Polar Affinity vs Surface Tension

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I have a Bronco that sits all week and is only driven on weekends. I've often wondered how much oil is left on the critical surfaces after a week of sitting, and then a "dry" start-up. It's been stated here that synthetic oil has a "polar affinity" to the metal, and clings to the parts. My thinking is that because syn flows better, it will also drain better, and drain off the engine surfaces. Since dino molecules have longer chains than syns, does this mean that dino oil also has a higher surface tension than syn (given the same weights)? The higher surface tension would help it to cling to bearing surfaces rather than drain off. I'm thinking that the dino oil with higher surface tension would protect better upon start-up than would syn oil with (higher) polar affinity.......? Thanks for all your opinions. Don
 

MolaKule

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Firstly, higher surface tension does not equate to better oil clinging. In the Global View, the macroscopic world, what makes oil cling is the tackifiers in the oil. What makes the oil spread is an additive called a surfactant, such as Schaeffer's penetro additive in 132 and their motor oils. The surfactant actually "reduces" the surface tension so the additives can move into the the small metal valleys. One the microscopic scale, chemically polar components of the addtitives, such as sulfur and phosphorus, cause the addtive to bond to the metal. If the addtive is ZDDP or MoTDC, the polar attractive chemicals cause the zinc or moly to "bind" to the metal's surface. Esters, which are more chemically polar than Group I,II or III mineral oils and PAO's, also imbed themselves into the metal in a similar fashion to ZDDP and MoTDC. A well formulated synthetic that contains esters will have better metal adhesion protection than will a dino.
 
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Every thing I've seen or read leads me to believe that the AW/FM have every thing to do with startup protection,once the AW/FM additives are absorbed to the wear surface they only come back off while protecting against wear,by that time oil should be back on the surface to protect with a hydrodynamic film and replenish the add's back on the surface.
 

TheLoneRanger

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Thanks for the explanations, Molakule and RB. I'm getting a better understanding of oil "clinging". I notice in the VOAs that most oils have much more zinc than moly, in some cases as much as 10 times more. Why is this, and does it have anything to do with "dry" starts? Also, after these AW additives come off the bearing surface (on a dry start), are they then depleted, or do they go back into the solution to be used again? Thanks again.
 

MolaKule

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ZDDP and Moly are multifunctional (many functions) additive. that has been around since about 1952 and 1982, respectively. Moly reacts at higher pressures and temps than does ZDDP. Think of the two AW/FM additives as a two-step protection system. ZDDP reacts first at the lower pressuures and temps, and then Moly kicks in for the tougher jobs. Some of the AW additives do get sheared but are replenished by the ZDDP or Moly in solution.
 
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