Tires made of short fiber reinforced rubber.

Recently, reduction of tire weight and rolling resistance has become one of the most important factors in tire performance, and like fuel economy of automobiles, is necessary for improving the global environment.

Tires are composed of composite materials topped with rubber such as carcass which is made with chemical cords e.g. polyester, rayon or aramid, and breakers made of plated steel cords. Other components are made of rubber compounds of NR, SBR or BR reinforced with carbon black.

There are several methods for reducing tire weight and rolling resistance. For rolling resistance reduction, energy loss reduction of a rubber compound is made by changing the polymer type or by reducing carbon black content in the compound, add, for tire weight reduction, the number of carcass plies and rubber gauges are reduced.

However, these methods accompany degradation of grip of tread rubber and stiffness necessary for the tire, giving negative effects on the handling stability basically required of the tire for a safe ride.

To cope with the degradation of grip, attempts have been made to compromise low rolling resistance and grip by developing solution SBR, etc. But in regard to tire stiffness decrease, no effective method for achieving low rolling resistance and weight reduction has been discovered until recently.

That is to say, if carbon black content is increased to improve rubber modulus, energy loss is increased causing poorer rolling resistance. And, even in the case where the rubber modulus could be increased by applying some other methods, the tire stiffness will increase in all directions, giving negative effects on other performance factors, e.g. ride, comfort, etc.

If short fibers in a rubber compound are oriented, the rubber compound developed an anisotropic property, consequently, it becomes possible to improve the modulus in the direction of short fibers only. In addition, being different from the case where modulus improvement is made by employing reinforcement agents such as carbon black, there is no energy loss increment.

So, experiments were conducted to help decide if it is possible to reduce tire weight and rolling resistance by employing short fiber reinforced rubber (SFRR) which could then be capable of improving rubber modulus in a required direction only. And, as a result, SFRR proved to be a very effective material for tires.

Experiments

The short fiber used for the test was nylon short fiber of 0.2~0.3 [mu]m in diameter and 100-200 [mu]m in length.

Short fibers were mixed with NR into masterbatches, and in the mixing process, short fibers were bonded to NR using a coupling agent.

Short fibers contained in the compound can be oriented to the direction of extrusion on calender rolls or extrusion machines usually used for ordinary rubber products.

Figure 1 shows the measurement results of complex modulus (E*) of the SFRR and its energy loss (tan [delta]). As it shows, E* in the short fiber's direction increases, and tan [delta] decreases in both directions.

Trial tire evaluation

Trial tires were made in 185/65R14 size incorporating SFRR components, and were tested on the following items:

* tire weight, gauge, etc.;

* rolling resistance;

* on-vehicle handling stability at the proving ground.

SFRR application to tread region

Tests were conducted to find whether it is possible or not to improve rolling resistance and other performances by improving the tread compound which occupies the largest component volume of a tire.

In making trial tires, a part of the carbon black in the tread compound was replaced with short fibers, orienting the short fibers in the tire's circumferential, radial and vertical directions.

Consequently, rolling resistance was reduced by about 10% in each case, and with regards to handling stability, tires with short fibers oriented to the radial direction showed improved lateral stiffness, therefore, improved steering response. However, due to the reduction of the carbon black content, absolute grip proved to be inadequate, and especially, the grip on wet road was too weak for practical use.

To cope with this situation, trial tires were made and tested adopting two layer construction tread (cap/base tread construction), with the cap tread made of ordinary tread compound, and the base tread composed of SFRR whose short fibers were oriented to the radial direction.

As a result, low rolling resistance and steering response improved, and the tire grip improved as well.

SFRR application to sidewall region

As in the case with SFRR application to the tread region, trial tires were made and tested replacing a part of the carbon black contained in the sidewall rubber with short fibers oriented to circumferential and radial directions of the tire.

The trial tire was made and tested without changing sidewall rubber gauges. Test result showed that tire stiffness contributes a great deal to the handling stability, and the tire whose short fibers were oriented to circumferential direction proved to have much improved tire stiffness and showed extremely improved steering response, but on the contrary, its lide comfort degradid.

A second trial tire was made reducing sidewall rubber gauge to improve the ride comfort and to decrease the tire weight. Consequent-ly, we could obtain tires compromising the steering response and the ride comfort. In addition, the sidewall rubber weight was reduced by about 40%.

SFRR application to bead region

Bead apex is a component which contributes a great deal to tire stiffness by holding the carcass firm with its hard rubber. So, compound formulations different from those of tread sidewall were adopted. In this case, instead of replacing carbon black with short fibers, they were added to the original bead apex compound formulation to get a harder compound, and orientations were given to the short fibers in the tire's circumferential and radial directions.

Trial tires that had short fibers oriented to the radial direction of the tire proved to have improved tire stiffness, consequently, improved steering response, but the ride comfort against shocks, etc., degraded.

Then, adjustment of tire stiffness was attempted by reducing bead apex volume. As a result, the steering stability the ride comfort were compromised, and the tire weight was decreased since the apex volume was reduced.

Combination

Based upon the test results mentioned, trial tires were made incorporating SFRR components with good test results (into a tire).

Results

* SFRR is capable of improving rubber modulus in the direction of the short fiber only.

* If applied to tire components, SFRR is capable of reinforcing the tire stiffness in the desired direction only, and the use of SFRR makes it possible to reduce the tire weight by changing tire, gauges, etc., while maintaining tire stiffness on a proper level.

* Being different from the reinforcement using carbon black, SFRR is capable of improving rubber modulus without increasing the tire's energy loss.

* By utilizing this property of low energy, loss it is possible to achieve low rolling resistance of tires.

Conclusion

If proper direction of the short fiber is selected and the SFRR is applied to various tire components, it becomes possible to control the tire's stiffness in the desired direction only, which enables us to reduce tire weight and rolling resistance. And at the same time, this makes it possible to develop safe and comfortable tires which are also excellent in handling stability, especially steering response, which used to have been thought of as contradictory factors to tire weight reduction and low rolling resistance.

[Figures 1 to 3 ILLUSTRATION OMITTED]