SCALE INHIBITION COMPOSITION

Abstract

A polyphosphate-based glass scale inhibition composition comprising from about 45 to about 55 mole percent P.sub.2O.sub.5, from about 35 to about 45 mole percent of an oxide of an alkaline earth metal, and from about 8 to about 12 mole percent of an oxide of an alkali metal.

Claims

1. A polyphosphate-based glass scale inhibition composition comprising from 45 to 55 mole percent P.sub.2O.sub.5, from 35 to 45 mole percent of an oxide of an alkaline earth metal, and from 8 to 12 mole percent of an oxide of an alkali metal.

2. The polyphosphate-based glass scale inhibition composition of claim 1, wherein the alkaline earth metal is selected from the group consisting of magnesium, calcium or strontium.

3. The polyphosphate-based glass scale inhibition composition of claim 1, where the alkali metal is selected from the group consisting of lithium, sodium or potassium.

4. The polyphosphate-based glass scale inhibition composition of claim 2, wherein the alkaline earth metal is calcium.

5. The polyphosphate-based glass scale inhibition composition of claim 3, wherein the alkali metal is sodium.

6. The polyphosphate-based glass scale inhibition composition of claim 1, wherein the P.sub.2O.sub.5 is present in the range from 48 to 52 mole percent.

7. The polyphosphate-based glass scale inhibition composition of claim 1, wherein the alkaline earth metal oxide is present in the range from 38 to 42 mole percent.

8. The polyphosphate-based glass scale inhibition composition of claim 1, wherein the alkali metal oxide is present in the range from 9 to 11 mole percent.

9. The polyphosphate-based glass scale inhibition composition of claim 1, wherein the P.sub.2O.sub.5 dosing in water at 22 C.3 C. is less than or equal to 2.5 ppm.

10. The polyphosphate-based glass scale inhibition composition of claim 1 having a surface area of at least 900 mm.sup.2.

11. The polyphosphate-based glass scale inhibition composition of claim 1 having a surface area of at least 2000 mm.sup.2.

12. Use of a polyphosphate-based glass scale inhibition composition of claim 1 as a scale inhibitor in a domestic appliance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0034] An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0035] The single FIGURE is a ternary graph showing the compositions of a variety of polyphosphate-based glasses according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The present invention will now be illustrated with reference to the following example.

[0037] Samples of a variety of polyphosphate-based glasses with the general composition of (P2O5)40-65(CaO)15-50(Na2O)5-40, (where subscript adjacent to parenthesis indicates the range of mole percent of the oxides within final glass composition) were produced by standard melt-quench techniques. One such technique is described below, but it will be appreciated that glasses according to the present invention can be made by a variety of techniques and from a variety of starting materials.

[0038] The appropriate raw materials were selected, CaCO3, NaH2PO4 and P2O5, and weighed according to the expected final compositions. Then, the starting materials were placed in a Pt/10% Rh crucible type 71040 (Johnson Matthey, Royston, UK) that was then placed in a furnace pre-heated at 700 C.

[0039] After 30 min at 700 C., the furnace temperature was increased to 1100 C. and maintained for 1 h. The glass was then poured into a graphite mould pre-heated to between 360-430 C. The mould was placed back in the furnace and left at the chosen temperature for 1 h. The furnace was switched off and the glass was left inside to slowly cool to room temperature to remove any residual stress.

[0040] The mould defined a cylindrical shape and the resulting cylindrical glass rods obtained from the mould were cut into discs of 15 mm diameter and 2 mm thickness, using a Testbourne diamond saw. The total surface area of each disc is approximately 450 mm2. The discs received no further polishing or surface treatment and were used as prepared in the subsequent procedures. Glasses prepared according to the present invention can be cast into different shapes and sizes, depending on the mould in use. The surface area of the individual glass units will vary accordingly to the respective mould shape and size.

[0041] Glasses made according to this process were tested to measure their hydrolysis products and their rates of hydrolysis. Ion exchange chromatography was used to detect the hydrolysis products and those glasses which released sufficient quantities of the polyphosphate ions, particularly P3O105-tripolyphosphate (TPP) were made up in larger quantities for further investigation.

[0042] In order to assess the effectiveness of the glasses as scale inhibitors real-time service tests were conducted to determine whether the products of hydrolysis from a particular glass would be able to prevent scale from forming on wetted surfaces where it has previously been found to form in the absence of a scale inhibition composition.

[0043] The service tests were conducted using standard commercially-available ultrasonic humidifiers, where it has been found that over time scale tends to form on the piezoelectric transducer and other wetted surfaces of the humidifiers. It had been found that with hard water and no scale control such ultrasonic humidifiers lost mist output very quickly. This performance measure was determined to be useful because measuring mist output (actually a weight loss of water in a product) is straightforward. Additionally qualitative visual assessments could be made of the surface of piezoelectric transducer and other wetted areas to determine the build-up of scale.

[0044] It was necessary to determine how much of the products of hydrolysis would be required to achieve the scale control necessary to prevent both loss of mist output and to prevent general nuisance build-up of scale on wetted surfaces. It was known that (keeping temperature constant) the concentration of the products of hydrolysis would be dependent upon the surface area of the immersed glass and the throughput of water through the device. In order to achieve effective scale control it was determined that a surface area of at least 900 mm2 was required.

[0045] Control tests conducted using untreated hard water (350 ppm CaCO3) resulted in mist output being lost after between 50-100 litres of water passing through the system. The reason for this is that scale builds up on the surface of the piezoelectric transducer and prevents it from operating to atomize the water.

[0046] As discussed above, the present invention sets out to strike a balance between solubility of the glass and the effective release of scale inhibiting polyphosphate species. A variety of glasses were made and tested as shown in FIG. 1.

[0047] The area within the polygon shown in FIG. 1 represents the glass compositions which fall within the scope of the present invention and the circles represent glasses which were made and tested in the service tests. All of the glasses tested were found to exhibit scale control and to significantly extend the life of the humidifier beyond the 50-100 litres water throughput achieved in the humidifier with hard water and no added scale inhibitor.

[0048] The glass which was found to have the optimal characteristics of solubility and species release was (P2O5)50(CaO)40(Na2O)10. Glasses made to this composition functioned consistently and sufficiently protected against scale formation on all wetted parts, including the piezoelectric transducer up to a throughput of over 1000 litres of hard water (350 ppm CaCO3). In addition to this, the glasses demonstrated properties which would make them commercially attractive. In particular they remained intact under a wider range of ambient storage conditions than commercially-available alternatives, such as Siliphos.

[0049] In addition to the glass compositions shown in FIG. 1 glasses were also tested which substituted the calcium for magnesium or strontium, and the sodium for lithium or potassium. These glasses exhibited good degradation rate whilst maintaining the most efficient release of TPP. It is envisaged that they could provide alternative polyphosphate-based glasses which may be preferred in certain applications.

[0050] The (P2O5)50(CaO)40(K2O)10 glass was as good a threshold inhibitor and was twice as soluble as the favoured (P2O5)50(CaO)40(Na2O)10 composition. However, the glass developed a crust in use. While this crust did not affect the performance of the glass it was deemed to be aesthetically unacceptable for applications in which it will be visible to a user. However, it is envisaged that there may be applications where the glass will not be visible in use and where the increased solubility may offer improved performance.

[0051] A (P2O5)50(SrO)25(Na2O)25 glass was also made according to the method described above. The glass was slightly more soluble than (P2O5)50(CaO)40(Na2O)10 and was an effective threshold inhibitor. It is envisaged that it may be more difficult to get safety approval for the use of glasses containing strontium, for example in domestic appliances, but there may be applications where this is not an issue.