THERMOGRAVIMETRY ANALYSIS [TGA] AS PER PCI

TGA THE KNOWN THE THERMOGRAVIMETRY ANALYSIS THE GLASS TRANITION BASED METHODS

1. THERMOGRAVIMETRY ANALYSIS (TGA)

2. THERMAL ANALYSIS TECHNIQUES are those in which a physical property of a substance or its reaction products is measured as a function of temperature. Usually, the substance is subjected to a controlled program during the analysis.

3. Sr.no Name of technique Abbreviation of the technique Instrument employed Parameter measured Graph 1. Thermogravimetry TG Thermobalance Mass Mass vs. Temp or Time 2. Differential Thermal analysis DTA DTA apparatus ∆T ∆T vs .Temp 3. Differential Scanning Calorimetry DSC Calorimeter dH . dt dH / dt vs. Temp

4. THERMOGRAVIMETRIC ANALYSIS (TGA) Universally accepted definition is: ‘‘It is a technique whereby the weight of a substance, in an environment heated or cooled at a controlled rate , is recorded as a function of time or temperature.’’ From the above definition , it follows that the basic requirements are a method of heating (or cooling ) and a means of weighing. There are three types of TGA:- a) Isothermal or static TGA: - In this technique the sample weight is recorded as a function of time at constant temp.

5. b) Quasi-static TGA:- In this technique the sample is heated to constant weight at each of a series of increasing temperatures. c) Dynamic TGA:- In this technique , the sample is heated in an environment whose temp is changing in a predetermined manner, generally at a linear rate.

6. In a TGA, the mass of a sample in a controlled atmosphere is recorded continuously as a function of temp or time as the temperature of the sample is increased (usually linearly with time). A plot of mass or mass percentage as function of time is called a thermogram or a thermal decomposition curve. TGA curves are characteristic of a given compound or material due to the unique sequence of physical transitions and chemical reactions that occur over definite temperature ranges.

7. The rates of these thermally induced processes are often a function of the molecular structure. Changes in weight result from physical and chemical bonds forming and breaking at elevated temperatures.

8. INSTRUMENTATION: It consists of :- The balance Sample holders The furnace Recorder Thermobalance

9. 1. THE BALANCE :- Requirements of a good balance- Its accuracy , sensitivity, reproducibility and capacity should be similar to those of analytical balance. It should have an adequate range of automatic weight adjustment. It should have a high degree of mechanical and electronic stability. It should have a rapid response to weight changes. It should be unaffected by vibration. The balance should be simple to operate and versatile. Recorder balances are of two types: 1. Deflection-type instruments and 2. Null-type instruments.

10. Deflection Balances: They are of following types: I. Beam Type- There occurs a conversion of beam deflection about the fulcrum. ii. Helical Type- There occurs an elongation or contraction of the spring with weight change. Transducers are used for recording these changes in length of the spring.  iii. Cantilevered Beam- One end of beam is fixed and the other end on which sample is placed is free to undergo deflections. iv. Torsion Wire- Beam is attached to a taut wire which acts as a fulcrum.

11. Null-Point Balances : It consists of a sensor which detects the deviation from the null point and restores the balance to its null point by means of a restoring force; either electrical or mechanical weight loading is applied to the beam.

12. 2.SAMPLE HOLDERS :- There are different varieties of crucibles used. Some differ in shape and size while some differ in materials used. They are made from platinum, aluminum, quartz or alumina and some other materials like graphite, stainless steel, glass, etc Different types of sample holders are as follows- 1. Shallow pans- These are used for such samples in which diffusion is the rate controlling step. Volatile substances produced during reaction must escape out which is determined as weight loss.

13. 2. Deep Crucibles: These are used in such cases where side reactions are required such as in study of industrial scale calcinations, surface area measurements, etc. 3. Loosely covered Crucibles: These are used in self-generated atmospheric studies. Rate of temperature or weight loss is not important because the studies are done isothermally. 4. Retort Cups: These are used in boiling point studies. It provides single plate of reflux for a boiling point determination. Different types of crucibles are used for different materials i.e. Flat crucibles with small lip are used for powdered sample whereas walled crucibles are used for liquid samples. Therefore, the form of crucibles used will determine the temperature gradients in sample.

14. 3. FURNACE:- The furnace and control system should be designed to produce a linear heating rate over the whole working range of furnace. The choice of furnace heating element and type of furnace depend the temp range being studied. For 1100˚C – Kanthal or Nichrome wire. For 1100˚C to 1500˚C- Platinum , alloy of platinum or rhodium For 1500˚C to 1750˚C-Platinum-rhodium alloy Above 1750˚C- Tungsten or molybdenum

15. 4. TEMPERATURE MEASUREMENT- i . Thermocouple is placed near the sample container and has no contact with sample container. This arrangement in not preferred in low-pressures. ii. The sample is kept inside the sample holder but not in contact with it. It responds to small temperature changes only. iii. Thermocouple is placed either in contact with sample or with sample container. This method is best and commonly employed. Position of thermocouple in a Thermobalance

16. 5. RECORDER : The recording systems are mainly of 2 types: 1. Time-base potentiometric strip chart recorder. 2. X-Y recorder. In some instruments, light beam galvanometer, photographic paper recorders or one recorder with two or more pens are also used. In the X-Y recorder, we get curves having plot of weights directly against temperatures. However, the percentage mass change against temperature or time would be more useful.

17. 6. THERMOBALANCE- A good thermobalance should have- Should be capable of continuously recording the weight changes. Should cover a wide range of temp. Temp should be recorded to an accuracy of better than ±1% Recorded temp should be ideally the sample temp. 5. The wt. loss should be recorded to an accuracy of ±1%.

18. FACTORS AFFECTING RESULTS- INSTRUMENTAL FACTORS :  HEATING RATE: If a substance is being heated at a fast heating rate, the temperature of decomposition will be higher than that obtained at a slower rate of heating. Eg .:- for a 10% decomposition of polystyrene, the temperatures are : 375˚C for a heating rate of 1˚C / min and 394 ˚C for a heating rate of 5˚C / min

19. EFFECT OF FURNACE ATMOSPHERE: The test samples are generally heated in the presence of an inert gas, in order to remove the gases formed during sample heating and also to prevent the occurrence of any undesirable reactions. The common atmospheres involved in thermogravimetry are as follows: 1. Static air: In this type air from atmosphere is allowed to flow through the furnace. 2. Dynamic air: In this type compressed air from a cylinder is allowed to pass through the furnace at a measured flow rate. 3. Inert atmosphere: Nitrogen gas (oxygen free) is used as inert environment.

20. SAMPLE HOLDER: The geometry of the sample holder can change the slope of TG curve. Sample holders range from flat plates to deep crucibles of various capacities. Materials used in their construction may vary from glass, alumina, and ceramic compositions to various metals and metallic alloys. When the atmosphere is solely the gas, the shape of crucible has no effect on the slope of the curve. Generally shallow dish is preferred as there is a rapid exchange of gases between sample and the surrounding atmosphere.

21. SAMPLE CHARACTERISTICS : Weight of the sample : If a large sample is used, there occurs a deviation from linearity as the temperature rises, especially for an exothermic reaction. Eg .: evolution of CO during decomposition of calcium oxalate to CaCO3. Sample particle size : With the particle size of smaller dimension the decomposition takes place earlier, while with greater particle size the decomposition proceeds only at higher temperatures. Previous history of the sample: Eg .: TG studies showed that Mg(OH)2 prepared by precipitation method has a different temperature of decomposition from that of the naturally occurring material. This factor shows that one should be sure

22. Heat of reaction : This effect was studied by Newkirk. The heat of reaction alters the difference between the sample temperature and the furnace temperature. If the heat effect is exothermic or endothermic, this will cause the sample temperature to lead or lag behind the furnace temperature. Compactness of the sample : A compressed sample will decompose at higher temperatures than a loose sample.

23. ADVANTAGES AND DISADVANTAGES:- • Any type of solid can be analyzed, with minimal sample preparation (e.g. powders, pellets, chunks, flakes etc) • Minimum sample size ( at least 0.1mg) • Qualitative or quantitative analysis • Solid (or initially solid) samples only • Data interpretation not always straightforward. Analysis in combination with other techniques is often helpful.

24. APPLICATIONS: • Thermal stability/degradation investigation of organic or inorganic materials, e.g. polymers, composites, glasses, metals, minerals etc. • Thermal stability/degradation investigations in inert or oxidative atmospheres, or in vacuum • Determination of organic/inorganic content of mixtures • Curing kinetics ( e.g , adhesives, polymers) • Chemical composition measurements (using appropriate reference standards, accurate quantification of sample composition can be determined • Phase transition measurement (e.g. glass transition, clustering, crystallinity , melting point)

25. Thermal Stability Material characterization Compositional analysis Used to analyze filler content in polymers; carbon black in oils; ash and carbon in coals. Kinetic Studies Corrosion studies Automatic Thermogravimetric Analysis Evaluation of gravimetric precipitates Evaluation of suitable standards Testing of purity of samples Curie point determination

26. THANK YOU!