TRENDS Ultrasonic welding has become an eflective and sometimes economically competitive way to join plastic parts. The weld produced by ultrasonic melting is occasionally even stronger than the base plastic and this fact may lead one to suspect that the material in the weld is chemically dzyerent from the base material. Some work reported by M . S. Akutin and his associates in Soviet Plastics, December 1967, 19 (just available in translated f orm) suggests strongly that ultrasonic melting can induce copolymerization in the weld when the pieces being welded are composed o f two diferent polymers having similar melting points. Even when the two welded pieces are identical plastics there is evidence o f further polymerization in the weld. Polystyrene and poly(methyl methacrylate) were the plastics used in the Soviet studies. Particle size distribution is a major-factor which determines the tensile strength o f a powder, reports D. C.-H. Cheng of England’s Warren Spring Laboratory [Chem. Eng. Sci., 23, 1405 (1968)l. The other major factor is density: the higher the density of the powder compact relative to particle density, the greater is its tensile strength. Cheng has developed a simpla$ed theory of powder breakage and has proposed an equation which relates tensile strength to size distribution, density, and interparticle f orces. The equation was tested for powders o f aluminum oxide, precipitated calcium carbonate, zinc dust and calcite, and promises to help engineers predict the tensile strength of any powder using a minimum of experimental data. Equations of state for liquids can be determined by an ultrasonic technique developed by P. L. M . Heydemann and J . C. Houck at the National Bureau of Standards, Gaithersburg, M d . The new technique, which can be used at pressures up to 650,000 psi, involves measuring as a function of pressure the time of travel o f an ultrasonic pulse through the liquid. The length, and hence the volume, of the sample are computed from the travel time. If as many as.jive reJections.from the-front and rear q f the sample are used, the travel time can be measured to within I nsec. The accuracy o f the method was established by measuring the density of distilled water at 12.5 Kbar, a system for which independently measured data were available,f o r comparison. Uniform suspension of solids in a liquid is frequently a necessity to ensure the ejiciency o f mass-tramfer processes such as dissolution or extraction. Of particular concern in this regard is how to arrange for proper suspension when a rotating impeller is used in a bafled or unbafled tank, at the same time minimizing speed and power requirements. A . W. Nienow o f University College, London, has.found that the clearance between the tip o f the impeller and the vessel wall is the single most important variable determining ejiciency of suspension. Large clearances produce quite dzflerent Jow patterns in the tank from small clearances, and large-clearance Jow patterns are not eflcient in promoting suspension. Nienow therefore strongly recommends the use o f large impellers with small clearances [Chem. Eng. Sci., 23, 1453 (1968)]. The fracture of glass as a result of a sharp impact usually takes place in the same way as breakage under a slowly increasing load. This slightly surprising conclusion has been reached by E. B. Shand, who reports on a series of glass fracture tests in A m . Ceram. SOC.Bull., 47, 1157 (1968). Most impacts, except those characterized by very high striking velocity (e.g., by a bullet), are in fact elastic in nature and are made up q f a series of successive impulses rather thanjust one. Under these conditions fracture originates at a j a w in the surface opposite that impacted, and impact resistance is dependent on the severity of flaws. When the impact is largely inelastic, impact resistance is dependent on the elastic energy stored in the glass and the extent to which the impaction force can thus be damped. VOL. 6 1
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