POLYMERS IN LUBRICANTS

The multi-grade oil in your car's engine is not just the gooey remains of some long-dead dinosaur sucked from the sands of Saudi Arabia. It also contains a polymer additive, usually about 1 percent in concentration, that modifies the viscosity-temperature relationship of the base oil. This esoteric-sounding bit of engineering is critical to your car's health. As those who have ever heated maple syrup for their pancakes know, the hotter a viscous liquid is, the runnier it gets. Unfortunately, what your car needs is an oil whose viscosity is essentially constant—low enough to be pumped through the engine, yet high enough to lubricate and protect the moving parts—no matter what the temperature is. But an unmodified light oil that is runny enough to pump when the engine is cold will be much too thin to lubricate it once it warms up. (Typical engine operating temperatures hover around 150°C.) Similarly, a heavy oil that lubricates well at 150°C is simply too viscous, like molasses in January, to work in a cold engine.

Fortunately, polymers have come to the rescue. Even low concentrations of a polymer in a solvent—including oil—can increase its viscosity quite a bit. It is possible to make a solution whose viscosity does not change very much with temperature. The mechanism for this ability of polymer additives to provide us with multi-grade motor oils is complex; in fact, quite a few mechanisms may be operating at the same time. Some of these polymers can simultaneously be used as dispersants for water and the sulfuric acid that gets into the motor oil from burning sulfur-containing fuels. These are usually graft copolymers of an oil-soluble polymer with a water-soluble polymer.

These principles say nothing about how much polymer to use, or how long its molecules should be, or what they should be made of. In practice, the chemically active environment of a hot engine, which is sufficient to break down many polymers, and the oil solubility and cost of manufacturing those polymers that can withstand a hot engine narrow the choices considerably, but there are more subtle considerations. The shearing force felt by a polymer molecule trapped between a static engine block and a plunging piston may be enough to tear the molecule apart. As the polymer gets longer, its susceptibility to shear increases, putting an upper limit on its size. However, smaller polymers have less thickening power. It takes more of a smaller polymer to produce the same increase in viscosity, and so some balance must be struck between the polymer's effect on viscosity and its length.

The polymer is not the only additive, either. Other chemicals, such as dispersants, antioxidants, and wax crystal modifiers, are also needed. These additives, unfortunately, interact with the polymer to diminish its effectiveness in cold engines. For this reason, and because it is easier and cheaper to add one thing to an oil instead of several things, there is considerable interest in creating multifunctional polymers that could take over the roles of the other additives, or at least not interact with them. There is plenty of exploration left in the oil business for creative polymer chemists.

aluminum in vehicle structures. Polymer composites are 5 to 7 times less dense than steel and 2 to 3 times less dense than aluminum. The advantages of weight reduction will continue to be most critical for automobiles and aircraft. Automobile weight is substantially in the vehicle itself, while for trucks and freight trains the weight is largely in the payload. Impressive fuel-saving reductions in the weight of automobiles over the last two decades are attributable to the substitution



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