11 replies to this topic

[Utkarsh]

Dear Experts,

 

I wish to compute melt viscosity of highly viscous psuedoplastic polymer.

 

Although the litretures which I have suggest dynamic viscosity:-

 

1. 12000 Poise @ certain temperature by Brookfied Rotational Viscometer

2. 130000 Poise @ same above temperature by Bohlin Oscillation Viscometer

 

Now I am in dilemma over selecting any one of them.

Based on this viscosity I have to do many further calculations.

 

So my question is that " Is there a correlation between measuring viscosity in rotational mode vs oscillation mode or it should be same whatever the method may be?"

 

I am not an expert into viscometers, forgive if any mistakes.

Thanks in advance.

Edited by Utkarsh, 14 December 2013 - 06:05 AM.

[PingPong]

highly viscous pseudoplastic polymer

That is a non-newtonian fluid, so its viscosity depends on the shear rate that is applied to it.

 

Pseudoplastic means: shear-thinning, so the more shear the polymer experiences, the lower its viscosity, which explains why the Rotational viscosimeter measures a much lower viscosity than the other viscosimeter.

 

Note also that the viscosity of a polymer will depend on its molecular length, so on its molecular weight. Therefor a literature value will not necessarily be correct for your polymer sample.

 

Based on this viscosity I have to do many further calculations.

It depends on the kind of calculations, what the shear will be, and consequently what the "correct" viscosity then is.

[curious_cat]

Important detail is what exactly are you trying to calculate using viscosity?

[JMW]

As is often the case with so many process measurements of viscosity, the more different viscometers you use the more "answers" you will get.

 

Curious cat asks the important question: "What are you trying to do?"

 

If you are trying to control a polymerisation process then who cares what the viscosity actually is? Most such processes are controlled by one of two methods: Using a PMI sensor (basically and usually a capillary with a dP measurement) to obtain the "Polymer Melt Index" at the process temperature or taking the finished polymer and dissolving in a solvent and measuring the viscosity of the solution. .

 

In either case the idea is to control the process so that the final product meets the quality specifications. For this what is required is a repeatable number. The actual viscosity is an abstract value which does indicate the mean molecular weight and most usually what is done is to standardise the sample and test procedure and base it around a single technology/sensor so that the PMI can be correlated with some lab value, whatever that is and whatever repeatable value it gives. 

 

The true or actual viscosity is dependent on too many things such as temperature and sheer stress etc. to be precisely determined and to what purpose? If what you want is to control the process then make sure you can obtain repeatable measurements.

[Utkarsh]

Dear JMW/Curious_cat/PingPong Sir,

 

I wish to select reactor/reactor viscometer/agitator type/motors & pump based on this viscosity data for a new product of Solid Poly Vinyl Acetate of molecular weight 100000.

 

Two different litretures from two different sources claim the above stated data @ 150C.

 

Actual practical data is doubtful due to such high viscosity measurement devices are not currently unavailable with us.

I wish to know that whether "two data can be somehow correlated or any one data can be wrong also".

 

Thanks in advance

[curious_cat]

I wish to know that whether "two data can be somehow correlated or any one data can be wrong also".

 

Both data can be right. Viscosity can and does depend on shear rate applied for such fluids. Also are you sure both studies used the same exact polymer in terms of MW & MW-distribution? 

 

Data is at 150 C, yes, but are you planning on pumping at 150 C too? Your polymer is ~100000 MW but was the data you looked up for the same MW too? Is it monodisperse? 

 

If you are selecting a pump you probably need a viscosity range and not a very accurate, sharp viscosity anyways. 

 

I'm not an expert. But I think @JMW is. He might have some tips for you.

 

You might get better help if you are more specific: What flowrates do you need to pump? How big a reactor do you need to agitate?

[JMW]

Ah! the need to know the actual viscosity is to size equipment. In that case I can be no help I'm afraid except to suggest that you are fortunate in that this is a niche market application and that you will find suppliers of equipment will be niche market specialists and therefore, hopefully, you will find some suppliers who can talk knowledgeably to you and with practical experience. Avoid the single source suppliers as you will find it very difficult to find  and talk with anyone knowledgeable.

 

I can only suggest a careful web search for the right suppliers of equipment, suppliers of PVA, etc.

For viscosity measurement start with the PMI sensors manufacturers.

e.g. (from a search) http://www.polymerso...ex-testing-mfi/

 

Talk to suppliers and make it their problem to come up with convincing data - if they want to sell you something, this they'll have to do.

They have the advantage of being able to talk with your competitors more freely than you.

 

Ditto pumps etc.

A search for low sheer pumps brings up this article from the pump school: http://www.pumpschoo...tions/Shear.pdf

and http://www.gd-transp...pumped product.

This is a case where I think you need to talk with those suppliers who have first hand experience of the application.

[Utkarsh]

Dear Curious_cat Sir,

 

Firstly thanks a lot for your valuable input.

 

Now comments on your queries- 

"Also are you sure both studies used the same exact polymer in terms of MW & MW-distribution?"-Yes 

 

Data is at 150 C, yes, but are you planning on pumping at 150 C too?- Yes, or probably 20C+ temperature otherwise pumping cost will increase & also effect pump selection

 

Your polymer is ~100000 MW but was the data you looked up for the same MW too?- Yes

 

Is it monodisperse?- Yes  

 

If you are selecting a pump you probably need a viscosity range and not a very accurate, sharp viscosity anyways- Agreed but since two above data vary a lot, I could not make a specific conclusion

 

What flowrates do you need to pump?- Minimum 2000Kg/Hr with gear pump

 

How big a reactor do you need to agitate?- Initially 5KL reactor; after everything goes correct will plan for 20KL 

 

Dear JMW Sir,

 

Thanks a lot for your valuable input. I got your point & will contact the suppliers.

 

Apart from above practical query; my question is as the http://en.wikipedia....wiki/Viscometer wikipedia suggest there are number of methods & concepts for different viscometers

"Can they be somehow correlated by any equation"

Thanks in advance

Edited by Utkarsh, 16 December 2013 - 01:05 AM.

[PingPong]

Utkarsh, you really should read about non-newtonian fluids, like your pseudoplastic polymer, because now this discussion is rather useless.

 

First get acquainted with the theory, and then you can understand why their viscosity depends on the shear that they experience, and understand why two different viscosimeters will give two different viscosities, both correct but nevertheless different simply because the two viscosimeters apply different amount of shear to the sample.

 

And no, there is no simple correlation formula for that.

[Utkarsh]

Dear PingPong Sir,

 

I told in the initial post itself that I am not an expert into viscometers.

Although I understand that viscosity depends on shear rate but the prime objective of the problem is to select a particular value of viscosity & hence do the further calculations on sizing of the equipments

 

Can you please help me in selecting any value or range for further calculations. 

[curious_cat]

Can you post your source for the viscosity information?

Edited by curious_cat, 16 December 2013 - 06:59 AM.

[Utkarsh]

Dear Curious_Cat Sir,

 

The 1st viscosity of 12000Poise from Brookfield is our R&D data with limited resources of viscosity measurement.

The 2nd viscosity of 130000Poise from Bohlin is our competitors data.

 

Both datas copies & names cannot be shared due to copyright issues & the forum moderator would immediately remove from this platform.

 

However on cheking other quality parameters both samples delievered similar results. 

 

My objective is to get a value or range of viscosity & proceed with further sizing.

Please help!!!