What TGA and DSC each measure — and why the distinction matters

Thermogravimetric analysis (TGA) measures the change in mass of a polymer sample as it is heated or held at a constant temperature, while differential scanning calorimetry (DSC) measures the difference in heat flow between a sample and an inert reference under identical temperature conditions. These two outputs—mass loss and heat flow—are complementary rather than interchangeable, and selecting the wrong technique for a given analytical question can lead to incomplete or misleading characterisation of polymer degradation behaviour.

The fundamental reason both techniques exist is that polymer degradation encompasses two distinct classes of event: those that involve mass loss (volatile product release, chain scission leading to off-gassing, depolymerisation) and those that involve only energetic change without any material leaving the sample (glass transition, crystallisation, melting, oxidative crosslinking). TGA is sensitive to the first class; DSC is sensitive to both, but is the only technique capable of detecting the second. According to guidance published by NIST, selecting the appropriate thermal analysis method requires matching the instrument’s measurand to the physical or chemical process under investigation.

In practical polymer R&D, the question is rarely “TGA or DSC?” but rather “what specific degradation information do I need, and which signal carries it?” A formulation team assessing long-term thermal stability of a polyolefin compound will reach for TGA first. A team characterising the morphological changes that make a semi-crystalline polymer susceptible to hydrolytic degradation will reach for DSC. Understanding the physical basis of each measurement is the prerequisite for making that choice correctly.

How TGA characterises polymer thermal degradation

TGA directly quantifies the rate and extent of mass loss as a polymer is heated, providing a continuous record of sample weight as a percentage of the original mass. The primary outputs used to characterise polymer thermal stability are the onset degradation temperature (Tonset), the temperature at which 5% of mass is lost (T5%), the temperature at which 50% of mass is lost (T50%), and the residual mass at the maximum programme temperature—often referred to as char yield.

These parameters allow polymer formulations to be ranked by thermal stability and enable direct comparison of neat resins, filled compounds, and additive-modified grades. A higher T5% indicates greater resistance to the onset of thermal decomposition. A higher char yield at 800 °C in nitrogen is characteristic of aromatic or ladder polymers and is relevant for flame-retardant applications, where residue formation can act as a barrier to heat and mass transfer. Standards bodies including ISO and ASTM have published standardised TGA protocols (for example, ISO 11358 for general polymer thermogravimetry) that specify heating rates, atmosphere conditions, and sample mass ranges to ensure inter-laboratory comparability.

Atmosphere selection critically affects TGA results for polymers. Running in an inert nitrogen atmosphere isolates thermal decomposition from oxidative pathways, revealing the intrinsic thermal stability of the polymer backbone. Switching to air or oxygen exposes oxidative degradation mechanisms, which typically proceed at lower temperatures than purely thermal decomposition. For polyolefins such as polyethylene and polypropylene, the difference in Tonset between nitrogen and air can exceed 100 °C, reflecting the dominant role of oxidative chain scission in real-world degradation. This atmosphere dependence means that TGA protocols must be specified relative to the intended end-use environment of the material.

TGA is also the standard method for quantifying filler and additive content in polymer compounds. Because inorganic fillers (glass fibre, carbon black, calcium carbonate, silica) are thermally stable at temperatures where the polymer matrix decomposes, the residual mass after complete polymer burnout in air directly reports filler loading. This makes TGA an essential quality-control tool for compounders and converters verifying incoming material specifications.

What DSC reveals that TGA cannot detect

Differential scanning calorimetry detects every thermal event that involves a change in heat flow—including transitions that produce no mass change whatsoever. This is the critical distinction: a polymer can undergo a glass transition, melt, crystallise, or oxidatively crosslink without losing a single milligram of mass, and all of these events are invisible to TGA. DSC is the only standard thermal analysis technique that can characterise these transitions quantitatively.

“A polymer can undergo glass transition, melting, crystallisation, or oxidative crosslinking without losing a single milligram of mass—events that are entirely invisible to TGA but fully resolved by DSC.”

The glass transition temperature (Tg) is arguably the single most important thermal parameter for amorphous and semi-crystalline polymers in the context of degradation susceptibility. Below Tg, polymer chains are in a glassy, rigid state with restricted mobility; above Tg, chain segments become mobile, dramatically increasing diffusion rates for oxygen, water, and other degradants. A polymer with a Tg close to its service temperature is far more susceptible to oxidative and hydrolytic degradation than one whose Tg lies well above the use environment. DSC measures Tg as a characteristic step change in heat capacity, typically reported as the midpoint temperature of the transition.

For semi-crystalline polymers, DSC provides two further parameters directly relevant to degradation: the melting point (Tm) and the degree of crystallinity, calculated from the enthalpy of fusion relative to a 100% crystalline reference value. Higher crystallinity generally correlates with lower permeability to degradants and slower hydrolytic degradation rates, because the crystalline regions restrict chain mobility and reduce accessible surface area for reactive species. DSC is therefore an essential tool for understanding how processing history—cooling rate, annealing, orientation—affects the morphological state of a polymer and, by extension, its long-term durability.

DSC is also used to measure the enthalpy of degradation reactions directly. Oxidative degradation of polyolefins is exothermic; hydrolytic degradation of polyesters and polyamides may show endothermic contributions associated with chain scission and dissolution of crystalline regions. The sign and magnitude of these enthalpy signals provide mechanistic information about the degradation pathway that mass-loss data alone cannot supply.

When to combine TGA and DSC: simultaneous thermal analysis

Simultaneous thermal analysis (STA) instruments measure TGA and DSC signals from the same sample in the same crucible under identical temperature and atmosphere conditions, eliminating the sample-to-sample variability that arises when the two techniques are run separately on different instruments. This is particularly important for heterogeneous polymer systems—blends, composites, and multi-phase copolymers—where small differences in sample preparation can produce measurably different results.

The principal advantage of STA for polymer degradation characterisation is the ability to correlate mass-loss events with their thermal signatures. A step in the TGA curve that coincides with an endothermic DSC peak indicates an evaporative or depolymerisation process; the same mass-loss step coinciding with an exothermic DSC signal suggests oxidative decomposition or crosslinking. This correlation is only unambiguous when both signals are collected simultaneously from the same sample, as advocated in thermal analysis guidance from ASTM International.

STA instruments can also be coupled to evolved gas analysis (EGA) techniques such as Fourier-transform infrared spectroscopy (FTIR) or mass spectrometry (MS) via a heated transfer line. This hyphenated approach—TGA-DSC-FTIR or TGA-DSC-MS—allows the chemical identity of volatile degradation products to be determined at the same time as the mass-loss and heat-flow signals are recorded, providing the most complete single-experiment characterisation of polymer degradation currently available.

Choosing the right technique for your polymer application

Selecting between TGA, DSC, and STA for polymer degradation characterisation requires matching the analytical question to the measurand each technique provides. The decision framework below maps common polymer R&D and quality-control objectives to the appropriate technique, based on the physical and chemical information each instrument is capable of delivering.

Use TGA when you need to:

• Determine onset degradation temperature (Tonset), T5%, or T50% for thermal stability ranking
• Quantify filler, ash, or residue content in a polymer compound
• Measure moisture, solvent, or plasticiser content by mass loss below the decomposition temperature
• Calculate degradation kinetics (activation energy) using multi-rate isoconversional methods (OFW, KAS)
• Compare degradation behaviour under inert versus oxidative atmospheres

Use DSC when you need to:

• Measure glass transition temperature (Tg) and its dependence on formulation or ageing
• Determine melting point (Tm), enthalpy of fusion, and degree of crystallinity
• Measure oxidative induction time (OIT) as a proxy for antioxidant efficacy (ISO 11357-6)
• Characterise exothermic or endothermic contributions to a degradation event
• Detect cold crystallisation or recrystallisation during heating of quenched samples

Use STA (TGA + DSC simultaneously) when you need to:

• Correlate mass-loss steps with specific thermal events (endothermic or exothermic) from the same sample
• Characterise heterogeneous or multi-component polymer systems where sample-to-sample variability is a concern
• Couple with FTIR or MS for evolved gas analysis to identify volatile degradation products
• Generate the most complete single-experiment thermal characterisation dataset for regulatory submissions or publication

For polymer scientists working at the intersection of materials innovation and intellectual property, understanding the capabilities and limitations of each thermal analysis technique is also essential for interpreting patent claims and designing around existing IP. Many patents in the polymer additives, biopolymer, and engineering thermoplastics spaces include TGA and DSC data as primary characterisation evidence. Tools such as PatSnap Eureka allow R&D teams to search and analyse the thermal characterisation data embedded in patent documents, connecting laboratory measurements to the broader innovation landscape. Patent databases maintained by EPO and WIPO contain extensive records of polymer thermal analysis innovations that can inform both research direction and freedom-to-operate assessments.