Improvement of Wet-Adhesion on Stainless Steels by Electrolytic

Chapter 28

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Improvement of Wet-Adhesion on Stainless Steels by Electrolytic Polymerization Treatment with Triazine Thiol Compounds

H. Yamabe

ABB Asea Brown Boveri Industry Κ. K., Shimada, Shizuoka, Japan

The surface treatment of stainless steels by electrolytic polymerization with 1,3,5-triazine-2,4,6-trithiol (TTN) in aqueous solution was investigated. This surface treatment improved the adhesion durability of epoxide resin adhesive joints in wet environment. Structural adhesive bonding for metallic materials is now established as indispensable industrial technology for construction of airplanes and cars (7) (2). Nevertheless, several technological problems of strategic importance are left unsolved in this important industrial technology. The most important problem includes "loss of adhesion strength in the presence of humidity." This unsolved problem is actually identical in quality to the problem of adhesion of the corrosion protective coating layer to a metallic substrate in a humid environment. Loss of adhesion between layers is believed to be a result of water to the adhered interface between two layers. In the case of corrosion protective coating, surface modification of the metal substrate is practiced to inhibit undesired loss of adhesion between coating layer and metal substrate in humid environments (3). It is well known that stainless steel exhibits inferior adhesion property to organic coatings as compared with other metals such as cold rolled steels or aluminum. Various pretreatments for stainless steel have been used to enhance the physical properties of the steel. Several different types of pretreatments for stainless steel have been used in aerospace industry (mechanical, chemical, and a combination of mechanical and chemical) (4). If a chemical pretreatment process is employed, a residue (smut) is deposited on the surface. This residue must be removed with another chemical treatment, a desmutting solution. For this reason such a chemical pretreatment process is environmentally not very attractive. In this study, the surface treatment of stainless steel by electrolytic polymerization with l,3,5-triazine-2,4,6-trithiol monosodium salt (TTN) in aqueous solution was investigated in an attempt to improve adhesion in humid environment. Three types of surface analysis techniques were employed in an attempt to ascertain the bonding mechanism with this surface treatment. ©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

345

346


Experimental Functionality of Triazine Thiol Compounds. This triazine thiol compound has various advantages as shown in Figure 1. For instance, if R represents a thiol group, this compound gives adhesion properties to polymeric materials such as epoxide resin, as the thiol can react with the glycidyl group of the epoxide resin (5). Thiol groups can polymerize radically with formation of disulfide bonding (SS). If one of the thiol groups is in the form of salt (SNa, SK), the triazine thiol compound is soluble in water (6). Therefore, l,3,5-triazine-2,4,6-trithiol monosodium salt (TTN) in water was employed as an adhesion promoting agent for stainless steel. Pretreatment Procedure. Figure 2 shows a schematic diagram of the pretreatment procedure. Before pretreatment with TTN, the stainless steel was degreased and rinsed by deionized water. Then the fresh stainless steel was pretreated electrolytically with TTN. Figure 3 shows a schematic diagram of the electrolytic polymerization process of TTN for stainless steel. The stainless steel at the anode was pretreated electrolytically in 5 mM TTN (99.9% purity, supplied by Toa Electric Co., Japan) in water at constant current. In this case, the current density was kept constant at 5 mA/cm . 2

Coating Thickness. The dry coating thickness of TTN was measured by ellipsometry. The coating thickness of TTN depends on the time of electrolysis. As shown in Figure 4, the almost linear relationship between time of electrolysis and coating thickness of TTN was shown. Adhesion Study. The pretreated specimens for lap joint were bonded using the epoxide (Epikote 828)/polyamide (Versamid 140) adhesive. The adhesive was cured for 7 days at room temperature and then for 1 hour at 100° C. The thickness of the adhesive layer was 100-150 μπι. The joints were then drawn in an Enstron Universal Testing Instrument. The load required to break the joints was determined in accordance to JIS K6850. Figure 5 shows the effect of the polymerization time on adhesion strength of stainless steel bonded by an epoxide polyamide adhesive. In this case, the optimal adhesion strength was obtained at the thickness between 10 and 20 Â. In the range of higher coating thickness, the cohesive destruction in pretreated layer was observed. From these results, the polymerization time was fixed to 20 seconds, which corresponds to the thickness of 100 Â. The effect of humidity exposure (70° C , 98% r.h.) on adhesion strength is shown in Figure 6. Significant improvement in adhesion durability can be achieved by the application of electrolytic polymerization treatment with TTN. Similar thiol compounds have also been developed for pretreatment of metallic materials to be subjected to adhesion bonding. For example, Schmidt and Bell (7) reported improved adhesion performance for steel achieved with an ethylenemercapto ester copolymer-coupling agent.



347

C

FUNCTIONALITY

^)

ADHESION TO POLYMERS : ANTIFOULING :

R=

R=

SH

NH(CF2)3CF3

REACTIVITY TO METALS SH + M —• S —M POL YMERIZABILITY s · + s - —•

s-s

J

Figure 1. Functionality of triazone thiol compounds (5).


348

STAINLESS C;0.06,

STEEL

Ni;8.89,

Cr;18.75,

Si;0.54,

Mn;1.59, P;0.03 S;0.005 Fe;balance y

y

J Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 18, 2017 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch028

ALKALINE

RINSING

DEGREASING

BY DE IONIZED

EL ECTROL Y TIC

WATER

POL Υ Μ Ε RIZ Τ10 Ν

TREATMENT

WITH

2,4,6-TRITHIOL

MONOSODIUM

1,3,5-TRlAZINESALT

(TTN) C0NC.;5mM,

CURR.

DENS.;5mAicm2

Figure 2. Procedure for sample preparation

electric source

1 mM TTN 0.1 MNa2C03 water

working electrode (anode)

counter electrode (cathode)

Figure 3. Schematic diagram of electrolytic polymerization equipment


349


700

Polymerization time ( s ) Figure 4. Effect of TTN polymerization on coating thickness.

Polymerization time ( s )

Figure 5. Effect of TTN polymerization time on the adhesion strength of bonded stainless steel.



350 Interface Characterization. As described above, the electrolytic polymerization of a layer of TTN improved adhesion durability of stainless steel. In order to characterize the interface between the TTN layer and the stainless steel, three surface characterization techniques were employed: • A Perkin-Elmer Model 257 spectrometer was used for reflectionabsorption Fourier transform infrared (RA-FTIR) spectroscopic analysis. • A Perkin-Elmer PHI Model 5500 was used for X-ray photoelectron spectroscopic (XPS) analysis. The exciting radiation was provided by magnesium Κ α X-ray source operated at a constant power of 200W (10 kV, 20 mA). • An ΑΤΟΜΙΚΑ Model A-DIDA3000 was used for static secondary ion mass spectroscopic (SIMS) analysis. In this technique, a 3 keV Argon ion beam irradiated an elliptical area and current densities were kept at ~10" A/cm . 6

2

RA-FTIR Analysis. RA-FTIR spectra from stainless steel treated with TTN are shown in Figure 7. As seen in this figure, in the case of immersion only, the IR spectrum of the TTN layer can not be observed. On the other hand, in the case of electrolytic polymerization treatment, the apparent spectrum of the TTN layer can be observed. Absorbance at 1510 and 1460 cm" can be assigned to the stretching vibration of -N=C< in the triazine ring. Absorbance at 1260 and 1220 cm" is assigned to the stretching vibration of C-N in the triazine ring. 1

1

XPS Analysis. XPS is employed as a meaningful analytical method in the field of surface characterization (7). As the chemical environment of an atom changes, the photoelectron spectrum undergoes changes in peak shape, position or intensity. Since the adhesion durability in a wet environment is a fundamental local phenomenon involving only a few atom layers of substrate and organic coatings, the resultant data from the XPS studies should contribute toward an understanding of chemical interaction behavior at the interface of TTN film and stainless steel. Figure 8 shows XPS high resolution examinations of the S(2p) region from stainless steel, which was electrolytically treated with TTN. The peak at a binding energy of 162 eV indicates the presence of covalent bonding between the thiol of TTN and the stainless steel surface. This kind of covalent bonding was also observed between n-decanthiol and mild steel by Stratmann (8) (9). He came to the conclusion that the thiol group has a relatively high reactivity with the metal surface and forms a covalent bond with metal such as Fe. This type of interfacial structure is responsible for the stability of TTN/stainless steel interface in a wet environment. The peak at binding energy of 165.5 eV may indicate the disulfide (S-S) bonding, from which a polymerized structure of TTN can be expected. The peak separation of thiol and disulfide is so difficult that further investigation is needed. However, the polymerization of TTN can be assumed because the TTN film formed was not soluble in tetrahydrofuran. Figure 9 shows XPS high-resolution examinations of the S (2p) region from fractured stainless steel after water vapor exposure for 72 hours. On both the adhesive and substrate sides, a polymerized TTN layer can be detected. This analysis also revealed the presence of the covalent bonding between thiol and metal. From



351

Figure 6. Effect of TTN polymerization treatment on the adhesion strength of bonded stainless steel after high-humidity test, φ : treated with TTN, Ο untreated. :

C=N C-N I

ι

3000

ι

2 000

ι

ι

I

1 5 0 0 1 0 0 0

W a v e n u m b e r

(cm

- 1

)

Figure 7. RA-FTIR spectra of stainless steel (SUS304 2B) treated with TTN. 1. Immersion 2. Polymerized film 3. TTN crystal/KBr



170

168

166

Binding

164

energy

162

(eV)

Figure 8. S XPS spectra of stainless steel (SUS304 2B) treated with TTN. 2P

\

A

S-Metal

\

Bond

i

Adhesive / s i d e /

Substrate side I

ι

4

1

174

17 0

16 6

16 2

Binding

energy

(eV)

Figure 9. S P X P S spectra of bonded stainless steel (SUS304 2B) fractured after high-humidity test for 72 hours. 2


353


these results it is concluded that thiol/metal bonding resists water penetration and that fracture occurred within the polymerized TTN layer. Static SIMS (SSIMS) Analysis. In SIMS, ionized particles are ejected from the surface by the action of an argon beam. They are analyzed according to their masses. As the current densities are very low in the static mode, only the surface layer can be investigated. Either atoms or molecules can be ionized, and thus, details about the chemical state of atoms in the surface can be inferred (10). It is evident from Figure 10 that the surface of the stainless steel contains traces of many elements. At 84 amu and 88 amu the molecular ions CrS+ and FeS+ can be observed, respectively. These two molecular ions may be associated with the covalent bonding between thiol of TTN and metallic elements of stainless steel, such as Cr and Fe. Gettings and Kinloch (77) investigated performances of several silanecoupling agents over a stainless steel sheet surface and observed that increased stability for the adhered interface with a coupling agent varied appreciably from one agent to another. They also used SSIMS for characterization of the bonding state of a silane-coupling agent/metal oxide interface. In this case, similar SIMS characterization was undertaken and ions indicative of the presence of FeOSi and CrOSi were detected. The silane-coupling agent, which gave such ions successfully, improved the adhesion performance for the stainless steel. As such, formation of a primary covalent bond in addition to a secondary bond seemed to be essential for enhancement of interface stability in a humid environment. Polymerization Mechanisms of TTN on Metal Surfaces. Figure 11 shows polymerization and adhesion mechanisms of TTN on a metal surface. The TTN molecules exist in ionized form as seen in this figure. This molecule can be oxidized and leaves an electron. The oxidized S radical reacts with another S radical. In this way, the polymerized layer will be formed on stainless steel. The terminal of polymerized TTN can form covalent bonding with metal atoms, such as Cr and Fe, on stainless steel surfaces. The outermost thiol can react with epoxide and combine stainless steel and epoxide by covalent bonding. Conclusion There are some important conclusions that can be drawn from this work: • •

•

The electrolytic polymerized TTN layers are excellent pretreatment for stainless steel/epoxide adhesive bonding. The results of RA-FTIR, XPS and SSIMS gave information regarding the polymerized layer. Especially, the evidence of polymerization and covalent bonding with stainless steel that was observed for XPS analysis. In conclusion, it is clear that the use of polymerized TTN allows a better understanding of surface treatments and allows the choice a priori of the best treatment of stainless steel to be bonded to the organic polymer.



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355 Literature Cited


1 2 3 4

Yamabe, H. Zairyo-gizyutu 1996, 14(10), 288. Yamabe, H. Japan Adhesion Society 1993, 29, 12. Yamabe, H.; Funke, W. Farbe und Lack 1990, 96, 497. Yamabe, H.; Hirahara, H.; Mori, K. J. Japan Society of Color Material 1995, 68, 404. 5 Yamabe, H. ibid. 1993, 66(10), 605. 6 Mori, K.; Nakamura, Y. J. Polymer Science: Polymer Chemistry Edition 1985, 23, 315. 7 Schmidt, R. G.; Bell, J. P. J. Adhesion 1988, 25, 85. 8 Stratmann, M. Farbe und Lack 1993, 99, 16. 9 Stratmann, M. Adv. Mater. 1996, 2, 191. 10 Yamabe, H.; Tsutsumi, A. Farbe und Lack 1993, 99, 16. 11 Gettings, M.; Kinloch J. Mater.Sci.1977, 12, 2511.


Improvement of Wet-Adhesion on Stainless Steels by Electrolytic

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