Thermogravimetric analysis

The thermogravimetric analysis ( TGA ), and Thermogravimetric called, is an analytical method or method of thermal analysis or thermal analysis , wherein said mass change is a sample as a function of temperature and time measured. For this purpose, the sample is heated in a small crucible made of temperature-stable and inert material (e.g. platinum or aluminum oxide ) in an oven to temperatures of up to 2400 ° C. The sample holder is coupled to a microbalance that registers the changes in mass during the heating process. A thermocouple near the crucible measures the temperature. Modern TGA devices allow the final temperature, heating rate, gas flow or similar to be set via a connected computer. During the analysis, the sample space is flushed with various gases as required. Mostly pure nitrogen is used to prevent oxidation of the sample. In some cases, however, it is also purged with air , oxygen or other gases. When heated, the sample can release volatile components into the environment through decomposition reactions or evaporation. B. absorb reactants by oxidation. The decrease or increase in weight and the temperature at which the change in weight takes place can be specific to an examined sample. From this, conclusions can be drawn about the composition of the substance.

Measuring principle

TGA measuring device (the oven has been pushed down to introduce the sample)

In thermogravimetry, the change in mass of a solid sample is observed during a known heating or cooling process. The most common application is heating the sample at a constant heating rate. Mass changes can be triggered by the following causes:

• Loss of mass through physical processes (e.g. evaporation , sublimation )
• Loss of mass of a sample through disintegration ( decomposition with the formation of volatile products)
• Loss of mass through reaction (e.g. reduction )
• Increase in mass through reaction (e.g. oxidation )

A thermal balance usually consists of the following components:

• An oven with adjustable temperature
• The scales
• Supply lines for hydrogen , nitrogen , oxygen and helium
• Evaluation devices for processing the measured values

Oven with temperature control

The most important property that the oven of a thermobalance must have is the generation of a homogeneous temperature field at the location of the sample, since even small temperature fluctuations during the experiment can have an influence on the output curve. It should be noted that the homogeneous area decreases with increasing temperature.

The scales

The most commonly used scales work on the principle of electromagnetic compensation. The metal balance beam is always held in the same position by two electromagnetic coils on both sides of the balance arms. Every deflection of the balance from its rest position is detected by a photoelectric sensor and the voltage of the magnetic coils is regulated in such a way that the balance is held in the initial position. In order to keep the current acting on the magnetic coils as low as possible, a counterweight is attached on the side opposite the sample arm. Its mass roughly corresponds to that of the crucible. During the measurement, the change in voltage is measured, which is linearly related to the change in mass.

The gas supply lines

The gas supply lines attached to the thermobalance allow the apparatus (furnace and balance head) to be charged with various gases and gas mixtures. Nitrogen is usually used as the inert gas . The connections and design of the thermobalance must be vacuum-tight so that sensitive samples do not react with ambient air (oxygen). When coupling the thermobalance with a mass spectrometer , the use of helium can make more sense, since it does not appear in the detection range of the mass of carbon monoxide like nitrogen. Possible reaction gases are synthetic air for oxidations or hydrogen for reductions .

Influences on the measurement

There are a number of technical and physical effects that have an influence on the test results. A blind measurement is therefore often taken before the start of the experiment, as the effects of temperature can change the device parameters such as the conductivity of the coils in the balance head, the density or the viscosity of the gases used.

Coupling methods

Additional analyzers can be connected to the exhaust pipes to analyze the substances volatilized in the furnace or the reaction and decomposition products. Mass spectrometers or IR spectrometers are most frequently used here , and recently more and more NMR spectroscopes have been used . There are also structures in which the thermobalance and the spectrometer are separated using gas chromatography (TG-GC-MS). A simple method of analyzing the exhaust gases is to use absorption tubes . Using special desorption ovens , the exhaust gases can then also be analyzed spatially separately on other measuring systems.

calibration

The temperature calibration of thermal balances can be carried out using metals or alloys that show a Curie transformation at a defined temperature . Suitable materials here can be nickel (Up. 360 ° C) and iron (Up. 768 ° C). In practice, the measuring cell must be in the area of ​​influence of a strong external magnetic field . The conversion is detected as an apparent change in mass. The temperature deviation is dependent on the heating rate. The temperature calibration must therefore be carried out for different heating rates. However, there is a linear relationship between temperature deviation and heating rate. The temperature deviations can depend on the temperature, which makes calibration at different temperatures and thus with several calibration substances necessary (multi-point calibration).

A mass calibration is carried out using suitable (calibrated) weights. A simple check of the detected mass differences can be done using calcium oxalate monohydrate. Under inert conditions (no oxygen ) the compound shows three defined degradation stages:

1. Release of the water of hydration: with Δ m = 12.33 Ma% ${\displaystyle \mathrm {CaC_{2}O_{4}*H_{2}O\longrightarrow \ } }$ ${\displaystyle \mathrm {\ CaC_{2}O_{4}+H_{2}O} }$
2. Release of carbon monoxide with formation of calcium carbonate : with Δ m = 19.17% by mass based on the initial mass of calcium oxalate monohydrate ${\displaystyle \mathrm {CaC_{2}O_{4}\longrightarrow \ } }$ ${\displaystyle \mathrm {\ CaCO_{3}+CO} }$
3. Release of carbon dioxide with formation of calcium oxide : with Δ m = 30.12% by mass based on the initial mass of calcium oxalate monohydrate. ${\displaystyle \mathrm {CaCO_{3}\longrightarrow \ } }$ ${\displaystyle \mathrm {\ CaO+CO_{2}} }$

TGA measurement on calcium oxalate monohydrate

precision

Thermogravimetric determination of proportions of plasticizers , polymer , carbon black and residue on ignition in synthetic polymers are ring experimental data is available. If the degradation stages are well separated, the comparative standard deviation s _{R is} around 0.7 g / 100 g regardless of the constituent measured. At low soot contents (2 - 3 g / 100 g) the s _R values ​​are around 0.3 g / 100 g. s _R is a good first estimate of the standard uncertainty.

literature

• DIN EN ISO 11358 - November 1997: Plastics - Thermogravimetry (TG) of polymers - General principles
• DIN 51006: Thermal analysis (TA) - Thermogravimetry (TG) - Basics (2005)
• ASTM D 3850: Testing of the reduction in the insulating effect of electrical insulating materials under the influence of heat using thermogravimetry (1994)
• ISO 9924-1: Rubber and rubber products - Determination of the composition of vulcanizates and unvulcanized compounds by thermogravimetry - Part 1: Butadiene, ethylene-propylene copolymer and terpolymer, isobutene-isoprene, isoprene and styrene-butadiene rubber ( 2000)
• ISO 9924-2: Rubber and rubber products - Determination of the composition of vulcanizates and unvulcanized compounds by thermogravimetry - Part 2: Acrylonitrile butadiene and halobutyl rubber (2000)
• ISO 21870: Raw materials for rubber - carbon black - Determination of weight loss by heating by means of thermogravimetry (2005)
• Gottfried W. Ehrenstein, Gabriela Riedel, Pia Trawiel: Practice of thermal analysis of plastics. Hanser, 2003, ISBN 3-446-22340-1 .
• WF Hemminger, HK Cammenga: Methods of thermal analysis. Springer, Berlin, ISBN 3-540-15049-8 .
• PJ Haines (ed.): Principles of Thermal Analysis and Calorimetry. The Royal Society of Chemistry, 2002, ISBN 0-85404-610-0 .

Individual evidence

1. ↑ Bruno Wampfler, Samuel Affolter, Axel Ritter, Manfred Schmid: Measurement uncertainty in plastics analysis - determination with round robin test data . Hanser, Munich 2017, ISBN 978-3-446-45286-2 , pp. 61-64 .