Optimization of barrier envelope
Optimization, design, and durability of Vacuum insulation panels G.Garnier (4,5), D.Quenard (1), B.Yrieix (2), M. Chauvois (3), L.Flandin (4), Y.Bréchet (5) (1) CSTB, 24 rue Joseph Fourier 38400 ST Martin d’Hères, France;
[email protected] (2) EDF, 77818 Moret sur Loing, France;
[email protected] (3) REXOR, La Feydelière 38850 Paladru, France;
[email protected] (4) LMOPS, Bâtiment IUT, 73376 Le Bourget du Lac, France;
[email protected];
[email protected] (5) LTPCM, BP75, 38402 Saint Martin d’Hères cedex, France;
[email protected]
The presentation is concerned with the optimization and the conception of protective packaging for vacuum insulation panels.
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The vacuum insulation panels
The vacuum insulation panels are generally made of a micro or nanoporous core (for example: foams with open cells). This core is encapsulated under vacuum by a welded film: the multilayer envelope (figure 1). This envelope primarily governs the durability of the vacuum insulation panels (VIP), it should in particular exhibit good barrier to water vapour and air.
Figure 1: Components of a VIP (foto: va-Q-tec)
The main goal of the present study consists in improving the performances of the protective envelope of the vacuum insulation panels. The Parameters to optimize in order to process better envelopes of PIV are: - The barrier properties: a combination of plastic film/metal layer with ultra high resistance to permeation of atmospheric gases and water vapour must be developed. - The thermal conductivity: a low thermal equivalent conductivity of the PIV must be preserved overtime. - The perforating strength: a sufficient level in mechanical properties is required, especially for the setup of the panel. 1
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One of the key issues for building application is to minimize failure in service and to ensure a service life of several decades under typical conditions. The main ageing factor for this application being temperature and hygrometry. In principle, to answer the requirements of the specifications, the multi-layer envelope of a VIP must be composed of at least 3 principal layers (figure 2, [S.Brunner et al. 2006]): - a polymeric film (PET or PP), used for its good mechanical properties - a metal film (Al) for the barrier properties - another polymeric layer (LDPE) for the weldability
Figure 2 : Exemples of multi-layer envelope of PIV
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Materials and methods
2.1
Materials
This study focused on “polymer + metal” model films which can be sorted out according to their structures. These films composed of a thin PET layer (12µm) metallised on to either 1 or 2 faces with various aluminium thicknesses. These films were first used to identify structural markers that may alter the physical properties of the structures. In a second step, accelerated ageing tests were performed in order to identify some mechanisms altering the structures and thereby, of the macroscopic properties. Two ageing conditions were applied, the first “70°C-90%RH” was generating in a climatic chamber whereas the second “70°C-75%RH” was applying in a temperature regulated room with a saturated solution of NaCl.
2.2
Characterization methods
The characterization of these materials is defined by an investigation at three scales: - the microstructural analysis, defined by the chemical interactions - the mesostructural analysis, corresponding to the size of the biggest heterogeneities, i.e. the holes in the present case - the end use properties, essentially that of the final product
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Optimization of barrier envelope
2.2.1
The microstructural analysis
Thermal analysis were mainly performed to identify the thermal transitions of within the polymers. In the case of quenched PET three transition may be observed during the first eating ramp namely the glass transition temperature (Tg) a cold crystallisation temperature (Tcc) and a melting temperature (Tm) (figure 3), [Y.Kong, J.N Hay. 2002].
PET
Figure 3: DSC thermogram of PET
The amount of semi crystalline phase in PET could be determined with the integrated signal of the second peak.
2.2.2
The mesostructural analysis (scanning electron microscopy and optic microscopy)
On these model films, it was found of interest to analyze the structure of the aluminium layer with SEM. The image of the figure 4 revealed the presence of numerous heterogeneities that can be sorted out in two types according to their gray level.
Holes
Agglomerates of aluminium or alumina
Figure 4: heterogeneities – Film PET metallised with 100nm
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On the one hand, white spits could be identified as Aluminium rich phase. These heterogeneities were assumed to be either Aluminium or Alumina that resulted from the process. On the other hand, dark areas were found to be polymer phases and revealed holes in the aluminium layer. In order to better characterize the ratio of the metallic layer, experiments were performed with an optical microscope in transmission-mode, figure 5.
Figure 5: Film metallised on 1 side_100nm-Al
Although the holes were found smaller than the wave length of incident light (figure 4), a diffraction phenomenon occurred that allowed some light to get through. The actual size of the holes could however not be directly with this technique.
2.2.3
The end use properties
Tensile testing machine equipped with force and displacement sensors permits to assess the classical mechanical parameters of the composites both in their initial state and after ageing. Permeability tests were also made to evaluate their barrier properties. Measurements of permeability use the cup method (figure 6) in which the moisture flow through the film exposed to a gradient of water vapour pressure is measured.
Figure 6: The cup method
The graph of the figure 7 presents the mass versus time for the PET control (in dark line) and for PET metallised 2 faces with 20nm (in light line) in order to evaluate the effect of metallisation.
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Optimization of barrier envelope
Figure 7: mesurements of permeability cup
The vapour flow was calculated from the slope of these curves. As a result, some basic properties/structure relationships could be proposed relating mechanical strength and permeability to the detailed architecture of multilayer.
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Results
The “polymers+ metal” films were characterized using the above described methods, it was thus possible to analyze the films from a point of view of their thermal, morphological, mechanical and barrier properties.
The size of the holes was close to 100-200nm and the amount of heterogeneities strongly varied from one film to the next (figure 7).
Figure 8: Holes in the metallic layer right: PET metallised 1 face-100nm Al – left: PET metallised 2 faces-40nm*2-Al
A statistical analysis with both SEM and optical microscopy is currently being performed. The first results show that in the case of metallised films in low thickness, the holes’ size increases whereas the number of holes decreases with the ageing time.
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Figure 9: Morphological modifications of holes with the ageing
In addition, it appears that the ageing tests realized with severe exposure does not really lead to morphological modifications on the PET metallised on both sides with 40nm. The results of this analysis will hopefully lead to a good understanding of the barrier properties.
The mechanical properties of materials were estimated both in the initial state and after ageing. When normalized the stress strain curves with the thickness, most of the composites exhibited a similar behaviour. The addition of polyethylene largely reduced the mechanical properties, the PET control presents a much more ductile behaviour. In other words, the addition of metal reduces the mechanical properties of the polymer. The results of the tensile tests show that an increase in the thickness of the aluminium film caused an embrittlement of the films (case of the PET metallised 1 face to 100nm). The ageing in severe conditions lead to an increase of the elastic limit and a decrease of the yield strain and stress.
Figure 10: Mechanical properties of materials
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Optimization of barrier envelope
Figure 11: Barrier properties of materials
A comparison of films of the range PET 12µm metallised 1 or 2 faces highlights better barrier properties in the case of PET metallised on both sides with 40nm (figure 10 & 11). The results on the permeability show, as expected, that the ageing reduces the barrier properties of materials. The better compromise overall between mechanical and barrier properties is found with PET metallised on both sides with 20nm.
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Conclusion
The characterization realised on the “polymers + metal “ model films have permit to identify the influencing markers on their physical properties, both in their initial state and after ageing. These mainly markers are: - the amount of aluminium and the method of addition - the number and the size of holes which are present in the metallic layer - the ageing realised with severe exposure, T = 70°C, RH = 90% or 75% The realization of morphological analysis has permited to study the surface state of the films and to follow the evolution of the holes after a time. The ageing leads an increase of the size of the holes and a decrease of their number. The analyse of the end use properties of these model films brings to light that the ageing realised for a temperature close to the PET glass transition temperature induced relaxation phenomenon. The extracted parameters values of the tensile curves, namely the tensile strength and the yield stress decreased with the ageing time whereas the yield strain increase. The ageing has also an impact on the barrier properties of these films; this one leads to an increase of their permeability. A comparison of the metallised films revealed that the optimal method is to metallise on the both sides of the polymer. Accelerated ageing tests were also performed in order to identify the mechanisms governing the evolution of the markers and thereby, of the macroscopic properties. The next step of the project 7
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consists in studying the properties and durability of complex multilayers protective envelope of the VIP and the behaviour of the zone assumed to be weak, like the welded areas.
[D.H.Jeon et al. 2004]
The effects of irradiation on physico chemical characteristics of PET packaging film, Radiation Physics and chemistry 71 (2004) 1059-1064
[S.Brunner et al.2006]
Investigation of multilayered aluminium-coated polymer laminates by focused ion beam (FIB) etching, Surface & Coatings Technology 200 (2006) 5908-5914
[Y.Kong,J.N.Hay.2002] The measurement of the crystallinity of polymers by DSC, Polymer 43 (2002) 3873-3878
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