Adhesive bonding, characteristics

Pocius AV, Aimer CJ, Wald RD, Wilson TH, Davidian BE (1984a) Investigation of variability in the adhesive bonding characteristics of 301 stainless steel. SAMPE J 20 11... 

Bond strength can vary from a temporary bond (non-curing compound) to a substrate tearing bond (using phenolic-modified curing products). Solvent-borne CR adhesives can be formulated to have very short open times for fast production operations or to retain contact bond characteristics for up to 24 h. Heat and solvent reactivation can be used to re-impart tack to dried surfaces. 

Dauksys, R.J. (1973). Graphite fiber treatments which affect fiber surface morphology and epoxy bonding characteristics. J. Adhesion 5, 211-244. 

Metals such as aluminium, steel, and titanium are the primary adherends used for adhesively bonded structure. They are never bonded directly to a polymeric adhesive, however. A protective oxide, either naturally occurring or created on the metal surface either through a chemical etching or anodization technique is provided for corrosion protection. The resultant oxide has a morphology distinct from the bulk and a surface chemistry dependent on the conditions used to form the oxide 39). Studies on various aluminum alloy compositions show that while the oxide composition is invariant with bulk composition, the oxide surface contains chemical species that are characteristic of the base alloy and the anodization bath40 42). 

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

Synthetic surfactants and polymers are probably most often used to modify the characteristics of a solid surface, i.e., they function at the solid - liquid interface, such as in the processes of detergency, lubrication, or the formation of adhesive bonds. The performance of modem FT - IR spectrometers is such that many new applications to the characterization of the solid - liquid interface, particularly in kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated total reflectance (ATR) techniques have been applied to "wet" interfaces, even the air - water interface, and have figured prominently in recent studies of "self -assembled" mono - and multilayers. 

To understand the popularity of adhesive bonding in the automotive industry, one needs to take a look at the characteristics that define the current market situation for autos. 

Primers and adhesion promoters work in a similar fashion to improve adhesion. They add a new, usually organic, layer at the interface. The new layer can be bifunctional and bond well to both the substrate and the adhesive or sealant. The new layer is very thin so that it provides improved interfacial bonding characteristics, yet it is not so thick that its bulk properties significantly affect the overall properties of the bond. 

The oxide layer that forms on aluminum is more complex than with other metal substrates. Aluminum is a very reactive surface, and oxide forms almost instantaneously when a freshly machined aluminum surface is exposed to the atmosphere. Fortunately, the oxide is extremely stable, and it adheres to the base metal with strength higher than could be provided by most adhesives. The oxide is also cohesively strong and electrically nonconductive. These surface characteristics make aluminum a desirable metal for adhesive bonding, and they are the reasons why many adhesive comparisons and studies are done with aluminum substrates. 

Kim, G., and Ajersch, F., Surface Energy and Chemical Characteristics of Interfaces of Adhesively Bonded Aluminum Joints, Journal of Materials Science, vol. 24,1994, pp. 676-681. 

These surface modifications were performed in "pure" micro-wave (2.45 GHz, "single-mode") or in combined microwave/ radio frequency (2.45 GHz/13.56 MHz, "dual-frequency") plasma. Important systematic changes of the surface composition, wettability, and adhesion of thin metal films were observed for different substrate bias values, and for the different gases. The modified surface-chemical structure is correlated with contact angle hysteresis of water drops this helps to identify which surface characteristics are connected with the wettability heterogeneity and with adhesive bonding properties, and how they are influenced by plasma-surface interactions. 

The reactivity of resorcinol with formaldehyde is essential for developing the cohesive strength of the interlayer and its bonding characteristics. Condensed tannins are known to be very reactive with formaldehyde (7-0), so these renewable phenolic polymers are good candidates as resorcinol replacements. Indeed, condensed tannins from wattle and pine bark extracts have been successfully used in cold-setting, wood-laminating adhesives, and the former are used extensively in the commercial production of laminated timbers in South Africa (Pizzi, A., National Timber Research Institute, Pretoria, South Africa, personal communication, 1982) (10-13). 

Adhesives made from blood and casein were some of the first used in the wood products industry. They have been replaced in many applications by petroleum-based adhesives, which show improved performance or better economics. Despite this competition, they remain important in certain specialty areas due to their unique curing and bonding characteristics. 

For bulk specimens, Kic and Gic convey the same information. However, for adhesively bonded structures, such as the specimens described in this report, Kic and Gic differ. E in Equation 3 is the modulus of the adherend (the PMMA) rather than the adhesive, and so Kic is not characteristic solely of the PS layer but of the composite as a whole. The other expression for fracture behavior is the effective fracture surface energy, y. It is related to the fracture energy by... 

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