Chemi-mechanical pulping with selective fiber separation using 2D correlation near-infrared spectroscopy

In chemi-mechanical pulping process, selective fiber separation strategies directly affect pulp property. The secondary wall (SW) separation strategy and middle lamella (ML) separation strategy expose more chemical structures from polysaccharides and lignin on the pulp fiber surface, respectively. Hydrogen-containing groups in these chemical structures generate characteristic spectral changes in response to water perturbation, which makes it possible for surface property analysis of pulp fibers based on near-infrared spectroscopy (NIR). Here, two-dimensional correlation spectroscopy (2DCOS) was obtained by the dynamic NIR spectra of pulp fibers with various equilibrium moisture content (EMC). The 2DCOS-NIR can provide richer structural and distributional information about hydrogen-containing groups which characterize the differences in surface chemical properties due to selective separation of the fibers. In synchronous map, the fibers separated at SW have significant water perturbation-induced spectral changes at the bands due to polysaccharides, and the fibers separated at ML have characteristic spectral changes at the bands due to hydrogen-containing groups from lignin. Furthermore, the 2DCOS spectral features can also accurately reflect the variation in the distribution of hydrogen bonding on the fiber surface under different refining conditions, which directly affects the strength properties of pulp fibers. The PLSR model based on 2DCOS spectral features exhibits excellent and robust predictive performance for internal bond strength with RMSE_p of 7.17 J/m2, R of 0.9378 and RPD value of 4.11.