Bond failure, adhesives

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

A.H. Muhr, A.G. Thomas, and J.K. Varkey, A fracture mechanics study of natural rubber-to-metal bond failure, J. Adhesion Set TechnoL, 10, 593-616, 1996. 

The extent of adhesive bond failure under corrosive environments is greatly accelerated when cyclic mechanical stresses are imposed on the adhesive bond during exposure. Three to four orders of magnitude reduction in fatigue life of adhesive bonds is observed for bonds exposed to environment prior to fatigue testing. 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

The effect of the HRH system on adhesion is further illustrated by the micrographs (Figures 7-11) of the same rayon-natural rubber composite with and without HRH. Figures 7-9 show a thin section of the composite without HRH stretched to various elongations with the force applied parallel to the direction of orientation. Many voids form as the strain is increased owing to fiber-matrix bond failures. Both the number and size of voids increase with increasing strain. 

With time (under increased temperature and humidity) the crack tip continues to a weaker region which for this surface treatment appears to be near the oxide/alloy interface. Figure 11 summarizes the analysis of the bond failure for this particular surface treatment. The important aspect here is that under identical conditions, different surface preparations show different modes of failure. Weak boundary layers are not developed using some treatment/bonding combinations. Processes have been developed in which the locus of failure remains in the adhesive ("a cohesive failure") and it is necessary to use a mechanical test in which even more stress is placed on the interfacial region (19). 

Adhesives are used in the electrical and electronic industries in a variety of different ways, from holding microcomponents in place on a circuit board to bonding coils in large power transformers. Reliability is always a concern, since bond failure could lead to component failure, which in turn leads to equipment failure and then possibly to a massive system failure. A system in this industry could be a commercial aircraft s electrical system or the power distribution system in an urban city. 

The plastic surface, at the time of bonding, may be well suited to the adhesive process. However, after aging, undesirable surface conditions may present themselves at the interface, displace the adhesive, and result in bond failure. These weak boundary layers could come from the environment or from within the plastic substrate itself. Plasticizer migration and degradation of the interface through uv radiation are common examples of weak boundary layers that can develop with time at the interface. 

Both the formulating of adhesives and bonding with adhesives are complex, multiple-part processes, complete with interacting and sometimes unexpected parameters that may contribute to success or failure of the final product. Thus, it is important that the quality control process consider the entire operation from receipt of materials to final product testing. 

The adhesive-substrate bond may also be subject to thermal stresses resulting from differences in the coefficients of thermal expansion of the hard tissue and the resin. Ideally the values of these two coefficients should be the same (in practice the coefficient of thermal expansion of the adhesive is usually much higher) to avoid the build-up of stresses which eventually may lead to bond failure. This is especially important for dental adhesives since the temperature range in the mouth may vary from 2 C to 55 C. 

Although the inherent chemical resistance of the rubber chosen is quite suitable for the given duty conditions, many a times the bond or adhesion failure is the cause of lining failure. Therefore much care is taken while applying the lining especially at joints, seam, corners and flanges. 

It generally applies that the quality of an adhesive depends on the further processing of the adhesive itself and the adherends to be bonded. Failures are mostly attributable to improper processing conditions and not to the adhesive itself... 

The mode of bond failure in the unexposed films was mainly cohesive. However, when a cross-linked polymer matrix was formed with UV exposure, the mode of bond failure was interfacial at all testing temperatures. We believe that by cross-linking the polymer with UV radiation, the modulus of the polyester was increased. This in turn caused the cohesive strength to be much greater than the interfacial bond strength. As a result, the mode of bond failure became interfacial, or adhesive (10) (Figure 8). 

Loss of adhesion may be caused by permeation problems. However, Internal formation of volatile species (outgasslng) and primary bond failure can also contribute to loss of adhesion. All polymers are Inherently permeable, but to widely varying degrees. Oxygen, moisture, air pollutants, etc., can penetrate polymer films and attack underlying reflector metallzatlon, conductors, or other functional elements. Furthermore, these gases... 

Differences between the hydrophilic wood and the hydrophobic thermoplastic during processing can limit bond development, resulting in poor adhesion between wood fibers and the plastic polymer. Without chemical or physical bonding, failures on the wood-polymer interface and interfacial voids can develop because of poor processing or as a result of external factors, such as moisture uptake or UV degradation. These failures in the wood-polymer interface... 

A larger number of failure modes is thought to be possible with composite materials bonded by adhesive joints than with equivalent metal joints. Failure in the adherends can be tensile, interlaminar or transverse. The last two can be either in the resin or in the interface. Finally, cohesive failure can occur in the adhesive. 

The fracture stress of adhesive-bonded joints (adhesion strength) is a consequence of processes that occur in the course of their formation, but all attempts to formulate a fundamental relationship between formation and failure of adhesive-bonded joints have so far been unsuccessful. This is mainly due to the lack of methods for meastuing adhesion that would permit determination of the failure equilibrium work. Accordingly, the relationship between the experimentally determined value of the adhesion strength and the thermodynamic characteristics can be one of correlation only. 

The failure loads of the reference beam GL24h on the one hand, and those of the bipartite beam bonded by adhesive 027 on the other, are listed in Table 5. 

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