Rigid polyurethane foams: A model of the foaming process

A model of the manufacture of rigid polyurethane foams by free rising is presented. The extent of cream and rise periods as well as the amount of blowing agent necessary to give the desired foam density are theoretically predicted. The rate of blowing agent evaporation is calculated from an experimental boiling temperature vs. composition curve. Experimental runs were carried out with a formulation consisting of a polymeric isocyanate, a polyether polyol based on sorbitol, a silicone-polyol block-copolymer as surfactant, dibutyltin dilaurate as catalyst, and trichlorofluoromethane as blowing agent. Mixing was performed in situ in the mold using a commercial foaming machine. Experimental results gave a satisfactory agreement with model predictions. A diagram containing all the relevant information may be theoretically built and used for the selection of adequate operating parameters for a given formulation.     

Oxidation of Manganese Ferrite

The oxidation of manganese ferrite (MnFe₂O₄) in air has been studied in the temperature range 750° to 1150°C. Between 1050° and 1150° a homogeneous oxidation takes place, resulting in a spinel phase of a defect type containing up to 10% of the manganese in the trivalent state. At 1050° and below, bixbyite (Mn₂O₃) and hematite (Fe₂O₃) are precipitated. The kinetics indicate that initially a diffusion-limited homogeneous reaction takes place, followed by precipitation and grain growth. At lower temperatures, well-defined Widmanstätten precipitates are formed; at higher temperatures, recrystallization alters these precipitates. The oxidation of manganese ferrite can be strongly retarded by a ceramic glaze, offering a practical method of preparing the material by air firing.     

The Determination of Surface Development in Cement Pastes by Nuclear Magnetic Resonance

Pulsed proton NMR was used to determine the specific surface of hydrating cement pastes. The method is based on the fact that the measured proton spin-lattice relaxation rate (l/ T₁ ) of water in cement pastes is (in view of the fast exchange between water molecules in the adsorbed and bulk phase) proportional to the fraction of water molecules covering the solid surface and thus proportional to the NMR surface of the newly grown hydration products. In general, the method can be used for powders, fibrous and porous materials in contact with liquid water, or other fluids containing protons.     

Leaching of Polyphase Nuclear Waste Ceramics: Microstructural and Phase Characterization

The leaching of complex polyphase nuclear waste ceramics is described in the context of the geochemically established dissolution behavior of the constituent phases. Static leach tests conducted on ceramic monoliths in deionized water, in simulated silicate, and in brine groundwaters, conforming to Materials Characterization Center standards and an accelerated, microscopic leach test, were used to identify the processes. Dissolution and formation of surface passivation layers are discussed in terms of hydrolysis and the adsorption of the metal hydroxocomplexes onto the monolith surface. The factors observed to affect dissolution are pertinent to the leaching of other polyphase nuclear waste ceramics.     

The preparation and properties of some urethane foams from castor oil and elaidinized castor oil

Summary The preparation and properties of two series of castor oil urethane foams, one from castor oil and the other from elaidinized castor oil, were investigated. The first series of foams was made from prepolymers containing 60% of castor oil prepared at increasing temperature levels to vary the degree of crosslinking in the final foams. These foams had lower tensile strengths than observed for a previously prepared foam of 60% castor oil and did not show significant differences in water resistance as crosslinking varied. They were increased nearly 100% in compressive strength with increased crosslinking and had very good shrinkage characteristics as values of only 1 to 2% were obtained. A second series of foams was prepared from 50, 60, 70, and 80% of elaidinized castor oil to compare with foams from a similar series from castor oil. This series of foams of 50 to 80% elaidinized castor oil contents was similar in density (1.7 to 6.7 lbs./cu. ft.), had improved shrinkage characteristics (11, 1, 3, and 4%, respectively), showed increased compressive and tensile strengths (up to 12.1 p.s.i. at 50% compression modulus and 34.7 p.s.i. ultimate tensile for the 60% foam formulation), and had better water-resistance properties (411 to 155%vs. 515 to 170% water absorption) than the analogous foams from castor oil. In general, humid aging only slightly affected the values obtained for the foams and was significant in only a few instances,e.g., decreased tensile in the elaidinized castor oil series. Thus increasing crosslinks in the foam apparently did not improve water resistance but did improve shrinkage characteristics in addition to some increased strength properties, as would be anticipated. Foams from elaidinized castor oil, while similar in density and foaming characteristics to analogous foams from castor oil, exhibited less shrinkage and improved water-resistance. Presented at the 50th Annual Meeting of the American Oil Chemists' Society, New Orleans, La., April 20–22, 1959. Ono of the laboratories of the Southern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture.     

Zur Selbstdiffusion von Benzol und Azeton in Polymethylmethacrylat

The selfdiffusion coefficients of acetone and benzene are studied in the systems acetone/poly(methylmethacrylate) and benzene/poly(methylmethacrylate) by means of the spin echo technique as function of concentration and temperature. From the dependence of temperature, the activation energies are calculated as function of concentration. The dependence of concentration of the selfdiffusion coefficients is discussed using the theory of Free Volume according to FUJITA. The Free Volume of the poly(methylmethacrylate) is calculated as function of temperature.     

Grafting onto polyester fibers. III. Electrokinetic properties of acrylic acid and acrylonitrile-grafted polyester fibers in the presence of cationic dyes

The electrokinetic properties such as zeta potential, surface charge density, and surface conductivity of polyester fibers grafted with acrylic acid and acrylonitrile were measured when cationic dye solutions were streamed through. The presence of the cationic dyes on the surface of the fibers and their specific adsorption at the carboxylic groups in the acrylic acid graft copolymer produce lowering of zeta potential. The decrease in surface charge density as the percent graft increases is due to the decrease in surface area of the fibers due to the adsorption of the cationic dyes. The same trend is observed with acrylonitrile-grafted fibers. The surface conductivity of the acrylonitrile-grafted polyester fibers increases with increase in dye concentration of the streaming solution. The results for the 27.4% grafted sample differed from those of the 7.32% and 12.1% grafts, which is indicative of the formation of a three-dimensional network causing change in both the physical structure as well as the chemical nature of the surface of the substrate.     

Gas-Liquid chromatography of fatty derivatives. I. Separation of homologous series ofa-olefins,n-hydrocarbons,n-nitriles,and n-alcohols

Summary Gas-liquid chromatography has been shown to be applicable to the analysis of several fatty derivatives. The difficulties inherent in conventional separation schemes such as liquid-liquid chromatography, fractional distillation, and crystallization can be overcome through the use of gas-liquid chromatography. It becomes the preferred procedure for several reasons: a) fractions are more nearly pure, b) analysis time is a fraction of the time required for conventional separations, c) operational procedures are simple and highly reproducible, d) sample size requirements are low, e) the cost per unit analysis is low. These factors suggest that gas-liquid chromatography is highly useful as an analytical tool in fat research. In many cases the determination of chainlength distribution has already become a routine function. There remain areas in which gas-liquid chromatography has not been completely successful. Until very recently the separation of fatty methyl esters of similar chain length and varying degrees of unsaturation has been time-consuming and tedious. Several new partitioning agents hold out great promise in the separation of the isomeric C₁₈ fatty acids. More selective partitioning agents and refined techniques undoubtedly will equally improve the resolution of unsaturated alcohols and other unsaturated fatty derivatives. Presented at annual meeting, American Oil Chemists' Society, Memphis, Tenn., April 21–23, 1958. Technical Paper No. 160, Archer-Daniels-Midland Company.     

Deformation Bands in Ceria-Stabilized Tetragonal Zirconia/Alumina : II, Stress-Induced Aging at Room Temperature

A stress-induced aging phenomenon is observed to occur at room temperature in deformation bands introduced into a 8.5 mol% ceria-stabilized tetragonal zirconia/alumina (CeTZP/A1₂O₃) composite by flexural loading. The aging occurs with time after unloading and in laboratory air. Over a period of 100 days, the concentration of monoclinic zirconia within a deformation band increases and, in addition, the wedge-shaped deformation band grows with time. Accompanying these two changes are an increase in the tensile stress in the remaining tetragonal zirconia within the deformation band and a consequential increase in the overall compressive stress within the band. The average value of the monoclinic concentration within the deformation band is found to increase parabolically with time, suggesting the mechanism responsible for the observed aging is diffusion limited. Away from the deformation bands, no aging is observed to occur, suggesting aging is stress dependent. Although a water-vapor-mediated mechanism cannot be ruled out, it is proposed that the observed aging is in fact due to a tensile stress assisted chemical reduction of Ce⁴⁺ to Ce³⁺ whose rate is controlled by the indiffusion of oxygen vacancies driven by the tensile stress gradient. It is further proposed that the deformation band grows with time, since the region ahead of the band is under tension and hence subject to an enhanced rate of reduction     

Cavity Development During Creep Deformation in Alumina with a Bimodal Grain Size Distribution

An alumina sample, codoped with equimolar proportions of magnesia and zirconia, exhibited a bimodal grain size distribution after hot-pressing. Flexural creep experiments were performed on this material at temperatures of 1673 and 1773 K in air. Inspection of the deformed specimens revealed extensive creep cavitation, with cavities developing preferentially in the coarse-grained regions. The nucleation, growth, and interlinkage of the cavities led to the formation of cracks. Crack growth occurred in the coarse-grained regions by the linkage of cavities with the crack tip. However, several cracks were observed to terminate after extending up to a fine-grained region of a specimen. A model has been developed to rationalize the observation that preferential cavitation occurs in the coarse-grained regions of a specimen undergoing creep deformation.