Thursday, August 02, 2012

"Improvements" to the Melt Flow Test that Aren't

The melt flow test is THE workhorse test for the polymer industry. Despite it being a test that academics and other well-educated people love to hate, it is pretty much the entire basis for the sale and purchase of billions of pounds of thermoplastics annually. That one little number pretty decides if a new batch of polyethylene is in spec or out.

The test in a nutshell is this: The sample is placed into a heated bore, and the molten plastic is driven out the bottom by a piston that descends because of weights placed on it. The temperature and the mass of the weights are prescribed by ASTM D1238 or agreement. The mass of material that is extruded through a small hole at the bottom over a known time interval is then converted to grams/10 minutes. And that's it. It really no different than measuring how long it takes for water to drain from a bucket [*].

So given a test like this that has been around for decades, well before computers and electronics were around, there is an endless temptation for people to bring the test into the modern/computer age by adding copious amounts of sophistication to the simple test. In some cases, it is helpful. Anything that can help put on and remove the weights is great. The weights are in the range of 1 - 21.6 kg (!) and need to be lifted up high over hot equipment, so any mechanical assistance is desirable particularly for short operators or anyone with weak arms.

But in other cases, the additions are non-productive or even counterproductive, such as when a computer calculates the shear stress, shear rate, viscosity, phase of the moon, and tonight's winning lottery numbers. Any number for the shear stress, shear rate and viscosity are meaningless. The die through which the polymer leaves the piston is too short - it's only 4 times the die's diameter. The length needs to be at least 20 times the diameter in order for the polymer to have "forgotten" its previous flow conditions, such as when it underwent extensional flow going from the large diameter piston down to the low diameter exit die. The flow in the melt flow die is shear that still has remembrance of extension, and yet all the equations used to in the calculations assume that there is only shear flow and no extensional flow.

If I ever see those numbers associated with a melt flow test, I ignore them. That's why I earlier called them "non-productive". But for too many people, they numbers become real and they put faith in them as being representative of the polymer. That's counterproductive or even dangerous. Ignore the shear stress, shear rate and viscosity. The test was designed to measure melt flow rate and only melt flow rate and that's all the more it will ever measure, and no computer-base calculations will ever change that.

[*] Please don't think for a minute that draining water from a bucket or the like is a moronic test. Glass capillary viscometry is also draining water from a bucket, but it is a test with plenty of theoretical support - and the results show it. However, that's another story for another day.

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