1. INTRODUCTION:

The study of thermal stability and flammability of materials is essential for determining their performance and ensuring safety, particularly in environments involving high temperatures or fire hazards^1-10.

To characterize these properties, a range of analytical methods is employed, spanning from small-scale thermal assessments conducted in controlled laboratory settings to more complex, full-scale fire testing scenarios^11-20.

Each method offers specific insights into material behaviour under thermal or combustion conditions, such as heat release, combustion efficiency, decomposition profile, ignition delay, and material mass loss.

1.1. Small-scale thermal analysis tests:

Multiple thermal analysis methods are utilized to evaluate the thermal stability of different substances^21-27. These approaches involve measurement of variations in physical or chemical properties of a substance as a function of time or temperature, while the material is subjected to a precisely controlled heating program in a specific gaseous environment. Depending on the desired testing conditions, atmospheres such as nitrogen, helium, argon, or oxygen can be employed. Extensive literature exists on the principles and applications of thermal analysis techniques^5,13,28-30. These techniques are typically classified based on the type of property measured during thermal treatment. The most commonly used thermal methods include thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC).

1.2. Flammability Tests:

In order to measure different fire response characteristics of materials, various industrial flammability tests have been designed such as UL-94 flammability test, limiting oxygen index test (LOI), and Cone-Calorimetry test etc.

(i) Pass/fail tests (UL-94 and Limiting Oxygen Index (LOI)):

The LOI and UL-94 methods serve as standard procedures to assess the flammability behaviour of substances. These tests offer a basic qualification either pass or fail based on amount of oxygen required and flame resistance, making them suitable for preliminary evaluation but not for generating detailed quantitative data^9,31-34.

UL-94 test:

The UL-94 standard has been developed by Underwriters Laboratories is a defined criterion to evaluate how materials respond to ignition, flame spread, and extinguishment after exposure to a controlled flame. This assessment helps to determine whether a material could be safely incorporated into appliances or electronic devices from a fire safety perspective¹¹.

Limiting Oxygen Index (LOI) test:

The Limiting Oxygen Index (LOI) test determines the lowest concentration of oxygen in a nitrogen–oxygen atmosphere that could sustain the combustion of a sample. Materials with higher LOI values indicate enhanced resistance to ignition. The materials satisfying LOI values above 28% are classified as flame retarded or self-extinguishing.^32,33,35,36

(ii) Bench-Scale Test (Cone Calorimetry):

Cone Calorimetry is considered one of the most reliable and widely used bench-scale method for the quantitative evaluation of material flammability. It is designed to simulate real fire scenarios in a controlled laboratory setting and provides critical data on parameters such as heat release rate (HRR), time to ignition (tig), total heat released (THR), and mass loss rate (MLR). These parameters are essential for understanding the combustion characteristics of materials. Due to its accuracy and reproducibility, cone calorimetry is often used as a standard technique for predicting fire performance and assessing fire safety.^1,15,37,38

This review aims to provide a comprehensive overview of widely used thermal and flammability testing techniques, emphasizing their principles, applications, and relative advantages in evaluating the thermal stability and fire performance of materials.

2.1 THERMAL ANALYSIS:

2.1.1 Thermogravimetry (TGA):

TGA determines how the mass of a sample alters with temperature changes under a stable and regulated atmospheric environment. This method helps in understanding the thermal behaviour of materials. The results are displayed as a curve that shows the percentage of weight remaining on the vertical axis and temperature or time on the horizontal axis^7,39.

A standard TGA system (Figure 1) includes a high-accuracy balance, a controlled heating furnace, and a unit for directing the flow of gas. The sample, usually in milligram quantity, is placed in a small crucible or pan made of inert material such as platinum or alumina. This pan is suspended from the balance, which continuously measures the mass of the sample as it is heated.

The furnace provides a controlled heating environment, often programmed with a specific temperature ramp rate (e.g., 10°C/min). The analysis is usually conducted under a flow of inert gas such as nitrogen or argon to produce a non-reactive atmosphere that prevents oxidative degradation unless oxidative conditions are specifically required. The gas flow system efficiently removes volatile decomposition products, which help in maintaining the precise measurement of sample mass. Mass changes in the sample are tracked throughout the heating process, correlating with temperature or elapsed time. In complex materials like composites or multi-component systems, different constituents degrade at characteristic temperatures. This results in distinct steps on the TGA curve, each corresponding to the breakdown of a particular component. These mass loss events provide valuable information about the composition, thermal stability, moisture content, and filler content of the material^{5,10,13,38-43}. To gain better insight into the decomposition stages, a Derivative Thermogravimetric (DTG) curve is employed. This curve is obtained by plotting the change in weight loss per unit temperature (dW/dT) on the vertical axis against temperature (T) on the horizontal axis. When the sample's weight remains unchanged, the DTG line remains flat. Sharp signals on the DTG curve give the temperatures at which significant decomposition events occur.

Modern TGA instruments may also be coupled with evolved gas analysis (EGA) techniques such as FTIR or mass spectrometry to identify the gases released during decomposition, offering deeper insight into degradation mechanisms^40,44,45. However, one limitation of TGA is that it cannot detect thermal changes where there is no weight loss, such as melting, glass transition, or crystallization. For those types of transitions, other methods like Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC) are used.

Figure 1. Schematic diagram of TG instrument⁴³.

2.1.2. Differential thermal analysis (DTA)

In differential thermal analysis (DTA), the temperature difference between a test sample and an inert reference material is monitored while both are exposed to the same heating or cooling conditions under a controlled temperature rate. The technique works on the principle that the sample when heated undergoes various physical or chemical transformations that involve either heat absorption or heat release. This leads to a temperature difference between the sample and the reference material. The data obtained is displayed in the form of a DTA thermogram, which plots the temperature difference (∆T) against either time or temperature. When the sample undergoes an endothermic event, it absorbs heat, resulting in a temperature lower than the reference, which appears as a downward peak on the thermogram. Conversely, during an exothermic reaction, the sample releases heat, causing its temperature to exceed that of the reference and generating an upward peak on the graph. These thermal signals are indicative of various transitions in the material, such as glass transition, oxidation, sublimation, melting, phase changes, crystallization, or decomposition^39,41-43,46. Although DTA is one of the earlier techniques developed for thermal analysis, it is mainly suitable for qualitative evaluation. This is due to the limited sensitivity of its components, particularly the sample holders, which are normally made from alumina (Al_₂O_₃) and contain built-in thermocouples.

Nowadays, TGA-DTA systems are used where both weight changes and thermal transitions need to be monitored simultaneously^{38,39,41,42,47}. In addition to the standard TGA components such as a sensitive microbalance, sample holder, and furnace, the combined TGA-DTA setup includes several additional elements to enable differential thermal analysis. One main addition is the inert reference holder placed alongside the sample holder, which is essential for detecting temperature differences between the sample and a thermally stable reference. The system also includes dual thermocouples near the sample and the reference to accurately measure the temperature difference (∆T) that occurs during heating or cooling. To support simultaneous TGA and DTA analysis, the data acquisition system is also improved. It records both the mass change and the differential temperature signal in real time, generating overlapping TGA and DTA curves for more detailed interpretation.

2.1.3 Differential Scanning Calorimetry (DSC):

Differential Scanning Calorimetry (DSC) is one of the most widely adopted techniques for studying the thermal behaviour of materials^{12,30,41,48,49}. Compared to Differential Thermal Analysis (DTA), DSC offers higher sensitivity and more advanced capabilities. It operates primarily in two configurations: power-compensated DSC and heat-flux DSC.

The power-compensated mode involves placing the sample and reference in distinct furnaces, each having its own electric heater. The system records the difference in electrical power supplied to both pans to maintain equal temperatures, and this difference is tracked with respect to temperature or time. In contrast, the heat-flux DSC places both the sample and reference within a single furnace, allowing heat to flow through a low-resistance path between them. Here, the difference in heat flow needed to keep both at the same temperature is measured over time or temperature.

The difference in heat flow observed between the sample and the reference corresponds to the quantity of heat gained or lost by the sample throughout thermal events. Among the two modes, the power-compensated DSC generally delivers more precise results than the heat-flux mode^48,49.

DSC is used to evaluate the heat changes^12,48 that occur during various transitions such as glass transition (T_g), crystallization, melting, curing, sublimation, decomposition, and oxidation (Figure 2). The resulting data is usually shown as a curve with heat flux on the vertical axis and either temperature or time on the horizontal axis. This enables direct measurement of important properties like specific heat capacity, enthalpy changes etc.

The change in enthalpy during a transition is obtained by applying the formula ∆H = K × A, in which the peak area (A) on the DSC curve is multiplied by the calorimetric constant (K). This constant varies depending on the instrument and is typically established by testing standard materials with known transition enthalpies¹².

Figure 2. The possible transitions in a DSC curve ^12,29,48,50

2.1.4. Comparison of Thermal analysis techniques: TGA, DTA, and DSC:

Table 1 presents a comparison of three important techniques used in thermal analysis: Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Differential Scanning Calorimetry (DSC). Each method is explained individually below, including how it works, where it’s used, and what makes it useful.

Thermogravimetric Analysis (TGA): TGA is used to find out how the weight of a material changes when it is heated over time. As the temperature rises, the sample may lose moisture or other components, which shows up as a loss in weight. This technique is helpful in checking thermal stability, identifying when materials break down, and measuring the amount of fillers or ash present. The weight changes are recorded with respect to temperature to show how the material degrades step by step.

Differential Thermal Analysis (DTA): In DTA, both the sample and a reference substance are heated together under the same conditions. When the sample undergoes a physical or chemical change like melting or reacting, its temperature becomes different from the reference. These temperature differences are shown as peaks on a graph. DTA helps in finding the temperatures at which these changes occur, but unlike DSC, it doesn’t directly measure how much heat is absorbed or given off.

Differential Scanning Calorimetry (DSC):

DSC measures the heat taken in or released by a sample during heating, cooling, or while holding it at a constant temperature. It gives accurate data about thermal transitions like melting, crystallization, or the glass transition. Because it directly tracks heat flow, DSC is more precise and sensitive than DTA. It is commonly used in industries and research involving polymers, medicines, and food materials.

2.2.2. Limiting Oxygen Index (LOI):

The Limiting Oxygen Index (LOI) test is widely regarded as a reliable technique for assessing the flammability characteristics of substances. Originally proposed by Fenimore and Martin in 1966, this method has been standardized internationally under ASTM D2863 (USA), NFT 51-071 (France), and ISO 4589.^32,33,36,37

The Limiting Oxygen Index (LOI) represents the minimum percentage of oxygen in an oxygen–nitrogen (O₂/N₂) mixture required to sustain combustion of a vertically oriented sample. This value is determined based on either of the following criteria: (a) the specimen must burn through at least 5 centimeters when ignited from the top, or (b) it should maintain continuous downward burning for a minimum duration of three minutes.

The test apparatus consists of a heat-resistant glass column mounted on a metallic base usually made up of brass. A clamp is used to suspend the sample vertically in the middle of the tube. At the top, a flame source is placed to ignite the specimen. The system is also equipped with a timer, gas flow meter, and control valves to regulate the gas mixture. During the test, a slow stream of an oxygen-nitrogen mixture is introduced from the bottom of the tube. Once ignited, the oxygen concentration is gradually adjusted to identify the minimum level at which the material continues to burn under the test conditions. This lowest level of oxygen that supports sustained burning is measured as the Limiting Oxygen Index (LOI). The apparatus configuration for the LOI experiment is schematically shown in Figure 4.

Figure 4. Schematic diagram of LOI apparatus.

The LOI value is calculated as:

Materials with higher Limiting Oxygen Index (LOI) values demonstrate enhanced flame resistance. Based on their LOI values, substances are categorized into different flammability classes. Since atmospheric air contains approximately 21% oxygen by volume, any material with LOI value less than 21% is considered highly flammable, as it can easily ignite and sustain combustion in ambient air. Materials exhibiting LOI values in the range of 21% to 28% are generally considered to have low flammability and burn slowly, whereas those with LOI values exceeding 28% are regarded as self-extinguishing, reflecting enhanced resistance to combustion^32,33,35,36.

2.2.3. Cone–Calorimeter:

The cone calorimeter is widely regarded as one of the most important bench-scale instruments for quantitatively assessing the flammability characteristics of materials^2,14,15,37. The heat release rate (HRR) during combustion is the principal parameter measured using this equipment, which is standardized in accordance with ISO 5660-1.

The HRR is determined based on the principle that heat released is directly proportional to oxygen consumption during burning⁵³. The instrument also analyzes combustion gases and quantifies the amount of smoke generated from a specimen subjected to a specific external heat flux. The setup includes a conical radiant heater, typically set to deliver a specific heat flux in the range 25–100kW/m², which uniformly irradiates the surface of a horizontally placed sample. The entire system operates under a controlled airflow to simulate real fire ventilation conditions, and the data collected provides key information about the flammability and fire growth characteristics of the tested material. Key parameters evaluated by the Cone-Calorimeter include heat release rate (HRR), time to ignition (tig), total heat release (THR), and mass loss rate (MLR)^15,37. Additionally, the HRR data is further assessed by calculating values such as peak HRR, average HRR, and time to peak HRR. The cone calorimeter also provides measurements of smoke production and the release rates of carbon monoxide (CO) and carbon dioxide (CO₂).

2.2.4. Comparison of Flammability Tests: LOI, UL-94, Cone-calorimeter:

Each flammability test is designed to assess specific fire-related properties, and their results may not always align.

The Limiting Oxygen Index (LOI) test measures the lowest concentration of oxygen that can support burning. It helps identify how easily a material catches fire in a controlled oxygen environment. Materials with LOI values below 21% are considered easily flammable in air, while those with higher values may resist burning. While easy to perform, it doesn’t reflect real fire situations where temperature and airflow vary.

The UL-94 vertical burning test checks whether a material continues to burn after flame exposure and whether it drips flaming particles. It gives ratings like V-0, V-1, and V-2 based on how long the flame lasts and whether drips ignite cotton placed below. However, it does not provide data on heat release, smoke, or gases produced during burning.

The Cone Calorimeter test is more detailed. It exposes the sample to a fixed heat source and measures key fire-related properties like time to ignition, heat release rate, total heat released, smoke production, and emission of gases such as CO and CO₂. It offers a more detailed picture of how a material behaves in a real fire-like environment although it is more complex and expensive to perform.

Each test has its advantages and limitations. LOI and UL-94 are useful for basic classification and screening, especially in quality control, but they lack the ability to predict real fire performance. The Cone Calorimeter is more advanced and provides comprehensive data needed for accurate fire risk assessment. For this reason, materials are often evaluated using a combination of these tests to get a complete understanding of their flammability and fire safety performance. Table 3 outlines the essential parameters and comparative differences among the LOI, UL-94, and Cone Calorimeter techniques.

Table 3: Comparison of flammability tests: LOI, UL-94, Cone Calorimeter^36-38,54

Feature	LOI Test	UL-94 Test	Cone Calorimeter
Test Scale	Small-scale	Small-scale	Bench-scale
Orientation	Vertical	Horizontal and Vertical	Horizontal
Flame Source	Varies oxygen level	Small flame applied twice	Radiant heat, constant exposure
Measured Parameters	Oxygen concentration to sustain flame	After-flame time, dripping, rating	HRR, ignition time, THR, smoke, CO/CO₂
Ventilation	Limited	Poor	Well-ventilated
Simulation of Real Fire	Low	Low	High
Dripping Behaviour Effect	Not considered	Affects rating	Dripping not relevant
Usefulness	Basic classification	Safety screening	Detailed fire performance analysis

3. CONCLUSION:

This review has outlined the principles, instrumentation, and applications of widely used analytical techniques (TGA, DTA, and DSC) and flammability tests (LOI, UL-94, and Cone Calorimeter). Together, these methods provide complementary information essential for understanding how materials respond to heat and fire. Through a comparative examination of both analytical and flammability test methods, this review underlines the role of thermal and fire behaviour assessments in material research and development. Techniques such as TGA, DTA, and DSC help analyse the thermal transitions and decomposition characteristics of materials, while LOI, UL-94, and cone calorimeter tests provide essential data on ignition, flame spread, and heat release. Integrating thermal analysis with fire testing is crucial for designing safe, high-performance materials suited for demanding applications. Recognizing the strengths and limitations of each method enables researchers and engineers to choose appropriate testing strategies based on safety requirements and practical needs.

4. CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this manuscript.

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Received on 04.08.2025 Revised on 04.09.2025

Accepted on 22.09.2025 Published on 06.11.2025

Available online from November 11, 2025

Asian J. Research Chem.2025; 18(6):392-400.

DOI: 10.52711/0974-4150.2025.00060

This work is licensed under a Creative Commons Attribution-Non Commercial-Share Alike 4.0 International License. Creative Commons License.

Parameter	TGA	DTA	DSC
Measured Property	Measures weight change as a function of temperature or time (e.g., due to decomposition, oxidation)	Detects temperature differences between sample and reference due to phase transitions or reactions	Heat flow associated with thermal transitions (endo-/exothermic)
Type of Information	Thermal stability, composition, moisture, decomposition patterns	Phase transitions, reaction temperatures, thermal events	Glass transition, melting, crystallization, curing behaviour
Sample Requirement	Solid or liquid (few mg)	Solid or liquid (few mg)	Solid or liquid (few mg)
Data Output	Mass vs. temperature or time curve	Temperature difference vs. temperature or time	Heat flow vs. temperature or time
Reference Material	Not required	Required	Required
Sensitivity	High for mass change	Moderate	High for heat flow
Quantitative Capability	Yes (mass change)	Mostly qualitative	Both qualitative and quantitative
Coupling with Other Tools	Easily coupled with FTIR, MS, or GC to analyze evolved gases	Often used with TGA	Often combined with TGA for advanced analysis
Application Examples	Thermal stability of polymers, degradation kinetics, filler content	Detection of phase transitions in ceramics and polymers	Determination of glass transition temperature (Tg), melting point, crystallization
Output Units	Weight (%), temperature (°C)	Temperature difference (°C), temperature (°C)	Heat flow (mW), temperature (°C)

UL-94HB	UL-94V
UL-94HB	Criteria conditions	V-0	V-1	V-2
(i) Specimen must stop burning before 100mm mark.	Afterflame time (t₁) after I^st application of flame	≤ 10 sec	≤ 30 sec	≤ 30 sec
	Afterflame time (t₂) after 2^nd application of flame	≤ 10 sec	≤ 30 sec	≤ 30 sec
	t₁+t₂ (for the five specimens)	≤ 50 sec	≤ 250 sec	≤ 250 sec
(ii) Flame spread rate must be less than 40 mm/min for thickness between 3 to 13 mm.	Afterflame plus afterglow time (t₂+ t₃) after 2^nd application of flame	≤ 30 sec	≤ 60 sec	≤ 60 sec
(iii) Flame spread rate must be less than 75 mm/min for thickness less than 3 mm.	Flaming drips ignite cotton	No	No	Yes
	Afterflame or afterglow combustion up to holding clamp	No	No	No