Weak adhesive systems

For weakly adhesive systems, the pair energy between two identical droplets, m, can be written as [109] ... 

Failure can occur in metal/epoxy adhesion systems in any one or more of a number of different regions. The fracture may propagate through the bulk metal or epoxy, the metal oxide layer, the metal oxide/epoxy or metal/metal oxide interfaces, or through weak boundary layers (WBL s) very near the interfaces. Some workers -78,153) be]ieve that most failures that have been claimed to be interfacial have actually... 

Miscible systems invariably form a single phase and are transparent. Their physical properties are directly related to the properties of the individual components and the mole fractions of these in the mixture, the glass temperature (Tg) composition diagram of such mixtures is Unear between the two Tg of the individual components. Since, however, the majority of would be interesting polymer combinations are immiscible, they separate into two phases and become practically uninteresting because of the relatively weak adhesion between the two phases. 

When used in epoxy adhesive systems, solvents are generally employed for reducing the viscosity for formulation or application purposes. Once the adhesive is applied to the substrate, the solvent must evaporate prior to cure. Otherwise, bubbles or vapor pockets could form in the bond line, causing a physically weak joint with poor adhesion. The solvent in the adhesive formulation must not adversely affect the substrate to which it is applied. Plastics, elastomers, and polymeric foams are especially sensitive to certain solvents used in epoxy adhesives. 

Various substrate surface treatments suggested for use with a common epoxy-substrate joint and service environment combinations are discussed in this chapter. Surface preparation processes for a range of specific substrates and detailed process specifications are provided in App. F. The reader is also directed to several excellent texts that provide prebond surface treatment recipes and discuss the basics of surface preparation, the importance of contamination or weak boundary layers, and specific processes for adhesive systems other than epoxy.1,2,3... 

The results show that the friction of a gel on a weak adhesive substrate can be minimized even in the high velocity region, depending on substrate roughness. This result provides some essential ideas for designing a soft gel system with low friction over a wide velocity range, which is important in bioengineering applications where low friction is required, such as in artificial articular joints and artificial hearts. 

Figure 18.7 gives scanning electron micrographs of cuts made under liquid nitrogen of PA6 blends containing 30 wt% of pure LDPE (Fig. 18.7a) and high viscous LDPE-g-lA (Fig. 18.7b) as well as HDPE-g-IA (Fig. 18.7c). The effect of functionalization onto blend morphology is rather strong. In blends with ptrre PE, the latter aggregates into droplike particles of up to 5 mm in diameter. They have a smooth surface. As mentioned above, similar morphology is typical of blended systems with distinct immiscibUity of polymer components and weak adhesion between the phases (1,9,20,66).

Thus, the hardening of the toluene diisocyanate-based polyurethane system in the presence of microquantities of the surface-active agent KEP-2 occurs at all stages of the process in conditions different from the ordinary hardening mixture of TMP-TDI-oligoglycol. These differences become apparent in kinetic and structural studies of the hardening process. The final product is a polymer with very weak adhesion compared with the initial specimen (hardening temperature 353 K). 

When solvents are used in the adhesive or coating formulation, they must be completely evaporated from the bondline before cure. Otherwise, bubbles could form in the bond line, causing a weak joint. In the early 1970s, during the time of the petroleum crisis, water-based adhesives were thought of as a possible replacement for solvent-based adhesives systems. However, water-based adhesives never met the lofty expectations, primarily because of the time and energy required to remove water from the bond line, the corrosion that the water causes in drying ovens, and the poor moisture resistance of the final product. 

The system which was chosen for this study was one in which the shrinkage of the adhesive was very small. Even so it was found that the effect of the shrinkage stresses was to reduce the crack extension force by over 40%. It may readily be seen that in many commercial adhesive systems the reduction in strength below that which is theoretically possible by the operation of shrinkage stresses may be very much greater. It is suggested that shrinkage stresses may often be the major cause of weakness in an adhesive joint. 

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

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