SEMICONTINUOUS EMULSION COPOLYMERIZATION OF BUTYL ACRYLATE-STYRENE†

Butylacrylate (BA)-Styrene (St) copolymers have been synthesized via a semicontinuous emulsion process in which the monomers feeding rate and weight ratios (BA/St) were varied. The feed consisted of only pure monomers which were added to an aqueous solution that contained the emulsifier and initiator

The experimental results show that monomer consumption is governed by the feeding rate as well as by the monomer's reactivity, The average molecular weight decreased as the feeding rate increased independent of the BA/St ratio

Measurement of the latex average particle size and particle number, as a function of the reaction time. showed a complex sequence of particle formation and agglomeration. In general, the average particle diameter increased in direct proportion to the feeding rate. This fact suggests that most probably homogeneous nucleation is preferred over monomer diffusion to the previously formed particles. Additionally, the monomer reactivity ratios (r ₁, r ₂) decreased as the monomer feeding rate was increased

The experimental copolymer compositions determined by gas chromatography and NMR-H⁺ showed that the resulting copolymers were closely homogeneous.     

Microstructural effect on tensile and fatigue behaviour of C–Mn steel

Stress–strain behaviour in tension and in torsion was studied in ferrite–pearlite and ferrite–bainite microstructures of C–Mn steel. The fatigue tests were performed under reverse torsional loading on hour-glass shape specimens. The cyclic plasticity in torsion was found at about 37% of the monotonic yield stress in both ferrite–pearlite and ferrite–bainite microstructures. The ferrite phase in the direction of maximum shear stress was the preferable site for crack nucleation. The ferrite–bainite microstructure showed better fatigue properties than the ferrite–pearlite microstructure.     

Hot isostatic pressing of metal reinforced metal matrix composites

Metal reinforced Metal Matrix Composites (MMMCs) made by combining an aluminium alloy matrix with stainless steel reinforcing wires are potentially cheaper and tougher than continuous fibre ceramic reinforced Metal Matrix Composites (MMCs). Although they do not give as great enhancements in stiffness and strength, worthwhile gains are achieved. Such MMMCs can be produced by Hot Isostatic Pressing (HIPping), which reduces interfacial reactions in comparison with liquid metal routes. Here, stainless steel (316L) and commercial purity aluminium wires were used to make bundles which were inserted into mild steel cans for HIPping at 525 °C/120 min/100 MPa. Some stainless steel wires were pre-coated with A17Si, to examine the effect of coatings on mechanical properties. Specimens were evaluated in terms of their tensile and fatigue properties. During HIPping, cans collapsed anisotropically to give different cross-section shapes, and for larger diameter cans, there was also some longitudinal twisting. Wires tended to be better aligned after HIPping in the smaller diameter cans, which produced material having higher modulus and UTS. Higher volume fractions of reinforcement tend to give better fatigue properties. Composites with coated stainless steel wires gave higher composite elongation to failure than uncoated wires. Both uncoated and coated wires failed by fatigue during fatigue testing of the composite. This contrasts with ceramic reinforced MMCs where the fibres fracture at weak points and then pull out of the matrix.     

INTERACTIVE EFFECT OF TWO COPLANAR CRACKS ON PLASTIC YIELDING AND COALESCENCE

A theoretical study of the conditions for coalescence of two fatigue cracks in a multi-crack system is presented. As both cracks grow, the near-side plastic zone sizes of the interacting cracks increase more rapidly than the off-side zone sizes which results in higher overall crack propagation rates for an interacting crack growth system than the simple addition of individual crack growth rates. The use of a single crack growth model to predict failure in the presence of two or more cracks may therefore underestimate the interaction between cracks and lead to un-conservative predictions. Some of the theoretical predictions were confirmed by published results of experimental measurements of crack interaction distances in multi-crack systems.     

A fatigue damage map for 2024-T3 aluminium alloy

The propagation of a fatigue crack from an initial defect of the same order as the scale of the microstructure through to failure has been modelled using a representation of fatigue damage according to the Navarro–de los Rios (N–R) model. The fatigue processes are presented in the form of a fatigue damage map (FDM). It is shown how the map can be used to create a traditional S–N curve and to provide information suitable for estimating fatigue lifetimes under damage tolerant conditions.     

Mechanisms of Salt Tolerance Conferred by Overexpression of the HAL1 Gene in Saccharomyces cerevisiae

Overexpression of the HAL1 gene improves the tolerance of Saccharomyces cerevisiae to NaCl by increasing intracellular K⁺ and decreasing intracellular Na⁺. The effect of HAL1 on intracellular Na⁺ was mediated by the PMR2/ENA1 gene, corresponding to a major Na⁺ efflux system. The expression level of ENA1 was dependent on the gene dosage of HAL1 and overexpression of HAL1 suppressed the salt sensitivity of null mutants in calcineurin and Hal3p, other known regulators of ENA1 expression. The effect of HAL1 on intracellular K⁺ was independent of the TRK1 and TOK1 genes, corresponding to a major K⁺ uptake system and to a K⁺ efflux system activated by depolarization, respectively. Overexpression of HAL1 reduces K⁺ loss from the cells upon salt stress, a phenomenon mediated by an unidentified K⁺ efflux system. Overexpression of HAL1 did not increase NaCl tolerance in galactose medium. NaCl poses two types of stress, osmotic and ionic, counteracted by glycerol synthesis and sodium extrusion, respectively. As compared to glucose, with galactose as carbon source glycerol synthesis is reduced and the expression of ENA1 is increased. As a consequence, osmotic adjustment through glycerolsynthesis, a process not affected by HAL1, is the limiting factor for growth on galactose under NaCl stress. © 1997 John Wiley & Sons, Ltd.     

SUBLIMATION OF PURE SUBSTANCES IN GAS FLUIDIZED BEDS AT ATMOSHPERIC PRESSURE.

This paper deals with the sublimation of large bodies, or “objects”, made up from a pure substance in a bubbling gas fluidized bed of considerably smaller particles, or “fines”. The influence of such parameters as the gas velocity, the bed temperature, the size and the adsorption capacity of the fines has been investigated.

The results obtained clearly show that the rate of sublimation in fluidized beds is far higher than in air alone. It increases with increasing bed temperature, decreasing particle size, increasing powder mass capacity, and roughly varies as a parabolic function of time. It has also been observed that the temperature difference between the bed and the object surface, or “temperature depression”, depends on the fines characteristics as well as on bed temperature, but is independent of gas velocity when good solid mixing conditions are achieved.

Bed-to-object heat and mass transfer coefficients have been deduced from data points and attempts have been made to provide a reasonable theory to account for them. After a complete examination, the idea of interpreting transport phenomena based on a well-adapted “surface renewal model” has been proposed.     

FREE OBJECT MOTION IN A GAS FLUIDIZED BED

The presence probability, the characteristic motions and the velocity of various “light” objects freely immersed in a gas fluidized bed of “heavy” fine particles have been investigated as a function of superficial fluid velocity, powder characteristics and bed height. The displacement of objects through the bed has been shown to result from both a “deterministic” dynamic force equilibrium and a “stochastic” stirring effect due to bubbles. A specific gas velocity exists at which a deep change in mixing occurs: above it the stirring effect is fairly noticeable, while below it pseudo-hydrostatic effects prevail. A method has been proposed to estimate this characteristic value.