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Synthetic cutting fluids
Synthetic cutting fluids A brief history and state of the art J. Edwards and E. Jones*
There have been many attempts, including several elsewhere in this publication, to define synthetic cutting fluids in clearcut terms. But, as with so many terms in industrial processing, there are grey areas. Much of the confusion exists because of a general tendency in the engineering industry to include all non-petroleum based fluids under this classification, including the long established chemical grinding solutions.
While the service to industry of soluble cutting oils has been, and continues to be, substantial, the problems of bacterial attack have never been totally overcome. This attack not only can produce the well-known unpleasant odours present in some machine shops on a Monday morning, but, ironically, can lead to splitting of the emulsion and lowering of the pH value, which in the end brings us right back to the old problem - corrosion.
Our company has, both in the UK and through its foreign associates, been heavily committed to fluids development within the metalworking field. We believe there is a clear distinction between the modern complex non-petroleum fluids, designed primarily for general metalworking applications, and the long-established chemical grinding fluids, whose primary function is that of rust control, allied to the greater cooling properties of water to aid grinding operations. It is the former category which we generally refer to as 'synthetics' and it is within this category that the majority of current development is taking place.
Incorporation of biocides and the instigation of rigorous maintenance schedules can certainly allay the potential problems of bacterial attack on soluble oils, but the hankering doubts present in many users' minds necessitated the development of a different approach during the 1950s. Before examining this approach in some detail, a parallel development to that of soluble oil emulsions had been taking place, and it was the state of the art in this field which really gave the lead into synthetics. This development is worth dwelling on.
History of synthetics In theory, water should be a good metalworking fluid. The vast majority of energy dissipated in the metalcutting operation presents itself as heat. Consequently cooling is of the essence. At the end of the nineteenth century water applied as a stream at the cutting tool was in fairly regular use. However, straight water gives two very obvious and serious problems - corrosion and lack of lubrication for both the machining operation and the machine itself. The first attempts to overcome this involved mixtures of fatty oils with water, loosely coupled by alkali. While working in a fashion, rusting was not really controlled satisfactorily, and the inherent instability of the mixture caused great problems.
In grinding operations the need for cooling is paramount, while the other requirements of a general metalcutting product, such as lubricity etc are obviously not so relevant. The corrosion problems of straight water had tended to be overcome, particularly in surface grinding operations, by the addition of soda ash. However, over a period of time the soda ash tended to deposit out on machine working surfaces, leading to jamming. Consequently, in the early 1950s alternatives to soda ash solutions were developed, mostly based on alkali nitrites and organic amine mixtures in aqueous concentrates. These fluids can be occasionally subject to bacterial attack, particularly by nitro-bacteria, and, due to reaction with
During the Great War true soluble oils, as we know them now, began to be used in industry. These are basically mineral oils or mineral fatty oil mixtures emulsified with soaps or sulphonates. They often incorporate additional additives such as solvents or phenolic materials for stabilisation, and corrosion inhibitors. As the sophistication of these products grew, variants were developed, including the fine particle translucent emulsions commonly used in some grinding applications. Neat cutting oils were also in widespread use then, and in fact the extreme pressure additives incorporated into these products led to the development of ep soluble oils. This development enabled aqueous cutting media to start taking over some of the more demanding metalcutting applications from their neat counterparts. * Edgar Vaughan and Co. Ltd, Legge Street, Birmingham B4 7EV, Uh
Water miscible chemical coolant used in a heavy planing and chamfering operation
TRIBOLOGY international February 1977
29
metallic fines, this can give rise to ammoniacal odours, but further developments have overcome the initial problems of crystalline build-ups on machines. Although there are now modern alternatives to these fluids, they have been in general use in industry in large volumes for the past twenty-five years. While these fluids were undoubtedly the first chemically conceived coolants, they are better described as non-oily grinding media or even chemical coolants, rather than as the synthetic cutting fluids which followed. These really required many more complex characteristics than the chemical solutions could provide. Regardless of the pedantics involved in defining what is or is not a synthetic, there is no doubt that the desire to overcome the problems being encountered with soluble oils, together with the increasing comparatively troublefree use of chemical grinding coolants, led naturally to the concept of the modern synthetic cutting fluid. We believe that Hocut 237t was the first of these synthetics. It was based on the idea that, if we could formulate without mineral oil, fatty oil and the usual emulsifier systems, we would have a coolant which, when diluted in application, could resist destruction by the type of bacterial attack normally encountered in machine shops. The idea was proven in practice, and Hocut 237, despite the development of many successive products, continues as probably the biggest selling synthetic cutting fluid world-wide. Much research and development work is currently geared to synthetic cutting fluids. The truly universal product, despite claims to the contrary, has never really arrived and with the ever-increasing permutations of application, tooling, material, cost, water and environment, we are somewhat sceptical of its feasibility. Nevertheless, there is now a range of synthetics available to the user in the market place which can embrace a variety of applications that would have been inconceivable fifteen years ago. There is no doubt that the shortages and price increases of mineral oil during the past few years have given a tremendous boost to the acceptance of synthetics. In addition, health and safety legislation together with environmental pressures have also given new development leads, the latter making products which can often be discharged to sewers without chemical treatment particularly relevant. However, the danger from this market pressure was, if anything, the tendency to look on synthetics as a panacea. They cannot, in general, be used as straight replacements for neat oils without first taking stock of maintenance procedures, machine conditions, etc, using specialist advice. Used on the right applications, in the right manner, synthetics can show real advantages to the user. A large number of the horror stories abounding in industry regarding the use of water-based products result from incorrect usage. If machines which have been run on neat oils for years are suddenly switched to synthetics without the proper investigations and procedural examinations, a whole string of problems can arise such as machine corrosion and seizing, seal failure, attacking of paintwork, foaming of the fluid and operator skin problems. But such has been the spur of neat otl prices, availability, cancer scares,
t Developedby E. F. Houghton and Co, Philadelphia, USA
30
TRIBOLOGY international February 1977
misting, fire risks and disposal problems that switches such as this have indeed taken place. These sorts of problems have, in their turn, frightened many potential users away from synthetics, when in fact real benefits could have been achieved. Now, with the early euphoria reasonably tempered, realistic appraisals of the potential for synthetics are far more common. We certainly believe that the gradual trend for synthetics to increase their share of the metalcutting fluid market will continue, although there are undoubtedly many areas of application where neat oils are unlikely to be successfully replaced in the foreseeable future. With this history of the development of synthetics behind us, it is a good time for a rational appraisal of the pros and cons of these fluids in line with the state of the art at this time.
Advantages of synthetics Coolant life: In general the user can depend upon a longer fluid life with synthetics, particularly compared with conventional solubles. This not only manifests itself in the obvious economy advantage, but also in minimising machine downtime for coolant changeover and reducing disposal problems. Easing of odour problems: again, in comparison with soluble otis. However, confusion often arises because of the occasional tendency of synthetics to pick up tramp oil from lubrication systems, eventually yielding odour problems from bacteria. Regular skimming of the coolant will generally maintain the advantage. Working conditions: There should be improvements in visibility of work, cleanliness of components, machines and work area, and removal of 'oily' smells. Safety: Operating conditions should be made safer by: removal of misting often prevalent in large machine shops running on neat cutting oils; removal of possible fire risks with neat oils; and reduction of operator contact with mineral oil, which has a stigma related to cancer associations not really so relevant nowadays, since most suppliers are utilising solvent-refined mineral oils for cutting oils. Environmental problems: The latest types of synthetic fluids, after allowing any solids to settle out and skimming off tramp oil, are generally more acceptable to sewage authorities than conventional products. Performance improvements: Depending on application and fluid, quite dramatic improvements in machining performance and tool life have been achieved with synthetics. Coolant drag-out: the nature of the fluid means that less of it is lost in use, not only yielding obvious economy, but also indirectly leading to cleaner swarf.
The disadvantages Machine lubrication: Since synthetics, by definition, do not contain mineral oil, they cannot provide the residual lubrication for which conventional cutting oils are sometimes relied. However, relying on a coating of oil from the machining fluid often covers up inadequate machine lubrication procedures. Although it might be said that this characteristic is a bonus from the use of mineral oil products, a review of the machine manufacturer's recommen-
dations will generally suggest routine lubrication procedures that will give more than adequate cover when synthetics are used. The introduction of semi-synthetics was partly aimed at this problem but, as with so many attempts to give 'the best of both worlds', these products are certainly not the ultimate answer. Whilst they do have merit in some applications, many of the problems with conventional soluble oils are reintroduced.
Removal o[ slideway lubricants: Many synthetics, particularly some of the early types, do have a tendency to emulsify or purge lubricating oil from slideways, etc because of their general 'wetting-out' characteristics. However, the more recent developments do not emulsify mineral oil, and the problems have been eased. In addition, improvement can also be achieved by the use of the more adhesive slideway lubricants, modifying coolant flow direction, and institution of the regular lubrication procedures referred to previously.
Component protection: Products containing mineral oil obviously tend to leave a film of oil on the component after machining, which can provide useful inter-operational protection. The newer synthetics do, however, incorporate good rust preventive qualities. On the other hand, there is an increasing trend towards the use of dewatering rust preventives for inter-operational corrosion protection in the metalworking industry, which obviously eases the requirements anyway.
Operator skin problems: Synthetics inherently incorporate a degree of detergency, and consequently can tend to remove the natural oils from the skin, in the same way that washing-up liquids do. This can, of course, lead to skin problems of varying degrees. However, insistence on the use of the proper barrier creams will generally remove this problem.
Paint and seal attack: It is certainly true that standard machine tool paint and seal materials can be susceptible to attack by modern synthetic fluids. However, this problem is of decreasing relevance, since many machine tool manufacturers are now constructing their machines with a view to running with synthetics, and hence are utilising paints and seal materials which are immune to attack. This does not apply to all manufacturers. However, the changing of seals and repainting of machines with the fight materials is not a major job, if it is found to be necessary, and can be a small price to pay for benefits elsewhere.
Synthetics in application From the pros and cons listed above, it can be seen that the
decision whether or not to use synthetic cutting fluids can involve a complex equation. The user is not dealing with a straight forward commodity or specification material, as with some oil purchases, but with complex chemical formulations. Consequently a far greater onus falls on the supplier to provide specialist advice to the user, and so, in his turn, the user must assure himself that his supplier has the resources to provide this. The user should certainly beware of a supplier who has a product line heavily committed to either straight oils or synthetic products alone, since it will obviously be difficult to obtain a totally unprejudiced opinion.
A specialist supplier with a product line containing a metalcutting range from straight mineral otis through solubles to synthetics should have few axes to grind, and should be able to aid the user in evaluating the only true measure of effectiveness - cost in application. The cost per litre of the cutting fluid is only one small part of the equation, although even the most expensive synthetic will generally show a cost saving over its neat oil counterpart when in its diluted form. The total cost in application must embrace machining times, component quality, tool life, sump life, disposal costs, top-up rate, maintenance cost and health, safety and environmental considerations. While neat oils are certainly easier to run from the maintenance standpoint we, as suppliers with no forced prejudice in either area, believe that there is a fundamental shift occuring in this equation in favour of synthetics. Whilst this shift may be slower, and hence far longer-term, than some have predicted, and is certainly unlikely to be 100% in the foreseeable future, much product development is committed to this shift in the equation.
Case histories Massey Ferguson Limited, Manchester, were carrying out a variety of machining operations with a neat cutting oil, but were concerned about fuming and fire risks. A modern synthetic (Evcol 70 at 40:1 ) was recommended. Massey Ferguson's Industrial Engineering Department lists among other benefits received from this changeover: reduction of atmospheric poilu-
tion, significant improvement in operating conditions, marked improvement in tool life and significant reduction in total cutting fluid bill. A number of potential machine problems were spotted, but these have generally been overcome by the tightening of maintenance schedule. Perkins Engines Limited, Peterborough, used to use straight paraffin for honing cast iron cylinder bores. The potential hazards were obvious and, having first used an imported high cost synthetic fluid, they now carry out the same operation with a UK produced synthetic (Evcol V). This performs well on a somewhat difficult operation at a substantially reduced cost. Girling Limited, Bromborough, had purchased a Hoffman Vertical Broaching Machine for machining cast iron brake caliper bodies. This is a fairly demanding operation with quite complex coolant requirements. The neat oil used was found very expensive in application, due to excessive drag-out. The decision was made to investigate a number of synthetics within the following requirements:Freedom from build-up of cast iron 'cake'. Good tool life. Freedom from corrosion. Good stability and sump life. Freedom from bacterial degradation. Freedom from foaming, Girling is now using Evcol V with excellent results, in an application generally considered to be the forte of mineral oil products.
TRIBOLOGY international February 1977
31
There have been many attempts, including several elsewhere in this publication, to define synthetic cutting fluids in clearcut terms. But, as with so many terms in industrial processing, there are grey areas. Much of the confusion exists because of a general tendency in the engineering industry to include all non-petroleum based fluids under this classification, including the long established chemical grinding solutions.
While the service to industry of soluble cutting oils has been, and continues to be, substantial, the problems of bacterial attack have never been totally overcome. This attack not only can produce the well-known unpleasant odours present in some machine shops on a Monday morning, but, ironically, can lead to splitting of the emulsion and lowering of the pH value, which in the end brings us right back to the old problem - corrosion.
Our company has, both in the UK and through its foreign associates, been heavily committed to fluids development within the metalworking field. We believe there is a clear distinction between the modern complex non-petroleum fluids, designed primarily for general metalworking applications, and the long-established chemical grinding fluids, whose primary function is that of rust control, allied to the greater cooling properties of water to aid grinding operations. It is the former category which we generally refer to as 'synthetics' and it is within this category that the majority of current development is taking place.
Incorporation of biocides and the instigation of rigorous maintenance schedules can certainly allay the potential problems of bacterial attack on soluble oils, but the hankering doubts present in many users' minds necessitated the development of a different approach during the 1950s. Before examining this approach in some detail, a parallel development to that of soluble oil emulsions had been taking place, and it was the state of the art in this field which really gave the lead into synthetics. This development is worth dwelling on.
History of synthetics In theory, water should be a good metalworking fluid. The vast majority of energy dissipated in the metalcutting operation presents itself as heat. Consequently cooling is of the essence. At the end of the nineteenth century water applied as a stream at the cutting tool was in fairly regular use. However, straight water gives two very obvious and serious problems - corrosion and lack of lubrication for both the machining operation and the machine itself. The first attempts to overcome this involved mixtures of fatty oils with water, loosely coupled by alkali. While working in a fashion, rusting was not really controlled satisfactorily, and the inherent instability of the mixture caused great problems.
In grinding operations the need for cooling is paramount, while the other requirements of a general metalcutting product, such as lubricity etc are obviously not so relevant. The corrosion problems of straight water had tended to be overcome, particularly in surface grinding operations, by the addition of soda ash. However, over a period of time the soda ash tended to deposit out on machine working surfaces, leading to jamming. Consequently, in the early 1950s alternatives to soda ash solutions were developed, mostly based on alkali nitrites and organic amine mixtures in aqueous concentrates. These fluids can be occasionally subject to bacterial attack, particularly by nitro-bacteria, and, due to reaction with
During the Great War true soluble oils, as we know them now, began to be used in industry. These are basically mineral oils or mineral fatty oil mixtures emulsified with soaps or sulphonates. They often incorporate additional additives such as solvents or phenolic materials for stabilisation, and corrosion inhibitors. As the sophistication of these products grew, variants were developed, including the fine particle translucent emulsions commonly used in some grinding applications. Neat cutting oils were also in widespread use then, and in fact the extreme pressure additives incorporated into these products led to the development of ep soluble oils. This development enabled aqueous cutting media to start taking over some of the more demanding metalcutting applications from their neat counterparts. * Edgar Vaughan and Co. Ltd, Legge Street, Birmingham B4 7EV, Uh
Water miscible chemical coolant used in a heavy planing and chamfering operation
TRIBOLOGY international February 1977
29
metallic fines, this can give rise to ammoniacal odours, but further developments have overcome the initial problems of crystalline build-ups on machines. Although there are now modern alternatives to these fluids, they have been in general use in industry in large volumes for the past twenty-five years. While these fluids were undoubtedly the first chemically conceived coolants, they are better described as non-oily grinding media or even chemical coolants, rather than as the synthetic cutting fluids which followed. These really required many more complex characteristics than the chemical solutions could provide. Regardless of the pedantics involved in defining what is or is not a synthetic, there is no doubt that the desire to overcome the problems being encountered with soluble oils, together with the increasing comparatively troublefree use of chemical grinding coolants, led naturally to the concept of the modern synthetic cutting fluid. We believe that Hocut 237t was the first of these synthetics. It was based on the idea that, if we could formulate without mineral oil, fatty oil and the usual emulsifier systems, we would have a coolant which, when diluted in application, could resist destruction by the type of bacterial attack normally encountered in machine shops. The idea was proven in practice, and Hocut 237, despite the development of many successive products, continues as probably the biggest selling synthetic cutting fluid world-wide. Much research and development work is currently geared to synthetic cutting fluids. The truly universal product, despite claims to the contrary, has never really arrived and with the ever-increasing permutations of application, tooling, material, cost, water and environment, we are somewhat sceptical of its feasibility. Nevertheless, there is now a range of synthetics available to the user in the market place which can embrace a variety of applications that would have been inconceivable fifteen years ago. There is no doubt that the shortages and price increases of mineral oil during the past few years have given a tremendous boost to the acceptance of synthetics. In addition, health and safety legislation together with environmental pressures have also given new development leads, the latter making products which can often be discharged to sewers without chemical treatment particularly relevant. However, the danger from this market pressure was, if anything, the tendency to look on synthetics as a panacea. They cannot, in general, be used as straight replacements for neat oils without first taking stock of maintenance procedures, machine conditions, etc, using specialist advice. Used on the right applications, in the right manner, synthetics can show real advantages to the user. A large number of the horror stories abounding in industry regarding the use of water-based products result from incorrect usage. If machines which have been run on neat oils for years are suddenly switched to synthetics without the proper investigations and procedural examinations, a whole string of problems can arise such as machine corrosion and seizing, seal failure, attacking of paintwork, foaming of the fluid and operator skin problems. But such has been the spur of neat otl prices, availability, cancer scares,
t Developedby E. F. Houghton and Co, Philadelphia, USA
30
TRIBOLOGY international February 1977
misting, fire risks and disposal problems that switches such as this have indeed taken place. These sorts of problems have, in their turn, frightened many potential users away from synthetics, when in fact real benefits could have been achieved. Now, with the early euphoria reasonably tempered, realistic appraisals of the potential for synthetics are far more common. We certainly believe that the gradual trend for synthetics to increase their share of the metalcutting fluid market will continue, although there are undoubtedly many areas of application where neat oils are unlikely to be successfully replaced in the foreseeable future. With this history of the development of synthetics behind us, it is a good time for a rational appraisal of the pros and cons of these fluids in line with the state of the art at this time.
Advantages of synthetics Coolant life: In general the user can depend upon a longer fluid life with synthetics, particularly compared with conventional solubles. This not only manifests itself in the obvious economy advantage, but also in minimising machine downtime for coolant changeover and reducing disposal problems. Easing of odour problems: again, in comparison with soluble otis. However, confusion often arises because of the occasional tendency of synthetics to pick up tramp oil from lubrication systems, eventually yielding odour problems from bacteria. Regular skimming of the coolant will generally maintain the advantage. Working conditions: There should be improvements in visibility of work, cleanliness of components, machines and work area, and removal of 'oily' smells. Safety: Operating conditions should be made safer by: removal of misting often prevalent in large machine shops running on neat cutting oils; removal of possible fire risks with neat oils; and reduction of operator contact with mineral oil, which has a stigma related to cancer associations not really so relevant nowadays, since most suppliers are utilising solvent-refined mineral oils for cutting oils. Environmental problems: The latest types of synthetic fluids, after allowing any solids to settle out and skimming off tramp oil, are generally more acceptable to sewage authorities than conventional products. Performance improvements: Depending on application and fluid, quite dramatic improvements in machining performance and tool life have been achieved with synthetics. Coolant drag-out: the nature of the fluid means that less of it is lost in use, not only yielding obvious economy, but also indirectly leading to cleaner swarf.
The disadvantages Machine lubrication: Since synthetics, by definition, do not contain mineral oil, they cannot provide the residual lubrication for which conventional cutting oils are sometimes relied. However, relying on a coating of oil from the machining fluid often covers up inadequate machine lubrication procedures. Although it might be said that this characteristic is a bonus from the use of mineral oil products, a review of the machine manufacturer's recommen-
dations will generally suggest routine lubrication procedures that will give more than adequate cover when synthetics are used. The introduction of semi-synthetics was partly aimed at this problem but, as with so many attempts to give 'the best of both worlds', these products are certainly not the ultimate answer. Whilst they do have merit in some applications, many of the problems with conventional soluble oils are reintroduced.
Removal o[ slideway lubricants: Many synthetics, particularly some of the early types, do have a tendency to emulsify or purge lubricating oil from slideways, etc because of their general 'wetting-out' characteristics. However, the more recent developments do not emulsify mineral oil, and the problems have been eased. In addition, improvement can also be achieved by the use of the more adhesive slideway lubricants, modifying coolant flow direction, and institution of the regular lubrication procedures referred to previously.
Component protection: Products containing mineral oil obviously tend to leave a film of oil on the component after machining, which can provide useful inter-operational protection. The newer synthetics do, however, incorporate good rust preventive qualities. On the other hand, there is an increasing trend towards the use of dewatering rust preventives for inter-operational corrosion protection in the metalworking industry, which obviously eases the requirements anyway.
Operator skin problems: Synthetics inherently incorporate a degree of detergency, and consequently can tend to remove the natural oils from the skin, in the same way that washing-up liquids do. This can, of course, lead to skin problems of varying degrees. However, insistence on the use of the proper barrier creams will generally remove this problem.
Paint and seal attack: It is certainly true that standard machine tool paint and seal materials can be susceptible to attack by modern synthetic fluids. However, this problem is of decreasing relevance, since many machine tool manufacturers are now constructing their machines with a view to running with synthetics, and hence are utilising paints and seal materials which are immune to attack. This does not apply to all manufacturers. However, the changing of seals and repainting of machines with the fight materials is not a major job, if it is found to be necessary, and can be a small price to pay for benefits elsewhere.
Synthetics in application From the pros and cons listed above, it can be seen that the
decision whether or not to use synthetic cutting fluids can involve a complex equation. The user is not dealing with a straight forward commodity or specification material, as with some oil purchases, but with complex chemical formulations. Consequently a far greater onus falls on the supplier to provide specialist advice to the user, and so, in his turn, the user must assure himself that his supplier has the resources to provide this. The user should certainly beware of a supplier who has a product line heavily committed to either straight oils or synthetic products alone, since it will obviously be difficult to obtain a totally unprejudiced opinion.
A specialist supplier with a product line containing a metalcutting range from straight mineral otis through solubles to synthetics should have few axes to grind, and should be able to aid the user in evaluating the only true measure of effectiveness - cost in application. The cost per litre of the cutting fluid is only one small part of the equation, although even the most expensive synthetic will generally show a cost saving over its neat oil counterpart when in its diluted form. The total cost in application must embrace machining times, component quality, tool life, sump life, disposal costs, top-up rate, maintenance cost and health, safety and environmental considerations. While neat oils are certainly easier to run from the maintenance standpoint we, as suppliers with no forced prejudice in either area, believe that there is a fundamental shift occuring in this equation in favour of synthetics. Whilst this shift may be slower, and hence far longer-term, than some have predicted, and is certainly unlikely to be 100% in the foreseeable future, much product development is committed to this shift in the equation.
Case histories Massey Ferguson Limited, Manchester, were carrying out a variety of machining operations with a neat cutting oil, but were concerned about fuming and fire risks. A modern synthetic (Evcol 70 at 40:1 ) was recommended. Massey Ferguson's Industrial Engineering Department lists among other benefits received from this changeover: reduction of atmospheric poilu-
tion, significant improvement in operating conditions, marked improvement in tool life and significant reduction in total cutting fluid bill. A number of potential machine problems were spotted, but these have generally been overcome by the tightening of maintenance schedule. Perkins Engines Limited, Peterborough, used to use straight paraffin for honing cast iron cylinder bores. The potential hazards were obvious and, having first used an imported high cost synthetic fluid, they now carry out the same operation with a UK produced synthetic (Evcol V). This performs well on a somewhat difficult operation at a substantially reduced cost. Girling Limited, Bromborough, had purchased a Hoffman Vertical Broaching Machine for machining cast iron brake caliper bodies. This is a fairly demanding operation with quite complex coolant requirements. The neat oil used was found very expensive in application, due to excessive drag-out. The decision was made to investigate a number of synthetics within the following requirements:Freedom from build-up of cast iron 'cake'. Good tool life. Freedom from corrosion. Good stability and sump life. Freedom from bacterial degradation. Freedom from foaming, Girling is now using Evcol V with excellent results, in an application generally considered to be the forte of mineral oil products.
TRIBOLOGY international February 1977
31