Increasing the strength and wear resistance of friction composite materials by modifying them with boric acid polymethylene-p-triphenyl ester

Transcription

1 Plasticheskie Massy, No. 10, 2011, pp Increasing the strength and wear resistance of friction composite materials by modifying them with boric acid polymethylene-p-triphenyl ester D.V. Korabel nikov, 1 M.A. Lenskii, 1 M.S. Nekrasov, 2 R.N. Kondrat ev, 2 and I.E. Kartavykh 2 1 Biisk Technological Institute (Branch), I.I. Polzunov Altai State Technical University 2 Barnaul Mechanical Asbestos Goods Factory Open Joint Stock Company Selected from International Polymer Science and Technology, 39, No. 2, 2012, reference PM 11/10/39; transl. serial no Translated by P. Curtis Summary The possibility is shown of controlling the strength and wear resistance of polymer friction materials based on rubbers by adding boric acid polymethylene-p-triphenyl ester without altering the basic production technology of the composites. The greater heat resistance of the modified friction composites is established. With the development of technology, average vehicle speeds are increasing, which leads to an increased number of braking cycles. In view of this, increasingly stringent requirements are being placed on the composite materials of the friction units heavily loaded in braking processes. To control the speed of motor vehicles, etc., use is made of the surface friction effect by pressing a specially designed friction composite material (brake lining or shoe) against the metal surface of the brake drum or disc. Trouble-free operation of brake systems of motor vehicles depends on the properties of the composite material from which the brake lining or shoe is made. During service, the composite material is subjected to the action of different factors, for example: high temperature depressions (rapid heating from 50 C to 300 C, and in some cases to 500 C), different mechanical effects during use of the vehicle (driving on ruts or ditches during braking, which entails shock to the brake unit), and furthermore deformation loads (resistance to shear force, compression, and so on). On the strength of this, a number of stringent requirements are placed on friction composite materials in order to ensure correct and trouble-free operation of brake systems, in particular a high friction coefficient and its small variation at temperature, strength, good thermal conductivity, and low toxicity of components [1]. Modern brake linings (shoes) generally have a large number of different components, for example a binder (rubbers, phenolic resins), fillers (asbestos, wollastonite, barytes, mineral wool, metals, etc.), process additives (carbon, graphite), and a curing system [2]. The main shortcomings of the composite materials used for the manufacture of brake linings (shoes) are their low physicomechanical characteristics at elevated temperatures, which is due to the low heat resistance of the polymer binder, and also the considerable variation in the friction coefficient as a function of temperature, which often leads to a reduction in braking efficiency of the lining (shoe) or to failure of the brake system. The noted shortcomings can be eliminated in two ways: by creating fundamentally new heat-resistant binders or by modifying existing friction composite materials. The economic expediency of the second approach, in our view, is obvious. To increase the physicomechanical characteristics and heat resistance of friction composites, use was made of organoboron polymer resins [3]. However, their use has not been widespread on account of the low quality of the resins themselves (a wide molecular weight distribution and a high content of toxic low-molecular-weight impurities), which has led to considerable variation in 2013 Smithers Rapra Technology T/51

2 the strength of the composite materials, depending on the resin batch. The given problem was solved by Lenskii [4] by synthesising boric acid polymethylene-p-triphenyl ester, which belongs to the class of polymethylene esters of phenols and boric acid, of satisfactory quality (a narrow molecular weight distribution, content of main substance >99%). In this context, the aim of the present work was to study the properties of boric acid polymethylene-p-triphenyl ester, belonging to the class of polymethylene esters of phenols and boric acid, and also to investigate it as a modifier of the strength of friction composites. The first stage of the investigations was to study the interaction of boric acid polymethylene-p-triphenyl ester with a sulphur curing system under different conditions. Investigations were conducted by the sol-gel method at temperatures used for the manufacture of asbestos and asbestos-free friction composites. The results are presented in Figure 1. As can be seen from Figure 1, at a temperature of 180 C the maximum value of the gel fraction content is achieved at 6 h curing and amounts to 90%. Here, at more than 6 h curing, a plateau is observed on the kinetic curve. The induction period at the given temperature amounts to about 1 h. At a temperature of 190 C the maximum value of the gel fraction content is again achieved at 6 h curing and amounts to 99.5%, and the induction period is 30 min. In the course of curing, at temperatures of 200 C and above, the maximum value of the gel fraction content reaches 99.5% and is achieved at h curing. The induction period amounts roughly to 15 min. Thus, the gel fraction content amounts roughly to 99.5% at curing temperatures above 200 C. The induction period amounts to less than 15 min, and a plateau is observed at more than 1.5 h curing. It is important to note that the gel fraction content does not decrease with increase in the curing time to 8 h, which makes it possible to judge whether thermal degradation of the cured system is absent. The next stage of the investigation was to study the physicomechanical characteristics of the composite material modified with the addition of boric acid polymethylene-p-triphenyl ester. Modification was conducted by introducing polydispersed powder of boric acid polymethylene-p-triphenyl ester in excess of 100% to a model base composite, the composition of which is presented in Table 1. The polymer composite material was prepared on a VK-6 laboratory mill. The obtained composite was pressed in a mould and cured at different temperatures for 30 min. Investigations were conducted according to GOST The obtained results of physicomechanical tests are presented in Figure 2. Figure 1. Effect of temperature on the gel fraction content during the vulcanisation of boric acid polymethylene-ptriphenyl ester Figure 2. Effect of the boric acid polymethylene-p-triphenyl ester content on the bending stress at the moment of failure, s f Table 1. Composition of the model composite material No. Component Content (wt%) 1 Wollastonite Voksil 100, TU Barytes, GOST SKD polybutadiene rubber, GOST SKI-3 polyisoprene rubber, GOST Sulphur, GOST Graphite, GOST I-20 industrial oil, GOST Carbon black, GOST Tetramethylthiuram disulphide, GOST Zinc oxide, GOST Mercaptobenzothiazole, GOST Boric acid polymethylene-p-triphenyl ester over 100% T/52 International Polymer Science and Technology, Vol. 40, No. 1, 2013

3 From Figure 2 it can be seen that the maximum increase in bending strength is achieved by introducing boric acid polymethylene-p-triphenyl ester in a quantity of 5 wt% into an asbestos-free composite based on SKI-3 polyisoprene and SKD polybutadiene rubber. Here, the bending strength at the moment of failure increases at a curing temperature of 230 C from 17.9 to 31.1 MPa (by 70%), at 220 C from 16.7 to 26.7 MPa (by 60%), and at 210 C from 16.4 to 22.9 MPa (by 40%) in relation to the model composite. Reduction or increase in the content of modifier additive leads to a proportional reduction in the strength of the composite material. A model composite with maximum bending strength was selected for linear wear tests on a 2070 SMT 1 machine, the friction pair being an St45 steel roller and a sheet of the tested polymer composite material. Abrasion was assessed from the area of the wear crater of the modified and base composite. For the base specimens, the average area of the wear crater S av amounted to mm 2, and for modified specimens (5 wt%) S av amounted to mm 2. Thus, the wear resistance of the modified composite is 25% higher than that of the base composite. To assess the effect achieved on the model composite, jointly with the Barnaul Mechanical Asbestos Goods Factory Open Joint Stock Company (OAO Bz ATI), investigations were conducted on standard brake linings BATI 231 (designed to ensure the necessary effectiveness of braking in brake units of MAZ-5440 lorries and MAZ-103 buses, TU ) and asbestos composites (a/c) (designed to ensure the necessary frictional force in the brake units of BelAZ and Mogilev Motor Factory lorries, TU ), the component composition of which is presented in Table 2. Modification was conducted by introducing 5 wt% polydispersed powder of boric acid polymethylene-ptriphenyl ester in excess of 100% of the composition indicated in Table 2. The modified composites were notated with the symbol M. Mixing of the components was carried out on a laboratory mixer of the type VN 400 3A by the following regime: 1. Compressed air pressure 4-6 atm 2. Rotor speed during mixing r/min 3. Current strength during mixing A 4. Temperature of preparation of mixture C 5. Charge 4.5 kg 6. Time of preparation of mixture min From the obtained mixtures, dense briquettes of certain shapes, dimensions, and weight were formed in coldforming compression moulds on SVI-250 hydraulic presses without heating. The briquettes were vulcanised by hot forming on SVI-500 hydraulic presses with electric heating. Briquettes of BATI 231 were formed at t = 200 C for 24 min, and briquettes of a/c at t = 190 C for 24 min. For the obtained composites, the Brinell hardness was determined. Tests showed that, with modification, the hardness of the finished products hardly differs from that of standard products and corresponds to the specifications (HB for BATI 231 and a/c ). The friction wear properties of the obtained articles were assessed from test results on a SIAM friction machine in accordance with VNIIATI procedure No F-90 Determination of the friction coefficient and abrasion intensity of friction articles on a laboratory friction machine of the SIAM type during cyclic interaction of friction surfaces with change in temperature from 50 to 350 C, a nominal pressure of 2 ± 0.02 MPa, a slip velocity of 10.5 m/s, and 90 braking cycles and are presented in Figure 3. From Figure 3 it can be seen that, at the initial stages, with a temperature in the friction zone of 50 C, the friction coefficient is equal to 0.6 for both specimens tested. For the base composite, further increase in temperature from 100 to 250 C leads to a practically linear reduction in Figure 3. The temperature dependence of the friction coefficient Table 2. Component composition of BATI 231 and brake linings Specimen Linear wear Wear intensity Friction coefficient f (mm) (10 12 m 3 /J) Average Stability (%) BATI BATI 231 M BATI BATI M Smithers Rapra Technology T/53

4 the friction coefficient to After this, the value of the coefficient stabilises and remains constant up to 350 C. In contrast to the base composite, for a specimen of modified composite, the high value of the friction coefficient is retained from the start of the tests up to 200 C and amounts roughly to Starting from 200 C to 300 C, there is a linear reduction in the friction coefficient to Here, the values of the friction coefficient at the control points for specimens of BATI 231 are times lower than for specimens of modified composite. It was not possible to heat specimens of BATI 231 M above 300 C, which, in our view, is due to increase in the specific heat of the composite by the addition of a boron-containing polymer having high heat resistance and consequently heat dispersal. The results of tests of asbestos-containing composites ( and M) on a SIAM machine are presented in Figure 4. From Figure 4 it can be seen that, at initial temperatures in the friction zone (up to 150 C), both for the base composite and for the modified composite, high stable values of the friction coefficient are retained. Here, specimens of M have a higher friction coefficient Figure 4. The temperature dependence of the friction coefficient value. It must be pointed out that, in contrast to asbestosfree composites, in tests of and M the number of braking cycles is not the same and amounts to 30 and 90 respectively, which is due to degradation of the base composite. With increase in temperature, the friction coefficients of both specimens tested have practically identical values. For specimens obtained, wear tests were also carried out on the SIAM machine, the results of which are presented in Table 3. From Table 3 it can be seen that the modified composites have twice the abrasion resistance. Furthermore, the intensity of wear of modified composites is twice as low as that of specimens of standard brake linings of BATI 231 and Thus, from the data presented it is evident that, with the addition of 5 wt% boric acid polymethylene-p-triphenyl ester, there is an increase in the strength characteristics of polymer friction composites based on rubbers. For the model composite, the maximum increase in bending strength amounts to 70%. In our view, on the one hand a considerable increase in strength occurs by interaction of the polymer modifier with sulphur, with the formation of a cured three-dimensional insoluble network, and on the other hand the presence in the composition of the polymer of a boron atom presupposes increase in interaction at the polymer-filler boundary by adhesion. Also, the formation of interpenetrating three-dimensional networks is not ruled out. The modification of standard brake linings leads to a considerable increase in wear resistance both of asbestoscontaining and of asbestos-free composites. Furthermore, modified composites have higher friction coefficient values and stable friction coefficient values with increase in temperature. It is important to point out that the given method of modification does not lead to any change in the manufacturing technology of the finished articles. Table 3. Wear and wear intensity of base and modified specimens of asbestos-free and asbestos-containing brake linings No. Components Content (wt%) BATI SKMS-30ARKM-15 butadiene methylstyrene rubber, GOST SKN-26SM butadiene acrylonitrile rubber, TU SFP-011L phenolic resin binder, TU Asbestos, GOST Baryte concentrate, GOST Carbon black, GOST Basalt wool, GOST Copper-containing filler Tetramethylthiuram disulphide, GOST Sulphur, GOST Mercaptobenzothiazole, GOST T/54 International Polymer Science and Technology, Vol. 40, No. 1, 2013

5 REFERENCES 1. Bratukhin A.G. and Sirotkin P.F., Materials of the Future and their Surprising Properties. Mashinostroenie, Moscow, 128 pp. (1995). 2. Andreeva A.V., Principles of the Physical Chemistry and Technology of Composites. IPRZhR, Moscow, 192 pp. (2001). 3. Hoefel H.B. et al., West German Patent (1974). C.A., 83:80240e (1975). 4. Lenskii M.A., Polyesters and boric acid polymethylene esters synthesis, structure, properties, application. Author s Abstract of Cand. Chem. Sci. Dissertation, Biisk, 20 pp. (2007). 5. Bartenev G.M., Strength and Mechanism of Failure of Polymers. Khimiya, Moscow, 280 pp. (1984) Smithers Rapra Technology T/55