GB2425075A

GB2425075A - Concrete roof tile or wall cladding element

Info

Publication number: GB2425075A
Application number: GB0607539A
Authority: GB
Inventors: Eric Hutton
Original assignee: MARLEY ETERNIT Ltd
Current assignee: MARLEY ETERNIT Ltd
Priority date: 2005-04-15
Filing date: 2006-04-13
Publication date: 2006-10-18
Anticipated expiration: 2026-04-13
Also published as: GB0607539D0; GB2425075B; GB0507663D0

Abstract

A concrete roof tile comprises one or more base layers and a topmost layer of discrete photocatalytic particles. The particles may comprise a binder or diluent such as Portland cement or other hydraulic cement, inorganic silicate, phosphate, sulphate or sorel cement or an organic composition such as furan, alkali phenolic, phenolic urethane, acrylic or epoxy resins. The particles may be fixed to the base layer by a slurry adhesive comprising Portland cement or other hydraulic cement or by means of a thermoplastic or thermo setting organic adhesive such as polyurethane, phenol formaldehyde, cellulose nitrate, epoxy resin or acrylic polymer. The photocatalytic layer is capable of oxidising pollutants such as NOx and SOx. The initial activity of the fresh photocatalytic layer or layers may be enhanced through etching which may be mechanical such as by blasting with abrasive grits or synthetic abrasives or solid CO2 (dry ice) or chemically.

Description

CONCRETE ROOF TILES AND WALL CLADDING ELEMENTS

This invention is in the field of concrete roof and wall tiles, specifically a concrete roof or wall tile having a photocatalytic layer capable of oxidising pollutants such as NO and SO, in the atmosphere to soluble nitrate and sulphate.

The problem of high levels of atmospheric pollution from road traffic has long been recognised, one aspect of that being the production of nitrogen oxides known collectively as NON. Recently, various proposals to reduce NO levels by means of photocatalytic layers on road and/or pavement surfaces have been put forward, see for example EP-A-0786283 disclosing paving blocks incorporating titanium dioxide and EP-1020564 where the titanium dioxide layer is applied to a road surface.

In these prior art documents the mode of action of the photocatalytic layer is discussed in some detail. Briefly, the NO contacts the photocatalyst and is oxidised to soluble nitrate, with ultraviolet radiation from sunlight as the energy source. Thus the catalytic reaction takes place during daylight hours, although EP-A-078283 also proposes adsorbing NO during the night for decomposition during the day.

The nitric acid which is the immediate product of the catalytic reaction needs to be neutralised, e.g. with calcium carbonate or other alkaline components of the concrete base material, so that the final product is soluble calcium nitrate which can be washed away by rainwater. The need to neutralise the nitric acid has been a constraint when photocatalytic technology of this type has been applied as a paint layer, but is less of a problem when incorporated into a concrete paving block or road surface.

Nonetheless there remains a need for alternative ways of locating photocatalytic layers in areas of high atmospheric NO pollution, in robust structures, with high NO oxidation capacity, long lifetimes and ease of access for replacement when necessary. Ease of manufacture of the photocatalytic layer- coated article and the lowest possible cost, given that the titanium dioxide or other photocatalyst will probably be the most expensive component of the article, are also desirable. The present invention seeks to address many, and in some embodiments all, of these objectives.

Thus one embodiment of the invention provides a roof tile or wall cladding element, for example a wall tile, comprising a base layer or layers and a topmost layer of discrete photocatalytjc particles.

Preferably the tile comprises a concrete base layer comprising cementitious material and aggregate. The cementitious material is conveniently a hydraulic cement such as Portland Cement or an aluminate cement, which may be combined with pozzolanic materials such as blast furnace slag, pulverized fuel ash or silica fume. Aggregates are conveniently provided by sand, crushed stone or hard crushed waste materials. The base layers can be coloured through the incorporation of additions of pigments such as iron oxides, chromium oxides or carbon black. To enhance durability and physical properties the base layer concrete may be modified by the addition of polymers such as acrylics, styrene acrylics, styrene butadiene rubbers or vinyls. The photocatalytic granules are conveniently fixed or adhered to this base layer using an organic adhesive such as thermoplastic or thermosetting organic adhesive, for example polyurethane, phenol formaldehyde, cellulose nitrate, epoxy resin or acrylic polymer, or an inorganic binder such as an alkali metal silicate or phosphatic cement.

In a second embodiment of the invention an intermediate layer is provided between the concrete base layer or layers and the photocatalytic particles.

Preferably this intermediate layer is formed from a fine-grained slurry adhesive comprising a cementitious material, which may be combined with additions such as fine sand, calcium carbonate or lime, and water. A weight ratio of water to cementitious material of 0.3 to 0.6 is usually preferred and more preferably the weight ratio of water to cementitious material is just sufficient to provide adequate viscosity for application onto the base layer by spray or brush. The photocatalytic granules are embedded into this slurry whilst it is in the green state, becoming firmly fixed as it cures. The slurry may be pigmented to generate the desired surface colour and advantageously may itself be rendered photocatalytically active by the addition of a photocatalyst such as titanium dioxide.

In a third embodiment of the invention the photocataltyic particles themselves do not contain an acid soluble alkali material to neutralize the nitric acid product of NO oxidation; instead this is achieved through components within the cementitjous matrix in the base or intermediate slurry layers.

The photocatalytic particles will conveniently contain a photocatalyst semi conductor such as ZnO, Sn02, CdS or preferably titanium dioxide or a doped version thereof, e.g. nitrogen doped, carbon doped, Nd-, Pd-, Pt- or Fe-doped titanium dioxide. Amongst the titanium dioxides, both anatase and rutile are suitable. Particle sizes of the titanium dioxide can suitably be in the range 75tm to l.Onm, more preferably 30jtm to lOnm.

The photocatalytic particles will also generally comprise a binder or diluent, for example a concrete matrix into which the photocatalyst is embedded. The matrix can be formed from any of the concrete materials described above for the one or more base layers. Alternatively the matrix may consist of an inorganic silicate, phosphate or sulphate binder or a polymer resin such as furan, alkali phenolic, phenolic urethane, acrylic or epoxy resins. An acid soluble alkali component may be added to such a binder, for example calcium carbonate, magnesium carbonate or sodium carbonate. The resulting composite of photocatalytic particles and binder is conveniently formed into granules or particles by known methods. Thus, for example, simple mixing of the photocatalyst and the concrete matrix ingredients is possible, followed by setting of the concrete and grinding or crushing to the desired particle size. Typically this particle size can be in the range 7.0 to 0.1mm, preferably 5 to 0.5mm.

The photocatalytic particles are conveniently coated onto the topmost surface of the roof tile or wall cladding element at a coating weight of 3000 - 300g!m2, preferably 2000 to 400g/m2, prior to curing to give a finished roof tile or wall cladding element. Coating can be effected for example by a flicker brush, pneumatic spray or gravity-fed applicator.

The advantages of applying the photocatalytic particles to the surface of the tile in granule form, rather than incorporating them into the body of the tile or wall cladding element, are that the maximum surface area of catalytic particle can be exposed to the air and thus carry out its function of NO reduction. This approach also generates a traditional "sanded" surface appearance. In one embodiment of the invention the use of a concrete matrix containing Portland Cement to form the granule matrix enables rapid neutralization of the nitric acid formed through the catalytic reaction. In a second aspect this activity can be enhanced through incorporation of the photocatalyst within a cement slurry adhesive holding the granules onto the base layer. In a third aspect the granules act as an inert carrier for the titanium dioxide, neutralization of the nitric acid being affected when this comes into contact with the adhesive slurry or base layer. By contrast simple application of the photocatalyst to the base layer of the tile would result in a lower active surface area, require greater quantities of titanium dioxide therefore increased cost, plus potentially impairing the overall performance and durability of the roofing tile or wall cladding element.

In a further advantageous step the initial activity of the fresh photocatalytic layer or layers may be enhanced through etching the surface. This is believed to expose a greater area of the photocatalyst to the atmosphere and thus increase NOx removal. Etching may be conducted mechanically, for example by blasting with known abrasive grits such as sand, coal or smelter slags, or synthetic abrasives, such as silicon carbide or aluminium oxide. Blasting with solid CO2 (dry ice) may also be used. More preferably, etching will be carried out chemically with an etching solution. Suitable etching solutions include solutions of acids, especially strong acids, such as mineral acids. Preferred acids include nitric, phosphoric or particularly hydrochloric or sulphuric acids, or mixtures thereof. Other acids including acetic, citric or carbonic acids are also suitable, either alone or in mixtures with other acids. Any effective concentration of acid may be used and will depend upon the strength of the acid and the contact time. A typical range would be 0.1 to 1OM (such as 1-99%) preferably 0.3 to 5M (e.g. 3 to 30%, especially 5 to 20%).

The acid may be rinsed off after a certain contact period (e.g. 30 seconds to 10 minutes), or more preferably may be left in contact with the tile to be gradually neutralised by reaction therewith. A method for the formation of a photocatalytic roof tile or wall cladding element, including a method comprising the above- described etching step, is thus provided by the present invention, as are roof tiles or wall cladding elements formed or formable thereby.

EXAMPLES

The invention will now be illustrated by means of a number of examples.

Example 1

Measurements of photocatalytic activity of laboratory produced granules.

A photocatalytic concrete mix was prepared to the formulation as shown below (parts by weight). The ingredients were weighed into a mixing bowl and prepared using a laboratory mixer. The mix was placed into a mould and cured in a chamber for 16-hours at 45 degrees centigrade and 95% relative humidity.

The dry concrete mix was crushed with a pestle and mortar. The resultant crushed concrete was graded into particulate granules of 1.18 mm-600jtm, using sieves to screen to the correct size.

A quantity of 200 grams of photocatalytic granules was placed evenly onto a tray of area 2 00cm2, which was placed in the test bed of an environmental chamber.

A simulated polluted gas containing lppm of NO2 was supplied into the vessel at a flow rate of 3 litres per minute. The concentration of the outlet gas was measured to determine the NO removal coefficient when the samples were subjected to ultra- violet light.

Cement is Portland Cement, complying with BS EN 197-1 CEMI 52.5R.

Sand is Chelford 30 as supplied by WBB minerals, grading between 1mm and 125pm.

The ultraviolet light was supplied using a 6-Watt lamp at 365nm wavelength.

The NO gas concentration was measured using a chemiluminescent nitrogen oxide analyser.

When irradiated with ultra-violet light the granules reduced the NO concentration of the atmosphere within the chamber.

Table 1.1 - Granule Formulation Formulation Cement Sand TiO Water Black Red NO Pigment Pigment Reduction L ____ __________ _____ _____ (%) L 1 1. 0 2.5 0.50 0.60 - - 42

Example 2

The addition of pigment did not significantly affect NO absorption.

Table 2.1 - Granule Formulation [mulation Cement Sand Ti02 Water Black Red NO Pigment Pigment Reduction _____ _____ _____ _____ _____ _____ (%) L 2 1.0 2.5 0.50 0.60 0.05 - 42 [ 3 1.0 2.5 0.50 0.60 - 0.05 45

Example 3

To optimize the mix, a variation in cement content was investigated. A 50% increase in cement reduced the NOx absorption as compared to formulation I. A 50% reduction in cement gave comparable results to formulation 1.

Table 3.1 - Granule Formulation Formulation Cement Sand Ti02 Water Black Red NO Pigment Pigment Reduction _____ _____ _____ _____ (%) 4 1.5 2.5 050 0.90 - - 35 0.5 2.5 0.50 0.30 - - 41

Example 4

An acrylic emulsion copolymer was added to further aid durability. This resulted in a small reduction of NO absorption compared to formulation 1.

Table 4.1 - Granule Formulation Formulation Cement Sand hO2 Water Black Polymer NO I Pigment Reduction _____ _____ (%) L 6 1.0 2.5 0.50 0.45 0.05 0.30 37

Examples

Measurements of photocatalytic activity on test coupons.

Extruded concrete tiles were prepared using a small tile-making machine, and were covered with photocatalytic granules.

Pigmented granules were prepared by a similar method as described in Example 1, using the formulations as shown in table 5.1.

Table 5.1 - Granule Formulations [mulation Cement Sand Ti02 Black Polymer Water Pigment L 7 1.0 2.0 0.25 0. 05 - 0.60 L 8 1.0 2.0 0.25 0.05 0.30 0.45 The granules were graded to 2. 36nmi - l.l8mm using sieves to screen to the required granule size.

A standard cement slurry adhesive and a cement slurry adhesive incorporating Ti02 were prepared. The slurry was sprayed onto freshly extruded tiles using a wet coating weight of 800g1m2.

OPC is Portland Cement complying with BS EN 197-1 CEMI 42.5N.

Table 5.2 - Slurry Formulations spersing Antifoam OPC Ti02 Water Comments agent _____________ 4.23 ml 0.35 ml 707.1 g - 297.0 g Standard ____________ slurry 4.23 ml 0.35 ml 707.1 g 81.7 g 331.3 g Slurry with _________ _________ Ti02 Granules of formulations 7 and 8 were applied onto the two formulations of cement slurry adhesive, coating the test tiles at an application weight of 1 540g/m2. The coated tiles were cured at 45 degrees centigrade and 95% relative humidity for 16- hours. Performance was assessed by a similar method as used for Example 1 by placing a single tile on the test bed of the environmental chamber. The measured NOx reduction per tile is shown in table 5.3.

Table 5.3 - NOx reduction coefficient for test tiles Formulation Tile with Ti02 in Tile without TiO2i1 slurry adhesive slurry adhesive

L_

43% 38%

Example 6

Measurement of the photocatalytic activity of concrete tile incorporating photocatalytic slurry and granule coating.

Extruded concrete tiles were prepared on a full-scale production machine and coated with a photocatalytic cement slurry adhesive and granules made to the formulations shown in Tables 6.1 and 6.2. The granules were screened to size range of 2.36mm to 600tm.

Photocatalytic activity was measured on a cut section of the tile with a surface area of 20,000mm2 which was surface treated using a 10% hydrochloric acid solution.

After washing and drying this was enclosed in an air-tight container having a borosilicate UV transmitting cover glass positioned 3mm above the surface.

A simulated polluted gas containing O.9ppm of NO at a controlled humidity level of 50% was supplied into the container at a flow rate of 2 litres per minute. The concentration of the outlet gas was measured to determine the NO removal coefficient when the samples were subjected to ultra-violet light.

The ultraviolet light was supplied using a 30-Watt lamp at 365nm wavelength.

When irradiated with ultra-violet light the tile section reduced the NO concentration of the atmosphere within the container as indicated in Table 6.3.

Table 6.1 - Slurry Formulation I opc Ti02 Sand Water Iron oxjd7 Pigment 1 0.2 0.5 0.43 0.04 OPC is Portland Cement complying with BS EN 197-1 CEMI 42.5N.

Sand is Congleton 80 as supplied by WBB minerals, grading between 25Oiim and 63 pm.

Table 6.2 - Granule Formulation I RHPC Sand Ti02 Water Polymer Iron oxide Pigment L 1 2.5 0.3 0.25 0.15 0.12 RHPC is Portland Cement complying with BS EN 197-1 CEMI 52.5R.

- 10 - Sand is Chelford 30 as supplied by WBB minerals, grading between 1mm and I 25pm.

Table 6.3 - NOx reduction coefficient for production tile sample following acid atment Initial NOx NOx Concentration after 15 NOx Reduction Concentration minutes UV exposure (%) 905 ppb 263 ppb 71

Claims

- 11 - Claims 1. A concrete roof tile or wall cladding element comprising

one or more base layers and a topmost layer of discrete photocatalytic particles.
2. A concrete roof tile or wall cladding element according to claim 1 wherein the photocatalytjc particles comprise a binder containing Portland Cement or other hydraulic cement.
3. A concrete roof tile or wall cladding element according to claim 1 or claim 2 wherein the photocatalytic particles are fixed to the base layer by a slurry adhesive comprising Portland Cement or other hydraulic cement.
4. A concrete roof tile or wall cladding element according to any one of claims 1 to 3 wherein the slurry adhesive further comprises photocatalytically active material.
5. A concrete roof tile or wall cladding element according to claim 1 wherein the photocatalytic particles comprise a binder containing an inorganic silicate, phosphate, sulphate or sorel cement or an organic composition.
6. A concrete roof tile or wall cladding element according to claim 5 wherein an acid soluble alkali component is added to the binder such as calcium carbonate, magnesium carbonate or sodium carbonate.
7. A concrete roof tile or wall cladding element according to claim 1 or claim 2 wherein the particles are fixed to the base layer by means of a thermoplastic or thermosetting organic adhesive.
8. A concrete roof tile or wall cladding element according to any one of claims 1 to 7 wherein the surface layer containing photocatalytic particles is etched.
9. A method for the formation of a photocatalytic concrete roof tile or wall cladding element comprising fixing a topmost layer of discrete photocatalytic particles to a concrete base layer.

- 12 -
10. The method according to claim 9 further comprising etching the topmost layer.
11. The method according to claim 10 wherein said etching is carried out by means of an acidic solution.
12. A photocatalytic concrete roof tile or wall cladding element formable by the method of any of claims 9 to 11.