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10 DesIGnInG WItH PoLYMeRs Measuring kinematic viscosity, which indicates how much force is required to get a liquid to flow elements35 Issue 2|2011 At low temperatures, the comb polymers reduce the kinematic viscosity of the oil compared to PAMA by about one third. At high temperatures, they achieve the same good values as PAMA polymers. Bottom line: comb polymers reduce the temperature dependency of viscosity Log log (KV [cst+0.8]) Contracted PAMA Collapsed comb Base oil 333 The type of polymers and, above all, their molecular mass, is key to the effectiveness of these viscosity index improvers (adding 3 to 7 percent corresponds to an additive content of 2.5 percent): the larger the molecules, the more they swell with rising temper atures and keep the lubricant sufficiently thick, even at high operating temperatures. Large molecules do have one weak point, however: mechanical stress in the thin lubrication gap can easily tear the polymer chain. For this reason, the second key parameter for the suitability of a lubrication oil is the shear stability of the polymers used. High shear stability means that the molecular chains are split slowly, even with heavy mechanical loading, so the polymer breaks down only after several years of service. Gear oils are expected to retain their lubricating effect for about 20 years, while engine oils are supposed to work reliably for approximately 30,000 kilometers. VI improvers ensure the viscosity stays as constant as possible Evonik has supplied VI improvers based on polyalkyl(meth)acrylate (PAMA) for decades under the VISCOPLEX® trademark. The molecules consist of a long polymethacrylate chain with alkyl side chains that ensure solubility in the base oil. In solution, the PAMA chains form a ball, which wells up with increasing temperature. When this happens, the balls expand, thereby increasing the viscosity of the oil. To be more exact, VISCOPLEX® ensures that the viscosity of the solution remains as similar as pos sible compared to that of the pure base oil without additive. What the chemists are doing, then, is thwarting physics: The effect is contrary to the natural behav- Log T [K] Expanded PAMA Expanded comb Oil-insoluble PAMA backbone oil-soluble polyolefin arms
ior of a liquid, which always becomes thinner when the temperature rises. The additives reduce the loss in viscosity and expand the temperature window in which the oil displays optimal lubrication. In principle, the thinner the oil, the easier it is for an engine to run, and the less fuel it consumes. The art lies in keeping an oil‘s viscosity as stable as possible within the highest possible temperature range. Today’s engines and transmissions are becoming increasingly compact and powerful. This means increasing standards for lubricants. For this reason, the chemists at Evonik have searched for molecular structures that hold the flow properties of the lubricant nearly constant without becoming too thick in cold temperatures and thin in hot temperatures. Even more powerful: comb polymers The specialists from Darmstadt have developed a completely new architecture for the molecules of the polymers. The backbone consists of extremely long, polar molecule chains that carry non-polar polyolefins as side chains at regular intervals. Chemists refer to these as “comb polymers” because their structure resembles a comb. The building blocks of the long backbone consist of short-chained methacrylates, and other co-monomers. By varying the percentages of the monomer mixture, the polarity of the chain and the number of side chains can be selectively controlled during polymerization. About 100 monomers have an aver age of 0.8 to 1.6 molecular teeth, each with some 400 carbon atoms. The modified structure results in completely new properties. The long side chains ensure extremely good solubility in the base oil over a broad temperature range. The stiffness of the backbone is designed in such a way that the large molecules „collapse“ at low temperatures by forming very small units, so that the lubricant remains adequately free-flowing. If the temperature rises, the long side chains push apart and the comb polymer wells up, which results in the desired thickening effect. Comb polymers have proven their outstanding properties as lubricant additives on a number of engine test stands. Compared to conventional PAMA additives, they show significantly better values for all key parameters. The shear stability of the molecules is many times higher, and the flow properties of the oil are optimized during cold start. The kinematic viscosity, measured at 40 degrees Celsius, is about one third lower. This means that the lubricant is easy to pump at relatively low operating temperatures, and the movement of the engine parts and gear wheels in the transmission slows only a little—an effect that has a direct and positive impact on fuel consumption. Lubricants must be precisely coordinated to operating conditions. Because thermal stress on the polymers is particularly high in the engine, comb polymers for engine oils contain fewer side chains than those for gear oils. Shear stability is the most 333 ResouRCe effICIenCY DesIGnInG WItH PoLYMeRs 11 Politicians demand economical vehicles with low CO 2 emissions Today, engine developers and car manufacturers not only focus their attention on the design and performance of their products but on fuel consumption and emissions. The pressure is coming primarily from the political arena: Over the next few years, the EU will be reducing the permissible fuel consumption of new vehicles in several stages. By 2015, manufacturers will have to reduce fuel consumption to the point that exhaust emissions are, on average, below 130 grams of CO 2 per kilometer. In 2010, the average CO 2 value of newly registered cars in Germany was 151 g/km. Car manufacturers and importers who do not comply with EU limits in the future will have to shoulder millions of euros in fines. Engineers use an array of methods to reduce fuel consumption: lower vehicle weight, improved aerodynamics, more efficient engines and drive trains. Most of these methods are technical in nature. But the more sophisticated the component, the more expensive and time-consuming it is to increase its efficiency and performance even more. In addition to technical optimization, selecting a high-performance lubricant can also reduce a vehicle‘s fuel consumption and emissions. Thanks to their modified chemical structure, the new additives from Evonik each show optimal viscosity over a broad temperature range, and guarantee highly efficient operation of engines and transmissions. Test-stand results have shown that additives based on comb polymers achieve fuel savings of about 1.5 percent. The benefit corresponds more or less to that obtained through high-efficiency wheel bearings or an electronic start-and-stop system for the engine. For engine developers and car manu facturers, then, this margin is a giant leap. elements35 Issue 2|2011
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Page 3 and 4: All set for the future If your car
Page 5 and 6: Fiscal year 2010: the best result s
Page 7 and 8: Plans for isophorone plants in Chin
Page 9: no MACHIne, PLAnt, engine would be
Page 13 and 14: Influence of the engine oil on gas
Page 15 and 16: Figure 1 Application areas of olefi
Page 17 and 18: Almost 5,000 km across Australia Pi
Page 19 and 20: Study verifies effectiveness of con
Page 21 and 22: PLAstIC oR GLAss? This is the quest
Page 23 and 24: The façade of the Photovoltaics In
Page 25 and 26: its partners, Sunovation has develo
Page 27 and 28: foR YeARs, It was impossible to ima
Page 29 and 30: 333 particularly since the situatio
Page 31 and 32: Robust, flexible, and fast drying:

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