pH of polypropylene

hi, can somebody tell me how to evaluate the pH of polypropylene sample?

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pH is a measure of the amount of free hydrogen in water. (The true techical definition goes way beyond this to the point of being pretty arcane and almost makes you wonder if pH means anything at all.) Since polypropylene in insoluble in water, it makes it pretty tough to measure the pH. Additionally, the hydrogens in polypropylene are not known for leaving the molecule easily - hence the general nonreactivity of the polymer. The ionization potential of methane is 12.98 eV 50 kJ/mole, which is a huge number. The pH would be correspondingly high, say 35 or some other meaningless number.
John Aspen Research - www.aspenresearch.com "Turning Questions into Answers"
Opinions expressed herein are my own and may not represent those of my employer.
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actually I meant the pH of PP originated from its impurities. some impurites like catalyst residuals.
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John you are correct in your response to this person. However, the industry has other more creative ways to relate this issue to cause and effect and I have answered this question based on those methods over the last 25 years.
Regards JRW
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If filling out some stupid form just say "not established".
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As the others have indicated pH is not really relevant when talking about polymers. If yougetting at is polypropylene a polar polymer or does it contain acidic or basic side groups, the answer would be that polypropylene is a fairly neutral, non-polar polyolefin polymer.
Larry

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moh3en wrote:

John Spevack of Aspen is correct when he says that PP is not soluble and you cannot make a pH measurement like a soluble salt. However, it is done all the time routinely in the PP industry from the plant level to the research level. What is typically done is to slurry the PP in a IPA water solution and either titrate the free chloride from the surface of the powdered resin made at the plant but in pellets this is not reasonable so the pH is not measured and either x ray fluorescene or neutron activation is used to measure chloride levels. Clearly neutron activation is superior while x ray is very misleading due to penetration of the xray on the pellet that is melt molded and looses chloride from the resin during the process. Others ignore pH and run corrosivity levels on metal surfaces by one or more procedures to test for the resins pH affect. Polypropylene gets it acidity from the residual catalyst and the various treatments used by the multiple old and new processes for making PP today. Remember catalyst systems change process alterations sometimes do not entirely change around the catalyst. No direct method can be definitive in getting a pH measurement so the industry has been creative in relating the acidity by other methods which are correlated to effect on stability and reactivity of additives by the residues. The only reason to measure pH is to know how this will affect corrosion on molding and on stabilization of the resin to LTHA,UV,Oxidative stabiity during processing. There is a hugh foundation in 25 years established using the above alternative methods to measure pH of PP.
JRW
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I think the distinction between pH and acidity is starting to get blurred. Ignoring molal activity coefficients and other such details, pH is the negative log of the concentration of hydrogen ions in water and is used for Bornsted acids/bases that donate/accept a hydrogen ion. The Lewis acids/bases accept/donate electron pairs, which is why chlorine, being a base, is able to make inert polypropylene acidic. To stretch this into pH is, in my humble opinion, making quite a leap, although I have seen even worse transgressions made because "everybody" understands pH and is comfortable working with that concept.
The story is told (I'd love to confirm this but have been unable to) of a professor testifying before Congress of the work he had done on a project, and how proud he was that he how lowered the pH of the waste from 13 down to 7. The congressperson then asks him: "If you took the pH from 13 down to 7, why didn't you just take it all the way down to 0?"
John Aspen Research - www.aspenresearch.com "Turning Questions into Answers"
Opinions expressed herein are my own and may not represent those of my employer.
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