Within the network of special cells of the Hevea brasiliensis tree, there’s a thick, creamy white, milky emulsion—you might know it as latex. The applications of this material spread across a range, but its most important usage is rubber (90% of all natural rubber derives from the Brazilian rubber tree). Rubber car tires, as the basic machines that support and propel automobiles, bear an iconic appearance marked by their black color and channels of treads. Considering the white color of natural rubber compared to the vastly different appearance of tires, one might wonder:
Why Are Car Tires Black?
The answer to this question is quite simple, as the loss of the natural white color of rubber comes from the addition of carbon black, which plays a key role in the manufacturing and production of automobile tires.
How Tires Are Made
While rubber is an essential ingredient to car tires, around two hundred different materials, including special oils, carbon black, pigments, antioxidants, silica, and other additives help determine the specific characteristics of a tire. After planning and selecting these materials, the tire is assembled. This begins with the innerliner, followed by the body plies and belts, then bronze-coated strands of steel wire in the sidewall, and, lastly, the tread and sidewalls. Once pressed together, these materials undergo vulcanization.
Rigorous testing assures that tires meet their desired characteristics, and standards provide the best practices for these procedures. For example, SAE J 2710-2017 outlines modal testing and identifying lower order natural frequencies of radial tires, the common type of tire on the market today. ASTM E1136-19 and ASTM F2493-20 offer additional specifications pertinent to radial tires. There are many standards pertinent to automobile tires, together outlining a range of interests. SAE J 1106-2012 details a laboratory procedure for measuring the steady state force and moment properties of car tires, and ASTM F1426-20 acts as the standard practice for identifying irregular wear patterns on tire treads.
Car tires owe their signature black color to the additive carbon black. This material makes an appearance in the pigment and reinforcing phase of tire creation, and it is valued for enhancing surface durability. By reducing thermal damage, carbon black extends the lifespan of tires on the road.
Carbon black retains these characteristics because it is virtually pure elemental carbon in the form of colloidal particles. Resulting from the incomplete combustion or thermal decomposition of gaseous or liquid hydrocarbons under controlled conditions, usually through the furnace black process, most forms of carbon black contain greater than 97% elemental carbon arranged as aciniform (grape-like cluster) particulate.
90% of the 18 billion pounds of carbon black produced annually is used in rubber applications. To help evaluate the properties of such an integral material to the functioning of car tires and other products, standards offer guidance, such as:
ASTM D3849-22: Standard Test Method For Carbon Black—Morphological Characterization Of Carbon Black Using Electron Microscopy
ASTM D4122-17e1: Standard Practice For Carbon Black – Evaluation Of An Industry Reference Black
ISO 4659:2020 – Styrene-Butadiene Rubber (Carbon Black Or Carbon Black And Oil Masterbatches) – Evaluation Procedure
In the early days of cars, tires were white. Since natural rubber was not ideal for tires, they were fortified with zinc oxide, another white material.
The superior carbon black later replaced zinc oxide as a means to strengthen tires, but, at the time, it was quite expensive. Considering this hurdle, carbon black was used only on the treads to fortify the surface of the tires. The black outer surface complementing the white inner tire birthed what became known as whitewall tires.
Initially, black tires were a luxury. However, whitewall tires quickly grew in popularity and became more expensive than black tires. In 1934, Ford introduced whitewall tires as an $11.25 option to all its new cars, a steep price at the time.
By the 1960s, whitewall tires dwindled in popularity. Radial tires superseded their stance as the common tire, as they remain today. Some companies still offer whitewall tires in the modern world, although their applications typically fall with retro-styled cars and restorations.