We want to conclude on the interest of the “crimping” process used to produce the glass wool and to make a comparison for anisotropic factor obtained from structural property (air permeability) as well as thermal property (thermal conductivity and diffusivity). The main structural (densities, porosity, specific surface, air permeability) and the thermal (conductivity, diffusivity, heat capacity) characteristics of this glass wool are presented. Thermal results are determined by using several methods (Hot disc (HD), Heat Flow Meter (HFM) and Guarded Hot Plate).
The industrialized countries have engaged, since the energy crisis of 1974, a policy of energy saving which has caused a considerable development of the heat insulation sector in the construction industry. Nowadays, the energy consumption of building accounts over 40% of final energy consumption in the European Union. The main solution for reducing the energy consumption is to increase the thickness of the thermal insulation inside the building envelopes. But the knowledge of the insulating materials thermal behaviour is very imperfect in spite of the interest [
There is a need of studying material structural properties in order to analyze and interpret thermal behavior. The present work presents three main characteristics of glass wool structure: volume ratio of the solid phase to the total volume (porosity), degree of fragmentation in the solid phase (specific surface), and spatial organization of the solid phase (anisotropic factor). Glass wool object of this study is not an ordinary insulation material. This material has been produced by an innovated crimping process described by Bergonnier et al. [
Glass wool intended for the insulation of roofs has been studied. Porous fibre materials contain three components: fibre, air and binder. The content of the components and their properties and arrangement determine the material structure. Before the curing stage, the fiber mat is processed to produce a structure endowing the material with a stronger mechanical resistance (to compression, tearing, or shear). This step is called “crimping”. It consists in breaking the naturally layered structure of the mat, through an on-line transverse and axial compression. The denser layers are buckled during the axial compression in order to produce a more isotropic fiber orientation (
croscopy (SEM, Quanta 200 FEG Environmental) shows heterogeneity of the geometry of fibers and the random distribution of fibers (
The bulk density ρ0 can be determined from the expression:
where εs and ε are the part by volume of, respectively, solid and gas (porosity) in the highly disperse systems and ρf is the true density of the fiber (usually equal to 2500 - 2800 kg∙m−3) for glass. To assess the total porosity (if non-accessible porosity like closed porosity is neglected), for materials like masonry, a specimen of the material is submitted to a vacuum pressure to extract air, before immersing it into distilled water. But, for fibrous specimen, this method could not be used because water is not retained and an expansion of the bulk structure is observable. If the true density of the fiber is known, Equation (1) can be used. In this work, we have preferred applying relation (2).
Specimen has been dried inside an oven regulated at the temperature of 40˚C during 3 hours. The mass of the specimen is measured by a Mettler Toledo (LJ16) weighting machine in order to check the moisture content of the specimen. Bulk density ρ0 is determined from mass measurement of a known volume of material. True density ρv is determined by a helium pycnometer (Accupyc 1330). Helium is an inert fluid for the fibrous structure. A specimen is putting inside a cylindrical cell. A volume (V = 10 cm3) has been chosen because of the elementary volume representative (EVR) of the material function principally here of the fiber thickness. Results are presented in
Bulk density ρ0 [kg∙m−3] | True density ρv [kg∙m−3] | Total porosity ε [%] | |
---|---|---|---|
Glass wool | 68.6% ± 3% | 2620.3% ± 1.6% | 97.4 |
The experimental deviation obtained for the true density measurement is 41.4 kg∙m−3 (or 1.6%). Deviation for determining bulk density can be estimated to 3% with apparatus accuracies. As bulk density is higher (
Specific surface (Sp) expresses the degree of division of the solid phase. For heat and mass transfer, a shape with higher specific surface value is more efficient. For a given volume V (or material mass), different geometric shape generate different amounts of surface area (Asg) by which to interact with the environment. Volumetric surface (SV) and specific surface (Sm) can be determined as:
An Accelerated Surface Area Porosimeter (ASAP, Micromeritics’s ASAP 220) has been used to determine the specific surface experimentally. Method is based on the fact that materials present open porosity which adsorbs surrounding gas. ASAP used with krypton’s content lower than 35% automatically determines by applying BET model specific surface. Important value (Sm= 0.2332 m2∙g−1) is obtained showing that material is finely divided.
An air permeability tester (TEXTEST FX 3300) has been used to determine the permeability of the glass wool samples [
in the third direction,
The anisotropic factor AF is determined by the ratio of the parallel air permeability to perpendicular permeability:
A series of five measurements for each direction has been performed. An anisotropy factor (AF) of 1.55 is obtained. This value is lower than common glass wools [
There are a variety of methods for the measurement of the thermal conductivity. They are broadly classified as steady-state methods and transient methods. Due to important density (more than 75 kg∙m−3), total heat transfer in glass wool is principally due to transfer in gas (
The most adapted technique for insulation materials (American ASTM C 177-97 and European ISO 8302 Standards) taking into account the total heat transfer is the steady-state methods and particularly the guarded hot plate (GHP). Another traditional steady-state method is the Heat Flow Meter (HFM) method (American ASTM C 518-98 and European ISO 8301 Standards). The GHP as other steady-state methods suffer from major drawbacks. They require a long time to establish a steady-state temperature gradient across the sample, and this temperature gradient is required to be large. Sample size is also required to be large and contact resistance between the thermocouple and the sample surface is considered a major source of error. Some material properties can be altered during the time to achieve steady state.
Transient techniques measure a response (temperature, flow) to thermal signal (flow). Therefore, these techniques are distinguished mainly by the short time required to obtain the desired results. Hot strip can be used to measure the thermal diffusivity and conductivity of solid non-electrically conducting materials. The most recent development of the hot strip method is the Hot Disc (HD) technique. The basic principle of this method relies on a plane element which acts both as temperature sensor and heat source [
depth
to optimize the best combination of sensitivity coefficients for the estimation of the thermal conductivity and thermal diffusivity property. Temperature increased must be also lower or equal to 1˚C. Considering the thermal characteristics of glass wool in literature, a radius sensor of 14.63 mm is used. Sample thickness and diameter is over respectively 30 mm and 90 mm in order to respect the infinite media hypothesis.
According to ISO 8302, guarded hot plate (GHP) has been used to determine thermal conductivity of glass wool L1 against sample mean temperature (
Bulk anisotropic module of Hot Disc method has been used to determine thermal conductivity and thermal diffusivity in axial and radial directions (
λA// (mW∙m−1∙K−1) | aA//2 (mm2∙s−1) | |||||
---|---|---|---|---|---|---|
Sample 1 | 53 | 0.83 | 41 | 0.53 | 1.31 | 1.31 |
Sample 2 | 53 | 0.81 | 42 | 0.67 | 1.20 | 1.20 |
Sample 3 | 54 | 1.07 | 38 | 0.75 | 1.43 | 1.43 |
dard ISO 22007-2. In the case of the flow, recommended value of 0.1 W induces a temperature increase greater than 1. We have chosen 20 mW the minimal value can Hot Disc accepts but it creates an increased temperature of 1.24˚C. Due to the low thermal conductivity of glass wool, it is very difficult to respect the maximal increase of 1˚C: one of the conditions for a good test. Results are obtained for a characteristic time of 0.528 s and a probing depth of 21.3 mm.
Axial conductivity (la) is referred to as through-plane in sample perpendicularly of the thermal probe. In our case, axial conductivity is in the same direction of kA//1 and kA//2 (
Hot disc method allows determining thermal conductivity in three principal directions of the sample. As before by air permeability, an anisotropy factor is defined as the ratio of conductivity corresponding to a heat transfer along the plane of stratification with the conductivity corresponding to a heat transfer perpendicular to the plane of stratification. The ratio axial conductivity against radial conductivity is equal to 1.32 in the dry state (
These remarks confirm thermal conduction and diffusion are mainly sent through the pores by the air inside pores.
Glass wool tested is not an ordinary one because it has been produced by a crimping process. Consequently a lower anisotropic factor (equal to 1.4) is obtained by air permeability measurement instead of a factor equal to 2
kA//1 (m/s) | kA//2 (m/s) | |||||
---|---|---|---|---|---|---|
Glass wool | 3.03 ´ 10−4 | 2.39 ´ 10−4 | 1.75 ´ 10−4 | 1.73 | 1.37 | 1.55 |
usually found. We have determined other structural parameters in order to characterize high porosity (97% from true and bulk densities) and finely divided structure (high specific surface of 0.2126 m2∙g−1). A comparison between experimental results from air permeability and thermal conductivity has been done. The same anisotropic factor is obtained showing the coherence between results from structural and thermal parameters. Thermal conductivities are measured from several methods: Guarded Hot Plate (GHP), High Flow Meter (HFM), Hot Disc. Coherent values have been obtained between these methods although needed establishment of steady state and Hot Disc is transient method. However, we have noticed that radial values are coherent but not axial ones. Anisotropy in thermal conduction and diffusion is not taking into account for the moment in the heat calculations in the case of French thermal rules.
The authors would like to express deep gratitude to J-B Rieunier ISOVER as well as the technical staff of UPJV microscopic platform Amiens, ENSAIT-GEMTEX Roubaix and LASIE La Rochelle University.
Laurent Marmoret,Hussein Humaish,Anne Perwuelz,Hassen Béji, (2016) Anisotropic Structure of Glass Wool Determined by Air Permeability and Thermal Conductivity Measurements. Journal of Surface Engineered Materials and Advanced Technology,06,72-79. doi: 10.4236/jsemat.2016.62007