Photoyellowing of thermomechanical pulps: Looking beyond alpha-carbonyl and ethylenic groups as the initiating structures

Photoyellowed and control unbleached, borohydride bleached, and bleached and diimide-reduced spruce thermomechanical pulps (TMPs) were studied using near-IR excited FT Raman, DRIFT, and diffuse reflectance UV-Vis. spectroscopies. In addition, extracts of yellowed pulps were analyzed spectroscopically. FT Raman spectroscopy was used for the first time in the study of the photoyellowing phenomenon. Post color number data of the yellowed TMP handsheets suggested that alpha-carbonyl and ring-conjugated ethylenic groups did not play a significant role in the yellowing process. This suggestion was further supported by not only the similarity of the FT Raman and DRIFT spectra of the three yellowed pulps but by the similarity between the spectra of the pulps' yellow extracts. Raman spectra of the TMPs provided evidence for the following: (1) diimide-reduced TMP was indeed free of aromatic ring-conjugated ethylenic bonds; (2) photoexposure of unbleached TMP caused degradation of coniferaldehyde and coniferyl alcohol structures; and (3) photoexposure of bleached TMP caused the degradation of coniferyl alcohol structures. In addition, yellowing resulted in changes in the 1600 cm(-1) Raman band and a new broad feature appeared at 1675 cm(-1). In the IR spectra of pulp extracts, the latter vibrational mode was detected at 1673 cm(-1). IR spectra of the yellowed pulps and their extracts showed increased absorption at 1727 cm(-1). Upon yellowing, contribution to this band was primarily from the photoproducts of those lignin structures that did not contain a conjugated-ethylenic bond. These results suggest that there are yellowing initiating structures, other than alpha-carbonyl and ethylenic groups, that are responsible for most of the yellowing in unbleached, borohydride bleached, and diimide-reduced TMPs.