Paper No. 14C DuPont Advanced Fibers Systems

KEVLAR Engineered Elastomer for Tire Reinforcement

by

C. W. Tsimpris*DuPont Advanced Fibers Systems

Richmond, Virginia

J. P. JakobDuPont Advanced Fibers Systems

Geneva, Switzerland

and

G. P. VercesiDuPont Advanced Fibers Systems

Geneva, Switzerland

Presented at theInternational Tire Exhibition and Conference

Akron, OhioSeptember 10-12, 2002

* Speaker

H-89939 10/02

2002 E.I. du Pont de Nemours and CompanyThe DuPont oval logo, DuPont and KEVLAR are trademarks or registered trademarks of E. I. du Pont de Nemours and Company.

Technical Paper

DuPont Advanced Fibers Systems ■ 5401 Jefferson Davis Highway ■ Richmond, VA 23234 ■ (800) 453-8527

KEVLAR Engineered Elastomer for Tire Reinforcement

ABSTRACT

KEVLAR engineered elastomer, a composite of KEVLAR pulp and elastomer,enables incorporating high-surface-area para-aramid reinforcement to tirecompounds. Extensive laboratory studies of the effects of this material on typicaltire compounds demonstrate its potential for reinforcement of several tirecomponents.

INTRODUCTION

DuPont KEVLAR brand fiber was introduced in the 1970’s. KEVLAR , the world’sfirst para-aramid fiber, is known for its high strength to weight ratio, high modulus, andexcellent chemical and thermal stability. Initially, it was offered in continuous filamentform, and soon found applications in tires, mechanical rubber goods, bullet resistantvests, and composites. In the 1980’s, short forms of the fiber - staple, floc, and pulp -were introduced and quickly found acceptance in cut-resistant protective apparel,gaskets, and friction materials. Photographs of these three product forms are shownin Figure 1.

Once short forms of KEVLAR and Akzo’s (now Teijin Twaron’s) para-aramidTWARON were introduced, they were evaluated for rubber reinforcement. Usingshort fibers (such as cellulosics, cotton, denim, polyester, and nylon) to reinforce rubberis common in rubber goods. They improve green strength, provide dimensionalstability prior to cure, and improve mechanical properties of the vulcanizate.Compounders found that they could incorporate para-aramid floc (we define floc asshort fiber less than 6 mm long) into rubber using an internal mixer or a roll mill,often with difficulty. An electron photomicrograph showing floc in a rubbercompound may be found in Figure 2. Incorporating the high-surface-area pulpproduct (Figure 3) proved to be exceedingly difficult. Only a few people were ableto adequately disperse pulp into a rubber compound. However, their work diddemonstrate the superior reinforcement potential of aramid pulp once thedispersion limitation was overcome. An electron photomicrograph showing pulp ina rubber compound may be found in Figure 4.

DuPont and KEVLAR are trademarks and registered trademarks of E. I. du Pont de Nemours and CompanyTWARON is a registered trademark of Teijin Twaron B. V.

DuPont initiated studies to define a method to disperse para-aramid pulp intorubber, and this effort led to development of a unique new technology platform fordispersing pulp into an elastomer matrix. Products produced via this technologyshowed superior dispersion of aramid pulp in rubber. Samples of rubbercompound of identical composition were analyzed using an ultrasonic scanningtechnique that measures relative porosity are shown in Figure 5. In the sample onthe left, the pulp was introduced using engineered elastomer; in the sample on theright, the pulp was added directly into the rubber. A uniform color indicates ahomogenous mixture. The sample made by compounding pulp directly into therubber shows significant color differences indicating relatively poor fiber dispersion;the sample prepared using engineered elastomer is nearly a uniform color,demonstrating its excellent dispersion.

Product made via this new technology enabled dispersion of pulp into rubber sowell that it was given a new name, ‘KEVLAR engineered elastomer.’ Initialevaluation in the rubber industry confirmed that engineered elastomer was fareasier to process than dry para-aramid pulp, and confirmed the improveddispersability of pulp made possible by using this offering. Initial adoptions forengineered elastomer were in power transmission belts; the PT belt industry hadexperience with short-fiber reinforcement; a common part of a belt formulation.

The tire industry had less experience with short-fiber reinforcement; we initiated afundamental study to determine the effect of para-aramid pulp reinforcement onproperties of typical tire compounds. The study included both static and dynamictests. A second study was initiated to determine the effect on compound propertiesby varying the concentrations of para-aramid pulp and carbon black reinforcement.

The two studies were quite extensive. Only a summary will be included in thispaper.

EXPERIMENTAL

Fundamental study:

A ‘hard’ compound formulation was selected for the fundamental study. A‘precompound’ of the following composition was prepared in an internal mixer:

SMR 10 89.96Renacit 11 0.10ZnO 7.00Stearic Acid 2.00TMQ 1.006PPD 0.75N326 60.00Durez 32333 1.00Resorcinol 1.25Aromatic Oil 5.00

Compounds for testing and evaluation were then prepared by adding the para-aramid (via KEVLAR engineered elastomer) and remaining natural rubber on aroll mill, then adding the cure package on the mill. The final compositions were:

Sample Identification Reference F1NR F3NR

Precompound (above) 89.96 89.96 89.98Engineered Elastomer 0.00 4.35 13.04SMR 10 10.04 6.70 0.00

Cristex OT33AS 4.00 4.00 4.00HMMM 3.00 3.00 3.00TBBS 0.40 0.40 0.40DCBS 0.50 0.50 0.50PVI 0.20 0.20 0.20

Total 186.2 187.2 189.2

(para-aramid pulp concentration) 0 1 3

Test work was conducted by the MRPRA (now Rubber Consultants) in the UK.

Compound formulation study:

A heavy-duty tread formulation was selected to determine the effect of pulp andcarbon black concentrations on compound properties. All mixing was done on aninternal mixer. A two-stage mixing protocol was employed:

Stage 1

Add ½ NR, fibers, then remaining ½ NRAdd remaining ingredients

Stage 2

Add ½ stage 1, cure package, then remaining ½ stage 1

The detailed formulations were:

SBR 1502 60.00 53.30 39.90 46.60 39.90 53.30PB 1207 40.00 40.00 40.00 40.00 40.00 40.00EE 1F724 8.70 26.10 17.40 26.10 8.70CB N-110 24.00 24.00 24.00 16.00 8.00 8.00CB N-234 20.00 20.00 20.00 14.00 7.00 7.00Aromatic Oil 8.00 8.00 8.00 8.00 8.00 8.00Zinc Oxide 4.00 4.00 4.00 4.00 4.00 4.00Stearic Acid 2.00 2.00 2.00 2.00 2.00 2.00Agerite Resin D 1.00 1.00 1.00 1.00 1.00 1.00Antozite 67 1.00 1.00 1.00 1.00 1.00 1.00Vantard PVI 0.25 0.25 0.25 0.25 0.25 0.25Amax 1.00 1.00 1.00 1.00 1.00 1.00Unads 0.20 0.20 0.20 0.20 0.20 0.20Sulphur 1.90 1.90 1.90 1.90 1.90 1.90

Total 163.35 165.35 169.35 153.35 140.35 136.35

phr Carbon Black 44.0 44.0 44.0 30.0 15.0 15.0phr Aramid 0.0 2.6 7.8 5.2 7.8 2.6

Compounding and testing were conducted by the Akron Rubber DevelopmentLaboratory.

FUNDAMENTAL STUDY

Static stress-strain:

A typical stress-strain curve for a compound reinforced with para-aramid pulp isshown in Figure 6. The tensile modulus at up to 50% strain is about three timesthat of a corresponding non-fiber reinforced compound. Above 50% strain, thestress-strain response of a compound reinforced with para-aramid pulp is similar tothe reference compound. The stress-strain response is not affected by strain rateas shown in Figure 7.

When subjected to shear, the fiber in a para-aramid pulp reinforced compound canbe oriented; and the properties of the rubber compound are different in thedirection of fiber orientation. The effects of fiber orientation and concentration areshown in Figures 8 and 9. Tensile modulus in the direction of fiber orientation(machine direction - MD) is higher than that perpendicular to the direction oforientation (cross-machine direction – XMD.) The MD/XMD modulus difference(anisotropy) of the compounds peaks near 50% strain. In Figure 10, shows therelationship between modular anisotropy, strain and fiber content where anisotropyis calculated from the absolute stress values at a given strain. Anisotropy peaks atabout 60% strain for the compound reinforced with 1 phr of para-aramid pulp, andat about 50% strain for the compound with 3 phr reinforcement. Figure 11 showsan identical plot where modular anisotropy is calculated by the tangential stiffnessat a given strain. Anisotropy peaks at about 40% for the compound with 1 phr ofpulp, and at about 30% for the compound containing 3 phr pulp.

An indication of the additive effects of carbon black and para-aramid pulpreinforcement is shown in Figure 12. The lower stress-strain curve shows theresponse of a ‘gum rubber’ compound. The next three curves show the effect ofaddition of carbon black (N330) at 30, 45 and 60 phr. The following two curvesillustrate the effect of adding 1 and 3 phr pulp to a compound containing 60 phrcarbon black. The large increase in modulus obtained by low concentrations ofpulp is readily apparent.

The stress-strain curves of compounds reinforced with para-aramid pulp are nearlylinear with ‘high modulus’ at low strain, and at again with ‘lower modulus’ at highstrain. The transition or inflection in the stress-strain curve occurs at around 50%strain. Acoustic emission tests were conducted in triplicate on the threecompounds prepared in this study. Specimens were pulled at a constant rate of 2inches per minute, and the acoustic output monitored throughout. Keyobservations made during the testing included:

• Each material (non-pulp-reinforced and pulp-reinforced) showed detectableacoustic activity.

• The amount of acoustic activity, as measured by the total number of events,was roughly proportional to the amount of pulp present (Figure 13).

• The amplitude (intensity) of the acoustic events was similar; that is, the fibercompounds reinforced with pulp did not produce louder events, just more ofthem.

The peak acoustic activity was determined by plotting the data as ‘hits’ per straininterval (Figures 14 and 15.) We found that peak acoustic activity occurs in therange 40-60% strain; this corresponds to the region where the stress-strain curvechanges slope. The onset and peak of acoustic activity for the three compounds issummarized below:

Pulp concentration (phr) 0 1 3

Onset of acoustic activity% Strain 14-38 26-36 24-37Stress (lbs) <10 16-18 26-30

Peak of acoustic activity% Strain 51-85 60 47-54Stress (lbs) 8-15 22-23 38-40

We hypothesize that reinforcement of elastomers by para-aramid pulp involvesassociation between charged groups on the fibril surfaces and those in theelastomer(1), a mechanism similar to bound rubber theories for carbon black(2). It isour belief that acoustic emission testing is recording the disruption of theassociation between the charged groups on fibrils and elastomers.

Cycled Stress Stain Measurements:

Compounds reinforced with para-aramid pulp are used in dynamic applications;cyclic and dynamic properties are those important in the end-use.

The cyclic stress-strain behavior of para-aramid pulp reinforced compounds wasobserved over 10 cycles up to 2.5, 10, and 50% strain. In these tests, the firstcycle was at the given strain, and subsequent cycles at the (constant) forcerequired to achieve this strain in the first cycle. Some of the test details aredescribed in Figure 16. Typical curves from the test at 10% ultimate strain areshown in Figure 17; the curves for the first and tenth cycle are shown.

The energy loss fraction (fraction of energy dissipated in each cycle as shown inFigure 18) was calculated from the stress-strain curves for each cycle (Figure 19.)The energy loss fraction was:

• Nearly constant after the first cycle;• Nearly the same in the machine and cross-machine direction, and • Nearly independent of the fiber concentration in the compound (Figure 20.)

The stress-strain behavior of para-aramid pulp reinforced compounds after cyclingwas also measured. These tests were conducted at 10, 30, 50 and 100% strain,and after 2, 10 and 100 cycles. Measurements were made in both the machineand cross-machine direction (with and against fiber orientation.) A description ofthe test is shown in Figure 21. The stress-strain curves in the machine directionfor the reference (no fiber compound) after 2, 10 and 100 cycles at 10% strain andfor the compound containing 1 phr para-aramid pulp are shown in Figures 22 and23. The effect of cycling on stress-strain behavior is nearly identical for bothcompounds. Similar nearly identical results were obtained with samples cycled toother strains, and those measured in the cross-machine direction.

Short Term Dynamic in Compression:

Short-term dynamic properties in compression were measured using plied-upcylinders (disks.) The specimens were precompressed to 10%. Samples weremeasured from 0.1% to 5% dynamic strain at 1, 10 and 100 Hz, and at 20° and100°C.

Curves for compound containing 1 phr aramid are shown in Figures 24 (20° C)and Figure 25 (100°C). Stiffness at 1 and 3 phr aramid content at 1% dynamicstrain is shown in Figure 26. Stiffness, as expected, increases with increasingfiber content. Loss angle was almost totally independent of fiber content as shownin Figure 27.

Short term dynamic properties in tension:

Short-term dynamic properties in tension were measured using molded stripsprestressed to 5%. Samples were measured at 0.1% to 5% dynamic strain at 1, 10and 100 Hz, and at 20° and 100°C.

Results were quite similar to those of the short-term dynamic properties incompression. Curves at 20° and 100°C are shown in Figures 28 and 29. Stiffnessincreased with increasing fiber content (Figure 30) while loss angle was relativelyindependent of fiber content (Figure 31.)

Both dynamic tests, compression and tension, showed that loss angle was nearlyindependent of fiber content. This is in contrast to carbon black reinforcementwhere loss angle is quite dependent upon concentration Figure 32.

COMPOUND FORMULATION STUDY

A series of compounds were prepared to determine the interaction between carbonblack and para-aramid pulp concentrations on compound properties. A two-levelfactorial type design with a center point was used.

60 X X

Carbon Black 45 X

15 X X

2.6 5.2 7.8

Fiber

The effect of carbon black and fiber content on low-strain modulus is shown inFigure 33. Modulus in the machine direction increased dramatically withincreasing fiber content, as expected. Little effect was seen in the machinedirection. Modular anisotropy, as mentioned before is a consequence of fiberorientation. The degree of anisotropy – fiber orientation – is dependent upon theamount of shear in processing. Higher anisotropy and orientation are achieved bycalendering the stock to a thin gauge, or extruding to a thin profile.

Tear resistance, as measured by both trouser tear and die C tear improved withincreasing fiber content. Surprisingly, the improvements were seen in bothmachine and cross-machine direction (Figures 34 and 35.) This isotropicimprovement in tear properties suggests that para-aramid pulp reinforcement maylead to improved life in off-road tires. Formulations are possible showingimprovement in tear resistance and higher modulus without dramatic increase inhardness (Figure 36.)

CONCLUSIONS

• Para-aramid pulp can be used to significantly reinforce rubber compounds forengineering applications.

• The reinforcement efficiency is significantly greater than that of other commonlyused reinforcing materials such as carbon black and silica.

• A high level of anisotropy can be introduced to a compound by conventionalprocessing techniques.

• Hysteretic properties are nearly unaffected by the concentration of para-aramidpulp used in the compound.

• Stress-strain and acoustic emission data suggest that association betweenelastomer and fiber exists up to ~40% strain.

• Tear resistance can be improved by incorporation of para-aramid pulp into a tirecompound.

• Para-aramid pulp reinforcement should be considered for low strain – constantstress tire components.

REFERENCES

1. C. W. Tsimpris, et. al. “KEVLAR Brand Engineered Elas tomer – An Enablerfor the Rubber Industry”, Paper 19, ACS Rubber Division Meeting, Cincinnati,October 2000.

2. J. L. Leblanc, J. Applied Polymer Science, 66, 2257 (1997).

H-89939 10/02

Product safety information is available upon request.

This information corresponds to our current knowledge on the subject. It is offered solely to providepossible suggestions for your own experimentations. It is not intended, however, to substitute for anytesting you may need to conduct to determine for yourself the suitability of our products for your particularpurposes. This information may be subject to revision as new knowledge and experience becomesavailable. Since we cannot anticipate all variations in actual end-use conditions, DUPONT MAKES NOWARRANTIES AND ASSUMES NO LIABILITY IN CONNECTION WITH ANY USE OF THISINFORMATION. Nothing in this publication is to be considered as a license to operate under or arecommendation to infringe any patent right.

United States and South America:

DuPont Advanced Fibers SystemsCustomer Inquiry Center5401 Jefferson Davis HighwayRichmond, VA 23234Tel: (800) 453-8527 (804) 383-4400Fax: (800) 787-7086 (804) 383-4132E-Mail: afscdt@usa.dupont.com

Europe: Canada:

DuPont Engineering Fibres DuPont Canada Inc.P.O. Box 50 Advanced Fibers SystemsCH-1218 Le Grand-Saconnex P. O. Box 2200Geneva, Switzerland Streetsville Postal StationTel. ++ 41-22-717 51 11 Mississauga, Ontario L5M 2H3Fax: ++ 41-22-717 60 21 Tel. (905) 821-5193

Fax: (905) 821-5177

Asia: Japan:

DuPont (Thailand) Limited DuPont Toray Company, Inc.6-7th Floor, M. Thai Tower, All Seasons Place 1-5-6 Nihonbashi-Honcho,87 Wireless Road Chuo-ku, Tokyo 103Lumpini, Phatumwan JapanBangkok 10330, Thailand Tel. 81-3-3245-5080Tel: 662-6594060 Fax: 81-3-3242-3183Fax: 662-6594002

Web Address: www.kevlar.com

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