GB2249556A - Water-in-oil emulsions - Google Patents

Abstract

An invert emulsion metal working fluid in which stability is gained by restriction of the particle size. Preferably the particles have a peak size less than one micron, at least 10 % and preferably 20 % of the volume of water being present in particles of less than 0.4 microns in diameter. The emulsion is mixed by introduction of the water phase into the oil phase with agitation and through a high shear screen.

Description

METAL WORKING FLUID This invention relates to metal working fluids and especially to machining fluids.

During mechanical machining fluids are utilized to provide cooling of both the work piece and the cutting or working tool and to provide lubricity in order to optimize tool life and reduce friction and the resulting heat generation.

Commercial metal working fluids have generally been selected from three groups: neat oils which provide high lubrication, oil-in-water emulsions which provide higher cooling but less lubrication than neat oils, and chemical coolants.

In recent years there has been development of a machining technique known as creep feed grinding in which large quantities of material are removed in a single pass of the tool with respect to the workpiece instead of smaller cut depths and several passes being required. This process involves high pressure cutting and generates considerable heat, being an extreme example of an arduous machining process. The demands on the machining fluid are high as both high lubricity and high cooling are required and it is generally found that the usual existing machining fluids do not enable desirable cutting rates and tool life.

The present invention proposes using water-in-oil (or 'invert') emulsions as a machining fluid. Invert emulsions are known as hydraulic fluids and have been proposed for some metal processing applications but they are also well known for exhibiting instability compared with the more usual oil-in-water emulsions and have not previously been successful as machining fluids. In particular they were regarded as unsuitable for demanding machining operations such as creep feed grinding. Successful development of an invert emulsion machining fluid has been achieved by restriction of the emulsion particle size.

Accordingly the present invention provides an invert emulsion metal wroking fluid comprising a continuous oil phase and a water phase dispersed in the oil phase, in which the particle size of the water phase is controlled to a fine distribution.

The invention also provides a method of making an invert emulsion metal working fluid comprising the steps of introducing water phase components into oil phase components and urging the water phase components to pass through a shear screen to establish a fine particle size.

Within the context of the present invention 'fine particle size' means having a significant proportion of the volume of the water contained in particles of less than 1 micron in diameter, as measured with laser techniques.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 illustrates particle size versus volume distribution for a preferred composition according to the invention; Figure 2 illustrates an invert emulsion without a fine particle size, and Figure 3 is a similar particle size/volume distribution for a further emulsion without a fine particle distribution.

The metal working fluids described may be used for a variety of machining operations, including those where material is removed (such as in grinding) and those where material is worked but not necessarily involving material removal (such as pressing) but are particularly useful for those operations requiring both high lubricity and high cooling which in general relate to operations in which large amounts of material are removed (such as in creep feed grinding) or in which cutting speeds are high or the tool is cutting in a restricted area.

The invert emulsion composition of the invention, described in more detail later, preferably generally comprises 50% oil, 40% water, about 5% antifreeze (ethylene glycol), about 2% emulsifier (e.g. sorbitan ester) with the 3% balance comprising various other additives such as biocides and corrosion inhibitors. The viscosity of the final emulsion is controlled by selecting the viscosity of the base oil blend and the emulsion particle size and distribution is controlled by use of a fine, high shear screen during the mixing process and conducting the mixing so that a significant proportion of the volume of the water is distributed in fine, sub-micron particles. The mixing process is described later.

This preferred fine particle composition was tested for performance characteristics and compared with the same charateristics measured for invert emulsions not having fine particle size.

The samples tested were as follows: Sample A - preferred fine particle composition, with particle distribution substantially as shown in Figure 1.

Sample B - invert emulsion with particle distribution as shown in Figure 2.

Sample C - invert emulsion with particle distribution as shown in Figure 3.

To test the performance of the samples under working conditions, each sample was pumped through a gear pump and in a seven minute period the total flow and temperatures reached were measured.

Sample Temperature Flow A 350C 42 litres B 550C 57 litres C 400C 39 litres This demonstrates that sample A, with controlled particle size, did not reach as high a temperature despite similar or lower flow rates. Blends of samples of A with samples B and C also produced higher temperatures than sample A alone.

The pumped samples were retained and used in a subsequent extended stability test along with unworked samples.

Extended temperature stability was measured by leaving samples to stand for seven and twenty-one days at a temperature of 70 degrees Celsius. Each of the samples was monitored for separation.

Extended Stability Table Sample 7 Days 21 Days Free Free Free Free Oil % Water % Oil % Water % A worked 1.5 Nil 2.0 0.5 B worked 4.0 Trace 17.0 28.0 C worked 6.0 0.5 24.0 7.5 A unworked 1.5 Nil 2.0 Trace B unworked 4.0 0.5 11.0 3.0 C unworked 6.0 1.0 30.0 9.0 Sample A showed substantially less separation for both the worked and unworked samples.

It is believed that the superior performance of sample A is due to the fine particle size. It will be observed from Figures 1 to 3 that for sample A the peak in the distribution is below 1 micron, but more significantly that the area under the curve, below 1 micron, is greater than the area under the curve above 1 micron. Also, there are significant percentages of particles, about 20% by volume, having a particle diameter less than 0.4 microns or more generally in the range of 0.1 to 0.4 microns. The volume percent of these small particles will vary with percentage water composition in the emulsion, but it is preferred to have at least 10% of the water volume in particles of less than 0.4 microns diameter.

Particle size was measured using a laser particle size analyser. It was found that using optical microscopy to measure particle size, the sub micron particles were not fully observed. In optical analysis the average particle size for sample A was 2 microns and larger for the other two samples, reflecting the difference in their peak positions but otherwise inaccurate.

Using the preferred composition for machining it is found that swarf removal from the area of the workpiece is more efficient than with other machine fluids. It is believed that this is a consequence of the restricted, fine particle size providing a fluid with non-Newtonion viscosity and greater buoyancy characteristics preventing swarf from redepositing in the immediate work area.

Turning now to more detail of the compositions of invert emulsions according to the invention, these generally comprise the following compounds: A. a continuous oil phase; B. an emulsifier; and C a water phase.

Optionally, the invert emulsions used in the invention may also include: D. anticorrosion and metal passivation agents; E. lubricity additives to augment the antifriction and antiwear properties of the oil phase; and F. other additives, for example of a conventional type for emulsion systems.

Each component type will now be described in turn. The following abbreviations are used in the description: KV - Kinematic viscosity determined in accordance with ASTM D-445.

C A - % of aromatic carbon atoms ; N % of naphthenic carbon atoms Cp - % of paraffinic carbon atoms The CA, CN and Cp values herein are determined in accordance with ASTM D-2140. Percentages in the following description are by weight unless otherwise indicated.

A. The Continuous Oil Phase The oil phase may be based upon mineral oil, vegetable oil, or animal oil, or upon a blend of two or more thereof. The oil phase provides the lubricity of the system and is desirably selected for its contribution to the reduction of friction at the tool face, thermal stability and compatability with the emulsifier, and the final viscosity of the system.

The preferred oils are mineral oils, optionally in the form of blends of mineral oils. We have found those possessing KV values of from 1 to 20 centistokes (1-20 x 10-6 m/s) at 40 degrees Celsius to be particularly suitable, and more desirably those with a KV of 3 to 20 cSt (3-20 x 10 6 m2/s), especially 5 to 15 cSt (5-15 x 10 6 m2/s). Most preferred are oils with a KV at 40 degrees Celsius of from 4 to 9 cSt (4 to 9 x 10 6 m2/s) and especially of from 7-9 cSt, e.g. 8 cSt (7-9 and 8 x 10 6 m2/s).

The viscosity of the oil is not critical to the invention and, for example, a viscosity of greater than 20 cSt (20 x 10 6 m2/s) may be desired, for example, up to 60 cSt (60 x 10-6 m2/s).

We have further found that mineral oils with a C A from 1 to 10, CN from 15 to 50 and Cp from 40 to 80 are very suitable.

More preferably the oils have a CA value of from 2 to 5, a value of from 40 to 50 and a C P value of from 45 to 60.

Most preferred are oils with a CA value of about 1 to 3 (e.g. 2), a CN value of about 44 to 48 (e.g. 46) and a Cp value of about 50 to 54 (e.g. 52). From a technical point of view, naphthenic oils (e.g. with a C N value of 40 or more) are particularly desirable because they are considered to improve emulsion stability, but cost factors may require that an oil with a lower naphthenic content be used. In general, it would appear that CA, CN and Cp values are less important than the KV and aniline point of an oil.

The mineral oil suitably has an aniline point of from 75 to 120 degrees Celsius, more preferably of from 80 to 90 degrees Celsius and most preferably of 83 to 86 degrees Celsius, especially 84 degrees Celsius. The aniline point is a measure of the solvency of the oil and is determined in accordance with ASTM D-611.

One oil sharing all the most preferred characteristics is that sold under the trade mark 'Shell Oil 60 Solvent Pale'.

Another preferred oil is sold under the trade mark GULFPAR 4P. This oil has a C A of 4%, C N of 26% and Cp of 70%; it has a KV of 3.84 cSt (3.84 x 10 6 m2/s) at 40 degrees Celsius an aniline point of 85 degrees Celsius.

The oil preferably forms up to 60% by weight of the emulsion, e.g. 10 to 60%. More preferably it constitutes from 40 to 55%, especially about 50% (e.g. 47 to 53%) of the composition.

The emulsion normally has a viscosity of from 10 to 120 cSt (10 to 120 x 10-6 m/s) at 40 degrees Celsius, more especially from 15 to 60 cSt (15 to 60 x 10-6 m/s) and most preferably from 20 to 50 cSt (20 to 50 x 10 The quantity and viscosity of the oil are appropriately selected to achieve the desired viscosity of the emulsion.

Invert emulsions have been used as hydraulic fluids, but such hydraulic fluids have not had the fine particle size of the machining fluid, and furthermore had a relatively high viscosity (over 70 cSt). For the present invention lower viscosities are preferred, with a kinetic viscosity of from 10 to 70 cSt (10 to 70 x 10 6 m2/s). Preferably, the viscosity of such emulsions is no more than 60 cSt and is most desirably no more than 50 cSt; the preferred minimum viscosity is 15 cSt, more preferably 20 cSt (60, 50, 15 and 20 x 10 6 m2/s, respectively).

B. The Emulsifier An emulsifier is incorporated to maintain the water phase as a homogenous dispersion of fine particles. The action of the emulsifier is to stabilise the water particles as they form, and in principle a wide range of surfactant types and surfactant blends could achieve such stabilisation, and examples of surfactants are given later in the description.

Surfactant systems which are appropriate for specific emulsions and uses may be determined empirically. The one or more surfactants used as the emulsifier system -in the emulsion also aid wetting of the emulsion on the workpiece and tool, and increase the flushing ability of the fluid, for instance, in grinding operations where open-structure (porous) wheels are used.

The surfactants may also contribute to the anticorrosive action of the emulsion, preventing attack of the water phase on ferrous metals.

The emulsifier employed may be selected from one or more of the following types: (I) amphoteric surfactants, for example fatty acid betaine and sultaine derivatives, imidazoline-carboxylates, sulphonated-imidazolines, amphoteric carboxyl and amino-glycinates and propionates, amine oxides and protein surfactants; (II) anionic agents, for example alkylaryl sulphonates, alcohol sulphates, ether sulphates, phosphate esters, sulphosuccinates, sulphosuccinamates, paraffin sulphonates, olefine sulphonates, taurates and isethionates, sarcosinates, fluoroalkyl carboxylates and sulphonates, and salts of fatty acids, for example sodium, potassium, calcium and zinc soaps of lauric and stearic acids; (III) cationic surfactants, for example fatty acid amines, quaternary ammonium chlorides and quaternary imidazoline derivatives; and (IV) non-ionic agents, for example alkoxylates, alkyl phenol ethoxylates and propoxylates, alcohol ethoxylates and propoxylates, amine ethoxylates and propoxylates, ester ethoxylates and propoxylates, castor oil ethoxylate and propoxylate, amide ethoxylates and propoxylates, block copolymers of ethylene oxide and propylene oxide, alkanolamides, esters derived from mono and polyhydric alcohols and fatty acids, fluoroalkyl esters, glucosides and ethoxylated derivatives thereof, lanolin and wool wax derivatives.

We have found that blends of surfactants are preferred in order to obtain the balance of properties desired. An ester of a polyhydric alcohol is one component of a preferred blend, specifically a mono-oleate ester of sorbitol. The ester of the polyhydric alcohol preferably has an acid value of 3 to 10 mgKOH/g and more preferably about 6 to 7 (e.g.

6.5) mgKOH/g, a saponification number of 130 to 180 mgKOH/g, and most preferably of 145 to 155, e.g. 150 mgKOH/g, and a hydroxyl value in the range 180 to 220 and more preferably of about 200 mgKOH/g. The sorbitan monooleate is added at levels between 1 and 5% and most preferably at about 2% (e.g. 1.75 to 2.25%) of the total composition.

A further component of this surfactant blend is an ethoxylated ester of a polyhydric alcohol, principally a trioleate ester of sorbitol ethoxylated to a preferred ratio of 1 mole of ester to 15 to 25 moles, more preferably 20 moles, of ethylene oxide. Preferred are grades with acid values of up to 5 mgKOH/g and most preferably of about 2 mgKOH/g, a saponification value of 70 to 100 mgKOH/g and most preferably about 90 mgKOH/g, and a hydroxyl number ranging from 30 to 70 mgKOH/g with a value of about 45 being most suitable. The ethoxylated ester may be conveniently added at 0.5 to 2% of the formulation but a level of about 1% (e.g. 0.95 to 1.05%) is most preferred.

A sodium sulphonate is also desirably added as part of the emulsifier package. This is an oil soluble sodium salt of an alkylaryl sulphonic acid and may be conveniently carried in a mineral oil for ease of dispersion. Especially useful are blends of sodium sulphonates in mineral oil containing of from 61 to 63% sodium sulphonate.

The weight average molecular weight of the sulphonate is preferably in the range of from 400 to 600, and preferably is about 420. The SO3 content of the sulphonate is preferably in the range of from 15 to 25% with the most preferable SO3 level being about 19% or 20%.

The sodium sulphonate normally constitutes from 0.5 to 2% but is more typically included at about 0.8% (e.g. 0.75 to 0.85%) by weight of the total composition.

C. The Water Phase The water phase of the emulsion is normally present at a level of from 30 to 80% by weight of the emulsion and is preferably formulated such that it provides the desired balance of cooling, viscosity, and stability.

Generally these properties are optimised when the water forms 35 to 45 and especially 40% of the total composition, and this is the preferred level in the present invention.

D. Anticorrosion and Metal Passivation Agents Anticorrosion and metal passivation ingredients may also be conveniently included. These are selected on the basis of their performance with the work metals and tools, their contribution to the stability of the emulsion, the viscosity of the system, and thermal stability characteristics.

Another important feature is the toxicity of the additive since recent and pending legislation does not permit the use of nitrite, borates, phenols, and agents such as nitrilotetra-acetic acid.

Suitable anticorrosives and metal deactivators are thiazole and triazole derivatives, amine and metal sulphonates (that is, salts of alkylaryl sulphonic acid), and alkanolamines.

Particularly preferred are calcium sulphonate for ferrous protection and benzotriazole as a multi-metal corrosion inhibitor/passivator. These are conveniently added at 0.01 to 5% by weight of the formulation but are incorporated preferably at levels of about 0.05% for the benzotriazole and 0.025% for#the sulphonate.

E. Lubricity Additives Lubricity additives designed to augment the anti friction and antiwear properties of the mineral oil component can also be used. Extreme pressure (EP) additives may be incorporated, if arduous metal working operations are anticipated.

Some lubricants, especially those possessing bound chlorine, phosphorus or sulphur, offer high levels of lubricity by formation of a layer of solid. lubricant by reaction of the additive with the metal surface. These solid lubricants remain effective at temperatures up to their melting points and are used as EP additives. By careful selection and blending of these EP additives, it is possible to formulate metal working fluids for a wide working range of materials and operations. Additives may be selected from sulphurized fatty oils, elemental sulphur, chlorinated paraffins, chlorinated oils, and sulpho-chlorinated oils, metal dithiocarbamates and phosphorodithioates Preferred EP additives are zinc diaryl-and dialkyldithio phosphates, however, and especially preferred is a zinc diaryldithiophosphate, containing 3 to 4% zinc, 3% phosphorus and 6 to 7% sulphur.

Other lubricating additives which may also be incorporated are esters, particularly trimethyloylpropane esters of fatty acids.

F. Other Additives It may also be advantageous to add ingredients to preserve the metal working fluid in storage from the effects of frost. Accordingly, anti-freeze compositions may be incorporated.

Typically, glycols may be added and a preferred anti-freeze additive is monoethylene glycol incorporated at levels of up to 10%, normally 1 to 10%, but about 5% is preferred in the present case.

Although cleanliness and good housekeeping in machine shops do much to avoid bacterial and fungal infection of water based machining fluids, chemical sterilization provides a convenient safeguard against emulsion breakdown brought about by the presence of fungi and aerobic and anaerobic bacteria. Since batericides vary in their effectiveness, they must be carefully selected. To be acceptable, a biocide must fulfil several requirements. It must produce a persistent biocidal effect under service conditions in which continuous reinfection may occur, and it must be compatible with the total emulsion, particularly with the emulsifier system. The chosen biocide or biocides must also provide acceptable toxicological hazards.

Additionally, the biocide should not significantly detract from the corrosion protection performance of the metal working fluid nor induce foaming in use.

Biocides that may be employed in this sytem, either solely or in blends, include oxazolidines, triazine derivatives including hexahydrotriazines, isothiazolinones, 0 and N formals, 0 and N acetals, halogenised acid amines, and an omadine, e.g. sodium-2-pyrithinethiol-1-oxide also known as sodium omadine or sodium pyrithione.

Preferred however, is a blend of 1, 3, 5 triazine - 2, 3, 5 (2H, 4H, 6H) - triethanol and an omadine providing broad spectrum biocidal activity. This is usually included at levels from 0.05 to 0.5% but a preferred dosage is 0.2%.

Defoamers may also be conveniently added to improve the flushing ability of the fluid and prevent the formation of froth, which inhibits the removal of swarf and tramp oil from the system. Sequestrants are also possible ingredients where water hardness salts are present at high levels, to prevent their interaction with the emulsifier system and subsequent instability of the emulsion.

G. Preparation of the Invert Emulsion The major determining factors in preparing a water-in-oil emulsion are the agitation rate in the agitator, the oil water feed ratio in the agitator and the type of agitator.

An emulsifying screen is used to obtain a fine particle size emulsion.

The invert emulsion is preferably made by mixing preblends of (1) the emulsifier and other amphiphilic or oleophilic (but non-oil) components, (2) the water and components soluble therein and, if a blend of oils is being used, (3) the oils. These blends are then agitated at high speed to form an invert emulsion.

The detailed preparation of invert emulsions according to the invention is illustrated by the following examples.

Emulsions were prepared from formulations 1, 2 and 3 whose compositions are set out in Table 1 below. The emulsions were evaluated in various metal working operations.

Preparation of the Emulsions The preparation of the emulsions was as follows: pre-blend A is made by mixing the ingredients with gentle agitation at 40 to 50 degrees Celsius until homogenous.

Preblend A is added to the mineral oil component B with agitation at 20 to 25 degrees Celsius.

Preblend C is prepared at 20 to 25 degrees Celsius and then added to the mixture of A and B at 20 to 30 degrees Celsius using a high speed agitator (Silverson).

The mixing of pre-blend C, which is essentially the water phase components, into the oil phase previously mixed from pre-blends A and B is achieved by introducing the water phase to a high speed mixing head (impeller) immersed in the oil phase. The rate of addition of the water phase is preferably 16 litres per minute, with an impeller speed of 970 revolutions per minute. The impeller is surrounded by a screen. The screen is not the usual screen used for making emulsions, which would be a round apertured screen, but is a square holed shear screen, with aperture dimension in the range of 1 to 3mm, most preferably 2mm.

The resultant fluids are opaque white emulsions.

Characteristics of the Emulsions The characteristics of the emulsions are shown in Table 2 below. Corrosion resistance is assessed in accordance with the procedures of IP135. IP135 tests a neat fluid, not an emulsion, but obviously our tests involve an emulsion.

Emulsion stability is assessed by a modified version of IP 290/84. IP 290/84 requires storage of liquid for 1000 hours at ambient temperature but in the modified version the test is accelerated by storing the emulsions for 48 hours at 70 degrees Celsius. The separation values in Table 2 refer to the amount of oil and water breaking free from the emulsion after the test period.

Table 1 In Table 1, the content of each component is expressed as the weight percentage based on the total weight of the formulation.

Formulation 1 2 3 Preblend A Sorbitan ester 1.88 1.88 1.88 Ethoxylated sorbitan ester 1.00 1.00 0.99 Sodium sulphonate (a) 0.80 0.80 0.79 Calcium sulphonate (b) 0.26 0.26 0.26 Zinc diaryldithiophosphate 1.00 1.00 1.08 Preblend B Mineral seal oil (c) 40.00 14.75 Mineral oil 60 SP 49.75 6.73 35.00 Preblend C Water 40.36 43.07 40.30 Ethylene glycol 4.70 5.01 4.70 Benzotriazole 0.05 0.05 0.05 Biocide 0.20 0.20 0.20 (a) The sodium salt of an alkylaryl sulphonic acid.

(b) Included as a corrosion preventative and based upon alkylaryl acids. The salt has a free alkalinity of 45 mg KOH/g and an SO content of 6.0 w/w.

(c) Sold as GULFPAR 4p.

Table 2 Characteristics of the Emulsions Formulation 1 2 3 Appearance White White White Emulsion Emulsion Emulsion S.G at 15.5 degrees Celsius 0.93 0.93 0.93 Kinematic Viscosity at 40 degrees Celsius/cSt 45 22.8 32.7 6 Corrosion Protection No Rust No Rust No Rust Emulsion Stability oil sep oil sep oil sep 3 ml 3 ml 3 ml water sep water sep water sep 1 ml 1 ml 1 ml Application of the Emulsions An emulsion made to formulation 1 was prepared and used with a creep feed grinding operation involving alumina wheels on a variety of nimonic alloys. Compared to a conventional soluble oil, surface finish was improved (examined by NDT) and cracking eliminated. Improvements were also recorded in reduced foaming, smoking, and wheel 'glazing'. Moreover the system was run successfully on shifts for seven days with no fluid maintenance.

The above-described invert emulsion has therefore been shown to provide advantageous properties as a metal working fluid for creep feed grinding and can be expected to be a useful working fluid also for other metal working operations where a fluid with a balance of lubricity and coolant properties is required. Further, invert emulsions have been shown in experiment to be effective as a metal working fluid in pressing processes.

Claims (9)

1. An invert emulsion metal working fluid comprising a continuous oil phase and a water phase dispersed in the oil phase, in which the particle size of the water phase is controlled to a fine distribution.

2. An invert emulsion metal working fluid according to claim 1 in which the particle size is controlled so that at least 10% of the volume of the water is in particles of less than 0.4 microns diameter.

3. An invert emulsion metal working fluid according to claim 2 in which at least 20% of the volume of the water is in particles of less than 0.4 microns in diameter.

3. An invert emulsion metal working fluid according to any preceding claim in which the peak average diameter of the particles is less that 1 micron.

4. A method of making an invert emulsion metal working fluid comprising the steps of introducing a water phase component into an oil phase component and urging the water phase component to pass through a shear screen to establish a fine particle size.

5. A method according to claim 4 in which the shear screen has square apertures.

6. A method according to claim 4 or claim 5 in which the apertures have a 2mm diameter.

7. An invert emulsion metal working fluid made according to the method of any of claims 4 to 6.

8. An invert emulsion metal working fluid according to any of claims 1 to 3 or claim 7 in which the kinetic viscosity lies in the range of 10 to 70 cSt at 40 degrees Celsius.

9. An invert emulsion metal working fluid according to any of claims 1 to 3 or claim 7 or claim 8 including at least one of a mono-oleate ester of sorbitol, an ethoxylated trioleate ester of sorbitol and an oil soluble sodium salt of an alkylaryl sulphonic acid as an emulsifier.