Characterization of Elastomers Using Thermal Analysis Techniques
On Demand Webinar

Characterization of Elastomers by Thermal Analysis

On Demand Webinar

TA is used for the characterization of elastomers whose properties are temperature-dependent

Characterization of elastomers
Characterization of elastomers

Thermal analysis is an excellent method for the characterization of elastomers because their properties are strongly dependent on temperature.

Elastomer behavior

Elastomers can be stretched or compressed but return to their original dimensions or shape when the applied stress is released. The term elastomer is derived from the words elastic polymer and is often used interchangeably with the term rubber.

At ambient temperatures, elastomers are relatively soft and deformable. Their main industrial uses are for seals, tires, engine mounts, and flexible molded parts.

Characterization of elastomers by thermal analysis

DSC is used to measure events in which a change in enthalpy occurs. These include the glass transition, reaction enthalpy, vulcanization reactions, melting, crystallization, and thermal stability.

TGA measures weight changes and is widely used for the analysis of elastomers. The main applications have to do with compositional analysis, fillers and additives as well as chemical reactions such as decomposition, oxidation and vulcanization, and last but not least evaporation, desorption and vaporization.

TMA and DMA are used to measure the mechanical behavior and properties of materials. TMA applications include expansion, shrinkage, softening and the glass transition. DMA is the method of choice for characterizing the viscoelastic behavior of materials, the glass transition, and the frequency-dependence of effects.

English

Thermal Analysis of Elastomers

Slide 0: Thermal Analysis of Elastomers

Ladies and Gentlemen,

Welcome to the METTLER TOLEDO webinar on the Thermal Analysis of Elastomers.

 

As you know, polymers can be classified according to their mechanical properties into three main classes, namely: thermoplastics,thermosets, and elastomers.

 

Elastomers can be stretched or compressed but return to their original dimensions or shape when the applied stress is released. The term elastomer is derived from the words elastic polymer and is often used interchangeably with the term rubber.

Thermal analysis is an excellent method for identifying and characterizing elastomers because their properties are strongly dependent on temperature.

 

The thermal analysis of thermoplastics and thermosets is covered in separate webinars.

 

Slide 1: Contents

The slide lists the topics I would like to cover.

 

First, I want to discuss the basic properties of elastomers and describe the four main thermal analysis techniques that are employed to investigate them.

The techniques are:

Differential Scanning Calorimetry, or DSC;

Thermogravimetric Analysis, or TGA;

Thermomechanical Analysis, or TMA;

and Dynamic Mechanical Analysis, or DMA.

 

I will then present a number of examples that illustrate how thermal analysis can be used to investigate the behavior of different elastomers.

 

Finally, I will summarize the thermal analysis techniques and their application fields and list a number of useful references for further information and reading.

 

Slide 2: Introduction

At ambient temperatures, elastomers are relatively soft and deformable. Their main industrial uses are for seals, tires, engine mounts, and flexible molded parts.

 

The diagram shows glass transition curves of styrene-butadiene rubber measured by DSC, TMA, and DMA, and its decomposition profile measured by TGA.

 

Elastomers can be easily analyzed and characterized by thermal analysis. In contrast, spectroscopic techniques such as infrared spectroscopy are often difficult to apply or do not provide enough information.

 

Slide 3: Typical Properties

The glass transition temperature (Tg) of an elastomer is a very important characteristic property.

During the glass transition, the material changes from a hard, glassy state to a soft, rubbery state on heating, or vice-versa on cooling. An elastomer is expected to be rubbery-soft and flexible and is therefore normally used above its glass transition temperature. Below this temperature, the material is brittle and can easily break. The glass transition temperature therefore defines the lower temperature limit for practical usage.

 

The diagram on the left of the slide shows the DSC curves of various elastomers. The glass transition is indicated by a step in the DSC curve. The glass transition temperatures of the different elastomers are displayed in the table on the right and are normally well below ambient temperature.

 

Slide 4: Structure of Elastomers

Elastomers are lightly crosslinked long-chain polymers of natural or synthetic origin.

 

Historically, the most well-known elastomer is natural rubber obtained from rubber trees as latex which is then crosslinked with sulfur in a vulcanization reaction as shown schematically in the diagram. The term rubber is generally used when referring to elastomers produced by vulcanization.

 

Unvulcanized rubber consists of long chains of molecules. Above the glass transition, natural rubber can be processed and deformed like a thermoplastic material. Vulcanization creates crosslinks between the polymer chains. The resulting elastomer is more rigid and can no longer be thermally processed. When a mechanical stress is applied the material deforms but quickly recovers its shape when the stress is removed.

 

In most commercial applications, elastomers are used in complex formulations and are referred to as “elastomer systems”. The properties of elastomeric materials are determined by the choice of polymer or polymer blend, the cross-linking system, as well as the type and content of additives such as fillers, stabilizers, flame retardants, and plasticizers.

 

Slide 5: Thermal Analysis

What exactly do we mean by thermal analysis? The ICTAC definition is given in the slide:

“A group of techniques in which a physical property of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature program”.

The schematic diagram on the right shows a simple linear temperature program.

 

The lower half of the slide illustrates typical events that occur when a sample is heated. For example, initial melting, in which the sample changes from the solid to the liquid state. If the sample is exposed to air or oxygen, it will start to oxidize and finally decompose. Thermal analysis techniques are widely used in research and development to investigate these effects.

 

Slide 6: Thermal Analysis                                                                      Techniques

The slide shows the most four most important thermal analysis techniques that are used to characterize elastomers, namely:

 

Differential Scanning Calorimetry, or DSC is the most widely used technique. The picture shows a DSC sensor with a crucible containing a sample (colored red), and a reference crucible.

 

Thermogravimetric Analysis, or TGA is frequently used in elastomer analysis. The picture shows part of the TGA ultra-micro balance with its automatic internal ring weights.

 

Thermomechanical Analysis, or TMA. The picture shows the sample area with a sample (colored red) and the quartz probe and sensor.

 

And finally, Dynamic Mechanical Analysis, or DMA. The picture shows one of the several different sample-clamping assemblies.

 

Slide 7: Industries and Applications

The slide summarizes some of the many industries in which elastomers are used.

 

The main applications have to do with the measurement of thermal stability, glass transitions, the investigation of vulcanization reactions, the study of the mechanical properties and behavior of materials.

 

Slide 8: DSC, HP DSC, Flash DSC

Let’s begin with DSC. This technique allows us to determine the energy absorbed or released by a sample as it is heated or cooled. There are several different types of DSC instruments:

 

The standard METTLER TOLEDO DSC 1 measures from minus one hundred and fifty degrees Celsius (–150 °C) to plus seven hundred degrees (700 °C) at heating rates of up to three hundred Kelvin per minute (300 K/min). Samples are normally measured in small crucibles made of aluminum, alumina or other materials, using two to twenty milligrams of sample.

 

The METTLER TOLEDO Flash DSC 1 expands the heating rate to an amazing two million four hundred thousand Kelvin per minute (2,400,000 K/min) and the cooling rate to two hundred and forty thousand Kelvin per minute (240,000 K/min). To achieve this, the Flash DSC 1 uses very small sample sizes of about one hundred nano-grams (100 ng) and no sample crucibles - the sample is in direct contact with the sensor. The ultra-fast heating and cooling rates allow industrial process conditions to be simulated in which materials undergo extremely rapidly cooling.

 

Another powerful DSC technique is high-pressure DSC, or HPDSC for short. The METTLER TOLEDO HP DSC 1 can analyze samples under inert gas or reactive gas atmospheres at pressures of up to ten mega-pascals (10 MPa). This suppresses undesired vaporization of samples or enables the stability of samples to be studied under increased oxygen pressures.

 

The schematic curve on the left shows a typical DSC measurement curve of a semi-crystalline polymer. Exothermic effects point in the upward direction and endothermic effects downward. The different effects are numbered next to the curve:

One, is the initial deflection or start-up transient of the DSC;

Two, is the baseline where no thermal effects occur;

Three, is a glass transition with enthalpy relaxation;

Four, is cold crystallization;

Five, is melting of the crystalline fraction; and finally

Six, is exothermic oxidative decomposition.

 

Slide 9: Differential Scanning Calorimetry (DSC)

The table summarizes the main analytical applications of DSC for elastomeric materials.

 

DSC is used to investigate events and processes such as glass transitions, melting, crystallization, chemical reactions, and thermal stability in which a change in enthalpy occurs.

 

The information obtained can be used to identify polymers, investigate the working range of materials, and for compositional analysis. The method can be employed in quality control and for the analysis of raw materials, additives and fillers as well as to characterize vulcanization and other types of reactions.

 

The picture on the right of the slide shows a view of an open DSC furnace with sample and reference crucibles. The standard crucibles are made of aluminum.

 

Slide 10: Application 1: DSC                              Identification of polymer blends

The first application shows DSC curves of elastomer blends containing two different polymers and illustrates how the results can be used to identify the individual polymers. NBR stands for acrylonitrile–butadiene rubber, CR for chloroprene rubber, NR for natural rubber, and SBR for styrene-butadiene rubber.

In this example, the polymers are incompatible - the polymer phases are separate and the DSC curve of each blend exhibits two separate glass transitions. The glass transition temperatures allow you to identify the individual components of the elastomer blend. The ratio of the step heights can be used to estimate the percentage content the individual polymers.

 

Slide 11: Application 2: DSC                                          Reaction kinetics by MFK

DSC measurements can also be used to analyze and predict the kinetics of vulcanization reactions using the Model Free Kinetics or MFK software program. The method is based on the evaluation of heating curves measured at three or more different heating rates.

The example shown here is the vulcanization reaction of acrylonitrile–butadiene rubber (NBR).

In Step 1 in the upper left diagram, three dynamic DSC curves were measured at different heating rates in the temperature range in which the crosslinking reaction occurs.

The results were then used to calculate the conversion curves shown in Step 2 by evaluating the reaction enthalpy at different temperatures.

The apparent activation energy curve of the reaction was then calculated from this data as a function of conversion as shown in Step 3. The curve indicates that the vulcanization reaction takes place in of two main stages.

The activation energy curve is the basis for predicting the kinetics and course of reactions at isothermal temperatures. The continuous green curve in Step 4 displays the conversion curve predicted by the Model Free Kinetics program for the reaction at one hundred and thirty degrees Celsius (130 °C). The prediction was checked and verified by comparing it with data measured under the same conditions shown by the solid black squares.

Model Free Kinetics is an ideal method for quickly obtaining information on the kinetics of reactions and for predicting the course of reactions.

 

Slide 12: Thermogravimetric Analysis (TGA) 1

Now let’s move on to thermogravimetric analysis, or TGA.

In this technique, the mass of a sample is continuously recorded as it is heated or cooled in a defined atmosphere. The method requires only a few milligrams or less of sample.

 

The schematic curve on the left shows a typical TGA measurement curve of a polymer. The steps are numbered next to the curve. They relate to effects that can occur during heating and include:

 

One: the evaporation of volatile components in the formulation;

Two: the pyrolysis of organic substances and polymers. This large step relates to the main pyrolysis process of the polymer. The step can be used to determine the amount of a particular component. The temperature at which pyrolysis occurs is characteristic for the type of polymer investigated.

Three, at six hundred degrees Celsius (600 °C) the atmosphere is switched from nitrogen to air to obtain oxidative conditions;

Four, remaining products such as carbon black or carbon fibers burn;

Five, inorganic fillers such as silicates that are left behind as a residue.

 

Slide 13: Thermogravimetric Analysis (TGA) 2

The table summarizes the main analytical applications of TGA for elastomeric materials.

The technique is mainly used for the quantitative investigation of vaporization and decomposition processes, compositional analysis, as well as the investigation of carbon black content and activity, filler content and oxidative stability.

 

Evolved gases can be analyzed online using hyphenated techniques such as TGA-MS or TGA-FTIR.

 

The picture on the right shows a view of the open furnace and the sample holder with positions for sample and reference crucibles. The standard crucibles are made of alumina.

 

I would now like to discuss some TGA applications in detail.

 

Slide 14: Application 1: TGA/DSC                                  Compositional analysis

The first application describes the TGA analysis of styrene butadiene rubber (SBR), and illustrates the basic principles of compositional analysis by TGA.

In this experiment, a cube of SBR weighing about fifteen milligrams (15 mg) was first heated in nitrogen to six hundred degrees Celsius (600 °C), and then in air from six hundred to eight hundred and fifty degrees (600 to 850 °C). The black TGA weight-loss curve shows three distinct steps.

The first step, below about three hundred degrees (300 °C) corresponds to the loss of a small amount of relatively volatile compounds.

Pyrolysis of the elastomer then takes place between three hundred and five hundred degrees (300 and 500 °C). The step height corresponds to the polymer content. The carbon present in the sample burns on switching from nitrogen to air. The height of this combustion step can usually be taken as a measure of the carbon black filler content of the sample.

The red DTG curve is the first derivative of the TGA curve. It enhances weaker details and helps you to set evaluation limits.

 

Slide 15: Application 2: TGA/DSC                                              Blend analysis

The METTLER TOLEDO TGA/DSC instrument simultaneously measures both the weight loss and the DSC heat-flow signal. The DSC curve provides additional information that can be used to identify thermal events.

The slide displays the measurement curves of blend of acrylonitrile–butadiene rubber and chloroprene rubber (NBR/CR) heated in a nitrogen atmosphere.

The top curve shows mass loss versus temperature, the middle curve the DTG curve, and the bottom curve the DSC signal.

The first weight loss step between one hundred and three hundred and twenty degrees Celsius (100 and 320 °C) is due to evaporation of an oil used as a plasticizer.

Steps 2 and 3 are related to the thermal decomposition of the polymer components. A reaction product of the decomposition of the chloroprene rubber component is hydrochloric acid. This reaction occurs in Step 2 and produces an exothermal peak in the DSC curve, while the main decomposition reaction in Step 3 produces a relatively weak endothermic peak.

Under these conditions, the pyrolysis reaction of chloroprene rubber produces a certain amount of carbon black. This can be analyzed by switching to an oxidative atmosphere and subsequent heating but is not shown here. Two different combustion steps are observed: the first is due to the carbon black pyrolysis product and the second due to the carbon black filler.

Evaluation of the step heights yields the content of volatiles, polymers, carbon black and ash.

 

Slide 16: Application 3: TGA/DSC                      Identification using TGA-FTIR

This application summarizes the TGA-FTIR analysis of acrylonitrile-butadiene rubber (NBR) and butadiene rubber (BR).

These two elastomers are difficult to distinguish from one another by TGA because their weight-loss profiles are very similar, as can be seen in the upper diagram.

In such cases, a TGA-FTIR combination is very useful. In contrast to butadiene rubber, acrylonitrile-butadiene rubber contains a nitrile group. This group absorbs infrared light in the wavenumber range between two thousand two hundred and sixty and two thousand one hundred and eighty wavenumbers (2260 and 2180 cm-1).

The lower diagram displays the simultaneously recorded chemigrams of NBR and BR. A chemigram is a curve generated from the infrared spectrum by integrating the infrared absorption intensity over a selected wavenumber range. The chemigram of NBR shows a peak during the pyrolysis process, indicating that the pyrolyzed material contained a nitrile group.

 

Slide 17: Application 4: TGA/DSC          Plasticizer separation under vacuum

Sometimes, the evaporation process of the plasticizer and the decomposition process of the polymer overlap one another. In such cases, measurements in vacuum at reduced pressure often allow the two processes to be separated. The oil content can then be determined with much better accuracy.

In this example, styrene-butadiene rubber was measured at a pressure of one thousand millibar (1000 mbar) and under partial vacuum at ten millibar (10 mbar). At normal pressure, the oil evaporated in almost the same temperature range as that in which the polymer decomposed. In vacuum, the evaporation was clearly shifted to lower temperature.

 

Slide 18: TMA/SDTA

The next technique I want to discuss is thermomechanical analysis, or TMA.

In contrast to DSC, TMA measures the dimensional properties of a sample as it is heated or cooled under the action of a defined force or load.

 

The schematic curve on the left shows the typical TMA curve of a polymer measured under low force conditions. The effects are numbered next to the curve:

One, is gradual expansion below the glass transition;

Two, is the glass transition point at which the rate of expansion changes;

Three, is expansion above the glass transition;

Four, is softening with plastic deformation; unvulcanized elastomers flow at higher temperatures.

 

Slide 19: Thermomechanical Analysis (TMA)

The table summarizes the most important analytical applications of TMA for elastomeric materials.

The main application is the determination of the expansion behavior and the coefficient of thermal expansion, or CTE, as a function of temperature.

The technique is also ideal for determining the glass transition temperature, and for studying vulcanization reactions, softening behavior, creep, and swelling in solvents.

The so-called dynamic-load mode can be used to detect softening and detect weak post-curing reactions.

The picture on the right shows the typical experimental setup with a ball-point probe in contact with the sample specimen.

The following slides describe some typical application examples.

 

Slide 20: Application 1: TMA/SDTA                    Expansion coefficient

The diagram displays the thermal expansion curves of two different elastomers – a styrene-butadiene rubber (SBR), and a blend of styrene-butadiene rubber and butadiene rubber (SBR/BR). The corresponding curves of the coefficient of thermal expansion are plotted in the inset diagram.

At the glass transition, the TMA curves exhibit a marked change in slope. The corresponding curves of the expansion coefficient show a step change. This is particularly clear in the red curves recorded for SBR.

TMA is a very sensitive method for the analysis of polymer blends. The TMA curve of the SBR/BR blend shows two glass transitions because the polymers are present as separate phases. This is more difficult to measure by DSC.

 

Slide 21: Application 2: TMA/SDTA                    Dynamic Load TMA

The second TMA application is an example of Dynamic Load Thermomechanical Analysis, or DLTMA.

In DLTMA, an alternating force at a particular frequency is applied to the sample. The technique can measure weak effects, expansion, and the elasticity or Young’s modulus of samples. The higher the stiffness of the sample, the smaller the amplitude of the oscillation in the DLTMA curve.

DLTMA is particularly useful for detecting partial vulcanization in elastomers. Insufficient vulcanization is one of the main reasons for material failure.

 

The application shows measurement results obtained from the vulcanization of ethylene-propylene-diene rubber (EPDM) using DLTMA. The force applied to the sample was switched between zero point one and zero point nine newtons (0.1 and 0.9 N) every 30 seconds. The upper half of the diagram displays the DLTMA curve as a function of temperature.

 

The measurement curve shows a downward tendency up until about one hundred and twenty degrees Celsius (120 °C) indicating that the sample is more and more compressed. This is due to the relatively low degree of vulcanization. At the same time, the amplitude of the measurement signal increases slightly with temperature. This indicates that the sample becomes softer. From one hundred and twenty to one hundred and seventy degrees (120 °C to 170 °C), the amplitude decreases significantly and the curve begins to move in an upward direction.

The DLTMA measurement results can be used to calculate the Young’s modulus curve shown in the lower half of the diagram. The modulus increases from about one hundred and twenty degrees (120 °C) onward. This is due to the post-vulcanization reaction in which the degree of crosslinking continuously increases.

Dynamic load TMA can therefore be used to follow the crosslinking reaction of a sample during post-vulcanization. This is a good method for quality control and for failure analysis.

 

Slide 22: Application 3: TMA/SDTA                                Creep behavior

TMA can be used to measure the response of an elastomer to mechanical stress by performing a so-called creep and recovery experiment.

In this experiment, a force is suddenly applied to the sample by the TMA probe, held constant for a certain time, and then quickly removed. The deformation, in this case the percentage change in thickness, is recorded as a function of time and comprises three components: the initial almost instantaneous reversible elastic response, the slower viscoelastic relaxation and the more-or-less constant viscous flow.

The diagram shows the behavior of two different sealing rings in a creep and recovery experiment performed at room temperature. The initial thickness of the sample was measured using a negligibly low force of zero point zero one newton (0.01 N). This ensured good contact between the TMA probe and the sample but was low enough to prevent sample deformation. The force was then suddenly increased to one newton (1 N), held at this value for 60 minutes and then reset to zero point zero one newton (0.01 N). The almost immediate elastic response is followed by slow viscoelastic relaxation. The remaining deformation at the end of the experiment shows the extent to which the sealing rings were permanently deformed through viscous flow.

 

Measurements like this characterize materials much better than simple hardness measurements. The flow behavior depends on the degree of vulcanization.

 

Slide 23: Application 4: TMA/SDTA                                            Swelling behavior

The swelling behavior of elastomers in different solvents is another application that can easily be investigated by TMA.

The sample specimen is equilibrated at the temperature of interest in the instrument and the thickness measured. The TMA furnace is then opened briefly and the glass vial containing the sample is filled with solvent at the same temperature using a syringe. The TMA probe measures the change in thickness of the specimen as it swells.

 

In the diagram, the normalized TMA swelling curves of four different elastomers in toluene are compared as a function of time.

FPM is a fluroelastomer and swells only about 2% in toluene. The material is clearly very resistant toward toluene. It can therefore be used as a sealing ring for applications in which the sealing ring is exposed to toluene or similar solvents. The situation is very different with the other elastomers. In particular, the silicone rubber MQ swells by more than 35%.

The method can be used to select elastomers and sealing rings for particular applications.

 

Slide 24: DMA and DMA/SDTA

The final technique I would like to discuss is Dynamic Mechanical Analysis, or DMA.

 

Dynamic mechanical analysis is used to measure the mechanical properties of a viscoelastic material as a function of time, temperature and frequency when the material is deformed under a periodic oscillating stress.

 

The schematic diagram on the left shows the results of a DMA measurement of a shock-cooled polymer in the shear mode. The curves display the storage modulus, gee prime (G′), the loss modulus, gee double prime (G″),and the damping factor, tan delta, as a function of temperature.

The different effects are numbered next to the curve:

One, is secondary relaxation, observed as a peak in tan delta;

Two, is the glass transition, seen as a decrease in the storage modulus;

Three, is cold crystallization;

Four, is recrystallization accompanied by a peak in tan delta;

Five, is melting of the crystalline fraction with a decrease in the storage modulus;

 

DMA measurements can be performed in different clamping modes such as shear, tension and bending, depending on the information required and the behavior and geometry of the sample.

 

Slide 25: Dynamic Mechanical Analysis (DMA)

The table lists the main analytical applications of dynamic mechanical analysis for elastomers.

Typical applications include the measurement of the glass transition and the investigation of viscoelastic behavior as a function of oscillation frequency.

 

The picture on the right shows a sample installed in a DMA instrument ready for measurement in the bending mode.

 

Slide 26: Application 1: DMA                  Multi-frequency temperature scan

The first application shows the measurement of the glass transitions of an acrylonitrile-butadiene rubber (NBR) chloroprene rubber (CR) blend at different frequencies.

The experiment was performed in the shear mode at a heating rate of two Kelvin per minute (2 K/min). The curves in the upper and lower diagrams display the storage and loss moduli and tan delta recorded simultaneously at frequencies of 1, 10, 100 and 1000 hertz (Hz) in the temperature range minus ninety to plus sixty degrees Celsius (–90 to +60 °C).

Two separate glass transitions are observed because the two components of the blend are incompatible. The first glass transition at about minus forty-five degrees (–45 °C) is due to the chloroprene rubber and the other at about zero degrees (0 °C) is due to the NBR.

The frequency dependence of the two glass transitions is very interesting. For example, at ten degrees (10 °C) and a frequency of one hertz (1 Hz), the glass transition is practically completed - the modulus is small and the material is elastic. In contrast, at the same temperature but at a frequency of one thousand hertz (1000 Hz), the modulus is about one order of magnitude higher.

Clearly, the properties exhibited at high frequencies are different compared with those at low frequencies. It is therefore very important to test materials in the same frequency range as that used for their practical application.

 

Slide 27: Application 2: DMA                                                      Filler content

In this application, the mechanical properties of four natural rubber samples containing different amounts of filler were investigated. The abbreviation, phr, means parts by weight of carbon-black per hundred parts of rubber.

The upper diagram shows the results of experiments performed in the shear mode at room temperature at a frequency of one hertz (1 Hz). The displacement amplitude was varied in steps between thirty nano-meters (30 nm) and one millimeter (1 mm) and the force measured.

In the lower diagram, the green curve shows that the unfilled material has a storage modulus of about zero point five mega-pascals (0.5 MPa). This value is practically independent of the displacement amplitude. The storage modulus increases with increasing carbon-black content. The modulus for a particular filler content however decreases as the displacement amplitude increases. This indicates non-linear behavior and is due to the reversible destruction of carbon black clusters.

The upper diagram shows that a large force of up to forty newtons (40 N) is essential for measurements like this. The data obtained provides information about polymer and filler interaction within the polymer matrix. The more active the filler, the larger the deviation from linear behavior.

 

Slide 28: Application 3: DMA                                                      Master curves

As we saw in a previous slide, the mechanical behavior of viscoelastic materials depends on frequency and temperature. In general, there is equivalence between frequency and temperature behavior during relaxation processes.

This phenomenon is known as the Time-Temperature Superposition principle and can be used to construct so-called master-curves from a series of isothermal frequency sweeps measured at different temperatures. The curves measured at temperatures below a particular reference temperature are shifted horizontally to higher frequencies so that the end sections of the curves overlap. Similarly, curves measured at higher temperatures are shifted to lower frequencies.

This slide shows master curves of the storage and loss modulus of unvulcanized and vulcanized SBR over an extremely large frequency range.

The curves of the unvulcanized sample exhibits different processes numbered 1 to 4 next to the curve. 1 is a region in which flow occurs; 2 is due to flow relaxation; 3 is the rubbery plateau; and 4 the glassy region.

Master curves describe the temperature- and frequency-dependent mechanical behavior of materials over frequency ranges that are much wider than those accessible by direct measurement.

 

Slide 29: Summary 1

The table summarizes the most important events and properties that characterize elastomers as well as the thermal analysis techniques recommended for their measurement. A red box means that the technique is strongly recommended; a blue box indicates that the technique can also be used for some applications. 

In general, DSC is used to measure events in which a change in enthalpy occurs. These include the glass transition, reaction enthalpy, vulcanization reactions, melting, crystallization, and thermal stability.

TGA measures weight changes and is widely used for the analysis of elastomers. The main applications have to do with compositional analysis, fillers and additives as well as chemical reactions such as decomposition, oxidation and vulcanization, and last but not least evaporation, desorption and vaporization.

TMA and DMA are used to measure the mechanical behavior and properties of materials. TMA applications include expansion, shrinkage, softening and the glass transition. DMA is the method of choice for characterizing the viscoelastic behavior of materials, the glass transition, and the frequency-dependence of effects.

 

Slide 30: Summary 2

 

This slide summarizes the temperature ranges of the METTLER TOLEDO DSC, TGA, TMA, and DMA instruments.

 

DSC experiments are performed at temperatures between minus one hundred and fifty degrees Celsius (–150 °C) and plus seven hundred degrees (700 °C). If special equipment or accessories are used, the temperature ranges, and heating and cooling rates may be different.

TGA measurements normally begin at room temperature. The maximum possible temperature is about sixteen hundred degrees (1600 °C). A combination of different furnace sizes and sensors is available. In addition to automatic operation, the instrument can be coupled to vacuum, sorption, FTIR and mass spectrometer systems.

TMA experiments can be performed between minus one hundred and fifty degrees (–150 °C) and plus eleven hundred degrees (1100 °C). Several modes of operation are possible depending on the information required.

DMA samples are normally measured in the range minus one hundred and fifty degrees (–150 °C) to plus six hundred degrees (600 °C). A number of different clamping assemblies are available for measuring samples in the shear, bending, tension and cantilever modes.

 

Slide 31: For More Information on Elastomers

Finally, I would like to draw your attention to information about elastomer analysis that you can download from the Internet. METTLER TOLEDO publishes articles on thermal analysis and applications from different fields twice a year in UserCom, the well-known METTLER TOLEDO biannual technical customer magazine. Back issues can be downloaded from www.mt.com/TA-usercoms as shown at the top of the slide.

 

 

Slide 32: For More Information on Thermal Analysis

In addition, you can download details about webinars, application handbooks or information of a more general nature from the Internet addresses given on this slide.

 

Slide 33: Thank You

 

This concludes my presentation on the thermal analysis of elastomers. Thank you for your interest and attention.

 
 
 
 
 
 
 
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