Monday, October 17, 2011

How to (NOT!) Determine PVC Rheology

We all know that the internet is filled with lots of bad information. A good example is this page entitled "How to Determine PVC Rheology". The strangest part of this is that this was published on "ehow.com", a site that is normally gives advice for general consumers (gardening tips, financial advice,...). Why anyone on the site would be interested in PVC rheology is beyond me.

Overlooking all this, the advice given is still awful. You can tell that there is a trainwreck coming anytime an article starts out with false compositional breakdowns of a material such as this:
"Polyvinyl chloride (PVC) is a synthetic thermoplastic resin made of 57 percent chlorine from industrial salt and 43 percent carbon in the form of ethylene from oil and gas sources.
First off, PVC is made from vinyl chloride monomer (VCM), which is what in fact is made from chlorine and ethylene. You don't just dump NaCl and oil/gas (or even ethylene) in a reactor, start it up and voila! - out comes PVC.

Unfortunately, the article has not yet bottomed out.
"Once formed, PVC can be resoftened by heating, with melting occurring at approximately 180 degrees Fahrenheit.
180 oF??? Only if it is greatly plasticized. The melting temperature of PVC is all over the map, probably more than any other plastic, and depends greatly on the formulation. 200 oC is not out of the question in some cases. But wait, there's still more:
" To properly manufacture PVC products, manufacturers study its flow properties when melted to learn how to successfully pour it into injection-molding machines."
Pouring molten PVC into an injection molding machine? Certainly the rheology of the pellets that are normally used to feed an injection molding machine could be what the author is referring to, but once you read the next sentence, you find out that that is not what was meant.
" Rheology is the study of how matter flows. So when you determine the rheology of PVC, you determine how it flows when melted."
This is only half true as the rheology of non-molten materials are also studied too.

At long last, the article then finally gets to the information on how to use make a measurement using a torque rheometer. The details are so insufficient that no one can possibly run it correctly even if they have access to the instrument. The final kicker is written in the "Tips and Warning" section:
"There are three typoes [sic] of rheometers: capillary, parallel plate and torque rheometers. Torque rheometers are the rheometer of choice when determining the flow proerties [sic] of polymers."
All of this is wrong - the types of rheometers are endless, and a torque rheometer would be the rheometer of choice in only a few situations, (such as when a test specifically calls it out, or if the operators are not trained or skilled in other techniques or...).

If this isn't enough of a laugh to start your morning, then check out the article that is listed in the "Related Articles and Videos" heading: "How to Use a UV Light to Evaluate PVC Rigidity" I kid you not! Throw away your tensile testing machine - all you need is a UV light!

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