Dimensionally stable polyesters replacing rayon in European tire market.

Current worldwide trends in tire yarn consumption predict significant future growth in the demand for PET (polyethylene terephthalate), or polyester, industrial yams for passenger tire applications. During the five year period ending in 1995, worldwide consumption of PET tire yam is predicted to grow by as much as 30% at the expense of rayon (especially in Europe, where 90% of the rayon market is situated).

Rayon provides good adhesion to rubber and, provided moisture pick-up can be mitigated, a good combination of strength and modulus, especially at elevated temperatures. However, the environmental and increasing cost concerns have led tire manufacturers to search for an acceptable alternative to rayon as the primary passenger tire carcass reinforcement. Extensive research and development has yielded new generations of dimensionally stable PET (DSP) engineered for high performance passenger tire bodies.

In the mid-1980s, the first generation of DSP fibers for tire reinforcement were introduced by AlliedSignal. These fibers raised performance standards for tire reinforcement in the U.S. to a higher level. At the same time, new European Community directives, aimed at reducing environmental pollution, made rayon costlier to produce than ever before. A second generation of environmentally-friendly DSP fibers with modulus/shrinkage characteristics equal to those of rayon, but which also provide much greater tenacity, was developed. These advanced (1X40/1X43) DSP fibers are rayon replacements, allowing tire makers to essentially "drop" them into their existing manufacturing programs which now use rayon.

An intrinsic advantage of polyester is that the cord treatment can be tailored to achieve maximum tire uniformity by adjusting the shrinkage levels. The new DSP fiber is engineered to be a rayon replacement; tire makers do not need new manufacturing equipment to use it, making the conversion to DSP even easier.

In addition to matching rayon performance, this DSP fiber offers weight savings (and therefore cost savings) of 10% to 30%, depending on the specific tire application.

The following addresses predictions about tire sidewall indentations (SWIs), growth during running, uniformity and handling, based upon laboratory simulations which duplicate the conditions which tire cords must endure. They can be used as guidelines for tire manufacturers and engineers who can now consider polyester as the major substitute for rayon as tire reinforcement.

Materials

Four 1100 decitex polyester yams produced by AlliedSignal Fibers were used for this study: IW70, a conventional polyester fiber; 1X90, the first generation of DSP fibers for tire reinforcement; 1X30, the second generation material with high tenacity; and 1X40, the second generation advanced polyester with ultra-high dimensional stability tailored especially for the European tire market. All were twisted into 3-ply greige cords (315 tpm), dipped in adhesive, heated and stretched. The rayon yarns (1850 decitex) were twisted into 2-ply greige cords (480 tpm) and were similarly dipped, heated and stretched. These are widely used commercial constructions for polyester and rayon cords, so they are valid for comparison.

Treated cord properties

Dimensional stability of PET treated cords directly affects the tire manufacturing process and the resulting tire uniformity and appearance (figure 1). The curve was generated by treating cords to different net stretches, thereby varying the extensional stiffness (LASE, or load at specified elongation; LASE-X represents the stress at X% elongation). As expected, increasing extensional stiffness for a given material by treating to higher stretches also increases the tendency to shrink when exposed to curing temperatures. Meaningful comparisons should therefore be made on the basis of either constant shrinkage or LASE. For example, by treating to obtain 1.5% free shrinkage, 1X40 DSP fiber provides higher extensional stiffness (23 cN/tex) than a standard polyester material such as 1W70 (11 cN/tex).

Dry rayon has a LASE-5 of 26 cN/tex. When compared at similar shrinkage, 1X40 DSP fiber has 50% higher LASE than 1W70 but still has lower LASE than dry rayon. However, polyester's tenacity is superior to rayon's (figure 2).

While dried rayon treated cord properties can be effectively used for quality control purposes, they are not representative of the cord in tires where moisture is present. Tenacity and LASE are maximized when rayon is perfectly dry, but decrease markedly as moisture content increases. This effect is not observed with polyester.

In-tire cord properties

Cured tire

A cure/run tire (C/RT) simulation was developed to assess changes in properties from treated cord to cured tire which arise due to cord shrinkage during the curing process. In the C/RT test, a treated cord bears a specified load within a tubular oven. The load and temperature are adjusted to reflect the particular portion of the tire curing process or service conditions to be simulated: curing, cure ejection and tire service under a specified inflation pressure. The length of the cord is monitored and the resulting cord properties measured after key steps (figure 3). The proper load can be determined by comparing cord properties after the simulation steps with the properties of cords from cured tires. Post-cure inflation (PCI) was not included because of the industry trend in the U.S. towards eliminating it and the general lack of such facilities in Europe. The simulation is performed as follows:

* Curing - load corresponding to the cord retractive force generated at 177 [degrees] C (typical curing temperature) is applied for 20 min.

* Cure ejection - load removed, oven turned off and allowed to cool for 30 min.

* Cords in splice and adjacent non-splice regions have undergone similar length reductions due to shrinkage (consistent with analyses of cords removed from cured tires).

After exposure to the simulation, cord modulus will decrease relative to the original treated cord/fabric. Depending upon the specific location in the tire (crown or sidewall) and the modulus of the uncured rubber stock, the degree of shrinkage and resulting property changes will vary. There are several mechanisms through which this can occur:

* Shrinkage during curing can occur uniformly along the entire length of the cord (figure 4a). If the cord is pulled around the beads, this can be detected by shortening of the portion of the cord in the turnup. If the curing pressure is maintained until the tire cools, cord modulus in the as-cured tire will be uniform along the entire length. Normal forces acting on the bead during curing will reduce the contribution of this mechanism.

* Alternatively, shrinkage can occur without pulling around the beads. Instead, the cord is pulled through the innerliner (figure 4b). Again, cord modulus is uniform along the length of the cord if the curing pressure is maintained. Visual analysis of the tire cross-section provides insight into the contribution from this mechanism.

* If the tire is ejected from the curing presses while excessively hot, the cords can shrink because curing pressure no longer constrains the tire to a given shape. Assuming that the degree of ply skin curing is sufficient to restrict cord movement within the rubber matrix, the residual shrinkage of the cords in the thin sidewall may be sufficient to cause an overall shortening of the sidewalls. The mass of the bead and tread regions will tend to prevent contraction of these regions. Consequently, the resulting cord modulus in the sidewall will be lower than the modulus under the crown (figure 4c).

* Belt pantographing during the green tire expansion in the mold can have a significant effect on the loss in modulus under the crown. A small degree of pantographing causes cord modulus under the crown to decrease because the cord is mechanically forced to "shrink." If pantographing is excessive, cords can actually be compressed during expansion and cause significant loss in strength as well as modulus. The modulus under the tread will be lower than in the sidewalls in this case.

* At elevated temperatures, polyester cords can lose modulus via internal stress relaxation. Experimentally, it has been shown that this mechanism is not a major contribution to modulus loss.

* Treating polyester to various net stretches will result in different shrinkage levels. The LASE after free shrinkage stays fairly constant unless the cord experienced too low a net stretch (figure 5). For example, while the retractions (shrinkages) during free shrinkage are different for 1X90 DSP fiber, the LASE values after free shrinkage are all similar (12 cN/tex), regardless of the original net stretch (stretch/relax). However, if the original treated cord was treated to 10 cN/tex, for example, there is no driving force to increase the LASE after shrinkage up to 12 cN/tex. This is important in determining optimum polyester treating conditions. The intrinsic advantage of polyester is that the cord treatment can thus be tailored to achieve maximum tire uniformity by adjusting the shrinkages. Unlike polyester, rayon shrinkage is fairly independent of treating conditions.

Studies have shown that the moisture content of rayon cords in cured tires is 4%-6%. Therefore, it is appropriate to compare polyester properties with rayon cords containing moisture (figure 6). Under these practical conditions, the dimensional stability of 1X40 DSP fiber compares favorably with rayon. The rayon removed from a tire compares with the rayon treated cords containing 6% moisture, confirming previous studies.

Run tire

The C/RT simulation can be continued to assess the effect of tire service on cord properties. After the cord cools, a load is applied which simulates the tension it experiences upon inflation of the tire. In order to study splice effects, it is assumed that, for a monoply tire, the cord load in a splice is one-half the cord load for a non-splice cord (from netting analysis). After "inflation," the oven is heated to 100 [degrees] C to simulate running the tire at ultra-high speeds (pressure is assumed to remain constant). After five hours, the power to the oven is turned off and the cord allowed to gradually cool; this simulates stopping and parking the car after five hours of running (figure 7). Alternatively, one could have subjected cords which have experienced the ASTM or free shrinkage tests to this simulation, but similar results were anticipated.

Predicted tire performance

Sidewall indentations (factory)

The extension upon "inflation" tension depends upon whether the splice or non-splice is being simulated; their difference in elongation is indicated as a differential D figure 4). This difference should be directly related to the SWI level measured at the factory. In order to translate the elongation difference D to a SWI depth, a simple model can be used. In the model, the sidewall away from the splice is approximated by a semicircle of radius R2. At die splice, the radius is R1. SWI is equal to the difference in radii R2-Rl. The elongation difference D is equivalent to the difference in arc length for the two semicircles. Since the radius is proportional to the arc length,

SWI = R2 - R1 = DL/P where DL = D * (sidewall arc length w/o inflation). For a given tire construction, D is proportional to the predicted SWI depth (table 1).

Table 1 - delta elongations and SWI
improvement

  Material   D (%)   % Improvement
    1W70      0.71         0
1X90 DSP      0.60        15
1X30 DSP      0.54        24
1X40 DSP      0.45        37

The advanced 1X40 DSP fiber is predicted to have over 37% shallower SWIs as compared to the standard polyester for an as-cured and inflated tire.

Sidewall indentations (run tire)

After running, SWIs increase significantly for all materials. However, DSP fibers start at lower levels and retain their advantage over standard 1W70 polyester. Tire test wheel experiments indicate that this SWI increase is representative of what actually occurs in tires. Tire tests also indicate that SWIs grow during service to an equilibrium level which is reached after approximately 1500 km.

The delta obtained immediately after running (before cooling down) is identical to that obtained after cooling, so SWIs should remain unchanged. Tire test wheel experiments confirm that SWIs do not change after a car is parked and the tires have had a chance to cool down.

Uniformity

Tire uniformity is measured in terms of radial force variation around the circumference of a tire in contact with a test wheel. Thus, it is primarily the measurement of deformability (radial modulus) of the tire section in contact with the test wheel. Factors such as rubber and carcass ply material nonuniformities and ply misalignment during building can contribute to excessive radial force variations in the final tire. Carcass ply nonuniformities may include variable cord spacing in the fabric or even cord property variations as a result of nonuniform treating conditions. However, a number of adjacent cords need to differ to significantly increase the modulus over a sufficiently wide area in order to be detected.

Experiments suggest that temperature differences around the tire during curing and after cure ejection can cause differential shrinkage from within the tire; hotter sections tend to produce low spots on the radial force curve. Therefore, samples with increased dimensional stability (i.e., lower shrinkage for a cord with a given starting LASE) should have better uniformity and decrease tire downgrades and rejects. Since 1X40 DSP and rayon possess similar dimensional stability, tire uniformity should also be comparable.

Bruise resistance/plunger energy

For safety reasons, tire bruise or impact resistance is important. A tire's energy absorption qualities determine its resistance to failure due to high-speed impact with obstacles on the road surface or travel over rough terrain. The U.S. Department of Transportation requires the static determination of this resistance via plunger energy measurements on the tire tread. Some tire manufacturers also determine sidewall bruise resistance by forcing a plunger against the sidewall and measuring the force and displacement characteristics. This measurement is dominated by the energy absorbing characteristics of the body cords.

For both tread and sidewall impact, relative performance of different body cords can be independently assessed by toughness measurements, especially for cords with similar LASE at intermediate strains. Cord toughness is measured after free shrinkage, which most closely represents the conditions of cords in tires (here, toughness is defined by the total energy to cord failure). 1X40 DSP fiber has 7% greater toughness than the standard polyester and 29% greater toughness than rayon on an equal weight basis. Therefore, 1X40 DSP fibers should provide tires with better impact resistance than other polyesters and especially rayon.

While tenacity for 1X40 DSP fiber is somewhat lower than the standard polyester, its toughness is higher (toughness and tenacity of all of the polyesters are considerably higher than rayon). Historically, tires were designed with the body cords required to meet certain minimum strength requirements. This monolithic approach may have been necessary for conventional cords where there existed a direct correlation between ultimate cord strength and modulus under intermediate loads (e.g., LASE-5). With new generations of advanced polyesters like DSP fibers, however, a choice exists between cords with high tenacity and modulus and cords with slightly lower tenacity but even higher modulus, toughness and elongation to failure.

Conclusions

In light of environmental concerns, performance goals and the increasing cost of rayon, advanced polyester fibers offer a potential alternative to rayon. By virtue of their high dimensional stability, polyesters such as 1X30 DSP and 1X40 DSP fibers can offer tires excellent uniformity, durability (bruise resistance) and physical appearance (in terms of reduced SWIs and growth resistance). Furthermore, predictable and controllable response to treating conditions allows custom tailoring of the cord properties to match the particular needs of a given tire manufacturer.