Characterising the deformation of a polymer melt is an important task to model its processing behaviour. Knowing only shear flow properties is not sufficient to explain processing behaviour of polymers in many processes such as blown film extrusion, thermoforming, fibre spinning or blow moulding that involve stretching. As Meissner (1979) realised on different LDPE resins using only shear data can not always sufficiently explain the deformation behaviour and therefore the processing behaviour of polymers. Since the 1980s the extensional or elongational viscosity has been recognised as a very important property for processes like blown film extrusion, thermoforming or fibre spinning (Baird 1999, Wagner et al. 2002, Bhattacharya 2004). Obtained results show that the elongational viscosity is also a very sensitive indicator for long-chain branching. Measuring the extensional viscosity means getting a value for the materials resistance to stretching flow or elongational deformation. Since the first theoretical works have been made on this topic by Trouton in 1906, different measuring methods have been developed. Apart from simple methods which are not well defined and perform only indirect measurements (like the melt strength test or capillary rheometry), some more complex and precise methods have been established. The widely accepted and commercial available and therefore the most important ones are the Munstedt-type and the Meissner-type, both introduced in the 1970’s. The Munstedt-type rheometer operates via translating clamps suspended in an oil bath (Munstedt 1979) and the Meissner-type uses counter rotating belts (Meissner 1981). Both have been commercialised by Rheometrics, the Munstedt-type as the Rheometrics Extensional Rheometer (RER) in 1981 and a modified Meissner-type as the Rheometrics Melt Extensional Rheometer (RME) in 1994. The RME technique was used for various experiments such as Meissner et al. (1981, 1982), Meissner (1985), Meissner and Hostettler (1994), Levitt and Macosko (1997), Wagner et al. (1998, 2000, 2002), Schulze at al. (2001).
Nevertheless there are different problems occurring with the RME device discussed in various papers e.g. by Meissner (1994), Schweizer (2000), Schulze at al. (2001) and Barroso et al. (2002). [...]
Table of Contents
1.1 Background
1.2 Objectives
2. Materials and equipment
2.1 Materials
2.1.1 Polystyrene
2.1.2 Polypropylene
2.1.3 PPNC
2.2.1 ARES rotational rheometer
2.2.2 CEAST Modular Melt Indexer
2.2.3 RME extensional rheometer
2.2.4 MDSC 2920 calorimeter
3.1 Introduction to rheology
3.2 Shear rheology
3.2.1 Sample preparation
3.2.2 Steady shear measurement
3.2.3 Dynamic measurement
3.3 Melt Density
3.4 Extensional rheology
3.4.1 Sample preparation
3.4.2 Measurement
3.4.3 Visual analysis
3.5 DSC
4. Results & discussion
4.1 shear testing
4.2 DSC
4.3 Extensional testing
4.2.1 Error analysis
4.2.2 Error calculation for video analysis
4.2.3 Comparison of PP and PPNC
5. conclusions
Objectives and Topics
This study aims to identify the factors influencing the accuracy of extensional viscosity measurements using the Rheometrics Melt Extensional Rheometer (RME) and to provide improved experimental and analytical techniques to enhance data precision for polymers and nanocomposites.
- Analysis of error sources in extensional rheology measurement.
- Evaluation of sample preparation and instrumentation impacts.
- Application of visual measurement techniques to determine true strain rates.
- Comparative rheological study of Polystyrene, Polypropylene, and Polypropylene nanocomposites.
- Development of correction methods for thermal expansion and time delays.
Excerpt from the Book
3.4.3 Visual analysis
In addition to the measurement automatically processed by the RME software a visual measurement technique is used to determine the actual, rather than just the commanded strain rate. Therefore special spherical markers sized 160-180 µm are placed on the samples before inserting them in the RME chamber. Capturing the stretching progress with a video camera the test is then manually analysed via computer and VIMPS software. The captured videos can be uploaded into the program and the processed data can be exported into Excel. The resolution of the program is limited due to the 25 frames per second on the captured video format and the maximum video resolution that can be displayed. The developed RME video tracking software (Rohr 1996) was not available for the analysis of this work but is recommended to use as it is especially designed for usage with the RME. This is due to abandoned support by the supplier Rheometrics.
Summary of Chapters
1.1 Background: Discusses the necessity of characterizing polymer melt deformation beyond shear flow to model processing behaviors like extrusion and thermoforming.
1.2 Objectives: Outlines the goal of identifying factors influencing extensional viscosity measurements and providing techniques to improve their accuracy using the RME.
2. Materials and equipment: Details the materials (Polystyrene, Polypropylene, PPNC) and the instrumentation (ARES, CEAST, RME, DSC) used in the experiments.
3.1 Introduction to rheology: Provides a theoretical foundation of rheology, defining viscosity and non-Newtonian flow behaviors through various mathematical models.
3.2 Shear rheology: Covers the experimental setup and theoretical calculations for both steady shear and dynamic measurement modes.
3.3 Melt Density: Explains the procedure for determining melt density, which is essential for accurate extensional rheology testing.
3.4 Extensional rheology: Describes the sample preparation, the RME operating principles, the mathematical models for calculating strain, and the visual analysis method.
3.5 DSC: Discusses the use of Differential Scanning Calorimetry to identify thermal transitions and determine sample crystallinity.
4. Results & discussion: Presents the analysis of test results, focusing on temperature dependencies and comprehensive error evaluation.
5. conclusions: Summarizes the challenges of RME measurement accuracy and suggests future enhancements like improved time correction modeling.
Keywords
Extensional rheology, RME, Polymer melt, Viscosity, Shear rheology, Polystyrene, Polypropylene, PPNC, Strain rate, Error analysis, DSC, Thermal expansion, Hencky strain, Melt density, Rheometer
Frequently Asked Questions
What is the core focus of this research?
The work focuses on identifying and mitigating measurement errors in polymer melt extensional rheometry using the Rheometrics Melt Extensional Rheometer (RME).
What are the primary themes covered?
The primary themes include instrumentation errors, sample preparation, rheological modeling, and the thermal analysis of different polymer grades.
What is the main objective of the thesis?
The objective is to establish factors influencing extensional viscosity measurements and to provide practical techniques to improve the accuracy of these measurements.
Which scientific methods are utilized?
The research employs both instrumented and visual measurement methods, alongside Differential Scanning Calorimetry (DSC) and shear rheology testing, to calibrate and compare results.
What is addressed in the main body?
The main body covers the theoretical background of rheology, detailed experimental setups, calculation models for strain and stress, and an extensive analysis of measurement errors including time delays, force deviations, and strain rate inaccuracies.
Which keywords characterize this work?
Key terms include extensional rheology, polymer melt, RME, strain rate, viscosity, and error analysis.
Why are Polystyrene and Polypropylene compared in the study?
They serve as models for different structural complexities: Polystyrene is a rheologically simple, amorphous material, while Polypropylene is a semicrystalline material, allowing for a broader evaluation of the RME's performance.
What role does the visual analysis play in correcting RME results?
Visual analysis, using markers on the sample, provides a means to determine the actual, rather than the commanded, strain rate, which is necessary because the RME's internal settings may deviate from reality during the test.
How does thermal expansion affect RME testing?
Thermal expansion leads to an excess length of the sample between the clamps, causing it to corrugate or stick to the table, which results in a measurable time delay during the initial phase of the stretching test.
- Quote paper
- Dipl. Wirtschaftsing. Guido Krebs (Author), 2006, Improved measurement and analysis techniques for melt extensional rheometry, Munich, GRIN Verlag, https://www.grin.com/document/58506
