Tuesday, May 08, 2007

MFI - One Really Bad Test

The “melt flow index” is about as bad of a test method as you can get. In fact, I would rate it as # 2 on the list of all-time bad test methods. (I’ll save the discussion for #1 for a later date.)

Background: See ASTM 1238 for all the gory details. Basically, you put some resin in a tank with a floating lid, heat it to a specified temperature that is sufficient to melt it, add a specified mass to the lid and then let it drain out a small circular hole at the bottom. The mass of material (in grams) that flows out in a specified time (usually 10 minutes) is the melt flow index. The softer, less viscous materials will flow out faster and have a higher melt index. End of background.

So what’s the problem? It’s an easy test to run, no advanced education is needed, and so the test results are used on specification sheets throughout the industry. 100’s of millions of pounds of xxPE, PP, PVC and other resins are bought and sold largely on this single test value. So what is the problem?

For starters, the test does not tell you anything fundamental about the resin. It is not a viscosity test even thought it is commonly thought to be one. In fact, the test is deceptively close to a capillary viscometer test, in which the mass flow rate through a small circular tube is recorded along with the pressure drop. The big difference is that the capillary viscometer tube is much longer than that of the mass flow indexer. This is necessary for the flow in the tube to be fully developed and free of any entrance effects (irregular flow patterns on the upstream side of the die such as eddies that result from the flow trying to squeeze into the small capillary). These entrance effects are retained by the polymer until they can relax out, something that takes time which means a longer flow tube. Typically a length/diameter ratio of 20 is needed. The MFI index has a L/D of 4.

But wait, there’s more. Viscosity changes with the shear rate. Drastically in some cases, and in a nonlinear fashion. The MFI measures the “viscosity” at only one shear rate. How many curves can you draw though a single point? Plenty. Measuring the MFI at a second shear rate goes a long ways towards understanding if two lots of resin really have the same or different flow characteristics. Again, assuming that the MFI really does measure “viscosity”.

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