Chemical Pulping Advantages of Zip‐lignin Hybrid Poplar

Hybrid poplar genetically engineered to possess chemically labile ester linkages in its lignin backbone (zip-lignin hybrid poplar) was examined to determine if the strategic lignin modifications would enhance chemical pulping efficiencies. Kraft pulping of zip-lignin and wild-type hybrid poplar was performed in lab-scale reactors under conditions of varying severity by altering time, temperature and chemical charge. The resulting pulps were analyzed for yield, residual lignin content, and cellulose DP (degree of polymerization), as well as changes in carbohydrates and lignin structure. Statistical models of pulping were created, and the pulp bleaching and physical properties evaluated. Under identical cooking conditions, compared to wild-type, the zip-lignin hybrid poplar showed extended delignification, confirming the zip-lignin effect. Additionally, yield and carbohydrate content of the ensuing pulps were slightly elevated, as was the cellulose DP for zip-lignin poplar pulp, although differences in residual lignin between zip-lignin and wild-type poplar were not detected. Statistical prediction models facilitated comparisons between pulping conditions that resulted in identical delignification, with the zip-lignin poplar needing milder cooking conditions and resulting in higher pulp yield (up to 1.41 % gain). Bleaching and physical properties were subsequently equivalent between the samples with slight chemical savings realized in the zip-lignin samples due to the enhanced delignification.