The present invention provides a process for the preparation of improved friction dust useful as friction modifier in brake / clutch facings. As the friction dust prepared from phosphorylated cashew nut shell liquid prepolymer by the process of the present invention possesses properties superior to that of the conventional CNSL- formaldehyde resin in terms of improved resistance to flame, high temperature stability, lower fade value, better durability etc., it can be used as a friction modifier in brake linings substituting the existing brands that are known to give rise to serious problems. So the industries to which the invention can apply are brake lining and clutch facing industries.
Friction dust is generally prepared from CNSL- formaldehyde resin or from the residue known as residol obtained after distillation of cardanol from CNSL. ( A.R.R. Menon, C.K.S. Pillai, J.D. Sudha and A.G.Mathew, J.Scient.lnd.Res., 44,324 (1985); J.H.P.Thyman, Chem. Soc.Rev.,8, 499,(1979)). The CNSL resin is then cross-linked with hexamine or paraformaldehyde and cured, further thermohardened and ground to a coarse dust (Cashew nut Shell liquid Patents U.S.Voll and UK, Indian and Japan , Vol. 2 , Cashew Export, June Cashew Export Promotion Council, Ernakulam, 1964; Cashew Nut Shell Liquid-Extraction and Uses- a Survey of World Patents upto 1976 , Cashew Export Promotion Council, Ernakulam, 1978; Indian Cashew Nut Shell Liquid- A Versatile Industrial Raw Material of Great Promise -Regional research laboratory, Trivandrum and Cashew Export Promotion
Council, Ernakulam., 1983,). The residol route is mostly preferred by the industry as it is more cost effective. The preparation, properties and specification of friction dust are trade secrets and published literature are rare( W.Knop and A.Sheib, Chemistry Applications of Phenolic Resins, Springer verlag, Berlin, 1979). Addition of friction dust is expected to modify frictional and wear properties of the brake linings (A.R.R.Menon, C.K.S.Pillai, J.D.Sudha and AGMathew, J.Scient.Ind.Res. , 44, 324 (1985); J.H.P.Thyman, Chem. Soc.Rev.,8, 499,(1979)). An increase of 30-40 % in wear improvement is expected. It also helps in reducing the fade value and lowering the noise level thereby giving the brake lining enhanced life and a quieter braking action. However, the conventional material was found to give serious problems: The friction dust prepared through this route is suspected to catch fire during shipments and hence its export market has eroded greatly. So it was thought necessary to develop a process to prepare friction dust having improved thermal stability and better resistance to flammability.
The main object of the invention is to provide a process for the preparation of friction dust useful as a friction modifier in brake / clutch linings, Another object of the present invention is to provide a process for the preparation of friction dust from phosphorylated cashewnut shell liquid prepolymer having improved resistance to flame and better high temperature stability and better fade and other friction modifying ptoperties than the conventional friction products.
In our patent No. 176069 (1157/DEL/88) we have described and claimed a process for the preparation of phosphorylated cashewnut shell liquid prepolymer.
The main finding underlying the present invention is our observation that phosphorylated CNSL prepolymer (PCNSL) obtained by the phosphorylation of CNSL has been found to give cross -linked products (on curing) having comparable thermal stability and better resistance to flame than that of CNSL-formaldehyde resins or residol. The Limiting Oxygen Index ( a measure of the flame resistance, the higher the value the better the flame resistance) value of the cross-linked PCNSL product is found to be 38 in comparison to 21 of the CNSL-formaldehyde resins indicating that the PCNSL based product possesses superior resistance to flame to that of the product obtained from CNSL- formaldehyde. The brake lining prepared with the addition of the former gave better fade value than that of the latter indicating that the former will have a longer life time. The friction dust obtained by the process of the present invention also posseses better high temeprature stabilty above 500 °C than that of the product obtained through the CNSL- formaldehyde process.
The process of the present invention has essentially the following steps: mixing a slurry of PCNSL, curing agent and organic solvent in a sigma mixer whereby PCNSL is directly converted to friction dust by a one-shot process. The coarse particles are pulverised once again and sieved to get the friction dust.
Accordingly, the present invention provides a process for the preparation of improved friction dust useful as friction modifier in brake / clutch facings which comprises preparing a slurry of phosphorylated cashewnut shell liquid prepolymer and a crosslinking agent in an organic solvent such as herein described, mixing the said slurry in a conventional mixer of a period ranging 20 to 30 minutes, pulverizing the resultant dust, sieving and thermohardening the obtained dust at a temperature in the range of 80 - 13°C for a period of 20 to 30 minutes to get the desired improved friction dust.
The crosslinking agent used may be such as paraformaldehyde, hexamine, formaldehyde.
The organic solvent used may be such as carbontetrachloride, carbondisulphide, toluene.
The crosslinking reactions may be effected at temperature of above 80°C.
The invention is described in detail in the following examples which are provided by way of illustration only and should not be construed to limit the scope of the invention.
Example 1
In a 4 Kg sigma mixer ( a common mixer having z type blades), 2.5 Kg of phosphorylated CNSL resin taken and the mixer run for 10 min.. A slurry of 125 gm of paaformaldehyde in carbon tetrachloride ( 500 ml) was slowly added to the resin while mixing was continued for a period of 15 mins. The mixing was continued for another 20 min. and the powdery product was cured at 80°C for 30
min.and further thermohardened at 150 °C for 30min. The product was sieved and those particles that lie above the +36 size are ground in a pulveriser. The upper and lower regions of+36 and -100 are rejected.
Example 2.
In a 4 Kg sigma mixer, 2.0 Kg of phosphorylated CNSL resin taken and the mixer run for 10 min.. A slurry of 100 gm of paraformaldehyde in carbon -disulphide ( 400 ml) was a slowly added to the resin while mixing was continued for a period for 15 min.. The mixing was continued for another 20 min. and the powdery product was cured at 80 °C for 30 min. and further thermohardened at 150 °C for 30min. The product was sieved and those particles that lie above the +36 size are ground in a pulveriser. The upper and lower regions of+36 and -100 are rejected.
The advantages of the present invention are the following:
The present process provides a one-shot process whereby friction dust is obtained by a unit operation involving cross-linking and conversion to powder the form simultaneously whereas the existing processes for conversion of CNSL-formaldehyde involves two unit operation involving cross-linking and powdering or pulverisation separately. The friction dust obtained by the present process based on phosphorylated CNSL resin shows superior thermal stability above 500 °C and better resistance to flame to those of friction dust obtained by the existing

We Claim:
1. A process for the preparation of improved friction dust useful as
friction modifier in brake / clutch facings which comprises preparing
a slurry of phosphorylated cashewnut shell liquid prepolymer and a
crosslinking agent in an organic solvent such as herein described,
mixing the said slurry in a conventional mixer of a period ranging 20
to 30 minutes, pulverizing the resultant dust, sieving and
thermohardening the obtained dust at a temperature in the range of
80 - 13°C for a period of 20 to 30 minutes to get the desired
improved friction dust.
2. A process as claimed in claim 1 wherein the crosslinking agent
used is selected from hexamine, paraformaldehyde.
3. A process as claimed in claim 1-2 wherein the organic solvent used
is selected from carbontetrachloride, carbondisulhide, toluene.
4. A process for the preparation of improved friction dust useful as
friction modifier in brake / clutch facings substantially as herein
described with reference to the examples.