Elastomeric foams

This chapter will briefly discuss elastomeric foams (other than urethane foams). The reader will note that the emphasis of this handbook is on plastic foams (cellular plastics), but it was felt by the author that some discussion of the more well-known elastomeric foam types would be helpful. 

Rubber products with a cellular structure have been used widely for many years. The earliest developments of these products predated World War I. The two forms of natural rubber—raw rubber, and latex, form the basis for different product types, one being blown dry rubber, and the other foamed and dried latex. Blown sponge and latex foam are distinctly different materials, although the end-products may appear simUar and have some overlapping applications. 

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Linear-elasticity, of course, is limited to small strains (5% or less). Elastomeric foams can be compressed far more than this. The deformation is still recoverable (and thus elastic) but is non-linear, giving the plateau on Fig. 25.9. It is caused by the elastic... 

Fig. 25.11. When on elastomeric foam is compressed beyond the I inear region, the cell walls buckle elastically, giving the long plateau shown in Fig. 25.9.

The feasibility of using these elastomeric foams as fire retardant thermal insulation has been demonstrated by a Department of the Navy-National Bureau of Standards Test Program (54). 

A family of elastomeric foams has been developed by Rand 129) for use as stress relief coatings on electronic components in encapsulated electronic assemblies. Polysulfide, silicone and polyurethane elastomers blended with glass and phenolic microspheres have been used to formulate syntactic foams (Fig. 10) These foams are used to minimize the stress caused by differential thermal expansion between the component and the encapsulant. 

Three types of silicone foams are discussed briefly below — silicone elastomeric foams, silicone rubber sponge, and room-tempera-ture-foaming silicone rubbers. 

The information given in this brief chapter covers plastic foams, both thermoplastic and thermosetting, and elastomeric foams, although very little discussion is given in the latter subject. 

Considerable useful and up-to-date information on plastic and elastomeric foams is available from journals, manufacturers bulletins, technical conferences and their published proceedings, seminars and workshops, standardization activities, trade associations, consultants and information centers (such as PLASTEC), in addition to books, many of which have been cited in the previous chapters. Some of these sources will be listed and commented upon briefly in this chapter. 

This specification, prepared by the Navy, covers polyphosphazene elastomeric foam material for thermal insulation on piping, in either sheet or tubing form. Polyphosphazene foam has excellent fire-retardant properties and is suitable for use in the range -20 to 180°F (-29 to 82.TC) in tubular form (Form T). Form S covers sheet form. 

This book is intended to be useful to anyone working with plastic foams (cellular plastics), and to a lesser extent, elastomeric foams. The emphasis is on practical, rather than theoretical aspects. The books should prove helpful to materials engineers, chemists, chemical engineers, sales personnel. It may also find use as a textbook or reference source in materials engineering courses. The book is a comprehensive technical treatment of plastic foams and covers information not available in any other single source. 

Chapter 3 and all subsequent chapters were prepared by the editor, A.H. Landrock. Chapter 3 covers all types of thermoplastic foams, including rigid, semi-rigid, and structural foams. Chapter 4 briefly discusses elastomeric foams. Chapter 5 discusses a number of miscellaneous and specialty foams, many of which were also covered in Chapter 2. 

FIGURE 41.8 Representative stress-strain curve for a cellular solid. The plateau region for compression in the case of elastomeric foam (a rubbery polymer) represents elastic buckling for an elastic-plastic foam (such as metallic foam), it represents plastic yield, and for an elastic-brittle foam (such as ceramic) it represents crushing. On the tension side, point A represents the transition between cell wall bending and cell wall alignment In elastomeric foam, the alignment occurs elastically, in elastic plastic foam it occurs plastically, and an elastic-brittle foam fractures at A. 

The strength for crushing of a brittle foam and the elastic collapse of an elastomeric foam is given, respectively, by... 

S.L. Shenoy, P. Kaya, C. Erkey, and R.A. Weiss, Synthesis of conductive elastomeric foams by an in situ polymerization of pyrrole using supercritical carbon dioxide and ethanol cosolvents, Synth. Met., 123(3), 509-514 (2001). 

Elastomeric PPs are a new class of materials with a higher thermal stability than polyethylenes and a good elasticity without crosslinking. Elastomeric PPs have a low abrasion resistance and a high resilience at small elongations. Futhermore, new foamed materials based on thermoplastic olefmic elastomers (TPO) are important for the future. TPO consists of 50% of polypropylene and completes the range of PP foams for elastomeric foam applications. 

In the case of these complex zinc hexacyanocobaltate initiators, the telogen could be a diol such as propylene glycol, a triol such as glycerine, or a higher functionality polyol such as pentaerythritol or sorbitol, and the product could then be a polyol of the type used extensively in making polyurethanes in elastomeric, foam, or... 

In addition to standard adhesion tests or failure tests that are explained in the previous chapters of this handbook, various types of special tests are also performed for specific purposes in industry or in scientific research. In this chapter, some special mechanical tests are treated, such as blister tests for membrane/adhesive/coating, tensile tests and shear tests for sealants and elastomeric/foam adhesives, and indentation tests and scratch tests for characterizing coating adhesion. Most of these are designed for testing macroscopic specimens in a macroscopic scale, but several micro- or nanometric test methods have recently been developed to measure mechanical properties of small specimens in a microscopic or nanometric scale. This chapter also introduces recently developed microscopic methods. 

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