JP2024521939A - Modified aramid pulp and friction material containing modified aramid pulp - Google Patents

Abstract

The present invention relates to an aramid pulp containing polyoxazoline. It also claims a method for producing an aramid pulp containing polyoxazoline, comprising: mixing an aramid short cut, a partially fibrillated aramid short cut, or an aramid pulp with a polyoxazoline in an aqueous solution to form a mixture; and subjecting the mixture to a beating step to form an aqueous slurry of aramid pulp. The present invention also relates to a paper containing an aramid pulp containing polyoxazoline, and a friction material containing said paper and/or said aramid pulp containing polyoxazoline.

Description

The present invention relates to an aramid pulp containing polyoxazoline, a paper containing the pulp, a friction material containing the pulp or the paper, and a method for producing an aramid pulp containing polyoxazoline.

Aramid pulp is known and is used in a variety of application areas, for example paper, friction materials, especially friction paper. Aramid is known in the art for its high strength and resistance to high temperatures.

Friction paper, 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 a support member for use in mechanical energy transfer applications. Although friction papers are manufactured by traditional papermaking methods, these materials are actually sophisticated composite structures that contain pulp, fillers, binders (usually thermosetting resins), and optionally further ingredients such as fibers and friction additives.

Friction papers are composite materials formulated to provide the appropriate friction, noise control, temperature resistance, and wear characteristics for each specific application.

Pulp wood is present to increase mechanical strength, adjust the porosity of the paper, and ensure retention of the filler during papermaking. Aramid pulp is often used as pulp due to its very good mechanical and thermal properties. Para-aramid pulp in particular shows good heat resistance, friction performance, and durability. In addition, it shows good properties in terms of noise and vibration behavior (NVH) and shows no chemical interaction with automatic transmission fluids (ATF). It also has good compressive and shear strength properties as well as good flexibility compared to metal fibers.

WO 2006/012040 describes acrylic and para-aramid pulps for use as reinforcing materials in products such as seals and friction materials.

WO 2018/037015 describes modified aramid pulp for friction paper. The aramid pulp is provided with PVP (polyvinylpyrrolidone) and is used in friction paper. The PVP modified aramid pulp improves friction performance.

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

The present invention provides a solution to this problem.

The present invention relates to aramid pulp containing polyoxazoline (also referred to in this specification as "modified aramid pulp").

Modified continuous aramid yarns are described for use in ballistic fabrics and thermoplastic composites.

US 5,266,076 describes continuous aramid fibers coated with a finish, e.g. fluorinated polyoxazoline. The continuous fibers are used in fabrics for bulletproof applications. US 5,266,076 does not mention aramid pulp, paper, or friction materials, especially friction paper. WO 01/34385 discloses a thermoplastic composite material comprising fibers coated with a poly-2-oxazoline polymer and a polymer resin. WO 01/34385 does not mention pulp or paper. The fibers in WO 01/34385 are continuous fibers (i.e. essentially endless fibers) and are selected from fibers such as glass fibers, carbon fibers, nickel-plated carbon fibers, and aromatic polyamide fibers that are coated with poly-2-oxazoline, cut into short cuts, and then coated with a thermoplastic resin. Continuous and short cut fibers are distinct from pulp.

It has been found that aramid pulp modified with polyoxazoline provides a combination of high strength and high porosity when used in papers and friction materials, e.g., friction paper. Increasing the porosity of paper often results in a decrease in mechanical properties (tensile strength, tear strength). Therefore, friction papers that combine improved mechanical properties without adversely affecting the level of porosity, or that combine increased porosity with improved or uniform mechanical properties, are of interest. Surprisingly, aramid pulp containing polyoxazoline provides such properties to papers, particularly friction paper. In addition, the modification with polyoxazoline can improve the filler retention of the paper.

Pulp is an irregularly shaped fibrous structure. It consists of short fibres that have been subjected to shear forces to form fibrils, most of which are connected to the "stem" of the original fibre, while thinner fibrils have detached from thicker ones. These fibrils are curled, sometimes ribbon-like, and show variations in length and thickness. Pulp is obtained by fibrillating short fibres (also called short cuts), for example in a refiner. Thus, pulp contains fibre stems and fibrils. Due to the fibrillation, pulp has a different morphology and different properties compared to continuous or short cut fibres. In particular, pulp is very short and has a large specific surface area.

In the context of this specification, aramid refers to an aromatic polyamide that contains or consists of aromatic fragments directly linked to each other via amide fragments. Methods for synthesizing aramids are known to those skilled in the art and typically involve polycondensation of aromatic diamines with aromatic diacid halides. Aramids can exist in meta and para forms, both of which can be used. Preferably, the aramid pulp of the present invention is a para-aramid pulp.

For purposes of this application, the term para-aramid refers to a class of fully aromatic polyamide polymers and copolymers having at least 60%, preferably at least 80%, and more preferably at least 90% para-oriented bonds between 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'-biphenylenedicarboxamide), and poly(para-phenylene-2,6-naphthalenedicarboxamide), 5,4'-diamino-2-phenylbenzimidazole, or poly(para-phenylene-co-3,4'-oxydiphenylene terephthalamide), or copolymers thereof.

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

In the context of the present invention, polyoxazolines relate to polymers based on oxazoline moieties, sometimes also called oxazoline polymers. Polyoxazoline polymers are distinct from non-polymerized oxazoline compounds, which may be used, for example, as hardeners for 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 in the ring. 2-Oxazolines are used to prepare polyoxazolines.

Preferably, the polyoxazoline is based on (possibly substituted) N-acyl ethyleneimine units (linear, obtained after ring opening of 2-oxazoline monomers), the main chain carbons being preferably substituted with hydrogen and the acyl groups being substituted with hydrogen or C ₁ -C ₄ alkyl (where R is H or C ₁ -C ₄ alkyl), preferably ethyl groups (where R is C ₂ 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 having 1 to 4 carbon atoms.

Preferably, the polyoxazoline is substantially free of halogen groups, especially fluorine groups. By substantially free, it is meant that the polyoxazoline polymer contains less than 5 mol % halogen groups, preferably less than 1 mol % halogen groups, especially fluorine groups.

Preferably, the polyoxazoline has a molecular weight in the range of 1000 to 1,000,000 g/mol, preferably 5,000 to 750,000 g/mol, more preferably 10,000 to 600,000 g/mol, and even more preferably 200,000 to 500,000 g/mol. In some embodiments, increasing the molecular weight of the polyoxazoline can improve the strength of paper containing pulp modified with the polyoxazoline.

Aramid pulp containing polyoxazoline typically has a length (LL0.25) in the range of 0.5 to 1.5 mm, particularly in the range of 0.60 to 1.4 mm, and in some embodiments in the range of 0.7 to 1.3 mm. This parameter is determined by a Valmet fiber image analyzer known as a Valmet FS5, calibrated on samples of pulp of known length. The length-weighted length LL0.25 [mm] is the length-weighted average length determined according to ISO 16065-2 and includes particles having a length greater than 250 μm, i.e., greater than 0.25 mm.

Aramid pulps containing polyoxazolines usually have a Schopper-Rigler (SR) in the range of 15-80° SR, in particular in the range of 16-60° SR, more particularly in the range of 17-40° SR. SR is a parameter often used in the field of pulp and paper technology. It is a measure of the drainability of a water suspension of pulp. SR can be determined according to ISO 5267/1 by dispersing 2 g (dry weight) of pulp in 1 L of water during 600 counts in a Lorentzen and Wettre disintegrator.

Aramid pulps containing polyoxazolines usually have a Canadian Standard Freeness (CSF) in the range of 15-700 mL, particularly in the range of 100-670 mL, and more particularly in the range of 200-650 mL. CSF is a parameter often used in pulp and paper technology. Like SR, it is a measure of the drainability of a water suspension of pulp. CSF can be determined according to TAPPI T227.

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

The specific surface area ( ^m2 /g) is determined using nitrogen adsorption by the BET specific surface area method using a Micromeritics Tristar 3000. The pulp is pre-dried in an oven at 105°C for at least 3 hours and then degassed at 200°C for 30 minutes under flowing nitrogen before the specific surface area is measured.

Non-fibrillated fibers (eg, continuous or short cut fibers) have a much smaller specific surface area, in the range of 0.1-0.2 m ² /g.

Preferably, the polyoxazoline is present (only) on the surface of the pulp. Preferably, the polyoxazoline coats at least a portion of the surface of the aramid pulp, or the entire surface of the aramid pulp. Preferably, the polyoxazoline is not used in the manufacture of a shortcut for producing pulp, 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.

The present invention also relates to paper containing aramid pulp containing polyoxazoline in the above-mentioned embodiment.

The paper may be, for example, friction paper, separator paper, or honeycomb paper.

In particular, the present invention relates to a friction paper comprising aramid pulp containing the polyoxazoline in the above-mentioned embodiment.

Friction paper is usually a composite material containing many different materials, each of which contributes to the properties of the paper.

Reinforcing fibers are often present to increase the mechanical strength and durability of the system. These also serve to impart a porous structure that helps ensure proper absorption of the resin.

Fillers are added to perform a variety of functions, such as, for example, to aid in resin absorption, to facilitate oil flow through the paper to reduce temperature drop during use, to ensure adequate friction performance, and/or to reduce noise.

The resin is present to ensure good dimensional stability, good friction performance, and good heat resistance.

Preferably, the paper of the present invention comprises 2 to 70% by weight of modified aramid pulp, more preferably 5 to 55% by weight of modified aramid pulp, and even more preferably 10 to 35% by weight of modified aramid pulp, based on the weight of the paper.

In one embodiment, the paper of the present invention comprises aramid pulp containing polyoxazoline, filler, and resin.

Preferably, the paper of the present invention contains, based on the weight of the paper, 5 to 55% by weight of filler, more preferably 20 to 40% by weight of filler, and 5 to 50% by weight of resin, more preferably 15 to 40% by weight of resin.

In the context of this specification, the term filler is intended to encompass all particulate materials, preferably other than fibers or resins, that affect the frictional 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, zeolites, alumina, silica, barium sulfate, barite powder, and particles derived from renewable resources such as powdered cocoa shells and cashew dust. Other examples of suitable filler particles include diatomaceous earth, graphite particles, and copper particles, although the use of the latter has generally been discontinued due to HSE considerations.

It may be preferred that the (friction) paper contains diatomaceous earth and/or graphite particles.

The filler is preferably present in an amount of 5-55% by weight. If the filler proportion is too low, its effect on the frictional properties of the paper is not obtained. If the amount of filler is too high, the amounts of the other components are too low. It may be preferred that the amount of filler be in the range of 10-50% by weight (based on the weight of the paper), more specifically in the range of 20-40% by weight.

The paper of the present invention comprises a resin as a binder. Suitable resins are known in the art. The resin is usually present in an amount of 5-50% by weight, especially 15-40% by weight (based on the weight of the paper). If the amount of resin is too low, the structural integrity of the paper is affected. If the amount of resin is too high, the content of the other components is too low. The resin is preferably a thermosetting resin. Preferably, the resin is selected from phenolic resins, vitrimer resins (so-called malleable thermosetting resins), polythiourethane resins, melamine resins, silicone resins, and epoxy resins.

Preferably, resins are used that allow the removal or reprocessing of the resin in order to allow the components of the paper to be separated for recycling. Suitable vitrimer resins are described, for example, in WO 2020/051506. EP 3149065 describes thermomechanically reprocessable epoxy resins and WO 2019/063787 describes reprocessable polythiourethane resins.

Suitable silicone resins are, for example, the organopolysiloxane resins described in EP 3 473 883 A1.

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

The paper of the present invention may contain further components.

In one embodiment, the paper comprises additional reinforcing fibres such as carbon fibres, mineral fibres, ceramic fibres, glass fibres, basalt fibres and mineral wool, or polymer fibres such as acrylic fibres, polyimide fibres and polyamide fibres. Organic fibres such as cotton and cellulose are also often used as (short cut) fibres or pulp. The paper may also comprise unmodified aramid pulp, i.e. aramid pulp without polyoxazoline or other surface modification. Thus, the paper may comprise a combination of unmodified aramid pulp and pulp modified with polyoxazoline. It may be preferred that the friction paper according to the invention comprises one or more of cellulose, cotton or carbon fibres. Reinforcing fibres are often used to improve the durability and mechanical strength of the paper. When used, they are usually present in an amount of 2-40% by weight, in particular 5-35% by weight. Reinforcing fibres and their uses are known in the art.

Preferably, in the resin impregnated paper, the total amount of all reinforcing fibers and pulp, including the aramid pulp with polyoxazoline (referred to as the fiber amount of the friction paper), accounts for 25-45% by weight of the paper, preferably 30-40% by weight. In one embodiment, the paper contains a fiber amount of 25-45% by weight, 25-45% by weight of filler and 25-45% by weight of resin. Preferably, the amounts of fiber, filler and resin are each (approximately) 1/3 of the paper weight. The aramid pulp with polyoxazoline may be present in the range of 20% to 100% by weight, preferably 30% to 80% by weight, based on the weight of the fiber.

The paper of the present invention preferably has a basis weight in the range of 100 to 800 g/m ² , in particular in the range of 200 to 600 g/m ² .

The paper preferably comprises aramid pulp at least partially coated with polyoxazoline. Preferably, the paper comprising the polyoxazoline modified pulp and the filler is not coated or covered with polyoxazoline either before or after incorporation of the resin. Preferably, only the aramid pulp contained in the friction paper is at least partially coated with polyoxazoline, and no other paper components are coated or covered with polyoxazoline.

The use of aramid pulp containing polyoxazolines in paper improves the properties of the paper. In particular, the paper exhibits a combination of high mechanical strength and high porosity, especially high wet strength, shear strength (and associated Z-axis strength), and tensile index, combined with high air permeability and good filler retention. This makes the paper particularly suitable as a friction paper. These properties make the friction paper particularly suitable for use in transmission systems.

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

Friction paper can be manufactured by a process that typically includes the steps of producing a paper containing aramid pulp, which contains polyoxazoline, resin, and filler, and heating the paper under conditions such that the resin cures. In one embodiment, in a first step, all the components of the paper, except the resin, are mixed in an aqueous medium to form a slurry. This can be done in any order, and the various compounds can be added simultaneously or sequentially. The resulting slurry is applied to a screen and the water is removed. This is conventional in papermaking and does not require further explanation. The resulting paper is dried. The dried paper is contacted with the resin. Typically, 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, a curing step can be performed on the impregnated paper to harden the resin. The exact processing conditions depend on the nature of the resin and typically include temperatures in the range of 100-300°C and pressures of 0.1-10 MPa.

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

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

Such friction or seal materials can take a variety of forms, such as multi-plate wet clutches that include multiple layers of friction paper or paper-based gaskets. Multi-plate wet clutches have proven to be ideal torque transfer devices for high energy applications. Multi-plate clutches contain alternating friction and steel plates that interact in an oil-cooled tribological system to transmit the desired torque. Multi-plate "wet" clutches are used in a wide range 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 present invention provides a method for producing an aramid pulp containing a polyoxazoline, comprising the steps of:
- combining an aramid short cut, a partially fibrillated aramid short cut, or an aramid pulp with a polyoxazoline in an aqueous solution to form a mixture;
- subjecting the mixture to a beating step to form an aqueous slurry of aramid pulp.

It has been found that the method according to the invention makes it possible to obtain aramid pulp containing polyoxazoline efficiently using an easy to operate process. Furthermore, it has been found that the above-mentioned process in which polyoxazoline is present during fibrillation improves the surface coverage by polyoxazoline and the pulp properties compared to a process in which unmodified pulp is first obtained by fibrillation and then the pulp is coated by exposing it to a polyoxazoline solution.

As starting material for this process, aramid short cut, partially fibrillated aramid short cut, or aramid pulp (or a combination thereof) can be used.

In this specification, the term aramid short cut refers to aramid fibers cut to a length of, for example, at least 0.5 mm, particularly at least 1 mm, more particularly at least 2 mm, and in some embodiments at least 3 mm. The length is usually up to 80 mm, particularly up to 10 mm, more particularly up to 8 mm. The thickness of the short cut is, for example, in the range of 5 to 50 microns, preferably in the range of 5 to 25 microns, most preferably in the range of 6 to 18 microns. Aramid fibers, particularly para-aramid fibers, from which such short cuts can be produced are commercially available, for example from Teijin Aramid. The length of the short cut refers to LL0.25, which is the length-weighted average length at which particles having a length of more than 250 μm, i.e., more than 0.25 mm, are included.

Such aramid short cuts can be obtained by cutting or chopping the continuous fibers into pieces of equal or random length using a cutting device.

Partially fibrillated aramid short cut refers to aramid short cut that has been partially fibrillated, for example by cutting and grinding (e.g. in a knife mill) or by cutting and subjecting to a short refiner process.

In the first step of the method according to the invention, the aramid short cut, partially fibrillated aramid short cut or aramid pulp is mixed with the polyoxazoline in an aqueous solution to form a mixture. This can be done in various ways. For example, the dry aramid short cut, partially fibrillated short cut or pulp can be added to an aqueous solution or suspension of the polyoxazoline, or the polyoxazoline can be added to a suspension of the short cut, partially fibrillated aramid short cut or pulp in water, or the polyoxazoline and the short cut, partially fibrillated short cut or pulp can be added together to an aqueous medium.

Aramid short cuts can be obtained by cutting continuous aramid yarns to lengths of up to 80 mm. The aramid short cuts can be further shortened in length, for example with a knife mill, before use in the process of the present invention. The aramid short cuts can be suspended in water to form a suspension, which is subjected to a first beating step without the addition of polyoxazoline, to further shorten the length of the aramid short cut. If the beating step is extended or the suspension is subjected to an additional beating step, partially fibrillated fibers or pulp are obtained. Any of these fiber types (aramid short cuts, partially fibrillated fibers, or pulp) or a combination thereof can be used as starting material for the process.

Preferably, an aqueous solution of polyoxazoline (stock solution) and an aqueous suspension of fibers are prepared separately and then mixed to form a mixture. The aqueous solution of polyoxazoline can be prepared at elevated temperature, for example at a temperature in the range of 20-60°C, preferably 30-50°C. The aqueous solution of polyoxazoline preferably has a concentration of up to 30% by weight, preferably 15-25% by weight.

Preferably, the aqueous polyoxazoline stock solution is added to the fiber suspension, for example by using a dosing system that adds the desired amount of aqueous polyoxazoline solution to the container in which the fiber suspension was prepared. The resulting mixture can be properly mixed by stirring and is subsequently conveyed to the equipment for the beating process.

The aramid short cut, partially fibrillated short cut, or pulp is usually present in the mixture in an amount ranging from 0.1 to 7% by weight, especially 1 to 5% by weight. This range of amounts has been found to be suitable for successful beating operations.

The concentration of polyoxazoline in the mixture depends on the amount of polyoxazoline desired in the final product. At higher concentrations, small amounts of polyoxazoline may remain in suspension. The amount of polyoxazoline in the final product typically ranges from 0.1 to 10% by weight of polyoxazoline, based on the weight of the dry pulp (including polyoxazoline). The amount of polyoxazoline present in the aqueous mixture, i.e. the aqueous mixture subjected to the beating step, varies between 0.1 and 15% by weight, preferably between 0.5 and 12.5% by weight, calculated based on the dry weight of the aramid short cut, partially fibrillated short cut, and/or pulp, in particular between 1 and 10% by weight or 2 and 5% by weight, calculated based on the dry weight of the aramid short cut, partially fibrillated short cut, and/or pulp.

The aqueous mixture is subjected to a beating step to form an aramid pulp containing the polyoxazoline. The beating process is known in the art. In general, in beating, the slurry is exposed to a high shear environment, for example by passing between disks moving against each other. The effect of the beating step is to shorten the length of the short cuts and to fibrillate the short cuts to form a pulp (or to further fibrillate the partially fibrillated short cuts or pulp). In fibrillation, fibrils are formed, resulting in a "stem" of interlocking fibrils and detached fibrils. Furthermore, the pulp stem may become twisted during the beating process. It is possible to carry out a single beating step, but it is also possible to subject the beaten pulp to one or more further beating steps carried out at the same or different conditions as the first beating step. In one embodiment, the pre-fibrillated aramid short cuts or pulp is further beaten in an aqueous solution containing the polyoxazoline.

The pulp slurry resulting from the beating process containing the polyoxazoline can be treated as required. For example, a dewatering step can be performed in which the slurry is dewatered, typically by placing it on a sieve or other filter material, optionally including a press section. This forms a dewatered pulp. The dewatered pulp typically has a moisture content in the range of 40-80% by weight, particularly 50-70% by weight. The dewatering step can be repeated to further reduce the moisture content of the pulp. The dewatered pulp can be in the form of a cake (as it leaves the filter) or the cake can be broken to form individual pieces, also called crumbs.

The dewatered pulp in the form of cake or crumb or any other form can be the final product and can be further processed as required. The dewatered pulp may be dried.

Drying of the dewatered pulp can be carried out in a conventional manner, for example by contacting the pulp with a dry atmosphere, optionally at elevated temperature, resulting in the formation of dry pulp. The dry pulp usually has a moisture content in the range of 2-20% by weight, in particular 3-10% by weight. Preferably, the moisture content of the dry pulp is in the range of 3-8% by weight.

The dry pulp can optionally be subjected to an opening process. Pulp opening is known in the art. It involves mechanically impacting the dry pulp using, for example, an impact mill, a mill using turbulent air, or a high shear/high agitation mixer. The opening process reduces the bulk density of the pulp material (i.e., makes the pulp material more "fluffy"). Opened pulp is easier to disperse and therefore easier to apply. In general, the opening process does not substantially change the properties of the pulp.

For further processing into friction paper or friction material, either wet (i.e. dewatered) pulp, preferably having a moisture content in the range of 50-70% by weight, or dry pulp, preferably having a moisture content in the range of 3-8% by weight, can be used. Preferably, the polyoxazoline-modified pulp is applied in the form of wet (i.e. dewatered) pulp. For use in friction paper or friction material, it may be advantageous to process dewatered pulp that has not been dried, for example dewatered pulp having a moisture content in the range of 40-80% by weight, preferably 50-75% by weight, more preferably 60-70% by weight.

As will be apparent to one skilled in the art, the various preferred embodiments described above may be combined as long as they are not mutually exclusive.

The invention is further described using the following non-limiting examples.

EXAMPLES a) Determination of Basis Weight The basis weight (also called basis weight) of paper was measured according to ISO 536:1995 and expressed in grams per square meter (g/m ² ).

b) Determination of Air Permeability Air permeability is a measure of the porosity of the paper and is an indication of its oil permeability.

The air permeability of the impregnated papers was determined according to ASTM D737 using a Textest type FX3030-LDM. Air permeability is expressed in units of liters/m ² /second (L/m ² /s).

c) Determination of Z-Axis Strength The Z-axis strength of paper (also called internal bond strength) correlates well with the shear strength. The Z-axis strength of impregnated sheets was determined according to Tappi T541.

d) Determination of Wet Strength Paper sheets were immersed in isopropanol for 1 minute. The wet sheets were then subjected to a tensile test to determine the tensile index. The determination was carried out according to ISO 1924-2.

e) Filler Retention Filler retention is a measure of the extent to which the pulp retains the filler during papermaking, with a value of 100% meaning that the filler is completely retained, i.e. no filler is lost during the papermaking process. Filler retention of paper was determined using diatomaceous earth as a filler. Filler retention is determined by dividing the amount of filler in the final sheet (calculated based on actual basis weight, sheet surface area (having a φ of 20 cm), minus the amount of pulp in the sheet [5.5 g]) by the amount of filler used (corrected for moisture content) and multiplying by 100.

f) Determination of Tensile Strength The tensile index of the dry paper before and after resin impregnation was determined according to ISO 1924-2.

Example 1 : Pulp Production 4 kg of 6 mm long para-aramid chopped fibers (6 mm short cut based on Twaron® type 1000 1680f1000) were added to 200 liters of aqueous PEOX solution. The PEOX had a molecular weight of about 500 kg/mol. The resulting suspension contained 2 wt. % aramid short cut and 0.07 wt. % (Pulp A) or 0.1 wt. % (Pulp B) PEOX depending on the amount of PEOX added to the suspension (weight percent per volume of suspension). The resulting suspension was passed through a Sprout-Bauer 12 inch lab refiner to reach the target fiber length of 0.95 mm ± 0.1 mm. The beaten suspension was dewatered on a sieve table to obtain a dewatered cake. The PEOX modified pulp designated Pulp A contained 3.4 wt% PEOX and the PEOX modified pulp designated Pulp B contained 4.8 wt% PEOX.

As a reference, the same procedure was followed without the addition of PEOX, resulting in an aramid pulp without any coating or covering. This pulp is called Pulp C.

As an additional comparison, the same procedure was followed for pulp B, but with the addition of PVP (molecular weight approximately 50 kg/mol) instead of PEOX. This pulp is called pulp D.

Example 2 : Preparation of friction paper with filler, resin and aramid pulp 24.48 g of PEOX-containing pulp A from Example 1 with a dry solids content of 22.45% (thus corresponding to 5.50 g of dry aramid pulp) were suspended in 2 L of water and mixed in a Lorentzen & Wettre disintegrator for 100 counts (20 s at 3000 rpm). 6.0 g of diatomaceous earth (Transcend ND-1, as filler) were then added to the suspension and mixed for a further 500 counts (100 s at 3000 rpm). This mixture was used to produce paper sheets in a Rapid Koethen lab sheet former according to ISO 5269-2. The resulting paper sheets were dried in a plate dryer at 105° C. between two pieces of blotting paper for at least 20 minutes. The resulting paper sheet was targeted to consist of 50% pulp and 50% diatomaceous earth with a basis weight of 350±16 g/ ^m2 .

The same procedure was followed for Pulps B, C, and D, except that slightly different amounts of pulp and filler were used to obtain the same final target sheet weight (see Table 1 for amounts used). The amount of pulp was adjusted to compensate for the moisture content of the different pulp samples so that the same amount of dry pulp (dry solids) was used.

The wet and dry strength of the paper was determined.

The thus produced paper sheets based on pulp samples A, B, C and D were also impregnated with phenolic resin (Bakelite PF0229RP). For papers containing pulps A and B (according to the invention), the resin was diluted to the desired concentration using a mixture of 22 mL of 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 onto the sheet. The tray was run for 1 minute, after which the paper sheet was transferred onto a Teflon sheet. Excess resin was removed by passing the paper sheet twice through a custom-made wringer (turning the paper over between passes). Residual solvent (isopropanol) was then evaporated in a ventilated oven at 90°C for 20 minutes. The target sheet weight after impregnation is 500±16 g/ ^m2 . The dilution of the resin is adjusted to reach the target sheet weight for papers containing pulps C and D shown in Table 1.

In the final step, the paper sheets were cured in an oven at 180°C for 60 minutes.

Example 3 : Dry and wet strength of base paper (before impregnation)
The pulp of the present invention is very beneficial for improving the dry strength of the paper. In addition, the wet strength required during resin impregnation of the base paper (50/50 pulp/diatomaceous earth by weight in these examples) is also improved when using the pulp of the present invention. This is shown by the tensile properties of the wet and dry paper produced in Example 2 shown in Table 2 below.

Based on these results, it is clearly seen that the wet strength of papers containing pulps A and B according to the invention is significantly increased (25-30 times higher) in comparison to the comparative paper containing pulp C. Paper containing comparative pulp D (PVP pulp) also shows an improvement compared to pulp C, but less than observed for papers containing pulps A and B.

Example 4 : Comparison of (impregnated) friction papers Various properties of the friction paper of Example 2 were determined, such as filler (diatomaceous earth) retention, air permeability, tensile strength, and Z-axis strength. Filler retention and air permeability were determined on two paper sheets (designated as sheets 1 and 2). Mechanical properties (Z-axis strength and tensile strength) were determined on one of these sheets each (as the test would destroy the sheets). The results are shown in Table 3.

The data show that the filler retention of papers made with pulps A and B according to the invention is significantly higher than that of paper made from comparative pulp C. Clearly, the polyoxazoline modified pulp according to the invention has a higher capacity to retain filler and resin particles than the unmodified pulp. Papers containing pulps according to the invention also have higher filler retention than papers containing PVP modified pulp (pulp D).

In friction applications it is important that the friction paper is as open as possible, so that oil can penetrate the friction paper during use, for example in a clutch. The object of the present invention is to provide a paper, in particular a friction paper, which combines high strength with high porosity. The air permeability of a paper is a measure of its porosity.

The air permeability of papers containing either pulp A or B (PEOX modified pulp) is at a similar level to the comparative pulp C (unmodified aramid pulp), while pulp D (PVP modified pulp) shows a clear decrease in air permeability.

In friction applications, paper strength is also an important property. Due to the high shear forces that paper is subjected to during operation, shear strength is the most relevant strength property. Shear strength correlates well with the so-called Z-axis strength or internal bond strength.

The results in Table 3 show that the strength of the model friction papers based on pulps A and B according to the present invention is significantly improved compared to the strength of the friction paper based on the comparative pulp C.

The great advantage of the pulps of the present invention is the combination of high strength, especially Z-axis strength, with high porosity. Comparative papers containing either unmodified pulp (Pulp C) or PVP-modified pulp (Paper D) do not exhibit this combination of properties, only reaching comparable or even lower values for either strength or porosity, but not both properties.

Example 5 : Comparison of commercial pulp samples and polyoxazoline modified pulps and corresponding papers. Polyoxazoline modified pulps were produced on a production scale by adding PEOX solution to a suspension of partially fibrillated aramid short cuts. The suspension contained 2.5 wt% partially fibrillated aramid short cuts and 0.09 wt% (weight percent per volume of suspension) PEOX. The resulting suspension was circulated through a refiner to reach a target fiber length of 0.98 mm ± 0.2 mm. The molecular weight of PEOX was 500 kg/mol. The PEOX modified pulp (Pulp E) contained about 3.3 wt% PEOX (dry weight basis) and has an SSA of about 4.8 ^m2 /g.

As comparative samples, the commercially available Twaron® pulp 1092 (called 1092, a less fibrillated pulp type, with an SSA of about 6.6 m ² /g) and Twaron® pulp 1094 (called 1094, a more fibrillated pulp type, with an SSA of 12-15 m ² /g) are used. Generally, a more fibrillated pulp increases the strength of the paper but reduces the air permeability of the paper.

Paper was produced based on 1092, 1094, and pulp E as described in Example 2.

The wet strength of the paper was then determined.

The paper sheets thus produced, based on pulp samples 1092, 1094, and pulp E, were impregnated with phenolic resin as described for Example 2. The air permeability, Z-axis strength, filler retention, and tensile strength of the impregnated papers were determined as described for Example 4.

The properties (average values) of the base paper and impregnated paper are shown in Table 4.

The data in Table 4 show that the use of polyoxazoline modified pulps improves the mechanical properties of base and impregnated papers compared to commercial pulp types that do not contain polyoxazoline. Papers based on pulp type 1092 have high air permeability, while papers based on pulp type 1094 have high filler retention. Papers based on pulps according to the invention combine high filler retention with high air permeability. In addition, papers based on pulps according to the invention have the highest wet strength of the base papers and the highest Z-axis strength and tensile strength of the impregnated papers.

Claims (15)

Aramid pulp containing polyoxazoline. The aramid pulp according to claim 1, comprising 0.1 to 10% by weight of polyoxazoline, preferably 0.25 to 7.5% by weight of polyoxazoline, more preferably 0.5 to 5% by weight of polyoxazoline, based on the weight of the dry pulp. The aramid pulp according to claim 1 or 2, wherein the polyoxazoline is a poly-alkyl-2-oxazoline, preferably poly-2-ethyl-2-oxazoline. The aramid pulp according to any one of claims 1 to 3, wherein the polyoxazoline covers at least a portion of the surface of the aramid pulp. The aramid pulp according to any one of claims 1 to 4, comprising a fiber trunk and fibrils. Aramid pulp according to any one of claims 1 to 5, having a specific surface area in the range of 2 to 20 m ² /g, preferably 3 to 15 m ² /g, more preferably 4 to 10 m ² /g and/or a length LL _0.25 in the range of 0.5 to 1.5 mm, preferably 0.6 to 1.4 mm. Paper containing the aramid pulp according to any one of claims 1 to 6. The paper of claim 7, comprising 2-70% by weight of the aramid pulp, more preferably 5-55% by weight of the aramid pulp, and even more preferably 10-35% by weight of the aramid pulp, based on the weight of the paper. The paper according to claim 7 or 8, which contains a filler and a resin, and preferably contains 5 to 55% by weight of the filler, more preferably 20 to 40% by weight of the filler, and 5 to 50% by weight of the resin, more preferably 15 to 40% by weight of the resin, based on the weight of the paper. The paper according to any one of claims 7 to 9, wherein the resin is a thermosetting resin, preferably a thermosetting resin selected from phenolic resins, vitrimer resins, polythiourethane resins, melamine resins, silicone resins, and epoxy resins. The paper according to any one of claims 7 to 10, which is friction paper, separator paper, or honeycomb paper. A friction material comprising aramid pulp containing the polyoxazoline according to any one of claims 1 to 6 or the paper according to any one of claims 7 to 11. A method for producing an aramid pulp containing a polyoxazoline according to any one of claims 1 to 6, comprising the steps of:
- combining an aramid short cut, a partially fibrillated aramid short cut, or an aramid pulp with a polyoxazoline in an aqueous solution to form a mixture;
- subjecting said mixture to a beating step to form an aqueous slurry of said aramid pulp. The method of claim 13, further comprising subjecting the aqueous slurry of aramid pulp to a dewatering step to form the dewatered pulp having a moisture content in the range of 40 to 80% by weight, preferably 50 to 70% by weight, based on the weight of the dewatered pulp. The method of claim 14, further comprising subjecting the dewatered pulp to a drying step to form the dry pulp having a moisture content in the range of 2 to 20% by weight, preferably 3 to 10% by weight, based on the weight of the dry pulp, and optionally subsequently subjecting the dry pulp to an opening step.