Silicones viscosity behavior

Figure 17-2 shows the influence of temperature on the viscosity of representative examples of synthetic fluids of various chemical types. Direct comparisons can be made when individual fluids have the same viscosity at a given temperature e.g. white oil and polyoxyalkylene glycol at 301 K (83 F). The viscosity of the glycol fluid at 372 K (210 F) is 6 Saybolt seconds higher than that of white oil, which is a significant difference that immediately shows the better viscosity behavior of the synthetic fluid. In similar fashion, u,u -di(chlorophenyl)pentane, di(2-ethylhexyl) sebacate and a dimethyl silicone all have a viscosity of 120 Saybolt seconds at 340 K (153 F) but at 372 K there is a spread of 15.7 Saybolt seconds in the viscosity behavior of these fluids. 

Dilatant Flows Krieger and Choi [1984] smdied the viscosity behavior of sterically stabilized PMMA spheres in silicone oil. In high viscosity oils, thixotropy and yield stress was observed. The former was well described by Eq 7.41. The magnimde of Oy was found to depend on ( ), the oil viscosity, and temperature. In most systems, the lower Newtonian plateau was observed for the reduced shear stress value = Oj d / RT > 3 (d is the... 

The ability of a material to flow is usually associated with liquids but whether a stressed body behaves as a solid or a liquid can depend on the period of time over which the stress is applied. With some materials this distinction is obvious. For example, silicone putty can be poured slowly from a container but also be bounced as a rubber ball. In other cases, the flow process can be very slow and the difference is less obvious. The physical distinction between a solid and a liquid is therefore arbitrary but a viscosity of 10 Pa s (100 TPa s) is sometimes used. Figure 5.1 shows schematically the fluidity (=l/viscosity) behavior with changes in temperature of simple molecular materials. The viscosity increases by approximately 30 orders of magnitude as one moves from a gas to a solid. The viscosity of a gas is usually about 10" Pa s (10 xPa s) and changes to 10 Pa s (1 mPa s) on condensing as a liquid. On further cooling, the viscosity rises. 

The melt-dripping behavior of various ethylene-acrylate formulations with chalk and silicone have also been addressed by exposing these formulations to a Bunsen burner. It has been demonstrated that viscosity plays a crucial role in this process by affecting the transport of volatile gases, eventual dripping, and the formation of the intumescent structure.32... 

Figure 3.6 Flow behavior of silicone oils of different viscosities as a function of time period as indicated. The blue line connects flow statuses corresponding to ri t...

Figure 3.21 shows the influence of extensional viscosity on the flow behavior of an aqueous PEO solution. The high extensional viscosity in comparison with the relatively low shear viscosity can cause the upper container to empty when pouring, even when the liquid surface is below the glass rim. This is visible in Fig. 3.21 by the liquid level markings. In the case of the viscous silicone oil however, the liquid thread breaks as the pouring container is tipped back (not shown here). 

Finally, some fluids that undergo viscosity changes on shearing are elastic as well. These are termed viscoelastic fluids. These materials have properties of both a liquid and a solid. An excellent example is silicone putty (e.g., Silly Putty), which shows three different types of behavior depending upon the shear rate. If a piece of this material is suspended (gravity, a low shear force), it will slowly flow downward like a very viscous fluid. If it is sheared fasten it has rubbery behavior. You can observe this by rolling some of it into a ball... 

C with two feed compositions are shown in Tables I and II. Surface tension has been measured as a function of time, and the viscosity of the solutions are shown along with surface tension. The data clearly show that as the viscosity increases with time, surface tension increases, and the higher the rate of increase of viscosity, the higher the rate of increase of surface tension. It has been shown for silicone polymers that as the viscosity increases from an increase in molecular weight, the surface tension increases (27). A step growth copolymerization mechanism, as mentioned earlier for the sulfur-DCP solutions, will have an increase of molecular weight with time, and the surface tension behavior appears to support this mechanism. 

The properties of thermosetting and thermoplastic resin systems are continually improved to meet increasing performance requirements of end users. One way to enhance material properties is to incorporate nano-modifiers, based on elastomeric silicone particles, which are optionally grafted with other (acrylic) polymers to control dispersibility, viscosity, and other parameters. As an example, epoxy resin formulations have been modified with silicone nanospheres to improve low-stress behavior. Table 1 shows the outstanding fracture toughness improvement of silicone coreshell nanospheres, even at very low particle loading levels. 

Figure 17-2. Viscosity-temperature behavior of various types of synthetic lubricants. 1 White oil, VI 101. 2 4 -Undecy1-m-terphenyl. 3 Fluoroalkane. 4 -Di (chlorophenyl )pentane. 5 Polyphenyl ether. 6 Polyoxyalkylene glycol. 7 Di(2-ethylhexyl) sebacate. 8 Dimethyl silicone.

Table 17-6 indicates that concern with the intrinsic influence of the specific lubricant on wear is valid. The fluids listed there are of low viscosity because they were investigated for low-temperature hydraulic service [14], However, the extent of wear shows no direct relation to viscosity the silicate fluid, which permits the most wear, is not the least viscous of the fluids tested, and the silicone, which is the most viscous, is not the one which gives the best antiwear rating. Figure 17-4 shows intrinsic lubricant behavior in a different version of... 

The flow behavior of silicone oils [5] and silicone oil/glass sphere suspensions [14] was studied by several authors. One of the most used rheological material parameters to characterize the flow behavior is the zero-shear-rate viscosity tIq. The t]o value of the linear silicone oils studied are correlated with the relevant weight-average molecular weight Af by Eq. 3 [16], where a = 3.58. 

The question is whether the viscosity can be predicted also for an unknown sample. This was the motivation for another evaluation of the data of Fig. 4. Figure 5 contains the same data as Fig. 4 but in contrast to Fig. 4 values at the same temperature (-20, 20 and 70 °C) are fitted with a potential function. The results of the fitting and the corresponding correlation coefficients are shown. Figure 5 demonstrates that for a constant temperature Tio correlates with T2. This can be expected as both the flow and the NMR relaxation behavior depend on the ipobility of the molecules in the silicone oils and, thus, on the molecular weight. 

The viscosities of the three compounds are approximately on the same level over the whole detection range. Only a slight decrease in viscosity is observed when the shear rate (Fig. 1) is increased all three compounds exhibit no shear thinning behavior [5]. In contrast, the viscosity levels are quite different. Whereas the carbohydrate-modified silicone exhibits a high viscosity (100 Pa s) the viscosity of the polyether-modified silicone is much lower (1.8 Pa s). The value observed for the silicone with mixed substituents is in between (50 Pa s). 

The higher viscosity of self-lubricating liquid silicones — it is roughly twice that of equally hard standard products containing no silicone fluid — is evident firom their thixotropic behavior. 

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