Liquid substances applied to the airfoil surfaces of aircraft to prevent their freezing and being covered with ice in inclement weather are well-known and are important to ensure safe and proper take off of aircraft in winter. It is also well-known that aircraft departures are often delayed under such conditions and the anti-icing formulations must often be reapplied if the plane must wait an extended period. These liquids must also be stable not only through temperature extremes, but also during the taxiing phases of the take off procedure and thus must adhere to the wing surfaces during travel and and ground winds. The fluids may also be diluted somewhat through intentional dilution with water, such as during particularly cold conditions below the expected operating temperatures of the fluids to enable their application, or incidentally as through freezing rain conditions. In addition to these demands, the anti-icing materials must also change characteristics, becoming much less viscous, and flow off the wing surfaces during the relatively high shear conditions of take off. Such compositions are also useful in the de-icing or anti-icing of other surfaces besides air foils.
Other anti-icing compositions are known. For example, U.S. Pat. No. 4,744,913 describes an anti-icing and de-icing agent, based on glycols and water and having crosslinked acrylic polymers useful a thickener, and also containing customary corrosion inhibitors, surfactants belonging to the group of alkali metal alkylarylsulfonates and a neutralizing agent to adjust the pH to a basic value. The agent contains, as the thickener, two selected crosslinked acrylic polymers in a specific ratio by weight to one another, namely a selected crosslinked acrylic acid or alkali metal acrylate homopolymer and a selected crosslinked acrylic acid/acrylamide or alkali metal acrylate/acrylamide copolymer in a ratio by weight of 2:1 to 10:1. The neutralizing agent may be three compounds, each in a selected amount, namely ammonia, monoethanolamine, diethanolamine and/or triethanolamine as the first alkaline compound, potassium hydroxide as the second alkaline compound and a further alkali metal hydroxide as the third alkaline compound. The new agent was found to exhibit the particular advantage that it has a relatively low viscosity even at arctic temperatures and low shear rates, which ensures rapid and complete runoff of the agent at the take-off of the aircraft even under extreme conditions.
Partially neutralized polymers of acrylic acid form water swellable networks with high water retention capability and very fast kinetics of volume change have been reported by F. Schosseler, et al. in J. Dispersion Sci. Technol., Vol. 8, p. 321, 1987. Further information about polyacrylic acids may be found in R. B. Lochhead, et al., "Polyacrylic acid thickener: The importance of gel microrheology and evaluation of hydrophobe modified derivative as emulsifier," Polymers in Aqueous Media, J. E. Glass, Editor, Advances in Chemistry Series, #223.
Publications which describe similar, but perhaps less relevant substances, include U.S. Pat. No. 3,940,356 which discusses ice release agents formulated from alkylene glycols having two to three carbon atoms, water, an alkanol and a salt of crosslinked polyacrylic acid.
Another liquid agent for de-icing and protecting against icing-up is described in U.S. Pat. No. 4,358,389, by means of which it is possible, in particular, to free the metal surface of aircraft rapidly and completely from ice, hoar-frost, snow and the like, and to protect the surface against further build-up for a relatively long period. The agent is essentially composed of several components, namely of (a) glycols, (b) water, (c) thickeners, (d) substances insoluble in water, (e) surface-active agents, (f) corrosion inhibitors and (g) alkaline compounds. The quantities are very specific in each case, the quantity of the components (a) and (b) being at least 94%, relative to the total weight of the agent. Its pH value is 7.5 to 10. The component (c) thickeners are crosslinked polyacrylates described in the patent in further detail and also in U.S. Pat. No. 2,923,692.
U.S. Pat. Nos. 3,635,756 and 3,488,215 describe a transparent, nonfogging coating which may be applied to a normally fogging transparent or reflecting substrate, such as vehicle windows, glasses, lenses, mirrors, masks, and the like. The nonfogging coating preferably comprises a hydrophilic acrylate or methacrylate polymer. The polymer can be modified by copolymerization with a monobasic or polybasic unsaturated carboxylic acid or partial ester thereof. The polymer can be cross-linked with a polyepoxide to increase its hardness.
An anti-icing fluid suitable for ground treatment of aircraft, being a glycol-based solution containing a gel-forming carrageenan, in an amount of less than 5 wt. % is described in U.S. Pat. No. 4,698,172. The carrageenan is present in the glycol-based solution in an amount sufficient to thicken the fluid to promote its adherence to aircraft surfaces when applied to a stationary aircraft. Use of this thickened deicing fluid does not adversely affect airfoil lift characteristics during takeoff, because the fluid exhibits shear thinning and readily flows off the aircraft surfaces when exposed to wind shear during the aircraft's takeoff run.
There is also U.S. Pat. No. 4,283,297 which discusses products for rapidly liberating areas covered by ice and/or snow, which products are composed in the following wt.%, relative to the total weight of all components: 60 to 79% of propylene glycol and 0 to 5% ethylene glycol or 70 to 85% ethylene glycol and 0 to 10% propylene glycol. In both cases, there are added 5 to 15 wt. % water; 5 to 20 wt. % urea and 1 to 5 wt. % monohydric aliphatic alcolhols having from 1 to 7 carbon atoms. The products are harmless, biologically degradable, non-corrosive and low flammability.
There remains a need for a thickened aircraft anti-icing composition that exhibits hightly pseudo plastic rheology and which under low shear conditions is relatively insensitive to temperature changes. It would also be advantageous if the fluid retained a high viscosity as the composition is diluted with water. The anti-icing fluids should also remain stable for relatively long periods of time to delay or avoid reapplication of the compositions during departure holdovers.