Robust Polyurethane Coatings with Lightly Cross-Linked Surfaces for Ice Shedding

A highly cross-linked coating tends to have a high shear modulus and ice adhesion strength, while a lightly cross-linked rubber lacks wear resistance but sheds ice readily. This paper reports a polyurethane (PU) coating that is heavily cross-linked in the bulk but is lightly cross-linked near its surface (LICROS) for ice shedding. To achieve this, two methacrylate polyols (P1 and P2) are synthesized by free radical polymerization. Of the polyols, P1 is rich in cross-linkable 2-hydroxyethyl methacrylate (HEMA) units and P2 is rich in non-cross-linkable 2-ethylhexyl methacrylate (EHMA) units and also contains 4.1 mol % of 2-(perfluorohexyl)ethyl methacrylate (FC6-MA) units. Solvent evaporation from a solution mixture of P1, P2, and a hexamethylene diisocyanate trimer (HDIT) causes P2 to stratify to the coating’s surface due to the low surface tension of FC6-MA. After thermal curing and lubrication with dibutyl phthalate (DBP), the resulting LICROS coatings feature ice adhesion values (τ) that are around one-fourth of those measured on the homogeneous PU coating derived from curing P1 with HDIT. However, the τ values of the DBP-lubricated coatings quickly increase with the number of icing/de-icing cycles presumably due to the shedding of the highly swollen surface layer with ice removal. The lubrication of an optimized LICROS coating with acetyl tributyl citrate (ATBC), which swells the coating much less than DBP, sustains low τ values of ∼60 kPa for 15 icing/de-icing cycles without showing signs of deterioration. LICROS coatings have nanoindentation hardness and wear resistance comparable with those of the cured homogeneous P1 coating but much better than those of the cured P2 coating. Since a robust LICROS coating can be made from P1 and P2 at a mass ratio of 97.5/2.5, the weight fraction of the expensive FC6-MA component used in the final coating is only 0.12%. Thus, this paper reports an approach to making inexpensive coatings with low ice adhesions but high wear resistance.