I guess almost everybody know that the tyres are made of rubbers/ elastomers. Some people think that the tyres are black because rubber is black in colour which is not true. Let me tell you that raw rubbers are almost very light colored and some times transparent also.
This brings us to the question why tyres are black. This is mainly because of the addition of carbon black,which is black in color as the name suggests. Carbon black are added more than 50% in the tyres and so the tyres becomes black. Its completely useless even if you put any pigment or coloring agent to change the color, because black has more covering property than any other color.
Now the question is why carbon black is added to the tyres, is it just to make the tyres black??.
Let’s just learn a little about rubbers before explaining why carbon black is mandatory for tyres. Rubbers, unlike metals or plastics, cannot be used as it is. Rubbers need to be mixed with several ingredients like fillers (reinforcing and non-reinforcing), curatives, antioxidants, plasticizers, etc to be useful. Without these chemicals, rubbers are almost useless.
Leaving aside the other chemicals, lets learn a little about fillers and why fillers are added.
Fillers are nothing but the solid particles of varying particle 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 will improve the various physical properties of the 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 and it is 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 been able to replace carbon black only partially.
But many researchers and even tyre companies say that it’s possible to completely replace carbon black with silica. If it’s done, then we can have tyres in various colors.