Bonds cleanliness

Surface characterization of materials prior to bonding cleanliness, surface contamination, oxide thickness, and so forth... 

The degree of surface cleanliness or even ordering can be determined by REELS, especially from the intense VEELS signals. The relative intensity of the surface and bulk plasmon peaks is often more sensitive to surface contamination than AES, especially for elements like Al, which have intense plasmon peaks. Semiconductor surfaces often have surface states due to dangling bonds that are unique to each crystal orientation, which have been used in the case of Si and GaAs to follow in situ the formation of metal contacts and to resolve such issues as Fermi-level pinning and its role in Schottky barrier heights. 

Cause, R.L., A nonconiacting scanning photoelectron emission technique for bonding surfaces cleanliness inspection. NASA Technical Memorandum NASA TM-100361, 1989. Schirato, R.C., Polichar, R.M. and Shreve, D.C., In Proc JANNAF Nondestructive Evaluation Subcommittee Meeting. Chemical Propulsion Information Agency, Columbia, MD, 1992. 

The priming coat provides the bond between the metal and subsequent coats. It gives electrochemical control of corrosion. Adhesion is dependent largely on the nature of the binder and the cleanliness of the metal surface. 

Metallic adherends were cut from. 032" (.08 cm) thick sheets into lx4-inch (2.54 cm x 10.16 cm) specimens. Cleanliness of the surface, is required in order to facilitate good adhesion. Steel, as bonded within the automotive industry, often experiences a variety of surface contaminants which are not removed prior to bonding. 

Surface Tension. Testing for surface tension is sometimes referred to as the contact-angle test (see Fig. 13.6). Surface free energy is defined by surface tension which is directly related to surface cleanliness. If an adsorbate (dirt) is present on a surface, the free energy-surface tension is reduced as energy is spent in bonding the adsorbate to the surface. In other words, the surface has become less clean... 

Since thiophenol was formed in the reaction, this by-product trapped the intermediate cation to give the bis(phenylthio)vinylcyclopropane 244 and so limited the formation of the desired cyclobutanone. To overcome this problem, a substitution pattern providing electronic acceleration for the cyclopropyl bond migration but also a steric bulk to inhibit the nucleophilicity of the thiol was required. For this purpose, l-(2,6-di-methoxyphenylthio)vinylcyclopropanes such as 242b were prepared the yield and cleanliness of the reaction were effectively increased, allowing by this route the isolation of pure cyclobutanones 243 63). 

The Bonding Process. Before the actual assembly operation, the cleanliness of the shop and tools should be verified. The shop atmosphere should be controlled as closely as possible. Temperature in the range of 18 to 32°C and relative humidity from 20 to 65 percent are best for almost all bonding operations. All parts should be fitted together without adhesive or sealant to indicate possible production problems due to fit. The suitability of fit is established by either visual inspection or direct measurement with gauge or shim. It is desirable that the extremes in mechanical tolerances also be noted and that test specimens be made with the worst possible fit to ensure that the bonding process will always provide reliable joints. 

The use of 2,6-dimethoxyphenyl as the aryl group of the cyclopropyl sulphide provides electronic acceleration for the migration of the cyclopropyl bond and steric bulk which reduces the nucleophilicity of the thiol. Consequently, the cleanliness and the yield of the reaction are improved ... 

Hydrosilation chemistry is presently having, and will continue to have, an important impact on certain areas of materials chemistry. The rapidity and cleanliness of the reaction, and the stability of the resulting Si-C bonds, have been used to produce dendritic polymers and block copolymers with well-defined structures [21]. The reaction can also be employed to attach molecules to surface Si-H groups, notably the Si-H present on the surfaces of aqueous etched single crystal, or nanoparticular Si [22], or other siliceous substrates, with substantial modification to their physical and chemical properties [23]. 

Cleanliness—the presence of contaminants such as grease, dust, dirt can reduce adhesive-adherent contact, hence weaken the bond. 

Looking at water-metal interactions as reflected by the zeta potential, one cannot draw any exact conclusions, though the ordering of the potentials follows the results here. In recent work, values of 49.0, 61.0, and 60.8 mV were reported for Ag, Au, and Pt, respectively, in distilled water. The metals were allowed to remain in water for a considerable length of time, however, and oxide layers could have formed. The low value for Ag is explained by the author as related to the weaker metal-oxide bond and to differences in cleanliness of the metal prior to the experiment. In an older work the zeta potential is reported for suspended Ag, Au, and Pt in water as 34, 32, and 30 mV, respectively. ... 

The cleanliness of the supported copper and silver samples of Kavtaradze and Sokolova 200) has also been questioned 192). The suggestion of these authors 104) that the failure to observe stretching frequencies below 2000 cm" for CO adsorbed on the group Ib elements is a consequence of the electronic environment of the adsorbed species, and need not signify any structural bonding differences between adsorbed CO on these metals and on the transition metals is still a valid one. 

The irreversible adsorption layer of aromatic species was investigated by means of a specially constructed UHV and electrochemistry system where surface structure is observed by low-energy electron diffraction (LEED), surface elemental composition and cleanliness are monitored by Auger spectroscopy (AES). The vibrational bonds of the adsorbed species is observed by high resolution electron energy loss spectroscopy (EELS). 

A variety of two component adhesives and sealants are currently available for applications requiring the ultimate in bonding and sealing efficiency and performance. Included among these conventional systems are epoxies, urethanes, polyesters, silicones, polysulfides and some modified phenolics and acrylics. Although their performance can be and usually is quite satisfactory, most are limited by pot life and fixturing requirements - and the need for thorough substrate treatment and cleanliness. 

The surface of the concrete substrate has to be prepared to provide sufficient bond of the repair material. Factors that may affect the bond are the strength and integrity of the substrate, the cleanliness of the surface, and the roughness. The surface should be rough and dust or incoherent residues should be removed (for instance by sandblasting or waterblasting). This operation is usually not necessary if hydro-demohtion has been used. If the cementitious repair material is appHed directly on the surface of the concrete, the surface should be saturated by water in order to avoid absorption of water from the substrate and subsequent plastic shrinkage and incomplete hydration of the repair material, which will result in loss of bond. 

The bonded repair can take the form of either an external patch, internal patch or a flush scarf or stepped repair as described in M1L-HDBK-17-3F 3. The internal patch usually is not an option due to accessibility. For simplicity the external lap is commonly used on internal component repairs such as bulkheads and inner skins. To maintain aerodynamic cleanliness and to minimise moment-induced failure modes, however, the flush scarf repair is preferred [1]. Furthermore, on composite control surfaces (flaps, ailerons etc.) which have critical mass balance limitations, the lighter weight flush scarf repair is often the only acceptable means of repair, but a high skill level and longer time is required to prepare the damaged area for repair. 

Surface preparation. The surface of the rebar is blasted to a surface cleanliness at least as good as Swedish Standard ASa 2.5. A surface texture of around 70 jxm depth is aimed for. This is typical of steel surface preparation requirements for other structural bonding applications (see Chapter 6). 

Testing is important in all aspects of materials science and engineering, but it is especially so in adhesives. Such tests evaluate not only the inherent strength of the adhesive, but also the bonding technique, surface cleanliness, effectiveness of surface treatments, etchings of surfaces, application and coverage of the adhesive, and the curing cycle. 

---