Document NDr5vJENDE494OQvEN73vaNy

FEATURED TOPIC Elucidation of Molecular Structure and Adhesion State of Cross-Linked Fluororesin Kazuaki IKEDA*, Yugo KUBO, and Kentaro OKAMOTO ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Fluororesin, represented by polytetrafluoroethylene (PTFE), is an excellent polymer that has the lowest level of friction performance among solids and the highest level of heat resistance, chemical resistance, weather resistance, and electrical properties among resins. In contrast, it has poor wear resistance and hardly adheres to substrates when used as a composite material. Through electron beam irradiation under special conditions, we promoted the cross-linking reaction of fluororesin and developed a new material that features wear resistance improved nearly 1,000 times compared with the conventional material and good adhesion to substrates. This paper investigates the cross-linking state of the new material using solid-state nuclear magnetic resonance, and elucidates its adhesion state to substrates utilizing SPring-8 hard X-rays. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: cross-linking, fluororesin, electron beam irradiation, wear resistance, adhesion 1. Introduction Fluororesin, typified by polytetrafluoroethylene (PTFE), is an excellent polymer, having the highest levels of stability in heat resistance, chemical resistance, and weather resistance among commercially available resins due to its structure wherein carbon molecular chains are closely surrounded by fluorine molecules as shown in Fig. 1, in addition to the strongest C-F bond among single bonds (Fig. 2). Further, fluororesin exhibits excellent non-adhesiveness and sliding performance (due to its coefficient of friction being lower than that of any other solid). The reason is that fluorine atoms with a small van der Waals radius are arranged in the surface, and this reduces the surface free energy--which indicates the magnitude of interaction with substances approaching the surface--to the lowest value among resins. However, when viewed from another point, the nonadhesiveness and low friction of fluororesin make it difficult for it to adhere to or combine with metals and other materials, thereby preventing its use in industrial products. Such stable characteristics of this resin hinder its chemical modification by a chemical agent, or physical modification by ionizing irradiation. Fluorine molecule (Carbon molecular chain atthe center) Fig. 1. Schematic illustration of PTFE molecule Since the fluorine molecules are bonded very weakly to each other in fluororesin, they are easily separated from the surface by an external force as shown in Fig. 2. This Fig. 2. Molecular structure and feature of PTFE degrades the wear resistance of the resin. Sumitomo Electric Industries, Ltd. has developed cross-linked fluororesin from which shortcomings are removed by electron beam irradiation under special conditions, and has supplied it under the tradename "FEX" (fluoro ethylene crosslinking (X-linking)). The effect of adhesiveness improvement by crosslinking was evaluated by a cross-cut tape peeling test (in compliance with JIS-K-5400) by peeling the tape 500 times. Since non-cross-linked fluororesin was easily separated from the substrate and therefore useless for the evaluation, Sumitomo Electric's SUMIFLON coating, whose adhesiveness was improved to a practical level by etching the metal surface, was used for comparison. After the test, approximately half of the cross-cut SUMIFLON coating was separated (white portions in the picture on the left of 46 Elucidation of Molecular Structure and Adhesion State of Cross-Linked Fluororesin Table 1), while no separation of the cross-linked fluororesin FEX coating was observed. Product name Adhesion method Table 1. Adhesion test result SUMIFLON Etching Cross-linked fluororesin FEX Electron beam irradiation Cross-cut test result The wear resistance of cross-linked fluororesin FEX was evaluated using a ring-on-disk friction and wear test (in compliance with JISK-7218) (Photos 1 and 2, and Fig. 3). With the test sample pressed against a S45C (carbon steel) at 10 MPa, the rotation speed of the sample was gradually increased, and the weight loss of the sample was measured. The measurement results are shown in Fig. 4. The cross-linking of the fluororesin greatly enhanced its wear resistance to a level far exceeding that of resins commonly used as slide members (poly-ether-ether-ketone (PEEK), polyacetal (POM), and polyphenylene sulfide (PPS)). This paper reports on the analysis and examination results for the molecular structure of cross-linked fluororesin and its adhesion state to a substrate. Photo 2. Ring-on-disk friction and wear test equipment Test load (10 MPa) S45C ring High-speed rotation Test sample Fig. 3. Wear test method S45C ring Test sample Photo 1. Wear test sample set on the test equipment 2. Analysis of the Molecular Structure of Cross-Linked Fluororesin A change in the molecular structure of PTFE by electron beam irradiation was evaluated by solid-state nuclear magnetic resonance (NMR). Figure 5 shows the solid-state 19F MAS NMR spectra of PTFE before and after electron beam irradiation. Before the electron beam irradiation, the spectrum of PTFE had only one peak at = -122 ppm, but after the irradiation, multiple small peaks were detected. Polyether Ether Ketone (PEEK) Polyactal (POM) Polyphenylene Sulfide (PPS) Non-Cross Linked PTFE Cross Linked PTFE (FEX) Fig. 4. Ring-on-disk friction and wear test result SEI TECHNICAL REVIEW NUMBER 90 APRIL 2020 47 The molecular structure assigned to each peak is shown in Fig. 5. It was confirmed that electron beam irradiation not only generates an end group ( = -82 ppm) by breaking the molecular chains of the tetrafluoroethylene unit, but also produces branched structures ( = -72, -100 to -120, -185 ppm).(1) PTFE usually breaks only molecular chains when irradiated with an electron beam. However, the above results suggest that, under such special irradiation conditions as in this experiment, PTFE not only causes the molecular chain to branch, but also forms a cross-linked structure. the inherently low cohesive force between the molecules of PTFE and to have dramatically improved its wear resistance. Molar ratio of binding amount (%) a) FF CC FF Fig. 6. Change in chemical bond amount due to electron beam irradiation b) CF2 CF3 CF2 CF CF2 CF2 CF3 CF CF2 CF CF2 FF CF3 CC F3C CC FF CC *Spinning sideband Fig. 5. Solid-state 19F MAS NMR spectra of cross-linked PTFE a) Before irradiation b) After irradiation It was found that electron beam irradiation causes both molecular chain breakage and cross-linking reactions, as described above. To stoichiometrically investigate the degree of progress of each reaction, the change in the bonding amount of each bonded structure associated with electron beam irradiation was evaluated based on the area ratio of each assigned peak (which is equivalent to the molar ratio of a molecular structure). Figure 6 shows the dependence of the change in the chemical bond amount ratio on the irradiation dose. As this figure shows, increasing the irradiation dose increased dramatically branched -CF = bonds compared to -CF2-CF3 bonds at the end of the molecular chains. The molecular structures at the branched ends were also compared in terms of a short chain that becomes a trifluoromethyl group (-CF3) and a long chain having two or more carbon atoms. As shown in Fig. 7, electron beam irradiation had a tendency to preferentially recombine long chain molecular structures with the branch points. It was found from these changes that PTFE breaks its molecular chains when exposed to an electron beam (Fig. 8). However under special conditions, the ends of the generated molecular chains are recombined with the original molecular chains, and this promotes the branching of the molecular chains as shown in Fig. 9. The above-described molecular structural change to a cross-linked polymer is considered to increase Fig. 7. Change in bond structure due to electron beam irradiation Fig. 8. Breaking of molecule by electron beam Fig. 9. Molecular structure of cross-linked fluororesin 48 Elucidation of Molecular Structure and Adhesion State of Cross-Linked Fluororesin 3. Elucidation of the State of Adhesion between Cross-Linked Fluororersin and a Substrate A hard X-ray photoelectron spectroscopic (HAXPES) analysis was carried out to elucidate the mechanism of improving the adhesiveness of cross-linked fluororesin to a metal substrate by electron beam irradiation (Table 1). Since the maximum analyzable depth of HAXPES is several tens of nanometers, the depth of the interface to be analyzed was required to lie within the above limit. We developed a method for fabricating an analytical sample that can be used to simulate the actual process (resin coating on a metal substrate) while satisfying the above requirement for analysis depth. An outline of the sample fabrication and analysis methods is shown in Fig. 10. Fe and fluororesin layers were sequentially laminated on a thin SiO2 film layer formed on a smooth-surfaced GaAs substrate, and only the GaAs substrate and the SiO2 film layer were removed from the sample after it was irradiated with electron beams under special conditions.(2) The morphology of the cross section of the sample was observed with a transmission electron microscope (TEM). The results revealed that a thin Fe film layer about 20 nanometers thick had been formed on the fluororesin, which was several tens of micrometers thick, verifying that the sample had the expected structure (Fig. 11). The chemical state of the Fe/resin interface in this sample was analyzed using the HAXPES installed in the SPring-8, a large synchrotron radiation facility. In the analysis, an X-ray with an incident X-ray energy of about 8 keV was delivered to the top surface of the thin Fe film to detect the photoelectron signal of the interface. In this process, electrostatic charge was neutralized with an electron gun to prevent the shifting of photoelectron peaks due to charging. Figure 12 shows the HAXPES analysis results (C1s spectrum) for two types of samples with and without electron beam irradiation. Fig. 12. HAXPES analysis result for Fe/fluororesin sample (The figure on the bottom shows analysis results for fluororesin alone (C 1s spectrum)) Fig. 10. Outline of sample fabrication and analysis methods Fig. 11. TEM observation result for a cross section of Fe/fluororesin sample and schematic illustration of sample layer structure SEI TECHNICAL REVIEW NUMBER 90 APRIL 2020 49 Fig. 13. Estimation of mechanism for improving adhesiveness of Fe/resin interface by electron beam irradiation One of the two peaks observed, the peak near 291 eV, was also found in the spectrum of fluororesin alone (bottom figure of Fig. 12), suggesting that this peak was derived from C of -CF2-. However, the peak near 285 eV is not observed in the case of fluororesin alone. Furthermore, since the height of this peak increased as the X-ray take-off angle decreased, this suggests that this peak was attributable to a bond being present near the interface between the Fe and the fluororesin. This peak also suggests the presence of a chemical bond (Fe-O-C) at the interface between the Fe and the resin. For a combination of various metals and resins, many papers report the bonding mechanism between the metal elements and the carbon in the resin at the metal/resin interface via oxygen in the metal oxide.(3) When the HAXPES analysis results are taken into account in conjunction with the conclusions of these papers, fluororesin is considered to adhere to a metal via the oxide film on the metal surface (Fig. 13). 4. Conclusion The characteristics of the fluororesin (PTFE) and cross-linked fluororesin (FEX) are shown in Table 2. The fluororesin is excellent in stability and electrical properties, but has a disadvantage in terms of wear resistance and adhesiveness to substrates. The cross-linked fluororesin FEX has eliminated the disadvantage of PTFE and expanded its application range in various industries by taking advantages of the intrinsic excellent characteristics of fluororesin. FEX is commercially available in the forms of crosslinked fluororesin sheets and tapes processed from the sheets (Photo 3), aluminum and steel substrates coated with cross-linked fluororesin, and products made by stamping these substrates. In practice, slide bearings (Photo 4), oil pumps (Photo 5), sliding packing, nip plates for office automation (OA) equipment, and various types of molds and jigs are coated with FEX. Table 2. Summary of characteristics of PTFE and FEX PTFE W(liemairt rPeVsi-svtaanlucee MPam/min) About 1 Adhesion to substrate Difficult (essential to use primer or other suitable coat) Coefficient of kinetic friction L(0o.0w8e)st level among solids Heat ressitance 260 Chemical resistance Inactive (except with hot aqua regia) Flame resistance (limiting oxygen index) 95% min. Weather resistance Can be used outdoors for a long period Dielectric constant (103 - 106 Hz) D150 Lowest level among solids (2.1) Dielectric dissipation factor (106 Hz) D150 Lowest level among solids (2 10-3) Pencil hardness HB FEX 1,000-1,200 Possible to adhere Same as on the left Same as on the left Same as on the left Same as on the left Same as on the left Same as on the left Same as on the left Same as on the left Photo 3. Cross-linked fluororesin tapes Photo 4. Slide bearing Photo 5. Oil pump 50 Elucidation of Molecular Structure and Adhesion State of Cross-Linked Fluororesin Sumitomo Electric Fine Polymer, Inc. has introduced various fluororesin products into the market. These products include fuser rollers for OA equipment, porous membranes (POREFLON), and rice cooker pots (SUMIFLON), as well as heat-shrinkable tubing made using an electron beam irradiation technique (SUMITUBE). Cross-linked fluororesin, FEX, is a new product developed by combining two core technologies of the company: fluorine processing and electron beam irradiation. We will work to cultivate the application of this product and spread it, thereby developing it as a new pillar of our business subsequent to the current pillars. 5. Acknowledgments Part of the sample fabrication was supported by the Nanotechnology Platform Project launched by the Ministry of Education, Culture, Sports, Science and Technology (Kyoto University Microfabrication Platform). The HAXPES experiments were carried out using BL16XU and 46XU installed in SPring-8 (Research task numbers: 2015B5030, 2015B1870, 2016A5031, 2016B5030). F EX, SUMIFLON, POREFLON, and SUMITUBE are trademarks or registered trademarks of Sumitomo Electric Industries, Ltd. References (1)Radiation Physics and Chemistry 54 (1999) 165-171 (2)Y. Kubo, H. Tanaka, Y. Saito, A. Mizoguchi, ACS Appl. Interfaces 2018, 10, 51, 44589-44602 (3)S. Maeda, J. Jpn. Soc. Colour Mater, 2005, 78, 3, 131-140 Mater. Contributors The lead author is indicated by an asterisk (*). K. IKEDA* A ssistant General Manager, Advanced Automotive Systems R&D Center Y. KUBO Ph.D. Assistant General Manager, Analysis Technology Research Center K. OKAMOTO Ph.D. Assistant Manager, Analysis Technology Research Center SEI TECHNICAL REVIEW NUMBER 90 APRIL 2020 51