Protective coating compositions for photocatalytic layers on substrates

Abstract

A coated substrate including a substrate including a treated layer, a photocatalytic layer, and a protective layer for impeding photocatalyst derived degradation of the treated layer, the protective layer being provided between the photocatalytic layer and the treated layer, the protective layer comprising colloidal particles distributed in a matrix comprised at least partly of an organosilicon phase which is oxidizable by the reactive oxygen species to form a non-volatile inorganic phase, wherein the organosilicon phase includes a surfactant incorporating an organosilicon component.

Claims

1. A photocatalytic, self-cleaning, coated building product including: a metal substrate including a treated layer comprising a painted layer, a photocatalytic layer, and a non-photocatalytic protective layer for impeding photocatalyst derived degradation of the treated layer, the protective layer being provided between the photocatalytic layer and the treated layer, the protective layer consisting of: (a) discrete spherical colloidal particles and (b) a matrix; wherein the particles have a narrow particle size distribution comprising a standard deviation of less than 20% of the average particle diameter, wherein the particles are in a lattice formation and are distributed in the matrix, which is comprised of: (i) an organosilicon phase which is oxidisable by reactive oxygen species to form a non-volatile inorganic phase, and/or (ii) one or more inorganic silicates formed by the oxidation of at least one organosilicon phase by reactive oxygen species, wherein the organosilicon phase comprises a surfactant incorporating an organosilicon component.

2. The coated building product according to claim 1, wherein the organosilicon phase is oxidisable by the reactive oxygen species to form a network silicate.

3. The coated building product of claim 1, wherein the silica colloid particles include a coating of an organosilicon compound.

4. The coated building product of claim 1, wherein the surfactant incorporating an organosilicon component is selected from the family of ethoxylated heptamethyltrisiloxane surfactants or polyalkyleneoxide modified heptamethyltrisiloxanes.

5. The coated building product of claim 1, wherein the colloidal particles are selected from one or more oxides of metallic and/or non-metallic elements selected from silicon, aluminium, boron, titanium, zirconium and phosphorous.

6. The coated building product of claim 1, wherein the colloidal particles comprise nanoparticulate silica colloids.

7. The coated building product of claim 1, wherein the colloidal particles have a particle size in the range from 0.4 to 400 nm.

8. The coated building product of claim 1, wherein the colloidal particles are composed of at least two materials with different particle size distributions.

9. The coating building product of claim 1, wherein a gloss difference, measured at a 60° angle of incidence according to ASTM D523, between the coated substrate and an uncoated substrate is not more than 20%.

10. The coated building product of claim 1, wherein the protective layer has a thickness of from 25 to 1000 nm.

11. A photocatalytic, self-cleaning coated building product comprising: a metal substrate including a treated surface comprising a painted layer; a non-photocatalytic barrier layer on the treated surface, the barrier layer comprising discrete spherical colloidal particles having a narrow particle size distribution comprising a standard deviation of less than 20% of the average particle diameter in a lattice-like formation and distributed in a matrix comprised of one or more organosilicon phases which are oxidisable by reactive oxygen species to form one or more inorganic silicate phases, and/or one or more inorganic silicates formed by the oxidation of at least one organosilicon phase by reactive oxygen species, wherein said one or more organosilicon phase is a surfactant incorporating an organosilicon component; and a photocatalytic layer on the barrier layer, wherein the barrier layer does not include elongate particles.

12. A method for protecting a metal substrate from degradation by reactive oxygen species, the method including the steps of: providing a metal substrate including an treated layer comprising a painted layer; applying on the treated layer a coating of a composition consisting of: (a) discrete spherical colloidal particles and (b) a medium; wherein the particles have a narrow particle size distribution comprising a standard deviation of less than 20% of the average particle diameter, wherein the particles are dispersed in the medium comprising at least one organosilicon phase which is oxidisable by reactive oxygen species to form an inorganic silicate phase, the at least one organosilicon phase comprising a surfactant incorporating an organosilicon component; converting the coating to form a protective layer.

13. The coated building product of claim 1, wherein the surfactant incorporating an organosilicon component is 2-[methoxy(polyethyleneoxy)-propyl]heptamethyltrisiloxane.

14. A method for protecting a metal substrate from degradation by reactive oxygen species, the method including the steps of: providing a metal substrate including an treated layer comprising a painted layer; applying on the treated layer a coating of a composition comprising discrete spherical colloidal particles, wherein the particles have a narrow particle size distribution comprising a standard deviation of less than 20% of the average particle diameter, wherein the particles are dispersed in a medium comprising at least one organosilicon phase which is oxidisable by reactive oxygen species to form an inorganic silicate phase, the at least one organosilicon phase comprising a surfactant incorporating an organosilicon component, wherein the coating does not include elongate particles; converting the coating to form a protective layer.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Preferred embodiments of the present invention are hereinafter described by way of example only, with reference to the accompanying drawings, wherein:

(2) FIG. 1 is a cross-sectional schematic view of a first embodiment of a coated substrate with a separate photocatalytic layer.

(3) FIGS. 2A and 2B are schematic magnified views of silica network formation from reaction between reactive oxygen species and organo-silicon-containing surfactant.

(4) FIG. 3 is a cross-sectional schematic view of a second embodiment comprising coated colloidal particles.

DETAILED DESCRIPTION

(5) One form of a coated substrate generally denoted as 10 is illustrated in FIGS. 1, 2A and 2B. In this form, the oxidisable phase is in the form of a surfactant dispersed matrix.

(6) The coated substrate 10 includes paint layer 12 disposed on metal substrate 13, a protective layer 14 on the paint layer 12 and a photocatalytic layer 18 on the protective layer. The protective layer 14 comprises colloidal particles 16 of one or more oxides distributed in a matrix 22. The matrix 22 is comprised of an oxidisable phase which is oxidisable by reactive oxygen species to form a non-volatile inorganic phase. The photocatalytic layer 18 includes photocatalytic particles 20 comprising of a metal oxide such as, but not limited to, one of nanoparticulate titanium dioxide or derivatives of titanium dioxide such as titanium dioxide doped with metal cations such as iron, vanadium, and other transition or rare earth metals, nanoparticulate zinc oxide, nanoparticulate tin oxide, or nanoparticulate cerium oxide (see FIG. 1).

(7) The colloidal particle size ranges from 10 to 40 nm and may be derived from oxides of silicon, aluminium, boron, titanium, zirconium and phosphorous. The colloidal particles may also be derived from oxides of other single elements or a mixture of metallic and/or non-metallic elements.

(8) If necessary, the colloidal particles may be first stabilised in an alkaline solution before forming the coating composition. The concentration of colloidal particles in the coating composition ranges from 0.1 to 10 wt %. The colloidal particles may associate with soluble cations including lithium, sodium, potassium, ammonium and alkyl ammonium ions.

(9) The protective layer 14 is formed by application of the coating composition onto the paint layer 12. The coating composition comprises a suspension of colloidal particles 16 in an aqueous or organic solvent in which organosilane-based surfactants 24, comprising silicon-based portion 24A and organic portion 24B, are also added (see FIG. 2A).

(10) The photocatalytic layer 18 includes photocatalytic particles 20 which are present in the layer at a concentration ranging from 0.1 to 100%, depending on the activity of the photocatalyst (with the concentration of the photocatalyst being inversely proportional to its catalytic activity). Before application onto protective layer 14, the photocatalyst particles are dispersed in a liquid medium at a concentration from 1 to 3 wt %. The medium can be aqueous or organic-based and includes alkaline solutions, alcohols of the general formula HOC.sub.nH.sub.2n+1, where n=1 to 8, aromatic hydrocarbons, aliphatic hydrocarbons, ketones, ethers or halogen compounds such as chloroform and methylene chloride.

(11) Upon application of the coating composition onto paint layer 12, the particles adopt a lattice formation in which adjacent particles are able to contact each other within matrix 22 which acts as a barrier to chemical diffusion by reducing the space within protective layer 14 in which the reactive oxygen species may diffuse to the paint layer (see path A in FIG. 1). The surfactants are distributed throughout matrix 22 between the colloidal particles (see FIG. 2A). However, there will still be some capacity for chemical diffusion of the reactive species to paint layer 12 through the interstitial spaces between the particles within the matrix (see path B in FIG. 1).

(12) When the photocatalyst particles 20 are activated by electromagnetic radiation such as ultraviolet and visible radiation, they produce reactive oxygen species such as hydroxyl and superoxide ions. The reactive oxygen species oxidise any organic material deposited onto the outer surface of the coating into carbon dioxide and water.

(13) When these reactive oxygen species diffuse into the protective layer, they are impeded from reaching the underlying paint layer 12 by colloidal particles 16. However, diffusion through the matrix 22 may still be possible. In this regard, when these reactive oxygen species encounter surfactant 24 within the matrix, it is believed that the organic portion of the surfactant is oxidised to carbon dioxide and water while the silicon-based portion is converted to solid silica. A stable silica network 26 is thereby formed between the colloid particles, which impedes diffusion of these reactive species through the interstices between the particles (see FIG. 2B).

(14) The surfactant also functions as a processing aid by decreasing foaming of the coating mixture during application of the coating on the substrate.

(15) Advantageously, when titanium dioxide is used as the photocatalyst, it can be combined with silica to form a superhydrophilic surface. The easy wetting and water mobility properties of the surface provide enhanced self-cleaning benefits to the coating by increasing the ability of water on the coating surface to wash away any organic material.

(16) FIG. 3 illustrates a further embodiment of a coated substrate in which, again, like reference numerals refer to like parts. In this form, colloidal particles 216 include an organosilicon layer 228, which is non-rigid and has a higher silicon content that the surrounding matrix including surfactant 224.

(17) As a result of the fluidity of organosilicon layer 228, interpenetration of the organosilicon layers of adjacent particles fill a greater proportion of the interstitial volume between the particles. Accordingly, when the reactive oxygen species react with layer 228, the density of the silica network is increased compared to a matrix including rigid particles.

EXAMPLE

(18) A panel, which had been coil coated with a melamine cured polyester paint, was treated with a protective coating composition formulated as follows. A LUDOX® HS-40 silica colloid suspension, having a nominal 12 nm particle size, was diluted with water to 2% w/w SiO.sub.2 and 0.4% v/v of the surfactant 2-[methoxy-(oligoethyleneoxy)propyl] heptamethyltrisiloxane was added. The coating composition was applied to the paint panel using a number 10 drawdown bar. After drying, the calculated average protective coating thickness was 270 nm. This coated panel was then further treated with a 2% w/w solution of P25 titanium dioxide photocatalyst in water using a number 10 drawdown bar. After drying, the calculated average photocatalyst layer thickness was 270 nm. This provided a high concentration of active photocatalyst on the surface of the panel. The specifications of the coated panel are shown below in Table 1 as Sample 5.

(19) A melamine cured polyester paint was used rather than a polyvinylidenefluoride paint because the effects of photocatalytically driven oxidation would be more readily apparent on this system than on polyvinylidenefluoride paint.

(20) Comparison Samples 1 to 4 were prepared in a similar way according to the specifications given in Table 1 below.

(21) Each Sample was exposed to UV radiation for discrete periods of time up to 2000 hours. After each exposure period a test piece was removed from the panel, washed to remove the protective and photocatalytic layers, and the surface gloss of the underlying paint was measured using a BYK GARDNER® Trigloss glossmeter. The results are presented in Table 2.

(22) TABLE-US-00001 TABLE 1 Coating Compositions Sample Paint Protective Photocatalyst No. system Coating Surfactant Treatment 1 MF None None None Polyester 2 MF None None 2% P25 in Polyester water, 270 nm 3 MF Ludox ® HS None 2% P25 in Polyester 30, 270 nm water, 270 nm 4 MF Ludox ® HS Triton X100, 2% P25 in Polyester 30, 270 nm 0.2% v/v water, 270 nm 5 MF Ludox ® HS 2-[methoxy(oligoethyl- 2% P25 in Polyester 30, 270 nm eneoxy)propyl]hepta- water, 270 nm methyltrisiloxane 0.4% v/v

(23) TABLE-US-00002 TABLE 2 Surface Gloss Measurement Exposure Time (light hrs QUVA) Sample No. 112 224 336 448 560 672 784 2000 1 ◯ ◯ ◯ ◯ ◯ ◯ ◯ .circle-solid. 2 .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. 3 .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. .circle-solid. 4 ◯ .square-solid. .square-solid. .square-solid. .square-solid. .circle-solid. .circle-solid. .circle-solid. 5 ◯ ◯ ◯ ◯ ◯ ◯ .square-solid. .circle-solid. ◯ - full retention of surface gloss .square-solid. - partial retention of surface gloss .circle-solid. - complete loss of surface gloss

(24) Sample 1 did not include a photocatalytic layer nor a protective layer and therefore did not exhibit self cleaning behaviour. Samples 2 to 5 each included a photocatalytic coating formed from a 2% w/w solution of P25 titanium dioxide photocatalyst in water. Sample 2 did not include a protective (barrier) coating between the paint layer and photocatalyst layer. Sample 3 did include a protective layer which comprised colloidal particles, but no surfactant. Sample 4 includes a protective layer including colloidal particles and a surfactant comprising TRITON® X-100 (C.sub.14H.sub.22O(C.sub.2H.sub.4O).sub.n). Sample 5 included a protective layer including colloidal particles and a surfactant incorporating an organosilicon component.

(25) It is evident from a comparison of Sample 1 (no photocatalytic layer nor protective layer) with Sample 5 (including a photocatalytic layer and a protective layer comprising colloidal particles distributed in a matrix of a surfactant incorporating an organosilicon component) that similar gloss levels are retained at least up to an exposure time of 784 hours. This indicates that the protective layer prevents degradation of the paint layer by radicals generated by the photocatalytic layer.

(26) A comparison of the gloss level results for those samples that did include a photocatalytic layer (ie, Samples 2 to 5) shows that optimum results were achieved when a protective layer was included, and where that protective layer contained a surfactant incorporating an organosilicon component. Where the surfactant did not include an organosilicon component (such as in Sample 4) gloss levels were only partially retained. In contrast, in Sample 5, where the surfactant comprised 2-[methoxy(oligoethyleneoxy)propyl] heptamethyltrisiloxane, there was full retention of surface gloss until in excess of 700 hours exposure time.

(27) In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.