Strength Degradation of Thermally Tempered Glass Plates

An earlier theory of contact-induced strength degradation of brittle materials is extended to include plates in residual surface compression. The scale of the strength-controlling flaw is predicted by indentation fracture mechanics, with the modifying effect of the residual field incorporated into both indentation and strength equations. Experimental verification of the predictions is obtained from diamond-pyramid indentation tests on thermally tempered glass plates. As with untempered plates, the theory accounts for the load dependence of the strength loss; it also explains the insensitivity of the degradation characteristics to initial flaw distribution and identifies toughness as the controlling material parameter. Most significant, however, is the demonstration that surface strengthening can produce dramatic improvements in degradation resistance. The possibility of obtaining all parameters necessary for a complete degradation analysis of a given tempered inaterial entirely by routine indentation/strength testing is discussed.     

Hydrolytic degradation of ionically cross-linked polyphosphazene microspheres

The hydrolytic degradation of gel microspheres based on calcium cross-linked phosphazene polyelectrolytes, poly[di(carboxylatophenoxy)phosphazene] (PCPP) and poly[(carboxylatophenoxy) (glycinato)phosphazene] (PCGPP), was investigated. These microspheres are of importance as carriers in protein and cell encapsulation. Both PCPP and PCGPP ionotropic polyphosphazene hydrogels are degradable in an aqueous environment (pH 7.4, 37°C). The degradation rates can be increased by incorporation of hydrolysis sensitive glycinato groups as the pendant structures in the polymer (PCGPP). Hydrolysis of these polymer hydrogels led to low molecular weight (<1,000 Da) products. The erosion and molecular weight profiles varied also according to the molecular weight of the polyphosphazene constituting the gel beads. Another approach to affect the degradation rates consists of coating microspheres with poly-L -lysine. Ionotropic polyphosphazene hydrogels have potential as biodegradable devices for controlled drug delivery systems. © 1994 John Wiley & Sons, Inc.     

Modeling of Filtration Combustion of Carbon

The kinetics of the overall reactions of carbon gasification at temperatures up to 1600 K is studied. The kinetic characteristics of the reactions are taken into account in a distributed mathematical model of filtration combustion of carbon. By computational experiment at different values of process control parameters (the gas flow rate, the fraction of the oxidant in the gas, and the mass ratio of carbon to the inert material in the combustible mixture), it is found that, in the wave of filtration combustion of carbon under superadiabatic conditions, carbon gasification can predominately yield either CO₂ or CO.     

Diffusion-controlled adsorption processes

Numerical solutions of differential equations for diffusion-controlled adsorption processes in spherical particles are presented for slow diffusion of gaseous molecules followed by rapid adsorption according to the Langmuir isotherm. As the parameter in the denominator of the Langmuir equation is increased the rate increases and the generalized rate curves shift progressively from that for the linear isotherm, the Langmuir parameter = 0, to the curve of the parabolic law. Adsorption and desorption cycles are considered and parameters are provided for relating any of these processes to the generalized curves. A simple linear equation was shown to approximate the rate curves with moderate accuracy.     

Spontaneous combustion regimes

Moscow. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 5, pp. 13–27, September–October, 1992     

Esters of hydroxystearic acids as primary low-temperature plasticizers for a vinyl chloride—Vinyl acetate copolymer

Summary Thirty-one acyloxy or aryloxy esters prepared from hydroxystearic acids have been evaluated as plasticizers for a vinyl chloride-vinyl acetate copolymer (95∶5). Many of them were found to be primary plasticizers, having outstanding low-temperature performance when employed at the 35% level. Formulations with these compounds compared quite favorably in tensile properties with those containing the di-2-ethylhexyl esters of phthalic, sebacic, azelaic, and adipic acids. Volatility losses were similar to those of the four di-2-ethylhexyl esters. The loss of plasticizer through migration was equal to or less than that from compositions containing the esters of sebacic, azelaic, and adipic acids but was greater than that of the phthalate ester. A mechanistic scheme of plasticizer-polymer interaction has been presented, proposing that the rate of diffusion of plasticizer through the polymer mass is a controlling factor in both good low-temperature performance and the resulting high migration losses. Methyl esters, some aromatic esters, and esters containing three or more polar centers have improved permanence but show a more rapid change in torsional modulus as the temperature is lowered during the determination of the Clash-Berg stiffening temperature. Eastern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.     

The effect of heat on metal incorporated silica gels

Silica gel prepared under varying conditions indicated that, in addition to the pH and kind of acid, the mode of adding the solutions govern the evolution of surface area of the produced xerogel. Co-precipitation with the oxides of iron, copper, or nickel showed a marked effect in decreasing the surface area of the dry gel. Xerogels incorporated with iron oxide suffered from a sharp sintering that appeared upon heating to 900 °C. Copper oxide-incorporated silicas showed a small decrease in surface area at the same temperature. However, silica incorporated with nickel oxide seems to be resistant to heat treatment at 900 °C, and exhibits only a small change in surface area. Acid treatment of the incorporated silicas resulted in negligible variations in surface area, probably due to the low content of oxides.     

The Role of Cations in Some Inorganic Hydrolysis Reactions

Cations which shift certain equilibria from left to right by forming a compound with one of the reaction products were investigated with respect to their kinetic role in the process. Silver ions have no influence on the rate of the hydrolysis of cyanogen iodide. Similarly, cations which form sparingly soluble carbonates do not influence the rate of hydration of carbon dioxide. On the other hand, the hydrolysis of chlorine is accelerated by Ag⁺ and, to a lesser extent, by Cd²⁺ and Fe(III). It is shown that the effect is not due simply to a bimolecular reaction between chlorine and the cation. In the case of Ag⁺, heterogeneous catalysis may play some role, in analogy with the solvolysis of organic halides. In the case of Cd²⁺, there is spectrophotometric evidence for compounds between Cl₂ and HOCl on the one hand, and the cation on the other. It is suggested that these compounds interconvert more rapidly than do the parent substances. A similar mechanism may be operative in the case of Fe(III), and, possibly, also in the case of Ag⁺.