ISO 11357-2 Plastics - Differential Scanning Calorimetry (DSC) - Part 2: Determination of Glass Transition Temperature and Step Height

The standard “ISO 11357-2 Plastics - Differential scanning calorimetry (DSC) - Part 2: Determination of glass transition temperature and step height”, published by the International Organization for Standardization (ISO), describes methods for determining the glass transition temperature and glass transition step height of amorphous and partially crystalline plastics.

For the purposes of this standard, the following definitions apply, in addition to the terms and definitions given in ISO 472 and ISO 11357-1:

• Glass transition temperature: This refers to the characteristic value of the temperature range at which glass transition occurs. The assigned glass transition temperature can vary depending on the specific property and the method and conditions chosen to measure it.
• Glass transition state height: This refers to the difference in specific heat capacity between the upper and lower extrapolated baselines at the glass transition temperature. For partially crystalline polymers, the glass transition state height is proportional to the amorphous content.

In principle, the difference in heat flow rate between the sample and the reference crucible is measured as a function of temperature and time, while the sample and reference are subjected to the same temperature control program in a specific atmosphere. Two types of differential scanning calorimetry (DFS) can be performed: heat flux DSC and power-compensated DSC. The change in heat flow rate is recorded as a function of temperature, and the glass transition temperature and step height are determined from the resulting curve.

In the fields of polymer science and materials engineering, precise thermal analysis is essential for characterizing the behavior of plastics under temperature variations. One of the critical properties is the glass transition temperature, which is the point at which an amorphous polymer transitions from a hard, glassy state to a soft, rubbery state. Accurate measurement of the glass transition temperature ensures product reliability in applications ranging from packaging films to automotive components.

ISO 11357-2, part of the broader ISO 11357 series of standards on plastics and differential scanning calorimetry (DSC), provides a standardized method for determining glass transition temperature using differential scanning calorimetry.

This standard applies to amorphous and semi-crystalline polymers (excluding fully crystalline polymers without a glass transition temperature). It covers glass transition temperature measurements in a temperature range between -100 degrees and +600 degrees, depending on the capacity of the differential scanning calorimetry device. However, melting points (ISO 11357-3), crystallization (ISO 11357-4), or oxidation induction time (ISO 11357-6) are outside the scope of this standard.

The ISO 11357-2 standard is consistent with other parts of this standard and supports a unified differential scanning calorimetry framework for thermal characteristics.

Differential scanning calorimetry (DSC) measures the differences in heat flow between a sample and a reference as the temperature changes at a controlled rate. In heat flux DSC (the most common), both are in the same furnace, while in power-balanced DSC, separate heaters maintain equal temperatures.

The glass transition temperature is not a sharp peak like melting, but a second-order transition marked by a step change in heat capacity. It appears as a baseline shift on the DSC thermogram due to increased molecular mobility. The glass transition step height, the vertical shift in heat flow, quantifies the change in specific heat capacity. Influencing factors include heating rate, sample history (e.g., annealing), moisture, and molecular weight. The ISO 11357-2 standard standardizes these to minimize variability.

The ISO 11357-2 standard includes a rigorous protocol:

• Sample preparation: Mass, 5-20 mg (usually 10 mg for accuracy). Thin films, powders, or granules are hermetically sealed in aluminum containers to prevent moisture ingress. If hygroscopic, dry samples are conditioned, optionally by preheating, for thermal history erasure purposes.
• Instrument calibration: Temperature is adjusted using high-purity standards (e.g., indium at 156,6 degrees). Basic correction is performed using empty vessels for heat flow.
• Test conditions:
• Heating rate: 10-20 K/min (20 K/min is recommended for standard glass transition temperature, slower rates such as 10 K/min for precision).
• Atmosphere: Inert gas (nitrogen or helium) at 20-50 mL/min.
• Temperature program: Initial heating from 50 degrees below to 50 degrees above the expected glass transition temperature. Cooling at a minimum of 20 K/min to erase past temperature changes. Second heating is repeated to achieve a repeatable glass transition temperature (mandatory for semi-crystalline polymers).
• Data analysis:
• Methods for determining glass transition temperature (midpoint preferred):
• Starting point (extrapole): The intersection of the tangents to the baselines after the previous or subsequent step.
• Midpoint: Half the height of the step (most common according to ISO).
• Turning point: Point of maximum slope.
• Glass transition step height calculation: The heat flow difference between the extrapolated baselines is normalized according to mass and velocity.
• Software usually automates this process, but manual verification is recommended.
• Reporting: The method, along with glass transition temperature values, thermogram with marked points, and uncertainties (typically plus/minus ±1-2 degrees for glass transition temperature) are reported.

The main application areas of the ISO 11357-2 standard are as follows:

• Material Selection: The glass transition temperature determines the service temperature.
• Quality control: Batch variations or spoilage are detected.
• R&D: Additives, mixtures, or curing processes are investigated (e.g., increasing the epoxy glass transition temperature through cross-linking).
• Industries: Automotive (instrument panels), electronics (PCBs), medical devices (sterilizable plastics).

In short, the ISO 11357-2 standard is an indispensable tool for reliable glass transition temperature measurement by promoting global consistency in polymer characterization. This leads to the development of safer products.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services to businesses in accordance with the "ISO 11357-2 Plastics - Differential scanning calorimetry (DSC) - Part 2: Determination of glass transition temperature and step height" standard.