WO1988008018A1

WO1988008018A1 - Ice release surfaces

Info

Publication number: WO1988008018A1
Application number: PCT/GB1988/000285
Authority: WO
Inventors: Edgar Harold Andrews; Norman Anthony Lockington
Original assignee: Qmc Industrial Research Limited
Priority date: 1987-04-14
Filing date: 1988-04-14
Publication date: 1988-10-20
Also published as: GB8923238D0; KR890701704A; CA1329743C; JPH02503090A; GB2224030A; GB8708876D0; EP0356447A1; AU1620088A; GB2224030B

Abstract

An ice release system comprises a substrate and an ice release coating on said substrate, said system remaining effective for multiple ice release events. The coating comprises a two component polymeric mixture. The first component of the coating is so firmly bonded to the substrate that it cannot be completely removed with a solvent and the second component is substantially completely compatible with the polymeric material of the first component and is homogenously dissolved or dispersed therein. The second component is substantially unbonded to the substrate, is not water leachable, has a glass transition temperature (Tg) less than 0°C and is sufficiently viscous that the second component will not exhibit substantial flow from the mixture over a vertically arranged coated substrate. In an ice release event a surface portion of the second component is removed with the ice, whereafter migration of the second component in the mixture re-establishes homogeneity to form a new surface portion of the second component.

Description

ICE RELEASF SURFACES

This invention relates to the production of an ice-release surface.

The formation of ice on engineering structures during winter weather creates obvious hazards and inconvenience. The problem is particularly acute when ice forms on aircraft control surfaces, on radar installations and on ships. It has long been desired, therefore, to find an efficient means of preventing ice formation on surfaces or of shedding ice from surfaces once it has formed.

Several approaches have been taken.

(1) Electrical and other heating means have been used to maintain the surface at a temperature above that at which ice will form. This is highly effective but is costly in terms of capital and running costs and is not always practicable.

(2) De-icing fluids and pastes. These are applied to surfaces to prevent ice formation or assist ice shedding and work by lowering the freezing point of water by the solution of salts and/or by creating an oily or grease-like barrier between the ice and the solid surface, thus minimising ice adhesion. These means of protection are only temporary, typically requiring re-application every 20 to 40 minutes.

(3) Low surface energy polymeric coatings. The adhesion of ice to non-polar polymers such as PTFE (polytetrafluorethylene), polyethylene, acrylics and certain polyurethanes is considerably lower than to polar materials such as metals and glass, sometimes by a factor of 10 to 20. Ice shedding from such surfaces is therefore considerably easier than from uncoated structural materials. However, the residual adhesion of ice to even these materials is normally strong enough to create icing problems. Furthermore, the adhesion tends to increase with successive ice release events. In quantitative terms, these materials have adhesion energies (defined below) to ice at -20°C in the region of 0.1 to 0.3J/m² compared with around 1J/m² for ice adhering to metal and glass (the latter figure is actually the cohesive strength of ice since attempts to remove ice at -20°C from high energy surfaces normally results in fracture in the ice rather than, at the interface). For effective ice-release it is believed that an adhesion energy of less than 0.1J/m² is required. Spontaneous release ("fall off") occurs at around 0.01J/m² depending on the conditions. These very low adhesion energies cannot be achieved directly with any known solid surface.

(0.1J/m ² corresponds to a shear stress of 0.5N/cm² for ice removal).

Jellenick et al, U.S. Patent No. 4301208 discloses a method for reducing the adhesion of ice to surfaces, especially locks which comprises coating the surface with a composition carrying a block copolymer of a polycarbonate and dimethyl siloxane preferably with the addition of a silicone oil. The composition is simply applied as a viscous solution for example by spraying. The composition allows multiple ice release events, up to about 7, but the release energy of ice is only reduced by a factor of approximately 10 to at best 2.2N/cm².

In addition to the silicone oil, when present, being gradually removed during ice release the block copolymer is also removed from the surface so that the whole coating process must be repeated at intervals.

(4) Flexible surfaces. It has been found that ice adhering to a flexible substrate can be removed by smaller applied forces than are needed to remove ice from a rigid surface of a substrate of the same material. This is because in a flexible system the application of a given force stores more energy than in a rigid system and this energy is potentially available to drive the debonding process. For a given adhesion energy, therefore, a lower applied force will generally be needed to remove the ice if the system is flexible.

(5) Mechanical removal. Vibration or flexing of a surface to which ice has adhered may be effective in removing the ice. This means is of limited value because mechanical deflection of a surface requires additional equipment and may not be acceptable or feasible in any given application.

The purpose of the invention described herein is to create a durable system to which ice has a very low adhesion energy ( 0.1J/m²) and which retains its ice-shedding capabilities for long periods of time in the presence rain, dust, and under conditions of multiple ice formations and releases. This is achieved according to the invention by creating on the surface of a base material such as a polymer film or a metal surface a polymeric layer containing at least one component which is sacrificial in a controlled manner.

That is, each ice release occurs with the removal of a small amount of the sacrificial material in such a way that the sacrificial component of the layer is not exhausted preferably until as many as fifty ice formation and release events have taken place.

The layer is preferably such that the adhesion energy between it and ice at -20°C is initially less than 0.1J/m² and remains below this value for successive ice release events until the sacrified component is exhausted. This means of ice release may be used in combination with other ice release means, for example with heating and/or with a flexible substrate or underlayer.

This invention provides an ice release system comprising a substrate and an ice release coating on said substrate, said system remaining effective for multiple ice release events, wherein said coating comprises a two component polymeric mixture, a first component of the coating being so firmly bonded to the substrate that it cannot be completely removed by a solvent and a second component being substantially completely compatible with the polymeric material of the first component and being homogenously dissolved or dispersed therein, the second component being substantially unbonded to the substrate and not being water leachable and having a glass transition temperature (T_g) less than 0°C and being sufficiently viscous that the second component will not exhibit substantial flow from the mixture over a vertically arranged coated substrate, whereby in an ice release event a surface portion of the second component is removed with the ice, whereafter migration of the second component In the mixture re-establishes homogeneity to form a new surface portion of the second component.

The invention also provides methods for producing the ice release coatings on substrates.

The two polymeric components of the coating may be identical or different polymeric species but the two components must be compatible to the extent that the second component is physically or mechanically bonded, for example by molecular entanglements, to the first component such that partial removal of the second component occurs during an ice release event without removal of the whole of the second component.

The polymeric material of the first component is preferably a polymeric species that can be physically or chemically grafted for example covalently bonded onto the substrate, for example by the action of a free radical initiating means or the use of a difunctional "linking" chemical such as a di-isocyanate or the use of reactive groups on the molecules of the first component which can under suitable conditions be caused to react with the substrate. If the first component is not already crosslinked or polymerised bonding of the first component may be accompanied by crosslinking or polymerisation or further cross-linking or polymerisation to increase its molecular weight and viscosity and optimise its physical or mechanical properties. This polymerisation or crosslinking of the first component may be achieved by the same chemical or physical means as the grafting step or by different means which may include any known method of polymerisation or crosslinking. The second component may be formed simultaneously by the same crosslinking or polymerisation reaction as the first component without bonding to the substrate.

Alternatively, the second component may be absorbed into an already existing layer of the first component, preferably from solution or a melt, and then may be subjected, if necessary, to a cross linking action to increase its viscosity to the desired level.

This latter method may be used to regenerate an ice release surface after exhaustion of the second component following repeated ice release events.

The second component is preferably of an oily, greasy or waxy consistency.

As examples of polymeric species that can be used as first and/or second components in the process of the invention there may be mentioned, for example, mineral oils, organic polyalkylsiloxanes, for example in the form of polysiloxane oils, fluorinated polymers, especially perfluorinated polymers, ethylene/propylene copolymers and polyurethanes. Especially when used to form the first component, these polymers may be modified by the incorporation of reactive end or side groups for example hydroxyl, vinyl or acrylic groups to promote grafting, polymerisation or crosslinking.

If the substrate does not have a surface that is receptive to the polymeric species to be bonded thereto the surface may need to be subjected to a preliminary treatment. Such treatment may, for example, comprise chemical etching or vapour phase reaction including oxidation of the substrate surface, exposure of the surface to electric discharge, plasma, corona, or ion bombardment, the exposure of the surface to energetic radiation, such as electron beams, ultra violet rays or gamma rays; absorption onto the surface of reactive species, for example hydroxyl groups from atmospheric water: or by applying an intermediate layer of a material that will bond both to the substrate surface and to the grafted component. Combinations of such treatment methods may be used.

When the first component is a cross-linked polymer there is the advantage that that a lesser number of points of bonding to the substrates are required so that any preliminary treatment of the substrate may be milder. The bonding step may be achieved by any of the known techniques, for example, by heating the component to be bonded and the substrate, alone or together with initiators, catalysts, accelerators or other additives to generate free-radicals or to promote other kinds of reaction between the molecules of the first component and with the substrate surface or by the use of physical free-radical initiating means, such as, for example, electron beam, ultra violet or gamma radiation. Termination of the reaction which may be necessary to ensure that an unbonded second component remains may be achieved by any of the conventional chain stopping techniques. Combinations of grafting techniques may be used.

The ratio of the two polymeric components in the coating is not critical provided that there is sufficient of the bonded component to retain the second component so that it is not all removed in a single ice release event and similarly, that there is sufficient of the second component that it is not all removed in a single ice release event.

It is possible by the method of the invention using polydimethylsiloxane oil to produce a surface for which the ice adhesion energy is less than 0.1J/m² for at least 50 ice removals. Generally the ice adhesion energy with a fresh coating is around 0.01J/m² this value rising gradually with repeated ice release events.

The substrate may be, for example, a massive structure such as a part of an aeroplane or ship but is preferably a film or sheet of metal or plastics that can be attached to such a part. In a preferred form of the invention the substrate is an adhesive tape, sheet or film. The substrate may for example be a polymeric material such as polyvinylidene fluoride, polyvinyl fluoride, polyethylene tercphthalate or polyethylene, or a material such as aluminium or an alloy thereof or steel.

Ice adhesion energy, θ, for the purposes of this application, is defined as the energy required per unit area to propogate debonding along an adhesive interface. It may be measured in various ways but always requires an initially debonded region to be propagated (like a spreading crack) along the interface by the application of forces or deflections to the bonded specimen. The method used in the examples to measure the ice release energy comprises fixing the substrate of interest to a circular metal base-block with a central hole running through both the base-block and substrate. A circular non-adhering disc of radius r is placed over the hole and ice is cast on to the substrate so as to enclose totally the disc and form a cylinder of height 'h' above the plane of the disc. Gas pressure is applied to the encapsulated disc (which acts as a circular crack or debonded zone at the interface) through the hole and increased until failure occurs at the interface at a critical pressure P_c.

Theory then gives

where E is the Young's modulus of the adhesive (e.g. ice) and f is a known function of (h/r). (Ref .E.H.Andrews & A.Stevenson, J.Materials Sci.13 (1978) 1680-1688)

Since θ is by its nature a function of the rate of debonding we specify that a rate of pressurisation between 10³ and 10⁴kNm s be used, preferably around 3.10 ³kNm^-2s^-1. Also results quoted in this specification refer to a base block diameter of 29mm and a value of h of 6mm.

The following examples Illustrate the invention: Example 1

A plasma treated polyvinylfluorlde (PVF) film (DU Pont "Tedlar" grade 150BL 30WH) is used as the base material. A silicone oil (polydimethylsiloxane) (PDMS) of viscosity 350cs is used to form the ice release layer. 10% by weight of dicumy peroxide (DCP) (a free radical inditiator) is dissolved in the oil by warming and the resulting solution is spread liberally on the PVF film which is then heated at 150°C for 2 hours. Some of the PDMS becomes grafted to the plasma treated surface by free radical reactions and cannot be removed by solvent treatment. The PDMS also crosslinks to form a grease, bordering on a week gel. The surface was blotted to remove excess free oil. The crosslinked layer is resistant to prolonged exposure to running water and has excellent ice-release priorities.

Ice was cast on this surface at -20°C and removed at this temperature after 1 hour manually. This was repeated a number of times on the same surface and θ was then measured using the method described earlier and in the literature (Ref.E.H.Andrews & N.A.Lockington, J.Materials Sci. 18 (1983) 145W465.) A graph of θ versus number of ice removals was plotted to show that 6 remained below 0.1J/m² for 50 or more ice removals (Fig.l). Ice removed in these tests, when melted, could be seen to have a thin floating oily layer on its surface indicating the sacrificial character of the ice-release surface. This oily material was collected from numerous tests and analysed by gel permeation chromatography. The results showed that the sacrificed material consisted of PDMS of molecular weights ranging from 1200 to 150000 with an average of 14000. The amount of sacrificial material removed at successive ice release events was measured for many samples and averaged to give the graph of Fig. 2.

The sacrificial component comprised about 21% of the total coating weight. Example 2.

The procedure of example 1 was repeated using PDMS of different viscosities, namely 20cs, 350cs and 12,500cs. The initial value of θ for ice release at -20°C was of the order of 0.02Jm^-2 in each case.

Immersion in cold running water simulating rainfull for 160 and 500 hours, followed by determination of θ, gave the following results:

PDMS viscosity 20cs 350cs 12,500cs

160 hours washing 0.10Jm^-2 0.03Jm^-2 0.07Jm^-2

500 hours washing 0.25Jm^-2 0.03Jm^-2 0.20Jm^-2

Example 2B.

The tests described in example 2A were repeated, but with addition of 20% DCP to the PDMS, instead of 10% DCP. The initital values of θ for ice release at -20°C were again of the order of 0.02Jm^-2

Exposure to running water for 160 and 500 hours, followed by determination of θ, gave the following results.

PDMS 20cs 350cs 12,500cs

160 hours washing 0.02Jm^-2 0.02Jm ^-2 0.05Jm^-2

500 hours washing 0.02Jm ^-2 0.02Jm ^-2 0.10Jm ^-2

Example 2C.

Example 2A was repeated, but using only 5% DCP added to the 350cs

PDMS. The initial value of θ for ice release at -20°C was about 0.03Jm^-2, but after : 160 hours exposure to running water, θ had increased to 0.10Jm^-2 Example 2D

The procedure of example 1 was repeated , but using temperatures lower than 150° C for grafting the 350cs PDMS to the PVF surface; 10% DCP was dissolved in the oil . The values of θ for ice release at -20° C were determined for the treated surface, and after brief cleaning with a solvent (hexamethyl disiloxane) , which leaches out the sacrifical component, as an Indication of durability. The results were :

Temperature of treatment 110°C 120°C 13ϋ°C 140°C 150°C

for as treated surface: 0.05Jm^-2 0.03Jm^-2 0.02Jm^-2 0.02Jm^-2 0.02Jm^-2

after solvent treatment: 0.75Jm^-2 0.72Jm^-2 0.57Jm^-2 0.35Jm^-2 0.11Jm^-2

Example 2E

The procedure of example 1 was repeated with 10 and 20% DCP with heating, but at 150°C, for different times. The number of ice releases before the value of θ rose to 0.1Jm^-2 was found for each condition of treatment.

350cs PDMS + 10% DCP Heating time (min) No. of ice releases

30 2

60 20

120 50

350cs PDMS + 20% DCP 30 3

45 20

60 25

120 50 Example 2F

The procedure of example 1 was repeated, but heating was carried out in a vacuum oven for 1¾ hours at 140°C. Using the same assessment methods, the surfaces produced were found to have similar ice-release properties to those produced in an air oven in 2 hours at 150°C.

Example 2G

The procedure of example 2A was substantially repeated using a fluorinated polysiloxane oil (FS 1265/10000cs) instead of PDMS.

After 100 h of washing in running water a 6 value of 0.01 j/m² was recorded for first ice release.

Example 3

Example 1 was repeated, but high density polyethylene film was used as the base material and the ice release layer was established using 200/350cs PDMS with 10% w/w added dicumyl peroxide at 150°C for 2 hours. The film was then immersed in cold running water for 94 hours to simulate the effects of rain before testing. A first ice-release value of θ of 0.003 to 0.005J/m² was obtained at -20°C.

Example 3A

The procedure of example 3 was repeated but using as a substrate a film of polyethylene terephthalate (PET) supplied by 3M Co., USA which has been surface treated by a proprietary method to improve adherability. θ for the sixth ice release was 0.06 J/m² and the same for the twentieth ice release.

Example 3B

As exampl e 3A but using a film of polvinylidine fluoride (PVDF) as the substrate. Initial θ values were around 0.04 J/m².

Example 3C

As example 3A but the PET film surface had first been coated with a thin layer of a diacrylate polymer (coated film supplied by 3M Co., USA). Initial θ values were around 0.04 J/m².

Example 3D

As example 3A but the PET film had first been coated with a thin layer of a siloxane-PET copolymer (coated film supplied by 3M Co., USA), θ for first ice release was 0.05 J/m².

Example 4.

Example 1 was repeated but using various chemical etching treatments instead of plasma treatment of the PVF. Du Pont "Tedlar" PVF film grade 200SG 40TR was surface treated in various ways as detailed below

Solvent Treatments i) The PVF film surface was swabbed with acetone, trichlorethylene, carbon tetrachloride, chloroform or dimethyl formamide, for between

30 sec.and 30 minutes. Alternatively, the film was immersed in one of these solvents for up to 24 hours at room temperature. DMF dissolves PVF so that only mild treatments with this solvent were possible.

Improved ice release characteristics were obtained after some of these pre-treatments. For example, a 2 min. soak in trichlorethylene gave θ <0.1J/m²for up to 14 ice-release events whereas the untreated film gave no more than 6 ice release events below this θ value. ii) NaOH/tetrabutyl ammonium bromide (TBAB)

30% NaOH solution with 0.5% w/v TBAB added was subjected to constant agitation and the PVF film was Immersed in it for approximately 10 minutes and washed. Pre-treatment at 25°C improved the reproducibility of ice-release data, but at higher temperatures (up to 95°C), ice release required higher energies. This pre-treatment gave acceptable results when a rubber underlayer was included (see example 5).

iii) Sodium/ammonia

The PVF film was treated in a solution of 1.5g sodium metal in 100ml liquid ammonia at -33°C and washed in distilled water. When combined with a pre-soak in C₂HCI₃ this treatment gave up to 17 ice releases at θ <0.1J/m²

iv) Sodium/naphthalene

5.75g of sodium and 32g of naphthalene were dissolved in 250ml of tetrahydrofuran, and the PVF film immersed therein at 25°C for 5 to 30 minutes followed by rinsing with acetone. When combined with a pre-soak in C₂HCl₃ this treatment gave up to 13 ice release events at θ <0.1J/m²

Example 5

PVF film (du Pont "Tedlar" grade 200SG 40TR) is used as the base material as in example 4 but a soft layer of vulcanized natural rubber is placed between the metal base-block of the test apparatus and the PVF film and secured in place by a rubber latex adhesive ("Copydex") to provide a flexible base. The ice release layer is established exactly as in example 1. The following results were obtained for the various pre-treatments of the PVF film indicated.

Pre-treatment of PVE Behaviour of θ J/m²

None 1st ice release 0.02 25th ice release 0.08

NaOH/tetrabutylammonium bromide 1st ice release 0.02 25th ice release 0.08

Na/naphthalene 1st ice release 0.05

25th ice release 0.07

35th ice release 0.04

51st ice release 0.02

Example 6

Example 5 was repeated in all respects except that the silicone oil was replaced by a monoacrylic-terminated polydimethyl siloxane oil supplied by 3M Co.U.S.A. With Na/naphthalene pre-treatment of the PVF film, and 1mm rubber underlayer, the following results were obtained

1st ice release 6 = 0.04j/m² 41st ice release θ = 0.085J/m²

Without a rubber underlayer the values were 3rd ice release θ=0.016J/m² 40th ice release 9=0.045J/m².

Example 6A

As example 6 but using an as-received film of PVDF as the substrate and without a rubber underlayer. θ for the fourth ice release was 0.05J/m².

Example 7A

Stainless steel is used as a base material having adsorbed hydroxyl groups on its oxide surface. A di-hydroxy terminated ABA block copolymer of (A) polyethylene glycol and (B) PDMS is used to form the release layer. The block copolymer is heated in contact with the steel surface at 200°C for 5 minutes. Water is eliminated and some grafting occurs to the surface, while net thermal crosslinking linear also occurs leaving a surface layer with a rubbery feel. The resulting layer is partially leachable by solvent but a permanent layer remains, θ values for ice release are not as low as in Examples 1-6 but values in the range 0.06 to 0.16 are obtained for up to 11 ice release events.

Example 7B

As 7A but a mono-hydroxy functional PDMS was used to form the ice release layer. Similar θ values were obtained.

Example 7C

Stainless steel was used as the substrate and the ice release layer was formed from silicone oil of 350 cs viscosity cured thermally for 2 h at 150 deg.C. The surface was washed with running water for 95 h. For first ice release θ was in the range 0.05 to 0.08 j/m².

Example 7D

As example 7C but the substrate was a commerical grade aluminium sheet material. Otherwise all details were as example 7C. A 9 value for first ice release of 0.04 J/m² was obtained. When 10%

DCP was included in the oil a θ value of 0.065 J/m² was obtained for first ice release.

Example 8

Different base polymer films were spread with 350cs sil icone oil with and wi thout 10% added dicumyl peroxide and exposed on a moving web to el ectron beam irradiation under ni trogen. Excess oil was removed by blotting. The Irradiation doses given are indicated below together with the θ values obtained at -20°C for first ice release.

Treatment Dose MR Film θ J/m²

350cs/DCP 30 PVF 0.085

30 PVDF 0.11

350cs oil 35 PVDF 0.23

35 PE 0.20

35 PET 0.35

Key: PVF = polyvinyl fluoride; PVDF= polyvinylidine fluoride PE = polyethylene; PET = polyethyleneterephthalate

Example 9

Example 1 was repeated but using a multigrade mineral motor oil in place of PDMS. First ice release gave θ = 0.05J/m².

Example 10.

Samples were prepared and tested as in Example 1 but having the sacrificial component exhausted through multiple ice release or solvent washing and with 6 values which had risen to 0.3 to 0.6 J/m² were soaked for 30 minutes in 20cs siloxane fluid and allowed to drain overnight.

Remaining excess oil was removed by blotting. Resulting θ values of 0.02 to 0.04 J/m² were recorded for 6 ice release events.

Claims

1. An ice release system comprising a substrate and an ice release coating on said substrate, said system remaining effective for multiple ice release events, wherein said coating comprises a two component polymeric mixture, a first component of the coating being so firmly bonded to the substrate that it cannot be completely removed with a solvent and a second component being substantially completely compatible with the polymeric material of the first component and being homogenously dissolved or dispersed therein, the second component being substantially unbonded to the substrate and being not water leachable and having a glass transition temperature (T_g) less than 0°C and being sufficiently viscous that the second component will not exhibit substantial flow from the mixture over a vertically arranged coated substrate, whereby in an ice release event a surface portion of the second component is removed with the ice, whereafter migration of the second component in the mixture reestablishes homogeneity to form a new surface portion of the second component.

2. A system according to claim 1 wherein the first component is chemically bonded to the substrate.

3. A system according to claim 2 wherein the first component is covalently bonded or grafted to the substrate.

4. A system according to any one of claims 1 to 3 wherein the first component is a cross linked polymer.

5. A system according to any one of claims 1 to 4 wherein the second component is of oily or greasy consistency.

6. A system according to any one of claims 1 to 5 wherein the second component is a cross linked polymer.

7. A system according to any one of claims 1 to 6 wherein the first and second components are identical polymer species in different states of polymerisation or cross-linking.

8. A system according to any one of claims 1 to 7 wherein the polymer material of the first and second component Is selected from polyalkylsiloxanes, fluorinated polymers, ethylene/propylene copolymers, polyurethanes and polyalkylene glycol polyalkylsiloxane block copolymers which may be modified by the incorporation of additional active groups.

9. A system according to any one of claims 1 to 7 wherein the polymer material of the first and second component is a polydimethyl siloxane.

10. A system according to any one of claims 7 to 10 wherein the second component is a residue remaining after a treatment to chemically bond the first component to the substrate.

11. A system according to any one of claims 1 to 10 wherein the substrate is a sheet or film of metal.

12. A system according to any one of claims 1 to 10 wherein the substrate is a polymeric film or sheet.

13. A system according to claim 12 wherein the substrate is selected from polyvinylidine fluoride, polyvinyl fluoride, polyethyleneterephthalate or polyethylene films or sheets.

14. A system according to any one of claims 1 to 13 wherein the surface of the substrate has been treated to promote or facilitate bonding of the first component.

15. A system according to any one of claims 1 to 14 wherein the substrate is a laminar substrate that can be bonded to a surface of an article from which ice release is desired.

16. A system according to claim 15 wherein the suface of the substrate opposite to the ice release surface is provided with an adhesive coating.

17. A system according to any one of claims 1 to 16 wherein the adhesion energy of ice to the ice release surface at - 20°C is less than 0.1 J/M² after 10 ice release events.

18. Aprocess for producing an ice release coating on a substrate which comprises forming on the substrate a coating of a two- component polymer mixture, the polymer mixture comprising first and second components as defined in claim 1.

19. A process for according to claim 18 which comprises applying to the substrate a cross-linked or cross-linkable polymeric material or a precursor therefor and subjecting the coated substrate to a treatment to promote the formation on the substrate of a two component coating comprising cross-linked polymer firmly bonded to the substrate, and having homogenously dispersed or dissolved therein a second polymeric component unbonded to the substrate, the second component forming a sacrificial component of the ice release coating which remains active for multiple ice release events.

20. A process according to claim 18 which comprises first applying to the substrate a firmly bonded coating of said first component and subsequently applying said second component to said first component whereby said second component is sorbed into the coating of said first component.

21. A process according to any one of claims 18 to 20 wherein before application of the coating the substrate surface is treated to produce active sites thereon to promote bonding to the cross-linked polymer.

22. A process according to claim 21 wherein said treatment comprises chemical etching or vapour phase reaction of the substrate surface, exposure of the surface to electric discharge, plasma ,corona or ion bombardment or to energetic radiation, absorption onto the surface of reactive species or application of a layer of a material that will bond both to the substrate surface and to the cross-linked polymer or a combination of any such methods.

23. A process according to any one of claims 18 to 23 wherein the cross-linked polymer is covalently bonded or grafted to the substrate.

24. A process according to claim 23 wherein bonding is achieved by heating the component to be bonded to generate free radicals or to promote other forms of chemical reaction or by the use of chemical or physical free radical initiating means.

25. A process according to claim 24 wherein the bonding reaction is terminated prior to completion to leave a residue of unbonded polymer as the second polymeric component of the ice release coating.

26. A process according to any one of claims 23 to 25 wherein the polymeric species is a cross linkable polymer or a precursor or a cross linkable polymer and polymerisation of a precursor and/or cross linking is carried out simultaneously with the bonding reaction.