How is ZDDP calculated?

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Mar 13, 2009
Northern California
How is the ZDDP number calculated for oils? For example, simply because there is a chart readily available... From the Mobil 1 FAQs:
Mobil 1 0W-40 already contains a higher level of ZDDP (1000 ppm) that could benefit your flat tappet engine. We also have a Mobil 1 High Mileage 10W-40 (1000 ppm); see our table listing the phosphorous levels for all Mobil 1 motor oils.
Both have ZDDP levels of 1,000PPM. Onto their handy chart... Mobil 1 0W40 has 1,000PPM of phosphorus and 1,100PPM of zinc. Mobil 1 High Mileage 10W40 has 900PPM of phosphorus and 1,000PPM of zinc. What am I missing?
Your question got my interest. I called an oil company and did some Google searches. We need someone who knows A LOT MORE ABOUT CHEMISTRY than I do. Even after talking with my chemistry professor friend, I am still very fuzzy, but: ZDDP=zinc dialkyldithiophosphate. When zinc and phosphorus come together they form something called an orbital compound Oil companies report this as an average based upon the compound, thus ZDDP of 800, 1000, 1200, etc. Phosphorus is the anti wear part, not the zinc. 1200 ZDDP does not give any more protection than 800 ZDDP, it give longer protection. If you look at a VOA, it may say something like 760 zinc and 690 phosphorus, but the oil data sheet would likely say 800 ZDDP. The 800 is a compound average for a zinc/phosphorus mix. We really need the help of a chemist here. I mean, I had to use spell check to even be able to spell cheam....chream....cemest...chemistry.
Yes, so in theory, we could run the SM oils out of the bottle and then dribble in a little zddp booster every 1000 miles or so to maintain a constant zddp level, eh?
That sounds feasible, but, I simply do not know. I found several references to higher levels of ZDDP protecting longer than lower levels of ZDDP. I could not get a straight answer if longer meant for more milliseconds at boundary failure or for more miles/hours. I know that Valvoline for sure, and a couple others, are talking of a new improved ZDDP that stays in service longer, protects longer, but, I still do not know what longer means.
I have seen both phosphorus and zinc being listed as anti-wear additives. I'm sure they both are. I was also thinking that it was some sort of average, but am not sure what exactly goes into it.
I'm a chemist, and I'm not sure why they don't match either. Each ZDDP molecule should have one zinc and one phosphorus, no reason for them to differ. But I would guess they're just listing approximate numbers so folks (hopefully) don't complain if their formula fluctuates a bit.
I am not a chemist but I have seen references to different types of ZDDP. Some are more bias to AW and some are more bias to AO. Maybe one of the real pros will chime in.
The issue is not how much ZDDP you have to start with. It is how much is depleted during your OCI. This is based on the whole makeup of the motor oil you use.
I think it is safe to say that the ZDDP molecules themselves in our motor oils have 1:1 ratio of Zinc to Phosporous. I think we see more zinc than phosphorous in almost every motor oil because during the manufacture of the ZDDP molecules, there was an excess of Zinc Oxide reacted with the dithiophosphate. See the Wiki article here:
Ok, now I'm confused. Maybe someone can figure this out. I realized that UOAs and VOAs measure the percent weight of the elements, so atomic weights have to be considered if it is a given that the ZDDP molecule has one zinc and one phosphorus atom. Well, the atomic weight of zinc is 65.4 and it is ~31 for phosphorus, so a zinc atom weighs over twice as much as phosphorus. As an example, if the only sources of zinc and phosphorus were from ZDDP (Zn and P do not also exist as free atoms nor are they bound up in other molecules), we should see something like 2100 PPM Zinc and 1000 PPM phosphorus. That 2.1 ratio of Zn to P is unheard of to me in motor oils, so either my thinking is flawed somewhere or there are a lot more phosphorus atoms that are either free or part of molecule(s) in addition to ZDDP.
PPM is parts per million, and the weight of those parts is irrelevant. If you have 1000 bowling balls, 1000 tennis balls, and 998,000 other balls, they are each 1000 ppm despite their different weights. It is possible to report percent by weight, but I don't think UOAs do this. I don't analyze oil, but I analyze mining samples every day, and we report in ppm, not by weight. I'd assume UOAs are the same.
Interesting. Well, I suppose it's up to the individual labs on how they want to report their results. And I discovered the reason for all this confusion... I had assumed each ZDDP molecule had one zinc and one phosphorus, but they have TWO phosphorus. So there's twice as many phosphorus atoms as zinc atoms, but since phosphorus weighs almost exactly half as much as zinc, they both end up giving the same "mass ppm". Problem solved! And I think this part from your link is worth note for this thread... "For phosphorus, repeatability is specified to be less than 71 ppm and reproducibility is 140 ppm. For zinc, repeatability is specified to be less than 65 ppm and reproducibility is 170 ppm." So results can wiggle around by 100 ppm or so, which nicely explains M1's discrepancies in zinc vs phosphorus. I solved two mysteries tonight. Can I go now?
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