June 5, 2013.
Tires are one of the important parts of modern vehicles. Tires are the only parts of the vehicles that have contactsto the road surface. Thus, the quality of tires directly affects the performance of the traction and maneuvering vehicles, and more importantly the safety of the people on the road. Technology in the design and production of automotive tires has greatly been improved. It is not far from reach to tune the performance of tires to fit the specific needs of different vehicles, such as passenger cars, truck, vans, 4-wheel drive vehicles and buses.
In tire manufacturing processes,various materials are used to achieve the desired properties. Most important ingredients are natural and synthetic rubbers. Sulfur is essentially used to vulcanize the rubbers to improve mechanical properties of tires. Carbon black powder is used to allow molecules of rubber holding tight to the tire more resistant to wear and resistant to scratches.Zinc oxide is normally used to slow down the degradation induced by ultraviolet or UV from the sun.In addition, tires are reinforced with layer of canvas made of nylon yarn or polyester, and steel wires.
Although the design and manufacture of tire technology has developed enormously, modern tires still wear and degrade over time depending on the conditions of usage. Many people may be wondering when to change the tires. Some people believe that the newly produced tires should only be used and that those remain kept in the stock for long time are badly degraded. Answers or explanations for such a variety of questions and concerns are varied depending on who provide the answer. However, there is an absolute need for replacing the tire when the tire is swelling, cracks or wounded. It is recommended to replace tire when the tread wears out to reach the point indicated on the tire called TWI (Tread Wear Indicator) is reached. In the general case, many people choose to change tires according to the distance or duration of usage by the standard of the tire manufacturer. Some people choose to change tires according to their feeling, such as the noise from the tires become too loud or the braking distance become longer than normal.
It is generally understood that the temperature is one important factor that makes the tires wear out. Deterioration of the rubber part is due to oxidation of sulfur. Sulfur is a chemical that is added to a mixture of rubber in the manufacturing process of rubber tires for use as substances that crosslink between the molecules of rubber, resulting in better mechanical properties of rubber. When the conditions are right, oxygen chemically reacts with sulfur in tires. The rate of chemical reaction between sulfur and oxygen increased with temperature. For normal weather conditions in Thailand,a vehicle moving at speed of 120 km/h, the temperature of the tire may goes up to about75 degrees Celsius. The temperature will raiseto higher values when the vehicle speed increases or when the pressure in the tire is less than normal pressure. The more sulfur reacts with oxygen, the mechanical properties of the tire become worst. For an example,the rubber part of tires becomes harder, resulting in less comfort driving, less traction and maneuvering performance, and louder noise. Thus, determination the amount of sulfur bonded to oxygen in tires is one method that can indicate the quality of the tires.
A powerful techniques used for determination of sulfur speciation is the X-ray absorption spectroscopy (XAS) using synchrotron light. The technique is particularly useful in the chemical analysis of materials, providing information about the chemistry of materials, such as the type of the element of interest and, more importantly, the type, quantity and location of adjacent elements.
Figure 1. Schematic of a cross-section of automotive tire (from internet)
Figure 2.Sulfur K-edge XAS spectral taken from the different positions of a new tires; i.e. atthe tire shoulder, tiresidewall and tire tread. The spectra were taken at BL8 of the Synchrotron Light Research Institute, NakhonRatchasima. The reference spectrum of FeSO4 is also shown.
Figure 2 shows the sulfur K-edge XAS spectral taken from a new tire made by a international branded name company with manufacturing factory based in Thailand. The spectra shown are the spectra from the different positions of the tires; i.e. atthe tire shoulder, tiresidewall and tire tread. The XAS spectrum FeSO4, a reference chemical, is also shown for comparison. The vertical axis of the graph in Figure 2 is the absorption of X-rays, and the horizontal axis is the photon energy or the wavelength of X-rays. In this investigation, we are interested in the absorption of X-rays by the sulfur atom as a function of photon energy. It is clearly visible that there are two peaks at the photon energy of around 2472 eV and 2481 eV. Both peaks indicated that there main different species of sulfur, absorbing X-rays at different photon energies. The peak at about 2472 eV represents sulfur atoms act as crosslink between rubber molecules. These sulfur atoms are bonded either with sulfur or carbon atoms. The peak at approximately 2481 eV photon energy is sulfur that bonds with oxygen. This can easily be pointed out by comparing the XAS spectra of tires with that ofthe reference chemical. From the spectra shown in the figure, it was found that even in new tires, a number of sulfur atoms already formed oxide during curing in the tire manufacturing process.
Figure 3.Sulfur K-edge XAS spectra taken at the shoulder, sidewall and tread of a new tire and a 12,000-km used tire.
Figure 3 shows the XAS spectra of the shoulder, sidewall and tread of a new tire and a used tire with the same model. The used tire has actually been used on the real road for approximately 12,000 kilometers. It was found that the oxidation of sulfur increased significantly in the shoulder and sidewall of the used tire. The oxidation certainly contributes to the increasing hardness of these parts of the tire. Interestingly, the amount of sulfur oxide measured at the tread of the used tire is similar to that of a new tire. This may be due to the fact that the surface of the tread is continuously removed. It is interesting to point out that if the tread is designed to be worn over or equal to the rate of oxidation, the material of the tread remains more or less the same mechanical properties.
It is well known that temperature is the main factor to cause oxidation of sulfur in tire, and thus degrade the tire properties.In real life, tires have to bear the load or force exerted on the load and the change over time. Under such conditions, combined with the increasing temperature of the tire,the microscopic composition in tires changed. As a result,the physical properties of the tires are inevitably affected. If the tire has not been used and arekept in an environment that is not conducive to oxidation, for example, at room temperature and low humidity, tire could be kept for a long time.
Knowledge in molecular science and nanotechnology has greatly enhanced the development of automotive tires. Particularly in the last 10 years, new measurement techniques such X-ray absorption spectroscopy and other synchrotron techniques have been developed greatly. This allows us to understand the mechanisms of chemical additives in the latex on the properties of rubber and to be able to track changes that affect the performance of the tire. XAS is especially suitable for investigations of the deterioration of tires due to oxidation of sulfur in the tires.
School of Physics
Suranaree University of Technology