Preparation of high-strength silicon-containing elastomer via coordination cross-linking reaction and its multiple recycling performance

Abstract

<p indent=0mm>It is a critical challenge to prepare elastomers with both high mechanical properties and multiple recycling properties to date. Due to cumbersome and difficult to control, it is difficult to realize both high mechanical properties and industrial production with current methods reported in numerous literatures. Herein we demonstrate a new method for the preparation of high-strength silicon-containing elastomer based on the coordination crosslinking reaction between functionalized polysiloxane and rare earth salts (CeCl₃·7H₂O, EuCl₃·6H₂O and TbCl₃·6H₂O, respectively). The functionalized polysiloxane, named poly(dimethyl-methyl (<italic>N</italic>-nitrile ethyl aminopropyl)) siloxane (PANPMS), was syntheiszed by the aza-Michael reaction which having the molecular weight of about 100000 and containing 8% of nitrile groups. The crosslinking reaction occurs in the molding stage, making the preparation process is simple and easy to realize industrialization. These elastomers could achieve high tensile strength <sc>(2.58 MPa)</sc> and high elongation at break (500%). These data exceed the properties of EA 2900 Sealant of Dow Chemical Company which are <sc>2.1 MPa</sc> and 400%, respectively. What’s more, narrow-band red luminescence is observed in EuCl₃·6H₂O and green luminescence is observered in TbCl₃·6H₂O when these elastomers are exposure to ultraviolet light. This means that the high-strength elastomer could be used as luminescent materials. The coordination process was detected by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR) and vulcanization curves. The vulcanization curves show that the optimum vulcanization conditions is 90°C and <sc>25 min.</sc> The torque value in vulcanization curves increase with the content of rare earth salts which means that the vulcanization is closely related to rare earth salts. XPS analysis and FTIR show that the coordinations between nitrile group and rare earth ion take place. Therefore, a conclusion could be drawn that the coordination process plays a role of vulcanization and crosslinking. In addition, it can be noticed from the analysis that there are still a large number of nitrile and rare earth salts in the vulcanized elastomer not participating in coordination reaction. From scanning electron microscopy micrographs, it can be observed that all of the rare earth salts are evenly dispersed in functionalized polysiloxane to form a sea-island structure. This island structures indicate that a large number of rare earth salts are contained in the particales and cannot participate in the coordination-crosslinking process. This phenomenon is perfectly verified with XPS and FTIR data. To our surprise, we happened to find that the elastomer’s powder can be reprocessed into a new elastomer material when the elastomer is crushed. This means that this elastomer could be recycled. The tensile strength of the new elastomer can still reach up to <sc>1.48 MPa</sc> and the elongation at break is 228% after three times of recycling. This data exceeds the standard of 3165 Fast Tack RTV Adhesive Sealant of Dow Chemical Company which are <sc>0.9 MPa</sc> and 185%, respectively. This means that this kind of material can still be used as sealant after three times vulcanization. Further analysis show that there are a lot of excess rare earth ions and nitrile groups which are not coordinated in the elastomer. When the elastomer is crushed and reused, these excess nitrile group and rare earth ions can participate in the coordination process and form new crosslinks, achieving high mechanical properties and multiple recycling properties. If these elastomers are to be recycled for more than four times, it is necessary to optimize the molecular design to make up for the adverse effect of polysiloxane main chain fracture. This research work is expected to provide a reference for the development of materials with high mechanical properties and multiple recycling properties.