Syntactic foams properties

Table 6.23 High-performance syntactic foams property examples...

Interest in the use of syntactic foam as a shock attenuator led to studies of its static and dynamic mechanical properties. Particularly important is the influence of loading rate on stiffness and crushing strength, since oversensitivity of either of these parameters can complicate the prediction of the effectiveness of a foam system as an energy absorber. 

For information,Table 6.23 shows some property examples of syntactic foams with thermoset matrices. 

Polymers such as polyethylene, which do not have polar groups, are excellent insulators of heat and electricity. The thermal insulating properties may be improved by foaming or by the incorporation of hollow glass spheres (syntactic foams). A low-density polyethylene foam will have a thermal conductivity in the order of 0.3 BTU/ft2 h F in. 

Abstract— The use of organosilanes as adhesion promoters for surface coatings, adhesives and syntactic foams is described and reviewed in the light of published work. Data are presented on the beneficial effect of silanes, when used as pretreatment primers and additives, on the bond strength of two pack epoxide and polyurethane paints applied to aluminium and mild steel. It is shown that silanes when used as additives to structural epoxide and polyurethane adhesives are less effective than when used as pretreatment primers on metals but are highly effective on glass substrates. The compressive properties of glass microballoon/epoxide syntactic foams are shown to be markedly improved by the addition of silanes. 

Syntactic foams manufactured from hollow glass or silica microspheres and an epoxide, phenolic or other matrix resin represent a class of lightweight structural materials used for buoyancy purposes, insulation and packaging. The effect of silanes on the mechanical properties of syntactic foams at a nominal density of 0.35 g/cm3 is shown in Tables 14-16. The Proportional Limit is defined as the greatest stress which the foam is capable of sustaining without any deviation from proportionality of stress to strain (Hooke s Law). 

Effect of silane addition level on the compressive properties of an anhydride cured epoxide silica microballoon syntactic foam (APES, cured at 100 C for 4 h, nominal density 0.35 g/cm )... 

In other words the area under the stress/strain curve is a measure of the energy absorbing characteristics. The effect of APES additions on the stress/strain properties of an anhydride cured epoxide syntactic foam at a nominal density of... 

The most important factor determining the mechanical properties of a syntactic foam is the distribution of its microspheres with respect to size, shape, and strength, the best properties being attained if these distributions are homogeneous19). Micro-spheres with the same shape and size can be obtained by sieving or flotation. Their mechanical strengths still differ considerably, even after sorting, due to wall thickness... 

Irrespective of the method used, the quality of a syntactic foam depends substantially on the rheological properties of the initial mixture with the microspheres68 70). The investigation of the rheology of syntactic compositions by Petrilaenkova et al. 71)... 

The properties and densities of the mixtures and their resultant syntactic foams not only depend on the binder/filler ratio but also on the microspheres themselves, their size, sphericity, polydispersity, apparent and bulk density, the thickness and uniformity of their shells. Thus, at a given binder/filler ratio, the fluidity of a mixture depends on the size of the microspheres (Fig. 2) and the apparent density depends on their bulk density (Fig. 3)l). As the bulk density of the microspheres increases (the filler particles become larger), the final strength of the material decreases3 76>. 

It follows that, when formulating a manufacturing composition for a syntactic foam, all of the binder and filler properties, as well as the process parameters have to be taken into account. Special computer methods have been proposed for selecting the best formulations whilst accounting for economic efficiency82... 

Table 8. Composition and Properties of Carbon/Carbon Three-phase Syntactic Foam 86)...

Most epoxy syntactic foams are vacuum molded at 70-120 °C, and vibrating stirrers are used to degas the mixture40 . Recent formulations can be processed without external heating (cured for 30 days at 20 °C) and yet yield syntactic foams with properties comparable to those of hot-cured foamsthe shrinkage is less than 1 % for both. 

Soviet scientists have developed a cold-cure EDS material87). Note that the epoxy binders used are in fact generally hot-cure oligomers (from the point of view of the functional groups used and the completion of cure). It turned out that the forced elastic limit and the compression elastic modulus of the matrix are the same for both hot and cold cures. The good mechanical properties of a cold cured syntactic foam seem to be due to strong intermolecular bonds. 

Table 9. Formulation and Properties of Carbonized Syntactic Foams Using BJOa Phenolic Microspheres...

The search for ways of improving the properties of syntactic foams has led to a number of new materials based on some unconventional binders. 

Three-phase syntactic foams with unique properties have been developed during recent years (see also Chapter 3.6 and Table 8). 

The properties of syntactic materials are influenced by several factors including the binder/filler ratio, the process and hardening conditions, and the physicochemical processes at the binder/filler interface 12,76,99). The best syntactic foams, at given apparent densities of 680-700 kg/m3, have a compression strength of 10 MPa, shear and tension elastic moduli of 2500—3000 MPa ultimate bending strengths of 40 to... 

Table 12. Properties of Cold-Cured Syntactic Foams...

Until recently, the materials made from epoxy binders and glass microspheres were believed to be the strongest syntactic foams. However, several papers 26,39) have shown that, when carbon microspheres replace those of glass, the material becomes stronger, more water resistant, and more capable to withstand hydrostatic pressure (for the same filler concentration) (Table 13). The smaller the carbon microspheres, the stronger are the resulting foams, 9 135). Carbon microspheres also improve the mechanical properties of phenolic and resol syntactic materials (Table 14) 38). 

Table 15. Properties of carbonized syntactic foams (Novolac phenolic oligomer and carbon microspheres) S9)...

Almost none of the cited papers dealing with water absorption by syntactic foams deals with the mechanism by which water or other small molecules penetrate into these materials. Filyanov et al.148,149) undertook one of the first attempts using the ED-20 epoxy oligomer—glass microsphere system. Water absorption by a filled polymer is known to depend on the sorptive properties of the binder, the stability of... 

Table 23 shows the thermophysical properties of some syntactic foams 2). Polyimide syntactic foams have a thermal expansivity of 20 xio-6 deg-1 and thermal conductivities of 0.085 W/mK at 22 °C and 0.111 W/mK at 370 °C 10S-106). The low thermal conductivity of the carbonized foams (Fig. 17) is due to their open pore structure113). 

Syntactic foams have good dielectric properties (Table 24)2) they can be varied over a broad range by changing the binder and filler, as well as the filler concentration 8 32). 

The dielectric properties of the syntactic foams used for electric insulation (SPAB-1 and ENS-6T) are very good. Even an increase in temperature in humid environments does not raise their s or tan 8 values by more than 20 %. The electric resistance of a SPAB-1 sample did not exceed 400-500 MOhm even after 24 hours in an environment of 95 % ( 3 %) relative humidity u. 

The most noticeable difference between syntactic foams with the same filler but different binders is seen in the tangent of the dielectric loss angle (Table 25)11(. If glass microspheres replace organosilicon ones for the same binder, not only tan 8, but also e decrease 1). But also the dielectric properties and the concentration of the binder affect the final foam s e (Fig. 18) n). 

Table 24. Dielectric properties of EDS-6 syntactic foam at different temperatures 21...

Table 25. Dielectric properties of syntactic foams with glass microspheres111 ...

Carbon microspheres yield syntactic foams with resistivities that are astonishingly low for these materials. Novolac syntactic foams with carbon microspheres have resistivities of 0.02-0.5 Ohm m (depending on the filler concentration)77 this is ten orders of magnitude lower than for glass microspheres in the same binder For materials made from carbon microspheres and silicone rubbers, the resistivity depends exponentially on the temperature, viz. 0.08 Ohm m at 20 °C, 0.2 Ohm m at 60 °C, and 200 Ohm m at 95 °C 1). Consequently, carbon microspheres make it possible to produce syntactic foams with electric properties appropriate for semiconductors. 

Their good superhigh-frequency dielectric properties (Table 26) explain why syntactic foams have been so widely used in radar blast furnaces and lenses, and for absorbing electromagnetic waves15e). 

Table 26. Superhigh-Frequency Dielectric Properties of Epoxy Syntactic Foams using Glass Microspheres...

In order to improve the properties of gas filled, particularly of syntactic, foams, a general theory covering their deterioration and deformation should be developed, along with continued technological research. The absence of reliable techniques for... 

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