As others may know, polyethylene's MFI, after processing will be decreased. but I want to know that how could we increase polyethylene's MFI during our process.
I assume, your wording is not precise. If you say "the MFI is decreased", that would mean the molecular weight has increased so the polymer flow is slower.
I assume you wanted to say "MFI has increased" which is a reflecxtion of molecular weight degradation.
What exactly do you want to achieve? a certain MFI? or an MFI as low as possible? Please let me know first, then I can give you some more precise answer / recommendation.
Thanks for all attentions.
I think,my question needs more explanation.
we have a reactive extrusion for silane grafting of polyethylene(silane+peroxide+polyethylene). its screw design, create high shear rates.more over, because of using screen(mesh 80),considerable back pressure created too.
according to theory,this situation will be resulted in more silane grafting and subsequently MFI decreasing. but what we observe in practice is different. when screen changes and back pressure reduced , MFI decreasing more!
It means that in this situation by shear rate reduction, MFI decreasing more!!!
why?why by shear rate increasing, our MFI would not decrease!
Dear Johan.
Thanks again for your attention.
it's a conventional determination that involves taking a sample of extruded and evaluate in a Plastometer. It is a part of our QC procedure.
both grafting and processing should resulted in MFI reduction for all kinds of PE. and so, What parameter which helps these factors should resulted the same result.
But what we observe is different!
when our screen be dirty, back pressure would be increased,so shear rate increasing too. in this situation and in opposite of what we learned, MFI starting to increase!
This is all.
you did not clearly answer my question, so now I assume that "MFI reduction" is really a reduction = higher viscosity.
Therefore I would assume that with the peroxide you somehow result in having created some crosslinking ==A higher molecular weight.
Increasing molecular weight (with cross linking) will increase the zero-shear viscosity of the melt. If you have any regions of extensional flow you may find the branched structures lead to extension-hardening (increase in extensional viscosity).
Dear Maziar,
Processing/extrusion polyethylene with a peroxide will cause long chain branching and crosslinking. Each of these changes will decrease MFI, that is increased viscosity. Long chain branching of polyethylene will increase shear thinning as entanglements between long branches are reduced at high shear rates. Long chain branching will increase melt strength, assisting with film production.
Addition of a reactive silane should decrease branching and crosslinking, since radicals on polyethylene can react with the silane instead of reacting with other polyethylene molecules. The contribution of grafted silane is uncertain, since if it is a crosslinking silane, then further decrease in MFI is likely due to silane crosslinks. The changes in polyethylene could be reduced by using less peroxide and/or more silane, since grafting of silane could minimise self-reaction of polyethylene. Polypropylene would should an opposite trend since attach by radicals caused polypropylene chain scission predominately, instead of long chain branching as with polyethylene.
Dear Maziar
Have you examined the nature of the materials on your filter and are you doing any thing to the slide on the filter such as adding high temperature lubricant on the sliding filter holder?
Dear all,
Actually I found you guys here very interested and expert in polymers and I believe my question has some relevance with the question above. I am a beginner in polymer area so I am sorry if my question may not suit you. My question is as below:
I have a problem dealing with Melt flow index (MFI) of master-batch. As far as I know, generally we can divide a master-batch content in two broad groups; polymer and solid phase powders. How can we define MFI for a material that is no longer pure polymer? What is the role of solid powders in evolving the flow of polymer (MFI value)? Do we have any parameter to control the MFI value when we are adding for example TiO2 or calcium carbide powders?
Thank you in advance,
Mohammad
You need add wax and increase the extrusion tempearture for better mixing and hig MFI.
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