How Do Viscosity Index Improvers Work?

[MolaKule]

Originally Posted By: kschachn
Originally Posted By: Red91
The Chevron Oronite link puts it in basic, understandable terms. If I'm interpreting correctly, the VII allows more flow at lower temperatures because it shrinks the oil's molecules, no?

No. Nothing is happening with the oil. The oil stays the same and it is the polymer molecules that are changing size and volume which influences the viscosity.

Quote:
When the oil is heated, the polymer molecule expands. A larger coil volume impedes the free movement of the oil more than a small coil, which causes the observed viscosity. The thickening impact on the oil’s viscosity at high temperatures is therefore greater than the impact at low temperatures, leading to the “Viscosity Index Improver” effect.

 

[SonofJoe]

Viscosity Index Improvers (VIIs) are strange beasts...

Above 0°C, they perform like 'regular' hydrocarbons. VIIs have very high Molecular Weights (think MWs of 40,000 to 80,000) and are extremely viscous; far more so than the heaviest base oil you could ever imagine. If you add VII to a base oil, it significantly increases the KV40 & KV100 on the resultant mix. Above 0°C, you can expect VIIs to 'thin' as you increase temperature (again, just like any other hydrocarbon).

However, it's what happens at low temperatures that marks out VII polymers as being 'different'. When you cool stuff like base oil and most oil additives (ZDDPs, dispersants, etc) to -15°C and below, they just keep on getting thicker and thicker and thicker. If you plot lnlnKV vs Temp for these 'normal' hydrocarbons, you will find that they all tend to conform to the same straight line relationship, with the oil's viscosity doubling for every 5°C you drop the temperature.

However at low temperature, VIIs break this relationship because they have a tendency to crystallise, with the long molecules 'folding' in and over on themselves. The oil 'sees' these 'folded over' molecules as having a lower viscosity so they directionally thin the oil as the temperature drops. Neat!!

Managing this propensity to crystallise is key to getting an acceptable VII. A 40,000 MW all Ethylene monopolymer will have a very strong tendency to crystallise at low temperatures, so much so that you drop-out crystallised solids from the oil (not good!). A 40,000 MW all Propylene monopolymer would on the other hand, not be crystalline enough and just get impossibly thick at low temperature. However 40,000 MW 50:50 Ethylene Propylene copolymer would have the right degree of crystallinity (such that the polymer folds over on itself) but without reaching that point where the crystals drop out of solution.

It all gets far more complicated than this but in essense, this is why VIIs work and why even today, they are an essential component in multi grade oils.

 

[Oro_O]

Quote:
VIIs have very high Molecular Weights (think MWs of 40,000 to 80,000) and are extremely viscous;


So if I have a wide multi-grade oil with a significant percentage of these massive molecules in it, how do they, on their own, provide lubrication to offset the amount of base oil they displace? Also, how do they protect themselves from shearing, and the whole solution falling out of grade? If I'm not routinely operating in the -10C or lower range, I'm failing to see how they make for a better lubricating oil.

 

[Shannow]

Originally Posted By: Red91

Come on guys, I'm serious. I thought it was a legitimate question.


Merk is pretty close, just you need the mental imagery to go with it.

Imagine oil on a really large scale.

Now imagine it filled with tiny cotton wool balls, and imaging how that would effect it's viscosity...see how it's "thicker"

Now imagine that the cotton wool balls as they get cooler shrink into themselves, and take up less space.

That's how they work, much like Merk's rolling ladder.

When cold, the straggly ends aren't as soluble in the oil, so retreat into itself, when hot, the ends are more soluble, and so waggle out and about, thickening the oil.

Now the thing that's not really good about them...think of the oil filled with these molecules, then passing through an area of really high shear like a bearing...the "fibres" get strung out in the direction that they are being pulled. So if you think of the cotton fibres getting stretched in the direction of shear, they aren't doing a lot of thickening at that point...that's the reason for HTHS, to find out how the oil behaves in the bearings and things.

Edit...just for giggles...that block of white rubber pencil eraser is potenitally a Viscosity Index Improver of sorts.

 

[SonofJoe]

Originally Posted By: Or
Quote:
VIIs have very high Molecular Weights (think MWs of 40,000 to 80,000) and are extremely viscous;


So if I have a wide multi-grade oil with a significant percentage of these massive molecules in it, how do they, on their own, provide lubrication to offset the amount of base oil they displace? Also, how do they protect themselves from shearing, and the whole solution falling out of grade? If I'm not routinely operating in the -10C or lower range, I'm failing to see how they make for a better lubricating oil.


You're raising some good points...

Most oil formulators don't really like VII polymers but see them as a necessary evil. VIIs are one of the major contributors to piston deposits (especially in diesel engines) and as you rightly point out, they can have a tendency to shear. If, like me, you don't live in an especially hot or cold climate, don't drive like the Devil possessed and want to do your bit for global warming, put a VII-free 10W20 in your car, not a VII-laden wide cross-grade like a 5W50. It should also be significantly cheaper!

However it's pertinent here to mention the viscometric limitations of modern base oils.

In the beginning there were Vacuum Distilled/Solvent Extracted (Group I) base oils & typically, the heaviest base oil you can make by this means will only have a KV100 of about 11.5 cst (say 600SN). So the heaviest oil you can make here is a 30 grade. You can derive a heavier mineral oil by Propane De-Ashphalting vacuum residue. This process yields a base oil called Bright Stock which typically has a KV100 of around 30 cst. Before the advent of VIIs this was used to help make 40 & 50 grade oils. However Bright Stock is horrible stuff, has awful cold-flow properties and oxidises very easily. Using VII polymer as an alternative to Bright Stock, to make heavier oils yields a much better overall oil.

Today, in the US, Group I base oils have been displaced by Group II oils. There is no Group II equivalent to Bright Stock and the heaviest base oil you can get from Group II refining is typically 500N with a KV100 of about 10 cst. If you want a 40 or 50 weight Group II oil, VII is your only option.

The situation with Group IIIs (what constitutes 'synthetic' in most oils) is even worse. Typically the heaviest Group III base oil will have a KV100 of just 7.5 cst so the heaviest 'natural' oil you can make is a 20 grade.

So to answer your question, VII polymers do not necessarily 'displace' base oil because for a lot of oil systems, VIIs are the ONLY viable option to make the oil in the first place. Nor would I assume that VII polymers do not 'lubricate' an engine in exactly the same way that a base oil would. At the end of the day, the engine is only really seeing multiple linked units of H-C-H and in any oil there will be a wide spectrum of chain lengths.

Hope this helps...

 

[Popsy]

Thank you to this thread's contributors, very interesting read.

Originally Posted By: SonofJoe
If, like me, you don't live in an especially hot or cold climate, don't drive like the Devil possessed and want to do your bit for global warming, put a VII-free 10W20 in your car, not a VII-laden wide cross-grade like a 5W50. It should also be significantly cheaper!
Give me some, and I'll use it

Interesting to learn that 40 or 50 grades aren't possible without VII...so a Sae40 is prone to shear, while a Sae30 isn't?
You mention "modern base oil". Any difference with oils from the beginning of automobile? Many old car specify Sae 30 oil, sometimes with kerosene to have it thinner in the winter, I guess judging by your explanation, it makes sens.

Not really related (or is it?), but what about gear oils? For example my uncle's 1939 Daimler specify SAE140 oil in the rear axle. What base can it use? VII or not?

 

[SonofJoe]

Popsy,

I am on a one man global quest to popularise the use of 10W20 PCMO. So far I think I've had absolutely zero success but I'll keep plugging away because it makes such logical & economic sense.

I don't know but suspect I could make a Group II SAE40 oil that did not contain VII polymer. You take say a Group II 500N and then add a decent amount of DI additive to it (especially Ashless Dispersant) until the KV100 just tops 12.5 cst (so qualifying the oil as 40 grade). You can do this to a mono-grade because you're not worried about the impact that the DI pack is having on the low temperature viscosity (CCS) of the oil.

The other 'trick' with making heavy mono-grades is to add loads of pour point depressant which is a VII is all but name.

Gear oils aren't my bag but I sort of recall that they do tend to contain very shear stable PMA VIIs which confer good low temperature Brookfield viscosity performance to the oil. In a transmission, you're not going to get the VII-related deposits you get in a fired engine environment.

 

[Popsy]

Were you by any chance a very sneaky oil formulator? Haha, just kidding. Many ways to get around the problems, I see!
I suppose Sae50/60/70 grades work the same way, since low temp viscosity isn't a problem, as long as you can pour it out of the jug at ambient temperature!

Sae140 gear oil wasn't fun to get out of the oil syringe though.

 

[Shannow]

Originally Posted By: SonofJoe
I am on a one man global quest to popularise the use of 10W20 PCMO. So far I think I've had absolutely zero success but I'll keep plugging away because it makes such logical & economic sense.


I'm carrying your sigil...

 

[SonofJoe]

Originally Posted By: Popsy
Were you by any chance a very sneaky oil formulator? Haha, just kidding. Many ways to get around the problems, I see!
I suppose Sae50/60/70 grades work the same way, since low temp viscosity isn't a problem, as long as you can pour it out of the jug at ambient temperature!

Sae140 gear oil wasn't fun to get out of the oil syringe though.



Looking back, I'd rate myself as possibly THE sneakiest oil formulator on the planet! I'd also say I was one of the more honest (or most honestly dishonest?) people to ever play the game.

Being 'sneaky' is sort of a way of life in lubes. Everyone knows the industry revolves around specs. What people don't necessarily realise is that while all of these specs are relatively easy to deal with in isolation, they were never ever designed to be combined together.

So when someone tells you that you have to design a system to cover SL/CF/A3/B4/MB229.1/VW505 which simultaneously covers 10W30, 10W40, 15W40, 15W50, 20W50 and all monogrades in ten different base oils (ranging from the reasonable to the unbelievably atrocious) and the additive treat can't be greater than x% or cost more that y $/MT (where x & y are both infeasibly small numbers)...well you have a massively complex problem that requires a degree of 'creativity' (or sneakiness) to arrive at an solution that is acceptable.

That hereby concludes the case for the defence M'lud...

 

[SonofJoe]

Originally Posted By: Shannow
Originally Posted By: SonofJoe
I am on a one man global quest to popularise the use of 10W20 PCMO. So far I think I've had absolutely zero success but I'll keep plugging away because it makes such logical & economic sense.


I'm carrying your sigil...


Thank you Shannow! That's doubled the size of the 10W20 supporters club!

 

[Popsy]

 

[Red91]

The rolled ladder visual actually did help me understand the physical aspect of the polymer, my question more pertained to the catalyst which causes the polymer to operate. It seems to me the answer was obvious in that temperature is the catalyst, and I was searching for a chemical aspect of operation.

In reference to 10W-20; is a modern ILSAC 10W-30 not very close in grade and operation to such grade, or is 10W-20 more of a pseudo mono with minimal VII?

Another question I have, especially for Shannow and SonOfJoe, is that you guys don't seem to fond of 10W-40. It's my understanding that modern 10W-40 is not the VII laden concoction it was in the seventies. What about it do you guys consider to be less than desirable? I've personally had good service out of it in old cars and OPE. What I've used in OPE never seems too affected by fuel dilution or shear upon draining, albeit that is completely based on visual aspect and not UOA analysis.

 

[alarmguy]

The more the spread, the more the index improver, only oil company marketing will try to change your thinking.
With that said, any modern oil is more stable then it was in the 70s.
Just look at the API category from then to now.

 

[kschachn]

Originally Posted By: SonofJoe
Most oil formulators don't really like VII polymers but see them as a necessary evil. VIIs are one of the major contributors to piston deposits (especially in diesel engines) and as you rightly point out, they can have a tendency to shear. If, like me, you don't live in an especially hot or cold climate, don't drive like the Devil possessed and want to do your bit for global warming, put a VII-free 10W20 in your car, not a VII-laden wide cross-grade like a 5W50. It should also be significantly cheaper!

But you said in another thread that VII are not nearly as shear prone as people imagine them to be, and are typically stable. Is that true?

 

[Red91]

I understand the wider viscosity spread, but a 10W-40 should have similar viscosity spread to a 20W-50, and the latter seems to be regarded as more shear stable than the former.

 

[Shannow]

Originally Posted By: Red91
Another question I have, especially for Shannow and SonOfJoe, is that you guys don't seem to fond of 10W-40. It's my understanding that modern 10W-40 is not the VII laden concoction it was in the seventies.



Bear in mind that GM had wording that warranty would be affected (voided) with 10W40 use back in the day.

Couple of points on the way that they used to be (long time ago).
* Were prone to turn to rubber in the sump...I read about that on SAE microfiche at the uni library.
* We basically the reason that HTHS was investigated...they didn't live up to the "40" in terms of protection.
* When HTHS WAS added to J300, they only had to achieve 2.9, same as a 10W30.

So the grade has long been one of my least favourites.

(Confession time... while it was my least favourite grade, up until 15 years ago I always chased the wide spread synthetics...drove 100 miles one way to get some Redline 10W40).

What's changed ?
* VIIS have changed
* J300 has changed so that they now need to get 3.5+ HTHS

So it's not like it used to be.

But in a world where there's so many 5W30s that meet A3/B4, C3, and Dexos 2 I really don't see 10W40 as being all that relevant.

Considering the "shear stable" grades
20W50 per Doug Hillary was designed around the problems of shared sump BMCs
15W40 the trucking industry took some time to adopt it.
10W30
5W20

 

[Red91]

Noted and taken into consideration.

 

[BrocLuno]

SAE 30 HD is all oil, no VII's and has worked for 100 years. Today's SAE 30 is the best of a long line of oils. You wanna stay away from VII's, try a mono grade

 

[CR94]

Originally Posted By: SonofJoe
... Most oil formulators don't really like VII polymers but see them as a necessary evil. VIIs are one of the major contributors to piston deposits ... and they can have a tendency to shear. If, like me, you don't live in an especially hot or cold climate, don't drive like the Devil possessed and want to do your bit for global warming, put a VII-free 10W20 in your car, not a VII-laden wide cross-grade ...
Based largely on your similar comments earlier threads, I've been buying 5W-20, instead of the manufacturer's recommendation of 0W-20, for my car, which never has to start in extreme cold, and is rumored to be highly susceptible to piston ring coking. That's as close as I can get to your hypothetical ideal of 10W-20. So far, so good. Thanks!