| The synthesis of strengthening agents about polyurethane-lignocellulosic fiber copolymers and their applications on paper making were investigated in this paper.The prepolymer of waterborne polyurethane was synthesized with a modified method. In order to pretect the isocyanate groups (NCO) which were easy to react with the active hydrogen, the end groups were blocked with imidazole. In the process of synthesis about polyurethane prepolymers, it was found that the purity of materials and the end-point selection of synthesis affected the final products. In addition, the purity of TDI, the hydroxyl group content of polyethylene glycol (PEG), the change of NCO groups with time and the content of blocked polyurethane were measured. At the same time, we found that imidazole was a good sealingagent to block the isocyanate groups (NCO)of TDI.Reaction mechanism between imidazole-TDI-PEG and fiber was studied by FT-IR analysis, NMR determination and scanning electron microscope observation. It is found that the blocked -NCO groups can be unblocked when coated paper is heated in drying process. The ester bonds can be formed between the unblocked -NCO and -OH in fiber. The chemical bonds are much stabler and stronger than the hydrogen bonds between the fiber so that the physical strength of coated paper is enhanced obviously.Paper were treated with the polymers so that the traction mechanism of polyurethane could be investigated. Furthermore, the applications in coating and wet-end on paper making were also investigated in this paper. It is found that the treatment can enhance physical strength of paper. When the coating weight of the filter paper reaches 7g/m2, the physical properties of the filter paper are enhanced significantly, especially the wet strength and folding resistance. Wet strength index of the paper reaches 28.80N·m·g-1, 19.59 times of the wet strength index of the original base paper. Dry strength index of the paper is enhanced to 54.40N·m·g-1, which is 1.74 times of the dry strength index of the original base paper. The folding number was 894, which is 47 times of the original base paper. The reason was that the blocked NCO groups could be unblocked when they were heated. And the unblocked NCO groups could react with the hydroxyl groups in paper fiber so that they could be linked by the chemical bonds. As a result, the hydrogen bonds could be changed into chemical bonds which enhanced the physical strength of paper.The metheods of synthesis of polyurethane by using paper and lignin as raw materials were investigated. The surface feature and physical characteristics of the polyurethane materials were also analyzed. It was found that carbomate bond was formed between lignin-TDI-PEG telechelate polymer and cellulose in paper. Because unblocked polyurethane could react with hudroxyl of cellulose in fibers and lignin.Carbomate bond not only can link the distant fibers in paper together, but also can connect fiber with lignin. Therefore the fiber-lignin-TDI-PEG type polyurethane is formed which enhances the paper strength. The physical strength of the treated fibreboard reaches maximum, especially the wet tensile index , dry tensile index, tear strength , folding endurance. The wet tensile index, dry tensile index , folding endurance, tear strength , stiffness strength of the treated fibreboard reaches 12.95 times , 1.28 times, 11.4 times, 1.17 times and 1.22 times of the original base paper.In addition, beatability of cellulose to influence of paper polyurethane was researched. It was found that Along with the beating degree enhancement, paper's physical property is also improved and the enhancement along with it. The polyurethane effect is also better. The wet tensile index, dry tensile index , folding endurance, tear strength , stiffness strength of the treated fibreboard reaches 2.52 times , 1.24 times, 4.57 times, 1.23 times and 1.22 times.
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