Rheological properties thixotropic

Newtonian flow, and their viscosity is not constant but changes as a function of shear rate and/or time. The rheological properties of such systems cannot be defined simply in terms of one value. These non-Newtonian phenomena are either time-independent or time-dependent. In the first case, the systems can be classified as pseudoplastic, plastic, or dilatant, in the second case as thixotropic or rheopective. 

Newton s law states that for a liquid under shear, the shear stress T is proportional to the shear rate. In this sense, most of the unpigmented vehicles used in the paint and printing ink industries are considered ideal or Newtonian liquids. The ratio of the shear stress t to the shear rate D is thus a constant t), dependent only on temperature and pressure. This is not true for specialized gel varnishes and thixotropic systems, which are designed to have special rheological properties. 

Fermented milk products exhibit thixotropic rheological properties, i.e. the viscosity (resistance to flow) decreases as the rate of shear increases a typical relationship is shown in Figure 10.33. The rheological properties are major parameters of quality and are controlled by varying the total solids content of the milk, the heat treatment and homogenization of the milk and the use of hydrocolloids, e.g. gelatin or carageenan. 

Brush marks. Lines of unevenness that remain in the dried paint film after brush application. Brush marks and ropiness are associated with poor flow and sticky application. These defects are more often encoimtered in highly pigmented products and in certain latex paint formulations. Tbo rapid recovery of consistency in a thixotropic system will also cause these defects. The remedy may be the addition of a flow promoter, reduction in consistency, or modification of the rheological properties. 

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. 

Fillers are used in these products to improve mechanical properties or impart flammability resistance. Fillers are frequently silane-treated to further improve mechanical properties. Fillers must have a low moisture (below 0.1%), a low absorption of resin, and are expected to impart thixotropic properties. There are special cases. For example, if peroxyketals are used as initiators, basic fillers have to be used because acidic fillers interfere with cure times and the shelf-life of the composition. Shape and particle size distribution must be considered in filler selection to impart the desired rheological properties. Calcium carbonate is the most popular filler but aluminum trihydrate, anhydrous calcium sulfate, and silica are also frequently used. Barite is well suited to this application, especially if acid... 

Time-Dependent Rheology. The rheological properties of suspensions are often time-dependent. If the apparent viscosity continuously decreases with time under shear with a subsequent recovery of the viscosity when the flow is ceased, the system is called thixotropic. The opposite behavior is called antithixopy or rheopexy. Figure 2 shows the time-dependent behaviors of suspensions. Curve 1 in Figure 2 illustrates a hysteresis produced by a thixotropic suspension, where con-... 

These help to avoid sedimentation of the abrasive and bring the desired rheological properties to the product, such as easing the flow from the tube while ensuring it ranains firm on the brush. Thickeners structure the liquid phase of the product, holding the abrasive in suspension. This structure disappears when pressure is applied to the tube, only to reform when the pressure stops (thixotropic behavior). 

The rheological properties of concentrated suspensions often depend not only on the shear rate but also on the time. When the viscosity decreases with time under shear but recovers to its original value after flow ceases, the behavior is known as thixotropic (Fig. 4.36). This type of behavior is more often observed in flocculated suspensions and colloidal gels. When the suspension is sheared, the floes are broken down leading a distribution of floe sizes. Often the regeneration of the floes is slow which causes the resistance to flow to decrease. The opposite behavior, when the viscosity increases with shear rate and is also time dependent, is known as rheopectic. 

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