FRICTION MATERIAL COMPRISING ARAMID

Abstract

A friction paper including filler, para-aramid pulp, and resin, characterised in that the para-aramid pulp includes 0.1 to 10 wt. % of polyvinyl pyrrolidone (PVP), the paper having a grammage in the range of 100 to 800 g/m2. It has been found that the use of a PVP-containing para-aramid pulp leads to improved friction performance as compared to the use of para-aramid pulp not containing PVP. Effects obtained include improved friction properties, improved strength properties, and improved filler retention.

Claims

1. Friction paper comprising: filler, para-aramid pulp, and resin, wherein the para-aramid pulp comprises 0.1 to 10 wt. % of polyvinyl pyrrolidone (PVP), the paper having a grammage in the range of 100 to 800 g/m2.

2. Friction paper according to claim 1, wherein the para-aramid is poly(paraphenylene terephthalamide).

3. Friction paper according to claim 1, wherein the para-aramid pulp comprises 0.5 to 6 wt. % of polyvinyl pyrrolidone (PVP).

4. Friction paper according to claim 1, wherein the PVP-containing para-aramid pulp has a length (LL.sub.0.25) in the range of 0.7 to 1.5 mm.

5. Friction paper according to claim 1, wherein the PVP-containing para-aramid pulp has a Schopper Riegler (SR) in the range of 15 to 80 SR.

6. Friction paper according to claim 1, wherein the PVP-containing para-aramid pulp has a Canadian Standard Freeness (CSF) in the range of 15 to 700 mL.

7. Friction paper according to claim 1, wherein the PVP-containing para-aramid pulp is prepared by a process comprising the steps of combining para-aramid short-cut with PVP in an aqueous solution to form a mixture, subjecting the mixture to a refining step to form a para-aramid pulp comprising PVP.

8. Friction paper according to claim 1, which has a grammage in the range of 200-600 g/m2.

9. Friction paper according to claim 1, wherein the PVP-containing aramid pulp is present in an amount of 5 to 60 wt. %.

10. Friction paper according to claim 1, wherein the filler is present in an amount of 5 to 55 wt. %.

11. Friction paper according to claim 1, wherein the resin is present in an amount of 5 to 50 wt. %.

12. Friction paper according to claim 1, wherein the resin is a phenolic resin.

13. Friction paper according to claim 1, wherein the paper further comprises reinforcing fibers, generally in an amount of 2 to 40 wt. %.

14. Method for manufacturing a friction paper according to claim 1, comprising the steps of manufacturing a paper comprising aramid pulp comprising PVP, resin, and filler, and heating the paper under such conditions that the resin is cured.

Description

EXAMPLE 1: PULP MANUFACTURE

[0047] 4 kg of para-aramid chopped fibers of 6 mm in length (6 mm short-cut based on Twaron type 1000 1680f1000 from Teijin Aramid BV, NL) was added to 200 liter of an aqueous solution of PVP. The PVP had a molecular weight of approximately 50 kg/mol. The resulting medium contained 2 wt. % of aramid short-cut and 0.1 wt. % of PVP. The resulting suspension was passed through a Sprout-Bauer 12 lab refiner until the desired fiber length was obtained. Intermediate samples were taken during the process. The refined suspension was dewatered on a sieve table to yield a dewatered cake. This PVP-pulp is denoted as Pulp A. It contains 5 wt. % of PVP.

[0048] As reference, the same procedure was followed without the addition of PVP. This pulp is referred to as Pulp B.

EXAMPLE 2: MANUFACTURE OF A FRICTION PAPER COMPRISING FILLER, RESIN AND PVP-ARAMID PULP ACCORDING TO THE INVENTION

[0049] 13.86 g of PVP-containing Pulp A with a dry solids content of 19.83%, thus equaling 2.75 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, 3.54 g of diatomaceous earth (Celatom MN-23) and 2.97 g of phenolic resin (BAKELITE PF 0229 RP) were added to the suspension and mixed for an additional 500 counts (100 s at 3000 rpm). This mixture was used for paper sheet preparation on a Rapid Kothen lab sheet former in accordance with ISO 5269-2. The resulting paper sheets were dried between two blotting papers in a plate drier at 90 C. for at least 30 minutes. The dried sheet was then cured in a press between two Teflon sheets. This was performed at 150 C. and a pressure of 0.5 MPa for 3 minutes in total. During the press program, after 1 and 2 minutes, the press shortly opens to avoid pressure build up due to possible release of vapour. The properties of this resulting model friction paper are discussed in Example 4.

EXAMPLE 3: MANUFACTURE OF A COMPARATIVE FRICTION PAPER COMPRISING FILLER, RESIN AND ARAMID PULP

[0050] The same procedure as described in Example 2 was followed for pulp B, with the exception that now 15.23 g of the pulp was used at a dry solids content of 18.04%, resulting in the same amount of 2.75 g of dry aramid pulp. The properties of the resulting model friction paper with comparative pulp B are discussed in Example 4.

EXAMPLE 4: COMPARISON OF FRICTION PAPERS

[0051] Various properties of the friction papers of Examples 2 and 3 were determined.

[0052] 1. Paper Yield (Filler Retention)

[0053] Paper yield is calculated by dividing the weight of the final cured paper by the dry weight of the initial raw materials (pulp, resin and fillers) and multiplying by 100%. The paper yield is thus a measure of how much filler and resin is lost during the paper-making process (the loss of pulp during the paper-making process on a sheet former is negligible). The paper yield is based on the average of two sheets. The results are presented in the table below.

TABLE-US-00001 Sheet Paper yield Paper yield Average paper based on sheet 1 [%] sheet 2 [%] yield [%] Pulp A 72.2 71.4 71.8 Pulp B 58.0 57.3 57.7

[0054] From the table it can be seen that the paper yield for the paper prepared with Pulp A according to the invention is substantially higher than the paper yield for the paper prepared from comparative Pulp B. Apparently, Pulp A according to the invention is more capable of retaining the filler and resin particles than comparative pulp B.

[0055] 2. Strength Properties

[0056] For friction applications, shear strength and Z-strength (as measure for internal bonding) are the most important strength properties, due to the high shear forces during operation. Samples have been taken from the sheets obtained from pulp A and Pulp B in Examples 2 and 3, and have been subjected to both a shear test and a Z-test. Tests were carried out in accordance with Tappi T541 and ASTM D-5868-95. The results are presented in the following table.

TABLE-US-00002 Z-strength Shear strength (tensile Sheet (Fmax) [kPa] stress at max load) [kPa] based on Avg SD Avg SD Pulp A 254 4 563 73 Pulp B 110 7 250 23

[0057] From this table it can be seen that that the strength of the model friction paper based on Pulp A according to the invention is greatly improved as compared to the strength of model friction paper based on comparative Pulp B.

[0058] 3. Friction Coefficients

[0059] Friction properties for Pulp A and Pulp B were obtained using Bruker's UMT Tribolab, equipped with a rotary drive module, a temperature chamber for the rotary drive, and special wet-friction clutch testing kit. Samples were prepared by punching rings out of the friction papers described in Examples 2 and 3, and fixing them to steel sample holders using a resin, similar to the process used in industry to prepare friction plates. For both friction papers described in Examples 2 and 3, respectively, three samples (i.e. friction paper on steel sample holder) were prepared for a friction test.

[0060] Fresh automatic transmission fluid (Pentosin FFL-2) was used at the beginning of each friction test and was pumped around at constant flow. The friction test of a single sample consisted of repeating a programmed sequence (in which e.g. pressure and speed was varied) 10 times at 40 C. The first 5 times were considered as run-in, as evidenced by decreasing friction coefficients from one run to the next. The measured friction coefficients of the 14-step-speed test at 2 different pressures of the last 5 runs are averaged at each speed for each sample. The results of the three different samples measured for each friction paper type are averaged at each speed, and are presented in the FIG. 1.

[0061] From FIG. 1 it can be seen that the friction coefficient of friction paper based on pulp A (invention) is higher than for friction paper based on pulp B (reference) for almost all speeds at both pressures. Moreover, the static friction coefficient (as speed approaches zero) of friction paper based on pulp A (invention) is lower than the dynamic friction coefficient (i.e. positive slope at low speeds), which is generally considered positive for driving comfort (reduced shuddering).