# Method Development for Generation Testing

28 October 2010

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# Outline

• Introduction • Overview • Test Protocol • Observations • Fluoropolymer chemistry • Possible sources • Context • What did we learn? • References

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# Method Development Introduction

A joint working group composed of Envi ronmental Protection Agency (EPA) and fluoropolymer industry representatives developed the basis for a method development project in 2005/2006.
The goal of this project was to develop a test method that would determine if perfluorooctanoic acid (PFOA) is generated as a degradation byproduct of fluoropolymers when heated.

Progress on this work has been communicated to EPA and documented in three progress reports to EPA.

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# Method Development Overview

• The first two reports covered development of sample handling techniques and the development of a test method to measure PFOA residuals at sub-ppb levels. • During the most recent method development work, the test method was further refined and the fluoropolymer test substance, a composite of PTFE samples manufactured without using PFOA, was subjected to solvent extraction (ASE) and thermal tests . • In both instances, PFOA, and in some cases other homologues, were detected at low levels (low ppb). • Data suggest a finite, low ppb source of these PFCAs, i.e., an impurity from external contamination and not from the thermal degradation of the polymer • Based on these method development results no additional work is contemplated at this time.

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# Method Development Test Protocol

• Initial work reported in previous progress reports established testing parameters which allowed good reproducibility at low levels of quantification (< 1 ppb). • During the most recent method development work, – The method was refined to improve sensitivity

. – Commercial PTFE samples (made without using PFOA) were received from member companies. – Samples were physically blended into a single sample (test substance) at another facility. – Some portions of the test substance were subjected to ASE. – Some portions of the test substance were subjected to ASE before and after the thermal test. – Some portions of the test substance were subjected to one or more 24 hour high temperature exposures (up to 315°C, which is higher than normal use temperatures).

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# Method Development Observations

• PFOA was detected at low levels (low ppb) on “as received” test substance portions using a solvent extraction technique (ASE).

– PFOA was not detected following subsequent ASE extractions of the same test substance portion. • PFOA was also detected at low levels (low ppb) in “as received” and in previously ASE extracted testsubstance portions using the thermal test.

– PFOA was detected at reduced levels following a second thermal test of the same test substance portion, under the same test conditions. – After additional thermal tests, levels of PFOA continued to decrease, or drop below the method’s lower leve l of quantification (<0.5 ppb). • In some tests, trace levels of other homologues of PFOA (PFHxA, PFNA, PFDA) were observed – The same pattern of level reduction was almost always observed for these substances after repeated ASE and thermal tests

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# Method Development Observations

• The pattern of low and decreasing levels of PFOA (and other PFCAs) with each subsequent 24 hour high temperature exposure is consistent with a finite, low ppb
source.

• This pattern suggests that PFOA (and other PFCAs) are present as impurities in the test substance rather than being continually generated during the thermal test or ASE extraction.

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# Fluoropolymer Chemistry

• Fluorocarbon and hydrocarbon chemistries are inherently different, reflecting the C-F and C-H bonds. The C-F bond is both stronger and has reversed polarity.

– C-F bond ~ 130 kcal/mol – C-C bond ~ 80 kcal/mol – C-H bond ~ 100 kcal/mol • Although H* abstraction is common in hydrocarbon systems, “F* abstraction” does not occur in these fluoropolymers. • This changes the available reaction paths both during polymerization and degradation. • Monocarboxylic acids (e.g., PFOA) have both one -CF 3 end and one -COOH end.

– Thus need a pre-existing source of -CF 3 endgroups to generate PFCAs

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# Fluoropolymer Chemistry
Possible Sources of -CF 3 

### Endgroups • Polymerization – During polymer termination reaction, R-CF 2 * Æ R-CF 3 can only occur in the presence of F* which cannot exist in an aqueous medium - Test Substance made by aqueous polymerization – Exceptional sources of -CF 3 perfluoroalkyl end groups • Perfluorinated initiators like perfluoroacyl peroxide initiators used in nonaqueous polymerizations; not relevant to these fluoropolymers. • Perfluoropropyl vinyl ether comonomer; see next slide. • Post fluorination of fluoropolymers – done with F 2 gas to improve polymer

stability since the -CF 3 end group is more stable than other end groups; not relevant to these fluoropolymers. – Conventional aqueous polymerized perfluoropolymers (like PTFE, FEP, PFA, ....) contain very few -CF 3 end groups • Degradation – The C-F bond is not susceptible to radical electrophilic attack (the C-C bond breaks, not the C-F bond) – Degradation of perfluoro polymers cannot yield R-CF 3

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### • Modified PTFE (test substance ingredient) has very small amount of PPVE in polymerization recipe • During polymerization, perfluoropropyl vinyl ether (PPVE) can be radically cleaved to form a perfluoropropyl radical and propagate a polymer chain

R* + CF 2 =CF-O-CF 2 -CF 2 -CF 3 ---> RCF 2 -FC*-O-CF 2 -CF 2 -CF 3 ---> RCF 2 -CF=O + *CF 2 -CF 2 -CF 3 

### • This is a low frequency event ; about 1% of the PPVE will follow this pathway under standard polymerization conditions. • The resulting -CF 3 

### end groups are expected to be observed at low ppm levels in PFA polymers made using PPVE (with 1-5 mol% PPVE) and at even lower levels in modified PTFE (with le ss than 0.1 mol% PPVE). 10 

# Fluoropolymer Chemistry
Possible Sources of -CF 3 

### Endgroups

# Fluoropolymer Chemistry

• -CF 3 

### end groups are not observed according to FTIR spectroscopy in PFA polymers made using PPVE (with 1-5 mol% PPVE).
– Observed end groups are: carboxylic acid –CF 2 -CO 2 H, amide – CF2 -CON 2 H, perfluorovinyl –CF 2 -CF=CF 2 , acyl fluoride –CF 2 - COF, difluoromethyl –CF 2 -CF2 H, ethyl –CF 2 -CH 2 -CH 3 – see Pianca et al., J Fluorine Chem 93 (1999) 71-84 

### • Modified PTFE (with <0.1 mol% PPVE) would have lower levels of -CF 3 

### end groups. • The -CF 3 

### end group when present, is very stable (nonreactive). Note, some co mmercial fluoropolymers are post-fluorinated to create -CF 3 

### end groups which further increase polymer stability and purity for specialty applications (i.e., semiconductors).

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# Fluoropolymer Chemistry

• Fluorocarbon and hydrocarbon chemistries are inherently different, reflecting the C-F and C-H bonds. The C-F bond is both stronger and has reversed polarity. • Although H* abstraction is common in hydrocarbon systems, “F* abstraction” does not occu r in these fluoropolymers. • This changes the available reaction paths both during polymerization and degradation.
• There are paths during polymerization to -CF 

### 3 end groups. As previously noted, the relevant one is a low frequency side reaction. • There is no path during degradation to -CF 

### 3 end groups from fluoropolymers that don’t have -CF 

### 3 end groups to begin with. • Monocarboxylic acids (e.g., PFOA) have both one -CF

### 3 end and one -COOH end. • Therefore one would not expect to observe monocarboxylic acids as either polymerization byproducts or degradation products.

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# Method Development Possible Sources

• PFOA is not intentionally used in the manufacturing and finishing of the commercial products used in this project.
• One of the samples in the test substance was a modified PTFE. Modified PTFEs are manufactured using a small amount of comonomer (e.g., less than 0.1 mol% of a perfluoropropyl vinyl ether).

– During polymerization this comonomer can produce some -CF3 end groups. This is a low frequency side reaction.
• The fluoropolymers sampled howeve r, were produced in facilities, and/or equipment, where PFOA was routinely used in the manufacture of other commercial fluoropolymers, or in facilities where fluorotelomers were manufactured. This could lead to possible low levels of contamination by PFOA or other homologues.

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# Method Development Context

• In order to create useful articles of commerce e.g. chemical tanks, seals, gaskets, etc. fluoropolymers such as these commercial samples are generally processed at high temperatures (~ 400°C). • These processing temperatures exceed the method development conditions and the thermal stability of PFOA. So any PFCA impurities present in processed articles will be lower than those found during method development.
• Given the volume of fluoropolymers in commerce and the low levels quantified in this method, the maximum amount of PFOA (and other PFCAs) that could be released from this source is low.

– Based on the global fluoropolymer production reported by CEFIC, the quantity of PFOA from this source (20 ppb x 40,000 MT/year = 800g/year) would present a low potent ial for exposure even if the fluoropolymer was not subject to high temperature processing (sintering/melt processing).

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# What did we learn?

• The method developed is able to quantify low levels of PFOA (<1ppb) in this test substance due to an improved, dual-path technique. • Purification of test substance was more difficult and complex than originally expected. • Thermal treatment was more efficient than ASE in extracting PFCAs from test substance. – Average amount of PFOA was 8 ppb, total PFCAs were about 25 ppb – The detected amount ofPFCAs almost always decreased as time at temperature increased

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# What did we learn?

• Data indicate a finite, low ppb source of PFCAs (impurities). This is consistent with what is known about fluoropolymer chemistry.

• Based on these conclusions, no additional work is contemplated.

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# References

• CEFIC. Global Fluoropolymer Production Data, 2 nd quarter 2009 (available to members) • Drobny, J George; Technology of Fluoropolymers; CRC Press (2001) • Ebnessajjad, Sina; Fluoroplastics; vol 1 & 2, PDL (2000, 2003) • Encyclopedia of Material Science & Engineering (Bever); vol 3, MIT Press (1986) • Encyclopedia of Polymer Science & Technology (Mark, 3 rd ed); vol 3, Wiley (2003) • Encyclopedia of Chemical Technology (Kirk-Othmer, 5 th ed); vol 18, Wiley-Interscience (2007) • Pianca et al., J Fluorine Chem, vol 93 (1999) pp 71-84 • Scheirs, John; Modern Fluoropolymers; Wiley (1997)

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