glass-resin interface. The purpose of a coupling agent is to produce a 

 better bond between the glass and the resin. In formulation ML-B3A the 

 spheres were treated with a solution containing gamma Aminopropyltriethoxy- 

 silane coupling agent. In formulation ML-B3B, 0.5 per cunt of this si lane 

 coupling agent was added to the resin. In formulation ML-B3C both the 

 hollow glass spheres, as in A, and the resin system, as in B, were treated 

 with the coupling agent. The ML-B3 without coupling agent, shows a 5.7 

 per cent loss in compressive strength after 1000 hours in oil immersion. 

 After treatment with coupling agent, there was negligible deterioration 

 in compressive strength after oil immersion. 



Lower Density Syntactic Foam 



Hollow spheres of lower density are now becoming available. These 

 should permit the development of lower density foams. TJiese spheres have 

 lower specific gravities than the Type 45 high-strength spheres used in 

 ML-B3 foam, and will thus permit the development of lower density foams. 

 Table 6 shows four foam formulations made with the same resin system but 

 with different types of hollow sphere filler. As the specific gravity of 

 the spheres decreases, as may be expected the density of the foam decreases 

 and the compressive strenght is reduced. Formulation NASL-B13 was made 

 with the same spheres as formulation NASL-BI2 except that the spheres were 

 pressure screened by subjecting them to 2000 psi hydrostatic pressure. 

 The unbroken "floaters", which were the stronger spheres, were used in 

 formulation NASL-B13. The resulting foam shows a slight increase in den- 

 sity over the NASL-B12 foam but a compressive strength comparable to the 

 NASL-Bll foam made with the higher density spheres having a nominal speci- 

 fic gravity of 0.4. 



FORECAST OF FUTURE (1970) PROPERTIES 



Compressive Strength 



The strength properties of syntactic foams are dependent on the 

 strength of the resin and filler components and on fabrication procedures. 

 Values for existing foam in Tables 3A and 3B are based on formulations 

 containing a resin system having an ultimate compressive strength of 

 22,000 psi and hollow glass spheres which are reported to show approxi- 

 mately 50 per cent breakage at W,000 psi. Formulations for normal 44 pcf 

 density foam are based on optimum resin to glass ratios. The resin has 

 greater compressive strength than the filler; therefore, as the percentage 

 of lightweight filler is increased, the compressive strength of the foam 

 as well as its density will decrease. Development of higher strength 

 resin systems and higher strength glass spheres would be expected to 

 improve the compressive strength of the nominal 44 pcf density foam as 

 shown in Figure 7, 



336 



