Short para aramid fiber reinforcement.

Para aramid fibrillated short fibers (pulp) and para aramid dipped chopped fibers (DCF) are used as reinforcing agents in rubber applications such as hoses, v-belts and tires. Twaron is Akzo's organic man-made high-performance para-aramid fiber. Its chemical designation is poly(para-phenylene terephthalanude). Dependent on the application, para aramid can be used as a continuous filament yam, short fiber and pulp fiber. Para aramids have a strongly crystalline structure, a high strength, a high decomposition temperature and a high resistance to elevated temperatures and most organic solvents. Table 1 shows a comparison of the physical properties of para aramid with other synthetic fibers. [TABULAR DATA 1 OMITTED]

When using pulp and short fiber to modify the elastomer properties, it is especially important to take care of:

* The adhesion of the fiber to the elastomer;

* An optimal dispersion of fibers in elastomer.

When the short fiber-elastomer composite is exposed to stress or compression, the fiber-elastomer interface is experiencing relatively high shear forces due to the enormous difference in Young's modulus of fiber and elastomer. Two adhesion types can be recognized: Mechanical and physico-chemical. Physicochemical adhesion can be achieved by applying an adhesive layer on the fiber surface. In case of para-aramid, adhesive activated fiber is treated with formulations containing RFL (resorcinol-formaldehyde-latex).

A proper dispersion of the fiber is necessary to achieve uniform composite properties. For instance, a cluster of short fibers, instead of uniform dispersed fibers, will only weaken the composite due to the fact that fibers on the inside of the cluster are not contributing to the elastomer's properties. These clusters may even act as actual failure sites.

Experimental

Twaron para aramid pulp is produced from chopped filament yarn by shearing a fiber suspension in water in a refining process. Under these conditions, the fibers become shorter and fibrillated. After isolation and drying, the pulp is obtained. Para-aramid pulp is characterized by its specific surface area and its fiber length distribution (table 2).

[TABULAR DATA 2 OMITTED]

The production of DCF starts with the dipping of yams on a filament level. Next, the yarns are assembled into strands that can be fed into the fiber cutter. The fiber cutter produces defined length DCF. DCF are characterized by their dip-pickup and the fiber length.

Compounds for determining the effects of short fibers on EPDM, CR and NR/BR compound properties were mixed in an internal mixer using a five minute cycle, 77 rpm rotor speed and an initial rotor temperature of 50[degrees]C. The formulations are shown in table 3. Curatives were added to the stock on a two-roll mill. The mill was run with a roller friction ratio of 1: 1,22 and a front speed of 25 rpm. The initial roll temperature was 50[degrees]C. Various characteristics were determined using the following test methods:

* Viscosity and scorch characteristics - ISO 3417;

* Cure characteristics - using an MDR 2000E with a 0.5[degrees] rotor arc at 160[degrees]C;

* Stress-strain results - ISO 37/2 using a dumbbell;

* Hardness - ISO 2783 ([degrees]Shore A);

* Abrasion - DIN 53.516;

* Compression set - ISO R 815; three days/23[degrees]C;

* Friction coefficients were determined against aluminum.

Table 3 - reference compound formulations
                      BR/NR         CR          EPDM
                    compound      compound     compound
                      phr          phr          phr
Neoprene WXJ           -           100           -
NR SMR 5              80            -            -
BR Buna CB 10         20            -            -
Keltan 778             -            -           100
N550                   -            40           80
N375                  40            -            -
N660                  10            -            -
Vestenamer V8012       5            -            -
X-100 oil              -            12           -
Ar. oil Ingralen 150   8            -            -
Sunpar 2280            -            -            50
Stearic acid           2             1            1
Magnesium oxide        -             4            -
Antigene 3C            -             1            -
Antigene PA            -             1            -
Zinc oxide             -             5            5
Soxinol TS             -             1            -
Soxinol D              -             1            -
Premanax 6PPD (R)      2             -            -
Perkacit MBTS c       0.6            -            -
Perkacit MBT c         -             -            1
Prkacit TMTD           -             -           0.5
Perkacit ZDBC c        -             -            2
Sulfur                2.5            1           1.5

The following model test was used to dynamically test fiber loaded compounds.

A sample of 20x20x20 mm, with fibers oriented MD or on-end were loaded with a sinus shaped stress on an Instron 8501. The resulting strain was measured. The phase angle 8 between stress and strain was used to calculate the heat generation per cycle. The samples were loaded as follows:

* Pre-conditioning during 100 cycles 0.05-1 MPa, frequency 1 Hz;

* 5 x 15 cycles 0.05-1 MPa, 1 Hz, last five of each run was measuring cycle;

* 15 cycles at a higher load of 2, 3 or 5 MPa

* Repeat of second step.

Theory of short fiber reinforcement

Analysis performed to predict mechanical properties such as the modulus of short fiber reinforced composites for small deformations have proven quite successful in many cases. This can simply be reflected by the Halpin-Tsai equation (ref 10) which predicts composite modulus in the direction of fiber alignment.

[E.sub.11] = 1 + [2n.sub.L][V.sub.f]*(1/d) 1 - [n.sub.L][V.sub.f]*[E.sub.m]

[n.sub.L] = [E.sub.f]/[E.sub.m]) - 1 ([E.sub.f]/[E.sub.m]) + 2(L/d) with: [E.sub.f] = fiber modulus; [aE.sub.m] = matrix modulus; L/d = aspect ratio or fiber length to diameter ratio; [V.sub.f] = volume fraction fibers.

In order to visualize the effect of a number of variables on the composite modulus, the Halpin-Tsai equation has been used to construct some graphs. Figure 1 shows the effect of the fiber modulus on composite modulus for various aspect ratios. It is clear that a high fiber modulus in combination with a high aspect ratio can lead to a tremendous increase in composite modulus. In case of para aramid the modulus is about 80 Gpa and the fiber diameter is 12 micron. For DCF the L/d ratio is a constant number that typically can range from 80 to 250. This corresponds with fiber lengths ranging from 1 to 3 mm. For pulp the average L/d is around 100.

Some important consequences of the Halpin-Tsai model are:

* An almost linear relationship between composite and matrix modulus. This makes the matrix modulus an important design parameter.

* For low volume fractions of short fiber, a linear dependence on the composite modulus is expected.

When short fibers are mixed with the rubber compound in an internal mixer and the processed, the fibers are not only uniformly dispersed in the matrix but are also oriented in the rubber flow direction. The ease of fiber orientation usually depends on the magnitude of shear in the last preparative step. The properties of the rubber will then be strongly anisotropic. The three main directions in which the rubber properties can be tested are machine, cross machine and on-end. The machine direction is the direction in which the fibers are normally oriented. The cross machine direction is perpendicular to this direction. On-end orientation means that fibers are perpendicular to the surface tested.

To provide maximum reinforcement, short fibers need to be properly oriented in the ultimate application. For parts where improved mechanical performance is desired, the fibers must be oriented in the direction of action. Wear resistance, on the other hand, is improved if the fibers are oriented perpendicular to the wear surface. Twaron short fibers are oriented in machine direction by conventional elastomeric compounding techniques, such as extruding, milling or calendering.

Results and discussion

A uniform dispersion is the most important requirement since fiber clumps may act as actual failure sites. Para aramid pulp fibers are highly fibrillated and are strongly mechanically interlocked, which makes a uniform dispersion difficult. To facilitate an even dispersion it is necessary to pre-open the pulp. This can be done by high shear mixing techniques. There is a positive effect of pre-opening of the pulp on reducing the internal mixer time and dispersability of the pulp fiber. However, shipping pre-opened and thus voluminous pulp is not an attractive economic activity.

Another possibility is to wet every pulp fiber. It is clear that a low viscosity polymer can do this job. However, shear forces must be high enough to wet every fiber. Experiments showed that mixing para aramid pulp with carbon black and the low viscosity polymer gave a pulp masterbatch that was relatively easy to mix into the rubber compound and gave the desired even dispersion of the pulp fibers. There is a positive effect of masterbatch technology on the modulus of an EPDM rubber compound. The masterbatch consisted of 40% pulp, 40% carbon black and 20% low viscosity polymer. Figure 2 compares the effect of pulp, a combination of pulp and carbon black, and masterbatch on the modulus of an EPDM compound. In all cases the fiber load is 5 phr. It is clear that the masterbatch technology gives the biggest increase in modulus.

In contrast, due to the fact that DCF's have a smooth RFL wetted surface, they are dispersed instantaneously into elastomeric compounds on the condition that the length does not exceed 5-6 mm.

Figure 3 shows the effect of fiber load on a stress-strain curve of a BR/NR tire compound in and across fiber direction. For both directions the presence of those relatively low fiber loads is lowering the ultimate tensile strength of the compound. In fiber direction an increase in modulus can be noticed. As expected, a higher concentration of pulp fiber gives a higher modulus. In fiber direction a pronounced yield point is visible. At the yield point the shear forces on the interface of fiber and rubber are becoming too big, and the fiber starts to slip in the matrix and is losing its reinforcing effect. More or less the location of the fiber is becoming a void and thus a potential failure point, when the stress increases. After reaching the yield point, the stress strain curve of the composite is becoming parallel to the one of the original BR/NR compound. The reinforcing effect has disappeared. In the direction perpendicular to the fiber the stress-strain curves of the fiber reinforced compounds are more or less equal to one of the compounds. This is an indication that the pulp fibers have a high degree of orientation.

The effect of pulp fiber reinforcement on other elastomers was also investigated. Table 4 shows the results. The same trends as above were observed, including a huge increase of the elastomer modulus in machine direction (MD) and a much smaller effect in cross machine direction. Although at room temperature the tensile strength of the compound is lowered by the presence of fibers, at elevated temperature (120[degrees]C) a gain can be observed. In case of tear strength a small increase can be found and also "on-end" abrasion resistance is improved. A negative effect on compression set can be expected, since the stresses at the same deformation are higher for the fiber loaded compounds.

Table 4 - effect of Twaron p-aramid pulp on
properties of elastomeric compounds
                     BR/NR        CR          EPDM
                   compound    compound     compound
                    10 phr       7.5 phr     10 phr
                    Twaron      Twaron       Twaron
                     pulp        pulp         pulp
                   MD   CMD     MD   CMD     MD    CMD
Modulus 25%        10x  2x      6x   1.2x    6x    2.5x
Modulus 50%        4x 1.8x     6x   1.2x    3x    2x
Tensile strength
-23[degrees]C     0.8x 0.7x   0.8x  0.7x    0.7x  0.6x
-120[degrees]C    1.3x 1.2x   1.2x  1.1x    1.2x  1.1x
Tear strength     1.2x 1.1x   1.3x  1.2x    1.3x  1.2x
Abrasion resistance
"on-end"          2x          1.7x          1.1x
Compression set
-24th; 125[degrees]C 1.1x                    1.1x
 -72h; 23[degrees]C           1.2x           1.9x

Figure 4 shows the stress strain curves for pulp and 3 mm DCF and the reference CR compound. It is clear that the DCF reinforced compound has a less pronounced yield point. It shows a very abrupt failure point. Furthermore in the low elongation range (until 5%) the modulus is extremely high. This behavior reflects the optimal physicochemical bonding between fiber and rubber. Although the shear forces on the fiber-rubber interface are high, the fiber will not easily slip in the rubber compound. The higher stiffness of the DCF reinforced compound translates in a lower friction coefficient and an improved on-end abrasion resistance. The effect of pulp and DCF on the former is illustrated in figure 5. The latter is also improved because the fibers that stick out of the surface are abraded away very slowly, which was shown by electron microscopy.

DCF showed a stronger interaction with the rubber than pulp or untreated fiber. However, this does not directly correlate with the heat build-up under constant load. Roughly pulp shows a somewhat lower heat build-up in most cases. This is illustrated in figures 6 and 7. The data are obtained by cycling between 0.05 and 1 MPa after intermediate cycles at higher pressures to simulate shock-loads. In case of MD orientation the direction of action is perpendicular to the fiber. In the fiber direction the deformation of the compound is strongly restricted. However, on the other direction perpendicular to the direction of action the modulus is low and the compounds can be deformed easily. In case of on-end orientation the fibers are seriously prohibiting deformation because of their high compression modulus and this leads to significant lower heat build-up on the test. When increasing shock-loads are used, the fibers can debond from the matrix and the energy loss is increased. For the on-end orientation this effect is much less present.

The use of short fibers in the rubber industry is not new. Various materials have been used in the last decades. Figure 8 shows the effect of short fibers made out of traditional reinforcing material on the modulus of EPDM rubber. Due to the high modulus, aramid pulp fiber has the most pronounced effect. In practice, this means small aramid fiber loads can replace relatively high other fiber loads in applications. An additional advantage of small fiber loads is a less permeable compound, the preservation of flexibility (or low modulus) in the CMD and processability of the compound. Another common reinforcing material In the rubber industry is glass fiber. Glass fiber has a high modulus and is heat resistant, but it is also very brittle. When mixing short glass fibers with a defined length in a rubber compound in an internal mixer, the fibers will break easily and become shorter and thus less effective.

When to use para aramid pulp or DCF is not purely an economic choice. As shown, both compounds are different in properties and only one compound may be suited for an application. Besides, the behavior of the compound in manufacturing is also important. It was found that in extrusion the pulp fiber reinforced compound shows a better processing behavior than the DCF. Unlike the DCF compound, extruded pulp reinforced compound showed smooth surfaces and edges.

Applications

Transmission belts

Transmission belts have short fiber reinforced rubber under the cord layer. The short fibers are oriented perpendicular to the surface that transfers the power. The increased hardness of the rubber in fiber direction gives the transmission belt:

* A lower friction coefficient and thus;

* A reduced noise level when in service;

* A lower heat build-up during the cyclic compression;

* An increase in transmission capability.

Both aramid pulp and DCF are suitable for this application. The low fiber load makes the belt compound flexible in the direction perpendicular to the short fiber and takes care about a low permeability. Real short DCFs (1-2 mm) are used in low-width belts where, with the low fiber load constraint, optimal adhesion and dispersability are both very critical to achieve the biggest change in properties. Higher DCF lengths are interesting for striking a balance between desired properties and cost.

Low pressure hoses

For example, an automotive heater hose can be reinforced with a knitted (para aramid) continuous filament yam construction. Two reasons for using a low amount of knitted reinforcement are to control the volumetric expansion and the bursting pressure of the hose. A small para aramid pulp fiber load (2-10 phr) has shown to:

* Control the volumetric expansion;

* Improve the coupling retention of an all rubber hose.

In this case the ultimate strength does not go up, but the load at a specific low elongation is controlled. For a hose with 19 mm (3/4") diameter, a bursting pressure of 2 MPa can be obtained. For a hose without other reinforcement a circumferential orientation of the pulp fiber is necessary. By using a so-called expanding mandrel die in hose extrusion, the expansion of the rubber compound in the extruder head forces the fibers to change to the desired orientation. Due to high hose service temperatures (>20[degrees]C), desired low permeability and extrusion properties, low para aramid pulp loadings are preferred in this application. DCFs are not suitable for this application because of the bad extrusion properties of a DCF compound.

High pressure hoses

The use of p-aramid pulp in the inner liner of braided high pressure hoses has the advantage of:

* An increased green strength of the liner; and therefore

* An improved production stability;

* An improved coupling retention;

* A better fatigue resistance.

The improved fatigue resistance can be explained by a reduction of the probability that pin-holing occurs because of the increased modulus of the liner compound. The fiber loaded compound can be applied between two plies in order to reduce the abrasion of the braiding layers. In view of the extrusion properties, DCFs are not suitable for this application, and low para aramid pulp loadings are to be preferred.

Tires

In the bead area, aramid short fibers give fewer mixing problems than high levels of high surface area carbon blacks like HAF N330. Advantages are offered by the high anisotropy and the increased dynamic modulus, leading to:

* A lower heat build-up; which will

* Extend the life of the bead compound; and

* Preserve the adhesion between bead wire and bead compound.

* When short fibers are mixed in a tread compound, this will lead to:

* A lower rolling resistance of the fire;

* Better water drainage;

* More uniform wear and probably;

* Less noise.

For off-the-road tires, like those used in trackless mining, the higher tear resistance and chunking resistance can be a big advantage. Both pulp and DCF are suitable for this application.

Seals

Para aramid pulp has already been used for a long time in the production of gaskets. Also for rubber seals and oil well packings the use of para aramid pulp or DCF may bring advantages. Some examples are:

* Better mechanical properties at elevated temperatures;

* Less creep;

* Higher abrasion resistance;

* Less swelling by solvents.

Summary and conclusions

To make short fibers an effective reinforcement in the rubber = maximal enhancement of properties), it is extremely important to take care about an even dispersion in the rubber and adhesion between fiber and rubber. Two morphological forms meet those two requirements. A masterbatch consisting of 40% fibrillated aromatic polyamide fiber, 40% carbon black and 20% low viscosity polymer shows favorable processing properties and mechanical adhesion to the rubber compound. Another form is RFL treated chopped fiber, where the RFL facilitates an even dispersion in the rubber and takes care about an optimal chemical adhesion.

* In fiber direction short para aramid (pulp) fibers in a rubber compound give a large increase in modulus at low elongation, e.g., in case of 5 phr in a CR compound 16 to 32x.

* Small loads of para aramid fiber can replace larger loads of other synthetic fibers. The typical replacement ratio is 2.5 or more. In case of strong physicochemical adhesion between fiber and rubber this ratio can easily go up to five or more.

* At 120[degrees]C P-aramid short fiber loaded rubber compounds show a higher tensile strength than reference compounds.

* Para aramid short fiber loaded rubber compound shows improved "on-end" abrasion.

* DCF loaded rubber compounds show poor extrusion behavior. Pulp fiber loaded compound has satisfying extrusion properties.

* The energy loss of fiber loaded compounds is 30-60% lower than reference compounds, also after shock-loads.

References

[1.] LI. Rueda, Influence of the adhesive system on composite consisting of an elastomeric matrix and short fiber, Kautschuk + Gummi Kunststoffe, 45, 1061 (1992). [2.] A.P. Foldi, Reinforcement of rubber compounds with short, individual fibers, Rubber World (May), 19 (1987). [3.] Y. Uchiame et al, Friction and wear of short fiber reinforced rubber, Journal of Applied Polymer Science: Applied Polymer Symposium 50, 283 (1992). [4.] N. Wada et al, Dispersion and orientation of short fiber reinforced rubber composites, Kautschuk + Gummi Kunststoffe, 44, 1142 (1991). [5.] L.A. Goettler, Short fiber-rubber composites; Handbook of elastomers, new developments and technology, chapter 7, Ed. A.K. Bhownick and H.L. Stephens, New York (1988). [6.] Technical applications of Santoweb fibers in the rubber industry - a survey, International Polymer Science and Technology, 10, T/35 (1983). [7.] U.S. Patent 1,493,836, Monsanto Company, Extrusion process with expanding mandrel die (1974). [8.] J. W. Rogers, The use of fibers in v-belt compounds, Rubber World (March), 27 (1981). [9.] D. C. Prevorsek et al, Prospects for short fiber reinforcements in tire and rubber technology, Proceedings of Rubbercon 92, Brighton, 211 (1992). [10.] J.E. Ashton, et al., Primer on Composite Materials, Technomic Publishing Company, Westport, CT (1969).