Novel Monomers and Polymers for Ice Mitigation

NASA is developing polymer coatings that reduce impact ice adhesion strength. Current coating compositions are based on epoxy resins due to their availability and ease of fabrication. It is anticipated that a successful composition could be used in other polymer classes such as polyurethanes. Initial molecular modeling studies of silanes terminated with various functionalities suggest that chain mobility effects the interface between ice and the surface. To that end, surfaces coated with these compounds were applied to aluminum substrates and the resulting monolayer coating investigated to assess the effect of chemical functionality and chain length under simulated in-flight icing conditions. As shown in the figure below, these molecular coatings demonstrated reduced ice adhesion strength, presumably as a result of the molecular flexibility imbued by the aliphatic chains that has been incorporated into polymers either within the polymer backbone or as pendant groups. Compared to an untreated aluminum alloy surface, a test polymer coating employing in-chain molecular flexibility exhibited a 56% reduction in ice adhesion strength at -16 C. Similarly, another test polymer coating employing pendant group molecular flexibility exhibited a 19% reduction and a 63% reduction in ice adhesion strength at -16 C compared to an aluminum alloy surface and a rigid epoxy control surface, respectively. The reduced ice adhesion strength may lead to decreased energy requirements for active ice mitigation strategies currently used on aircraft when used in conjunction. These anti-icing coatings are a passive approach that are anticipated to be applied to the aircraft surface either as a topcoat or as a constituent of aircraft paint. The coatings need to be optimized for durability, with the goal of achieving a reapplication frequency consistent with routine aircraft maintenance and painting requirements.

Low ice adhesion strength coatings are only useful insofar as they remain on the surface of interest, and aircraft leading edges experience extreme environmental conditions during flight. Ensuring durability while maintaining performance – in this case, reduction of impact (i.e., accreted in-flight) ice adhesion strength – is critical to meeting the needs of the aviation industry and other commercial applications. To that end, NASA engineers investigated coating compositions comprised of epoxy resins, including aromatic and aliphatic resins, and aromatic diamine hardeners. Several nonreactive additives were incorporated and tested. The first was holey graphene, a unique nanomaterial made by partly oxidizing areas of graphene that already have defects. This creates high energy functionalities that result in good dispersion throughout the matrix, enabling the mechanical properties of graphene to be imparted throughout the coating. Secondly, micrometer-sized core-shell rubber particles were dispersed throughout the epoxy resin to increase toughness. Finally, a series of polyhedral oligomeric silsequixones (POSS) were used for mechanical reinforcement. Several different coating formulations were development and tested, each incorporating different relative amounts of additives, with good results. Thus, the coatings can be tailored to meet different application-specific requirements. NASA's coating formulations, with further development, may be suitable for in-flight (i.e., impact) ice adhesion reduction on aircraft leading edges and other platforms exposed to harsh environments.

Accumulation of undesirable species such as insect residue is mitigated by use of copolyimide coatings incorporating fluorine and silicon surface modifying agents (SMAs) in a novel chemical formulation. The coating minimizes adhesion while at the same time maintaining the bulk properties of the polyimide coating. The specific SMAs used here are designed to be thermodynamically drawn to the coating surface. Further, the SMAs react chemically with the polymer backbone and are thus chemically bonded, so the SMAs will not evaporate or migrate out of the coating material. The coating adheres well to a range of relevant materials, including aluminum, composites and plastics.

This innovation utilizes a tri-layered structure, comprised of solid plastic front and back layers sandwiching a viscous, reactive liquid middle layer. Combined, this system provides rapid self-healing following high velocity ballistic penetrations. Self-healing in the front and back layers occurs when the puncture event creates a melt state in the polymer materials and the materials melt elasticity snaps back and closes the hole. The viscous middle layer augments the self-healing properties of the other layers by flowing into the gap created by a ballistic puncture and concurrently solidifying due to the presence of oxygen. Thus, this innovation has two tiers of self-healing: a puncture-healing mechanism triggered by the projectile and a second mechanism triggered by the presence of oxygen.

The technology is a method of mitigating insect residue adhesion to various surfaces upon insect impact. The process involves topographical modification of the surface using laser ablation patterning followed by chemical modification or particulate inclusion in a polymeric matrix. Laser ablation patterning is performed by a commercially available laser system and the chemical spray deposition is composed of nanometer sized silica particles with a hydrophobic solution (e.g. heptadecafluoro-1,1,2,2-tetrahydrodecyltriethoxysilane) in an aqueous ethanol solution. Both topographic and chemical modification of the substrate is necessary to achieve the desired performance.