What is Silica in Tires?

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Have you ever seen the description of "silica compound" in an article introducing a tire?
AndI don't know what it is, but it's got something amazing in it, and it works!Do you usually skip over it because you're like, "Oh, my God.

Shh!
What is silica? What happens when it is added to the mix? If you know the following, you can understand what's so great about it, and it may help you decide when choosing a tire.

WebiQ is here to bring you a little happiness and motorcycle knowledge.
In this article, we will explain the mysterious substance "silica" that is used in tires.

Review 1: There are two basic types of tire materials

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Tires are made of rubber, but there are actually two main types of rubber. There are two main types of rubber: natural rubber and synthetic rubber.

Natural rubber is made by scratching rubber trees and collecting the resin that comes out. It has the characteristics of high strength and low heat generation, so it is used in tires for heavy vehicles (buses, trucks, work vehicles, etc.).

But since motorcycles are lightweight, tires made of natural rubber material are almost non-existent, and synthetic rubber derived from petroleum is used.

The advantage of using synthetic rubber material is that the characteristics of the material can be changed depending on the part of the tire. Synthetic rubber is used in different parts of the tire, such as the tread part that touches the ground, the sidewall part that supports the weight and load of the vehicle, and the liner part that prevents air leakage.

Review 2: What is vulcanization?

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Synthetic rubber was not developed out of the blue, but only naturally occurring natural rubber was used in the beginning. When the sap from the trees was solidified, a lump of natural rubber was formed, but when sulfur was added to the natural rubber and heated, it was discovered by chance that the elasticity was dramatically improved. It's amazing that it was an accident. Elasticity is the ability of a tire to be soft, to deform, and to return to its original shape after deformation, so this is a wonderful discovery that improves all the aspects of tire performance that we want to improve.

This method of improving performance by adding sulfur was discovered by accident, and is called "vulcanization" because of the addition of sulfur. Nowadays, this vulcanization process is mandatory for both natural and synthetic rubbers.

The vulcanization process is completed when the rubber material is mixed with sulfur and heated, but it is difficult to control the temperature and time for the vulcanization of rubber used as tire material, so it is a challenge for each company to show their know-how.

Review 3: Why tires are black

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Michelin Man (formerly known as Bibendum) is the famous mascot of Michelin, but have you ever wondered why he is white when he is the mascot of a tire manufacturer? The reason is that originally, tires were usually white. Natural rubber has a milky white color, so it was normal for tires to be white when they were made of natural rubber.

Now, one of the characteristics of rubber is that vulcanization improves its elasticity, and this occurs when the rubber molecules are tightly bound together by vulcanization. This is because the rubber molecules are tightly bound by the vulcanization process, which means that the elasticity of the material can be further improved by strengthening the bonds on.

The "substance to make the bonds stronger" is carbon powder called carbon black. Since it is carbon, it is completely black.

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However, carbon black is so black that it is used in ink, so just a little bit of carbon black in the mix will turn the tire black.

The ratio of carbon black in the mix is a trade secret of each company, so we don't know, but we can be sure that a large amount of carbon black is mixed in. That's why the tires are so black.

What is the silica in question?

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Carbon black had long been the absolute best additive for rubber, but around 2000, a substance was found to replace carbon black. That substance is silica.
So it's a rather recent development. It has only been about 20 years since tires containing silica became commonplace in the market.

Silica is the second most abundant substance on earth after oxygen, and is found in soil and rocks. It is so common that silica is the main ingredient of foundation in cosmetics.
While carbon black is a black powder, silica is a white powder, just the opposite.

Another name for silica is silicon. The chemical symbol for silica is SiO2, which means "silicon dioxide" to be exact. You may have seen it in the description of glass coating agents, etc. It is the same thing. There are many cool names for it, such as quartz, quartz glass, crystal, and so on, but in short, it's all silica, and what's used in tires is actually super fine silicon powder.

What are the benefits of adding silica to the product?

So, what is the advantage of using silica instead of carbon black?

Silica, like carbon black, is an additive that strongly binds rubber molecules together, but the speed of recovery from deformation is faster than when carbon black is used to strengthen the bond.

However, the speed of recovery from deformation is faster than when carbon black is used to strengthen the bond. Tires that have been deformed by uneven surfaces quickly return to their original shape (a perfect circle), resulting in less rolling resistance.
This is why it is often used in eco-friendly tires that claim to be fuel-efficient.

Moreover, while it recovers quickly from deformation, it generates less heat than carbon black, which is a bonus. There is no needless heat loss.

What's more! It has been found that it has a high frictional force on wet surfaces, which means it has excellent braking power on rainy days.

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The reason why all the touring tires use silica is to achieve wet grip. In fact, the wet grip of recent touring tires is astonishing, and silica must be a big part of that.

And more! It is a versatile additive that does not lose its elasticity at high temperatures (i.e., it provides stable resilience at high temperatures) and does not harden at low temperatures (i.e., it provides stable resilience at low temperatures). It is a wonderful additive for tires.

Weaknesses of Silica (Part 1)

Silica is such a versatile material, but it is not without its weaknesses. The biggest weakness is that it is not conductive, so static electricity generated in the tire cannot be released to the ground. Carbon black is carbon, so it can conduct static electricity, but silica is like glass powder. 

But don't worry about the rubber!

When tires are manufactured, there are thin stripes in the tread that do not contain silica, and these stripes are designed to discharge static electricity.

If you look closely at the tread of a tire, you may have seen a black line running all the way around. That thin, black wire is the ground wire that discharges static electricity to the ground. Some tires for 4-wheeled vehicles with a wide tread surface have two or three ground wires!

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Weaknesses of Silica (Part 2)

The second weakness relates to manufacturing technology. The silica used in tires is in the form of a very fine powder (originally stone powder), but it is extremely difficult to mix silica evenly into rubber and mold it.

Furthermore, silica does not inherently bond to rubber. In order to force such silica to bond with the rubber, another additive has to be added. As with vulcanization, the temperature and time are precisely controlled to bond with silica.

However, if you make a small mistake, the vulcanization reaction of the rubber will start before the silica and the additive have a bonding reaction, and the rubber and silica will not be able to bond.

The image is that the rubber that has been vulcanized first becomes lumpy and cannot bond with silica. In this case, the rubber that could not be bonded with silica would simply be vulcanized rubber, and its performance would be lower than rubber reinforced with carbon black (which easily bonds with rubber).
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More specifically, there are wires inside the tire to begin with, but metal wires and rubber are a very poorly bonded combination. Normally, these materials would separate, but by vulcanizing the rubber with sulfur, it reacts with the plating on the surface of the wire to form a strong bond.

In other words, after controlling the temperature and time for bonding the silica and rubber, it is necessary to control the temperature and time for vulcanizing the rubber (by mixing sulfur) to bond it to the wire, while bonding the rubber molecules to each other.
This series of temperature and time control is said to be a very difficult technology, and we ordinary people cannot know the whole story (it is a very important trade secret of each manufacturer, so it is not allowed outside the company), but I can imagine that it is not easy.

Fortunately, the weaknesses of silica are not directly related to us, the general public who use silica tires. We can only benefit from the technology of the tire manufacturers, thanks to them. Thank goodness for that.

Modern tires are great!

That's how we finally achieved this silica compound tire. The result is a tire that doesn't get hard in the cold, doesn't waste away in the heat, has high grip in the rain, and has low rolling resistance for good fuel economy.

If you see the words "silica compound" in a tire's literature, you may wonder if the tire has these basic characteristics. I hope you will think so. If I tried to explain it, it would be a long story like this article, so manufacturers just write it lightly in their tire catalogs, but it is actually an amazing piece of technology.

Bonus: Are your tires turning white?!

Instead of the black carbon black, the latest technology is using white silica, but you can't tell from the outside if it is rich in silica or not. The tires are still black.

However, it is true that carbon black content is decreasing due to the large amount of silica. Some products even claim to use 100% silica as a rubber binding agent without carbon black.

Tires are black because they contain carbon black, but if all the carbon black were replaced by white silica, the tires would turn white. But why do tires that claim to contain silica still remain black?

There seem to be two reasons for this. The first is that the tires look weak when they are white, so a black dye is added to make them black, which sounds like a lie. The second reason is that a small amount of carbon black is sometimes better than 100% silica.

The latter reason seems to be increasing these days. In this case, the carbon black content is small, so the tire will not be completely black, but will be slightly grayish.

There are many motorcycle tires that have different tread compounds near the center and on both sides, but when you use them, you can clearly see the difference, right?
That's because the difference in carbon black content becomes visible as a color change (probably). Of course, manufacturers don't disclose the difference in carbon black content between different compounds, so this is just speculation, but looking at the color of the tires, I don't think we're way off.

In general, high-grip touring tires have more silica near the center for all-weather use, and more carbon black on the sides for dry grip rather than rain performance. So, if you look carefully, the center will look more grayish and the sides will look darker.

If you are curious, please observe the surface of your own tire tread. If the tread surface is not too rough from heavy driving, you should be able to see the ground wire for static discharge!

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