Z. Tadmor

Dispersive Mixing in Internal Mixers—A Theoretical Model Based on Agglomerate Rupture

Abstract The mixing of solid additives into a matrix of rubber or plastics is an energy intensive process. Such a mixing operation generally involves rupture of agglomerates formed by the solid phase, separation of closely packed particles after rupture, and distribution of the separated particles throughout the polymeric matrix. Dispersive or intensive mixing refers primarily to the first two steps, whereas, the third step is by and large an extensive mixing process. Depending upon the polymer-additive system and mixer machine design, each of these steps may be rate determining. The most important and most investigated system is the carbon black-elastomer system. Mixing of such a system is generally carried out in batch Banbury type internal mixers (Figure 1). The dispersive mixing process in internal mixers is quite complex with many subtle features. A detailed review of the subject is outside the scope of this paper. However, careful reading of reviews and publications in the field, indicate that a gre...

Melting phenomena and mechanisms in polymer processing equipment

This article examines the heating and melting phenomena taking place when individual polymer particulates or compacted polymer particulate systems are subjected to stressing that forces them to deform and flow. The heating/melting behavior in compression experiments of single polymer cylinders and melting in batch internal mixers, as well as in corotating twin-screw extruders, was examined. Different polymers and different polymer particulate solid systems were used, over a range of processing conditions. The results of this work shed light on the important roles that solid dissipative deformation and interparticle frictional phenomena play in generating the heat necessary to melt polymer particulate systems. Also, an attempt is made to deal with the modification of the thermal energy balance equation, so that it includes the heat-generating dissipative source terms.

Dispersive mixing in rubber and plastics

Abstract Most useful plastics and rubber products are mixtures of pure polymers or copolymers with a variety of additives such as pigments, stabilizers, antioxidants, flame retardants, crosslinking agents, fillers, reinforcing agents, plasticizers, foaming agents, etc. Moreover, much attention is being placed recently on the preparation of blends of polymers, leading to products of unique properties. The distribution of additives into the polymeric matrix and the detailed topological structure of the blends depend on the nature of the mixing mechanisms, hence they are intimately related to the mixing equipment configuration and mixing operating conditions. It is convenient to distinguish between two mixing mechanisms extensive and intensive or dispersive. The former, in the very viscous polymeric systems, is achieved by and large by convection. It may either be distributive or laminar. Distributive mixing may involve an ordered or random rearrangement process. Laminar mixing is achieved by imposing on the...

Scale-Up of Internal Mixers

Abstract In this paper, a new criterion for scaling Banbury type internal mixers is proposed. The new criterion is a dimensionless number Xt*, representing the product of the fraction of broken agglomerates during one pass through the high shear zone of the internal mixer and the average number of passes through this zone, for a given mixing time. It can be calculated for a given system of polymer-additive from a knowledge of machine geometry and operating conditions. The dimensionless number Xt* is uniquely related to the fraction of undispersed agglomerates, ψ, which is a frequently encountered mixing quality criterion. Experimental results reported in literature, for a broad range of mixer sizes, fit, within practical acceptable error, the theoretical curve ψ versus Xt*, lending support to its validity. Moreover, the dimensionless number Xt* correlates almost linearly with the presently used scaling-up criteria, the unit work input and the total shear strain, which in turn proved to be interrelated to ...

The Formulation of a Mathematical Model for Plasticating Screw Extrusion

Abstract The theory of plasticating extrusion has reached the stage where scientific design methods can replace the empirical ones. It is possible today to build a mathematical model comprising the whole plasticating extruder. This model predicts the temperature and pressure profiles along the extruder, the solids content everywhere in the extruder, the total power requirements and even the temperature fluctuations of the extruder. These developments were made possible by the recent mathematical formulation of the melting mechanism in the extruders, by a more accurate formulation of the melt conveying theories and by the availability of high speed digital computers. The development of such a mathematical model is discussed in this paper. The model has been built by a stepwise process of improving existing theories and eliminating critical simplifying assumptions and undue complexities. This has been accomplished by comparing the computer simulated results with their experimental counterparts. The applicat...

Proactive higher education for high technology

Foreseeing a need for far more engineers and scientists than its universities were turning out, Israels government, military, and academia worked in concert to entice students into technical fields. Appeals to patriotism were part of the mix. The national initiative clarified to high school graduates and young people who had done their military service the understanding that both their personal and the nations future success lies in technology and science, and it urges everyone with the qualifications to study engineering and science. Special emphasis was placed on attracting women, who form Israels largest untapped human resource in these fields. The national drive proved a resounding success. Today, enrollment in the Technion-Israel Institute of Technology in Haifa, the countrys major and leading technological research university, is 40 percent higher than just two years ago. The other two engineering schools in the country, at the Tel-Aviv University and Ben Gurion University in Beer Sheva, enjoyed similar surges in enrollment. At the same time, many colleges in Israel began educating students in disciplines pertaining to high-tech industries. In view of not one but two consecutive years of far greater enrollment nationwide, we would appear to have succeeded in reversing the decade-long decline in young peoples interest in engineering and related studies.