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Elucidation of Molecular Structure and Adhesion Stateof 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 amongresins. In contrast, it has poor wear resistance and hardly adheres to substrates when used as a composite material. Throughelectron beam irradiation under special conditions, we promoted the cross-linking reaction of fluororesin and developed a newmaterial that features wear resistance improved nearly 1,000 times compared with the conventional material and good adhesion tosubstrates. This paper investigates the cross-linking state of the new material using solid-state nuclear magnetic resonance, andelucidates 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 levelsof stability in heat resistance, chemical resistance, andweather resistance among commercially available resinsdue to its structure wherein carbon molecular chains areclosely 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 isthat fluorine atoms with a small van der Waals radius arearranged in the surface, and this reduces the surface freeenergy—which indicates the magnitude of interaction withsubstances 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 othermaterials, 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 fromthe 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 areremoved 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 (incompliance 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, whoseadhesiveness 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 of46 · 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.
Table 1. Adhesion test result
Product nameAdhesion method

SUMIFLON
Etching

Cross-linked fluororesin
FEXElectron 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, andFig. 3).
With the test sample pressed against a S45C (carbonsteel) at 10 MPa, the rotation speed of the sample wasgradually increased, and the weight loss of the sample wasmeasured. 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
Test sample
High-speed rotation
Fig. 3. Wear test method

2. Analysis of the Molecular Structure ofCross-Linked Fluororesin

S45C ring

Test sample

Photo 1. Wear test sample set on the test equipment

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, thespectrum of PTFE had only one peak at δ = −122 ppm, but
after the irradiation, multiple small peaks were detected.
Cross Linked PTFE (FEX)

Polyether Ether Ketone (PEEK)Polyactal (POM)
Polyphenylene Sulfide (PPS)Non-Cross Linked PTFE

Fig. 4. Ring-on-disk friction and wear test result

SEI TECHNICAL REVIEW · NUMBER 90 · APRIL 2020 · 47a)

F F
C C
F F

CF 2 CF 3b)

CF 2 CF CF 2
CF 3

F 3C

C C

CF 2 CF CF 2

the inherently low cohesive force between the molecules ofPTFE and to have dramatically improved its wear resistance.

Molar ratio of binding amount (%)

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 (δ = -82 ppm) by breaking themolecular chains of the tetrafluoroethylene unit, but also
produces branched structures (δ = -72, -100 to -120, -185ppm).(1) PTFE usually breaks only molecular chains whenirradiated with an electron beam. However, the above
results suggest that, under such special irradiation conditions as in this experiment, PTFE not only causes themolecular chain to branch, but also forms a cross-linkedstructure.

Fig. 6. Change in chemical bond amount due to electron beam irradiation

CF 2 CF 3
F FC CCFF FC C

*Spinning sideband

Fig. 7. Change in bond structure due to electron beam irradiation

Fig. 5. Solid-state 19F MAS NMR spectra of cross-linked PTFEa) 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 thedegree of progress of each reaction, the change in thebonding amount of each bonded structure associated withelectron beam irradiation was evaluated based on the area
ratio of each assigned peak (which is equivalent to themolar ratio of a molecular structure).
Figure 6 shows the dependence of the change in thechemical bond amount ratio on the irradiation dose. As thisfigure shows, increasing the irradiation dose increased
dramatically branched -CF = bonds compared to -CF2-CF3
bonds at the end of the molecular chains. The molecularstructures at the branched ends were also compared interms 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 atendency to preferentially recombine long chain molecular
structures with the branch points. It was found from thesechanges 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 asshown in Fig. 9. The above-described molecular structuralchange to a cross-linked polymer is considered to increase

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 ofimproving the adhesiveness of cross-linked fluororesin to ametal substrate by electron beam irradiation (Table 1).
Since the maximum analyzable depth of HAXPES isseveral tens of nanometers, the depth of the interface to beanalyzed 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 (resincoating 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. Feand fluororesin layers were sequentially laminated on athin SiO2 film layer formed on a smooth-surfaced GaAs
substrate, and only the GaAs substrate and the SiO2 filmlayer were removed from the sample after it was irradiatedwith 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 thatthe sample had the expected structure (Fig. 11).
The chemical state of the Fe/resin interface in thissample was analyzed using the HAXPES installed in theSPring-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 toprevent the shifting of photoelectron peaks due to charging.
Figure 12 shows the HAXPES analysis results (C1sspectrum) 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 wasderived from C of -CF2-.
However, the peak near 285 eV is not observed in thecase of fluororesin alone. Furthermore, since the height ofthis 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 chemicalbond (Fe-O-C) at the interface between the Fe and theresin.
For a combination of various metals and resins, many
papers report the bonding mechanism between the metalelements 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).

Table 2. Summary of characteristics of PTFE and FEXPTFEFEX

Wear resistance
About 1(limit PV-value MPa•m/min)

1,000–1,200

Adhesion to substrate

Difficult(essential to use primer orother suitable coat)

Possible to adhere

Coefficient of kinetic friction

Lowest level among solids(0.08)

Same as on the left

Heat ressitance

260 ℃

Same as on the left

Chemical resistance

Inactive(except with hot aqua regia)

Same as on the left

Flame resistance(limiting oxygen index)

95% min.

Same as on the left

Weather resistanceCan be used outdoors for along period

Same as on the left

Dielectric constant(103 – 106 Hz) D150

Lowest level among solids(2.1)

Same as on the left

Dielectric dissipation factor(106 Hz) D150

Lowest level among solids(2 × 10-3)

Same as on the left

Pencil hardness

HB

Same as on the left

4. Conclusion
The characteristics of the fluororesin (PTFE) andcross-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 andadhesiveness to substrates. The cross-linked fluororesin
FEX has eliminated the disadvantage of PTFE andexpanded its application range in various industries bytaking advantages of the intrinsic excellent characteristicsof fluororesin.
FEX is commercially available in the forms of crosslinked fluororesin sheets and tapes processed from thesheets (Photo 3), aluminum and steel substrates coated withcross-linked fluororesin, and products made by stamping
these substrates. In practice, slide bearings (Photo 4), oilpumps (Photo 5), sliding packing, nip plates for officeautomation (OA) equipment, and various types of molds
and jigs are coated with FEX.

Photo 3. Cross-linked fluororesin tapesPhoto 4. Slide bearing

50 · Elucidation of Molecular Structure and Adhesion State of Cross-Linked Fluororesin

Photo 5. Oil pump

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 wellas heat-shrinkable tubing made using an electron beam irradiation technique (SUMITUBE).
Cross-linked fluororesin, FEX, is a new productdeveloped by combining two core technologies of thecompany: fluorine processing and electron beam irradiation.
We will work to cultivate the application of thisproduct and spread it, thereby developing it as a new pillarof our business subsequent to the current pillars.

5. Acknowledgments
Part of the sample fabrication was supported by theNanotechnology Platform Project launched by the Ministryof Education, Culture, Sports, Science and Technology(Kyoto University Microfabrication Platform). The
HAXPES experiments were carried out using BL16XU and46XU installed in SPring-8 (Research task numbers:
2015B5030, 2015B1870, 2016A5031, 2016B5030).
• ‌FEX, SUMIFLON, POREFLON, and SUMITUBE are trademarks or registeredtrademarks 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. Mater.
Interfaces 2018, 10, 51, 44589-44602(3)	S. Maeda, J. Jpn. Soc. Colour Mater, 2005, 78, 3, 131-140

Contributors The lead author is indicated by an asterisk (*).

K. IKEDA*• ‌Assistant General Manager, Advanced Automotive
Systems R&D Center

Y. KUBO• ‌Ph.D.
Assistant General Manager, Analysis TechnologyResearch Center

K. OKAMOTO• ‌Ph.D.
Assistant Manager, Analysis Technology ResearchCenterSEI TECHNICAL REVIEW · NUMBER 90 · APRIL 2020 · 51