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CHARACTERISATION OF DISPERSIONS FOR PROCESS CONTROL IN THE PRODUCTION OF PARTICULATE MAGNETIC MEDIA

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A computational model of the behaviour of strongly interacting colloidal particulate dispersions has been developed. A molecular dynamic approach has been used, which allows the dynamic properties of the dispersions to be calculated. Standard hydrodynamic expressions are used to model the interaction between the particle and the fluid. The model can be used to calculate physical microstructures within the particulate dispersion and in particular its response to an applied magnetic field. It should be stressed that although the model has been specifically developed for a magnetic system, its application to non-magnetic colloidal dispersions, or dispersions of electrically active materials, is relatively straightforward. Overall, the microstructure of the particulate dispersion consists of small strongly bound aggregates of particles which themselves interact to give an extensive particle network which is the likely cause of the observed viscoelastic behaviour of particulate dispersions. In particular the yield point of the dispersion is likely to be associated with the break-up of the network itself and represent the energy of interaction between the aggregates. The application of an aligning magnetic field gives rise to extensive chain structures having some features of smectic ordering. We have also used the model to develop a simulation of the coating process of magnetic recording media. An important feature of the simulation is that clustering in the high magnetic field phase leads to channels of depleted particle density which can provide channels through which solvent can escape, thus speeding up the drying process. These channels are shown to persist into the final coating as voids which themselves could give rise to noise in the final recorded signal. The model has reached a high level of technical sophistication and has the advantage that it can be used to make a study of parts of the production process for which measurement is difficult, for example the high shear coating process itself.
Research was carried out in order to help improve the efficiency of producing magnetic media and to expedite the development of new products. Success was achieved in designing and building a new type of rapid rise-time variable pulse-width magnetometer with which to measure the properties of dispersions on time scale below 1 ms and down to approximately 5 µs. At the faster time scales there is not time for the particles to move before their magnetization changes and the measurement therefore gives a kind of 'flash picture' of the momentary state of the dispersion. The longer time scales approach those found in certain orienting systems and thus provide design information for those systems. The ability to distinguish stages in the factory dispersion process was demonstrated. By devising an improved theory, the analysis of filter blocking has with success been brought into use in formulation development. An improved form of dispersion magnetometry was found to provide more information. In particular, the monitoring of the kneading stage of new metal particle formulations has been advanced and quantified, and, in parallel the process of orienting such dispersions better understood and improved. The experimental results were used to develop and test the mathematical model. After a Monte-Carlo simulation was unable, because of the difficulty in handling dynamic systems, to simulate laboratory magnetic measurements on dispersions, a molecular dynamics model was produced which gave very good agreement. Putting in the experimental time responses, the model can calculate dynamic processes such as orientation.