Silicone-based process aids play a pivotal role in helping tire manufactures obtain ideal tire balance and sustainable quality. Overlooking the use of silicone process aids in tire production negatively impacts sustainable tire quality.
Thanks to its ability to withstand high temperatures and pressures, silicone is already widely used in the automotive industry. In our cars we find massive amounts of silicones in the form of heat-resistant engine seals, rubber hoses, electronics and cable insulation. However, silicones are also present in smaller quantities as they streamline processes by acting as release agents for steering wheels, dashboards, bumpers and a variety of plastic body parts. Even the creation of motor blocks, gear boxes, alternators, water pumps and other mechanical parts require silicones in the production process.
In short, without silicones it would actually be impossible to build modern cars!
Yet, you may be asking yourself: how can silicone help to produce perfect tires during the manufacturing process? It all comes down to the protective, anti-stick and slip promoting properties of silicones. But before we get there, let’s take a pit stop to look at how tire technology has developed over the years.
In the past, tires were manufactured with an inner tube to hold the air pressure. This system soon proved to be inconvenient in many ways but mainly because tube tires tend to deflate more quickly after a puncture and do not sit as securely on the wheel. This means that the tire bead has a greater chance of separating from the wheel, which will make vehicle drivers lose control at high speeds. When repairing punctures, air leaks are hard to pinpoint and the entire tire and tube assembly needs to be disassembled. When the tire and the tube are poorly mounted, air can be trapped between the two and cause havoc when heated as the car reaches high speeds. Furthermore, the rolling resistance of tube tires is high, which negatively impacts consumption and ultimately the environment. Hence, tires for high-speed capable vehicles are now tubeless as they result in safer and more efficient automobiles.
Today, tubeless tires are formed with layers of various types of rubber, compounds and reinforcing materials and then cured in a metal mold to give them their shape. With the help of a rubber bladder inflated by steam, the tire rubber is pushed into the grooves of the metal mold to shape the tire tread and sidewall down to the last detail. While this process works great, it could not be properly performed without the help of one essential component — and that is silicone.
Tires are made from natural rubber or polyurethane (low speed industrial tires), two materials that stick to metal. If they were pushed directly into a hot metal mold without some form of protective layer, it would be nearly impossible to remove them.
In the past, talcum powder was used as a mold release agent. Now this is quite messy as it leaves white traces on the outside of the tire, as if flour had been scattered on the surface of a chocolate cake, rendering unaesthetically and unappealing tire optics. In terms of operations, using talcum is also a very demanding process as it has to be applied to each tire.
Therefore, a silicone film, with its anti-stick properties and superb surface slip, when applied in a thin layer (5 to 10 microns) on the metal mold, allows for tires to be easily separated from the metal mold. As the film is formulated to be semi-permanent, multiple tires can be cured before it has to be reapplied.
Not only is the release function of silicone film important but the slip function of the release layer allows rubber to flow correctly into all the grooves of the mold, ensuring perfect tire molding with clean tire optics. Perfect rubber flow molds tires and intricately copies all the details of the mold to the tire sidewall and tread surface. The slip function then allows the tire to effortlessly slide out of the hot tire mold.
Caution oncoming traffic: Bladder rubber versus tire rubber
A rubber bladder is inflated with steam and heated to 170°C. Under such high temperatures and pressure, the bladder rubber risks cracking or sticking to the tire surface. How then can you retrieve a tire, which has been cured at such high temperatures, from the curing press when a chemical bond forms between the inner liner of the tire and the bladder, without damaging either?
The solution is a silicone-based separation layer between the tire and the bladder. Resistant to heat and pressure, flawless film forming silicones effectively provide a thermal protective layer around the tire curing bladder and separate both rubber parts. A thin layer of silicone between the inner side of the tire and the bladder suffices to prevent the bladder from sticking to the tire, oxidizing, overstretching and cracking.
After the cure, thanks to the slip-effect of the silicone layer, the bladder can be easily removed from the tire rubber without either of them being damaged. This ensures the longevity of bladders used in the tire curing process. On top of that, the silicone film acts as a lubricant thereby allowing for optimal bladder centering, perfect tire geometry, and thus tire balance.
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No U-turn for air bubbles
Last but not least, a performing silicone-based process aid ensures the evacuation of air (known as air bleed) during bladder inflation. When the bladder inflates, the air it displaces must be evacuated to avoid trapped air. Any trapped air between the tire and the bladder forms air bubbles, leading to defects in the inner liner and thereby causing air leaks in the tire. This could result in unsafe tires as layers may separate and the inflated tire risks exploding during a speedy ride.
Since the inner liner replaces the inner tube, it must be airtight. This function can only be maintained if there are no defects in the inner liner layer. The presence of air bubbles during curing would create a discontinuous inner liner and destroy air tightness. Performing silicone-based process aids help to avoid such defects thereby facilitating the production of safe tires.
We are nearing our destination and along the way we have seen how silicone-based process aids have many advantages when it comes to making tires. Their slip effect and anti-stick properties ensure tires can be smoothly molded and removed from the bladder and mold. Performing silicone-based process aids prevent bladders from cracking and guarantee adequate evacuation of air bubbles.
All in all, silicone-based process aids help create perfect tire manufacturing conditions resulting in significant quality improvements as well as time and cost savings.
Photo credits: Freeimages / A. Carlos Herrera