Co-relation between reinforcing textile properties and radial tire performance.

Since radial tires were introduced about 55 years ago, a series of developments has taken place. These developments were mainly in the area of:

* Product quality improvements resulting in better performance of tires; and

* upgrading manufacturing technology.

The product quality improvements were essentially driven by the emerging needs of the customers. The constant demand from car makers compelled the tire manufacturers to develop high performance tires suitable for new generation cars. The manufacturing technology also had to be upgraded continuously to produce high precision tires.

The developments in radial tire technology were primarily focused towards higher speed, safety at higher speeds, fuel efficiency and comfort. As a consequence of higher speed, heat generation/rolling loss also increases, which has an adverse impact on performance and durability of radial tires (ref. 1). Thus, reduction of heat generation/rolling loss was one of the key areas of the design and development of radial fires. Two of the most predominant indicators of the development of passenger radial tires are:

* Evolution of the high speed rated tires (table 1); and

* improvements in tire design to reduce rolling loss, which has a direct influence on improving fuel efficiency. The average rolling loss coefficient of a passenger radial tire was reduced by over 25% from the level of 12.6 N/ KN in 1975 to 9.3 N/KN in 1989 (ref. 2).

Reinforcing textiles have a significant role in the performance f passenger radial tires. Various tire characteristics that control the performance of passenger radial tires are strongly influenced by the attributes of the reinforcing textiles. These tire characteristics are dimensional stability, uniformity, rolling loss and durability.

Reinforcing textiles are used as carcass ply (both ply) and cap ply in passenger radial tires. Various reinforcing textiles. e.g., Nylon-6, Nylon-66, rayon and polyester (it refers to dimensionally stable polyester--DSP) are used in the manufacture of passenger radial tires. An attempt has been made to understand and present all of these major reinforcing textiles used in passenger radial tires and their role in the development of high-speed radial tires. Various aspects of this review, include the following:

* Characteristics of passenger radial tires and their linkage with the properties of the reinforcing textiles;

* comparison of various attributes of each of the commonly used reinforcing textiles and their relative advantages/ drawbacks:

* recent developments in tire reinforcements; and

* present and future trends.

Characteristics of radial fires and their linkage with the properties of reinforcing textiles

The properties of reinforcing textiles have significant influence on the performance of passenger radial fires and the manufacturing processes of radial tires. The relationship between tire characteristics/tire manufacturing processes and the properties of reinforcing textiles are given in table 2.

Dimensional stability

Dimensional stability of a tire is the resistance against deformation. A tire is susceptible to deform at higher speed and at elevated temperature. It is the most critical tire characteristic that influences the performance of the radial tires. Passenger radial tires are designed to run at 180 Km/hr. or above, and hence the tires need to always he dimensionally stable in the operating range of conditions for consistent performance.

"Flat spoiling" of a tire occurs due to poor dimensional stability. The mechanism of flat spotting of tires is described as when a passenger radial tire comes to rest after prolonged running, and the inside tire temperature may go up to 110[degrees]C. Reinforcing textiles at the footprint region of the tire get cooled under lesser stress than the textiles in the other parts of the tire (figure 1). This cause deformation of the tire at the footprint region due to differential shrinkage of the textiles present in this region compared to the other parts of the tire. When the tire starts moving again, the ride becomes a bumpy ride. In passenger radial tires, which run at higher speed, this non-uniformity can eventually result in service failure (refs. 3-5).

[FIGURE 1 OMITTED]

As described above, the reinforcing textile has a significant role in the dimensional stability of the tire. Two important properties of the reinforcing textiles which influence the dimensional stability of the radial tire are initial modulus and thermal shrinkage. Higher initial modulus and lower thermal shrinkage of the reinforcing textile provide better dimensional stability of the tire. These two properties of the reinforcing textiles are described in greater detail in the following sections.

Initial modulus

The initial modulus of a reinforcing textile is obtained from the slope of the initial portion of the stress strain curve generated in tensile testing. A reinforcing textile having higher initial modulus would elongate to a lesser extent under stress and thus provide more dimensional stability of the tire.

During its service condition, the deformation of tire normally varies between 5-8%. Thus, the initial modulus of the reinforcing textiles has the most significant role in the dimensional stability of radial tires.

Stress strain curves (figure 2) of various reinforcing textiles indicate that both Nylon 6 and Nylon 66 have an advantage of higher strength, but they suffer from the disadvantage of lower initial modulus (ref. 6).

[FIGURE 2 OMITTED]

Rayon stands out to be the first with respect to initial modulus. The decreasing order among various reinforcing textiles with respect to initial modulus is: Rayon > polyester > Nylon 66 > Nylon 6.

Dimensionally stable polyester (DSP) has an initial modulus that is higher than Nylon 6 and Nylon 66, but less than rayon. A series of developments has been taken place to increase initial modulus and reduce the shrinkage of polyester.

Compared with standard polyester, DSP is over 1.5 times higher in modulus and half in shrinkage value while maintaining the same level of tenacity and toughness. DSP is, in fact, tailor made to satisfy the needs of high speed passenger radial tires.

Although polyester has lower modulus than rayon, it has some other advantages. These will be discussed in the subsequent chapters.

Shrinkage

Reinforcing textiles have the property to shrink on heating. This is the result of release of the internal stress of the polymer molecules, which is induced at the yarn stage (spinning) during manufacturing of the reinforcing textile.

Due to the differences in the basic polymer molecules and the morphology of yarn, different reinforcing textiles shrink to a different extent on heating viz. Rayon < polyester < Nylon 66 < Nylon 6.

During the vulcanization process, passenger radial tires undergo thermal treatment at around 175[degrees]C. and during service the inside tire temperature goes to around 110[degrees]C upon prolonged running. The cords inside the tire tend to shrink under these conditions. Therefore, reinforcing textiles having lesser shrinkage cause lesser deformation of the tires, which means better dimensional stability of the tires.

Rayon is the preferred material with respect to shrinkage characteristics, followed by polyester, Nylon 66 and Nylon 6.

However, free shrinkage does not have relevance in tires. Shrinkage three is the more accurate and relevant parameter to evaluate the performance of reinforcing textiles.

Uniformity

Uniformity refers to the uniform force distribution on a tire during running conditions. Since passenger radial tires are designed for higher speeds, uniformity plays a crucial role for the consistent performance and durability of the tires.

A tire is a composite material consisting of various materials and components. Thus, the uniformity of a tire depends on the uniformity of weight distribution of its various inputs and the uniformity of properties of its various components. Reinforcing textiles have a significant role on uniformity of passenger radial tires.

Shrinkage force is the single most important property of the reinforcing textiles that influences uniformity of the passenger radial tires. Lower shrinkage force provides better uniformity of the tire.

Shrinkage force

Shrinkage force is the force generated within a rubber-textile composite when it is exposed to heat. As tires get heated upon prolonged running, the reinforcing textiles inside the tire tend to shrink. Since it is in composite form with a rubber compound, it cannot shrink freely. As a result, shrinkage force is developed within the composite, which tries to deform the tire. Therefore, shrinkage force plays a significant role on the uniformity of radial tires.

Quite often, free shrinkage values (testrite shrinkage) are used for comparison of thermal characteristics of various reinforcing textiles. Tire manufacturers normally specify "free shrinkage" (e.g., at 177[degrees]C, two minutes, 0.05 gpd pretension) as one of the properties of the reinforcing material. However, there is no relevance of free shrinkage of the reinforcing material either in the tire manufacturing process or during service. Shrinkage force is a more accurate and relevant parameter to evaluate the performance of reinforcing textiles (ref. 10). Free thermal shrinkage is not directly proportional to the shrinkage force of reinforcing textiles. As an example, Nylon 6 has considerably higher free shrinkage than Nylon 66, but their shrinkage forces are comparable, which is illustrated in figures 3-5 (refs. l0 and 11). (Note: LASE = load at specified elongation.)

[FIGURES 3-5 OMITTED]

Polyester has a distinct edge over Nylon 6 and Nylon 66 with respect to thermal characteristics because of its lower shrinkage force. Among various reinforcing textiles, rayon has the least shrinkage force, followed by polyester and nylon (both 6 and 66). Thus, tire manufacturers should consider specifying shrinkage force rather than free thermal shrinkage.

Rolling loss and heat generation

The tire and road may be considered a system that receives mechanical energy from outside and dispenses it in the form of mechanical energy and heat. Rolling loss may be defined as heat generated by the tire per unit distance travelled (ref. 12). The rolling loss has a very important role in tire performance, because it has a direct relationship with fuel efficiency (power loss) and heat generation inside the tire.

The inside temperature of a passenger radial tire goes up to 110[degrees]C on prolonged running. Heat generated inside the tire is one of the common causes of tire failures during service. Adhesion failure/separation in service is generally caused by excessive heat (ref. 12) generated inside the tires. This also causes degradation of reinforcing textiles and resulting fatigue/premature failure of the tires.

There are many causes of rolling loss/heat generation within the tire, including hysteresis of various tire components; tire non-uniformity: tire vibration: low inflation pressure: road curvature and roughness: slippage between road and tire: aerodynamic drag: etc.

Rolling loss itself is tin independent subject on which a lot of papers have been published in the last two decades. Laboratory experiments indicate that the reinforcing textile contributes around 40cA of the total rolling loss in a passenger tire (refs. 13 and 15). Thus, reinforcing textiles have a significant role in rolling loss and heat generation characteristics of passenger radial tires. The viscoelastic property of a polymeric material is responsible for heat generation during dynamic loading.

Dynamic property/loss tangent

Dynamic properties of a material are associated with dynamic loading during its service condition. During the running condition, tires are subjected to dynamic loading and unloading with each rotation of the tire, and thereby all components inside the tire are subjected to dynamic loading.

A reinforcing textile is neither a truly elastic material (stress strain curve does not follow Hooke's law) nor a viscous liquid. It has a property falling between an elastic and a viscous material, its "viscoelastic" property. The morphology of yarn, which consists of crystalline and amorphous regions (figure 6). is responsible for the viscoelastic behavior of the reinforcing textiles during dynamic loading.

[FIGURE 6 OMITTED]

The crystalline parts of the yarn morphology behave like an elastic material (time independent), whereas the amorphous regions behave like a viscous liquid (time dependent). When a cyclic load is imparted (say at a frequency of ~ 10 cycles per second), the crystalline part of the yarn responds immediately, whereas the amorphous part does not. As a result, there is always a phase lag between strain and stress. It is described as shown in figure 7a.

[FIGURE 7 OMITTED]

The phase lag between the dynamic strain ([epsilon]) and the stress ([sigma]) is [delta] (delta). The stress can be resolved into two components (figure 7b) including: [sigma] Cos [delta] which is in phase with the strain ([epsilon]). The modulus due to this is called "storage modulus."

E' = [sigma] Cos [delta]/[epsilon]

The other component of stress, i.e., [sigma] Sin [delta], lags the strain ([epsilon]) by 90[degrees]; it does not contribute to the strain. The modulus due to this is called "loss modulus."

E" = [sigma] Sin [delta]/[epsilon]

In each cycle of loading and unloading, the loss stress ([sigma] Sin [delta]) is converted into heat, which is the root cause of heat generation by a viscoelastic material during a dynamic loading cycle.

A dynamic mechanical analyzer (DMA) measures various dynamic properties, e.g., storage modulus, loss modulus and tan [delta] (loss tangent).

(Loss modulus/storage modulus) = E"/E' = [sigma] Sin [delta]/[sigma] Cos [delta] = tan [delta].

Tan [delta] can be measured over a wide range of temperatures (closed chamber) with a desired frequency and amplitude of strain. A "tan [delta] vs. temperature" curve is widely used to evaluate the viscoelastic property/heat generation characteristics of a polymeric material like a reinforcing textile. The tan [delta] of yarn/cord generally increases with an increase in temperature until it attains the peak. Tan [delta] values indicate the rate of heat generation of the material under dynamic loading. The temperature associated with the tan [delta] peak is the glass transition temperature (Tg) of the material. Therefore, a lower tan [delta] value and higher peak temperature (Tg) are the preferred characteristics of a reinforcing material, since these are associated with lower heat generation under dynamic conditions. Tan [delta] vs. temperature curves of nylon and polyester are shown in figure 8.

[FIGURE 8 OMITTED]

The tan [delta] behavior of Nylon 6, Nylon 66 and polyester over a range of temperatures indicates that:

* Both Nylon 6 and Nylon 66 have peak temperature at around 100[degrees]C (ref. 16).

* Above 110[degrees]C, the tan [delta] of Nylon 66 is more than Nylon 6, thus Nylon 66 generates more heat than Nylon 6 above 110[degrees]C. However, in a passenger radial tire, the performance of both Nylon 6 and 66 are comparable, since the peak temperature rise inside passenger radial tires is around 110[degrees]C.

* Polyester has the tan [delta] peak temperature at around 120[degrees]C. This is an advantage of polyester over Nylon 6 and Nylon 66 (ref. 6). The peak temperature difference of around 20[degrees]C makes polyester a superior material over nylon in passenger radial tires.

* Around 120-130[degrees]C, there is significant heat generation by polyester, which leads to a sudden increase in tire temperature (ref. 13). However, this temperature is beyond the operating range of passenger radial tires; and hence, it does not affect the performance of polyester in passenger radial tires.

Durability

Durability, that is the service life of the tires, is the outcome of the integrated performance of all components inside the tire. Of course, it is also affected by various other factors. such as road condition, overloading, inflation pressure, maintenance, etc.

The reinforcing textile has a significant role in durability of the tires. The two most important properties of reinforcing textiles that influence the durability of the tires are rupture energy and fatigue characteristics.

Rupture energy

Rupture energy is defined as the energy to break. It is measured from the area under the stress strain curve generated in tensile testing of the reinforcing textiles. Higher rupture energy of the reinforcing textiles is associated with higher plunger energy and longer tire life. From the stress strain curves (figure 2) of various reinforcing textiles, it is evident that Nylon 6 and Nylon 66 have the advantage of higher breaking strength and elongation at break, and thus higher rupture energy.

However, tires never deform up to the breaking elongation of the reinforcing textile at the time of failure. Reinforcing textiles inside the tires fail at a much lower elongation than the elongation at break determined in tensile testing. This failure is related to the fatigue properties of the reinforcing textiles, which is described below in greater detail.

Fatigue resistance

Fatigue resistance of the reinforcing material is the resistance against loss of strength due to cyclic loading and unloading. Tires undergo cyclic loading and unloading, which causes a progressive loss of tensile strength of the reinforcing material with mileage. The loss of cord strength is centered mainly in the flex zone (ref. 17) of the tires. The principal mechanism of fatigue failure is the progressive loss of cord strength due to filament rupture. Flexing causes cords to become weaker until the residual strength of the carcass is less than the tensile force in the tires; then the remaining filaments break abruptly, causing failure of the tires (ref. 18). Therefore, the fatigue resistance characteristics of the reinforcing textile have a strong influence on tire life/durability.

In a laboratory evaluation, the fatigue resistance is either expressed in terms of retention of strength alter flexing a certain number of cycles or as the number of flexing cycles required for failure of the cord in a rubber-cord composite.

Apart from the type of reinforcing textile, there are a few important factors which significantly influence fatigue performance of a reinforcing textile, including twist, dipping condition and adhesion.

The twist on the cord has the most significant role on fatigue resistance of a reinforcing textile. Studies on twist vs. fatigue of a reinforcing textile (ref. 19) indicate that fatigue improves with an increase in twist up to a certain extent, beyond which it decreases. The optimum twist level is different for different constructions of cord, which is higher for lower denier plies and vice versa. The general relationships of twist with fatigue resistance, greige cord tenacity and EASL (elongation at specified load) are shown in figure 9.

[FIGURE 9 OMITTED]

SCEF (shear-compression-elongation-flex) comparative fatigue studies (ref. 20) on various reinforcing textiles indicate (figure 10) that Nylon 6 and Nylon 66 are both superior to polyester with respect to fatigue life, and rayon has poor fatigue resistance compared to nylon and polyester.

[FIGURE 10 OMITTED]

Tire manufacturing process--post cure inflation

Post cure inflation (PCI) is the process to keep the tire under high pressure after curing and allows the tire to cool down to obtain a constant geometry of the tire. When a tire is released from the mold, the inside tire temperature is around 175[degrees]C. A high degree of force is developed inside the tire due to the shrinkage force of the carcass cords, which tries to deform the tire. PCI is applied to counter the deformation of the tire. Therefore, PCI has a significant influence on the uniformity of the tire, which is very critical for the performance of passenger radial tires.

The type of reinforcing textile used in the tire has a direct relationship with the PCI process in tire manufacturing. A reinforcing textile with lower shrinkage force requires less time on the PCI cycle. Generally, the PCI cycle allows the tires to cool down to a temperature that is close to the glass transition temperature (Tg) of the reinforcing textile used for the carcass ply. A few studies were conducted on cord tension during tire curing and PCI (ref. 21). These indicates that:

* PCI does not have much influence on the tensile strength of the cord inside the cured tire: however, it has a significant role on the cord modulus and the dimensions of the final tire. There is a significant increase in the modulus of the cord inside the tire that has undergone a PCI cycle compared to the tire without a PCI cycle. The result of an experiment on EASL measured on polyester cords from a cured tire (with and without PCI) is shown in figure 11.

[FIGURE 11 OMITTED]

* Tires made with rayon need little or no PCI because of its very low shrinkage/shrinkage force.

* A reinforcing material having a higher Tg needs less duration in the PCI cycle, which is a favorable situation in a tire manufacturing process. The temperature profile of carcass cords inside the tire during curing and a PCI cycle is shown in figure 12.

[FIGURE 12 OMITTED]

* Tires made with Polyester need a PCI process, but the duration required is less than the tires made with nylon, because polyester has higher Tg (by around 20[degrees]C) and lower shrinkage force than nylon.

* Tires made with Nylon 6/Nylon 66 require similar PCI treatment, since their Tg and shrinkage forces are comparable.

With respect to the PCI process in tire manufacturing, the order of performance of various reinforcing textiles is--rayon > polyester > Nylon 6 and Nylon 66.

Cap ply

A cap ply is made of a reinforcing textile, oriented in the tire circumferential direction over the belt of a radial tire. It helps to maintain the steel belt integrity at higher speeds. The role of the cap ply is to prevent the belt separation from the body ply, thereby enhancing tire durability and reducing the risk of catastrophic failure/accident.

The inextensible steel belt rotates from its bias angle towards the tire circumferential direction. The sharp steel cord edges may cut the adjacent rubber and initiate cracks. These cracks could grow during tire service and finally cause belt separation, resulting in catastrophic failure of the tire. The usage of cap ply prevents belt separation in a passenger radial tire.

Two major requirements of the cap ply materials are higher retractive force/shrinkage force and moderately high modulus.

For the carcass ply application, a higher shrinkage force of the reinforcing material is detrimental to tire performance, due to its adverse impact on dimensional stability. Cap ply is the only application where higher shrinkage force/retractive force of the reinforcing material is explored to retain the position of the belt at high speed. Nylon 66 is generally used for cap ply applications due to its higher shrinkage force. Many tire manufacturers use Nylon 6 since it has shrinkage force comparable to Nylon 66 and is cheaper. Generally, lower denier constructions, e.g., 840/2, 1,260/2, 1,260/1 are used for cap ply applications.

A newly developed polyester (ref. 23) is under evaluation for cap ply application in passenger radial tires. The new tailor-made polyester has higher shrinkage force, which is comparable to nylon. Moreover, its relatively high modulus gives polyester an edge over nylon, due to its lower flat spotting characteristic. However, very high modulus is not desired in cap ply applications, since the cord may pull through its rubber coating during green tire expansion in the curing press and result in tire non-uniformity.

In brief, Nylon 6 and Nylon 66 are the preferred materials for cap ply application. Polyester is being developed for this application as well. Therefore, cost is going to be a deciding factor for choosing the appropriate type of reinforcing textile for this application.

Recent developments

Reinforcing textiles for tire applications is an area of continuous development to meet the emerging needs of the new generation tires and also to reduce cost of the tires.

The recently developed polyethylene naphthalate (PEN) is under evaluation for carcass ply in radial tires (ref. 24). PEN cord is claimed to be superior to polyester, having modulus over three times higher with a similar range of specific gravity (1.36 against 1.38 of polyester). It has the potential to be used as bell in place of steel. There has always been a drive to replace steel with a lighter weight textile. Aramid could have been the right alternative, but its higher cost restricted its use in tires. PEN has good fatigue characteristics. It may find a place as belt for passenger radial tires in the future. Tires with PEN belts may have additional positive attributes of lower rolling loss due to lighter weight. PEN cord is also under evaluation as cap ply in passenger radial tires. However, its success as bell/carcass ply/cap ply in radial tires finally depends upon the cost competitiveness with respect to polyester, rayon and steel.

Present and future trends

Literature is available on the global consumption of various reinforcing textiles in tire applications as a whole. No authenticated data are available on the consumption of reinforcing textiles based on various categories of tires. The consumption trend of various reinforcing textiles for tire applications is shown in figure 13. The highlights of the trends are:

* Polyester is growing at a faster rate than nylon. Before 1996, the consumption of polyester was a little below 200 kta, and it grew by over 25% in the next two years. The growth is primarily in passenger radial tire applications. The major consumption of polyester is in North America, the Far East and West Europe.

* The usage of rayon is restricted mainly to West Europe. It is consumed primarily in very high-speed passenger radial tires.

* Bulk quantities of Nylon 6 and Nylon 66 are used in truck tire applications. Significant quantity is also consumed in radial tires as carcass ply material (LCV and passenger). Cap plies of radial tires are mostly made of Nylon 66 and Nylon 6. Newly developed polyester is under evaluation for cap ply applications.

* In India, polyester has grown at a fast pace during the last 3-4 years due to the revolution of passenger radial tires on Indian roads. In the passenger tire sector, presently radial tires contribute around 75% of the total market (~9 million tires/year) which is likely to cross 85% in the next three years. Polyester is the predominant reinforcing textile for the carcass ply of passenger radial tires. A few tire manufacturers also use Nylon 6 in the carcass ply. Nylon 6 is mostly used in cap ply applications of passenger radial tires in India.

* The future relies more on cost effectiveness of the reinforcing textiles, since the performance required is a kind of passing score that is to be satisfied first, without which it would not be even considered for the specific application.

Conclusion

Each reinforcing material has its own positive and negative attributes. There is no single reinforcing textile available having the best characteristics in all aspects.

Among four different types of reinforcing textiles, broadly two categories have emerged. The first group consists of rayon and polyester, which have superior dimensional stability, uniformity and rolling loss characteristics. These are the key characteristics for the performance of high speed passenger radial tires. The second group consists of Nylon 6 and Nylon 66, which have superior strength and fatigue characteristics over rayon and polyester. However, the attributes possessed by polyester and rayon are more critical for the application as carcass ply in high speed passenger radial tires.

An attempt has been made to give a relative ranking with respect to each attribute of the reinforcing material (table 3).

Overall, polyester is preferred to rayon primarily due to its better cost effectiveness. Polyester is cheaper than rayon on a weight basis. Moreover, polyester has higher strength and lesser density compared to rayon. Thus, tires made with polyester require less weight of reinforcing textiles than the tires made with rayon. This gives an additional advantage of better fuel efficiency of the tires made with polyester due to their lighter weight. For these reasons, polyester is gradually replacing rayon in carcass ply applications for high speed passenger radial tires due to the lower cost of textiles per tire (refs. 7-9).

Between Nylon 6 and Nylon 66, all attributes desired for passenger radial tires are comparable. However, Nylon 6 is cheaper than Nylon 66. Nylon is still used as carcass ply material in relatively slower speed rated passenger radial tires by many tire manufacturers, and this will continue as long as the cost of nylon per tire is cheaper than polyester.

Low shrinkage and increased modulus may be the areas of interest for future developments of nylon to sustain its position in radial tire applications. Benchmark studies (ref. 25) among various sources of Nylon 6 yarns produced all over the world revealed that a wide range of Nylon 6 yarns exists with respect to dimensional stability, shrinkage and other attributes. Nylon 6 yarns produced with a few technologies have significantly higher dimensional stability compared to other yarns. This may be explored further towards developments of Nylon 6 to meet desired needs of specific applications in passenger radial tires.

Nylon is the preferred material for cap ply applications in passenger radial tires, since it satisfies the specific needs of cap ply material. However, a tailor-made polyester has been developed for this application and is under evaluation.

The recently developed polyethylene naphthalate (PEN) is under evaluation, which has three times higher modulus than polyester. It has the potential to be used as the belt in place of steel. However, its success in passenger radial tire applications depends upon its cost competitiveness with respect to polyester, rayon and steel.

Table 1--speed rating of
passenger radial tires

 Tire      Limiting speed
rating        (Km/hr.)

S rated         180
T rated         190
H rated         210
V rated         240
Z rated      Above 240

Table 2--relationship between tire characteristics
and properties of reinforcing textiles

Tire characteristics/tire    Properties of             Remarks
  manufacturing process        reinforcing textiles

Dimensional stability        Initial modulus           Higher is better
                             Shrinkage                 Lower is better
Uniformity                   Shrinkage force           Lower is better
Rolling loss and heat        Loss tangent              Lower is better
  generation                 Tan [delta] peak          Higher is better
                               temperature
Tire life/durability         Breaking strength/        Higher is better
                               rupture energy
                             Fatigue resistance        Higher is better
Post cure inflation          Shrinkage force           Lower is better

Table 3--relative ranking of various reinforcing
materials

                                                         Relative
                                                          ranking
Tire characteristics/    Properties of
  need of tire             reinforcing                   Polyester
  manufacturers            textiles

Dimensional              Shrinkage force                    II
  stability              Modulus                            II
Uniformity               Shrinkage force                    II
Rolling resistance       Dynamic proper-                     I
                           ties (up to 110[degrees]C)
Durability               Toughness                          II
                         Fatigue                            II
PCI                      Shrinkage force                    II
Lower cost of            Strength and cost/                 II
  textile per tire         kg and density
Overall                                                      I

                            Relative ranking
Tire characteristics/
  need of tire           Rayon    Nylon    Nylon
  manufacturers                     6       66

Dimensional                I       III      III
  stability                I       III      III
Uniformity                 I       III      III
Rolling resistance         I       II       II

Durability                III       I        I
                          III       I        I
PCI                        I       III      III
Lower cost of             IV        I       III
  textile per tire
Overall                   II       III      IV

References

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