Epoxy adhesives adhesive thickness effects

The use of sealants to coat the edges of exposed joints has some merit in constituting a barrier to liquid water, but not water vapour. Most sealants are in fact more permeable than epoxy adhesives, so that a very thick layer would have to be applied in order to be effective. The usual joint-edge adhesive spew provides a useful barrier, as well as reducing stress concentrations. 

Figure 2.10 Effect of bond line thickness on the peel strength of a toughened epoxy-based adhesive. Although not directly proportional, the effect is substantial and of considerable practical importance because bond line thickness is often difficult to control. In most cases the overall result of an increase in bond line thickness is beneficial, even though the shear strength may fall most joints fail because they cannot meet peel and cleavage overloads. Generally shear overloads are rare.

Kinloch and Shaw [106] have investigated the effect of joint width by employing a TDCB specimen consisting of mild-steel substrates bonded with a simple brittle epoxy or a rubber-toughened epoxy adhesive. For the former, low toughness adhesive no effect was observed. However, for the tough adhesive the relationships between Gic and adhesive layer thickness, /la,... 

The coating is applied to protect the steel from corrosion due to the acid or alkaline condition of the soil surrounding the pipe in service. Usually, the process requires three layers. First, an epoxy powder is applied to achieve adhesion to the pretreated metal and therefore resistance to cathodic disbondment. Second, a tie layer of polyolefin copolymer is applied and third a thick layer of polyethylene is cascaded, which in effect protects the epoxy from physical damage. 

Organic primers formulated with corrosion inhibitors are typically applied to pretreated metal surfaces to protect the surfaces prior to adhesive bonding and during environmental exposure. Pike [7-11] found that inorganic primers, such as sec-butyl aluminum alkoxide, improved the durability of aluminum-epoxy bonds when applied to both porous and nonporous aluminum oxide surfaces. It was shown that the effective thickness of the inorganic primer was directly related to the degree of oxide porosity and the depth of the porous oxide layer resulting from the normally used pretreatments for aluminum [10,11]. 

The effect of the HBPs and dendrimer PAMAMs on the strength characteristics of aluminum-, PEI-, and magnesium-bonded joints based on epoxy and PU adhesives is very significant. This result can be attributed to the chemical interactions between the peripheral end groups of the branched PAMAMs and the epoxies and PUs at the joint interface. Furthermore, the interactions of the branched PAMAMs with the metallic or organic substrates may be physical or chemical in nature. Finally, the amount of the branched PAMAMs at the interface may not exceed a certain threshold, since plasticization of the epoxy and high primer thickness may cause inferior interfacial strength. 

In a typical case where a silicon IC is attached to an alumina ceramic substrate that, in turn, is attached to the inside of a metal or ceramic package, the two epoxy interfaces can easily contribute 2.5 °C/watt to the total resistance. However, some silver-filled epoxies are reported to have high thermal conductivities, thus contributing 0.6 °C to 1 °C/watt. Actual measurements may differ considerably from calculated values because of reported thermal conductivities that differ from the actual, differences in the thicknesses of bond lines, voids in the adhesive, and incomplete mating of surfaces. Further in the analysis, the effects of lateral flow of heat and interactions of heat flow among adjacent components are often neglected. 

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