Method of manufacturing silica currently used in the industry. The major drawbacks of the previous studies can be seen in the properties of their final product. Normally, the silica that was produced had small pore size, poor dispersion in rubber, and the synthesis method was non-economical. That is why silica is not yet widely accepted as filler for tire to completely replace the carbon black which is commonly used. In addition to the sol–gel strategies, a careful design of the manufacturing apparatus can be employed to advance the properties of the final products that are desired for a particular application. Thus, there are more strategies to improve the properties of silica to make it more suitable as green tire filler.
Silica is a colourless, white, and has chemical compound. Silica is known as silicon dioxide or SiO2 because the name given is composed from silicon and oxygen. Silicon and oxygen is made up from silica because of common elements on earth. It is the main component of most sand and the primary ingredient in glass. Silica also make up the mineral called quartz. Pure silica is colorless, but if contaminants are present in a sample of quartz, it may be colored. For example, rose quartz is silica with trace amounts of iron and Milky quartz is simply silica with air bubbles or inclusions of liquid that make the mineral appear white.
Silica is found commonly in the amorphous state and crystalline state. Silica is composed from two atoms of oxygen and one atoms of silicon resulting in chemical formula SiO2. Silica has been known to humans since ancient times, long before we knew it was made of silicon and oxygen. The art of making glass objects with silica dates back centuries. Today, there are many industrial uses for silica. These include abrasives, building materials, fillers, electronics, and water filtration.
Other than that, silica is environmentally used in green tyre technology as a filler. According to Rodhia (2008), silica can increases its hardness and makes it less susceptible to deformation, thereby the rolling resistance also can decreases from 25% to 5% because of reduction in fuel consumption and vehicle carbon dioxide. Silica has been used for tread of tyre for many twenty years ago in order to reduce the fuel consumption of vehicle hence will contribute of greenhouse gases with reduction in vehicle.
Now, most tyre users is the silica tyre technology because of their performance that offer optimum level of traction that minimize the fuel consumption of vehicle, with no associated loss in tyre performance. The filler was added in the silica tyre to increase hardness, strength and abrasion resistance that is known as reinforcing filler. If the tyre without reinforcing fillers, tires would need to be replaced considerably more often so it can resulting in an increased burden on end-of-life-tire (ELT) management and tyre manufacturing energy requirements and greenhouse gas emissions per vehicle will increase. Filler has their benefit which consist of high strength of filler that has ability to transmit forces through the tread area of the tyre. The most important aspect of this is improving traction and reducing stopping distance for the vehicle. Without well adapted reinforcing fillers, vehicles would not be able to stop as quickly as they currently can, causing safety concerns for drivers.
By far, carbon black is the most common filler that is used in the rubber industry due to the ability to improve physical properties combined with its desirable effect on compound unit cost relative to that of the raw elastomer. The colour of carbon black is an excellent because it can absorb light and absorbs most of the ultraviolet components of sunlight, which can otherwise initiate oxidative degradation of the rubber. But, it has a technology that offer the compounder that work in tyre industry has chance to improve traction, abrasion resistance and rolling resistance simultaneously is likely to be immediately attractive. So, silica has been used as filler to make properties of tyre is more better and make new tyre technology when use the silica as filler compared to the carbon black.
Nowdays, tyre technology was changed dramatically when coupling agent was introduced in the silica. Silica can form chemical bond itself to elastomeric polymers which is basic of the tyre compound. Coupling agent is used in the silica because to improve the reinforcing filler like silane coupling agent. Low molecular weight and viscocity make the improvement of coupling agent. The presence of sliane coupling agent can enhances the mechanical properties of the vulcanizates due to the combined effects of better filler dispersion, better rubber–filler interaction and increased the crosslink density. Silica are quite different compared to the carbon black because silica is the only or majority filler type used in the compound.
The uses of silica in green tyre can improve tyre performance which has ability of the tyre to grip the road, that influenced by high frequency distortions of the tyre tread when it impacts on the texture of the road surface. Tyre has major problem that must be solve which is compromise between low rolling resistance, high wet grip and high wear resistance. Tyre can be high or low performance that depend to the rolling resistance. Rolling resistance is the amount of energy a tyre absorbs as it revolves and deflects. High hysteresis can achieve by improve traction a tread rubber compound that can absorb high levels of energy. The rolling resistance of the tyre depends on low frequency while the tyre is rotates. So, the tyre try to conserve as much energy as possible that required a tread compound does not absorb more energy. Hence, hence low rolling resistance treads are known as low hysteresis compounds. If the rolling resistance is lower, the less fuel is required to propel the vehicle forward. Lowering the rolling resistance make reduction in wet grip performance, which of course is unacceptable. So, in tyre technology it has some ways to solve the problems of tyre on the road that can be achieved by the replacement of carbon black by silica in the tyre’s tread compound. Manufacturers has produce tyres which provide improved wet grip properties, better winter performance and lower rolling resistance all at the same time.
In green tyre technology it has some reasons why silica is used in green tyre technology. This occur because of it has grip in tyre that affected by the degree to which a tyre is distorted at high frequencies. In other words, grip is the best serves by rubber compounds which absorbs high level of energy or called high hysteresis compound. The use of silica is more effective compared to carbon black that contributed 20% reduction in rolling resistance of tyre. Moreover, tyre pressure are maintained and making allowance for varying speeds. A 20% reduction in rolling resistance corresponds to a 5% fuel saving, which according to Michelin in their promotion of the Michelin “Energy Tyre”, can save the average cost of the tyres. Other than that, the uses of silica also can improve wet skid performance. Silica also provides benefits to the winter tyre and all season tyres. Compound that is used in the silica is most elastic and flexible at lower temperature that can allow better performance for grip and braking during wintry weather.
However, silica has many advantage compared to the carbon black as reinforcing filler. This is because carbon black is natural rubber or synthetic rubber which has weak properties that has lack of lack of hardness, strength properties and wear resistance. Filler is important in silica in order to improve the properties of rubber compounds.
The graph shows the effect of reinforcing fillers on different compound properties. Nowdays, silica are mostly used in the tyre manufacturing that mainly used to decrease the rolling resistance. Low rolling resistance can be achieved by increased attitude of protecting the environment gives rise to a demand for tyres combining a long service life with driving safety and low fuel consumption. The change of carbon black to silica have difficulty because of technical problems was involved. The mixing of rubber with pure silica is difficult, because of the polarity-difference between silica and rubber. In order to solve this problem coupling agent is used in the production of the tyre to bridge polarity difference. Silica as the main filler that have formulations consisting of a medium or high vinyl solution polymerized styrene-butadiene rubber (S-SBR) copolymer as the main elastomer, and silane is the coupling agent as the silica modifier. Styrene butadiene rubber is more preferred in silica reinforced tyre as it provides favourable balance of rolling resistance and wet skid resistance in combination with silica. Precipitation process of silica uses as the reinforcing filler has the advantages such as better control of particle size, porosity and specific surface area and cost benefit.
Surface of silica become polar with the presence of silanol groups (hydroxyl groups) thus will hinder good dispersion and wetting of silica in nonpolar rubber like styrene butadiene rubber. Therefore, the surface characteristics of silica has to be modified for using in rubber compounds. Silane is the one effective coupling agent that has been widely used modifiers for silica in tire tread compounds. The use of coupling agent in silica tyre is growth worldwide. Sulfur is containing silanes are particularly desired in diene-based rubber compounds as the sulfur atom in silane can form chemical bond with rubber molecule during vulcanization. Silane and 3-octanoylthio-1- propyltriethoxysilane (NXT) 7 are widely used sulfur-containing commercial silanes. The application of the silane is also the key factor for silica being the successful replacement of carbon black as the main reinforcing filler in the tread compound of the green tyre. The NXT silane is a blocked mercaptosilane, where the sulfur atom of the mercaptosilane part is blocked with an octanoyl group. The blocking of the mercaptosilane part by octanoyl group reduces premature that release of sulfur from the coupling agent to cause some premature vulcanization of the compound. It has been reported that a natural rubber (NR) compound with NXT silane is more resistant to scorching before curing when compared with an NR compound with silane.
Figure below is the structure of silane coupling agent and have some comparison effect of four new silane coupling agents in silica reinforced tire tread rubber compound against the two well-known commercial silanes Si69 and NXT. The four new silane coupling agents are 1-3-(octanoylthio) propyl-1,1,3,3,3-pentaethoxy-1,3-disilapropane (S1), bis3-(octanoylthio)-1-propyl- diethoxysilane (S2), 5-triethoxysilylpropylthia-2-potassium-1,3,4-thiadiazolate (S3) and 2,5-(triethoxysilylpropylthia)-1,3,4-thiadiazole (S4).
The structure of silane coupling agent
As shown in Figure above, the chemical structure of S1 and S2 are related to NXT in such a way that the S1 contains one more Si atom and two more ethoxy groups compared with the NXT for improved bonding between the silane and silica, whereas in the case of S2 silane, one more 3-(octanoylthio) propyl group is attached with the Si atom in place of one of the ethoxy groups of the NXT silane, in an effort to increase the bonding between the silane and rubber molecule. The chemical structures of new silanes S3 and S4 are also related to each other as the S4 is a bifunctional or dipodal silane-containing similar functional groups as that of the monofunctional silane S3.
graph of the temperature dependence of tan ? for a silica (?) and a carbon
black (?) reinforced compound
The figure above is the additional effect of temperature dependence of tan ? for reinforced rubber compounds is the result of the characteristics of reinforcing fillers to form a filler network. This is due to the main reasons of greater reinforcing power of silica compared to carbon black. The tan ? values at different temperatures for silica and carbon black was compared, as shown in the graph lower tan ? value is found for silica at lower temperatures. In the rubbery state at temperatures beyond 20 °C the tan ? and hysteresis is still higher for carbon black. Replacement of carbon black by silica therefore results in a decrease of tan ? at higher temperatures, and thereby in a reduction in the rolling resistance. The use of silica in addition leads to a comparable tan ? at lower temperatures, providing a comparable ice- and wet grip of a tyre.
The higher hysteresis of carbon black at higher temperatures is mainly due to the energy dissipation during repeated destruction and reconstruction of the filler network, which leads to a rapid decrease of tan ? with increasing temperature primarily due to a reduction of filler-filler interaction as well as filler-polymer interaction. The hysteresis of the silica-filled rubber increases with increasing temperature, finally showing a crossover point with carbon black at about 90 °C. As the temperature increases, it will shown filler-filler interaction are weaker would result in an increase in the portion of filler network, which can be broken
down and reformed during cyclic deformation at low strain amplitudes. There are some different effect between silica and carbon black towards different tyre properties:
The effect of silica and carbon black on different tyre properties
(+ = better; – = worse; ~ = same/comparable)
Properties Carbon black Silica
Rolling resistance – +
Wet grip ~ ~/+
Wear resistance + –
Compounding with silica enables tyre technicians to reduce the filler content, because of the greater reinforcing power of silica. A decrease in filler content corresponds to a higher amount of elastic rubber in proportion to the damping filler phase in the compound and is an effective way to reduce rolling resistance.
In conclusion, silica is most important used in green tyre technology because it has lower the rolling resistance while maintaining good wet traction. Currently, silica is used for tyre manufacturing especially for marketing in green tyre product due to changes in economic factors and increased environmental awareness. Thus, silica is the best way use as the main filler for tyre tread compound so green tyre properties can be achieved.