Reinforcing materials for tyres and their treatment

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1 Kauchuk i Rezina, No. 2, 2001, pp Reinforcing materials for tyres and their treatment I. L. Shmurak, V. Ya. Genin, T. V. Kuleikina and Z. P. Podkopaeva* Scientific Research Institute of the Tyre Industry (NIIShP) State Unitary Enterprise, Moscow Selected from International Polymer Science and Technology, 28, No. 7, 2001, reference KR 01/02/22; transl. serial no The principal design of car, lorry, and bus tyres is the radial design. The breaker of these tyres must ensure dimensional stability, high impact strength and puncture resistance, and a low level of rolling losses; the carcass must possess high durability and creep resistance and ensure comfort of travel. These requirements can be satisfied by using metal cord in the breaker: in the multiply carcass of lorry and bus tyres, depending on their size and designation nylon-6 or nylon-66 cord; in a singleply carcass metal cord or cord of aromatic polyamide [1]; in the carcass of car tyres nylon-66, polyester, or viscose cord. For cord used in the carcass of high-speed car tyres, it is particularly important to combine high modulus and fatigue strength and low shrinkage. Of the modern reinforcing materials, polyester cord corresponds most to this requirement [2]. In recent years, in the United States there has been a sharp reduction in the consumption of viscose cord (Fig. 1), trends have begun to show of a reduction in consumption of polyamide cord, and by the year 2000 an increase was noted in the consumption of polyester cord and metal cord. The same trend is being observed in Western Europe and Japan. It appears that the era of viscose cord has ended, and the leading companies producing this material are ceasing developments associated with improving its properties and in individual cases are even shutting down the production of viscose cord [3]. Nonetheless, viscose cord of super-3 fibre, which has a higher modulus and lower shrinkage than polyester cord, continues to be used in the carcass of radial car tyres [4]. The proportion of aramide cord in the total volume of consumption of reinforcing materials is small, and there is no trend towards any increase in it. This is due to the high cost of this material, although, according to data of the company Akzo, the reduction in the mass of tyres for trailers by replacing metal cord in the single-ply carcass with aramide cord, and for some designs also by partial replacement in the breaker, and consequently the reduction in fuel consumption, can compensate for the increased cost of tyres with aramide cord [1]. In the former USSR, the expediency was shown of replacing metal cord with aramide cord in the breaker of 8.25R20 and 7.50R20 tyres [5, p.23], but the break-up of the USSR and the state of the economy in the CIS countries did not enable Russian aramide cord to be introduced into production. Improvement in the consumer properties of textile cord can be achieved as a result of improvement in the existing Fig. 1 Consumption of viscose (A), polyamide (B), polyester (C), and aramide (D) cord and metal cord (E) in USA in 1970 (a), 1980 (2), 1990 (3), and 2000 (4) * T. A. Serebrier, N. V. Kropin, N. V. Litvinova, R. N. Mitropol skaya, and E. A. Sal nikova took part in the work T/20 International Polymer Science and Technology, Vol. 28, No. 12, 2001

2 fibre-forming polymers. At the Grodno Khimvolokno Production Association (Belarus ), a unit of the Zimmer Ritter company has been assimilated, with combined forming and stretching, making it possible to produce nylon-6 fibre with a strength of the order of 90 cn/tex [6]. This makes it possible to increase the breaking load of strands of 187 tex x 1 x 2 structure from to 295 N, or to reduce the linear density of the cord with a breaking load of N from 187 tex x 1 x 2 to tex x 1x 2, and the thickness of the strands from 0.70 to mm. As a result of the reduction in linear density, a saving of cord is achieved and there is a reduction in the rubber mix consumption and consequently in the tyre mass. The use of high-strength nylon-6 cord of 187 tex x 1 x 2 structure (cord 30 KNTS) instead of standard-production cord of similar structure in lorry tyres will make it possible to reduce the number of plies in the tyre carcass and to obtain, on this account, a considerable saving of cord and rubber with a considerable reduction in tyre mass. The use of high-strength cord of traditional structure (187 tex x 1 x 2) is much more effective than the use of cord with a strand strength of 300 N but a 210 (or 230) tex x 1 x 2 structure. Besides the higher breaking load of cord produced on the Zimmer Ritter unit, the coefficients of variation for breaking load and elongation are 8 30% lower, the heat resistance is 6% higher, and the linear shrinkage is 5% lower, but the breaking elongation of high-strength cord is 7% higher. Consequently, to achieve the necessary level of properties of dipped and heat-treated cord, it is necessary to adjust the treatment schedules for the cord. Furthermore, cord produced by the classical method was treated with lubricants Teprem and Sintoks 20M, while cord produced on the Zimmer Ritter unit was treated with lubricants Filapan TS-26 and Limanol 35F, which ensure better processability but lower the rubber-to-cord bond strength, which requires an improvement in the adhesion properties of the dipping compositions. On the basis of the real situation in Russia as regards reinforcing materials for car tyres, the most promising direction is the organisation of industrial plants for the manufacture of polyester cord with the use of technical adhesive polyester cord (111 tex) of the Khimvolokno Production Association (Mogilev, Belarus ). Work by NIIShP has shown that, according to data of bench and laboratory road tests, the main characteristics of tyres with polyester and nylon-66 cord in the carcass were equivalent, while, as regards vehicle stability at high speeds, especially on a wet road surface, tyres with polyester cord had an advantage. A decision was made concerning the fitting of VAZ automobiles, which are primarily high-speed vehicles, with tyres with polyester cord in the carcass. In the near future, an increase is expected in the consumption of polyester cord in Russia to 500 t/year. Abroad, the properties of polyester cord increased modulus and reduced shrinkage (HMLS or DSP cord)* were improved as a result of changing the production technology for fibres, making it possible to ensure the optimum combination of the degree of orientation of polymer chains, the size of the amorphous and crystalline phases, and their volume ratio [2]. The main directions in improving the properties of metal cord are to use high-carbon and microalloyed steel, as a result of which the strength and fatigue strength are increased (HT metal cord ) and to improve the design of the metal cord and brass coating [7, p.10; 8 10]. The use of NT metal cord makes it possible, while retaining the strength reserve of the tyre components at the level of normal cord, to reduce the content of metal cord in the tyre and consequently to reduce its mass, rolling losses, and fuel consumption. The higher fatigue strength of NT wire opens up the possibility of switching to simplified designs with the wire diameter increased to mm, since this will not lead to a reduction in the fatigue strength of the metal cord. The special design of metal cord of the type, in which two interwound wires are twisted with two parallel unwound wires, and a simple design, in which the metal cord strand is produced by winding two or three elementary wires of increased diameter, ensure practically complete filling of the space between the wires by rubber mix. By way of examples, it is possible to cite special-design cord x 0.25NT, x 0.28NT, and x 0.30NT, and simple-design cord 1 x 3 x 0.28NT and 1 x 3 x 0.30NT [8,9]. Cord of simple design is now being used by tyre works of the CIS in the breaker of radial car tyres. Another means of retaining openness of the metal cord for the breaker of radial car tyres is the BETRU (BEkaert Total RUbber penetration) design [11]. For example, in the 1 x 4 metal cord structure, the one (or more) elementary wire does not have a round but an oval cross-section, as a result of which, after winding, the cross-section of the metal cord strand acquires the shape of a polygon, while very large forces are required to close up the microgaps formed in this case. This leads to deep inflow of rubber mix into the metal cord strand, the fatigue strength of which in the rubber cord specimen increases on account of the elimination of fretting. For the breaker of radial lorry tyres, it is recommended that metal cord of one-sided winding, 3 x 5 x 0.18 and 3 x 5 x 0.245, be used [8,9]. In breaking load, the former is comparable with 22L15 cord (7 x 3 x ) and the latter with 28L18 cord (7 x 4 x 0.18) and 29L18/15 cord (7 x 4 x ). However, the fatigue strength of cord of one-sided winding is higher on account of linear interstrand contacts, while the cost is lower. Metal cord 3 x 5 x 0.18 has been introduced into industrial * HMLS high-modulus, low-shrinkage; DSP dimensionally stable polyester. HT high-tenacity. International Polymer Science and Technology, Vol. 28, No. 12, 2001 T/21

3 production, while 3 x 5 x metal cord has passed industrial tests (with positive results) in the breaker of radial lorry tyres. The adhesion properties of metal cord depend on the integral and layer-by-layer composition, the thickness and defectiveness of the brass coating, and the presence on its surface of oxides and non-metallic impurities. The optimum copper content in the entire coating amounts to 60 68%, and in a surface layer of mm to 25 60% [12]. With a reduction in the copper content of the coating, the initial rubber-to-metal cord bond strength can be reduced or remain unchanged, but after ageing at elevated temperature and humidity it increases. This is due to a reduction in the electrical conductivity of the zinc oxide layer on the surface of the coating with decrease in the copper content, which prevents its depletion under the action of moisture and increased temperature, as a result of which the diffusion of copper and zinc ions and the formation of loose boundary layers of ZnO/Zn(OH) 2 and excess Cu x S slow down. The same outcome is brought about by the introduction of % cobalt or 2 4% nickel [13,14]. Industrial technology for applying a threecomponent coating to metal cord is now being introduced at the Belorussian Metallurgical Works. Reduction in the thickness of the brass coating increases the stability of rubber-to-metal cord adhesion, since in this case the diffusion of iron ions through the brass layer into the zinc oxide layer is ensured, the properties of which change in the same way as when the coating is alloyed with cobalt or nickel [15]. Excessive reduction in the thickness of the coating leads to a reduction in the rubberto-metal cord bond strength, which is due to stripping of the coating during drawing and to uncovering of the steel base. The optimum thickness of the brass coating is µm [7,p.137]. Change in the structure of consumption of textile tyre cord in recent years increase in the use of nylon-66 cord and the start of the use of polyester cord has required an improvement in the adhesion properties of the dipping compositions. In 1997, the production of butadiene methylvinylpyridine latex BMVP-10Kh was stopped, the use of which had ensured an acceptable level of adhesion properties of dipped nylon-66 and polyester cord. This led to the appearance of butadiene nitrileamide latex (BNA-22 produced by the SK-Prem er Open Joint Stock Company (OJSC), Yaroslavl ) and butadiene butyl acrylate methacrylamide latex (DBA-1 produced by SK- Prem er and the NII Yarsintez OJSC) [16]. At certain tyre works, use is made of latex BNS-5, which is a blend of latices BNA-52 and SKD-1s in a 1:1 mass ratio (in terms of solids content). At NIIShP, a dipping composition formulation has been developed using these latices, ensuring a bond strength of nylon-66 and polyester cord with rubber at the level of compositions containing latex BMVP-10Kh (Table 1). However, in adhesion properties these latices are inferior to butadiene styrene 2-vinylpyridine latex, for example Pliocord VP-107 (Goodyear Chemicals Europe). Furthermore, in the production of latices with methacrylamide units, for example BNA-52, it is possible for polymethacrylamide to get into the latex, reducing the stability of the latex and of adhesives containing it. The possibility was shown of eliminating methacrylamide from polymers of latices BNA-52 and DBA-1 by using an adhesive formulation with which amide groups are formed as a result of the hydrolysis of acrylonitrile units and the ammonolysis of butyl acrylate units at the stage of adhesive preparation. The rubber-to-metal cord bond strength is not reduced in this case by comparison with adhesives in which latices BNA-52 and DBA-1 are used [17]. Bond strengths of nylon-66 and polyester cord with rubber that are similar to the corresponding indices of imported butadiene styrene 2-vinylpyridine latex can be obtained by using in the adhesives latices with reactive nitrogen-containing groups in the polymer, for example latex BSA with amino groups in the polymer (the Kauchuk OJSC, Sterlitamak) (see Table 1). Table 1 Influence of type of latex in adhesive on bond strength of nylon-66 and polyester cord with rubber compound based on SKI-3 polyisoprene rubber Latex SKD-1s BMVP-10Kh BNA-52 DBA-1 BSA Pliocord VP-107 Reactive groups or units nylon-66 Bond strength by H method, N cord 25A Polyester cord 23 C 120 C 23 C 120 C Carboxyl, units of methacrylic acid Units of 2-methyl-5-vinylpyridine Nitrile, amide Amide, units of butyl acrylate Amine Units of 2-vinylpyridine T/22 International Polymer Science and Technology, Vol. 28, No. 12, 2001

4 Improvement in the process of heat treatment of polyamide and polyester cord can be achieved by optimising the treatment schedule and modernising the equipment. An improved heat treatment schedule for polyamide cord has been developed and introduced at a number of tyre works, and can easily be realised on existing KLK and LPK cord lines [18]. This schedule can be recommended for all types of nylon- 6 and nylon-66 cord, besides single-twist cord. According to data of the Nizhnekamskshina OJSC, heat treatment of 25KNTS cord by an improved schedule has no effect on the breaking load and elongation of cord of rubberised material but makes it possible to reduce its shrinkage from 7.2 to 6.0%, to increase stretching from 4.0 to 5.4%, and to reduce heat-treated cord waste. As a result of this, at the Voltair OJSC (Volzhsk) the coefficient of use of the area of the cord increased by It was shown that increase in the concentration of the dipping composition leads to an increase in rubber-topolyamide cord bond strength as a result of improvement in its wetting by the dipping composition. This improvement is indicated by a reduction in the wetting angle of model nylon-6 film with increase in the concentration of the composition (Fig. 2). Increase in the heat treatment temperature of polyester cord from 170 to 230 C leads to a 15 20% increase in its bond strength with rubber and roughly to a 1% reduction in its shrinkage. The positive influence of an increase in the heat treatment temperature on the adhesion properties and shrinkage was also established for nylon-66 cord. However, increase in the concentration of the dipping composition to over 15% on Russian cord lines causes the formation of adhesions on the dipped cord. This is due to the fact that removal of excess dipping composition from cord fabric after the dipping bath on Russian equipment is achieved by blowing with compressed air, in contrast to the foreign Litzler and Repiquet lines, where squeezing and a vacuum pump are used. The temperature in the heat treatment chambers of most Russian lines in the best case can be brought to C, and only in the upper part of the chambers. On foreign lines, heat treatment of polyamide and polyester cord is carried out at C, which is achieved as a result of heating of the cord material by a mixture of gas combustion products and air. On Russian cord lines, heating of the cord material is carried out with hot air. OOO Kontinent (Tambov) developed an LPTK line for the dipping and heat treatment of cord, in which use is made of squeezing and a vacuum pump to remove excess dipping composition from the cord and heating of cord in drying and heat treatment chambers using low-inertia electric heaters [19]. Such a heating method makes it possible to raise the heat treatment temperature to 250 C. On account of the smaller productivity compared with existing LPK and KLK lines, there is a considerable reduction in the overall size of the line and its mass. Such a line may prove to be promising for the production of radial car tyres, for which a large consumption of reinforcing materials is unnecessary. Modernisation of existing cord lines by the realisation of technical solutions for the removal of excess dipping composition from the cord and the heating of cord material in heat treatment chambers available in the LPTK line is also possible [19], which will also make it possible to produce cord whose quality corresponds to the advanced level. Thus, promising directions in the development of reinforcing materials for tyres are as follows: the use of high-strength polyamide cord, mainly of traditional structure, which makes it possible to reduce the number of carcass plies in lorry tyres; the use of polyester cord in the carcass of radial car tyres; the use of metal cord of high-alloy and microalloyed steel of special, simple, and compact designs with a brass coating containing 60 65% copper or alloyed with cobalt or nickel. For textile cord, adhesives based on latices containing amino groups are promising, as a temporary alternative for which it is possible to use latices with nitrile, acrylate, and amide groups in the polymer. Improvement in the parameters of cord heat treatment on existing equipment, its modernisation, and the creation of a new cord line will make it possible to improve the mechanical, processing, and adhesion properties of treated cord. Fig. 2 Influence of concentration of dipping composition on (1, 2) bond strength at 23 C (1) and 120 C (2) of nylon-6 cord of 187 tex x 1 x 2 structure with rubber compound based on SKI-3 polyisoprene rubber and (3) wetting angle of model nylon-6 film by dipping composition REFERENCES 1. B. Ielsma, Tire Technol. Int., 1994, pp B. Lang, Kautsch., Gummi, Kunstst., 39, No. 2, 1986, pp International Polymer Science and Technology, Vol. 28, No. 12, 2001 T/23

5 3. Eur. Rubb. J., 178, No. 1, 1966, p F. Elrink and E. Steyn, Tire Technol. Int., 2000, pp V. Ya. Genin et al., State and prospects of development of textile cord fabrics for tyre industry. Topical symposium, TsNIITEneftekhim, Moscow, 1990, 64 pp 6. A. A. Bogoslavskii, Symposium of Papers of 4th Rubber Industry. Present and Future (Moscow, 1997), NIIShP, Moscow, 1997, pp Yu. G. Alekseev and N. A. Kuvaldin, Metal cord for automobile tyres, Metallurgiya, Moscow, 1992, 192 pp 8. V. P. Fetisov et al., Abstracts of Papers of 6th Rubber Industry. From Materials to Products (Moscow, 1999), NIIShP, Moscow, 1999, pp A. S. Zheltkov, Abstracts of papers of 7th Moscow, 2000, pp B. A. Biryukov, Abstracts of papers of 7th Moscow, 2000, pp O. Arkens, Prostor, No. 10, 1997, pp V. Van Oi and V. Vining, Zhurn. Vses. Khim. Ob-va im. D. I. Mendeleeva, 31, No. 1, 1986, pp S. A. Matyukhin et al., Prom-st SK, Shin, RTI, No. 7, 1986, pp S. A. Matyukhin et al., Proizv-vo, Ispol z. Elast., No. 1, 1992, pp W. Van Ooij et al., Rubb. Chem. Technol., 54, No. 2, 1981, pp A. S. Doronin et al., Abstracts of papers of 6th Rubber Industry. From Materials to Products (Moscow, 1999), NIIShP, Moscow, 1999, pp I. L. Shmurak, Abstracts of papers of 7th Russian Scientific and Practical Rubber Workers Moscow, 2000, pp N. V. Kropina et al., Kauch. i Rezina, No. 1, 1990, pp I. L. Shmurak, Symposium of Papers of 11th Symposium on Problems of Tyres and Rubber Cord Composites (Moscow, 2000), NIIShP, Moscow, 2000, Vol. 2, pp (No date given) T/24 International Polymer Science and Technology, Vol. 28, No. 12, 2001

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