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Biological Deterioration of Water-Miscible Metalworking Fluids

July 01, 2012
Dr. Frederick Passman
Online Only Articles

 

To paraphrase W. E. Deming – the famous guru of statistical process control – you can’t manage what you don’t understand.  In order to manage water-miscible metalworking fluids (MWF) effectively, a fundamental understanding of some MWF microbiology basics is invaluable.  In a companion article, it was noted that:

“Microbial metabolism can cause irreversible chemical changes to the fluid itself, impede the flow of fluid through the system, damage surface finish of the worked parts and induce health problems among the workers.  The ultimate result of such uncontrolled growth is loss of productivity and increased costs.”

This article will shed some light on what microbes are and how they cause the types of damage listed above.

Microbes are organisms that are too small to be seen with the naked eye.  Although microbiologists estimate that there are billions of different types of microbes, in MWF management we are concerned with bacteria and fungi.  Bacteria comprise the most ancient of the living kingdoms and despite their tremendous diversity, share the common feature of not having distinct internal structures (organelles). Fungal cells have cell structures that are very similar to those of plant and animal cells.  In fact, genetically, the fungi are much more closely related to humans than they are to bacteria.  This is important in terms of microbicide selection – a topic for another article.

Microbes use MWF as food.  Like all organisms, they need carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorous (CHONS + P) to grow.  The base fluids and functional additives used to formulate MWF are, of course, primarily CHONS.  Phosphorous is a key component of commonly used lubricity additives.  MWF components vary in the easy with which they are digested by microbes.  This phenomenon is reflected in the selective depletion of smaller, less complex molecules; typically functional additives.  Consequently, MWF formulation performance can suffer long before there’s obvious evidence of fluid failure. 

The process of using MWF molecules as food is called metabolism.  Just as with human metabolism, after food is ingested and metabolized, waste is generated.  Microbial waste metabolites include a cornucopia of one to 6-carbon (C1 to C6) weak organic acids and other volatile organic chemicals (Microbial VOC, or MVOC) with nasty odors.  The weak organic acids degrade MWF by creating a demand on pH buffers and causing the pH to drop.  This, in turn, reduces emulsifiable-oil and semisynthetic emulsion stability.  Within biological masses (biofilms) weak organic acids react with salts to form strong, corrosive inorganic acids (hydrochloric, nitric, nitrous and sulfuric).  

In addition to waste products, microbes also produce metabolites that help them to survive.   Two, in particular, have a significant negative effect on MWF performance.  Biosurfactants are metabolites that act as dispersants.  The microbes depend on these surfactants to gain direct access to organic molecules as food.  However biosurfactants also change micelle size and distribution; often adversely affecting surface finishes. These molecules help to disperse tramp and way oils into non-emulsion synthetic MWF that would otherwise reject these oils.   Extracellular polymeric substance (EPS) is a combination of polysaccharides and polypeptide molecules that form the structure of biofilm communities.  Microbes embedded within biofilm EPS are protected from microbicide exposure and other adverse environmental conditions that might otherwise prevent their growth.  Moreover, EPS acts like glue; binding metal fines (swarf) and other inanimate MWF debris and thereby causing flow restriction and premature filter plugging. 

Direct metabolism of MWF components, production of acids, biosurfactants and EPS add up to a family of MWF deteriogenic activities that can have a substantial negative impact on annual fluid maintenance costs. Increased MWF concentrate consumption, water treatment costs, downtime for corrective maintenance and part rejection rates combine with decreased MWF performance and tool life conspire to increase operational control.  Timely, effective condition monitoring and contamination control efforts can reduced annual operational costs by 5 to 20%.

Definitions

  • Bacteria are any of a class of single-cell microorganisms characterized by the absence of defined intracellular membranes that define all higher life forms. 
  • Biofilm is a mass of cells adhering to each other and attached to a surface. It’s generally slimy.   (Free-floating individual microbial cells are called “planktonic.”)  Dental plaque is a type of biofilm. 
  • Biosurfactants are biologically produced molecules with charged (polar) heads and uncharged (non-polar) tails that make water and oil miscible.
  • Fungi are neither animals nor plants and are classified in a kingdom of their own. Fungi can be single-cell microscopic organisms (yeasts) or multi-cellular, macroscopic (visible to the naked eye) molds, mushrooms, and puffballs.
  • Laminar fluid flow is smooth, non-turbulent.
  • Polypeptides are chains of amino acids; typically having 50 or fewer amino acids linked together.   Longer amino acid chains are called proteins.
  • Polysaccharides are chains of sugar molecules.  The number of individual sugar molecules in a polysaccharide can range from as few as 4 sugar to >1,000. Polysaccharide molecules can be simple chains or complex networks of sugar molecules.  
  • Swarf is the very fine metal particles removed during a metalworking operation such as grinding.

Other articles from this issue:

Editor's Note: You can get more information on this subject by attending the Metalworking Fluid Management Certificate Course, or by attending the 2013 Annual Meeting Education Course, Metalworking 115: Basic Metal Removal Fluids.

 
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