Cohesive failure degree

In the case of conventional construction sealants, the poly sulfides, polyurethanes, epoxies, and acrylics have all shown various degrees of sensitivity to moisture. Hydrolysis causes the breaking of bonds within the sealant. Thus, the bond strength decreases and cohesive failure results. However, before this occurs, the sealant usually swells and may cause deformation or bond failure before hydrolysis can completely take action. 

In his test, a thin film of adhesive on a glass microscope slide or a metal coupon is cured and soaked in hot water until the film can be loosened with a razor blade. There is usually a sharp transition between samples that exhibited cohesive failure in the polymer and those which exhibited more of an interfacial failure. Since the diffusion of water into the interface is very rapid in this test, the time to failure is dependent only on interfacial properties and may differ dramatically between unmodified epoxy bonds and epoxy bonds primed with an appropriate silane coupling agent. The time to debond in the hot water for various silane primers differed by several thousandfold when used with a given epoxy. In parallel tests, a thick film of epoxy adhesive on nonsilaned aluminum coupon showed about the same degree of failure after 2h in 70°C water as a silaned joint exhibited after more than 150 days (3600 h) under the same conditions. 

Cohesive failure of the adhesive bonds was observed for slides pretreated with AS. The results can be explained in terms of the different structures of the amines and the reactions that can occur with the amines. We conclude that just like in the work of Eckstein and Dreyfuss referred to above (4-8), chemical bonds form from the surface through the amine to the substrate The degree of enhancement of adhesion is related to the number of bonds that can form between the surface and the adhesive. Aniline does not lead to reinforcement because it is monofunctional and the ring does not become part of the backbone. 

In all cases, more than one specimen was tested for sustained load durability and the legend for the bar-graphs is as follows. The ordinate shows the sustained load applied to the samples while the abscissa shows the days of exposure. A white bar with an arrow at the end indicates no failure of the specimens, while a white crosshatched bar indicates that at least one sample is still in test. A white bar with a blunt end indicates that all of the samples had failed. In this case, the average of the days to failure of all the tested specimens is given. When possible, the degree of cohesive failure is shown at the end of the failed specimen bar-graph. For example, 0.1 C means 10% cohesive failure and 90% apparent adhesion failure. 

While cohesive failure is interesting and desired in many applications, materials that fail cohesively do not generally exhibit a high degree of tack adhesion. For the polymer gel materials discussed here, cohesive failure is not observed in any case. 

Figure 1.6 shows the correlation between the complex Jenike failure function ff with the simple FR (Table 1.4). These results indicate that for values of FR less than 1.25, the industrial powders tested were free flowing. Above an FR value of 1.55, the powders could be classifled as cohesive with intermediate degrees of flowability between FR values of 1.25 and 1.55 as illustrated in Table 1.5. 

The role of the deformation energy can best be demonstrated by peeling a pressure-sensitive tape from a rigid substrate. During separation typically the adhesive becomes filamented and the legs are stretched to such degree that they break or separate from the surface. The former is a cohesive and the latter an adhesive failure. The energy required to achieve this is the peel force. 

A second reasonable postulate might be that the detachment will occur at a critical strain or degree of deformation. Such a postulate would suggest that the peel force should increase with increasing rate but would not predict the transition from cohesive to interfacial failure as the rate increases. 

The above comments are seen to be reinforced by observations on the failure path in joints before and after environmental attack. The locus of joint failure of adhesive joints when initially prepared is usually by cohesive fracture in the adhesive layer, or possibly in the substrate materials. However, a classic symptom of environmental attack is that, after such attack, the joints exhibit some degree of apparently interfacial failure between the substrate (or primer) and the substrate. The extent of such apparently interfacial attack increases with time of exposure to the hostile environment. In many instances environmental attack is not accompanied by gross corrosion and the substrates appear clean and in a pristine condition, whilst in other instances the substrates may be heavily corroded. However, as will be shown later, first appearances may be deceptive. For example, to determine whether the failure path is truly at the interface, or whether it is in the oxide layer, or in a boundary layer of the adhesive or primer (if present), requires the use of modern surface analytical methods one cannot rely simply upon a visual assessment. Also, the presence of corrosion on the failed surfaces does not necessarily imply that it was a key aspect in the mechanism of environmental attack. In many instances, corrosion only occurs once the intrinsic adhesion forces at the adhesive/substrate interface, or the oxide layer itself, have failed due the ingressing liquid the substrate surface is now exposed and a liquid electrolyte is present so that post-failure corrosion of the substrate may now result. 

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