ELAPSED TIME INDICATOR

Abstract: An elapsed time indicator which provides a visual indication after a fixed period of time has elapsed, the visual indication being brought about, in use, by dissolution of a soluble glass part which is opaque and initially obscures a non-soluble coloured portion of the indicator in aqueous based solvent, such as water, which is present at the beginning of the period.

Description: Elapsed Time Indicator This invention relates to elapsed time indicators which provide a visual indication that a fixed period of time has elapsed, and in particular to elapsed time indicators which contain soluble glass. Known types of elapsed time indicators are operated by a large variety of mechanisms, including mechanisms which are triggered by a manufacturing or packing step and are used to indicate when a product has become too old to fulfill its intended purpose. In one particular example a dye which undergoes a chemical reaction and therefore changes colour has been used to indicate the useful life of an insecticide. In a second example, as described in EP 0227167. a product in a packaging is determined to be time expired when evaporation of an opaque liquid from the package causes a coloured part of the package to become visible. All of these known elapsed time indicators suffer from the drawback that they do not conveniently start to time in response to first use of an article. This is a considerable drawback when the time period to be monitored relates to the effective life of a product in use, for example a plant stick for providing nutrients to pot plants or a filter for a domestic water jug. The present invention is concerned with providing a solution to the above-mentioned drawback. In accordance with the present invention there is provided an elapsed time indicator which provides a visual indication after a fixed period of time has passed, the visual indication being brought about, in use, by dissolution of a soluble glass part which is opaque and initially obscures a non-soluble coloured portion of the indicator, in an aqueous based solvent which is present at the beginning of the period. Preferably, the elapsed time indicator comprises a shaped piece of soluble glass which is placed over a backing plate, wherein the piece of glass and the backing plate are of contrasting colours. In this preferred example the glass is soluble and as it is dissolved into the aqueous based solvent the contrasting colour of the backing plate becomes visible to the human eye. In one embodiment of the present invention the backing plate is made from a plastic material and the glass is shaped in the form of a disc. Preferably, the elapsed time indicator is for use with an aqueous based solvent which is water. However, the elapsed time indicator may be adapted for use with all kinds of aqueous based solvents. The soluble glass for use with the present invention may comprise anyone of the known soluble glass systems, silicates, borates, phosphates or organic glass. However, preferably, the soluble glass is a phosphate glass having a phosphorous pentoxide (P-CL.) concentration in the region of 40-48^ mol. With the addition of alkali metal ions and modifier ions such as calcium, magnesium and zinc the glass solubility can be varied. In this way the solubility of the shaped piece of glass can be controlled, and with the ability to vary the thickness of the glass the elapsed time for exposure of the backing plate can be adjusted. It should also be noted that the soluble glass can be coloured by the addition of colouring agents such as cobalt, or colour can be generated by gamma irradiation which could be used to sterilise the device. Devices in accordance with the present invention have several uses, for example, they can be used to indicate the exhaustion of the working life of a filter in a domestic water jug. The actual selection of the glass composition for use in the present invention is made on a number of criteria, including: (a) that the properties of the glass are such as to make forming the glass piece acceptable for the production process to give high product yields. (b) that the solubility of the glass should be substantially independent of the hardness of the water in contact with the glass. (c) that by the combination of glass piece thickness and glass solubility the correct dissolution profile can be achieved to expose the backing plate in the target time. The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: fig 1 is a cross section through a water jug filter exhaustion indicator before use; and fig 2 is a cross section through a water jug filter exhaustion indicator when it is indicating that the fixed time has elapsed. There is shown in Figures 1 and 2 of the accompanying drawings an elapsed time indicator in accordance with the present invention. The elapsed time indicator comprises cylinder 40 having at its base a brightly coloured plastic disc 41. The disc is covered with a layer of opaque soluble glass 42, so that, when water is poured into the cylinder and overflows through outlets 43. a layer of water 44 forms in contact with the soluble glass and commences the dissolution process. After a fixed time the glass has been removed to reveal the disc 4l as shown in figure 5. This type of indicator is particularly useful for indicating in devices that are frequently topped up with water, such as water jug filters. The soluble glass layer of the elapsed time indicator may comprise any one of the following compositions: Table 1 - Glass Composition in Mol % 8 10 11 12 Phosphorous Pentoxide P-0_ CaO MgO ZnO Na20 13 14 15 16 17 18 19 20 21 22 23 To illustrate that the solubility of the glass is substantially independent of the hardness of water in content therewith two glasses (11 & 13) were formed in discs and dissolved in deionised water, soft tap water and hard tap water. The results are shown below: It can be seen that the rate of dissolution of glass 11 in particular is substantially independent of water hardness. The results given below show that good reproducibility from disc to disc can be achieved and that the dissolution rate can be reduced to close to that required for a 30 day glass of about 12 mg/day; assuming a disc having a thickness of 1-2 mm and a weight of 360 mg of soluble glass. Composition 9 11.7. 11-7. 11-9. 11-5. 11-9. 12.1, 11.9. 12.2, 11.3, 11.0 Ave = II.72 mg/day over 12 days 4 6.1, 6.2, 5-8, 5-4, 5-5. 6.0, 5-2, 5.3. 6.7. 6.0 Ave = 5-82 over 13 days

Claims: Claims 1. An elapsed time indicator which provides a visual indication after a fixed period of time has elapsed, the visual indication being brought about, in use, by dissolution of a soluble glass part which is opaque and initially obscures a non-soluble coloured portion of the indicator in aqueous based solvent which is present at the beginning of the period. 2. An elapsed time indicator as claimed in claim 1 which comprises a shaped piece of soluble glass which is placed over a backing plate, wherein the piece of glass and the backing plate are of contrasting colours. 3. An elapsed time indicator as claimed in claim 2, in which the backing plate is made from a plastics material and the glass is shaped in the form of a disc. 4. An elapsed time indicator as claimed in any one of claims 1 to 3. in which the elapsed time indicator is for use with an aqueous based solvent which is water. 5. An elapsed time indicator as claimed in any one of claims 1 to 4 in which the soluble glass is a phosphate glass having a phosphorous pentoxide concentration in the region of 40-48J. mol.

Inventors: DODD STEPHEN FRANCIS, MELLING RICHARD, DODD, STEPHEN, FRANCIS, MELLING, RICHARD
Applicants: PILKINGTON CONTROLLED RELEASE, PILKINGTON CONTROLLED RELEASE SYSTEMS LIMITED
Application year: 1992
Main classification: G07C1
IPC codes: G04F1, G04F13, G07C1