Study On Strengthening Technology Of High Yield Pulp

Paper made by High-yield Pulp (HYP) has some great properties including low density, high opacity and light scattering coefficient. However, surface of HYP fibers is almost covered by lignin and hydrogen bonds among them are very few in HYP paper, so the paper strength is rather low. Paper strength is one of the most important paper properties mainly including tensile index, Z direction strength, tear index and folding strength, which are mainly relevant to fiber materials, types and dosage of strengthening agents and fillers.Under the condition of rarely affecting excellent properties of HYP paper, for increasing hydrogen bonds among fibers and improving paper strength, the effects of CS with different DSs and CMC, as well as CS, PEI and self-made NFC deposition on HYP fibers and their impacts on HYP paper strength properties, density and optical properties were studied emphatically. Besides, the impacts of MFC or NFC on above properties were analysed comparatively.MFC prepared by PFI mill was relatively long. The images screened by computer photography biomicroscope showed that MFC could form bridge-network structure with HYP fibers, which obviously improved paper strength properties, especially for tear index, but it had little effect on HYP paper density and optical properties. Energy consumption of prepareing MFC was significantly reduced through the method of Cellulase-PFI mill. This type of MFC was relatively short, and most of them filled interspace and increased hydrogen bonds between HYP fibers in paper. Therefore, tensile index, Z direction strength and tear index were obviously enhanced. However, it was not beneficial to bulk and optical properties of HYP paper.The performances of NFC prepared by the method of Chloroacetic acid-high-speed shearing was studied. Being Observated by SEM and AFM. the width of NFC was about100-200nm. Its Zeta potential was about-87mV and carboxyl content was2.78mmol/g. NFC arranged densely in the film. The film was transparent and its tensile strength as well as folding strength were very high. Comparing with above two MFCs, the improvement of HYP paper strength is not obvious. Because both HYP fibers and NFC with smaller particle size had high negative charge, it was difficult for NFC to be automatically adsorbed to HYP fibers. Consequently, its retention was very low in paper, which decreased the number of hydrogen bonds among fibers.Because of high negative charge of HYP, the effects of CS with high, moderate and low substitution degree and CMC deposition on HYP fibers was first studied. The images of SEM showed that CS/CMC could be well deposited on HYP fibers. With deposited layers increasing, the retention of CS decreased, but it had little concern with DS of CS. HYP paper strength was significantly improved by CS with different DSs and CMC depositing on HYP fibers. However, the best comprehensive effect of improvement on HYP paper come from CS with moderate substitution degree.The deposition effects of CS with moderate DS(DS=0.22), PEI and CMC were further studied. By AFM. it was found that PEI and NFC could be well deposited on HYP fibers. Although only a small quantity of NFC was added, the deposited layers of both PEI/NFC and CS/NFC on HYP fibers greatly improved paper strength, especially for Z direction strength. However, the bulk and optical properties of HYP paper were influenced adversely.Besides, Zeta potential of HYP showed that driving forces of cationic/anionic polymers adsorption to HYP fibers became smaller and smaller with the deposited layers increasing.Summary, MFC prepared by PFI mill had the largest contribution to tearing index of HYP paper; MFC prepared by the method of cellulase-PFI mill had the largest contribution to tensile index of HYP paper; PEI/NFC and CS/NFC deposition on HYP fibers had the largest contribution to Z direction strength of HYP paper.