MODIFIED ARAMID PULP AND FRICTION MATERIAL COMPRISING MODIFIED ARAMID PULP

Abstract

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

Claims

1. An aramid pulp comprising polyoxazoline.

2. The aramid pulp according to claim 1 comprising 0.1 to 10 wt % of polyoxazoline, based on the weight of the dried pulp.

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

4. The aramid pulp comprising polyoxazoline according to claim 1 wherein the polyoxazoline covers at least part of the surface of the aramid pulp.

5. The aramid pulp according to claim 1 comprising fiber stems and fibrils.

6. The aramid pulp according to claim 1 having a specific surface area in the range of 2 to 20 m.sup.2/g.

7. A paper comprising the aramid pulp according to claim 1.

8. The paper of claim 7 comprising 2-70 wt % of the aramid pulp, based on the weight of the paper.

9. The paper of claim 7 comprising a filler and a resin, based on the weight of the paper.

10. The paper of a claim 7 wherein the resin is a thermoset resin.

11. The paper of claim 7 being a friction paper, a separator paper or a honeycomb paper.

12. A friction material comprising the aramid pulp comprising polyoxazoline according to a claim 1.

13. A method for manufacturing the aramid pulp comprising polyoxazoline according to claim 1 comprising: combining aramid short-cut, partly fibrillated aramid short-cut or aramid pulp with polyoxazoline in an aqueous solution to form a mixture, subjecting the mixture to a refining step to form an aqueous slurry of the aramid pulp.

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

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

Description

EXAMPLES

a) Determination of Grammage

[0085] The grammage of the papers (also referred to as the areal weight) was measured according to ISO 536:1995 and expressed in terms of grams per square meter (g/m.sup.2).

b) Determination of Air Permeability

[0086] The air permeability is a measure for the porosity and an indication for the oil penetrability of the paper.

[0087] The air permeability of the impregnated paper was determined according to ASTM D737 using a Textest type FX3030-LDM and is expressed with a unit of liter/m.sup.2/second (L/m.sup.2/s).

c) Determination of Z-Strength

[0088] The Z-strength (also referred to as internal bonding strength) of the paper correlates well with the shear strength. The Z-strength of the impregnated sheets was determined in accordance with Tappi T541.

d) Determination of Wet Strength

[0089] Paper sheets were immersed in isopropanol for 1 minute. Afterwards, the wet sheets were subjected to a tensile test to determine the tensile index. The determination was done in accordance with ISO 1924-2.

e) Filler Retention

[0090] The filler retention is a measure of the extent to which pulp retains a filler during paper making, where a value of 100% means complete retention of the filler, i.e. no loss of filler during the paper making process. The filler retention of the papers was determined using diatomaceous earth as filler. The filler retention is determined by dividing the amount of filler in the final sheet (calculated based on the actual grammage, the sheet surface area [having a ? of 20 cm] and subtracting the amount of pulp in the sheet [5.5 g]) by the amount of filler used (corrected for the moisture content) and multiplying by 100.

f) Determination of Tensile Strength

[0091] The tensile index of the dried papers before impregnation and the papers after resin impregnation was determined in accordance with ISO 1924-2.

Example 1: Pulp Manufacture

[0092] 4 kg of para-aramid chopped fibers of 6 mm in length (6 mm short-cut based on Twaron? type 1000 1680f1000) was added to 200 liter of an aqueous solution of PEOX. The PEOX had a molecular weight of approximately 500 kg/mol. The resulting suspension contained 2 wt. % of aramid short-cut and 0.07 wt % (Pulp A) or 0.1 wt. % (Pulp B) of PEOX, depending on the amount of PEOX added to the suspension (weight percentage per volume of suspension). The resulting suspension was passed through a Sprout-Bauer 12 lab refiner to reach the target fiber length of 0.95 mm?0.1 mm. The refined suspension was dewatered on a sieve table to yield a dewatered cake. The PEOX-modified pulp denoted as Pulp A contains 3.4 wt % PEOX, the PEOX-modified pulp denoted as Pulp B contains 4.8 wt % of PEOX.

[0093] As reference, the same procedure was followed without the addition of PEOX, resulting in an aramid pulp free of any coating or covering. This pulp is referred to as Pulp C.

[0094] As additional comparison, the same procedure as for Pulp B was followed with the addition of PVP (having a molecular weight of approximately 50 kg/mol) instead of PEOX. This pulp is referred to as Pulp D.

Example 2: Manufacture of a Friction Paper Comprising Filler, Resin and Aramid Pulp

[0095] 24.48 g of PEOX-containing Pulp A of Example 1 with a dry solids content of 22.45%, thus equaling 5.50 g of dry aramid pulp, was suspended in 2 L of water and mixed for 100 counts (20 s at 3000 rpm) in a Lorentzen & Wettre disintegrator. Then, 6.0 g of diatomaceous earth (Transcend ND-1, as filler) was added to the suspension and mixed for an additional 500 counts (100s at 3000 rpm). This mixture was used for paper sheet preparation on a Rapid K?then lab sheet former in accordance with ISO 5269-2. The resulting paper sheets were dried between two blotting papers in a plate drier at 105? C. for at least 20 minutes. The resulting paper sheets were targeted to have a grammage of 350?16 g/m.sup.2, consisting of 50% pulp and 50% diatomaceous earth.

[0096] The same procedure was followed for Pulp B, C and D, with the exception that slightly different amounts of pulp and filler were used to obtain the same final target sheet weights (see Table 1 for amounts used). The amount of pulp was adjusted to correct for the water content of the different pulp samples such that the same amount of dry pulp (dry solids content) was used.

[0097] The wet strength and the dry strength of the papers was determined.

[0098] The paper sheets prepared in this way and based on pulp samples A, B, C and D were also impregnated with phenolic resin (Bakelite PF 0229 RP). For the paper comprising pulps A and B (according to the invention) the resin was diluted to the desired concentration using a mixture of 22 mL of the resin and 78 mL of isopropanol. The paper sheet was placed in a tray covered with a plastic liner and the resin mixture was poured over the sheet. The tray was moved around for 1 minute, after which the paper sheet was transferred onto a Teflon sheet. Excess resin was removed by passing the paper sheet through a custom made wringer twice (flipping the paper in between the passages). The residual solvent (isopropanol) was then evaporated in a ventilated oven at 90? C. for 20 minutes. After impregnation, the targeted sheet weight is 500?16 g/m.sup.2. The resin dilution is adjusted accordingly to reach the desired sheet weight for the papers comprising pulp C and D as indicated in Table 1.

[0099] In a final step, the paper sheet was cured in an oven at 180? C. for 60 minutes.

TABLE-US-00001 TABLE 1 Materials used for preparation of impregnated paper sheets based on pulp samples A-D Dry solids Modifica- content of the Resin/IPA tion of dewatered pulp Pulp Diatomaceous solution pulp with [%] [g] earth [g] [mL] Pulp A 3.4 wt % 22.45 22.48 6.0 22/78 PEOX Pulp B 4.8 wt % 22.89 24.01 6.0 22/78 PEOX Pulp C none 17.48 31.44 7.2-7.6 23/77 Pulp D 4.8 wt % 25.17 21.83 6.4 24/76 PVP

Example 3: Dry and Wet Strength of the Base Paper (Before Impregnation)

[0100] The pulp of the current invention is highly beneficial to improve the dry strength of the paper. In, addition, the wet strength, required during resin impregnation of the base paper (in these examples 50/50 pulp/diatomaceous earth by weight), is also improved when the pulp of this invention is used. This is illustrated by the tensile properties of the wet and dry paper prepared in example 2 given in Table 2 below.

TABLE-US-00002 TABLE 2 Dry and wet strength of base (not impregnated) papers comprising pulps A-D Sheet comprising pulp with Dry strength Wet strength based on a modification of [Nm/g] [Nm/g] Pulp A 3.4 wt % PEOX 1.05 ? 0.07 0.356 ? 0.019 Pulp B 4.8 wt % PEOX 1.43 ? 0.07 0.430 ? 0.013 Pulp C none 0.20 ? 0.03 0.014 ? 0.004 Pulp D 4.8 wt % PVP 1.22 ? 0.02 0.229 ? 0.016

[0101] Based on these results, it can clearly be seen that the wet strength of papers comprising Pulp A and B from the current invention results in a strong increase in wet strength compared to a comparative paper comprising Pulp C (25 to 30 times higher). The paper comprising comparative Pulp D (PVP pulp) also shows an improvement compared to Pulp C, but less than observed for papers comprising Pulp A and B.

Example 4: Comparison of (Impregnated) Friction Papers

[0102] Various properties of the friction papers of Example 2 were determined, including the filler (diatomaceous earth) retention, the air permeability, the tensile strength and the Z-strength. The filler retention and the air permeability were determined on two paper sheets (indicated as sheet 1 and 2). The mechanical properties (Z-strength and Tensile strength) were each determined on one of those sheets (because the testing destroys the sheet). The results thereof are shown in Table 3.

TABLE-US-00003 TABLE 3 Properties of impregnated friction papers comprising pulps A-D comprising Air Z- Filler Sheet pulp with a permeability strength retention Tensile based modification Sheet 1 / 2 (F.sub.max) Sheet 1 / 2 strength on of [L/m.sup.2/s] [kPa] [%] [Nm/g] Pulp A 3.4 wt % 26.7 / 34.4 329 ? 10 94 / 95 11.6 ? 0.4 PEOX Pulp B 4.8 wt % 27.2 / 24.2 292 ? 9 94 / 94 12.4 ? 0.5 PEOX Pulp C none 28.6 / 33.6 135 ? 15 70 / 70 7.0 ? 0.3 Pulp D 4.8 wt % 16.4 / 15.0 315 ? 6 85 / 84 13.7 ? 0.3 PVP

[0103] The data show that the filler retention for the papers prepared with Pulp A and B according to the invention is substantially higher than the filler retention of the paper prepared from comparative Pulp C. Apparently, polyoxazoline-modified pulp according to the invention is more capable of retaining the filler and resin particles than unmodified pulp. Papers comprising the pulp according to the invention also have a higher filler retention than papers comprising PVP-modified pulp (pulp D).

[0104] For friction applications, it is important for the friction paper to be as open as possible, so that oil can penetrate into the friction paper during e.g. clutch application. An aim of current invention is to provide a paper, in particular a friction paper, combining high strength and high porosity. The air permeability of a paper is a measure for its porosity.

[0105] The air permeability of papers comprising either Pulp A or B (PEOX-modified pulp) is at comparable level as comparative Pulp C (unmodified aramid pulp), whereas Pulp D (PVP-modified pulp) exhibits a clear decrease in air permeability.

[0106] For friction applications, paper strength is also an important property. Shear strength is the most relevant strength property due to the high shear forces to which the papers are subjected during operation. The shear strength correlates well with the so-called Z-strength or internal bonding strength.

[0107] The results in Table 3 show that that the strength of the model friction paper based on Pulp A and B according to the invention are greatly improved as compared to the strength of a friction paper based on comparative Pulp C.

[0108] The large benefit of pulp of the invention is the combination of high strength, in particular in Z-strength, combined with a high porosity. Comparative papers comprising either unmodified pulp (Pulp C) or PVP-modified pulp (paper D) do not show this combination of properties and only reach comparable or even lower values for either strength or porosity, but not for both properties.

Example 5: Comparison of Commercial Pulp Samples and Polyoxazoline Modified Pulp and the Corresponding Papers

[0109] Polyoxazoline modified pulp was produced on production scale by adding a PEOX solution to a suspension of partly fibrillated aramid short-cut, wherein the suspension comprised 2.5 wt % of partly fibrillated aramid short-cut and 0.09 wt % of PEOX (weight percentage per volume of suspension). The resulting suspension was circulated through a refiner to reach the target fiber length of 0.98 mm?0.2 mm. The PEOX had a molecular weight of 500 kg/mol. The PEOX-modified pulp (Pulp E) contains about 3.3 wt % of PEOX (based on dry weight) and has an SSA of about 4.8 m.sup.2/g.

[0110] As comparative samples, commercially available Twaron? pulp 1092 (referred to as 1092, less fibrillated pulp type, having an SSA of about 6.6 m.sup.2/g) and Twaron? pulp 1094 (referred to as 1094, more fibrillated pulp type, having an SSA of 12-15 m.sup.2/g) are used. Usually, a more fibrillated pulp will increase paper strength but decrease air permeability in the paper.

[0111] Papers were prepared based on 1092, 1094 and Pulp E as described in Example 2.

[0112] Subsequently, the wet strength of the papers was determined.

[0113] The paper sheets prepared in this way and based on pulp samples 1092, 1094 and Pulp E were impregnated with phenolic resin as described for Example 2. The air permeability, Z-Strength, filler retention and tensile strength of the impregnated papers was determined as described in Example 4.

[0114] The properties (average values) of the base papers and the impregnated papers are shown in Table 4.

TABLE-US-00004 TABLE 4 Properties of base and impregnated friction papers comprising pulps 1092, 1094 and Pulp E. Wet strength Air Z-strength Filler Tensile Sheet [Nm/g] permeability (F.sub.max) retention strength based Base [L/m.sup.2/s] [kPa] [%] [Nm/g] on paper After impregnation 1092 0.02 37 81 79 7.1 1094 0.04 13 136 92 10.6 Pulp 0.28 41 200 92 11.6 E

[0115] The data of Table 4 show that using polyoxazoline modified pulp improves the mechanical properties of the base paper and the impregnated paper in comparison to the commercially available pulp types which do not contain polyoxazoline. While papers based on pulp type 1092 have a high air permeability, papers based on pulp type 1094 have a high filler retention. The paper based on the pulp according to the invention combines a high filler retention and a high air permeability. In addition, the paper based on the pulp according to the invention has the highest wet strength as base paper and the highest Z-strength and tensile strength as impregnated paper.