Polymer Analysis with MCR Rheometers. MCR Series

Transcription

1 Polymer Analysis with MCR Rheometers MCR Series

2 Polymer Analysis Today, polymers are among the most important materials in existence as the properties of polymers can be adapted in a very wide range to fit the field of application. Some polymers are hard and brittle or tough and shock-resistant, while other polymers are soft and flexible. The manufacturing and characterization of polymers is therefore the focus of activity for numerous industrial companies and research institutes. Introduction Plastics are organic or semi-organic substances with a high molecular weight. The length of the molecular chains and the entanglement between them are decisive parameters which influence the properties of the material. Many of the relevant properties can be characterized using rheological tests. Polymers have complex chemical and morphological structures and a wide range of variation in the composition and possibility of modifying the material. Therefore they show complex behaviors which need to be taken into consideration when using or manufacturing these materials, e.g. the viscoelasticity, non-newtonian flow behavior, anisotropy (dependent on orientation or modification), complex aging behavior and much more. Describing the properties of polymers requires versatile procedures in order to obtain the needed information. Many methods are used for processing and manufacturing plastics. The majority are forming and reforming procedures (compression molding, calendering, film extrusion, blow & injection molding, etc.). Optimizating these procedures and their quality control is therefore extremely important in the production of plastics.

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4 Rheological Tests on Polymers Process Simulation Correlation to manufacturing conditions Measurements at low shear rates are mainly used for analyzing manufacturing problems. Whereas manufacturing processes such as extrusion or injection molding occur at high shear rates, differences between the materials are usually seen at low shear rates. Manufacturing problems often occur at low shear rates, e.g. delayed die swell with extrusion or delay due to irregular relaxation during the cooling phase of injection molded parts. Polymer melts show pronounced shear thinning behavior, i.e. the viscosity decreases with increasing shear rate. Flow curves are important for the manufacturing of polymers to determine the energy required for the process. Oscillatory measurements also reveal information about the elasticity of the melt, which can be correlated with die swell. Material characterization Molar Mass Distribution Recently, mathematical models have been developed which allow the determination of the molar mass distribution via rheological measurement. Correlations to molar mass distribution or material branching can be seen in the viscoelastic behavior, which influence both, the manufacturing process and the properties of the end product. The molar mass is the most important structural parameter which affects the flow behavior of polymers. The viscosity curve becomes flatter with decreasing shear rate and the polymer melt shows Newtonian behavior with a constant viscosity. This region at low shear rates is called the terminal relaxation zone or the 1st newtonian plateau. The constant viscosity in this range is called the zeroshear viscosity η 0 and represents an important temperature dependent material parameter. For most technical polymers, the zero-shear viscosity is directly proportional to the average molar mass. The rheological measurement therefore clearly shows small differences in the molar mass. At a constant average molar mass, the energy required for shear thinning in the manufacturing process can be correlated with the molar mass distribution. Polymers with a wide molar mass distribution have more of a tendency to shear thinning, even at low shear rates, than more narrowly distributed materials with the same average molar mass. Broadening the molar mass distribution aids extrusion and shaping. This means, for example, that the surface quality of molded plastic parts can be improved by varying the distribution width. The width of the molar mass distribution correlates with the cross-over point between the storage modulus G and the loss modulus G in a frequency sweep.

5 Branching The number, length and mobility of side chains influence the rheological properties. If the side chains are not very long, this leads to increased viscosity at low shear rates and more pronounced shear thinning compared to the corresponding linear polymer. If a polymer has long-chain branching, it will display low viscosity at low shear rates. The extent of branching can therefore be used to control manufacturing and product characteristics. Fillers Fillers also influence the manufacturing process and the properties of the end product. Important factors are size, form and concentration of the fillers and the interactions between the particles. Fillers usually lead to an increase in the melting viscosity and a reduction of die swell. From a rheological standpoint an increasing filler content results in a smaller so-called linear visco-elastic (LVE) range, which can be determined in an amplitude or strain sweep. Measurements on solids With the appropriate accessories, a rheometer can be used to perform dynamic mechanical analysis (DMA) on solid samples by measuring the samples in torsion and tension. The solid properties are usually determined as a function of the temperature and the results give insight into the morphological properties and behavior of the polymer when in use. Measurement of the glass transition temperature (T g ) and storage modulus (G or E ) below the glass transition temperature gives information on the maximum service temperature and the impact strength, embrittlement and stiffness of the material. For crystalline or partially-crystalline polymers the melting temperature (T m ) is another important material parameter accessible with such a DMA test. Conversion and analysis methods

6 Applications Fig. 1: GLASS TRANSITION TEMPERATURE ANALYSIS finding the application temperature of thermoplastics Glass transition temperature with DMA can be determined with three different analysis methods. The onset temperature of G (red) shows the beginning of the decreasing storage modulus. The maximum of G (black) shows the maximum change in polymer mobility and is therefore the chemical definition of T g (grey). The maximum of tan(δ) shows the maximum of mechanical damping and is often used due to historical reasons. Further methods are the midpoint and endpoint of the G. All analysis methods are included in the Anton Paar software. Fig. 2: TIME-TEMPERATURE SUPERPOSITION (TTS) AND MASTERCURVE FOR SOLIDS how to determine the storage and loss moduli at high frequencies Polymers are used in high-frequency applications. It is not possible to measure the storage and loss moduli at very high and low frequencies with DMA and rheology devices. Polymer behavior at low temperatures is equal to the behavior at high frequencies. Due to this reason a time-temperature superposition is used to generate a mastercurve for a broad frequency range over several decades. Fig. 3: EXTENSIONAL RHEOLOGY CURVE how to determine branching of thermoplastics Extensional rheology can be used to characterize the polymer in terms of its molecular branching. This is one of the critical properties in polymer manufacturing, e.g. blown-film extrusion. Furthermore, extensional rheology can be used for polymer identification in quality control. High stress in large macromolecules leads to rearrangement of the molecular entanglements. The strain-hardening effect of polymers at higher elongation shows evidence of branched molecular chains because rearrangement is disabled, as shown by the red curve.

7 Fig. 4: FLOW AND VISCOSITY CURVE or how to get information about the flowability of a thermoplastic: Flow and viscosity curves give information about the flowability of thermoplastics under different shear and process conditions. The zero-shear viscosity η 0 at low shear rates is an important material property and is directly proportional to the average molar mass M w. In order to determine a viscosity curve over a broad range of shear rates a master curve can be constructed using time temperature superposition in combination with the conversion method according to Cox Merz. In addition, conversions from transient tests and a direct measurement with controlled shear rate provides the whole spectrum of shear rates. Powerful regression methods may help to calculate the zero-shear viscosity η 0 and the infinite-shear viscosity η inf in a shear range where all the molecules are totally disentangled and oriented. Fig. 5: TIME TEMPERATURE SUPERPOSITION TTS or looking deep into the macromolecular structure of a polymer melt: The Dynamic mechanical analysis (DMA) in torsion, determined via time temperature superposition TTS, provides shear and time dependent information about the viscoelastic properties of a material. Predictions regarding Die Swell are possible. All the information about the polymeric macro-structure and its short and long term behavior is already included in the Master Curve. Comparative average molar mass and molar mass distribution, as well as the calculation of their absolute values, are supported. Conversions into transient and oscillatory material functions are applicable. Fig. 6: CROSS OVER POINT G x or how to compare the molar mass of thermoplastics within 10 minutes: Within 10 minutes, the molecular structure can be analyzed with respect to the average molar mass M w and the molar mass distribution MMD. A powerful and model free method is given for a relative comparison of thermoplastics. A qualitative measure for the average molar mass is expressed by the horizontal position of the cross-over point G x while the vertical position of G x indicates the MMD. In addition the degree of branching can lead to a horizontal shift of G x while comparing polymers of the same type.

8 Applications Fig. 7: MOLAR MASS CALCULATION a method used not only for short or narrow distributed polymer melts: Processability and product performance depend very much on the molecular structure of the polymer melt, so it is important to analyze thermoplastics with respect to their molecular structure. A rheological dynamic mechanical measurement (DMA) in combination with the latest sophisticated MMD analysis methods has major advantages and is still easy to use. The thermoplastic material can be measured as molten material and does not need to be diluted in an aggressive solvent. The method has no upper limits regarding length and distribution of the molecules; rather there are advantages due to the higher sensitivity with increasing average molar mass M w. Input data for the method include a frequency sweep showing zero-shear viscosity and cross-over point G x. Fig. 8: DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA) in TORSION or how to determine phase transitions of thermoplastics, thermosets and elastomers: This test provides the essential information about the material s phase transitions. Glass transition (T g ), melting (T m ) and crystallization temperature (T c ) can be determined with high precision using the environmental chamber CTD 450. Information about the degree of crystallinity or cross-linking is expressed in the slope of the material functions G and G. With the film and fibre fixture, DMA and DMTA tests in tension can be performed even on soft polymeric films and fibres. In figure 5, a multiwave experiment is presented with the determination of the glass transition temperature, expressed as the maximum in tan(δ). As can be seen, T g is a function of the applied frequency. Fig. 9: TRANSIENT TEST TYPES (Creep, Stress Relaxation and Stress Growth Tests) find out more about the time response of your material: Step stress (creep & recovery), step strain (stress relaxation) and step rate (stress growth / start up flow) experiments are typically performed to measure the time (transient) response of a material to a given constant shear stress, shear strain or shear rate. Analysis methods enable the calculation of important material constants such as zero-shear viscosity, plateau modulus, creep compliance and the conversion from the transient material functions to oscillatory material functions - G (ω), G (ω). In Figure 6, a stress growth experiment is presented.

9 Measurement Systems for Polymer Testing Parallel-Plate System Shear rheology of melts A flow curve over a wide range of shear rates can also be generated from a mastercurve using the Cox-Merz transformation. Regression models such as Carreau-Yasuda allow the calculation of important material parameters such as the relaxation time and power law index. The polymer s zero-shear viscosity is directly proportional to the molar mass. Solid rectangular fixture (SRF) - Solid circular fixture (SCF) DMA in torsion A main application of the system is the dynamic mechanical thermal analysis of thermoplastics, thermosets or elastomers in torsion. The measurement is performed in the destruction-free linear viscoelastic range using the SRF (Solid Rectangular Fixture). The SRF is used to determine the shear moduli G and G and the loss or damping factor tan(δ) of solid bars. Universal Extensional Fixture (UXF) DMA in extension The UXF (Universal Extensional Fixture) enables the measurement of the storage and loss moduli E and E as well as the damping factor tan(δ) of thin film sheets and fibers in the glass transition region. The system can be used to characterize multi-layer laminates as well as coating films. Shrinkage and expansion measurement By applying a constant tensile stress or strain to a film or fiber, its shrinkage or expansion is measured depending on time and temperature. Sentmanat Extensional Rheology (SER) Extensional viscosity Extensional rheology is an excellent method for morphologically investigating elastomers or polymers. By measuring extensional viscosity under a preset constant strain rate, a distinction between linear and branched polymers is easily made.

10 MCR Accessories for Polymer Testing MCR: The 2-in-1 solution - Rheometry and Dynamic Mechanical Analysis Whatever your rheological and dynamic mechanical requirements are and will be in the future, MCR rheometers are efficiently and comfortably adapted to meet your needs. The patented normal force sensor inside the air bearing measures the normal force. This enables a separate pretension and compensation of thermal expansion by the stepper motor and oscillatory signal measured by the EC motor, without superposition of a pretension force. The optimal measuring signal can be achieved, especially in borderline areas at the extremely low temperatures below the glass temperature (below T g ) and the high temperatures close to the melting point. Changing a parallel-plate measuring system for a solid rectangular fixture system is just as easy as integrating a new temperature device or extending your rheometer s testing capabilities with a wide range of application-specific accessories. MCR 302 and MCR 502 The EC motor technology of the MCR rheometer series Air bearing and normal force sensor Nanorheometry: Nano torque and strain resolution TruStrain - Real-time position control oscillation Toolmaster - Automatic recognition of measuring systems and accessories TruGap - The innovative and patented gap measurement system Polymer Package SRF Convection Temperature Device: CTD 450 TD ready Solid rectangular fixture (SRF) Parallel-plate system: PP25 Polymer analysis software package SRF sample package Polymer accessory box Polymer Package UXF Convection Temperature Device: CTD 450 TD ready Universal Extensional Fixture: UXF L-PP50/SS/CTD Parallel-plate system: PP25 Polymer analysis software package Polymer accessory box

11 Environmental Systems Since temperature has a great influence on the rheological behavior of all polymeric samples, precise temperature control is crucial to obtaining reliable rheological data. Convection Temperature Device CTD 450 TDR/CTD 180 Temperature control based on combined convection and radiation Suitable for DMA in torsion, DMA in extension, Photo DMA (UV curing), extensional rheology (SER) Digital Eye CCD camera option for capturing images and videos during the measurement TruGap support T-Ready feature Gas consumption heating: 25 L/min Total gas consumption: 48 L/min Liquid nitrogen consumption: 3 L/h to 6 L/h Humidity option Helpful accessories Included: Filling ring for melting and molding polymer granuls Simple to remove before measurement Included: Scraper for fast and effective cleaning Included: Trimming tool for easy sample trimming on all sides Option: Stamping press This press enables the production of sample discs in a thickness up to 2 mm and in diameters of 25, 12 and 8 mm.