Improving the thermal properties of aircraft cabin interiors with the integration of vacuum insulation panels

Commercial aircrafts require insulation to protect passengers in the cabin from thermal and acoustic loads. The conventional insulation in aircrafts consists of blankets made from layers of glass wool wrapped in foil that keeps the glass wool from being adversely affected by the environment. There is a potential to improve the thermal and acoustic properties of the cabin by replacing the interior panels with conventional secondary insulation by new panels combined with vacuum insulation panels (VIP). This article is focusing on the study of the VIP integration into the interior panels. First, the new structure solutions are defined on the basis of a requirement analysis for interior panels and VIP and theoretical analysis. Second, the manufacturing feasibility study for the new solutions is performed. The results show that the new structures can be manufactured. Third, the thermal properties of the new structure solutions are measured. The test results show a decrease of thermal conductivity of the new panels by a factor of 3–6 compared to the conventional solutions. Finally, the impact of the hot molding press on the vacuum maintaining inside the VIP is investigated. The trials demonstrate that the high barrier films can withstand high-temperature and pressure conditions and that the thermal conductivity of the test specimens didn’t worsen after 1 year.


Introduction
Passengers of commercial aircrafts are spending an increasing amount of time in the aircraft cabin during travel. Therefore, they pay more attention to the comfort of the cabin. To achieve a satisfied passenger experience, the aircraft industry is seeking to improve the cabin comfort continuously. Some of the main factors that affect passenger comfort are cabin climate and noise.
To protect passengers from thermal and acoustic loads, insulation such as glass wool is used between fuselage and interiors [1], as shown in Fig. 1. Typical thermal conductivity values for the conventional insulation materials are between 0.030 and 0.040 W/m K [2].
Another important comfort parameter in the aircraft cabin is space. With the intention of space increase and simultaneously improvement of thermal and acoustic situation in the aircraft cabin, the search for better insulation materials led to vacuum insulation panels (VIP). VIPs are ultra-thin insulants that can be up to 5-10 times more effective than traditional insulation materials. Such materials are used in building construction, refrigeration units, and insulated shipping containers and other applications requiring low energy loss from heat transfer [2][3][4].
The first investigations with replacement of conventional glass wool insulation by VIP attached on cabin interiors showed that the VIP has a potential to reduce heat transfer. However, the theoretical values of the heat transfer coefficient could not be achieved due to air leakages between VIP and the surface of the interior panel in addition to a thermal bridge effect of VIP (Fig. 2) [5]. 1 3 To minimize the deviation between theory and practice, the idea of VIP integration into the interior panel was initiated. The aims of this study are to develop new robust structure solutions for interior panels, which already include vacuum insulation inside, to verify the aviation-specific requirements for these solutions and to investigate its effectiveness in the aircraft cabin (Fig. 3).
The use of VIP panels leads to an enlargement of the interior cabin diameter, which in turn brings more comfort for the passengers. As an alternative, during development of new aircrafts a reduced fuselage diameter can be defined. It reduces the weight and drag of the aircraft and as a consequence diminishes the fuel consumption.
Another major advantage of a well heat insulated cabin is a compensation of its heat loss that can lead to a reduction in energy consumption.
Besides the lining panels (sidewall, door frame lining and ceiling) the new structure can also be used in crew rest compartments, galleys, lavatories, stowages, doors and other applications, which require good thermal and/or acoustic insulation properties.

State of the art
This chapter includes the current state of knowledge of the elements, which are intended to integrate into the new combined structure solutions for the interior panels.

Aircraft insulation
The conventional insulation in the aircraft (primary and secondary) fills the space between the interior panels and the fuselage of the aircraft [1].
The main functions of the insulation in the cabin are [6]: • Damping against heat or cold; • damping against noise; • moisture barrier; • preventing propagation of an in-flight fire;  The entire fuselage is lined with primary insulation, except for the bilge. The insulation packages are arranged between the frames on the inner side of the aircraft skin and on the stringers [1] (Fig. 4).
The frames, which protrude far into the cabin and thus serve as thermal bridges, are covered by a second insulating mat. This insulation is called frame insulation. The secondary insulation is a part of the sidewalls or other lining panels in the cabin. Both primary and secondary insulations have a thickness of approximately 40 mm in the case of Airbus A340 aircraft. There is an air gap between the primary and secondary insulation, which is a part of the ventilation system of the aircraft [1].
Besides the glass wool material, thermoplastic foams such as polyimide foam (PI), elastomeric foam and polyethylene foam (PE) are also used in aircraft cabins as thermal and acoustic insulation. The thermal conductivity of such insulation is in the range of 30-50 mW/m K.
The moisture, which comes from the cabin, can penetrate through the interior lining panels into the glass wool packages and condense or freeze inside the insulation. During the landing and on the ground, the ice melts. Most of the water runs out through drainage vents into the bilge in the lower part of the aircraft, where it can be pumped out, while it is on the ground. But some of the water accumulates in the insulation blankets. Due to the short time of the aircraft turnarounds, the blankets don't have enough time to dry out before the next flight. The accumulation of the water between the cabin lining and the structure can cause problems, such as "rain in the plain" effect, a significant increase in the total weight of the aircraft, a degradation of the thermal and acoustic properties of the glass wool packages, a corrosion of the primary structure and a growth of mold and bacteria [1,7].

Cabin interior sandwich panels
The cabin interiors cover the fuselage structure with installed thermal and acoustic insulation, the components of the electrical, air conditioning and ventilation systems from cabin side. The linings shall withstand the impact from passengers and luggage. Therefore, the interior panels should have a robust structure with the sufficient mechanical properties.
In this regard, sandwich structures have found their wide application for the cabin interiors. A sandwich structure is a three-layered structure, composed of two face sheets with a thick core layer in-between. The core thickness is many times greater than the thickness of the face sheets. The facings are generally very thin and stiff, which can be constructed from a variety of different kinds of materials. In aviation industry there are two types of commonly used cores: honeycomb and foam. The honeycomb-type core constructions are lightweight, flexible, fire-retardant and have a good impact resistance and a great strength-to-weight ratio [8] (Fig. 5).
The fiber-reinforced thermosetting pre-impregnated materials (Prepreg) are typically used as the face sheets. The predominate material for sandwich cores is NOMEX ® honeycomb [9].
This structure fully meets requirements regarding weight, mechanical loads, environmental conditions and flammability. The secondary insulation blankets are attached to the rear side of the interior panels (Fig. 6). The insulation is commonly fixed by adhesive or hook and loop tapes on the lining panels. The production of the insulation blankets and its fixation mean additional steps in the production chain, which is time consuming and requires an enormous amount of manual work.

Vacuum insulation panels
VIP is an ultra-thin and high-performing insulation structure, which consists of a rigid porous core and a gas-and water vapor-tight envelope. The panel is evacuated and sealed to prevent outside gases and humidity from entering into the panel (Fig. 7).
The combination of porous material and vacuum results in an extremely high thermal resistance. The VIPs have thermal conductivity from 0.0035 to 0.008 W/m K at center of panel after production and possess long-term thermal performance [3].
The VIPs have many applications, such as appliances, temperature-controlled transportation and buildings [2,4]. So far, the VIPs have not found any function in the aircraft industry except for cargo containers, where clinical and pharmaceutical goods are transported [10].
The core material shall have a sufficient high compressive strength to withstand the mechanical pressure load up to 10 t/m 2 and open-porous structure to enable the evacuation [11]. In this regard, such materials as fumed silica [12,13], aerogel [12,14], glass fiber [14][15][16] or polyurethane foam [17] can be used as the VIP core material. Moreover, artificial structures such as staggered beams [18] or honeycomb [19][20][21] are proposed as an alternative for the VIP core.
The cover-foil bag is responsible for maintaining the vacuum inside the panel. It prevents permeation of gas and   [2] moisture into the VIP. The barrier materials must combine several properties. They should offer high resistance to gas transfer, like metals do, and at the same time should have resistance to heat transfer through the barrier material itself, like plastics do. In addition, it must be relatively easy to seal the cover-foil bag [12]. The current high barrier multilayer films, which are used in VIP, consist of an inner sealing layer (polyethylene PE), a middle barrier layer (aluminum foil AF or metalized polymer film multilayer laminates MF) and an outer protective layer. Expanded polystyrene (EPS) or extruded polystyrene (XPS) can be applied as the outer layer to improve fire resistance properties or panel robustness, and the polyethylene terephthalate (PET) to withstand general handling through transportation and installation [22].
The permeability of air and water vapor through a barrier layer depends on two different mechanisms. The gas permeation occurs at macroscopic defects in the range of 0.1-1 μm 2 , which form in the seams of VIP. The water vapor permeation appears even at microscopic defects. Therefore, the moisture penetrates through the seams as well as through the barrier layer [23]. Fumed silica cores, enveloped in high barrier foils, show a gas pressure increase of less than 1 mbar/year. Thus considered, the thermal conductivity would increase from 4 to 7 mW/m K in 100 years [24].
Thermal bridge effect belongs to the most important research subjects of the VIP. With such low values of thermal conductivity of the VIP, the thermal bridges can worsen the overall heat transfer of the system. Therefore, the term of the effective thermal conductivity was introduced, which includes the thermal conductivity of the panel center and also the contribution of the thermal bridge. Three kinds of thermal bridges are considered for VIPs: a metallic barrier layer in the foil cover, an air gap or foam between adjacent panels and a thermal bridge caused by the mounting system [25]. The investigations for the envelope of the VIP showed that the laminated aluminum foils have a high thermal bridge effect. Therefore, the aluminum-coated multilayer foils have been used in current VIPs, which have a factor 10 smaller thermal bridge effects compared to laminated aluminum foils [23].
In addition, hybrid envelopes, formed from metalized laminate on one side of the bag and Al foil on the other, show excellent results in comparison with aluminum foils [15].
Gas and water vapor, permeated to the core, can be sorptioned by adding getters and desiccants. It enables maintaining of the inner vacuum for a longer time and an increase of service life of VIPs. Radiation through the VIP core can be decreased by adding the radiation shields between core sheets or opacifiers [23,26].
The service lifetime of a VIP can be defined as the time at which the required thermal conductivity has been surpassed [26]. The combination of the material, the envelope and the adsorbents has a great influence on the inner gas pressure and water content, which in its turn define the service life of VIP. The increase of the gas pressure is caused by a number of such factors as gas permeation through the envelope surface and sealing edge, outgassing from the core material and inner surface of the envelope and effectiveness of the getter or desiccant [27]. The investigations showed that the outgassing rate can be reduced to a negligible level by thermal pre-treatment [28].
The VIPs can be shaped in different forms depending on the core material. It is possible to manufacture 3D shapes, folded VIPs, panels with cut-outs and cylindrical panels, as shown in Fig. 8 [10].
Despite the huge advantages in thermal properties, VIPs also have several disadvantages: • High barrier foils are sensitive and VIPs lose their thermal effectiveness after damages; • decrease of the thermal performance over the years due to gas and moisture permeability; • appearance of thermal bridges between adjacent VIPs.

Theoretical background
Thermal conductivity of the interior panels is in the focus of this study. Before the new solutions will be presented in the article, the important theoretical aspects are summarized in this chapter.
Thermal conductivity is determined by four components: radiation transfer between internal surfaces, solid conduction within the material skeleton, gas conduction within the material pores and air, and coupling effect [4].
where λ core is VIP core thermal conductivity [W/m K], λ S is solid thermal conductivity [W/m K], λ R is radiative thermal conductivity [W/m K], λ G is gaseous thermal conductivity [W/m K], λ coupling is coupling effect thermal conductivity [W/m K].
After evacuation, heat transfer in VIP occurs mainly by means of solid conduction and radiation.
Nevertheless, the gaseous thermal conductivity can have a huge increase after a certain time due to an inaccurate combination of pressure reduction and pore size of the core. VIPs, enveloped in high barrier foils, show a core = S + R + G + coupling , typical gas pressure increase of approximately 1 mbar/year [24]. Depending on the pore size of the core, the correct pressure reduction during evacuation shall be obtained. As shown in Fig. 9, the best core for VIP is the nano-porous core that requires evacuation down to 10 mbar. In this case, the pressure increase of 1 mbar/year doesn't significantly influence the gaseous thermal conductivity.
The gaseous thermal conductivity achieves lower values in nano-structured core materials due to the so-called "Knudsen effect". While reducing the gas pressure in a material, the gas conductivity of the non-convective gas remains almost unaffected until the mean free path of the gas molecules reaches values in the same order of size as the largest pores in the medium. When the pore diameter of the material becomes less than the average free length of path of gas molecules, the air molecules will only collide with the pore surfaces without transferring energy by this elastic impact [23].
Considering the "Knudsen effect", all new solutions were defined in such matter that the core of the interior panel possesses micro or nano pores to maintain vacuum during required 25-30 years.
The new structure solutions consist of several layers of different materials. In such cases the total thermal resistance of the panel is a summation of the resistances of each layer [29]: Fig. 9 Thermal conductivity of air as a function of the air pressure and the average pore diameter of the core material [23] where R is total thermal resistance [m 2 K/W], α f1 and α f2 are convective heat transfer coefficients of fluids 1 and 2 [W/(m 2 K)], d i is thickness of the ith layer [m], λ i is thermal conductivity of the ith layer [W/m K] (Fig. 10).

Structure definition
The intention of this research project was to combine two elements, interior panel and VIP, to develop the advantages of these elements and diminish the disadvantages. While doing this, it is mandatory that all airworthiness requirements, applicable to these elements, are met.
Cabin interior panels must fulfill such requirements as mechanical strength, low weight, fluid susceptibility, resistance against applicable environmental conditions, fire resistance, aesthetics and other requirements. In addition to these requirements, new structure solutions must fulfill requirements on VIP, such as [2]: • Resistance against moisture and gas absorption; • puncture resistance; • open-porous structure of core material to allow evacuation; • mechanical requirements on core material to withstand the environmental pressure load; • low thermal conductivity of core and envelope materials.
Based on the requirement and theoretical analysis for interior panels and VIP, four structure solutions were proposed for further investigation (Fig. 11): 1. Evacuated honeycomb core filled by micro-or nanoporous powder; 2. sandwich structure with an evacuated VIP in the middle, which is covered by rigid foam and face sheets from both sides; 3. evacuated aerogel core; 4. double sandwich, which consists of two core elements, an evacuated VIP and honeycomb.
All structures are constructed in such a manner that the sensible evacuated VIP panel is protected by means of additional prepregs, honeycomb or foam core materials. Within this construction the disadvantage of VIP losing its thermal effectiveness after damage can be eliminated.
Furthermore, the usage of the VIPs, which have gas and water vapor tight envelopes, could minimize the accumulation of the water inside of the insulation blankets. It can eliminate the problems, such as "rain in the plain" effect, a degradation of the thermal and acoustic properties, a growth of mold and bacteria and other problems mentioned in chapter 2.1.

Manufacturing process
The feasibility study tests were performed for all four structure solutions to analyze its manufacturing process and verify its robustness.
The manufacturing process for the solution 1 with the honeycomb core consisted of several steps as shown in Fig. 12.
Structure 1 was manufactured using a honeycomb core, made of phenolic resin impregnated polyamide paper. The core was filled with pyrogenic silica powder to achieve nano pores in the core, which led to a reduction of mean free path of air molecules, penetrated through high barrier foil ("Knudsen effect") mentioned in Sect. 3.1. The prepregs, which are used in the aviation industry, were applied as upper and lower face sheets. For the envelope, the high barrier foil MF4 (three-layer metalized film) was taken.
The bottom of the honeycomb core was covered by a light weight prepreg to avoid scattering of the silica powder (Fig. 12a).
After the filling process of the half-sandwich, an upper side of the panel was covered with the thermoplastic nonwoven layer, which had two functions. First, it allowed an evacuation of the honeycomb core due to its open-porous structure. Second, after the evacuation it converted into an adhesive layer by a heating process in the molding press [30].
Heat treatment of the silica powder for 30 min with the temperature of 140 °C was performed before evacuation to desorb moisture already absorbed by the silica.
In the first tests the evacuation up 200 mbar was investigated (Fig. 12c). After the achievement of satisfying results, the lower vacuum with the inner gas pressure of 5 mbar was reached. As the last step, the evacuated core was covered by the face sheets from the prepreg material in a hot molding press (Fig. 12d).
The feasibility tests demonstrated the achievement of the evacuation of the honeycomb core panel.
The second structure with VIP, which is covered by different rigid foams, was manufactured in one step, using a molding press (Fig. 13).
The finished vacuum insulation panels va-Q-pro were used in these structure solutions. To achieve a robust structure, two layers of rigid foam with the thickness of 2 mm were added to lower and upper sides of VIP. Different materials of foam, which meet aviation-specific requirements for the cabin, were applied during feasibility tests: polyether sulfone (PES), polyvinyl chloride (PVC) and polymethacrylimide (PMI).
During the tests, the bonding between all elements was investigated. As a result, no adhesive was required between the foam and prepreg. The pre-impregnated materials already have a pre-catalysed resin system, which is impregnated into reinforcement fabrics and has good adhesive properties after curing in the hot molding press. In turn, the bonding between VIP and rigid foam was achieved by application of the thermoplastic co-polyester adhesive.
Several flat panels with the size of 400 × 400 × 8 mm were manufactured, where the VIPs with a thickness of 4 mm were used. Because of an uneven distribution of the powder on the edges of the VIP, the first panels demonstrated insufficient surface quality on its edges. With the purpose to improve this issue, the panels were pressed until the nominal thickness of 7 mm, that led to good results. Besides that, the VIPs were compressed up to 2.5 mm, while the layers of rigid foam kept its thickness. To not worsen an effective thermal conductivity of the panel, the VIPs with the thickness of 5 mm were used in the further tests, which were compressed up to 4 mm during a molding process. The polyurethane-based aerogel insulation board was applied as a core in structure 3. Before the manufacturing of this structure solution, the evacuation possibility for the aerogel board, with 90% air consistency, was tested. The experiments demonstrated that the aerogel panel can be evacuated, but should be heat treated before or during the evacuation process. The aerogel board was covered by a thermoplastic nonwoven layer (Fig. 14a) as in the manufacturing process of the solution 1 to enable an evacuation and following bonding between aerogel and high barrier foil.
Structure solution 4 "Double Sandwich-VIP bonded on Honeycomb Core" also could be manufactured in one step during hot press molding, the same as solution 2. The feasibility of this solution was proved (Fig. 15).
Similar to the second proposal, the finished VIPs va-Q-pro with the thickness of 5 mm were applied to achieve sufficient surface on the edges and to obtain a minimum of 4 mm of the insulation material after the compression. The pregreg was chosen as a middle layer between the VIP and honeycomb core with the purpose to get the required bonding and to protect VIP from damage by stiff cells of the honeycomb core during the compression process.
Panels from all four solutions demonstrated robust structure, good surface quality and sufficient bonding between the core and face sheets. Therefore, the next step after the manufacturing tests was the performance of thermal conductivity tests.

Thermal conductivity test
The thermal conductivity tests were performed for all new structure solutions.
The solutions were compared to conventional sandwich structures without and with glass wool insulation (conventional 1 and 2) to show a thermal potential of new proposals. In addition, the vacuum insulation panels used in solutions 2 and 4 were tested to analyze possible improvements later.
The thermal conductivity measurements were carried out in a guarded hot plate two-specimen apparatus (Taurus TLP 300-DTX) in accordance with the international standard ISO 8302. The test specimens had an overall size of 300 × 300 mm. Two samples were tested for each solution. The thickness of the panels that were tested for thermal conductivity varied. The solutions 2 and 4 had the thickness of 8 mm, whereas the solutions 1 and 3 had 7 mm and 10 mm, respectively. The conventional sandwich structure had the thickness of 6 mm in addition to the glass wool insulation with 20 mm. The mean values of the results, which were obtained during the tests, are shown in Fig. 16.
Solutions 2a, 2b, 2c, 3 and 4 showed similar results in range between 10.3 and 12.6 mW/m K. The thermal conductivity of solution 1 had the value of 24.5 mW/m K. The higher value was obtained due to the cell size of the honeycomb core (4.8 mm) used in the structure. To achieve lower values till 10 mW/m K, it is necessary to work with bigger cell sizes, e.g., 12 mm. Proposals 2 and 4 can achieve lower values, if the VIP has a thermal conductivity in the range of 4-5 mW/m K. The current VIPs used in the new solutions had the value of 6.7 mW/m K.

Maintaining of vacuum in VIP after hot press molding
The maintaining of the vacuum inside the VIP is an important factor for the service life of the panel, which is intended to be used for long-term applications. The barrier performance of the envelope of the VIP should not deteriorate after a manufacturing step in a hot press molding. The metalized film MF4, which was used in the VIPs during the feasibility manufacturing tests, has three metalized polyethylene terephthalate (PET) and one sealable linear low-density polyethylene (LLD-PE) laminates.
The PET film has outstanding properties such as tensile strength, chemical resistance, light weight, elasticity and stability over a wide range of temperatures (− 60° to 220 °C) Fig. 13 Manufacturing process of the structure solution 2 "VIP covered by foam and prepreg": a panel in molding press before heat processing, b manufactured panel with PES foam, c manufactured panel with PMI foam, d manufactured panel with PVC foam [31]. The melting temperature T m of PET is in the range of 240-265 °C. The inner sealing layer of LLD-PE has the melting temperature T m of 122-124 °C [32].
Further tests were performed to investigate the impact of the press (3.5 bar) and heat treatment (140 °C) for the curing time of 75 min. Three flat panels of 400 × 400 × 5 mm, which consisted of the VIP (va-Q-vip) as a core and the prepreg from both sides as face sheets, were manufactured (Fig. 17).
After the manufacturing of the test samples, the thermal conductivity of the panels was measured. After 1 year, the Fig. 14 Manufacturing process of the structure solution 3 "Evacuated Aerogel Core": a aerogel foam covered by nonwoven before putting into the high barrier foil, b evacuation process, c evacuated aerogel board, d manufactured panel with bonded face sheets Fig. 15 Manufacturing process of the structure solution 4 "Double Sandwich-VIP bonded on Honeycomb Core": a panel before press molding before heat processing, b manufactured panel test was repeated to see the difference between the thermal conductivity values. The obtained test results are shown in Table 1.
Thermal conductivity slightly changed in range from − 4 to + 2%. The "-" values can be explained by tolerance of measurement in the test facility.
The results demonstrated that the thermal performance of the test panels didn't worsen after 1 year. As a conclusion, it can be stated that the barrier performance of the VIP envelope didn't deteriorate after the manufacturing process of the flat panels in a hot molding press with a temperature of 140 °C and pressure 3.5 bar.

Conclusions and outlook
In this research project the integration of highly efficient insulation VIP into cabin interior panels was analysed. Four structure solutions were proposed for further feasibility study tests. The experiments demonstrated that all new solutions can be manufactured.   The new solutions showed a big difference in thermal properties in comparison to conventional structures. The thermal conductivity of new structures is lower by the factor of 3-6. Even lower values can be achieved with the appropriate modifications.
The investigation of the maintaining of the vacuum in the VIP demonstrated that the manufacturing process in a hot molding press doesn't have an impact on the barrier performance of the envelope. The tested high barrier film can withstand high-temperature treatment while keeping a certain level of vacuum. The thermal conductivity tests, which were performed after the manufacturing of the test samples and 1 year later, showed no degradation of the insulation performance.
In addition to the analysis of thermal properties for the new solutions, further investigations for mechanical properties, acoustics, fire properties and others are intended to get a solution that fulfills the aviation-specific requirements. Furthermore, feasibility study for a manufacturing of curved panels on example of sidewall and an analysis of different leakage factors such as the gap between interior panels, thermal bridges and transition areas etc. for the cabin will be performed.