Lead, bond strength

The most common use of curing agents is with carboxylic latices. Isocyanates and melamines can be used but zinc oxide is the most common curing agent. Zinc oxide cross-links carboxylated latices and improves bond strength by ionomer formation [78]. Carboxylated polychloroprene reacts slowly with zinc oxide in dispersed form, causing a gradual increase in adhesive gel content. This can lead to restricted adhesive shelf life. Resin acid sites compete with the polymer acid sites for Zn(II). The more resin acid sites, the more stable the adhesive. 

PMDI is produced on an industrial scale by the phosgenation of diamin-odiphenylmethane. Structure and molar mass of PMDI depend on the number of aromatic rings in the molecule. For PMDI the distribution of the three monomeric isomers has a great influence on the quality, because the reactivities of the various isomers (4,4 -, 2,4 - and 2,2 -MDI) differ significantly. The greater the portion of the 2,2 - and 2,4 -isomers, the lower is the reactivity. This can lead to different bonding strengths as well as to residual isomers in the produced wood-based panels. 

Simulation models describe the various conditions occurring during a press cycle (gradients of the temperature, the moisture content, the steam pressure and the formed bond strengths) which lead both to microbuckling of the wood cell walls by their moisture and temperature-induced densification (Fig. 6) [215-218]. 

How close are the two nuclei in the H2 molecule If they are too close, they will repel each other because both are positively charged, yet if they re too far apart, they won t be able to share the bonding electrons. Thus, there is an optimum distance between nuclei that leads to maximum stability (Figure 1.9). Called the bond length, this distance is 74 pm in the H2 molecule. Every covalent bond has both a characteristic bond strength and bond length. 

Agreement with the electrostatic valence rule is satisfactory except for the oxygen atoms G (Fig. 4), common to only three oc-tahedra. It is seen, however, that these atoms occur in groups of four, which can be combined to tetra-hedra by placing aluminum ions in positions 4e, the total bond strengths then becoming 2 . The four chlorine ions occupy positions 4 c, 4 b being ruled out by the small Cl -0= distance it leads to (2.72 A, sum of radii 3.21 A). 

Raevsky, 0. A. Hydrogen bond strength estimation by means of HYBOT. In Computer-Assisted Lead Finding and Optimization, Van de Waterbeemd, H., Testa, B., Folkers, G. (eds.), Wiley-VGH, Weinheim, 1997, pp. 357-378. 

Raevsky, O., Hydrogen bond strength estimation by means of the HYBOT program package, in Computer-assisted lead finding and optimization, H. van de Waterbeemd, B. Testa and G. Folkers, eds, Verlag Helvetica Chimica Acta, Basel, 1997, 367-378. 

---