Autohesion bonding

The mechanism by which a thermoplastic matrix composite consolidates to form a laminate was attributed to autohesive bond formation between plies [13,18]. Autohesion, however, can... 

The mechanism governing the formation of interply bonds has been established as autohesion or self-diffusion [28], Autohesive bonding is controlled by two mechanisms (1) intimate contact between the interfacial surfaces, and (2) diffusion of the macromolecules across the interface. Figure 7.22 shows the phenomenon of autohesion for an amorphous thermoplastic polymer. At time zero, the two surfaces are pressed together. Providing the temperature is... 

Figure 7.22 Schematic diagram of autohesive bond strength development across an interface...

If wetting is instantaneous and the instantaneous wetting load at initial time is negligible, then the autohesive bond fracture stress, cr, is proportional to the fourth root of contact time and the fracture energy, GIC, is proportional to the square root of contact time as shown in the following equations ... 

The interply bond strength for thermoplastic matrix composites has been shown to be dependent upon the processing parameters, pressure, temperature, and contact time. If the temperature distribution in the composite is nonuniform during processing, the ply interfaces will bond (or heal) at different rates. Thus, for a specified processing cycle, it is important to know precisely the temperature and degree of autohesive bonding at every point in the composite laminate in order to estimate the required process time. 

The process by which a thermoplastic matrix composite consolidates to form a laminated structure has been attributed to autohesive bond formation at the ply interfaces. Autohesive bond formation is controlled by two mechanisms (1) intimate contact at the ply interfaces, and (2) diffusion of the polymer chains across the interface (healing). The rate of autohesive bond formation and hence the speed of the composite consolidation process is directly related to the temperature-pressure-time processing cycle. 

By a comparison of the forces of adhesion of particles Fad as calculated from Eq. (1.42) with experimental data on the detachment of a monolayer, it is easy to establish that ad corresponds to the force of the most weakly held particles of the monolayer, i.e., the initial section of the integral curves for adhesive force (see Fig. 1.2). Consequently, in the detachment of a powder layer by tilting a dust-covered surface, we measure the average force of adhesion of the readily removable particles. As they sUde, these particles produce an avalanchelike removal of the remaining particles. If the force of adhesion of the layer to the substrate is greater than the autohesion in the layer, the detachment will take place across the weakest autohesive bonds. 

Finally, it should be noted that the barriers to molecular contact may be different for adhesive tack and autohesion. Surface impurities, eg, from bloom, may readily redissolve into the bulk elastomer during autohesive bonding. On the other hand, impurities on common substrates, such as surface moisture, may not be readily displaced by an adhesive that is too stiff. Molecular mobility and microscopic flow are expected to aid displacement of impurities. 

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