"Synthetics" that contain no PAO or Esters

[Tom NJ]

Hi Scott,

Polyol esters of the type Redline use are much more oxidatively stable than PAOs and Group IIIs and therefore resist thickening and coking better. Whether your engine gets hot enough to see this difference I don't know.

While these base oils will handle the heat better, the additive system also plays a large role. I don't know what Redline uses in their additive package, but their reputation is very good and I believe they formulate for maximum performance.

Tom NJ

 

[labman]

As a chemist, I would expect the PAO's to be the least oxidatively stable of all the basestocks. They have tertiary hydrogens more likely to run off with stray oxygens, just as isooctane oxidizes easily. If you carefully refine out all the unsaturated molecules, the straight chain mineral oils are one of the most stable materials there is, in Greek para afins.

Polyol esters could also be highly stable if you avoid glycerine and stick to neopental glycol, tri menthylol propane, or even pentaerythritol. Branched acids such as 2 ethyl hexanoic also have the less stable tertiary hydrogens, but not near as many as the PAO's.

The esters used as oils have enough oxygen in them to give flatter viscosity to temperature curves, better additive solubility, more seal swelling, but I wouldn't expect them to be hygroscopic.

I wish I could remember more of what I was doing 40 years ago and how poly methacrylic esters made better paint than poly acrylics. The main ingredient in most ''acrylics'' is methyl methacrylate. I think once again, it was the tertiary hydrogens the acrylics had just like PAO's. At the time, I was working for Rinshed Mason. I think United Technologies still sells automotive paint under that name.

 

[Tom NJ]

Hi Labman,

You are correct that tertiary hydrogens are the least stable, but there are actually fewer of them in PAOs than mineral oils. Group II and III base oils are full of multi-branched hydrocarbons which have a tertiary hydrogen opposite each branch. The trimer of 1-decene (4 cSt PAO) only has two tertiary hydrogens. Straight chain hydrocarbons are indeed the most stable but these are refined out as waxes. Some base oil oxidation discussion is here:

http://www.oilanalysis.com/article_detail.asp?articleid=475&relatedbookgroup=OilAnalysis

Neopentyl polyol esters (POEs) are very oxidatively stable, much better than PAOs. POEs made from straight chain acids have no tertiary hydrogens and no beta hydrogens on the alcohol. In addition, the secondary hydrogens next to the carbonyl are about six times more stable than the secondaries down the chain.

In fact, certain branched acid POEs are much more stable than the straight chain acid types. For example, 3,5,5, trimethylhexanoic acid has four primary carbons with three primary hydrogens each, thus substantially reducing the number of secondary hydrogens. These primaries are about 17 times more stable than the secondaries they replace, which overcomes the single tertiary they contain. So the branched C9 acid above has 12 primary hydrogens, 2 regular secondaries, 2 stable secondaries, and 1 tertiary, while a linear C9 acid has 3 primaries and 12 regular secondaries, and 2 stable secondaries. The triple branched acid may also benefit from steric hindrance protecting its regular secondaries. This improved oxidative stability and reduced coking tendencies from certain multi-branched acids is the subject of two of my patents (http://www.freepatentsonline.com/5503761.html and http://www.boliven.com/patent/US20030104956)

2-Ethylhexanoic acid has one branch which gives it only three more primary hydrogens compared to the linear octanoic, but one tertiary. While this does not seem much of an improvement, POEs made from 2-EH acid do test more oxidatively stable. They are also much more hydrolytically stable because the branch is on the alpha carbon, which sterically hinders the ester linkage from attack by water.

Tom NJ

 

[JAG]

Thank you guys for the excellent education!

 

[Gary Allan]

For me it's like watching an "exciting" foreign film. You like it when they talk dirty, even if you can't understand a word that they said.

JAG? You got a consumer version? (j/k

)

 

[engineerscott]

Thanks for the info Tom,

Regarding air cooled VW oil temps, generally you hear people who measure their oil temps reporting in the range of 180 to 220 degrees F. I have seen some engines that ran in the 240 to 250 degree F range, but these engines had various combinations of problems; cooling tin modified or removed, improper valve clearance, overly lean air fuel ratios, incorrect timing, etc.. I am not set up yet to measure oil temperature, but when I am I hope to see temps below ~200 F. I think this is somewhat higher than most water cooled motors so the extra oxidative stability of polyol esters makes me feel a bit better about the oil system. Besides, as I understand it polyol esters have inherent cleaning properties and since the oil system is also part of the cooling system keeping it clean gives long term benefits.

Thanks once again,

Scott

 

[labman]

I yield to tom since it looks like he has experience directly in the field. Much of my experience is with resins.

Primary, secondary and tertiary hydrogens relate to how many hydrogens are on a carbon. Hydrocarbons have chains of CH2 groups. -CH2-CH2-CH2-CH2- Those hydrogens are secondary because the carbons are bonded to 2 other carbons. Add a third hydrogen to the end CH2, and you have 3 primary hydrogens because the carbon is only bonded to one other carbon. Replace one of the hydrogens in a CH2 group with a radical such as CH3 or Ch3-CH2 or longer group and the one hydrogen left is a tertiary hydrogen. It is less stable because its electrons can sort of get lost in the crowd. Replace both of them, and you have a stable branched hydrocarbon. Thus branched hydrocarbons may be either more or less stable than straight chain ones.

The polyols I mentioned all share the structure of one carbon bonded to 4 others. NPG has a hydrogen on 2 of the outer carbons replaced with an OH group. TMP, 3 of them and PE, all 4 have an OH group. I think PE is the cheapest and used extensively in paint resins. Combined with an acid with 2 acid groups on it, you can form long chains.

Steric hinderance? If an acid has a branch near the acid group, carboxyl, it gets in the way of say a molecule of water breaking down the ester linkage. Esters are formed by reacting an OH group with an acid to form an ester and water. It is reversible. Water can break down an ester linkage.

Esters can also react with other OH or acid containing compounds. I shudder to think of what might happen if you threw a polyol ester and a 2 or more acid based ester together in a crankcase. Acids with 2 acid groups are called dibasic, 3 tribasic. Succinates, phthalates, trimelitics are common ones. After my resin chemist days, I worked with vinyl where we added the 2 ethyl hexanol esters of them to make it flexible to use as electrical insulation.

 

[engineerscott]

In doing some further research it looks like I might have been a little optimistic in the max oil temperatures I expect to see in my particular engine. Given that I live in the Southeastern US and 100ish F days in the summer are not uncommon, running 65+mph speeds in that ambient it looks like a well tuned engine with all of the cooling tin in place (needed to properly separate cool/hot air and duct it accordingly), I could probably expect to see max oil temps around 210 degrees F, give or take a couple of degrees. Not well versed in water cooled engine oil temperatures, but it seems like I've heard 180F to 190F mentioned as common oil operating temperatures for water pumpers. So, my oil temps might be 20 to 30 degrees F hotter than your average water pumper. Maybe not a problem for a good Grp III or a PAO (not sure about that), but I kind of like the extra margin of the Polyol Ester based oils.

Labman - I'm intrigued by your knowledge of polymer resins. By training and profession I'm an electrical engineer in real life. I co-own a small firm that designs and manufactures electronic assemblies, some of which are housed in various plastic enclosures which are in unprotected outdoor environments. We typically use various polycarbonate and polycarbonate blends for these boxes as they have to be strong and UV resistant. The polycarbs are some of the more expensive resins (though they do have excellent properties). I know this is completely OT but if you have any ideas about cheaper materials that we might investigate I'd love to hear them. PM me if you do. Thanks,

Scott

 

[Tom NJ]

Originally Posted By: labman
I shudder to think of what might happen if you threw a polyol ester and a 2 or more acid based ester together in a crankcase.

Hi Labman,

While saturated polyol esters and diesters can in theory transesterify into larger complex molecules, this doesn't happen in the mild conditions of automobile engines. It is a concern in jet engines where sump temperatures are over 400F, but even here there was one approved formulation in the past that did contain a small amount of diester. Otherwise, virtually all jet engine oils are based entirely on polyol esters.

Now if you add a dibasic acid into the reaction mix when reacting a polyol ester (i.e. polyol alcohol/dibasic acid/monobasic acid), then the dibasic acid will easily enter into the reaction and form a complex ester, but transesterification of completed esters is much more difficult. Complexing POEs with dibasic acids during esterification is commonly done to make higher viscosity esters.

Tom NJ