Profile of premium rayon reinforcement for high performance tires. High-Tenacity Rayon Filament Yarn

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1 Profile of premium rayon reinforcement for high performance tires High-Tenacity Rayon Filament Yarn

2 Tough by Nature Rayon is defined as a chemical fiber based on renewable resources. This definition points out two fundamental aspects of rayon production: the renewable raw material base on the one hand and the defined industrial production process on the other. The result is a high-performance, renewable material which has well-defined properties and comprises a contemporary, sustainable product. Cordenka rayon, also referred to as high-tenacity multifilament viscose yarn, is a thermostable reinforcement fiber: it owes its resistance to heat and stress to its cellulose origin. Today, Cordenka yarns are applied mainly in the reinforcement of high-performance tires and hoses; it is here that they have proven their outstanding technical performance in safety-related components. Cordenka produces and sells some 32, metric tons of rayon yarns a year, and guarantees their consistent quality and secure supply to its customers. Carcass reinforcement The carcass is one of the most important parts of a radial tire, transferring all forces from the road surface to the rim, interacting with the belt package and providing flexible sidewalls. As the carcass greatly influences handling, comfort and safety, its reinforcement plays an essential role with respect to the quality of the tire and thus to its performance and durability. Each carcass reinforcing material, however, has its own physical properties and reacts differently to different influences. Cordenka, producer of premium rayon yarns for carcass reinforcement, tested and compared key properties in rayon (non-thermoplastic material based on cellulose from wood) and polyester (thermoplastic material based on oil). The results from this comparison, together with other information of possible importance for the application of rayon in tires, are suarized in this brochure. Rayon: built for lasting performance Rayon has been applied as reinforcing material for high quality tires since the nineteen forties. Due to its proven outstanding properties and lasting performance, rayon high tenacity yarns have not lost their attractiveness to the tire designer. Rayon s suitability for the reinforcement of high quality tires is not only apparent during the production process, but even more importantly, on the road and during the entire tire life - until the very last kilometer. 2 Dimensional stability Uniformity is a prerequisite for a high quality, high performance tire. Lasting properties are decisive for retaining the original optimal driving characteristics, also under heavy and enduring circumstances and after exposure to high temperatures. Dimensional stability, therefore is one of the basic requirements for carcass reinforcing material. Good dimensional stability includes low shrinkage and lasting high modulus (low elongation). High tensile modulus is required to keep the dimension of the pressurized tire and the depth of sidewall indentation at the splice within limits, and is also important for the ease of vehicle handling. The load/elongation behavior in the operational range (figure 1) strikingly shows the excellent, constant, dimensional stability of a standard rayon dipped cord. In the graphs the operational range is up to 2 of breaking strength. Comparison of the difference in elongation between 1 and 2 of the breaking load iediately indicates the indentation at an overlap splice of a carcass. Both at room temperature and at elevated temperatures rayon clearly outperforms polyester. 3

3 Creep Rayon stays far more constant over the time than polyester. Due its cellulose origin, the thermostable rayon has a significant lower creep. Low creep is necessary to prevent growth of the side wall indentation and to retain the initial tire dimensions. The latter is important because the contour affects the road tread contact area and pneumatic stiffness. The oil based thermoplastic polyester yarns, affected by heat and stress, show higher shrinkage, higher elongation and more creep. 4 Dynamic modulus The influence of the temperature on the dimensional stability (dynamic modulus - elongation during motion) can also be measured by a dynamical test. Figure 3 shows the results of this test carried out on rayon and polyester cords cycled between two load limits, with a frequency of 1Hz. During cycling the temperature was gradually increased. Whereas the polyester cord shows the typical behavior of a thermoplastic material with a quite abrupt decrease of modulus at the dynamic glass transition temperature, the thermostable rayon exhibits its much better and lasting modulus stability. Dynamic work loss (hysteresis, heat generation) Low dynamic work loss is required to achieve low rolling resistance and low temperature build-up in the tire. This is of particular importance under heavy and enduring driving circumstances. High temperatures accelerate degradation processes and may thus limit the tire s life. The same dynamic test as described above enables comparison of dynamic work loss - heat generation - of reinforcing materials. Figure 4 shows the significant effect of the higher work loss of the polyester cord on the heat build-up of a tire in use. This effect for rayon is negligible. 5

4 Cord requirements and characteristics The textile reinforcement of the carcass always consists of cords. Constructions of two or three yarns twisted around each other; 2-fold cords are more widely applied than 3-fold cords. 2-fold constructions allow manufacturing on modern and efficient direct cablers while these twisting machines cannot be used to make 3-fold constructions. Twisting yarns into cords is necessary to improve fatigue resistance. Twist level and properties Reinforcing cords, including rayon, are used with different twist levels. Increasing the twist level results in decrease of a cord s breaking strength as is shown in figure 6 for a 184 x 2 Cordenka 7 cord. The same holds for the modulus expressed in figure 7 as FASE 2 (Force at Specific Elongation). Adhesion The reactivity of the cellulose based rayon cords exceeds most other reinforcing materials. This allows the use of a relatively simple dipping process to reach the required adhesion to rubber. No highly reactive chemicals have to be added to the dip fluid to get the maximum possible adhesion in the strap peel test. Maximum adhesion according to this test is the adhesion achieved when the cords are fully covered with rubber. Figure 5 (also see picture) shows the excellent adhesion of a rayon cord to a rubber compound used by a tire manufacturer. Even testing at elevated temperatures keeps the rubber coverage at 1. This means that the decrease in separation force is caused by a decrease in shear strength of the rubber and not by adhesion failure. 6 7

5 Lasting performance of Cordenka rayon Fatigue resistance Fatigue resistance represents a very important property of any reinforcing material, allowing to predict the decrease in breaking strength during use of the tire. Obviously, the breaking strength should stay high enough throughout the entire tire life. Figure 8 shows the fatigue resistance tested under three different conditions according to the disc fatigue test. The test shows higher strength loss in accordance with growing compression and a lower twist level. If a tire designer can avoid carcass cords to come under unduly high compression, a high twist level is not required. It is then an advantage to use rather low twisted cords, reducing the loss in breaking strength and modulus compared to a high twisted cord. Comparison of the behavior of rayon and polyester in tires showed that rayon outperforms polyester with respect to the essential dimensional stability. Given its thermoplastic nature, polyester is incapable of performing better than thermostable rayon in this field. This also applies to the fact that, unlike rayon, polyester suffers from creep. In other words, once heated to a certain temperature, a polyester carcass will expand and will not return to its initial shape. Fatigue resistance at elevated temperatures Figure 9 shows the fatigue resistance of a Cordenka 7 cord, 244 x 2 (Z33/S33) tested according to the flex or shoeshine method. This testing device is more suitable for measuring the fatigue resistance at elevated temperature than the disc method. The test results confirm that temperature hardly affects the fatigue resistance of rayon. Cordenka product range Cordenka is supplied as yarn, cord, and cord fabric. The program includes Super 2 and Super 3 filament yarns in different titers and make-up. Generally the yarns are available from stock. Rayon s in-tire dimensional stability makes for efficient production of accurately sized tires safer monitoring by ensuring the permanence of properties and uniformity over the tire s life fine cornering behavior in the tires and better vehicle handling Cord distribution in tire cord fabrics Tire cord fabrics processed with highly regular distances between the cords significantly contribute to the tire uniformity. The cord distribution at the edges should be the same as in the remaining part of the fabric. By using modern weaving equipment, Cordenka succeeded in producing cord fabrics with an excellent even cord distribution. By adequate measures during dipping, the high regularity will be kept in the dipped fabric. 8 9

6 CORDENKA 61F Super 2 CORDENKA 7 Super 3 Yarn Specification Yarn Specification Linear Number of Linear Breaking Breaking Elongation EASF Force Tenacity at Break (45N) N mn/tex (Linear Number of Linear Breaking Breaking Elongation EASF Force Tenacity at Break (45N) N mn/tex Applications: Tires, Hoses, Strapping Applications: Tires, Hoses, Strapping Tensile testing is performed at a conditioned yarn with a twist of Z1 t/m Tensile testing is performed at a conditioned yarn with a twist of Z1 t/m Twisting and Make-Up Linear Number of Twist Package Tube Spool Spools Weight Dimension Ø per Weight L/Ø t/m Linear Number of Twist Package Tube Spool Spools Weight Dimension Ø per Weight L/Ø t/m Z 1 Z 1 Z 1 Package Type: Cyl. Bobbins (for all titers) Dimension: 126/15/11 cm /37 29/94 17/37 175/56 29/94 17/37 29/ Z 1 Z 1 Z /37 175/56 29/94 17/37 175/56 29/94 17/ Package Type: Cyl. Bobbins (for all titers) Dimension: 126/15/11 cm Average properties of cords from dippped CORDENKA fabrics CORDENKA 61F Cord construction Twist, nominal Diameter Linear density Breaking force* Elongation at break* EASF 45N* t/m N 184 x 2 48/ x 3 36/ CORDENKA x 2 36/ x 2 48/ x 3 38/ x 2 385/ *Tested oven-dry 1 Dipped cord fabrics Cordenka dipped cord fabrics are produced according to customer specifications and cover various cord constructions, end counts, and widths. The main cord properties of frequently produced fabrics are included in the table, which only present a brief selection of possible cord constructions. 11

7 CORDENKA GmbH & Co. KG Industrie Center Obernburg Obernburg Germany Phone Fax info@cordenka.com Reg. trademark Cordenka GmbH 1/213 The data contained in this sheet are for general purposes only. They reflect the state of knowledge at the time of publication. CORDENKA gives no guarantee in respect thereof and does not accept liability for deviations therefrom exhibited by their products.