Beam-Bending Viscometers

Experimental techniques such as those used to measure specific values of viscosity (e.g. softening point) are still in common use, but are not as powerful as those in which a range of viscosities can be measured. Hence, only the Margules (1 to 106 Pa-s), parallel plate (103 to 108), and beam bending viscometers (107 to 1014 Pa-s) will be discussed here. These devices are manufactured and marketed by Theta Industries. 

The beam bending viscometer is depicted in Figure 10.10. A glass beam of uniform cross section is extended across an alumina muffle. Using a sapphire or fused silica hook, a load is applied at the center of the beam. The deformation rate of the center of the beam is measured, and the viscosity is determined... 

A 2 mm diameter rod of the glass discussed in question 5 is placed in a beam-bending viscometer. The deflection rate of the center point of the rod is 10 jam s" . The span between the supports is 10 cm. The moment of inertia of a circular cross-section rod is given by Tcr", where r is the radius of the rod. What is the viscosity of this rod in Pa s (Watch the units)... 

The ease of sample preparation for the beam-bending method makes this technique particularly suitable for research studies. Any beam shape, including rods or tubing in addition to square or rectangular bars, can be used, provided the moment of inertia can be calculated. Viscometers have been designed which use samples as small as 1 X 1 X 10 mm, which allow measurements on compositions which can only be formed as glasses by rapid quenching. 

Mergules viscometer Fiber elongation Beam bending... 

High viscosities, more than 10 Pa s,canbesatisfactorilymeasuredinthecommercialTMA and it is in this region that the glass-forming liquid is typically shaped. Table 10.2 summarises the way in which TMA has been used for beam bending, indentation and parallel-plate viscometers to cover the viscosity range 10 - 10 Pa s. The TMA indentation and parallel-plate viscometry are described in detail below (Section 10.5.5 and 10.5.6, respectively). 

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