| Cellulosic fiber is the basic building-block for paper sheets.The characteristics of cellulosic fiber are significant for the formation and physical properties of paper sheets.As one of the most important characteristics of cellulosic fibers,the surface wettability affects the interaction of fiber and chemical additives as well as the inter-fiber bonding capability,which further influences the paper machine runnability and resulting paper sheet properties.In this work,the fiber surface wettability was characterized based on a novel method of the contact angle determination.Further,the effect of HYP fiber surface wettability on the inter-fiber bonding properties was studied systematically.Firstly,the hardwood pre-conditioning refiner chemical alkaline peroxide mechanical pulp(P-RC APMP)fiber and softwood thermo mechanical pulp(TMP)fiber were chosen as the raw materials,which were modified by two different methods,chemical one(NaSO3/NaOH)and mechanical one(PFI refining).The effects different modifications had on fiber surface properties,including fiber surface charge,surface lignin,surface roughness,and surface contact angle,were then analyzed.It was found that after the mechanical treatment,the fiber surface lignin decreased,fiber surface charge increased and surface functional groups changed.In this dissertation,compared with chemical modification,mechanically treated fibers showed a higher yield and better changes,which resulted in the following discussion based on mechanically treated fibers.Secondly,a novel method for the fiber surface contact angle determination,namely the"liquid-bridge" testing,was developed from the traditional sessile drop technique and used for the surface contact angle measurement of cellulosic fiber.The results of the determination of the "liquid-bridge" testing were then compared with the contact angle determined by the traditional sessile drop technique and it was proved that both methods had a similar accuracy with a distance less than 3.89°.In addition,the fiber surface contact angle was easier and more convenient to measure using the "liquid-drop" testing rather than the traditional sessile drop technique,indicating that the "liquid-bridge" testing is more suitable than the traditional sessile drop technique for the determination of cellulosic fiber surface contact angle.This is because that the plain constructed by the fiber group in the"liquid-bridge" testing could contribute to the formation of a stable liquid drop on the fiber,which is difficult for the traditional sessile drop technique since the fiber itself is a cylinder.Based on the determined surface contact angle,the fiber surface wettability was further evaluated,which was dramatically improved after the mechanical treatment.The influence of fiber surface roughness and surface pores on fiber surface contact angle was then studied.It could be concluded that with the increase of surface roughness and surface pore number and size,the fiber surface wettability was improved.At last,the relationship between fiber surface wettability and inter-fiber bonding properties was analyzed.It was found that while the surface contact angle decreased and fiber surface wettability was improved(surface free energy increasing from 46.63 mJ/m2 to 54.45 mJ/m2),both the inter-fiber relative bonded area(RBA)and the shear bond strength(b)increased.In addition,the bonding strength index of fibers,namely B,was more than doubled and the tensile index also increased dramatically.It was concluded that there was a quadratic function relationship between the bonding strength and bulk and fiber surface wettability(including surface free energy and surface charge).Furthermore,the bulk of the final products didn't decrease seriously with a small decrease from 4.95 cm3/g to 3.57 cm3/g,which meant that improving the surface wettability could benefit for the inter-fiber bonding strength without hurting too much of the bulk of the final product. |
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