Substrate Surface Pretreatment

To form a strong, integrally bonded, load-bearing structure, the surface of the adherend should be pretreated before application of the adhesive this is vital if good environmental or thermal durability is required. Such a procedure ensures that the surface is in as clean a condition as possible, removing weak boundary layers which could adversely affect the performance of the resultant joint. 

Bonded beam thickness Honeycomb thickness Relative  

This important topic is dealt with in the chapter entitled Surface Pretreatment for Structural Bonding , in this volume. 

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In particular, CVD of the derivatives Cu(hfac)(PMe3),1,2 Cu(hfac)(l,5-cod),3-6 Cu(hfac)(2-butyne),7,8 and Cu(hfac)(vtms),9-12 where 1,5-cod = 1,5-cyclooctadiene and vtms = vinyltrimethylsilane, has been studied in detail. These species can be used to deposit copper films either selectively or nonselectively on various surfaces depending on the nature of the precursor, the deposition conditions, and the substrate surface pretreatment. The syntheses of these species from a general salt elimination reaction according to eq. (2) is described here in detail.10,13,15-17 It should be noted that other general methods of preparation of this class of compounds have been reported elsewhere.18... 

Various substrate surface pretreatments have been reported to increase nucleation density. 

Figure 3. Dissimilar grit-size dependence of diamond nucleation densities on substrate surface pretreatments, (a) Nucleation density versus inverse abrasive paste mean-size used in polishing pretreatment.(b) Nucleation density versus single-particle mean-size used in ultrasounding pretreatment solid squares Ref. 317, blank squares Ref. 322, solid circles Ref. 323. (Reproduced with permission.)...

P. Ascarelli, and S. Fontana, Dissimilar grit-size dependence of the diamond nucleation density on substrate surface pretreatments, Appl. Surf. Sci., 64(4) 307-311 (1993)... 

Due regard should be paid to the practical difficulties of achieving a high standard of substrate surface pretreatment in repair and strengthening works on site. The stability of the adherend/adhesive interface is probably the most important factor in the durability of bonded joints. 

Several studies have examined the adhesive properties of LCTs. Ochi and Takashima, and Carfagna and co-workers both foimd an increase in the lap shear strength for an LCT epoxy compared to an isotropic LCT (85,86). However, Frich and Economy (87) found that the lap shear strength with a titanimn substrate for an LCT that cures by transesterification was lower than that for an isotropic resin. Given the limited data available, it is not clear whether these differences are due to the substrates, surface pretreatment, the type of cure reaction, the liquid crystalline phase, or some other factor. However, most of these studies did find... 

Figure 6. Hole injection efficiency figure of merit for substrate contacts of varying work function vs. energy step across the contact polymer interface estimated from published work function data and electrochemical redox potential data. The height of each bar reflects the variability in injection efficiency due primarily to variation in substrate surface pretreatment and for the particular case of Au, diffusion to the interface of metal atoms from underlying binder layers.

This poorer performance is often attributable to the lack of cleaning action of water in removing contaminants on the substrate surface, compared to that of organic solvents, again emphasizing the interaction between the choice of substrate surface pretreatment and that of the adhesive which was noted in the previous chapter. 

Primers are often used to help ensure an adequate service life and, indeed, the presence, and the chemical type, of any primer may play a crucial role in ensuring a long service life for the adhesive joint. For example, it will be shown later that the use of organometallic primers may be extremely beneficial and many commercial corrosion-inhibiting primers have been developed for use with epoxy aerospace adhesives following the above comments, it is probably not just a coincidence that many of the latter types of primers contain phenolic-based constituents. Examples of the effect of substrate surface pretreatments on joint durability were shown in Figs 4.16 and 4.17 and Table 4.15 further examples are illustrated in Fig. 8.8. 

Finally, the effect of cyclic stresses applied in the presence of a hostile environment has been studied in a limited number of investigations [8,9,12]. As might be expected, such a dynamic loading may accelerate any environmental attack but, by careful attention to the choice of substrate surface pretreatment, long service lives for the joints may be attained. 

Substrate Properties. It is clear from equation 5 that higher hardness of the substrate lowers friction. Wear rate of the film also is generally lower. Phosphate undercoats on steel considerably improve wear life of bonded coatings by providing a porous surface which holds reserve lubricant. The same is tme for surfaces that are vapor- or sandblasted prior to appHcation of the soHd-film lubricant. A number of typical surface pretreatments are given in Table 13 to prepare a surface for solid-film bonding (61). 

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. 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

Surface Morphology. The initial Integrity of an adhesively bonded system depends on the surface oxide porosity and microscopic roughness features resulting from etching or anodization pretreatments. (17) The SAA surface characterized in this study consists of a thick (9 ym), porous columnar layer which provides excellent corrosion resistance in both humid and aggressive (i.e., Cl ) media. I The thinner FPL oxide provides a suitable substrate surface for evaluating the candidate inhibitors. 

Substrate surface structure exerts a controlling influence upon monolayer electrodeposition, as can be seen by comparing deposition of Ag at I2-pretreated Pt(lll) and Pt(100), all else being equal [41,45] (Fig. 21). The UPD process at Pt(100)(v/2 x x/8)i 45°-I took place in two stages, rather than three, and the electrodeposit structures were related to the square mesh of the Pt(100) substrate in contrast to the hexagonal structures observed at Pt(lll). 

The most common technique involves abrading the substrate surface with hard powders (preferably diamond), or ultrasonic treatment in a slurry of an abrasive powder (preferably diamond grit) in an organic liquid for a prolonged period. This type of pretreatment leads to embedding of the abrasive particulates into the surface as well as mechanical damage to the substrate, both of which are believed to enhance the nucleation density. The maximum nucleation enhancement is observed when diamond grit is used, and this has been attributed to implantation of diamond particles... 

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