User Com. Thermal Analysis. Heat capacity determination at high temperatures by TGA/DSC Part 2: Applications. Information for Users.

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1 Thermal Analysis Information for Users 28 User Com Dear Customer, We are always pleased to receive feedback from you about individual articles. It helps us to select topics of common interest that we can include in future issues of UserCom to address current questions. We try to cover as wide a field as possible and hope the information will give you new ideas on how to use thermal analysis for solving analytical problems in your laboratory. The constant improvement in the performance and versatility of our instruments results in never-ending new application possibilities. We are sure the articles in this UserCom will help you in your work. Heat capacity determination at high temperatures by TGA/DSC Part 2: Applications Dr. Rudolf Riesen In Part 1 published in UserCom 27, we showed how c p values of materials can be measured up to 1600 C [1]. This second article presents various applications such as the determination of c p changes at glass transitions and in 2 nd order phase transitions. It describes how a change in mass that occurs in a decomposition reaction can be taken into account in c p determination and how the process enthalpy is determined. Contents 2/2008 TA Tip - Heat capacity determination at high temperatures by TGA/DSC Part 2: Applications 1 New in our Sales Program - QUANTOS automatic powder dosing for small sample sizes 5 Applications - The characterization of olive oils by DSC 6 - Tips on method development for DMA measurements in 3-point bending 9 - Elastomer seals: Creep behavior and glass transition by TMA 13 Tips and hints - Temperature and enthalpy adjustment of the high pressure DSC827 e 17 Dates - Exhibitions 19 - Courses and Seminars 19

2 TA Tip Figure 1. DSC curves of a glass sample and sapphire to determine the glass transition and the change in c p of the glass. Heating rate 10 K/min; crucible 30-µL Pt. Introduction Differential scanning calorimetry (DSC) offers several different measurement procedures for determining the specific heat capacity (c p ) [2]. The direct method and the sapphire method were discussed in Part 1. An important conclusion was that only the sapphire method yields reliable results. This second article presents a number of applications that have been performed with this method. Determination of c p changes The following examples describe several application possibilities of the sapphire method for the determination of c p at high temperatures. The measurements were performed using a METTLER TOLEDO TGA/DSC 1 equipped with an HSS2 sensor. A platinum crucible with lid was used in all the experiments. The curves are blank corrected. Change in c p at the glass transition The glass transition is accompanied by a noticeable change in the specific heat capacity. This change can be measured using the sapphire method, as shown in Figure 1 using a sample of glass as an example. While the characteristic glass transition temperature can be read directly from the DSC curve, the overall course of the c p and the magnitude of the change at the glass transition can only be obtained from the c p curve. Change in c p in second order phase transitions Second order phase transitions show a characteristic change in the specific heat capacity without additional energy (latent heat) being involved. The change is observed as an apparent endothermic effect on the DSC heating curve of a suitable sample. The temperature dependence of c p is measured using the sapphire method. The example in Figure 2 shows the c p function of cobalt around the Curie point at 1120 C (1393 K). The c p curve exhibits the typical l-shape. The measurement values lie in the range of literature values for cobalt and the standard deviation is about ±10%. Change in c p in decomposition reactions The determination of the specific heat capacity assumes that the sample does not change. Figure 2. Upper diagram: DSC curves of cobalt and sapphire using the sapphire method. Lower diagram: the c p curve, literature values: [4], + [5], [6]. Cobalt measured as five disks. Heating rate 20 K/min; crucible 30-µL Pt with lid. If this is not the case, for example in a decomposition reaction, the sapphire method measures an apparent specific heat capacity. To determine the c p values of the starting material and the end product, the change in mass must be taken into account. The combination of the TGA and DSC techniques in the TGA/DSC 1 allows mass and enthalpy changes to be simultaneously measured using the same test specimen. The thermal decomposition of calcium carbonate to calcium oxide is studied as an example. The heat flow measured during the decomposition of the calcium carbonate includes a latent heat component that is determined by the reaction enthalpy. The TGA and DSC curves are shown in Figure 3. The pure calcium carbonate decomposes in a nitrogen atmosphere to calcium oxide, which remains behind as a residue. Stoichiometrically, a mass loss of 43.97% occurs. The specific heat capacity of CaCO 3 measured before decomposition (which begins at about 600 C) agrees well with the 2

3 literature value (Figure 4). Afterward, c p increases due to the overlapping decomposition enthalpy. The curve corresponds to the apparent specific heat capacity. After the reaction is completed, the c p value determined is too low if normalization is performed as usual with respect to the original starting mass (dashed curve). Normalization with respect to the starting mass gives incorrect results because calcium oxide is left behind after the reaction and not calcium carbonate. The STAR e software allows the c p calculation to be performed with respect to the changing mass. The result of this calculation is shown in Figure 4 as the red curve (mass corrected). It shows that correct values are obtained for the calcium oxide. A second heating run of the same test specimen (Figure 3) yielded the c p curve of CaO for the entire temperature range as shown by the blue curve in Figure 4. Since no mass change occurs, the calculation can be made with constant starting mass. In the example shown, this corresponds to the residue from the first heating run. At 1100 C, the c p values of both measurements agree well. Determination of the process enthalpy from c p measurements The energy required in a thermal process (the process enthalpy) is the sum of the heat needed to heat the material (the sensible heat) and the reaction enthalpy (the latent heat). Both these values can be calculated from the c p curve, whereby the process enthalpy is always evaluated with respect to the initial mass. The enthalpy and its change as a function of temperature are determined by integrating the c p curve. This is explained using the calcination of CaCO 3 again as an example. Figure 3. TGA and DSC curves of sapphire, CaCO 3 and CaO measured according to the sapphire method. Heating rate 20 K/min; crucible: 150-µL Pt with lid. Figure 4. The c p curves of CaCO 3 and CaO calculated from the curves in Figure 3 using the sapphire method. Black dashed line: c p without mass correction; red line with correction. Literature data: + [4], r [5], [5, 6]. Figure 5. The c p curve of CaCO 3 (dashed, not mass corrected). The blue curve is the enthalpy as a function of temperature. Figure 5 once again shows the c p curve calculated with respect to the starting material (Figure 4). Integration with respect to temperature gives a process enthalpy of approx J/g for the calcination process between 450 and 1000 C. The enthalpy as a function of the temper- 3

4 TA Tip ature (blue curve) is calculated by partial integration of the c p curve, whereby the starting value is set to zero. The reaction enthalpy is obtained through the integration of the c p curve above the red baseline (Type: Integral Tangential [3]). This gives a value of approx J/g. The baseline is constructed on the assumption that the c p value of the sample changes in proportion to the conversion. If a chemical or physical change involving latent heat occurs, an apparent heat capacity is determined. In a decomposition reaction, the mass of the sample also changes. This mass change must be taken into account in order to determine correct c p values of the starting and end products. The integration of the c p curve with respect to temperature yields the enthalpy of a process. This was shown using the calcination of CaCO 3 as an example. Summary and conclusions In Part 1 it was shown that the sapphire method is ideal for determining the c p at temperatures up to 1600 C [1]. In Part 2, the change in c p that occurs during different thermal events is investigated. The glass transition of an inorganic glass and the second order phase transition of cobalt are presented as examples. The mass of a sample usually remains constant during c p determination. Literature [1] Heat capacity determination at high temperatures Part 1: DSC standard procedures, UserCom 27, 1 4 [2] G. W. H. Höhne, W. Hemminger and H.-J. Flammersheim: Differential Scanning Calorimetry, Springer Verlag, 1996, 127 [3] Choosing the right baseline, UserCom 25, 1 6 [4] Handbook of Chemistry and Physics, 70 th edition, , CRC Press Inc., Boca Raton, Florida [5] I. Barin, O. Knacke, Thermochemical properties of inorganic substances, 1973, Springer Berlin [6] D Ans. Lax, Taschenbuch für Chemie und Physik, Band 1, Makroskopische physikalisch-chemische Eigenschaften, 1967, Springer Berlin 4

5 QUANTOS automatic powder dosing for small sample sizes New in our Sales Program Figure 1. Optimize your process with QUANTOS, economize on sample size, and protect the environment and your workforce. Weighing with a spatula is one of the more tedious tasks in the laboratory. A new system automates the dosing of small quantities of free-flowing powdered substances directly into different sample containers (for example into a crucible). Laboratory technicians can concentrate on other important work. QUANTOS is the first system worldwide for the automatic dosing of small quantities of free-flowing powders. The heart of the system is an intelligent dosing head including storage container for dosing active or harmful powdered substances. Thanks to the high-precision mechanism and intelligent electronics, the dosing head and the dosing instrument are perfectly matched. QUANTOS doses directly and precisely to the target weight without exceeding set tolerances. The entire dosing process is controlled and monitored by a METTLER TOLEDO analytical balance. QUANTOS doses with the highest precision up to 20 times faster compared to manual processes and at the same time with much better safety! The highly compact dosing instrument is ideal for dosing small amounts of powders from 1 to about 250 mg. It is typically used in the pharmaceutical industry and in biotechnology for sample preparation for analytical methods such as HPLC. QUANTOS also improves working processes wherever small quantities of powder have to be repeatedly dosed with the highest accuracy and precision. For thermal analysis, QUANTOS allows you to weigh powders directly into crucibles. Figure 2. QUANTOS doses quickly, safely and economically into different containers. Based on METTLER TOLEDO s experience and know-how in precise weighing technology, QUANTOS offers unrivaled 220 g measuring performance with mg readout accuracy. This permits the typical minimum weight of 10 mg in accordance with USP. The entire process is controlled from the balance. Automatic dosing is not only faster and more economical, but also safer: QUANTOS protects laboratory personnel and the samples thanks to the closed system, hazardous substances remain sealed and aerosol formation is kept to an absolute minimum. For further product information, visit or ask your local sales representative. 5

6 The characterization of olive oils by DSC Dr. Angela Hammer Applications Figure 1. OOT measurements of the three different olive oils. Figure 2. OIT measurements of the three olive oils at 140 C. Can olive oils be used for frying? What are the optimal storage conditions for these products? These questions are discussed in the following article based on the results from different DSC experiments. OIT and OOT measurements were performed to characterize oxidation stability. Crystallization and melting behavior was also investigated. Introduction Oxidation stability is an important criterion for assessing the shelf life and quality of different products. Oxidation stability can be characterized by the OIT (Oxidation Induction Time) [1] or the OOT (Oxidation Onset Temperature) [2]. The OIT of a material is measured in an oxygen atmosphere at a particular isothermal temperature. It is the time from when the material is first exposed to oxygen up until the onset of oxidation. In contrast, OOT experiments can be performed more rapidly. The OOT of a material is measured in a dynamic measurement in oxygen. The OOT is defined as the temperature at which oxidation of the material begins, i.e. the onset temperature of oxidation. The advantage of OIT is that it offers better reproducibility than OOT. Furthermore, the values obtained at particular temperatures can be more meaningfully compared. High-pressure DSC can be used to increase the oxygen concentration. This allows reactions to be performed more rapidly at lower temperatures. OIT and OOT measurements can however be quite easily performed with standard DSC instruments. In this article, OIT and OOT measurements of edible oils used in the food industry (olive oils) were performed. The crystallization and melting behavior of the oils was also characterized. Table 1. Results obtained for the OIT and OOT measurements. Sample OIT at 140 C OOT Olive oil No min 175 C Olive oil No min 171 C Olive oil No min 185 C Depending on the definition used, olive oils are divided into up to nine different quality classes. The three olive oils investigated here all belonged to the first and best so-called Extra Virgin category. Oils of this class are distinguished by the fact that they are cold pressed (from the first pressing of the olives) and are carefully produced under mild conditions without use of excessive temperatures. Olive oil contains about 77% monounsaturated fatty acids, 9% polyunsaturated fatty acids and 14% saturated fatty acids. According to manufacturers specifications, cold-pressed oil can be heated to about 6

7 180 C. Refined oil (produced from the second pressing for longer shelf life) can be used to about 220 C. For users it is important to know whether an olive oil can be used for specific purposes such as frying or only as a salad oil at normal temperatures, and furthermore whether an olive oil decomposes at higher temperatures with consequent loss of some of the health-promoting ingredients. For example, above about 180 C, antioxidants of the phenol and tocopherol groups (Vitamin E) in olive oils decompose. Experimental details Three different olive oils (Extra Virgin) were investigated. The OIT and OOT measurements were performed using a METTLER TOLEDO STAR e System consisting of a DSC 1 with gas controller and air cooling. Samples of 0.5 to 3 mg were measured in 40-µL aluminum crucibles without a lid. The program used for the OIT measurements was as follows: Heating from 30 C to the isothermal measurement temperature at 40 K/min with a purge gas flow of 50 ml/min nitrogen; isothermal for 5 min at the measurement temperature with a gas flow of 50 ml/min nitrogen; then switching to oxygen to enable the oxidation reaction. The gas was automatically switched by the gas controller. The sample was held at the isothermal temperature until the oxidation reaction began. The OOT measurements were performed by heating the samples from 30 to 300 C at 5 K/min using oxygen as purge gas. The crystallization and melting experiments were performed using a DSC 1 equipped with an intracooler. Samples of 2 to 3 mg were sealed in 40-µL aluminum crucibles. These were cooled from 30 C to 75 C at 5 K/min and then heated from 75 C to 150 C at 5 K/min. Results OIT/OOT Figure 1 shows the results of the OOT measurements of the three olive oils. The OOT temperatures lie between 171 and 185 C. Although the oils belong to the same quality class, their stability is clearly different. Based on these results, an isothermal temperature of 140 C was chosen for the OIT measurements. Figure 2 shows the OIT values obtained for the three olive oils measured isothermally at 140 C. The values lie between 14 and 24 min and show the same tendency as the OOT results. Olive oil No. 3 is the most stable. These measurements allow olive oils to be differentiated and characterized with regard to stability. Table 1 summarizes the numerical results. Crystallization and melting behavior The crystallization and melting behavior of the oils was compared in further experiments. As a rule, olive oils consist of three main components (saturated, monounsaturated and polyunsaturated fatty acids), which result in three different melting peaks. The numerical values (peak temperatures of the melting peaks and melting range) are given in Table 2 and the crystallization and melting curves displayed in Figures 3 to 6. An overview of the curves of all three samples is shown in Figure 3. Sample T 1 T 2 T 3 Melting range DT Olive oil No C 5 C 5 C 38 to 14 C Olive oil No C 5 C 5 C 28 to 16 C Olive oil No C 6 C 6 C 32 to 30 C Table 2. Results obtained for the melting peaks. Figure 3. Comparison of the crystallization and melting curves of the three olive oils. Figure 4. Evaluation of the crystallization and melting curves of olive oil No. 1. 7

8 Applications The samples exhibit characteristic crystallization behavior. The crystallization depends very much on the chemical composition. The results clearly show that olive oil should be stored at room temperature and not in a refrigerator. Otherwise, partial crystallization of the natural product will occur because all three products melt between 40 and +30 C and are only liquid at room temperature or above. Summary The three olive oils studied showed clear differences with regard to stability. One can conclude that they should not be heated to above 150 C and are therefore unsuitable for frying. Other characteristic features are their crystallization and melting behavior. This provides information on their composition and on optimum storage temperatures. Since the olive oils begin to crystallize at room temperature, they should not be stored in a refrigerator. Literature [1] ASTM E-1858 Standard Method for Determining Oxidation Induction Time of Hydrocarbons by DSC. [2] ASTM E-2009 Standard Test Method for Oxidation Onset Temperature of Hydrocarbons by DSC. Figure 5. Evaluation of the crystallization and melting curves of olive oil No. 2. Figure 6. Evaluation of the crystallization and melting curves of olive oil No. 3. 8

9 Tips on method development for DMA measurements in 3-point bending Dr. Markus Schubnell In the 3-point bending mode, an offset force has to be specified in the method in addition to the maximum displacement and force amplitudes. In this article, we describe a possible procedure for determining the offset force and the other measurement parameters. We also estimate the uncertainty of the resulting 3-point bending modulus values. Introduction Ceramics, metals and composite materials are usually measured in DMA in the 3-point bending mode (see Figure 1). In this mode, the sample is held in position in the clamping assembly by an offset force (preload force) applied to the sample. The offset force must be greater than the amplitude of the force applied to the sample otherwise the sample will lose contact with the clamping assembly. In 3-point bending measurements, the offset force is usually provided as a constant current offset force (details are given in the next section). In this article, we explain how measurement parameters can be determined for 3-point bending experiments. We also estimate the uncertainty of the resulting 3-point bending modulus values. The different offset control modes in the DMA861 e Constant Current Offset control mode In the Constant Current Offset control mode, the drive motor generates a constant offset force, F cc, in addition to the dynamic force with amplitude F A. The force generated by the drive motor is applied to the sample via a membrane. The membrane and the sample (with the sample holder and the drive shaft) are connected in parallel. Part of F cc is therefore used to deform the membrane. If the stiffness of the sample is more than twice the stiffness of the membrane, then practically the entire offset force, F cc, is applied to the sample. The stiffness of the membrane is determined during the mechanical adjustment of the instrument and is typically about 6 N/mm for the 40-N drive motor and about 3 N/mm for the 12-N or 18-N drive motors. This means that from a sample stiffness of about 12 N/mm or more, the entire offset force is applied to the sample. If the sample stiffness is about the same as that of the drive motor membrane, the offset force actually exerted on the sample is reduced by half. With softer samples, it is correspondingly smaller. To a certain extent, this effect can be made use of when the sample softens during a glass transition. The offset force actually applied decreases and the predeformation of the sample is reduced making it possible to measure softer samples. The Force Offset measurement curve that can be displayed in the Evaluation Window is the static offset force actually exerted on the sample. The constant current offset mode is mainly used for experiments in 3-point bending. If the samples are sufficiently stiff (above 12 N/mm), the force specified in the method corresponds to the offset force acting on the sample. The force amplitude is not influenced by the membrane because if necessary it is controlled by the force sensor. The sample stiffness, S s, is given by where E is the modulus of the sample and g the geometry factor. For a rectangular bar, the geometry factor in 3-point bending is given by Here l, b and h are the dimensions of the sample (l = clamping length, b = width, h = thickness). Autooffset control mode In the Autooffset control mode, the offset force just sufficient to hold the sample in place during the measurement is first determined (autooffset = 100%; this corresponds to the condition F s = F A (see Figure 1)). The actual offset force to be applied is then given in units of this minimum offset force. In the autooffset mode, the offset force is determined using an iterative process. Compared to Force Force Figure 1. In the 3-point bending mode, the sample is held in the clamping assembly by the offset force, F s. Above: Without the offset force, there are times when no force is exerted on the sample. Below: The sample is only held in the clamping assembly when F s > F A. 9

10 Applications the constant current offset method, this increases the time needed to measure a modulus value. The advantage of the autooffset mode is that the offset adapts under control to the sample stiffness as it changes with temperature. The autooffset mode is mainly used with soft samples measured in tension. Typical values are %. Example: Autooffset = 150%. Assumption: for a displacement amplitude of 100 µm a force of 0.5 N is required. It follows that the Autooffset force is 150% 0.5 N = 0.75 N. Determination of measurement parameters for 3-point bending measurements Comments on sample geometry In 3-point bending measurements, the maximum displacement amplitude, the maximum force amplitude, and the offset force must be specified in the measurement method. To determine reasonable values for this, the stiffness of the sample must be estimated. The stiffness of a sample is determined by the sample geometry and the modulus of the sample according to equations (1) and (2). The sample stiffness should be at least five times smaller than the sample holder stiffness. In the 3-point bending assembly, the sample holder stiffness depends on the clamping length. Typical values for the sample holder stiffness of the 3-point bending assembly in the DMA861 e are given in Table 1. If the sample stiffness turns out to be less than five times smaller than the sample holder stiffness, the measurement results become increasingly unreliable. In this case, the sample stiffness should be reduced by changing the sample geometry. In 3-point bending, this is most easily done by using a longer clamping length. Once one has decided on a suitable sample geometry using the above-mentioned criteria, the next step is to determine the method parameters referred to earlier on. A suitable procedure is illustrated in the following using a sample of a glass-fiber reinforced epoxy resin (printed circuit board). Example: Determination of the DMA measurement parameters for a printed circuit board The modulus of this material is typically about 25 GPa. The sample size was 100 x 7.51 x 1.51 mm. The same sample was used for all the measurements. The experiments were performed with a DMA/SDTA861 e (40-N drive motor) at room temperature and a frequency of 1 Hz. We decided to use a clamping length of 50 mm. In this case, the sample stiffness is 20.7 N/mm, which is far below the sample holder stiffness (2464 N/mm). We then investigated how different displacement amplitudes and offset forces affected the measurement results. This was done by creating a method in which the offset force was increased every four minutes while maintaining the displacement amplitude constant at a certain value. The force amplitude was set to an unrealistically high value (20 N). This means that the experiment was controlled by the preset displacement amplitude. Figure 2 shows results for such measurements for displacement amplitudes of 80 and 120 µm. The curve for the offset force is the same for both displacement amplitudes. The displacement amplitude chosen cannot be applied until the offset force exerted on the sample is sufficiently large. For a displacement amplitude of 80 µm this is about 2 N or more, and for a displacement amplitude of 120 µm about 4 N or more. The figure shows an apparent (slight) dependence of E * and tan d on the offset force. The reason for this is the contact of the sample with the clamping assembly. To obtain good contact between the sample and the clamping assembly, high offset forces are necessary, especially with samples that have a rough surface. Under optimum measurement conditions, tan d should be as small as possible and independent of the displacement amplitude. Likewise, under optimum measurement conditions, the modulus should be as large as possible and independent of the method parameters. In the example, this is the case with offset forces of about 6 N or more. If the modulus of the printed circuit board is to be determined at room temperature, a constant current offset force of 14 N and a displacement amplitude of 80 µm are thus possible parameters for the measurement method. In a temperature-dependent DMA experiment, the modulus of the material becomes smaller during heating. For the printed circuit board investigated here, the modulus after the glass transition, which occurs around 90 C, is about 8 GPa. With a high offset force, for example F cc = 14 N, the sample would be predeformed by about 1.1 mm after the glass transition due to the reduced stiffness. This predeformation is however much too large, the material would be unnecessarily stressed. For this reason only about 6 N should be used as a constant current offset force for this sample. Table 1. Stiffness of the sample holder for different clamping lengths (typical values). Clamping length in mm Sample holder stiffness in N/mm A constant current offset force of 6 N also produces a predeformation of about 475 µm in the rubbery state. In addition to the static predeformation, there is also the dynamic deformation. In this case, the question arises as to whether the linearity range of the sample is exceeded. To answer this, we must estimate the strain that the sample undergoes. 10

11 In 3-point bending, the strain, e, can be estimated from the equation Here F is the sum of the forces exerted on the sample (static offset, F s, plus the dynamic force amplitude, F A ), l is the clamping length, E * the modulus of the sample material, b the sample width, and h the sample thickness. For the printed circuit board under investigation (l = 50 mm, b = 7.51 mm, h = 1.51 mm), the strain in the glassy state (E * = 25 GPa) is about 0.1% and in the rubbery plateau (E * = 8 GPa) about 0.26% (assumption: F cc = 6 N, displacement amplitude = 80 µm, i.e. the dynamic force amplitude is about 1.6 N in the glassy state and 0.5 N in the rubbery state). Estimation of the uncertainty of the modulus values The uncertainty of the measured modulus values in 3-point bending depends mainly on the uncertainties in the sample geometry, contact between the sample and the clamping assembly, clamping of the sample, and reproducibility of the measurement. In the following sections, we estimate the individual contributions for the sample investigated here and to determine a value for the combined uncertainty for the modulus value in the glassy state. Sample geometry If the uncertainties involved in determining the dimensions of the sample are known, the relative standard deviation of the geometry factor, s g, can be calculated from the equation Here D h and D b are the uncertainties of the width b and the thickness h of the sample. The uncertainty resulting from the measurement of the clamping length can be neglected because b and h are typically at least about ten times smaller than the clamping length. The uncertainties in the width and thickness originate mainly from the deviations of the actual sample geometry from an assumed ideal sample geometry (as a rule a rectangular bar). They are difficult to quantify. For the printed circuit board measured here, we assume values of 0.05 mm for both D h and D b (from our experience, these are very good values). For our sample, this gives a relative standard deviation for the geometry factor of s g = 10%. This also corresponds to the relative standard deviation of the modulus. With a modulus of 25 GPa, this gives a standard deviation of 2.5 GPa. It should be noted that the error in the geometry factor arises mainly from the uncertainty of the sample thickness. Contact of the sample with the clamping assembly This uncertainty is difficult to determine because the measurement conditions (displacement amplitude, offset force) influence the measurement results in a systematic way. If we assume that the sample is measured under favorable conditions (in the example with a displacement amplitude of 80 µm and an offset force of F cc = 6 N), then the uncertainty (standard deviation) of the modulus value would be about 0.9 GPa (an estimate using the data in Figure 2). Clamping the sample This influence can be relatively easily estimated by performing a series of measurements in which the sample is repeatedly clamped and measured using the same method. Based on ten measurements of our printed circuit board, we obtained a standard deviation of 0.4 GPa, whereby the sample was purposely not always mounted exactly in the middle of the clamping assembly. The standard deviation of 0.4 GPa determined is therefore somewhat too large. The magnitude of the standard deviation however shows that the clamping of the sample in the 3-point bending holder is not so important for the quality of the measured values. Reproducibility of the measurements The reproducibility can be estimated as a standard deviation from the modulus values measured under isothermal measurement conditions. For our sample, we obtained a value of 0.04 GPa. This low value confirms the excellent performance of the DMA/SDTA861 e regarding the reproducibility of DMA measurements. Combined uncertainty The different uncertainty components for the modulus in the glassy state (here about 25 GPa) are summarized in Table 2. The data applies to a particular sample at a certain temperature (printed circuit board at room temperature (modulus about 25 GPa), clamping length: 50 mm, width: 7.51 mm, thickness: 1.51 mm)). Figure 2. E * modulus, tan d and the offset force, F s, applied to the sample with displacement amplitudes of 80 and 120 µm. The values for the specified constant current offset force in the measurement segments are also given. The curve for the offset force is the same for both displacement amplitudes. 11

12 Applications Table 2. Standard deviation for different uncertainty components in 3-point bending measurements. Figure 3. Creation of a DMA method for estimating the optimum constant current offset force. The table shows that the uncertainty in the determination of the modulus with 3-point bending measurements is mainly due to the sample geometry (in particular the sample thickness). The uncertainty that arises from the performance of the instrument is in comparison negligible. For the sample measured here, a combined uncertainty of about 2.7 GPa or 11% (standard deviation) was obtained. Conclusions and recommendations 1. The optimum displacement amplitude is typically between about 20 µm (for samples with smooth flat surfaces, e.g. metals) and 100 µm (for samples with rough surfaces, e.g. composite materials such as the printed circuit board measured here). Sources of uncertainty 2. The constant current offset force specified in the measurement method, F cc, is shared between the membrane and the sample. If the sample stiffness is larger than about 12 N/mm, F cc is applied practically completely to the sample. With samples whose stiffness is about the same as that of the stiffness of the membrane of the drive motor, the offset force actually applied to the sample is about half of F cc. The other half is used to deform the membrane. In this case, with temperature-dependent measurements, the offset force exerted on the sample automatically adapts to the sample stiffness during the experiment. For the constant current offset mode the sample stiffness should ideally Standard deviation in GPa Standard deviation in % Sample geometry Contact of the sample (measurement method) Clamping of the sample Reproducibility (instrument specific) be between about 12 N/mm and one fifth of the sample holder stiffness (see Table 1). 3. To determine the optimum constant current offset force, F cc, a trial experiment should be performed under isothermal conditions (e.g. at room temperature). In this experiment, F cc is varied under constant dynamic measurement conditions. A typical method is shown in Figure 3. Depending on the stiffness and the nature of the surface of the sample, F cc is typically about two to ten times larger than the dynamic force amplitude. To provide this relatively large static offset force, it is best to use a 40-N drive motor for 3-point bending. It should be noted that the 40-N drive motor provides a maximum static force of about 25 N (the 40 N relates to the maximum dynamic force amplitude). Similarly, with the 12-N and 18-N drive motors, maximum constant current offset forces of about 8 N and 12 N are available. 4. Quality criterion for optimum parameters: the tan d values should be as small as possible and independent of the constant current offset force. 5. Although the predeformation of the sample might be clearly visible, the sample strain that actually occurs is in the per thousand range. 6. The accuracy with which the modulus value can be determined depends mainly on the accuracy with which the sample geometry can be specified. Even with geometrically well defined samples (standard deviation for clamping length, width and thickness each 0.05 mm) standard deviations for the modulus value of the order of 10% are obtained. 12

13 Elastomer seals: Creep behavior and glass transition by TMA Ni Jing This article describes how thermomechanical analysis (TMA) can be used to characterize the creep and viscous flow behavior of two different types of elastomers. This was done by means of different isothermal creep and recovery measurements and thermally stimulated creep (TSC) experiments. These methods allow the glass transition and other relaxation processes (e.g. reversible flow relaxation) to be measured with high sensitivity. Elastic deformation and the viscous flow of elastomers can also be determined. The elastomers studied were SBR (styrene-butadiene rubber) with different degrees of vulcanization and EPDM (ethylene-propylene-diene rubber) containing different amounts of carbon black. Introduction Hardness, glass transition, creep and the viscous flow component are some of the more important properties that have to be taken into account when elastomers are used for sealing applications. The hardness of a material is determined by its elasticity and modulus and predicts the deformation capacity under pressure or load. The determination of the glass transition and the temperature retraction method (ASTM D1329) are often used to characterize the low-temperature sealing performance of such elastomers. The term creep refers to the time- and temperature-dependent elastic and plastic deformation of a material when it is subjected to a load or stress. Creep deformation consists of two components: reversible creep relaxation and irreversible viscous flow. The time-independent elastic deformation that also occurs under load is not considered as being part of creep deformation. The creep deformation caused by reversible creep relaxation recovers over time when the stress is reduced or removed. This is a positive factor in sealing applications. Viscous flow however causes permanent deformation and geometry change and often leads to product failure. butadiene rubber) with different degrees of vulcanization and from EPDM (ethylene-propylene-diene rubber) containing different amounts of carbon black are discussed in detail. Isothermal creep and recovery In an isothermal creep and recovery experiment, the sample is held isothermally at a specified temperature. A mechanical stress (in this case, the TMA force) is applied, held constant for a certain period, and then quickly removed. The strain (i.e. the relative change of sample thickness) is recorded as a function of time. Thermally stimulated creep The measurement principle of thermally stimulated creep (TSC) is shown schematically in Figure 1. Step 1: The sample is subjected to a mechanical stress (the TMA force, F 0 ) at a given temperature T p for a specified time. This results in a certain amount of orientation of relaxation units (such as polymer chain segments). Step 2: The sample is then cooled under control to T 0, causing molecular mobility and rearrangements to freeze. Step 3: The mechanical stress is removed at T 0. Step 4: The sample is heated at a constant rate, which allows the relaxation units to lose their orientation and recover. Retardation, or time-delayed reorientation, occurs. This appears as a step in the sample thickness and as peak in the first deriva- Figure 1. Typical force and temperature program in a thermally stimulated creep experiment. These properties can be readily investigated using thermal mechanical analysis (TMA) by performing isothermal creep and recovery experiments and thermally stimulated creep (TSC) measurements. The results obtained from SBR (styrene- 13

14 Applications Figure 2. Creep and recovery curves of SBR samples with different degrees of vulcanization measured at 30 C. The time is displayed on a logarithmic scale to show the details of the segments. tive curve. The step height is a measure of the intensity of this retardation. If the temperatures and force are suitably chosen, the TSC technique even allows the time distribution of retardation processes to be studied. Experimental details The measurements were performed using a TMA/SDTA841 e equipped with a 3-mm ball-point quartz probe and liquid nitrogen cooling. Isothermal creep and recovery: The samples were held isothermally at 30 C throughout the experiment. A force of 0.01 N was applied for 10 min (EPDM) or 15 min (SBR), followed by 1 N for 60 min, and finally 0.01 N for 120 min. TSC measurements: The samples were held isothermally at 30 C and subjected to a force of 1 N for 60 min. They were then cooled down to 120 C at 5 K/min and maintained at this temperature for 5 min with the force still applied. The force was then quickly reduced to 0.01 N. The samples were left at 120 C for a further 30 min and finally heated to 60 C at 5 K/min. Sample preparation: Cubes of approximately mm were cut from the elastomers. A quartz disk was placed between the upper surface of each sample and the TMA probe in order to distribute the force uniformly over the sample. Analysis of vulcanized SBR Four samples of SBR with different degrees of vulcanization were measured with the TMA. The degree of crosslinking of each SBR sample was controlled by using different concentrations of crosslinking agents (Vulkacit CZ/C and sulfur) in the formulation. SBR0 did not contain any crosslinking agent and is therefore unvulcanized, while SBR1, SBR2 and SBR3 contained increasing amounts of sulfur: 0.16%, 1.79% and 3.45% sample mass, respectively. Figure 2 shows the isothermal creep and recovery curves obtained for the four SBR samples. The thickness of each sample was measured by the TMA at the beginning and end of each experiment using a negligibly low force of 0.01 N. This force is so low that no significant sample deformation occurs; its purpose is to ensure good contact between the probe and the sample. In contrast, the force of 1 N is the force that actually deforms the sample. The deformation comprises three components: 1. Elastic deformation: an immediate change in thickness, marked by the four horizontal dashed lines. 2. Viscoelastic relaxation: a gradual time-dependent reversible change in thickness. 3. Viscous flow: a gradual time-dependent irreversible change of sample thickness. When the force is removed at 75 min, a sudden change in the thickness of the sample (the elastic component) is observed. This corresponds exactly to the change that occurred when the force of 1 N was applied at 15 min. The larger the relative elastic deformation, the smaller the elastic modulus of the elastomer. This initial deformation is followed by a period in which viscoelastic relaxation takes place. This part of the deformation is reversible and is desirable for sealing applications. The remaining deformation (marked with an arrow on the right in Figure 2) corresponds to the viscous flow. It is the difference between the initial thickness of the sample (the thick dashed line) and the sample thickness at the end of the experiment. As can be seen in Figure 2, the unvulcanized SBR0 sample shows the largest elastic deformation, while the deformation of the vulcanized SBR samples 1 to 3 gradually decreases. The higher the degree of crosslinking, the smaller the elastic deformation and the larger the elastic modulus. The unvulcanized SBR0 sample also shows the largest degree of viscoelastic relaxation while in contrast the relaxation of the crosslinked SBR samples is much less pronounced. The SBR0 sample also exhibits the largest deformation component. The creep recovery segment would however have to be appreciably longer to observe the complete recovery because the curve is still rising slowly even 120 min after the force has been removed. The SBR1 and SBR2 samples still show a certain amount of viscous flow whereas in the SBR3 sample this deformation component is practically no longer observed due to the high degree of crosslinking. 14

15 Figure 3 shows the TSC curves of the four SBR samples. The TMA curves exhibit steps that are displayed as peaks in the first derivative curves. The steps can be assigned to the glass transition of SBR. With increasing vulcanization, the glass transitions are shifted from 12.2 C for SBR0 to C for SBR3. The step change and peak height indicate the intensity of the reorientation of the relaxation units. Above the glass transition, the sample thickness continues to increase, but the shapes of the curves are somewhat different. The most noticeable effects are the weak step-like increase of SBR0 and SBR1. This can be more clearly seen in the first derivative curves where the steps are observed as broad weak peaks. These effects can be attributed to the flow relaxation process. The unvulcanized SBR0 exhibits pronounced flow relaxation whereas the lightly vulcanized SBR1 shows very little flow relaxation. In SBR2 and SBR3, flow relaxation can no longer be seen because the high crosslinking density prevents such relaxation. Figure 3. TSC curves of SBR with different degrees of vulcanization during the heating segment. Figure 4. Creep and recovery curves of EPDM samples with different contents of carbon black measured at 30 C. The time scale is shown logarithmically to highlight the details of the individual segments. Analysis of EPDM with different carbon black contents Three samples of EPDM containing 30 phr, 60 phr and 90 phr of carbon black were measured (phr means parts per hundred parts rubber). As can be seen in Figure 4, EPDM1 containing 30 phr carbon black exhibits the largest elastic deformation when subjected to the force of 1 N. This means it has the smallest elastic modulus of the three samples. With increasing carbon black content, the elastic deformation decreases and the elastic modulus of the EPDM increases. It is of interest to note that the viscous flow (the irreversible deformation component) is not influenced by the carbon black content. Figure 5. TSC curves of EPDM samples with different contents of carbon black during the heating segment. Figure 5 shows that EPDM1 with the lowest carbon black content (30 phr) exhibits a significant step-increase in sample thickness due to the glass transition process. This step is observed as a large 15

16 Applications peak in the first derivative curve of the TMA curve. With increasing carbon black content, the step change and peak height decrease dramatically, which indicates that the reorientation of the mobile units involved in the glass transition process weakens. For EPDM3, the step can hardly be recognized only the increase in slope can still be seen. This is observed in the first derivative curve as a small step. The temperature range of the glass transition process is not influenced by the carbon black content. EPDM1 exhibits a shoulder in the range 40 C to 20 C next to the peak in the first derivative curve. This can be attributed to the melting process of the crystallites in the EPDM. The melting process is influenced by the carbon black content; for EPDM3 it can hardly be detected. Conclusions The most important physical properties of elastomer used for sealing applications are the elastic modulus, glass transition and the creep and viscous flow behavior. These properties should be measured and compared to ensure adequate and consistent sealing performance. TMA is a powerful technique that can be used for this purpose. The elastic modulus, creep and flow behavior of materials can be determined in isothermal creep and recovery experiments. Increasing the degree of crosslinking of elastomers through vulcanization not only increases the elastic modulus but also reduces creep relaxation and the undesired viscous flow. Although the addition of carbon black to the elastomer formulation increases the elastic modulus, it does not lead to a reduction of the viscous flow. The latter is often responsible for the failure of sealing rings. The problem cannot therefore be solved through the addition of carbon black. Due to its sensitivity, the thermally stimulated creep method is ideal for measuring the glass transition of highly filled elastomers and other weak relaxation processes such as the flow relaxation of unvulcanized or lightly vulcanized elastomers. Literature [1] C. Lacabanne, D. Chatain and J. C. Monpagens, J. Appl. Phys. 1979, 50(4),

17 Temperature and enthalpy adjustment of the high pressure DSC827 e Tips and hints Figure 1. The melting behavior of indium at different pressures (nitrogen). Dr. Markus Schubnell Introduction The high pressure METTLER TOLEDO DSC827 e is usually adjusted at standard atmospheric pressure. The question is whether the adjustment is still valid at higher pressures. Heat exchange within the DSC cell is strongly pressure dependent, so it seems reasonable to assume that enthalpy adjustment is also pressure dependent. But to what extent? And what about temperature adjustment? This article presents measurements of the enthalpy of fusion and melting temperature of pure metals performed at different pressures. Experimental results The measurements described here were performed with a high pressure DSC827 e that had been adjusted at standard atmospheric pressure using the total calibration method. The melting peaks of indium, tin and zinc under nitrogen were then measured at different pressures. The corresponding curves for indium together with the values for the enthalpy of fusion and the melting temperature are displayed in Figure 1. The results show that with increasing pressure the enthalpy of fusion shows a slight decrease and the melting temperature a slight increase. In a DSC, increased pressure results in better thermal contact between the crucible (and therefore the sample) and its surroundings (furnace, furnace lid). As a result, the heat flow measured by the sensor decreases, which is equivalent to a reduction in sensor sensitivity. One therefore expects the measured enthalpy of fusion to decrease with increasing pressure. This is confirmed by the results presented in Figure 1. Figure 2 shows the relative enthalpies of fusion of indium, tin and zinc measured at different pressures using the enthalpy of fusion at standard pressure as the reference value (100%). Figure 2. Relative enthalpies of fusion of indium, tin and zinc at different pressures. 100% corresponds to the enthalpy of fusion at standard pressure. The error bars marked in the diagram correspond to the standard deviations. The data shows that the influence of pressure only becomes noticeable at pressures above about 50 bar (5 MPa). The enthalpy of fusion is of course also pressure dependent for physical reasons: increased pressure changes the lattice dimensions slightly, which in turn affects the enthalpy of fusion. For example, for indium, a pressure increase of 1 kbar causes the enthalpy of fusion to change by about 0.24% [1]. Such a small effect cannot be detected in the pressure range within which the HP DSC827 e operates and can therefore be neglected. Figure 2 shows that if the enthalpy adjustment is performed at standard pressure, then at high pressures, enthalpy values are obtained that are systematically about 5% too low, depending on the particular pressure and temperature. 17

18 Tips and hints Figure 3 shows the differences between the pressure-dependent melting temperatures (literature values) and the melting temperatures measured at different pressures for In, Sn and Zn. For example, from the literature it is known that the melting temperature of indium increases by about 4.7 mk when the pressure is increased by 1 bar [1]. The diagram shows that the melting temperature is also systematically influenced by the change in heat flow in the DSC due to increasing pressure. Quantitatively, the measured melting temperatures compared with the true melting temperatures are systematically too low by about 1.5 mk per bar overpressure. Conclusions The adjustment of the high pressure DSC827 e turns out to be slightly pressure dependent both with respect to enthalpy and to temperature. An analysis of melting temperatures showed that the melting temperature measured at a particular pressure compared with the true melting temperature at the same pressure was about 1.5 mk/bar too low. If measurements are performed at maximum pressure (100 bar = 10 MPa) in the DSC827 e, the temperature is consequently about 0.15 K too low. For most intents and purposes, this deviation is of no practical importance. At pressures up to about 50 bar, enthalpy adjustment is also practically independent of pressure. Above 50 bar, pressure dependence increases significantly. Depending on the temperature and pressure, the measured enthalpies are systematically too low by up to about 5%. Figure 3. Difference between the pressure-dependent melting temperature and the measured melting temperature for In, Sn and Zn at different pressures. The results presented here refer to nitrogen as the fill gas. Compared with nitrogen, other gases may have markedly different temperature-dependent and pressure-dependent thermal conductivity properties, for example helium. The results found here therefore cannot be directly applied to other gases. Literature [1] G. W. H. Höhne, K. Blankenhorn, Thermochimica Acta, 238, ,

19 Exhibitions, Conferences and Seminars Veranstaltungen, Konferenzen und Seminare Fakuma International Trade Fair for Plastics Processing October 13 17, 2009 Friedrichshafen, Germany Dates International and Swiss TA Customer Courses: TA Customer Courses and Seminars in Switzerland Information and Course Registration: TA-Kundenkurse und Seminare in der Schweiz Auskunft und Anmeldung: Ms Esther Palma-Andreato, Mettler-Toledo AG, Analytical, Schwerzenbach, Tel: , Fax: , Courses / Kurse SW Basic (Deutsch) 04. Mai Nov SW Basic (English) May 11, 2009 Nov 09, 2009 TMA (Deutsch) 04. Mai Nov TMA (English) May 11, 2009 Nov 09, 2009 DMA Basic (Deutsch) 04. Mai Nov DMA Basic (English) May 11, 2009 Nov 09, 2009 DMA Advanced (Deutsch) 05. Mai Nov DMA Advanced (English) May 12, 2009 Nov 10, 2009 TGA Basic (Deutsch) 05. Mai Nov TGA Basic (English) May 12, 2009 Nov 10, 2009 TGA Advanced (Deutsch) 06. Mai Nov TGA Advanced (English) May 13, 2009 Nov 11, 2009 DSC Basic (Deutsch) 06. Mai Nov DSC Basic (English) May 13, 2009 Nov 11, 2009 DSC Advanced (Deutsch) 07. Mai Nov DSC Advanced (English) May 14, 2009 Nov 12, 2009 TGA-FTIR (Deutsch) 07. Mai Nov TGA-FTIR (English) May 14, 2009 Nov 12, 2009 SW Advanced (Deutsch) 08. Mai Nov SW Advanced (English) May 15, 2009 Nov 13, 2009 TGA-MS (Deutsch) 08. Mai Nov TGA-MS (English) May 15, 2009 Nov 13, 2009 Kinetics / TMDSC (Deutsch) 08. Mai Nov Kinetics / TMDSC (English) May 15, 2009 Nov 13, 2009 TA- Kundenkurse und Seminare in Deutschland und der Schweiz Local TA Customer Courses: Nähere Informationen unter oder durch: Frau Petra Fehl, Mettler-Toledo GmbH, Giessen, Tel: , labtalk@mt.com Anwenderworkshop DSC (kostenpflichtig) 03. u Giessen 23. u Giessen Anwenderworkshop TGA (kostenpflichtig) 05. u Giessen Anwenderworkshop STAR e - Software (kostenpflichtig) 10. u Wittenberge Weiterbildungsseminar Thermische Analyse Excellence Neuss Darmstadt (kostenfrei) München Hannover Stuttgart Berlin Weiterbildungsseminar Thermische Analyse und Düsseldorf Greifensee (CH) Rheologie in Forschung u. QS (kostenfrei) Hamburg Lausanne (CH) Leipzig Cours et séminaires d'analyse Thermique en France Renseignements et inscriptions par: Christine Fauvarque, Mettler-Toledo S.A., Av. de la pépinière, Viroflay Cedex, Tél: , Fax: , christine.fauvarque@mt.com Principe de la TMA/DMA 5 octobre 2009 Viroflay (France) Principe de la TGA 8 octobre 2009 Viroflay (France) DSC: notions de base 6 octobre 2009 Viroflay (France) Logiciel STAR e : perfectionnem. 9 octobre 2009 Viroflay (France) DSC: perfectionnement 7 octobre 2009 Viroflay (France) Corsi e Seminari di Analisi Termica in Italia Per ulteriori informazioni Vi preghiamo di contattare: Simona Ferrari, Mettler-Toledo S.p.A., Novate Milanese, Tel: , Fax: , simona.ferrari@mt.com DSC base 3 Febbraio Giugno Settembre 2009 Novate Milanese DSC avanzato 4 Febbraio Giugno Settembre 2009 Novate Milanese TGA 5 Febbraio Giugno Settembre 2009 Novate Milanese TMA 6 Febbraio Giugno Settembre 2009 Novate Milanese L Analisi Termica per la caratterizzazione dei materiali polimerici 10 Febbraio 2009 Bologna 17 Febbraio 2009 Milano 3 Marzo 2009 Napoli 4 Marzo 2009 Roma L Analisi Termica nell industria farmaceutica 10 Marzo 2009 Milano TA Customer Courses and Seminars in the UK For details of training courses and seminars, please contact: Lester Troughton, Mettler-Toledo Ltd, Leicester, Tel: ++44 (0) , Fax: , lester.troughton@mt.com Thermal Analysis October 27, 2009 Leicester 19

20 TA Customer Courses and Seminars: Thermal Analysis training based on the STAR e System is available at various locations. For information, please contact: Australia and New Zealand: Kha Nguyen, Tel: , kha.nguyen@mt.com Belgium: Annick Van Hemelrijck, Tél: , annick.vanhemelrijck@mt.com China: Lu LiMing, Tel: , liming.lu@mt.com España: Francesc Catala, Tel: , francesc.catala@mt.com India: Mahesh Tripathi, Tel: , Fax: , mahesh.tripathi@mt.com Japan: Tomoko Wakabayashi, , tomoko.wakabayashi@mt.com Korea: Kihun Lee, Tel: , kihun.lee@mt.com Korea: HoeWoon Jeong or JungWon Kim, Tel: , hoewoon.jeong@mt.com, jungwon.kim@mt.com, Homepage: Latin America: Francesc Català, Tel: (Spain), francesc.catala@mt.com Malaysia: Ms Candace Leong, Tel: (603) , candace.leong@mt.com Netherlands: Hay Berden, Tel: , hay.berden@mt.com Österreich: Frau Geraldine Braun, Tel: , geraldine.braun@mt.com Ceská Republika: Helena Beránková, Tel: , helena.berankova@mt.com Singapore: Chelsea Low, Tel: , chelsea.low@mt.com Slovenia: Keith Racman, Tel: , keith.racman@mt.com Sweden: Fredrik Einarsson, Tel: , fredrik.einarsson@mt.com Thailand: Chatriya Suamsung, Tel: , chatriya.suamsung@mt.com USA and Canada: Michael Zemo, Tel: , michael.zemo@mt.com For further information regarding meetings, products or applications, please contact your local METTLER TOLEDO representative and visit our home page at Editorial team Dr. A. Hammer Ni Jing Dr. D. P. May Dr. R. Riesen Dr. J. Schawe Dr. K. H. Hassdenteufel Chemist Chemist Chemist Chem. Engineer Physicist Material Scientist Dr. M. Schubnell Dr. M. Wagner J. Widmann M. Zappa K. von Euw U. Jörimann Physicist Chemist Chemist Material Scientist Technical Writer Electr. Engineer Mettler-Toledo AG, Analytical Postfach, CH-8603 Schwerzenbach Phone Fax Contact: urs.joerimann@mt.com For more information 11/2008 Mettler-Toledo AG ME , Printed in Switzerland MarCom Analytical