Dynamic Mechanical Analysis (DMA)
Differential Scanning Calorimetry (DSC)
Thermal Gravimetric Analysis (TGA)
Dielectric Analysis (DEA)
Temperature Monitoring
Cure Monitoring
Optic Fibre Sensing
Fibre Bragg Gratings (FBG)
Ultrasonics
References
This section briefly examines a number of thermal analysis techniques that can be used for characterising physical and chemical changes in polymeric materials through exposure to aqueous environments.
Typical DMA plot of a 2-part epoxy adhesive
Dynamic Mechanical Analysis (DMA), also known as Thermal Mechanical Analysis (TMA) obtains information about the mechanical properties of a material by applying a sinusoidal load to a specimen and measuring the resultant deformation, whilst the sample is subjected to a controlled temperature programme. This technique (often used in conjunction with DSC) enables the determination of mechanical (storage and loss modulus) and thermal (e.g. glass transition temperature Tg) properties of polymeric materials over a wide range of temperatures (-150°C to 800°C) and frequencies (0.01 to 200 Hz) by free vibration, and resonant or non-resonant forced vibration methods (ISO 6721 [1])—see also [2]. (ISO 6721 [1])—see also [2]. Tension, compression and flexure loading configurations can be employed, although flexural samples are generally most popular. Three-point flexure specimens can be up to 50 mm in length, 15 mm wide and 7 mm thick. The sample is mounted on a clamp and then subjected to sinusoidal changes in strain (or stress) while undergoing a change in temperature. DMA is suitable for polymeric materials with stiffness ranging from 1 kPa to 1,000 GPa. DMA can be used to measure heat distortion temperature, and thermal expansion and contraction (i.e. coefficient of thermal expansion) under dynamic or isothermal heating conditions.
DMA has many advantages, including:
Typical DSC plot of a 2-part epoxy adhesive
DSC is used to determine the heat flow associated with material transitions as a function of time and temperature (-60°C to 300°C) or changes in heat capacity using minimal amounts of material. The technique provides quantitative and qualitative data on endothermic (heat absorption) and exothermic (heat evolution) processes of materials during physical transitions caused by phase changes, melting, oxidation and environmental degradation. DSC can be used to measure the glass transition temperature Tg and degree of cure. The technique involves slowly heating a small sample of material and measuring the heat absorbed or emitted by the sample as a function of temperature compared to a reference material. DSC can be used to measure changes in Tg with increasing moisture content for neat resins and reinforced plastics. ISO 11357 [3] specifies methods for thermal analysis of polymeric materials using DSC.
The technique has many advantages, including:
A small sample may also be a disadvantage as it may be unrepresentative of the bulk material; particularly if the material is heterogeneous. The sample for testing must also be removed from the manufactured item after the cure has occurred, which means that the test can not be performed in real-time or on-line. For some materials it can be hard to assign a Tg value, either because the wrong thermal transition has been assigned as the Tg or because there may be only a small change in heat flux as the Tg is passed through during the temperature ramp.
Temperature Modified DS (TMDSC) allows the differentiation of overlapping transitions. A sinusoidal modulation is overlaid on the linear heating ramp of the DSC. Depending on the settings, it results in improved resolution and sensitivity not possible using other techniques. Typical reversing events are glass transitions and crystalline melting; and examples of non-reversing events are cold crystallisation, evaporation, thermoset cure and decomposition.
Elevated Pressure DSC is a useful technique for analysing pressure dependant transitions, separation of overlapping thermal events or the study of reactions dependent on atmospheric composition. Pressures up to 7 MPa (1000 psi) are possible.
TGA can be used to monitor weight changes in a sample as a function of temperature. The technique is primarily used for studying degradation processes, providing information on thermal oxidative degradation rates and thermal degradation temperatures of polymeric materials. The technique is not particularly sensitive to the changes in adhesives and resins due to different states of cure and this is the main reason it has not been developed further for cure monitoring.
DEA measures the capacitive and conductive properties of materials as a function of temperature, time and frequency under controlled environments. It provides information on rheology (flow) and molecular mobility (relaxation) of materials, and can measure permitivity, loss factor and ionic conductivity of solids. Ionic conductivity is a useful measure of moisture ingression and leaching of chemicals, such as uncured monomer and additives from composite materials (see Dielectrometry for more information).
Thermocouples and other temperature sensitive devices can be used to monitor the temperature during the curing process. The monitoring process compares the measured temperature with the specified cure temperature of the material. If the temperature is too low the resin/adhesive will be uncured and if the temperature is too high then the material can be degraded. The specified cure temperature for the material must be known beforehand; otherwise the comparison cannot be made between measured and required temperatures. The technique is simple to implement and use, but it does not make any direct measurement of a physical property of the material, and is therefore reliant on good batch-to-batch consistency of the resin/adhesive.
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