Technology for Wood and Natural Fiber-Based Materials

Research Project

Quick determination of the thermal conductivity of insulating materials made from renewable raw materials

A crucial goal in today’s world is protection of the environment, in particular the considerate handling of the limited energy deposits remaining in and on the Earth. This necessitates challenging and efficient insulation measures in buildings in which the most heating energy is consumed. In an ecological evaluation, insulating materials from renewable raw materials definitely achieve a higher score. 

© Fraunhofer WKI
THB-Messeinrichtung mit leerer Messzelle und Brückensensor
© Fraunhofer WKI
THB-Messeinrichtung mit Polystyrol-Hartschaumplatte in der Messzelle

Despite the ecological advantages, insulating materials from renewable raw materials must possess thermal properties which are similar to those of conventional materials, in order to remain competitive. Optimised processes and meticulous supervision of the manufacturing process are thereby required in order to ensure the quality of the product.

A measuring technique is therefore required in which it is possible to very accurately determine the thermal transportation characteristics of insulation materials, including situations where they are influenced by high material moisture.

Currently, thermal conductivity is measured using the so-called one or two-board process, in accordance with DIN 52612, whereby sample sizes of more than 500mm edge length, sample thicknesses of more than  5mm and measuring periods of several hours are necessary. In particular the long measuring periods make this technique unsuitable for a fast industrial testing. Additionally, the measurement of the thermal conductivity dependent on the material moisture results in unreliable values, as the moisture distribution within the material changes under the influence of the temperature gradient and time.

At the WKI, a new technique has been tested, in co-operation with the Physikalisch-Technische Bundesanstalt (PTB), in which the thermal and temperature conductivity of insulation material, dependent on thickness and material moisture, can be determined.

These extremely fast techniques, known as Transient Hot Strip (THS) or, as a further development, Transient Hot Bridge (THB), are based on a transient principle. Due to the short measuring periods, determination of the thermal conductivity is also possible dependent on the moisture, without the material moisture being significantly altered by the measuring process.

For THS, a very thin (0.005mm) metal strip is clamped between two cube-shaped samples in a temperature-controlled environment. An electrical heating current is then used to warm it to a constant measuring temperature which is held for several minutes. During this time, its temperature-dependent voltage drop is simultaneously registered as the gauge for the sought transportation properties. The strip thereby serves simultaneously as heat source and resistance thermometer. The thermal and temperature conductivity can subsequently be calculated. 
The further development of this sensor uses a Wheatstone bridge circuit (see Fig. 1) instead of a heating strip, allowing the influence of the considerably lower bridge current on the measurement results to be ignored.

The measuring cell itself is constructed on the basis of a centring bench vice which, when closed using a torque wrench, exerts the same amount of pressure on both sides of the material being examined (Fig. 2).  The sensor, which is flexibly hung in the centre, is therefore subjected to an equal contact pressure from all sides. 
In order to enable the measurement of bulky materials, a perspex container is fitted around the sensor. By closing the two clamping jaws, the thermal conductivity can also be determined dependent on the bulk density.
Currently, not only insulation materials from renewable raw resources such as cellulose, coniferous wood chips, flax, hemp and cotton but also conventional insulation materials such as mineral wool, fibreglass and polystyrene are being measured at 30%, 50%, 70% and 85% relative humidity.

The uncertainty analysis and the first experimental comparison with other measurement methods showed that a measurement uncertainty of less than 3% was achieved, which can be regarded as highly satisfactory as regards the examination of insulation materials.