Now the tire builder adds the steel belts that resist punctures and hold the tread firmly against the road. The tread is the last part to go on the tire. After automatic rollers press all the parts firmly together, the radial tire, now called a green tire, is ready for inspection and curing.

The curing press is where tires get their final shape and tread pattern. Hot molds shape and vulcanize the tire. The molds are engraved with the tread pattern, the sidewall markings of the manufacturer and those required by law. Tires are cured at about 300 oF for 12 to 25 minutes, depending on their size. The tires are popped from their molds and taken to final finish and inspection. If anything is wrong with the tire, it should be rejected. An inspector’s trained eyes and hands catch some flaws; specialized machines find others.

Some tires are pulled from the production line and X-rayed to detect any hidden weaknesses or internal failures. Also, quality control engineers regularly cut apart randomly chosen tires and study every detail of their construction that affects performance, ride or safety .

Figure 2: Cross-Section

D. The Chemistry of Tires

Introduction

Vulcanization, or the process by which rubber is heated with sulfur to create a network of chemical cross-links, was invented by Charles Goodyear in 1839. It produces a finished product that is not sticky like raw rubber, does not harden with cold or soften much except with great heat, is elastic, springing back into shape when deformed instead of remaining deformed as unvulcanized rubber does, is highly resistant to abrasion. The process, a key advancement during its time, has been refined and enhanced since.

Natural rubber, also known as isoprene, when vulcanized will form a three dimensional network of mono-, di-, and polysulfide bridges which give the rubber its characteristic strength and elasticity. It is also important to note that the cross-links that give the tires these properties are not just sulfide linkages. They can be ionic clusters, polyvalent organic clusters, or polyvalent metallic ions. The process increases retractile force of the material, while decreasing the amount of permanent deformation occurring with the removal of a load.

The other major chemical process associated with tire manufacturing is the process by which brass is coated onto the steel belts, which are used in tire reinforcement. The brass coating adheres better to the rubber, and also helps to increase the retractile force of the composite material .

Vulcanization

The process of vulcanization profoundly changes the molecular structure of rubber, with the average distance, in terms of molecular weight, between linkages being approximately 4000-10000. Hard rubber is vulcanized rubber in which 30 – 50 % sulfur has been mixed before heating; soft rubber contains usually less than 5 % sulfur. After the sulfur and rubber (and usually an organic accelerator) are mixed, the compound is usually placed in a mold and subjected to heat and extreme pressure A vulcanized material cannot be processed in an extruder, mixer, or any device, which requires the material to flow. Therefore, the vulcanization is done after the material has taken its final shape or form .

Figure 3: Sulfide Network Formation

The characterization of polymers starts with certain properties such as hysteresis, tear strength and tensile strength all of which can be plotted as a function of cross-link density within the polymer. This is shown in the figure below:

Figure 4: Vulcanizate Properties as Function of Cross Link Density

Hysteresis represents the history dependence of physical systems. If you push on something, it will yield: when you release, does it spring back completely? If it does not, it is exhibiting hysteresis, in some sense. In the figure above, hysteresis decreases with increasing cross-links. This is because the cross-links give the material some strength and rigidity, which allow it to return to its original shape when the loading is relieved. A material with no cross-links would remain permanently deformed .

There exist many types of vulcanization: with and without accelerator, phenolic curatives, benzoquinone derivatives, metal oxide, organic peroxide, and dynamic. This report will focus mainly on the chemistry of vulcanization with and without accelerators .

Vulcanization without the use of an accelerator was commonplace until 1906 when Oenslager found the first useful accelerator (aniline) for use in the process. The unaccelerated process utilized elemental sulfur at 8 parts per 100 parts of rubber (phr) and required a temperature of 140 oC for 5 hours. The common reaction mechanism for unaccelerated vulcanization is the free-radical method, given below :

Figure 5: Unacclerated Vulcanization Mechanism via Free-Radical Polymerization

The reaction is a basic free-radical polymerization between isoprene and a sulfur radical. Since this scheme has a long curing time, it is not practical for use in designing a mass-production plant around. As discussed in the next section, accelerators can greatly increase the rate of reaction (hence, the name accelerator) and thus unaccelerated vulcanization is generally not used except for certain specialty products.

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