This invention relates to insulation materials. The insulation materials disclosed are of particular use in the insulation of vehicle data recorders, but the materials disclosed are not limited to such applications.

Background

Flight data recorders, popularly known as "black boxes", are of importance in aviation safety.

Frequently, when an aircraft disaster occurs, the best and most complete information on the events leading up to the incident is to be found in the data recorder. Data recorders are beginning to be used in vehicles other than aircraft.

Naturally, it is of important that the data recorder is provided with as much protection as is reasonably possible to maximize its odds of surviving a crash with its data intact. As well as the physical shock of crashing, a data recorder may have to survive being submerged in sea water, or exposure to fire or explosions.

US patent nos. US5804294 and US6045718 describe microporous materials with a range of industrial applications, including as a thermal insulation material to protect a data recorder from a thermal event (such as a fire or a flash of heat from an explosion). The materials of these patents comprised an inorganic particulate material, an endothermic compound (including, for example, alumina trihydrate, magnesium carbonate, melamine, and water), an opacifier, an inorganic fiber and a binder. These materials were pressed to shape and then further machined as required for the application. US5804294 and US6045718:-

• have a maximum endothermic material content of 60%;

• include inorganic particulate material, particularly microporous material, to provide

insulation; and

• include an opacifier to inhibit radiative heat transmission.

The materials of US5804294 and US6045718 could be used in dry form or, where the application permitted, in water-soaked form to obtain higher endothermic effect through the latent heat of vaporisation of water. Where free water was not acceptable, the dry form of the materials of US5804294 and US6045718 could be used.

In dry form endothermic materials such as alumina trihydrate could be used which evolve water (dehydrate) on being heated. Alumina trihydrate [AI(OH) ₃ - also described as AI ₂ 0 ₃ -3H ₂ 0] decomposes on the application of heat at around 220°C [430°F] to produce about 35% of its weight as water vapor. [Alumina trihydrate is also known as a component of concretes and cements, for example US4983220 and EP0030662].

A typical formulation of microporous materials such as disclosed in US5804294 and US6045718 is given in Table 1:- Table 1

Fumed Silica 10.0%

Resin Binder 5.0%

Alumina Trihydrate 49.0%

Alumina Oxide C 30.0%

1/2" E-Glass 1.0%

SW607 alkaline earth silicate fiber 5.0% and on exposure to heat such a material gives off about 112kg. m ^"3 (7pcf) of water released from the alumina trihydrate. A typical specific heat for such a material is around SkJ.g ^{' 1} .

A constraint to using more alumina trihydrate (and thereby a greater endothermic effect) is that the specific heat of the alumina (that results on dehydration of the alumina trihydrate) is relatively high such that heat retained at the surface of the insulation after exposure to temperature can percolate through the insulation.

Additionally, the resin binder will give off heat through combustion adding to the heat load that the material has to cope with in the event of a fire.

For a given insulation material, the insulation provided to a flight data recorder or other device protected by the material can be increased by increasing the thickness of material provided:

however, increasing the thickness increases the weight and size of the insulation material. Any particular aircraft will have a limit on how much weight it can carry and how much space it can provide for the flight data recorder and any protecting thermal insulating material for it. Thus, a trade-off exists between the competing considerations of keeping the weight and volume of insulating material protecting a flight data recorder within acceptable limits while providing appropriate thermal insulation effect.

Summary of disclosure The present disclosure provides a thermal insulator configured to house and protect an article against fire, the thermal insulator comprising one or more bonded bodies of thermal insulation material comprising one or more endothermic materials comprising chemically bound water and capable of liberating water on heating to form an at least partially dehydrated residue wherein:- • the bonded body is or comprises a hydraulically set inorganic material; and

· wherein chemically bonded water is liberated on heating to a temperature of 260°C, and preferably wherein the amount of chemically bonded water liberated from the endothermic materials is greater than 400 kg.m ^"3 of the insulation material.

Preferably the insulation materials further comprise fillers having a lower heat capacity per unit volume than the at least partially dehydrated residue. The invention is as set out in the appended claims and as further exemplified in the following illustrative, but not limiting, description and appended drawings. Description of Drawings

Fig. 1 shows a thermal profile of a prior art thermal insulation material under testing in

comparison with a bonded body comprising 100% alumina trihydrate formed by hydration and hydraulic bonding of a hydratable alumina;

Fig. 2 shows thermal profiles of a prior art thermal insulation material in comparison to bonded bodies comprising varying proportions of alumina trihydrate formed by hydration and hydraulic bonding of a hydratable alumina with a vermiculite filler;

Fig. 3 shows thermal profiles of a prior art thermal insulation material in comparison to bonded bodies dried under different conditions, and comprising 95% by weight alumina trihydrate formed by hydration and hydraulic bonding of a hydratable alumina, with 5% vermiculite filler;

Fig. 4 shows thermal profiles of a prior art thermal insulation material in comparison to a bonded body comprising 95% by weight alumina trihydrate formed by hydration and hydraulic bonding of a hydratable alumina, with 5% vermiculite filler, under more extreme conditions that the test of Figs 1 to 3;

Fig. 5 shows a flight data recorder can being filled with a wet mix preparatory to forming a

hydraulic bond;

Fig. 6 shows temperature profiles for materials produced by setting a slurry comprising hydratable alumina and phase change material beads;

Fig. 7 shows temperature profiles at several points inside a module insulated using insulation formed from a material in accordance with a further embodiment of the invention;

and

Fig. 8 shows temperature profiles at several points inside a module insulated using insulation formed by pressing a material in accordance with a further embodiment of the invention.

Detailed Description

The present disclosure provides bonded bodies of thermal insulation material. The constituent materials of this thermal insulation material are at minimum one or more endothermic materials, comprising chemically bound water and capable of liberating water on heating to form an at least partially dehydrated residue, the bonded bodies are or comprise a hydraulically set inorganic material. The endothermic material is preferably but not essentially provided in amounts such that on heating to a temperature of 260°C the amount of chemically bound water liberated from the endothermic materials is greater than 400 kg.m ^"3 of the insulation material.

Endothermic materials not comprising chemically bound water may additionally be present [e.g. phase change materials, or materials liberating gases on heating]. "Free water" (water in physical contact with the material which is free to flow away or evaporate) is not excluded from being present but is not necessary. The endothermic materials may comprise endothermic materials set with an additional material providing a hydraulic bond, or the endothermic materials may themselves provide a hydraulic bond. Suitable materials for use in the insulating material and capable of forming a hydraulic bond include (without limitation) hydratable alumina, high alumina cement, Portland cement, and plaster. For example, the materials may comprise alumina trihydrate bonded with hydrated hydratable alumina: or hydrated hydratable alumina may provide both the hydraulic bond and the endothermic material.

Hydratable aluminas are finely divided microporous alumina particles that readily hydrate and bond with each other or other particles to form a hydraulic bond. They are sometimes known as transition aluminas or activated aluminas and are formed by thermal decomposition of aluminum hydroxides or oxyhydroxides. Suitable materials include Alphabond ^® from Alcoa, or Dynabond 3 activated alumina binder, from AluChem, Incorporated.

The bonded bodies may further comprise fillers. The fillers may comprise inorganic fillers. The inorganic fillers may comprise materials of lower heat capacity per unit volume than the at least partially dehydrated residue of the one or more endothermic materials. Suitable inorganic fillers include (without limitation), clay, fumed silica, perlite, vermiculite, zeolite, diatomaceous earth, hollow glass microspheres, and mixtures thereof. The presence of low density fillers can reduce the density of the bonded bodies which can be useful in aerospace applications.

Optionally, the formulation of the thermal insulation material may include an air-setting binder, and/or minor additives such as flow aids, dispersants, pressing aids, aids for the gelling of colloidal silica (when colloidal silica is used as an air-setting binder material) and setting agents. Suitable air-setting binders for use in the insulating material include colloidal silica, potassium silicate, sodium silicate, lithium silicate, and a mixture of lithium and potassium silicate. Suitable endothermic materials that may form all or part of the endothermic materials include alumina trihydrate, magnesium hydroxide, zinc borate powder, gypsum, borax, magnesium carbonate, and Dawsonite.

Suitable flow aids for use as minor additives in the insulating material include polyethylene glycol, sodium tetraborate, sodium phosphates, lithium sulphate, and lithium citrate.

Suitable setting agents for use as minor additives in the insulating material include lithium carbonate.

Suitable pressing aids for use as minor additives in the insulating material include clays.

Suitable aids for the gelling of colloidal silica for use as minor additives in the insulating material when colloidal silica is used as an air-setting binder include MgO and CaO. The materials may be made by forming a wet mixture, forming the resultant mixture into shape by casting or pressing, and permitting the mixture to undergo a hydraulic reaction, causing the mixture to set. If necessary the resultant material may be machined and tooled. Preferably, prior to final assembly the hardened material is dried to a sufficient extent to remove free water, but not to remove chemically bound water. Other forming methods are not precluded (for example, spraying, troweling, or rolling).

A typical process for producing a thermal insulation material according to the present invention may follow the following steps:

Appropriate quantities of the solid components are weighed out and combined as required.

The solid components are dry mixed [for instance, but not necessarily, in a Hobart mixer]. Once a suitably homogeneous mix is attained, continue the mixing process and add the water.

Continue to mix the wet mixture for an appropriate period sufficient to provide good blending but not so long as to cause premature setting. Pour the mix into a suitable container of cast for the shape you want the mixture to dry into. For instance, a flight data recorder "can" (external container for flight data recorder apparatus) may be appropriate for flight data recorder applications [Fig. 5].

Allow the insulation material to air set.

Using a temperature controlled oven, finish drying the part.

The flow and set time of the wet mix can be altered through the use of flow and set time additives. As an alternative, the shaped material may be produced via pressing instead of casting; if so additives may be added to improve pressing behaviour. It has been discovered that if pressing substantially lower pressures are required compared to pressing of prior art microporous based- materials. Comparative Example 1

For the purpose of testing the properties of thermal insulation materials according to the invention, the materials were compared to an existing prior art material currently supplied to the industry for the purpose of flight data recorder protection. This formulation is as set out in Table 1 above.

Testing Method

The test standard for thermal insulation materials for the protection of flight data recorders is EU Civil aviation test procedure ED-212. The US Federal Aviation Authority requires that solid state recorders must survive a minimum of 60 minutes under testing conditions. The test entails setting up a flight data recorder can housing the thermal insulation, and directing three propane burners at the can to maintain a temperature at the surface of the can within the range of 933°C-1100°C (1712°F to 2012°F), with 1093°C (2000°F) being the target. For the flight data recorder data module to survive the test the temperature at the core of the thermal insulation material ["inside can temperature"] must not reach or exceed 260°C (500°F).

All of the tests were done with essentially the same geometry of can and insulating material. Example 1

Figure 1 shows the thermal profiles measured in a test comparing the performance of the prior art material of Table 1 and a sample of 100% alumina trihydrate (ATH) formed by hydration of hydratable alumina with water using the method outlined above.

As can be seen, both meet the requirement that the inside can temperature remains below 260°C (500°F) at 60 minutes. However it can also be seen that after the burners are switched off, the temperature inside the can continues to rise as heat stored in the outer part of the can percolates inwards as the outer part cools. For the prior art material the maximum temperature approaches the 260°C (500°F) at around 90 minutes whereas for the 100% ATH material the maximum temperature is significantly below the 260°C (500°F) at around 90 minutes.

It can also be seen that the endothermic plateau resulting from the evaporation of steam from the material is much shorter for the prior art material of Table 1 than for the 100% alumina trihydrate (ATH) material, keeping the core below about 149°C (300°F) only up to about 35minutes; whereas the 100% ATH material keeps the core below 149°C (300°F) for the entirety of the 60 minutes providing a wide safety margin and enabling the use of less temperature sensitive components in the data recorder. Example 2

Fig. 2 shows the result of using vermiculite as a filler in varying amounts in materials formed by the method outlined above and comprising alumina trihydrate (ATH) formed by hydration of hydratable alumina with water using the method outlined above. Compositions and characteristics of the examples are set out in Table 2

As can be seen, for vermiculite loading of 15% and 25% by weight, the material behaves worse than the standard material: whereas with only 5% it is similar to the 100% ATH product of Example 1; again shows the maximum temperature significantly below the 260°C (500°F) level at around 90 minutes; and shows that the core is kept below 149°C (300°F) for the entirety of the 60 minutes.

The material with 25% vermiculite loading evolved about 240 kg.m ^"3 [15pcf]; that with 15% vermiculite loading evolved about 336 kg.m ^"3 [21pcf]; and that with 5% vermiculite about 528 kg.m ^"3 [33pcf].

Thus the presence of fillers (in this case vermiculite) comprising materials of lower heat capacity per unit volume than the at least partially dehydrated residue of the endothermic material (in this case alumina, following degradation of alumina trihydrate) can reduce the "carry over" of heat after the heat source is removed. However, as the filler is substituting for endothermic materials, provision of too much filler reduces the effect of the endothermic.

Example 3

Figure 3 shows the thermal profiles measured in a test comparing the performance of the prior art material and samples of thermal insulation materials produced according to the method outlined above. Each time, hydratable alumina, vermiculite, and water were used in the production of the thermal insulation material in such proportions that, after drying, the thermal insulation material comprised 95% of alumina trihydrate and 5% vermiculite (all percentages by weight). The samples of thermal insulation material varied in the temperature at which the materials were dried.

As can be seen from Figure 3, not only do all these samples of thermal insulation material produced according to the method of the present invention provided a superior performance to the prior art material: but also increasingly superior thermal insulation is produced as the drying temperature decreases, with the best results arising when the thermal insulation is dried at room temperature (said sample not quite reaching a core temperature of 149°C (300°F)). Advantageously therefore drying takes place below 50°C, below 40°C or preferably at 30°C or below.

Example 4 Figure 4 shows the thermal profile measured in a test comparing the performance of the prior art material (plain line) and a sample of thermal insulation material prepared as in Example 3 and air dried (line with marker□). However in this test the burners operated for 90 minutes instead of the

60 minutes required by the standard test. It can be seen from this figure that the prior art material exceeds the maximum permitted temperature by a substantial margin under these conditions, whereas the thermal insulation material according to the present invention never reaches 260°C (500°F). It is apparent too that this material remains below 149°C (300°F) even after 90 minutes of exposure to flame. Example 5

The use of phase change materials in addition to endothermic materials comprising chemically bound water, permits absorption of heat through phase change. Phase change materials typically are materials having a high heat of fusion, absorbing heat in the process of melting.

It has been found that among the many phase change materials that can be used, waxes provide a particularly useful material in this respect, since they may start to absorb heat through melting at relatively low temperatures, and can further absorb heat through vaporization at higher temperatures. The vapors produced can dissipate and take heat away from an article being protected.

Fig 6 shows temperature profiles for materials produced by setting a slurry comprising hydratable alumina and phase change material beads in the proportions for casting Mix A and casting Mix B set out in Table 3 below.

The materials were dried as set out in Table 4 below and tested using the burner times there indicated:-

The phase change material used (Microtek Laboratories MPCM 43D - microencapsulated phase change material), comprised a paraffin wax core material encapsulated in a polymer shell, having mean particle size 17-20μιη and with a melting point in the region of 43°C.

As can be seen from Fig. 6, despite being exposed to the burners for 60 minutes, the standard material approaches the 260°C (500°F) limit at around 90 minutes, whereas the formulations containing phase change materials go nowhere near the temperature limit even after 90 minutes burner exposure. Example 7

Fig 7 shows temperature profiles at several points inside a module insulated using insulation formed by casting from the components of Table 5:

As can be seen, even after 120 minutes with the burners on, these materials never reach the maximum allowed temperature the260°C (500°F). This formulation provides a hydraulic bond through the hydratable alumina, with the zinc borate and hydratable alumina (once hydrated to ATH) providing the chemically bound water. The sodium carbonate, although anhydrous as an ingredient in the original mix, is water soluble, and the applicants hypothesize (without wishing to be bound to this hypothesis) that when the mixture dries the sodium carbonate forms hydrates with the water supplied, so providing further chemically bound water. In addition, sodium carbonate evolves carbon dioxide endothermically at the elevated temperatures experienced in a fire, and the carbon dioxide produced can carry heat away, so both endothermic cooling and gas transfer will contribute to removing heat from the insulation material. This shows the advantage of having endothermic materials that liberate gases other than water vapor.

The presence of phase change materials having a phase change at temperatures between 25 ^Q C and 260°C provides additional absorption of heat in the core of the insulation, so lessening the amount of evolved water that needs to be evolved to maintain a given temperature. More than one phase change material may be used to provide cooling over a range of temperatures.

The material of Table 5 shows endothermic peaks at a number of temperatures attributable to:-

• Melting of the phase change material

• Loss of chemically bound water of hydration from sodium carbonate

• Loss of chemically bound water from ATH

• Loss of chemically bound water from zinc borate hydrate

• Decomposition of the sodium carbonate

So providing absorption of heat at a range of temperatures, and hence at a range of depths through the insulation in the event of fire. Example 8

A pressing formula would ideally have significantly less water than a cast mix. Casting uses more water than is necessary to produce a hydraulic bond and this excess water can impair mechanical properties and require excessive drying times. By using water at a level such that the process of hydraulic bonding takes up all or most of the water during the setting process, detrimental effects of excessive water can be reduced. For example, the formulation shown in Table 5, with the amount of water reduced to 25 parts by weight can be pressed, and Fig. 8 shows in like manner to Fig. 7, temperature profiles at several points inside a module insulated using insulation formed by pressing the modified formulation.

It should be noted that the mixture is fluent enough that it can be pressed around an article to be insulated, providing that article has strength enough to resist the pressures involved.

Typical set and dried compositions for materials falling within the present invention are set out in Table 6:-

For the materials comprising zinc borate, sodium carbonate, hydratable alumina, and phase change materials typical compositions might be as set out in Table 7. Table 7

Component Amount (parts by weight)

Zinc borate 30-70

Sodium carbonate 5-40

Hydratable alumina 5-40

Phase change material 5-40

Water 5-40

Other (e.g. flow aids, dispersants and setting 0-10

agents)

It is apparent, that thermal insulation materials according to the present invention can pass substantially more stringent testing conditions than currently apply, conditions which would otherwise cause prior art thermal insulation materials to fail catastrophically.

In the alternative, the improved thermal protection provided by the materials of the present invention will permit less insulation material (by volume) to be used than previously, while still meeting the current regulatory tests. This will permit manufacture of data recorders occupying less space [and mass] than is currently required; or to allow for a greater volume of recording apparatus while retaining the same outside dimensions.

The methods and products described above possess numerous advantages. A short time after water is added during the forming process, the hydraulic binder [which may be or form part of the endothermic material and is preferably hydratable alumina]) undergoes a hydraulic reaction - (so hydratable alumina will form alumina hydrates, including alumina trihydrate). This hydraulic reaction hardens the shape after forming, allowing it to be machined and painted as required. This replaces the need for organic, heat-set binders, which give off heat when oxidized. As shown in the above examples, thermal insulation materials produced according to the present invention undergo less heating at the core after external heat sources are deactivated than the prior art materials. One factor contributing to this is the use of filler material having a lower volume heat capacity than the at least partially dehydrated residue of the endothermic materials, minimizing the extent of core heating after the heat source is removed by minimizing thermal mass.

Such materials [e.g. vermiculite, perlite) also reduce the mass of the as-made product, weight being an important factor in aerospace applications. Tested formulations show a 50% improvement in thermal insulation behavior over the prior art material with only a 20% increase in mass and this weight difference may be further minimized, for instance by optimizing the formulation density to balance thermal mass, thermal conductivity/insulation and endothermic ability.

Examples 9 and 10

Compositions comprising the following components of Table 8 were cast and dried:- Table 8

Component 10% Mix - parts by weight 30% Mix - parts by weight

Zinc borate 80 60

Perlite 10 30

Hydratable alumina (Dynabond) 10 10

Water 194.6 88

Both compositions on exposure to the burners for 60 minutes were capable of keeping the peak temperature within the insulation below 180°C (356°F) even after the burners were turned off. This provides a large margin of safety for temperature sensitive components.

Formulations according to the present invention also do not require (but do not preclude) opacifiers or the use of insulation fibers for product performance, unlike prior art materials.

Another factor that contributes to less heating at the core after external heat sources are deactivated than the prior art materials, is use of endothermic fillers, and phase change materials.

Endothermic fillers may be added to work in conjunction with the hydrated hydraulic binder to increase endothermic potential and the timing/temperature at which the heat absorption occurs during a thermal event. Air-setting binders may be added to add strength after drying, in order to prevent cracking and improve machinability. Their inclusion may reduce the amount of hydraulic binder needed if so desired.

Including a glass former into the formulation helps reduce open porosity, thereby slowing the convective transfer of hot air from the surface to the core of the thermal insulation material. Such glass formers may either come from water glass air setting binders (silicates) or through

decomposition of the endothermic fillers based on borates such as zinc borate or borax.

Other materials to improve strength of the thermal insulation (e.g. inorganic fibers) may be required in some applications.

Since the thermal insulation material may be shaped by casting or pressing under light load, it is even possible to incorporate the flight data recorder electronic data collection module into the lining as a monolithic piece, eliminating the need for an access port to allow for placement into the center of the module after forming and machining. Other forming methods can be used to apply the materials, for example by spraying, troweling, or rolling.

An important advantage of the present materials, which are set hydraulically, is that they do not require exposure to high temperature during manufacture so enabling the retention of high quantities of endothermic materials that might otherwise degrade (for example at temperatures as might occur in curing a resin binder).

The above description has focused on flight data recorders, but other articles requiring protection against fire can be protected using the materials of the invention. For example, energy storage devices (including but not limited to lithium ion batteries or cells) can be protected against fire (from the general environment or from an adjacent energy storage device in an assembly of energy storage devices) using the materials of the invention. In addition the materials of the invention can be used to house and protect metal (e.g. steel or alloy) structural members in buildings or other structures. For this application the materials can be sprayed or applied by any other suitable method.

The above description is illustrative of the invention and modifications and variants will be apparent to the person skilled in the art without departure from the scope of the appended claims.