Boron-modified high-ortho phenolic resins (BPRs) were prepared under normal pressure by using phenol and formaldehyde as raw materials, zinc acetate, and oxalic acid as catalysts, and boric acid as a modifier. Boron-modified phenolic fibers (BPFs) were prepared by melt spinning and curing in a mixture of formaldehyde and hydrochloric acid, followed by a heat treatment under high temperature. The structure, ortho–para ratio (O/P), molecular weight and distribution, spinnability, thermal stability, fiber strength, and morphology of the resins were characterized by Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and fiber strength testing. The results showed that the addition of boric acid reduced the ortho reaction of the synthetic resin and the O/P value of phenolic resin. When the content of boric acid was 3 wt%, the thermal stability was the best, the O/P value was up to 3.26, and the weight average molecular weight (Mw) was 18745 g/mol. In Compared with the unmodified resin, the mass loss was increased by 33.7%, and finally the carbon yield was 51.2%. The tensile strength of the fibers reached 187.2 MPa and the elongation at break was 10.5%. By introducing boron into the molecular chain, the structure of the resin was improved, and the thermal stability and mechanical properties of the fibers were improved.

Surplus lignin, which is inefficiently used, is generated in the forestry industry. Currently, most studies use lignin instead of phenol to synthesize thermosetting resins which cannot be reprocessed, thus affecting its application field. Thermoplastic phenolic resin has an orderly structure and excellent molding performance, which can greatly improve its application field and economic value. Herein, phenol was partially replaced with enzymolysis lignin (without treatment), generating lignin-based high-ortho thermoplastic phenolic resins (LPRs), and then lignin-based phenolic fibers (LPFs) were prepared by melt spinning. FTIR, ¹³C-NMR and GPC were used to characterize the ortho–para position ratio (O/P value), molecular weight and its distribution (PDI), and rheological properties of the resin. TG, XRD, SEM and tensile property studies were used to determine the thermal stability, orientation, and surface morphology of the fiber. Lignin addition resulted in the decline of the O/P value and molecular weight of the resin. For the 10% LPR, the O/P value, Mw, and PDI were 1.28, 4263, and 2.74, respectively, with the fiber exhibiting relatively good spinnability. The tensile strength and elongation at break of the 10% LPF were 160.9 MPa and 1.9%, respectively. The addition of lignin effectively improved the thermal properties of the fiber, and the carbon yields of 20% LPF before and after curing were 39.7% and 53.6%, respectively, which were 22.2% and 13.7% higher than that of the unmodified fiber, respectively.

In order to improve the application range of lignin phenolic resin, lignin thermoplastic phenolic resin was prepared by controlling the molar ratio (formaldehyde: phenol) with liquefied lignin, phenol and formaldehyde as raw materials, and thermoplastic phenolic fiber was obtained by melt spinning. The effects of different curing bath acid proportion (40%, 50%, 60%) on the mechanical property, thermal property and morphology of the fiber were investigated, and the curing behavior of lignin phenolic fiber was studied. The structure and properties of the resin and fiber were characterized by FT-IR, NMR, GPC, TG, DSC, XRD, SEM and fiber strength testing. The results showed that when the molar ratio was 0.7:1, the resin had the best performance, the O/P value was 1.75, Mw was 8298, and PDI was 2.92. After curing, the fiber continued cross-linking reaction, and the diameter of the cured fiber was thicker than that of the original fiber. The most suitable curing process for the original fiber was that formaldehyde and hydrochloric acid were prepared into curing bath according to the mass ratio of 1:1, the heating rate was 20 ℃ h⁻¹, the curing termination temperature was 170 ℃. The tensile strength of the fibers reached 176.64 MPa and the elongation at break was 1.48%, and finally the carbon yield at 800 ℃ was 37.2%.