What attaches to the spare "bonds" when an oil shears ?

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It's something that's been bugging me for ages, when you hear of oils and additives hsearing, is when a long hydrocarbon molecule is cut in half, there's at least two loose ends that don't have a hydrogen or whatever taking up it's bond. What does it pick up instead ? Is this the fuinction of the additives to put something "safe" in it's place ? Is this the site that oxidises ?
 
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Those bonds that don't regenerate--organics oxidized under heat--can become acids, alcohols, and ketones, among others. Not good stuff in the crankcase.
 
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Molecule, are there any tests that can comment on the "shear stability" from mechanical shearing for different PAO oil grades? Is shear stability related to the size of the PAO molecules? ie. is a 50 weight MORE shear stable than a 40 or 30 etc.? That is, is a synthetic oil's shear stability determined both by the viscosity spread and the size of the PAO molecules? Secondly, does an oil thin out when the V.I's are sheared? Or does it do both....thin out at high temp. and get progressively THICKER at low temps?
 
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quote:
Originally posted by TheLoneRanger: I wonder at what point the peroxides will turn into sludge particles big enough to get trapped in a filter?
Wouldn't the sludge form on the metal parts inside the engine rather than remaining as "floating particles" in the oil?
 

MolaKule

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"are there any tests that can comment on the "shear stability" from mechanical shearing for different PAO oil grades? Is shear stability related to the size of the PAO molecules? ie. is a 50 weight MORE shear stable than a 40 or 30 etc.? That is, is a synthetic oil's shear stability determined both by the viscosity spread and the size of the PAO molecules? Secondly, does an oil thin out when the V.I's are sheared? Or does it do both....thin out at high temp. and get progressively THICKER at low temps? " As a rule of thumb, the greater the viscosity for PAO, EOP's or esters, is that the higher the base viscosity, the greater the VI. The shear stability is related to the composition and geometry of the synthetic molecule. Initially, the oil thins when the VII's become sheared, resulting in a thinner oil. Some of the VII molecules do reattach. Later, as more and more VII molecules shear, those large molecules contribute to a viscosity increase. [ January 12, 2003, 09:58 PM: Message edited by: MolaKule ]
 

MolaKule

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Shannow, Good question! First a little background and then on to your question. There are four basic mechanisms for destroying molecules, which are specific arrangements of atoms; in this case mostly atoms of hydrogen and carbon in specific geometric patterns. 1. Heat 2. Mechanical shearing 3. Oxidation 4. Radiation (which we will disregard here) Raising the temperature of a molecule causes it to vibrate and rotate and if enough energy of the correct frequency is present, it will blow it apart. Mechanical shearing places shear-type stresses on the molecule and pressure in one or more directions can cause the molecule to separate at certain bonding sites. Synthetic oil molecules are stronger than petroleum molecules and thus are more "shear stable." Oxidation occurs when an oxygen molecule replaces one or more hydrogen or carbon bonding sites (usually due to increased temperatures acting as a catalyst), resulting in peroxides, causing sludge. Petroleum oils are less stable than are synthetics in this case. In most cases, and due to the geometry of the smaller oil hydrocarbon molecules, mechanical shearing is reversible; that is, once the shear stress (pressure) is relieved, there is enough coulombic (electrostatic energy) to help the ends find each other. There will always be a few molcules that do not find each other and remain sheared. Reagrding additives, the very large Viscosity Index Improving (VII) additive molecules are more prone to shearing than are oil molecules, mainly because of their molecular arrangement and size. VII molecules are "coiled" when cold and thus present very little resistance to movement (flow). When heated up, the VII molecules "un-coil" and are more resistant to flow, which is why VII's cause an oil to thicken when hot and, and thin when cold. VII molecules shear more easily under mechanical shearing forces (because they are larger) and are more prone to "permanent" shear, leaving the large unattached methacrylate molecules to thicken the oil over time. If heated and oxidized, these remnants also contribute to sludge. The more shear stable oils with the least amount of VII's are preferred for the longer drain intervals. [ January 12, 2003, 12:59 PM: Message edited by: MolaKule ]
 
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