Research and Development I Protection of Vacuum Insulation Panels

Under high dynamic load Vacuum insulation panels – short VIP – are predestined for mobile use in applications with confined space. Can their long term insulating quality be guaranteed over the lifetime period of e.g. refrigerated trucks? To answer this question, a development team examined the technical solutions for the protection of vacuum insulation panels by adhesive bonding among other. The protective effects of various techniques were compared in puncture tests.

Eduard Kraus, Ulrich Hack, Benjamin Baudrit, Peter Heidemeyer, Martin Bastian, Thomas Wollheim, Roland Caps, Werner Wiegmann, Marc Fricke

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Figure 1 > Example of a vacuum insulation panel – short VIP

© SKZ

The demand for energy resources is increasing worldwide. In this area of conflict, energy efficiency is a key to successful energy policy. However, unused energy must not be generated, transported or stored. A significant contribution improving energy efficiency can be achieved by vacuum insulation panels (VIP) and their high insulation effect (Figure 1). A typical VIP consists of an evacuated core, which is wrapped by a gas- and water vapor-tight multilayer foil. The core of this evacuated insulation panel often consists of an open-pore, compression loadable material with relatively low thermal conductivity (e.g. polyurethane). The interior air pressure, depending on the size of the open pores in the core, is reduced to values between 10.00 and 0.02 mbar to suppress the thermal conductivity of the air. Depending on the core material, thermal conductivities in the range of 1.5 to 8.0 mW/(m∙K) can be reached for a VIP. Compared to the commonly used insulating polystyrene foam, this value is up to twenty times better. In numerous applications the insulation solutions based on VIP help saving valuable energy without requiring much space. Today VIP find their use in various fields of thermal insulation (for example, in containers, high performance thermal packing etc.), in household appliance industry (i.a.

refrigerators), in building insulation and increasingly in technical areas.

Sensitive outer cover Due to the mechanically sensitive outer cover the uses of particular VIP in dynamic technical applications are currently still limited. The current integration technologies for VIP cannot withstand the high dy

namic loads. This leads to premature failure of the panels and an insufficient service life. But the area of mobility with confined available space is predestined for the use of high performance insulation with VIP and needs these efficiency components to fulfill the forthcoming framework prescribed by law. However, this very interesting field of mobile applications could not be broadly developed due to the occur-

Name

UHU Ultra Montageklebstoff

SE-Polymer 690.00

Teroson 8597 HMLC

Otto Coll S610

Manufacturer

Uhu

Jowat

Henkel

Otto Chemie

Components

1C

2C

1C

2C

Chemical base

Silane-modified polymer

Silane-modified polymer

Polyurethane (PU)

Alkoxy-Silicone special adhesive

Table 1  > Adhesives used

ring high dynamic loads (e.g. in passenger and goods transport). The aim of the present investigations included in exploring and comparing of diverse techniques for VIP protection, so that the long-term insulation quality of these panels (especially no failure of outer cover) can be guaranteed over the entire period of use. Possible VIP applications are isolation techniques for thermo-packaging, thermotransportation and logistics area (for example, trains, shipping containers, refrigerated vehicles, aircrafts). Such protection can be achieved by various techniques. As part of these studies, the protection through foaming, pouring and adhesive bonding were examined.

Material used In the research project, the protection of VIP panels was performed by various adhesives. To compensate the dimensional tolerances, which can arise in the manufacturing of VIP, flexible adhesives with high gap filling up to 3.0 mm were preselected. The preselection of adhesives was also carried out according to other important criteria such as processing and pot life time for a large-area adhesive application. The selected adhesives are therefore suitable for relatively large automotive com-

ponents. Since high temperature differences between the outside and the inner wall may occur in the application, the adhesives have to withstand and balance these temperature differences without losing the mechanical and adhesion properties. For easy application and large-scale processing, the adhesive curing should take place at room temperature without external influences such as e.g. temperature. Table 1 provides a summary of the preselected represent adhesives. The preselected adhesives were Uhu Ultra Montageklebstoff (Uhu) and SE polymer 690.00 (Jowat) as one (1C) or two-component (2C) silane-modified room temperature curing adhesives. They exhibit very flexible and elastic properties at the specified service temperature. Teroson 8697 HMLC (Henkel) is a 1C room temperature curing Polyurethane adhesive and Otto Coll S610 (Otto Chemie) as a 2C room temperature curing silicone adhesive. Both 1C-adhesives cure by reaction with air moisture. The 2C-adhesives cure without external moisture influences.

Testing methods For the measurement of the internal gas pressure, the company Va-Q-Tec uses by default the so-called Va-Q-check method, which is based on heat flow measurement

/1/. Here, a heat flow measuring device is directly placed on the built-in panel mounted sensor and the temperature gradient between measuring device and sensor is being measured. The resulting heat flow depends on the internal pressure of the panel. The internal pressure can be accurately determined with the Va-Q-Check process in the relevant range of ± 1 mbar. This method allows not only a rapid and accurate determination of the current internal pressure but also a 100% inspection. Since the used protective methods partly or completely enclose the panels by foaming, pouring or adhesive bonding, the internal gas pressure cannot be measured due to missing contact to the sensor with the Va-Q-check method. Therefore, protected panels feature a novel measuring system (based on RFID Sensors). To read the internal gas pressure with the introduced RFID sensors, the panels do not need to rest on the sensor during the measurement. The gas pressure can be measured in a distance of several centimeters. Hence, this new RFID system allows contactless measurements of the internal gas pressure.

Experimental The used multi-layer aluminum-laminated foils for panel wrapping were investi35

Research and Development I Protection of Vacuum Insulation Panels

© SKZ

Figure 2 > Maximum tensile shear strengths of the film-film bonding

gated regarding their surface properties with respect to the adhesion. The surface energy (SFE) of unmodified foils was determined with accordance to DIN 55660-2 /2/. The SFE was determined by measuring the contact angle by means of three test liquids (water, ethylene glycol and diiodomethane) on a Drop Shape Analysis instrument DSA30 (Kruss). The evaluation of the measured contact angle values was carried out according to the Owens Wendt

Rabel and Kaelble (OWRK) method /3/ with determination of disperse and polar components of SFE. Based on the obtained findings the possible adhesives and protective materials (resins and plastics) were pre-selected. Subsequently, numerous overlapped specimens with the foil F1 (according to DIN EN 1465 /4/) were produced. For this, foil strips with dimensions of 100 mm x 25 mm were punched. Afterwards, the sur-

Foil F1

Surface free energy [mN/m]

Disperse part [mN/m]

Polar part [mN/m]

Outer side

56.1 ± 0.9

29.2 ± 0.4

26.9 ± 0.5

Inner side „sealing layer“ 31.3 ± 0.2

31.0 ± 0.1

0.3 ± 0.1

Table 2 > Measured surface free energy of the inner and outer sides for foil F1

Tensile direction

Tensile strength [MPa]

Transverse to the extrusion direction

0.29 ± 0.01

Longitudinal to the extrusion direction

0.35 ± 0.02

Table 3 > Tensile strengths for foil F1 36

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faces were cleaned with a suitable organic solvent (isopropanol). After sufficient venting the foil strips were overlapped bonded with overlapping in a clamping device. The adhesives were applied with a pneumatically operated dispensing gun. During dispensing, an attention was paid to homogeneous adhesive layers (0.3 mm). For reproducible adhesive bonding, the samples were fixed with a defined weight for at least 24 h. The overlapped bonded square in the specimens was 20 x 25 mm. After the complete curing of the adhesives under standard conditions (23°C and 50% rel. H., according to the technical data sheets), the specimens were tested on a testing machine Z010 (Zwick) with 100 mm/min (according to DIN 55529 /5/). Subsequently, the failure evaluation was made according to DIN EN ISO 10365 /6/. The panels were protected with diverse methods against the failure of the outer cover. Techniques like foaming (PU foam with density 80 g/L, BASF Polyurethanes), pouring with a PU hotmelt (Stages 70931, stages), pouring with a cold-curing PU protective layer (PU cast resin System Elastic PU HS 655/PUH 855, Baku Plast Faserverbundtechnik) and adhesive bond-

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ing of panels with GRP panels (1.5 mm thickness) were examined. The protection effect was subsequently compared by examination with puncture tests (according to DIN EN ISO 6603 /7/).

Surface energy of foils Adhesive joining requires the surface wetting with adhesive. A statement about the wettability can be made with information about the surface free energy of preselected the films. Furthermore, the basic ability for adhesive bonding of films in combination with a specific adhesive can be examined through the SFE. Lower substrate SFE often leads to a worse bonding ability. The measured SFE values of foil F1 are shown in Table 2. The surface energies of the outer and inner foil sides differ significantly. The inside layer consists of a polyolefin film with a significantly lower surface energy. However this side is not relevant for adhesive bonding and is used in the production of the panels for thermal joining of the foils. On the outside polyester layer much higher SFE values > 55 mN/m could be measured. These values lay in a

non-typical range for this material group (σPET = 41 mN/m, [8]). Probably, the measurements were affected by the relatively thin layer (about 16 microns) and the underlying aluminum layers. For these relatively high SFE values, a generally good bonding ability without necessary surface preparation can be expected. Since the wetting represents only a necessary but not sufficient condition for stable adhesive joints /8/, implementation of the application-related adhesion tests is always advisable.

Tensile shear strength of overlap bondings Table 3 shows the tensile strengths of base material for foil F1. To evaluate the adhesion as well as the mechanical properties of the bonded foils, specimens in accordance with DIN EN 1465 /4/ were prepared. After complete curing of the adhesive (according to the manufacturer reccomendations) at standard conditions (23 °C, 50 % rel. H.), the specimens were tested. The test speed was 100 mm/min. At least five specimens per test series were tested. Figure 2 shows

the achieved maximum tensile shear strengths for the bonded foil-foil-joints. The tensile shear strengths of foil-foilbonded joints were between 0.29 ± 0.01 MPa and 0.46 ± 0.04 MPa. The fracture study in accordance with DIN EN ISO 10365 for foil-foil-bonds showed a substrate failure (SF) for all bonds, which indicates a good adhesion of the adhesive to the foil surface. These calculated values of bonding strength can therefore be considered as minimum strength values. The different values of the bond strength can be explained by the various adhesive strengths and resulting differences in stress distributions in an adhesive bond during the test (caused by various cross contractions of joints) /9/. The variance for the measured values for all attempts was ≤ 0.097.

Mechanical protection of VIP panels The mechanical short-time properties of protected panels were determined by several methods. In the framework of the implemented investigations, the puncture test has been found the most appropriate and application-oriented test method. The 37

Research and Development I Protection of Vacuum Insulation Panels

© SKZ

Figure 3 > Measured puncture energy and maximum forces of protected and unprotected VIP

implementation of puncture tests on protected panels was carried out in accordance with DIN EN ISO 6603 /7/. For testing, the VIP panels were clamped between two steel plates with a defined drilling. The subsequent load on the panels was caused by a vertically falling impactor. The testing machine allowed the determination of the fracture energy and the maximum occurring penetrate force. At least three specimens were tested per series. A comparison of the puncture energies achieved during the test and the maximum forces are shown in Figure 3. As reference for these measurements, the puncture energy and penetrate force of the nonprotected panels (in Figure 3 named "Without protection") were used.The best mechanical protection of VIP can be achieved by covering the panels with a protective material, for example, with glass fiber reinforced plastics (adhesive independent, compare Figure 3). Here, a significant increase in the puncture energy and the maximum force can be observed. By foaming the panels with PU foam, similar puncture energies as with cold PU cast resin are accessible. These protection techniques increase the penetration energy of about 100 %. The pouring of PU hotmelt to cover the panels at higher processing temperatures (> 140 °C) and thick layers 38

adhesion 2 I 16

> 3.0 mm can lead to a failure of the panels. This failure is caused by the thermally sensitive outer cover (foil) of the VIP. Application of this technique can be performed in several thin layers. This protection technology resulted in an increase of achieved puncture energy of up to ≥ 200 %.

Summary and Outlook Various studies on vacuum insulation panels have shown that protection of the panels by foaming and by pouring cold PU casting resin increase the puncture energy of ≥ 100% compared to an unprotected panel. By pouring hotmelt on the panels, an increase in penetration energy of ≥ 200% can be achieved. The best mechanical properties were reached by protecting the panels with adhesive bonding with protective material (e.g., with GRP). Here, a noticeably increase in puncture energy by factor 5 can be considered. The use of VIP with their numerous advantages and potentials is still in its initial stage in many areas. It can be expected that the VIP have a great technical future in applications with limited space and the simultaneous need for very good insulation properties. Conceivable are truck applications, where the safe protection of VIP should be presupposed (Figure 4). The

presented protective techniques for VIP make a significant contribution to this. //

Acknowledgments The presented results were funded by the German Federal Ministry of Education and Research (BMBF) in the project "VIP – Fit for Mobility" (FKZ: 01LY1210B). The following partners were involved in the project: SKZ-KFE gGmbH, va-Q-tec AG and BASF Polyurethanes GmbH. We thank BMBF for the funding of this research project. Thanks also go to Henkel, Jowat, Herman Otto and Uhu for providing materials and equipment.

References / 1 / Va-Q-check: A unique VIP quality control method, Internetaufruf http://www.va-q-tec. com/en/va-Q-check-95.html, 2015 / 2 / DIN 55660-2:2011-12: Bestimmung der freien Oberflächenenergie fester Oberflächen durch Messung des Kontaktwinkels, DIN e.V., Berlin, 2011-12 / 3 / D. K. Owens, R. C. Wendt: Estimation of the surface free energy of polymers, Journal of Applied Polymer Science, 13 (8), 1741-1747, 1969 / 4 / DIN EN 1465:2009-7: Klebstoffe – Bestim-

© SKZ

Figure 4 > Thermally insulated trailers, a possible future mobile application for VIP

mung der Zugscherfestigkeit von Überlappungsklebungen, DIN e.V., Berlin, 2009 / 5 / DIN 55529:2012-09: Verpackung – Bestimmung der Siegelnahtfestigkeit von Siegelungen aus flexiblen Packstoffen, DIN e.V., Berlin, 2012 / 6 / DIN EN ISO 10365:1995-08: Klebstoffe – Bezeichnung der wichtigsten Bruchbilder, DIN e.V., 1995 / 7 / DIN EN ISO 6603-1:2000: Kunststoffe – Bestimmung des Durchstoßverhaltens von festen Kunststoffen / 8 / G. Habenicht: Kleben – Grundlagen, Technologien, Anwendungen, 6. Auflage, SpringerVerlag, 2008 / 9 / E. Kraus, B. Baudrit, P. Heidemeyer, M. Bastian: Fast detection of mechanical strength, Innovative Methods for Testing of Bonded Plastic Joints, 11 (4), 31 - 34, 2014 / 10 / H. Domininghaus, P. Elsner, P. Eyerer, T. Hirth:

Kunststoffe – Eigenschaften und Anwendungen, 8. Auflage, Springer-Verlag, 2012

The Authors M.Sc. Eduard Kraus (Tel.:+49 9314104-480, [email protected]), is Deputy Business Unit Manager "Joining of Plastics" in SKZ-KFE (research and development) gGmbH, Wuerzburg;

Dr.-Ing. Peter Heidemeyer is Managing Director and Prof. Dr.-Ing. Martin Bastian is an Institute Director. Dr. rer. nat. Thomas Wollheim operates as a Manager at the Va-Q-Tec AG, Wuerzburg, in Research and Development in the field "Thermal Memory"; Dr. rer. nat. Roland Caps is a member of the Management Board of this company.

Dipl.-Ing. (FH) Ulrich Hack is Project Manager.

Werner Wiegmann is Business Development Manager at BASF Polyurethane, Lemfoerde;

Dr. rer. nat Benjamin Baudrit is Business Unit Manager "Joining of Plastics";

Dr. rer. nat. Marc Fricke works here as team leader.

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