Use of 1-year saltwater plant fibers to produce a bio-based ablative thermal protection material for aerospace.

Abstract

Use of 1-year saltwater plant fibers to produce a bio-based ablative thermal protection material for aerospace.

Description

[0001] The invention relates to thermal protection using materials made from renewable raw materials, whereby, in addition to heat protection, good mechanical properties or the ability to absorb loads also play a role, so that use in space travel is possible.

[0002] Regarding thermal protection, there are various principles: ceramic heat protection tiles, passive cooling, active cooling and heat protection using ablative thermal protection materials (TPS = thermal protection systems).

[0003] Ablative thermal protection means that the material sacrifices itself under heat, thereby dissipating the heat energy by, for example, burning up, flaketing off, or releasing cooling gases. This form of thermal protection is of great interest in aerospace applications.

[0004] Cork materials are widely used for ablative thermal insulation. These are usually applied as flexible, pliable sheets glued to substrates and serve solely as thermal insulation. Such materials are made from cork granules with the addition of a binder. Furthermore, cork is only available in very limited quantities.

[0005] Cork-based TPS do not yet meet the mechanical requirements for structural materials in aerospace.

[0006] Seagrass fibers have a long tradition in the insulation of buildings. They are used in their dried form.

[0007] DE 199 54 474 C1 discloses insulation materials made from biogenic raw materials for interior building construction – specifically aimed at thermal and acoustic insulation. Mechanical stabilization plays no role in this application. DE 20 2005 012 457 U1 discloses seagrass noise protection elements. DE10017202A1 also describes seagrass for thermal and acoustic insulation in buildings. The stated strength values ​​are very low.

[0008] German patent DE 1 695 391 U describes sound pressure plates made of porous material, e.g., wood or seagrass. Here, too, cavities are filled, using a porous sound-absorbing material whose flow resistance is lower than that of the plate material.

[0009] DE 10 2020 001 467 A1 discloses insulating bricks with an insulating material, such as wood fiber or seagrass fiber. The insulating bricks preferably do not contain a binder.

[0010] CH452885A describes a process for manufacturing a coated synthetic board and synthetic boards obtained by the process. A polyurethane coating is applied to a board, causing the board to exude moisture. This board can be a wood fiberboard. Synthetic boards contain, for example, wood fibers (see description) with or without a synthetic binder. In addition to wood particles, seagrass, for example, can also be used as a lignocellulose material.

[0011] German patent applications DE 103 41 205 A1 and DE 20320953 U1 describe a molded body made of biodegradable material and a process for its production, in which seagrass is fibrillated by mechanical or thermomechanical fiberization and pressed into materials with the addition of synthetic or natural binders. The molded body is intended for use in furniture manufacturing, the packaging and automotive industries, or in residential interior design.

[0012] EP3540027A1 describes the use of tannin-based flame retardants in natural fiber materials.

[0013] DE 100 17 202 A1 discloses an insulating material component consisting of a mixture of seagrass fibers, binders and aggregates, wherein the proportion of seagrass in the component is between 20 and 95 wt% and the seagrass used contains up to 10 wt% NaCl.

[0014] EP 1 247 916 B1 describes insulating body, in particular an insulating board, based on lignocellulose-containing fibers bonded with a binder, wherein the fibers in the insulating body have a preferred orientation parallel to a plane extending transversely to the front surface and transversely to the rear surface and / or at least one internal adhesive surface extending parallel to the plane of the preferred orientation is provided, on which the pressing surfaces of two wood fiber boards are bonded together, and that each wood fiber board has an edge increase of its bulk density relative to its mean bulk density of at least 20% in the area of ​​its pressing surfaces.

[0015] The combination of wood fibers and seagrass has been known for a long time. However, existing materials lack sufficient heat resistance combined with mechanical strength. Their use in space travel has been inconceivable until now.

[0016] The use of wood-based or seagrass-based materials as TPS or structural materials is not yet known in space travel.

[0017] The object of the invention is to provide a material, largely derived from renewable raw materials, that protects against or shields from heat while simultaneously exhibiting mechanical strength. In particular, the invention is intended to enable its use in aerospace or other fields where ecology, thermal protection, or mechanical protection are important. This includes, for example, rocket stages and their components, which are not recovered after use and remain in the environment.

[0018] The invention aims to enable the use of materials containing wood and seagrass as ablative and structural materials in aerospace applications. The proportion of necessary binding agents should be kept to a minimum.

[0019] Structural materials are defined as materials that meet the mechanical strength requirements for components.

[0020] In addition, it should enable the replacement of fiberglass-reinforced plastics or metals such as stainless steel.

[0021] The invention is solved by the features of the independent main claim. Preferred embodiments are clarified in the dependent claims.

[0022] The invention relates to the use of 1-year saltwater plant fibers for the production of a bio-based ablative thermal protection material with increased strength for the aerospace industry, in particular the thermal protection material described below.

[0023] In embodiments, a bio-based (i.e., largely produced from renewable raw materials) ablative thermal protection material with increased strength for aerospace applications is used, comprising a mixture of a binder and natural fibers, wherein the natural fibers are selected from: a) wood fibers and b) 1-year saltwater plant fibers, wherein • the binder, preferably an isocyanate-based binder, with a proportion of 6-12% dw Faser (i.e., 6-12 wt% based on the dry mass of the fibers) is contained in the mixture, and • the weight ratio of wood fiber : 1-year saltwater plant fiber is 30 : 70, with a maximum deviation of ±20 wt.% (this means that the weight ratio, with maximum deviation, is in the range of 10-50 : 50-90).

[0024] "Mixture" means that the natural fibers and the binder are mixed almost homogeneously.

[0025] "Annual saltwater plants" are marine plants with cellulose structures and a high salt content. They are annuals. Examples include seagrass or plants of the Zosteraceae family, specifically the genus Zostera. All plants of the Zosteraceae family, specifically the genus Zostera (names in bold are accepted names according to Govaerts, R., Dransfield, J., Zona, S., Hodel, DR & Henderson, A. (2022). World Checklist of Zosteraceae. Made possible by the Royal Botanic Gardens, Kew. Published online; http: / / wcsp.science.kew.org / qsearch.do?page=quickSearch&plantName=Zosteraceae&page=quickSearch): ◯ Alga Tourn. ex Lam., Fl. Franc. 3:539 (1779). ◯ Alga marina (L.) Lam., Fl. Franc. 3:539 (1779). ◯ Heterozostera (Setch.) Hartog, Verh. Kon. Ned. Akad. Wetensch., Afd. Natuurk., Sect. 2, 59(1): 114 (1970). ◯ Heterozostera chilensis J.Kuo, Aquatic Bot. 81: 126 (2005). ◯ Heterozostera nigricaulis J.Kuo, Aquatic Bot. 81: 110 (2005). ◯ Heterozostera polychlamys J.Kuo, Aquatic Bot. 81: 124 (2005). ◯ Heterozostera tasmanica (M.Martens ex Asch.) Hartog, Verh. Kon. Ned. Akad. Wetensch., Afd. Natuurk., Sect. 2, 59(1): 116 (1970). ◯ Nanozostera Toml. & Posl., Taxon 50: 432 (2001). ◯ Nanozostera americana (Hartog) Toml. & Posl., Taxon 50: 432 (2001). ◯ Nanozostera capensis (Setch.) Toml. & Posl., Taxon 50: 432 (2001). ◯ Nanozostera capricorni (Asch.) Toml. & Posl., Taxon 50: 432 (2001). ◯ Nanozostera japonica (Asch. & Graebn.) Toml. & Posl., Taxon 50: 432 (2001). ◯ Nanozostera mucronata (Hartog) Toml. & Posl., Taxon 50: 433 (2001). ◯ Nanozostera muelleri (Irmisch ex Asch.) Toml. & Posl., Taxon 50: 433 (2001). ◯ Nanozostera noltii (Hornem.) Vol. & Posl., Taxon 50: 433 (2001). ◯ Nanozostera novaelandica (Setch.) Vol. & Posl., Taxon 50: 433 (2001). ◯ Phyllospadix Hook., FI. Bor.-Amer. 2: 171 (1838). ◯ Phyllospadix × choshiensis Miyata & H.Ohba, Seagrasses Japan: 10 (2007), and Latin descr. ◯ Phyllospadix iwatensis Makino, J. Jap. But. 7: 15 (1931). ◯ Phyllospadixjaponicus Makino, Bot. Mag. (Tokyo) 11: 137 (1897). ◯ Phyllospadix juzepczukii Tzvelev, News Syst. Vyssh. Right. 18: 55 (1981). ◯ Phyllospadix ruprechtii Tzvelev, Novosti Syst. Vyssh. Right. 18: 56 (1981). ◯ Phyllospadix scouleri Hook., FI. Bor.-Amer. 2: 171 (1838). ◯ Phyllospadix serrulatus Rupr. ex Asch., Linnaea 35: 169 (1868). ◯ Phyllospadix torreyi S.Watson, Proc. Bitter. Acad. Arts 14: 303 (1879). ◯ Zostera L., Sp. PI.: 968 (1753). ◯ American Zostera Hartog, Verh. Kon. Ned. Akad. Wetensch., Afd. Natuurk., Sect. 2, 59(1): 74 (1970). ◯ Zostera angustifolia (Hornem.) Rchb., Icon. FI. Germ. Helv. 7:3 (1845). ◯ Zostera angustifolia Loser, Oesterr. Bots. Z. 13: 383 (1863), sensu auct. ◯ Zostera Asiatica Miki, Bot. Mag. (Tokyo) 46: 776 (1932). ◯ Zostera bullata Delile, Descr. Egypte, Hist. Nat. 2(Mem.): 289 (1813). ◯ Zostera caespitosa Miki, Bot. Mag. (Tokyo) 46: 780 (1932). ◯ Zostera capensis Setch., Proc. Natl. Acad. Sci. USA 19:815 (1933). ◯ Zostera capensis f. elatior Setch., Proc. Natl. Acad. Sci. USA 19:810 (1933). ◯ Zostera capricorni Asch., Sitzungsber. Jesus. Naturf. Freunde Berlin 1876: 11 (1876). ◯ Zostera caulescens Miki, Bot. Mag. (Tokyo) 46: 779 (1932). ◯ Zostera chilensis (J.Kuo) SWLJacobs & Les, Telopea 12: 422 (2009). ◯ Zostera ciliata Forssk., Fl. Aegypt.-Arab.: 157 (1775). ◯ Zostera dubia EHLKrause in J.Sturm, Deutschl. Fl. Abbild., ed. 2, 4: 70 (1905), not validly publ. ◯ Zostera emarginata Ehrenb. & Hemprich ex Asch., Linnaea 35: 167 (1867). ◯ Zostera geojeensis H.Shin, SKCho & YSOh, Algae 17: 71 (2002). ◯ Zostera hornemaniana Tutin, J. Bot. 74:227 (1936). ◯ Zostera hornemannii Rouy in G.Rouy & J.Foucaud, Fl. France 13: 290 (1912), nom. superfl. ◯ Zostera japonica Asch. & Graebn. in HGAEngler (ed.), Pflanzenr., IV, 11: 32 (1907). ◯ Zostera japonica subsp. Austroasiatic Ohba & Miyata, Seagrasses Japan: 9 (2007), no Latin descr. ◯ Zostera latifolia Morong, Mem. Torrey Bot. Club 3(2): 63 (1893). ◯ Zostera marina L., Sp. Pl.: 968 (1753). ◯ Zostera marina var. angustifolia Hornem., Fl. Dan. 9(3): t. 1501 (1807). ◯ Zostera marina proles angustifolia (Hornem.) Asch. & Graebn., Syn. Mitteleur. Phl. 1:298 (1907). ◯ Zostera marina var. atam TWHBackman, Canada. J. Bot. 69:1368 (1991). ◯ Zostera marina subsp. hornemaniana (Tutin) Rothm., Excurs. Deutschl., ed. 7, 4:19 (1963). ◯ Zostera marina proles hornemannii Rouy in G.Rouy & J.Foucaud, Fl. France 13: 290 (1912). ◯ Zostera marina var. izembekensis TWHBackman, Canad. J. Bot. 69: 1368 (1991). ◯ Zostera marina var. latifolia Morong, Bull. Torrey Bot. Club 13:160 (1886), nom. illeg. ◯ Zostera marina f. latifolia (Morong) Setch., Bull. Torrey Bot. Club 54:3 (1927). ◯ Zostera marina var. latifolia Gray, Nat. Arr. Brit. Pl. 2: 37 (1821 publ. 1822). ◯ Zostera marina var. major Roth, Enum. Pl. Phaen. Germ. 1(1): 8 (1827). ◯ Zostera marina var. phillipsii TWHBackman, Canada. J. Bot. 69:1368 (1991). ◯ Zostera marina var. stenophylla (Raf.) Asch. & Graebn., Syn. Mitteleur. Phl. 1:297 (1897). ◯ Zostera marina f. sulcatifolia Setch., Proc. Natl. Acad. Sci. USA 19: 813 (1933). ◯ Zostera marina f. typica Setch., Bull. Torrey Bot. Club 54:3 (1927), not validly publ. ◯ Zostera maritima Gaertn., Fruct. Sem. Pl. 1:76 (1788). ◯ Zostera mediterranea DC. in JBAMde Lamarck & APde Candolle, Fl. Frang., ed. 3, 3: 154 (1805). ◯ Zostera minor Nolte ex Rchb., Icon. Fl. Germ. Helv. 7:2 (1845). ◯ Zostera mucronata Hartog, Verh. Kon. Ned. Akad. Wetensch., Afd. Natuurk., Sect. 2, 59(1): 91 (1970). ◯ Zostera muelleri Irmisch ex Asch., Linnaea 35: 168 (1867). ◯ Zostera muelleri subsp. capricorni (Asch.) SWLJacobs, Telopea 11: 128 (2006). ◯ Zostera muelleri subsp. mucronata (Hartog) SWL Jacobs, Telopea 11: 128 (2006). ◯ Zostera muelleri subsp. novazelandica (Setch.) SWLJacobs, Telopea 11: 128 (2006). ◯ Zostera nana Roth, Enum. Pl. Phaen. Germ. 1(1): 8 (1827), nom. illeg. ◯ Zostera nana var. latifolia Harmsen, Ned. Kruidk. Arch. 46:871 (1936). ◯ Zostera nana var. muelleri (Irmisch ex Asch.) Kirk, Trans. & Proc. New Zealand Inst. 10:392 (1878). ◯ Zostera nigricaulis (J.Kuo) SWLJacobs & Les, Telopea 12: 422 (2009). ◯ Zostera nodosa Ucria, Nuova Racc. Opusc. Sicilian Author. 6:256 (1793). ◯ Zostera nodosa Guss., Fl. Sicul. Syn. 2:565 (1844), nom. illeg. ◯ Zostera noltii Hornem. in GCOeder & al. (eds.), Fl. Dan. 12(35): 1, t. 2041 (1832). ◯ Zostera novazelandica Setch., Proc. Natl. Acad. Sci. USA 19:816 (1933). ◯ Zostera oceanica L., Mant. Pl.: 123 (1770). ◯ Zostera oregana S.Watson, Proc. Amer. Acad. Arts 26: 131 (1891). ◯ Zostera pacifica S.Watson, Proc. Amer. Acad. Arts 26: 131 (1891). ◯ Zostera polychlamys (J.Kuo) SWLJacobs & Les, Telopea 12: 423 (2009). ◯ Zostera pumila Le Gall, Congr. Sci. France 16(1): 96 (1850). ◯ Zostera serrulata (R.Br.) Targ.Tozz., Cat. Veg. Mar.: 90 (1826). ◯ Zostera stenophylla Raf., Amer. Monthly Mag. & Crit. Rev. 2:175 (1818). ◯ Zostera stipulacea Forssk., Fl. Aegypt.-Arab.: 158 (1775). ◯ Zostera tasmanica M.Martens ex Asch., Sitzungsber. Jesus. Naturf. Freunde Berlin 1867: 15 (1867). ◯ Zostera tridentata Solms in G.Schweinfurth, Beitr. Fl. Aethiop.: 196 (1867). ◯ Zostera trinervis Stokes, Bot. Matt. Med. 4:319 (1812), nom. illeg. ◯ Zostera uninervis Forssk., Fl. Aegypt.-Arab.: 157 (1775). ◯ Zostera uninervis Vahl ex Rchb., Fl. Germ. Excurs.: 137 (1831), nom. illeg.

[0026] „dw Faser“ means percentage by weight based on the dry weight of all fibers in the mixture of natural fibers. It is therefore expressed as a percentage by weight.

[0027] A weight ratio of 30 : 70 is the same as a ratio of 30 wt.% to 70 wt.%.

[0028] Examples of suitable binders include: phenolic resins, urea-melamine resins, mineral-based binders (such as water glass, silanes), isocyanate-based binders (such as PMDI), and bio-based binders (such as starch, proteins, lignin). An isocyanate-based binder is particularly preferred.

[0029] An "isocyanate-based binder" is one that hardens (through-)due to the isocyanate groups it contains. This also includes the possibility that the binder may contain other reactive groups besides the isocyanate groups, which also contribute to the hardening process. In embodiments, however, at least 50% of the reactive groups involved in the hardening are isocyanate groups.

[0030] It can be assumed that the isocyanate groups react with the functional groups on the wood and the saltwater plant, and that, due to the chosen ratio between the natural fiber materials, a bond is formed with the binder, producing the special properties. Exactly how these advantageous properties were achieved could not be fully elucidated.

[0031] In embodiments, plates containing the bio-based ablative thermal protection material with increased strength are used, with a bulk density of 300 to 1200 kg / m³. 3 , particularly preferred in the range of 850-900 kg / m³ 3 .

[0032] In embodiments, 1-year saltwater plant fibers are used to produce the bio-based thermal protection material with increased strength, particularly in the following process.

[0033] In embodiments, a process for producing the bio-based thermal protection material with increased strength is carried out, comprising the following steps: • Provision of natural fibers wood fibers and 1-year saltwater plant fibers in a weight ratio of 30:70, each with a maximum deviation of ±20 wt.%, • Provision of a binder, preferably an isocyanate-based binder, in an amount of 6-12% dw Faser and • Mixing both natural fibers together and with the binder.

[0034] Ideally, the resulting finished mixture (of natural fibers and binder) is then treated to ensure that the isocyanate-based binder fully hardens. The methods for this are known to those skilled in the art and can be selected according to the specific isocyanate-based binder.

[0035] It was surprisingly observed that the flexibility of a mixture of 1-year saltwater plant fibers, in a suitable ratio to the rigid wood fibers and with an optimal proportion of a binder, results in the material exhibiting not only high strength but also thermal resistance (e.g. up to 2500°C).

[0036] Surprisingly, the suitability of one-year saltwater plant fibers for use as external heat protection in the aerospace industry (up to 2500°C) has proven to be significant. Experience has shown that low oxygen levels are common in these environments. This thermal protection material is suitable as cladding for components exposed to high thermal stress, such as in space applications. The ratio between wood fiber and the fibers of the one-year saltwater plants, as well as the proportion of binder, have a particularly positive influence on its performance.

[0037] A surprising advantage of this combination has also emerged: its strength is higher than that achieved when using only wood fibers, for example. This results in a thermal insulation material—a material with heat-shielding properties—that also exhibits increased strength, all without the use of cork.

[0038] Another advantage is that the thermal protection material, and seagrass fiber materials in general, have significantly lower densities compared to glass fiber reinforced plastics or metals such as stainless steel, thus significantly reducing the component weight while still meeting the mechanical requirements.

[0039] In a preferred embodiment, the binder comprises a proportion of 9-12% d WFaser contained in the mixture of the thermal protection material, particularly preferably with 11-12% dw Faser .

[0040] The greatest advantage is the combined effect of thermal protection and mechanical strength.

[0041] In another preferred formulation, the one-year saltwater plant fibers are selected from seagrass fibers and fibers of the Zosteraceae plant family. Seagrass is an annual plant. Its salinity is advantageous because it is optimal for use in thermal insulation materials, yet the salt does not interfere with the hardening of the isocyanate-based binder at the appropriate proportion of 6-12% dw. Faser .

[0042] Seagrass of the variety Zostera marina (at least 70% by weight) is particularly preferred, especially at least 95% by weight. It can also consist exclusively of this variety.

[0043] A preferred embodiment uses PMDI as the binder. PMDI (polymeric diphenylmethane diisocyanate = technical-grade MDI) is a technical mixture of methylenediphenyl isocyanates and aromatic polyisocyanates.

[0044] In another preferred embodiment, the proportion of the binder (in the mixture) is in the range of 9.5-10.5% dw Faser The binder PMDI is particularly preferred.

[0045] In a preferred embodiment, the wood fibers have a fiber length of 0.5–6 mm, and in particular even 1–3 mm. This fiber length of the wood fibers is particularly advantageous in combination with the annual saltwater plants in a ratio of 30:70.

[0046] In another preferred embodiment, the thermal protection material also contains a tannin-based flame retardant in a proportion of 6-12 wt.%.

[0047] For all (including those listed below) possible additional ingredients, these ingredients are sensibly distributed homogeneously in the mixture. In the process for manufacturing the bio-based thermal protection material, they are therefore added during the mixing step.

[0048] Also preferred is an embodiment in which the thermal protection material contains a hydrophobic agent (for example, oil, wax, paraffin or paraffin-based hydrowax) and a tannin-based flame retardant, wherein the 1-year saltwater plant fibers are seagrass fibers, and wherein the binder is PMDI, and wherein the ratio of PMDI:hydrophobic agent:tannin-based flame retardant is 10:1:10 wt% based on dw Faser (i.e., based on the dry weight of all fibers, especially wood fibers and seagrass fibers), with a relative deviation of ±5% relative to the respective value of the individual component. This corresponds to a wood:seagrass ratio of 30:70 and to 8.26 wt% tannin-based flame retardant in the thermal protection material.

[0049] By individual component, we mean the wood fiber, or the seagrass fiber, or the PMDI, etc. For example, in the case of the number 10 regarding PMDI, this means a range limit of 10 ± 0.5, for 1 regarding water repellenant, 1 ± 0.05, and for 10 regarding flame retardant, 10 ± 0.5. All these weight ratios are given in wt.%.

[0050] In another advantageous embodiment, the thermal protection material contains a paraffin-based hydrophobing agent.

[0051] In another preferred embodiment, it contains cork and / or bark, with bark comprising 15-20% by weight. The addition of bark, in particular, enhances its high thermal resistance.

[0052] In a preferred embodiment of the plates, they have a bulk density in the range of 300 to 1200 kg / m³. 3 , particularly preferred in the range of 850-900 kg / m³ 3 .

[0053] In a preferred embodiment, the thickness of the plates can range from 0.5 to 200 mm, and is particularly preferably in the range of 0.5 to 30 mm. This thickness has proven advantageously suitable for providing sufficient heat protection combined with sufficient mechanical strength.

[0054] The natural fibers are preferably obtained by shredding and processing debarked wood chips, e.g., using the TMP (thermo-mechanical pulp) process. Seagrass, for example, can be purchased or collected on the beach and then dried before being shredded or ground.

[0055] Fig. Figure 1 shows a test specimen (exemplary embodiment 1) during a thermal load test in the Huels arc heater (left: general test setup, top right: tip of an oak test specimen, bottom right: tip of a test specimen according to exemplary embodiment 1 after 120 seconds).

[0056] Fig. 2a and Fig. Figure 2b shows the temperature profile at the tip of these two test specimens. Fig. 1.

[0057] For the realization of the invention, it is also advantageous to combine the aforementioned inventive configurations, embodiments and features of the claims. Examples of implementation

[0058] The invention will now be explained in more detail using exemplary embodiments and comparative examples. Tables 1, 2, and 3 show the corresponding measured values.

[0059] The bulk density was tested according to DIN EN 323. The flexural strength and flexural modulus were determined according to DIN EN 310:1993. The transverse tensile strength was determined according to DIN EN 319:1993. The tensile strength was determined according to DIN 52377. Example 1:

[0060] A bio-based thermal protection material is produced from 30 wt% wood fiber and 70 wt% seagrass fiber. The binder PMDI is added at 10 wt%, as well as a paraffin-based hydrophobic agent (Sasol Hydrowax Pro A18) at 1 wt%. A tannin-based flame retardant (according to patent EP 3540027 B1, embodiment 1) is also added at 10 wt%. The dosage of additives and binder is based on the oven-dry mass (absolutely dry) of the fibers. The fibers are mixed in a plowshare mixer. The two additives and the binder are sprayed onto the individual fibers (fluidized bed) during the mixing process. The material is then hot-pressed into sheets with a bulk density in the range of 800 to 950 kg / m³. 3The plates are processed. They are 20 to 22 mm thick. Afterwards, they are ground to a thickness of 20 mm and a leading edge geometry is milled out. The fin leading edge is installed in control surfaces and, during its service life, is subject to both thermal loads (temperatures up to 2500 °C) and mechanical loads (multi-axial bending stress) due to deformation of the control surfaces. Example 2: Analogous to example 1 + additionally bark

[0061] A bio-based thermal insulation material is produced from 30 wt% wood fiber and 70 wt% seagrass fiber. The binder PMDI is added at 10 wt%, bark (ground oak bark) at 15 wt%, and a paraffin-based hydrophobic agent (Sasol Hydrowax Pro A18) at 1 wt%. A tannin-based flame retardant (according to patent EP 3540027 B1, embodiment 1) is also added at 10 wt%. The dosage of additives and binder is based on the oven-dry mass (absolutely dry) of the fibers. The fibers are mixed in a plowshare mixer. The two additives and the binder are sprayed onto the individual fibers (fluidized bed) during the mixing process. The material is then hot-pressed into sheets with a bulk density in the range of 800 to 950 kg / m³. 3The plates are processed. They are 20 to 22 mm thick. Afterwards, they are ground to a thickness of 20 mm and a leading edge geometry is milled out. The fin leading edge is installed in control surfaces and, during its service life, is subject to both thermal loads (temperatures up to 2500 °C) and mechanical loads (multi-axial bending stress) due to deformation of the control surfaces. Use 2:

[0062] The material, produced according to a mixture from embodiment 1, is processed into a sheet and used as cladding material / coating material on the surfaces of components to protect substructures from thermal stress or as a solid material, for example on tank structures, nose cones, housings and bases. Comparison examples 3: wood fiber only, no wood fiber, seagrass + paper only

[0063] A comparative example using 100% wood fiber was also investigated. In this example, the following components were added to the wood fiber: PMDI : water repellent : flame retardant in a ratio of 10:1:10 (each wt%), based on dw. Faser added.

[0064] An analogously produced bio-based thermal protection material without wood fiber does not achieve the desired strength.

[0065] Another comparison example, without wood fiber but with seagrass fiber and shredded paper, also fails to achieve the desired strength.

[0066] These various comparative examples showed significantly worse thermal protection properties as well as significantly lower mechanical stability. Characterization of the bio-based thermal protection material according to embodiment 1 and comparative example 3:

[0067] The bulk density was tested according to DIN EN 323. The flexural strength and flexural modulus were determined according to DIN EN 310:1993. The transverse tensile strength was determined according to DIN EN 319:1993. The tensile strength was determined according to DIN 52377. (Results and comparison in Table 1) Table 1: Bio-based thermal protection material compared to cork-based thermal protection material Key figure Thermal protection material Example of implementation 1 : See example 3 Further comparative examples : MDF (70% seagrass / 30% wood fiber) MDF (100% wood fiber) Cork material P45 Cork material P50 Cork material P60 Cork material NORCOATLIÈGE Bulk density [g / cm³] 3 ] 0,89 0,87 0,32 1 @20°C 0,48 1 0,45 1 0,47 2 Thermal conductivity [W / mK] 0,252 0.353 (level) 0,06 1 0,07 1 0,08 1 0,09 2 / 20 °C Specific heat capacity [J / kgK] 1493,2 1467,7 2500 1 2100 1 1900 1 1800 2 / 150°C Flexural strength [N / mm²] 2 ] 15,07 40,02 - - - - Bending modulus [N / mm²] 2 ] 559,35 1652,30 - - - - Tensile strength [N / mm²] 2 ] 10,43 9,87 0,86 3 1,5 3 1,1 3 0,7 2 -2,5 3 1 http: / / www.amorim-sealtex.com / AMORIM / UploadFiles / 2013924152058965.pdf 2 https: / / www.ariane.group / wp-content / uploads / 2019 / 01 / 19_NORCOAT®LIEGE.pdf 3 https: / / ec.europa.eu / research / participants / documents / downloadPublic?documentlds=080166e5b86a9ef4&appId=PP GMS

[0068] The flexural modulus (E-modulus) is the ratio of the applied stress to the deflection in a bending test. It is a measure of the material's stiffness in the elastic range. Tensile strength describes the maximum mechanical tensile stress a material can withstand before failure.

[0069] “Mechanical stability / strength” means a greater ability to withstand acting forces; a lower modulus of elasticity also makes the material resilient to different coefficients of thermal expansion of the surrounding materials.

[0070] Interim conclusion: Cork materials have only low strength. Thermogravimetric analysis (TGA)

[0071] Additionally, a thermogravimetric analysis (TGA) was performed in the measuring range of 25 °C to 1000 °C. Thermogravimetric analysis determines the change in mass of a test specimen as a function of temperature and time. It provides information about the carbonization yield and the pyrolytic behavior of a material. The test specimens are arranged symmetrically in a test crucible within a test carousel located in the oven of the LECO TGA 701 thermogravimetric analyzer.

[0072] Before testing, the specimens were pre-dried in an oven for approximately 18 hours to allow any residual moisture to escape. The specimens were then placed in crucibles, ensuring they were symmetrically positioned within the test carousel. In the first step, the specimens were heated in the TGA tester from 25 °C to 105 °C at a rate of 15 K / min. In the second step, the specimens were heated from 105 °C to 1000 °C at a rate of 40 K / min. During this process, the mass of the specimens was determined by the tester. The measurement was performed under a nitrogen atmosphere with a flow rate of 3.5 l / min. Table 2: TGA measurements of the bio-based thermal protection material from embodiment 1 compared to a conventional thermal protection material made of cork and one made of oak bark Mass m [%] Temperature [°C] Example of Implementation 1 See example: Cork material P50 See also: Oak bark (ground) 25 100 100 1 100 100 100 ~ 97 1 ~ 97 200 ~ 99 ~ 88 1 ~ 92 300 ~ 81 ~ 76 1 ~ 80 400 ~ 60 ~ 56 1 ~ 56 500 53 ~ 26 1 ~ 36 600 49,5 ~ 23 1 ~ 32 700 48 ~ 22 1 ~ 30 800 46 ~ 21 1 ~ 28 900 44 ~ 20,5 1 ~ 25 1 000 ~ 41 ~ 20 1 ~ 23 1 Paixäo et al. „RETALT_TPS design and manufacturing", CEAS Space Journal, 2021

[0073] Conclusion: Compared to comparative values ​​for TPS-cork P50 from the literature, the material produced according to application 2 regarding embodiment 1 with seagrass / wood fiber shows a significantly lower thermal degradation.

[0074] For fin leading edges, GRP (glass fiber reinforced plastic) or stainless steel are currently used (Table 3). Due to their high density, high weight, and high thermal conductivity, their substitution is being pursued. Table 3: Bio-based thermal protection material from embodiment 1 compared to previously used materials: Key figure Thermal protection material Previous fin leading edge material Example 1 GRP Stainless steel (1.4301) Bulk density [g / cm³] 3 ] 0,89 2,0 ± 0,1 1 7,9 3 Thermal conductivity [W / mK] 0,252 0,484 2 15 3 Specific heat capacity [J / kgK] 1493,2 910 2 500 Flexural strength [N / mm²] 2 ] 15,07 >200 1 - Bending modulus [N / mm²] 2 ] 559,35 >20 000 1,* 200 000 3 Tensile strength [N / mm²] 2 ] 10,43 - 500 - 700 3 1 Laminate, DIN 53479, Krempel Group * Fiber-dependent parameters are converted to 50 vol% according to EN 3783, EN 63, warp. 2 S. Assenheimer, 'Determination and evaluation of thermophysical parameters of the leading edge composite material of stabilizing fins of rockets', Duale Hochschule Baden-Württemberg (2021) 3 https: / / www.google.com / url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwizrPh68_5AhXtVPEDHW8rCa8QFnoECAgQAQ&url=https%3A%2F%2Fwww.hsmstahl.de%2Ffileadmin%2Fuser_upload%2Fdatenblatt%2FHSM_Datenblatt_1.4301.pdf&usg=AOvVaw2qVrel1pd0B5CNfYQo1jZg Comparisons regarding its application in space travel:

[0075] Tests were conducted in an arc-heated wind tunnel.

[0076] The arc-heated test facilities LBK of the DLR Cologne are certified by the ESA (European Space Agency) as a key European facility for the testing and qualification of thermal insulation systems.

[0077] The L2K system is equipped with a Huels-type electric arc furnace for gas mass flows between 5 and 75 g / s. With a maximum power output of 1.4 MW, moderate specific enthalpies of up to 10 MJ / kg are achieved at a gas mass flow of 50 g / s, corresponding to a pressure of approximately 1500 hPa. Working gases include air, nitrogen, argon, or mixtures such as those used for Martian (CO2 / N2) or Titanian (N2 / CH4) atmospheres. Hypersonic free jet velocities are achieved through a convergent-divergent nozzle. The nozzle's expansion section is conical with a half-angle of 12°. Various neck diameters from 14 mm to 29 mm are available and can be combined with nozzle outlet diameters of 50 mm, 100 mm, and 200 mm. This allows the system configuration to be effectively adapted to the requirements of a specific experimental campaign.

[0078] The test conditions were chosen to simulate the case of maximum total enthalpy, which occurs during a projected sounding rocket trajectory at an altitude of approximately 41.5 km and a flight Mach number of 8.6. At this point, the rocket experiences a total enthalpy of approximately hν = 4.048 MJ / kg at a dynamic pressure of ps = 254 hPa. With these values, the parameter is obtained. h0*square root of ps~64.5 as a reference value for thermal stress, if ho is in the unit MJ / kg and p sThe pressure is entered in hPa. This parameter can be used as a similarity parameter for determining the wind tunnel conditions. Furthermore, it should be considered that the flow in L2K is in thermal and chemical non-equilibrium with a relatively high proportion of atomic oxygen from dissociation in the arc heater when air is used as the test gas, whereas in actual flight the proportion of atomic oxygen is very low (<< 1%).

[0079] The test specimens produced from the selected materials consist entirely of solid material. The materials exhibiting the least deterioration were tested three times. This increased the reliability of the test. The samples tested multiple times include the following materials: • a (100% wood fiber, flame retardant-free) • b (100% wood fiber with bio-based flame retardant, corresponds to material according to comparison example 3) and • Execution example 1 (corresponds to the sample from embodiment example 1).

[0080] The test specimens have no temperature sensors integrated into their structure. The specimen geometry is determined by the defined leading-edge geometry. During testing, the specimens are clamped in a cooled specimen holder, which is fixed within the L2K test chamber. A water-based cooling system ensures uniform heat dissipation from the specimen holder during measurements and guarantees a secure hold under the applied thermal loads.

[0081] Fig. Figure 1 shows the boundary layer that forms around the test specimen. In the upper half of the image, the mechanical surface erosion of the ablation, the flaking and removal of a material particle carried away from the test specimen by the free plasma stream, can be seen.

[0082] The test specimen tips from embodiment 1 ( Fig. 1 bottom right) and solid oak ( Fig. 1 (top right) are in Fig. Figure 1 on the right shows the glowing, continuously degrading specimen tip and the formation of melt beads from the contained resins and mineral components. The pointed shape and straight leading edge were maintained throughout the entire test period.

[0083] The tip of the oak test specimen degrades irregularly compared to embodiment 1 and does not form a straight leading edge. The pointed shape of the oak test specimen cannot be maintained for the test duration of 120 s. Detachment of porous carbon and solid material is visible on the upper part of the test specimen.

[0084] Fig. 2a and Fig. Figure 2b shows the corresponding temperature profiles (measured using a 2-color pyrometer) at the sample tip ( Fig.2a: Test specimen with oak tip, Fig. 2b: Exemplary embodiment 1). In exemplary embodiment 1, the temperature initially rises from 1500 °C to a maximum of 2300 °C and remains relatively constant over the test period.

[0085] In contrast, the temperature measurement of the calibration sample shows significant fluctuations. This is due to more pronounced degradation of the material at the sample tip, where the focus of the temperature measurement spot lies after the material has been baked on. Material is lost, and the focus of the measurement spot becomes blurred, or the measurement spot may even disappear entirely from the sample. From t = 20 s onwards, flaking of material due to strong temperature fluctuations can be observed. After 110 s, the two-color pyrometer no longer registers any results and drops abruptly from 2400 °C back to its initial range of 1000 °C. This can also be explained by the loss of the measurement spot on the material surface.

Claims

[1] Use of 1-year saltwater plant fibers to produce a bio-based ablative thermal protection material for aerospace. [2] Use according to claim 1 for the production of the bio-based ablative thermal protection material as external heat protection for the aerospace industry. [3] Use according to one of claims 1 or 2, wherein the produced bio-based ablative thermal protection material comprises: a mixture of a binder and natural fibers selected from a) wood fibers and b) 1-year saltwater plant fibers, wherein • the binder with a proportion of 6-12% d WFaser is included, and • the weight ratio of wood fiber : 1-year saltwater plant fiber is 30 : 70, with a maximum deviation of ±20 wt.%. [4] Use according to claim 3, wherein • the binder is an isocyanate-based binder. [5] Use according to one of claims 3 or 4, wherein the 1-year saltwater plant fibers are selected from seagrass fibers and fibers of the plant family Zosteraceae. [6] Use according to any one of claims 3 to 5, wherein the binder is PMDI. [7] Use according to claim 6, wherein the binder comprises 9.5-10.5% dW Faser is included. [8] Use according to any one of claims 3 to 7, wherein the wood fibers have a fiber length of 0.5-6mm. [9] Use according to any one of claims 3 to 8, wherein the thermal protection material contains a tannin-based flame retardant in a proportion of 6-12 wt.%. [10] Use according to claim 9, wherein the thermal protection material contains a hydrophobizing agent, wherein the 1-year saltwater plant fibers are seagrass fibers and wherein the binder is PMDI, and where the ratio of PMDI:hydrophobic agent:tannin-based flame retardant is 10:1:10 wt% based on d WFaser lies, each with a relative deviation of ±5% based on the respective value of the individual component. [11] Use according to any one of claims 3 to 10, wherein the thermal protection material contains a paraffin-containing hydrophobing agent. [12] Use according to any one of claims 3 to 11, wherein the thermal protection material comprises cork and / or bark, wherein bark is present to a proportion of 15-20% by weight.

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