(College of Petrochemistry, Logistical Engineering University, Chongqing 401311)

Lubricant Biodegradation Enhancers: Designed Chemistry and Engineered Technology

Chen Boshui; Gao Lingyue; Fang Jianhua; Zhang Nan; Wu Jiang; Wang Jiu

(College of Petrochemistry, Logistical Engineering University, Chongqing 401311)

In recent decades, a growing worldwide trend of developing the biodegradable lubricants has been prevailing to form a specific field of green chemistry and green engineering. Enhancement of biodegradability of unreadily biodegradable petroleum-based lubricants has as such become an urgent must. For over a decade the authors have been focusing on the improvement of biodegradability of unreadily biodegradable lubricants such as petroleum-based lubricating oils and greases. A new idea of lubricant biodegradation enhancer was put forward by the authors with the aim to stimulate the biodegradation of unreadily biodegradable lubricants by incorporating the enhancer into the lubricants in order to turn the lubricants into greener biodegradable ones and to help in situ bioremediation of lubricant-contaminated environment. This manuscript summarizes our recent efforts relating to the chemistry and technology of biodegradation enhancers for lubricants. Firstly, the chemistry of lubricant biodegradation enhancers was designed based on the principles of bioremediation for the treatment of hydrocarbon contaminated environment. Secondly, the ability of the designed biodegradation enhancers for increasing the biodegradability of unreadily biodegradable industrial lubricants was investigated through biodegradability evaluation tests, microbial population analysis, and biodegradation kinetics modeling. Finally, the impact of biodegradation enhancers on some crucial performance characteristics of lubricants such as lubricity and oxidation stability was tested via tribological evaluation and oxidation determinations. Our results have shown that the designed chemistry of nitrogenous and/or phosphorous compounds such as lauroyl glutamine, oleoyl glycine, oleic diethanolamide phosphate and lauric diethanolamide borate was outstanding in boosting biodegradation of petroleum-based lubricants which was ascribed to increase the microbial population and decrease the oil-water interfacial tension during the biodegradation process. Lubricants doped with the biodegradation enhancers exhibited much better biodegradability and higher biodegradation rate in the surrounding soils which could be well modeled by the exponential biodegradation kinetics. Furthermore, as lubricant dopants, the biodegradation enhancers also provided excellent capability in reducing friction and wear and in retarding oxidation of lubricants. In the nature of things, lubricant biodegradation enhancers, which are multi-functional not only in the improvement of biodegradability, but also in the fortification of lubricity and in the inhibition of oxidation of lubricants, are expected to be promising as a new category of lubricant additives.

lubricant; biodegradation enhancer; biodegradability; biodegradation kinetics; lubricity.

1 Introduction

Lubricants are indispensable ingredient for meeting the industrialization requirements. They can facilitate the effective operation of machinery by reducing the friction and wear and bringing forth other effects including heat transfer or corrosion protection. However, lubricants and the environment do not always go together very well. It has been estimated that millions of tons of lubricants end up in the environment worldwide every year as a result of leaks, emissions, spillages, or other problems[1-6]. Contamination of eco-systems caused by conventional petroleum-based lubricants, which consist predominantly of hydrocarbons and subsidiarily of performance additives, has posed great environmental problems mainly owing to their unreadily biodegradable nature[7-15]. In spite of the large number of works on petroleum and hydrocarbon biodegradation, successful treatment of the lubricant contaminated environment remains a challenge. Since the mid-1970s, the focus on health, safety and preservation ofthe environment has turned the searchlight to the effects of mineral lubricants on the environment and has stimulated the development of lubricants that showed more or less compatibility with the environment. The ecologically responsive technology and lubricant design seek to meet both the performance and environmental needs in a bid to harmonize the technical performance and the ecological requirements. As the world turns “green”, a growing worldwide trend of developing the biodegradable lubricants, which represent nowadays an ingenious achievement in lubrication engineering, has been created and has become a specific field of green chemistry and green engineering[16-22].

As we know, biodegradation is the major route by which lubricants can be removed from the eco-systems. Over the last decades, biodegradation has been the predominant yardstick to measure how lubricants can be rated as being environmentally friendly. Consequently, biodegradability is of most extensive concern for environmentally friendly lubricants and has become one of the most important design criteria on the overall formulation of a lubricant[23-25]. In practice, the state-of–the-art biodegradable lubricants are formulated so that the good biodegradability can reduce their negative impact on the environment. Therefore the key issue in formulating biodegradable lubricants is the choice of reliable base oils and suitable performance additives so as to harmonize the technical performance and the ecological requirements. Nowadays, many base fluids such as vegetable oils and synthetic ester oils have been extensively investigated and have found practical applications in the formulation of biodegradable lubricants because of their excellent biodegradability[26-43]. Even though the choice of mineral base oils in biodegradable lubricant formulations has as far never been recommended, the improvement of their environmental safety such as better biodegradability is indeed imperative since conventional petroleum-based lubricants have been dominating the lubrication system for more than a century. A question thus naturally occurs: can a petroleum-based lubricant be turned into a biodegradable one by doping with the so-called “biodegradation enhancer” in the lubricant formulation? The answer is yes! It is well known that biodegradability is not only an exact property or characteristic of a substance, but is also a “system” concept, i.e. the conditions within an entire system can determine whether or not a substance within it is biodegradable. Factors influencing the biodegradability of a substance mainly include the molecular structure, the chemical properties and the environmental conditions. For example, the treatment of petroleum hydrocarbon contamination has been generally done ex-situ using bioremediation following bio-stimulation of the indigenous microbial community by means of fertilizers[44-46]. Therefore it may be possible to deliberately formulate additive blends in which the additives might supply the necessary nutrients for base oil biodegradation. This approach can provide a hint for the creation of lubricant biodegradation enhancers.

For over a decade the authors have been concentrating on the improvement of biodegradability of unreadily biodegradable lubricants. The new idea of lubricant biodegradation enhancer was first proposed by the authors. As the name implies, biodegradation enhancers aim at accelerating the biodegradation of unreadily biodegradable lubricants such as petroleum-based lubricating oils and greases, thus turning the lubricants into greener biodegradable ones. This is a problem which has not been considered by earlier investigators. No work has so far been published with respect to the effect of biodegradation enhancers on the biodegradability of lubricants except those published previously by the authors[47-57]. The present manuscript demonstrates the review of some of our work on the chemistry and technology of biodegradation enhancers for lubricants.

2 Designed Chemistry of Lubricant Biodegradation Enhancers

The chemistry of lubricant biodegradation enhancers was designed based technically on the principles of bioremediation related with the treatment of hydrocarbon contaminated environment. As it has been mentioned above, biodegradation of petroleum hydrocarbons is a natural process controlled by the temperature, the pH value, the scarcity of nutrients and the bacterial inoculation. Successful bioremediation is usually achieved through bio-stimulation, a process in which the natural biodegradation capacities of hydrocarbons are enhanced by nutrient addition[58-63]. Nitrogen and phosphorus are well known microbial nutrients and have been used in the bio-stimulation applications. Therefore in our design schemes, nitrogen and phosphorus were the primary considerations as the backbones of the chemistry of lubricant biodegradation enhancers. Furthermore, as lubricant dopants, lubricant biodegradation enhancers were also designed following the general chemistry for biodegradable lubricant additives such as good biodegradability and good compatibility with lubricant base oils and additives. Thus the chemistry of lubricant biodegradation enhancers followed three basic design concepts: nitrogen and/or phosphorus containing, readily biodegradable, and well compatible substances.

For years of careful and rigorous ‘trial-and-error’ attempts, many efforts have been exerted by the authors to explore the possibility of lubricant biodegradation enhancers which are chemically and technologically available. Our investigations have demonstrated that two categories of nitrogenous and/or phosphorous chemicals are extremely efficient as lubricant biodegradation enhancers, viz.: those of amino acid basis and those of amide phosphate/borate basis, respectively. Important biodegradation enhancers of amino acid basis mainly include lauroyl glutamine (a white solid compound prepared by reacting lauroyl chloride with glutamic acid under alkaline and then acidic conditions, abbreviated as LGA) and oleoyl glycine (a brownish-yellow greasy compound prepared by reacting oleoyl chloride with glycin under alkaline and then acidic conditions, abbreviated as OGN), while those of amide phosphate/borate basis mainly include oleic diethanolamide phosphate (a reddish-brown greasy compound prepared by reacting oleic acid with diethanolamine and phosphorus pentoxide under alkaline condition, abbreviated as ODAP) and lauric diethanolamide borate (an ambercolored greasy compound prepared by reacting lauric acid with diethanolamine and then boric acid under alkaline conditions, abbreviated as LDAB). The designed chemistry of the biodegradation enhancers is presented in Figure 1.

Figure 1 Designed chemistry of biodegradation enhancers

3 Engineered Technology of Lubricant Biodegradation Enhancers

3.1 Ability in enhancing biodegradation of lubricants

As it is well known, biodegradation is a process of chemical breakdown or transformation of a material caused by microorganisms or their enzymes. Biodegradability is a measure of the breakdown of a chemical or a chemical mixture by microorganisms. Lubricant biodegradability is the extent to which the material can be broken down by microorganisms into innocuous products such as carbon dioxide and water. In our researches, laboratorial biodegradability tests were performed on a biodegradation tester devised by the authors, the principle and operating processes of which are specified in the reference[47,64]. In short, the principle of this method is based on measurement and comparison of carbon dioxide evolution of a tested lubricant and a readily biodegradable reference substance oleic acid by parallel biodegradation reactions of a tested lubricant and oleic acid over a period of biodegradation durations. The biodegradability index (BDI), a comparative parameter of the percentage ratio of the net amount of carbon dioxide evolved by a tested lubricant versus that evolved by oleic acid, was thus introduced to evaluate the biodegradability of the lubricant. Thereforeas an indicator of biodegradability of lubricants, higher BDI indicates better biodegradability for different lubricants.

Our investigations have demonstrated that the ability of the biodegradation enhancers as lubricant additives in promoting biodegradation of unreadily biodegradable lubricants is outstanding. Figure 2 shows the effect of the biodegradation enhancers on biodegradability of a petroleum-based fully formulated industrial hydraulic oil (with a kinematic viscosity of 48.54 mm2/s at 40 ℃) over a period of 15 days of biodegradation process.

Figure 2 Effects of biodegradation enhancers on biodegradability of a lubricant.

It can be observed clearly from Figure 2 that incorporation of the biodegradation enhancers into the mineral lubricating oil, at whatever contents, obviously enhanced the biodegradability of the lubricating oil, especially at the contents ranging from 1.0% to 2.0%. The apparent increase of lubricant biodegradability that was achieved with the addition of biodegradation enhancers was very encouraging. A possible explanation on the nitrogenous and phosphorous biodegradation enhancers that could accelerate the biodegradation of unreadily biodegradable lubricants was likely ascribed to their effect as microbial nutrients for promoting the microbe production. On the other hand, since oil biodegradation mainly took place at the oil-water interface, the amphiphilic biodegradation enhancers also decreased the oil-water interfacial tension by accumulating at the interface of immiscible fluids, and increasing the oil-water interfacial areas, thereby leading to increased mobility, bioavailability and subsequent biodegradation. Increase of microbial population and decrease of oil-water interfacial tension in the biodegradation processes via incorporating nitrogenous and phosphorous biodegradation enhancers into unreadily biodegradable mineral lubricating oils have been observed by the authors[48,51-52], as shown in Figure 3 and Figure 4, respectively.

Figure 3 Microscopic images of microbes in the sewage after biodegradation (×500)

Figure 4 Oil/water interfacial tensions versus biodegradation durations for neat oil and enhancer-doped oils

3.2 Biodegradation kinetics modeling

The effectiveness of biodegradation enhancers in accelerating biodegradation of unreadily biodegradable petroleum-based lubricants was further testified by simulation of biodegradation kinetics in the surrounding soils[50,65-67].

The results indicated that the first-order exponential kinetics model could well fit the biodegradation of petroleumbased lubricants. The biodegradation rates for lubricants doped with biodegradation enhancers were much higher than those achieved by neat lubricants.

In our studies of lubricant biodegradation kinetics in the surrounding soils, the soil specimens were deliberately contaminated with the neat oil and the enhancer-doped oils, respectively. Lubricants in the soils were biodegraded under the environmental conditions fortified by continuous supply of oxygen flow through pumping air into the soils. After a specified biodegradation duration, the total residual concentration of oil in soils was determined, and the lubricant biodegradation kinetics was simulated based on the first-order exponential kinetics model, with the integral equation of which presented below:

in which Stis the residual concentration of oil in the soil after t days of biodegradation and k is a biodegradation rate constant. Upon using the biodegradation enhancer lauroyl glutamine as an example, Table 1 shows the variations of the residual contents of lubricating oils in soils and the natural logarithms of (St/S0) with biodegradation duration.

Table 1 Variations of contents of lubricating oils in soil and ln(St/S0) with duration.

Figure 5 shows the regression curves of biodegradation rate equation for neat oil and the oil doped with lauroyl glutamine, respectively.

Figure 5 Regression curves of biodegradation rate equation for lubricating oils.

It can be seen from Figure 5 that the regression curves for biodegradation rates of both neat oil and the formulated oil were well linearized, indicating that the biodegradation of lubricating oils in soils could be well simulated by the exponential kinetics model. The slope of the regression lines, which corresponded to the biodegradation rate constant k, was 0.015 5 mg·g-1·d-1for neat oil and 0.023 5 mg·g-1·d-1for the formulated oil, respectively. Thus the biodegradation half-life (t1/2= ln(2/k)) for neat oil and the formulated oil was 44.72 days and 29.50 days, respectively. The greater biodegradation rate constant and shorter biodegradation half-life for the biodegradation enhancer-doped oil indicated that the biodegradation enhancer could increase the biodegradation rate of lubricants.

3.3 Impact on Lubricity Performance of Lubricants

The lubricity of a lubricant denotes its capacity in reducing friction and wear. Investigations by the authors also demonstrated that the biodegradation enhancers were efficient in improving the lubricity of lubricants.

In our investigations, lubricity tests were conducted on a four-ball friction and wear tester following the ASTMD2783 procedures. By taking oleoyl glycine as an example, Figure 6 and Figure 7 show the friction coefficients and the morphologies of scanning electron microscope (SEM) images of the worn surfaces under a load of 392N for a petroleum-based industrial lubricating oil, which had a kinematic viscosity of 98.16 mm2/s at 40 ℃ and contained 1.0 wt.% of oleoyl glycine in the oil formulation. It can be clearly seen from Figure 6 and Figure 7 that during the whole test duration, the oil containing oleoyl glycine all along provided lower friction coefficient than neat oil, and the surface lubricated with the formulated oil was characterized by milder wear and smaller wear scar diameter than that lubricated with neat oil. The results indicated that oleoyl glycine acted as an efficient friction and wear reducer. Improvement of lubricity by the biodegradation enhancers was attributed to the tribo-adsorption and tribo-reactions of biodegradation enhancers on the rubbing surfaces, thus forming a composite boundary lubricating film[47,51,53,68].

Figure 6 Friction coefficients versus test duration

Figure 7 SEM images of the worn surfaces (×100)

3.4 Impacts on oxidation of lubricants

Oxidative degradation reduces the effectiveness of lubricants, increases their acidity and viscosity, and contributes to the formation of insoluble sludges. In the course of oxidation study, the oxidative stability of a petroleumbased industrial lubricating oil (with a kinematic viscosity of 98.16 mm2/s at 40 ℃) and the oils formulated with 1.0 wt.% of lauroyl glutamine, oleoyl glycine, oleic diethanolamide phosphate and lauric diethanolamide borate, respectively, was tested with the introduction of oxygen flow and under the catalysis effected by iron and copper at 125 ℃ for 8 hours. At the end of the test, the net increases in viscosity and total acid number, as well as the amount of oil-insolubles, were measured to characterize the oxidative stability of the tested lubricating oil. Table 2 shows the oxidation test results of the neat oil and the oil samples doped with 1.0% of biodegradation enhancers.

Table 2 Oxidation test results for neat oil and the doped oils

It can be seen from Table 2 that the four biodegradation enhancers, after being incorporated into the lubricating oil, could to some extent improve the oxidative stability of the oil by producing smaller amount of insolubles and exhibiting less increase in acidity and viscosity as compared to those of the neat oil. The effectiveness of biodegradation enhancers in retarding the oxidation of lubricants is of important practical significance but therelated mechanisms still need further investigations.

4 Conclusions

A fire-new idea of lubricant biodegradation enhancers was proposed aiming at stimulating the biodegradation of unreadily biodegradable lubricants such as petroleum-based lubricating oils and greases. The chemistry of lubricant biodegradation enhancers was designed based technically on the principles of bio-stimulation during the bioremediation of hydrocarbon contaminated environment. The engineered technological investigations demonstrated that, as lubricant additives, the nitrogenous and phosphorous biodegradation enhancers such as lauroyl glutamine, oleoyl glycine, oleic diethanolamide phosphate and lauric diethanolamide borate, were prominent in stimulating the biodegradation of unreadily biodegradable petroleumbased lubricants as evidenced by the enhanced biodegradability and biodegradation rate in the surrounding soils. They were also excellent in improving the performance characteristics of lubricants such as lubricity and oxidative stability. The designed and engineered biodegradation enhancers are chemically and technologically practicable. Lubricant biodegradation enhancers are thus expected to be favorable to the in situ bioremediation of lubricantcontaminated environment, and are in the nature of things expected to be promising as a new category of lubricant additives.

Acknowledgements: The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation of China (project Nos.50975282 and 50275147) and the Natural Science Foundation of Chongqing, China (project No. CSTC 2008BA4037). Special thanks go to many doctoral and master’s candidates for their arduous and fruitful efforts.

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date: 2015-06-23; Accepted date: 2015-08-24.

Prof. Chen Boshui, Telephone: +86-023-86731415; E-mail: boshuichen@163.com.