Pentadecafluorooctanoic-acid-free polytetrafluoroethylene and mechanism of PFOA formation by γ-irradiation

Abstract

Low-molecular-weight (M w) polytetrafluoroethylene (PTFE) micropowder is added to wax for use in automotive equipment and printing machines and is produced by radiation-initiated degradation under atmospheric conditions. However, pentadecafluorooctanoic acid (PFOA) is produced as a by-product in concentrations greater than 25 ppb, which is problematic because PFOA does not degrade in the environment. Herein, we clarify the PFOA-formation mechanism and develop a manufacturing process for a novel low-M w PTFE micropowder that does not contain PFOA (less than 5 ppb). The process uses combined irradiation and heat treatment in an oxygen-free atmosphere. Furthermore, PFOA-free PTFE micropowder can be produced on the 10-kg scale. Pentadecafluorooctanoic acid (C 7 F 15 COOH, PFOA), its salts, and PFOA-related compounds have excellent surfactant properties and are used to give textiles water repellency and antifouling properties, as well as to prevent the scorching of kitchenware. However, they are not degraded in the natural environment and have high bioaccumulation properties. Thus, they are a cause of concern in relation to environmental pollution and human health damage. The Registration, Evaluation, Authorisation and Restriction of Chemical (REACH) legislation in the European Union (EU) will apply to the manufacture and sales of PFOA-containing products in the EU from July 4, 2020; hence it is necessary to reduce PFOA contamination to less than 25 ppb and that of PFOA-related compounds to less than 1,000 ppb 1. Moreover, in the Stockholm Convention on Persistent Organic Pollutants (POPs), regulations concerning PFOSs are under consideration by the POPs Review Committee 2 and will be added to Annex A at the ninth meeting of the Conference of the Parties to the Stockholm Convention (COP9) 3. Low-molecular-weight (low-M w) polytetrafluoroethylene (PTFE) micropowder is a powerful additive used in diverse industrial applications. With its high surface lubricity and anti-blocking properties, its micropowder is becoming accepted in industries that include food, textiles, pharmaceuticals, and electronics, and its global market was worth US$ 400 million in 2018 4. PTFE micropowder is produced industrially by the chain scission of its polymerised powder or moulded scraps induced by 60 Co γ-rays, electron beam (EB) from accelerator or EB conversion X-rays irradiation in air. However, PFOA is produced at concentrations of more than 25 ppb as a by-product of radiation processing 5. For this reason, PTFE micropowder does not comply with regulations and may cause serious problems in industrial applications. Therefore, to comply with regulations, a method for the manufacture of low-M w PTFE with low PFOA contents (< 25 ppb) is required. Furthermore, its formation mechanism needs to be elucidated. In this paper, we clarify the formation mechanism and report a manufacturing process for a novel low-M w PTFE with < 25 ppb PFOA. Note that 'PFOA-free' means below the detection limit (< 5 ppb) when analysed by liquid chromatography/mass s...

Trapped radical behavior of electron beam irradiated polytetrafluoroethylene fine powder at various temperatures

“…The observed spectra showed asymmetric signals, and they were assigned as peroxide macroradicals. These are a superposition of major asymmetric signals assigned from the middle-chain type peroxide macroradical and minor symmetric signals from the end-chain type peroxide macroradical, as already reported 16 – 18 . It was noticed that the signal intensity was more intense when the irradiation temperature was higher.…”

“…It was reported previously that the radical decay of PTFE accelerated with heat treatment 17 , 18 , the thermal stability of the fluoroalkyl and chain-end radicals are different, and the chain-end radicals were decayed faster 18 . It is suggested that the difference in the spectral shape in Fig.…”

“…PTFE fine powder has a crystallinity of 93%. The trapped radicals gradually decay, even when stored for a long time at 297 K 18 . The decay of radicals is accelerated by heat treatment (see Supplementary Fig.…”

“…As the hyperfine splitting of F-atoms has a state of anisotropy and a relatively large value, the ESR spectrum shows their specific broad patterns originating from these radical species. It has already reported that both fluoroalkyl and end-chain radicals react easily with oxygen and convert to the peroxide macroradicals 16 – 18 . That is, the former one is an fluoroalkyl-based chain type peroxide macroradical –CF 2 –CF(OO·)–CF 2 –: middle-chain) and the latter one is an end-chain type peroxide macroradical (–CF 2 –CF 2 OO·: end-chain), which are mainly induced in the crystalline region 16 , 17 .…”

“…Although the lifetime (half-decay time) of trapped peroxide macroradicals is estimated to be 1000 h when stored at 297 K under an oxygen-free atmosphere, radicals are gradually annihilated by heat relaxation 17 , 18 .…”