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Rheology is a central science that can be applied successfully to material systems as seemingly dissimilar as peanut butter and tire rubber, and the use of rheometric instrumentation in development and quality control is well established in highly competitive, low-margin industries, such as plastics and inks. In recent years, increasing numbers of researchers and manufacturers in ceramics and glass have also begun to incorporate rheology into their research and operations.
At the most basic level, rheology can be described as the study of how materials flow or, in a broader sense, how materials respond to the application of deformational energy. This definition could obviously encompass the entire field of solid and fluid mechanics. In practice, both the field of rheology and the development of rheometric instrumentation have been linked to the study and development of viscoelastic material systems, which may be simply defined as material systems whose properties are time and/or temperature dependent.1 Some material systems, such as commercial thermoplastics and food products, may display viscoelastic behavior across their entire range of application—from raw material to finished product. Other material systems, including many ceramics and glasses, may only display these tendencies toward time and temperature sensitivity during specific phases in their manufacture. Rheometric analyses can therefore be invaluable in characterizing the effects of formulation and processing variables on both the processability and end-use or performance properties of these material systems.
How Rheometers WorkThe most basic and widely used form of rheometric instrumentation is the simple steady shear viscometer. A wide variety of existing devices have been developed for the measurement of steady shear viscosity, many of which are specific to a particular industry or material. In principle, however, all of these devices share a common goal: to measure the bulk viscosity of a material as it flows in a steady or continuous fashion. In fact, the definition of the term “viscosity,” which is the ratio of the shear stress applied to a material and the resulting shear rate, suggests the design of most of these simple viscometers.
As a comprehensive materials characterization tool, however, the traditional viscometer is of limited use. The most powerful and versatile form of rheometric instrumentation currently in use may be described in general terms as a dynamic shear rheometer or simply a dynamic rheometer. The modern dynamic rheometer shares a basic common concept with the simpler viscometer in that it uses well-defined geometries, such as cone plates, parallel plates or concentric cylinders, to isolate and deform the material in a controlled fashion (see Figure 1). A fully functional dynamic rheometer will include steady shear rate capabilities that enable it to be used as a viscometer to measure steady shear or bulk viscosity.
The real power of a dynamic rheometer, however, lies in its unique ability to apply very small amounts of rotation or deformation in a dynamic or oscillatory fashion. It is often useful to visualize this type of dynamic shear testing as if the sample were being “vibrated” between parallel plates or concentric cylinders, as opposed to being sheared in a continuous fashion. The components of a modern dynamic rheometer enable this “vibratory” measurement to be applied to a sample in a controlled fashion while also controlling the sample temperature.
The significance of this dynamic testing method is that the resulting measurement is delivered in terms of discrete components of the material’s viscosity or shear modulus, as opposed to the simple bulk viscosity reported by traditional viscometers. As mentioned previously, viscoelastic materials display time- and temperature-dependent properties. When analyzed using a dynamic rheometer, the viscosity or shear modulus of a viscoelastic material may be resolved into components parts referred to as the “elastic” and “viscous” components: 2
G* = [(G’)2 + (G’’)2]1/2
where G* is the dynamic shear modulus, G’ is the elastic or storage modulus, and G’’ is the viscous or loss modulus.
These component parts of the bulk viscosity or modulus have specific meaning in the context of the bulk properties of the material and are individually very sensitive to specific events occurring in the morphology or microstructure, or even the nanostructure, of the material system. These same structural effects or phenomena are often invisible to traditional, steady-shear viscometry.
A Dynamic Rheometer in ActionAn illustrative example can be drawn from the experience of a product manager of a sol-gel ceramic used in an optical application. The existing combination of product and process created a manufacturing process that was too time-consuming, thereby increasing the unit cost of the finished product. Less expensive competitive products were eroding this company’s market share. After reviewing the range of options available for reducing the cost of the product, it was determined that the manufacturing process could not be easily changed. Instead, the company decided to change the formulation of the sol-gel ceramic to optimize the material’s response to the process conditions. The primary formulation change involved the evaluation of different catalyst packages formulated into the sol-gel ceramic to initiate and control the gel time.
A dynamic rheometer* was used to monitor the effect of various catalyst packages on the gelation behavior of the sol-gel ceramic. The gelation process is an excellent example of material behavior that can be uniquely characterized using a dynamic rheometer. With the appropriate temperature control systems, the rheometer can simulate the process conditions very accurately.
The gel point can be easily isolated by locating the point at which the G’ and G’’ are equal or cross one another. A more rigorously accurate gel point can be determined by isolating the time at which the ratio of G’’/G’ is frequency-independent, but a majority of practicing rheologists use the aforementioned G’ = G’’ criterion to estimate the onset of gelation.3 Conceptually, this is point in the gelation process at which the complete three dimensional network has been formed within the ceramic—from this point forward, the ceramic will become increasingly elastic and less viscous or viscoelastic.
Note also that the final modulus of the newly formulated ceramic was almost an order of magnitude lower than the original formulation. This reduction in modulus was determined to be the result of the faster gelation process, which resulted in a lower final crosslink density and a “looser” network. For this product’s application, it was determined that the reduced modulus met existing performance criteria and had the unexpected benefit of enhancing the ceramic’s ability to accept the diffusion delivery of other constituents into the ceramic system. This diffusion-controlled enhancement of the product was an important final stage of the product’s manufacturing process, which was unexpectedly improved by the change in product formulation. The use of a dynamic rheometer in this project was critical in both the speed and the degree of success with which it was completed. 4
Beyond Basic RheologyIn addition to providing insight into the effects of formulation on processing behavior, a fully capable dynamic rheometer can provide analysis of the final morphology and properties of the material system. The most advanced of today’s rheometer systems couple standard rheometric analysis with simultaneous analysis features, such as optical, dielectric or high-voltage measurements. Figure 4 displays the use of an optical analysis system** in conjunction with a parallel plate rheometric experiment to measure the birefringence and dichroism of the sample material.5
Modern rheometric instrumentation offers unique capabilities to today’s ceramic and glass manufacturers, and is a sound investment in improving product quality and profitability.
For More InformationFor more information about rheometric analysis, contact Rheometric Scientific, One Possumtown Rd., Piscataway, NJ 08854; (732) 560-8550; fax (732) 560-7451; or visit www.rheosci.com.
*Rheometric Scientific’s ARES, Advanced Rheometric Expansion System
**Rheometric Scientific’s OAM, Optical Analysis Module