Source: BACTERIA – Bing images
June TLT Sounding Board Question 1: “Do you think that biolubricants are more sensitive to microbiological attacks?”
In Talmudic style, my response to that question is another question: is biolubricant susceptibility to microbial attack – i.e., biodeterioration – a matter of conjecture?! Most of the respondents apparently thought so. However, one responded: “Depends on the composition and overall resistance to microbial attack. I would imagine testing on microbial resistance would be part of the testing for new products.” This individual chose not to take the bait. They recognized that “sensitivity to microbial attack” depended on the finished stock’s chemistry rather than its source.
Having it both ways?
As ASTM D8324 Standard Guide for Selection of Environmentally Acceptable Lubricants for the U.S. Environmental Protection Agency (EPA) Vessel General Permit points out, the concept of an environmentally acceptable lubricant (EAL) aligns with Humpty Dumpty’s comment: “When I use a word,’ Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’” Figure 1’s exchange between Humpty Dumpty and Alice is relevant for several reasons.
1. There are numerous EAL labels and the criteria for each label is unique. Germany’s Blue Angel, EU’s Ecolabel US EPA’s EAL, respective criteria are similar but different criteria. Thus, the concept of what defines an environmentally friendly lubricant is a matter of debate.
2. Alice’s response is a question all too commonly asked by marketers. One common trope among companies selling water-miscible metalworking fluids (MWF) is the claim that their product is “biocide-free” when it most likely contains an unregistered component that’s more toxic than most registered biocides (see What’s New, February 2022).
Fig 1. Humpty Dumpty’s perspective on definitions.
3. Humpty Dumpty’s retort is again relevant to the extent that folks can use meaningless generalizations to support any point of view. The fact is that biolubricant biodegradability can and has been tested.
Lubricant marketers want to be able to claim that their products are environmentally acceptable, but are considerably less enthusiastic about also claiming that readily biodegradable products are – well – also readily susceptible to biodeterioration.
All of the environmental label issuers require biodegradability testing. Typically testing is performed in accordance with Organisation for Economic Co-operation and Development (OECD) Test 301 OECD Guideline for Testing of Chemicals – Ready Biodegradability. OECD 301 has a number of variants – 301A through 301F. As illustrated in Figure 2, the first step is to determine a product’s partition coefficient (OECD 117). If the product’s n-octanol/water partition coefficient – POW (POW = ; Log POW = KOW) – is in the ≤3 KOW ≤ 7 range, then the substance is potentially bioaccumulative. Bioaccumulative substances are those that can be captured irreversibly in the tissues of various organisms. When KOW is in the 3 to 7 range, the next step is to test for biodegradability. Lubricants are biodegradable if > 95 % (w/w) of the formulation is biodegradable (>70 % of each molecular species is mineralized within 28 days). If >5 % of the formulation is not biodegradable, the product is then tested in accordance with OECD 305 Bioaccumulation in Fish. The key points here are:
1. Biobased feed stocks and lubricants can readily be tested for their partition coefficient, and
2. If KOW is in the 3 to 7 range, the lubricant’s water accommodated fraction can be further tested for biodegradability.
There is no need to speculate about biobased lubricant biodegradability (i.e., susceptibility to microbiological attack). The OECD test methods are standardized, making it possible to compare how readily any finished lubricant or lubricant stock might be.
Fig 2. A very simplified flow chart for assessing lubricant or lubricant base stock biodegradability and bioaccumulative properties.
Keep in mind that if there is no water present, then neither lubricants nor base stocks – regardless of their chemistry – will be susceptible to microbiological attack. Similarly, if none of the lubricant or base stock partitions into the aqueous phase when water is present, it is not going to be susceptible to microbiological attack.
Here’s the having it both ways conundrum: by definition, the more biodegradable a substance by OECD 301, the more susceptible it is to microbiological attack. Ecological good news is necessarily operational bad news unless you keep water out of the lubricating system!
Are all biobased stocks equal?
Fatty acid methyl ester (FAME) is used to blend biodiesel fuels. Consequently, FAME biodegradation susceptibility has been studied quite extensively. It is not unreasonable to use FAME biodegradation knowledge as a starting point to speculate about biolubricant basestock resistance to microbial attack.
Generally speaking, biodegradability inversely related to oxidative stability. The primary factors affecting both oxidative stability and bioresistance are carbon chain length and saturation. FAMEs with shorter chain length (i.e., <C 15) tend to be more biologically and oxidatively stable than those with chain lengths > C15. Also, saturated molecules (i.e., those with no carbon-carbon double bonds – C=C) are more stable than mono- or polyunsaturated carbon chains (Figure 3). Note, for example that coconut FAME (C12-C14; >85 % saturated) is quite stable. It is even used as a topical antimicrobial agent. Conversely soy and canola FAME (are composed of primarily C16-C18 and C24-C26 mono- and polyunsaturated acids – soy is ~14 % saturated and ~canola is 5 % saturated). Both are readily biodegradable.
Fig 3. Relative biodegradability of selected biobased oils and fatty acid methyl esters.
Biobased lubricants and lubricant stock susceptibility to biodegradation should not be assessed by polling. As my friend and STLE Metalworking Fluid Education Committee colleague John Burke frequently observes, “without data, you are just another person with an opinion.” Everybody has an opinion, but objective data are relatively easy to develop. The question should have been the basis of a research effort rather than an opinion poll.
What do you think? As always, please share your comments and questions with me at firstname.lastname@example.org.