GB1583433A

GB1583433A - Production of coated structural elements comprising inorganic binders

Info

Publication number: GB1583433A
Application number: GB5815/78A
Authority: GB
Original assignee: Bayer AG
Current assignee: Bayer AG
Priority date: 1977-02-19
Filing date: 1978-02-14
Publication date: 1981-01-28
Also published as: DE2707228B2; AU520070B2; IT1102368B; NL7801757A; JPS53104623A; FR2381002A1; AU3335578A; DE2707228A1; SE7801805L; BE864041A; DE2707228C3; IT7848088A0

Description

(54) PRODUCTION OF COATED STRUCTURAL ELEMENTS COMPRISING INORGANIC BINDERS (71) We, BAYER AKTIEN GESELLSCHAFT, a body corporate organised under the laws of the Federal Republic of Germany, of 509 Leverkusen, Germany do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the production of coated structural elements comprising inorganic binders, and is an improvement in the invention of our British Patent Specification No. 1,555,526.

It is known that prefabricated concrete sections can be produced by introducing a generally pigmented concrete mixture into a suitable mould, followed by hardening, -optionally after a surface treatment with a pigmented cement slurry. Concrete roof tiles are produced, for example, by machine-extruding the generally pigmented concrete mixture in moulds (sp-called pallets). The extruded concrete roof tile may then be coated over its surface with a thin, similarly pigmented cement slurry into which coloured sand granulate has been strewn (R. E. Paasch: Eigenschaften und Herstellung farbiger Betondachsteine unter Verwendung anorganischer Pigmente (Properties and Production of Coloured Concrete Roof Tiles Using Inorganic ments), Betonstein-Zeitung, No. 10, H. E.Schulz: Der Betondachstein und seine Herstellung in Deutschland (The Concrete Roof Tile and its Production in Germany), Betonstein-Zeitung, No. 5, 1964). However, it is also possible to use the extruded concrete roof tile without any further treatment. The pallets are automatically stacked and introduced into a so-called curing chamber for setting the concrete. After setting, the tiles are separated from their moulds (which are returned immediately to the production process), while the tiles themselves are either sprayed with an emulsion paint, for example based on acrylate, or are immediately stored.

It is also known that asbestos cement sheets can be coated with a glaze-like coating, preferably based on silicate. In addition to SiO2, silicate-containing coatings of this kind contain other constituents such as, for example, oxides, carbonates with magnesium, lead, calcium, boron and zinc preferably being present as cations. In general, an aqueous colour paste, consisting essentially of waterglass and, for example, zinc oxide as the oxidic component, is sprayed onto the already hardened asbestos cement sheets in such a quantitative ratio that glaze-like zinc silicates can be formed. Hardening of the coating to form silicates is carried out either in an autoclave under pressure at elevated temperatures or by a purely thermal treatment under normal pressure.

The base sheet and the coating are therefore produced in two operations separated from one another by a time interval.

Any attempt to produce prefabricated concrete sections and concrete roof tiles with a glaze-like coating in the same way gives very unsatisfactory results because the glaze is only of inadequate quality.

Although the coating of the concrete does not involve any particular problems, the porosity and unevenness of the surface make it absorbent to different extents with the result that, after the waterglass coating has hardened, a film of irregular thickness and varying composition is formed. After the coating has hardened, this results in crack formation which is clearly discernible after weathering tests. After boiling tests, for example, (boiling of coated samples in distilled water for short-term testing of the coating), those areas of the concrete roof tiles which have been coated too thinly are found to develop a number of cracks in the coating, the calcium hydroxide liberated from the concrete being deposited to a large extent on the surface of the waterglass coating.Frost tests (freezing and thawing cycles which are intended to simulate the stresses to which the tiles are subjected in winter time (also produce clearly discernible crack formation.

Because of the above-mentioned deficiencies in quality, which are clearly visible after the short-time weathering tests alone, the coating of hardened structural elements, such as concrete roof tiles and prefabricated concrete sections, by conventional methods is inadequate for practical application.

Accordingly, an object of the present invention is to develop a simple process for applying glaze-like coatings to be preformed, hardened structural elements, such as prefabricated concrete sections and concrete roof tiles, which show adequate strength, adhesion, uniformity and resistance to weather.

The present invention provides a process for forming and coating structural elements based on inorganic binders, which comprises producing a workable composition from the inorganic binder and water and optionally at least one additive, forming structural elements from this composition and hardening them, wherein (a) at least one water-soluble inorganic salt is initially introduced into the workable composition or into a facing layer up to 50 mm thick applied to the formed structural elements in a minimum quantity of 0.5 % by weight, based on the inorganic binder, after which the formed structural elements are hardened on their own (when a facing layer is present) together with the facing layer, or (b) a facing layer up to 50 mm thick is optionally applied to the formed structural elements, the formed structural elements are hardened on their own or (when a facing layer is present) together with the facing layer, and an aqueous 5 to 15 % inorganic salt solution is then applied to the hardened structural elements in a quantity of from 40 to 100 g/m2 and dried for 3 to 15 minutes, and after step (a) or (b) an aqueous paste containing waterglass together with at least one metal oxide is applied in a relatively thin layer to the hardened structural elements, converted into a gel-like, non-fluid state, and itself hardened.

The composition which is used as a starting material for producing the structural elements preferably has a water-binder (cement) factor (ratlo by weight of water to cement) of about 0.3 to 0.7, more preferably 0.3 to 0.5. In addition, it may contain the usual additives such as extenders, for example sand, pigments such as, for example, iron oxides, sealing agents such as, for example, calcium stearate, plasticizers such as lignin sulphonate for example, in the usual quantities (Albrecht, Mannhertz, Zusatsmittel, Anstrichstoffe, Hilstoffe fur Beton und Mortel (Additives, Coating Compositions and Auxiliaries for Concrete and Mortar), 1968, pages 38 etseq, Bauverlag GmbH, Wiesbaden).

A few embodiments of the process according to the invention will now be discussed in detail.

In one embodiment, water-soluble inorganic compounds are added to the cementcontaining compositions from which the structural element is formed. The inorganic compounds convert a paste applied to the preformed structural elements after they have hardened and consisting of an aqueous mass containing waterglass and at least one metal oxide and, optionally, at least one pigment and/or at least one fillers, into a gel-like non-fluid state. The inorganic salts are used in quantities of at least 0.5% by weight, preferably from about 0.5 to 5 % by weight, based on the cement. In another embodiment of the process according to the invention, a facing layer which, depending upon the method of compaction applied, reaches a layer thickness usually of from about 1 to 50 mm, preferably about 1 to 20 mm, is applied as uniformly as possible to the preformed structural element before it hardens.The facing layer may be compacted in the absence of pressure, for example by vibration or, for example, by rubbing in or by extrusion. This facing layer preferably contains inorganic binders, optionally the usual additives and/or water and also the soluble inorganic salts which convert an aqueous paste based on waterglass and metal oxide applied to the facing layer on the preformed hardened structural elements into a gel-like non-fluid state. In this embodiment, the water-soluble inorganic salts are added in quantities of at least 0.5%, preferably from about 0.5 to 5 % by weight (based on the quantity of cement in the facing layer). About 1 to 2 % by weight, based on the quantity of cement in the facing layer, of soluble inorganic salts are preferably added to the facing layers.

In another embodiment, it is not necessary, for obtaining a hard and weatherproof glaze-like coating according to the invention on the structural elements, to add the inorganic salts to the composition or to the facing layer. Instead, the composition used as a starting material for producing the structural elements can be prepared in the same composition as indicated above with the difference that no soluble inorganic salts are added to the composition or to the facing layer. In this case, a workable composition is prepared from the binder (cement in the case of concrete roof tiles and prefabricated concrete sections) and water and conventional additives, and structural elements are formed from this composition and subsequently hardened. An aqueous salt solution in a concentration of from about 5 to 15 % is applied, for example by spraying, in a quantity of from 40 to 100 g/m2 to the hardened structural elements, and dried for about 3 to 15 minutes. An aqueous paste containing waterglass and at least one metal oxide and, optionally, pigments and/or fillers is then applied in a relatively thin layer to the surface of the hardened structural element treated with salt solution and is also hardened after conversion into a non-fluid state.

The aqueous paste containing waterglass and at least one metal oxide and, optionally, pigments and/or fillers is applied to the preformed structural element, preferably in quantities of from about 200 to 500 g/m2, resulting in the formation of a thin layer.

According to the invention, suitable inorganic salts which are added either to the workable compositions containing the inorganic binder in homogeneous distribution or to the facing layer, or are applied in aqueous solution to the surface of the hardened structural element, are alkaline earth metal salts such as, for example, calcium formate, calcium chloride, calcium nitrate, magnesium nitrate or magnesium chloride, or alkali metal salts such as, for example, potassium chloride, sodium formate or sodium acetate, for example. The alkali metal salts are generally added in relatively large quantities, preferably in quantities which correspond to the upper limits referred to above. The preferred compounds added are calcium formate and/or chloride.

These compounds may be added to the composition or to the facing layer either as such or in the form of aqueous solutions.

In the embodiment in which a facing layer is applied to the structural element before hardening, the facing layer may in addition contain the usual additives as already described in connection with the composition of the preformed structural elements.

The composition from which the structural elements are formed and the facing layer preferably contain the same inorganic binder, namely cement.

In the case of facing layers based on cement, the water/cement factor may amount to between 0 and about 0.5 and preferably amounts to between 0 and about 0.4.

The preformed structural elements which either contain the soluble inorganic salts in homogeneous distribution or to which a facing layer containing the inorganic salts is applied, are subsequently hardened. Hardening is normally carried out in a steam chamber at temperatures of up to about 65"C, preferably 22 to 65"C, and at relative air humidity levels of from about 95 % to 100 %. The hardening time is generally about 6 to 8 hours. Another method of hardening the formed structural elements is to harden them conventionally in air over periods ranging from about 14 to 28 days.

An aqueous paste containing waterglass and at least one metal oxide and, optionally, at least one pigment and/or filler is then applied in a thin layer to the preformed hardened structural elements. The composition of the pastes with which the preformed hardened structural elements with or without a facing layer can be coated in accordance with the invention may vary within relatively wide limits. Preferred coating pastes contain alkali metal silicate, for example sodium silicate in aqueous solution (waterglass), metal oxides, for example ZnO, MgO, PbO, CaO or B203, either individually or in any combination, the SiO2-content amounting to between about 42 and 63 mole %, the Na2-content to between about 11 and 27 mole % and the total metal oxide content to between about 19 and 42 mole % (based on the total weight of these components).

Oxide-containing compounds such as, for example, carbonates may also be used for the necessary metal oxide content of the paste. The paste may also contain pigments, for example TiO2, red, yellow or black iron oxides and/or iron oxide hydroxides, chromium oxide pigments, and standard fillers such as, for example, kaolin and calcium carbonate. The pigments, fillers and water are added in such quantities that a readily processible, sprayable and spreadable paste is obtained. In addition, the pigment and/or filler content of the paste should not exceed about 25 % by weight and should preferably be between about 10 and 15 % by weight.

Pastes such as these based on waterglass are known, for example, from J. G. Vail "Soluble Silicates", Vol. II, pages 322 et seq (1952), Reinhold Publishing, New York; US Patent No. 2,354,350 and German Offenlegungschrift No. 1,571,579.

On typical example of a waterglass paste which can be used according to the invention has the following composition: Sodium waterglass, 37 to 40 Be 70 parts by weight Zinc oxide 15 parts by weight Kaolin 5 parts by weight Pigment 5 parts by weight Water 5 parts by weight.

The process according to the invention will now be described with reference to a preferred coating paste based on waterglass.

The various components of the waterglass paste are processed into a homogeneous paste, for example in a dissolver or in ball mills, and are thinly sprayed or coated (in a layer thickness of about 40 to 80 ,um) in this form onto the preformed structural elements themselves or, if they contain a facing layer, onto the facing layer. Preferably about 200 to 500 g and more preferably from 300 to 400 g of waterglass paste are used per square metre of surface area of the preformed structural element to be coated.

After the waterglass paste has been applied to the preformed structural element, it is first left to harden into a gel-like nonfluid state. In the case of structural elements coated in accordance with the invention, this gelling process takes about 30 minutes to 1 hour. After gelling, the gelled coating is also hardened.

This second hardening process may take place in an indirectly electrically heated autoclave at temperatures of about 150 to 210 C and preferably at temperatures of about 170 to 1800C under pressures of about 4 to 19 bars and preferably under pressures of about 7 to 10 bars. At the temperatures indicated, the hardening time is about 4 to 8 hours.

However, it is not necessary to carry out the second hardening process in an autoclave in order to obtain a hard weather roof glaze-like coating on the prefabricated concrete sections.

Thus, the second hardening process may be carried out in a dry atmosphere. The structural elements coated with waterglass may be hardened in a drying cabinet at a temperature of about 200 to 3000C, preferably at a temperature of about 250"C, in the absence of pressure over a period ranging from about 12 minutes to 4 hours.

By virtue of the process according to the invention, structural elements can be provided in a simple manner with a firmly adhering, uniform coating which combines extreme hardness with resistance to weather.

The process according to the invention is illustrated by the following examples: EXAMPLE 1 Production of coated concrete roof tiles Portland cement and Rhine sand (particle size o to 3 mm) were intensively mixed in a ratio (by weight) of 1:3 in the presence of water (water/binder factor for cement 0.37).

The mixture was then processed into concrete roof tiles measuring 20 x 30 cm in a type 270065 test concrete roof tile machine (built in 1970 by Messrs. Ing. Kurt Schade).

The concrete roof tiles were hardened: a) for 48 hours at 280C/90 % relative air humidity in a condition in chamber, b) for 6 hours at 65"C/95 to 100 % relative air humidity in a steam chamber.

After hardening, the tiles were left standing for about 5 minutes to 1 hour so that any moisture adhering to them could evaporate.

A 10 % aqueous calcium chloride solution was sprayed onto the surface of the hardened concrete roof tiles in a quantity of about 80 g per square metre and, after preliminary drying for about 5 minutes at room temperature, a waterglass paste having the following composition was applied to the surface thus treated of the concrete roof tiles: Sodium waterglass, 37 to 400 Be 70 parts by weight ZnO 15 parts by weight Kaolin 5 parts by weight Pigment (iron oxide) 5 parts by weight Water 5 parts by weight.

The waterglass paste was thinly sprayed on in a quantity of 400 g per square metre of concrete roof tile surface.

After standing in air for about 30 minutes, the paste on the surface of the concrete roof tiles was no longer fluid and had gelled to such an extent that the coating could be hardened. Hardening was carried out first over a period of 1 hour at 80"C, then over a period of 1 hour at 1300C and finally over a period of 4 hours at 2500C in a drying cabinet. Alternatively, instead of being carried out over a period of 4 hours in a drying cabinet, the subsequent hardening process was carried out in a jet dryer with a conveyor belt (manufacturers: Messrs.

Werner & Pfleiderer) at temperatures of around 250"C and with four additional heaters (output 1900 watts per heater) for a throughflow time of at least about 3 x 4 minutes. Hardening was then complete and the concrete roof tiles had a coherent, uniform coating.

EXAMPLE 2 Production of coated concrete roof tiles Portland cement and Rhine sand (particle size 0 to 3 mm) were intensively mixed in a ratio (by weight) of 1:3 in the presence of water (water/binder factor for cement 0.37) with the following addition: a) 2 % by weight of calcium chloride, based on cement, b) 2 % by weight of calcium formate, based on cement.

The same waterglass paste as described in Example 1 was applied in a uniform layer thickness to the concrete roof tiles thus produced.

After standing in air for about 30 minutes, the paste on the surface of the concrete roof tiles was no longer fluid and had gelled to such an extent that hardening of the concrete roof tile and coating could be carried out. Hardening was carried out in the same way as described in Example 1.

The hardened concrete roof tiles had a coherent uniform coating.

WHAT WE CLAIM IS: 1. A process for forming and coating structural elements based on inorganic binders, which comprises producing a workable composition from the inorganic binder and water and optionally at least one additive, forming structural elements from this composition and hardening them, wherein (a) at least one water-soluble inorganic salt is initially introduced into the workable composition or into a facing layer up to 50 mm thick applied to the formed structural elements in a minimum quantity of 0.5 % by weight,

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Claims

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gelling process takes about 30 minutes to 1 hour. After gelling, the gelled coating is also hardened.

This second hardening process may take place in an indirectly electrically heated autoclave at temperatures of about 150 to 210 C and preferably at temperatures of about 170 to 1800C under pressures of about 4 to 19 bars and preferably under pressures of about 7 to 10 bars. At the temperatures indicated, the hardening time is about 4 to 8 hours.

However, it is not necessary to carry out the second hardening process in an autoclave in order to obtain a hard weather roof glaze-like coating on the prefabricated concrete sections.

Thus, the second hardening process may be carried out in a dry atmosphere. The structural elements coated with waterglass may be hardened in a drying cabinet at a temperature of about 200 to 3000C, preferably at a temperature of about 250"C, in the absence of pressure over a period ranging from about 12 minutes to 4 hours.

By virtue of the process according to the invention, structural elements can be provided in a simple manner with a firmly adhering, uniform coating which combines extreme hardness with resistance to weather.

The process according to the invention is illustrated by the following examples: EXAMPLE 1 Production of coated concrete roof tiles Portland cement and Rhine sand (particle size o to 3 mm) were intensively mixed in a ratio (by weight) of 1:3 in the presence of water (water/binder factor for cement 0.37).

The mixture was then processed into concrete roof tiles measuring 20 x 30 cm in a type 270065 test concrete roof tile machine (built in 1970 by Messrs. Ing. Kurt Schade).

The concrete roof tiles were hardened: a) for 48 hours at 280C/90 % relative air humidity in a condition in chamber, b) for 6 hours at 65"C/95 to 100 % relative air humidity in a steam chamber.

After hardening, the tiles were left standing for about 5 minutes to 1 hour so that any moisture adhering to them could evaporate.

A 10 % aqueous calcium chloride solution was sprayed onto the surface of the hardened concrete roof tiles in a quantity of about 80 g per square metre and, after preliminary drying for about 5 minutes at room temperature, a waterglass paste having the following composition was applied to the surface thus treated of the concrete roof tiles: Sodium waterglass,

37 to 400 Be 70 parts by weight ZnO 15 parts by weight Kaolin 5 parts by weight Pigment (iron oxide) 5 parts by weight Water 5 parts by weight.

The waterglass paste was thinly sprayed on in a quantity of 400 g per square metre of concrete roof tile surface.

After standing in air for about 30 minutes, the paste on the surface of the concrete roof tiles was no longer fluid and had gelled to such an extent that the coating could be hardened. Hardening was carried out first over a period of 1 hour at 80"C, then over a period of 1 hour at 1300C and finally over a period of 4 hours at 2500C in a drying cabinet. Alternatively, instead of being carried out over a period of 4 hours in a drying cabinet, the subsequent hardening process was carried out in a jet dryer with a conveyor belt (manufacturers: Messrs.

Werner & Pfleiderer) at temperatures of around 250"C and with four additional heaters (output 1900 watts per heater) for a throughflow time of at least about 3 x 4 minutes. Hardening was then complete and the concrete roof tiles had a coherent, uniform coating.

EXAMPLE 2 Production of coated concrete roof tiles Portland cement and Rhine sand (particle size 0 to 3 mm) were intensively mixed in a ratio (by weight) of 1:3 in the presence of water (water/binder factor for cement 0.37) with the following addition: a) 2 % by weight of calcium chloride, based on cement, b) 2 % by weight of calcium formate, based on cement.

The mixture was then processed into concrete roof tiles measuring 20 x 30 cm in a type 270065 test concrete roof tile machine (built in 1970 by Messrs. Ing. Kurt Schade).

The same waterglass paste as described in Example 1 was applied in a uniform layer thickness to the concrete roof tiles thus produced.

After standing in air for about 30 minutes, the paste on the surface of the concrete roof tiles was no longer fluid and had gelled to such an extent that hardening of the concrete roof tile and coating could be carried out. Hardening was carried out in the same way as described in Example 1.

The hardened concrete roof tiles had a coherent uniform coating.

WHAT WE CLAIM IS: 1. A process for forming and coating structural elements based on inorganic binders, which comprises producing a workable composition from the inorganic binder and water and optionally at least one additive, forming structural elements from this composition and hardening them, wherein (a) at least one water-soluble inorganic salt is initially introduced into the workable composition or into a facing layer up to 50 mm thick applied to the formed structural elements in a minimum quantity of 0.5 % by weight,

based on the inorganic binder, after which the formed structural elements are hardened on their own or (when a facing layer is present) together with the facing layer, or b) a facing layer up to 50 mm thick is optionally applied to the formed structural elements, the formed structural elements are hardened on their own or (when a facing layer is present) together with the facing layer, and an aqueous 5 to 15 % inorganic salt solution is then applied to the hardened structural elements in a quantity of from 40 to 100 g/m2 and dried for 3 to 15 minutes, and after step (a) or (b) an aqueous paste containing waterglass together with at least one metal oxide is applied in a relatively thin layer to the hardened structural elements, converted into a gel-like, non fluid state, and itself hardened.
2. A process as claimed in Claim 1, wherein the structural elements are prefabricated concrete sections or concrete roof tiles.
3. A process as claimed in Claim 1 or 2, wherein the aqueous paste also contains at least one pigment and/or at least one filler.
4. A process as claimed in any of Claims 1 to 3, wherein, in the case of cementcontaining workable compositions, the ratio by weight of water to cement is adjusted to a value of from 0.3 to 0.7.
5. A process as claimed in any one of Claims 1 to 4 wherein alkali metal and/or alkaline earth metal salts are introduced into the workable composition in quantities of from 0.5 to 5 % by weight, based on cement.
6. A process as claimed in any of Claims 1 to 5, wherein alkali metal and/or alkaline earth metal salts are introduced into the facing layer in quantities of from 0.5 to 5 % by weight, based on the cement content of the facing layer.
7. A process as claimed in any of Claims 1 to 6, wherein a 5 to 15 % aqueous solution containing an alkali metal and/or alkaline earth metal salt is sprayed onto the hardened surface of the mass or the facing layer in a quantity of from 40 to 100 g/m2 of surface area and is dried for 3 to 15 minutes.
8. A process as claimed in Claim 7, wherein a 10 % aqueous solution is used.
9. A process as claimed in any of Claims 1 to 8, wherein the following salts are used individually or in admixture as the soluble inorganic salts: calcium formate, calcium chloride, calcium nitrate, magnesium nitrate, magnesium chloride, potassium chloride, sodium formate, sodium acetate.
10. A process as claimed in any of Claims 1 to 9, wherein, in facing layers consisting of cement and additives, the ratio by weight of water to cement is adjusted to a value of from 0 to 0.5.
11. A process as claimed in Claim 10, wherein the ratio of water to cement is from 0two0.4.
12. A process as claimed in any of Claims 1 to 11, wherein aqueous alkali metal silicate-containing pastes containing 42 to 63 mole % of SiO2, 11 to 27 mole % of alkali metal oxide and 19 to 42 mole % of metal oxide, based on the total weight of these components, are applied to the preformed and hardened structural elements or the structural elements provided with a facing layer.
13. A process as claimed in any of Claims 1 to 12, wherein from 200 to 500g of paste are applied per square metre of surface to be coated.
14. A process as claimed in any of Claims 1 to 13, wherein, after it has been applied to the preformed structural element or to the facing layer, the paste is hardened at ambient temperature over a period of from 0.5 to 1 hour.
15. A process as claimed in any of Claims 1 to 14, wherein, before coating with the paste, the preformed cement-containing structural elements are hardened alone or, when they contain a facing layer, together with the facing layer by standing in air for 14 to 28 days or in moisture and steam for 6 to 48 hours at temperatures of from 22 to 65 "C.
16. A process as claimed in any of Claims 1 to 15, wherein the preformed, hardened paste-coated structural elements are hardened at temperatures of from 150 to 210 C under pressures of from 4 to 19 bars.
17. A process as claimed in Claim 16, wherein the temperature is from 170 to 1800C and the pressure is from 7 to 10 bars.
18. A process as claimed in Claim 16 or 17, wherein the hardening time is between 4 and 8 hours.
19. A process as claimed in any of Claims 1 to 15, wherein the preformed, hardened paste-coated structural elements are hardened in a drying cabinet at a temperature of from 200 to 3000C.
20. A process as claimed in Claim 19, wherein the hardening time is about 4 hours.
21. A process as claimed in any of Claims 1 to 15, wherein the preformed, hardened paste-coated structural elements are hardened in a jet dryer with a conveyor belt at a temperature of about 250"C.
22. A process as claimed in Claim 21, wherein the hardening time is at least 12 minutes.
23. A process as claimed in Claim 1, substantially as hereinbefore described with reference to Example 1 or Example 2.
24. Structural elements formed and coated by a process as claimed in any one of claims 1 to 23..