Effect of twist level on tyre cord performance

Fibers and Polymers 2009, Vol.10, No.2, 221-225 DOI 10.1007/s12221-009-0221-7 Effect of Twist Level on Tyre Cord Performance Ayse Aytac*, Berrin Yilmaz1, and Veli Deniz Department of Chemical Engineering, Engineering Faculty, Kocaeli University, 41040 Kocaeli, Turkey KORDSA Global Industrial Yarn and Tyre Cord Fabric Manufacturing and Trading Inc., Kocaeli, Turkey 1 (Received June 6, 2007; Revised October 30, 2008; Accepted January 20, 2009) Abstract: The effect of twist level on the mechanical and thermal properties of nylon 66 and polyethylene terephthalate (PET) tyre cords has been studied. Effects of the twist on some critical cord properties such as tensile properties, shrinkage, shrink force, adhesion and fatigue have been evaluated. Breaking strength was decreased between 3.1 and 7.3 twist factor values, whereas breaking elongation was increased, on both nylon 66 and polyester cords. The tensile behaviour of high twist factor PET is similar to that of low twist factor nylon cords. This is an advantage for the possibility to get closer the properties of different materials by adjusting theirs twist factors. The shrinkage values increase with increasing twist factor, whereas shrinks force values decrease for greige nylons and polyester cords. Adhesion and fatigue resistance is increased with increasing twist factors. Keywords: Tyre, Cord, Mechanical property, Twist factor, Polyester, Nylon 66 Introduction tyre and can be changed according to the customer requests [8]. It is well known that cords in the tyre are continuously flexed, extended and compressed tyres while tyre is running. Therefore, the reinforcing materials must withstand to a large number of fatigue cycles keeping the initial properties [9]. Nkiwane and Mukhopadhyay investigated flex fatigue life nylon 66 tyre yarns and cords at different stress levels at standard atmospheric conditions [10]. Naskar et al. have reported the physico-mechanical and fatigue characteristic of polyester, nylon 6 and nylon 66 cords [1]. In this paper PET and Ny 66, mainly used cords types in the tyre industry, have been studied. Mechanical and thermal properties of nylon and polyester cords at different twist factors were investigated. Effect of the twist on some critical cord properties such as on the tensile strength, shrinkage, shrink force, adhesion and fatigue has been evaluated. Tyre is a composite matter of reinforcing materials and rubber compounds. The reinforcing materials used are mainly textile cords, steel cords and steel bead wire. These materials carry the major part of the structural load of the automobiles and should exhibit excellent dimensional stability, tensile and fatigue properties [1-3]. Cord fabric is the basic textile material, which is used to reinforce the pneumatic tyre. It consists of parallel warps and rare wefts. It can be produced with different type of yarns. The most widely offered yarns at the market are Polyamide 6 and 66, Polyester and Rayon. Due to the high cost of the complete developing and getting approval of new yarns for use in tyre application, polyester and nylon yarns are still predominantly used in the tyre industry worldwide [4,5]. The total synthetic fiber production in 2000 was 51.6 million tones in the world. Over two-thirds of the synthetic fiber produced was polyester. Approximately 4 million tones of nylon were produced in the world. In terms of production volume, nylon ranked third among the major fibers [6]. Various efforts have been made to investigate the effects of the twist level on the tyre cord performance. Fristsch had investigated rayon, PET, and Polyethylene Naphthalate (PEN) tyre reinforcement materials with different twisting conditions [7]. Hockenberger and Koral had investigated the effect of twist on the cord performance of PEN, dimensionally stable polyester and high tenacity polyester cords [3]. For optimum tyre performance, adequate adhesion between reinforcing materials and rubber compound is essential. Conventional resorcinol-formaldehyde-latex (RFL) adhesion systems provide required adhesion for nylon cords whereas polyester cords require the use of reactive chemicals [2]. RFL formulation depends on to compound type to be used in Material Experimental The commercial nylon 66 (940 dtex) and polyester (1100 dtex) greige yarns supplied from KORDSA Global (Turkey) were used for the study. Tyre cords were prepared on an industrial ring twister machine by twisting the yarns into two-ply construction with 200, 350 and 470 turns/m. Then, the same twist levels were applied to the single yarns to keep the filaments together. The dipped cords were dried in a series of oven to obtain the required rubber adhesion and the tensile properties. The greige cords were then treated with RFL adhesive solutions under controlled tension and temperature. Consequently, the excessive tension and the heat setting on the dipped cords were relaxed. and wound up to the rolls. Standard rubber compounds required for the fatigue and H-adhesion tests were obtained from KORDSA. *Corresponding author: aaytac@kocaeli.edu.tr 221 222 Fibers and Polymers 2009, Vol.10, No.2 Twist The twists (in turns per meter, tpm) of the greige and dipped cords were measured using a Zweigle twist tester (Germany), according to ASTM D885. Twist level of the cords were then transformed in to the twist factor (TF) values, in order to compare the properties of commercial tire cords with different linear densities. The twist factor was calculated by using the following equation (1). = ( /1000) (1) Where, : twist in turns per meter : linear density in tex t strips. The samples were vulcanized at 153 C and under the pressure of 3.2 MPa for 25 min. Then the products were cut into H-shaped samples. Static adhesion was evaluated by measuring cord pull out force in Instron tester 4502 at 25 C, (ASTM D4776). An average of 8 test runs has been reported for each type cord. o Method TF Ayse Aytac et al. LD 1/2 t LD Tensile Tests The tensile tests were performed by using Instron tester 4502, with cross head speed of 300 mm/min and gauge length of 254 mm according to ASTM D885. Averages of 5 test runs have been reported for each type cord. o Fatigue Test Fatigue properties of the cords were measured using Wallace test equipment. The equipment has five hubs and is capable of testing up to five specimens at once. The prepared test specimens are mounted to the hubs. Then the equipment runs up to 100 000 cycles. The number of cycles in each test is being recorded by a counter affixed to each rocker arm (ASTM D 430). Optical Microscopy Studies Olympus SZ6045 Model, Automatic Trinoculer Stereo Zoom Microscope, was used for optical microscopic analysis. Shrinkage and Shrink Force The hot shrinkage of the greige cords was measured using Testrite shrinkage tester at 177 C for 2 min. The pretension used for the thermal and free shrinkages measurements was 0.05 g/denier. An average of 3 test runs has been reported for each type cord. o H-adhesion Test The rubber strips were placed in the channels of a stainless steel die. Then dipped cords were placed on rubber strips. The ends of cords were stretched by 50 g weights. The cords were then covered completely by the second layer of rubber Results and Discussion Tensile Properties The optical microscope pictures of two ply nylon and polyester cords with three different twist levels are shown in the Figures 1-2 respectively. The number of turns per unit length increases with increasing twist factor. Therefore the amount of yarn per unit length increases. The filaments are tightened and the contact area of the plies in unit length increases as the number of twist increases. Since, the changes on the surfaces and the direction of the plies depend very Figure 1. Polyester cords with different twist levels (a) 200 tpm (b) 350 tpm, and (c) 470 tpm. Figure 2. Nylon cords with different twist levels (a) 200 tpm (b) 350 tpm, and (c) 470 tpm. Fibers and Polymers 2009, Vol.10, No.2 Effect of Twist Level on Tyre Cord Performance Stress-strain curves of nylon and polyester greige cords with different twist levels. Figure 3. much on the twist factor of the cord, the mechanical and the thermal properties of the cord should also be affected with increasing twist factors. Two different regions of the stress-strain curves can be easily distinguished (Figure 3). These regions are the elastic region including yield point and the strain-hardening region. It has been observed that the first region, i.e., elastic region did not change significantly with increasing twist factor. But, the yield points appear at lower loads as a result of decreasing modulus values. The modulus of nylon 66 cords is lower than that of polyester cord (see Figure 3 and Table 1). The initial modulus values of nylon and polyester cords decreases with increasing twist values. The change in the second region, on the other hand, is more obvious with increasing twist factor. The tensile behaviour of high twist factor PET cords approaches to that of low twist factor nylon cords. Tensile test results of greige cords Twist level Cord type Twist factor (tpm) 200 2.9 350 5.0 Nylon 66 470 6.7 200 3.1 350 5.4 Polyester 470 7.3 223 This result gives us the possibility to get closer to the properties of different materials by adjusting theirs twist factors. As it can be seen from Figure 3, stress-strain behaviour of nylon cords with 200 tpm approached to that of the polyester cord with 470 tpm. In other words, stress-strain behaviour of two different cords, but at different twist factors is similar. The tensile test results of the greige cords are given in Table 1. It has been observed that the breaking strength was decreased and breaking elongation was increased for twist factor values between 3.1 and 7.3 in both types of greige cords. It is well known that when the twist is applied to any textile yarn the breaking strength of the yarn increases initially up to an optimum twist level, and then decreases independent type materials. As twist increases, the helix angle (between cord axis and filament axis) of cord increases. This is the reason of the fact that cord with higher twist level has low breaking strength but high fatigue resistance. Therefore, in order to obtain better fatigue resistance and breaking strength, twist level of tire cord is kept in a certain range. It is known that twist also affects the breaking energy. Breaking energy, measured as the area under the stress-strain curve, is greater for nylon 66 than that of PET. This property provides greater resistance to impacts from road hazards such as rocks, curbs, debris or potholes. It has been also observed that breaking energy was increased with increasing twist factors for both nylon and polyester cords. The tensile test results of the dipped cords are given in Table 2. The tensile strengths of the cords are slightly decreased respect to that of greige cords values. Dipping and treatment operations cause also reduction in the elongation at break values of dipped cords. For that reason, breaking Table 1. Tensile and adhesion test results of dipped cords Breaking strength Cord type Twist factor (N) 2.9 151 5.0 147 Nylon 66 6.7 146 3.1 152 5.4 145 Polyester 7.3 137 Breaking strength (N) 156 155 152 151 145 139 Breaking elongation (%) 19.1 22.4 26.1 11.5 14.3 16.8 Breaking energy (Joule) 3.91 4.57 5.11 2.72 3.34 3.35 Initial modulus (N/mm) 5.9 5.1 4.2 6.4 5.4 4.5 Breaking elongation (%) 19.4 21.9 22.0 12.7 13.5 13.9 Breaking energy (joule) 3.77 4.33 3.88 2.68 2.73 2.62 Initial modulus (N/mm) 5.04 4.50 4.15 5.66 5.07 4.61 H-adhesion (N) 107 123 121 102 93 112 Table 2. 224 Fibers and Polymers 2009, Vol.10, No.2 Ayse Aytac et al. energy of the dipped cords is lower than that of greige cords. Adhesion Properties During the tyre cord dipping processing, an adhesive is applied and fabric is treated under the controlled conditions of time, temperature and tension (parameters called traditionally as 3T). An adhesive- that is RFL- is loaded to fabric in order to adhere the fabric and tyre compound in the tyre manufacturing process. The fabric is passed through a dip solution tank, and then is dried in an oven. The treatment parameters are very critical for getting optimum rubber compound-cord adhesion. The effect of twist on adhesion was also studied for both nylon and polyester tyre cords. The results of H-adhesion test are given in Table 2. An increase in cords’ surface area due to the increase in twist factor is thought to be responsible for better adhesion to rubber. Shrinkage and Shrink Force A comparison of shrinkage-shrink force values of greige and dipped cords are shown in Table 3. It was observed that nylon 66 tyre cords have higher shrinkage than that of the polyester tyre cords. As the twist increases, linear density, i.e. the weight of material in unit length increases (Table 3). Therefore, shrinkage values of nylon and polyester cords are increased with increasing twist factors. It has also been observed that shrinkage values of dipped cords are lower than those of greige cords for both nylon and polyester (Figure 4). However, if different dipping conditions are applied, these values can be changed as shrinkage and shrink force values are controlled by different dipping conditions. Shrinkage and shrink force test results Linear density Cord type Twist factor (tex) 2.9 192 5.0 198 Nylon 66 6.7 204 3.1 226 5.4 235 Polyester 7.3 242 Figure 4. cords. Changes in the shrinkage values for nylon and polyester Low shrinkage value of polyester is an advantageous for the tyre production process. It is known that shrinkage force is a combination of pretension force and the force that is developed in the specimen as a result of heating. Shrinkage force increases with total shrinkage for single fiber or untwisted yarn. But this relationship becomes invalid with the increasing twist factor due to the increase in the helix angle. In this study, it was observed that shrinkage force values were decreased with increasing twist factors for all greige cords and dipped nylon cords, but was increased slightly for dipped polyester cords. Fatigue Resistance The flex fatigue resistance test results for dipped nylon Table 3. Greige cords Shrinkage (%) Shrink force (N) 6.7 5.14 7.6 4.71 8.6 4.13 4.8 5.06 5.3 4.58 5.7 3.72 At different twist factor fatigue resistance for nylon and polyester cords Adhesion (N) Cord type Twist factor Unflexed sample Flexed sample (N) %Residual adhesion (N) (100 000 cycle ) 2.9 205 181 88 5.0 307 256 83 Polyester 6.7 299 248 83 3.1 376 300 80 5.4 363 358 97 Nylon 66 7.3 370 328 89 Dipped cords Shrinkage (%) Shrink force (N) 2.8 3.23 3.0 3.00 3.6 3.07 1.2 0.87 1.4 1.77 1.8 2.29 Table 4. Breaking strength (N) Unflexed sample Flexed sample %Residual strength 141 147 137 117 145 140 67 92 110 113 144 137 48 63 80 97 99 98 Effect of Twist Level on Tyre Cord Performance Figure 5. Residual strength curve after fatigue test for nylon and polyester cords. and polyester tyre cords are given in Table 4. Three sets of fatigue test samples which are simulations of tyres were prepared for fatigue resistance measurements. Two sets of samples were flexed applying 100 000 cycles at test equipment. A set of sample is not flexed and is kept for comparison. The evaluation of the results has been made by comparing residual breaking strengths and adhesion values of flexed samples and unflexed samples. It has been observed that fatigue resistance was improved significantly with the increasing twist factors for polyester cords. The residual strength curves at three different twist factors are shown in Figure 5. It is clear that residual breaking load and adhesion values are increased with the increasing twist factors for the polyester cords. On the other hands, no changes are observed in the residual breaking strengths and adhesion values for nylon 66 tyre cords. The authors have concluded that fatigue resistance of Ny 66 cords is better than that of polyester cords for the different twist factors studied. The higher cycles, are required for nylon 66 tyre cords to obtain deteriorations. It has been observed that although breaking strength was decreased with the increasing twist factors, an increased was recorded for fatigue resistance. The fibers with lower twist factors are subjected to the destructive interactions during the fatigue testing due to their low extensibility. When the twist factor is increased, the fiber extensibility increases as well, as a function of increased helix angle of yarns. This results in, the protection of fibers from any kind of deformation. Additionally, tyre cords act like a spring at the higher twist factor and consequently can flex. Therefore the fatigue resistance is improved when twist factor of the cord is increased. Conclusion Twist is a parameter affecting directly the tensile and Fibers and Polymers 2009, Vol.10, No.2 225 indirectly the thermal properties of tyre cords. Breaking strength was decreased between 3.1 and 7.3 twist factor values, whereas breaking elongation was increased, for both nylon 66 and polyester cords. Initial modulus values of nylon and polyester cords were decreased with increasing twist factor. Breaking energy was increased with increasing twist factor of nylon cords and polyester cords. The adhesion increases for both nylon and polyester cords with increasing twist factor. Shrinkage values of greige nylon and polyester cords were increased with increasing twist factors. Polyester cords have very low shrinkages that are advantageous for the tyre production process. Increasing twist factor of polyester cords significantly improved the fatigue resistance. Nylon 66 tyre cords showed excellent fatigue resistance. Acknowledgement The authors are grateful to KORDSA GLOBAL Industrial Yarn and Tyre Cord Fabric Manufacturing and Trading Inc. of Turkey for their support and the permission to use of their laboratories. References 1. A. K. Naskar, A. K. Mukherjee, and R. Mukhopadhyay, Polym. Deg. Stabil., 83, 173 (2004). 2. R. S. Bhakuni, G. W. Rye, and S. J. Domchick, “Adhesive and Processing Concepts for Tyre Reinforcing Materials”, ASTM Symposium on Tyre Reinforcement and Tyre Performance, Ohio, 1978. 3. A. S. Hockenberger and S. Koral, Ind. J. Fibre Text. Res., 29, 19 (2004). 4. A. M. Azizo o lu, “Developments in the Cord Fabric Industry”, Tyre Technology International, 1995. 5. W. L. Jing, J. Appl. Polym. Sci., 95, 859 (2005). 6. J. E. Mclntyre, “Synthetic Fibres: Nylon, Polyester, Acrylic, Polyolefin”, CRC Press, England, 2005. 7. J. F. 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