Photon Factory Activity Report 2016 #34 (2017)

BL-13B/2016G529

Chemical State Analysis of Rubber−Brass Adhesive Interface
by Soft and Hard X-ray Photoelectron Spectroscopy

2 EXPERIMENTAL
The HAXPES measurements were carried at BL47XU
of SPring-8 using a 7.94-keV hard X-ray for the in situ
chemical state analysis of a buried rubber−brass interface.
The rubber−brass interface was fabricated by heating a
brass plate, whose surface was covered with a thin rubber
film, at 170oC under pressurization of 8 MPa. The XPS
measurements using soft X-rays were done at BL-13B.
Interface-exposed samples were prepared by a filter-paper
method [2] and were measured to assess the trend of thecompositional change upon adhesion degradation.

(a) S 1s spectra(h = 7.49 keV)(b) S 2p spectra

10 min40 min10 min50 min2475
2470
Binding Energy (eV)

2475
2470
Binding Energy (eV)Intensity (arb. units)

1 INTRODUCTION
Automotive tires require a mechanical strength while
keeping flexibility of rubber. The mechanicalreinforcement is achieved by steel cords, which areembedded in the rubber. To realize strong rubber−cordadhesion, brass-plated steel cords are used, since
rubber−brass bonding is superior to rubber−steel bonding.
Since pioneering works by van Ooij [1], it has beenrecognized that copper sulfides, which are formed at therubber−brass interface, act as an adhesive agent. However,details of the agent, especially its chemical composition,
have not been clearly understood.
X-ray photoelectron spectroscopy (XPS) and Auger
electron spectroscopy are powerful tools to investigate
chemical composition of solids. However, these electron
spectroscopy techniques are sensitive to the surface
because of short inelastic mean free paths (IMFPs) of theelectrons in solids. If one wants to study a buriedinterface, the interface must be exposed to the surface forthe measurements. However, the chemical state of theadhesive interface may be affected upon interface
exposure procedures. Therefore, bulk-sensitive techniquessuch as X-ray absorption spectroscopy and X-ray
emission spectroscopy are more suitable for an in situ
investigation of the buried interface. XPS utilizing hard
X-ray (HAXPES) is another bulk-sensitive technique,since emitted photoelectrons have relatively high kinetic
energies so that large IMPFs are expected. In the presentstudy, we have examined the chemical composition of therubber−brass adhesive interface by XPS utilizing soft Xray as well as HAXPES to clarify the chemicalcomposition of the adhesive interface as well as the effectof a compositional change on the adhesion strength.

Intensity (arb. units)

Kenichi Ozawa1,*, Takashi Kakubo2, Naoya Amino2, Kazuhiko Mase3,4, Eiji Ikenaga5
and Tetsuya Nakamura5
1
Tokyo Institute of Technology, Meguro, Tokyo 152−8551, Japan
2
The Yokohama Rubber Co., Ltd., Hiratsuka, Kanagawa 254−8601, Japan
3
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305−0801, Japan
4
SOKENDAI, Tsukuba, Ibaraki 305−0801, Japan
5
Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679−5198, Japan(h = 1060 eV)

165
160
Binding Energy (eV)Figure 1 (a) S 1s HAXPES spectra of the rubber-bonded brass
samples which were subjected to 10- and 40-min vulcanization.
(b) S 2p XPS spectra of the interface exposed samples which were
subjected to 10- and 50-min vulcanization. Arrows indicate acharacteristic structure which suggests the existence of CuS.

3 RESULTS and DISCUSSION
Fig. 1a shows S 1s spectra of the rubber-bonded brass
samples which were subjected to vulcanization for 10 and40 min. The 10-min vulcanization time is optimum forstrong adhesion, whereas adhesion is degraded at longer
times. The spectral shape of the 40-min sample isreproduced by two Voigt-shaped peaks, whereas anadditional peak is requisite for the 10-min sample toreproduce a characteristic shoulder structure, indicated byan arrow. A similar shoulder structure is also seen in the S
2p spectrum obtained from the interface-exposed surface(Fig. 1b). However, these structures are no more
observable on the surfaces subjected to the longer
vulcanization times (40 and 50 min). It is known that CuS
gives S core-level photoemission peaks, each of which iscomposed of two components with an intensity ratio of 2to-1, and that the lower binding-energy component isresponsible for the shoulder structure [2]. Thus, thevulcanization time dependence of the spectral feature
indicates the variation of the chemical state from a CuSrich to CuS-deficient state. This change should be
responsible for the degradation of rubber−brass adhesion.
References[1] W.J. van Ooij, Rubber Chem. Technol. 52, 605 (1979).
[2] K. Ozawa et al., Appl. Surf. Sci. 264, 297 (2013).
* ozawa.k.ab@m.titech.ac.jp