Лако-красочные материалы — производство

In this section some selected examples of XPS, AES, and SIMS are discussed, which may illustrate the capabilities of these techniques for adhesives-related applications.

In the example shown in Fig. 7, a thin film of plasma-polymerized trimethylsilane had been deposited on cold-rolled steel as a pretreatment for improved adhesion and corrosion [30]. The film thickness was determined by ellipsometry to be 500 A. The composition was characterized by AES, XPS, and TOFSIMS. AES gave information on the bulk composition, surface enrichment, and interfacial oxide (Fig. 7b). Note that the C/Si ratio of the bulk of the film, after equilibrium sputtering conditions have been reached, is approximately 3, i. e., identical to that of the monomer from which the film was

made. XPS was performed on the same samples at two different takeoff angles. Lowering the takeoff angle increases the surface sensitivity (sampling depth) of the technique. Shown in Fig. 7a are the Si2p lines from which the conclusion can be drawn that the surface of the film is enriched in Si-O bonds, whereas the bulk has a higher concentration of Si-C bonds. In Fig. 7c, parts of the TOFSIMS spectra are shown of the same film before and after solvent cleaning. This rinse was performed to check on the presence of low molecular weight materials at the film surface, which are known to form in plasma polymerization.

This rinsing treatment had practically no effect on the XPS and AES results, but the TOFSIMS spectra before and after rinsing are quite different. Before rinsing, the spectrum resembles that of polydimethylsiloxane [19,20]; after cleaning the surface is similar to that of SiO2. The spectrum indicates a high concentration of silanol groups, as can be con­cluded from the high intensity of the peak at +45 amu (SiOH+). The peak identification and the fit between observed and calculated masses are shown in Table 3 for a silane film on CRS [31]. Only those compositions were accepted that had a deviation of less than 0.01 amu from the calculated mass. It is clear from this example that application of the three

techniques discussed here yields complementary information, enabling a more detailed description of the film structure than any of the three techniques alone.

An example of the characterization of a thin film of a coupling agent adsorbed on a metal surface is given in Fig. 8 [24]. The coupling agent was vinylbenzylaminoethyl

aminopropyl trimethoxysilane (SAAPS). Following hydrolysis in a mixture of water and alcohol, the silane was applied by dipping the metal in a very dilute solution. The figure shows a very high intensity of the peak at +117 amu after short immersion times, and a high Si+ and lower +117 amu intensity after longer immersion times. The peak at +117 amu was uniquely identified as CH2 = CH-C6H4-CH2+, i. e., the end group of the coupling agent. This example thus demonstrates that the silane is highly oriented if applied as a monolayer, but this preferred orientation is absent after deposition of several monolayers. Knowledge of this orientation is important for the optimization of surface pretreatments by means of coupling agents.

An example of the use of deuterated materials in TOFSIMS studies is given in Fig. 9. Blends of polystyrene (PS) and fully deuterated polystyrene (d8-PS) were prepared in experi­ments in which segregation effects were investigated [32]. Variables were the ratios between the two polymers and the molecular weights. The most characteristic peak in polystyrene is +91 amu (C7H7+, tropyllium) and the corresponding ion for d8-polystyrene is at +98 amu. The spectrum in Fig. 9 illustrates that in this example of 1:1 PS/d8-PS ratio of equal molecular weights, the deuterated material is enriched at the surface. This type of application, i. e., monolayer surface sensitivity with organic structural information capability and separation of all isotopes, is unique to SIMS. There appears to be no other technique, except perhaps SERS (to be discussed below), that could identify this phenomenon.

The final example is a combined application of TOFSIMS and XPS, which was used to characterize the interface between a metal and a polymer system [33]. The

polymer system was a cathodic electroprimer that is widely applied in automotive applica­tions over the zinc phosphate conversion coating. Knowledge of the chemistry at the interface between the phosphate and the primer is important for the understanding and optimization of the adhesion and corrosion performance of the entire paint system. One aspect is, for instance, the degree of the cure of the primer, which may vary among different parts of the automobile. Therefore, an example is also given of TOFSIMS analysis of the primer surface/interface after undercure and overcure conditions.

In Fig. 10, XPS maps are shown of the C1s photoelectron line and ZnLLM Auger line recorded at the metal surface of a paint-galvanized steel system following exposure in a corrosion test. The panels showed several small circular spots where corrosion had occurred. The paint had been removed after the test [33]. The distributions of the two elements in the small corrosion spots are complementary, indicating that in these local areas organic debris was covering the metal surface. SIMS analysis detected in these areas high concentrations of Na+ ions. Areas with high sodium concentrations are normally the cathodes of the corro­sion cells, the counterions being the cathodically generated OH“ ions. This experiment thus demonstrated that the local areas shown in Fig. 10 were formed at cathodic sites, probably as a result of decomposition of the polymer by alkaline attack. This is a good example of a system where the XPS mapping capability is useful. AES, despite its higher spatial resolu­tion, is not very useful for organic surfaces and SIMS, with its much higher surface sensi­tivity, would also detect mainly organic material in the Zn-rich regions.

In Fig. 11, TOFSIMS spectra are presented of the two sides of the interface between a cathodic E-coat and a phosphated cold-rolled steel substrate. A variable in this experi­ment was the cure temperature of the paint [24]. The paint system is epoxy-based and can, after the cure, be described as an epoxy-urethane as the crosslinker is a blocked diisocya­nate. The spectra indicate that in both cases the separation is very close to the metal (i. e., zinc phosphate) surface, but the difference in the spectra indicates differences in the cure conditions of the paint. Several high mass peaks, indicated on the spectra, decrease or increase with cure temperature. Other peaks that are marked demonstrate the presence of the cross-linker at the interface. Other conclusions drawn from these paint studies using TOFSIMS and imaging XPS were that the degree of cure is generally higher at the metal/ coating and that the surface of the paint is always in a lower state of cure as a result of interfering oxidation reactions [33].

In similar studies in which the metal/coating interface was investigated as a function of immersion time in a dilute salt solution, it was found by TOFSIMS that paint degradation always occurred in regions with high Na+ concentration [34]. The counterions Cl“ were not detected at the interface. These results are very important in that they prove that cation diffusion through an organic coating is a critical step in the complex series of events leading to corrosion beneath organic coatings and that alkaline paint attack plays a role in the propagation mechanism. The high sensitivity of SIMS, along with the mapping and peak identification capabilities, makes this technique very powerful for the study of metal-organic interfaces.