A Fundamental Study On The Devulcanization Of Waste Tire Rubber In Supercritical Fluids

Waste tire rubber could turn into the available resources after devulcanization. Through this technology, not only the limited rubber and fossil resource can be saved, but also the environmental pollution from waste rubber could be avoided. The devulcanization has gradually become the research focus in the field of waste transformation.In this paper, the waste tire rubber with unsaturated butadiene units was studied object, and three kinds of devulcanization process were proposed. Firstly, the waste tire rubber was devulcanized in SCCO₂. And then, the waste tire rubber was devulcanized in ScE. Finally, the devulcanization of waste rubber and the degradation of dye wastewater were carried out in subcritical or supercritical water simultaneously. The devulcanization reaction was investigated in the three processes, and the influencing mechanism of different reaction factors was studied in depth. Furthermore, the reaction mechanisms of devulcanization processes were obtained. The research results could be used as the basis for developing the new processes for recycling waste tires, and improving the supercritical fluid technology toward industrialized application. The grinding capability of devulcanized rubber was studied, the research result could provide a new process for producing higher-value product from processed waste tire rubber.Using ScCO₂ as reaction medium, the devulcanization of actual waste tire rubber was studied. Waste tire rubber composition includes SBR, BR and NR. SCCO₂ could bring the devulcanizing agent into the inside of crosslinking network, breaking down the crosslinking network to facilitate the devulcanizaiton process. At the same time, the crosslinking reaction from unsaturated butadiene units would be inhibited. The concentration of devulcanizing agent and temperature were the significant factors on devulcanization, while the reaction pressure and time had only slight influence. For the high-quality devulcanizing rubber, the reaction temperature should be below 200 ℃, and the concentration of devulcanizing agent should be above 10 g·L^-1 and 8 g·L^-1 for tread and sidewall rubber respectively. The rubber compositions of tread and sidewall rubber are different. And the corresponding devulcanization reactions show different characteristic, too. However, no mutual influence was shown when the two types of rubber were simultaneously devulcanized in simultaneously.Using ScE as solvent, the devulcanization of actual waste tire including butadiene rubber and nature rubber was studied. ScE could break down the crosslinking network, and then achieve devulcanization process. The key factor for sol fraction was reaction temperature, while that was reaction pressure for crosslinking density of gel. The sol fraction had no marked increase with the addition of devulcanizing agent at 270 ℃, it demonstrated that ScE could replace devulcanizing agent in higher temperature. Comparing the devulcanizing reaction in ScE and ScCO₂, the reaction temperature in ScCO₂ devulcanizing process was lower. While ScE devulcanizing process had an advantage that devulcanizing agent was not necessary. From 210 ℃ to 260 ℃, the apparent activation energy of devulcanizing reaction in ScE and ScCO₂ were 3.06×104 J·mol^-1 and 6.92×103 J·mol^-1 respectively, it indicated that the devulcanization in ScCO₂ was easier to carry out.Using sub/supercritical water as solvent, the compound technology combining devulcanizaiton of waste tire rubber and degradation of dye wastewater was studied. The optimum reaction temperature was 360 ℃. At this temperature, the sol fraction was above 30% and the COD removal rate of wastewater was above 90%. The sol fraction increased with the increase of temperature, but the COD removal rate decreased at the same time. This result was due to the excessive degradation of the rubber for the supercritical waster. The sol increased to above 50% at 380 ℃, but the COD removal rate decreased to 44%. In the devulcanizing reaction in dispersed dye and acid dye wastewater, the apparent activation energy showed little difference, which were 7.3×104 J·mol^-1 and 4.65×104 J·mol^-1. This result showed that the type of wastewater had no influence on the apparent activation energy of devulcanizing reaction.The research of cryogenic grinding capability was conducted for devulcanized rubber by a jet mill. The greater the devulcanization for rubber degree was, the smaller particle size was for product. The reason was twofold:（1） The sol fraction was increased and the crosslinking density of gel was decreased; （2） The glass transition temperature of rubber was increased. When the concentration of devulcanizing agent was increased to 6 g-L"1, the aggregation of ground product was enhanced. This made the mass fraction of product with the particle size between 32 μm and 72 μm decrease. The result of grinding kinetic analysis indicated that the devulcanized rubber powder was more likely to be ground. The particle size of ground product was mainly between 72 μm and 546.6 μm for devulcanized rubber powder. However, it was mainly between 162 μm and 820.1 μm for rubber powder. Thus it can be seen that the particle size of ground product from devulcanized rubber powder was smaller.The waste tire rubber was devulcanized in different supercritical fluids, the experimental analysis, model calculation and theoretical study were conducted. The influence of different factors on the devulcanization process and the reaction mechanism were studied. A compound technology that allowed two types of waste to be disposal at the same time in one supercritical fluid process was researched. In this technology, the attempt was made to use one type of waste to treat another one, and the process conditions and mechanism were explored. The new method and process for recycling waste tire with multiple rubber components in the supercritical fluid were developed.