The reliability of a specific technique to measure thermal properties is determined by several factors, such as the speed of operation, the required accuracy and performance under various environmental conditions, the physical nature of material, and the geometry of the available sample. However, in most techniques the main concern is to obtain a controlled heat flow in a prescribed direction, such that the actual boundary and initial conditions in the experiment agree with those assumed in the theory. There are several methods (techniques) used to measure the effective thermal conductivity of composites. Indeed, the choice of technique depends on the type of application, size of the sample and the available sample geometry; however they are divided into two groups, the steady state and the nonsteady state (transient) methods.
1.1 Steady state techniques
In the 1st group, the sample is subjected to a constant heat flow. In the past, much attention has been focused on the problems of this group. Examples of steady state are the well – known guarded hot plate and heat flow meter techniques for thermal conductivity (ISO 8302, 1991), (Gawin et al., 2004) and (Sombatsompop, 1997) which are typically suited for bulk applications that require large and thick samples. For these methods, testing times can be long due to the need for thermal equilibrium, which can be of the order of 24 hours, and temperature gradients across the sample can be large The principle of the heat flow meter method is based on mounting the test-sample between two plates, a ‘hot’ and a ‘cold’ plate, then a heat a heat flux transducer is used to measure the heat flow through the sample. Using measurements of the heat flux, temperature difference across the sample and sample thickness the thermal conductivity can be found. The method is a quasi-steady state method and is a variant of the guarded hot plate technique thus the instrument and sample must be allowed to reach isothermal condition (thermal equilibrium) before measurements are made. It should be mentioned that this method is a comparative method and thus the instrument must be calibrated using a specimen of known thermal conductivity. More details and analysis can be found in ASTM E1530.
On other hand, these steady state methods may not be suited to testing molten composites which include soft materials such as plastics and polymers. In this type of composites, in addition to other practical factors related to the molten state of the soft materials, they are vulnerable to degradation during performing the measurements and before the required thermal equilibrium is attained. The thermal properties of such composites are influenced by the level of crystalline and molecular structure of the molten which are directly related to its thermal history (Sombatsompop, 1997).