Modified aramid pulp and friction material containing modified aramid pulp

Abstract

The present invention relates to an aramid pulp containing polyoxazoline. Also claimed is a method for producing an aramid pulp containing polyoxazoline, comprising: 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; and subjecting the mixture to a beating step to form an aqueous slurry of aramid pulp. Furthermore, the present invention also relates to a paper containing the aramid pulp containing polyoxazoline, and a friction material containing said paper and / or said aramid pulp containing polyoxazoline.

Description

[Technical field]

[0001] The present invention relates to an aramid pulp containing a polyoxazoline, a paper containing the pulp, a friction material containing the pulp or the paper, and a method for producing the aramid pulp containing a polyoxazoline.

[0002] Aramid pulp is known and is used in a variety of applications, such as paper, friction materials, especially friction paper, etc. Aramid is known in the art for its high strength and high temperature resistance.

[0003] 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 a support member for use in mechanical energy transmission applications. Although friction papers are manufactured by traditional papermaking methods, these materials are actually sophisticated composite structures that include pulp, fillers, binders (usually thermosetting resins), and optionally further components such as fibers and friction additives.

[0004] Friction papers are composite materials formulated to provide the appropriate friction, noise control, temperature resistance, and wear characteristics for each specific application.

[0005] Pulp wood is present to increase mechanical strength, adjust the porosity of the paper, and ensure the 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.

[0006] WO 2006 / 012040 describes acrylic and para-aramid pulps for use as reinforcement materials in products such as seals and friction materials.

[0007] 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.

[0008] Nevertheless, it has been found that the performance of aramid pulp in paper, especially in friction paper, still needs to be improved.In particular, there is a need for aramid pulp that provides papers and materials, especially friction paper, with high shear strength and high tensile strength combined with high porosity.In addition, high filler retention is desired to improve the homogeneous distribution of filler in paper to obtain homogeneous paper.

[0009] The present invention provides a solution to this problem.

[0010] The present invention relates to aramid pulp containing polyoxazoline (also referred to herein as "modified aramid pulp").

[0011] Modified continuous aramid yarns are described for use in ballistic fabrics and thermoplastic composites.

[0012] US 5,266,076 describes continuous aramid fibers coated with a finish, such as 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 different from pulp.

[0013] It has been found that aramid pulp modified with polyoxazoline combines high strength with high porosity when used in papers and friction materials, for example friction paper. When the porosity of paper increases, the mechanical properties (tensile strength, tear strength) often decrease. Therefore, friction paper that combines improved mechanical properties without adversely affecting the level of porosity, or increased porosity and improved or uniform mechanical properties, is of interest. Surprisingly, aramid pulp containing polyoxazoline provides such properties to paper, especially friction paper. In addition, the modification with polyoxazoline can improve the filler retention of the paper.

[0014] 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 the thinner fibrils have detached from the 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] Preferably, the aramid pulp comprises 0.1-10 wt. % polyoxazoline, preferably 0.25-7.5 wt. % polyoxazoline, more preferably 0.5-5 wt. % polyoxazoline (based on the weight of the dry pulp). In one embodiment, the aramid pulp comprises less than 6 wt. % polyoxazoline, preferably up to 4 wt. % polyoxazoline (based on the weight of the dry pulp). 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.

[0019] In the context of the present invention, polyoxazoline refers to a polymer based on an oxazoline moiety, sometimes referred to as an oxazoline polymer. Polyoxazoline polymers are different from non-polymerized oxazoline compounds, which may be used, for example, as hardeners for epoxy resins. Polyoxazoline is based on an oxazoline moiety. Oxazoline is a five-membered heterocycle (3×C, O, N) with three different structural isomers depending on the position of the double bond in the ring. 2-oxazoline is used to prepare polyoxazoline.

[0020] 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 hydrogen or C 1 ~C 4 Alkyl (where R is H or C 1 ~C 4alkyl), preferably an ethyl group (R is C 2 is substituted with: [ka]

[0021] 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.

[0022] 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 % of halogen groups, preferably less than 1 mol % of halogen groups, especially fluorine groups.

[0023] 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, 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 the polyoxazoline-modified pulp.

[0024] Aramid pulps containing polyoxazolines typically have a length (LL0.25) in the range of 0.5-1.5 mm, in particular in the range of 0.60-1.4 mm, and in some embodiments in the range of 0.7-1.3 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.

[0025] Aramid pulps containing polyoxazolines usually have a Schopper-Rigler (SR) in the range of 15-80° SR, especially in the range of 16-60° SR, more specifically 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.

[0026] 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 the field of 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.

[0027] Aramid pulp containing polyoxazoline is 2 to 20 m 2 / g, preferably 3 to 15m 2 / g, more preferably 4 to 10m 2 / g, or 5-8m 2 The specific surface area (SSA) may range from 1 / g.

[0028] Specific surface area (m 2 / 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, after which the specific surface area is measured.

[0029] Non-fibrillated fibres (e.g. continuous or short cut fibres) have a much smaller specific surface area, between 0.1 and 0.2 m 2 / g range.

[0030] Preferably, the polyoxazoline is present (only) on the surface of the pulp. Preferably, the polyoxazoline covers at least a part of the surface of the aramid pulp, or the entire surface of the aramid pulp. Preferably, the polyoxazoline is not used in the production 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 production of the modified pulp, as described below.

[0031] The present invention also relates to a paper comprising the aramid pulp containing the polyoxazoline in the above-mentioned embodiment.

[0032] The paper may be, for example, a friction paper, a separator paper, or a honeycomb paper.

[0033] In particular, the present invention relates to friction papers comprising aramid pulp containing the polyoxazoline in the above-mentioned embodiments.

[0034] Friction papers are typically composite materials containing many different materials, each of which contributes to the properties of the paper.

[0035] Reinforcing fibers are often present to increase the mechanical strength and durability of the system. These also serve to provide a porous structure that helps ensure proper absorption of the resin.

[0036] Fillers are added to perform a variety of functions, such as, for example, aiding in resin absorption, promoting oil flow through the paper to reduce temperature drop during use, ensuring adequate friction performance, and / or reducing noise.

[0037] The resin is present to ensure good dimensional stability, good friction performance, and good heat resistance.

[0038] 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, even more preferably 10 to 35% by weight of modified aramid pulp, based on the weight of the paper.

[0039] In one embodiment, the paper of the present invention comprises aramid pulp that includes polyoxazoline, filler, and resin.

[0040] Preferably, the paper of the invention comprises, 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.

[0041] In the context of this specification, the term filler is intended to include all particulate materials, preferably other than fibers or resins, that affect the friction performance of paper. Fillers suitable for friction paper 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 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.

[0042] It may be preferred that the (friction) paper contains diatomaceous earth and / or graphite particles.

[0043] 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 other components are too low. It may be preferred that the amount of filler is 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.

[0044] 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, in particular 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 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.

[0045] Preferably, resins are used that allow the removal or reprocessing of the resin so that the components of the paper can 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.

[0046] Suitable silicone resins are, for example, the organopolysiloxane resins described in EP-A-3 473 883.

[0047] The phenolic resin may optionally be modified, for example with silicone, melamine, epoxy, cresol, or cashew oil. The resin is present to improve the heat resistance, dimensional stability, and friction and wear performance of the paper.

[0048] The paper of the present invention may contain further ingredients.

[0049] 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 pulps. 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.

[0050] Preferably, in the resin-impregnated paper, the total amount of all reinforcing fibers and pulp, including the aramid pulp containing polyoxazoline (referred to as the fiber amount of the friction paper), accounts for 25-45% by weight, preferably 30-40% by weight, of the weight of the paper. 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 containing polyoxazoline may be present in the range of 20% by weight to 100% by weight, preferably 30% to 80% by weight, based on the weight of the fiber.

[0051] The paper of the present invention preferably has a density of 100 to 800 g / m 2 In particular, the range of 200 to 600 g / m 2 The composition has a basis weight in the range of 1000 to 2000 g.

[0052] The paper preferably comprises aramid pulp at least partially coated with polyoxazoline. Preferably, the paper comprising the polyoxazoline modified pulp and filler is not coated or covered with polyoxazoline either before or after incorporating the resin. Preferably, only the aramid pulp contained in the friction paper is at least partially coated with polyoxazoline, and other paper components are not coated or covered with polyoxazoline.

[0053] The use of aramid pulp containing polyoxazoline 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 friction paper. These properties make the friction paper particularly suitable for use in transmission systems.

[0054] The paper can be manufactured by methods known in the art.

[0055] 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.

[0056] 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.

[0057] The present invention also relates to friction or sealing materials comprising the described aramid pulp containing polyoxazoline and / or comprising the described paper.

[0058] 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.

[0059] 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 process to form an aqueous slurry of aramid pulp. The present invention also relates to a method comprising the steps of:

[0060] 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 an unmodified pulp is first obtained by fibrillation and then the pulp is coated by exposing it to a polyoxazoline solution.

[0061] As starting material for this process, an aramid short cut, a partially fibrillated aramid short cut, or an aramid pulp (or a combination thereof) can be used.

[0062] In this specification, the term aramid short cut refers to aramid fibers cut to a length of, for example, at least 0.5 mm, in particular 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, in particular 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, and most preferably in the range of 6 to 18 microns. Aramid fibers, in particular 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.

[0063] 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.

[0064] Partially fibrillated aramid short cut refers to aramid short cuts that have been partially fibrillated, for example, by cutting and grinding (for example in a knife mill) or by cutting and subjecting to a short refiner treatment.

[0065] In the first step of the method according to the invention, the aramid short cut, the partially fibrillated aramid short cut or the 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, the partially fibrillated short cut or the 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, the partially fibrillated aramid short cut or the pulp in water, or the polyoxazoline and the short cut, the partially fibrillated short cut or the pulp can be added together to an aqueous medium.

[0066] 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 being used in the process of the 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.

[0067] 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.

[0068] 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 the aqueous polyoxazoline solution to the container in which the fiber suspension is prepared. The resulting mixture can be properly mixed by stirring and then conveyed to the equipment for the beating step.

[0069] 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, in particular from 1 to 5% by weight, which has been found to be suitable for successful beating.

[0070] 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 is usually in the range of 0.1-10% 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 0.1-15% by weight, preferably between 0.5-12.5% ​​by weight, calculated based on the dry weight of the aramid short cut, partially fibrillated short cut, and / or pulp, in particular between 1-10% by weight or 2-5% by weight, calculated based on the dry weight of the aramid short cut, partially fibrillated short cut, and / or pulp.

[0071] The aqueous mixture is subjected to a beating step to form an aramid pulp containing 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" where the fibrils are connected and detached fibrils. Furthermore, the pulp stem may be 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 under 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 polyoxazoline.

[0072] The pulp slurry resulting from the beating process containing 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 comes out of the filter) or the cake can be broken down to form individual pieces, also called crumbs.

[0073] The dewatered pulp in the form of a 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.

[0074] Drying of the dewatered pulp can be carried out in a conventional manner, for example by contacting the pulp with a drying atmosphere, optionally at elevated temperature, resulting in the formation of a dry pulp. The dry pulp usually has a moisture content in the range of 2 to 20% by weight, in particular 3 to 10% by weight. Preferably, the moisture content of the dry pulp is in the range of 3 to 8% by weight.

[0075] 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 pulp 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 pulp opening process does not substantially change the properties of the pulp.

[0076] 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.

[0077] As will be apparent to those skilled in the art, the various preferred embodiments described above can be combined unless they are mutually exclusive.

[0078] The invention is further illustrated by the following non-limiting examples.

[0079] Working Example a) Determination of basis weight The basis weight of paper (also called basis weight) is measured in grams per square meter (g / m2) according to ISO 536:1995. 2 ) is expressed as

[0080] b) Determination of air permeability Air permeability is a measure of the porosity of the paper and is an indication of the oil permeability of the paper.

[0081] The air permeability of the impregnated papers was determined according to ASTM D737 using a Textest type FX3030-LDM. Air permeability is expressed in liters / m 2 / sec(L / m 2 It is expressed in units of Hz ( / s).

[0082] 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.

[0083] d) Determination of wet strength The paper sheets were immersed in isopropanol for 1 minute. The wet sheets were then tensile tested to determine the tensile index. The determination was made according to ISO 1924-2.

[0084] e) Filler retention rate 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 in 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.

[0085] f) Determination of tensile strength The tensile index of the dry paper before and after resin impregnation was determined according to ISO 1924-2.

[0086] 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 PEOX aqueous 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 dewatered cakes. The PEOX modified pulp designated Pulp A contains 3.4 wt. % PEOX and the PEOX modified pulp designated Pulp B contains 4.8 wt. % PEOX.

[0087] As a reference, the same procedure was followed without the addition of PEOX, resulting in an aramid pulp without any coating or covering, which was designated Pulp C.

[0088] As an additional comparison, the same procedure as for pulp B was followed, but with the addition of PVP (molecular weight approximately 50 kg / mol) instead of PEOX. This pulp is called pulp D.

[0089] Example 2 Manufacture of friction paper containing 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 between two sheets of blotter paper in a plate dryer at 105°C for at least 20 minutes. The resulting paper sheets consisted of 50% pulp and 50% diatomaceous earth and had a basis weight of 350±16 g / m 2 The goal was to become.

[0090] 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.

[0091] The wet and dry strength of the paper was determined.

[0092] 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 was 500±16 g / m 2The resin dilution is adjusted to reach the target sheet weight of the paper containing pulps C and D shown in Table 1.

[0093] In the final step, the paper sheets were cured in an oven at 180°C for 60 minutes.

[0094] [Table 1]

[0095] 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 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.

[0096] [Table 2]

[0097] 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.

[0098] Example 4 : Comparison of (impregnated) friction papers Various properties of the friction paper of Example 2 were determined, including 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 destroyed the sheets). The results are shown in Table 3.

[0099] [Table 3]

[0100] The data show that the filler retention of the papers made with pulps A and B according to the invention is significantly higher than that of the paper made from the comparative pulp C. Clearly, the polyoxazoline modified pulps according to the invention have a higher capacity to retain filler and resin particles than the unmodified pulp. Papers containing pulps according to the invention also have a higher filler retention than papers containing a PVP modified pulp (pulp D).

[0101] 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.

[0102] 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.

[0103] In friction applications, the strength of paper 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.

[0104] The results in Table 3 show that the strength of the model friction papers based on pulps A and B according to the invention is significantly improved compared to the strength of the friction paper based on the comparative pulp C.

[0105] A 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 show this combination of properties, only reaching comparable or even lower values ​​for either strength or porosity, but not both properties.

[0106] Example 5 : Comparison of commercial pulp samples and polyoxazoline modified pulps and the corresponding papers Polyoxazoline modified pulp was produced on a production scale by adding a 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 the PEOX was 500 kg / mol. The PEOX modified pulp (Pulp E) contained approximately 3.3 wt.% PEOX (on a dry weight basis) and had a length of approximately 4.8 m. 2 / g SSA.

[0107] As a comparative sample, commercially available Twaron® pulp 1092 (a type of pulp with low fibrillation, called 1092, approximately 6.6 m 2 / g) and Twaron® pulp 1094 (a type of highly fibrillated pulp called 1094, 12-15 m 2 / g) is used. Generally, a more fibrillated pulp increases the strength of the paper but reduces the air permeability of the paper.

[0108] Papers were made as described in Example 2 based on 1092, 1094, and Pulp E.

[0109] The wet strength of the paper was then determined.

[0110] 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 in Example 4.

[0111] The properties (average values) of the base paper and the impregnated paper are shown in Table 4.

[0112] [Table 4]

[0113] The data in Table 4 show that the use of polyoxazoline modified pulps improves the mechanical properties of base and saturated papers compared to commercial pulp types not containing polyoxazoline. Paper based on pulp type 1092 has high air permeability, while paper based on pulp type 1094 has high filler retention. Paper based on pulp according to the invention combines high filler retention with high air permeability. In addition, paper based on pulp according to the invention has the highest wet strength of the base paper and the highest Z-axis strength and tensile strength of the saturated paper.

Claims

1. An aramid pulp containing polyoxazoline.

2. The aramid pulp according to claim 1, containing 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.

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

4. The aramid pulp according to claim 1 or 2, wherein the polyoxazoline covers at least a part of the surface of the aramid pulp.

5. The aramid pulp according to claim 1 or 2, containing fiber trunks and fibrils.

6. 2 to 20 m ２ / g, preferably 3 to 15 m ２ / g, more preferably 4 to 10 m ２ Specific surface area in the range of / g and / or length LL in the range of 0.5 to 1.5 mm, preferably 0.6 to 1.4 mm ０．２５ The aramid pulp according to claim 1 or 2, having the above characteristics.

7. Paper containing the aramid pulp according to claim 1.

8. The paper according to claim 7, containing 2 to 70% by weight of the aramid pulp, more preferably 5 to 55% by weight of the aramid pulp, even more preferably 10 to 35% by weight of the aramid pulp, based on the weight of the paper.

9. The paper according to claim 7 or 8, containing a filler and a resin, preferably 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.

10. The paper according to claim 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.

11. The paper according to claim 7 or 8, which is friction paper, separator paper, or honeycomb paper.

12. A friction material containing the aramid pulp containing polyoxazoline according to claim 1 or the paper according to claim 7.

13. A method for producing an aramid pulp containing polyoxazoline according to claim 1 or 2, comprising: - Mixing an aramid shortcut, a partially fibrillated aramid shortcut, or an aramid pulp with polyoxazoline in an aqueous solution to form a mixture; - Performing a beating process on the mixture to form an aqueous slurry of the aramid pulp. The method as described above.

14. The method according to claim 13, wherein a dehydration step is performed on the aqueous slurry of the aramid pulp to form the dehydrated pulp having a water content in the range of 40 to 80% by weight, preferably 50 to 70% by weight, based on the weight of the dehydrated pulp.

15. The method according to claim 14, wherein a drying step is performed on the dehydrated pulp to form the dried pulp having a water content in the range of 2 to 20% by weight, preferably 3 to 10% by weight, based on the weight of the dried pulp, and optionally, subsequently, a fibrillation step is performed on the dried pulp.