The Study Of Wet Refining Method NR/nSiO₂and NR/N234Composites: Preparation、Structure And Properties

Only after the rubbers, especially the rubbers used for preparing car tires, reinforcedwith a reinforcing agent, the performance of the vulcanizates could meet the requirement forapplication. The carbon black is the most important reinforcing agent, and has the bestreinforcement effect to rubber and the maximum consumption. The white carbon black （SiO₂powder） is followed. The reinforcement effect of SiO₂is not better than that of carbon black,and the application of SiO₂was not universal too. But the rubber tire with silica as reinforcingagent had a significant advantage of a high wet skid resistance and low rolling resistance. Thestudy on rubber and silica composites being used to prepare tire tread has become a hot topicat present.Since the birth of the rubber industry, the solid rubbers and the powder fillers were usedas raw material, and the mechanical refining method, or dry refining method was used topreparing masterbatch stock along with the rubber processing industry. The dry refiningmethod has the shortcomings of complex process, big investment for equipment and highlabor intensity. Reinforcing agents, especially carbon black, could be caused the blackpollution. The carbon black and the silica used to reinforcing rubber were nano-powders withhigh surface energy, so it is difficult to disperse the carbon black or the silica particles torubber matrix with mechanical shearing. The nano-particles often aggregated with severalmicrons in the rubber matrix, causing to damaging the physical and mechanical properties ofvulcanizates. The wet refining method is a novel technology for preparing masterbatch usingthe rubber latex and filler slurry as raw material. The filler particles were dispersed uniformlybetween the rubber latex particles by liquid mixing, and the wet refining system were formed.After coagulation-coprecipitation, the rubber/filler composites were obtained. The wetrefining method could easily overcome the shortcomings which are difficult to be overcomeby dry refining method.In this article, firstly, the NR/nano-silica composites were prepared by the in situgeneration-wet refining method. Using the character of silica sol easy forming silica gel whenheated, the natural rubber latex （NR） was mixed with silica sol （silica precursor）, and then thewet refining system was heated until the silica sol generating in-situ the nano-silica particles （nSiO₂） in the NR latex. The NR/nSiO₂nano-composites were obtained by means ofcoagulation. The effects of wet refining conditions and the different vulcanization recipeswere studied on the physical and mechanical properties of the NR/nSiO₂vulcanizates. Thewet refining conditions were involved the types and the contents of the surface active agentfor the silica sol particles and the nSiO₂particles, the type particle size and concentration ofthe silica sol, the concentration of NR latex, the reaction temperature and reaction time for theconversion of the silica sol into the nSiO₂gel particles, the types and the dosages offlocculant and the nSiO₂contents of the materbatch stock and so on. The optimiumconditions of wet refining and vulcanization recipe were determined with which thevulcanizates obtained had the best physical and mechanical properties. The results obtainedwere compared with the NR/SiO₂vulcanizates based on traditional dry mixing of bale naturalrubber and white carbon black. The results showed that the key factors affecting the physicaland mechanical properties of the vulcanizates were the type of the surface-active agent andthe type of the silica sol. When using the hydroxylether as the surface-active agent and aneutral silica sol as SiO₂precursor, the vulcanizates had excellent physical and mechanicalproperties with the SiO₂contents50⁹0phr. In particular,300%modulus, tensile strength,tear strength, abrasion resistance, compression fatigue heat and permanent deformation of thevulcanizates achieved a good level. The composites and its vulcanizates were analyzed byFourier transform infrared spectroscopy （FTIR）、differential scanning calorimetry （DSC） andthermal gravimetric analysis （TG、DTG）. The bound rubber content of the NR/nSiO₂composites and the cross-linking degree of its vulcanizates were measured. The resultsindicated that silica sol in wet refining system was completely converted into nSiO₂. With theSiO₂contents increasing, the Tg of the composites and its vulcanizates did not changesignificantly and the initial thermal decomposition temperature gradually increased. TheNR/SiO₂vulcanizate was studied using rubber processing analyzer （RPA） and dynamicviscoelastic analysis （DMA）. The results showed that, with the SiO₂content increasing, thePayne effect, or the SiO₂network architecture of the vulcanizates increased, the wet skidresistance decreased and rolling resistance increased. The scanning electron microscopy（SEM） analysis showed that, in the NR latex, the silica sol converted in-situ into SiO₂ surfaces of the vulcanizates shown a "valves" structure. This novel structure had not beenreported before. There were no bare nSiO₂aggregates and cracks existing on the tensilefracture surfaces. It was indicated that reinforcement mechanism of the NR/nSiO₂vulcanizates was NR matrix shear yield with the character of the "valve" structure. A goodpre-dispersity of nSiO₂particles in NR matrix and the reinforcement mechanism of matrixshear yield charactered by the special "valves" structure were the main reasons forvulcanizates having excellent physical mechanical properties and dynamic mechanicalproperties.The NR/DnSiO₂composites and its vulcanizates were successfully prepared by the wetrefining method with natural latex and silica nano-dispersion （DnSiO₂） liquid as the main rawmaterial, under the condition without adding any surface-active agent for nSiO₂particles. Thevariation of mechanical properties and the microstructure of the vulcanizates was studiedwith the SiO₂contents increasing, and compared with the NR/nSiO₂vulcanizates prepared byin situ generation-wet refining method under the condition of the same SiO₂content. Theresults showed that, when the SiO₂contents were varied from50phr to80phr, the NR/DnSiO₂vulcanizates had slightly higher300%modulus, lower rolling resistance, significantly higherpayne effect and the SiO₂existed in the morphology of aggregates with average particle sizeof150²10nm in the NR/DnSiO₂composites, when the SiO₂contents at50phr and60phr, theNR/DnSiO₂vulcanizates had better tear strength, higher flex cracking resistance performance,similar other physical and mechanical properties and the "valve" structure without bare nSiO₂aggregates showed on the tensile fracture surface of the vulcanizates, which meant that thereinforcement mechanism was matrix shear yield, so that the vulcanizates had bettercomprehensive performance. But when the SiO₂contents at70phr and80phr, there was no"valve" structure on the tensile fracture surface of the NR/DnSiO₂vulcanizates in which theSiO₂particles were poorly dispersion and existed in aggregates with particle diameter of23μm²8μm. As a result, the overall performance of the vulcanizates was significantlyreduced.The comparative analysis above showed that under the same SiO₂contents and samevulcanization recipe. among the three types of vulcanizates, the NR/nSiO₂vulcanizates hadthe best comprehensive performance, followed by the NR/DnSiO₂vulcanizates and the NR/silica vulcanizates, the worstFinally the NR/N234composites prepared by wet refining method and traditional dryrefining method. The physical and mechanical properties and the structures of the two typesof vulcanizates were investigated. The results showed that compared with the vulcanizates ofthe NR/N234prepared by dry refining method, the NR/N234vulcanizates prepared by wetrefining method had better physical and mechanical properties, in particularly significantlyhigher300%modulus, tensile strength, tear strength and resilience value, and significantlylower compression fatigue temperature rise, compression permanent deformation, andpayne effect, better wet skid resistance and similar the rolling resistance.The results showed that the comprehensive performance of three types of vulcanizes,involving the NR/nSiO₂vulcanizes prepared by in situ generation-wet refining method withSiO₂content of50⁷0phr, NR/DnSiO₂vulcanizes prepared by wet refining method with SiO₂contents of50⁶0phr and the NR/N234vulcanizes prepared by wet refining method withcarbon black N234contents of45⁵5phr, met the requirements for preparation of the radialtruck tire tread, and were the excellent composites of NR to prepare radial truck tire tread.