A Multi-Layered Coating

Abstract

There is provided a multi-layered coating having a plurality of cavities therein, the multi-layered coating comprises a first layer comprising an oxide-containing polymer; and a second layer disposed on said first layer, said second layer comprising an oxide. There is also provided a process for forming such multi-layered coating, an article comprising the multi-layered coating and uses thereof.

Claims

1. A multi-layered coating having a plurality of cavities therein, said multi-layered coating comprising: a. a first layer comprising an oxide-containing polymer; and b. a second layer disposed on said first layer, said second layer comprising an oxide.

2. The multi-layered coating according to claim 1, wherein a size of said cavities is in a nanometer scale.

3. The multi-layered coating according to claim 2, wherein said size of said cavities is in a range of 380 nm to 740 nm.

4. The multi-layered coating according to claim 1, wherein said cavities have a shape selected from a dimple shape, a cylindrical shape, a conical shape, and a conical frustum shape.

5. The multi-layered coating according to claim 1, wherein the oxide of said oxide-containing polymer and the oxide in said second layer are the same.

6. The multi-layered coating according to claim 5, wherein said oxide is selected from the group consisting of silica (SiO.sub.2), titanium oxide (TiO.sub.2), and zinc oxide (ZnO).

7. The multi-layered coating according to claim 1, wherein said polymer is a UV-cured resin.

8. The multi-layered coating according to claim 7, wherein said UV-cured resin comprises a monomer selected from the group consisting of (meth)acrylates, esters, epoxy resins, urethanes, silicones, ethers, and vinyl ethers.

9. The multi-layered coating according to claim 1, wherein said first layer has a thickness in a range of 1 μm to 100 μm and said second layer has a thickness in a range of 1 nm to 50 nm.

10. The multi-layered coating according to claim 1, wherein said multi-layered coating is substantially optically clear.

11. A process of forming a multi-layered coating having a plurality of cavities therein, the process comprising the steps of: a. applying an oxide-containing UV-curable resin onto a substrate to form a first layer; b. contacting said oxide-containing UV-curable resin with a mold to imprint a plurality of cavities in said first layer; c. polymerizing said oxide-containing UV-curable resin while in contact with said mold; and d. oxidizing said first layer to form a second layer disposed on said first layer, said second layer comprising an oxide.

12. The process according to claim 11, wherein said contacting step is undertaken at a room temperature under pressure of 5 bars to 20 bars.

13. The process according to claim 11, wherein said polymerizing step is undertaken at a room temperature under pressure of 5 bars to 20 bars.

14. The process according to claim 11, wherein said oxidizing step is undertaken at a room temperature of under pressure of less than 1 mbar.

15. An article comprising a multi-layered coating thereon, said multi-layered coating having a plurality of cavities therein and comprises: a. a first layer comprising an oxide-containing polymer; and b. a second layer disposed on said first layer, said second layer comprising an oxide.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0060] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0061] FIG. 1 is a schematic diagram illustrating (A) the cross-section and (B) the top view of the nanostructured coating.

[0062] FIG. 2 is a schematic diagram indicating the effects of shear force applied to (A) a hole or (B) pillar type structure.

[0063] FIG. 3 is a number of scanning electron microscope (SEM) images of nanostructured coating, comparing (A) hole type and (B) pillar type structures (i) and (iii) before and (ii) and (iv) after rubbing with a spectacle cloth, 320 revolutions with approximate 200 g to 300 g pressure.

[0064] FIG. 4 is a schematic diagram showing the gradient refractive index (GRIN) created by an array of nano-scale dimple structures.

[0065] FIG. 5 is a graph showing the percentage of reflected light measured across the visible light spectrum, comparing a coating with pillar type and hole type structures coated on acrylic as substrate.

[0066] FIG. 6 is a graph showing the percentage of reflected light measured across the visible light spectrum, comparing the present invention coating with a commercially available anti-reflective coating (Shamir) and a control substrate with no anti-reflective coating. The control substrate is made of material CR-39, which is a type of polycarbonate used for making spectacle lenses.

[0067] FIG. 7 is a schematic diagram showing how the (A) silicon content of the polymer coating can be concentrated at the coating surface to form (B) an oxide surface, by a process of strong oxidation, increasing the hydrophilicity of the coating.

[0068] FIG. 8 is a number of X-ray photoelectron spectroscopy (XPS) spectra of (a) silicon coating, (b) silicon containing coating after surface oxidation, (c) non-silicon containing coating, and (d) non-silicon containing coating after surface oxidation.

[0069] FIG. 9 is an overlay image of the XPS silicon 2p narrow scans over a range of oxidation exposure times for the current invention coating (silicon containing) and control coating (non-silicon containing).

[0070] FIG. 10 is a number of images taken of the anti-fogging test showing steps (i) a sample before steam exposure, (ii) sample initiating steam exposure, (iii) sample being exposed to steam, and (iv) sample immediately after steam exposure; and showing (a) the apparatus used to produce a steady supply of steam, (b) (i-iv) a sample with current invention coating having undergone surface oxidation, (c) (i-iv) a sample with identical coating as in (B) (i-iv) which has not undergone surface oxidation and (D) (i-iv) same sample as in (B) (i-iv), 14 days after undergoing surface oxidation.

DETAILED DESCRIPTION OF FIGURES

[0071] Referring to FIG. 1, there is provided a schematic diagram illustrating (A) the cross-section and (B) the top view of a multi-layered coating in the form of a nanostructured coating 2 provided on a substrate 4. The nanostructured coating 2 is shown as having a plurality of cavities in the form of dimples 6. The nanostructured coating 2 is one that is made up of a first layer comprising an oxide-containing polymer and a second layer disposed on the first layer, where the second layer comprises an oxide.

EXAMPLES

[0072] Non-limiting examples of the invention will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1

A Multi-layered Coating

[0073] A multi-layered coating with an array of dimples was formed by the method of nanoimprint lithography. Here, a UV-curable resin was made from a mixture of the following chemicals: MA0736—Acrylo POSS Cage Mixture (Hybrid Plastics, Mississippi, USA), 1,6-hexanediol diacrylate (Sigma-Aldrich, Missouri, USA), Pentaerythritol tetrakis(3-mercaptopropionate) (Sigma-Aldrich, Missouri, USA), isobornyl acrylate (Sigma-Aldrich, Missouri, USA), 3-(Trimethoxysilyl)propyl methacrylate (Sigma-Aldrich, Missouri, USA) and 2-Hydroxy-2-methylpropiophenone (Sigma-Aldrich, Missouri, USA). Among the above, MA0736—Acrylo POSS Cage Mixture (Hybrid Plastics, Mississippi, USA) and 1,6-hexanediol diacrylate (Sigma-Aldrich, Missouri, USA) were used for providing the coating. Pentaerythritol tetrakis(3-mercaptopropionate) (Sigma-Aldrich, Missouri, USA) is a monomer used to increase cross-linking and to reduce oxygen inhibition of the polymerization. Isobornyl acrylate (Sigma-Aldrich, Missouri, USA) is a monomer which increases adhesion of the cured polymer to the substrate. 3-(Trimethoxysilyl)propyl methacrylate (Sigma-Aldrich, Missouri, USA) is a silicon containing chemical, which increases the concentration of silicon at the surface of the polymer and it can increase adhesion to oxide substrates. 2-Hydroxy-2-methylpropiophenone is the photo-initiator. The as-prepared UV curable resin was applied onto a substrate 4, as shown in FIG. 1. A stamp with a topographical pattern was used to imprint the resin to form the negative of the stamp's topography 6, where the array of dimples 6 was formed. Whilst the stamp was in contact with the resin, UV radiation was used to cure the resin in order to create the array of dimples 6. When the array of dimples 6 was consolidated, the stamp was removed.

[0074] The formed multi-layered coating is then subjected to a number of characterization processes.

Shear Force

[0075] In order to show that cavities were more resistant to shearing as compared to projections, the multi-layered coating with the plurality of cavities was subjected to a shear force test. Conventionally, arrays of pillar type projections were used to confer the anti-reflection properties to the coating. However, such pillar type structures cannot cope with abrasive forces because the pillars were brittle and fragile to shear forces. Here, it can be seen that cavity structures 10 on nanostructured coating 11 were much less susceptible to damage by shear force 8 as compared to pillar type structures 12 as illustrated in FIG. 2. FIG. 2 provides a schematic diagram indicating the effects of shear force 8 applied to (A) a cavity structure 10 and (B) pillar type structure 12 on a nanostructured coating 11. The cavity structure 10 was relatively unaffected by the shear force 8, whereas the pillar structure 12 was easily broken and removed by the shear force 8. FIG. 3 shows experimental data, which compares SEM images of cavity-type and pillar-type nanostructured coatings before and after 320 cycles of abrasive rubbing. The images of the surfaces after rubbing clearly show that the cavity type structures remained undamaged and intact, whereas the pillar type structures were almost totally destroyed. The anti-abrasion property was tested by using a spectacle cloth to rub the surface in a circular motion with the thumb at an applied load of between 200-300 g. The sample was placed on a scale whilst being rubbed to ensure the pressure remained between 200-300 g. The pillar coating was made from the same material as the hole structure. Comparing (A) hole type and (B) pillar type structures (i) and (iii) before and (ii) and (iv) after, a result shown the coating with of hole structures are well maintained in FIG. 3.

Anti-Reflection

[0076] The anti-reflection properties were acquired using an array of structures with dimensions smaller than the wavelength of visible light. As show in FIG. 4, the cavity type structures can create the gradient of refractive index (GRIN) that is required for the anti-reflective effect. Although there was a loss of anti-reflection performance as compared to that of pillar type projections, as shown in the refection spectrum (see FIG. 5), the level of anti-reflection produced by the cavity structures was still significant and highly competitive with anti-reflective coatings commercially available as shown in FIG. 6.

Anti-Fogging

[0077] Anti-fogging properties can be obtained by creating a superhydrophilic surface that will facilitate the immediate formation of a water film of the surface, avoiding the negative appearance of fog. As seen in FIG. 7, a superhydrophilic surface 14 was created by oxidizing 20 a silicon contained polymer polyoctahedral silsesquioxanes (POSS) 18 with O.sub.2 plasma to concentrate the silica 16 at the surface through removing the organic components, as illustrated in FIG. 7. The SiO2 formed at the surface produced a superhydrophilic surface with a water contact angle of less than 10°.

[0078] XPS characterization of the coating surface taking (a) before and (b) after oxidation treatment, and a control surface (non-silicon containing coating) taken (c) before and (d) after oxidation treatment is shown in FIG. 8. The control surface was made by the same resist formulation as used in the invention coating, except for the Si containing monomers. The resist only contained the following chemicals: 1,6-hexanediol diacrylate, Pentaerythritol tetrakis(3-mercaptopropionate), isobornyl acrylate and 2-Hydroxy-2-methylpropiophenone. Comparison of the spectra before and after oxidation of the coating (FIG. 8 (a) and Fig. (b)) showed a dramatic reduction in carbon peak intensity and a significant increase in silicon peak intensity, which was consistent with the removal of organic material and subsequent increase in the concentration of silicon material at the surface of the coating. The change in elemental composition of the surface is further clarified in Table 1, which shows that when the oxidation times was increased from 0 seconds to 120 seconds, the carbon % decreased from 65.9% to 10.8% while the oxygen and silicon % increased from 29.0% and 5.1% to 64.7% and 24.0%, respectively. Conversely, the % composition for carbon and oxygen in the control samples showed very little change. FIG. 9 shows the overlay of the Si 2p peaks, with their binding energies tabulated in Table 2. The peaks shift from 101.9 eV to 103.2 eV with increased oxidation, which was consistent with the transformation of organo-silicon to SiO.sub.2.

TABLE-US-00001 TABLE 1 Element composition % at increasing oxidation times Sample O.sub.2 Exposure/s C1s (%) O1s (%) Si2p (%) 1 0 65.9 29.0 5.1 2 15 30.0 52.5 17.0 3 30 20.1 59.6 20.0 4 60 13.7 63.0 22.1 5 120 10.8 64.7 24.0 Control 1 0 74.6 25.4 0.0 Control 2 120 79.4 20.6 0.0

TABLE-US-00002 TABLE 2 Si 2p peak position at different oxidation times O.sub.2 Exposure/s Si 2p/eV 0 101.9 15 102.8 30 103.0 60 103.0 120 103.2

[0079] To demonstrate the anti-fogging properties, samples were exposed to a flow of steam to simulate a fogging environment. FIG. 10 shows a series of images taken of the fogging test. The tests were carried out using a home-made setup to produce a concentrated supply of steam as shown in FIG. 10 (a). The tested samples consisted of square pieces of acrylic with an area of imprinted coating in the centers. The sample shown in FIG. 10(b)(i-iv) had undergone surface oxidation and displayed no detectable fogging or any other visual impairment to the coated area of the sample when exposed to the steam. Conversely, the sample shown in FIG. 10(c)(i-iv) had not undergone any surface oxidation treatment, and the sample experienced extensive fogging when exposed to the steam. The same sample shown in FIG. 10(b)(i-iv) was tested again 14 days later to determine any loss in the anti-fogging effect as shown in FIG. 10(d)(i-iv), in which no measurable loss was detected.

[0080] Therefore, this shows that the coating of the present application is able to have anti-fogging properties that remain even after a period of time.

INDUSTRIAL APPLICABILITY

[0081] The multi-layered coating may be used in optical applications such as eyewear (which can include spectacle lenses, visors or goggles) as well as in digital displays, camera lenses or photovoltaics. This may be due to the advantageous properties of the multi-layered coating such as anti-reflective, anti-fogging and abrasion resistant. Depending on the application required, the multi-layered coating may be optically clear or transparent so that where the multi-layered coating is used on devices, this does not impede the eyesight or vision of a user using such devices.

[0082] The multi-layered coating may find application in various optoelectronic equipment, aeronotical displays and sensors, automotive displays and sensors, space technologies and head-up display (HUD) devices as emitters and in displays.

[0083] The multi-layered coating may also be applied to soft substrates, creating a hard, non-scratch surface on softer polymer substrates.

[0084] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.