Tuesday, May 29, 2007

One last comment on polyurethanes, I promise!

Given the wild diversity in the physical properties of polyurethanes, they are my default answer whenever someone tries to play “Stump the Polymer Expert” with me.  (“Hey John, what’s this plastic?”) PU may not be the right answer, but it never is a wrong answer.  That can go a long ways in establishing instant credibility.

Friday, May 25, 2007

Why I hate polyurethanes

I can’t imagine a more nondescript name for a polymer.  (To be fair, polyureas, polyesters and all other condensation polymer suffer the same flaw, except that the options for the intra-urethane segments of a polyurethane are much more diverse than for any of these others.) I am constantly barraged by coworkers and clients bringing me a sample of a “polyurethane” highly expecting me to know exactly what it is. You can make bowling balls, sponges, pressure sensitive adhesives and much, much more from polyurethanes: how am I suppose to know the exact chemistry when all you can tell me is it is a polyurethane?

Thursday, May 24, 2007

Why I love polyurethanes

Polyurethanes are a most versatile polymer. Between the urethane linkages lay potentially hundreds of different segments ranging widely in polarity, flexibility and connectiveness.  The result is bowling balls, sponges, pressure sensitive adhesives and much, much more.

Monday, May 14, 2007

To Infinity and Beyond

How can measurements of an infinitely dilute polymer have any meaningful purpose at all, especially since nobody works with such dilute solutions in the real world?   It turns out that the intrinsic viscosity can be related to the molecular weight by the Mark-Houwink equation: [η] = K Mα  [η] is the intrinsic viscosity, K is a constant, M is the molecular weight and α is another constant.  You can do some theoretical development of this equation and find that α should be 0.5, which is usually not the case.  But it is still an important value.


Polymers in solution do not exist as long straight molecules, but instead are coiled up in a random coil.  The size of this coil can be related to α with larger values of α indicating that random coil is larger than would be expected. A larger coil will interact with more solvent molecules and drag these along in the flow field creating a larger viscosity.  So a quick look at the value of α will tell you how good the solvent is that you are working with. α = 0.8 means it’s a terrific solvent, α =0.5 means that it’s a lousy solvent.  α less than 0.5 means that the polymer will soon be coming out of solution.


In an incredibly mind-blowing relationship, these values determined at infinite dilution can in fact be related to a pure polymer.  Molten polymers also exist as a random coil, and the size of this coil is the exact same size as when α = 0.5. How cools it that? An infinitely dilute solution and an 100% pure polymer have something in common.

Friday, May 11, 2007

A much better QC test

So as you can tell, I don’t like MFI much as a QC test. Another less common test is IV – which in this case means inherent viscosity. Polyesters are commonly spec’ed on this basis. Here again, running the test at two different conditions can provide a huge increase in the data and what you can conclude from it.

Background: This test is also run with the polymer flowing through a capillary, but with big differences. First off, the capillary is glass. (A variety of different glasses are available each with different advantages and disadvantages.) Also, instead of being a melt (100% polymer), the polymer is dissolved in a solvent and the concentration is so low that each polymer molecule does not interact with any other one because the distance between them is greater than the size of the molecules. The polymer solutions flows down under the pull of gravity and a stopwatch is used to measure the time that it takes for a certain volume of fluid to drain. The same measurement is also made for the pure solvent. It’s a very easy test to run – I’ve had high school students get fantastic results – and even automated systems exist. End of Background

The ratio of the drain time for the polymer solution divided by the drain time for pure solvent is the relative viscosity. Take the natural log of this, divide it by the concentration and you have the IV. There also another “IV” which is called the intrinsic viscosity (Inherent/intrinsic viscosity – very confusing. I didn’t name these things, I just live with them.) The intrinsic viscosity is found by rerunning the inherent viscosity at a different concentration and then extrapolating the inherent viscosity to zero concentration. The neat thing is that this can be self correcting by doing a second set of independent calculations. Going back to the relative viscosity, subtracting “1” from it gives you the specific viscosity. Dividing this by the concentration gives you the reduced viscosity. Extrapolating the reduced viscosity to zero concentration also gives you the intrinsic viscosity. (The IV line has a negative slope while the reduced viscosity line has a positive slope. If they don’t cross at the y axis, something is wrong with the test.)

Like the MFI, measuing IV at just one point is of dubious value, as there are a large number of lines that can be drawn through that point. I know that it is an infinite number of lines, but for practical purposes, it is much less than this. First we know that the lines have to have a negative slope. Also, intrinsic viscosity is determined to two decimal places, so lines that lead to changes in the intrinsic viscosity of less than 0.01 are of grouped together. Nonetheless, there still is a large number of lines that can be drawn though a single IV measurement. Having a second IV measurement lets you calculate the slope AND the intrinsic viscosity AND also lets you calculate the reduced viscosity line too. All that from a second measurement. It's a heckofa deal.!

The intrinsic viscosity is magical. Despite being a measurement made at infinite dilution (commonly called “a single molecule in a sea of solvent”), you can actually learn much about the melt properties of the polymer. But that will have to wait for the next post.

Thursday, May 10, 2007

Chemical Comics

I always seem to be the last to know about these things, but if you are further out of the loop than I am, both of these comics can be terrific, namely xkcd and nearingzero. They both have a clever approach to all the sciences as well as life in general.

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”.