The reason for difference of die-swell in metallocene PE(L-LDPE)

Dear Everyone
I would like to know why there is a big difference in die-swell of L-
LDPE which is produced by metallocene catalyst.
In fact, we polymerized m-PE and tested die-swell with other company's
m-PE products.
We got 2.5 times higher die-swell values in our m-PE than other's.
They have similar Mw and density, comonomer(Hexene-1) content(about
7~8wt%).
Would you please explain this situation?
I am looking forward to your helps.
Thank you.
Reply to
hyh
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Unless you used the exact same metallocene cataylyst (and cocatalyst and...) and thereby infringed on your competitor's patents, I would not be surprised that there are differences. These would show up in NMR analysis (the diads, triads, ...).
As for why these differences show up as differences in die swell, I don't know. Die swell can be related to the memory of the material (i.e., it was in a larger diameter prior to entering the die and this state is remembered) but that does not explain all. Die swell decrease is the die length increases, but it never reaches zero. In addition, if the flow is shut off for a longer time than the relaxation time of the polymer and then restarted, die swell is immediately observed. I've never read of anything that related die swell (or for that matter, normal forces) to any fundamental properties of a polymer, but I certainly do not claim complete knowledge of the literature.
John Aspen Research, -
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Opinions expressed herein are my own and may not represent those of my employer.
Reply to
john.spevacek
You said "They have similar Mw". What do you mean by Mw: Mn or Mw, or something else? What are the molecular weight distributions of the two polymers? How did you measure Mw?
Compare melt viscosities at very low, and very high shear rates.
Ernie
Reply to
Ernie
Thank you Ernie
Mw mean weight of molecular weight(Mw) measured by PL-220GPC at 160=A1=C9. These polymers have 2.7 of molecular weight distribution.
Thank you again.
Reply to
hyh
Have you compared the degree of molecular branching between the various PEs? Normally, die swell increases with more branching, I guess ...
hyh wrote:
Reply to
Rolf Wissmann
Thank you
How can measure the degree of molecular branching in PE? Could I get the method to measure the degree of molecular branching in PE?
Thank you again.
Reply to
hyh
Die Swell of m-PE. The answer to your question needs to assume first that you have done the same testing on the same equippment under the same conditions of shear, temperature;etc. This being said as John at Aspen Research points out the conditions for this measurement are fixed variables. Those factors that affect die swell of polymers include molecular weight distribution and the higher Mz avg. , and how stable the polymer is during processing so the additives have a profound factor on keeping Mw in check especially the use of more powerful secondary antioxidants and lastly degree of branching of the PE. Branching is not a factor in PP. The rheology of metallocenes is predicated on the secondary antioxidant levels used and their compatabilty. Density determines compatability in this case. Traditional phosphites are less compatible than phosphonites and therefore limit the levels necessary to provide proper stability of Mw and IV or Melt flow of the resin. As this changes so does die swell. Some metallocene resins are more susceptible to degradation than others and only a few techniques are sensitive enough to differentiate this mechanism.
Joe
Reply to
magicjoe
Thank you Ernie
Mw mean weight of molecular weight(Mw) measured by PL-220GPC at 160¡É. These polymers have 2.7 of molecular weight distribution.
Thank you again. ------------ hyh,
Assuming that the two resins were processed under identical conditions, die-swell implies the presence of a high Mw fraction. Compare the high Mw ends of the GPC plots. Does the resin with die-swell have a higher Mw tail, perhaps as little as 1%?
Is there a insoluble high Mw (cross-linked?) fraction in the resin with die-swell? If it is not soluble, it will not be detected by GPC.
You may also want to run GPC on the extrudate; did the Mw increase, or cross-link during extrusion?
Is there a "melt lubricant" that reduces die-swell in the other resin?
Ernie
Reply to
Ernie
Good question!
it looks like for short chain branching (SCB) H1- & C13-NRM, on the basis of model compounds, are most prefered. For analyzing long chain branching (LCB) it requires a combination of H1- & C13-NRM and GPC and rheology.
The following pdf-documents ought to give you some info on the subject:
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>> Have you compared the degree of molecular branching between the
Reply to
Rolf Wissmann

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