Fireproof phosphorylated kraft lignin/polyester based composites: Green material for rocket propellant thermal protection systems

Presented study was aimed to investigate the influence of surface functionalization of the industrial kraft lignin (KL) on mechanical and thermal properties of high-performance and fireproof composites based on unsaturated polyester (UPe) resin. In order to improve flame retardant properties, surface of the KL was modified by phosphorylation method in two-step process. First, direct grafting of phosphorus chloride on KL phenolic hydroxyl groups was performed and phosphoric acid ester of KL was formed. In the second step, phosphor ester of KL was recovered by precipitation in cooled isopropyl alcohol and the final obtained product, phosphorylated industrial kraft lignin with methyl terminal groups (KLP) was obtained. KLP mass contents in UPe based composites varied from 0.5 wt.% to 5.0 wt.%. Structural characterization was done via Fourier Transform-Infrared Spectroscopy (FT-IR). The effects of KLP surface functionalization and KLP mass contents on the UPe tensile properties were studied. Tensile strength (σ), elongation (ε) and Young’s modulus of elasticity (E) and Shore A (Sh A) hardness of the UPe/KLP composites were analyzed in relation to the structure of KLP modification. Tensile testing results showed an increase in σ of 31 % for UPe/KL sample with 1.0 wt.% KL particles compared to the bare UPe matrix. The origin and cause of fracture that occurred during uniaxial tensile testing were analyzed using stereo microscopy. The obtained composites were tested on fire retardant properties in accordance with the standard test method UL-94V, and composites with 5.0 wt.% loaded KLP achieved V-1 category