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Why are tires made of rubber? Professors of Zhejiang University Deciphered the Hundred Years of Problems
A laboratory in the Yuquan Campus of Zhejiang University, a research group has studied rubber reinforcement for 17 years, and finally has a new achievement - the "Song-Zheng" two-phase model theory. In September 2016, the review article invited by the research group was published in the famous journal “Progress in Materials Science”. This theory, named after the researcher's surname, jumped out of the frame of Einstein's viscosity theory, ending the situation where the previous theoretical models were contradictory and could not effectively guide production, and it is expected to have an important impact on the production of rubber materials.
"Necklace" and "Grape" in the tire
Song Yihu, a professor of polymer science at Zhejiang University, is one of the main members of the "Song-Zheng" two-phase model theory group. There is a transparent "plasticine" in his office, which he says is the raw rubber necessary to make a tire. "Rubber is the largest molecular weight of polymer materials. If the rubber molecule is likened to a string of molecular necklaces, then millions of 'pearls' are strung together to form a rubber molecule. Long molecular necklaces entangle each other to form a network of necklaces. The longer the necklace, the better the elasticity."
Why is the tire black? Song Yihu said that because it has a substance called carbon black. "It is a form of carbon, the structure is like a bunch of grapes, and each 'grape' is a carbon particle with a diameter of about 20 nanometers. A soot that can be discerned by the naked eye is 100 nanometers (0.1 micron). ) The 'grape bunch' around."
For more than 100 years in the tire industry, carbon black has proven to be the best filler. In a tire tread rubber, rubber accounts for about 50%, and carbon black accounts for more than 40%. Under the action of more than 50 kinds of auxiliaries, the two are fused together in the mixing process, and after being vulcanized, they become a firm and wear-resistant tire tread rubber.
The combination of “Necklace” and “Grape Bunch” provides strong mechanical properties for the tire. Professor Zheng Qiang, another major member of the research group, said: "Many carbon black 'grape bunches' form a three-dimensional network structure, and rubber molecules 'necklaces' are intertwined and vulcanized, forming a dead knot between them. There is a group of 'disorders' that can't be solved. We call it 'cross-linking'. Just as raw eggs are cooked, the protein is denatured."
There are at least five theoretical schools in the world.
Both high strength and good elasticity are the basic requirements for a good tire, especially for heavy truck tires. Different use occasions, different requirements for the tires.
"Especially in the big trucks on the mine, when the tires run over sharp stones, the tread is not pierced." Zheng Qiang said that during the research process of more than ten years, the research group was most interested in "carbon black network." How does the rubber network interact with how the filler carbon black affects the viscoelasticity of the rubber network."
This issue has appeared in textbooks and in the papers of various researchers. Song Yihu and Zheng Qiang successively analyzed more than 700 articles and more than 100 theoretical models that were commonly used and not commonly used. It was found that at least five theoretical genres appeared for the rubber modulus (note: hardness parameters), but between the models. Contradictory, or contradictory to existing research results.
To this end, between industry and academia, you say yours, I am my. This disconnection makes the advanced tire manufacturing still belong to the “secret recipe” of the major tire factories, who can “try” out who makes money.
Found that there have been more than 100 years of "prejudice"
"Internationally, there is a consensus on the formation of a three-dimensional network of carbon black and providing mechanical support. We have also confirmed this through experiments." Song Yihu said that what makes them more confused is that all theoretical models only focus on the carbon black network. The role of rubber molecules in it is not considered. These models are too simplistic, causing prejudice against facts. “Industrial tires use rubber as the basis, but no theory can express the role of rubber in the inside. So we think while working. If the theoretical model does not have a parameter that can indicate the effect of rubber on the elasticity of the material, then this The theory is definitely immature."
Academically classified, this research belongs to the category of viscoelasticity of polymer nanocomposites. The earliest study of this system in the world is said to be Einstein. He published several papers on the viscosity of suspensions around 1911 and analyzed the effects of particles on the flow field of liquids. Although the material system used in the subsequent research tires has changed, the most successful and widely used viscosity prediction theory still belongs to the modification and improvement of the Einstein equation.
“A basic question is why we must use filler-filled rubber to make tires without using water suspension systems to make tires. Therefore, the impact of rubber on material properties cannot be ignored.” Song Yihu said that it should be re-examined for more than 100 years. Prejudice".
Rubber molecules that cause the tires to heat up
The research team used solvent soaking method to extract rubber from unvulcanized rubber compound material and obtained a complete carbon black network structure. This network is strong and flexible. The research team adopted a high-temperature ablation method and also obtained a complete carbon black network structure.
The reporter saw a sample of the block carbon black network obtained by the ablation method. When the hand reached out, the carbon black network was immediately broken like a waffle. Song Yihu told reporters that the surface of the carbon black particles has an adsorption effect. When the long rubber molecules are close, the carbon black adsorbs one of the chains and forms a very stable interface structure. “Even if you pull out the individual 'necked' that has not been adsorbed, the structure will not collapse.” However, the high-temperature ablated carbon black network does not have a rubber necklace “necklace”, which is unstable and brittle.
“The previous model only considered the carbon black network, but the experimental evidence is not strong. The two-phase model we proposed considers the rubber factor.” Song Yihu said that this model can provide a new theoretical explanation for the rolling heat of the tire: fever It is not carbon black, but rubber molecules.
On the highway side of the summer, we often see heavy truck drivers carrying buckets to cool the tires. If it does not cool down, the tire will accelerate aging. The conventional wisdom is that the carbon black particles in the tire rub against each other to generate heat. "Our experimental conclusion is the opposite. It is the long friction of rubber molecules that generate heat. The carbon black particles only accelerate the friction between them."
Expectation theory can be used to guide actual production
According to Zheng Qiang, the key to the two-phase model is to reposition the effects of carbon black and rubber on tire performance, and explain the mechanism of tire reinforcement: filler particles such as carbon black form a network structure, which slows the activity of rubber molecular chains. Increased the elasticity of the tire tread rubber.
The research team also used other fillers, including silica, to prepare the rubber compound. Its rheological properties are consistent with the two-phase model, which is named by the peer scientists as the “Song-Zheng two-phase model”.
"I dare not say that our theory has ended the existing debate, but one thing we can see is that the research on the previous model in the world has been slowly decreasing, and the papers citing our theory are increasing year by year." Song Yihu said that the research team will also do some experiments to explore how to reduce the friction between polymer chains. “We expect our theory to be used to guide actual production and to produce high-efficiency tires with high strength, good elasticity and low heat generation.”