COATING LEAD SURFACES

Abstract: A method of forming a barrier coating on a lead surface comprises treating the surface with an aqueous solution of a film forming ionomer having -COO<-> or -SO3<-> groups which is such that on drying the ionomer is converted to a water insoluble form, and drying said surface to form the coating thereon. The method is particularly useful for coating the inside surface of lead service pipes, e.g. water pipes.

Description: COATING LEAD SURFACES The present invention relates to the coating of lead surfaces and more particularly, but not exclusively, to the coating of the inner surfaces of lead pipes. The drinking water supply to many houses in the United Kingdom is via service pipes which are of lead and/or which have lead soldered joints. It is well known that lead from such pipes and joints may dissolve in the water and pose a health hazard. The problem is particularly acute in areas which have a soft, acidic water supply having "plumbosolvent" characteristics. In such areas, the lead levels in drinking water may considerably exceed the standard of 10μg/l established in October 1992 by the World Health Organisation. Clearly there is a need to reduce lead levels in many areas. Although replacement of all lead-based pipes and plumbing in the water supply system is an obvious long-term solution to this problem, there will be many instances where immediate replacement is impractical. A short- to medium-term approach to the problem is to provide existing service pipes with an inner, impervious barrier to prevent lead becoming dissolved in the water. The use of linings as the barrier is seriously restricted by the very narrow bore of most service pipes although there has been a proposal to use an expandable PET insert [1] . The internal coating of large-diameter water pipes (e.g. with epoxy resin) is a well established technology. We have however found very few examples of this approach being directed towards the re-lining of lead service pipes. Techniques are described by Harada et al. [2! whereby lead water pipes are internally coated with epoxy resin by filling the pipe with the resin and forcing a ball through the pipe to expel excess resin and produce a coating. A similar technique is described using molten polyethylene, [31 but it seems likely that these techniques are in fact designed for manufacture of pre-lined pipe rather than rehabilitation of existing systems. A simpler method, and one explicitly designed for buried pipework, [41 involves feeding an epoxy resin, pre-mixed with hardener, into a pipe at high pressure, expelling excess resin from the other end, and maintaining a high airflow through the pipe until the resin remaining on the walls has hardened sufficiently to be self- supporting. A brief experimental test confirmed that this method can work, and is probably most suitable for the narrowest bore of pipe. A closely related technique involves applying a vacuum rather than a positive pressure to remove excess resin from the pipe. [51 The potential for lead service pipes to vary in diameter means that using a ball or pig of fixed dimensions to drive excess epoxy rein from the pipe is not likely to be consistently successful. An alternative is to use an elastically compressible pig, [6] which can not only cope with a varying cross-section of pipe but can be used to create a thicker or thinner coating by varying the driving pressure used to move it along the pipe. These polymer coating techniques all suffer from one major disadvantage - the epoxy must be pre-mixed with hardener before being introduced into the pipe. The resin is thus continuously increased in viscosity during the lining process, with an ever-present danger of the pipe blocking before the excess resin is expelled. An alternative idea is to use a moisture-curable resin such as a silicone, epoxy, or cyanoacrylate. The pipe is first drained, then dried with a flow of air, then filled with a low-viscosity moisture-curable resin, and finally the excess resin is expelled with tap water which simultaneously initiates cross-linking of the polymer film remaining on the walls [7] . A somewhat different approach involves the use of a light-cured resin. [8] Like the moisture-cured system, this puts the resin in place before initiating polymerisation and cross-linking, but it is not easy to see how a light-pipe could be inserted into a narrow pipe whose walls are already coated with a viscous resin. It is clear that little serious effort appears to have been made so far to develop a genuine technology for in-situ polymer-lining of lead service pipes. Indeed, in a recent US survey [91 of "Monitoring and Control Experience under the Lead and Copper Rule" the only actions contemplated to control lead levels in drinking water were (i) replacement of all lead-based pipes and connections, and (ii) control of water chemistry to minimise lead corrosion. According to the present invention there is provided a method of forming a barrier coating on a lead surface the method comprising treating the surface with an aqueous solution of a film forming ionomer having -COO " or -SO 3 " groups which is such that on drying the ionomer is converted to a water insoluble form, and drying said surface to form the coating thereon. The method of the invention thus utilises an aqueous solution (which may be a colloidal solution) of a film forming ionomer containing -COO " or -SO 3 " groups (i.e. the ionised form of carboxylic acid and sulfonic acid groups respectively) and which is such that the ionomer is converted to a water insoluble form on drying. Such ionomers may be used to provide coherent and tenacious barrier coatings which, if applied within a lead service pipe, prevent or inhibit the dissolution of lead into water contained within the pipe. More specifically, we believe that the coherence and tenacity of the coatings are derived at least in part by reaction of the COO " or SO 3 " groups with lead so as to produce a form of the polymer in which the chains thereof are cross-linked by -COO-Pb-OOC- or -SO 3 -Pb-O 3 S- groups. The lead may be derived from either metallic lead or PbO on the surface being treated. The method of the invention has the further advantage of being easy to effect since apart possibly from removing scale or other loose deposits from the surface to be treated further preparation thereof may not be necessary. The ionomer used in the method of the invention should have a good film- forming capability, and should be tough but not elastomeric. A low softening point (preferably between about 50 and 150°C) is preferably as this enables coalescence of the polymer film to be achieved during drying without using excessive temperatures. The ionomers used in the method of the invention are preferably salts of co- polymers (preferably random co-polymers) of an alkene (preferably a C 2 -C 4 alkene) and an unsaturated aliphatic carboxylic or sulphonic acid. It is however possible to use polymers incorporating aromatic rings in the backbone. The co-polymer should have sufficient acidic groups to allow a soluble salt form to be produced but not so many acidic groups that the salt form cannot be converted to a water insoluble form on drying. Generally the polymer should contain 5% to 15% by mole of carboxylic or sulfonic acid groups so that the ionomer correspondingly contains 5% to 15% by mole of -COO " groups or -SO 3 " groups.. It is preferred that the alkene is ethylene and that the unsaturated carboxylic acid is acrylic or methacrylic acid. Particularly preferred polymers are those obtained by random polymerisation of 75% to 85% (most preferably about 80%) by weight of ethylene and 15% to 25% (most preferably about 20%) by weight of acrylic acid. These levels of acid functionality not only aid the cross-linking density which is achievable but also dispersion of the polymer in water. In a preferred embodiment of the invention, the ionomer is converted to its water insoluble form by virtue of a chemical conversion of the cations associated with the ionomer. The cations may, for example, be ammonium ions which on drying are released as ammonia and therefore no longer contribute to the solubility of the polymer. A further possibility are cationic groups derived from volatile amines such as triethylamine. In this embodiment of the invention, the salt form of the polymer is most preferably the ammonium salt. The aforementioned polymers obtained by copolymerisation of an alkene and a carboxylic acid may be converted to the ammonium form by refluxing with concentrated aqueous ammonia (S.G. 0.91), usually at a temperature of 110°C-115°C. We have found that the best coatings are achieved if the volume of the refluxing solution is maintained constant by slow addition of aqueous ammonia during refluxing and also if the solution is filtered prior to use. Furthermore we have found that the use of the ammonium salt produces a "self-priming" effect resulting in dissolution of surface lead oxide. The dissolved lead ions formed in this way provide a degree of additional cross-linking and hence further stability for the coating. In a further embodiment of the invention, the ionomer is one which is normally cold water insoluble (due to the proportion of -COO " or -SO 3 " groups in the polymer) but which is soluble in superheated water to yield a solution in which, on cooling, the polymer remains dissolved. During the drying step employed in the method of the invention, the loss of water from the solution causes "precipitation" of the ionomer onto the lead surface to provide the coating. Such ionomers will generally contain a relatively low (e.g. 5-8% by mole) proportion of -COO " or -SO 3 " groups. In this embodiment, the ionomer solution may be obtained by dissolving the ionomer in superheated water or by reacting the free acid form of the ionomer with a superheated aqueous solution of a base (e.g. KOH/NaOH). The ionomer solution may be as described in J.Am.Chem.Soc. 1998, 120, 4215-4221 ("Experimental Section"). The ionomer solution will generally be used at a concentration of 10% to 20% by weight and may be applied to the surface by any convenient technique. After application of the ionomer solution to the surface (and, if necessary, removal of excess) the surface is dried. This may be effected by simply allowing the surface to dry or more preferably heating the surface (e.g. to a temperature of 70°C to 150°C, preferably 100°C to 140°C). We have found that such heating improves the coherency of the coating, possibly by softening the ionomer and allowing it to flow. It will generally be preferred to apply the method of the invention a plurality (e.g. 2 or 3) times to a surface to provide the required barrier properties for the final coating. The method is applicable particularly to the formation of barrier coatings on the inner surfaces of service pipes which are of lead and/or which have lead soldered joints. For such pipes it is a relatively simple matter to pass a solution of the ionomer through the pipe, drain the ionomer solution (so that a film of the solution clings to the pipe walls) and then drying the ionomer so that the water insoluble coating is formed. Such drying may conveniently be effected by passing hot air at the required temperature along the pipe. The invention is illustrated by reference to the following non-limiting Examples. Example 1 This Example utilised a copolymer of ethylene and acrylic acid available from Aldrich Chemical Co., Cat. No. 18, 104-8. This is a random copolymer comprised of 80%) by weight ethylene and 20% by weight acrylic acid. Three solutions A,B and C with respective solids contents of 15.0%>, 18.2% and 23%> by weight were produced using the following procedures. For preparation of each solution, 300 cm 3 of 0.88 aqueous ammonia were introduced into a one litre flange flask having a five-necked head fitted with a high speed stirrer, a 600mm reflux condenser and a thermometer. The aqueous ammonia was vigorously stirred (ca 400 rpm) and the appropriate amount of polymer added. The mixture was then refluxed for 5 hours using an oil bath heated to 110°C. For solution A, "top-up" aqueous ammonia (to restore original volume) was continually added and the final solution was filtered using grade 1 filter paper. The resultant product was a translucent liquid of low viscosity. For solution B, "top-up" aqueous ammonia (to restore original volume) was added at the end of the reflux period to aid cooling. The resultant product was an opaque liquid of low viscosity. For solution C, no further ammonia was added. The resultant produce was an opaque liquid of high viscosity. Each of solutions A, B and C were applied to respective milled flat lead slabs (two for each solution) with a 24 μm "K"-bar and dried either at room temperature or 70°C for one hour to produce coatings. The lead slabs were then used for adhesion, thickness and water sensitivity measurements. The adhesion measurements were effected using a cross-hatch and lift-off method in which a cross-hatch pattern of lines was made on the coating with a diamond knife. The cross-hatch pattern was such that a square of 20 mm x 20 mm was cut with the knife and cross-hatched at 1 mm intervals (to yield 400 squares). The value of "Cross-Hatch as shown in the table represents the number of 1 mm squares remain attached after turning the sample upside down. The "Lift-Off test followed and involved sticking a piece of Scotch tape over the cross-hatched area and lifting the tape off. The value of "Lift-Off is the number of squares remaining attached to the metal surface. The water sensitivity measurements were obtained by visual inspection after 24 hours immersion of the sample in water. The results are shown in Table 1. Table 1 It can be clearly seen that the degree of neutralisation, indicated by the viscosity and opacity of the fluid, has a direct effect on the adhesion of the polymer to the surface of the lead. Also the adhesion is consistently better with the samples dried at 70°C rather than at room temperature. This may be a function of film thickness as this response is consistently lower at 70°C. Example 2 An 18 inch (45 cm) length of used lead pipe of internal diameter 0.5 inch (1.25 cm) was carefully bent into a "U" tube shape and then cleaned with warm water and a bottle brush until deposits stopped issuing from the pipe. The pipe was then filled with polymer solution A which subsequently drained from the pipe. The pipe was allowed to drain and then dried using a hot-air blower such that the ingoing drying air was at a temperature of ca 150°C and the outgoing air was at a temperature of 90- 100°C. A lead leaching test was then conducted. For the purpose of this test, a 50 ml sample of tap water was poured into the pipe and both ends were covered to avoid contamination. The pipe was left for 24 hours and the water was then carefully poured from the pipe into a Perspex sample bottle. The water was analysed for lead content which was found to be 9.2 micrograms of per litre. In contrast, a repeat of the lead leaching test but using a pipe which had not been coated with polymer gave a lead value of 394 micrograms per litre. REFERENCES 1. WRC Report, UM 1291, March 1992. 2. JP 02,122,870, May 1990. 3. JP 02,122,869, May 1990. 4. JP 02,123,181, Nov. 1988. 5. JP 58 06,272, July 1981. 6. JP 63 31,578, Feb. 1988; also JP 04,187,269, Jul. 1992. 7. JP 61, 181, 574, Aug. 1986. 8. JP 60 28, 858, Jul 1983. 9. J. Am. Water Works Assocn., 1993, 85 (2), 64.

Claims: CLAIMS 1. A method of forming a barrier coating on a lead surface the method comprising treating the surface with an aqueous solution of a film forming ionomer having -COO " or -SO 3 " groups which is such that on drying the ionomer is converted to a water insoluble form, and drying said surface to form the coating thereon. 2. A method as claimed in claim 1 wherein the ionomer comprises 5% to 15% by mole of -COO " or -SO 3 " groups. 3. A method as claimed in claim 1 or 2 wherein the ionomer is derived from a copolymer of C 2 -C 4 alkene and an unsaturated aliphatic carboxylic acid. 4. A method as claimed in claim 3 wherein the alkene is ethylene. 5. A method as claimed in claim 3 or 4 wherein the aliphatic carboxylic acid is acrylic acid or methacrylic acid. 6. A method as claimed in any one of claims 3 to 5 wherein the co-polymer is a random copolymer. 7. A method as claimed in claim 3 wherein the ionomer is derived from a copolymer of ethylene and acrylic acid. 8. A method as claimed in claim 7 wherein the co-polymer comprises 75% to 85% by mole of ethylene and 15% to 25% by mole of acrylic acid. 9. A method as claimed in claim 8 wherein the copolymer comprises about 80%) by mole of ethylene and about 20% by mole of acrylic acid. 10. A method as claimed in any one of claims 1 to 9 wherein the ionomer is converted to its water insoluble form by virtue of a conversion of the cations associated with the ionomer. 11. A method as claimed in claim 10 wherein the cations are ammonium cations. 12. A method as claimed in any one of claims 1 to 9 wherein the cations are derived from a volatile amine. 13. A method as claimed in any one of claims 1 to 9 wherein the ionomer is one which is normally cold water insoluble but which is soluble in super heated water to yield a solution in which, on cooling, the polymer remains dissolved. 14. A method as claimed in any one of claims 1 to 13 wherein the drying is effected at elevated temperature. 15. A method as claimed in claim 14 wherein the drying is effected at a temperature of 100°C to 150°C. 16. A method as claimed in claim 15 wherein the drying is effected at a temperature of 120°C to 140°C. 17. A method as claimed in any one of claims 1 to 16 wherein the steps of claims 1 are repeated at least once. 18. A method as claimed in any one of claims 1 to 17 wherein the lead surface is the inner surface of a lead pipe. 19. A method as claimed in claim 18 wherein the lead pipe is a lead service pipe. 20. A method as claimed in claim 18 or 19 wherein drying is effected by passing heated air along the pipe.

Inventors: COLQUHOUN HOWARD M, EYRES ROY, COLQUHOUN, HOWARD, M., EYRES, ROY
Applicants: COLQUHOUN HOWARD M, EYRES ROY, UNITED UTILITIES PLC, COLQUHOUN, HOWARD, M., EYRES, ROY
Application year: 1999
Main classification: B05D7
IPC codes: C09D201, C23C22, E03B7