GB1579530A - Method of treating the surface of an article - Google Patents

Description

(54) METHOD OF TREATING THE SURFACE OF AN ARTICLE.

(71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF a British Company 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 an anti-fouling treatment for surfaces and particularly to a treatment for surfaces such as the hulls of marine vessels, buoys, and structural members of piers which are to be exposed to, and especially immersed in, aqueous media containing living organisms, to prevent or at least substantially inhibit fouling of the surfaces by the living organisms. A particular feature of the invention is an anti-fouling treatment for surfaces to be subjected to marine environments, where fouling by barnacles, shells, seaweed, and hydroids is a major problem.

We have now found that fouling of surfaces by living organisms can be inhibited or Prevented by providing those surfaces which are to be exposed to the media containing the iving organisms with a coating of a hydrolytically soluble glass, for example a phosphate glass.

According to the present invention there is provided a method of treating an article intended for prolonged immersion in an aqueous environment comprising applying to the surface of the article a coating of a glass which coating has a rate of dissolution in pure water at 20"C of at least x 10-6ssm depth of coating dissolved per minute and not greater than 1 mm per year the average molecular weight of the material dissolved from the coating being at least 300 when the glass-coated article is immersed in water at 200C.

The rate of dissolution is preferably at least 1 x 10-5 Fm/min and especially at least 6 x 10-5 Fm/min and the molecular weight of the dissolved material is preferred to be at least 400. The measurement of molecular weight may be conducted by the method given in "Solution Silicates" Vol. I pp. 98-101 J.G. Vail, Reinhold, New York 1952.

It will be apparent that if the rate of dissolution of the glass coating is very high the coating will soon dissolve away and therefore for prolonged exposure to aqueous conditions glasses are normally selected which will dissolve very slowly.

The depth of coating dissolved is calculated from a measurement of the weight (and hence the volume) dissolved away from a known surface area of glass.

Examples of glasses which will fulfil the conditions required and provide a useful anti-fouling (or alternatively an effect which may be termed an anti-scaling effect if the potential deposition is of a non-living nature) are glasses having a structural network of a predominantly "3-connective" type. (L Holliday, "Ionic Polymers", Applied Science London 1975). For example in a high phosphate glass, the especially preferred material for use in this invention, the structure consists predominantly of a network in which each phosphorus atom is joined via a covalently bonded oxygen atom to three other phosphorus atoms. Examples of glasses which also may be used in this invention include vanadate, phospho vanadate, alkali borate, alkali zinc/lead borate, sulphato phosphate, boro phosphate, tellurite, arsenite and beryllium fluoride glasses.Certain soluble silicate glasses may also be used; those having a formula Nazi.. SiO, wherein X is 3.4 to 4.0 are known to fulfil the required conditions hereinbefore specified for use in this invention. The surfaces to be coated with the glass may be those of an article designed for movement in or through the media containing the living organisms, for example the hulls of sailing vessels, or those of a stationary article, for instance buoys. the structural members of piers and oil rigs, or they may be the walls of a water tank or other vessel containing aqueous fluids.

By the term high phosphate glass as used throughout this specification we mean a glass derived from glass-forming mixtures wherein the glass-forming oxide phosphoric oxide, P2Os is present in an amount of at least 50 mole percent of the total glass-forming oxides.

Other glass-forming oxides may be present, for example silica (SiO2) and boron oxide (B2O), and also cations including lithium, sodium, potassium, rubidium, caesium, cobalt, magnesium, calcium, strontium, barium, cadmium, zinc, lead, silver, copper, gold, tungsten, vanadium, chromium, molybdenum, and aluminium, as is well known in the glass-making art.

Whilst many of the so-called phosphate glasses may be employed, as hereinbefore stated, we have found that particularly suitable glasses are those derived from mixtures which contain boric oxide in addition to phosphorus oxide. Additional preferred oxides are sodium, lithium, magnesium and calcium oxides; and an especially preferred composition contains also either lead or cobalt oxides. The proportion of boric oxide in the glass-forming mixture preferably is in the range 1 mole % to 5 mole %, , especially from 2 mole % to 3.5 mole %.

The glasses specified for this invention are those slowly dissolved by water, and when they are in contact with aqueous media the surface is continuously and gradually dissolved.

whilst it is to be understood that the invention is not limited by any particular theory, it is believed that the molecular size of the dissolved species as it comes away from the surface of the glass plays an important part in preventing or inhibiting fouling of the surface of the glass. The rate of dissolution of a particular glass determines its suitability for any particular anti-fouling application. Thus a more soluble glass may be required for coating surfaces in environments which are very susceptible to fouling than may be required for coating surfaces in environments which are only slightly susceptible to fouling.The rate of dissolution of a phosphate glass in water or in any particular aqueous medium can be controlled within a fairly wide range by varying the amounts of alkaline earth metal oxides such as MgO and CaO, and by varying the proportion of boric oxide in the mixture from which the glass is derived, and/or by controlling the degree of cross-linking introduced into the glass as described by N H Ray and C J Lewis in Journal of Materials Science, published by Chapman and Hall Limited, No. 7 (1972) pages 47-51. In general, increasing the proportion of divalent cations such as calcium and increasing the proportion of boric oxide results in a decrease in the rate of dissolution of the glass in water and other aqueous media.

The temperature of the aqueous medium with which the glass will be contacted affects both the solubility and the rate of dissolution of the glass in that medium, and hence the suitability of a particular glass for a particular anti-fouling application. In general, the glass will dissolve more rapidly in a hot medium than in a cold medium. However, as explained hereinbefore, the solubility of the glass is easily controlled and it is a matter of simple experiment to design a glass of optimum solubility for any particular environment.

In the case of surfaces such as the hulls of sailing vessels which may encounter various conditions of water temperature in a single voyage, it is possible to select a glass having a rate of dissolution which is a compromise between the optimum rates for the extremes of temperature conditions likely to be encountered. Since the rate of dissolution of any particular glass varies according to the ambient temperature, the rate of dissolution of that glass will to some extent be self-compensating for changes in ambient temperature.

Whilst the rate of and the products of dissolution of the glass in water are believed to be the major factors contributing to the anti-fouling properties of surfaces coated with a glass, other contributing factors are believed to be the very smooth surface of the glass coating and the inhibition of corrosion of metal articles provided by the glass coating. It is well known that fouling of surfaces such as the hulls of sailing vessels is enhanced by irregularities in those surfaces, for example minute cracks and scratches. In general, the smoother and more perfect the surface, the less will be the tendency for fouling of that surface. Coasting the surface with a phosphate glass provides a very smooth surface and has a streamlining effect on that surface, the smoothness of the resulting surface being beneficial in inhibiting fouling.Corrosion of surfaces is prevented by the coating of phosphate glass. This eliminates corrosion as a source of surface irregularity.

The coating of the glass chosen e.g. suitably a phosphate glass, may be applied to the appropriate surfaces to be protected in any convenient manner, for example by dipping, brushing, flame-spraying. enamelling. doctoring, or transfer from rollers, or molten glass may be caused to flow over the surface to form a film of the glass. Flame-spraying is the preferred method of application to large surfaces e.g. hulls of ships and it is convenient with this method to feed the glass continuously to the spraying tool (e.g. a torch or gun) from a powder hopper preferably, or from a sheathed cord having a core of the powdered glass. If desired, the metal, wood or other material from which the article is to be constructed may be provided with the coating of a suitable glass prior to fabrication into the article.It will be readily appreciated that re-coating or relining of surfaces is possible whenever an existing coating or lining has been degraded to the extent that it no longer affords efficient anti-fouling protection.

We have found that in general the release of potentially toxic materials from a phosphate glass into the aqueous environment is insufficient to kill living organisms even in the immediate vicinity of the glass surface. Thus for instance toxic materials such as boron and lead are released as the glass slowly dissolves but in general the rate of their release does not result in killing of organisms in the aqueous environment. If desired, however, a toxin, notably copper, may be included in a phosphate glass specifically to provide sufficient toxicity to kill organisms on or closely adjacent the surface of the glass, so enhancing the anti-fouling protection afforded by the glass.

When borophosphate glasses are exposed to a moist atmosphere, they develop a viscous liquid coating on the surface which is clearly a polymeric species dissolved in adsorbed water. Also it may be noted that when glasses of the high phosphate type, dissolve in water a polymeric solute is observed to be produced because there is a considerable increase in viscosity of the liquid surrounding the glass.

The sodium silicate glass Na2O/3.5 SiO2 dissolves in water to give a solution whose solute has a molecular weight average of about 400, signifying a silicate polymer chain length of between 5 and 6 silicate units.

The invention is illustrated but not limited by the following examples in which the glass coatings possess properties in accordance with the invention.

Examples 1 to 7 Seven phosphate glasses with the compositions given in Table 1 were prepared by (a) heating a mixture of the metal oxides with phosphoric acid in a "Pyrex" beaker at 5000C for 5 hours in a vertical tube furnace, (b) cooling and solidifying the melt and then breaking and removing the beaker, (c) founding the glass at 700-8000C in a 250 ml high-alumina Purox crucible heated in a resistively-heated muffle furnace, and (d) pouring the founded melt onto a stainless steel plate and cooling it to form a plate of glass. The samples thus prepared all possessed properties as specified in claim 1.

Glass Composition - (mole %) wt % Example Tg No. ( C) P2O5 B203 Na2O Li2O MgO CaO PbO CoO 1 66.1 2.3 7.1 7.1 2.4 2.4 12.6 - 160 2 66.1 2.3 7.1 7.1 2.4 2.4 12.6 - 194 3 62.5 2.5 2.5 2.5 2.5 2.5 25 - 177 4 62.5 2.5 2.5 2.5 2.5 2.5 25 - 203 5 55.0 3.0 15.0 7.5 8.5 8.5 3.0 - 309 6 62.5 2.5 2.5 2.5 2.5 2.5 25 1 170 7 63.1 2.25 18 7.65 4.5 4.5 - 1 322 "Pyrex" is a Registered Trade Mark.

4-inch square samples of the glass plates together with an 8-inch square sample of a conventional silicate plate glass were separately clamped on a frame and suspended from a raft at a depth of about 4 feet in sea water in the Menai Straits. The samples remained suspended in the sea for about 5 months, through the months May to September. When the samples were removed and examined at the end of September, the phosphate glass samples were found to be completely free from fouling by living vegetable or animal organisms, whereas the sample of silicate glass was uniformly fouled with weed and the metal frame and brackets used to suspend the samples were fouled with patches of barnacles and weed.

It was noted that some of the phosphate glass samples when dried had deposits on their surfaces, but these deposits were very readily removed by gently wiping or rinsing with a gentle flow of water, showing that the deposits were not attached to the glass surface and that the surface was in fact completely free from fouling.

Example 8 A stainless steel bar of dimensions 6" x 1" x 114" was coated completely with the phosphate glass identified in Example 6 in Table 1.

The coated bar was suspended as described above in the Menai Straits for a period of 3 months after which time no trace of fouling could be detected. By comparison a similar stainless steel bar which had not been coated with phosphate glass was completely fouled in patches when suspended alongside but separated from the coated bar for the 3 months period.

Example 9 40-45 Kg. batches of a mixture of phosphoric acid and the oxides of boron and various metals (as shown in the Table 2 below) were founded in a high alumina crucible in an electric furnace at 800"C. When the glass had attained the required glass-transition temperature the molten mixture was allowed to flow into a covered stainless steel trough. It may be noted that the compositions contain the oxides of phosphorus, boron and magnesium together with at least two other oxides selected from lithium, sodium, calcium, lead, copper tungsten and cobalt oxides.

TABLE 2 glass No.

composition 1 2 3 4 5 mole % P2O5 63.1 63 63 63 66.2 13203 2.25 4 4 4 2.3 Li2O 7.65 16.5 13 7.1 Na2O 18 20 7.1 MgO 4.5 16.5 16 2 2.4 CaO 4.5 2.4 PbO 6 12.5 CuO 4 WO3 5 CoO (in wt %) 1 1 1 Tg "C 232 331 331 200 160 Rate of dissolution 14x10-5 0.3x10- 0.5x10-j 8.0x10-5 t 130x 10-5 Example 10 Testing of A ti nfoulitig efficac of bo tvphosph ate glasses Glasses 1-5 of Example 9 were flame sprayed onto both mild steel and stainless steel plates and the coated plates were bolted onto the bilge keel of the research ship of the University of Wales, Bangor. After one season at sea i.e. April - September, (a season which was considered to be particularly heavy for fouling by barnacles) the hull of the vessel, even where treated with commercially available antifouling paints showed weed and particularly barnacle growth adhering to it. However, the glass-coated plates were very different in appearance after the test; the plate with composition 1 was totally free of fouling, the others showed the residual marks of 3-5 barnacles per square foot which had become detached in the course of the test. The rest of the hull showed heavy barnacle growth still strongly adhering to the surface.

Example 11 Two glasses were prepared containing no phosphate groups and tested for their ability to inhibit the collection of unwanted matter on the surface. The test applied was to suspend test pieces of the glass in a saturated solution of pentaerythritol in water at 1000C and allow them to equilibrate and cool slowly to room temperature. Crystallisation occurs and the extent of the crystallisation deposit on the test pieces of glass allows an evaluation of their ability to inhibit deposition. Samples which behave well in this test are likely to be effective also for the inhibition of growth of living organisms.

The glasses tested were: (i) A glass having the composition Na2O/3.5 SiO2 and (ii) a glass having the composition 30 Na2O 70V205.

The test pieces were observed to be entirely free of crystallisation deposit except just in the crevices of glass (ii) which had a rough surface compared with the fire-polished surface of glass (i). Test pieces of "Pyrex" glass and soda-lime glass compared in the same pentaerythritol solution were observed to be heavily scaled with crystals.

WHAT WE CLAIM IS: 1. A method of treating an article intended for prolonged immersion in an aqueous environment comprising applying to the surface of the article a coating of a glass which coating has a rate of dissolution in pure water at 20"C of at least 1 x 10-6 um depth of coating dissolved per minute and not greater than 1 mm per year, the average molecular weight of the material dissolved from the coating being at least 300 when the glass-coated article is immersed in water at 20"C.

2. A method as claimed in claim 1 wherein the glass coating has a rate of dissolution of at least 1 x 10-5 llm/minute.

3. A method as claimed in claim 1 wherein the glass coating has a rate of dissolution of at least 6 x 10-5 ttm per minute.

4. A method as claimed in any one of the preceding claims wherein the average molecular weight of the material dissolved from the coating when the glass-coating is immersed in water at 20"C is at least 400.

5. A method as claimed in any one of the preceding claims wherein the glass is selected from vanadate, phosphovanadate, alkali borate, alkali zinc/lead borate, sulphatophosphate, boro-phosphate tellurite, arsenite and beryllium fluoride glasses.

6. A method as claimed in claim 1 wherein the glass is a silicate glass having a composition Na2O.XSiO2 where X is from 3.4 to 4.0.

7. A method as claimed in any one of claims 1 to 4 wherein the glass is a high-phosphate glass as hereinbefore defined, a glass in which phsophorus pentoxide constitutes at least 50 mole per cent of the glass-forming oxides present.

8. A method as claimed in claim 7 wherein the high phosphate glass contains boric oxide in addition to phosphorus pentoxide.

9. A method as claimed in claim 8 wherein the phosphate glass contains from 1 mole % to 5 mole % of boric oxide.

10. A method as claimed in claim 9 wherein the proportion of boric oxide in the glass is from 2 mole % to 3.5 mole %.

11. A method as claimed in any one of claims 8-11 wherein cations are also present in the glass.

12. A method as claimed in claim 11 wherein the cations present in the glass are selected from the cations of lithium, sodium, potassium, rubidium, caesium, cobalt, magnesium, calcium, strontium, barium, cadmium, zinc, lead, silver, copper, gold, tungsten, vanadium, chromium, molybdenum and aluminium.

13. A method as claimed in any one of claims 7 to 12 wherein the glass comprises a glass forming mixture of the oxides of phosphorus boron, sodium, lithium, magnesium and calcium.

14. A method as claimed in claim 13 wherein the glass also contains either lead oxide or cobalt oxide.

15. A method as claimed in any one of claims 7 to 12 wherein the glass comprises a glass forming mixture of the oxides of phosphorus, boron and magnesium together with at least two other oxides selected from lithium, sodium, calcium, lead, copper, tungsten and cobalt oxides.

16. A method as claimed in claim 1 wherein the phosphate glass has a composition substantially as described herein with reference to any one of examples 1 to 10.

17. An article treated by a method as claimed in any one of the preceding claims.

18. An article treated to inhibit fouling of its surface in an aqueous environment the article having a surface coated by a method substantially as described herein with reference to or as shown in examples 1 to 8 or 10.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (19)

**WARNING** start of CLMS field may overlap end of DESC **. Example 11 Two glasses were prepared containing no phosphate groups and tested for their ability to inhibit the collection of unwanted matter on the surface. The test applied was to suspend test pieces of the glass in a saturated solution of pentaerythritol in water at 1000C and allow them to equilibrate and cool slowly to room temperature. Crystallisation occurs and the extent of the crystallisation deposit on the test pieces of glass allows an evaluation of their ability to inhibit deposition. Samples which behave well in this test are likely to be effective also for the inhibition of growth of living organisms. The glasses tested were: (i) A glass having the composition Na2O/3.5 SiO2 and (ii) a glass having the composition 30 Na2O 70V205. The test pieces were observed to be entirely free of crystallisation deposit except just in the crevices of glass (ii) which had a rough surface compared with the fire-polished surface of glass (i). Test pieces of "Pyrex" glass and soda-lime glass compared in the same pentaerythritol solution were observed to be heavily scaled with crystals. WHAT WE CLAIM IS:

1. A method of treating an article intended for prolonged immersion in an aqueous environment comprising applying to the surface of the article a coating of a glass which coating has a rate of dissolution in pure water at 20"C of at least 1 x 10-6 um depth of coating dissolved per minute and not greater than 1 mm per year, the average molecular weight of the material dissolved from the coating being at least 300 when the glass-coated article is immersed in water at 20"C.

2. A method as claimed in claim 1 wherein the glass coating has a rate of dissolution of at least 1 x 10-5 llm/minute.

3. A method as claimed in claim 1 wherein the glass coating has a rate of dissolution of at least 6 x 10-5 ttm per minute.

4. A method as claimed in any one of the preceding claims wherein the average molecular weight of the material dissolved from the coating when the glass-coating is immersed in water at 20"C is at least 400.

5. A method as claimed in any one of the preceding claims wherein the glass is selected from vanadate, phosphovanadate, alkali borate, alkali zinc/lead borate, sulphatophosphate, boro-phosphate tellurite, arsenite and beryllium fluoride glasses.

6. A method as claimed in claim 1 wherein the glass is a silicate glass having a composition Na2O.XSiO2 where X is from 3.4 to 4.0.

7. A method as claimed in any one of claims 1 to 4 wherein the glass is a high-phosphate glass as hereinbefore defined, a glass in which phsophorus pentoxide constitutes at least 50 mole per cent of the glass-forming oxides present.

8. A method as claimed in claim 7 wherein the high phosphate glass contains boric oxide in addition to phosphorus pentoxide.

9. A method as claimed in claim 8 wherein the phosphate glass contains from 1 mole % to 5 mole % of boric oxide.

10. A method as claimed in claim 9 wherein the proportion of boric oxide in the glass is from 2 mole % to 3.5 mole %.

11. A method as claimed in any one of claims 8-11 wherein cations are also present in the glass.

12. A method as claimed in claim 11 wherein the cations present in the glass are selected from the cations of lithium, sodium, potassium, rubidium, caesium, cobalt, magnesium, calcium, strontium, barium, cadmium, zinc, lead, silver, copper, gold, tungsten, vanadium, chromium, molybdenum and aluminium.

13. A method as claimed in any one of claims 7 to 12 wherein the glass comprises a glass forming mixture of the oxides of phosphorus boron, sodium, lithium, magnesium and calcium.

14. A method as claimed in claim 13 wherein the glass also contains either lead oxide or cobalt oxide.

15. A method as claimed in any one of claims 7 to 12 wherein the glass comprises a glass forming mixture of the oxides of phosphorus, boron and magnesium together with at least two other oxides selected from lithium, sodium, calcium, lead, copper, tungsten and cobalt oxides.

16. A method as claimed in claim 1 wherein the phosphate glass has a composition substantially as described herein with reference to any one of examples 1 to 10.

17. An article treated by a method as claimed in any one of the preceding claims.

18. An article treated to inhibit fouling of its surface in an aqueous environment the article having a surface coated by a method substantially as described herein with reference to or as shown in examples 1 to 8 or 10.

19. An article as claimed in claim 17 or claim 18 whenever used in contact with sea-water.