Influence of Silicon Content on the Thermal/Mechanical Performance of Urea–Formaldehyde Fibers

Silicon-modified urea–formaldehyde fibers (SiUFF) were prepared by adding different proportions of phenyltriethoxysilane to urea–formaldehyde resin, which were then subjected to dry-spinning and curing treatment. The SiUFF were characterized by Fourier infrared spectroscopy, micro-infrared analysis and ¹³C nuclear magnetic resonance analysis (¹³C NMR). It was found that the element silicon existed in the form of Si–O bond in the SiUFF, and the hydroxymethyl group cross-linked with the amino group to form methylene at the curing stage, while internal thermal conduction caused surface crosslinking density to exceed that of the fiber core. Optimal performance of SiUFF was achieved with 4 wt % Si-modifier addition, cured at 190°C for 30 min, exhibiting an elongation at break of 7.1%, a tensile strength reaching 389 MPa and a char yield of 30.01% at 1000°C, which represented significant improvements of 96, 92, and 14.2%, respectively compared to the unmodified urea–formaldehyde fiber.