Tribology and Lubrication Technology October 2010 : Page 10
TECH BEAT Dr. Neil Canter / Contributing Editor New type of polyalkylene glycol These lubricant basestocks are soluble in mineral oil and have the potential to be widely used in industrial applications. P KEY CONCEPTS A new series of PAGs are readily soluble in mineral oil. Oil soluble PAGs display excellent deposit control characteristics when blended as a co-basestock with Group I mineral oils. Oil soluble PAGs are best used as a basestock when combined with other basestocks such as mineral oil or PAO and as an additive to contribute friction-reducing proper-ties to a lubricant. olyalkylene glycols (PAGs) have been used as basestocks in synthetic lubri-cants for more than 50 years. Applications using PAGs have been primarily compressor fluids, gear oils and hydraulic fluids. STLE-member Dr. Govind Khemchandani, senior technical specialist for The Dow Chemical Co. in Freeport, Texas, says, “PAGs have provided good low-tem-perature properties, high viscosity indexes, excellent control of deposits and hydro-lytic stability. The fact that every third atom of a PAG is an oxygen atom provides the polarity needed to prevent varnish formation.” STLE-member Dr. Martin Greaves, technology leader for Dow Europe in Hor-gen, Switzerland, discusses the synthetic versatility of PAG chemistry. He says, “As derivatives of ethylene oxide and propylene oxide, there are virtually a limitless number of possible combinations that can be developed.” In a previous TLT article, PAGs were used to boost the performance of biode-gradable lubricants through the preparation of blends with high oleic canola oil.1 Sludge and varnish formation were significantly reduced with these blends as com-pared to high oleic canola oil without sacrificing biodegradability. But PAGs have faced some challenges that limit their use in lubricant appli-cations. Khemchandani says, “Three of the largest challenges in using PAGs are miscibility with mineral oil basestocks, hygroscopicity and compatibility with elas-tomers.” Greaves adds, “The inability of conventional PAGs to be compatible with mineral oil basestocks makes it very difficult, time-consuming and costly for lu-bricant end-users to convert from mineral oil-based fluids to a lubricant based on PAGs.” Development of a PAG that is soluble in mineral oil could facilitate the wider use of this lubricant basestock in additional applications and enable conversions to be done more efficiently. Such a PAG type has not been available until now. OIL SOLUBILITY Dow has just introduced a new series of PAGs that are readily soluble in mineral oil. They are known as OSP™ PAGs and are available in viscosity grades ranging from ISO 32 to ISO 680. Greaves says, “The oil soluble PAGs are based on downstream derivatives of butylene oxide as one of the precursors.” Khemchandani says, “The use of higher alkylene oxides such as butylenes oxide increases the ratio of carbon to oxygen in the PAG, which boosts oil solubility.” Solubility testing was conducted with the ISO 46 oil soluble PAG in a wide range of mineral oil basestocks including Group I, II, III and naphthenics. Syn-thetic basestocks such as esters, polyalphaolefins (PAOs) and PAG-propylene oxide homopolymers were also evaluated. Testing was done at -10 C, ambient temperature and 80 C for 168 hours at the following basestock ratios: 10:90, 50:50 and 90:10. In all cases, except for 40 cSt PAO, complete miscibility is seen at all temperatures. Aniline point is a measure of the effectiveness of the solubility of a basestock. 10 TECH BEAT
Dr. Neil Canter
<b>New type of polyalkylene glycol
These lubricant basestocks are soluble in mineral oil and have the potential to be widely used in industrial applications.</b>
Polyalkylene glycols (PAGs) have been used as basestocks in synthetic lubricants for more than 50 years. Applications using PAGs have been primarily compressor fluids, gear oils and hydraulic fluids.
STLE-member Dr. Govind Khemchandani, senior technical specialist for The Dow Chemical Co. In Freeport, Texas, says, “PAGs have provided good low-temperature properties, high viscosity indexes, excellent control of deposits and hydrolytic stability. The fact that every third atom of a PAG is an oxygen atom provides the polarity needed to prevent varnish formation.”
STLE-member Dr. Martin Greaves, technology leader for Dow Europe in Horgen, Switzerland, discusses the synthetic versatility of PAG chemistry. He says, “As derivatives of ethylene oxide and propylene oxide, there are virtually a limitless number of possible combinations that can be developed.”
In a previous TLT article, PAGs were used to boost the performance of biodegradable lubricants through the preparation of blends with high oleic canola oil.1 Sludge and varnish formation were significantly reduced with these blends as compared to high oleic canola oil without sacrificing biodegradability.
But PAGs have faced some challenges that limit their use in lubricant applications. Khemchandani says, “Three of the largest challenges in using PAGs are miscibility with mineral oil basestocks, hygroscopicity and compatibility with elastomers.” Greaves adds, “The inability of conventional PAGs to be compatible with mineral oil basestocks makes it very difficult, time-consuming and costly for lubricant end-users to convert from mineral oil-based fluids to a lubricant based on PAGs.”
Development of a PAG that is soluble in mineral oil could facilitate the wider use of this lubricant basestock in additional applications and enable conversions to be done more efficiently. Such a PAG type has not been available until now.
Dow has just introduced a new series of PAGs that are readily soluble in mineral oil. They are known as OSP™ PAGs and are available in viscosity grades ranging from ISO 32 to ISO 680. Greaves says, “The oil soluble PAGs are based on downstream derivatives of butylene oxide as one of the precursors.”
Khemchandani says, “The use of higher alkylene oxides such as butylenes oxide increases the ratio of carbon to oxygen in the PAG, which boosts oil solubility.”
Solubility testing was conducted with the ISO 46 oil soluble PAG in a wide range of mineral oil basestocks including Group I, II, III and naphthenics. Synthetic basestocks such as esters, polyalphaolefins (PAOs) and PAG-propylene oxide homopolymers were also evaluated.
Testing was done at -10 C, ambient temperature and 80 C for 168 hours at the following basestock ratios: 10:90, 50:50 and 90:10. In all cases, except for 40 cSt PAO, complete miscibility is seen at all temperatures.
Aniline point is a measure of the effectiveness of the solubility of a basestock. This parameter is measured by checking the solubility of aniline in the basestock and measuring the temperature at which the solution turns hazy. A desired result is as low a temperature as possible.
Khemchandani says, “The aniline point for the oil soluble PAGs is less than -30 C. In contrast, the aniline point for a typical trimethylolpropanebased polyol ester is 8 C, naphthenic oil is 71 C and an 150 SN paraffinic oil is 96 C.” The low value of the oil soluble PAGs is an indication of how effective this basestock is as a solvent for oil soluble materials as well as additives.
A series of performance tests have been conducted to determine the performance of oil soluble PAGs when used in conjunction with other oil soluble basestocks. The ability of an ISO 46 oil soluble PAG to act as an additive in reducing the friction of an 8 cSt PAO was determined through use of a mini-traction machine in which a load is applied on a steel ball as it rolls on a steel disc.
Under a contact pressure of 0.9 gigapascal, a slide roll ratio of 10% and a temperature of 80 C, the coefficient of friction for a blend of 10% of the oil soluble PAG in the PAO dropped to less than 0.1 and remained at that level for the 30-minute duration of the test. The coefficient of friction for straight PAO rose steadily until it reached a value above 0.3 at the end of the test. Greaves says, “The polarity of the PAG enables it to reach the metal surface and significantly reduce the coefficient of friction.”
Deposit control characteristics of the oil soluble PAGs were evaluated by blending 20% of the ISO 46 grade in a Group I mineral oil and evaluating it for 70 days using a modified version of ASTM D2893. Greaves says, “Air is blown through a vessel containing the blend of oil soluble PAG and Group I mineral oil at a fixed rate at 120 C.”
As shown in Figure 1, no deposits are detected in the PAG Group I mineral oil mixture while deposits are seen with the Group I mineral oil. Khemchandani says, “Degradation of mineral oil produces incompatible polar byproducts that form insoluble deposits. But the polarity of oil soluble PAGs enables them to solubilize these deposits.”
Greaves adds, “The complete absence of insolubles means there is good potential for using oil soluble PAGs in applications such as turbine oils, which have been plagued with deposit problems.”
Further data generated indicates that oil soluble PAGs exhibit excellent deposit control characteristics down to treat rates as low as 5%. This means that a small increment of an oil soluble PAG added to a lubricant will minimize the generation of insolubles.
The hygroscopicity of conventional PAGs is of concern because they can absorb water into a lubricant that can lead to fluid degradation over time. Under conditions of 80% relative humidity and 50 C, ISO 46 oil soluble and conventional PAGs were evaluated for water absorption. After 16 days, the oil soluble PAG absorbs much less water than its conventional counterpart.
Khemchandani says, “This study is even more significant because no demulsifier was used with the PAGs.”
Both Greaves and Khemchandani believe that oil soluble PAGs have the potential to be used as basestocks in lubricant products, especially when combined with other basestocks such as mineral oil or PAO to upgrade them, and as an additive to impart friction-reducing characteristics to a finished lubricant.
This versatility should enable oil soluble PAGs to be used in a variety of new applications. Further information can be found in a recent presentation given at STLE’s 2010 Annual Meeting & Exhibition2 or by sending an e-mail to OSP@ dow.com.
1. Canter, N. (2009), “Upgrading Biodegradable Lubricant Performance,” TLT, 65 (6), pp. 14-15.
2. Greaves, M., Voorst, R., Carn, C., Khelidj, N., Meertens, R. , Johnson, L. and Hoozemans, E. (2010), “Oil Soluble Polyalkylene Glycols,” Presented at the STLE 2010 Annual Meeting & Exhibition, May 16-20, Las Vegas, Nev.
<b>Bacteria: A new power generator
Researchers develop a procedure that uses bacteria as a power source for running mechanical micromachines.</b>
In development of new sources of power, researchers are looking to emulate how nature uses energy in an efficient manner. One case in point is the process of photosynthesis in which plants capture sunlight and transform it through a series of pathways into chemical energy that can be used as a food source or fuel.
Many of us in the lubricant field are familiar with dealing with the negative characteristics of microbes such as bacteria. For example, bacteria are known to proliferate in metalworking fluid systems because this medium is an excellent nutrient. The result is that the metalworking fluid is decomposed, leading to inferior product performance and the potential for health and safety concerns.
But if harnessed in the proper manner, bacteria could be used as a source of power for running mechanical micromachines. One reason for this potential is that bacteria swimming in a specific medium have been found to reduce the viscosity of the fluid.
Dr. Igor Aronson, physicist and principal investigator at Argonne National Laboratory in Argonne, Ill., says, “We suspended bacteria in a free-standing liquid film that was prepared by stretching two supporting crossed pairs of fibers into a square window approximately 200 micrometers thick with dimensions that were 7 millimeters-by-7 millimeters.”
“Through the analysis of the decay time of a macroscopic vortex created in the film by a moving magnetically actuated probe and the measurement of torque generated on a rotating magnetic particle immersed in the film, we measured the viscosity of the liquid film by itself and in the presence of swimming bacteria.”
The result is that the use of swimming bacteria reduces the viscosity of the liquid film by a factor of seven.1 This work indicates that if placed in the proper environment, the mechanical energy generated by swimming bacteria can be harnessed over a long timeframe as a power source without fluid viscosity becoming a problem.
An experiment demonstrating the potential for using bacteria in this fashion has not been conducted until now.
Researchers at Argonne National Laboratory and Northwestern University developed a procedure to enable swimming bacteria to turn gears with teeth of varying number, shape and arrangement. Aronson says, “Laws of thermodynamics prohibit extraction of useful work from the Brownian motion of particles in equilibrium, which means that it is impossible to create directed motion of the gears in water.”
“When placed in close proximity, molecules that collide randomly with gears Will not turn them in a certain direction. Bacteria that are swimming near gears are self-propelled particles that can convert chemical energy of the nutrient into mechanical energy that turns gears.”
The researchers placed gears that are 380 micrometers in diameter and 50 micrometers thick into a liquid film suspension with bacteria and their nutrient, bacterial broth. Each gear was made from SU-8 photoresist using conventional photolithography.
The gears were twice as dense as the film, which meant that they sank to the lower fluid/air interface. Bacillus subtilis (also known as hay bacillus or grass bacillus), a common gram-positive bacteria, was used in this study. Aronson says, “This type of bacteria swim very well in this medium and can move an incredible 20 times their body length per second. They also do not present any harm to people.”
The bacteria move gears with asymmetric teeth by sliding along until they are trapped in the v junction. Due to the asymmetry, they are trapped in one direction and will move the gear in that direction. Gears with symmetric teeth were used as a control, and no movement was seen with bacteria because of their orientation. Aronson says, “In the case of symmetric gear, bacteria pushed from both sides, which led to zero momentum. The symmetric gears did not rotate but, rather, just fluctuated in the film.”
A variety of different gears being turned by bacteria is shown in Figure 2. Diagram “I” and “J” show that arrangements can be prepared to have two gears engage each other by rotating in opposite directions.
The rate of gear rotation is a function of the concentration of bacteria present in the film. Aronson explains, “Gear movement does not occur until the bacteria concentration reaches 1x1010 per cubic centimeter. We believe that some sort of friction is present in the surface film that can only be overcome by a critical force that requires a certain number of bacteria.”
The optimal concentration for bacteria is between 1 and 4x1010 per cubic centimeter. Above this range, the gears will Stop rotating. Aronson says, “At a very high concentration, bacteria literally cannot swim because there are too many of them crowded into a finite space. This condition also prompts bacteria to secrete certain chemicals, which are the precursor for the generation of a biofilm.”
Gear movement can be regulated by adjusting the concentration of air or oxygen that is placed in contact with the bacteria. Aronson says, “We found that gear rotation stops when the oxygen in the atmosphere is completely replaced with nitrogen, leading to an anaerobic environment. Without access to oxygen, bacteria cannot undergo respiration and will stop moving.”
This experiment also served to prove that the motion of the bacteria is the sole reason why gears are rotating. Other factors controlling the rate of rotation are the size, number and shape of the gear teeth.
In the initial experiments, gear rotation occurred for six to eight minutes and was restricted by the level of nutrient. Eventually, a high enough concentration of metabolic products is generated from the bacteria to increase the viscosity of the film and reduce gear rotation. Aronson says, “Sustaining the motion for a longer period of time is feasible if fresh fluid containing nutrient can be constantly pumped into the system while metabolic byproducts are removed.”
The ability of an ordinary species of bacteria to turn gears means there is potential for injecting energy generated by microorganisms into a system to power micromachines in the future. Further information can be found in a recent article2 or by contacting Aronson at email@example.com.
1. Sokolov, A. and Aronson, I. (2009), “Reduction of Viscosity in Suspension of Swimming Bacteria,” Physical Review Letters, 103 (14), 148101.
2. Sokolov, A., Apodaca, M., Grzybowski, B. and Aronson, I. (2010), “Swimming Bacteria Power Microscopic Gears,” Proceedings National Academy of Sciences, 107 (3), pp. 969-974.
<b>High-efficiency gasoline engines
New technology is helping to improve fuel economy and lower emissions.</b>
The emphasis on increasing fuel economy and reducing emissions is placing greater demands on engine manufacturers. As a result, engine sizes have been reduced to improve fuel economy, and measures have been taken to protect the automobile’s emission system.
Last month, TLT featured a special report on the development of the most recent ILSAC gasoline engine oil specification, ILSAC GF-5.1 Among the issues is a tradeoff between using friction modifiers to improve fuel economy and being able to pass engine deposit tests. Two key tests that are challenging to pass are the Sequence VID fuel economy test and the TEOST 33C. The latter is used to evaluate the effectiveness of the engine to minimize turbocharger deposits.
Light-duty diesel engines have traditionally exhibited superior fuel economy performance compared to gasoline engines. Dr. Terry Alger, manager of the Advanced Combustion and Emissions section in Southwest Research Institute’s (SwRI) Engine, Emissions and Vehicle Research Division in San Antonio, Texas, says, “In 2001, SwRI’s Clean Diesel consortium was looking at reducing emissions From heavy-duty diesel engines. As part of this process, we examined the use of gasoline technology with a 3-way catalyst as a low-cost option. We found that cooled exhaust gas recirculation (EGR) could suppress knock in gasoline engines, allowing them to meet heavy-duty performance and efficiency goals.”
Attention turned to how the performance of a gasoline engine can be improved to achieve improved fuel economy and lower emissions. Alger explains, “We believe there is an opportunity to utilize a comparable approach to enable a gasoline engine to be as efficient as a diesel engine.”
A technology is in the process of being implemented to meet this goal.
SwRI initiated a program known as High-Efficiency, Dilute Gasoline Engine (HEDGE) to develop a more effective gasoline engine. The objective involves utilizing cooled EGR to achieve the goals noted in Figure 3.
Alger says, “We found that using EGR at a rate between 25% and 50% suppresses engine knock, improves fuel economy between 5% and 30% and reduces emissions. A heat exchanger is used to drop the temperature of the EGR stream from 700 C to between 90 C and 150 C.”
This result can be achieved with a higher compression ratio between 11:1 and 15:1, which enables the engine to obtain more mechanical energy from the air, fuel mixture and, therefore, be more efficient.
Another benefit of using cooled EGR is the elimination of a phenomenon known as low-speed pre-ignition (LSPI). Alger says, “LSPI is a random and sporadic pre-ignition event that can occur in a cylinder for a few cycles and then go away only to return at a later time. For example, LSPI events occur in an alternating fashion with a pre-ignition cycle followed by a normal cycle for up to 15 or 20 engine cycles and then disappear only to return as many as 30,000 cycles later. A typical frequency for LSPI is between 4 and 20 events per 30,000 engine cycles, depending on the engine speed and torque.”
Alger indicates that the origin of LSPI is not known, but it has become a major problem since turbochargers have been used to boost the performance of gasoline engines. He adds, “LSPI causes heavy engine knock and can increase the peak pressures in the cylinder by two to three times. This effect can lead to holes in the piston and break other components, which can cause complete engine failure in one to two cycles.”
While the HEDGE consortium was established eight years ago to investigate the effect of cooled EGR on engine efficiency, the attention turned to LSPI over a year ago. Alger says, “In engines showing 4-5 LSPI occurrences per 30,000 cycles in the absence of EGR, the use of 5% EGR reduced the Number of LSPI episodes to 1 per 30,000 cycles. If EGR is raised to 10%, then LSPI is eliminated at that load. At higher loads, up to 15% cooled EGR is required.”
Alger suspects that fuel and the engine lubricant also contribute to LSPI. A study was done comparing 87 octane gasoline to blends of this fuel with 10% ethanol, a high level of aromatics and a low level of aromatics.
Alger says, “The highest frequency of LSPI was observed with 87 octane gasoline. Introduction of ethanol reduced the occurrence of LSPI by 50%, while the presence of 87 octane with a high level of aromatic content also generated some LSPI. No LSPI was seen with 87 octane gasoline prepared with a low level of aromatic content.”
Alger indicates that engine oil is a big contributor to knock as it displays a cetane number similar to diesel fuel. He indicated that some engine manufacturers and other researchers believe that the oil is also causing LSPI.
Alger says, “We are launching a new consortium to evaluate the effect that engine oil and fuel composition have on triggering LSPI. The goal is to come up with a test that engine manufacturers can use to evaluate the impact of the lubricant or fuel in contributing to LSPI.”
Alger believes that the engine manufacturers then will include such a test in their lubricant specification. The new consortium kicks off on Jan. 1, 2011.
Further information on HEDGE can be found at http://www.hedge.swri.org or by contacting Alger at firstname.lastname@example.org.
1. Canter, N. (2010), “Proper Additive Balance Needed to Meet GF-5,” TLT, 66 (9), pp. 10-18.
Read the full article at http://onlinedigitalpublishing.com/article/Tech+Beat/504396/47703/article.html.
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