The Investigation of Viscometric Properties of the Most Reputable Types of Viscosity Index Improvers in Different Lubricant Base Oils: API Groups I, II, and III

Round 1

Reviewer 1 Report

The authors should be congratulated on an interesting and useful paper. I have a few comments below which may help the authors improve the paper still further: 

1. Page 2 - there is a comment about the recommended oil viscosity for spur/helical/bevel gears. In practice, it is usually the gearbox manufacturers that recommend an ISO grade for their gearbox (and this is usually based on the VI of a mineral grade oil, around 100) so although the authors' calculations are based on the same viscosity fluid, in practice, some of these alternative fluids would not have the manufacturer's recommendation
2. Page 3 - the authors use the abbreviation PB a number of times, should this be PIB ?
3. Page 3 - a word seems to be missing in the following sentence (last sentence on page: "The disadvantages are poor shear stability and some .... if ethylene content is too high"
4. Page 4: The authors comment that developments in VII polymer chemistry include the use of "blends" of polymers. Although this may have be done in a research environment, I don't think many commercially available lubricants use a blend of VIIs in their formulations.  
5. Pages 7-8: c is not defined, but is used in equations (3)-(6). In fact I found later that c was defined to be the concentration on page 10, but should be defined on page 7 when it was first used
6. Page 14: Many lubricant formulators add a small amount of ester base stock (around 5%) to Group III base oils to improve their solvency. It would be interesting for the authors to repeat the VI work with a Group III + 5% ester blend to see if this would affect the behaviour of VIIs (compared to Group III alone) 

Author Response

Dear Reviewer

Hi, hope you be very well.

First of all, thank you very much for the complete review and useful and valuable comments of your Excellency.

 Regarding the comments, I offer the following explanations:

1. You are right. This statement is taken from reference 10 and, although somewhat abstract, indicates that oils with different viscosities at 40 degrees can be recommended for a fixed gearbox, provided that those oils have different viscosity indices. In that case, the viscosity of all those oils will be similar and close in operating conditions. However, we could delete this paragraph if you see fit.
2. Yes.By all PB, we mean polyisobutene and it was corrected everywhere.
3. Corrected.
4. Infineum Company, as the most famous manufacturer of SIP polymers, recommends the use of polymer blends. In addition, this has been mentioned in some sources and references, but as you said, this is not common. In our company, we have done this only as research, and if necessary, we will publish the results later, but we have not used this suggestion in the formulations in practice.
5. You are right. The letter C is synonymous with concentration and has been modified in the text.
6. Yes, you are right. Adding ester as a solubility aid is of course more common for industrial oils. Checking VI Improver in the base oil mixture is one of our future plans, and if you allow, your Excellency's proposal will be reviewed at that time.

Thank you again for your attention and kindness. Your valuable feedback shows that you have excellent knowledge and experience in the field of lubricants and the lubrication industry. Have a nice time and best wishes.

          Best Regards

        Seyed Ali Khalafvandi

Reviewer 2 Report

This manuscript discusses variability of lubricating oils viscosity with temperature which is usually quantified by viscosity index calculated from intrinsic viscosity (IV). This is an important problem in the lubricating field and as such it fits the profile of this journal.  However, in m my opinion, the authors failed to collect sufficient data and/or present them in such a way that would make their paper worth publishing. In fact, the manuscript reads more like an industrial report (albeit somewhat enriched by a general discussion, which is largely trivial or well known) than a scientific paper. Indeed, they consider their “primary finding”, as stated in the end of Results and Discussions section (p.14), that intrinsic viscosity of all polymer/oil combinations they tested drops with temperature. But the fact that it does so for majority of polymers is well known since Muller (1977) and more recently was extensively discussed by Covitch (2015). The latter work has provided some supporting data to explain this behavior (SANS), while the authors of this manuscript simply performed viscosity measurements. Secondly, the authors state (on the same page) that another “finding” is a positive correlation of IV with molecular weight. But this is simply trivial for the same type of polymers. For polymers of different solubility (like SIP1 and 2) this is different, but in order to seriously discuss this difference one needs to show some actual data on the coil size, either by light (DLS) or neutron (SANS) scattering or some other method. Again, simply reporting viscosity with some hand-waving arguments does not qualify a collection of data for a scientific paper.

There are some minor problems too. Section numbering is obviously wrong, many misprints in the References section, units lack in the tables. English definitely needs to be improved. For instance, I do not understand what the authors were trying to say in the 2^nd paragraph of the Conclusions. Are the authors referring to their work or others? But in their work there is no data on molecular size at all.

In summary, I think that this paper may be reconsidered for publication only if the authors extend their experimental base adding some data on polymer coil size (I’d suggest DLS as relatively simple and easily accessible) and/or other methods to assess solubility.

Author Response

Dear Reviewer

Hi and hope you are very well.

     First of all, thank you very much for your careful review and comment. I would like to say the following about the objection:

   As you said, and in Ms. Martini's most recent review article on VI Improver (Reference No. 11), there are three ways to measure or estimate the size of polymer molecules in solution. Direct method such as DLS or SANS, indirect method or the same viscometric method that we have used and the newer MD method.  All three methods are common and valid, and as you mentioned, most of the articles have used two methods. But we did not seek to repeat the works of others and reach their results or reach trivial.

   The disadvantage of all previous works in this field is that they did not simulate or did not cover the operating conditions in the formulations of lubricants, and although they considered one or more practical aspects, the other aspects were selected and studied only theoretically or hypothetically. But the novelties in our work are the following four cases:

• Examining three types of polymers OCP, SIP and IP that cover more than 90% of the lubricants market.
• Investigating and comparing the behavior of these polymers at two standard temperatures of 40 and 100 degrees, because the viscosities at these two temperatures are used to calculate VI and the VI enhancement is the most important and perhaps the only reason for using these polymers with such a large volume in lubricants.
• Investigating the behavior of these polymers in three most conventional types of base oils, namely API group I, II and III base oils.
• Our concentration range covers the practical concentration range in the formulation of lubricants, i.e. 0.7 to 0.85.

While in none of the previous works, even two above aspects have been considered and studied together. It should be noted that in our future work, we intend to examine other types of VI Improver, especially those common in industrial oils and study them in base oil blends.

 Please note that we work in an oil research laboratory that has many kinds of equipment for study of lubricant properties, but we do not have DLS and SANS devices, and in this short opportunity, we are not able to prepare these equipment and check by DLS and SANS methods, but in Next work, We will try hard to make your valuable point of view.  

We mean in the second paragraph of the conclusions section as follows: “All of the above well-known polymers increase the VI of lubricants, while the molecular size of all of them decreases with increasing temperature. Then improvement of VI by VII is not necessarily due to the expansion of the polymer molecule with temperature.” We replaced this.

Anyway, thank you again for your careful review and valuable point of view and have a nice time.

Yours sincerely

Seyed Ali Khalafvandi

Round 2

Reviewer 2 Report

Since the authors did not make aby substantial changes to their manuscript, I can only repeat my conclusion from the original review: this paper may be publishable in a scientific journal only if the authors add direct measurements of polymer coil size. Otherwise it remains what it is: an industrial research of no particular interest to wider scientific community.