Para-aramid is used in high-performance tires due to its excellent mechanical properties such as high strength, high modulus, fatigue resistance, and low creep.
Since the first pneumatic tire of Michelin was officially produced in 1895, along with the rapid development of the automobile industry, the tire industry has continuously improved technology and brought forth new products. Now a mature and complete tire production chain has been formed worldwide. For tires, skeleton materials are key materials to ensure tire strength, load bearing and maintain tire dimensional stability. With the increasing popularity of radial tires, high-performance radial tires and green environmentally friendly tires are gradually developing, which puts higher requirements on the performance of skeleton materials. Para-aramid is gradually used in high-performance tires due to its excellent material properties.
Classification and performance of tire skeleton materials
Skeleton materials are the main stress-bearing layer of rubber products, which play a decisive role in the performance, service life and use value of rubber products. Ideal skeleton materials must have mechanical properties such as high strength, high modulus, fatigue resistance, low creep, and physical and chemical properties such as low density, high and low temperature resistance, corrosion resistance and flame retardancy. There are four main categories of tire fiber skeleton materials, each with its own advantages and disadvantages.
(1) Rayon cord has excellent high-temperature modulus retention and low shrinkage characteristics, and its dimensional stability is far better than that of nylon and polyester cords. It can be used in radial tire carcasses to enable tires to obtain excellent handling performance, but rayon cord has low wet strength and serious production pollution.
(2) Polyester cord has the advantages of high modulus, high strength, low elongation, low heat shrinkage, good dimensional stability, and roughly the same dry and wet strength. Its fatigue resistance and impact resistance are better than rayon cord, but its temperature rise will cause aminolysis at high temperatures.
(3) The advantages of nylon cord are high strength, low relative density, low hysteresis loss, low moisture absorption rate, high wet strength, good elasticity, 10 times higher flex resistance than rayon cord, and better fatigue resistance than other fiber cords. The main disadvantages are large heat shrinkage, poor thermal stability and dimensional stability.
(4) Aramid cord has gradually been expanded in recent years due to its high temperature resistance, high strength, high modulus and small deformation. Studies have shown that aramid cord can not only reduce tire mass and rolling resistance, but also help improve tire puncture and cut resistance.
The performance comparison of commonly used tire skeleton materials is shown in Table 1.
| Items |
Para aramid |
Steel Wire |
Rayon |
Nylon 66 |
Polyester |
| Density/（Mg∙m^-3） |
1.44 |
7.85 |
1.53 |
1.14 |
1.38 |
| Decomposition temperature (nitrogen)/°C |
>500 |
1600 |
200 |
255 |
260 |
| Tensile strength/MPa |
2830 |
2550 |
780 |
960 |
1150 |
| Specific strength/(mN∙tex^-1) |
1970 |
330 |
510 |
840 |
830 |
| Initial modulus/GPa |
80 |
160 |
18 |
6 |
14 |
| Specific modulus/(N∙tex^-1) |
55 |
20 |
12 |
5 |
10 |
| Thermal expansion coefficient ×10⁶/K^-1 |
-2.2 |
3.7 |
- |
- |
- |
| Elongation at break/% |
3.6 |
1.9 |
13.0 |
20.0 |
13.5 |
| Strength retention rate after 200 °C×48 h/% |
90 |
100 |
20 |
45 |
55 |
| Air dry heat shrinkage (160°C×4min)/% |
<0.1 |
0 |
1.0 |
3.8 |
5.0 |
The specific strength and specific modulus of para-aramid are nearly 6 and 3 times that of steel wire, and 2 and 10 times that of nylon 66. The performance of para-aramid materials can still maintain normal levels in the temperature range of -200 to 200°C. Under the premise of the same tensile strength, the mass of rubber products can be greatly reduced, and it can meet the use under extreme climatic conditions. Para-aramid has excellent low creep, fatigue resistance, low heat shrinkage and chemical corrosion resistance, which can significantly improve the dimensional stability of rubber products and extend the service life. Therefore, para-aramid is an extremely ideal tire skeleton material.
Application of para-aramid in tire skeleton materials
The first application of aramid in tire structure was as a radial tire belt layer. Due to its special properties, it is now being used in an increasing number of different tire components. From a technical point of view, aramid can be used as a skeleton material for any part of the tire, and can even be combined with resin to replace steel wire to make tire beads, greatly reducing the weight of the tire. At present, there are examples of aramid application in exposed edge belt layer, folded belt layer, circumferential belt layer, radial tire carcass, bead cover, wire ring, racing bias tire carcass, and bias tire buffer layer.
The application of 1680dtex/2 aramid cord instead of 2×0.30HT steel cord in the belt layer of passenger car tire can reduce the weight of the tire by 6.4% to 7.5% and the rolling resistance by 12.5%. Using 1680dtex/2 aramid cord for carcass and belt layer can reduce the weight of the tire by 15%, and other tire properties meet the standard requirements, and improve the ride comfort of the car.
At the same mass, its strength and modulus far exceed those of materials such as polyester, polyamide and steel wire, and it is extremely excellent in safety and durability. The most significant advantage of using aramid cord instead of steel cord for foldable racing tires is that it reduces the weight of the tire and makes it foldable. The lightweight characteristics of aramid are very suitable for high-performance racing tires. While reducing the weight of the car, it can achieve faster braking and better cornering performance.
Application of aramid short fiber in the filling rubber of all-steel load-bearing radial tires. The special structure and properties of aramid short fiber itself can give rubber products excellent properties such as high modulus, high hardness, high strength, dimensional stability, and wear resistance. The test results show that aramid short fiber reduces the Mooney viscosity of the rubber compound, shortens t90, reduces temperature rise, reduces loss factor, reduces Payne effect, and has good durability of the finished tire, indicating that aramid material is very effective in improving tire performance.
| Items |
1100dtex/2 |
1680dtex/2 |
| Breaking strength/N |
≥330 |
≥450 |
| Uneven rate of breaking strength/% |
≤3.5 |
≤3.5 |
| 66.6 N Elongation at rated load/% |
1.8±0.6 |
2.0±0.6 |
| Elongation at break/% |
4.5±1.5 |
6.0±1.5 |
| Elongation at break unevenness/% |
≤5.0 |
≤5.0 |
| Dry heat shrinkage/% |
≤0.5 |
≤0.5 |
| H Extraction force/（N•cm^-1) |
≥100 |
≥130 |
Although aramid cord has many advantages, as a rubber skeleton material, it also has problems such as poor compression and bending fatigue resistance, difficulty in bonding, and high production cost, which limit the wide application of aramid cord. Therefore, aramid/high-strength nylon composite cord fabric has been developed. The composite cord fabric obtained by spinning, twisting, dipping and heat treating aramid and nylon in a certain ratio can eliminate the shortcomings of nylon and aramid and combine the advantages of both, realize the complementary material properties, and meet the stringent requirements of high-performance tires. In the future, composite cord fabric will be used more and more widely in tires.
