Polyol Esters for Lubrication

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Molakule has asked me if I would contribute a question for this forum related to my field of esters, so here goes.... A polyol ester is made by reacting a polyol alcohol with one or more acids. Technically the term "polyol" could refer to any alcohol with multiple alcohol groups, (poly = multiple and ol = alcohol), however, in the realm of lubricant basestocks the term "polyol" generally refers to a specific subset of polyol alcohols. What is the full name of this subset, what does each word mean, and why are they superior to other polyols for high temperature lubrication? This question is open to all, but I ask professionals to wait a couple of days to allow others to research and provide answers. Tom NJ
 

JAG

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That's a tough one. I did some research and have answers but I'm not sure they are correct. Here goes: Subset: trimethylol...CH3OH Meanings: tri: 3; methyl: alkyl radical; ol: alcohol or phenol. Reason for high temp. suitability: they allow a sterically hindered ester, protecting the ester from radical attack. I based my reason largely on the following quote from the following patent: https://patents.google.com/patent/US3282971 "In view of the symmetrical nature of the trimethylol type of alcohols, each of the ester groups are spaced the same distance from the center carbon atom. Because each of the three ester groups are quite close to the center of the tetrahedron defined by the structure of the trimethylolalkane, it is believed that any ester linkage which is directly sterically hindered by the no alpha hydrogen carboxylic acid, will in itself tend to protect the other two adjacent ester linkages by steric hinderance."
 

Tom NJ

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Thank you for responding JAG! After 10 days and over 300 views without an attempt to answer the questions I was beginning to think the question was either too difficult, too specific, or just not of interest (or all three!) Your answer to the subset of polyol alcohols used in synthetic lubricants was well thought out but too specific. The subset of polyol alcohols I was looking for is "neopently polyols" of which trimethylol (propane) is a type, as are pentaerythritols and neopentyl glycols. So the remaining questions are what does "neo" and "pentyl" mean with respect to the chemical structure of these alcohols, and why does this structure offer better performance at high temperatures.
 

JAG

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Thanks Tom. After more research, I say the neo prefix means an alkane that has two methyl groups attached to the second to last carbon in the continuous chain. Pentyl means a five-carbon alkyl functional group with molecular structure -CH2CH2CH2CH2CH3. The better high temperature performance is due to the absence of a labile (easily changed) hydrogen on the β-carbon (central carbon).
 

MolaKule

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I'll attempt to offer an explanation in Layman's terms: Neo normally means "new," but in the context of organic chemistry it usually means a molecular structure with a central carbon atom surrounded by 4 carbon atoms. Penta refers to "5" or in this context, a total of 5 carbon atoms in the totality of the basic structure. So a pentyl is essentially a five-carbon alkyl functional group (substituent) with a general chemical formula of C5Hxx. Neopentyl polyols are thermally stable because of 1) the absence of a labile hydrogen (usually a beta hydrogen) on the polyol component and 2), the steric protection afforded the molecules by virtue of the shielding of the ester groups in the neopentyl configuration. Now "labile" in chemistry refers to or describes a relatively unstable and transient chemical species. So the "absence" of a labile hydrogen atom allows for a more stable molecule. E.g., the absence of a third party, boisterous troublemaker makes for a better dialogue. Steric shielding occurs when a molecular group is so large that it protects nearby reactive groups from contact with things that would normally react with them. Similar to the shielding effect of a large group of security personnel from say an attacker.
 

Tom NJ

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Bingo! JAG nailed it and Mola explained it beautifully. "Pentyl" referring to a five carbon group arranged in a "neo" structure (a central carbon atom surrounded by 4 carbon atoms) thus eliminating the weak hydrogen on the beta (center) carbon, and "polyol" indicating more than one alcohol group (-OH) attached to some or all of the four outer carbons (CH2OH). These alcohol groups are then "esterified" by reacting with fatty acids, usually 5-10 carbons long, to make the final POE. This lack of a "beta hydrogen" substantially increases the thermal stability of polyol esters compared to diesters, making them suitable for temperatures of up to 100+°F over diesters. The polarity of esters also allows them to dissolve much more anti-oxidants compared to PAOS which further improves their high temperature capability. Combined with excellent low temperature properties (no wax), polyol esters are the only base oil chemistry (93-96%) used in jet engine oils. Thanks JAG for spending the time to research what was apparently a fairly difficult (or boring) question, and MolaKule for the detailed explanation.
 

JAG

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Thank you Tom and Molakule! It was certainly a learning experience for me. While struggling to find the answers, it reminded me of why I changed my college major from chemistry when I took organic chemistry. It's not my strong suit!
 

MolaKule

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Thank you Tom for submitting the question and looking forward to more. cool I hesitated to make a post until the last minute for fear that other's might not post an answer but was glad to see JAG jump on it. thumbsup
 
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over my head but interesting + the comment on the only base used for jet engines satisfied some curiosity as i had seen posted that esters were the only ones to survive jet engine heat + are used for years before changing!!!! surely $$$$$$
 

Tom NJ

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Originally Posted by benjy
over my head but interesting + the comment on the only base used for jet engines satisfied some curiosity as i had seen posted that esters were the only ones to survive jet engine heat + are used for years before changing!!!! surely $$$$$$
Yes all commercial and military jets fly with POE based oils in the engines and APUs. Hydrocarbon oils will cause coking at the high temperatures encountered in the engines while POEs will withstand the high temperatures and flow at -65°F.
 
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