Making adhesive bond

So far most of the discussion has centred on mechanisms of making adhesive bonds and their various details. It has already been indicated that real joints are nothing like as strong as theories suggest - indeed discrepancies are of several orders of magnitude. 

Adhesive bonding presents several distinct advantages over conventional mechanical methods of fastening. There are also some disadvantages that may make adhesive bonding impractical. These are summarized in Table 7.1. 

Earlier portions of this review dealt with aspects of making adhesive bonds. We now consider methods of measuring adhesion. Many test methods have been developed but not all of them are amenable to analysis, i.e., to determining the relation between the breaking load, the dimensions of the components, and the properties of the adhesive and adherends. Even for the simplest cases this problem has proved to be quite difficult. Indeed, it is only recently that general fracture criteria have been developed and applied to simple joints. 

Oxane bonds, M—O—Si, are hydroly2ed during prolonged exposure to water but reform when dried. Adhesion in composites is maintained by controlling conditions favorable for equiUbrium oxane formation, ie, maximum initial oxane bonding, minimum penetration of water to the interface, and optimum morphology for retention of silanols at the interface. The inclusion of a hydrophobic silane, such as phenyltrimethoxysilane [2996-92-17, with the organofunctional silane increases thermal stabiUty of the silane and make the bond more water resistant (42). 

In contact adhesives, the so-called tack open time is important. This can be defined as the time available after the adhesive is applied during which the surface remains tacky enough for the application of the adherend. It can be easily measured by applying a thin layer of fresh adhesive on Kraft paper and making a bond at different times until no bond is obtained. 

The crystallization kinetics defines the open time of the bond. For automated industrial processes, a fast crystallizing backbone, such as hexamethylene adipate, is often highly desirable. Once the bond line cools, crystallization can occur in less than 2 min. Thus, minimal time is needed to hold or clamp the substrates until fixturing strength is achieved. For specialty or non-automated processes, the PUD backbone might be based on a polyester polyol with slow crystallization kinetics. This gives the adhesive end user additional open time, after the adhesive has been activated, in which to make the bond. The crystallization kinetics for various waterborne dispersions were determined by Dormish and Witowski by following the Shore hardness. Open times of up to 40 min were measured [60]. 

As mentioned above, there are many aspeets of adhesive bonding whieh make it attraetive for use on aerospaee vehieles. Listed below are the primary advantages noted by aerospaee designers when eonsidering bonding as a fabrieation teehnology. 

Volume I of Adhesion Science and Engineering dealt with the mechanics of adhesive bonds and the rheology of adhesives. Volume II deals with the other two disciplines that make up adhesion science, surfaces and chemistry. In addition, this volume describes several applications of adhesion science and engineering. 

For most adhesive bonded metal joints that must see outdoor service, corrosive environments are a more serious problem than the influence of moisture. The degradation mechanism is corrosion of the metal interface, resulting in a weak boundary layer. Surface preparation methods and primers that make the adherend less corrosive are commonly employed to retard the degradation of adhesive joints in these environments. 

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. 

Elastomer materials specifications usually do not focus on the adhesive properties, but mainly address the chemical and physical properties of the rubber. Thus, the supplier has wide latitude within the specification to make changes in the compound formulation that could be disastrous to the adhesive bond. One solution is to qualify every new lot of elastomer material for adhesion as well as the more standard properties. 

A simple, but not very quantitative, hardness test has been used for hundreds of years— the fingernail indentation test. The indentation that a fingernail makes in the edge of an adhesive bond or in the body of a sealant can often be used as an approximate indication of hardness of the material. 

Other NDT Methods. Radiography (x-ray) inspection can be used to detect voids or discontinuities in the adhesive bond. This method is more expensive and requires more skilled experience than ultrasonic methods. The adhesive must contain some metal powder or other suitable filler to create enough contrast to make defects visible. This method is applicable to honeycomb sandwich structures as well as metal and nonmetal joints. 

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