No abstract
SynopsisBy measuring tack energy using a modified probe tack testing procedure, the interrelation of bulk energy and surface energy effects in pressure-sensitive adhesives was studied. Tack energy was strongly influenced by the solvent used in the preparation of the adhesive film. A procedure was empirically derived which reduced the number of variables to a single variable, yielding a single master curve in which the independent variable was the speed of probe withdrawal expressed on a logarithmic scale. The form of the curve was a simple exponential function, y = A exp ( m x ) , where A and m are constants and y and x are the dependent and independent variables, respectively. The constant rn was found to be a unique function of the type of adhesive used. A theoretical interpretation of the devised procedure was based on bulk viscoelastic effect.s and a combined activation energy-free volume concept of adhesive bonding. The wider implications of this are briefly discussed.
The systematic study of agitation, as a chemical engineering operation, has been undertaken in most of the categories possible, but certainly not in the high shear section of the technology. Even the usual very empirical studies are lacking. The reasons typically offered for neglecting this area are lack of adequate theoretical background, limited scope of application, plethora of variables involved, and extreme diversity of equipment types available. The state of the art has improved little since Scoville (12), in 1895, reported that "five minutes of very rapid trituration will accomplish more in emulsifying an oil or balsam than an hour of slow trituration."High shear agitation as discussed here refers to the classes of application known as emulsification, dispersion, homogenization, etc. It is the processing area that lies between the agitation intensity produced by conventional impellers operated at high power per unit volume and the shear forces generated by homogenizers or colloid mills. It is a relatively narrow range, at present served by a variety of high speed impellers, most of them being modified disks or cones or the rotor-stator type.The term high shear may be defhed in several ways. First to be considered should naturally be a theoretical one. Discussions of the fundamental hydrodynamics such as those by Calderbank (3) and Hinze (7) do much to help understand the forces controlling dispersion processes. But the mechanism of shear involved in fluid agitation has not yet been satisfactorily established in terms that can be applied to practice. Although theories in favor of cavitation, velocity change, direction change, and mechanical forces have been offered, none has been verified.A better understanding of the process is obtained from consideration of the impeller action. The shear/flow characteristics of various standard mixing impellers in the conventional range of use have been evaluated by a number of investigators (1, l o ) , and the general concept of shear is established as a function of impeller geometry, system geometry, and power-speed relationships. It is mathematically improper to say, as is often done, that a high shear impeller invests the majority of the total energy via shear and a minority in flow. But it can be stated that performance and power are adjusted to maximize the head term (WD') and reduce the flow ( N D'), and that is done by means of a relatively small D / T ratio, a high speed, and a small opposed blade area.SpecSc literature on high shear agitation of liquid systems is predominately qualitative in nature. Case history publications have been universally testimonial in nature and reflect the proprietary designs involved. Patents abound but add nothing of technical value. A good general treatment of the subject is found in the book by Clayton ( 5 ) . Several investigators (6, 9) reported quantitative agitation data on emulsification problems, and the papers by Brothman ( 2 ) and Miller and Rushton (8) discuss possible factors controlling shear. All of these sources recog...
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