WO2005075112A1

WO2005075112A1 - Surface coating with anti-dew and ice non-stick properties

Info

Publication number: WO2005075112A1
Application number: PCT/DK2005/000088
Authority: WO
Inventors: Thomas Zweig
Original assignee: MULTI SIGN AS
Current assignee: MULTI SIGN AS
Priority date: 2004-02-10
Filing date: 2005-02-10
Publication date: 2005-08-18
Anticipated expiration: 2006-08-10

Abstract

The present invention relates to a heterogeneous surface structure having hydrophobic and simultaneously hydrophilic properties adapted to maintain high optical property of the surface under different conditions. The surface structure comprises at least a first surface area having a first surface energy level, wherein the energy level is such that hydrophobic property is obtained at said at least first surface area, and at least a second surface area having a second surface energy level, wherein said second surface energy level is higher than said first surface energy level, and wherein the energy level is such that hydrophobic property is obtained at said at least second surface area, wherein the energy level difference between said first and second surface areas is adapted to prevent the generation of a compact ice layer, wherein said at least first and second surface areas are hybrid layers, containing covalently bounded organic and inorganic phase.

Description

SURFACE COATINGS WITH ANTI-DEW AND ICE NON-STICK PROPERTIES

FIELD OF THE INVENTION

The present invention relates to surface coatings with anti-dew, dirt repellent, anti-graffiti and ice non-stick properties.

BACKGROUND OF THE INVENTION

In the last years there has been some development in developing surfaces where high optical requirements are needed under various whether conditions. A typical example of such surfaces is traffic signs. It is well known that dew, ice, dirt and graffiti layers at surfaces often cause serious problems where the optical requirements are important.

Dew is created when a surface temperature is low and the air chills to the dew point so that the water vapour condenses to a liquid. Typically, very small water droplets are formed at the surface structure. The contact angle and drop diameter depend strongly from the surface energy at the surface structure. In case of surfaces with special optical properties, the negative effect of dew formation at surfaces is due to the water droplets created and their influence on the optical properties. One way to undergo dew formation at technical surfaces is by heating these surfaces up to more than 3 °C above the actual dew point. This may be difficult to realize in practise and is very uneconomical. Another way to undergo dew formation at such technical surfaces is to use high energetic surfaces with hydrophilic property, so that the droplets formation becomes evenly distributed on the surface and therefore do not affect the contact angle. Ice is formed at a surface when the surface temperature is below the freezing point, and the ambient temperature is close to or below the freezing point too. The characteristics and form of ice depend on the surface temperature, the super cooling rate, the ambient temperature, the relative humidity and the air convection around the surfaces. Hydrophobic surfaces with low surface energy have often been selected for the design of surfaces with ice non-stick property. In the last years a relative large amount of coating systems have been developed, wherein mainly water repellent materials with a very low surface tension, such as PTFE (Teflon) or Polyethylene have been chosen. Besides from being ice repellent materials, they have also shown to have dirt and graffiti repellent properties.

US 6,696,225 and WO 99/57185 describe surfaces which consist of a combination of surface areas having hydrophilic and hydrophobic properties, where the hydrophilic areas are embedded into the pre-fabricated surface layers having hydrophobic properties. Through such a combination the surfaces have both anti dew as well as anti ice properties, where the hydrophilic areas act as a nucleation sites with anti-dew property, and the hydrophobic areas there between as anti ice areas. The problem with the surfaces as dis- closed in these references it the weak binding energy between the areas having the hydrophilic properties and the substrate since these are embedded into the surface. Therefore, when localized ice formation takes place on the hydrophilic areas, the bounding between the localized ice layers and the hydrophilic areas can easily exceed the bounding between the hydrophilic areas and the surface. The result is that when the localized ice layer falls off, it can rip the embedded hydrophilic areas from the surface with it, thereby leaving the surface with a shortage of hydrophilic areas. In that way the anti- dew property of the surface is reduced and therefore the initial property of the surface. OBJECT AND SUMMARY OF THE INVENTION

It is therefore an object to solve the above mentioned problems.

According to the first aspect, the present invention relates to a heterogeneous surface structure having hydrophobic and simultaneously hydrophilic properties adapted to maintain high optical property of the surface under different conditions, comprising:

• at least a first surface area having a first surface energy level, wherein the energy level is such that hydrophobic property is obtained at said at least first surface area, and

• at least a second surface area having a second surface energy level, wherein said second surface energy level is higher than said first surface energy level, and wherein the energy level is such that hydrophobic property is obtained at said at least second surface area, wherein the energy level difference between said first and second surface areas is adapted to prevent the generation of a compact ice layer,

wherein said at least first and second surface areas are hybrid layers, containing covalently bounded organic and inorganic phase.

By creating such surface structure the formation of a closed ice layer at the whole surface is disturbed, thereby keeping the protected surface still visible. Since said first and second surface areas are synthesized as hybrid layers, a strong covalent bounding can be obtained between said second areas and the surrounding first areas during manufacturing. Therefore, the hydrophilic areas are very strongly bound to the surface, wherein the binding energy is much stronger than the binding energy, or adhesion energy, between the localized ice crystals at the hydrophilic areas. This prevents that the hydro- philic areas will be ripped of when localized ice crystals formed at said surfaces fall off and therefore enhances significantly the lifetime of said heterogeneous surface structure.

Preferably the energy level of at least a first surface area is significantly or much higher than the surface energy of water, 72,8mJ/meter, thereby having anti-dew property since the dew will be uniformly distributed on the surface. The negative effect of dew water droplets onto optical properties of surfaces may therefore be avoided. If the surface has a retroreflective pattern, the property of said pattern is obtained. It is essential that the distance between the nucleation areas is large enough so that the ice crystals at the nucleation areas do not get in contact and form a compact ice layer.

Preferably, the surface energy of at least a second surface area is signifi- cantly lower than the surface energy of water, thereby having ice non-stick property with simultaneous dirt repellent and anti-graffiti property.

In one embodiment the arrangement of said areas is predefined, e.g. has a matrix like structure, so that a homogeneous property of the surface structure is obtained.

In one preferred embodiment said at least second surface area is a heterogeneous surface with nucleation points having higher surface energy than at the area between said nucleation points, preferably higher than the surface energy of water, 72,8mJ/meter. The ice crystallisation at said areas is thereby further localized and the possible ice adhesion reduced.

In an embodiment, said at least one first surface area is comprised in a sur- face layer having said hydrophilic property onto which said at least one second surface area is coated. In that way, as strong bounding is obtained be- tween the hydrophilic areas since these areas are a part of a complete hydrophilic layer.

Depending on different weather conditions, the area ratio between the first and the second areas can play an important role. As an example, where ice formation is the dominating problem, the area with hydrophobic property (ice non-stick) is larger than that having hydrophilic property (anti-dew). In the same way, where dew formation is the dominating problem, the area with hydrophilic property is larger than the area having hydrophobic property.

According to the second aspect, the present invention relates to a method for coating a heterogeneous surface structure having hydrophobic and simultaneously hydrophilic property adapted to maintain high optical property of the surface under different conditions, the method comprising the following steps:

• coating a surface area with a first material such that a surface layer is obtained having a surface energy level such that hydrophilic property is obtained at said surface layer, and

• coating a surface area with a second material having a surface energy lower than said first surface energy level such that hydrophobic property is obtained at said at least one area, wherein the energy level difference between said at least first and second surface areas is adapted to cause a local ice crystallization at said nucleation areas and thereby preventing a generation of a compact ice layer at the whole surface,

wherein said at least first and second surface areas are hybrid layers cova- lently cross linked to each other.

In one embodiment coating the surface area with said first material comprises coating the whole surface area with said first material and subse- quently coating the surface with said second material such that said heterogeneous surface structure is obtained. Preferably, said first material is a material having high surface energy and anti-dew property, and said second material is a material having low surface energy having ice non-stick prop- erty.

In another embodiment the coating comprises coating the surface area with said first material such that a predefined area of said first material is obtained, and subsequently coating the surface with said second material such that a predefined area of said second material is obtained, thereby defining said heterogeneous surface structure.

In still another embodiment the coating comprises coating the surface areas with said first and second materials simultaneously.

According to the third aspect, the present invention relates to a printing device adapted for printing and coating a surface of a material such that a heterogeneous surface structure having hydrophobic and simultaneously hydro- philic property is obtained, comprising:

• a printing section adapted for printing on said surface, and

• a coating section for coating said surface such that said heterogene- ous surface structure is obtained.

In one embodiment a computer system is adapted to control said printing and said coating sections based on instructions relating to the printing and the coating of the surface.

In one preferred embodiment the printing section is a laser printer and the coating section is based on an ink jet printing, whereby the inkjet patrons comprise a material having a low and high surface energy level, and thereby ice non-stick and anti-dew properties. As mentioned before, the surface of said material can be a metal surface or any other kind of material, such as all types of plastic material, and may have a retroreflective pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following preferred embodiments of the invention will be described referring to figures, where

Figs. 1-3 show heterogeneous surface structures having both ice nonstick, and simultaneously anti-dew properties in connection with dirt deposition and graffiti,

Figs. 4-6 show the surface structures from Figs. 1-3, wherein the structures are implemented as traffic signs, and

Fig. 7 shows a printing device adapted for printing and coating a surface structure with anti-dew and ice non-stick property

DESCRIPTION OF PREFERRED EMBODIMENTS

Figs. 1-3 show three different types of heterogeneous surface structures having both ice non-stick properties, and simultaneously anti-dew properties in connection with dirt deposition and graffiti. This property is obtained by creating a surface structure comprising surface areas of different surface energy levels by means of adding different type of coatings on the surface.

By creating a surface structure with surface areas 2-7 having a high or very high surface energy in combination with creation of surface areas having a low or very low surface energy, nucleation areas are created where ice crys- tallization mainly takes place. At these nucleation areas the ice crystallization becomes localized. This is because the Gibbs free energy is lower at the areas having higher surface energy, which makes the ice crystallization more favorable. Here, a low or very low surface energy is lower or much lower than the surface energy of water, 72,8mJ/meter, whereas a high or very high surface energy is higher or much higher than the surface energy of water. Due to the localization of the ice crystals, the adhesion of the generated ice crystals is reduced both in strength and contact area compared to the generation of a compact ice layer. The areas of high surface energy cause the formation of a thin water layer instead of forming water droplets under dew conditions, which prevents the negative effect of the dew water droplets on the optical properties of the surfaces.

In Fig. 1 the different areas of different energy levels 2, 3 are distributed in a predefined way forming a matrix-like surface structure, wherein area 2 could have a second energy level and area 3 a first energy level having a higher energy than that at the first energy levels, and thereby defining the nucleation areas. Preferably, the energy level at area 3 has a surface energy being significantly or much higher than the surface energy of water, 72,8mJ/meter, thereby having an anti-dew property since the dew will be uniformly distributed on the surface, whereas the surface energy at area 2 is lower or much lower than the surface energy of water, thereby having ice non-stick property. Through this combination of said areas with high and low energy levels, both the ice non-stick properties, and simultaneously anti-dew properties, are ob- tained simultaneously.

As an example the surface energies at the surface areas 3 could be in the range of 500-5000mJ/meter, whereas the surface energy at areas 2 could be in the range of 5-40mJ7meter. Other energy levels are of course also possi- ble. In one preferred embodiment the surface area 2 is a heterogeneous surface with a plurality of very small nucleation points with dimensions less than 10 microns. Thereby, a stress is created in this area 2 which enhances the ice repelling property. Besides from that it makes the ice crystals at the nuclea- tion areas more localized and thereby hinders a generation of a compact ice layer at the surface.

The surfaces can be designed in their anti-dew and ice non-stick, dirt and graffiti non-stick behavior in dependency of the necessary actual conditions. If dew formation is the dominating problem, the area having high surface energy is larger. If ice formation, dirt deposition or graffiti is the dominating problem, the area having high surface energy will be reduced in size and localized in order to achieve the described non-stick effect.

Figs. 2 and 3 show similar surface structures as shown in Fig. 1 , wherein in Fig. 2 the areas 5, 6 have different symmetry, but are arranged in a predefined way, whereas in Fig. 3 the areas 6, 7 are distributed randomly. In Fig. 2 the ice formation at the surfaces is, as an example, the main problem. Therefore, the total anti-dew area 5, having a high energy level, is larger than the low energy level for ice non-stick area 6. Other symmetries of the anti-dew or non-stick areas are of course also possible.

In the embodiments shown in Fig. 1-3 the energy levels have been chosen such that surface structures each have two different energy levels. The num- ber of energy levels may also be more than two in the same surface structure.

Figs. 4-6 show the surface structures from Figs. 1-3, wherein the structures are implemented as traffic signs, showing Aarhus 39 km 8. According to the invention the coatings can, in one embodiment, be synthesized by sol-gel technology, involving hydrolysis, condensation and additional reactions from a combination of polymerized molecular metal alkoxides Me(OR)4, silicon alkoxides Si(OR)4, organic modified silicone alkoxides (R- Si(OR)3) and furthermore organic polymers. By that hybrid layers will be created, forming an inorganic-organic coating network at the surfaces. The metal atom can be found in the group of Ti, Al, Zr.

In a first step, different sols with functionalized nanoparticles from different functionalized silanes are synthesized.

In order to achieve a hydrophilic surface area or segments, metal alkoxides and silicon alkoxides with reactive hydrophilic functional end groups like amino, carboxyl or hydroxyl are applied. The hydrophobic surface areas are achieved with silicone alkoxides with non-polar hydrophobic functional end groups like alkyl, aryl or fluoralkyl.

In order to obtain the heterogeneous surface with the different physical-and chemical properties, such that anti-dew and ice non-stick property are obtain, the following methods can as an example be applied:

In a first method and as the second step after the synthesis of the sols, the different sols are blended together in a certain amount and time. Blending different sols together can be done with different ratios, e.g. ratios 100/1 , 10/1 , 1/1 , 1/2, 1/10 etc., depending on whether dew or ice is the main problem. As an example, if dew is the main problem it would be preferred to have sol with dominating hydrophilic property, whereas where ice is the main problem it would be preferred to use sol having dominantly hydrophobic property (e.g. ratio 10/1 of sols with hydrophobic property and sol with hydrophilic property). Afterwards, the sol blending is cross-linked by condensation reaction due to coating the surface with the blended sols. As an example this can be done by spraying the surface with said blended sols, by rolling the surface with said blended sols, by printing it on the surface or by using a brush. Further, cross-linking with additional polymers, with other reactive functional groups, hydrophilic or hydrophobic is possible.

Another method to obtain such a surface structure is by using a printing technology e.g. laser printer or inkjet printer.

Fig. 7 shows a laser printer adapted for printing on metal surfaces and coat- ing the surface structure with anti-dew and ice non-stick property, comprising a printing section 10 and a coating section 11. If the surface structure is the traffic sign shown in Fig. 4, the first step would be to print the sign, i.e. Aar- hus 39 km in the printing section. The second step is coating the surface at the coating section 11 of the printing device, so that the surface becomes said structure having anti-dew and ice non-stick property. As an example this could be done by using ink jet printing. The coating could be based on different kind of criteria. One criterion could be that the surface is to be used as a traffic sign. Another criterion could be the size of the sign, the number of letters in the sign, the size of the letters and the location of the letters on the surface.

In one embodiment the surface is initially coated with a material having anti- dew property (hydrophilic property). Subsequently, the material having ice non-stick property (hydrophobic property) is printed on the anti-dew layer, such that a surface structure having a predefined structure is obtained, such as the one shown in Fig. 1-3.

In another embodiment the materials having anti-dew property and ice nonstick property are printed on the surface in a predefined way, either sepa- rately or simultaneously. The thickness of the applied coatings is in one embodiment less than 10 microns. Due to the formation of an inorganic-organic network covalent cross- linked to the metal or polymer surface, a very good mechanical and abrasion resistant surface is realized. Also, due to the covalent bonding, a coating de- laminating will not occur.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the ap- pended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word 'comprising' does not exclude the presence of other elements or steps than those listed in a claim. The invention can be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In a device claim enumerating several means, several of these means can be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A heterogeneous surface structure having hydrophobic and simultaneously hydrophilic properties adapted to maintain high optical property of the surface under different conditions, comprising:

2. A heterogeneous surface structure according to claim 1 , wherein said at least second surface area having said hydrophobic property further comprises nucleation points having higher surface energy than said at least one hydrophobic area.

3. A heterogeneous surface area according to claim 1 , wherein said at least one first surface area is comprised in a surface layer having said hydrophilic property onto which said at least one first surface area is coated.

4. A surface structure according to any of the preceding claims, wherein said at least one surface area having said hydrophobic property is larger than that having hydrophilic property.

5. A surface structure according to any of the preceding claims, wherein said at least one surface area having said hydrophilic property is larger than that having hydrophobic property.

6. A surface structure according to any of the preceding claims, wherein the arrangement of said areas is distributed in a predefined way.

7. A surface structure according to claim 1 , further comprising an additional dirt and graffiti non-stick property through a hydrophobic or an oil phobic property, or a combination of both.

8. A method for coating a heterogeneous surface structure having hydrophobic and simultaneously hydrophilic property adapted to maintain high optical property of the surface under different conditions, the method comprising the following steps:

• coating a surface area with a second material having a surface energy lower than said first surface energy level such that hydrophobic property is obtained at said at least one area, wherein the energy level difference between said at least first and second surface areas is adapted to cause a local ice crystallization at said nucleation areas and thereby preventing a generation of a compact ice layer at the whole surface, wherein said at least first and second surface areas are hybrid layers covalently cross linked to each other.

9. A method according to claim 8, wherein coating the surface area with said first material comprises coating the whole surface area with said first material and subsequently coating the surface with said second material such that said heterogeneous surface structure is obtained.

10. A method according to claim 8, wherein the coating comprises coating the surface area with said first material such that a predefined area of said first material is obtained and subsequently coating the surface with said second material such that a predefined area of said second material is obtained, thereby defining said heterogeneous surface structure.

11. A method according to claim 8, wherein the coating comprises coating the surface areas with said first and second materials simultaneously.

12. A printing device adapted for printing and coating a surface of a material such that a heterogeneous surface structure having hydrophobic and simul- taneously hydrophilic property is obtained, comprising:

• a printing section adapted for printing on said surface, and