THERMOMECHANICAL ANALYSIS

INTRODUCTION

Many materials are used under conditions that subject them to forces or loads. Examples: polymers used in molded automobile bumpers, and polymer fibers in “stretch” clothing.

The mechanical properties of these materials are very important;

Mechanical properties include: ductility, stiffness, hardness and strength.

The mechanical properties of a material are a function of the applied load, the duration of the load, environmental conditions, including temperature.

Thermo Mechanical AnalysisThe response of mechanical behavior as a function of

temperature is studied using thermo mechanical methods of analysis.

Two important classes of thermo mechanical methods of analysis are :

Thermo mechanical analysis (TMA) and Dynamic mechanical analysis (DMA).

TMA

Thermo mechanical Analysis (TMA) can be defined as:

the measurement of a specimen’s dimensions (length or volume) as a function of temperature whilst it is subjected to a constant mechanical stress.

In this way thermal expansion coefficients can be determined and changes in this property with temperature (and/or time) monitored.

TMA

TMA measures changes in the physical dimensions of a sample as a function of temperature under an applied external load or stress. The dimensional change is generally measured in one direction (the length of the sample) with respect to the applied load;

The applied load is a static load in TMA. If the applied load is zero (i.e., no external load is applied), the TMA technique is called thermodilatometry.

TMA is used to measure expansion, compression, softening point, bending properties and extension (elastic and plastic deformation) for materials, especially polymers, composites, ceramics, and glasses

TMA

Schematic Diagram

Instrumentation

Probes: The probe is usually also made of quartz or steel. Probes come in a variety of shapes with the shape determining the type of dimensional change measured.

Loads and Temperature employed

Loads of 0.1–200 g, or forces of 0.1–100 N may be selected

and furnace temperatures may be programmed from -150oC to 1000oC, depending on the particular instrument.

Calibration

TMA instruments must be calibrated for both temperature and dimensional motion. High-purity In, Sn, Zn, and Al are often used

The LVDT is calibrated by using a standard with a well-known linear coefficient of thermal expansion, such as Al.

The initial length of the standard is determined at room temperature and then the change in length of the standard with temperature is measured.

The linear coefficient of thermal expansion, α, is defined by:

The calculated value of α is compared to the literature value for the material and a correction factor is applied to the data if needed.

APPLICATIONS

Tg Coefficient of Thermal Expansion Evaluate the effect of Processing

changes Textile Industries Softening Point

Measurement of Tg of epoxy printed circuit board

Expansion behaviour of printed Circuit Boards

Textile Industries: Effect of Processing

Strength of the composites

Thermal Expansion Measurements and Softening Temperatures

Co effficient of thermal expansion

The CTE is a quantitative assessment of the expansion of a material over a temperature interval.

When manufacturing products that contain two different materials, it is oftentimes critical to ensure that the materials will have CTE values that are identical to avoid the build-up of thermal stresses or to prevent leaks or component malfunctions.

glass transitions of coatings on a substrate

TMA penetration results generated on a wire sample with two coatings. The wire is used to produce electrical motor coils and the inner coating prevents electrical contact between adjacent wires and the outer coating is used to bond the coil.

The TMA penetration results show that the outer coating has its Tg at 128 C and that of the inner coating occurs at 176 C. The decomposition of the resin is observed at approximately 260 C.

TMA penetration results on crosslinked and non-crosslinked polyethylenes.

The sample with crosslinking exhibits a smaller degree of penetration due to the higher viscosity in the liquid region above the melting point.