elastomer. The second criterion was novel applications—a polymer that is promising for one or more purposes but not yet of conunercial importance—for example, because of its electrical conductivities, its nonlinear optical properties, or its suitability as a preceramic polymer. The hope is that some readers wiU become interested enough in these newer materials to contribute to their further development and characterization. Finally, the handbook includes some polymers simply because they are unusually interesting—for example, those utilized in fundamental studies of the effects of chain stiffness, self-assembly, or biochemical processes. 

A major factor in the selection of elastomers for automotive applications is the extent to which they absorb petrol or oil. When an elastomer absorbs diluent (vapour or liquid) it is said to swell (this is illustrated in Figure 3.12). The increase in volume shown is X4. The increase in length of any... 

Unlike the changes that result fi om exposure to high temperatures, changes brought about by low-temperature exposure are generally not permanent and can often be reversed once heat returns. For example, extended exposure to low temperatures will increase an elastomer s hardness, but the material will soften again when the temperature rises. Perhaps the most important consideration related to low temperatures involves seals which must also work in a low-pressure environment. Unless the selected seal compound is sufficiently soft and resilient, the combination of low temperature and low service pressure can cause leakage and failure (Hudson 2011). 

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