RAIRS adhesion

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

In order to characterize the surface regions of a sample that has been modified in some way, as is usually the case in adhesion-related investigations, some sort of a reflection experiment is required. Two types of experiments, attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIR),... 

Many applications of XPS to problems in adhesion science have been reported in the literature. One interesting example is provided by the work of Tsai et al. on the use of XPS to investigate reactions between model rubber compound and plasma polymerized acetylene films that was discussed above [22,23], Consideration of that system permits some interesting comparisons to be made regarding the type of information that can be obtained from RAIR and XPS. 

Many investigations of the molecular structure of thin films formed by y-APS deposited onto inorganic substrates from aqueous solutions have been carried out. Ondrus and Boerio [2] used reflection-absorption infrared spectroscopy (RAIR) to determine the structure of y-APS films deposited on iron, 1100 aluminum, 2024 aluminum, and copper substrates from aqueous solutions at pH 10.4. They found that the as-formed films absorbed carbon dioxide and water vapor to form amine bicarbonate salts which were characterized by absorption bands near 1330, 1470, 1570, and 1640 cm-1. y-APS films had to be heated to temperatures above about 90°C in order to dissociate the bicarbonates, presumably to free amine, carbon dioxide, and water. Since the amine bicarbonates failed to react with epoxies, the strength of adhesive joints prepared... 

The technique of choice for studying thin films on metals (or certain other substrates) directly is single reflection RAIR [47-54]. The limitation here is that the substrate must be very smooth, but this can be easily achieved by polishing the metal before deposition of the film. Characterizations of thin organic layers on metal (oxide) surfaces, such as occur in lubricants, corrosion inhibitors, adhesives, polymers, paints, and so forth, are specific applications of this rather recent form of FTIR. It should be noted that the relative band positions and shapes may be different in this technique than in conventional transmission IR. The spectrum may also change with the thickness of the organic film, which implies that polymer/metal interactions are in principle observed [47,51]. The teehnique is so surface sensitive that oxidation of metals can be determined in situ [51] and the packing... 

Infrared spectroscopy, including Fourier-transform infrared (FTIR) spectroscopy, is one of the oldest and most widely used analytical techniques in adhesion-related research. Transmission infrared spectroscopy has been used to identify compounds used in formulating adhesives and to follow curing reactions. Attenuated total reflection (ATR) (see Infrared spectroscopy attenuated total reflection) has been used to probe the surface composition of polymers that have been surface modified by an etching process or by deposition of a film. More recently, reflection-absorption infrared spectroscopy (see Infrared spectroscopy RAIR) has been used to characterize thin films on the surfaces of reflecting substrates. 

Since the angle of incidence is very large in RAIR, a large area of the substrate (typically several square centimeters) is usually examined and the spatial resolution of the technique is limited. However, several manufacturers have developed objective lenses that enable RAIR spectra to be acquired using an infrared microscope. These lenses make it possible to obtain RAIR spectra from areas as small as about 25 p,m in diameter and to use RAIR for failure analysis of adhesive bonds. 

Fig. 2. RAIR spectra of plasma-polymaized siUca-like films deposited onto aluminium substrates for diffoent times and thus at diffaent thickness. Copyright (2002) from (Journal of Adhesion) by (F. J. Boaio). Reproduced by permission of Taylor Francis, Inc., http //www.taylorandfrancis.com...

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