Compounding with para-aramid fiber engineered elastomers.

DuPont Kevlar brand fiber, the world's first para-aramid fiber, was introduced in the 1970s. It is known for its high strength to weight ratio, high modulus, and excellent chemical and thermal stability. Initially, Kevlar was offered in continuous filament form, and soon found applications in tires, mechanical rubber goods, bullet resistant vests and composites. In the 1980s, short forms of the fiber - staple, floc and pulp - were introduced, and quickly found acceptance in cut-resistant protective apparel, gaskets and friction materials.

Once short forms of Kevlar and Acordis' para-aramid Twaron were introduced, they were evaluated for rubber reinforcement. Using short fibers to reinforce rubber is common in rubber goods. They improve green strength, provide dimensional stability prior to cure and improve mechanical properties of the vulcanizate. Cellulosics, cotton, denim, polyester and nylon are commonly used in the industry. Compounders found that they could incorporate para-aramid staple or floc (we define floc as staple less than 6 mm long) into rubber using an internal mixer or a roll mill, often with difficulty. Incorporating the high surface area pulp product proved to be exceedingly difficult. Only a few people were able to adequately disperse pulp into a rubber compound. However, their work did demonstrate the superior reinforcement potential of aramid pulp once the dispersion limitation was overcome.

DuPont initiated studies to define a method to disperse para-aramid pulp into rubber, and this effort led to development of a unique new technology platform for dispersing pulp into an elastomer matrix. Products produced via this technology showed superior dispersion of aramid pulp in rubber. Samples of rubber compound of identical composition were analyzed using an ultrasonic scanning technique that measures relative porosity and are shown in figure 1. In the sample on the left, the pulp was introduced using engineered elastomer; in the sample on the right, the pulp was added directly into the rubber. A uniform color indicates a homogenous mixture. The sample made by compounding pulp directly into the rubber shows significant color differences indicating relatively poor fiber dispersion; the sample prepared using engineered elastomer is nearly a uniform color, demonstrating its excellent dispersion.

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Product made via this new technology enabled dispersion of pulp into rubber so well that it was given a new name, Kevlar engineered elastomer. Initial evaluation in the rubber industry confirmed that engineered elastomer was far easier to process than dry para-aramid pulp, and confirmed the improved dispersibility of pulp made possible by using this offering.

Improved reinforcement efficiency

One of the first rubber chemists who evaluated engineered elastomer found it very effective in rubber reinforcement. This person described the offering as having "... an intimacy between the rubber and particle never before reached via conventional compounding."

We also learned that several other customers who had been able to achieve good dispersion of dry para-aramid pulp in rubber found better reinforcement when using engineered elastomer to introduce pulp into their compound. They obtained up to 20% higher modulus at the same fiber content when using engineered elastomer rather than using raw pulp.

We have proposed a hypothesis to explain the basis for this improved reinforcement; the hypothesis is based on:

* Superior dispersion;

* the microstructure of para-aramid fiber;

* the presence of `bound rubber' in engineered elastomer; and

* the process used to manufacture engineered elastomer.

The polymer base for para-aramid is poly(p-phenylene terephthalamide); a rigid rod molecule. When spun into fiber, the polymer becomes highly oriented and highly crystalline. The high orientation allows extensive hydrogen bonding between the carbonyl and `N-H' functionalities in the amide groups of adjacent polymer chains.

The fiber has a structure as shown in figure 2 (ref. 1). Morphological studies show the crystal structure is oriented radially, so the fiber has a backbone of strong covalent bonds along the fiber axis and hydrogen bonding in the radial direction. The hydrogen bonded radial sheets stack and are bonded together by Van der Waal's forces.

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The fiber may be viewed as having a fibrillar structure. Fibrils are very highly ordered regions oriented along the fiber axis. The fibrils are joined by bundles of tie fibrils that pass from one fibril to another.

This fibrillar filament structure can be mechanically abraded to form a high surface area pulp. Because of the fiber morphology, the pulp fibrils have a surface of aromatic rings and amide groups.

Kevlar fiber is spun from highly concentrated sulfuric acid. Free [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] in the solvent sulfonates some of the aromatic rings, and studies within DuPont suggest that the resulting sulfonic acid groups tend to be accessible at the fibril surface. Thus, the pulp fibril surface contains polar groups (amide and sulfonic acid on the polymer backbone, as well as amine and carboxylic acid end groups) that can associate with a group on an elastomer.

Evidence for association of an elastomer with charged groups on the fibril surface is provided by gravimetric determination of fiber content of engineered elastomer. Engineered elastomer grades in polychloroprene contain 23% fiber on a weight basis; gravimetric analysis averages about 26%. Gravimetric analysis of `apparent' fiber content of engineered elastomer in the more polar NBR matrix averages 29.1% fiber, at the nominal fiber concentration of 23 weight percent. We propose that the gravimetric analyses provide the evidence for `bound rubber' in engineered elastomer, similar to bound rubber in carbon black.

Bound rubber theories for carbon black assume that segments of elastomer molecules adhere to active sites or reactive sites on the filler particles. Leblanc describes this theory in a recent publication (ref. 2). A similar mechanism could certainly be operative in engineered elastomer.

A key requirement for our hypothesis is high accessibility of the surface of the pulp to the elastomer. The patented latex coagulation process used to prepare engineered elastomer presents the pulp to the elastomer in a way that the fiber is fully open to allow the elastomer to completely wet the fibrils.

The engineered elastomer process maximizes the wetting of the pulp, allowing it to reinforce with maximum efficiency. If pulp is mixed directly in an internal mixer or roll mill, or if a masterbatch of pulp in rubber is made by other technologies, the pulp can become compacted to some degree, and its reinforcing potential can not be fully realized. The process by which engineered elastomer is manufactured creates the intimacy between the rubber and particle. Engineered elastomer is more than a simple masterbatch or dispersion of pulp in elastomer.

Experimental

Compound formulations used in this work were based on those published in the Vanderbilt Handbook (ref. 3). In all cases, the aramid pulp was introduced to the compound using Kevlar engineered elastomer.

All compounds were mixed in a lab scale internal mixer. The mixing procedure is important to achieve proper mixing. Neat elastomer and engineered elastomer were mixed at low speed with maximum cooling for about 1.5 minutes, followed by addition of fillers and process aids. When the dump temperature appropriate to the base elastomer being used was reached, the batch was dropped to a mill, sheeted off and cooled. Curatives were added on the mill, or in a second mixer pass. Testing was done using standard ASTM or ISO test methods.

Reinforcement with aramid pulp

Achieving high compound modulus with traditional stiffening agents (carbon black, silica, resins) typically requires high loadings. At high loadings, processing becomes difficult because of high compound viscosity, and dispersion becomes a challenge. Since short fibers outperform simple particulates as reinforcing agents, they are often used when high modulus is required (ref. 4).

Para-aramid pulp builds modulus more readily than other short fibers, leading to lower compound viscosity for a given modulus. Figure 3 compares reinforcement by aramid pulp with other short fibers in a polychloroprene GRT power transmission belt compound. Reinforcement with three parts pulp gives the same modulus as 10 parts of 6-mm polyester; however, compound viscosity is nearly 15 Mooney units lower. Reinforcement with 7.5 parts pulp gives about twice the modulus of 10 parts of 6 mm nylon; again, Mooney is about 10 units lower.

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A generalized plot of the effect of para-aramid pulp on modulus is shown in figure 4. Modulus ratios at 25% and 50% elongation (measured in the machine direction) are shown as a function of parts fiber. The modulus ratio is defined as:

Modulus of compound reinforced with pulp/ Modulus of compound without pulp

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The data points are for a number of different compounds in several different elastomers. The aramid pulp was introduced to the compound via engineered elastomer in all cases. Compound modulus at 25% elongation can be increased by about 10x with addition of only 5 phr pulp.

Para-aramid pulp has a high L/D aspect ratio. This geometry makes possible an orientation of the particle when sheared in processing. Calendering or extruding compounds reinforced with aramid pulp leads to modular anisotropy - a difference in modulus between the machine direction and cross machine direction. This modular anisotropy is illustrated in the data shown in figure 5. MD modulus is about 5x that of XMD modulus in 2 mm test pieces of this NR/SBR fire tread compound. Samples calendered or extruded to thinner sheets will display even higher anisotropy. Some users of engineered elastomer routinely achieve MD/XMD ratios greater than 10.

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The ability of para-aramid pulp to build modulus very efficiently, and the relative ease by which it can be dispersed into rubber via engineered elastomer, affords the rubber chemist opportunities to go beyond present limits in designing rubber compounds. Modular anisotropy is an important characteristic in many product designs.

Applications

Belt body compounds (power transmission belts)

Aramid pulp is used as a reinforcement in a number of high performance, high load power transmission belts. It provides stiffness to the body in a v-belt, and can increase tooth shear resistance in synchronous and timing belts.

The high modulus and modular anisotropy, made possible with pulp reinforcement, are used in design of high performance belts. Belts can be designed to better withstand distortion at high loads. One advantage of using aramid pulp for reinforcement in a belt body is illustrated in figure 6. Compounds reinforced with aramid pulp have lower hysteretic properties than those reinforced with higher loadings of other short fibers or high loadings of carbon black. Lower hysteresis leads to lower heat build up, which can extend service life of the belt. Belt body compounds reinforced with aramid pulp also have lower Mooney, as mentioned earlier.

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Tensile gum compounds (power transmission belts, conveyor belts, tires)

Aramid pulp is used in two ways in the cord (gum or adhesion) layer of a belt or fire.

It is used at low loadings as a process aid to improve in green strength, which facilitates more efficient processing.

At higher loadings, it can improve belt life. The compound in the cord (adhesion) layer must have sufficient flowability (low compound viscosity) to adequately penetrate and coat the cords; consequently, the compound modulus is limited. The belt body compound must have high stiffness to prevent distortion or deformation as described above. The modulus mismatch between the two rubber stocks can lead to cracking and delamination at the interface. Increasing the stiffness of the rubber in the cord layer by use of short fibers or higher loadings of carbon black has a negative impact on adhesion or flowability of the rubber. Para-aramid pulp reinforcement of the cord layer is a solution; it can build modulus with minimal effect on flowability. Higher modulus of the cord compound also reduces the difference in modulus between it and the tensile member - a concept we call modulus bridging. The feed rubber for the aramid pulp is best prepared using the latex coagulation technology used to manufacture engineered elastomer; "... the aforementioned rubber layer is made of a feed rubber prepared by solidifying a rubber latex in which short fibers are blended and dispersed" (ref. 5).

Fabric reinforcement or replacement (diaphragms, tires)

Aramid pulp reinforced rubber has successfully been used as a fabric replacement in several low to medium pressure applications. The high modulus attainable in thin sections (by calendering or extrusion) can eliminate the need for fabric. Coating a fabric with a compound containing aramid pulp reinforcement can prevent delamination of the fabric by the modulus bridging concept described above.

Roll covers

Aramid pulp reinforcement of rubber used in roll covers has been shown to improve wear and abrasion resistance of the roll cover. One mechanism leading to abrasion of a rubber covered roll (ref. 6) results from the pressure on the working roll, which causes a stress on the rubber covering. This stress leads to a strain, or deformation of the rubber. The strain causes a bulge in the rubber as it approaches the pressure point (roll nip.) The shape of the roll cover changes from the dotted line to the solid line, as shown in figure 7.

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The rubber moves rapidly in the entry bulge as the turning roll approaches the point of maximum stress. The rubber is subjected to a tangential shear stress and a normal friction stress. Since the shear stress is greater than the friction stress between points A and B, the rubber slips in this region. This slipping under stress can lead to abrasion of the roll cover. A similar situation exists in the exit bulge and slipping occurs between points C and D.

Using aramid pulp to reinforce the roll cover compound can help reduce wear due to slipping and abrasion. By designing the roll so fibers are aligned in the circumferential direction, the modulus or stiffness of the compound can be increased, reducing strain at a given stress. However, because of the modular anisotropy possible with aramid pulp reinforcement, radial and axial moduli are increased to a lesser extent.

Increasing compound modulus using traditional stiffening agents typically results in an increase in compound hardness. All too often, this increase in hardness means a sacrifice in roll grip, since a harder roll may have less desirable frictional properties. Engineering the rubber compound by balancing the relative content of aramid pulp and other reinforcing agents allows for increased modulus without an increase in hardness. This is illustrated in figure 8. Compounds were prepared at different loadings of silica and aramid pulp. The control (no fiber compound) has a durometer hardness of 81A. A compound of 80 durometer was prepared with [is greater than] 6x higher modulus (13.1 vs. 2.1 MPa) by addition of 9 phr aramid pulp, while decreasing silica from 45 to 15 phr.

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The data from this compound study illustrate the importance of re-engineering a compound to make the best use of aramid pulp reinforcement. It is highly unlikely that one can achieve property goals by simply dropping aramid pulp into an existing formulation.

Additional data from this study are shown in figure 9. Incorporating aramid pulp into the rubber compound enabled an improvement in both tear and abrasion - the two key properties for improved performance in a rubber covered roll. As with the belt compound properties shown in figure 3, aramid pulp enabled desirable improvements in modulus, tear and abrasion resistance, without affecting hardness or affecting processability, as shown in figure 9.

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Subtread/cap ply compound (tires)

Aramid pulp is used as a reinforcing agent in a number of premium bicycle tires. A stable, stiff subtread reduces squirm of the tire, and improves handling of the bicycle. Engineering a subtread compound with aramid pulp reinforcement is a route to lower rolling resistance, better cornering performance, reduced abrasion and a greater resistance to punctures (ref. 7).

Aramid pulp reinforcement is used effectively in the subtread and cord layers of several high-performance motorcycle tires to improve their handling characteristics (ref. 8).

Aramid pulp reinforcement was evaluated in several areas of passenger car tires (ref. 9); it is particularly effective in the apex and crown of the tire. Recent work has shown advantages of an aramid pulp reinforced ply support strip (ref. 10). Most recently, an aramid pulp reinforced subtread was engineered to eliminate the cap ply of a high-performance tire (ref. 11).

Footwear (soling)

Footwear soling compounds have been formulated to improve friction and slip properties, improve wear and abrasion and resistance to stitch tear without sacrificing sole flexibility. The key has been the ability of aramid pulp to enable reformulation of the compound to achieve the desirable properties.

Extruded and molded components

Aramid pulp reinforcement increases the modulus of molded and extruded goods. The increased stiffness improves the resistance to deflection, and enables thinner cross sections, lowering the weight of the component.

The increased stiffness of rubber compounds reinforced with aramid pulp is of particular advantage in seals and gaskets, improving the resistance to blowout.

Conclusions

Kevlar engineered elastomer provides the compounder a vehicle to bring the benefits of para-aramid pulp reinforcement to rubber; it overcomes the dispersion limitation that had been a barrier to successful use of pulp in rubber. The process by which engineered elastomer is manufactured presents the pulp to the elastomer in a way that the fiber is fully open; the elastomer can completely wet the fibrils. The engineered elastomer process maximizes the wetting of the aramid pulp, allowing it to reinforce with maximum efficiency.

Para-aramid pulp reinforcement of rubber enables opportunities for the rubber chemist to go beyond present limits in designing rubber compounds. Compounds reinforced with aramid pulp can be developed which have:

* Increased green strength;

* improved processability;

* improved dimensional stability;

* improved flow;

* increased tensile modulus;

* potential for anisotropic properties;

* increased dynamic modulus;

* lower tan delta;

* increased tear resistance;

* increased abrasion resistance; and

* improved penetration resistance.

Engineered elastomer is used to reinforce a variety of finished articles including belts, tires, footwear, roll covers, seals and molded components.

References

(1.) H.H. Yang, Aromatic High-Strength Fibers, Wiley-Interscience, New York, 1989.

(2.) J.L. Leblanc, J. Applied Polymer Science, 66, 2,257 (1997).

(3.) R.F. Ohm, editor, The Vanderbilt Rubber Handbook, 13th edition, R.T. Vanderbilt Company, Inc., Norwalk, CT.

(4.) S.K. De and J.R. White, Short Fiber - Polymer Composites, Woodhead, Cambridge, 1996, Chapter 9.

(5.) M. Ogino, Japanese Patent Application HEI 10[1998]-103413 (1998).

(6.) P. Metlikovic, "Stresses, slip and abrasion of rubber covered conveyor rollers, a review," paper 66, ACS Rubber Division Meeting, Louisville, 1996.

(7.) Specialized Bicycle Components, "Specialized vindicated in tire re-test," Press Release, Morgan Hills, CA., July 17, 1998.

(8.) G. Armellin, U.S. Patent 5,975,175 (1999).

(9.) R.J. Brown and R.M. Scriver, U.S. Patent 4,871,004 (1989).

(10.) P.R. Appleton, European Patent Application EP 0 931 676 (1999).

(11.) M. Nahmias Nanni, A. Brunacci and C. Zanichelli, World Patent Application WO 00/24596 (2000).