Thermal Analysis of Tires
On Demand Webinar

Thermal Analysis of Tires

On Demand Webinar

"Thermal Analysis of Tires" details the main thermoanalytical techniques used in the tire industry

Thermal Analysis of Tires
Thermal Analysis of Tires

A modern tire consists of tread and body; its constituents encompass synthetic rubber, natural rubber, fabric and wire, along with carbon black and other chemical compounds. The purpose of the tread is to provide traction, while the body contains compressed air and provides comfort. The investigation of tread and body samples, by thermal analysis allows the prediction of tire behavior and is important for compositional analysis and quality control.  

In this Webinar, we will show how thermal analysis is applied to investigate tire materials. We will present some typical examples of samples measured by DSC, TGA, TMA, or DMA.

1:00:35 min
English

The webinar covers the following topics:

  • Introduction
  • Properties of tire treads
  • Typical questions
  • Thermal analysis
  • Instrumentation and applications
    - Differential Scanning Calorimetry (DSC) and Flash DSC
    - Thermogravimetry (TGA)
    - Dynamic Mechanical Analysis (DMA)
  • Summary

In the webinar titled "Thermal Analysis of Tires", we describe a number of techniques and methods that can be used to characterize the physical properties of tire tread compounds.

Tire tread

The tread of the tire is in direct contact with the surface of the road. The tread formulation and the design of the tread pattern are therefore decisive factors for most tire properties such as low rolling resistance, good wet traction and high resistance to abrasion.

The ingredients of the tire tread have a significant impact on these properties. These are investigated using various thermal analysis techniques.

 

Thermal analysis of tires

The most important effects that can be analyzed by DSC are the melting point, melting range, and melting behavior. DSC is also used to determine the heat of fusion, the glass transition, and oxidative stability.

TGA measures weight changes. The main applications of TGA are content determination, thermal stability, decomposition kinetics, and compositional analysis.

DMA is used to determine the modulus and damping behavior of materials. It allows tire properties such as rolling resistance or grip behavior to be directly predicted.

Thermal Analysis in the Automotive Industry

Slide 0: Thermal Analysis in the Automotive Industry

Ladies and Gentlemen,

Welcome to the METTLER TOLEDO webinar on “Thermal Analysis in the Tire Industry”.

During the course of the webinar, I would like to describe a number of interesting application examples that demonstrate the use of thermal analysis techniques in the tire industry. These have mainly to do with the measurement and testing of physical properties of tire tread compounds and the prediction of tire properties.

Typical examples include:

- the identification of polymers by their glass transition temperature;

- crystallization and melting processes;

- compositional analysis and decomposition behavior;

- and the determination of dynamic mechanical properties such as the loss factor or the complex modulus as a function of temperature, frequency or the displacement amplitude.

 

Slide 1: Contents

This slide lists the main topics I plan to cover in the seminar.

 

I would like to begin with some comments about important properties of tires such as rolling resistance, wet grip behavior, and abrasion that can be investigated by thermal analysis and in particular by dynamic mechanical analysis. I will then discuss the techniques used to characterize the properties of tire treads.

The techniques include:

Differential Scanning Calorimetry, or DSC;

Ultra-fast scanning DSC, the so-called Flash-DSC;

Thermogravimetric Analysis, or TGA;

and Dynamic Mechanical Analysis, or DMA.

 

After this, I wll then present several applications that illustrate the use of these techniques in the tire industry.

Finally, I want to summarize the different thermal analysis techniques and their applications.

 

Slide 2: Introduction - The Magic Triangle

The tread of the tire is responsible for contact between the vehicle and the road. The contact area is typically about the same as the surface area of a mobile phone.

The contact area affects all aspects of the physics of driving such as rolling, braking, and the resistance to abrasion.

 

The complexity of the requirements of a tire tread is often demonstrated by the so-called magical triangle of tire properties. It illustrates the fact that the improvement of one particular tire property leads to a deterioration of other properties.

The example shows that improving the rolling resistance leads to poorer wet grip behavior and poorer resistance to abrasion.

 

The simultaneous optimization of multiple tire properties is nevertheless possible but requires a fundamental understanding of the behavior of the materials used and their mutual interaction.

 

An outstanding improvement of tire properties was the development of the so-called green tire by Michelin. It is based on an improved interaction between the polymer and the filler. This results in a reduction of the rolling resistance and an increase of the wet grip behavior with practically unchanged resistance to abrasion.

 

Two important prerequisites for this innovation were the development of silica-based fillers, which have the ability to form chemical bonds with the polymeric matrix via special silanes, and the development of new dynamic-mechanical testing methods for the characterization of these interactions.

 

Slide 3: Introduction - Design of a Tire

This slide shows the main components of a tire.

The carcass and the sidewall guarantee the stability of the tire during acceleration and braking, while the inner liner ensures that an air pressure of typically 2 bar is maintained inside the tire.

The tread of the tire is in direct contact with the surface of the road. The tread formulation and the design of the tread pattern are therefore decisive factors for most tire properties such as low rolling resistance, good wet traction and high resistance to abrasion.

 

Since the ingredients of the tire tread have a significant impact on these properties, I want to first briefly discuss the basic ingredients of a tire tread.

 

Slide 4: Introduction - Formulation of Tire Treads)

The main components of a tire tread are polymers, crosslinkers, fillers, plasticizers and antioxidants.

The polymer used has by far the largest influence on the dynamic properties of the tire tread. The differences in the tread formulations of summer and winter tires show this very clearly. Summer tires contain polymers with relatively high glass transition temperatures such as SBR, whereas the tread compounds of winter tires mainly consist of polymers with a low glass transition temperature such as natural rubber.

 

In the rubber industry, the amount of an ingredient in a formulation is usually specified in parts per hundred rubber or phr for short.

A typical tire compound consists of 100 phr rubber, 2 to 4 phr crosslinker, 50 to 80 phr filler, and 0 to 30 phr plasticizer and antioxidant.

 

After mixing the polymer with the other ingredients, the resulting compound has to be vulcanized with sulfur, accelerators and activators to establish a three-dimensional polymer network. This provides dimensional stability and is the origin of the reversible elasticity of the tire tread.

 

Active fillers such as carbon black and silica in the tread compound reinforce the mechanical properties and improve ultimate properties such as tensile strength and resistance to abrasion. This results in a significantly longer lifetime of the tire.

 

The first commercially available tires were developed 1904 by the Continental company. These tires had no fillers in the tread; the resulting lifetime was less than 100 kilometers. Today's tires with optimized polymer-filler interactions can cover up to 50,000 km.

 

The type of the filler and its interaction with the polymer also has major influence on the rolling resistance. Stable filler-filler and filler-polymer interactions reduce the amount of energy dissipated during dynamic deformation and result in reduced rolling resistance.

 

However, filler-filler and polymer-filler interactions that are too strong deteriorate the ultimate properties. The result is a reduction of grip and abrasion.

 

Choosing the right type and right amount of filler is therefore a delicate balance between the reduction of energy dissipation and the deterioration of the ultimate properties.

 

The production of the final tread involves several extrusion and milling steps. Plasticizers and processing aids are added to adjust the viscous properties of the rubber compound in order to facilitate and optimize processing.

 

The addition of a variety of antioxidants and UV stabilizers prevents aging of the tire tread and ensures lifelong uniform tire properties

 

Slide 5: Thermal analysis

Some of the properties of tires can be determined using thermal analysis techniques.

According to the International Confederation for Thermal Analysis, ICTAC, thermal analysis is:

“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 diagram illustrates the processes that a material undergoes when it is heated, and the characteristic properties involved, for example, physical properties such as the heat capacity, expansion coefficient, or modulus. Fully or partially amorphous materials undergo a glass transition. Crystalline or semi-crystalline materials melt. If the sample is exposed to air or oxygen, it will start to oxidize and finally decompose at higher temperatures. We use thermal analysis techniques to investigate these effects.

 

Slide 6: Thermal Analysis

The slide shows the most important techniques used in thermal analysis, namely:

 

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

 

Thermogravimetric Analysis, or TGA. This technique measures the mass of the sample as a function of temperature using a highly sensitive electronic balance.

 

Thermomechanical Analysis, or TMA, is used to measure dimensional changes of a sample. The picture shows the sample support with a sample colored red and the quartz probe.

 

and finally

Dynamic Mechanical Analysis, or DMA, which provides information on mechanical properties. The picture shows one of the different types of sample-clamping assemblies.

 

I will explain these techniques in more detail in the following slides and describe some application examples.

 

Slide 7: Thermal Analysis in the Tire Industry

This slide summarizes the applications of thermal analysis techniques in the tire industry.

DSC is mostly used to characterize the glass transition temperature of the elastomeric components of the tire. Another important application area is the analysis of the crystallization and melting behavior of tire ingredients.

DSC measurements can also be used to investigate the vulcanization of the compounds to the final rubber parts. Enhanced software programs such as the model-free-kinetics program even allow modeling of the vulcanization process.

The efficiency of antioxidants can be evaluated by measuring the oxidation induction time, OIT.

 

TGA measurements can be used to determine the composition of rubber compounds used in different parts of the tire, as well as to determine the moisture content in fillers and other ingredients.

Another important area of the application of TGA is the evaluation of the efficiency of antioxidants.

TGA is also used to characterize the activity of carbon black in filled rubber compounds

 

DMA measurements can be used to investigate the dynamic-mechanical properties of tire components such as the rolling resistance of the tire tread, as well as to determine the glass transition temperature, to characterize filler-filler and filler-polymer interactions, and to analyze vulcanization kinetics.

The resolution and sensitivity of glass transition measurements using DMA is better than by DSC. The technique therefore enables close-lying glass transitions to be detected.

The hardening due to vulcanization can be analysed by dynamic load TMA. Information about the cross-linking density can be obtained by measuring the swelling behavior in solvents.

 

Slide 8: Differential Scanning Calorimetry (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 instrument measures from minus one hundred and fifty degrees Celsius to plus seven hundred degrees Celsius at heating rates of up to three hundred Kelvin per minute. The samples are normally measured in small crucibles made of aluminum, alumina or other materials, using one to twenty milligrams of sample material.

 

The METTLER TOLEDO Flash DSC  expands the maximum heating rate to two-million-four-hundred-thousand Kelvin per minute and the cooling rate to two-hundred-and-forty thousand Kelvin per minute. To achieve this, the Flash DSC uses very small samples weighing about one hundred nanograms and no sample crucibles - the sample is in direct contact with the chip-sensor.

Another useful DSC technique is high-pressure DSC or HPDSC for short. The METTLER TOLEDO HP DSC  can analyze samples under inert-gas- or reactive-gas-atmospheres at pressures of up to one hundred bar or ten mega-pascals. This suppresses undesired vaporization of samples or enables the stability of samples to be studied at 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 proportional to the heat capacity of the sample;

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 on the left of the slide summarizes the main analytical applications of DSC in the automotive field.

 

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 technique 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.

DSC curves also contain information about the thermal history of a sample.

The stability of materials can be determined using the oxidation induction time or OIT method.

Other important applications include the investigation of the influence of stabilizers, plasticizers or other additives. Most of the events involved are related to enthalpy changes that occur when the temperature is raised or lowered.

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.

METTLER TOLEDO has also developed versatile optical techniques that allow us to observe changes that occur in a sample while it is heated or cooled. The techniques include DSC-Microscopy and DSC-Chemiluminescence.

 

Slide 10: Application 1: DSC                                              Glass transitions of tire treads  

This slide displays the DSC curves of samples take from the treads of typical summer and winter tires.

 

The DSC curve in the upper part of the diagram is from a winter tire and shows two glass transitions at minus 104 and minus 63 degrees Celsius. This suggests that the tread compound is a blend of two incompatible elastomers. A comparison with literature values indicates that these are cis 1,4-polybutadiene (or BR), and natural rubber. These two polymers are typical elastomeric components that are used for the treads of winter tires.

The broad melting process in the temperature range minus 60 to plus 10 degrees is a further indication for the existence of BR in the formulation of the tire compound.

 

The natural rubber is added to formulation because it exhibits low energy dissipation under mechanical deformation, which is a prerequisite for low rolling resistance.

 

The main reason for the use of BR is its ability to crystallize and melt almost instantaneously. This improves the ultimate mechanical properties and results in enhanced abrasion resistance.

An understanding of the kinetics of this crystallization process is therefore essential for the optimization of the low temperature- and abrasive-behavior of a tire tread.

 

Later on in my talk, I will describe a novel way to quantitatively characterize crystallization kinetics using an advanced new technique – the so-called Flash DSC.

 

But before doing this, I would like to discuss the result of the measurement of the summer tire displayed in the lower curve. The summer tire has only one glass transition at minus 28 degrees. This indicates that SBR was used as the polymer for the tread.

 

In summary, it seems that the glass transition temperature is the basic difference between the treads of summer and winter tires.

This reflects the average differences in temperature between summer and winter and ensures that the treads of summer and winter tires remain flexible when they are deformed.

 

If a summer tire were used in winter, a glass transition temperature of minus 28 degrees would be far too close to outside winter temperatures - the tread of the tire would become harder and it would lose its elasticity. This would result in increased braking distance and have a direct negative effect on road safety.

 

Slide 11: Application 2: DSC                                              Crystallization of polybutadiene  

The slide displays DSC heating runs of polybutadiene measured at a heating rate of 10 kelvin per minute after the sample had been cooled under different conditions.

 

The bottom curve was recorded after cooling the sample at 10 kelvin per minute. It shows the glass transition of polybutadiene at about minus 104 degrees Celsius and the melting of the crystalline regions in the range minus 80 to plus ten degrees. The curve does not exhibit any effects due to cold crystallization because the sample had sufficient time to crystallize during the slow cooling process.

 

The middle curve was obtained after the poybutadiene had been allowed to cool ballistically. The mean cooling rate was about 50 kelvin per minute. Once again, no crystallization effects are visible on the heating curve. As in the first measurement, the polybutadiene crystallized during cooling.

 

The top curve was measured after the polybutadiene sample had been quench-cooled in liquid nitrogen and then immediately inserted into a DSC that had been cooled down to minus 150 degrees. In contrast to the two other measurements, the heating curve now shows a pronounced peak due to cold crystallization at about minus 70 degrees followed by the melting of the crystallites in the temperature range minus 50 to plus 10 degrees.

 

The experiments show that the cooling rates of conventional DSC instruments are too low, to prevent the polybutadiene from crystallizing. This means that the crystallization kinetics cannot be investigated by conventional DSC instruments.

 

Recently, METTLER TOLEDO has developed a new instrument known as the Flash DSC for ultra-fast measurements. I would now like to briefly describe this instrument before discussing the crystallization kinetics of polybutadiene.

 

Slide 12: Flash DSC 1: Basic Principles

The picture on the left side shows the Flash DSC 1.

 

The Flash DSC 1 combines a DSC chip sensor based on MEMS technology, an innovative, patented measurement and control concept, and a functional ergonomic design.

 

In its development, great importance was placed on simple sample preparation and ultra-high-heating-and-cooling rates. The lower heating rates of the Flash DSC however should overlap the higher heating rates of a conventional DSC instrument. The overlap range is shown in the inset diagram.

 

The Flash DSC 1 provides typical heating rates of up to forty thousand kelvin per second, and typical cooling rates of up to four thousand kelvin per second.

 

This means that the Flash DSC 1 can perform measurements over a scanning rate range of 4 to 5 decades. Together, the Flash DSC and conventional DSC cover a scanning range of more than 7 decades.

 

The temperature range of the Flash DSC 1 is typically between minus 95 and plus 450 degrees Celsius.

 

Slide 13: The Flash DSC 1: The chip sensor  

The heart of the Flash DSC 1 is the chip sensor.

 

The Flash DSC 1 uses the MultiSTAR UFS 1 sensor. UFS stands for ultra-fast sensor. This sensor is a chip sensor based on MEMS technology. The heaters and temperature sensors are incorporated on a small, extremely thin membrane that is only 2 micrometers thick.

The construction is in fact a complete, miniaturized DSC furnace for the sample and the reference material.

 

The picture on the left shows the UFS1 sensor. The membrane with the actual sensor is mounted in a ceramic frame together with the electrical connections.

 

The picture in the middle is an expanded view of the actual sensor. It consists of two membranes with identical furnaces. Each sensor is in fact a complete DSC cell. In the picture, the sample is placed on the upper part of the sensor. The lower part is the reference, which usually remains empty. The high degree of symmetry of the differential sensor results in flat and extremely reproducible baselines. Even at the highest scanning rates, the measurement curves exhibit a degree of reproducibility never before achieved in a DSC instrument.

 

The picture on the right is an enlarged view of the sample side of the sensor with a sample mounted on it. To achieve a homogeneous temperature profile and to simplify sample preparation, the active part of the membrane area with a diameter of 0.5 millimeters is coated with aluminum. The picture also shows the eight thermocouples and four resistance heaters.

 

Slide 14: Application 3: Flash DSC                                   Measurement program

The slide shows steps 1 to 3 of the measurement program used for characterizing the crystallization kinetics of the sample.

 

In the first step, the sample is annealed for 3 seconds at 100 degrees Celsius in order to melt all the crystallites that are present.

The sample is then cooled at 1000 kelvin per second to the crystallization temperature Tx. This means that the sample spends less than one tenth of a second in the critical temperature range of minus 80 to plus 10 degrees Celsius. Test measurements showed that no crystallization occurred at this cooling rate.

 

In the second step, the sample is annealed for a defined time at the crystallization temperature and then cooled to minus 90 degrees at one thousand kelvin per second. Crystallization therefore only occurs during annealing at Tx. The annealing time at Tx is varied between 0.1 and ten thousand seconds.

In the third step, the sample is heated from minus 90 to plus 100 degrees at a heating rate of 100 kelvin per second and the heat flow measured as a function of the temperature. During heating, the polybutadiene content that crystallized in the annealing phase melts; the measured enthalpy of melting is a measure of the degree of crystallinity of the polybutadiene.

 

The crystallization kinetics of the sample was determined by allowing the sample to crystallize at different annealing temperatures between minus 90 and zero degrees.

 

Slide 15: Application 3: Flash DSC                   Crystallization of polybutadiene

The slide shows heating curves measured after annealing the sample for different times at a crystallization temperature of minus 35 degrees Celsius. The hatched peak areas correspond to the enthalpy of melting.

 

The top curve shows the measurement for an annealing time of 0.1 seconds. No melting peak is visible – the polybutadiene was not able to crystallize to any significant extent in this short time.

The bottom curve was measured after an annealing time of ten thousand seconds. The melting peak is now clearly visible.

The difference between the enthalpies of melting after annealing times of one thousand and ten thousand seconds is only 2 percent. This means that crystallization is almost complete after ten thousand seconds. The first measurable melting peak is visible after an annealing time of about 10 seconds.

The results show that the crystallization kinetics for an annealing temperature of minus 35 degrees can be completely determined from measurements with annealing times between 0.1 and ten thousand seconds.

 

A characteristic quantity used in crystallization kinetics is the halftime of crystallization, namely the time it takes for the crystallinity to reach half its maximum value. In this example, the halftime is about 63 seconds.

 

Slide 16: Application 3: Flash DSC                                   Kinetics of crystallization

The diagram on the left shows the crystallization halftimes of the sample at different crystallization- or annealing-temperatures. The table on the right lists the most important results.

For the polybutadiene sample studied, the minimum halftime was 25 seconds for a crystallization temperature of minus 50 degrees Celsius.

Above minus 50 degrees, the halftime increases. For example, at minus 10 degrees the halftime is 70 minutes. At temperatures higher than this, no crystallization was observed under the conditions used for the measurements.

 

At crystallization temperatures below minus 50 degrees, the crystallization halftime also increases, that is, the crystallization process becomes slower - for example, the crystallization halftime at minus 90 degrees is about 15 minutes.

 

Since the crystallization rate strongly depends on the microstructure of the polymer concerned, the method is clearly a very sensitive way to characterize the microstructure of polymers.

 

In the example described, the crystallization behavior of polybutadiene was measured for the very first time thanks to this new measurement technique for the ultra-fast characterization of thermal properties.

 

The results described offer a new possibility to derive structure-property correlations between the properties of tires and the microstructure of the polymers and hence to optimize the properties of tire treads.

 

Slide 17: TGA/DSC

Now let’s turn our attention to thermogravimetric analysis, or TGA.

In this technique, the mass of a sample is measured as it is heated in a defined atmosphere. We simply put a few milligrams of the sample into a crucible, heat it, and continuously record the mass change. This allows us to obtain information about the composition of the sample.

The schematic curve on the left shows a typical TGA measurement curve of a polymer. The different effects are numbered next to the curve and explained in the table. These are:

One: heating begins, and volatile components such as moisture and solvent residues vaporize;

Two: decomposition of the polymer;

Three: the atmosphere is switched from nitrogen to air;

Four: carbon black decomposes.

Five:  a residue consisting of ash, fillers, glass fibers, etc. is left behind.

 

The METTLER TOLEDO TGA/DSC  instrument can also simultaneously measure heat flow and thereby detect effects that involve heat exchange.

 

Slide 18: Thermogravimetric Analysis (TGA)

The table on the left summarizes the main analytical applications of TGA for automotive materials. TGA is used to investigate processes such as vaporization or decomposition. It allows us to measure thermal stability and the kinetics of reactions.

For elastomers and rubber, TGA is usually used for compositional analysis.

 

The combination of TGA with a mass spectrometer or with a Fourier-transform-infrared-spectrometer allows us to analyze gaseous compounds evolved from a sample. This provides information about the nature and composition of the sample.

 

The picture on the right shows a view of the open furnace of a TGA/DSC instrument, and in particular the sample holder with positions for the sample and reference crucibles. The standard crucibles are made of alumina.

 

Slide 19: Application 1: TGA/DSC                    Decomposition of tire treads

The slide displays the TGA curves of samples taken from the treads of the summer and winter tires presented earlier on. The measurements were performed at a heating rate of 10 kelvin per minute. The atmosphere was switched from nitrogen to oxygen at 600 degrees Celsius and the measurement continued up to 800 degrees.

 

The curves show that the samples lose mass in several steps. At temperatures up to 300 degrees, the tread of the summer tire loses about 9 percent of its mass and the winter tire about 4.6 percent. This step in the measurement curve is assigned to the loss of oils added as plasticizers.

 

In the second temperature range from 300 to 500 degrees, the polymers decompose.

The TGA curve of the summer tire exhibits only one mass loss step. As we saw earlier, the DSC curve also showed only one glass transition. From the decomposition temperature and the glass transition temperature, we conclude that the material involved is SBR. The step height is a measure of the SBR content of the tread compound.

In contrast, the TGA curve of the winter tire sample exhibits a two-step decomposition process. This agrees well with the results from the previous DSC measurement in which two glass transition steps were observed and assigned to NR and BR.

Since NR is thermally less stable than BR, the step height at the lower temperature corresponds to the NR content and the step height at the higher temperature to the BR content of the tread compound.

 

After switching from nitrogen to oxygen atmosphere, there is further weight loss between 600 and 700 degrees due to the combustion of carbon black. The step height corresponds to the carbon black content of the sample. The tread of the summer tire contains 7.6 percent carbon black and that of the winter tire about 20 percent.

At 800 degrees, the summer tire leaves an ash residue of about 30 percent and the winter tire only about 9.7 percent. The ash consists mainly of thermally stable mineral fillers. This is most probably silica because this substance is nowadays used in almost all tread compounds to reduce the rolling resistance and to improve grip.

 

The tread formulation can be completely derived from the individual steps of the TGA measurement. This is demonstrated on the following slide.

 

Slide 20: Application 1: TGA                                              Decomposition of tire treads

The table on the left summarizes the DSC and TGA results obtained for the summer and winter tire treads. The table on the right shows the amounts of each ingredient converted to parts per hundred rubber.

 

The summer tire contains 100 parts of SBR. Usually, 10 to 20 parts of BR are added to improve the grip. The exact ratio of the two polymers cannot be determined because the polymers are compatible and their decomposition occurs at similar temperatures.

Evaluation yields a filler content of 60 parts silica and 15 parts carbon black for the summer tire. The large silica content ensures low rolling resistance and good grip under wet conditions. The carbon black is really only used as a pigment. Ten parts of oil were added for better processing.

 

Evaluation of the results for the winter tire yields a filler content of 30 parts of carbon black and 15 parts of silica. The lower silica content has to do with the poor mixing behavior with natural rubber. If too much silica is added to the natural rubber, the abrasive silica particles break the molecular chains of the natural rubber. This leads to a significant deterioration of the properties of the compound. Processing was also improved by adding oil, in this case only 7 parts.

 

Slide 21: Dynamic Mechanical Analysis (DMA)

Let’s now discuss one of the most important thermal analysis techniques used in the tire industry, namely dynamic-mechanical analysis or DMA..

 

DMA allows the viscoelastic behavior of a material to be characterized over a wide temperature and frequency range.

 

The table on the left of the slide shows the different application possibilities of DMA, for example, the measurement of glass transitions, and the determination of melting, crystallization, and viscous flow.

In particular, the investigation of viscoelastic behavior as a function of the stress frequency and the determination of the influence of fillers on the viscoelastic properties of the rubber mixture are of great importance and interest in the tire industry.

 

Slide 22: Dynamic Mechanical Analysis (DMA)

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

The different effects are numbered next to the curve and explained in the table. These are:

One: secondary relaxation, observed as a peak in the tan delta and the gee double prime curves; gee prime decreases slightly

Two: the glass transition, seen as a peak in the tan delta and the gee double prime curves and as a decrease in the storage modulus curve;

Three: cold crystallization of the polymer. The stiffness and hence the storage modulus, gee prime (G′), increase.

Four: recrystallization accompanied by a peak in tan delta and gee double prime;

Five: melting of the crystalline fraction with a decrease in the storage and the loss moduli. After melting, the loss factor increases considerably. Gee double prime is then larger than gee prime.

 

Slide 23: Application 1: DMA                               Amplitude-dependent measurements

Most components made of rubber contain fillers that have been added to the polymers in order to strengthen mechanical properties. The improvement can be directly demonstrated by performing amplitude-dependent measurements of the complex modulus.

 

The diagrams on the left and right of the slide show the storage modulus and the loss factor of samples of a butyl rubber containing different amounts of a high-active carbon black as a function of the displacement amplitude. The measurements were performed at 25 degrees Celsius and a frequency of 1 Hertz.

 

The storage modulus increases rapidly with increasing filler content for small displacement amplitudes. This increase characterizes the effect of mechanical strengthening of the filler when there is no deformation.

 

The higher the filler content, the more the storage modulus and the loss factor change as a function of the displacement amplitude. At small amplitudes, (here less than 0.1 percent), the storage modulus is constant and independent of the displacement amplitude. With larger amplitudes, the storage modulus shows a marked decrease. At even higher amplitudes, in this example from 20 percent onward, the storage modulus approaches a constant value.

The loss factor increases with increasing amplitude, reaches a maximum at displacement amplitudes between 2 and 5 percent and then decreases at larger displacement amplitudes.

 

The amplitude dependence of the storage modulus and the loss factor of filled elastomers was first studied by the British rubber scientist A.R.Payne more than 50 years ago and is named the Payne effect after him.

Payne explained the amplitude dependence of the storage modulus and the loss factor as being due to the formation of a mechanically fragile filler network that is stable if the deformation is small, but is reversibly degraded if the deformation is larger. In the limiting case when the deformation is very high, the network breaks down completely. This explains the constant modulus.

 

This theory of Payne no longer corresponds to the current state of scientific knowledge. However, the measurement of amplitude dependence is an indispensable method for characterizing filler-filler- and/or filler-polymer-interactions, particularly in the development and optimization of dynamically stressed rubber components

 

In the tire industry, the amplitude-dependent measurement of the modulus is very important for characterizing the rolling resistance of tire treads.

 

Slide 24: Application 2: DMA                                             Payne effect of tire treads

To illustrate the determination of the rolling resistance, amplitude-dependent measurements of the complex modulus were performed using samples taken from the treads of a summer- and a winter-tire.

 

The measurements were carried out at two different temperatures to take into account the influence of summer and winter temperatures on the tread:

In summer, when average temperatures are between 20 and 30 degrees Celsius, the tread heats up to about 60 degrees during driving.

In winter, at temperatures between minus 20 and zero degrees, the tread reaches a temperature of about 20 degrees.

 

To compare the dynamic properties, amplitude-dependent measurements were performed on the tread samples at 20 and 60 degrees. The two diagrams display the resulting storage modulus and loss factor curves. The curves measured at 20 degrees represent conditions in winter, and the curves measured at 60 degrees conditions in summer.

 

To predict the rolling resistance, the maximum of the measured loss factor is usually correlated with the energy dissipated during rolling. The lower the amount of energy dissipated, that is, converted to heat by the tread during rolling, the lower the rolling resistance. In other words, a small loss factor is a direct measure for low rolling resistance.

 

This estimate is however only valid if the tread materials of the two tires have similar moduli. In this particular case, the tread of the winter tire is however significantly softer. This means that it will be more strongly deformed on rolling and will dissipate more energy. If one, however, used the loss factor as an indicator for rolling resistance, one would come to the wrong conclusion, namely that the winter tire has lower rolling resistance both in winter and in summer.

 

Slide 25: Application 3: DMA                                             Payne effect of tire treads

To correctly predict the rolling resistance, one must take into account the fact that both tire treads are stressed to about the same extent during rolling, that is, the force acting on the tread during rolling is constant.

For this reason, it is the loss compliance and not the loss factor that is the quantity proportional to the dissipated enregy

 

The two diagrams compare the loss compliances of the treads for summer and winter conditions. The double-headed arrow between the two diagrams indicates that greater loss compliance leads to higher energy dissipation and thus to higher rolling resistance. A reduction of the loss compliance therefore reduces the rolling resistance.

 

The diagram on the left shows the loss compliances of summer and winter tires for a tread temperature of 20 degrees Celsius, that is, for typical conditions in winter.

The smaller values for the loss compliance of the winter tire indicate that less energy is dissipated during rolling. In winter, the winter tire has the lower rolling resistance. Changing from summer to winter tires in winter not only improves performance on ice and snow but also helps to save fuel because of the reduced rolling resistance.

 

The diagram on the right displays similar measurements for summer conditions, that is, for a tread temperature of 60 degrees. The loss compliance of the tread of the summer tire and thus the rolling resistance is clearly lower than that of the winter tire.

If you forget to change from winter to summer tires in spring, you pay for this with higher rolling resistance and hence higher fuel consumption.

 

Slide 26: Application 4: DMA of Tire Treads (temperature-dependent measurement)

 

Dynamic-mechanical analysis can not only predict the rolling resistance of a tire tread compound but can also be used to estimate the grip of the tread, that is, the adhesion between the tread and the surface of the road.

 

The assumption is that good adhesion is due to good mechanical interlocking between the tread and the surface of the road. This is achieved when the tread can relax into areas of micro- and mesoscopic-roughness during contact with the surface of the road and so dissipate maximum energy.

An estimate based on a mean vehicle speed of about 100 kilometers per hour and a road surface roughness of several micrometers to several millimeters yields a characteristic contact time of several nanoseconds to several microseconds for adhesion.

From this, it follows that the adhesion between the tire tread and the road is good when the polymer dissipates high energy in a time interval of several nanoseconds to several microseconds.

 

However, in a dynamic-mechanical measurement, a time interval of micro- to nano-seconds would require measurement frequencies of the order of several kilohertz to several megahertz. Since direct measurements cannot be performed in this frequency range, the so-called principle of time-temperature superposition is applied using measurement conditions that can easily be realized in order to reproduce the adhesion between tire and road.

In this particular case, this means that the dynamic-mechanical behavior in kilohertz and megahertz region at tread temperatures of 60 degrees can be reproduced by measurements at 1 hertz and a temperature of about zero degrees Celsius, and for tread temperatures of 20 degrees, at 1 hertz and a temperature of about minus 40 degrees.

 

The diagram shows the loss factors of the treads of summer and winter tires as function of temperature. The loss factor of the summer tire has a maximum at about minus 5 degrees and the winter tire at about minus 45 degrees.

 

To a simple approximation, the loss factor can again be looked on as a measure of the energy dissipated.

In summer conditions, that is, with a tread temperature of about 60 degrees, the summer tire dissipates more energy than the winter tire in the temperature range relevant for adhesion of about 0 degrees and therefore has the better road grip.

If you drive with winter tires in summer, you not only use more fuel but you also need a longer braking distance.

 

In winter conditions, the tread of winter tire dissipates significantly more energy than tread of the summer tire and therefore has a significantly better grip.

If you drive with summer tires in winter, your braking distance is much longer than with winter tires.

 

Slide 27: Summary- Thermal Effects of Tire Treads

 

The table summarizes the thermal analysis techniques recommended for the measurement of tire properties and the information that can evaluated from these measurements.

The most important effects that can be analyzed by DSC are the melting point, melting range, and melting behavior. DSC is also used to determine the heat of fusion, the glass transition, and oxidative stability.

 

TGA measures weight changes. The main applications of TGA are content determination, thermal stability, decomposition kinetics, and compositional analysis.

 

DMA is used to determine the modulus and damping behavior of materials. It allows tire properties such as rolling resistance or grip behavior to be directly predicted.

 

 

Slide 28: Summary - Instruments

This slide presents an overview of the temperature ranges of the METTLER TOLEDO DSC, TGA, TMA, and DMA thermal analysis instruments.

 

Depending on the specific cooling accessories, DSC experiments can be performed at temperatures between minus one hundred and fifty degrees Celsius and plus seven hundred degrees.

 

TGA measurements normally begin at room temperature. The maximum possible temperature is sixteen hundred degrees.

 

TMA experiments can be performed between minus one hundred and fifty degrees and plus sixteen hundred degrees.

 

DMA samples are measured in the range minus one hundred and fifty degrees to plus six hundred degrees.

 

Slide 29: For More Information

Finally, I would like to draw your attention to information about application examples 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 technical customer magazine. Back issues can be downloaded as PDFs at www.mt.com/ta-usercoms as shown on the slide.

Our “Collected Applications” and “Thermal Analysis in Practice” handbooks contain many other examples and are recommended for further reading.

 

Slide 30: 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 and so ensure that you are always fully up-to-date in thermal analysis.

 

Slide 31: Thank You

This concludes my presentation on Thermal Analysis in the Tire Industry. Thank you for your interest and attention.

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