New platform technologies for cap ply.
For high performance speed rated tires and for larger tire sizes, a cap ply is generally introduced to hold the heavy steel belt in place and to minimize tire growth when operated at high speeds.
Cap plies reduce tire growth by covering the steel belt with a restraining fabric, called an overlay or a cap ply. A cap ply is an industrial textile reinforcement, typically nylon 6,6, which is oriented in the tire circumferential direction and helps maintain the steel belt integrity under high speed and high stress levels. The goal is to keep the belt from separating from the body ply, thereby enhancing durability, reducing the risk of catastrophic failure and providing a higher speed rating.
Occasionally, when more advanced performance is required, ultra high modulus reinforcements such as PEN or aramid are used. In addition to the previously discussed functions, these materials are known to improve tire uniformity versus nylon, enhance handling and, in the case of PEN, provide good noise dissipation.
There is a strong trend in Europe towards high speed rated tires. Today, 60% of passenger car tires have a cap ply vs. 40% in 1995. Additionally, there is increased popularity for light trucks, sport utility vehicles and luxury sedans. Vehicles are faster and heavier, and greater stresses are being put on the tires. To cope with these stresses, more tires will be equipped with cap plies, and greater performance will be demanded from those cap plies.
Cap ply as a key part of tire technology
During vehicle operation at high speeds, centrifugal force will stretch the tire belting, causing the inextensible steel belt cords to rotate from their bias angle toward the tire circumferential direction. During this displacement, the sharp steel cord ends may cut the adjacent rubber and initiate cracks, which could grow during tire service and ultimately result in tire belt separation. This growth also alters the tire shape, resulting in a non-uniform tire footprint. Uneven contact between the tire tread and the road leads to accelerated treadwear in localized regions. Since tire tread life is often judged on its most worn element, tire life is shortened (refs. 1 and 2).
In an attempt to restrict this problematic tire growth, cap ply cords are placed along the tire circumference. The modulus of the cap ply cord is critical, as it provides resistance to tire growth in the belt region.
To enhance tire durability, the cap ply width extends beyond the edge of the steel belts and thus mitigates stress concentrations at the interface between the sharp steel cord ends and the adjacent rubber. This is typically the place of maximum shear stress and usually the location of the initial failures that can lead to a full or partial belt edge separation. From a reinforcement perspective, good fatigue and adhesion are also important.
Cap ply construction
The overlay can be made from a full width fabric splice over the belt. This traditional manufacturing method involves calendering a wide reinforcement fabric similar to those used in tire sidewalls. This calendered fabric is then wrapped around the circumference of the tire, over the steel belts. This method has the disadvantage of forming a splice or seam after wrapping one revolution around the tire. Sometimes, the calendered fabric is slit into strips which are only placed over and around the steel belt edges, a common place for failure to initiate.
The contemporary method of applying the cap ply to the tire is to spiral wrap narrow (typically 12 mm) rubber coated strips of reinforcement in the circumferential direction, around the steel belts. These strips can be overlapped slightly during tire wrapping and, if required, they can be double wrapped in regions of high stress like the belt edges.
Due to the elimination of the splice associated with wrapping a single sheet of calendered fabric, the newer spiral strip method provides better tire uniformity. The preparation of the strips consists of slitting calendered fabric or extruding rubber reinforced cords.
The extrusion process can produce multiple cap ply strips, simultaneously wound on individual spools, for direct loading and running on cap ply applicators located at the tire building machines. Each strip consists of compounded rubber extruded around pre-tensioned treated tire cord, cooled and wound onto its spool.
With the spiral wraps in place, the "green" tire is expanded in tire curing mold. If the cap ply cord modulus is too high, the cord can either pull-through its rubber coating, or result in tire non-uniformity. In both instances, the final tire is rejected. This problem is more pronounced with clamshell molds than with the more modern segmented molds, where there is less expansion of the green tire.
Cap ply reinforcement materials
Before being considered for use in cap plies, reinforcement must possess characteristics, which make it compatible with the tire manufacturing and curing process. For example, the reinforcement must exhibit a melting point higher than the rubber curing temperature, good resistance to rubber chemicals and good thermo-chemical stability.
As evidenced by its long established performance in sidewalls, the PET based new technology cord can be expected to perform well in these categories.
Nylon 6,6 has been the primary overlay reinforcement because it provides a combination of reasonable cost, adequate adhesion and a modulus that is high enough for good tire performance and low enough for good tire manufacturability.
One of the features of nylon 6,6 that is often cited is high retractive force at the tire curing temperature of around 180[degrees]C. High retractive force will aid the cap ply in encapsulating and shrinking around the steel belt, a feature that may be valuable at the belt edge.
The new material is capable of achieving retractive force values comparable to nylon 6,6, with the added benefit of lower overall shrinkage levels for better tire uniformity.
The ever-increasing performance requirements of tomorrow's tires will challenge nylon's capabilities in a number of areas. First, nylon flat-spots because its glass transition temperature is below room temperature at the residual moisture levels in the tire. Second, it has an unfavorable balance between modulus and shrinkage at elevated temperature. Unless the temperatures in the tire curing press are uniform around the tire circumference, cord shrinkage may be greater in one tire section versus another. Since cord modulus goes down as the cord shrinks, one tire section will have a different stiffness than another. This results in greater radial-force variations during tire uniformity testing and a "bumpier" ride during tire service. Finally, while nylon cap plies do suppress belt edge separations, they still can occur during high speed testing.
The "ideal" cap ply material, then, would have:
* Sufficient modulus to restrain growth and eliminate belt edge separation;
* superior dimensional stability to resolve flat-spotting and tire non-uniformity; and
* still retain advantages in cost, adhesion and manufacturability.
A successful candidate will have to respond to the changing tire industry conditions delineated in the preceding section and will likely have to also increase tire safety margins.
A new polyester reinforcement for cap plies
Honeywell Performance Fibers has developed a new reinforcement with features specifically designed to enhance the performance of tires with cap plies. Incorporated into this reinforcement are advanced dimensional stability and a new engineered adhesion technology.
The advanced dimensional stability provides high modulus and low shrinkage, the same features which have made this material valuable as sidewall reinforcement. The engineered adhesion is a new proprietary feature that provides a significant improvement in adhesion to rubber.
The first steps involve wrapping the cap ply material around the building drum, under tension. Nylon undergoes significant stress relaxation, whereas this new cord retains its tensioning. The green tire is then placed in a curing press where it is expanded and heated to curing temperature. The new technology cord was tailored so that under the normal conditions of vulcanization (ASTM, 10 minutes at 177[degrees]C) it will have a similar restraining tension as nylon. Therefore, this cord does not increase the risk of displacing the hot rubber and causing tire non-uniformity.
The tire is then released and cooled to room temperature. The tension during tire service can be simulated by measuring in a shrinkage tester the total retractive force at 80[degrees]C and then 25[degrees]C to represent high-speed operation and parking. Figure 1 confirms a 70% superior restraining force of the new technology material at 80[degrees]C.
It is believed that adhesion is the "missing link" for successful use of polyester in cap plies. With this step change in adhesion, the inherent dimensional stability advantages of advanced polyesters can be utilized to provide design flexibility and improve tire performance. Improved adhesion is also required to fully benefit from this superior restraining force.
Figure 2 shows that adhesion tests carried out on nylon and the new cords are giving equivalent results. Standard polyester cords have a lower adhesion, particularly with the hot strip adhesion test, which is the most discriminating among these three adhesion testing methods. In the future, dynamic adhesion test methods will be studied to better replicate the actual conditions in the cap ply area. The figure shows BT cord provides adhesion performance more indicative of nylon than standard polyester.
This combination of improved adhesion and restricted tire growth can reduce or eliminate catastrophic belt edge separations caused by belt pantographing, particularly at high speeds. This superiority can be used by the tire manufacturer, either for improving the tire performance or for reducing the weight of overlay.
Other potential benefits of this increased dimensional stability are better handling, reduced flat-spotting and overall better tire uniformity.
When tires are used at high speed, the need for retractive force should be proportional to the squared velocity of the car. However, with nylon, figure 3 shows that the overall retractive force remains extremely low until the overlay warms up. This actually indicates that the structural properties of the tire may change with time (until the equilibrium temperature is reached) while the car is used at constant speed.
[FIGURE 3 OMITTED]
The BT cord is three times less sensitive to temperature and has a significantly higher retractive force over the overall range of temperature experienced during tire usage.
Honeywell has launched a series of technical qualification programs to validate these technologies with tire manufacturers. Tire results are now available for high speed testing (growth and failure mode), endurance and flat spotting.
In all cases, the BT cord is compared to nylon in side by side testing. Quite remarkably, the advantages to be demonstrated are for tires made with 21% less total BT material vs. nylon 6,6, except for the growth measurements which were for tires built to equal reinforcement weights. Both tires were designed with a one- or two-ply rayon casing, a two-ply steel belt and a cap ply made from single end dipped cords.
BT reinforcement was evaluated on 195/65 R15 H as well as 225/45 WR17 tires. A single layer of BT 1100x1 construction was used in substitution of nylon 1400x1 for the H-rated 195/65 R15 H tire. Also, BT cord (1100x2) was evaluated in two layers, in comparison to nylon 1400x2 on the W-rated ultra high performance 225/45 R17 W tire.
Tire growth was measured at high speeds on an indoor test wheel. Figure 4 demonstrates that the higher stiffness and modulus of BT cords helped reduce tire growth by nearly 20%. This result is particularly important since it should also produce a superior steering precision in the curves, thereby improving handling.
[FIGURE 4 OMITTED]
Speed testing is carried out to verily the compliance of tires with the actual performance that the car can achieve. This test is performed under specific conditions of inflation pressure, load and speed increments using the H-rated 195/65 R15 tires. The evaluation was performed under the SAE J1561 procedure at 80% of maximum load and 93% of maximum inflation. This test was carried out up to tire failure. The actual speed of failure is interesting, but the failure mode is even more indicative of tire performance for this particular situation.
Here, the tire evaluations carried out demonstrated that BT cord could easily surpass the tire's specified speed rating. Figure 5 shows that none of the five tires reinforced with BT overlay failed by separation of the belt. whereas each of the tires with the nylon overlay failed from belt edge separation.
[FIGURE 5 OMITTED]
The failure mode totals add to more than five in the figure because some tires failed in two places. Since the test was run to failure, it is important to examine how the failure mode shifts when a higher modulus material is used in the overlay.
BT cord reinforced tires did experience some failures in the tread and shoulder region, however they were localized "chunk-outs" as opposed to the potentially catastrophic belt edge separation that affected all of the nylon tires in this test.
Tire endurance is traditionally related to reinforcement fatigue and adhesion. The endurance test reflects the time to failure for a tire running under severe conditions. In this test, five W-rated tires of type 225/45 R17 W were tested to failure under conditions of 113 km/h, 120% load and maximum inflation.
None of the tires failed before 35,000 km, which is superior to the expected requirements. Furthermore, none of the failures was from belt edge separation.
Flat spotting is a key uniformity issue that can occur when a cap ply softens during operation, then cools and solidifies when the vehicle is parked. The tire tread region in contact with the road continues to extend (creep). The consequence is an unpleasant rumble at the beginning of the next trip. With the increasing usage of low profile passenger tires and the new popularity of sport vehicles and heavier luxury sedans, the need for improved tire uniformity is magnified.
Higher dimensional stability, the resistance to length changes at high temperatures, is a key to addressing flat spotting.
With nylon 6,6, the cap ply operates above the glass transition temperature of the cord and can soften, leading to the flat-spotting effect. The stiffer BT cords will not soften as much under normal tire operating temperatures. Figure 6 shows the comparative variation of radial force during standard operating condition and after stationary cooling. BT reinforcement provides nearly a 25% uniformity improvement in flat spotting.
[FIGURE 6 OMITTED]
The higher modulus of BT overlay brings a new level of design freedom, particularly with the possibility of using smaller gauge cords and calendered rubber in the cap ply area.
Table 1 suggests that a textile reduction of 25% can be reached with BT reinforcement, even while maintaining a dramatic advantage in modulus. This can he accompanied by proportional reduction of calendering compound by up to 30%.
For years, the tire manufacturers have been designing and manufacturing tires with nylon cap ply. With the advances described in this article, BT cord is positioned to take the leading position in cap ply reinforcement of high and ultra high performance tires. BT reinforcement was designed to meet today's need for higher speed ratings, increased awareness to safety and the requirements of heavier vehicles.
Table 1 Tire 195/65 R15 Nylon 6,6 Cap ply construction 1400x2 Cap ply density EPDM 115 Cord gauge mm 0.466 Cap ply layout 3-2-3 Cap ply weight g/tire 131 Cord properties Lase 3 23 Tensile strength N 145 Shrinkage force N 4.0 Shrinkage % 5.8 Cap ply properties Fabric lase 3% daN/dm 637 Fabric strength daN/dm 4,002 Shrinkage force daN/dm 111 Tire 195/65 R15 NT Cap ply construction 1100x2 Cap ply density 125 Cord gauge 0.396 24% rubber saving Cap ply layout 2,5-1,5-2,5 31 % textile weight saving Cap ply weight 91 Cord properties Lase 3 Tensile strength 42 Shrinkage force 133 Shrinkage 7.4 Cap ply properties 4.1 Fabric lase 3% 56% higher modulus Fabric strength 993 Shrinkage force 3,159 58% higher shrinkage force Figure 1 - comparative cord tension during tire usage Tension (cN/text) Removing from High speed Tire cooling mold @ 25[degrees]C 80[degrees]C at 25[degrees]C NBT cord 2.0 2.2 2.0 Nylon 6,6 0.5 1.0 0.5 Higher residual retractive force to better control tire performance. Figure 2 - comparative adhesion levels Force (N) H USA cold USA hot Polyester 110 158 83 NBT cord 125 210 125 Nylon 6,6 120 205 108 The engineered dip and optimized treating technology offer competitive adhesion levels with standard rubber compounds.
(1.) "DSP replacing rayon in European tire market," P.B. Rim, C..L Nelson and D.S. Liu, Rubber World, October 1993.
(2.) "PET substitution for rayon in European radial passenger tires," from P.B. Rim, C.J. Nelson and D.S. Liu, Kautschuk and Gummi Kunststoffe, 45, 268, 1992.
(3.) F.J. Kovac, "Tire Technology," 5th edition, Goodyear Tire & Rubber Company.
(4.) "Tire cord process simulation and evaluation," Carl Draves and Leonard Skolnick, Kautschuk and Gummi Kunststoffe, 22, (10), pp. 561-565, 1967.
(5.) Testrite, Ltd., Woodfield Works, Old Lane, Halifax, England.
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|Comment:||New platform technologies for cap ply.|
|Author:||Brown, Donald L.|
|Date:||Sep 1, 2002|
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