Pressure-sensitive theory

The principal theories can be divided into five groups plus an additional one that is quite different (1) mechanical theories (2) adsorption theories (3) chemical theories (4) diffusion theories (5) electrostatic theories plus pressure sensitive theories. Each of these will... 

The maximum superheat which can be achieved with a nonboiling liquid is definitely pressure sensitive. This is evident in Fig. 18 and also in Fig. 28. Both plots show that the possible superheat (and therefore the possible values of ATc) decreases to zero as the critical pressure is approached. This is in agreement with the equation-of-state theory and also the nucleation-rate theory. 

Since the development of the equation, it has been tried to derive further information from it. Rees and Rue [129] determined the area under the Heckel plot. Duberg and Nystrom [137] used the nonlinear part for characterization of particle fracture. Paronen [138] deduced elastic deformation from the appearance of the Heckel plot during decompression. Morris and Schwartz [139] analyzed different phases of the Heckel plot. Imbert et al. [134] used, in analogy to Leuenberger and Ineichen [14], percolation theory for the compression process as described by the Heckel equation. Based on the Heckel equation, Kuentz and Leuenberger [135,140] developed a new derived equation for the pressure sensitivity of tablets. 

It may be noted that no consideration of pressure-sensitive adhesives is included in the present discussions. The relevant theory is quite separate and as the type of adhesive is of no significance in structural applications, it is omitted. 

This theory was developed by another Russian, Deryagin, particularly for pressure-sensitive tape. The adhesive and the adherend are likened to the two plates of a capacitor, and the work of separation is equated to that required to separate the two charged capacitor plates. Again the theoretical basis developed correlates with experiment. 

Among branched polymers, star polymers represent the most elementary way of arranging the subchains since each star contains only one branching point, and as such, they serve as useful models for experimental evaluation of theories about solution properties and rheological behavior of branched polymers (Angot et al., 1998). Star polymers nd applications as additives in various areas such as rheology modi ers, pressure sensitive additives, etc. Besides serving as additives, star polymers can also be used as such to achieve sped c properties. For instance, star block copolymers with polystyrene-fc-polybutadiene (PSt-fo-PB) arms have better processability and me-... 

Experimental observations and measurements are interpreted by means of a theoretical framework that is capable of modelling the cold plastic deformation of polymers with pressure-sensitive yield surfaces. Qualitative evidence of the adequacy of the model to provide explanation of the results provides the link between theory and experimentation. 

The acrylic-type pressure-sensitive adhesives are often partly cross-linked, partly linear in composition. Theory and data available suggest that the cross-linked component should be only very lightly cross-linked, so that the Rg calculated from Me of the cross-linked portion is larger than the Rg based on Me of the linear portion see Section 10.2 (93,94), especially Figure 10.11. Then the linear polymer can enter the network structure through reptation, developing physical bonds. 

The theory that we will describe is intended for application to thermoplastic polymers. Its most immediate relevance is to pressure-sensitive tapes, to contact adhesives, and to hot-melt adhesives. But it is also a valid theory of the behavior of polymers whose glass transition temperature, T, is well above the ambient temperature. We will discuss such polymers explicitly, in the later parts of the chapter. 

THERMAL CONTROL MECHANISMS IN ADHESION ADIABATIC THEORY 4.1. Pressure-Sensitive Adhesives... 

Finally, we may look at the theory that we have developed from the viewpoint of catastrophe theory. Filament rupture is intrinsically a catastrophe, and so is interfacial separation. The peeling of a pressure-sensitive tape and the propagation of a fracture front are macroscopically continuous processes, but the change in behavior of individual filaments or fibers in the separation front is discontinuous either they detach from the solid or they rupture. Thus, the microscopic course of events is subject to a bifurcation, i.e., a choice between two catastrophes. 

This chapter provides a summary of the generalized theory and a discussion of the parameters which appear in its equations. The use of the theory is illustrated for several different adhesion situations, including the peeling of rubberlike and pressure-sensitive adhesives, adhesion to skin, and the moisture resistance of structural adhesives bonded to metals and glass. 

Other Theories. In addition to these theories, some special cases are discussed in the literature. They include adhesion by primary valence forces, for example, in the bonding of metals [8], [23], and so-called liquid adhesion. In the latter, a thin film of a liquid of extremely high viscosity produces adhesion through a process in which separation of adherent and substrate results in a flow in the narrow gap which is only possible by overcoming considerable resistance. Liquid adhesion is particularly assumed in pressure-sensitive bonding [16] and in the initial tack of a liquid adhesive. 

There are two other explanations which are relevant to some situations though neither is important in connection with structural adhesives. The first is the electrostatic theory which, as its title indicates, involves adhesive and substrate as the plates of an electric condenser, work being necessary to separate the plates. The second is the mechanical theory in which mechanical interlocking of adhesive and substrate involves fracture of parts of one or other before separation can be achieved. The former theory has some part to play in explanations of the adhesion of pressure sensitive tapes to substrates and the adhesion of sputtered metal in the surface decoration of plastics. The latter is important in the adhesion of polymers to textiles and papers and, to a much lesser extent, to wood. 

An isothermal theory of separation in (thermoplastic or pressure sensitive) polymer-solid adhering systems based on drawing of filaments is given by Good and Gupta (1988). 

Electron spectroscopic techniques require vacuums of the order of 10 Pa for their operation. This requirement arises from the extreme surface-specificity of these techniques, mentioned above. With sampling depths of only a few atomic layers, and elemental sensitivities down to 10 atom layers (i. e., one atom of a particular element in 10 other atoms in an atomic layer), the techniques are clearly very sensitive to surface contamination, most of which comes from the residual gases in the vacuum system. According to gas kinetic theory, to have enough time to make a surface-analytical measurement on a surface that has just been prepared or exposed, before contamination from the gas phase interferes, the base pressure should be 10 Pa or lower, that is, in the region of ultrahigh vacuum (UHV). 

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