Thixotropic flow advantage

Thixotropy is a rheological property that results in yield stress on standing. Thixotropic flow is defined as a reversible, time-dependent, isothermal gel-sol transition. Thixotropic systems exhibit easy flow at relatively high shear rates. However, when the shear stress is removed, the system is slowly reformed into a structured vehicle. The usual property of thixotropy results from the breakdown and buildup of floccules under stress. A small amount of particle settling takes place until the system develops a sufficiently high yield value. The primary advantage of thixotropic flow is that it confers pourability under shear stress and viscosity and sufficiently high yield stress when the shear stress is removed at rest. 

In practice, it is advantageous to be able to decrease the viscosity (thus decreasing the resistance to flow) by increasing pressure or velocity, as well as to modify the flow geometry in order to increase shear rates. In some cases, there is an effect of the history of previous deformation. This is exemplified by thixotropic liquids that show a low viscosity after a shear history (like stirring), while a high viscosity prevails at rest. This phenomenon is much desired in the case of paints and varnishes. 

It is well known that polymer melts exhibit thixotropic behavior, i.e, reversible structural break down with flow. This means that an opportunity exists to take advantage of this reduction in resistance to flow to process polymers with less expenditure of energy. Current studies of the time dependence of recovery of structure from previous shearing indicate that rather long times, up to ten minutes, are available before complete recovery . Ap-... 

Another method to measure flow properties after application involves the application of microdielectric measurements that have shown promise for characterizing fihn formation and cure (5,10-13). In this technique, the paint is applied to a thin, flat sensor connected to a frequency generator, an impedance analyzer and a computer. The sensor monitors the dielectric properties of the paint film at and near the interface between the sensor and the coating. The dielectric parameter used to estimate flow is called ion viscosity, but really is electrical resistivity rather than a trae viscosity. Ion viscosity is a measure of both the number and mobility of ions in a specimen. It is affected by changes in temperature, loss of solvent, crosslinking and other chemical reactions, formation of physical structure (such as by a thixotrope), ionic impurities, and ionic additives. The advantage of this technique is that measurements can be made in situ during application, flash, and the bake. 

---