The importance of dimensional stability in the tire carcass.

Reinforcement fibers with dimensionally stable properties - high modulus and low shrinkage - have made an important contribution to tire performance in recent years. Advances like improved cornering and handling, greater tire-curing uniformity, tread wear, prolonged endurance and better tire economics are the result of advances in dimensional stability.

For these reasons, improvements in modulus and shrinkage in tire yarn have been the focus of fiber suppliers' R&D in the past 15 years. Table 1 shows just how important dimensional stability is to a tire's performance, appearance and cost.

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The carcass of a pneumatic tire is responsible for the transmission of all forces that pass between a vehicle and the road surface over which it is driven. Factors such as ride comfort, braking ability, lateral/longitudinal acceleration and tire durability are intimately linked to the specification and performance of the carcass. Small changes in the construction of the carcass can have significant effects on the performance of the tire/vehicle combination. The selection and application of materials for this application is therefore extremely important.

Characteristics of dimensional stability

The dimensional stability of a thermoplastic fiber reflects its inherent tensile modulus. Several mathematical definitions exist, but one of the most common ones is the algebraic sum of the fiber's EASL (elongation at specified load) and thermal shrinkage. These parameters are important to the material development engineer, and to the tire process engineer and designer, because they define the ability of a fiber to be treated to a desired modulus without reaching an undesirably high level of thermal shrinkage. The level of shrinkage associated with a defined modulus as measured by the EASL has a profound effect on the tiremaking process and the final tire characteristics. These effects are associated with the changes that occur as the cord shrinks during tire curing and subsequent cooling, and can result in varying effects on the tires:

* Variations in the tire size/dimensions (differences in section width and circumference);

* differences in tire uniformity (due to uneven shrinkage around the tire);

* tire durability (due to cord/compound movement and tire geometry);

* carcass splice depressions (sidewall indentations due to shifting of in-tire carcass cord modulus);

* variations in tire performance (due to in-tire cord properties).

The continually improving dimensional stability and related properties of the advanced tire cords provide a number of benefits over conventional polyester to tire makers and tire consumers. Among these benefits are:

* Longer tire life and theoretical greater fuel economy due to lower hysteresis and resulting lower heat generation;

* tire manufacturing economies resulting from the elimination of the need for the post-cure inflation (PCI) process in radial tire production;

* improved tire uniformity;

* reduced sidewall indentations at the carcass splices to meet auto industry tire quality standards.

Standard polyester (PET) vs. advanced polyester

The first polyester yams to be commercialized during the 1970s and the early 1980s for applications such as passenger tire carcass reinforcement were of a quality that we today call "regular" or "standard" polyester.

During those early days of polyester application development for tires, the carcass splice depression (sidewall indentation, or SWI) was one of the main weak points of polyester reinforcement. Sidewall indentations may occur in monoply tire sidewalls adjacent to the body splice region (figure 1a). The high (double) cord density in the splice overlap yields half the stress per cord compared to the non-splice cord. Consequently, the overlapped cords undergo less elongation during tire inflation and the result is a localized radial depression (indent) in the sidewall (figure 1b). The tire is not affected structurally, but this cosmetic aspect is important in many markets.

Other problems with standard polyester for tire building could be resolved by:

* Adjusting fabric treating conditions to modify the relative levels of shrinkage and modulus;

* adjusting the tire building and curing processes.

It was difficult to treat standard polyesters such that they met minimal technical requirements for passenger radial tires, and fiber technology had not yet begun to develop alternatives in the form of advanced polyesters with improved dimensional stability.

Fiber suppliers' R&D efforts finally paid off when the first advanced polyester became commercially available in the early 1980s. The missing ingredient, i.e., dimensional stability, had finally been captured.

Understanding advanced polyester's growth for tires

The magnitude of the dimensional stability changes achieved with PET is clearly shown by the product line in table 2. In comparison, 1X30 and 1X40 DSP (dimensionally stable polymer) fibers exhibit 20% and 50% higher intermediate modulus compared to first generation DSP fibers (1X90/93), but with 1X40 slightly deficient in strength. 1X40 was designed to possess the dimensional stability to displace rayon at reduced cord weight/ tire and still have the strength to permit this substitution. The newly emerging 1X50 has both high strength and dimensional stability.

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Today, tire makers in the U.S., Japan and Australia are using advanced polyesters for virtually all passenger radial tires. In Europe, where polyester is rapidly replacing rayon as the reinforcement of choice, every tire maker is either using or evaluating polyester fabrics for tire reinforcement. A similar pattern is unfolding in developing countries of Central Europe, Africa and the Middle East, where the utilization of polyester has become more apparent in the last 18 months.

Two main technical reasons explain why polyester is being specified or evaluated so often for tire reinforcement today:

* The technical performance of polyester has now reached the point where its use in radial passenger tires can significantly improve tire performance. In addition to passenger radials, other tire types such as light truck, agricultural and recreational vehicle tires are now being produced with advanced dimensionally stable polyester.

* Sufficient experience has been gained to support the use of polyester in tires previously thought to be outside of its design range, i.e. H, V and Z speed rated tires. Indeed, current generation polyester fibers, when combined with sufficiently well developed conversion, tire design and tire processing technology, are well suited for use in the full range of radial passenger tires.

Manufacturing of tires with advanced polyester cord

The evaluation of a new reinforcement material is always considered as a heavy task requiring large internal development resources and involving tire building and intensive tire testing (tire measurements of sidewall indentations, handling, tide, uniformity, plunger energy, high speed and endurance performance, etc.). However, a tire manufacturer can still be interested to evaluate a new reinforcement like advanced polyester for two main reasons:

* Introduction of a new tire line (radialization, tires for export markets, original equipment markets, etc.);

* substitution of the current reinforcing material (rayon or nylon) to decrease the cost and/or increase the performance competitiveness of its tires.

In order to gain the maximum benefit from the available technology it is important that tire companies work closely with the fiber makers from the first stages of tire design in order to ensure that the correct decisions regarding fiber and fiber characteristic choices are considered. Fiber companies have technical expertise and can offer valuable assistance for the makers when making decisions regarding fiber type, strength requirements, replacement ratios, fabric treating and adhesive systems and even offer the services of extensive laboratory facilities and technical personnel to assist in evaluating and testing activities.

First steps in evaluating a new polyester

The first contact with polyester is obviously through laboratory evaluation. While it is unlikely that this evaluation can eliminate the need for tire performance testing, it should be able to "screen" for an easier selection of potential cord candidates. Polyester treated cord samples covering a wide range of physical properties, twist levels and material types can be easily and quickly prepared on a laboratory single-end treating unit. This unit reproduces the exact treating conditions experienced by a tire cord in a majority of industrial dipping units.

Realistic projections of the change in properties from treated cord to cured tire and from cured tire to run tire can also be realized in those laboratory simulations as well as the assessment of the inherent fatigue and adhesion properties of the reinforcement.

These preliminary results should bring interesting insights to the tire design engineer on the potential improvements in tire performance he or she might expect when using polyester and enable a fair comparison with existing reinforcements.

Fiber substitution

When using advanced polyester reinforcement, one aspect of the fabric testing process becomes of primary importance - the measurement of the cord shrinkage. Inexpensive and standard apparatus exist and should be part of the standard equipment of the raw material testing laboratory.

In addition to the laboratory evaluation of the material itself, it is important to study precisely its compatibility with the neighboring rubber compound. Several factors in rubber are important in controlling the chemical degradation of polyester cord and the adhesion of polyester cord to rubber. The amines released from some accelerators during vulcanization might lead to this degrading phenomenon called aminolysis.

Carcass stocks which minimize or eliminate the presence of amines retain good performance in polyester tires in severe tests. These stocks contain vulcanization accelerators that do not liberate amines or additives. Rubber chemical suppliers can recommend the best suited accelerators when polyester reinforcement is being used.

Provided that the laboratory results are satisfactory, at this stage the definition of a treated fabric specification should be considered in light of the reasons why polyester reinforcement is being envisaged (new tire or material substitution).

The choice of the polyester yam type and linear density, construction and twist level, fabric characteristics and treated cord properties is made through extensive technical exchanges and at its best when partnership between the tire manufacturer and the fiber supplier exist.

When substituting rayon or nylon for polyester, cost and performance considerations should always be kept in mind. The actual substitution will heavily depend on:

* The application of the tire (passenger, light truck or heavy duty);

* the utilization in the tire (carcass or belt);

* the market (original equipment, replacement, exports);

* the performance/economy trade-off.

As an example, table 3 shows some of the theoretical trade offs that are possible in a normal monoply tire when substituting rayon with polyester in the carcass. The two conversion strategies considered here are equal strength and equal endcount. When considering equal endcount, the replacement of 1650/2 denier rayon by 1000/2, 1300/2 and 1500/2 denier polyester has been calculated. The conclusions to be drawn from this exercise are two fold:

* On an equal strength basis the weight of fiber in the tire carcass can be reduced by over 30%.

* The availability of multiple constructions to replace the rayon construction at equal endcounts allows significant variations in strength/weight to enable the tire manufacturer to obtain the desired balance between fiber usage and carcass strength.

Table 3 - rayon to polyester conversion properties

Yarn (Decitex)      Strength   Strength   Weight
                    per cord   in tire    in tire

Base - rayon (*)      100         100       100
1840/2 SII

1100/2 PET (1X30)      97         100        65
at equal strength

1100/2 PET (1X30)      97          97        62
at equal epdm(**)

1440/2 PET (1X30)     124         125        82
at equal epdm(**)

1670/2 PET (1X30)     138         143        94
at equal epdm(**)

(*) Index to which other constructions are compared. Weight are related to dry rayon base (**) ends per decimeter

A typical substitution scenario would give higher carcass strength and toughness (110-130%) and lower fiber weight (80-90%) for overall reduced fabric cost.

One important consideration when designing tires for today's original equipment applications is the ever increasing pressure from the vehicle manufacturers to reduce tire weight, improve uniformity and reduce rolling resistance.

Less material - eliminating one body ply in the carcass

The significant tenacity advantage that polyester has over rayon gives the tire designer an opportunity to completely eliminate one body ply from some tire designs while maintaining tire strength. This opportunity might be combined with the fact that the total amount of rubber per square meter deposited on the fabric during the calendering process can be reduced substantially due to the difference in cord gauge between rayon and the polyester construction. However, the decision on the rubber gauge to be used still needs to be based on the compound viscosity and the tire building technology.

Apart from the close examination of the carcass compound, a first trial with advanced polyester tire cord does not usually necessitate any other heavy modification of the tire design critical parameters unless important endurance problems are already present. The optimization of these parameters will start as soon as the first plant trials have been completed and once the first results (indoor and outdoor) have been obtained.

One important aspect of this first exercise is obviously to become familiarized with the new reinforcement, from the calender to the ply cutting, from the tire building into the curing, and from the pulley wheel to the road (figure 2).

With the utilization of polyester, the potential to design tires with overall improved rolling resistance, lower weight and better performance becomes apparent. In this way, polyester gives the tire designer the flexibility and tools which allow the manufacturer to meet die requirements of his customer, i.e. the vehicle maker.

References

[1.] "Tire cord and cord to rubber bonding," T. Takeyama, J. Matsui and M. Hijiri, in Mechanics of Pneumatic Tires, U.S. Department of Transportation, 1981.

[2.] "Dimensionally stable PET fibers for tire reinforcement, "P.B. Rim and C.J. Nelson, Rubber World, Vol. 2, May 1991.

[3.] "An approach to laboratory fatigue testing on tire cords," A.L. Promislow, ISIFM Meeting, May 1991.

[4.] "PET substitution for rayon in European radial passenger tires, "P.B. Rim, C.J. Nelson and D.S. Liu, Kautschuk Gummi Kunstoffe., Vol. 45,1992, p. 268.

[5.] "The behavior and performance of PET reinforced passenger tires," Kautschuk Gummi Kunstoffe., Vol. 45, 1993, p. 297.

[6.] "Polyester breakthroughs in fiber performance, " G.S. Rogowski, Kautschuk Gummi Kunstoffe, April 1994, pp. 276-278.

[7.] "Polyester fiber for radial tire reinforcement, " J.D. Burrows, presented at the conference on the New Direction in the Rubber Industry, Zlin, Czech Republic, 22-23 November 1994.

[8.] "How polyester is changing the European tire-making business," J.D. Burrows, presented at TyreTech 95, Turin, October 1995.