Patent Application
20070269595
The present invention relates to the prevention of the leaching of metals into water in contact with an object comprising copper. In particular, the invention relates to the prevention of the leaching of metals such as copper and lead from a plumbing component for potable water.
A problem occurring with plumbing fixtures is the leaching of various metals from the material making up the surfaces contacting the water. Plumbing fixtures are generally manufactured from copper-containing alloys, containing for example zinc or lead in order to improve the workability and machinability of the metal. Also, solders and fluxes used in the manufacture of plumbing fixtures usually contain various metals, which are not fully inert in an aqueous environment. Thus, faucets, valves and related products for delivering potable water may have a tendency to release small amounts of metal, which are undesirable in water intended for consumption due to their toxic or potentially toxic properties. The amount of released metals is influenced by a number of factors, including pH and dissolved solids, and it may vary with time, often being relatively high after the installation of the fitting. Testing procedures and maximum metal release concentrations for various categories of plumbing fixtures, fittings and pipes for the US market are specified in ANSI/NSF Standard 61.
Attempts to reduce or eliminate this problem have involved various treatments and coatings of the inner surfaces of the fixtures. In German OS 35 15 718, a water faucet is disclosed having a plastic coated boring making up the water conduit, while the faucet body is manufactured from a zinc alloy which is less expensive than brass. Tin plating of the wetted surfaces of a fitting made of copper alloy is described in, for example, German patent 14 192 and U.S. Pat. No. 5,876,017. In U.S. Pat. No. 5,958,257, a treatment is disclosed in which a brass component is treated with a caustic solution, leached, and treated with carboxylic acid in order to remove leachable lead. According to U.S. Pat. No. 6,461,534, the treatment sequence is first acid, then alkali. In U.S. Pat. No. 6,656,294, a method is disclosed in which the surface is alkali treated and subsequently a chromate plating is applied. According to European patent application 1 548 155 A, a dilute solution of nitric and hydrochloric acids is used to remove lead and nickel and to passivate the copper surface.
The multilayer coating of copper-alloy objects, such as faucets, for decorative purposes and to improve wear resistance, is disclosed in e.g. U.S. Pat. No. 5,879,532, U.S. Pat. No. 6,221,231 and U.S. Pat. No. 6,399,219. Organic polymers, metals and their compounds are used; coating techniques include electroplating, dipping and various vapor deposition methods. However, these methods do not eliminate the leaching of unwanted material from the coated objects.
According to one aspect of the present invention, a method is provided for reducing or eliminating the leaching of undesirable metals by forming an inert, at least partial film comprising titanium and oxygen on copper or copper-alloy surfaces. Particularly, the surfaces are those of plumbing components such as faucets, valve components and the like, and more particularly those surfaces that are in water contact during use. Thus, the surfaces coated in accordance with the present invention are in particular the inner surfaces of a hollow object. The object in question may be a single component, e.g. a plumbing component, or an assembly of several such components.
According to a further aspect of the invention, plumbing components having an inert, at least partial film on copper or copper-alloy surfaces are provided.
The expressions “at least partial film” and “coated at least partially” in this context imply, that the film need not cover the copper or copper alloy surface completely. Discontinuities in the film may be due to, e.g., cracking caused by stretching or bending of the substrate material; to grain boundaries particularly in a crystalline material; to insufficient cleaning prior to the coating process; impurities or particles on the substrate surface; or to physical damage. Sections of the surface may also be left uncoated e.g. for technical reasons relating to the joining of parts.
Metal leaching is reduced considerably by using at least a partial film according to this invention, even if the film coating includes discontinuities as described above. Preferably, however, at least 30% of the surface is coated by a film according to this invention. According to a preferable embodiment of the present invention, the surface is completely covered by a film coating according to the invention. “Completely” should be taken as free from defects from a practical point of view.
A final film coating may include several layers with different functionality. Typical functional layers are primer layers, barrier layers and protective layers. The film coating formed according to the invention includes at least one layer comprising titanium and oxygen. In particular, this layer comprises titanium oxide. For the purpose of this text, “oxide” refers to all oxides (for example, titanium oxide, aluminium oxide, tantalum oxide) of various chemical composition, phase and crystalline structure. Correspondingly, where a stoichiometric chemical formula is used, as is common practice in the field, this does not necessarily imply that the layer in question has the corresponding absolute stoichiometric composition. Titanium oxide is commonly referred to as titanium dioxide, TiO2. Preferably, the film is formed by means of atomic layer deposition (ALD), also called atomic layer epitaxy (ALE). This method is particularly suitable for the relevant purpose, as it makes possible the uniform and reliable coating of rough or irregular surfaces, especially the inner surfaces of hollow or tube-shaped objects, to yield a tight, pinhole-free layer. A representative description of this technology may be found in e.g., Atomic Layer Epitaxy, Suntola, T. and Simpson, M., eds., Blackie and Son Ltd., Glasgow, 1990. A detailed description of TiO2 deposition using this technology may be found in the thesis of Mikko Ritala, Atomic Layer Epitaxy growth of titanium, zirconium and hafnium dioxide thin films, Annales Academia Scientiarum Fennica, Series A, II. Chemica 257, Helsinki 1994. Examples of patents relating to ALD are U.S. Pat. No. 4,058,430, U.S. Pat. No. 4,389,973, U.S. Pat. No. 4,413,022, U.S. Pat. No. 6,941,963, U.S. Pat. No. 6,907,897 U.S. Pat. No. 6,936,086 and FI 84980.
Other possible techniques include Chemical Vapor Deposition (CVD), Metal Organic Vapor Deposition (MOCVD) and sol-gel-type processes. Descriptions can be found in, e.g., Bradley, D. C., Mehrotha, R. C., Rothwell, I. P. and Singh, A., Alkoxo and Aryloxo Derivatives of Metals, Academic Press 2001.
The finished film may comprise several materials, for example silicon, in addition to titanium and oxygen. Contaminants, such as H, C, N or Cl from the manufacturing processes of the raw materials of the reagents used in the coating process, are typically present in a total amount below 20% by weight. The amount of impurities, e.g. a weight percentage of above 0.1 of Cl or H in the process for depositing titanium oxide may have a positive influence on the barrier properties of the resulting layer, e.g. by having an effect on the degree of amorphousness. Such impurities may be included in the precursors.
Titanium oxide is well suited for the coating of plumbing components, as titanium oxide is chemically stable in all relevant aqueous environments. It is widely used and considered physiologically safe. Further, there are a number of useful depositing methods for this material.
Amorphous, crystalline (e.g. anatase, brookite or rutile) or polycrystalline titanium oxide or mixtures of these are all preferred materials according to the present invention. An amorphous titanium oxide layer is particularly advantageous, as interfaces (e.g. grain boundaries) occurring in a crystalline structure may act as a channel for metals prone to leach through. For the formation of an amorphous layer, low temperatures are preferable. To keep production costs at a reasonable level, no excessive layer thicknesses should be used. Preferably, the total thickness of the coating according to the invention (that is, excluding any additional functional layers e.g. primer and protective layers) is less than 10 000 nm; more preferable, in the range 3-1000 nm; most preferable in the range 30-100 mn. A coating process according to the invention is preferably carried out at a temperature in the range 10° C.-500° C.; preferably 20° C.-150° C.; more preferably 60° C.-140° C. The expression substrate for the purposes of this text refers to the surface being coated, and the process temperature referred to is the substrate temperature. Inert carrier gases include nitrogen, argon, carbon dioxide and dry air. The process may be carried out at pressures up to atmospheric pressure, but reduced pressure levels are advantageous. Preferably, the process pressure is in the range 10-7000 Pa, more preferably in the range 25-3000 Pa. In a preferred method according to the invention, the gaseous precursors and purge gases flow through the same conduit that carries water during the final use of the object being coated.
FIGS. 5 to 7 show examples of surfaces partly coated according to the invention,
The surface, which is to be coated according to the invention, should be clean from organic contaminants like grease, as well as from inorganic dust and particulate matter. Cleaning methods known to those skilled in the art may be used, involving e.g. surfactants, acid or basic solutions, or ultrasonic cleaning.
To grow films by means of the ALD technique, objects the surfaces of which shall act as substrate are placed in a reaction chamber, in which process conditions, including temperature and pressure, are adjusted to meet the requirements of the process chemistry and the substrate materials. Once the substrate reaches a stable temperature and pressure, a first precursor vapor is directed over the substrates. Some of this vapor chemisorbs on the surface, resulting in a one monolayer thick film. In true ALD, the vapor will not attach to itself and this process is therefore self-limiting. A purge gas is introduced to remove any excess of the first vapor and any volatile reaction products. Subsequently, a second precursor vapor is introduced which reacts with the monolayer of the first chemisorbed vapor. Finally the purge gas is introduced again to remove any excess of the second vapor as well as any volatile reaction products. This completes one cycle. This procedure is repeated until the desired film thickness is achieved. A key to true ALD growth is to have the correct precursor vapors alternately pulsed into the reaction chamber. Another prerequisite in the ALD process is that each starting material is available in sufficient concentration for thin film formation over the whole substrate surface area and no extensive precursor decomposition takes place. The flow velocities and precursor concentrations may be optimized for optimal production economy and efficiency. In a process according to the invention, strict adherence to ALD principles may not be necessary. Thus, in a cost-efficient process according to the invention, the purge stages need not be perfect, but a degree of overlap of the precursor pulses (up to 10% of the total material amount) may be allowed, as the bulk (about 90%) of the film nevertheless grows according to ALD principles, and a sufficient degree of conformity and a sufficient lack of defects and pinholes is achieved. Metal leaching is reduced considerably by using a method according to this invention even if coating process does not strictly adhere to the ALD principle, or purge stages are not perfect.
FIGS. 5 to 7 show examples of cases where the film coating does not completely cover the surface.
The object selected for coating may be placed in the reaction chamber of a deposition device, or in the alternative the interior of the fitting, which is to be coated, functions as a reaction chamber, whereby the substrate is only the inner surfaces of the fitting. In the latter case, couplings for generating a diminished pressure and for conducting the required reagents into the object are connected to the ends of the fitting, and the coating sequence is carried out inside the fitting affecting the same surfaces that will contact water when the fitting has been installed for use. The substrate temperature may be controlled e.g. by placing the object in an oven.
Below, several possible precursors are listed for the deposition of titanium oxide in an ALD process.
Additionally, several organometallic titanium compounds exist which are suitable as precursors.
Preferably, the titanium and the oxygen originate from separate precursors.
As a titanium source, TiCl4 is the preferred choice, because of its low cost and availability from several vendors.
Useful precursors for oxygen include water, oxygen, ozone and alcohols. A particularly preferred combination is TiCl4 and water at a substrate temperature below 150° C. This yields a robust, amorphous layer of good quality. A Cl content of >0.1 percent by weight may provide enhanced protective properties and amorphousness. Examples of useful silicon and aluminium precursors for silicon oxide or for mixtures of silicon oxide and aluminium oxide are tris(tert-butoxy)silanol, tris(tert-pentoxy)silanol, tetrabutoxysilane, tetraethoxysilane, aluminium chloride and trimethylaluminium.
Examples of suitable devices for carrying out the invention are those commercially available from Planar Systems, Inc., e.g. the P400A ALD reactor.
As mentioned above, other possible processes for carrying out the invention include sol-gel processes. These involve subjecting a precursor compound to a series of hydrolysis and polymerisation reactions to form a colloidal suspension or sol. The sol may be deposited on a substrate, and by heat treatment a dense film is formed. Deposition of the sol may be effected by dipping, spraying or spinning.
The benefit of U.S. Provisional Application No. 60/739,931, filed Nov. 28, 2005 is claimed under 35 U.S.C. §119(e).
| Number | Date | Country | |
|---|---|---|---|
| 60739931 | Nov 2005 | US |
