November 20, 2013

How Particle Generation and Oil Oxidation are Like Thanksgiving
by Michael Holloway, CLS, OMA I
Business Development Manger

Before we talk Turkey, we have to set the table.  Often we can provide a visual to explain a concept.  Oil oxidation and equipment wear can be very costly. In order develop a corrective action we must first understand a root cause.  Contaminants are the leading source of system failure. While water is considered one of the most dangerous contaminants, particles can be just as bad.  Particles can come from many different sources and can bring about increased wear, viscosity changes and oil oxidation. The origin or particles include:

  • Implanted -  from manufacturing, handling, storage, equipment commissioning
  • Generated -  from wear debris, chemicals electrolysis (rust, corrosion), oxidation
  • Ingested – from breather issues, seal issues
  • Induced - from poor maintenance
  • Escaped - from filter

From these sources of particulate come fundamental types of wear that can produce particles from the following action:

  • Rubbing
  • Cutting
  • Rolling
  • Fatigue
  • Severe sliding
  • Combined Rolling and Sliding

Wear particles can occur in many different systems; any scenario where there is the potential for two surfaces to come in contact. Particles are typically the precursors to free radical development and oil oxidation (see earlier article on varnish development and testing). Particles from wear or external contamination offer an attractive surface for reactive compounds to migrate towards. The free radicals and reactive compounds that form when oil begins to breakdown have a charge.  This charge is attracted to metal surfaces. Microscopic (sub-micron up to 50-micron) contaminants such as wear metals, dirt, and debris always react faster than large chunks of solids. The small contaminants have a much greater total surface area than large chunks, so there will be many more chances for reactions to take place. In chemistry, it’s all about the opportunity for a reaction to occur. In order for a reaction to take place, molecules have to collide. There will be an increased number of successful collisions, so the reaction rate increases. Examining size and shape of particles, one could predict the opportunity for oil to oxidize. Only rubbing wear and early rolling fatigue mechanisms generate particles predominantly smaller than 15 microns.  These are particularly dangerous. The chemical nature of the contaminant can influence the development of deposits.  The contaminant can be a metal ion or particle or organic or inorganic particle or even a fiber from corrugated cardboard.  Different particulates provide a different reaction mechanisms with some having lower activation energies allowing a greater proportion of all the molecules to collide.  Solid contaminants or even metal surfaces can provide a surface on which the reactant molecules can temporarily stick in the correct orientation for an easy reaction to take place.  While a metal surface can make for an attractive site, the surface area and reaction opportunity is orders of magnitude less than particles.


Metal surfaces and contaminants can facilitate the oil oxidation process.  When two different molecules bump into each other, they might react to make new chemicals. How fast a chemical reaction occurs depends upon how frequently the molecules collide.  Metal particulate, inorganic or organic contaminants make a chemical reaction go faster by increasing the chance of the molecules to collide.  They set up a meeting place of sorts.  Metallic compounds may also act to catalyze a reaction.  This is done by the metal compounds producing what are called ions.  These ions are changed and provide a kick-start for a reaction to take place.  Often it doesn’t take many of these ions to facilitate the formation of deposits.  Consider the fact that when an oil oxidizes what is happening is that the oil molecule is breaking up.  At the ends that break there is a reactive side.  This reactive side is looking to link up with another reactive side to make it ‘comfortable’. When a molecule has found its comfort zone it is said that all the electrons are satisfied.  The molecule is now in a place where further reactions are not required. 

Think of these reactions like a Thanksgiving meal.  A full plate with Turkey and all the trimmings is very satisfying yet often to get the meal started there is an appetizer.  The appetizer gets the stomach ready for the big event.  The mind becomes engaged as well in anticipation.  All this occurs during the first half of a football game!  Once the dinner is finished and a slice of pumpkin pie is washed down with a cup of coffee, you are satisfied to the point where a nap is in order.  The reaction is complete.  Consider the metal ions the appetizer or catalyst, and a particle the dinner table. This is where all the reactive stuff meets.  All the great food (like reactive oil molecules), work in symphony to produce a huge mass found sleeping on the sofa during the fourth quarter. If you want the reaction to occur even faster, turn up the heat in the living room. The warmer the environment, the quicker reactions occur.

So when Uncle Phil is sawing logs on the La-Z-Boy, he is really no different than the sludge found at the bottom of an oil pan.  But don’t tell him that.     

 

 

 

 

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