WO2025209975A1

WO2025209975A1 - Resin-impregnated aramid-based honeycomb core and paper suitable for use therein

Info

Publication number: WO2025209975A1
Application number: PCT/EP2025/058705
Authority: WO
Inventors: Jan-Cees Tiecken; Edo Mugge; Yen Vu; Richard Visser; Frank DIEDERING; Antonius J. J. HENDRIKS
Original assignee: Teijin Aramid BV
Current assignee: Teijin Aramid BV
Priority date: 2024-04-03
Filing date: 2025-03-31
Publication date: 2025-10-09
Anticipated expiration: 2026-10-03

Abstract

A honeycomb core and a process for making it, comprising a plurality of interconnected walls having surfaces that define a plurality of honeycomb cells, wherein the cell walls are formed of a paper meeting the following requirements: - the paper comprises at least 90 wt.% of aramid material - the paper comprises 10-40 wt.% para-aramid fibrid and 40-90 wt.% meta-aramid fiber, - the paper has a thickness in the range of 10-200 micron, and a Gurley in the range of 2-1000 Gurley seconds, - the paper has a density in the range of 0.65-1.0 g/cm3.

Description

Resin-impregnated aramid-based honeycomb core and paper suitable for use therein

The present invention pertains to a resin-impregnated aramid-based honeycomb core and to a paper suitable for use therein. The invention pertains in particular to an aramid-based resin-impregnated honeycomb core and to an aramid-based paper suitable for use therein.

Honeycomb structures are used in many applications where good structural properties have to be combined with low weight. They are used in. e.g., aerospace applications, but also in other transport applications, such as in ships, trains, and cars, they are also attractive for use in temporary structures such as, among others, military structures.

Honeycomb structures based on meta-aramid paper have been used for many years. They have been found to be attractive in particular because they show good strength to weight ratio, a high stiffness, excellent stability to moisture, fire, and high temperatures, and excellent corrosion resistance.

Honeycomb cores comprising para-aramid have also been disclosed.

For example, W02009/108672 describes a honeycomb core with improved shear properties made from a para-aramid floc, 10-40 wt.% para-aramid pulp, and 5-30 wt.% of para-aramid fibrid.

US 8764941 describes a base paper for an aramid fiber honeycomb which comprises 11-90 wt.% of para-aramid fiber as structural fiber, 10-70 wt.% of meta-aramid fibrid, and 0-19 wt.% of polyester fiber as additive fiber.

US2011/0244175 describes a honeycomb core structure having a high compression modulus. The core structure comprises honeycomb cells of which the cell walls are formed from a nonwoven sheet, and a cured resin in an amount of at least 62 wt.%, calculated on the weight of the core structure. The non-woven sheet has a Gurley of less than 30 seconds per 100 ml. It comprises 70-100 wt.% of a high-modulus high-strength fiber, with para-aramid listed as example, and not more than 30 wt.% of a binder, which may be a fibrid binder.

US9976258 describes a fiber-reinforced honeycomb structure based on a paper sheet which comprises 30-70 wt.% para-aramid fiber and 30-70 wt.% aramid fibrid, which has a Gurley of at least 200 seconds per 100 ml. The paper is provided with a compression enhancing layer, formed into a honeycomb, and the honeycomb is provided with a resin. The compression enhancing layer is, e.g., an epoxy coating. Honeycomb cores based on para-aramid fibers generally show improved shear properties as compared to honeycomb cores based on meta-aramid fibers. A downside to para-aramid fiber based honeycomb cores is that their stiffness may be so high that they are difficult to process through thermoforming. Additionally, as para-aramid fiber based papers are more complicated to manufacture than meta-aramid fiber based papers, making para-aramid fiber based papers less cost-effective.

There is need in the art for a resin-impregnated honeycomb core with improved properties. In particular, there is need in the art for a resin-impregnated honeycomb core which shows at least one of a higher L-shear modulus and a higher W-shear modulus, preferably both, as compared with conventional resin-impregnated honeycomb cores based on meta-aramid, while other properties, in particular strength properties such as compressive strength, L- shear strength, and W-shear strength are maintained or improved.

The present invention provides a resin-impregnated honeycomb core which solves these problems.

The invention pertains to a resin-impregnated honeycomb core, wherein the honeycomb core comprises a plurality of interconnected walls having surfaces that define a plurality of honeycomb cells, wherein the cell walls are formed of a paper meeting the following requirements:

- the paper comprises at least 90 wt.% of aramid material, calculated on dry paper components,

- the paper comprises 10-40 wt.% para-aramid fibrid and 40-90 wt.% meta-aramid fiber,

- the paper has a thickness in the range of 10-200 micron, and a Gurley in the range of 2- 1000 Gurley seconds,

- the paper has a density in the range of 0.65-1.0 g/cm³.

The resin-impregnated honeycomb core according to the invention shows at least one of a higher L-shear modulus and a higher W-shear modulus, preferably both, as compared with conventional resin-impregnated honeycomb cores based on meta-aramid, while other properties, in particular strength properties such as compressive strength, L-shear strength, and W-shear strength are maintained or improved. As compared to resin-impregnated honeycomb cores based on para-aramid fibers, the honeycomb core of the invention shows better thermoforming properties, and is more cost-effective.

It is noted that US2021/140108 describes an aramid-based paper comprising at least 90 wt.

% of aramid material, the aramid material including at least one of aramid shortcut and aramid fibrid, the paper including at most 40 wt. % aramid pulp, calculated on the total amount of aramid material, wherein the paper includes 0.1-10 wt. % of polyamido-amine epichlorohydrin (PAE). It has been found that the incorporation of 0.1-10 wt. % of polyamidoamine epichlorohydrin (PAE) into an aramid-based paper of this composition, leads to a surprising improvement of the z-strength and the tear strength of the paper. As is explained in [0007] of US2021/140108, these features are of particular relevancy to the winding and unwinding steps which occur in paper manufacture and/or during manufacturing of the final product. They have no relationship to the L-shear modulus and a higher W-shear modulus of honeycombs obtained from the paper of the present invention.

Further advantages of the present invention and specific embodiments thereof will become apparent from the further specification.

The invention will be elucidated below.

The resin-impregnated honeycomb core comprises, in short, a honeycomb core impregnated with a thermosetting resin. Suitable resins should be capable of coating the honeycomb core, and, after curing, provide strength thereto. Suitable resins are known in the art and require no further elucidation here. They can, e.g., be selected from the group of phenolic resins, e.g., curable phenol formaldehyde resins, and epoxyresins. It is within the scope of the skilled person to select a suitable resin.

The resin will generally be present in the resin-impregnated honeycomb core in an amount of 5-90 wt.% calculated on the total weight of the resin-impregnated honeycomb core, in particular 5-85 wt.%. The exact amount of resin may vary within wide ranges, and may depend on the cell size, the paper weight and the core density of the honeycomb aimed for.

In one embodiment, a resin-impregnated honeycomb core with a core density of at least 100 kg/m³ has a resin content in the range of 40-90 wt.%, in particular 50-85 wt.%.

The resin-impregnated honeycomb core generally has a cell size in the range of 1.5-25 mm. The cell size is defined by a distance between 2 parallel edges in a hexagon. It is normally measured from 10 consecutive cells, on parallel edges where the paper is glued, and divided by 10 to get the average cell size of a single cell.

In one embodiment, the cell size is in the range of 2-10 mm, in particular 2.5 to 8 mm. In another embodiment the cell size is in the range of above 8 to 15 mm. In a further embodiment the cell size is in the range of above 15 to 25 mm, in particular 17-22 mm. The resin-impregnated honeycomb core of the present invention generally has a core density in the range of 10-200 kg/m³, in particular 20 - 150 kg/m³.

The resin-impregnated honeycomb core is based on a paper meeting the following requirements:

- the paper comprises 10-40 wt.% para-aramid fibrid and 65-85 wt.% meta-aramid fiber,

- the paper has a density in the range of 0.65-1.0 g/cm3.

These features will be discussed in more detail below.

The paper comprises at least 90 wt.% of aramid material, calculated on dry paper components. It may be preferred for the paper to contain at least 95 wt.% of aramid material, in particular at least 98 wt.% of aramid material. Aramid material refers to pulp, shortcut (also indicated as floc), fibrid, and fibrils. In the context of the present specification aramid refers to an aromatic polyamide which is a condensation polymer of aromatic diamine and aromatic dicarboxylic acid halide. Aramids may exist in the meta- and para-form, both of which may be used in the present invention.

Para-aramid is aramid in which at least 85%, in particular at least 90%, more in particular at least 95%, of the bonds between the aromatic moieties are para-aramid bonds. As typical members of this group are mentioned poly(paraphenylene terephthalamide), poly(4,4'- benzanilide terephthalamide), poly(paraphenylene-4,4'-biphenylenedicarboxylic acid amide) and poly (paraphenylene-2,6-naphthalenedicarboxylic acid amide or copoly(para- phenylene/3,4'-dioxydiphenylene terephthalamide). The use of poly(paraphenylene terephthalamide), also indicated as PPTA or PPD-T is particularly preferred.

Meta-aramid is aramid in which at least 85%, in particular at least 90%, more in particular at least 95%, of the bonds between the aromatic moieties are meta-aramid bonds. As typical members of this group are mentioned poly(metaphenylene isophthalamide) and poly(metaphenylene-3,3”-biphenylenedicarboxylic acid amide). The use of poly (metaphenylene isophthalamide), also indicated as MPIA or MPD-I is particularly preferred.

The paper comprises 10-40 wt.% para-aramid fibrid in combination with 40-90 wt.% metaaramid fiber. It has been found that the specific combination of para-aramid fibrid and meta- aramid fiber in the specified ranges is required to obtain the advantageous effects of the resin-impregnated honeycomb core of present invention. In particular, the use of para-aramid fibrid rather than meta-aramid fibrid in combination with meta-aramid fiber results in a substantial increase in at least one of a higher L-shear modulus and a higher W-shear modulus, preferably both, while maintaining or improving other properties, in particular strength properties such as compressive strength, L-shear strength, and W-shear strength. As compared to the use of a paper comprising para-aramid fiber, the resin-impregnated honeycomb core has improved thermoforming properties and is more cost-effective. If the para-aramid fibrid content is too low and the fiber content too high, the binding properties of the fibrid will be insufficient. This will detrimentally affect the paper strength, making the paper difficult to process. It may further make for a paper which is too porous, which may lead to issues in the manufacture of honeycombs. For example, when the paper is very porous, there is a risk that the resin used for nodeline printing is transferred through the paper, as a result of which the honeycomb cannot be expanded.

If the fibrid content is too high, the fiber content of the paper will be too low, detrimentally affecting paper strength and honeycomb properties.

It is preferred for the paper to comprise 15-35 wt.% of para-aramid fibrid in combination with 65-85 wt.% meta-aramid fiber. It may be particularly preferred for the paper to comprise 20- 30 wt.% of para-aramid fibrid in combination with 70-80 wt.% meta-aramid fiber.

Within the context of the present specification the term fibrid refers to small, non-granular, non-rigid film-like particles. The film-like fibrid particles have two of their three dimensions in the order of microns, and have one dimension less than 1 micron. In one embodiment, the fibrid used in the present invention have an average length in the range of 0.2-2 mm, and average width in the range of 10-500 microns, and an average thickness in the range of 0.001-1 microns. In one embodiment, the para-aramid fibrid comprises less than 40%, preferably less than 30%, of fines, wherein fines are defined as particles having a length weighted length (LL) of less than 250 micron.

The most suitable papers have been made from para-aramid fibrid with a Schopper-Riegler (SR) value between 50 and 90, preferably between 65 and 80. This fibrid preferably has a specific surface area (SSA) of less than 10 m2/g, more preferably between 0.5 and 10 m2/g, most preferably between 1 and 4 m2/g.

In one embodiment, para-aramid fibrid is used with a LL0.25 of at least 0.3 mm, in particular of at least 0.5 mm, more in particular at least 0.7 mm. In one embodiment the LL0.25 is at most 2 mm, more in particular at most 1.5 mm, still more in particular at most 1.2 mm. LL0.25 stands for the length weighted length of the fibrid particles wherein particles with a length below 0.25 mm are not taken into account. The para-aramid fibrid used in the present invention can, e.g., be obtained by high shear processes such as for example described in W02005/059247, which fibrid is also called jet- spun fibrid.

The meta-aramid fiber used in the present invention is generally obtained by cutting metaaramid fibers to the desired length, in general a length in the range of 0.5-25 mm. In a preferred embodiment the average length is at least 2 mm, in particular at least 3 mm. In some embodiments it may be at least 4 mm. The average length of the microfilaments preferably is at most 15 mm, in one embodiment at most 10 mm. Aramid fibers of this type is also known in the art as aramid shortcut or aramid floc. The aramid fiber generally has a titer in the range of 0.05-5 dtex. Fibers with titers below 0.05 dtex have been found difficult to process. Fibers with a titer above 5 dtex may result in paper with less attractive properties. It may be preferred for the fiber to have a titer of at least 0.3 dtex, in particular at least 0.6 dtex, in some embodiments at least 0.9 dtex and/or at most 3 dtex in particular at most 2 dtex.

The aramid materials discussed above are commercially available, int. al. from Teijin Aramid.

The paper used in the present invention comprises 10-40 wt.% para-aramid fibrid and 60-90 wt.% meta-aramid fiber. As long as these requirements are met, it is possible for the paper to comprise additional components. Such additional components may, e.g., be meta-aramid fibrid, para-aramid fiber, aramid fibril, and aramid pulp.

Meta-aramid fibrid will generally have the properties described above for para-aramid fibrid. Meta-aramid fibrid may, e.g., be obtained by shear precipitation of polymer solutions into coagulating liquids as is well known from U.S. Pat. No. 2,999,788. Fibrid of wholly aromatic polyamides (aramids) are also known from U.S. Pat. No. 3,756,908, which discloses a process for preparing poly(meta-phenylene isophthalamide) (MPD-I) fibrid.

Para-aramid fibers will generally have the properties described above for meta-aramid fibers. The paper of the present invention may comprise aramid fibril, in particular para-aramid fibril. Aramid fibril can, e.g., be obtained by direct spinning from solution, e.g. as described in W02004/099476. In one embodiment the aramid fibril has a structural irregularity expressed as the difference in CSF (Canadian Standard Freeness) of never dried fibril and dried fibril of at least 100, preferably of at least 150. In one embodiment fibrils are used having in the wet phase a Canadian Standard Freeness (CSF) value less than 300 ml, preferably less than 150 ml, and after drying a specific surface area (SSA) less than 7 m2/g, preferably less than 1.5 m2/g, and preferably a weight weighted length for particles having a length > 250 micron (WL 0.25) of less than 1.2 mm, more preferably less than 1.0 mm. Suitable fibrils and their preparation method are described, e.g., in W02005/059211. The paper of the present invention may comprise aramid pulp, in particular para-aramid pulp. In the present specification, the wording “aramid pulp” refers to aramid material comprising stems with a diameter of the order of 5-50 micron and a length of 0.5-6 mm with fibrils extending from the stem. The fibrils are fine, fiberlike extensions with a diameter which generally is in the submicron range. Aramid pulp is known in the art. It may be derived from aramid fibers which are cut to a length of, e.g., 0.5-6 mm, and then subjected to a fibrillation step, wherein the fibers are pulled apart to form the fibrils, whether or not attached to a thicker stem. Pulp of this type may be characterized by a length of, e.g., 0.5-6 mm, and a Schopper-Riegler of 15-85. In some embodiments, the pulp may have a surface area of 4-20 m2/g.

While, as indicated above, it is possible for the paper used in the present invention to comprise additional components, it is preferred for the paper to consist for at least 85 wt.%, in particular for at least 90 wt.%, more in particular for at least 95 wt.%, of the total of paraaramid fibrid and meta-aramid fiber, calculated on the dry weight of the paper. It has been found that it is the presence of these two components which makes for the desirable properties of the resin-impregnated honeycomb core of the present invention.

The paper used in the present invention has a Gurley in the range of 2-1000 Gurley seconds. As is known to the skilled person, the Gurley second is a unit that describes air permeability as a function of the time required for a specified amount of air to pass through a specified area of, in this case, an aramid paper, under a specified pressure. In the present specification the Gurley is determined in accordance with TAPPI T460. A Gurley of 2-1000 seconds places specific requirements on the porosity of the paper, and therewith on its impregnability for the resin. If the Gurley is above 1000 Gurley seconds, the paper will be insufficiently porous to be sufficiently impregnated with resin. If the Gurley is below 2 Gurley seconds, the paper is so porous that honeycomb manufacturing properties will be affected. In particular, there is a risk that the adhesive used in the nodeline printing will be absorbed into the paper. This would mean on the one hand that insufficient adhesive may be available on the surface to ensure that the papers are adequately adhered to each other. On the other hand, when the paper is very porous, there is a risk that the resin used for nodeline printing is transferred through the paper, which may interfere with the expansion of the honeycomb. It may be preferred for the paper to have a Gurley in the range of 3-400 Gurley seconds, more in particular 5-100 Gurley seconds.

The paper used in the presence invention has a thickness in the range of 10-200 micron. If the thickness is too low, the strength of the honeycomb core may be insufficient. If the thickness of the paper is above 200 micron, it may be difficult to manufacture an appropriate cell structure. It may be preferred for the paper to have a thickness in the range of 20-150 micron, in particular 30-120 micron.

The paper used in the present invention generally has a grammage in the range of 10-120 g/m², preferably in the range of 20-100 g/m², in particular in the range of 25-90 g/m².

The paper used in the present invention has a density in the range of 0.65-1.0 g/cm³. It has been found that if the density of the paper is below 0.65 g/cm³ or above 1.0 g/cm³, it is not possible to obtain the required combination of Gurley and paper thickness. It is preferred for the density of the paper to be in the range of 0.70-0.90 g/cm³. If the density of the paper is too high, the paper will be insufficiently porous to be sufficiently impregnated with resin. If the density of the paper is too low, the paper is so porous that honeycomb manufacturing properties will be affected, as is described above for the Gurley.

The paper used in the present invention can be manufactured by methods known in the art. In one embodiment, a suspension in a liquid medium, generally an aqueous suspension, is prepared comprising the various aramid materials as described above, and any further paper components. The suspension is applied onto a porous screen, so as to lay down a web of randomly interwoven material onto the screen. Water is removed from the web, e.g., by pressing and/or applying vacuum, followed by drying to make paper. To ensure that the requirements as regards density and Gurley are, the paper is subjected to a calendering step under pressure. In general, the pressure applied during the calendering step is at least 200 N/mm, in particular at least 350 N/mm. As a maximum value 1000N/mm may be mentioned.

It is preferred for the calendering step to be carried out at a temperature of at least 100°C, in particular of at least 150°C, more in particular of at least 200°C. A maximum temperature of 500°C may be mentioned, as the paper properties may start to deteriorate above that temperature.

Calendering generally involve passing the paper through one or more sets of rolls.

To arrive at the resin-impregnated honeycomb core of the present invention, the paper as described above will be converted to a honeycomb core. This can be done by methods known in the art.

In one embodiment, a honeycomb core is manufactured through an expansion process comprising the steps of: providing a stack of a plurality of papers as described above, which papers have been provided with adhesive node lines, the stacking being in such a manner that each paper is shifted with respect to its adjacent papers for a distance of half the interval of the adhesive node lines, pressing the stack under such conditions that the adhesive node lines connect the papers to each other, expanding the stack to form a honeycomb core and heat set to stabilize the core.

The invention also pertains to a honeycomb core suitable for use in the manufacture of a resin-impregnated honeycomb of the claimed invention, and to an aramid paper suitable for use in the manufacture of a resin-impregnated honeycomb core according to the invention. It will be clear to the skilled person that the various preferences pertaining to the nature of the paper as discussed above in the context of the resin-impregnated honeycomb core are also applicable to the honeycomb core, and to the paper suitable for use therein.

A honeycomb core can be converted into a honeycomb, also indicated as a resin- impregnated honeycomb core by impregnating the honeycomb with a liquid resin, following by drying and curing. The impregnation step can conveniently be carried out by dipping the honeycomb core in a bath of liquid resin and removing it when the desired degree of impregnation has been achieved. Drying can be carried out through methods known in the art, as can curing. In general, multiple sequences of dipping-drying-curing will be carried out.

It will be clear to the skilled person that various preferred embodiments described herein can be combined, unless they are mutually exclusive.

The invention is illustrated by the following example, without being limited thereto or thereby.

Example 1

A resin-impregnated honeycomb core according to the invention was prepared as follows: An aramid paper was provided comprising 71 wt.% of meta-aramid fibers (Teijinconex produced by Teijin Aramid) with a length of 6mm and a titer of 1.7dtex and 29 wt.% of paraaramid fibrid (type 8016 produced by Teijin Aramid). The dry paper had been calendered between two steel rolls at 150°C to a density of 0.75 g/cm3. The paper had a grammage of 42 g/m2, a thickness of 56 micron and a Gurley of 36 G.s.

From this paper a resin-impregnated honeycomb core was prepared having a cell size of 3.2 mm and a core density of 64 kg/m3. The resin content (phenolic resin) was estimated at 45 wt.%. This honeycomb was tested in compression according to ASTM-C365 and in shear according to ASTM-C273. The results are given in the table below.

The table also contains data on the properties of a resin impregnated honeycomb core available on the market which is based on a 100% meta-aramid paper which comprises meta-aramid fiber and meta-aramid fibrid. The honeycomb core has cell size, core density, and resin type according to the same standard. From this it is clear that the honeycomb according to the invention has an improves shear modulus, both in the L-direction and in the W-direction. The compression strength and shear strength in the L- and W-direction are maintained or improved. [Note that a difference in shear strength in the L-direction of 5% is not considered significant.]

Claims

1. Resin-impregnated honeycomb core, wherein the honeycomb core comprises a plurality of interconnected walls having surfaces that define a plurality of honeycomb cells, wherein the cell walls are formed of a paper meeting the following requirements:

- the paper has a density in the range of 0.65-1.0 g/cm³.

2. Resin-impregnated honeycomb core according to claim 1 , wherein the resin is present in an amount of 5-90 wt.% calculated on the total weight of the resin-impregnated honeycomb core, in particular 5-85 wt.%.

3. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the resin-impregnated honeycomb core has a cell size in the range of 1.5-25 mm, in one embodiment in the range of 2-10 mm, in particular 2.5 to 8 mm, in another embodiment in the range of above 8 to 15 mm, and in a further embodiment in the range of above 15 to 25 mm, in particular 17-22 mm.

4. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls comprises at least 95 wt.% of aramid material, in particular at least 98 wt.% of aramid material, calculated on dry paper components.

5. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls comprises 15-35 wt.% of para-aramid fibrid in combination with 65-85 wt.% meta-aramid fiber, in particular 20-30 wt.% of para-aramid fibrid in combination with 70-80 wt.% meta-aramid fiber.

6. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls consist for at least 85 wt.%, in particular for at least 90 wt.%, more in particular for at least 95 wt.%, of the total of para-aramid fibrid and metaaramid fiber, calculated on the dry weight of the paper.

7. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls has a Gurley in the range of 3-400 Gurley seconds, in particular 5-100 Gurley seconds.

8. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls has a density in the range of 0.70-0.90 g/cm3.

9. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls has a thickness in the range of 20-150 micron, in particular 30-120 micron.

10. Resin-impregnated honeycomb core according to any one of the preceding claims, wherein the paper forming the cell walls has a grammage in the range of 10-120 g/m², preferably in the range of 20-100 g/m², in particular in the range of 25-90 g/m².

11. Aramid paper suitable for use in the manufacture of a resin-impregnated honeycomb core according to any one of claims 1 to 10, wherein

- the paper has a density in the range of 0.65-1.0 g/cm³.

12. Honeycomb core suitable for use in the manufacture of a resin-impregnated honeycomb core according to any one of claims 1 to 10, the honeycomb core comprising a plurality of interconnected walls having surfaces that define a plurality of honeycomb cells, wherein the cell walls are formed of a paper meeting the following requirements:

- the paper has a density in the range of 0.65-1.0 g/cm³.

13. Method for manufacturing an aramid paper according to claim 11 , comprising the steps of - providing a suspension of solid paper components in a liquid medium, the solid paper components comprising at least 90 wt.% of aramid material, the solid paper components comprising 10-40 wt.% para-aramid fibrid and 40-90 wt.% meta-aramid fiber,

- applying the suspension onto a porous screen, so as to lay down a web of randomly interwoven material onto the screen,

- removing water from the web, e.g., by pressing and/or applying vacuum,

- drying the web from which water has been removed to make paper,

- subjecting the thus formed paper to a calendering step under pressure, preferably at a temperature of at least 100°C.

14. Method according to claim 13, wherein the calendering step is carried out at a pressure of at least 200 N/mm, in particular at least 350 N/mm, and at most 1000N/mm and a temperature of at least 100°C, in particular of at least 150°C, more in particular of at least 200°C, and at most 500°C.

15. Method for manufacturing a resin-impregnated honeycomb core according to any one of claims 1-10, comprising the steps of

- providing a honeycomb core comprising a plurality of interconnected walls having surfaces that define a plurality of honeycomb cells, wherein the cell walls are formed of a paper meeting the following requirements:

- the paper has a thickness in the range of 10-200 micron, and a Gurley in the range of 2-1000 Gurley seconds,

- the paper has a density in the range of 0.65-1.0 g/cm³,

- impregnating the honeycomb core with a liquid resin, following by drying and curing.