High performance biofluids - natural or synthetic?

Citation

Margaroni, D. (1999), "High performance biofluids - natural or synthetic?", Industrial Lubrication and Tribology, Vol. 51 No. 3. https://doi.org/10.1108/ilt.1999.01851cab.001

High performance biofluids - natural or synthetic?

High performance biofluids ­ natural or synthetic?

Keywords Biofluids, Lubricants

The global drive towards the use of lubricants having minimum impact on the environment is increasing, although with a higher degree of emphasis in the more developed countries. The overall environmental impact of a lubricant is determined by a number of factors, including resource renewability, the nature and quantity of emissions arising during manufacture, emissions during use and finally disposal issues, which can include energy (and emission) liberation, also biodegradability and ecotoxicity. Attention has inevitably been focused upon disposal issues, since it has been estimated that some 5-10 million tonnes of petroleum-based oleochemicals enter the biosphere every year. Of this, some 40 per cent emanates mainly from spills, industrial and municipal wastes, urban run-off, refinery processes and condensates from marine engine exhausts (Lal, 1994). Although the development of severe hydrotreatment refining processes has reduced some of the disposability hazards of the mineral oil derived basestocks traditionally used for lubricants, attention has been focused on lubricants derived either from vegetable oils or from synthetic fluids such as polyalphaolefins or esters.

Lubricants based on vegetable oils

Although the use of vegetable oils as lubricants' bases have undeniable advantages in terms of resource renewability, biodegradability, ecotoxicity and cost, they suffer from a number of performance shortcomings which have limited their uptake in this area. Types of vegetable oils include corn, soyabean, rapeseed (canola), sunflower, peanut, olive oil, etc. In the UK rape seed represents 50 per cent of the production, while in the USA, soya predominates. In the Far East, palm oil is the main crop while sunflower oil is preferred in France. The oils occur in their natural form primarily as triglycerides, and tend to suffer from poor thermal, hydrolytic and oxidation stability compared with mineral oils. Most of these oils are not able to withstand reservoir temperatures in excess of 80°C, also, the presence of even ppm levels of water can cause pronounced foaming and degradation problems. For some years, various research and development organisations such as the Centre for Agricultural Strategy, University of Reading, in the UK, have been involved in investigational programmes to improve the shortcomings of vegetable oils. At Reading, for example, much work has been carried out on the genetic modification of natural varieties of oilseeds to improve their properties for use as lubricant basestocks in addition to a number of other potential use areas. It was considered that those basic properties of e.g. oilseed rape, which adversely affected the suitability of rapeseed oil as a lubricant base, could possibly be "engineered-out" by suitable genetic transformation. Their findings (Carruthers, 1995) identified that, as far as lubricants were concerned, unsurprisingly the greatest potential was identified in total-loss systems, e.g. chain-saw bar oils, two-stroke marine engines, drilling muds, agricultural greases, etc.) and also in situations where the risk of loss was high (e.g. certain hydraulic systems). In these situations, the inherently high biodegradability of vegetable oil based products could outweigh the significantly increased costs of such products. Certain countries, such as Germany and Switzerland, have already introduced specific environmental legislation which dictated the use of such products in environmentally sensitive areas.

The main advantages of vegetable oils over mineral oils were identified by Carruthers et al. as follows:

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  High biodegradability. This occurs since the relatively unstable, linear-chain molecular structure of the fatty acids in vegetable oils allows enzymatic and bacteriological action. Mineral oils were cited as being relatively non-biodegradable since they are composed largely of ring structures which are more stable and slow to degrade. (This is disputed since the non-biodegradability of paraffinic mineral oils is due more to chain branching which interrupts the enzymic activity, although the end result is the same. Also, ring structures, due to strain within the molecule, are inherently less stable than straight chain molecules, which is evidenced by their decreased resistance to oxidation. They are, however, not amenable to enzymic attack. Editor.)

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  Low toxicity. Vegetable oils were quoted as seeming to pose less of a toxicity problem compared to mineral oils, although the precise extent was not clear.

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  High flash point. Vegetable oils are admittedly less volatile than mineral oils and have higher flash points.

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  High viscosity indices. Vegetable oils have higher VIs than mineral oils although in practice this is not a major benefit since the usable temperature range of vegetable oils is limited by their susceptibility to oxidation at high temperatures, and by their high pour points at lower temperatures.

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  High lubricity. The lubricity of vegetable oils, particularly under conditions of boundary lubrication, is superior to that of mineral oils.

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  Compatibility with paints and seals. Vegetable oils show good compatibility with all paints and seals used in standard industry tests.

The main disadvantages of vegetable oil based lubricants were identified as follows:

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  The double bonds in the unsaturated fatty acid chains provide sites for oxidative attack, which causes the oils to progressively increase in viscosity.

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  Poor low temperature properties. Vegetable oils have high pour points, which again limit their usable temperature range. Also, they do not respond as well to the addition of pour-point depressants as do mineral oils.

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  Poor hydrolytic stability. Vegetable oils, when subjected to high temperatures in the presence of water, are prone to hydrolysis.

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  Price. Lubricants based on vegetable oils are usually between 1.5 to 3 times the cost of an equivalent mineral base-lubricant.

The report detailed the traditional ways in which the various disadvantages may be at least partially overcome. Oxidative stability can be improved by the use of oils with a lower proportion of double bonds, by the incorporation of anti-oxidants, or by the conversion of the oils to their alkyl esters.

Low temperature performance can be improved by using oils with a high ratio of double bonds, by blending different oils, by the incorporation of pour-point depressants, or again by converting the oils into esters.

Hydrolytic stability can be improved by using oils with longer-chain, mono-unsaturated fatty acids.

Although the use of esters effectively extends the usable temperature range of vegetable oils, they are hydrolytically unstable and also cost between five and ten times the price of mineral oils. Nevertheless, since they are more akin to the properties of mineral oils, while retaining the essential biodegradability performance, they are finding increased use in certain areas.

However, as can be seen by the conflicting corrective methods, the improvement of one characteristic will often result in a deterioration of another. At present, very high oleic acid content oils appear to offer the best compromise.

As mentioned earlier, the uptake of vegetable oil-based lubricants has been more pronounced in other European countries, assisted in some cases by generous government funding into research and development, notably in Germany. Germany and Austria have now prohibited the use of mineral oil in chainsaw lubricants, and Germany has also prohibited the use of such lubricants on inland waterways.

The most significant potential markets for vegetable oil-based lubricants were identified as follows:

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  Chainsaw-bar lubricants. Being a total-loss system, a high degree of biodegradability is desirable. Thermal oxidation stability is relatively unimportant because of the very short working life.

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  Hydraulic systems. The benefits of using highly biodegradable lubricants in situations where inadvertent leaks could contaminate environmentally sensitive areas are self-evident. (However, the rapid biodegradation of such lubricants can pose a particular problem in the form of oxygen depletion in water systems. In the UK, any leakage of significant amounts of lubricant into the environment is regarded by the Environment Agency as a serious environmental incident, whether or not the lubricant is biodegradable. Editor.)

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  Offshore drilling. The substitution of mineral oil-based lubricants by vegetable oils in drilling muds is attractive because of the high loss rates into aqueous systems. There is a major research and development programme currently being conducted by the Institute of Offshore Engineering at Heriot-Watt University in Edinburgh.

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  Two-stroke systems. Again, because of the total-loss system of lubrication used in the simpler varieties of these types of engine, environmental emissions are significant, and a high degree of biodegradability is desirable.

Having identified a number of other areas in addition to lubricants where the use of vegetable oil-based products have potential advantages, the report detailed the methodology involved in genetically modifying the various crops.

Investigations into genetic modification were conducted in a number of different areas. These included modifications to the fatty acid synthesis process and to the triglyceride synthesis process.

Because designer oilseeds must be capable of thriving in the environments for which they are intended, the genetic modification of established crops was preferred to the development of new crops or of less widely grown species. The UK environment favours oilseed rape compared with other varieties, and accordingly most R & D has been centred on this particular crop.

The development of transgenic crop varieties comprises three critical stages:

1. 1.

   The identification and isolation of those genes imparting the desired properties.

2. 2.

   The introduction of the isolated "foreign" gene in to the host plant's DNA in order to implant the new property.

3. 3.

   Satisfactory regeneration of a viable whole plant. Alternatively, non-transgenic procedures may be used which make use of the considerable natural variations which can occur within the chosen variety, although the scope for significant modification is more limited in this case.

The investigations into transgenic modifications to oilseed rape have produced new varieties with novel fatty acid profiles. Oils rich in erucic acid, lauric acid, ricinoleic acid, petroselinic acid, gamma-linoleic acid, epoxy fatty acids, wax esters and polyhydroxybutyrate have been obtained.

Of particular interest to grease manufacturers, for example, were the investigations in attempting to clone the required genes from castor and transfer them to oilseed rape. If this is successful, a high ricinoleate oilseed rape could be made available, which could reduce the dependence of grease manufacturers on the supply of expensive hydrogenated castor oil imported from Brazil and India.

The Reading report concluded that the benefits of using biodegradable lubricants based on vegetable oils are clear, i.e. low toxicity, high biodegradability, renewable resourcing and in some cases certain technical advantages. It was anticipated that the current investigations into genetic modification into the favoured variety, namely oilseed rape, overcome many of the attendant disadvantages, also that prices will fall as volumes increase.

Although up-to-date figures are hard to come by, earlier reports (1993) from the US Department of Agriculture estimate that, of the 70 million tonnes of vegetable oil currently produced worldwide, only some 54,000 tonnes are used in lubricants, due to the technical limitations described earlier. Basic vegetable oil products are most ideally suited to applications where the lubricant is used on a once through total loss basis such as sawmill or chain drive lubrication. They may also be used in low to medium pressure hydraulic systems or for lightly loaded gear drives. However, in the USA Dupont has filed patent applications for two processes involving genetically modified soyabean oil, where the oleic acid content has been enriched to some 80-85 per cent as opposed to the normal level of 30 per cent, and is now being produced on a commercial scale.

A further area which has been cited as a potential growth area for the use of vegetable oil bases in lubricants has been the use in cutting oils, both neat and water-mix varieties, on account of their perceived occupational health advantages. They do not tend to de-fat the skin as do mineral oil products, and tend to produce lower mist concentrations than do mineral oils under the same conditions of use. Research by some companies has overcome earlier problems of deposit and gum residue formation when used as neat cutting oils (Jorsmo, 1994). Other companies have concentrated on the use in water-mix fluids, by preparing water-soluble derivatives of vegetable oils. Castor oil, which contains ricinoleic acid, is reacted with either maleic or phthalic anhydride to form semi-esters of the corresponding diacids. The alkanolamine soaps of these compounds are water-soluble. Alternatively, ricinoleic acid itself can react directly with the two anhydrides giving esters containing two carboxyl groups, which also form water-soluble alkanolamine soaps. It was found in practice that the concentrates obtained by these two routes could produce clear solutions in the case of the ricinoleic derivatives, or slightly opalescent solutions in the case of the castor oil derivatives (Balulescuet al., 1994). These fluids exhibited exceptional corrosion inhibition and antiwear/ extreme pressure performance, equalling or even surpassing the performance of conventional fluids containing sulphur and chlorine additives to boost performance.

There is no doubt that the use of more sophisticated naturally-derived base fluids together with a range of additives specifically tailored to improving the performance of these oils has considerably extended the scope of application of such vegetable oil products.

A possible indicator to future applications is the use of vegetable oil based oils as automotive crankcase lubricants, which have been a subject for investigation for some years in the USA. One test vehicle, a Ford Taurus sponsored by the Ohio Corn Growers Association, has run for 60,000 miles on corn-based lubricants, while an Oldsmobile has run for 45,000 miles on sunflower oil lubricants. Other well-publicised uses have including the lubrication of flexible fuel vehicles and some motor sport activities. Research is intensifying and a number of patents exist pertaining to use of vegetable oils as crankcase fluids, but very few papers have been published on the subject owing to the highly proprietorial nature of the investigations. However, it remains to be seen whether the rate of development of such lubricants can match the increasing performance expectations of the newer generation of crankcase oil specifications, which necessitate to a greater and greater extent the use of unconventional base oils or of synthetic lubricant bases. However, US investigators already consider that the main problem areas are not technical, but logistical, in that the development of substantial growth calls for massive coordination of a number of diverse groups, including farmers, consultants, research, production andmarketing entities as well as regulatory authorities.

A number of sources describe other specific case histories involving newly-developed products for specific applications, e.g. fully saturated esters have been used as a basis for high-performance hydraulic fluids inGermany (Omeis, 1998).

Lubricants based on synthetic fluids

Compared with lubricants derived from vegetable oils, lubricants derived from synthetic fluids are more costly, and also use up non-renewable resources. The range of candidate synthetic fluids is large and a number of possibilities exist, all with differing characteristics which may favour uses in particular applications. Additive solubility in some synthetic fluids is often a limiting factor in determining the performance of a lubricant. As with vegetable oil-based products, additive chemistry needs to be carefully optimised to ensure optimum benefit is realised. Although absolute comparisons in overall environmental impact depend on the actual manufacturing processes involved in producing the two types of fluid bases, it is also more than likely that the manufacturing route of the synthetic bases is more energy-intensive, and produces more in the way of emissions. However, this article concentrates on the disposal issues of lubricants, and not on the wider issues involved in life cycle analyses.

Polyalphaolefins (PAOs)

Low viscosity PAOs (i.e. from 2 to 4 cSt at 100°C) are readily biodegradable by all of the standard methods of assessment, although the higher viscosity versions (i.e. 6 cSt and above at 100°C) are not, presumably on account of their reduced solubility in water. Mammalian and aquatic toxicities of the low viscosity PAOs are also low, and these products are consequently used in a number of environmentally-acceptable lubricants, and are particularly suited for application as fire-resistant hydraulic fluids. Their water solubility is a disadvantage, since this can facilitate water contamination. Also, additive solubility is low in PAOs, restricting their potential for performance enhancement. However, such products have been successfully used in a number of other applications, including the lubrication of marine gears, where leakage into the vessel's bilge water is always a strong possibility, leading to subsequent contamination of the immediate marine environment.

Dibasic esters (diesters)

Diesters of adipic and other dicarboxlyic acids esterified with alcohols from hydroxylated petroleum fractions are being increasingly used as base stocks for, e.g. biodgradable hydraulic fluids with extended drain intervals. They are compatible with mineral oils, have good low temperature properties and high oxidation stability

Polyol esters (PEs)

PEs made form saturated fatty acids attached to an appropriate alcohol molecule have good oxidative and hydrolytic stability, as well as good low temperature performance. They find favour in applications calling for higher viscosity products.

Neopolyols esters (NPEs)

These products suffer from low viscosities; longer chain length versions with higher viscosities have poorer biodegradabilities and raised pour-points.

Complex esters (CXEs)

These are NPEs compounded to gain higher viscosities without the associated drawbacks described above.

Polyglycols

The best known of these are the polyalkylene glycols (PAGs) and the polyethylene glycols (PEGs). The polyglycol chemistry is capable of great flexibility, which enables the end product to be tailor-made for the intended application. Products can be formulated for high viscosity index (up to 280), for high temperature applications (up to 280°C), and for low temperature applications with pour points down to ­45°C. By suitably altering the respective contents of the ethylene oxide and of the butylene oxide constituents, the resultant polymers can be either water-miscible or water-immiscible. They are highly bio-degradable, have low toxicities and resist hydrolysis. The main drawbacks are their incompatibility with mineral oils and their effect upon some paints, lacquers and some elastomeric sealing materials.

Conclusion

The use of vegetable base oils as crankcase lubricants is unlikely to develop as a major application area for a number of reasons. Crankcase oils in practice are not in general released into the environment to any great extent during use. Also, the process of collection and non-harmful disposal has become more regularised in most countries, and there is little in the way of regulatory pressure to resort to the use of more environmentally-acceptable crankcase oils. As more emphasis is placed on other "green" issues, such as recycling, the use of vegetable oils would be further restricted since, unlike synthetic-based engine oils, they are not as amenable to conventional oil re-refining processes, and wastes containing vegetable-derived products would need to be separated from waste oil destined for re-re-refining. It would therefore appear that, although market growth is anticipated, the use of vegetable oil-based products will ultimately in the main be restricted to niche products used in environmentally-sensitive areas or in total-loss oiling systems.

The use of synthetic-based biodegradable oils is similarly likely to expand, and some may offer performance advantages over their vegetable oil-derived counterparts, but their use is likely to be further restricted by reasons of costs.

David Margaroni