Editorial Feature

Characterizing the Viscosity of Glass by Rotational Viscometry

Hot extrusion processes are popular techniques to develop objects with specified cross-sectional profiles, such as those that are required to achieve heat- and corrosion-resistance, as well as higher material strengths. This process can be applied to various metals, such as magnesium, aluminum, copper, steel, titanium, nickel and other refractory alloys, with operating temperatures ranging from 350° C (650° F) to 2000° C (4000° F).

As aerospace industries, offshore plants, desalination plants and a number of other industries are becoming increasingly interested in maximizing the potential of their metals; hot extrusion processes have been extensively used for this purpose.

To control product quality and efficiency, an appropriate lubricant is essential to minimize friction between the die and workpiece. It also guarantees insulation when extrusion temperatures exceed 800° C.

Glass lubricants, which are composed of a glass powder, solid lubricants, and an organic bonding material, have been shown to extend the life of metals by preventing excessive die chilling any unwanted heat conduction, by creating an insulating effect between the die and billet.

Characterizing Glass Lubricants

Since glass lubricants exhibit varying melting temperatures and viscosity characteristics as a result of the components that comprise the lubricant and the different functions the lubricant is responsible for. It is therefore imperative that the industries that utilize glass lubricants are equipped with the tools that allow for extensive characterization of the potential variations in the viscosities of glass lubricants. To this end, researchers have investigated a number of different techniques to characterize glass viscosities, of which include:

  • Parallel Plate Viscometry (PPV)
  • Ball Penetration Viscometry
  • Fiber Elongation Viscometry
  • Rotational Viscometry (RV)

Parallel Plate Viscometry (PPV)

PPV is a useful test to investigate glass viscosities within the range of 104 Pas to 108 Pas, as well as measuring the softening point of the glass. To perform this test, the experimental device referred to as a parallel plate viscometer determines any change in the thickness of a cylindrical specimen through the movement of parallel plates along their common central axis. This viscometer consists of a balancing weight, load bar, linear variable differential transformer (LVDT), a 500-gram load and heating chamber that controls the holding temperature and heating rate of the apparatus.

The LVDT accurately measures the displacement of the glass under the 500-g load as the temperature increases to a constant rate of 5 ° C/min. As the temperature increases, the initial glass pad will lose its resistance to the applied load as it transforms into its softened state. The thermal expansion in any aspect of the viscometer, including the load bar, plate, and lever, can affect the precision of measurement for the displacement of the molten glass as the temperature reaches 900° C.

Rotational Viscometry

RV is also a useful technique used to measure glass viscosity. However, its detection range is capable of measuring viscosities of 100 Pas to 106 Pas. Although this technique is useful for its ability to measure a wide range of viscosities, its inability to measure the viscosity surrounding the softening point of a glass material can cause damage to the machine when the tested material undergoes this transformation.

The viscometer used for RV analysis calculates the torque that is required to rotate a spindle that is immersed in a molten glass solution at a constant angular velocity through a computer-controlled rheometer. From this point in the procedure, the temperature is decreased from the melting point of the glass. Therefore the main principle of the RV test involves measuring the torque under a constant spindle speed with decreasing temperature.


Song, Y., Won, C., Kang, S., Lee, H., Park, S., Park, S. H., & Yoon, J. (2018). Characterization of glass viscosity with parallel plate and rotational viscometry. Journal of Non-Crystalline Solids. 486, 27-35. DOI: 10.1016/j.jnoncrysol.2018.02.003.

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Benedette Cuffari

Written by

Benedette Cuffari

After completing her Bachelor of Science in Toxicology with two minors in Spanish and Chemistry in 2016, Benedette continued her studies to complete her Master of Science in Toxicology in May of 2018. During graduate school, Benedette investigated the dermatotoxicity of mechlorethamine and bendamustine; two nitrogen mustard alkylating agents that are used in anticancer therapy.


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