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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