The present invention relates to the control of micro- organism growth in fluids during storage.

Fluids are frequently infected by micro-organisms and where the fluid is stored or used over a relatively long period of time, the presence of the micro- organisms may become significant. For example the micro-organisms can cause the quality of the fluid to deteriorate and may alter the composition of the fluid by degradation of a fluid component.

Additionally microbially infected fluids can be unpleasant to work, generate bad odours and may constitute a hazard to health especially with regard to skin irritations and respiratory problems, such as dermatitis, rhinitis and asthma. In addition contaminated fluids can cause increased costs due to the need for careful disposal of the contaminated fluid and also the increased need to replace contaminated fluid

For example, metal-working fluids (M Fs) suffer from frequent microbial contamination. MWFs are

essentially oil/detergent compositions which are emulsified with water and used in cooling, lubricating and cleaning machine tools and the workpiece material, and removal of swarf and debris. Microbial contamination of MWFs is a serious problem as discussed in Laboratory Practice, Vol 41, No 1, pages 19-23; and Metalworking Production, April 1992, pages 38 onwards.

Typically, the MWF is recycled back to the workpiece and so is in use for long periods of time. The conditions in the metal working machine favour bacterial proli eration and if left unchecked, the bacteria in the MWF produce an extremely unpleasant odour. Further the activity at the cutting surface causes an aerosol of the contaminated MWF which could be inhaled by a machine operator in the vicinity of the machine. This is unpleasant for the operator and may cause infection and have long term side effects.

In an attempt to control the microbial contamination, chlorine has been added to MWFs as a biocide. However chlorine is corrosive and can degrade the operating machinery. Chlorine is therefore an undesirable additive.

Alternatively borate has been included as an anti- microbial additive, but again there are disadvantages to borate addition.

Antibiotics may also be included in MWFs, but these are expensive and are not effective against resistant strains of bacteria. Moreover the continued addition of antibiotics can induce resistance in the bacterial population infecting the fluid.

An alternative solution put forward by the Ford Motor

Company is to use the known anti-microbial action of silver ions and to silver-plate surfaces of the metal working machine. However silver-plating is expensive and does not enable accurate and controlled delivery of the silver ions over a long period of time. Moreover it is difficult to monitor the degradation of the silver plating and, once the plating has degraded, to replenish the silver without expensive and time- consuming engineering, which is costly in terms of outlays and down-time of the machinery.

Similar problems with microbial contamination have also been encountered for other fluid storage systems, for example oil storage vessels, fountain and pool systems, cooling towers and ventilation systems, windscreen washer bottles, and the like.

The present invention thus provides the use of a controlled release glass (CRG) comprising an anti- microbial agent to combat microbial growth in a fluid which is in a fluid storage system. On contact between the CRG and the fluid, the glass dissolves or dissipates slowly, releasing the anti-microbial agent in a controlled manner.

The term ' fluid storage system' includes assemblies where the fluid is continually or intermittently recycled (for example fountains, machine tools requiring MWFs, etc) as well as fluid holding tanks (for example oil storage tanks, windscreen washer bottles , etc) .

Controlled Release Glasses (CRGs) are inorganic polymers, normally based on phosphates of sodium and calcium, which gave been converted into a glassy form by melting the constituents at about 1000 °C. CRGs may

dissolve in water completely leaving no solid residue.

The rate of dissolution can be selected by adjustment of the composition and the physical form of the device and is constant for as long as any of the material remains. The product can be produced in many physical forms; as a powder or granules, fibre or cloth, tubes, or as cast blocks of various shapes.

Suitable glasses are known, but particular mention may be made of the glasses disclosed in WO-A-90/08470 of Giltech Limited. Typically the glass compositions may comprise any one of the following (or a mixture thereof) :

Na ₂ 0 7-33 mole% K ₂ 0 0-22 mole% CaO 0-21 mole% MgO 0-22 mole% P ₂ 0 ₅ 40-49 mole%

A suitable CRG may contain B ₂ 0 ₃ at up to 5 mole % and/or Cu ^z* at up to 10 mole% .

A suitable CRG may achieve solution rates of from 0.03 to 3.0mgcm ^"2 h ^"1 .

Elements other than sodium and calcium, including most metals as their oxides and a limited number of inorganic anions, can be included in the composition of the glass. Examples include borate, silver, copper, zinc, cerium and bismuth. Combinations of any of these or other suitable agents can be used. Incorporation of silver into the Na ₇ O-Ca0-P ₂ 0 systems offers the possibility of producing a CRG capable of releasing the anti-microbial agent silver in a controlled release

manner.

In one embodiment of the present invention, a device comprising silver containing CRG is used to control bacteria in cutting fluids used as coolants in machine drilling, and especially in cutting fluids that are recycled. The CRG may be positioned at a convenient place in the recycling circuit, for example in a filter, or lining the circuit walls, so that as the fluid flows over the CRG, it dissolves and silver is released into the fluid.

In one embodiment the device is in the form of cast blocks, for example spheres, and may be incorporated into a filter which may optionally be reflllable.

The device can be manufactured in any form (eg rods, tubes, powder, granules). In one embodiment the device is a small tube. The tube dissolves equally from the inside and the outside thereby maintaining an almost constant surface area; the rate of the anti- microbial release is therefore maintained at an almost constant level throughout the device life.

The device may be incorporated into the MWF circulatory system at any point (eg the sump, the supply line, the return line). Tests will determine if the current composition performs best in high or low rate positions, suspended or dripped into the sump or placed in a flow line.

As an alternative to silver, other anti-microbial agents including borate, copper, zinc, cerium, bismuth and combinations thereof may be present.

Phosphate glasses have the added advantage that

phosphate is an environmentally safer extreme pressure (EP) additive than most such additives currently in use. Borate glasses have an advantage in that boron is used in existing MWFs as an anti-microbial.

In tests there has been no effect on the performance of the MWF due to the presence of the CRG ie. no increase in foaming, no decrease in ability to cool and lubricate, no decrease in efficiency of cutting tool and no effect on the workpiece.

The present invention will now be further illustrated by reference to the following, non-limiting, example:

Example 1

Summary

The evaluation of the release glass was terminated after 828 hours of testing.

Method

A laboratory mini-sump facility was utilised to provide a severe environment for the glass sample. The sump was filled with 20 litres of KLD549/2 (boron containing biostable product) at 4% which was then maintained at ±1% for the duration of the test. Alternating periods of strong flow then still were used to stimulate actual sump conditions, each state representing approximately 50% of the total test time. During flow periods temperatures of 30-40°C were normal. At the start of each test week the sump was reinfected with used dip- slides showing at least IO ⁵ bacteria and heavy fungi, the slide medium being dissolved into the emulsion to promote growth. Every week a dip-slide was taken and

incubated over the following 48 hours to see if infection was occurring.

The sample was initially weighed on the analytical balance then re-weighed at regular intervals during the test period. Weights were recorded along with test state and final sump temperature.

Results

Period Condition Hours Weight Temp (°C) 1 flow 64 112.2558 25 2 flow 88 111.6493 38 3 flow 112 111.4135 30 4 flow 136 111.2767 28 5 still 160 111.1882 11 6 flow 184 111.0914 24 7 still 254 110.9430 10 8 flow 326 9 still 398 110.7739 10 10 flow 446 110.7260 22 11 flow 492 12 still 588 109.3401 11 13 flow 612 108.9019 37 14 still 708 108.4707 11 15 flow 756 105.6350 38 16 still 828 terminated

The glass sample cracked during period 14 but showed no signs of break-up. During period 15 the cracks were significantly widened due to material loss leading to severe degradation of the sample during period 16. Large portions of an outer layer broke free leaving a core which showed no signs of fracture.

The dip-slide tests showed the sump remained clear of

infection during the full duration of the test.

Emulsion condition was not effected during the full duration of the test. After 700 hrs only 3% of the glass weight had been lost.

Discussion The concept of maintained biostability through slow release of silver ions by a slowly dissolving piece of water soluble glass was demonstrated. The emulsion remained free of bacteria under conditions which normally promote rapid growth and the emulsion condition remained constant. No severe scumming was observed. Slow dissolving of the glass is preferably maintained.