A Study On Triploid Clone Of Populus Tomentosa Carr. Lignin Structure And Its Changes In Pulping And Bleaching

Studies on lignin chemistry are very important to pulp and papermaking technology, bleaching technology and spent liquor treatment, and the analysis on lignin structure can provide basic data for the application of lignin. In order to gain information about the structural characteristics of lignin of Triploid Clone of Populus tomentosa Carr., and its changes in the process of pulping and bleaching, milled wood lignin(MWL), lignin-carbohydrate complex (LCC) and the residue lignin from kraft pulp(KP) and alkaline sulfite-anthraquionoe (AS-AQ)pulp were isolated and analyzed by Fourier transform infrared(FT-IR) spectrum, ¹³C and ¹H nuclear magnetic resonance (NMR). Experiments were made to explain the delignification theory in the process of KP and AS-AQ bleaching, xylanase, laccase and biomimetic system were used to treat the unbleached pulp, the changes of lignin and the reaction mechanism in the bleaching process were studied by FT-IR. The results showed that in the lignin of Triploid Clone of Populus tomentosa Carr., the ratio of p-hydroxyphenylpropane unit was 2.2%, guaiacylpropane unit(G) was 44.2%, and syringylpropane unit(S) was 53.6%, and the value of S/G was 1.21, which means the ratio of guaiacylpropane unit was lower than the common hardwood., it will make the lignin delignification more difficult during KP pulping and bleaching sequences. The linkage between lignin unit was mainly composed by β -O-4 and β-β , accompanied with a little α —O—4, β-5 and 5—5' . LCC was mainly composed by oxyloglucan and glucuronic acid, with a little arabinan. The lignin content in LCC increased with the wood age. Compared with the residue lignin in KP and AS-AQ pulp , it can be seen that in the process of AS-AQ pulping, guaiacylpropane unit was more easier to be dissolved than syringylpropane unit. So there was almost no β-O-4 linkage in AS-AQ pulp, but a little in KP pulp. In KP and AS-AQ spent liquor, there was almost no aralkyl ether linkage, which was dissolved in the process of pulping. After the bleaching sequence of O-D1ED2, the degree of whiteness increased, while ClO₂ was saved by 25%. After the bleaching process of O-D₁ED₂-P, the whiteness of KP pulp could reach 86.8%ISO, and AS-AQ pulp could reach 89.4%ISO. The treament of xylan enzyme and laccase had different effects on different bleaching sequences, which could increase the whiteness of the pulp and shorten the bleaching sequence, especially to KP pulp. Except that, lignin could be deligninficated by laccase on a certain extent, and carbon-hydrate could not be dissolved. After the treatment of biomimetic system, the degree of whiteness went up, while the viscosity went down greatly, compared with the process of O-GIF-P and O-P, the degree of whiteness of KP pulp was increased by 4.3%ISO, and 1.3%ISO for AS-AQ pulp. Compared with O-D₁ED₂ bleaching sequence, the degree of whiteness of KP pulp rose by 1.7%IS0, while AS-AQ pulp rose by 2.7%ISO in the process of O-GIF-D₁ED₂.The structure properties of Triploid Clone of Populus tomentosa Carr.lignin, lignin-carbonhydrate complex and lignin changes in the process of pulping and bleaching were discussed in this paper, and then xylanase enzyme, laccase and biomimetic system were used in the bleaching sequences, their effects on the pulp and lignin structure were analyzed, which can be the basis to the application of Triploid Clone of Populus tomentosa Carr. in pulp and papermaking industry.