FRICTIONAL AND WEAR PROPERTIES OF ASBESTOS SHORT FIBRE FILLED PHOSPHORYLATED CNSL POLYMERS FOR AUTOMOBILE BRAKELININGS APPLICATIONS
The frictional and wear properties of asbestos short fibre filled RBR 200 polymers used in automobile brakelinings were studied. RBR-200 polymer-based brakelinings gave a coefficient of friction in the range of 0.33-0.45 which was within the recommended value for medium range brakelinings. The values of fade and wear rate for the RBR-200 based brakelinings were 32.83% and 0.691 cm3/kwh, respectively, compared to 43.28% and 1.189 cm3/kwh, respectively, for the phenol-formaldehyde-based brakelinings prepared under similar conditions, indicating that RBR-200 polymer could work out to be an excellent matrix resin for brakelinings.
Cashew nut shell liquid (CNSL) obtained from the plant Anacardium occidentale L is a potential source of phenolic monomers for polymer production. One of the major application of CNSL polymer is in the area of brakelinings and clutch facings of automobiles. Brakelinings and clutch facings based on CNSL polymers are reported to exhibit extremely low fade and high recovery characteristics. CNSL polymer probably absorbs or dissipates the heat created during braking action. Thus, apart from improving the braking efficiency, the CNSL-based friction material shows only a low noise level and gives a quieter braking action than phenolics. It was thought that the use of RBR-200 polymers might improve these properties because of its higher bonding and heat-resistant properties. RBR-200 shows better bonding and heat-resistant characteristics than the conventional CNSL-formaldehyde resin. For example, RBR-200 resin has given 500 times higher value for lap shear strength compared to that of CNSL-formaldehyde. Also RBR-200 resin (cured product) has given a critical oxygen index value of 38.0 in comparison to 21.0 of CNSL-F resin. The literature on CNSL-based friction materials not withstanding a large number of patents, does not provide any idea of the use of such improved resin such as RBR-200 resins for brakelinings. The preparation and processing of the brakelinings based on asbestos short fibre filled RBR-200 CNSL polymers and the evaluation of their frictional and wear properties are reported.
Materials – RBR-200 CNSL prepolymer was prepared by condensing CNSL in acidic media.
Asbestos short fibre crysotile grade, Hexamethylene tetramine (Hexamine), Toluene and Methyl Ethyl Ketone are also used. Electrolytic grade copper powder was used as the metal additive.
Processing and preparation of samples for testing
A 4 Kg sigma mixer was used for blending and mixing. RBR-200 prepolymer was first masticated for 15-20 min. in the presence of toluene or methyl ethyl ketone to achieve a reduced viscosity level. Asbestos and other ingredients were added shortly. The whole batch was compounded for 90 min.
Known predetermined quantities from the compounded materials were taken for hot pressing in various moulds for different studies such as tensile, impact and frictional properties. Measurement of coefficient of friction and wear rate were prepared and tested. A constant torque-type friction testing machine based on “Scale Friction Rig ” method which simulates the road braking conditions to a greater extent than any other method as per the specifications mentioned, was used for the measurement of coefficient of friction, percentage fade, percentage recovery and wear rate.
The coefficient of friction was calculated from the equation –
T = 10–6mµ(P-A) ÿm
P = air pressure on the diaphragm
A = effective diaphragm area
m = coefficient of friction between the lining sample and the disc, and
T = torque applied on the shaft by hanger weight.
The conditions of testing were rubbing speed 1530 – 1550 rpm, mean radius of rubbing 32 mm, wrap angle of lining sample on the holder -210°, inner diameter of lining sample 50 mm, outer diameter of the lining sample 80 mm, torque arm length of 300 mm and preferred diaphragm area of 7800 mm2 effective. A pressure recording gauge of 0-700 Pa and a thermocouple for temperature recording at 0-600ºC were used. The test samples were 20 x 20 mm square.
The lining test consisted of (a) bedding of 1h duration on 30s cycling time and 4s braking time at a control load of 25N and disc temperature at (max) 180ºC (b) postbedding check of four successive applications from a control load of 25-40N in steps of 5N for 5s duration each. This was followed by five applications at 40N control load for 5s duration each at a disc temperature of 150ºC.
Fade percentage = (Average value of coefficient of friction during the previous postbedding check) – (lowest value of coefficient of friction during the fade test) ÷ (the average value of coefficient of friction during the previous postbedding check) expressed as a percentage.
The recovery percentage is calculated as follows:
The recovery percentage = (higher value of coefficient of friction during the recovery test) ÷ (the average value of coefficient of friction during the previous postbedding check) expressed as a percentage.
Wear rate is calculated on the basis of total work done during the test which included the first fade and recovery test, subsequent bedding and postbedding check, second fade and recovery cycle and subsequent bedding and postbedding check. The energy calculated for 40 applications of fade and recovery cycles are averaged. The lining holder was removed from the rig at the end of the test, allowed to cool and the loss in weight was recorded. The rate of wear was calculated in terms of unit loss in volume per unit of work done (mm3/kwh). Since variations from exact temperatures occur frequently during the cycle of operation, the value of wear rate is expected to be on the higher side by this method. The friction was measured after placing the test specimen in water for 2h. Density, hardness, cross-breaking strength, acetone extract, loss on ignition were also determined.
Tensile strength was measured using an Universal Instron testing machine at a cross-head speed of 20 mm/min. The impact strength was measured using an impact tester.
The formulations for the brakelinings consisted of six ingredients: RBR-200 CNSL resin, phenol-formaldehyde (Novolac) resin (PF), asbestos short fibres, hexamine, friction dust and electrolyte copper powder. Five formulations were made to study the overall contributions of RBR-200 in comparison to PF and also to study the effect of metal powder in the formulation. The resin content was in the range of 26-29% and metal powder in the range of 3.9-7.6%.
The friction and wear properties of the five test samples are given in Table 1. In all cases the coefficient of friction is between 0.33 and 0.45, the standard set for medium friction brake lining materials.
Table 1: Friction and wear properties of the five formulations containing RBR-200 PF and metal powder
The coefficient of friction also showed an improvement on wetting. It could be seen from this table that RBR-200 alone gave the lowest fade value (32.83%) (the lower the fade, the better the performance) compared to 43.78% of the PF system. Addition of metal powder raises the value of fade in the case of RBR-200, but reduces the value of fade in the case of PF. Another interesting observation is that the wear rate for RBR-200 alone, is also the lowest compared to PF and other formulations indicating possibly that RBR-200 alone could be used as a matrix resin for brake linings and costly metal powder could be eliminated. The data presented here also shows that the conventional CNSL-formaldehyde-PF blends could easily be replaced by RBR-200 which could ultimately turn out to be cost effective.
Table 2 gives a comparative evaluation of RBR-200- based brakelinings with two of the brakelinings from the market. One of the commercial samples had a percentage recovery of 117.85 and a coefficient of friction of 0.377. Otherwise the RBR-200-based brake linings show much better values for fade, wear rate and coefficient of friction than those of the commercial samples.
A typical set of properties of RBR-200-CNSL based brakelinings are given in Table 3. The impact strength of RBR-200 (1.63-2.07J) is much superior to that (1.35J) of the PF system.
The maximum variation in properties for the RBR-200-based samples is on the range of 3-11%. The value of water absorption (2-3%) and acetone extracts (3-4%) are also within the prescribed limits. (The values of wear rate shown here will be on the higher side since it was not always possible to achieve the temperature limits as prescribed). These values have been achieved without resort to any of the sophisticated processing techniques. Also, the samples were baked only for a few hours after hot pressing. In the industry, baking goes for as long as 30-40h. The properties of RBR-200 could be further improved by adopting improved processing techniques. For example, when a few samples were processed in one of the brake lining industries as a trial experiment, a value of fade as low as 19% was obtained.
Under normal use conditions, the brake friction interface experiences temperatures between 900ºC and 1125ºC. Organic constituents undergo rapid thermal degradation under these conditions affecting friction and wear. However, it is generally thought that for a stable wear there must be a positive gradient of mechanical properties at the interface. This is assured in the case of organic materials. Coupled with factors such as cost and the binding ability, organic resins will still have a major role in brakelinings.
Phenolics are the commonly used brakelinings resins. The use of cashew resins became prevalent when it was found that cashew resins behave like rubber and contribute to lining improvement. It gives rise to lower fade and lower noise levels. But its hot wear characteristics are not good. It appears that cashew resins allow faster heat dissipation. The main difference with phenolics is the presence of a C15 hydrocarbon side chain in cashew resin. The contribution to lower fade and heat dissipation arise from structural and conformation changes in the side chain. A detailed study is needed to arrive at the mechanism. It can be seen from Fig. 1 that the behaviour of the RBR-200 based brakelining is as good as that of phenolics. Moreover, RBR-200-based brake linings have shown lower fade (19%) compared to that (43%) of phenolics. Earlier cashew resins were used only in cold wear conditions, though it had lower fade than phenolics. Now RBR-200 based brakelinings could be used in hot wear conditions with added advantage of lower fade than phenolics.
The performance of friction materials depends on many factors such as the matting surface, operating conditions, history of the material, the composition and micro-structure of the materials, etc. The nature of the binder resin, the processing conditions, the cure schedule, the effect of additives, and the type of asbestos used affect the friction and wear properties. So a comprehensive study is needed to assess the true behaviour of a friction material. The interrelations of the various parameters that affect the friction and wear properties of RBR-200 resin-based brake linings were not studied and therefore at this stage, all that could be said is that based on criteria of cost and optimum performance, the RBR-200 polymer could be used as a good matrix resin for brake lining.