CHAPTER NEED FOR OPTIMIZING THE RESIN

Transcription

1 95 CHAPTER-5 OPTIMIZATION OF ALKYL BENZENE MODIFIED PHENOLIC RESIN IN A FRICTION COMPOSITE EFFECT ON THERMAL STABILITY, FRICTION STABILITY AND WEAR PERFORMANCE 5.1 NEED FOR OPTIMIZING THE RESIN The binder is the heart of the system which binds other ingredients in the formulation. The amount of resin in the brake pad is very critical in this respect. Right amount of resin imparts adequate integrity to the brake pad without sacrificing other important properties. Selection of resin with higher thermal stability and oxidation resistance is of immense importance in controlling fade. The friction and wear performance of brake pad containing two different kinds of phenolic resins reinforced with aramid pulp was studied by Kim 2007 and it was found that the modified resin improved the thermal stability. The influence of various types of modified commercial resins on the fade and recovery behaviour of the brake pad was studied by Bijwee 2006 and it was observed that it was not easy to get excellent strength, friction, fade and recovery, and wear resistance at the same time for one type modified resin. The fade and wear performance of brake pads made with Straight Phenolic resin, boron Phosphorous modified phenolic resin and polyamide resin was studied by Jang 2004 and it was observed that boron-phosphorous

2 96 modified resin was more resistant to wear and fade than the other two resins. The effect of CNSL resin content in a Carbon fibres reinforced paper-based friction materials was studied by Li 2011 and it was observed that the optimized resin amount in the range of 35-40% produced good mechanical performance, heat resistance, friction and wear performance comprehensively. In the present work, Alkyl benzene modified phenolic resin with suitable molecular weight is selected as the binder based on previous studies. They have a good combination of mechanical properties and very good wetting capability with most of the ingredients. If the binder amount is too less it results in material weakness and if too much is used, then there is a friction drop in high temperatures. Organic components like resin and organic fibers in non-asbestos friction composites are most vulnerable to thermal degradation, charring leading to glazing and subsequent deterioration in performance due to heat generated at the interface as a result of increase in the severity of braking. Hence it becomes necessary to find out the optimum weight % of the resin and their effect on thermal stability, friction stability and wear performance. Normally, to see significant differences in performance, one needs to tweak the ratios at much higher levels. A change of 1% or 2% does not necessarily bring out big performance differences. However, since the formulation is finalized in our previous work, fine tuning to the extent of 1% or even 0.5 or 0.2% are needed for the ingredients like resin which affects the performance. The objective of this work is to present the results and discuss an experimental study that considers the effect of weight% of the resin matrix in relation to fade and recovery of a Non Asbestos Disc brake pad. Three pads with same formulation, but differing in the weight% of the phenolic matrix were fabricated and used to evaluate the friction characteristics.

3 97 The various chemical and mechanical properties were evaluated following the Industrial standards. Tribological Testing was carried out on a Brake Inertia Dynamometer as per test schedule JASO C-406. Alkyl Benzene modified phenolic matrix resin suitable molecular weight was selected based on the previous work and the same was optimized in this present work. The properties of the selected AB modified phenolic resin are listed in the Table 5.1. Moreover, the effect of the amount of resin in relation to frictional stability is carried out in more vigorous condition in an Inertia Brake dynamometer Table 5.1 Details of the properties of the selected alkyl benzene modified phenolic resin taken from the previous study Properties Units Alkyl Benzene Modified Phenolic Resin Melting point 0 C 82 Gel Time at C ( ISO 8987 B) Sec Flow Distance at C( ISO 8619) mm Hexa content (Potentiometric method) % Moisture content % 1.4 Initial degradation temperature from TGA (T 1 ) Curing Temp from DSC 0 C C 154

4 THERMO GRAVIMETRIC ANALYSIS (RESIN DEGRADATION TEMPERATURE) The thermo gravimetric (TG) analysis is an important thermal analysis that shows the thermal stability of the materials. Also, the profile of the decomposition process and yield of the material associated with the thermal treatment can be obtained. The sample tested shows two stages of weight loss as shown in figure 5.1. In the first stage, the material showed a maximum weight loss of 50 percentage between C and 388 C. In the second stage, between 389 C and 798 C, the weight loss is 34 percentage, and this can be attributed to thermo-oxidative reactions, leaving a carbonaceous residue equal to 16percentage of the initial mass which reveals the weight loss and the withstanding temperature of the resin. Figure 5.1 TG of the selected resin in Zero air (20 ml/min). Heating rate 10 0 C/min sample weight = mg

5 DIFFERENTIAL SCANNING CALORIMETRIC ANALYSIS (CURING TEMPERATURE OF THE RESIN) With Differential Scanning Calorimetry (DSC) measurement is possible to get a thermal profile of the investigated sample under the conditions of thermal dynamic scanning. The results of the measurements produce the knowledge of the reaction behaviour, the beginning, the end and at which point the reaction reaches its maximum peak Fig 5.2 shows the DSC curve of the Alkyl benzene modified phenolic resin. The reaction starts at around C, with a peak in C, and ends at C. The heat of reaction is approximately J/g. Hence, curing temperature is set around 154 o C. Figure 5.2 DSC of Alkyl benzene modified phenolic resin (DSC 60 Thermal Analyzer (TA-60 WS)) The brake pads were fabricated in four steps which are mixing of the ingredients, preforming, curing in a Hydraulic Press and post baking. Fabrication of the brake pads was carried out on the basis of keeping all other ingredients (except the resin and the barytes). Varying the AB- modified

6 100 phenolic resin in 10.11, and 12.11% by weight and compensating it with space filler is shown in the table 5.2. Table 5.2 List of the varying ingredients in composites Ingredients DBL DBM DBH Resin (wt%) ca12.11 Barytes (wt%) For our reference it is designated as DBL (Disc brake pad made with wt% resin), DBM (with wt%) and DBH ( with wt%). The other ingredients are Kevlar, Cellulose fibre, Barytes powder, Lapinus and MCA Rockwool fiber, Vermiculite Dug, Steel wool, Green Chrome oxide, Syn Graphite Powder, Tyre Peels/Crumb rubber, Rubber NBR, Friction dust, China Clay, Yellow Iron oxide (natural) and Zinc oxide. The additions of ingredients are mixed in a drum mixer with feeder and chopper. 5.4 MOLDING IN HYDRAULIC PRESS Table 5.3 Detail of the processing condition for brake pad Procedure Conditions Sequential mixing Total duration 12 mins feeder RPM 300, Chopper RPM 3000 Sequence (a) Power ingredients (b) pulps and fibers Curing Temp. 160 ; Post- curing Compression 17 MPa; Curing time : 8 mins C, 8 hr.

7 BRAKE EFFECTIVENESS TEST AS PER JASO C-406 SCHEDULE In the present work, the schedule followed is JASO C 406, which is for passenger car pads (typical customer). The test is conducted on a double ended full-scale dynamometer. The specifications and the conditions of the test schedule are given in the Table 5.4 & 5.5. The test is conducted by first establishing the conformal contact between the mating surfaces (pad and the disc) for nearly four hours. The effectiveness tests are done at three different braking speeds viz., 50,100 and 130 Km/h and at the starting temperature of 80 0 C. Bedding is done at C. The tests are conducted at different decelerations (0.1 to 0.8 g). As the deceleration increased, the severity of the braking conditions also increased. The amount of deceleration is controlled by pressure, which was programmed to achieve a selected rate of deceleration depending on the friction level of the tested material. In the fade test process, the deceleration of 3m/s 2 and 6m/s 2 is repeated for an interval of 35s under the vehicle speed of 100 Km/h and measures the variation of temperature and friction coefficient under very harsh braking condition. The recovery test process is to simulate the state to recover from the fade and the deceleration of 3m/s 2 and 6m/s 2 is repeated for an interval of 120s under vehicle speed of 50 Km/h and measures the variation of temperature and friction coefficient under more relaxed braking condition than the fade state. With the brakes released, immersed the friction material surface in water for 120 seconds while rotating the brake slowly at 10 to 30 r.p.m to carry out the water recovery test. Finally the wear in the rotor and the pad is noted.

8 Schedule and Specifications The JASO schedule used for passenger cars along with the specification is given in the table 5.4 Table 5.4 Specification of Dynamometer Schedule JASO C 406 Brake Model PE54 C-14 Inertia 49 Kg ms 2 Rolling or Tyre Radius m Effective radius (Pad on disc sliding radius) 0.103m Conditions of Dynamometer Testing Mode Table 5.5 Test schedule as per JASO C-406 Procedure Speed (Kmph) Brake Deceleration (g) Initial Brake Temp 0 C No. of Applications Air Blower Bedding Test C 200 on Effectiveness I 50, to o C 20 on Effectiveness II 50,100, to 0.8 < 80 o C 24 on Effectiveness III 50,100, to 0.8 <100 o C 24 on Fade & recovery I Fade Cycles to o C for 1 st brake 10 off Recovery cycles to 0.8 >80 o C 10 on

9 103 The conditions for the all the effectiveness test (I,II,III) along with the fade and recovery is shown in table 5.5. Figure 5.3 Disc brake assembly and calliper Figure 5.4 Inertia Brake Dynamometer Setup for testing Brake performance

10 PHYSICAL, CHEMICAL AND MECHANICAL PROPERTIES EVALUATION The tests mentioned in this section were conducted at Technology Center, Hindustan Composites Limited, located at Aurangabad, India. Brake pads were characterized for physical (specific gravity, water swelling, heat swelling, porosity) chemical properties (acetone extraction) and mechanical properties (hardness, hot and cold shear strength) as per Industrial standards. All the results are shown in Table 5.6. Table 5.6 Physical and mechanical properties of the brake pads evaluated in the laboratory Properties Unit DBL DBM DBH Specific gravity , , , 2.15 Hardness HR S 105,110, 118,120, ,91,97, 98, 99 85,86,88, 91,95 Heat swell mm Acetone Extraction % 0.58, , , 0.90 Loss of Ignition % 32.22, , ,36.79 Cold Shear Strength (ISO 6312) Hot Shear Strength (ISO 6312) MPa 3.955, , , MPa 2.386, , , Porosity (JIS D-4418) %

11 EFFECT OF THE RESIN CONTENT ON THE PHYSICAL, CHEMICAL AND MECHANICAL PROPERTIES OF BRAKE PADS With an increase in the content of resin, the specific gravity and the hardness of the brake pad decreased. The reason is due to the removal of barytes which is having higher density. Acetone extraction reveals that DBL is the highest cured and DBH is the least cured under same process conditions. The shear strength of the all the three brake pads made with the three different weight percentage of resins exhibits values acceptable by Industrial standards. The variation in the resin content has not much significant variation in the shear strength. The increase of wt% in the loss of ignition with higher content of resin indicates more degradation due to heat showing its poor thermal stability. Porosity has great influence on the thermal properties of the friction materials since the size and distribution of internal pores can change the thermal conductivity (Jang 2004). The porosity decreased with the increasing resin content. This is because the resin flows through gaps inside the friction material during hot pressing and fills voids before it is consolidated during curing. The resin property which most influences porosity of friction materials is the resin inclined plate flow. 5.8 EFFECT OF RESIN CONTENT ON THERMAL STABILITY OF THE BRAKE PADS TGA test is normally used for testing of thermal stability of raw and finished materials. In this work TG-DTA is carried out to study the effect of resin content on the thermal degradation of the brake pad. Figure 5.5 shows thermo gravimetric analysis (TGA) results of the three friction composites which reveal the weight loss % and the degradation temperature when exposed to ambient temperature 800 C. The TG curves of all the three composites tested show three different regions of weight loss, which were

12 106 reflected on three major peaks in the DTA curve, showing that the friction materials (brake pads) had at least three stages of degradation. The first degradation stage was C; the second stage was around C; and the third stage was in the range of C. The temperature at the peak position at each stage of degradation corresponded to the maximum weight-loss rate at this stage. The amount of weight loss of the samples was almost same in the first stage and the third stage. The thermal stability of the brake pad in the second stage was mainly affected by the resin content, and their weight loss, increased for DBM and DBH when the resin content rose from 10.11% to 12.11%. Meanwhile, brake pad DBH got the maximum (a) (b)

13 107 (c) Figure 5.5 TG DTA curves of the samples with different resin contents. (a) DBH (b) DBM and (c) DBL Weight-loss rate at the temperature about 510 C. At the second stage, the main origin of the weight loss was the resin degradation by oxidation into volatile elements and the oxidation of other organic ingredients, namely crumb rubber and cellulose fibers by cracking, dehydrogenation and dehydration. The thermal stability of the friction materials decreased with the increase of the resin content. Table 5.7 Typical degradation temperatures and weight loss of the friction material Designation T 1max C) T 2max C) T 3max C) W 1 (%) W 2 (%) W 3 (%) DBL DBM DBH T 1max, T 2max and T 3max are the degradation temperature of the peak positions in the first, second and third stages, respectively.

14 108 stages, respectively W 1, W 2 and W 3 are the weight loss in the first, second and third 5.9 EFFECT OF RESIN CONTENT ON THE TRIBOLOGICAL PROPERTIES OF THE BRAKE PADS Performance Properties of Brake Pad on Dynamometer The data on the coefficient of brake pad are plotted as a function of deceleration (g) in figures 5.6(a-h). These horizontal lines (µ g variation) show the load sensitivity of µ. For an ideal material it should be a straight line. A Steeper the slope, higher is the sensitivity of µ towards the load. Similarly the distance between three lines at selected g value shows the speed sensitivity of µ. Higher the distance between these lines, the higher is the sensitivity of µ for speed variation and poorer is the behaviour. (a) (b) Figure 5.6 Performance I effectiveness at (a)50 & (b)100 KMPH

15 109 (c) (d) (e) Figures 5.6(c),(d) & (e) Performance II effectiveness at 50,100 &130 KMPH (f)

16 110 (g) (h) Figure 5.6 (f),(g) & (h) Performance III effectiveness at 50,100 &130 KMPH Effectiveness Studies (pressure speed sensitivity) Change in µ as a function of sliding speed and applied pressure is a very important issue during braking and it should show minimal changes because drivers expect the same level of friction force under various braking conditions. From the sensitivity of µ point of view (more the parallelism of the plot to the x-axis (abscissa) lower the sensitivity and better the performance) (Fig 5.6) the performance order was as follows: DBL >DBM >DBH (during first effectiveness) DBM >DBL >DBH (during second effectiveness) DBL >DBM >DBH (during third effectiveness) With an increase in speed, the DBL showed least slope with maximum µ showing its least sensitivity for speed. For rest of friction composites the slope is more showing their sensitivity of µ towards speed.

17 111 Even though there is minor variation with respect to µ in all the speeds at different effectiveness, the performance order is as follows: DBL >DBM >DBH. For mid-size passenger cars, in general, the required brake ranges from 0.35 to As seen from fig. 5.6, Composites DBL and DBM clearly fulfils these conditions, but DBH produced only borderline acceptable performance. from 0.38 to Performance of Friction composite DBH was poor and its µ varies Fade and Recovery Behaviour (Temperature sensitivity) Fig.5.7 shows the fade and recovery behaviour of composites. For an ideal performance µ should be in good range ( ) and fade curve (µ vs. number of brake applications) should be straight with less undulation. In case of recovery mode, the curve should be flat with low slope and µ should be in the range of pre-fade value. Figure 5.7 Fade and recovery behaviour of the brake pad

18 112 It was observed that addition of more amount of resin content decreased fade resistance and recovery. DBH showed the poorest fade behaviour as µ started to decrease from 3 rd braking itself. In the case of DBM, composite showed steady µ initially, but fade started by 5 th braking onwards and µ started to decrease and so the performance. For DBL, best fade behaviour is observed only after the seventh cycle. Fade and Recovery behavior as per JASO C 406 Table 5.8 Fade and recovery behaviour Fade & Recovery I Parameters DBL DBM DBH µ-fade Fade % fade ratio Max. disc temp ( o C) Recovery µ-recovery % recovery ratio µ- fade = lowest µ recorded during fade test % Fade ratio =( ( µ max - µ min )/ (µ max ))100 µ-recovery = highest µ recorded during recovery test % recovery ratio = µ min x 100: higher the better µ max Resistance to Fade (% fade ratio): Performance Order Lower the Better This is the most important parameter which reflects deterioration in µ when operating conditions are severe. Brake pads are normally rated by their resistance to fade. This fade behavior was due to increase in surface

19 113 temperature as the test was conducted at constant pressure and speed. Generally fade % ratio in the range of are acceptable as per industry norms. The % fade ratio (lower the better) and hence performance was in the following order for composites; DBL(20) > DBM(25) > DBH (34) The Fade is caused by thermal decomposition of ingredients due to accumulation of frictional heat on the surface. In the case of DBH, which has higher amount of resin, the degradation of organic resin is more, especially at higher temperatures. The worn out particles causes heat to accumulate that are developed during sliding causing the resin to change its properties above the glass transition temperature and transforms into a char at the thermal decomposition temperature. This weakens the binding force and causes the change in real contacts at the friction interface and hence µ is not consistent The Recovery Behavior (% recovery ratio) Performance order higher the better. Brake pads which recover their friction level considerably after fade cycles are rated as good friction materials. In general for acceptable composite, recovery should be in the range of %. The recovery % of composites is shown below. DBL(92) > DBH(75) > DBM (68) DBL showed highest recovery performance followed by DBM and DBH. In the recovery performance the temperature rise at the interface was reduced by using air blower. After cooling, retesting is carried out under the same condition with an existing friction film, which changes the rheology of surface layer during the next run. The loosely attached wear debris, which

20 114 forms the original film disintegrates and gets hardened and act as hard abrasives between the disc and rotor by rolling abrasion mechanism (Trezona 2007). Also, the deformation nature of the film has been reported to depend upon the composition (Trefilov 2007). Thus the in situ changes in the formation and deformation of friction films during the recovery cycle are responsible for the differences in the recovery and fade behavior EFFECT OF RESIN ON COUNTERFACE FRIENDLINESS The rise in the disc temperature during fade cycles decides the counterface friendliness of the pad materials. Lower the rise better is the friendliness. The performance of composites was in following order: DBH(412 C) > DBM(390 C) > DBL(376 C) The contact between the pad and the disc surface occurs at the plateaus formed by the high strength fibers rising over the irregular surroundings of organic components as a result of removal of fragments. (Ericson 2004). The removal of materials surrounding the contact plateaus is due to the decomposition of the organic pad constituents. The removal of organic binder material was due to the emission of CO, CO2 or other gaseous decomposition products. This deterioration of organic components causes the metal fibers (pad surface) to metal contact (disc surface) rising the temperature at the contact surface. Composite DBL proved best in this aspect followed by DBM. Here again, the porosity proved an influencing factor because it provided the passage for the developed heat at the interface to flow freely and led to lower temperature rise in the disc.

21 WEAR OF THE BRAKE PADS Wear resistance : DBH(7.4%) > DBM (9%) > DBL(9.3%) Hardness : DBL(115)> DBM(95) > DBH(89) Generally, the harder samples are supposed to have lower wear. But in this investigation, it is found that this postulation does not hold good. It also has been reported that hardness of brake materials cannot be simply related to the content of structural constituents, and there is no correlation between hardness and wear resistance (Mutulu 2006). In this study it is found that increase in resin content increased the wear resistance. This observation is in agreement with the findings of (Bijwee 2006) who reported that increase in resin content increased the wear resistance and lowered the fade resistance while studying the effect of NBR modified resin. Also, the composite, with highest fade showed best wear behavior and the composite with lowest fade showed poor behaviour. Wear mechanisms are extremely complex as the number of interactions and mechanisms (adhesive, abrasion and ploughing) are simultaneously operating which results in material removal/gain process from both pad and the rotor surfaces. These tend to form a third layer which depends on the composition at the real contact areas. Wear, in general, relies on many factors such as temperature at the interface, applied load, vehicle speed, and properties of mating materials. In the case of friction composites developed, the organic constituents, such as binder resin, crumb rubber, organic fibers like cellulose fibers, aramid fibers and other organic friction modifiers like cashew friction dust play crucial roles in the wear. This is because the heat generated during braking at the interface raises the temperature of the friction material beyond the transformation temperatures of these organic constituents as noticed from the TGA studies (Mutulu 2006).

22 116 Hence, TGA is carried out to find a correlation of the wear with thermal decomposition of the ingredients in the friction composite.it shows that a major weight reduction begins at 300 C and it continues up to 500 C before the second major weight reduction takes place near 600 C. The first weight reduction in the figure 5 is attributed to the thermal decomposition of rubber powders, and cellulose fibers. The second weight reduction is mainly due to the degradation of resin, cashew friction dust and aramid pulp. The third major reduction mainly represents the oxidation of graphite. The above mentioned point is drawn after finding the degradation temperature of softer to tougher ingredients individually and are marked in the graph to understand the wear. Figure 5.8 TG and DTA analysis of the friction material DBL used in this study Decomposition temperatures of the degradable ingredients were indicated. In the formulation matrix tested, DBH has a maximum amount of resinous matter and hence its degradation should also be maximized. This would necessarily lead to higher wear among the three tested composites. But from the report, the results observed are contrary. The histogram fig 5.9

23 117 shows the various tribological properties of the three brake pads made with varying the proportion of resin. Figure 5.9 Histogram showing various tribo-performances attributes Lack of correlation between the mechanical and tribological properties of the friction material provoked us to study the wear surfaces by scanning electron microscopy WORN SURFACE STUDIES BY SEM Organic matter, whether degraded, melted, or charred, gets transferred onto the disc surface easily. The tendency of this transferred material to get back-transferred onto the pad surface affects both friction and wear. If the surface is heavily covered with secondary plateaus, the wear damage to the underlying material may be reduced (Jacobson 2004).

24 118 Figure 5.10 Contact phenomenon (Courtesy Jacobson 2004) These secondary plateaus which are formed by the compaction of wear debris due to softer organic components, controls the wear behaviour and the primary plateaus formed by the broken hard fibres are responsible for the frictional characteristics of the composites (Satapathy 2006). The different interfacial boundaries between primary and secondary plateaus represent the sequential change of wear. Such topography of worn surface has been correlated with extent of wear of the friction composites (Gopal 2004). The deterioration in filler-matrix adhesion due to repetitive braking causes thermal and mechanical stresses on the asperities, as can be seen in the form of micro cracks and secondary plateaus on the surface of almost all the samples.

25 119 (a) DBH (b) DBM (C) DBL Figure 5.11 SEM images of (a) DBH, (b) DBM & (c) DBL In the case of DBL, the surface was covered with uneven micro cracks, filler matrix debonding, more amount of primary plateaus. It contained lesser amount of resin and hence extent of back transfer of debris in the form of secondary plateaus was minimized. It has been reported that an increase in porosity reduces the wear as there is a passage for heat dissipation (Ho Jang 2007). In the present study, in spite of increase in porosity and fade resistance, as the back transfer mechanism dominated, the composite DBL showed highest wear.

26 120 Composite DBM has smooth patches with minimum amount of secondary plateaus due to medium amount of resin content and hence showed modest wear. In case of DBH, the porosity is lesser causing heat to accumulate rather than to dissipate. During the case of elevated temperatures, due to heat accumulation and the poor thermal stability of resin as suggested in our earlier columns discussed about thermal stability, degradation of resinous matter is maximum. If back transfer on the pad is heavy, loss in weight of the pad is less resulting in less wear. Thus, even if material degradation is more, it would not necessarily lead to high wear as discussed in the earlier sections. But as these secondary plateaus are mainly due to charred materials, they significantly reduce the friction behaviour and hence results in a lower friction coefficient SUMMARY Since fade is the most undesirable phenomenon in friction composites, the composites were evaluated for Tribo behavior in fade conditions. Based on the studies, it can be concluded that the increase in amount of resin from to 12.11% influenced the performance properties as follows: Fade µ %Fade Max µ Recovery µ %Recovery : DBL>DBH>DBM : DBL>DBM>DBH : DBL>DBH>DBM : DBM>DBL>DBH : DBL>DBM>DBH Disc Temperature Rise : DBH>DBM>DBL Wear resistance : DBH>DBM>DBL

27 121 Resin percentage beyond 11.11% imparts unacceptable fade. Hence DBH was declared unsuitable even though it has the best wear behavior. Though the selection of material is a multiple criteria optimization problem, DBL(10.11%) in general was found to be superior to DBM in most of the important tribological properties. After optimizing the resin by wt%, the other organic component in the formulation namely the cashew dust is also optimized along with the resin. Since, the cashew dust is an organic friction modifier, its % by weight is more or less kept equal to that of the resin which was optimized in the chapter 5. Resin % is varied from 10.11% to 12.11% considering the wear aspect as the cashew dust can compensate for the fade resistance which is dealt in the coming chapter.