Syn base oil viscosity

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Nov 15, 2002
Given how PAO and ester base oils are made, what actually determines the viscosity of the neat base oil? Does a thinner synthetic base oil have "smaller" molecules than a thicker one? Is that what's referred to as "molecular weight?"
However, when you are talking hydrocarbons, where the two main ingredients are Carbon and Hydrogen (with a little Oxygen in some of them), viscosity increases with molecular weight.
The relative molecular mass (sometimes called "Molecular Weight") is the ratio of the average mass per molecule of an element or compound TO 1/12 the mass of a carbon-12 atom. I.E., it is equal to the SUM of relative atomic masses of ALL atoms that comprise a molecule. Generally speaking, the higher molecular weight products of an oil show higher viscosities, but that is only half of the story since the molecular "structure" also affects the molecular weight. Take the VII molecules of polymthylacrylates. These are high molecular weight structures that cause the formulated oil to become more viscous as temperature rises because of uncoiling at higher temperatures. To further illustrate, here is a quote by Shubkin ( aresearcher in PAO's): "The ability of PAO's to outperform petroleum-based products of similar viscosity at both ends of temperature extremes becomes readily apparent if one compares the gas chromotography traces. The PAO product is essentially decene trimer with a small amount of tramer present. The fine structure of the timer peak is attributable to the presence of a variety of trimer isomers (same molecular weight, different structure). The HVI oil on the other hand, has a broad spectrum of different moleclar weight products. The oil contains low-molecular materials that adversely affect the volatility and flash point characterists. It also contains high-molecular weight components that increase the low-temperature viscosity and linear paraffins that increase pour point...Compare a 6 cSt PAO with the same VHVI KV oil, the PAO has a well defined peak chemical composition consisting of decene trimer, tetramer, pentamer, and a small amount of hexamer. The VHVI fluid, like the HVI fluid in the previous example, contains a wide range of components that degrade performance at both ends of the temperature scale." Ester may contain high or low molecular weight components as well, and may have KV's of 2 cST to over 100 cSt at 100 C. In esters, the structures are more important than molecular weight (except of course in determining final base viscosity). When developing esters, particular attention is paid to ester structure. Short linear chains show better oxidative stability, whereas increasing the acid chain length of the molecule improves (decreases) the coefficient of friction. This why Di- and Pentaerithyritol esters (PE's) are better (more stable) than Trimethylpropane (TMP) esters, and why TMP is better than Neopentylglycol (NPG) esters. The PE's have short chains of linear acid chains with make the ester more oxidatively stable and exhibit lower coefficients of friction. For esters, if the ester is made from linear branched acids, the ester has higher flash points; increasing the molecular weight (making the ester molecule more compact) will aslo increase the flash point. Representative esters: Phthalates - Used mainly in air compressors; short fat molecule results in VI v.s pour point tradeoff. Trimellitates - Short, branched esters that have high flash points and low volatilities and good thermal stability. Used when you need to leave a soft film behind. Dimerates - Made from the acid of tallow oils and an alcohol (is three-branched); Has excellent lubricity and thermal and oxidative stability; used mainly in 2-stroke oils. Polyols - SPE's, PE's, TMP's, and NPG's. Three or more shortchain but fat molecules. Polyols are generally more oxidative and thermally stable by 50 C over diesters and 150 C over petroleum oils. Thes esters have lower coefficients of friction than either diesters or PAO's. By adding a polyol ester at least 5-10% to a PAO or mineral oil reduces base oil friction remarkedly. So esters are natural Friction Modifiers. Advanced esters can also BE USED AS VII improvers. Unlike long-chain polymers (such as methacrylates), complex polyols do NOT EXHIBIT the temporary loss of viscosity under forces exterted by shear, as in gears. Because complex esters are shorter chain molecules, they tend not to shear into smaller molecules. Adding amine "backbones" to ester molecules allows them to have better antioxidant capabilities. [ February 23, 2003, 08:17 PM: Message edited by: MolaKule ]
Originally posted by MolaKule: The relative molecular mass (sometimes called "Molecular Weight")...
MK, Thanks for taking the time to answer. I'm sure there are some here that understood your answer, unfortunately, I'm not one of them. [Smile] In laymen's terms, can you just give me some idea of what the difference would be between a 2 cst PAO and a 6 cst PAO where the only apparent difference is the viscosity of the neat oil. What makes the thicker PAO "thicker?"
Thanks for taking the time to write the post. Although I don't understand some of it-I did learn a few things. Couple of questions: You discussed polyols under "representative esters" and then you said they were more stable then diesters. I didn't see diesters represented under "representative esters". Where do they fit in-maybe I missed them. Also you discussed complex esters and complex polyols. Is the Polyol in Redline a "complex Polyol" ??
Let me describe the GC (Gas Chromatographg) traces and I think this will illustrate the point for different viscosity PAO's. When two or more compounds have the same formula, but different structure (same collection of atoms but arranged in different ways), they are called "isomers." In the 4.0 cSt trace one sees a narrow curve with a single peak. In the GC of the 6.0 cSt trace, one sees a curve of a series of three closlely spaced, but very narrow peaks. Each individual peak corresponds to the number of isomers, or isomer types. The greater the degree of isomer, the higher the viscosity of the fluid. For the 4.0 cSt fluid, you have mainly isomers, dimeters, and trimeters; for the 6.0 cSt fluid, you have trimers, tetramers, pentamers, and small amount of hexamers. In other words, the more repeating hydrocarbon chains you have, and the higher the CxHx numbers, the thicker the fluid. In a trimeter isomer, the there is C3H6 molecules, for a pentamer, there are C5H10 molecules, etc. [ February 23, 2003, 09:16 PM: Message edited by: MolaKule ]
Originally posted by MolaKule: Let me describe the GC (Gas Chromatographg) traces and I think this will illustrate the point for different viscosity PAO's.
Thanks. I think I'm getting a handle on it now. [Smile] Something you said in your first post got me to thinking. When it comes to hydrocrakced and isomerized Group III oils, no matter how high their purity levels, they still will have all sorts of differnet size hydrogen and carbon molecules, right? Hence, they could never duplicate certain attributes of PAO where the molecular uniformity is the key. Is that correct?
Al, The diester is an ester, just another class of esters. Polyols being the other class, so I should have included di-esters, but was mainly focusing on the three main types of polyol esters and their advantages. Redline uses complex polyol esters. See a similar discussion in this thread which classifies esters a bit more succinctly.;f=1;t=001406#000002 G-MAN, You are correct. If you examine a GC result of any non-synthesized hydrocarbon, and you will see a large curve similar to a bell curve (Gaussian distribution) with many small spikes in the curve representing hydrocarbon molecules of all different sizes and molecular configurations. Sorry I couldn't descibe it better; diagrams and charts speak volumes, as they say. [ February 24, 2003, 12:37 AM: Message edited by: MolaKule ]
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