Every passenger tire sold in the United States possesses a DOT number (serial number) molded or branded onto its sidewall. The DOT numbers certifies that the passenger tire meets or exceeds Department of Transportation Standard 109 (49 CFR 571.109 – Standard No. 109; New pneumatic and certain specialty tires). This standard specifies tire dimensions and laboratory test requirements for the testing of tires.
In addition to these tests, tire manufacturers employ a vast array of testing to the rubber compound used to construct the tire. Tire companies each have a skim stock formula that they guard with their lives; it is the equivalent of the Coca-Cola recipe. No company uses the same formula, but, generally, they all use a similar combination of polymer rubber, reinforcement agents like carbon black and/or silica, oils, sulfur, resins, waxes, antiozonants, and antioxidants. Tire companies compound these various ingredients and chemically cross-link them through a vulcanization process.
As tires are being constructed, tire-manufacturing plants should be following their own plant guidelines to ensure the tire is built to a very specific design. Design changes are made fairly frequently, but the plant’s quality control measures should be consistent. Strict adherence to these guidelines is imperative to ensure all the plants tires are built to prevent failure. Because a failed tire at highway speed is frequently catastrophic, the plant’s organizational structure should be crafted with emphasis on strict adherence to building protocol.
Even before a tire undergoes Standard No. 109 testing, tire manufacturers require testing of its compounded rubber processing and vulcanization. During processing, testing focuses on the plasticity or viscosity of the rubber, such as its resistance to flow and the time required for cross-linking. To withstand the stresses of a vehicle tire, the compounded rubber should be able to slightly flow. Tire manufacturers test this through a Mooney viscometer, which measures the torque required to rotate a disk at a fixed temperature. Vulcanization tests measure the performance of the rubber compound during the three phases of the curing process: induction period, crosslinking phase, and reversion or overcure phase.
Tire manufacturers also physically test the rubber through stress-strain tests, tensile and elongation tests. This test combination is the most widely used in the rubber industry. It tests the compound’s tensile stress and elongation at break. The compound’s hardness is tested by measuring its resistance to indentation prior to puncture. Differential thermal analysis reveals a compound’s thermal behavior during heating and cooling conditions.
Tire companies will test a compound through dynamic mechanical tests to evaluate its elasticity, resilience, hysteresis and damping (response to hysteresis). Resilience is tested through a Rebound Test. Tire manufacturers utilize vibration and heat on a variety of tests to analyze a compound’s hysteresis and damping. A compound’s ability to withstand cracking is vital to its ability to bond. After a calendar process, the compound is subjected to adhesion tests. Finally, the compound is tested against abrasions.
Even before a fully-built tire is subjected to Standard 109 testing, its rubber compound is subjected to a battery of tests. Only compounds that satisfy these tests are used to construct the tire.
Written by:
Guy L. Watts, II
Watts Guerra LLP
4 Dominion Drive, Bldg 3, Suite 100
San Antonio, Texas 78257
Phone (210) 447-0500
© Watts Guerra LLP 2015
