WO2022258516A1

WO2022258516A1 - Modified aramid pulp and friction material comprising modified aramid pulp

Info

Publication number: WO2022258516A1
Application number: PCT/EP2022/065180
Authority: WO
Inventors: Walter Nijhuis; Mikael VERTOMMEN; Adrianus VAN HAREN; Jan-Cees Tiecken; Frank DIEDERING
Original assignee: Teijin Aramid B.V.
Priority date: 2021-06-08
Filing date: 2022-06-03
Publication date: 2022-12-15
Also published as: KR20240021779A; EP4352300A1; CN117425756A

Abstract

Instant invention pertains to an aramid pulp comprising polyoxazoline. Also claimed is a method for manufacturing the aramid pulp comprising polyoxazoline comprising: - combining aramid short-cut, partly fibrillated aramid short-cut or aramid pulp with polyoxazoline in an aqueous solution to form a mixture, - subjecting the mixture to a refining step to form an aqueous slurry of the aramid pulp. Further, the invention is directed to a paper comprising the aramid pulp comprising polyoxazoline and a friction material comprising said paper and/or said aramid pulp comprising polyoxazoline.

Description

Modified aramid pulp and friction material comprising modified aramid pulp

Description:

The present invention is directed to an aramid pulp comprising polyoxazoline, to a paper comprising said pulp, to a friction material comprising said pulp or said paper and to a process to manufacture the aramid pulp comprising polyoxazoline.

Aramid pulp is known and is used for various application areas, including e.g. papers, friction materials and in particular friction papers. Aramid is known in the art for its high strength and high temperature resistance.

Friction papers, or paper-based friction materials, may be used in wet friction applications 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 manufactured by the conventional paper making method, but these materials are in fact elaborate composite structures comprising pulp, fillers, and a binder, usually a thermoset resin, and optionally further components such as fibers and friction additives.

Friction papers are composite materials that are formulated to give appropriate friction, noise control, temperature resistance, and wear properties in each specific application.

Pulp materials are present to increase the mechanical strength, to tune the porosity of the paper, and ensure the retention of fillers during paper making. Aramid pulp is often used as pulp due to its very good mechanical and thermal properties. In particular para-aramid pulp shows good heat resistance, friction performance, and durability. It further shows good properties as regard noise and vibration behaviour (NVFI) and does not show chemical interaction with the automatic transmission fluid (ATF). It also has good compressibility and shear strength properties, and good flexibility as compared to metal fibers.

W02006/012040 describes an acrylic and para-aramid pulp for use as reinforcement material in products such as seals and friction material.

A modified aramid pulp for friction papers has been described in WO2018/037015. The aramid pulp is provided with PVP (polyvinyl pyrrolidone) and used in friction papers. The PVP-modified aramid pulp leads to improved friction performance.

Nevertheless, it has been found that there is still room for improving the performance of aramid pulp in papers, especially in friction papers. In particular, there is need for an aramid pulp which provides papers and materials, in particular friction papers, with high shear strength and high tensile strength in combination with high porosity. In addition, a high filler retention is desired to improve the homogeneous distribution of the filler in the paper resulting in a homogeneous paper.

The present invention provides a solution to this problem.

The invention pertains to an aramid pulp comprising polyoxazoline (also referred to as “modified aramid pulp” herein).

Modified continuous aramid yarns have been described for use in ballistic fabrics and thermoplastic composite materials.

US5266076 describes continuous aramid fibers coated with a finish of e.g. fluorinated polyoxazoline. The continuous fibers are used for fabrics in ballistic applications. US5266076 is silent on aramid pulp, paper and friction materials, in particular friction paper. WO01/34385A1 discloses a thermoplastic composite material comprising a fiber coated with poly-2-oxazoline polymer and a polymeric resin. WO01/34385A1 is silent on pulp and on papers. The fibers of WO01/34385A1 are continuous fibers (i.e. basically endless fibers) and selected from fibers such as glass, carbon, nickel plated carbon and aromatic polyamide fiber which are after coating with poly-2-oxazoline cut into short cut and coated with a thermoplastic resin. Continuous fibers and short-cut fibers are different from pulp.

It has been found that polyoxazoline-modified aramid pulp results in a combination of high strength and high porosity when used in e.g. papers and friction materials, such as e.g. friction papers. Often, when porosity of a paper is increased, the mechanical properties (tensile strength, tear strength) will decrease. Hence, a friction paper combining improved mechanical properties without negatively affecting the level of porosity or a friction paper combining an increased porosity and improved or level mechanical properties is of interest. Surprisingly, aramid pulp comprising polyoxazoline provides such properties to paper, in particular to friction paper. In addition, the modification with polyoxazoline may improve the filler retention of papers.

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 fibrillating 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.

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 used. Preferably, the aramid pulp of the invention is a para-aramid pulp.

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.

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.

Preferably, the aramid pulp comprises 0.1 to 10 wt% of polyoxazoline, preferably 0.25 to 7.5 wt% of polyoxazoline, more preferably 0.5 to 5 wt% of polyoxazoline (based on the weight of the dried pulp). In one embodiment, the aramid pulp comprises less than 6 wt% of polyoxazoline, preferably up to 4 wt% of polyoxazoline (based on the weight of the dried pulp). Dried pulp has an equilibrium moisture content in the range of 3 to 8 wt%. The amount of polyoxazoline is relative to the total weight of the dried pulp including polyoxazoline and equilibrium moisture.

In the context of the present invention polyoxazoline pertains to a polymer based on oxazoline moieties and may also be referred to as oxazoline polymer.

The polyoxazoline polymer is different from a non-polymerized oxazoline compound, which may be used as curing agent for e.g. epoxy resins. Polyoxazolines are based on oxazoline moieties. Oxazolines are five-membered heterocycles (3xC, O, N) with three different structural isomers, depending on the position of the double bond within the ring. 2-oxazolines are used to prepare polyoxazolines. Preferably, the polyoxazoline is based on (potentially substituted) N- acylethylenimine units (linear, obtained after ring opening of the 2-oxazoline monomer), wherein the main chain carbons are preferably substituted with hydrogen and the acyl group is substituted with hydrogen or C1-C4 alkyl (R in below formula is H or C1 -C4 alkyl), preferably with an ethyl group (R is C2 alkyl):

Thus, preferably the polyoxazoline is a poly-alkyl-2-oxazoline, preferably poly-2- ethyl-2-oxazoline (PEOX). Alkyl means a monovalent saturated linear or branched hydrocarbyl radical with 1 to 4 carbon atoms.

Preferably, the polyoxazoline is substantially free of halogen groups, in particular fluor groups. Substantially free means that the polyoxazoline polymer comprises less than 5 mol% of halogen groups, preferably less than 1 mol% of halogen groups, in particular fluor groups.

Preferably, the polyoxazoline has a molecular weight in the range of 1000- 1000000 g/mol, preferably 5000-750000 g/mol, more preferably 10000-600000 g/mol, even more preferably 200000-500000 g/mol. In some embodiments, the increase of the molecular weight of the polyoxazoline may result in improved strength of a paper comprising the polyoxazoline-modified pulp.

The aramid pulp comprising polyoxazoline generally has a length (LL0.25) in the range of 0.5 to 1.5 mm, in particular in the range of 0.60 to 1.4 mm, in some embodiments in the range of 0.7 to 1.3 mm. This parameter is determined by the Valmet Fiber Image Analyzer, known as the Valmet FS5 which is calibrated with samples of pulp with known lengths. The length weighted length LL0.25 [mm] is a length-weighted average length, determined according to ISO 16065-2, wherein particles are included having a length > 250 pm, i.e. > 0.25 mm.

The aramid pulp comprising polyoxazoline generally has a Schopper Riegler (SR) in the range of 15 to 80 °SR, in particular in the range of 16 to 60 °SR, more in particular in the range of 17 to 40 °SR. The SR is a parameter often used in the art of pulp and paper technology. It is a measure of the drainability of a pulp suspension in water. The SR can be determined in accordance with IS05267/1 by dispersing 2 g (dry weight) of pulp in 1 L water during 600 counts in a Lorentzen and Wettre disintegrator.

The aramid pulp comprising polyoxazoline generally has a Canadian Standard Freeness (CSF) in the range of 15 to 700 ml_, in particular in the range of 100 to 670 ml_, more in particular in the range of 200 to 650 ml_. The CSF is a parameter often used in the art of pulp and paper technology. As the SR, it is a measure of the drainability of a pulp suspension in water. The CSF may be determined in accordance with TAPPI T227.

The aramid pulp comprising polyoxazoline may have a specific surface area (SSA) in the range of 2 to 20 m²/g, preferably 3 to 15 m²/g, more preferably 4 to 10 m²/g or 5 to 8 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 pulp 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.

The specific surface area of non-fibrillated fiber (e.g. continuous fiber or short-cut fiber) is much lower and in the range of 0.1 to 0.2 m²/g.

Preferably, the polyoxazoline is (exclusively) present on the surface of the pulp. Preferably, the polyoxazoline covers at least part of the surface of the aramid pulp or the entire surface of the aramid pulp. Preferably, the polyoxazoline is not used to make the short-cut from which the pulp is made, but is applied to the surface of the pulp. The polyoxazoline is provided to the surface of the aramid pulp during the manufacture of the modified pulp as described below.

Instant invention also pertains to a paper comprising the aramid pulp comprising polyoxazoline in the embodiments described above.

The paper may e.g. be a friction paper, a separator paper or a honeycomb paper.

In particular, the invention pertains to a friction paper comprising the aramid pulp comprising polyoxazoline in the embodiments described above.

Friction papers are composite materials that usually 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.

Fillers are added to fulfill various functions, e.g., to assist in resin absorption, to promote oil flow through the paper to control in-use temperature degradation, to ensure adequate friction performance, and/or to reduce noise.

A resin is present to ensure a good dimensional stability, a good tribological performance, and a good heat resistance.

Preferably, instant paper comprises 2-70 wt% of the modified aramid pulp, more preferably 5-55 wt% of the modified aramid pulp, even more preferably 10-35 wt% of the modified aramid pulp, based on the weight of the paper.

In one embodiment, instant paper comprises the aramid pulp comprising polyoxazoline, a filler and a resin. Preferably, instant paper comprises 5-55 wt% of the filler, more preferably 20-40 wt% of the filler and 5-50% of the resin, more preferably 15-40 wt% of the resin, based on the weight of the paper.

Within the context of the present specification the term filler is intended to encompass all particulate materials other than fiber or resin which preferably influence the friction performance of the paper. Suitable fillers for friction papers 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.

It may be preferred for the (friction) paper to comprise diatomaceous earth and/or graphite particles.

The filler is preferably 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 50 wt%, more in particular in the range of 20 to 40 wt% (based on the weight of the paper).

The 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% (based on the weight of the paper). 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 is preferably a thermoset resin. Preferably, the resin is selected from a phenolic resin, a vitrimer resin (a so-called malleable thermoset resin), a polythiourethane resin, a melamine resin, a silicone resin and an epoxy resin. Preferably, a resin is used that allows removal or reprocessing of the resin in order to enable separation of the components of the paper for recycling. Suitable vitrimer resins are e.g. described in W02020/051506A1. EP3149065A1 describes thermomechanically reprocessable epoxy resins and WO2019/063787A1 describes reworkable polythiourethane resins.

A suitable silicone resin is e.g. an organopolysiloxane resin as described in EP3473883A1.

The phenolic resin may optionally be modified with for example, silicone, melamine, epoxy, cresol, or cashew oil. The resin is present to improve the thermal resistance of the paper, its dimensional stability, and its performance in friction and wear.

The paper of the present invention may contain further components.

In one embodiment, the paper comprises additional reinforcing fibers, 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 (short-cut) fibers or as pulp. The paper may also comprise unmodified aramid pulp, i.e. not comprising polyoxazoline or another surface modification. Thus, the paper may comprise a combination of unmodified aramid pulp and polyoxazoline modified 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.

Preferably, in the resin-impregnated paper the combined amount of all reinforcing fibers and pulp (referred to as fiber amount of the friction paper), including the aramid pulp comprising polyoxazoline, makes up 25-45 wt%, preferably 30-40 wt% of the weight of the paper. In one embodiment, the paper comprises a fiber amount of 25-45 wt% and comprises 25-45 wt% of filler and 25-45 wt% of resin. Preferably, the fiber amount, the filler and the resin are each (approximately) 1/3 of the paperweight. The aramid pulp comprising polyoxazoline may be present in the range of 20 wt% to 100 wt% based on the weight of the fiber amount, preferably 30 to 80 wt%.

The paper of the present invention preferably has a grammage in the range of 100-800 g/m², in particular in the range of 200-600 g/m².

The paper comprises aramid pulp preferably at least partially covered with polyoxazoline. Preferably, the paper comprising polyoxazoline modified pulp and filler, either before or after resin incorporation, has not been coated or covered with polyoxazoline. Preferably, only the aramid pulp comprised in the friction paper is at least partially covered with polyoxazoline, the other paper components are not coated or covered with polyoxazoline.

The use of the aramid pulp comprising polyoxazoline in the paper leads to improved properties of the paper. In particular, the paper shows a combination of high mechanical strength and high porosity, in particular a high wet strength, shear strength (and related Z-strength) and tensile index in combination with a high air permeability and good filler retention. This makes the paper especially suitable as friction paper. Due to these properties, the friction paper is especially suitable for use in transmission systems.

The paper may be manufactured by methods known in the art.

Friction papers may generally be manufactured by a process comprising the steps of manufacturing a paper comprising aramid pulp comprising polyoxazoline, resin, and filler, 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. This is conventional in papermaking and requires no further elucidation. The resulting paper is dried. The dried paper is contacted with the resin. Generally the resin is provided in a solvent (preferably an alcohol such as ethanol or isopropanol) and the paper is impregnated with the resin solution. Depending on the type of resin, the impregnated paper can be subjected 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 0.1 to 10 MPa.

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.

The invention also pertains to a friction or sealing material comprising the described aramid pulp comprising polyoxazoline and/or comprising the described paper.

Such friction or sealing material may take various forms, e.g. multi-plate wet clutches comprising several layers of friction paper or paper-based gaskets. Multi-plate wet clutches have been proven to be ideal torque transfer devices for high energy applications. The multi-plate clutch comprises alternate friction and steel plates which interact within an oil cooled tribological system to transmit the desired torque. Multi-plate “wet” clutches are used in a broad spectrum of applications such as clutches in double clutch transmissions, torque converter lock-up clutches, clutches and brakes in automatic transmissions, wheel and axle brakes, differential locks, all-wheel drive transfer cases, power take offs, and master clutches.

The invention also pertains to a method for manufacturing the aramid pulp comprising polyoxazoline comprising:

- combining aramid short-cut, partly fibrillated aramid short-cut or aramid pulp with polyoxazoline in an aqueous solution to form a mixture, - subjecting the mixture to a refining step to form an aqueous slurry of the aramid pulp.

It has been found that the process according to the invention makes it possible to obtain aramid pulp comprising polyoxazoline in an efficient manner using an easy to operate process. Further, it has been found that the above described process where the polyoxazoline is present during fibrillation improves the surface coverage with polyoxazoline and the pulp properties compared to a process where first unmodified pulp is obtained by fibrillation and subsequently the pulp is coated by subjecting it to a polyoxazoline solution.

As starting material for the method, either aramid short-cut, partly fibrillated aramid short-cut or aramid pulp may be used (or combinations thereof).

Within the present specification the term aramid short-cut refers to aramid fibers cut to a length of, e.g., at least 0.5 mm, in particular at least 1 mm, more in particular at least 2 mm, in some embodiments at least 3 mm. The length generally is at most 80 mm, in particular at most 10 mm, more in particular at most 8 mm. The thickness of the short-cut is, e.g., in the range of 5-50 micron, preferably in the range of 5-25 micron, most preferably in the range of 6-18 micron. Aramid fiber, in particular para-aramid fibers from which such short-cut can be prepared are commercially available, e.g., from Teijin Aramid. The length of the short-cut refers to the LL0.25, which is a length-weighted average length wherein particles are included having a length > 250 pm, i.e. > 0.25 mm.

Such aramid short-cut may be obtained by cutting or chopping continuous fiber into either equal or random length pieces with a cutting device.

Partly fibrillated aramid short-cut refers to aramid short-cut that has been partly fibrillated, e.g. by cutting and milling (e.g. in a knife mill) or by cutting and subjecting to a short refiner treatment. In the first step of the method according to the invention, aramid short-cut, partly fibrillated aramid short-cut or aramid pulp is combined with polyoxazoline in an aqueous solution to form a mixture. This can be done in a variety of ways. For example, dry aramid short-cut, partly fibrillated short-cut or pulp may be added to a solution or suspension of polyoxazoline in water, polyoxazoline may be added to a suspension of short-cut, partly fibrillated short-cut or pulp in water, or polyoxazoline and short-cut, partly fibrillated short-cut or pulp may be added together to an aqueous medium.

The aramid short-cut may be obtained by cutting continuous aramid yarn to a length of at most 80 mm. This aramid short-cut may be further reduced in length before using it in instant process, e.g. in a knife mill. The aramid short-cut may be suspended in water to form a suspension which is subjected to a first refining step without the addition of polyoxazoline to shorten its length further. If the refining step is prolonged or the suspension is subjected to an additional refining step, partly fibrillated fiber or pulp is obtained. Either of these fiber species (aramid short-cut, partly fibrillated fiber or pulp) or a combination thereof may be used as starting material for the method.

Preferably, an aqueous solution of polyoxazoline (a stock solution) and a suspension of the fibers in water are prepared separately and then combined to form the mixture. The aqueous solution of polyoxazoline may be prepared at increased temperature, e.g. at a temperature in the range of 20 to 60°C, preferably at 30 to 50°C. The aqueous solution of polyoxazoline preferably has a concentration of at most 30 wt%, preferably of 15-25 wt%.

Preferably, the aqueous polyoxazoline stock solution is added to the fiber suspension e.g. by using a dosage system that adds the desired amount of the aqueous solution of polyoxazoline into the vessel where the fiber suspension has been prepared. The resulting mixture may be properly mixed by stirring and is subsequently transported to the equipment of the refining step. The aramid short-cut, partly fibrillated short-cut or pulp generally is present in the mixture in an amount of 0.1-7 wt%, in particular in the range of 1-5 wt%. An amount in this range has been found suitable for a successful refining operation. The concentration of polyoxazoline in the mixture depends on the amount of polyoxazoline desired in the end product. At higher concentrations, a low amount of polyoxazoline may remain in the suspension. The amount of polyoxazoline in the end product generally is in the range of 0.1-10 wt% polyoxazoline, based on the weight of the dried pulp (including the polyoxazoline). The amount of polyoxazoline present in the aqueous mixture, i.e. the aqueous mixture to be subjected to the refining step, varies between 0.1 and 15 wt%, preferably between 0.5 and 12.5 wt% calculated based on the dry weight of the aramid short-cut, partly fibrillated short-cut and/or pulp, in particular between 1 and 10 wt% or between 2 and 5 wt%, calculated based on the dry weight of the aramid short-cut, partly fibrillated short-cut and/or pulp.

The aqueous mixture is subjected to a refining step to form an aramid pulp comprising polyoxazoline. Refining processes are known in the art. In general, in refining, a slurry is subjected to a high shear environment, e.g., by passing it between discs which move with respect to each other. The effect of the refining step is to reduce the length of the short-cut, and to fibrillate the short-cut to form pulp (or to further fibrillate partially fibrillated short-cut or pulp). In fibrillation, fibrils will form, which will result in “stems” with fibrils connected thereto and loose fibrils. Further, the stems of the pulp may become kinked during the refining process.

It is possible to carry out a single refining step, but is also possible to subject refined pulp to one or more further refining steps, which are carried out at the same or different conditions as the first refining step. In one embodiment, previously fibrillated aramid short-cut or pulp is subjected to further refining in an aqueous solution including polyoxazoline.

The pulp slurry resulting from the refining process including polyoxazoline may be treated as desired. It can for example be subjected to a dewatering step wherein the slurry is dewatered, generally by bringing it onto a sieve or other filtering material, optionally involving a press section. This results in the formation of a dewatered pulp. Dewatered pulp generally has a water content in the range of 40- 80 wt.%, specifically 50-70 wt.%. The dewatering step may be repeated to further decrease the water content of the pulp. The dewatered pulp can be in the form of a cake (as it originated from the filter), or the cake can be broken to form individual pieces, also indicated as crumb.

The dewatered pulp, in the form of cake or crumb or any other form, can be an end product, which can be further processed as desired. The dewatered pulp can also be dried.

Drying of the dewatered pulp can take place in a conventional manner, e.g. by contacting it with a drying atmosphere, optionally at an elevated temperature, resulting in the formation of dried pulp. Dried pulp generally has a water content in the range of 2 to 20 wt%, in particular 3 to 10 wt%. Preferably, the dried pulp has a water content in the range of 3 to 8 wt%.

The dried pulp can, if so desired, be subjected to an opening step. Pulp opening is known in the art. It encompasses subjecting the dried pulp to mechanical impact, e.g., using an impact mill, a mill using turbulent air, or a high shear/high agitating mixer. The pulp opening step decreases the bulk density of the pulp material (i.e. , it makes it more “fluffy”). Opened pulp may be easier to disperse and therewith easier to apply. In general, the pulp opening step does not substantially change the properties of the pulp.

For further processing into a friction paper or friction material, either wet (i.e. dewatered) pulp, preferably having a water content in the range of 50 to 70 wt%, or dry pulp, preferably having a water content in the range of 3 to 8 wt%, may be used. Preferably, the polyoxazoline-modified pulp is applied in the form of wet (i.e. dewatered) pulp. For the use in friction papers and friction materials, it may be advantageous to process the dewatered pulp without drying, e.g. a dewatered pulp having a water content in the range of 40-80 wt.%, preferably 50-75 wt.%, more preferably 60-70 wt%.

As will be evident to the skilled person, the various preferred embodiments as described above can be combined unless they are mutually exclusive.

The invention will be further explained with the following, non-limiting examples.

Examples a) Determination of grammage

The grammage of the papers (also referred to as the areal weight) was measured according to ISO 536:1995 and expressed in terms of grams per square meter (g/m²). b) Determination of air permeability

The air permeability is a measure for the porosity and an indication for the oil penetrability of the paper.

The air permeability of the impregnated paper was determined according to ASTM D737 using a Textest type FX3030-LDM and is expressed with a unit of liter/m²/second (L/m²/s). c) Determination of Z-strength

The Z-strength (also referred to as internal bonding strength) of the paper correlates well with the shear strength. The Z-strength of the impregnated sheets was determined in accordance with Tappi T541. d) Determination of wet strength Paper sheets were immersed in isopropanol for 1 minute. Afterwards, the wet sheets were subjected to a tensile test to determine the tensile index. The determination was done in accordance with ISO 1924-2. e) Filler retention

The filler retention is a measure of the extent to which pulp retains a filler during paper making, where a value of 100% means complete retention of the filler, i.e. no loss of filler during the paper making process. The filler retention of the papers was determined using diatomaceous earth as filler. The filler retention is determined by dividing the amount of filler in the final sheet (calculated based on the actual grammage, the sheet surface area [having a 0 of 20 cm] and subtracting the amount of pulp in the sheet [5.5 g]) by the amount of filler used (corrected for the moisture content) and multiplying by 100. f) Determination of tensile strength

The tensile index of the dried papers before impregnation and the papers after resin impregnation was determined in accordance with ISO 1924-2.

Example 1 : Pulp manufacture

4 kg of para-aramid chopped fibers of 6 mm in length (6 mm short-cut based on Twaron® type 1000 1680f1000) was added to 200 liter of an aqueous solution of PEOX. The PEOX had a molecular weight of approximately 500 kg/mol. The resulting suspension contained 2 wt.% of aramid short-cut and 0.07 wt% (Pulp A) or 0.1 wt.% (Pulp B) of PEOX, depending on the amount of PEOX added to the suspension (weight percentage per volume of suspension). The resulting suspension was passed through a Sprout-Bauer 12" lab refiner to reach the target fiber length of 0.95 mm ± 0.1 mm. The refined suspension was dewatered on a sieve table to yield a dewatered cake. The PEOX-modified pulp denoted as Pulp A contains 3.4 wt% PEOX, the PEOX-modified pulp denoted as Pulp B contains 4.8 wt% of PEOX. As reference, the same procedure was followed without the addition of PEOX, resulting in an aramid pulp free of any coating or covering. This pulp is referred to as Pulp C.

As additional comparison, the same procedure as for Pulp B was followed with the addition of PVP (having a molecular weight of approximately 50 kg/mol) instead of PEOX. This pulp is referred to as Pulp D.

Example 2: Manufacture of a friction paper comprising filler, resin and aramid pulp

24.48 g of PEOX-containing Pulp A of Example 1 with a dry solids content of 22.45%, thus equaling 5.50 g of dry aramid pulp, was suspended in 2 L of water and mixed for 100 counts (20 s at 3000 rpm) in a Lorentzen & Wettre disintegrator. Then, 6.0 g of diatomaceous earth (Transcend ND-1, as filler) was added to the suspension and mixed for an additional 500 counts (100s at 3000 rpm). This mixture was used for paper sheet preparation on a Rapid Kothen lab sheet former in accordance with ISO 5269-2. The resulting paper sheets were dried between two blotting papers in a plate drier at 105°C for at least 20 minutes. The resulting paper sheets were targeted to have a grammage of 350 ± 16 g/m², consisting of 50% pulp and 50% diatomaceous earth.

The same procedure was followed for Pulp B, C and D, with the exception that slightly different amounts of pulp and filler were used to obtain the same final target sheet weights (see Table 1 for amounts used). The amount of pulp was adjusted to correct for the water content of the different pulp samples such that the same amount of dry pulp (dry solids content) was used.

The wet strength and the dry strength of the papers was determined.

The paper sheets prepared in this way and based on pulp samples A, B, C and D were also impregnated with phenolic resin (Bakelite PF 0229 RP). For the paper comprising pulps A and B (according to the invention) the resin was diluted to the desired concentration using a mixture of 22 ml_ of the resin and 78 mL of isopropanol. The paper sheet was placed in a tray covered with a plastic liner and the resin mixture was poured over the sheet. The tray was moved around for 1 minute, after which the paper sheet was transferred onto a Teflon sheet. Excess resin was removed by passing the paper sheet through a custom made wringer twice (flipping the paper in between the passages). The residual solvent (isopropanol) was then evaporated in a ventilated oven at 90°C for 20 minutes. After impregnation, the targeted sheet weight is 500 ± 16 g/m² The resin dilution is adjusted accordingly to reach the desired sheet weight for the papers comprising pulp C and D as indicated in Table 1.

In a final step, the paper sheet was cured in an oven at 180°C for 60 minutes.

Table 1: Materials used for preparation of impregnated paper sheets based on pulp samples A-D

Example 3: Dry and wet strength of the base paper (before impregnation)

The pulp of the current invention is highly beneficial to improve the dry strength of the paper. In, addition, the wet strength, required during resin impregnation of the base paper (in these examples 50/50 pulp/diatomaceous earth by weight), is also improved when the pulp of this invention is used. This is illustrated by the tensile properties of the wet and dry paper prepared in example 2 given in Table 2 below. Table 2: Dry and wet strength of base (not impregnated) papers comprising pulps A-D

Based on these results, it can clearly be seen that the wet strength of papers comprising Pulp A and B from the current invention results in a strong increase in wet strength compared to a comparative paper comprising Pulp C (25 to 30 times higher). The paper comprising comparative Pulp D (PVP pulp) also shows an improvement compared to Pulp C, but less than observed for papers comprising Pulp A and B.

Example 4: Comparison of (impregnated) friction papers

Various properties of the friction papers of Example 2 were determined, including the filler (diatomaceous earth) retention, the air permeability, the tensile strength and the Z-strength. The filler retention and the air permeability were determined on two paper sheets (indicated as sheet 1 and 2). The mechanical properties (Z- strength and Tensile strength) were each determined on one of those sheets (because the testing destroys the sheet). The results thereof are shown in Table 3. Table 3: Properties of impregnated friction papers comprising pulps A-D

The data show that the filler retention for the papers prepared with Pulp A and B according to the invention is substantially higher than the filler retention of the paper prepared from comparative Pulp C. Apparently, polyoxazoline-modified pulp according to the invention is more capable of retaining the filler and resin particles than unmodified pulp. Papers comprising the pulp according to the invention also have a higher filler retention than papers comprising PVP-modified pulp (pulp D).

For friction applications, it is important for the friction paper to be as open as possible, so that oil can penetrate into the friction paper during e.g. clutch application. An aim of current invention is to provide a paper, in particular a friction paper, combining high strength and high porosity. The air permeability of a paper is a measure for its porosity.

The air permeability of papers comprising either Pulp A or B (PEOX-modified pulp) is at comparable level as comparative Pulp C (unmodified aramid pulp), whereas Pulp D (PVP-modified pulp) exhibits a clear decrease in air permeability.

For friction applications, paper strength is also an important property. Shear strength is the most relevant strength property due to the high shear forces to which the papers are subjected during operation. The shear strength correlates well with the so-called Z-strength or internal bonding strength.

The results in Table 3 show that that the strength of the model friction paper based on Pulp A and B according to the invention are greatly improved as compared to the strength of a friction paper based on comparative Pulp C.

The large benefit of pulp of the invention is the combination of high strength, in particular in Z-strength, combined with a high porosity. Comparative papers comprising either unmodified pulp (Pulp C) or PVP-modified pulp (paper D) do not show this combination of properties and only reach comparable or even lower values for either strength or porosity, but not for both properties.

Example 5: Comparison of commercial pulp samples and polyoxazoline modified pulp and the corresponding papers

Polyoxazoline modified pulp was produced on production scale by adding a PEOX solution to a suspension of partly fibrillated aramid short-cut, wherein the suspension comprised 2.5 wt% of partly fibrillated aramid short-cut and 0.09 wt% of PEOX (weight percentage per volume of suspension). The resulting suspension was circulated through a refiner to reach the target fiber length of 0.98 mm ± 0.2 mm. The PEOX had a molecular weight of 500 kg/mol. The PEOX-modified pulp (Pulp E) contains about 3.3 wt% of PEOX (based on dry weight) and has an SSA of about 4.8 m²/g.

As comparative samples, commercially available Twaron® pulp 1092 (referred to as 1092, less fibrillated pulp type, having an SSA of about 6.6 m²/g) and Twaron® pulp 1094 (referred to as 1094, more fibrillated pulp type, having an SSA of 12-15 m²/g) are used. Usually, a more fibrillated pulp will increase paper strength but decrease air permeability in the paper.

Papers were prepared based on 1092, 1094 and Pulp E as described in Example 2.

Subsequently, the wet strength of the papers was determined.

The paper sheets prepared in this way and based on pulp samples 1092, 1094 and Pulp E were impregnated with phenolic resin as described for Example 2. The air permeability, Z-Strength, filler retention and tensile strength of the impregnated papers was determined as described in Example 4.

The properties (average values) of the base papers and the impregnated papers are shown in Table 4. Table 4: Properties of base and impregnated friction papers comprising pulps 1092, 1094 and Pulp E.

The data of Table 4 show that using polyoxazoline modified pulp improves the mechanical properties of the base paper and the impregnated paper in comparison to the commercially available pulp types which do not contain polyoxazoline. While papers based on pulp type 1092 have a high air permeability, papers based on pulp type 1094 have a high filler retention. The paper based on the pulp according to the invention combines a high filler retention and a high air permeability. In addition, the paper based on the pulp according to the invention has the highest wet strength as base paper and the highest Z-strength and tensile strength as impregnated paper.

Claims

Claims:

1. An aramid pulp comprising polyoxazoline.

2. The aramid pulp according to claim 1 comprising 0.1 to 10 wt% of polyoxazoline, preferably 0.25 to 7.5 wt% of polyoxazoline, more preferably 0.5 to 5 wt% of polyoxazoline, based on the weight of the dried pulp.

3. The aramid pulp according to claim 1 or 2 wherein the polyoxazoline is a poly- alkyl-2-oxazoline, preferably poly-2-ethyl-2-oxazoline.

4. The aramid pulp comprising polyoxazoline according to any of the preceding claims wherein the polyoxazoline covers at least part of the surface of the aramid pulp.

5. The aramid pulp according to any one of the preceding claims comprising fiber stems and fibrils.

6. The aramid pulp according to any of the preceding claims having a specific surface area in the range of 2 to 20 m²/g, preferably 3 to 15 m²/g, more preferably 4 to 10 m²/g and/or a length LL0.25 in the range of 0.5 to 1.5 mm, preferably 0.6 to 1.4 mm.

7. A paper comprising the aramid pulp according to any one of claims 1 to 6.

8. The paper of claim7 comprising 2-70 wt% of the aramid pulp, more preferably 5- 55 wt% of the aramid pulp, even more preferably 10-35 wt% of the aramid pulp based on the weight of the paper.

9. The paper of claim 7 or 8 comprising a filler and a resin, preferably comprising 5-55 wt% of the filler, more preferably 20-40 wt% of the filler and 5-50% of the resin, more preferably 15-40 wt% of the resin, based on the weight of the paper.

10. The paper of any one of claims 7 to 9 wherein the resin is a thermoset resin, preferably a thermoset resin selected from phenolic resin, vitrimer resin, polythiourethane resin, melamine resin, silicone resin and epoxy resin.

11. The paper of any one of claims 7 to 10 being a friction paper, a separator paper or a honeycomb paper.

12. A friction material comprising the aramid pulp comprising polyoxazoline according to any one of claims 1 to 6 or the paper of any one of claims 7 to 11.

13. A method for manufacturing the aramid pulp comprising polyoxazoline according to any one of claims 1 to 6 comprising:

- combining aramid short-cut, partly fibrillated aramid short-cut or aramid pulp with polyoxazoline in an aqueous solution to form a mixture,

- subjecting the mixture to a refining step to form an aqueous slurry of the aramid pulp.

14. The method according to claim 13, wherein the aqueous slurry of the aramid pulp is subjected to a dewatering step to form a dewatered pulp having a water content in the range of 40-80 wt%, preferably 50-70 wt%, based on the weight of the dewatered pulp.

15. The method according to claim 14, wherein the dewatered pulp is subjected to a drying step to form a dried pulp having a water content in the range of 2 to 20 wt%, preferably 3-10 wt%, based on the weight of the dried pulp, optionally followed by subjecting the dried pulp to an opening step.