Microstructural and chemical changes in perfluoroalkoxy polymer (PFA) improve lifetime of PFA-alumina composite materials by 10,000x

Mark A. Sidebottom1, Christopher P. Junk 2, Holly L.S. Salerno3, Heidi E. Burch3,Greg S. Blackman3, Brandon A. Krick4

1Mechanical and Manufacturing Engineering Miami University, Oxford, OH

2,4Material Science and Engineering, Lehigh University, Bethlehem, PA

3DuPont Company, Wilmington, DE

INTRODUCTION: Though it has similar mechanical, thermal, and chemical properties as polytetrafluoroethylene (PTFE), perfluoroalkoxy polymer (PFA) has rarely been evaluated in tribological (friction and wear) applications 1,2, which is peculiar considering PTFE is one of the most studied polymeric tribological materials. In addition to its unique physical, thermal, and chemical properties, PFA can be melt-processed instead of the sintering-based methods of PTFE. This allows filler particles or fibers to be melt mixed with PFA and this mixture can be used to produce complex geometries via screw-injection molding.

Recently, the wear rate of PFA was shown to improve nearly four orders of magnitude with the addition of 5-10 wt. % of α-alumina filler particles 3, highlighting the need for a better understanding of the fundamental aspects of how shear stress may affect the microstructure and chemistry of PFA. The objective of this work is to link the chemical and microstructural observations of unfilled PFA the observations seen in PTFE-alumina and PFA-alumina composite systems.

METHODS:  To evaluate the fundamental aspects of wear of unfilled PFA and PFA-alumina composites, differential scanning calorimetry (DSC), x-ray diffraction (XRD), transmission infrared spectroscopy, and attenuated total reflectance fourier transform infrared spectroscopy (ATR-FTIR) were used on bulk and worn samples of unfilled PFA and PFA-alumina composites were slid against  stainless steel counterfaces at 6.25 MPa of applied pressure and a sliding velocity of 50 mm/s.

RESULTS & DISCUSSION: Tribological experiments revealed a 10,000x improvement in wear of PFA-alumina composites (K~4.0x10-8mm3/Nm) compared to unfilled PFA (K~4.0x10-4 mm3/Nm). Additionally, PFA-alumina composites were found to have similar friction (µ~0.25) which is surprising considering unfilled PTFE (µ~0.12 has a friction coefficient nearly 2x lower than that of unfilled PFA (µ~0.24). Wear properties of PTFE-alumina composites (K~4.4x10-8 mm3/Nm) matched those observed in filled PTFE composites. The reduction in wear rate for PTFE-alumina and PFA-alumina composites has been attributed to tribochemical reactions of broken fluoropolymer chains, environmental constituents, and the metallic countersurface/aluminum oxide filler material. These reactions promote the development of running films and transfer films that protect the surface of the polymer composite and steel counterface.

To better understand this tribochemical mechanism maybe different in PFA compared to PTFE, the effects of sliding on the microstructure and chemical makeup of PFA were investigated (Figure 1). From observations of DSC endotherms, PFA wear debris had higher crystallinity compared to bulk PFA. Additionally, XRD of unworn and worn PFA showed an increase in crystallite size for the wear debris. Both these observations point to microstructural rearrangement of the PFA backbone due to stress applied during sliding, which have also been reported in the wear of unfilled PTFE  and drawing of PFA films 4,5. Higher concentrations of PAVE comonomer were seen in ATR-IR spectra of PFA wear debris, suggesting preferential crack propagation through the amorphous region of the microstructure of PFA during sliding. Similar repeated loading during MIT flex life experiments on PFA showed that PFA samples with lower crystallinity of PFA

(higher % amorphous region) exhibited improved fatigue performance, suggesting that cracks preferentially travel through the amorphous region of PFA 6. Transmission IR spectra of PFA wear debris revealed the presence of free carboxylic acid endgroups that were not present in the bulk PFA surface. The presence of free carboxylic acids is due to the breaking of C-C bonds in the PFA backbone reacting with environment constituents. The formation of free-carboxylic acids endgroups is a critical intermediate step of the tribochemical reaction necessary to form robust transfer films and running films proposed for PTFE-alumina composites 7.


Figure 1 – Effects of sliding on unfilled PFA against a stainless steel substrate

REFERENCES:

1. Bowers, R. C. & Zisman, W. A. Frictional Properties of Tetrafluoroethylene-Perfluoro(propy1 vinyl ether) Copolymers. 13, 115–118 (1974).

2. Crosby, J. M., Theberge, J. E. & Talley, K. L. Melt Processible Fluropolymer Composites. J. Elastomers Plast. 13, 149–155 (1981).

3. Sidebottom, M. A. et al. Ultralow wear Perfluoroalkoxy (PFA) and alumina composites. Wear 362–363, 179–185 (2016).

4. Khedkar, J., Negulescu, I. & Meletis, E. I. Sliding wear behavior of PTFE composites. Wear 252, 361–369 (2002).

5.  Fujimori, A. & Hayasaka, Y. Changes in arrangement of lamella and fine crystallite in fluorinated ‘crystalline’ transparent fibers with drawing. Macromolecules 41, 7607–7615 (2008).

6.  Lee, J. C., Namura, S., Kondo, S. & Abe, A. Miscibility and cocrystallization behavior of two melt-processable random copolymers of tetrafluoroethylene and perfluoroalkylvinylether. Polymer (Guildf). 42, 5453–5461 (2001).

7.  Harris, K. L. et al. PTFE Tribology and the Role of Mechanochemistry in the Development of Protective Surface Films. Macromolecules 150526153713007 (2015). doi:10.1021/acs.macromol.5b00452