Adhesive strength Adhesion energy

Fig. 17. Adhesion energy G measured as a function of the surface density of the interfacial chains. It may noted that the strength measured in a peel test (a) is about 5 times larger than that measured using the JKR method (b). Further, a maximum exists in the value of G as function of the surface chain density. This is because of swelling effects at larger values of surface chain density. The open symbols represent the data for elastomer molecular weight Mo = 24,000 and the closed symbols represent the data for Mo = 10,000.

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

The adhesive strength of a food fouling deposit may be related to the surface free energy of the substrate. Zhao et al. (2004) have developed a theory that gives the minimum adhesion energy between a deposit and a surface ... 

Tadepalli and Thompson focused on the bonding strength of copper-to-copper bonded structures using four-point bending characterization [66]. Adhesion energy was characterized for three different surface preparation... 

Adhesive curing starts from the outside and moves inwards. As the cured skin becomes thicker, the retention strength increases until it meets the requirements mandated by the FMVSS regulations. Due to the highly viscous nature of the adhesives, a thin cured skin is often sufficient to fulfill the needs. The principal characteristics of crash-suitable adhesives are strength and energy absorption (Fig. 73). 

Adhesion is the molecular force of attraction between unlike materials. The strength of attraction is determined by the surface energy of the materials. The higher the surface energy, the greater is the molecular attraction while the lower the surface energy, the weaker is the attractive force. The adhesion energy, or the work of adhesion, is defined as the work per unit area that needs to be provided to separate reversibly a solid-liquid interface so as to create solid-vapor and liquid-vapor interfaces [52]. Thus... 

FIGURE 18. Adhesion energy W (i.e., G) for polybutadienne crosslinked with 0.2% dicumyl peroxide against various glass surfaces. Treated with 0% vinylsilane (100% ethylsilane) (o) 50/50 mixture of ethyl and vinylsilanes ( ) 100% vinylsilane (o) clean glass (+). Dotted curve cohesive strength vs. rate of tear. (From Reference 69.)... 

The energy of attraction between neutral bilayers has been measured by this method to be 0.015mJm [20,22,26]. This attraction energy is well within the range for this technique that can measure adhesion energies up to 40 mJ m , i.e. limited only by the tensile strength of the adherent bilayers. The results are well modeled by the van der Waals power law attraction limited by a steep, close-in, exponential hydration repulsion, establishing adherent contact at an interbilayer separation of 25 A, as measured by X-ray diffraction, [64]. 

For CNTs not well bonded to polymers, Jiang et al. [137] established a cohesive law for carbon nanotube/polymer interfaces. The cohesive law and its properties (e.g., cohesive strength, cohesive energy) are obtained directly from the Lennard-Jones potential from the van der Waals interactions. Such a cohesive law is incorporated in the micromechanics model to study the mechanical behavior of carbon nanotube-reinforced composite materials. CNTs indeed improve the mechanical behavior of composite at the small strain. However, such improvement disappears at relatively large strain beeause the eompletely debonded nanotubes behave like voids in the matrix and may even weaken the composite. The increase of interface adhesion between CNTs and polymer matrix may significantly improve the composite behavior at the large strain [138]. 

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