In the 1990s, the concept of “green tires” was proposed. Green tires come with improved rolling resistance, reduced fuel consumption, and gas emissions without losing wet-skid resistance and wear performance. However, rolling resistance, wet-skid resistance, and wear performance are often contradictory, improving one aspect will result in other aspects deteriorating. To solve the constraints among the “magic triangle” of anti-skid performance, rolling resistance, and wear performance, in 1992, Michelin introduced a filler system with a silane coupling agent modified highly dispersed silica. In addition, traditional emulsion polystyrene-butadiene rubber (ESBR) was replaced with solution-polymerized styrene-butadiene rubber (SSBR), the final product showed both improved rolling resistance and better wet-skid resistance. At present time, green tire tread rubber is mainly still solution-polymerized styrene-butadiene rubber, rare earth cis-butadiene rubber or natural rubber, etc. and the filler is mainly silica.
In 2012, the EU introduced a tire labeling law that banned the use of high aromatic oil (DAE), a rubber processing plasticizer that has historically dominated the market. The environmentally friendly aromatic oil (TDAE) was selected as the replacement. The reduction of aromaticity of the processing oil causes negative impacts on the dynamic performance of rubber products, and tire manufacturers around the globe had to improve safety performance, rolling resistance, wear performance, and environmental friendliness all at the same time, despite contradicting with each other, to meet EU standards. Hence the introduction of resin, which meets environmental protection requirements while retaining the same performance compared to products manufactured with polycyclic aromatic hydrocarbon oil. Different types of resins added to the green tire tread compound plasticizes the rubber, which not only strengthens the fundamental mechanical properties of the tire but also balances the performance between rolling resistance and wet-skid resistance.
The tire tread rubber resin can be categorized as natural resin and synthetic resin. The commonly used natural resins are mainly rosin resin and terpene resin, and synthetic resins include coumarone resin, styrene resin and petroleum resin.
|Functional Resin||Main Features
|Rosin Resin||Can increase the glass transition temperature of the rubber compound. Improves the wet-skid resistance of the rubber compound while only having a slight impact on the rolling resistance. Usually, modified rosin (such as hydrogenated rosin, rosin ester, hydrogenated rosin ester, etc.) is used to prevent aging.|
|Terpene Resin||Can improve both wet-skid resistance and rolling resistance performance, reduces tread wear, and has good compatibility with the rubber.|
|Petroleum Resin||Has a competitive price and good comprehensive performance; can partially replace natural resins.|
|Coumarone Resin||The raw material of comes from coal chemical industry, which has serious pollution and limited development, and has been gradually replaced.|
|Styrene Resin||Suitable for the processing of styrene-butadiene rubber, not suitable for rubber with high saturation such as butyl rubber|
In the rubber industry, rosin is often used as a plasticizer and softener in elastomers such as styrene-butadiene rubber, nitrile rubber, chloroprene rubber, and ABS to improve its processing procedure, and it also brings improvements to physical properties. improvement. Rosin increases the rubber’s glass transition temperature, improves the wet-skid resistance while only have slight impact on rolling resistance performance. Due to the active double bond in the rosin molecule, rosin ages faster, modified rosin (such as hydrogenated rosin, rosin ester, hydrogenated rosin ester, etc.) is commonly used to prevent such problem.
Terpene resin is made of α-pinene and β-pinene monomers or polymerized with other monomers (such as α-methylstyrene, styrene, phenol). The double bond of β-pinene is outside the carbon ring, which determines β-pinene’s reactivity is better than α-pinene. The tan δ loss factor versus temperature was used as an indicator of wet-skid resistance and rolling resistance. Resin with higher glass transition temperature causes tan δ peak at a higher temperature, which improves wet-skid resistance. VLEUGELS conducted a study on silica-reinforced SBR-BR tire treads from the perspective of using different terpene resins and found that the maximum improvement rate in the wet zone (0°C–30°C) was about 35%. At 60 °C, the tan δ of rolling resistance is improved, indicating that the interaction between filler particles is reduced. Depending on the specific choice of resin, the maximum improvement over the rolling resistance temperature range is about 15%. This indicates that the terpene resin can improve both wet-skid resistance and rolling resistance. The performance of terpene phenol resin is better than polyterpene resin. But the overall performance of the resin depends on the specific rubber formulation and choice of rubber.