The potential of PEN for tire reinforcement: replacing PET, rayon and even steel.

Extensive research in fiber reinforcement has made a major contribution to advancing the design and performance of the modem automotive tire. The fiber reinforcements used in various parts of the tire contribute significantly to specific tire properties (table 1). High dimensional stability makes DSP polyester fibers (polyethylene terephthalate, or PET) one of the most advanced tire reinforcements available today. PEN (polyethylene naphthalate) fibers, with twice the dimensional stability of the most advanced DSP fibers, are now being evaluated for tire reinforcement. Interest in PEN as a tire reinforcement derives from its use in numerous demanding industrial applications in recent years.

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In particular, high performance PEN films have been commercialized or studied for use in magnetic recording tapes, electrical applications such as flexible printed circuit boards and motor insulation and photographic film for miniature cameras. PEN also has potential in packaging applications such as hot-filled bottles. These applications have been driven by PEN's high dimensional stability and modulus, as well as its enhanced chemical resistance, UV stability and barrier properties relative to currently-used materials like PET.

Research at AlliedSignal laboratories has been dedicated to evaluating PEN fiber for numerous industrial applications, especially as a tire reinforcement. In-rubber automotive applications such as v-belts and hoses are also being evaluated. Other manufacturers are researching specialty applications such as fabric for filtration media and papermakers' felt.

PEN's advantageous properties are derived from its chemical backbone containing both rigid and flexible components (figure 1). PEN is widely regarded as a high performance member of the polyester family, much as aramid is a high performance nylon. The naphthylene ring in PEN provides more rigidity and hence a higher Tg (glass transition temperature) and higher modulus than materials like PET. However, maintaining the ethylene units ensures melt processability of higher-molecular-weight polymers, unlike wholly aromatic polyesters or aramids. Melt processability provides the opportunity to keep processing costs low relative to gel or solution spun fibers, or fibers which require extensive heat treatment after spinning.

PEN fibers were not previously commercialized because the raw material cost was high, and therefore the cost/performance ratio did not compare well with existing reinforcements. In addition, reinforcement requirements were not previously as demanding as they are today, so that PEN's added performance capabilities were not as high a priority.

However, a number of environmental, performance, manufacturing and raw material trends have evolved which may provide the opportunity for PEN to become a commercially viable fiber for tire reinforcement.

Trends

Environmental

The primary environmental concerns in the tire industry are automotive fuel efficiency, tire recyclability and the impact of manufacturing processes on the environment. PEN is a lightweight reinforcement with improved properties, so it will allow tire weight reduction and reduced rolling resistance to increase fuel economy. This advantage would be even greater if it were possible to substitute PEN for steel in belts and truck tire sidewalls.

In the future, substitution for steel would also provide improved tire recyclability by permitting easier shredding and combustion of tires, especially if composite beads were used. With North American plants alone now capable of producing over one million tires per day, PEN would make a tremendous environmental impact if it were widely substituted for steel.

Like the closely related PET, PEN fibers can also reduce the environmental problems caused by rayon production. There is a great need for a material which can suitably substitute for rayon, especially in Europe, and which can be manufactured with fewer environmental concerns. It is estimated that for every kilogram (2.2 lbs.) of rayon produced, up to 757 liters (200 gallons) of water are required in the production process, and up to 10 times that quantity is required for supplementary processes. The prevalent rayon production method used in producing tire yam - the viscose process - uses sodium hydroxide, carbon disulfide, hydrogen sulfide, sulfuric acid and zinc. This process releases effluents such as H2S gas and CS2 vapor, which are extremely costly to treat.

These environmental problems associated with producing rayon have already contributed to increased usage of PET in Europe, especially advanced dimensionally stable materials suitable for demanding applications such as DSP fibers. As an extension of the PET family, PEN provides the same environmental benefits. In fact, PEN uses the same manufacturing process as PET, with only one key ingredient changed.

High performance

High modulus, dimensionally stable materials like PEN are currently needed to satisfy the increasing demand for high performance tires, and to provide a means for tire manufacturers to improve product quality. The goal is to eliminate defects by process improvements rather than by segregating, good from bad at the end of the process. The net effect is lower rework - for example, less need for regrinding operations. Improved manufacturing efficiencies have been gained by using PET fibers with improved dimensional stability, and further advances should result from PEN, with even greater dimensional stability.

Increased availability

The final trend pointing to commercial viability of PEN is related to the availability of raw materials. PEN is currently synthesized from the monomers ethylene glycol and naphthylene dicarboxylate (NDC). Amoco Chemical's commitment to build a new facility in Decatur, AL capable of manufacturing commercial quantities of the latter monomer is one big reason for the current surge in interest in PEN. With an anticipated 27,000 tons-per-year capacity (24,489 metric tons), the facility ensures that a suitable supply of raw materials will be available. Amoco says that when the plant goes into production (completion is scheduled for 1995), Amoco NDC will be sold at $1.50/lb. ($3.31/kg). Mitsubishi Chemical also manufactures NDC.

These trends favor the commercialization of PEN fibers for tire reinforcement, but the market is already dominated by PET, rayon, nylon and aramid. Therefore, PEN's performance compared to these other fibers will be the biggest factor in determining its commercialization potential.

Testing

To measure the comparative performance, testing was conducted to compare the tenacity, dimensional stability, flex fatigue, compressive modulus and retractive force of all these reinforcements.

Materials

Commercial fibers studied were rayon, nylon 6.6, standard aramid and a dimensionally stable PET made by Allied-Signal, IX30 DSP; as well as a developmental PEN also produced by AlliedSignal. These yams were twisted into tirecord form as shown in table 2, unless otherwise noted. The twist multiplier (TM) is included to provide a relative measure of helix angle.

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These cords were treated with a standard RFL adhesive dip. In the case of aramid, PET and PEN, a pre-dip typical of the industry was also used prior to the RFL.

Test procedures

* Cord strength - tensile measurements were performed using an Instron tester at a strain rate of 120% min.

* Dimensional stability - shrinkage was determined under a load of 0.05 g/d (0.04 cN/dtex) at 177 [degrees] C (35 [degrees] F).

* High temperature properties - dynamic modulus was measured on a Rheometrics RSAII analyzer.

* Fatigue - was measured on a Scott flexural fatigue tester at 250 cycles/min for 2.5 hours employing a 0.5 in. (1.27 cm) spindle for high-twist samples and a 1 in. (2.54 cm) spindle for low-twist samples.

* Cord compressive modulus - measurements were made by using a cord-reinforced rubber specimen of dimensions 1.25 x 0.5 x 0.5 in. (3.175 x 1.27 x 1.27 cm).

* Retractive force - values were taken as the maximum of the retractive force vs. time curve at 177 [degrees] C (351 [degrees] F).

Results

Cord strength

The strengths of treated cords were compared by stretching the cords to different lengths during the treating process and plotting the values versus tenacity (figure 2). Plotting the data in this fashion ensures that the materials are compared on an equal weight and shrinkage basis. PEN is higher in tenacity than typical tire carcass reinforcement materials like rayon and PET. Aramid, which is not plotted, has a high treated cord tenacity of 16 g/d (14 cn/dtex).

Dimensional stability

Dimensional stability, the resistance of a material to deform at elevated temperatures, can be best quantified for tire cords by measuring modulus at a given level of shrinkage. The secant modulus at 3% elongation or LASE-3 (load at specified elongation of 3%) is plotted versus shrinkage (figure 3). The rigid backbone structure of PEN provides high dimensional stability relative to the commodity fibers, but not as high as aramid, which is about 9 g/d (8 cn/dtex). Since dimensionally stable PET reinforcement greatly reduces sidewall indentations, this improvement should eliminate sidewall indentations, even in demanding applications. Furthermore, PEN should minimize rejects due to non-uniformity, even in the absence of post-cure inflation.

High temperature properties

Dynamic modulus is plotted versus temperature for treated cords of PEN, PET and rayon in figure 4. Rayon performs quite well in this test, and hence is quite extensively used in Europe for high performance applications. PET'S behavior is good for the majority of high performance applications, and this material is even used for Z-rated tires. However, the decline in modulus at roughly 100 [degrees] C (212 [degrees] F) sometimes is looked upon unfavorably for ultra-high-performance tire designs which traditionally have used rayon. PEN, with a Tg some 72 [degrees] C (162 [degrees] F) above PET, shows excellent behavior up to 150 [degrees] C (302 [degrees] F). Since this temperature is beyond what could be reasonably expected in actual tire performance, PEN could not only be an excellent replacement for rayon, but could also provide the opportunity for tire designs which are not possible with current reinforcements.

Fatigue

Flexural fatigue results for PEN are compared to aramid in figure 5, and rayon and PET in figure 6. For the latter figure, cord constructions were chosen to duplicate actual tire carcass usage. Twist multipliers are also included to provide a measure of helix angle and inherent fatigue resistance. For example, rayon requires a high twist multiplier due to its inherent poor flexural fatigue. The high retained strength of PEN is unique, since high modulus materials frequently possess low fatigue due to fibrillation under compression. For PEN, it is hypothesized that this does not occur because PEN's melt processability, which is derived from the flexible methylene backbone units, yields a more traditional crystalline/amorphous microstructure. Consequently PEN provides higher retained strength than aramid under low twist, fatigue sensitive conditions.

Compressive modulus

This parameter is important for belt package applications, which primarily utilize steel. As expected, all of the synthetic fibers are low relative to steel, but PEN is quite high relative to the other synthetics (figure 7). This behavior is a function of PEN chemistry, microstructure and macrostructure.

Retractive force

Comparison of this parameter indicates that nylon 6.6>PEN>PET (figure 8). For this test, PEN and PET were used in 1,000 denier (1,100 decitex)/2 ply 11x11 tpi (433x 433 tpm) construction to more closely duplicate nylon's construction. These value are in part determined by the inherent elastic nature of these materials at elevated temperature. However, since factors such as the orientation of the amorphous regions also play a role, process modifications could modify this behavior. For example, increasing the net stretch during treating would increase the retractive force. Also, actual in-tire values could differ from those provided due to shrinkage during the manufacturing process.

PEN tire reinforcement applications

PEN fiber and treated cord properties are compared to the other tire reinforcements in table 3. These test results position PEN between PET and aramid in terms of both properties and cost. The $4/lb. ($8.82/KG),PEN fiber cost is projected for large volumes. It is interesting to note that nylon and rayon, two materials of considerable historical production volume, do not compare well on a tensile property-to-cost basis, because of poor modulus and poor tenacity, respectively.

The success of these materials, and perhaps tire reinforcements in general, is determined by the special features the materials bring to specific tire applications. Examples would include nylon's excellent fatigue and adhesion, and rayon's excellent high temperature properties. Special features of PEN make it desirable as a tire reinforcement in high performance passenger tire carcass, truck carcass and passenger tire cap plies.

High performance radial passenger tire carcass

The two materials currently used for this application are primarily PET in the U.S. and rayon and PET in Europe. Modulus and strength properties of PEN are desirable in this application for two reasons.

First, for ultra-high-performance applications, good high temperature modulus is frequently desired. PEN's superior modulus performance provides the incentive to use this material.

Second, many high performance tires are made with two carcass plies. PEN's higher strength along with higher modulus may permit elimination of one carcass ply and the associated rubber. This would reduce total tire weight as well as simplify the manufacturing process.

Preliminary work on tires made using PEN reinforcement confirms some of the expected advantages. Table 4 compares relative cord modulus, strength and sidewall indentation information of high performance tires made from PET and PEN. Modulus and tenacity were measured on cords from tires run until failure under low inflation pressure and high loading. It was observed that failure occurred due to belt separation rather than carcass failure. PEN's superior modulus and tenacity characteristics were maintained throughout the tire manufacturing and testing processes. In addition, PEN's high dimensional stability is reflected in the reduction of sidewall indentations.

Truck carcass

Light truck applications are similar to high performance tires in terms of the materials currently used as carcass reinforcements, namely rayon and PET. The same observations about PEN described above remain valid, especially the opportunity to reduce the number of plies from two to one. Since sidewall indentations are more severe in one-ply tires, especially under low inflation pressure, PEN's dimensional stability is particularly valuable.

For medium truck tires, steel is often the material of choice since its high density allows great strength to be achieved with only one ply. The negative consequence is the extra cord weight necessary compared with synthetics. For example, if PEN were used instead of steel, a 50% weight reduction would be possible since PEN's tenacity is more than double that of steel. This is especially important since it is medium truck tires which are commonly used for tractor-trailer trucks on the highway today. The larger the truck tire size, the higher the strength requirement in order to "fit" enough strength into one ply.

This is illustrated in table 5 for light and medium trucks using two-cord constructions and assuming an 85% linear cord packing fraction. PEN, with a tenacity of 8.3-8.4 g/d (7.3-7.4 cN/dtex), should work well for both applications. PEN's excellent dimensional stability and fatigue properties should minimize concerns about tire growth and durability. These latter two issues arise because medium truck tires are frequently retreaded many times during their lifetime.

Cap plies and belt packages

This application can be broken into two segments. The cap ply segment is concerned with enveloping the steel belt component in order to control belt growth at high speeds and to secure the belt edges. Nylon 6.6 is primarily used in this application. The other segment is concerned with complementing the stiffness of the steel belt. This requirement is typically met with an aramid reinforcement, which can be over or under the steel belt and can best be described as a belt package.

One of the features of nylon 6.6 which makes this material valuable for cap ply applications is its high retractive force at elevated temperatures. During the manufacturing process, retractive force ensures the belt is enveloped by the nylon, and this ultimately keeps the belt from growing during high speed operation. The reinforcement property important for belt packages is compressive modulus, since it impacts the stiffness of the rubber/belt composite, which ultimately influences tire handling and treadwear.

PEN's high retractive force and compressive modulus will make this material an excellent candidate for these applications. In addition, since the fatigue requirements are less severe for belt compared to carcass applications, the opportunity to reduce cord twist and gain modulus and tenacity exists. This property can be particularly important for belt package applications. In this regard, PEN's good low-twist fatigue should allow a significant gain.

Summary

Technical arguments have been provided which give a good basis for studying PEN for tire applications. The important question remains whether PEN's performance characteristics and economics will provide what today's tire market needs, ultimately resulting in making PEN a viable commercial product. For example, how valuable is the significant weight reduction which can be achieved with PEN versus steel, and will that be enough incentive to pursue the required tire design changes?

Both the fiber and tire industries are becoming more dynamic, with product improvements occurring at a more rapid pace. This is a positive environment for PEN fiber. Another positive sign is that the tire industry is slowly evolving from a commodity toward a specialty industry. The trend to redefine value as the performance of component systems rather than looking at individual component cost/performance ratios will also help a higher performance/price reinforcement like PEN.

The viability of this material ultimately remains in the hands of the fiber maker, the tire designer and the tire manufacturer. The fiber maker must learn to rapidly tailor properties to specific applications. The tire manufacturer must learn how to use PEN to realize manufacturing efficiencies. And finally, the tire designer must effectively take advantage of the special performance features of PEN fiber.

References

[1.] G.S. Rogowski, C.J. Nelson and D.S. Liu, "Designing tire reinforcements to meet future market needs," Tire Technology International, May 1995, p. 126. [2.] Amoco Chemical Company, NDC Elements Newsletter, Vol 1, No. 2. [3.] Amoco Chemical Company, NDC Elements Newsletter, Vol 1, No. 3. [4.] D.S. Liu and C.J. Nelson, "The behavior and performance of PET reinforced passenger tires," Kautschuk Gummi Kunststoffe, Vol. 46, 1993, p. 297. [5.] Modern Tire Dealer 1995 Facts Issue, 29th Edition, January 1995, p. 37. [6.] Bob Ulrich, "Overrated? Speed helps classify performance tires," Modem Tire Dealer, February 1995, p. 26 [7.] Pat Reynolds, "Naphthalate-based bottles beckon," Packaging World, July 1995, p. 26. [8.] A.L. Promislow, "An approach to laboratory fatigue testing on tire cords," ISIFM Meeting, May 1991. [9.] Y.D. Kwon, R.K. Sharma and D.C. Prevorsek, "Measurement of nonlinear viscoelastic properties on polymers in cyclic deformation under relatively large strain amplitude," ACS Advances in Chemistry Series, No. 174, 1979, p. 35-52. [10.] Y.D. Kwon and D.C. Prevorsek, "Use of high strain dynamic viscoelastomer in the analysis on tire rolling resistance," SAE Technical Paper Series, #800242, 1980. [11.] K. Sarkan, Y.D. Kwon, and D.C. Prevorsek, "A new approach for the thermomechanical analysis on tires by the finite element method, " Tire Science Technology, Vol. 15, 1987, p. 261.