Evaluation of ice release coatings at cryogenic temperature for the space shuttle

The brackets that secure the liquid oxygen feed line to the external tank are known locations of frost and ice growth during the pre-launch period following fuel loading. This experimental program quantified the reduction in adhesion that resulted when ice phobic coatings were applied to test coupons that simulated the bracket surface. Double lap shear testing of coated and uncoated coupons provided robust test specimens and consistent load response patterns with exceptional resolution. Ice was grown as strong and consistently as possible for these tests and then subjected to temperature decreases comparable to those of the bracket. All tests were conducted at a constant temperature of − 112 °C (− 170 °F) and included uncoated controls with each group of samples. The program evolved in three phases. Phase 1 evaluated a wide range of coatings, and showed that Rain-X mixed with MP-55 powdered Teflon (RXM) was an outstanding coating to reduce ice adhesion to Koropon coated aluminum. Coating material retained on the ice surface following each test indicated failure in the coating and the potential for loss of effectiveness with repeated ice formation and release. Phase 2 evaluated modifications to RXM that might enhance durability while maintaining effectiveness. However, the modified mixtures did not improve coating durability or ice release. Phase 3 focused on the effects of mixing method, cure time, wiping off of excess coating, and coating resistance to weathering by water. A progressive coating loss again occurred through repeat test cycles, but performance generally improved. Results also indicated that cure times longer than 1.5 h prior to coating disturbance are needed for optimal performance. The MP-55 remaining on coupon surfaces after 3 cycles of testing and on one untested coupon from the same group was measured with XPS. The tested coupons retained slightly less MP-55 than the untested coupon, confirming the loss of coating. Contact angle analysis of these same coupons showed that the hydrophobic performance of the tested surfaces was largely preserved. Scanning electron microscopy with an energy dispersive spectroscopy elemental map indicated that the MP-55 was evenly dispersed throughout the coated surface, and abrasive wiping did not remove a significant portion of the Teflon. Test results suggest follow-up studies to refine the optimal coating formulation, mixing and application procedures, and understanding of reaction processes, cure time, and coating durability. Double lap shear testing and XPS analysis can quantify ice adhesion and coating profile thickness changes with cycling, and is capable of resolving these remaining issues.