Kriechen (Rheologie)Creep is one of the earliest “controlled stress” rheometer tests that quite literally “creeps” the material, i.e. we measure over a relatively prolonged period the small movement (the creep defined as creep compliance, J) of the sample by applying a small constant stress.Rheology is the science of studying the flow and deformation of materials rooted in the laws of elasticity and viscosity proposed by Hooke and Newton in the late 17th Century. Thermoplastic polymer melts are widely used in many modern industrial processes to manufacture a multitude of objects. Polymers are used because they are relatively cheap to form into complex shapes in the molten state. However, we need to understand how they flow when being processed .
Influencing factors on polymer flow behavior
Polymers are complicated materials to characterize rheologically because there are many factors that influence their flow properties. Examples of factors that influence the flow behavior may include processing temperature, rate of flow, residence time, etc.
Furthermore, the rheological properties of polymers are in between those of a liquid and a solid. This leads to time dependence of the flow properties and other important characteristics, some of which are discussed below .
Important rheological properties of polymers
Melt viscosity is well known to be critically dependent on temperature. By lowering the temperature of a mold until the part being produced has a matt finish, the process engineer can learn the minimum temperature (hence maximum resin viscosity) at which the process can be run without surface defects becoming apparent. Reducing the mold temperature saves energy and can reduce cycle times and so an understanding of the temperature dependence of melt viscosity is very useful.
Polymer melts are known to exhibit die swell when extruded. This phenomenon reveals itself as an increase of diameter of an extrudate after exiting a die. The amount of die swell is related to the amount of elastic deformation of the material at the inlet of the die. A further fact to be considered is that the degree of die swell (more correctly extrudate swell) is dependent on the length of the die when material is extruded at constant throughput. In other words polymer melts exhibit time dependency as the material forgets the elastic deformation applied at the entrance of the die, the more time the material spends within the die the less die swell.
Melt elasticity can also have profound implications for many other polymer processes such as:
- Blow Molding where the wall thickness of the blown component depends on the degree of swell that has taken place during the extrusion process prior to the mold being closed.
- Vacuum Forming or Thermoforming where the polymer must maintain a degree of elasticity to prevent the material sagging before it is pulled by vacuum over the cold forming die. If the material does not have sufficient elasticity, it is likely to come into contact with the chilled die before the vacuum or pressure is applied .
How to characterize polymer melt flow behavior
Polymer processing properties also depend on the concentration of lubricants, plasticizers, fillers and other components in the compound being processed. From this brief introduction, one can appreciate that proper characterization of polymer melt flow behavior is likely to require sophisticated and versatile instrumentation.
From the point of view of the rheologist, polymer flow behavior can be conveniently separated into three components: Shear and extensional flows, which are characterized by the corresponding viscosities and elastic behavior, which is characterized by measurement of modulus or swell ratios .
The right instrumentation is key
To fully characterize a material, instrumentation is required which has the capability of extracting these parameters over a range of temperatures and shear/extension rates. Modern laboratory rheological test apparatus can be divided into two broad categories of Kinexus rotational rheometers and Rosand capillary rheometers. Furthermore, thermal analysis instrumentation like Differential Scanning Calorimetry, Dynamic Mechanical Analysis and Thermomechanical Analysis also allows valuable insights into material properties.
In the next article, you will learn how our Kinexus Rotational Rheometer can be used to characterize thermoplastics.
Thanks to Dr Bob Marsh (former employee of Malvern Panalytical) as the original author of this article!