Phosphorylation of Kraft fibers with phosphate esters

• We present a new synthesis pathway to produce phosphorylated cellulosic fibers.
• The main phosphorylating reagent is a phosphate ester.
• Phosphorylated fibers show negligible degradation compared to previous phosphorylation processes.
• The phosphorus distribution (bulk vs. surface) is presented.
• The type of phosphate structures found on modified fibers is discussed.

Phosphate esters, derived from two different long-chain aliphatic alcohols, were used as phosphorylating reagents for Kraft pulp fibers. High phosphorus contents and almost non-degraded fibers were obtained by following this pathway. The phosphorylation efficiency was influenced by the alkyl chain length of PEs since the phosphorus content in modified fibers was higher for the shorter chain reagent. Due to the heterogeneous reaction environment, the amount of grafted phosphorus was found to be almost three times higher at the surface than in the bulk of the fibers. Analyses also indicated that the phosphorus was bonded to fibers as a phosphate-like structure. Furthermore, the situation seemed to be different for the fiber surface where significant amounts of phosphorus were present in more complex structures like pyrophosphate or even oligo-phosphate.