HK1097010B - Aramid fibrils - Google Patents

Description

Aramid fibrils

The present invention relates to aramid fibrils, a method for producing the fibrils, and paper made therefrom.

Pulp is defined as a highly fibrillated fibrous dry (fiber stem). The fibrillated parts, called fibrils, are highly entangled and have a high aspect ratio (>100) and a large surface area (8-10 m/g), which is about 40 times that of standard fibrils. Thus, aramid pulp is a fibrillated particle used to make paper, gaskets, brake linings, and the like. Pulp can generally be made from virgin fiber by subjecting it to cutting and fibrillation steps. However, it is advantageous to produce pulp directly without first spinning the polymer into fibers. Such direct pulping processes have been disclosed in the prior art, for example, U.S. Pat. No. 5,028,372. According to this process, an aramid pulp is produced as follows: forming a solution of para-aramid polymer, extruding said solution having an intrinsic viscosity of 1 to 4 onto a conveyor, incubating the solution on the conveyor until it forms a gel, and cutting the gel and separating the pulp therefrom. The polymer has a solution concentration of 6 to 13% by weight, and the pulp thus obtained has a specific surface area of more than 2 m/g.

It is contemplated that highly fibrillated pulp may be advantageous for certain applications. More advantageously, the polymeric material is completely (or substantially completely) in fibrillar form, i.e. no longer contains a significant amount of fibrous material. In other words, there is a need for a "pulp" that contains primarily fibrillated portions and no longer fiber dry. Such materials have not been known to date. These materials can be expected to have very useful properties such as high flexibility, high adhesion capacity in paper and good porosity of paper made therefrom. Furthermore, it can be expected that such materials have considerable hardness after drying and are therefore suitable for use in composite materials. For the purposes of the present invention, the material is defined as "fibrils".

It is well known in the art that the higher the Specific Surface Area (SSA) in a pulp, the lower the Canadian Standard Freeness (CSF). Thus, in the standard reference works by Yang, 1993, Wiley & Son, ISBN0471937657, p.156, it is explained that CSF decreases when SSA increases. It is an object of the present invention to provide materials which have many of the properties of pulp but which have a low SSA and at the same time a low CSF. Such materials can be expected to have unique properties for many applications including papermaking. Such materials are not known in the art.

Fibers having low degree of fibrillation and low SSA are known in the art. Fine denier pulp-like fibers have been disclosed in EP 381206. These fibers have been made by standard methods using high dope concentrations and using sulfuric acid as the solvent. These fibers have low SSA, but high CSF (i.e. values above 600 ml).

In EP 348996 and US 5,028,372, pulp has been made by a process in which partial polymerization is carried out after extrusion and orientation of the spinning dope. Pulp has a low SSA (e.g. 5.2 to 7.1 m/g) and therefore a high CSF, i.e. >450 ml according to Yang, p.156.

A first object of the present invention is therefore to provide an aramid polymer solution as a spinning dope, preferably exhibiting optical anisotropy, thereby obtaining a spinning dope that can be directly spun to produce fibrils without the application of high pressure and/or high spinning temperature. This object is achieved in that aramid fibrils of a predetermined length (as defined in the present invention) can be produced in one step. These fibrils are not only bent, but further contain kinks, where in each kink the direction of the fibril changes sharply to form an angle.

Accordingly, another object of the present inventionIt is to provide fibrils which loose most of their fluff shape when dry but remain multicoiled when wet. The fibrils of the present invention are aramid fibrils having a Canadian Standard Freeness (CSF) value of less than 300 ml in the wet state and having a Specific Surface Area (SSA) of less than 7 m/g after drying. Fibrils of the present invention versus length>The 250 micron particles have a weight Weighted Length (WL) of less than 1.2 mm, more preferably less than 1.0 mm_0.25). These fibrils are characterized by a lower SSA, a higher CSF.

The fibrils of the present invention, which are not redispersible after drying, result in paper with very high paper strength and very stiff material after drying.

Preferred fibrils of the present invention have a CSF value of less than 150 ml and an SSA of less than 1.5 m/g in the wet state.

The fibrils may be made from a solution of meta-and/or para-aramid polymer, such as poly (p-phenylene terephthalamide), poly (m-phenylene isophthalamide), copoly (p-phenylene/3, 4' -dioxodiphenylene terephthalamide), and the like, some of which may be available under the trade name p-phenylene terephthalamideAndcommercially available for fibers and pulp. The preferred aramid is para-aramid, more preferably poly (p-phenylene terephthalamide).

The para-oriented aramid is a condensation polymer of a para-oriented aromatic diamine and a para-oriented aromatic dicarboxylic acid halide (hereinafter referred to as "para-aramid") and has hitherto been known to be useful in various fields such as fibers, pulp, etc. due to its high strength, high elastic modulus and high heat resistance.

As typical para-aramid, there may be mentioned aramid whose structure has a poly-para-oriented form or a form close thereto, such as poly (p-phenylene terephthalamide), poly (4, 4 '-N-benzanilide terephthalamide), poly (p-phenylene-4, 4' -biphenylenedicarboxamide) and poly (p-phenylene-2, 6-naphthalenedicarboxamide). Of these para-aramids, poly (p-phenylene terephthalamide) (hereinafter abbreviated to PPTA) is most representative.

To date, PPTA has been made in a polar amide solvent/salt system in the following manner. For example, PPTA is made by solution polymerization in a polar amide solvent. The PPTA precipitated, washed with water and dried and isolated once as a polymer. The polymer is then dissolved in a solvent and made into PPTA fibers by wet spinning. In this step, concentrated sulfuric acid is used as a solvent for the spinning dope, because PPTA is not readily soluble in organic solvents. The dope usually exhibits optical anisotropy.

Industrially, PPTA fibers are produced from a spinning dope using concentrated sulfuric acid as a solvent, in consideration of properties as long fibers, particularly strength and rigidity.

According to the closest prior art EP 381206, a process for producing fine fibers from lyotropic liquid crystal dope is disclosed. The method comprises 1) extruding a stream of an optically anisotropic solution of a polymer into a chamber, 2) introducing a compressed gas into the chamber, 3) directing the gas in the direction of flow of the solution stream within the chamber and around and in contact with the solution stream, 4) passing both the gas and solution streams through orifices into a zone of lower pressure at a velocity sufficient to dilute and divide the solution stream into fibers, and 5) contacting the segmented solution stream within the zone with a stream of coagulated liquid droplets. The use of the proposed method prevents the formation of fine fibers and facilitates the formation of fibrils.

In order to rationalize the aforementioned processes, various other processes have been proposed to make pulp from liquid polymer dope without separating the polymerization and spinning steps from each other, including the aforementioned US 5,028,372, but none of these produce (non-fibrous) fibrils.

Another object of the present invention is to overcome the drawbacks of the conventional pulping processes and to obtain fibres with high relative viscosity by providing a process for producing stable polymer solutions and products with uniform quality according to an industrially advantageous and simplified process. In order to obtain a material with a high relative viscosity in one step, a polymer solution with a low dynamic viscosity is required to easily form fibrils.

These and other objects have been achieved by a method of making a polymer solution, comprising the steps of:

a. polymerizing an aromatic diamine and an aromatic dicarboxylic acid halide in a mixture of N-methylpyrrolidone or dimethylacetamide and calcium chloride or lithium chloride to form an aramid polymer, thereby obtaining a dope in which the polymer is dissolved in the mixture and the polymer concentration is 2 to 6% by weight,

b. converting the dope into fibrils using a jet spinneret under a gas stream, and

c. the fibrils are coagulated using a coagulation jet.

In a preferred embodiment, the polymerization step is carried out by at least partially neutralizing the hydrochloric acid formed. The process can obtain aramid polymers having η rel (relative viscosity) between 2.0 and 5.0.

According to a preferred embodiment of the present invention, para-aramid has been made in NMP/CaCl₂Non-fibrous polymer solution in a mixture of NMP/LiCl or DMAc/LiCl, wherein the polymer solution has η_rel>2.2 relative viscosity.

The dope is converted into fibrils of the present invention using an air stream. Suitable gases are, for example, air, oxygen, nitrogen, noble gases, carbon dioxide, and the like.

The aramid polymer solutions of the present invention are at temperatures up to about 60 deg.C and 100-000 seconds^-1Has a low shear rate rangeDynamic viscosity. Thus, the polymer solution of the invention can be spun at a temperature below 60 ℃, preferably at room temperature. Further, the aramid spinning dope of the present invention does not contain an additional component such as pyridine and can be advantageously produced from an industrial point of view because the production process can be simplified and the process is free from the problem of corrosion of equipment by concentrated sulfuric acid as compared with the existing spinning dope using concentrated sulfuric acid as a solvent.

Further, according to the method of the present invention, the polymer solution can be directly spun and the product can be made into fibrils, and therefore, the manufacturing method is greatly simplified as compared with the existing manufacturing method of aramid pulp, which is generally carried out by first manufacturing a yarn.

An aramid having a long breaking length can be produced from the aramid fibrils of the present invention. When used as a raw material for friction materials (including paper for automatic transmissions and the like), the performance is good. Fibrils are produced directly by spinning a polymer solution, and therefore do not need to be made into fibers.

The invention therefore also relates to fibrils having a CSF (canadian standard freeness) of less than 300, preferably less than 150, from never-dried fibrils. More preferably, the para-aramid fibrils have a relative viscosity (η) higher than 2.2_rel)。

In another embodiment, the invention also relates to aramid paper obtainable from the fibrils of the present invention. Such paper comprises at least 2 wt.%, preferably at least 5 wt.%, most preferably at least 10 wt.% of aramid fibrils.

The present invention will be explained in more detail below.

The stabilized dope has a para-aramid concentration of 2-6 wt% and a medium to high degree of polymerization to achieve high relative viscosity (. eta.)_relAbout 2.0 to about 5.0). Depending on the polymer concentration, the dope exhibits anisotropic (polymer concentration 2 to 6 wt%) or isotropic properties. Preferably, at 1000s^-1Dynamic viscosity at shear rate of_dynLess than 10pa.s, more preferably less than 5 pa.s. Neutralization is carried out during or preferably after polymerization of the monomers to form the aramid. No neutralizing agent was present in the monomer solution before the start of polymerization. Neutralization reduces the dynamic viscosity by at least two thirds. The neutralized polymer solution can be used for direct fibril spinning using a nozzle to contact the polymer stream with compressed air in a zone of lower pressure where the air expands to divide the polymer stream into droplets. The droplets are attenuated into fibrils. Using a suitable coagulant (e.g. water or water/NMP/CaCl)₂Mixture) to effect coagulation of the fibrils. Instead of CaCl, it is also possible to use other chlorides such as LiCl₂. By adjusting the polymer stream/air flow ratio, the fibril length and CSF can be varied. At high ratios long fibrils are obtained, while at low ratios short fibrils are obtained. The Specific Surface Area (SSA) of the fibrils decreases with decreasing Canadian Standard Freeness (CSF).

The fibrils of the present invention can be used as raw materials for para-aramid paper, friction materials (including automobile brakes), various gaskets, E-paper (e.g., for electronic applications because it contains a very low amount of ions compared to para-aramid pulp made from a sulfuric acid solution), and the like.

Examples of para-oriented aromatic diamines useful in the present invention include p-phenylenediamine, 4 '-diaminobiphenyl, 2-methyl-p-phenylenediamine, 2-chloro-p-phenylenediamine, 2, 6-naphthalenediamine, 1, 5-naphthalenediamine, and 4, 4' -diaminobenzanilide.

Examples of para-oriented aromatic dicarboxylic acid halides useful in the present invention include terephthaloyl chloride, 4' -benzoyl chloride, 2-chloroterephthaloyl chloride, 2, 5-dichloroterephthaloyl chloride, 2-methyl terephthaloyl chloride, 2, 6-naphthalenedicarboxylic acid chloride and 1, 5-naphthalenedicarboxylic acid chloride.

In the present invention, 0.950 to 1.050 moles, preferably 0.980 to 1.030 moles, more preferably 0.995 to 1.010 moles of the para-oriented aromatic diamine is used per 1 mole of the para-oriented aromatic carboxylic acid halide in the polar amide solvent in which 0.5 to 4% by weight of an alkali metal chloride or an alkaline earth metal chloride is dissolved, preferably 1 to 3% by weight, so that the concentration of the para-oriented aromatic polyamide produced therefrom is 2 to 6% by weight, preferably 2 to 4% by weight, more preferably 2.5 to 3.5% by weight. In the present invention, the polymerization temperature of the para-aramid is-20 ℃ to 70 ℃, preferably 0 ℃ to 30 ℃, more preferably 5 ℃ to 25 ℃. In this temperature range, the dynamic viscosity is in a desired range, and fibrils produced therefrom by spinning may have a sufficient degree of crystallinity and a sufficient degree of crystal orientation.

One of the main features of the present invention is that the polymerization reaction can be first enhanced and thereafter terminated as follows: the polymer solution or the solution forming the polymer is neutralized by adding an inorganic or strong organic base, preferably calcium oxide or lithium oxide. In this regard, the terms "calcium oxide" and "lithium oxide" include calcium hydroxide and lithium hydroxide, respectively. This neutralization removes hydrogen chloride formed during the polymerization reaction. Neutralization results in a reduction in dynamic viscosity by at least two-thirds (relative to the unneutralized corresponding solution). The chloride is preferably present in an amount of 0.5 to 2.5 moles, more preferably 0.7 to 1.4 moles, per mole of amide group formed in the polycondensation reaction after neutralization. The total amount of chloride may be derived from CaCl used in the solvent₂And CaO used as a neutralizing agent (alkali). If the calcium chloride content is too high or too low, the dynamic viscosity of the solution is excessively increased and is not suitable as a spinning dope.

The liquid para-aramid polymerization solution may be supplied to a spinning pump via a pressure vessel to be fed to a nozzle of 100-1000 μm for air-jet spinning into filaments. The liquid para-aramid solution is spun through a spinneret into a zone of lower pressure. For air-jet spinning, air of more than 1 bar, preferably 4-6 bar, is applied separately to the same zone through an annular channel, where an air expansion takes place. Under the influence of the expanding air stream, the liquid spinning dope is divided into droplets and simultaneously or subsequently oriented by drawing. The fibrils are then coagulated in the same zone by applying a jet of coagulant and the formed fibrils are collected on a filter and washed. The coagulant is selected from water, water/NMP/CaCl₂A mixture, and any other suitable coagulating agent.

The invention will now be illustrated by the following non-limiting examples.

The test and evaluation methods and judgment standards used in the examples and comparative examples are as follows.

Test method

Relative viscosity

The sample was dissolved in sulfuric acid (96%) at room temperature at a concentration of 0.25% (m/v). The flow time of the sample solution in sulfuric acid was measured in an Ubbelohde viscometer at 25 ℃. Under the same conditions, the flow time of the solvent was measured. The viscosity ratio was then calculated as the ratio between the two observed flow times.

Dynamic viscosity

Dynamic viscosity was measured at room temperature using capillary rheometry. The actual wall shear rate and viscosity were calculated using the power law coefficients and the Rabinowitsch correction.

Fiber length measurement

Using Pulp Expert^TMFS (ex Metso) fiber length measurements were performed. As the length, an Average Length (AL), a length-weighted length (LL), and a weight-Weighted Length (WL) are used. Subscript 0.25 refers to length>Corresponding value for 250 micron particles. The grain size refers to the length-weighted length (LL)<Fraction of particles of 250 μm.

The instrument needs to be calibrated with samples having known fiber lengths. Calibration was performed with the commercially available pulp shown in table 1.

TABLE 1

A 1F539, type 979

B 1095，Charge315200，24-01-2003

C 1099，Ser.No.323518592，Art.No.108692

CSF

During 1000 whipping cycles in a Lorentz and Wettre disintegrator, 3 grams (dry weight) of never-dried fibrils were dispersed in 1 liter of water. A fully opened sample was obtained. Canadian Standard Freeness (CSF) values were measured and the fines weight differences (Tappi227) were corrected.

Specific Surface Area (SSA) determination

The specific surface area (square meter/gram) was measured by the BET surface area method using nitrogen adsorption using Gemini 2375 manufactured by Micromeretics. The wet crude fiber samples were dried at 120 ℃ overnight and then purged with nitrogen at 200 ℃ for at least 1 hour.

Optical anisotropy evaluation (liquid Crystal State)

The optical anisotropy was evaluated under a polarizing microscope (sharp image) and/or observed as opalescence during stirring.

Strength of paper

From 100% fibrillar material or 50% fibrils and 50%6 mm fiber (C)1000) Making handsheets. Tensile index: (tensile index) was measured on dry paper (120 ℃ C.) according to ASTM D828 and Tappi T494 om-96Nm/g) wherein the width of the sample is 15 mm, the length of the sample is 100 mm, and the test speed is 10 mm/min under the conditions of 21 ℃/65% relative humidity.

Example 1

Polymerization of p-phenylene terephthalamide was carried out using a 2.5 cubic meter Drais reactor. After the reactor had been sufficiently dried, 1140 liters of CaCl having 2.5 wt.% were charged into the reactor₂Concentration of NMP/CaCl₂(N-methylpyrrolidone/calcium chloride). Then, 27.50 kg of p-phenylenediamine (PPD) was added at room temperature and dissolved. Thereafter, the PPD solution was cooled to 10 ℃ and 51.10 kg of Terephthaloyl Dichloride (TDC) were added. After addition of TDC, the polymerization reaction was continued for 45 minutes. The polymer solution was then neutralized with a calcium oxide/NMP slurry (14.10 kg CaO in 28L NMP). After the addition of the CaO slurry, the polymer solution was stirred for at least 15 minutes. This neutralization is carried out to remove hydrogen chloride (HCl) formed during the polymerization. A PPTA content of 4.5% by weight and a relative viscosity of 2.8 (at 0.25% H) was obtained₂SO₄In (b) is added. The resulting solution exhibits optical anisotropy and is stable for more than one month. The solution was diluted with NMP until a polymer concentration of 3.0% was obtained.

The spinning pump was supplied with 3% solution (120 l/h) to feed into a spinning head with 20 holes of 350 microns. The spinning temperature is ambient temperature. The PPTA was spun through a nozzle into a region of lower pressure. The same zone was separately supplied with 6 bar (160 Nm) perpendicular to the polymer flow through the annular channel³H) (normal cubic/hour) air jet, where an air expansion is generated. Thereafter, a coagulant jet (600 liters/hour) was applied through the annular channel at an angle to the direction of polymer flow, thereby coagulating the fibrils in the same zone (H)₂O/30％NMP/1.3％CaCl₂) And the fibrils formed are collected on a filter and washed.

The spun fibrils had a CSF value fibril character of 83 ml, whereas they had an SSA of only 0.63 m/g. When viewed under a microscope, very fine structures are seen, whichLow CSF values were confirmed. WL_0.25Is 0.76 mm.

Example 2

Polymerization of p-phenylene terephthalamide was carried out using a 160 liter Drais reactor. After the reactor had been sufficiently dried, 64 liters of CaCl having 2.5% by weight were charged into the reactor₂Concentration of NMP/CaCl₂(N-methylpyrrolidone/calcium chloride). Then 1487 g of p-phenylenediamine (PPD) were added and dissolved at room temperature. Thereafter the PPD solution was cooled to 10 ℃ and 2772 g of TDC were added. After addition of TDC, the polymerization reaction was continued for 45 minutes. The polymer solution was then neutralized with a calcium oxide/NMP slurry (776 grams CaO in NMP). After the addition of the CaO slurry, the polymer solution was stirred for at least 15 minutes. This neutralization is carried out to remove hydrogen chloride (HCl) formed during the polymerization. A PPTA content of 4.5% by weight and a relative viscosity of 2.7 (at 0.25% H) was obtained₂SO₄In (b) is added. The resulting solution exhibits optical anisotropy and is stable for more than one month. The solution was diluted with NMP until a polymer concentration of 3.6% was obtained.

The spinning pump was supplied with 3.6% PPTA solution (16 kg/h) to feed into a spinning head containing 4 holes of 350 microns. The spinning temperature is ambient temperature. PPTA is spun through a nozzle into a zone of lower pressure. An air jet of 7 bar (45Nm3/h) was applied separately to the same zone perpendicular to the polymer flow through the annular channel, where an air expansion was produced. Thereafter, water jets (225 liters/hour) were applied through the annular channels at an angle to the direction of the polymer flow, thereby causing the fibrils to solidify in the same zone, and the formed fibrils were collected on a filter and washed.

The collected fibrils exhibited higher SSA values, but as CSF values decreased, SSA decreased as well (see table 2).

TABLE 2

Example 3

Paper was made from never-dried fibrils of example 1. 50 percent ofThe paper strength of 10006 mm fibres and 50% fibrils was 23 Nm/g.

Example 4

Paper was made from never-dried fibrils of example 2. 50 percent ofThe paper strength of 10006 mm fibres and 50% fibrils was 18 Nm/g. The paper strength of the paper constituted by 100% of fibrils was 10.8 Nm/g.

Claims (10)

1. Aramid fibrils having a canadian standard freeness value of less than 300 ml in the wet state, a specific surface area of less than 7 m/g after drying, and particles having a length >250 microns having a weight-weighted length of less than 1.2 mm.

2. The fibrils of claim 1 wherein the CSF value in the wet state is less than 150 ml and the SSA after drying is less than 1.5 m/g.

3. The fibrils of any of claims 1-2, wherein the aramid is para-aramid.

4. The fibril of claim 3 wherein said aramid is poly (p-phenylene terephthalamide).

5. A process for preparing the fibrils of claims 1-4 comprising the steps of:

a. polymerizing an aromatic diamine and an aromatic dicarboxylic acid halide in a mixture of N-methylpyrrolidone or dimethylacetamide and calcium chloride or lithium chloride to form an aramid polymer, thereby obtaining a dope in which the polymer is dissolved in the mixture and the polymer concentration is 2 to 6% by weight,

b. converting the polymer stream of the dope into fibrils by separately applying air jets through an annular passage perpendicular to the polymer stream of the dope to a region of lower pressure into which the polymer stream is spun, and

c. the fibrils are coagulated using a coagulation jet.

6. The process according to claim 5, wherein at least a portion of the hydrochloric acid produced is neutralized to obtain a neutralized spin mass wherein the chloride is present in an amount of 0.5 to 2.5 moles.

7. The process according to claim 6, wherein the relative viscosity of the aramid polymer is between 2.0 and 5.0.

8. A paper made from a composition comprising at least 2% by weight of aramid fibrils according to any of claims 1-4.

9. Paper according to claim 8, made from a composition comprising at least 5% by weight of aramid fibrils according to any of claims 1-4.

10. Paper according to claim 8, made from a composition comprising at least 10% by weight of aramid fibrils according to any of claims 1-4.