METHOD OF PREVENTING ICE FORMATION

ON SURFACES

Field of the Invention The present invention relates to a method of preventing the accumulation of ice layers on solid surfaces of various geometries and materials exposed to humid and super-cool environment.

Background of the Invention The formation and growth of ice layers on solid surfaces exposed to very low temperatures may create serious functional and safety problems. The available means for removing or preventing the formation of ice layers include employing heating and/or mechanical devices, modifying the protected surface by nanostructuring or coating in order to repel water drops, or contacting the surface with antiicing chemicals. For example, US 6,809,169 relates to a polysiloxane coating applied on a surface to inhibit the ice accumulation. RU 2221002 describes a composition for preventing the ice deposition on public structures, such as buildings, containing at least sodium formate, urea, and potassium carbonate, and to be applied onto the surface to lower the freezing point of aqueous mixtures. US 7,037,442 describes a method for removing ice from the surface of an aircraft, comprising applying onto said surface an aqueous composition containing a glycol and a water- soluble sheet silicate.

However, the available means have various drawbacks. The heating and mechanical devices require energy supply, and they are usually quite robust. As for the surface modifications, although being quite expensive, they are not always efficient against accumulation of ice particles (being rather useful for water drop rejection and as anti-wetting agent), while applying antifreeze chemicals is not always feasible and usually is environmentally compromising. It is therefore an object of this invention to provide a method

for preventing the ice formation on the surfaces of various geometries and materials exposed to very low temperatures, without requirements for energy sources or for outer control.

It is another object of this invention to provide a method for preventing the ice formation on the surfaces exposed to low temperatures by low cost means.

It is still another object of this invention to provide a method for preventing the ice formation on the cool surfaces of transportation means, aerodynamic elements, communication elements, or electric installations, the method being environmentally friendly.

It was observed that airborne objects moving slowly through the clouds accumulate ice on the windows, and on plastic or metal surfaces. The ice buildup may form an undesired optical or electromagnetic shield, it leads to the mass increase, and it may change the aerodynamic parameters of the flying object. The supercooled water droplets impinge on the cool solid surface in high altitudes, loosing their latent heat energy, and they stick onto the surface being transformed into a thin film of ice which accumulates, subsequently forming a thick ice layer; the surface may comprise any material such as metal, plastic, glass, or composites, or even a water repellant material. It is therefore a further object of this invention to provide a method for preventing the ice formation on the surfaces of the aircraft or missile structures moving through rain, snow, or cloud.

Other objects and advantages of present invention will appear as description proceeds.

Summary of the Invention The present invention provides a method of preventing the formation of ice layer on a solid surface exposed to an environment comprising humidity and

a temperature lower than -2O ⁰ C, comprising covering said surface with a sheet impregnated with a salt having a eutectic freezing point of less than - 20 ⁰ C. Said sheet preferably comprises a fibrous or porous structure having a thickness of from 20 μm to 500 μm; the structure possibly comprising woven fabric, textile layer, stable layer of fibers, film comprising one or more layers, and a layer of polymeric foam. Said salt is in a mixture with water, being dispersed in said sheet in the form selected from anhydrous solid, hydrate, solid saturated mixture with water, aqueous suspension, aqueous solution, a mixture thereof. Water may be present in an amount corresponding to the salt saturation or less. For example, when impregnation comprises CaCh, hygroscopic hydrates may be involved, binding water in amounts of 70% or more. Said stable salt may comprise, for example, CaCb or MgCh.

Said surface to be protected against the accumulation of ice is associated with an item selected from transportation means, communication means, aerodynamic elements, electric installations, construction elements, etc. Said protected surface may comprise a material selected from metals, polymers, glass, ceramics, composites, and a mixture thereof. Said environment to which said protected surface is exposed may comprise a situation selected from high altitudes, high latitudes, winter season, and industrial refrigeration. Said humidity which condensates to form said ice layer, to be precluded by the method of the invention, may comprise water in various its forms, including drops or other liquid portions, fog, aerosols, vapor, clouds, supercooled droplets, snow flakes, ice crystals, and amorphous particles. Said temperatures, under which a method according to the invention typically works, is from about -45°C to about -5°C, and more typically from about - 3O ⁰ C to about -15°C. Said sheet in the method of the invention may be firmly attached to said protected surface, by any suitable known method. In a preferred embodiment of the invention, said method of preventing the ice formation comprises wrapping said impregnated sheet around an object, or around a part thereof, or adhering to it, comprising said surface to be protected. In a preferred embodiment of the invention, said impregnated

sheet has an essentially cylindrical shape and is pulled onto said object, to "dress" it and fit it, similarly as a stocking fits the leg; preferably the sheet is elastic and has a lower area than said protected surface, so that it can be in tight contact with the surface of the object after being put on it. When using the term "essentially cylindrical", intended is a shape that comprises a closed surface, such as cylinder, but including other closed shapes such as conical, and their combinations, enabling to fit the sheet well and tightly on protected surfaces of various geometries. In a preferred embodiment of the invention, a surface to be protected from the ice formation is associated with an airborne object. Said object is preferably an aircraft or a missile, particularly when moving relatively slowly through humid and cool environment. In an important aspect of the invention, a method is provided for preventing the formation of an ice layer on the surface of a missile moving through clouds, comprising wrapping the body or a part thereof of said missile into a sheet impregnated with said salt, or bonding said body or its part to said sheet. In a preferred embodiment of the invention, said salt comprises calcium chloride.

The invention aims at a method of inhibiting the ability of moisture or ice to adhere to a cool surface and to cause or promote the growth of ice layer on the surface. Further provided by the invention is a protective antiicing sheet for preventing the ice accumulation on surfaces exposed to humidity and a temperature lower than the water freezing point, preferably a temperature of from -15°C to -45°C, comprising a fabric or polymer layer impregnated with said salt. Said fabric or polymer layer preferably comprise a material selected from the group consisting of synthetic polymers, natural fibers, and a mixture thereof. Said sheet, which is inexpensive, may be used repeatedly for nearly unlimited time.

The invention relates to a method for manufacturing a protective antiicing sheet, comprising i) providing a porous polymeric layer or absorbing hydrophilic fabric; ϋ) wetting said fabric with a saturated solution of a salt

having a eutectic freezing point of less than -20 ⁰ C; and optionally iii) drying said wetted fabric. In a preferred embodiment, said salt is calcium chloride.

Detailed Description of the Invention It has now been found that a thin textile layer impregnated with calcium chloride provides surprisingly efficient protection against ice accumulation when covering a surface exposed to very low temperatures. When, for example, a plastic surface was protected with a cotton cloth impregnated with calcium chloride and equilibrated at -20 ⁰ C or -40 C, no icing (only slight wetting) was observed when the surface was sprinkled with water droplets at -25°C. The same plastic surface without said protection according to the invention showed a buildup of continuous ice layer firmly attached to the surface. When the same plastic surface was provided with a hydrophobic coating, water droplets, although not spreading on the surface, got frozen and formed hard icy bulges covering the entire surface.

The invention provides protection against the formation of ice layer on a cool surface, comprising covering the surface with a thin sheet impregnated with a stable inorganic salt having a eutectic freezing point of less than -20 ⁰ C, thereby inhibiting the ability of moisture or ice to adhere to a cool surface and to cause the growth, or to contribute to the growth, of an accumulated ice layer. A salt used in a method according to the invention should be stable at temperatures occurring both during the use of said thin sheet and during its manufacture. The salt preferably conforms to the safety and environment requirements. A preferred example of such salt is calcium chloride. The objects to be protected against the surface icing may include transportation or communication means, electric installations, construction elements, etc., while the situations may include high altitudes, high latitudes, winter season, or industrial refrigeration. In a preferred embodiment of the invention, provided is a method of protecting a solid surface exposed to a temperature much lower than the freezing point of water against ice

accumnlation, comprising covering said surface with a thin sheet impregnated with said suitable salt. Said solid surface may typically comprise metals, polymers, glass, and composites; said temperature comprises in preferred applications a temperature lower than -15°C; said sheet being capable of being impregnated, such as a film or fabric comprising porous polymer or woven fibers; said calcium chloride typically being a saturated water solution before the use of said sheet. For example, a silk cloth was soaked in saturated solution of CaCk, dried in air and hot pressed at about 150 ⁰ C, then cut to fit the shape of a polyetherimide base to which it was bonded with spots of polyurethane adhesive.

In a preferred embodiment of the invention, a method is provided of protecting an airborne object from ice buildup on its surface, comprising covering the parts of surface to be protected by a cloth impregnated with calcium chloride. It is a known phenomenon that a relatively slow movement through clouds is specially supportive for the ice buildup. The method of the invention provides, in a preferred embodiment, a particularly effective protection even for this situation: a method is provided of preventing the ice formation on the surface of airborne objects moving relatively slowly through clouds, an example of such objects being missiles, the method comprising covering said objects with a sheet impregnated with calcium chloride or other salt. Without wishing to involve any particular theory, it is believed that a thin layer comprising a freezing point depressant such as calcium chloride provides an efficacious protection against the deposition and accumulation of ice particles onto the missile surface; said super-cooled water droplets remaining in the liquid phase even at temperatures as low as -30 ⁰ C; said water droplets when contacting said protective sheet according to the invention, while said supercooled droplets seem to remain in the liquid phase. Other salts such as MgCb may provide the same effect, as a skilled artisan will appreciate. Whatever the mechanism, it was observed that the protective sheet according to the invention confers the desired protection. It was further observed that the efficiency of said sheet does not decrease with

water absorption and with time. It can be used over and over again without loosing its deicing properties.

The invention relates to a method of preventing the ice formation on solid surfaces exposed to temperatures lower than -20 ⁰ C, particularly lower than - 30 ⁰ C, comprising covering said surface with a porous or absorbing material impregnated with a salt depressing the freezing point of water by at least 25°C in eutectic solution. In a preferred embodiment said salt is calcium chloride. Said porous or absorbing material may be a knitted or woven fabric, comprising, for example, cotton, silk, polyester, and their mixtures. The material may have the form of dense continuous layer, or of a net-like composite, such as gauze. When using the term absorbing material, intended is a material, such as porous or fibrous, that can absorb efficient amounts of aqueous salt solutions, enabling the impregnation with concentrated aqueous salt solutions. Said porous or absorbing material thus may be also a hydrophilic polymer, or a material capable of absorbing sufficient volume of aqueous solutions into its pores. In a preferred embodiment the porous or absorbing material has the form of a thin sheet, sufficiently strong to be attached onto the surface and to withstand the planned working conditions, or a layer of elastic polymeric foam. The shape is designed according to the shape to be covered, the sheet is attached to the protected surface by any known method. Attaching a polymer sheet may comprise, for example, gluing or polymer melting. In the cases when the whole object should be surrounded by a protective sheet, said sheet may be provided as a closed shape, for example cylindrical, and may be "dressed" onto the object. In a preferred embodiment of the invention, a method is provided of preventing the ice formation on a surface exposed to temperatures lower than -20 ⁰ C, comprising covering the surface with a thin layer of a material impregnated with a salt or salt solution, wherein the surface is a closed surface, and the material has essentially cylindrical shape with a smaller surface area than the area of said closed surface, while being elastic and capable of being dressed onto said protected object. In a preferred embodiment of the invention, said closed

surface is a cylindrical surface, and the protective sheet is rolled onto it as a nylon stocking onto the leg. The thickness of said impregnated sheet may be designed in accordance with the component's demands and performances.

The invention, thus, provides a protective antiicing sheet for preventing the ice accumulation on surfaces exposed to temperatures lower than -20 ⁰ C comprising a fabric or polymer layer impregnated with a suitable salt, such as calcium chloride. The invention further provides a method for manufacturing a antiicing sheet, comprising i) providing a porous polymeric layer or absorbing hydrophilic fabric; ii) wetting said fabric with concentrated solution of a stable inorganic salt having a eutectic freezing point of less than -20 ⁰ C, preferably calcium chloride, and optionally iii) drying said wetted fabric. For example, silk cloth was soaked in a saturated solution of CaCh in diionized water, dried in air then hot pressed at about 150 ⁰ C, and then bonded to a surface to be protected. Said hydrophilic fabric may comprise synthetic or natural polymers, and may be knitted or woven or pressed fabric. Preferably, said fabric can absorb at least 70% water per square meter. Preferably said fabric has a thickness from about 50 μm to about 500 μm. Said fabric may comprise cellulose, polyester, polyamide, cotton or silk fibers. In a preferred embodiment, said fabric has the shape of a closed surface and can be either elastic or just bendable. In a preferred embodiment of the invention, the protective sheet is used repeatedly, without either removing water or re-impregnating. It has been observed that antiicing sheets according to the invention surprisingly do not lose their antiicing efficiency even when absorbing considerable volumes of water during their inhibiting activities. CaCh comprises a cryohydrate which is solid at about -55°C and melts at -53 ⁰ C. It attaches very strongly to the cloth and is hygroscopic. Thus it allows large volumes of water to be absorbed without a need to dry or re -impregnate.

The invention, thus, solves the functional and safety problems associated with the ice formation on objects exposed to very low temperatures and high water concentrations either on the ground or on the sea or during the flight. Said objects may comprise transportation means, aerodynamic elements, communication means, construction elements, or electric installation. The surface of said objects may comprise metals, plastics, glass, ceramics, composites, etc. The ice buildup, possibly forming an undesired optical or electromagnetic shield, or accretions on the windows, leading to the mass increase of the objects, and possibly changing the aerodynamic parameters of the flying object, and possibly blocking optical and electro-magnetic sight, is hindered by the invention, without spreading undesirable components in the environment.

The mass, shape, and surface changes of flying objects associated with the ice accumulation have deleterious effects on the flight performance and controllability. The invention particularly relates to preventing the ice formation occurring on the outer surfaces of aircrafts or missiles flying through clouds, rain, or snow. A technically simple, low cost, passive method of the invention hinders the ability of moisture, fog, supercooled droplets, snow, and ice to adhere to a cool surface and to initiate or promote the growth of accumulated ice layers.

The invention will be further described and illustrated in the following examples.

Examples

Example 1

A static test compared the behavior of 18 plastic boards made of polyetherimide (PEI) and 3 made of Teflon, all 5 mm thick, and each having a surface of about 400 cm ² . Three PEI boards and three teflon boards remained untreated, twelve PEI boards were coated with Siθ2, SiN, TiN, and

silane, and 3 boards were covered with a protective sheet according to the invention, the sheet being cotton cloth impregnated with saturated CaCb, dried for five hours in air, and then hot pressed at 15O ⁰ C, and finally glued with patches of polyurethane adhesive onto the plastic surface. The treated boards were equilibrated at -2O ⁰ C and at -4O ⁰ C for 4 hrs and then sprinkled with water having a temperature of about +25°C, using a pressurized spray bottle providing water fog (droplets size of about 10 μm), the amount of water deposited on each slab was about 100 ml within 30 seconds, building ice layers up to several mm high on a board 20 x 20 cm ² . The untreated boards provided a continuous ice layer firmly attached onto the board surface. Said 12 boards having various hydrophobic treatments provided separated ice bulges (up to 4 mm high) covering densely the entire area. The boards protected according to the invention remained wetted without traces of ice.

Example 2

The test as in Example 1 was effected on cones 400 mm high x 200 mm diameter, yielding similar results as Example 1.

While this invention has been described in terms of some specific examples, many modifications and variations are possible. It is therefore understood that within the scope of the appended claims, the invention may be realized otherwise than as specifically described.