Why are tyres black? This is mainly because of the addition of carbon black, which is black in color, as the name suggests. Carbon black is added more than 50% in the tyres, and so the tyres becomes black. It’s completely futile to add pigment or coloring agent to change the color because black has better coverage properties than any other color.
Let’s learn a little about rubbers before explaining why carbon black is mandatory for tyres. Rubbers, unlike metals or plastics, cannot be used as is. Rubbers need to be mixed with several ingredients such as fillers (reinforcing and non-reinforcing), curatives, antioxidants, plasticizers, and others to be useful. Without these chemicals, rubbers are almost useless.
Leaving aside the other chemicals, let’s learn a little about fillers and why they are added.
Fillers are nothing but solid particles of varying size, structure, surface area, etc. Based on this, fillers are of three types: reinforcing, semi-reinforcing, and non-reinforcing.
Reinforcing fillers are the type of fillers that improve the various physical properties of rubber products, like tensile strength, tear strength, ageing resistance, and also reduce the cost. Non-reinforcing fillers are fillers that are added just to reduce the cost of the product. One can say clearly that high performance products like tyres will have both reinforcing and non-reinforcing fillers, while low performance products like pencil erasers or balloons will have only non-reinforcing fillers.
Unfortunately, there is only one filler which can reinforce the rubber compound, and that is carbon black (until 20 years ago). So now you know why tyres are black.
Here are the properties which are improved by adding carbon black (when compared to raw rubbers):
- Tensile strength
- Tear strength
- Abrasion resistance (wear)
- Low rolling resistance
- Reduces heat buildup (Hysteresis)
- Ageing resistance
- Fatigue resistance
- and others.
Recently, a new reinforcing fillers has been developed and, surprisingly, it is not black in color: silica. More advantageously, it is said that silica gives better (lower) rolling resistance, and high abrasion resistance when compared to carbon black.
Tyre companies have started using silica nowadays. But one of the problems is that the chemistry behind silica reinforcement is very complicated. Because of this complexity, until now, silica has only partially been able to replace carbon black.
But many researchers and even tyre companies say that it’s possible to completely replace carbon black with silica. If that’s done, then we could have tyres in various colors.