and inorganic foams have the serious limitations of permeability and low 

 strength. The low density plastics have limited buoyancy as a class, but 

 they might be used as thick walled spheres or sections at moderate pres- 

 sures. However, in this form they are air filled structures rather than 

 buoyancy materials which are the subject of this paper. The syntactic 

 foams however, offer exceptional promise for use at deep submergence 

 pressures. 



Syntactic Foam 



Syntactic foam is defined as a material consisting of a resin 

 matrix containing a low density hollow sphere filler. This definition 

 describes a family of materials whose density and other properties de- 

 pend on the type of resin system, filler, percentages of each component, 

 and fabrication techniques. The specific system developed at the U. S. 

 Naval Applied Science Laboratory to provide buoyancy for the Submersible 

 Research Vehicle (SRV) planned by the Bureau of Ships consisted of hollow 

 glass spheres, with an outside diameter of 20 to 90 microns, in a rigid 

 epoxy resin matrix. Figure 1, shows an exploded view of a cross-section 

 of the NASL ML-B3 syntactic foam magnified 250 times. The hollow glass 

 spheres in a range of sizes are visible. The formulation consists of 

 approximately 40 per cent hollow spheres and 60 per cent resin by weight. 

 The principal properties of the foam are also listed in the figure. 

 The specimen shown in Figure 1 , and the specimens used for test had cut 

 or milled faces. The effect of a molded "skin" or a coating on specimen 

 properties will be discussed later. 



Physical Properties 



The syntactic foams when used in an equalized pressure design, 

 eliminate the disadvantages of liquid buoyancy materials used at present 

 and offer reliability and reasonable economy. Studies of syntactic foam 

 show that several of them are suitable for underwater use if the hydro- 

 static pressure does not exceed 10,000 psi. NASL formulation ML-B3 and 

 the best commercially available formulations meet this requirement and may 

 be expected to have represented physical properties such as listed in 

 Tables 3A and 3B. 



Bulk Modulus 



Bulk modulus characterizes the incompressibility of a material; 

 it is the ratio of the stress to the change in volume. Figure 2 shows 

 the bulk modulus of water and that of the low density liquid and solid 

 buoyancy materials listed in Tables 1 and 2; the values are representative 

 for the respective classes of materials. Bulk modulus for expanded plas- 

 tics and woods have not been included since at 10,000 psi these materials 



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