Method Development for 

Generation Testing

28 October 2010

1

Outline

• Introduction

• Overview

• Test Protocol

• Observations

• Fluoropolymer chemistry

• Possible sources

• Context

• What did we learn?

• References

2
Method Development

Introduction

A joint working group composed of Environmental 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.

3

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.

4
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). 

5

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 test substance 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 level 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 -CF3 end and one 

-COOH end. 

– Thus need a pre-existing source of -CF3 endgroups to generate PFCAs

8
Fluoropolymer Chemistry

Possible Sources of -CF3 Endgroups

• Polymerization

– During polymer termination reaction, R-CF2* Æ R-CF3 can only occur in 

the presence of F* which cannot exist in an aqueous medium - Test 

Substance made by aqueous polymerization

– Exceptional sources of -CF3 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 F2 gas to improve polymer 

stability since the -CF3 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 -CF3 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 perfluoropolymers cannot yield R-CF3

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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* + CF2=CF-O-CF2-CF2-CF3 --->

RCF2-FC*-O-CF2-CF2-CF3 ---> 

RCF2-CF=O + *CF2-CF2-CF3

• This is a low frequency event; about 1% of the PPVE will 

follow this pathway under standard polymerization 

conditions. 

• The resulting -CF3 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 less than 0.1 mol% PPVE). 10

Fluoropolymer Chemistry

Possible Sources of -CF3 Endgroups
Fluoropolymer Chemistry

• -CF3 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 –CF2-CO2H, amide –

CF2-CON2H, perfluorovinyl –CF2-CF=CF2, acyl fluoride –CF2- COF, difluoromethyl –CF2-CF2H, ethyl –CF2-CH2-CH3

– see Pianca et al., J Fluorine Chem 93 (1999) 71-84

• Modified PTFE (with <0.1 mol% PPVE) would have 

lower levels of -CF3 end groups. 

• The -CF3 end group when present, is very stable (nonreactive). Note, some commercial fluoropolymers are 

post-fluorinated to create -CF3 end groups which further 

increase polymer stability and purity for specialty 

applications (i.e., semiconductors). 

11

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 occur in these fluoropolymers. 

• This changes the available reaction paths both during polymerization and 

degradation.

• There are paths during polymerization to -CF3 end groups. As 

previously noted, the relevant one is a low frequency side reaction. 

• There is no path during degradation to -CF3 end groups from 

fluoropolymers that don’t have -CF3 end groups to begin with.

• Monocarboxylic acids (e.g., PFOA) have both one -CF3 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 however, 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 potential 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 of PFCAs 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, 2nd 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, 3rd ed); vol 

3, Wiley (2003)

• Encyclopedia of Chemical Technology (Kirk-Othmer, 5th 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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