Resistance to debonding

Organosilanes vary in their effectiveness, judged by such criteria as resistance of the glass reinforced resin to prolonged immersion in hot water, its resistance to debonding under stress, the magnitude of the... 

Similar considerations control the resistance to debonding of adhesives. In some cases, when the interface is very strong, the process is identical to that in bulk materials. The joint fails by cohesive failure within the adhesive layer. However, even when the crack propagates along an interface between an adhesive and a substrate, the ability of the system to resist the propagation of such a crack depends on the ability of the adhesive to absorb energy by a yielding deformation near the crack tip. 

An effective pressure-sensitive adhesive must provide resistance to debonding from the substrate. However, in many PSA applications, (masking tape, for example) it is desired that the adhesive be removable with some moderate force. Unlike structural adhesives, stronger is not always better. It is also generally desirable that the PSA separate from the substrate and not fail cohesively within the bulk of the adhesive. In tape applications, it is undesirable for the PSA to separate from the backing, leaving adhesive residue on the substrate. 

Different factors contribute to the mechanical properties of plant tissue cell turgor, which is one of the most important ones, cell bonding force through middle lamella, cell wall resistance to compression or tensile forces, density of cell packaging, which defines the free spaces with gas or liquid, and some factors, also common to other products, such as sample size and shape, temperature, and strain rate (Vincent, 1994). Depending on the sample properties (mainly turgor and resistance of middle lamella), two failure modes have been described (Pitt, 1992) cell debonding and cell rupture. 

There is a need for an accurate assessment of the mechanism of cathodic debonding of thick adherends. This would contribute to better control over the selection of formulations resistant to attack. One of the objectives of this work was to develop an accelerated screening test. The screening test was used to evaluate the relative performance profiles of a number of commercial adhesives which were recommended for marine applications. It is hoped that a screening test such as this one could be used to isolate measurable primer properties which can be related to the long term prospectus of the primer, and the variability of this prospectus under permutation of substrate type or additives. 

The achievement of enhanced toughness in a brittle polymer such as an epoxy resin is vital in applications such as adhesives, for which peel strength and resistance to interfacial debonding through crack propagation are important criteria for performance. This may be achieved by incorporation of elastomeric or other phase-separated particles into the network. These may be present before reaction or they may form during the process of... 

In a typical probe test, the probe is brought in contact with the surface of the adhesive at a velocity Va and subsequently removed at a constant velocity Vdeb-The stress applied to the adhesive film is at first compressive, then becomes tensile until failure is initiated, typically by the formation of cavities. The plateau stress is then representative of the high-strain deformation of the cavity walls in the tensile direction. This geometry is ideally suited to characterize the adhesive properties of a fibrillating PSA and in particular of two specific properties, namely its resistance to the formation of cavities under a nearly hydrostatic stress, and its ability to form stable fibrils upon debonding [26]. 

Two representative probe test curves for the detachment of an SIS adhesive from steel and from EP surfaces are shown in Pig. 22.17 while the initial portion of the curve is identical, the force drops rapidly to zero for the EP surface, and never forms the characteristic fibrillar plateau observed on steel surfaces. How does this happen As qualitatively described by Creton et al. [55] for a detachment from a polydimethylsiloxane layer, when the resistance to crack propagation is low, cavities are nucleated (around the peak stress) and then propagate as interfacial cracks at the interface between the probe and the adhesive, and eventually coalesce. This process of crack propagation and coalescence is responsible for the sharp drop in force observed in Fig. 22.17 for the EP surface and occurs at rather low values of nominal strain. In this case no formation of the characteristic foam stracture responsible for the high debonding energy is observed. 

Long-term durability of adhesively bonded joints may require resistance to a number of individual or combined degradation modes, including environmental attack, fatigue and time-dependent failures. Time-dependent failure mechanisms are often characterized nsing either a strength approach, involving creep and creep-rupture tests, or a fracture approach, in which debond rate is determined. In creep-rupture tests, adhesive joints are subjected to... 

Corrosion resistance can be improved in epoxy adhesives by the addition of suitable fillers. DGEBA-resin-based adhesives show improved resistance to anodic debonding when used to bond steel/polyolefin laminates by the incorporation of aluminum oxide.Levels of aluminum oxide can be varied from 3-40 weight % of the total adhesive composition. 

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