In this paper, the potential application of phosphorylated industrial kraft lignin (KLP) in improving flame
retardant properties of unsaturated polyester/KLP composites (UPe/KLP) was studied. KLP mass contents in UPe based
composites varied from 0.5 wt.% to 5.0 wt.%. The successfulness of the performed phosphorylation method, direct grafting
of phosphorus chloride on KLP surface hydroxyl groups, and formation of phosphoric acid ester were confirmed 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) of the UPe/KLP
composites were analyzed in relation to the structure of KLP modification. The obtained composites were tested on fire
retardant properties according to the standard test method UL-94V
