Two-photon autofluorescence spectral imaging applied to probe process-effects in thermomechanical pulp refining

Norway spruce wood pulps were produced in full industrial scale trials at different thermomechanical pulp refining conditions, such as plate gap, housing pressures and energy consumption levels. To investigate the effects of these refining conditions on the lingocellulosic matrix in fines and fibers, we applied high-resolution spectral imaging in a two-photon fluorescence microscope and compared with conventional pulp and paper analyses (strength, freeness etc.). The fluorescence spectra of lignin in native wood and fibers- and fines from pulps showed that spatial- and spectral heterogeneities can be observed using two-photon autofluorescence spectral imaging, and successfully probed on a microscopic level. Moreover, it was shown that wood autofluorescence depends on fiber morphology and becomes red-shifted by increased temperature, but the fluorescence spectrum of TMP long fiber fraction shifted towards blue by increased refining pressure.