33 CFR 183.516: Cellular plastic used to encase fuel tanks.

(a) Cellular plastic used to encase metallic fuel tanks must:

(1) Not change volume by more than five percent or dissolve after being immersed in any of the following liquids for 24 hours at 29 °C:

(i) Reference fuel B ASTM D 471 (incorporated by reference, see § 183.5).

(ii) No. 2 reference oil of ASTM D 471 (incorporated by reference, see § 183.5).

(iii) Five percent solution of trisodium phosphate in water; and

(2) Not absorb more than 0.12 pound of water per square foot of cut surface, measure under Military Specification MIL P-21929B.

(b) Non-polyurethane cellular plastic used to encase metallic fuel tanks must have a compressive strength of at least 60 pounds per square inch at ten percent deflection measured under ASTM D 1621 (incorporated by reference, see § 183.5), “Compressive Strength of Rigid Cellular Plastics”.

(c) Polyurethane cellular plastic used to encase metallic fuel tanks must have a density of at least 2.0 pounds per cubic foot, measured under ASTM D 1622 (incorporated by reference, see § 183.5), “Apparent Density of Rigid Cellular Plastics.”