Recycling of aramid honeycombs

Abstract

The invention pertains to a process for the recycling of honeycomb material, wherein the honeycomb material comprises a resin-impregnated paper comprising a resin and a paper comprising aramid fibrous material, comprising: - optionally subjecting the honeycomb material to a size reduction step, - subjecting the honeycomb material to at least one milling step to obtain an milled aramid fiber comprising the resin, wherein the milling step is carried out at a temperature above 5°C, wherein the honeycomb material comprises 10 to 80 wt% of resin based on the weight of the honeycomb material, wherein the resin comprises phenolic resin, and wherein the recycling process is carried out at a temperature in the range of 5 to 50°C, and to a milled aramid fiber and to process for the re-use of recycled honeycomb material in friction materials and the respective friction material.

Description

[0001] Recycling of aramid honeycombs

[0002] Description:

[0003] The present invention is directed to a process for the recycling of aramid honeycomb material, to a milled aramid fiber comprising phenolic resin, to a process for the re-use of the recycled honeycomb material in friction materials and to the friction materials.

[0004] Core structures for sandwich panels are based on sheet-like materials such as papers or nonwovens and cured resin. They are mostly in the form of honeycombs. These composite materials are widely used in many applications where a high strength and lightweight material is required, primarily in the aerospace industry. These composite materials are difficult to be recycled because the resin and the sheet-like materials are intimately bonded and separation is difficult. Hence, recycling is limited and honeycomb materials may end in landfills or being incinerated. Hence, it is desirable to provide an efficient recycling process for honeycomb materials.

[0005] EP0324910 describes how aramid cloth impregnated with about 20 wt% of a blend of phenolic and polyvinyl butyral resin is first cryogenically chilled to the point of embrittlement in a liquid nitrogen bath. Subsequently, the brittle material is fed into a chilled rotary impact mill or rotary cutter to comminute the pieces into small fibrous material. The starting materials are e.g. woven aramid fabrics derived from uncured or partially cured scraps from the production of protective helmets from laminates of resin-impregnated cloth. EP0324910 is silent on honeycombs and the process requires an energy-intensive cooling step. US2001 / 0023903A1 describes a process for recycling materials containing high- tenacity fibers impregnated with resin matrix (e.g. prepregs), but is silent on honeycombs. For the purpose, a hammer mill is employed. The recycling material is fed into the hammer mill and grinded such that at least 50%, preferably at least 80%, of the resin particles are separated from the fiber. Hence, US2001 / 0023903 aims at a separation of the fiber and resin.

[0006] JP2007321310A pertains to a process to make aramid fibrids from aramid waste for use in friction materials, where the aramid fiber is dissolved in an amide solvent, the resulting spin dope is subjected to shearing and precipitation.

[0007] EP0826896 is directed to a process for producing a friction lining mixture for a friction lining made of recycled material from asbestos-free brake and clutch linings, including the following steps:

[0008] - removal of the old linings from the lining carriers,

[0009] - sorting of the old linings and lining carriers,

[0010] - comminution of the old linings to form a recycling granular material,

[0011] - classifying of the recycling granular material and

[0012] - use of the recycling granular material as friction lining mixture. EP0826896 is silent on honeycombs, and describes the recycling of friction materials to be used again in friction materials.

[0013] WO201 2 / 118187 is directed to the recycling of calendered aramid paper using a dry pulverization process. These paper particles and new fibrids are then used in the making of papers after preparing a watery slurry. WO2012 / 118187 is silent on recycling resin-impregnated papers or honeycombs.

[0014] EP1508418A1 is directed to shreds for reinforcement allowing the effective recycling of waste fiber converted products. In the process, used fiber converted products are cleaned and subsequently resin is applied to them and this material is shredded. The starting materials are wound yarns, twisted thread, knit and cloth, e.g. working gloves, protective clothing and industrial materials. Honeycombs are not mentioned. The process is complicated since it comprises the cleaning and subsequent resin impregnation of the recycling material. The aim of EP1508418A1 is to prevent the resulting shreds from becoming too cotton-like, i.e. having a too high bulk volume.

[0015] W02014 / 020532A1 is directed to a process for recycling thermosetting composite materials mainly based on glass fibers. In the process, industrial processing waste and / or end-of-life products are triturated to very small size powder, subsequently mixed with a thermosetting resin and finally extruded into shaped or molded parts. A rheology modifier is added into the process. The aim of W02014 / 020532A1 is to produce very small parts to improve the mechanical performance of the extruded parts.

[0016] None of the prior art documents describes a simple process for the recycling of aramid honeycombs. In particular, it is desired that such process does not require and result in a separation of the resin and the paper component of the honeycomb. In addition, it is desirable to produce recycled materials that show the same or even improved properties compared to conventional, non-recycled materials in their final application, e.g. for friction materials. Further, it is of interest to produce milled aramid fiber having a relatively high bulk volume.

[0017] The present invention provides a solution for this problem.

[0018] The invention pertains to a process for the recycling of honeycomb material, wherein the honeycomb material comprises a resin-impregnated paper comprising a resin and a paper comprising aramid fibrous material, comprising:

[0019] - optionally subjecting the honeycomb material to a size reduction step,

[0020] - subjecting the honeycomb material to at least one milling step to obtain a milled aramid fiber comprising the resin, wherein the milling step is carried out at a temperature above 5°C, wherein the honeycomb material comprises 10 to 80 wt% of resin based on the weight of the honeycomb material, wherein the resin comprises phenolic resin, and wherein the recycling process is carried out at a temperature in the range of 5 to 50°C.

[0021] The common feature in honeycombs is a lattice of hollow, thin-walled cells. The cells are often hexagonal, but may also have other geometries such as square, over-expanded and flex-core cells. The lattice of cells is usually covered on one or both sides with a face sheet to form so-called sandwich panels.

[0022] Similar to honeycombs are folded cores. These are tessellated three-dimensional structures of folded geometric patterns folded from a relatively thin planar sheet material. In contrast to honeycombs, folded cores do not have separate, enclosed cells. However, as for honeycombs, the folded cores are usually used in form of a sandwich panel with face sheets on both sides. Such folded or tessellated sheet structures are discussed in US6935997 and US6800351 .

[0023] For the purpose of the application the term “honeycomb material” includes honeycombs, folded cores and the sandwich panels made from either of them or parts or pieces thereof.

[0024] The honeycomb material may be honeycomb production waste, e.g. vertical cutoffs after sawing honeycombs of a certain thickness from a lattice of a larger thickness, horizontal cut-offs, edges, or cut-offs generated by CNC-milling (computer numerical control milling) etc..

[0025] Also end-of-lifetime honeycombs, folded cores or sandwich panels comprising them may be used as honeycomb material for instant recycling process, i.e. honeycombs, folded cores or sandwich panels incorporating them as removed from airplane components such as e.g. parts from floors, walls, the rudder, aileron, spoiler, flaps and interior components.

[0026] The honeycomb material comprises a resin-impregnated paper which comprises a resin and a paper comprising aramid fibrous material. Various resins and binders may be employed in honeycombs, e.g. polymeric binders, in particular phenolic resin, as e.g. described in EP0739707A1. The resin present in the paper of the honeycomb material used in present invention preferably comprises a phenolic resin, but may also comprise other resins, e.g. epoxy resin, polyureas, polyurethanes, melamine formaldehyde resins, polyesters, polyvinyl acetates, polyacrylonitriles, alkyd resins, and the like, as described in EP0467286A1 . The honeycomb material may comprise more than one resin, depending on the production process of the honeycomb or folded core and the chosen resins. One common process for fabricating honeycomb structures involves bonding multiple sheets of papers together along specially oriented node lines. The node lines are offset between different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as "expansion" process. Suitable node-line resins are e.g. described in US6451406.

[0027] Another fabrication process or method commonly referred to as the "corrugation" process has been used to form high strength thermoset honeycomb structures in those situations where the expansion process cannot be used. The corrugation process involves initially shaping sheets of uncured thermoset or thermoplastic material into a corrugated configuration. The corrugated sheets are cured at high temperature to form stiff corrugated sheets which are then bonded together to form the honeycomb core. The honeycomb core is then optionally coated with a dip resin.

[0028] Depending on the source of aramid honeycomb material for the recycling, the phenolic resin present in the honeycomb material may originate from the resin used to impregnate the paper (or sheets), the dip resin and / or the node resin. In one embodiment, the paper comprises a phenolic resin, i.e. the paper of the honeycomb material is impregnated with phenolic resin. In one embodiment, the resin comprised in the resin-impregnated paper of the honeycomb may comprise a mixture of a phenolic resin and e.g. a polyamide resin, as described e.g. in EP1048446. In one embodiment, the resin consists of phenolic resin. The honeycomb material comprises a paper comprising aramid fibrous material. In one embodiment, the aramid fibrous material comprises aramid fibers, in particular aramid shortcut, aramid fibrils, aramid fibrids and / or aramid pulp. Preferably, the aramid fibrous material comprises fibrids. Fibrids are small, non-granular, non-rigid fibrous or film-like particles, wherein in films one of their dimensions is in the order of microns, and in fibers two dimensions are in the micron range, as described herein. The presence of fibrids in the honeycomb material, more particularly in the paper comprising aramid fibrous material may lead to the presence of (parts of) fibrids in the milled aramid fiber. This may improve the matrix formation in the friction materials in which the milled aramid fiber obtained from the recycling process may be used.

[0029] Besides aramid fibrous material the paper may comprise further fibrous material as paper components. Preferably, the paper contains at least 80 wt% of aramid fibrous material, calculated based on the fibrous components of the paper. It may be preferred for the paper to contain at least 90 wt% of aramid fibrous material, in particular at least 95 wt% of aramid fibrous material, or the fibrous components of the paper consist of aramid fibrous material. (Aramid) fibrous material refers to (aramid) fibers (including continuous fibers and shortcut, also indicated as floc), (aramid) pulp, (aramid) fibrid, and (aramid) fibrils. It may also be the case that the honeycomb material comprises resin-impregnated papers based mainly on aramid fibrous material (e.g. comprising at least 80 wt% of aramid fibrous material, more preferably at least 90 wt% or even at least 95 wt% of aramid fibrous material or even consisting of aramid fibrous material, calculated based on the fibrous components of the paper). The honeycomb waste may further (in part) comprise sheets or composites (e.g. originating from the face sheets of sandwich panels) based mainly on non-aramid fibrous material, e.g. based mainly on carbon fibers or glass fibers (e.g. such sheets or composites may comprise at least 80 wt% of carbon fiber or glass fiber material, more preferably at least 90 wt% or even at least 95 wt% of carbon fiber or glass fiber material or even consisting of carbon fiber or glass fiber material, calculated based on the fibrous components of such paper). The sheets of composites that serve as face sheets of sandwich panels will usually also comprise resin, such as e.g. phenolic resin or epoxy resin.

[0030] In one embodiment, at least 80 wt%, more preferably 85 wt% of the fibrous material present in the honeycomb material is aramid fibrous material. In one embodiment, all the fibrous material present in the honeycomb material is aramid fibrous material.

[0031] The amount of resin in the honeycomb material may vary widely, it may e.g. be in the range of 10 to 80 wt%, preferably 20 to 60 wt% or up to 70 wt% (based on the weight of the honeycomb material). In one embodiment, the honeycomb material comprises at least 30 wt%, preferably at least 35 wt% of resin. In one embodiment, the honeycomb material may comprise 30 to 40 wt% of resin, more preferably 35 to 40 wt% of resin. Alternatively, the honeycomb material may comprise at least 40 wt% of resin. The resin comprises phenolic resin and may consist of phenolic resin.

[0032] Hence, in one embodiment, the honeycomb material comprises at least 30 wt%, preferably at least 35 wt% of phenolic resin. In one embodiment, the honeycomb material may comprise 30 to 40 wt% of phenolic resin, more preferably 35 to 40 wt% of phenolic resin. Alternatively, the honeycomb material may comprise at least 40 wt% of phenolic resin.

[0033] The amount of (aramid) fibrous material in the honeycomb may also vary widely, and may generally be in the range of 5 to 60 wt% (based on the weight of the honeycomb material).

[0034] Within the context of the present specification the term aramid 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 fibrids used in the present invention have an average length in the range of 0.2-2 mm, and average width in the range of IQ- 500 microns, and an average thickness in the range of 0.001-1 microns. Meta-aramid fibrids 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. Fibrids 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) fibrids. Para-aramid fibrids can, e.g., be obtained by high shear processes such as for example described in W02005 / 059247, which fibrids are also called jet-spun fibrids.

[0035] Aramid shortcut, also known as aramid floc, is known in the art. It is generally obtained by cutting aramid fibers to the desired length, in general a length in the range of 0.5-25 mm.

[0036] Pulp is an irregularly shaped fibrous structure. Pulp consists of short fibers which have been subjected to a shearing force leading to the formation of fibrils, which are mostly connected to a “stem” of the original fiber, while thinner fibrils peel off from the thicker fibrils. These fibrils are curly and sometimes ribbon-like, and show variations in length and thickness. Pulp is obtained by fibri llati ng short fibers (also referred to as short-cut), e.g. in a refiner. Hence, pulp comprises fiber stems and fibrils. Due to the fibrillation, pulp has a different morphology and different properties compared to continuous fiber or short-cut fiber. In particular, pulp is much shorter and has a higher specific surface area.

[0037] Each of the aramid fibrous material used in the present invention may be paraaramid material, meta-aramid material, or a combination thereof.

[0038] The paper may comprise para-aramid fibrous material or meta-aramid fibrous material or a combination thereof.

[0039] Hence, the aramid fibrous material of the paper of the honeycomb material may comprise para-aramid shortcut, para-aramid fibrils, para-aramid fibrids, metaaramid shortcut, meta-aramid fibrids and / or meta-aramid fibrids. Preferably, the aramid fibrous material comprises at least para-aramid fibrids and / or meta-aramid fibrids.

[0040] Preferably, the paper of the honeycomb material that is not derived from face sheets is free of glass fibers and / or carbon fibers. In one embodiment, the honeycomb material is free of glass fibers, where “free of’ refers to comprising less than 5 wt%, preferably less than 2.5 wt% and more preferably less than 1 wt% of glass fibers and / or carbon fibers (based on the weight of the honeycomb material).

[0041] In the context of the present specification aramid refers to an aromatic polyamide comprising or consisting of aromatic fragments directly connected to one another via amide fragments. Methods to synthesize aramids are known to those skilled in the art and typically involve the polycondensation of aromatic diamines with aromatic diacid halides. Aramids may exist in the meta- and para-form, both of which may be present in the aramid fibrous material present in the paper of the honeycomb material. Aromatic copolymers for whose production the monomers p- phenylenediamine and / or terephthaloyl dichloride are partially or completely substituted by other aromatic diamines and / or dicarboxylic acid dichlorides may also be present in the aramid fibrous material.

[0042] For the purpose of this application, the term para-aramid refers to a class of wholly aromatic polyamide polymers and copolymers having at least 60%, preferably at least 80% and more preferably at least 90% of para-oriented bonds between the aromatic moieties. In one embodiment, at least 95% or all (i.e. 100%) of the bonds are para-oriented bonds.

[0043] Typical para-aramids are poly(para-phenylene terephthalamide) (PPTA), poly(4,4'- benzanilide terephthalamide), poly(para-phenylene-4,4'-biphenylene dicarboxamide) and poly(para-phenylene-2,6-naphthalene dicarboxamide), 5,4'- diamino-2-phenylbenzimidazole or poly(para-phenylene-co-3, 4' -oxidiphenylene terephthalamide) or copolymers thereof. For the purpose of this invention, the term meta-aramid refers to a class of wholly aromatic polyamide polymers and copolymers having at least 70%, preferably at least 80% and more preferably at least 90% of meta-oriented bonds between the aromatic moieties. In one embodiment, at least 95% or all (i.e. 100%) of the bonds are meta-oriented bonds. The amide bonds between the aromatic moieties are thus located substantially in the meta-oriented or nearly meta-oriented positions of the aromatic rings (as e.g. in a 1 ,3-phenylene group or 1 ,3-naphthalene group). Meta-aramids may be produced by polymerization of meta-type aromatic amine and meta-dicarboxylic acid halide.

[0044] Suitable aromatic meta-diamines are meta-phenylenediamine, 3,4'- diaminodiphenylether, and 3,4'-diaminodiphenylsulfone; and derivatives thereof having substituents such as halogen atoms and / or alkyl groups having 1 to 3 carbon atoms, attached to the aromatic cyclic structures thereof, for example, 2,4- toluylenediamine, 2,6-toluylenediamine, 2,4-diaminochlorobenzene, and 2,6- diaminochlorobenzene, may be employed. Preferably, meta-phenylene diamine or mixed diamines containing meta-phenylene diamine in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more are employed.

[0045] Suitable aromatic meta-dicarboxylic acid dihalides are isophthalic acid halides, for example, isophthalic acid chloride and isophthalic acid bromide; and derivatives thereof having substituents, for example halogen atoms and / or alkoxy groups having 1 to 3 carbon atoms, for example 3-chloroisophthalic acid chloride and 3- methoxyisophthalic acid chloride may be employed. Preferably, isophthalic acid chloride and mixed carboxylic acid halides containing isophthalic acid chloride in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more, are employed.

[0046] In one embodiment, the meta-aramid used in present invention is co-poly(m- phenylene isophthalamide) comprising at most 5 molar% of aromatic moieties other than m-phenylene. In another embodiment, the meta-aramid is poly(m- phenylene isophthalamide). The recycling process is carried out at a temperature in the range of 5 to 50 °C, more preferably at a temperature in the range of 15 to 40°C, even more preferably at room temperature. Preferably, all steps of the recycling process are carried out at the indicated temperature and the honeycomb material is not subjected to a treatment at higher or lower temperature, in particular no cooling. To avoid the waste of energy, the process is preferably carried out without any active cooling or heating. This includes that the starting material, i.e. the honeycomb material is not subjected to any active cooling or heating and all of the process steps take place without active cooling or heating.

[0047] The recycling process involves the milling of the honeycomb material. Prior to the milling step, different additional steps may be carried out. Especially in end-of-lifetime materials, a sorting and / or separation step may be applied, to separate honeycomb materials from other materials such as e.g. facings and metallic / non-metallic parts.

[0048] Optionally, the honeycomb material may first be subjected to a size reduction step, e.g. by cutting, sawing or chopping larger pieces of honeycomb material to a size of about 50 cm, preferably 25 cm (where the size refers to the largest dimension of a piece of honeycomb material). This step may be necessary to produce honeycomb material that fits in the mill.

[0049] Subsequently, the honeycomb material is subjected to at least one milling step. The milling step may take place in a knife mill or a hammer mill. Preferably, the milling step is a dry processing step, i.e. it takes place without the addition of water.

[0050] A knife mill is a size reduction machine which cuts material with the help of rotating knives that provide a shearing action rather than impact or attrition to convert a feed into fine size particles that are collected from a discharge chute. Material may be fed into the knife mill via a hopper. Due to the rotation of the rotor, the feed is forced between the knives and the stationary part, i.e. a screen with openings, which cuts down the material into finer particles. The screen retains the material in the mill until the desired product is formed. The size of the openings of the screen determines the particle size of the milled aramid fiber. The product that comes out of the knife mill is collected.

[0051] In one embodiment it is preferred that the screen has openings with a size in the range of 2 to 20 mm, preferably in the range of 4 to 10 mm.

[0052] A hammer mill has the purpose to crush or shred a material into smaller pieces by the repeated blows of small hammers. Usually, a hammer mill is in essence a steel drum containing a vertical or horizontal rotating shaft or drum on which the hammers are mounted and the material is processed between the hammers and the stationary screen. As in the knife mill, the stationary screen has openings. Preferably, the openings in the screen of the hammer mill have the same size as described for the knife mill. The rotor is spun at a high speed inside the drum while material is fed into it. The material is impacted by the hammers and thereby shredded and expelled through screens of a selected size.

[0053] Preferably, by applying shear action and cutting to the material by use of a knife mill or a hammer mill, in combination with a fine screen the incoming material is reduced in size and (at least partially) fibrillated.

[0054] The treatment in the mill leads to a further size reduction and at least partial fibrillation of the fibers present in the honeycomb material.

[0055] The honeycomb material may be passed multiple times through a mill.

[0056] Preferably, the milling step does not lead to a separation of the resin from the aramid fibrous material. Preferably, at least 70 wt%, more preferably at least 80 wt% and even more preferably 90 wt% of the resin present in the honeycomb material remains bound to the aramid fibrous material (based on the total weight of the resin present in the honeycomb material before the milling step).

[0057] Preferably, the recycling process does not include the addition of resin to the milled aramid fiber other than the resin present in the honeycomb material.

[0058] The milled aramid fiber resulting from the milling step may be subjected to a size selection step to obtain a milled aramid fiber having a particle size in the range of 50 to 6000 pm, preferably having a particle size in the range of 200 to 4000 pm. The size refers to the largest dimension of a particle and particles of the desired size are obtained by passing the milled aramid fiber through a sieve with an appropriate opening size, e.g. having openings of 5600 pm.

[0059] The particle size distribution of the milled aramid fiber may be determined by e.g. using an air-jet sieve apparatus according to ISO 4610:2001 (“Sieve analysis using air-jet sieve apparatus”). The principle of the analysis is based on application of vacuum under the sieve, thus forcing the fine fraction through the applied sieve. The sieve retention value can be determined for different mesh sizes, thus giving information on the particle size distribution. Typical sieves used have aperture sizes of 100, 200, 500 and 1000 pm. Preferably, 50-80 wt% of the milled aramid fibers obtained in the recycling process has a particle size >1000 pm, 60-90% has a particle size >500 pm and 70-100% has a particle size >125 pm.

[0060] Milled aramid fiber with a particle size above 6000 pm may be subjected to a further milling step.

[0061] One purpose of the sieving step is to remove very small dust particles that (mainly) consist of either only fibrous material or only resin.

[0062] The invention also pertains to a milled aramid fiber comprising a phenolic resin and aramid fiber wherein the aramid fiber comprises 10 to 80 wt% of resin, wherein the resin comprises a phenolic resin, and wherein 50 to 80 wt% of the milled aramid fibers has a particle size > 1000 pm, 60 to 90 wt% of the milled aramid fibers has a particle size > 500 pm and 70 to 100 wt% of the milled aramid fibers has a particle size > 125 pm determined by air jet sieving according to 1804610:2001 . Preferably, the 30 to 30 wt% of the milled aramid fiber has particles size > 2000 pm and 0 to 5 wt% of the milled aramid fiber has a particle size > 5600 pm.

[0063] The milled aramid fiber usually is a collection of irregularly shaped particles, which have a particle size in the range of 50 to 6000 pm, where the particle size is determined using an air-jet sieve apparatus as described above. The milled aramid fiber generally comprises short fibers, fibers stems and in part fiber stems with fibrils attached thereto and fibrids or parts thereof. The individual fibers form a dense network, connected by pieces of cured phenolic resin. The milled aramid fiber is less fibri Hated than pulp obtained by the refining of short cut fibers, but has a similar but more coarse morphology.

[0064] The milled aramid fiber preferably comprises 10 to 80 wt% of phenolic resin, more preferably 20 to 60 wt% or up to 70 wt% of phenolic resin, based on the weight of the milled aramid fiber including the resin.

[0065] In one embodiment, the milled aramid fiber comprises at least 30 wt%, preferably at least 35 wt% of resin, wherein the resin comprises phenolic resin. In one embodiment, the milled aramid fiber may comprise 30 to 40 wt% of resin, more preferably 35 to 40 wt% of resin. Alternatively, the milled aramid fiber may comprise at least 40 wt% of resin, wherein the resin comprises phenolic resin. The resin comprises phenolic resin and may consist of phenolic resin.

[0066] Hence, in one embodiment, the milled aramid fiber comprises at least 30 wt%, preferably at least 35 wt% of phenolic resin. In one embodiment, the milled aramid fiber may comprise 30 to 40 wt% of phenolic resin, more preferably 35 to 40 wt% of phenolic resin. Alternatively, the milled aramid fiber may comprise at least 40 wt% of phenolic resin.

[0067] In one embodiment, the milled aramid fiber has a bulk volume of at least 2.2 g / ml, preferably of at least 2.4 g / ml, more preferably of at least 2.5 g / ml.

[0068] A high bulk volume is advantageous for creating a more uniform and rigid matrix in the mixture of the brake pad formulation, enhancing the stability of friction levels and ensuring consistent braking performance.

[0069] The bulk volume is determined by determining the volume of the milled aramid fiber after settling. 19 to 21 g of milled aramid fiber is weighted into a graduated measuring cylinder of 250 ml (determine exact weight, which corresponds to mass (A) in g). Subsequently, the milled aramid fiber is shaken very softly until the upper surface of the sample is aligned horizontally. The volume (B) may be read from the scale of the cylinder with a preciseness of 2 ml. The bulk volume is calculated by calculating A(g) / B (ml).

[0070] In one embodiment, the milled aramid fiber has a specific surface area of less than 1 m2 / g, preferably less than 0.5 m2 / g or even less than 0.3 m2 / g. The specific surface area (m2 / g) is determined using adsorption of nitrogen by the BET specific surface area method, using a Tristar 3000 manufactured by Micromeritics. The milled aramid fiber is pre-dried in an oven at 105 °C for at least 3 hours, then degassed at 200°C for 30 minutes, under flushing with nitrogen and subsequently the specific surface area is measured.

[0071] The milled aramid fiber generally has a lower specific surface area than conventional pulp, which usually has a specific surface area in the range of 5-15 m2 / g. This may be due to the presence of the resin during milling and due to the different starting material (which is short cut fiber in the case of producing conventional pulp by refining).

[0072] Preferably, the milled aramid fiber has a green strength of at least 0.3, more preferably of at least 0.5 and even more preferably of at least 1 .0 mJ / mm2In some embodiments, the green strength of the milled aramid fiber may be at most 1.0 mJ / mm2The green strength [mJ / mm2] is determined as described in the following: a mixture of 97 % Kaolin (Laude SP20) and 3 % of the sample to be tested is prepared on a high speed vertical mixer. 10 g of said mixture are molded at 70 bar to a rod with a thickness between 7.5 and 11 mm and a width of 15 mm. The rod is fractured on a pendulum ram impact testing device perpendicular to its main axis and the areal-specific energy, which is necessary for said fracture, is determined as the green strength in units of [mJ / mm2].

[0073] The specific surface area and the green strength may be influenced by the milling step by choosing a suitable screen opening size. A smaller screen size will lead to a higher specific surface area. Preferably, the milled aramid fiber has a filler retention in the range of 0.5 to 30%, preferably 1 to 20% and even more preferably 2 to 10%. In some embodiments, the filler retention may be at most 8%.

[0074] The filler retention is a measure of the extent to which the milled aramid fiber retains a filler during further processing, where a value of 100% means complete retention of the filler, i.e. no loss of filler during the processing. For friction materials, mixtures are used based on numerous materials and the mixture should be homogeneous and stable for further processing. The ability of the milled aramid fiber to retain particulate materials is of importance for the stabilization of the mix. The filler retention of the milled aramid fiber was determined using kaolin as filler. A mixture of 90 g of milled aramid fiber and 2910 g kaolin is made (3 wt%) in a MTI M20 mixer and mixed 5 minutes at a speed of 2300 RPM. Subsequently, 20 g of the mixture is placed on a sieve (mesh size 250 pm) and rotated horizontally on a JEL 200 / 80 sieve machine for 3 minutes. The filler retention is determined by dividing the amount of filler remaining on the sieve by the total amount of filler used and multiplying by 100.

[0075] The milled aramid obtained by instant process for the recycling of aramid honeycomb material, i.e. the recycled honeycomb material, may advantageously be used for products which would otherwise require the separate addition of aramid pulp and resin, in particular phenolic resin.

[0076] Accordingly, instant invention is also directed to a process for the re-use of recycled honeycomb material in friction materials comprising:

[0077] - obtaining a milled aramid fiber according to the process described above,

[0078] - compounding the milled aramid fiber with a filler and a binder, and

[0079] - shaping a friction material.

[0080] In one embodiment, the process for the re-use of recycled honeycomb material comprises

[0081] - providing honeycomb material comprising a resin-impregnated paper comprising a resin and a paper comprising aramid fibrous material, - optionally subjecting the honeycomb material to a size reduction step,

[0082] - subjecting the honeycomb material to at least one milling step to obtain a milled aramid fiber comprising the resin,

[0083] - compounding the milled aramid fiber with a filler and a binder, and

[0084] - shaping a friction material, wherein the milling step is carried out at a temperature above 5°C, wherein the honeycomb material comprises 10 to 80 wt% of resin based on the weight of the honeycomb material, wherein the resin comprises phenolic resin, and wherein the recycling process is carried out at a temperature in the range of 5 to 50°C.

[0085] Preferably, the milled aramid fiber as obtained from the at least one milling step without any further treatments or addition of components (e.g. resin, modifiers) is directly used in the subsequent compounding step.

[0086] Friction materials are in fact elaborate composite structures comprising fillers, and a binder, usually phenolic resin, and optionally further components such as fibers and friction additives. These composite materials are formulated to give appropriate friction, noise control, temperature resistance, and wear properties in a specific application. They comprise a number of different materials, each contributing to the properties of the friction material. Since heat energy is a byproduct of the process of creating friction, manufacturers typically use materials that are resistant to heat to make friction materials. As reinforcing fibers aramid fibers are often present to increase the mechanical strength and durability of the material. They further help to provide a porous structure, which helps to ensure proper resin absorption.

[0087] Fillers are added to fulfill various functions, e.g., to assist in resin absorption, to promote oil flow through friction papers to control in-use temperature degradation, to ensure adequate friction performance, and / or to reduce noise. Within the context of the present specification the term filler is intended to encompass all particulate materials which influence the friction performance of the friction material. Suitable fillers are known in the art. Examples of suitable fillers include refractory organic and inorganic particles such as calcium carbonate, magnesium carbonate, silicon carbide, titanium carbide, activated carbon, clay, kaolin, zeolite, alumina, silica, barium sulphate, barite powder, and particles derived from renewable resources such as powdered cocoa nutshell and cashew dust. Other examples of suitable filler particles include diatomaceous earth, graphite particles, and copper particles, although use of the latter has generally been discontinued in view of HSE concerns.

[0088] A resin, preferably phenolic resin, is present to ensure a good dimensional stability, a good tribological performance, and a good heat resistance.

[0089] Pulp materials are present to increase the mechanical strength, the porosity of the material, and the retention of fillers. Typical friction materials are e.g. paper- or plate-based friction materials, as e.g. used in wet friction application such as clutch facings in automatic transmissions and hard friction materials such as brake blocks, brake pads and brake linings. The paper-type materials and brake pads and brake linings are typically bonded to support members (e.g. metal plates) for use in mechanical energy transfer applications.

[0090] A typical friction material may e.g. include 10 wt% binder, 10 wt% of cashew dust, other fillers such as barium sulfate (e.g. 25 wt%), zirconia (e.g. 2 wt%) and graphite (e.g. 8 wt%) and different reinforcing fibers, including e.g. metal fibers such as copper fibers, aramid fibers (e.g. 5-10 wt%) and potassium titanate fibers. Suitable binders are thermosetting resins such as phenolic resins (i.e. 100% phenolic resin or phenolic resin modified with rubber or epoxy resin), melamine resins, epoxy resins and polyimide resins, and mixtures thereof.

[0091] Hence, instant invention also pertains to a friction material comprising the milled aramid fiber as described above. The friction material may comprise 0.2 to 60 wt% of the milled aramid fiber described above based on the weight of the friction material. Preferably, the friction material is a brake pad, brake lining, brake block, dry clutch facings, clutch face segment or a friction paper (e.g. automatic transmission paper). Such brake pads may e.g. be used in vehicles, such as automotive and train applications. However, brake pads may also be used in various industrial applications, e.g. for wind mills.

[0092] Preferably, non-paper friction materials comprise 0.2 to 20 wt% of the milled aramid fiber described above.

[0093] Advantageously, 0.5 to 10 wt%, preferably 3 to 6 wt% of the milled aramid fiber comprising phenolic resin may be combined with 0 to 3 wt% of conventional aramid pulp, based on the weight of the friction material, to manufacture a friction material. In this case, the milled aramid fiber described herein is used to at least partially replace conventional aramid pulp in the non-paper friction material. In addition, the friction material may comprise the conventional ingredients, such as binders, friction additives, abrasives and fillers.

[0094] Brake pads may e.g. comprise (metal) powder and inorganic fibers, graphite, rubber, ceramic materials, abrasives, lubricant and filler in addition to organic fibers. For (at least part of) the organic fibers, the milled aramid fiber of this invention may be used. The mixture of ingredients may be bonded together by a thermosetting phenolic resin. Preferably, the brake pad comprises 0.5-10 wt%, more preferably 3-6 wt% of the milled aramid fiber described above, based on the weight of the brake pad.

[0095] Friction papers may comprise pulp, fillers, and a binder, usually phenolic resin, and optionally further components such as fibers and friction additives. The milled aramid fiber of instant invention may be used to at least partially replace the fibers and / or pulp.

[0096] Friction papers, or paper-based friction materials, are used in wet friction application such as clutch facings in automatic transmissions. These paper-type materials are typically bonded to support members for use in mechanical energy transfer applications. Friction papers are formulated to give appropriate friction, noise control, temperature resistance, and wear properties in each specific application. They comprise a number of different materials, each contributing to the properties of the paper. Reinforcing fibers are often present to increase the mechanical strength and durability of the system. They further help to provide a porous structure, which helps to ensure proper resin absorption.

[0097] In the friction paper, the milled aramid fiber will generally be present in an amount of 5 to 60 wt.%. It may be preferred for the amount of aramid milled fiber to be in the range of 6 to 55 wt.%, more in particular in the range of 8 to 45 wt.%, still more in particular in the range of 10 to 35 wt.%.

[0098] The friction paper further comprises a filler. It may be preferred for the friction paper to comprise diatomaceous earth and / or graphite particles.

[0099] The filler will generally be present in an amount of 5 to 55 wt.%. If the percentage of filler is too low, its effect on the friction properties of the paper will not be obtained. If the amount of filler is too high, the amount of other components will be too low. It may be preferred for the amount of filler to be in the range of 10 to 45 wt.%, more in particular in the range of 20 to 35 wt.%.

[0100] The friction paper of the present invention comprises a resin as binder. Suitable resins are known in the art. The resin is generally present in an amount of 5 to 50 wt.%, in particular in an amount of 15 to 40 wt.%. If the amount of resin is too low, the structural integrity of the paper will be affected. If the amount of resin is too high, the content of other components will be too low. The resin generally is a phenolic resin, which may optionally be modified with for example, silicone, melamine, epoxy, cresol, or cashew oil. Other suitable binder resins include epoxy resins, and melamines. The resin is present to improve the thermal resistance of the paper, its dimensional stability, and its performance in friction and wear.

[0101] The friction paper of the present invention may contain further components. In one embodiment, the friction paper comprises further reinforcing fibers in addition to the milled aramid fiber, such as carbon fibers, mineral fibers, ceramic fibers, glass fibers, basalt fibers, and mineral wool, or polymer fibers such as acrylic fibers, polyimide fibers and polyamide fibers. Organic fibers like cotton and cellulose are also often used, as fibers or as pulp. It may be preferred for the friction paper according to the invention to comprise one or more of cellulose, cotton, or carbon fiber. Reinforcing fibers are often used to improve the durability and mechanical strength of the paper. If used, they are generally present in an amount of 2 to 40 wt.%, in particular 5 to 35 wt.%. Reinforcing fibers and their use are known in the art.

[0102] The actual steps of shaping the friction material can vary, depending on the friction material desired. For example, methods for making molded friction materials generally involve combining the desired ingredients in a mold, curing the part, and shaping, heat treating and grinding the part if desired.

[0103] Friction papers can be manufactured by methods known in the art, in general by a process comprising the steps of manufacturing a paper comprising the milled aramid fiber, resin, and filler (i.e. compounding the milled aramid fiber with a filler and resin, and heating the paper under such conditions that the resin is cured. In one embodiment, in a first step, all components of the paper except for the resin are combined in an aqueous medium to form a slurry. This can be done in any sequence, and the various compounds can be added simultaneously or sequentially. The resulting slurry is applied onto a screen and water is removed to shape the friction paper. This is conventional in papermaking. The resulting paper is dried. The dried paper is contacted with the resin. Generally the resin is in liquid form and the paper is impregnated with the resin. Depending on the type of resin, the impregnated paper can be submitted to a curing step to cure the resin. Exact process conditions will depend on the nature of the resin, and generally include a temperature in the range of 100 to 300°C and a pressure of 0.5 to 10 MPa.

[0104] In another embodiment, solid resin particles are added to the aqueous medium with the other components, and the resulting slurry is processed to form a paper as described above. The paper is then dried and cured as described above.

[0105] Surprisingly, the recycling and use of the milled aramid fiber obtained from the honeycomb material leads to good or even improved mechanical properties of the friction materials. In particular, brake pads comprising the milled aramid fiber (and hence recycled material) described herein show good fade and wear properties. Brake fade is a temporary and sudden reduction in braking power, caused by excessive heat in the system from braking repeatedly, under high loads or at high speeds. Excessive heat build-up in the brake pad, which is not dispelled effectively, can result in brake fade.

[0106] Wear of a brake pad indicates the loss of friction material during repeated cycles of exposing the brake pad to friction.

[0107] In addition, brake pads including the milled aramid fiber according to the invention have improved friction properties, in particular a higher friction coefficient which remains at stable level during friction testing.

[0108] The brake fade, wear and friction coefficient are determined using a brake dynamometer according to SAE J2522.

[0109] For the fade test, the brake pressure is an indicator of the fade. During the repeated braking cycles during the test, the temperature in the brake pad increases. This can lead to an increase in the brake pressure and decrease of the brake performance. It is therefore desired, to provide a brake pad where the brake pressure remains basically constant. Brake pads comprising instant milled aramid fiber demonstrated a stable brake pressure throughout testing.

[0110] The measured friction coefficient also remains at a stable level, comparable to using commercially available materials.

[0111] Completion of the brake performance tests (wear, fade and friction coefficient) according to the SAE J2522 standard shows for the brake pads comprising the milled aramid fiber according to the invention that the wear on both the brake pad and the rotor is low.

[0112] In addition, the compounding of the materials used in friction materials may in some embodiments be improved when using the milled aramid fiber comprising phenolic resin in comparison to separately adding aramid pulp and phenolic resin.

[0113] The invention will be elucidated with reference to the following Examples, without being limited thereto or thereby.

[0114] Testing methods

[0115] 1. Specific Surface Area (SSA) of milled aramid fiber

[0116] The specific surface area (m2 / g) is determined using adsorption of nitrogen by the BET specific surface area method, using a Tristar 3000 manufactured by Micromeritics. The milled aramid fiber is pre-dried in an oven at 105 °C for at least 3 hours, then degassed at 200°C for 30 minutes, under flushing with nitrogen and subsequently the specific surface area is measured.

[0117] 2. Green strength of milled aramid fiber

[0118] The green strength [mJ / mm2] is determined as follows: a mixture of 97 % Kaolin (Laude SP20) and 3 % of the sample to be tested is prepared on a high speed vertical mixer. 10 g of said mixture are molded at 70 bar to a rod with a thickness between 7.5 and 11 mm and a width of 15 mm. The rod is fractured on a pendulum ram impact testing device perpendicular to its main axis and the arealspecific energy, which is necessary for said fracture, is determined as the green strength in units of [mJ / mm2].

[0119] 3. Filler Retention of milled aramid fiber

[0120] The filler retention of the milled aramid fiber was determined using kaolin as filler. A mixture of 90 g of milled aramid fiber and 2910 g kaolin is made (3 wt%) in a mixer. Subsequently, 20 g of the mixture is placed on a sieve (mesh size 250 pm) and rotated horizontally on a JEL 200 / 80 sieve machine. The filler retention is determined by dividing the amount of filler remaining on the sieve by the total amount of filler used and multiplying by 100.

[0121] 4. Fade, wear and friction coefficient of brake pads

[0122] The brake pads were tested according to the SEA J2522 on a AK master

[0123] Dynamometer from Link (number 3452) to determine the brake performance. The dynamometer had a rolling radius of 325.0 mm and a gross axle weight of 1136 kg, a wheel load of 568.1 kg and an actual inertia of 59.8 kg m2.

[0124] The complete brake performance test comprised 21 scenario’s with 5 to 20 brake applications each in which the friction coefficient is measured as a function of the applied brake pressure, temperature, brake speed and deceleration.

[0125] One of the 21 scenarios is called the Fade test in which the brake pad was subjected to 15 brake sequences, braking from 100 to 5 kph. During the test temperatures exceed 550°C. After and before the brake tests the thickness of the two pads (inboard and outboard brake pad) and the rotor was measured to determine the wear (difference in mm).

[0126] 5. Size determination of milled aramid fiber

[0127] The particle size distribution of the milled aramid fiber was determined using an air-jet sieve apparatus according to ISO 4610:2001 (“Sieve analysis using air-jet sieve apparatus”). The milled aramid fiber was placed in an air-jet sieve apparatus applying air flow and vacuum and consecutively using sieves with aperture sizes of 125, 200, 500 and 1000 pm respectively.

[0128] 6. Bulk volume

[0129] The bulk volume was determined using a graduated cylinder of 250 mL with a round bottom suitable for the STAV 2003 instrument. 19 to 21 grams of milled aramid fiber is placed in the graduated cylinder after which the exact mass is written down (A). Next, the cylinder is shaken very softly and carefully to avoid disintegration until the upper surface of the pulp material is aligned horizontally. The bulk volume is read from the scale of the cylinder with a preciseness of 2 mL (B). The bulk volume is calculated by dividing: A (g) / B (ml). Example 1

[0130] Honeycomb production waste consisting of both phenolic-resin impregnated para- and meta-aramid based paper was obtained (including saw dust, edge trim material, CNC-waste).

[0131] To obtain milled aramid fiber, this waste material was fed through a Pallman knife mill type PS3 containing 3 knifes which rotate at a speed of 950 rpm. The knife mill is equipped with a 10 mm screen. After the milling step the material was processed with a JEL shaker sieve (type Prufsieb JEL 200) having 4 sieves with a metal wire cloth according to ISO 3310-1 and a bottom pan, all with a size of 200 mm x 50 mm. From top to bottom the sieves have a 316L stainless steel mesh with a mesh size of 5600, 1000, 500 and 150 pm. The material was added to the top sieve and shaken with the JEL shaker for 10 min at 278 rpm. After 10 minutes the first sieve (5600 pm) contained a fraction of 32%, the second sieve (1000 pm) a fraction of 49%, the third sieve (500 pm) a fraction of 8%, the fourth sieve (150 pm) a fraction of 7% and the remaining 4% was found in the bottom pan. This corresponds to 63 wt% of the milled aramid fibers having a particle size of > 1000 pm, 74 wt% of the milled aramid fibers having a particle size of > 500 pm, and 84 wt% of the milled aramid fibers having a particle size of > 125 pm.

[0132] After sieving the material, the bottom fraction was disposed and the 4 other fractions were added together and mixed with a rod.

[0133] The specific surface area, filler retention, green strength and bulk volume of the milled aramid fiber were determined as described above. The properties are shown in below table 1 .

[0134] Table 1 Example 2

[0135] The milled aramid fiber produced in example 1 was used to produce two SS60 type non-asbestos organic (NAO) brake pads containing 6 wt% of the milled aramid fiber (brake pad 1 ).

[0136] The fade, wear and friction coefficient of the brake pad were tested according to SEA J2522 on a AK master Dynamometer.

[0137] The brake pad made with the milled aramid fiber of this invention shows a very stable friction coefficient of 0.25 to 0.35 and braking pressure of 35 to 45 bar during each cycle of the Fade test.

[0138] After the brake performance test the wear of the brake pad according to the invention was determined by measuring the thickness and the weight of the brake pads. The brake pad according to the invention shows low wear on both the inner and outer brake pad, as well as the rotor.

[0139] Table 2

[0140] Overall, the friction coefficient determined with the AK master Dynamometer shows the following:

[0141] Table 3

[0142] Example 3

[0143] Honeycomb panels (excluding face sheets) consisting of both phenolic-resin impregnated para- and meta-aramid based paper were obtained.

[0144] To obtain milled aramid fiber, the honeycomb panels were fed through a large scale knife mill. Subsequently, the particle size distribution of the milled aramid fiber of Example 3 was determined. 80 wt% of the milled aramid fiber had a size larger than 1000 pm, 86 wt% of the milled aramid fiber had a size larger than 500 pm, 96 wt% of the milled aramid fiber had a size larger than 200 pm and 97 wt% of the milled aramid fiber had a size larger than 125 pm.

[0145] The specific surface area, filler retention, green strength and bulk volume of the milled aramid fiber were determined as described above. The properties are shown in below table 4.

[0146] Table 4

[0147] As comparative example, para-aramid milled fiber made in a knife mill but based on para-aramid fabric not comprising any resin was made.

[0148] Also for the comparative example, the particle size distribution was determined: 30 wt% of comparative sample had a size larger than 1000 pm and 50 wt% of the comparative sample had a size larger than 500 pm.

[0149] The specific surface area, filler retention, green strength and bulk volume of the comparative example were determined as described above. The properties are shown in below table 5.

[0150] Table 5 Example 4

[0151] The milled aramid fiber of Example 3 and the comparative example was used to produce for each two SS60 type non-asbestos organic (NAO) brake pads containing 6 wt% of the milled aramid fiber (brake pad 2) and the comparative example (brake pad 3) respectively.

[0152] The fade, wear and friction coefficient of the brake pads were tested according to SEA J2522 on a AK master Dynamometer.

[0153] Brake pad 2 according to this invention shows a very stable average friction coefficient of 0.33 to 0.35 and an average braking pressure between 24 to 37 bar during each cycle of the Fade test. The brake pad made based on the comparative examples (brake pad 3) has an average friction coefficient of 0.34 to 0.35 and average braking pressure between 19 to 33 bar during each cycle of the Fade test.

[0154] After the brake performance test the wear of the brake pads was determined by measuring the thickness and the weight of the brake pads. The brake pad according to the invention shows low wear on both the inner and outer brake pad, as well as the rotor, while the comparative brake pad 3 shows slightly higher wear. This is advantageous for lowering the emission of dust from the brake pads and for increasing the life time of the brake disc.

[0155] Table 6

[0156] Overall, the friction coefficient determined with the AK master Dynamometer shows the following:

[0157] Table 7 The data demonstrate that the recycling method is simple and efficient and results in a milled aramid fiber that equals or even improves the performance of comparative friction materials. This is particularly surprising, since the properties of the milled aramid fiber as such are rather lower or worse than of the comparative material. In fact, the comparative example of milled aramid fiber as such has a larger surface area and a higher green strength, filler retention and bulk volume. Nevertheless and surprisingly, the milled aramid fiber according to the invention results in comparable or even improved properties when applied in friction material while providing a friction material containing recycled material and hence having a lower carbon foot print. On the other hand, the amount of unused waste material from honeycomb production can be lowered.

Claims

Claims:

1. A process for the recycling of honeycomb material, wherein the honeycomb material comprises a resin-impregnated paper comprising a resin and a paper comprising aramid fibrous material, comprising:- optionally subjecting the honeycomb material to a size reduction step,- subjecting the honeycomb material to at least one milling step to obtain a milled aramid fiber comprising the resin, wherein the milling step is carried out at a temperature above 5°C, wherein the honeycomb material comprises 10 to 80 wt% of resin based on the weight of the honeycomb material, wherein the resin comprises phenolic resin, and wherein the recycling process is carried out at a temperature in the range of 5 to 50°C.

2. The process of claim 1 , wherein the paper comprises a phenolic resin.

3. The process of claim 1 or 2, wherein the aramid fibrous material comprises aramid shortcut, aramid fibrils, aramid fibrids and / or aramid pulp, preferably aramid fibrids.

4. The process according to any of claims 1 to 3, wherein the honeycomb material is honeycomb production waste and / or end-of-lifetime honeycomb material.

5. The process according to any of claims 1 to 4, wherein the mill is a knife mill or a hammer mill.

6. The process according to any of claims 1 to 5, wherein the milled aramid fiber is subjected to a size selection step to obtain milled aramid fiber having a particle size in the range of 50 to 6000 pm determined by air-jet sieving according to ISO 4610:2001 , preferably wherein 50 to 80 wt% of the milled aramid fibers has a particle size > 1000 pm, 60 to 90 wt% of the milled aramid fibers has a particle size > 500 pm and 70 to 100 wt% of the milled aramid fibers has a particle size > 125 pm.

7. A milled aramid fiber comprising a phenolic resin and aramid fiber, wherein the aramid fiber comprises 10 to 80 wt% of resin, wherein the resin comprises phenolic resin, and wherein 50 to 80 wt% of the milled aramid fibers has a particle size > 1000 pm, 60 to 90 wt% of the milled aramid fibers has a particle size > 500 pm and 70 to 100 wt% of the milled aramid fibers has a particle size > 125 pm determined by air jet sieving according to ISO 4610:2001.

8. The milled aramid fiber of claim 7 having a bulk volume of at least 2.2 g / ml, preferably at least 2.4 g / ml, more preferably at least 2.5 g / ml.

9. The milled aramid fiber of claim 7 or 8 comprising 10 to 80 wt% of phenolic resin, preferably 20 to 60 wt% of phenolic resin.

10. The milled aramid fiber according to any of claims 7 to 9 having a specific surface area of less than 1 m2 / g, preferably less than 0.5 m2 / g even more preferably of less than 0.3 m2 / g as determined by the BET specific surface area method.11 . The milled aramid fiber according to any of claims 7 to 9 having a green strength of at least 0.3 mJ / mm2, preferably of at least 0.5 mJ / mm2, even more preferably of at least 1 mJ / mm2.

12. The milled aramid fiber according to any of claims 7 to 11 , which has a filler retention in the range of 0.5 to 30%, preferably 1 to 20% and even more preferably 2 to 10%, determined as described in the specification.

13. A process for the re-use of recycled honeycomb material in friction materials comprising:- obtaining a milled aramid fiber according to the process of any of claims 1 to 6,- compounding the milled aramid fiber with a filler and a binder, and- shaping a friction material.

14. A friction material comprising the milled aramid fiber according to any of claims 7 to 12.

15. The friction material of claim 14, being selected from a brake pad, brake lining, brake block, dry clutch facings, clutch face segment or a friction paper.

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