MFI value and decomposition of PC/ABS

[trxt]

Hi all,

I want to ask questions regarding MFI value and degrading of pc/abs.

Virgin PC/ABS's MFI value is 7 and PC/ABS molded part's value is 10. My customer insists on that increasing from 7 to 10 shows that there is a problem with my process and material was decomposed.
In my opinion, MFI testing is not enough by oneself to judge if the material is degraded or not.
Therefore i have it made some testing at the university polymer labaratory.
Performed testings are as follows;
***GPC (Gel Permation Chromatography)
***DSC ( Differential Scanning Calorimetry)

According to the results of these GPC and DSC testings there is no degradation.


I kindly want to ask, if MFI increasing from 7 to 10 absolutely indicates that decomposition has occured?

And MFI testing is enough by oneself for describing the decompositon?

Thank you for your attention..

Best Regards..

 

[patprimmer]

If the MFI increases substantially the molecular weight has been decreased. I was of the impression that for moulding grades of polymers, DSC was mainly used to determine crystallinity and crystalline melting points. I don't see where this is any more useful than a very cheap and simple MFI test to determine degradation in the range that would be expected during moulding.

Even the best processing will cause some degradation.

PC/ABS is prone to decomposition if it is overheated, kept to long at melt temperatures, is not dried properly prior to moulding, is overheated or kept at temperature to long during drying, or is subject to to much shear during moulding.

How much loss of molecular weight is acceptable is arguable, but 7 to 10 seems a lot if processed under ideal conditions and for a flow path that is not excessively tortuous. Degrading from MFI 7 to MFI 10 might be unavoidable if the flowpath is difficult

Regards
Pat
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[trxt]

Hello Pat,

Thanks for your helps.

During my process pc/abs is not overheated , dried properly before molding and stays only max. 8 min. in barrel which is acceptable according to the raw material supplier.
The mold is reverse mold and directly inject melting into the cavity and mold has cold runner. Mold temperature is 40 C degree around.

GPC testing showed that molded part has %5 lower moleculer weight than raw materials.

Product's thickness is 0.6mm and my customer says that the product is more brittle than expected.(Design responsibility belongs to my customer)

I couldnt evaluate moleculer weight loss and MFI increasing relation. Moleculer weight loss is just %5(which is acceptable and proves that there is no decomposition) but MFI increases from 7 to 10.

What is your opinion?

 

[Demon3]

This is complex. First, DSC won't tell you anything. The MFI increasing points to loss of molecular weight but the change you report is not enough to worry about in my opinion.

My best guess as to why it's brittle would be that the polybutadiene rubber in the ABS has degraded. That is by far the most heat sensitive component in there. You won't see any change in the rubber by GPC because the rubber is cross-linked and insoluble.

I've worked quite a bit with ABS and it does change properties even due to one short extrusion.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[Pud]

Try a sample of a higher impact grade of ABS (more polybutadiene) - if it's still brittle, it's as Demon3 suggested. If not, perhaps you have started with the wrong material grade/type.

I would not have thought an MFI shift of this degree would be obvious in the moulded part mechanical properties. The MFI shift may just be a problem with small sample sizes and testing variations.

Have the material manufacturers anything to say about this problem?

With the (relatively) cool tool temperature, you may have moulded-in stress, giving an apparent loss of material properties. e.g. Bayer give a tool temperature of 70 to 100C for Bayblend:

http://www.omnexus.com/tc/bayblend/index.aspx?id=processing-tips.

A higher tool temp will also enable (should) a lower injection pressure, further reducing moulded-in stress.

They also say 4-6mins residence time...


Cheers

Harry

http://www.tynevalleyplastics.co.uk

 

[patprimmer]

8 min is a long time in the barrel.

0.6mm is very thin unless it is a small part with short flow path. This may well induce stress in the moulded part.

40 deg C is a fairly cold mould.

I would try a mould temperature of 60 deg C and maybe reduced melt temperature.

I would also seriously consider trying to use a machine with a considerably smaller shot size.

I would try adjusting temperature profile to minimise degradation, but I cannot comment without seeing screw size, current profile, shot size, screw profile, screw speed, back pressure and if any temperature controllers are currently overriding. To be honest, one really needs to be there to assess that aspect.

Also is it holding a cushion and is the cushion small but consistent, like about 5mm

Are you adding anything, like a masterbatch or lubricant or mould release.

How do you know you are drying it thoroughly. One of the most common problems when moulding PC or its alloys is inadequate or defective drying despite the moulder being genuinely convinced that he dried it properly.

Excessive shear is a very common problem when moulding PC/ABS.

Overheating by excessive shear in the compression zone of the screw as indicated by the temperature controller over riding is a common cause of degradation.

A leaky ring check valve can also cause excessive shear and local thermal degradation.







Regards
Pat
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[Demon3]

Just FYI, you don't need to oxidize the rubber to get a loss of impact resistance. It also depends on the size (and therefore the dispersion / agglomeration) of the rubber particles.

You can get change the impact resistance you get by changing the processing and mold temperature. I've seen this effect in real life. By that I mean it's not a theoretical thing but proven by experiment.

Chris DeArmitt

"Knowledge has no value except that which can be gained from its application toward some worthwhile end."
Think and Grow Rich - Napoleon Hill

 

[trxt]

I am not adding anything , lubricant , masterbatch or any other chemicals.

Raw material manufacturer can not say anything. He just told me that he couldnt gurantee anything. Thats why i intend to change the raw material.

Now that , GPC , DSC testing is not working for determining the decomposition. What are the ways of determining material is decomposed or not?

thanks..

 

[patprimmer]

MFI is closely related to molecular weight.

IR can indicate a change in any part of the chemical composition.

Microscopic examination can indicate a change in the rubber dispersion.

If MFI is up and impact is down you obviously have a problem.

The solution may lay in looking for cures rather than intricate details of the cause.

A different tougher grade.

A different supplier who will provide good support. Note, they usually cost a little more.

careful monitoring of all relevant aspects of processing conditions and use of an appropriate machine.

Regards
Pat
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