Talk:Thermogravimetric analysis

Duplicate article?
There is also an article entitled "Thermogravimetry". Shouldn't these articles be merged, and one term be redirected to the other? -- Thomas.Hedden —Preceding unsigned comment added by Thomas.Hedden (talk • contribs) 16:04, 11 September 2008 (UTC)

I was going to make the same comment! I see that more than a year after the above comment was made, it has not been acted on and no-one has commented on it. I'm not an expert in this field, so I hope there's an expert out there who will do this work! Timothy Cooper (talk) 13:49, 3 October 2009 (UTC)

After almost 9 years the duplicate article question still seems open. Thermogravimetry is a branch of sciences while a thermogravimetric analysis is a very specific analytical procedure. Reading from the page on merging this article does qualify. The field of thermogravimetry is derived almost entirely from the single test of thermogravimetric analysis. --Agriculturist50 (talk) 18:03, 5 September 2017 (UTC)

Text of duplicate article
Thermogravimetry (also known by the acronym "TG"; alternative spellings include thermo-gravimetry and thermogravimmetry) is a branch of physical chemistry, materials research, and thermal analysis. It is based on continuous recording of mass changes of a sample of material, as a function of a combination of temperature with time, and additionally of pressure and gas composition.

Process
A sample of material (ranging from 1 mg to 100 mg, but sometimes as large as 100 g) is placed on an arm of a recording microbalance, also called thermobalance where that arm and the sample are placed in a furnace. The furnace temperature is controlled in a pre-programmed temperature/time profile (most commonly), or in the rate-controlled mode, where the pre-programmed value of the weight changes imposes the temperature change in the way necessary to achieve and maintain the desired weight-change rate. The most common temperature profiles are: jumping to isotherm and holding there for a specified time ("soak"); temperature ramping at constant rate (linear heating or cooling); and combination of ramp and soak segments. The profile "ORTA" ("oscillation-rate thermal analysis") is used in other methods of thermal analysis, but not in TG, due to unavoidable disturbance forces. The rate-controlled method is very time-efficient, but for some types of materials it produces incorrect results or "ghost effects"; since this method does not reveal the automatically imposed temperature profile, the users may be misled by their trust for the "sophisticated, computerized program, which saves the analysis time tremendously".

The gaseous environment of the sample can be: ambient air, vacuum, inert gas, oxidizing/reducing gases, corrosive gases, carburizing gases, vapors of liquids or "self-generating atmosphere". The pressure can range from high vacuum or controlled vacuum, through ambient, to elevated and high pressure; the latter is hardly practical due to strong disturbances.

The commonly investigated processes are: thermal stability and decomposition, dehydration, oxidation, determination of volatile content and other compositional analysis, binder-burnout, high-temperature gas corrosion etc. The kinetic data obtained by TG are reliable only for irreversible processes, whereas reversible ones are grossly affected by diffusion, and only special procedures can handle them. Although many industrial processes could benefit from thermogravimetric investigations, the industry is often discouraged by the natural discrepancies between the data produced by milligram-size samples, and those of the bulk processes. In this respect gram-size and larger TG samples are more suitable for optimization research of industrial processes.

The conventional TG focuses on various aspects of ANALYSIS of materials; the other facet of thermogravimetry is studying SYNTHESIS, e.g. using a thermobalance to monitor the making of materials. The industrial processes of chemical vapor deposition (CVD), chemical vapor infiltration (CVI), metallurgical carburization, synthesis of carbon-carbon composites can greatly benefit from modeling them with large-sample TG instruments.

Other Suggestions for edits
When searching for Thermogravimetric analysis using the search term "TGA," I am directed to a page unrelated to Thermogravimetric analysis. Can a disambiguation page be created for "TGA"?

The ‘high temperature oxidation’ hyperlink is a broken link so either it needs a new article written for that subject or the link itself should be removed. —Preceding unsigned comment added by 68.66.17.81 (talk) 07:53, 15 December 2010 (UTC)

Missing figures
it appears that the commons-delinker bot has removed all of the deleted figures used in this article.. making it pretty useless for anyone trying to learn how to interpret such data. there are several references to "figures" in the article which have been removed >< -176.26.107.213 (talk) 10:25, 3 April 2014 (UTC)

Major Rewrite
These are the notes from my rewrite.

At isothermal conditions (constant temperature) the proper term is a gravimetric analysis, not thermogravimetric analysis.

If changes in mass are measured it is DTG (differential thermal analysis) not TGA.

TGA can provide information on any kind of thermal phase change.

Information was withdrawn because it looks like original research - not many citations for article given its age
Some researchers have been studying ways in which to protect certain oligomers or polymers from oxidation processes. One example is inserting an oligomer into a multiblock copolymer. An example is the TGA traces of both the oligomer and the oligomer/multiblock copolymer in N2 and in air. When the TGAs were run under a nitrogen atmosphere, there is no oxidation of the substrate. When the TGA of the oligomer was run under air, an oxidation process can be seen between 200 °C-350 °C. This process is not seen for the oligomer/multiblock copolymer. The authors of this paper explained this disappearance by suggesting that the oxidative process involved hydroxyl end groups on the oligomer. The encasing of the oligomer by the multiblock copolymer prevented this from happening.

This information was pulled because the is citing trade literature and because it is too specific for a general article
Regardless of the furnace programming, the sample is placed in a small, electrically heated furnace equipped with a thermocouple to monitor accurate measurements of the temperature by comparing its voltage output with that of the voltage-versus-temperature table stored in the computer’s memory. A reference sample may be placed on another balance in a separate chamber. The atmosphere in the sample chamber may be purged with an inert gas to prevent oxidation or other undesired reactions. A different process using a quartz crystal microbalance has been devised for measuring smaller samples on the order of a microgram (versus milligram with conventional TGA).

This information was removed because it was either redundant or more applicable to an article on DTA.
If the identity of the product after heating is known, then the ceramic yield can be found from analysis of the ash content (see discussion below). By taking the weight of the known product and dividing it by the initial mass of the starting material, the mass percentage of all inclusions can be found. Knowing the mass of the starting material and the total mass of inclusions, such as ligands, structural defects, or side-products of reaction, which are liberated upon heating, the stoichiometric ratio can be used to calculate the percent mass of the substance in a sample. The results from thermogravimetric analysis may be presented by (1) mass versus temperature (or time) curve, referred to as the thermogravimetric curve, or (2) rate of mass loss versus temperature curve, referred to as the differential thermogravimetric curve. Though this is by no means an exhaustive list, simple thermogravimetric curves may contain the following features:


 * A horizontal portion, or plateau that indicates constant sample weight
 * A curved portion; the steepness of the curve indicates the rate of mass loss
 * An inflection (at which $$\frac{dw}{dt}$$ is a minimum, but not zero)

Certain features in the TGA curve that are not readily seen can be more clearly discerned in the first derivative TGA curve. For example, any change in the rate of weight loss can immediately be seen in the first derivative TGA curve as a trough, or as a shoulder or tail to the peak, indicating two consecutive or overlapping reactions. Differential TGA curves also can show considerable similarity to differential thermal analysis (DTA) curves, which can permit easy comparisons to be made.

This test is not specific to TGA. Any muffle furnace can be used to perform this test...
Ceramic yield is defined as the mass percent of starting material found in the end product. From this, stoichiometry can then be used to calculate the percent mass of the substance in the sample.

Metal aluminates (MAl2O4) are an important type of mixed-cation oxide ceramics that have many applications. The metal aluminate CaAl2O4 is used in the cement industry as a hydraulic material. Its precursor is CaAl2C18H37O9N3. The formation of CaAl2O4 occurs during the thermogravimetric analysis. This is how the theoretical ceramic yield is calculated for this example:


 * 1) Calculate molecular weight of CaAl2O4: $$ 40.078 + (2\times 26.982) + (4\times 15.999) = 158.038~\text{g/mol} $$
 * 2) Calculate molecular weight of CaAl2C18H37O9N3: $$ 40.078 +(2\times 26.982) + (18\times 12.011) + (37\times 1.008) + (9\times 15.999) + (3\times 14.007) = 533.548~\text{g/mol} $$
 * 3) Calculate the percentage that CaAl2O4 is of CaAl2C18H37O9N3: $$\frac{\text{molecular weight of CaAl}_2\text{O}_4}{\text{molecular weight of CaAl}_2\text{C}_{18}H_{37}\text{O}_9\text{N}_3} \times 100 = \frac{158.038~\text{g/mol}}{533.548~\text{g/mol}} \times 100 = 29.6\%$$

Therefore, the theoretical ceramic yield for the thermogravimetric analysis of CaAl2C18H37O9N3 is 29.6%. This correlates well with the experimentally determined ceramic yield of 28.9%.

As another example of calculating theoretical ceramic yield, take the TGA of calcium oxalate monohydrate. Using the same process detailed above, the theoretical ceramic yield can be calculated: the formula weight of calcium oxalate monohydrate is 146 g/mol. The final ceramic product is CaO, with a formula weight of 56 g/mol. The theoretical ceramic yield is therefore 38.4%. The actual yield from the TGA was found to be 39.75%. Some reasons for discrepancies between the theoretical and actual yields are trapped CO2 and the formation of metal carbides.

In the TGA trace of calcium oxalate monohydrate, the first mass loss corresponds to loss of water of hydration. The second mass loss corresponds to decomposition of dehydrated calcium oxalate to calcium carbonate and carbon monoxide and carbon dioxide. The last mass loss is due to the decomposition of calcium carbonate to calcium oxide and carbon dioxide.

The differences between thermograms can be seen in the example of four different chloro-polymers: (a) polyvinyl chloride, (b) chlorinated polyvinyl chloride, (c) chlorinated rubber, and (d) polyvinylidene chloride. There are two stages of degradation in these four polymers. The first stage is the loss of hydrogen chloride, and is complete around 250 °C. This first step occurs at lower temperatures for the polymers containing more chlorine (chlorinated polyvinyl chloride, chlorinated rubber, and polyvinylidene chloride), implying that these chloride groupings are less stable than in polyvinyl chloride.

The second stage is the carbonization of the polymer, and takes place between 250 °C and 500 °C. This is seen by the large loss of mass between 250 °C and 500 °C. Tar and simple gases, such as hydrogen and methane, are evolved and the carbon that remains loses very little mass between 500 °C and 900 °C. In this second stage, the higher the chlorine content of the polymer, the lower the yield of tar. This is because chlorine is able to remove hydrogen, which would otherwise be used in the compounds that form tar.

This information is certainly interesting; however, it seems to drift off topic. This is a highly specific example when many such examples exists.
High performance fibers can be compared using TGA as an evaluation of thermal stability. From the TGA, polyoxazole (PBO) has the highest thermal stability of the four fibers as it is stable up to ca. 500 °C. Ultra-high-molecular-weight polyethylene (UHMW-PE) has the lowest thermal stability, as it begins to degrade around 200 °C. Often the onset of mass loss is seen more prominently in the first derivative of the mass loss curve. High performance fibers used in bulletproof vests must remain strong enough mechanically so as to protect the user from incoming projectiles. The thermal and photochemical degradation of the fibers causes the mechanical properties of the vests to decrease, effectively rendering the armor useless. Thus, thermal stability is a key property when designing these vests.

Three ways a material can lose mass during heating are through chemical reactions, the release of adsorbed species, and decomposition. All of these indicate that the material is no longer thermally stable. Out of the four fibers shown in the previous example, only Terlon shows loss of adsorbed species, most likely water, as the mass loss occurs after 100 °C. Because the TGA is performed in air, oxygen reacts with the organic fibers which eventually degrade completely, evidenced by the 100% mass loss. It is important to link thermal stability to the gas in which the TGA is performed. PBO, which completely decomposes when heated in air, retains ~60% mass when heated in N2. Thus, PBO is thermally stable in nitrogen up to 630 °C, whereas in air, PBO has almost completely decomposed at that temperature.