Properties of Syntactic Foams

Effect of Matrix Resins on Physical Properties. Table 39 shows the effect of type of matrix resin on the physical properties of syntactic foams in which the glass microspheres employed were Emerson Cuming Company s product. 

The structure of syntactic foams is composed of closed-cell microspheres and matrix resins, and the resultant foam has the advantages of (a) isotropic properties not available in other types of foams (b) very low water absorption due to highly closed-cell structures and (c) very high compressive strengthAveight ratio. Table 40 shows properties of syntactic foams composed of epoxy resin and glass microspheres. 

The 42 Ib/ft foam in the table showed a hydraulic crush point of 17,000 psi (1,190 kg/cm ). This figure means that the foam can resist the hydraulic pressure at about 12,000 m (36,000 ft) of ocean depth. 

Phenolic microbubbles generally yield lower-strength syntactic foams than do glass microbubbles of equal density. As bubble density decreases, relative strengths also decrease (7). The most important factor for use as deep-submergence buoys is the rate of water absorption under a given hydrostatic load. 

The hydraulic crush point of a foam is also important in determining to what maximum hydrostatic pressure it can be subjected without rapid failure via high water absorption. Water absorption is another important factor. A high-quality syntactic foam displayed less than 3% water absorption after six weeks of exposure to its ultimate hydrostatic strength for 1-in-diameter by 2-in-long test specimens. The test pressure employed should be not greater than 75 to 80% of the crush point. 

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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). 

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

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

Let us examine some concrete examples of calculating the strength properties of syntactic foams. 

Table 39 I pical Properties of Syntactic Foams (Microballoons and Various Resin Binders) (12)...

Table 40 Properties of Syntactic Foams of Various Densities (7)... 

Guzman M E, Rodriguez A J, Minaie B and Violette M (2012), Processing and properties of syntactic foams reinforced with carbon nanotubes , J Appl Polym Sci, 124, 2383-2394. 

Wouterson, E. M. et ah. Effect of fiber reinforcement on the tensile, fracture and thermal properties of syntactic foam. Polymer, 2007b. 48(11) 3183-3191. 

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. 

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... 

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... 

Dementyev and Tarakanov 8 160) used another approach by adopting a macrostructural model of syntactic foam morphology161 to calculate the strength properties of an epoxy foam with phenolic microspheres. They made two restrictive assumptions, i.e. that the mechanical properties of the microsphere walls and the binder are the same, and that the volume fraction of filler is substantially smaller than that of the matrix. The macrostructural parameters of the syntactic foam are then defined in terms of the dimensions of the microspheres, and their displacements have the same nature as the deformations of the nodes and edges of an imaginary latice. We then get ... 

The main advantage of this type of syntactic foam lies in its thermal properties. The compressive strength retention of the foams at 288 C is in the range of 60 to 85%, depending upon the foam density. 

The major advantage of syntactic foams is the high strength-to-weight ratios. This advantage has ied to applications in deep-submergence vehicles for hydrospace use (7), aerospace applications such as interior floor panels of aircraft (8), nose cones, fins, and bodies of rockets, sonar windows (some acoustic properties of the foam are similar to those of sea water, radomes, etc. (11). 

The presence of cells marks the distinctive difference between bulk composite and composite foams. The level of porosity measures the amount of empty space within the matrix and varies with foam density. For the case of syntactic foams, different microstructures or levels of porosity can be created through varying the type and amount of microspheres. Details of the microspheres used by Wouterson et al. (2007b) to prepare the epoxy syntactic foams are listed in Table 2.1. In the fracture toughness assessment under quasistatic loading, SEND specimens were loaded in a three-point bend (3PB) geometry. Due to the difference in density between the various types of microspheres, densities of foams with equivalent amounts of microspheres vary. The difference in density makes the comparison of the properties of foams nonrelevant. In order to compare the performance of foams, the specific mechanical/fracture properties are used. 

Wouterson, E. M. et al., Specific properties and fracture toughness of syntactic foam Effect of foam microstructures. Composites Science andTechnology, 2005.65(11-12) 1840-1850. 

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. 

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... 

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