Rheology is a science. It focuses on the flow of matter, both liquids and solids and is based on the principles first established by Robert Hooke (1635 – 1703) and Sir Isaac Newton (1642-1727) in the late 1600s. It examines the processes and compiles data on what factors interact with and affect the flow of these forms of matter. Testing of these materials is helpful in providing information on how to process them successfully. The processing and development of polymers is one particular area where rheology testing is a very useful tool.
Rheology Tests
Rheology tests measure the flow of polymers. Polymers are a complex material that is a staple for many industrial applications. Thermoplastic polymer melts are widely employed to produce a variety of components for products. Relatively cheap, they are rather ductile, easily formed into the diverse shapes required by manufacturers. However, before using any polymer for production, technicians need to understand how various factors will influence their flow. They must discover how the material will flow while being processed in a specific manner.
To accomplish this, technicians in the lab first analyze the polymer by testing it. The rheology testing methods applied usually consist of one or even both of the following types of instruments. These are rotational rheometers and capillary rheometers.
Rotational Rheometers
Using a small piece of the polymer, this device – made up of a pair of parallel plates or two separate ones – an upper cone and a lower plate, is capable of providing the following:
- Creep Tests
- Flow Rates
- Sinusoidal Oscillatory Testing (Small amplitude only)
- Stress Relaxation
The shear rates are substantially lower than those for the other rheometer – the capillary rheometer. However, the rotational rheometer is the preferred instrument when the desire is to acquire data on the molecular structure of a polymer and how this will influence the characteristics during processing.
Capillary Rheometers
Capillary rheometers measure at higher shear rates than their rotational counterpart. They also provide a true measurement of viscosity as well as the melt properties. What is also of importance is the ability of technicians to use this method to obtain measurements of elongational or extensional properties at higher extension rates. More importantly, it is not restricted to lab applications but is employable in normal processing conditions.
Rheology Testing
Polymers are complex materials. Addressing their melt rheology is not an easy task. Yet, laboratory technicians and the industries they work for have to do so if they want to produce products and components capable of meeting the specifications of their customers and the demands of consumers. With little margin of error permissible, the answer lies in effective and precise rheology testing using either the rotational or the capillary rheometer.