Differential Scanning Calorimetry
Differential Scanning Calorimetry (DSC) is the measurement of heat flux with respect to a reference as a function of temperature.
Heat flux occurs in materials most normally when they experience a change in phase:
• melting and freezing
• vaporization and condensation
• sublimation
They may also experience a heat flux when:
• passing through a glass transition
• transitioning between amorphous and crystalline states
• changing polymorphic forms
• changing crystalline states of hydration or solvation
Some of these thermal events require energy or heat to occur and are categorized as being endothermic; e.g. melting, vaporization, sublimation, etc. Other thermal events give off energy or heat when they occur and are categorized as being exothermic; e.g. freezing, condensing, transitioning from an amorphous to a crystalline material, etc.
The transition between polymorphic forms may be either exothermic or endothermic. A glass transition, unlike the normal phase transitions, is considered a second-order transition. Thus, the first derivative of the free energy with respect to chemical potential is continuous, but the second derivative is discontinuous. This is believed to be because of the change in thermal expansion and heat capacity of amorphous materials as they transition from their hard, brittle state into a soft, rubber-like state.
DSC is a multipurpose tool. As such, it can be used to generate several types of data that have applicability to nearly any business sector or application. The most common sectors that use DSC include:
- polymers
- plastics
- rubber
- pharmaceuticals
- carbons (including carbon black, graphite, graphene, and carbon nanotubes)
- fire retardants
- electronics (including solder)
- proteins and biochemical substances
- cosmetics
Though most analysis performed on the DSC instrument involve the measurement of melting points or glass transition temperatures, the versatility of the technique lends itself to a broad array of additional analyses. For instance, plasticizers are often added to plastic in order to decrease its glass transition temperature. Thus, the effect of various plasticizers and their concentrations on the glass transition of a particular system can be studied.
Certain materials are often heated, extruded, and rapidly cooled. This rapid cooling locks the structure of the material in place, even if it is not fully equilibrated to a global minimum in energy. If this material is then placed in the DSC instrument, heated, and cooled slowly, it may assume a different structure, often at a lower energy level. Once the material is reheated, differences in melting points and glass transition temperatures may be evident and are often attributed to the “thermal history” of the sample.
As in TGA, one of the vital components is the gas composition within the calorimeter. Most commonly, an analysis is either performed in an inert nitrogen environment or in an oxidative environment of air. In certain sample types, various pathways or mechanisms that may be present in an oxidative environment are not present in an inert environment.
Another common application of DSC is the measurement of the specific heat capacity of a substance, which is detailed in ASTM E1269 and ISO 11357-4. The specific heat capacity is defined as “the amount of heat required to raise the temperature of a given mass of material a certain amount.” The specific heat capacity of a substance is useful for reactor and cooling system design as well as in research and development.
Advantages
- Small amount of sample needed
- Can measure small energy changes
- Highly customizable to client specific parameters
- Rapid and accurate
Considerations
- Typical heating rates: 10 or 20 K per minute. Other ramp rates can be requested, though a custom calibration may be required
- A standard DSC analysis consists of 2 heating cycles with a cooling cycle in between
- Typical environment: nitrogen, though oxidative environment can also be requested
- PTL cannot do modulated DSC or microcaloimetry
- The sample and decomposition products should not be corrosive
Sample Requirements
For dry materials, 20 to 50 mg minimum is requested for the first submission. For liquids, a minimum of 85 µL is required, 200 µL preferred.
Detection Range
This instrument has been qualified for use from -170 °C to 600 °C.
Data Reported
The glass transition, onset temperature, peak temperature, enthalpy of transition, and selected points of interest are included in the data outputs.
Instrumentation
The DSC instrument used at Particle Technology Labs is the Netzsch DSC 214 Nevio. The instrument has been qualified in compliance with the U.S. Food and Drug Administration’s (FDA) current Good Manufacturing Practices (cGMP) Regulations, 21 CFR Parts 210 and 211 and the United States Pharmacopeia (USP), General Chapters <891> Thermal Analysis and <1058> Analytical Instrument Qualification. When applicable, analyses may be performed per various methods including ASTM D3418, ASTM E2253, ASTM E967, ASTM E968, ASTM E2160, ISO 11357-1, and USP <891>.
