retention of compressive strength are attributed to the sudden release of 

 pressure from 10,000 psi which may have caused tensile failure of surface 

 hollow spheres. Tests using a longer, eight hour, cycle of exposure with 

 a slower rate of increase and decrease of pressure are now underway. 



Effect of Specimen Size 



Table 4 shows the relationship between specimen size and water 

 absorption. As the specimen size increases and therefore the surface area 

 to volume ratio decreases, the percentage of water absorbed decreases. The 

 syntactic foam modules that will be used for service applications are ex- 

 pected to have a volume of several cubic feet. Under similar conditions 

 of pressure, the percentage of water absorbed for these specimens which have 

 a much smaller surface area/volume ratio, should be substantially lower 

 than that absorbed by the small specimens. 



Effect of Coatings 



A molded block of foam when removed from its mold has a "skin" 

 or outer resin-rich surface on most or all of its faces. As reported 

 previously, all of the above data on water absorption was obtained from 

 specimens prepared with cut or milled faces. In order to obtain informa- 

 tion on the effect of the normal "skin", a slightly lower strength sample 

 of ML-K3 was selected. The original skin remained on some surfaces and 

 a thin coat of the ML-B3 resin was painted on the cut surface. In addi- 

 tion, to obtain information on the effect of a coating, a commercial 

 sample of syntactic foam with a proprietary coating (not the normal molded 

 "skin") was selected for test. The 1x1x1 inch specimens were exposed 

 to 1000 cycles of hydrostatic pressure between atmospheric and 10,000 psi. 

 In the NASL ML-B3 sample with the molded "skin" surface, the water 

 adsorption was reduced 66%. In the commercial specimens, the coating re- 

 duced the water absorption 92%. 



Exposure of Syntactic Foam to Mineral Oil 



Preliminary plans for the submerged research vehicle called for 

 mineral oil to be used as the fluid surrounding the syntactic foam. To 

 determine the stability of the syntactic foam in mineral oil (Humble Oil 

 Company Zerice 37 Brand) specimens were immersed at 10,000 psi for 1000 

 hours. Table 5A shows that 0.54 per cent was absorbed after the 1000 

 hours of immersion. By comparison 2.6% water was absorbed when similar 

 specimens were immersed in water for 1000 hours at 10,000 psi. The 

 specimens immersed in oil showed 0.5 per cent absorption when weighed 

 after a week of immersion, then oil absorption remained constant during 

 the next five weeks. This indicates that oil absorption is primarily a 

 surface phenomenon. Table SB shows the effect of coupling agents on the 



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