Aggressive molecules

A key feature of encapsulation processes (Figs. 4a and 5) is that the reagents for the interfacial polymerisation reaction responsible for shell formation are present in two mutually immiscible Hquids. They must diffuse to the interface in order to react. Once reaction is initiated, the capsule shell that forms becomes a barrier to diffusion and ultimately begins to limit the rate of the interfacial polymerisation reaction. This, in turn, influences morphology and uniformity of thickness of the capsule shell. Kinetic analyses of the process have been pubHshed (12). A drawback to the technology for some apphcations is that aggressive or highly reactive molecules must be dissolved in the core material in order to produce microcapsules. Such molecules can react with sensitive core materials. 

Carboxyhc acid dust and vapors are generally described as being destmctive to tissues of the mucous membrane, eyes, and skin. The small molecules such as formic, acetic, propionic, butyric, and acryUc acids tend to be the most aggressive (Table 11) (83). Formic, acetic, propionic, acryUc, and methacrylic acids have time weighted-average exposure limits of 20 ppm or lower. AcryUc acid showed an LD q of 33.5 mg/kg from oral adrninistration to rats. 

The left-hand end of the activated monomer is sealed off by the OH terminator, but the right-hand end (with the star) is aggressively reactive and now attacks another ethylene molecule, as we illustrated earlier in Fig. 22.1. The process continues, forming a longer and longer molecule by a sort of chain reaction. The —OH used to start a chain will, of course, terminate one just as effectively, so excess initiator leads to short chains. As the monomer is exhausted the reaction slows down and finally stops. The DP depends not only on the amount of initiator, but on the pressure and temperature as well. 

The evolution of hydrogen from the acid molecule can also occur in slightly dissociated weak acids such as HjCOj and HjS. In the case of only slightly dissociated weak acids, such as HjCOj and HjS, production of hydrogen can also occur from the acid molecules. In this case, the acid concentration rather than the pH value is a measure of the aggressiveness of the corrosion. In the same way, hydrogen can be evolved from HjO ... 

Surface analysis has made enormous contributions to the field of adhesion science. It enabled investigators to probe fundamental aspects of adhesion such as the composition of anodic oxides on metals, the surface composition of polymers that have been pretreated by etching, the nature of reactions occurring at the interface between a primer and a substrate or between a primer and an adhesive, and the orientation of molecules adsorbed onto substrates. Surface analysis has also enabled adhesion scientists to determine the mechanisms responsible for failure of adhesive bonds, especially after exposure to aggressive environments. The objective of this chapter is to review the principals of surface analysis techniques including attenuated total reflection (ATR) and reflection-absorption (RAIR) infrared spectroscopy. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and secondary ion mass spectrometry (SIMS) and to present examples of the application of each technique to important problems in adhesion science. 

Molecular hydrogen atmosphere is the less aggressive method but, as we shall see, decomposition of the H2 molecules at the surface is the limiting process for its introduction. In this section, we shall present mainly results concerned with hydrogen plasma introduction. Comparison will be made with other introduction methods. We shall present also the effect of nonin-tentional introduction of hydrogen in these semiconductors. 

Laser ablation of compounds of almost all elements in the periodic table will produce the bare ion M+. Laser ablation and other methods of producing bare metal ions have been discussed in Section II.C.5. The bare metal ion has a coordination number of 0 and for most elements these ions will aggressively seek molecules able to share or donate electrons. Thus most bare transition metal ions will increase their coordination number by reacting with any donor, this even includes the inert gas atoms such as Xe (96). 

Biotransformation pathways of nitroaromatic compounds are believed to result from nitroreductases that have the ability to use nitro as either one- or two-electron acceptors. One-electron acceptance by the nitro compounds results in the production of the nitro radical-anion. This nitro radical-anion becomes one of the most aggressive species in biological systems because of its reaction on endogenous molecules (DNA bases) and its well-known catalytic ability to transfer one electron to molecular oxygen with superoxide anion formation. 

---