Study On Fiber Properties And Paper Strength Based On Lignin Content Of Softwood Pulps

For paper producers, a further understanding of the development of pulp fiber propertiesand paper strength is of uttermost importance. Strong papers are important operators both inthe traditional paper industry as well as in new fields of application, such as fibre-basedpackaging, furniture and light-weight building material. The physical strength of paper andpaper products are determined mainly by fiber properties. According to the difference of fiberseparation methods, the process can be divided into chemical pulping and mechanical pulping.The properties of pulp fibers made from different pulping and bleaching processes were hugedifferent even if pulps from the same raw plant material. And the nature of different fibers isthe disparity of lignin content. The variation tendency of these fiber properties were alsototally different in the subsequent processing such as beating, wet pressing. Those ultimatelyaffect the strength properties of paper. Although it was already found that that fiber chemicalcomposition and physical morphology have great impacts on interfiber bonding strength, thecurrent theoretical findings of paper strength still can not explain the nature difference andrelationships between fiber properties from different pulps and paper strength properties infiber level. It is necessary to further perfect the theory of paper strength.The objects of this dissertation are softwood kraft pulps and chemi-thermal mechanicalpulps. The softwood kraft pulp was bleached by elemental chlorine free bleaching, and theMasson pine CTMP pulp was oxidized by chlorine dioxide. The paper studied the variationtendency of fiber characteristics and strength properties of pulp along with the change oflignin content and suffering the beaten treatments. The correlation and relation modelsbetween fiber characteristics and physical strength properties of softwood pulps werediscussed. The results were as follows.The morphology of fibers: With the decrease of lignin content, the fiber length and widthof kraft pulp had a linear decrease, while the fiber curl and kink index had an overall increase.The fiber morphological charaters except for fiber width had no obvious change during themultistage treatments of Masson pine CTMP pulp by chlorine dioxide. The effects of beating on fiber morphology were greatly different between kraft pulp and chemi-thermal mechanicalpulp. The fiber length, width, curl and kink indexes of kraft pulp and bleached kraft pulp hadno obvious change after beating. However, the fiber morphological charaters such as length,width, curl and kink indexes of Masson pine CTMP had a great decline during beating. Thefines content and water retention value of kraft pulp and Masson pine CTMP pulp bothincreased alone with the beating degree.The charge properties of fibers: The total charge and surface charge of kraft pulpdeclined linearly with the decrease of lignin content. On the contrary, the total charge andsurface charge of Masson pine CTMP increased linearly with the decrease of lignin content.The charge ratio (surface charge/total charge) of kraft pulp had no change, but the charge ratioof Masson pine CTMP increased linearly with the decrease of lignin content. Whatever kraftpulp or Masson pine CTMP, the total charge of fibers had no change after beating treatments.But the surface charge and charge ratio increased with the beating degree. The CTMPexhibited higher surface charge and total charge than kraft pulp. But the charge ratio and itsincrement of kraft pulp were higher than those of CTMP.The fiber strength and internal bond strength: The zero-span tensile strength and Scottbond of unbeaten kraft pulp showed a super logarithmic correlation with pulp kappanumber(R2=0.99), and the logarithmic regression coefficient for Scott bond was negative. Theregression equations were as follows: y1=6.76Ln(x)+92.57，y2=-1.46Ln(x)+131.48，where x=kappa number of pulps, y1=zero-span tensile strength and y2=Scott bond.However, the zero-span tensile strength and Scott bond of CTMP increased linearly with thedecrease of lignin content (R2=0.99). The regression equations were as follows: y1=-1.86x+140.40，y2=-5.77x+183.84，where x=lignin content, y1=zero-span tensile strength and y2=scott bond. The zero-span tensile strength of kraft pulps increased a little with the beatingdegree, while that of CTMP pulps had no any change. The relative bond area of sheet madefrom kraft pulps increased sharply in the initial stage of beating treatment, and then becameflattening. There was only a slightly increase of relative bond area for CTMP pulps duringbeating process. The variation of internal bond strength of all pulps was the same as relative bond area of pulps.The tensile, burst and tearing indexes of pulps’ handsheet: The tensile, burst and tearingindexes of unbeaten or beaten kraft pulps decreased with the decrease of lignin content. Incontrast, the tensile, burst and tearing indexes of CTMP increased linearly with the decreaseof lignin content. The tensile index and burst index of kraft pulp was significantly enhancedinitially by beating, then increase smoothly. The tearing index of kraft pulp and CTMP fellslightly with increase beating.The effect of fiber surface lignin precipitation on interfiber bonding and paper strength:The results showed that the surface lignin could reduce the internal bond strength of papersheet greatly, while the fiber intrinsic properties have no significant change during lignindeposition treatments. The decrease of sheet Scott bond was maily due to the decline ofinterfiber bonding strength after surface lignin precipitation, while the sheets density whichcould represent the relative bond area of sheet had showed only an slightly decline. The sheettensile strength index was also decreased linearly with the decline of Scott bond. The effectsof Scott bond on tear and burst strength index were relative small.The correlation analysis between tensile strength and individual fiber strength, internalbond strength: The overall decline of tensile strength of softwood kraft pulps during the ECFbleaching sequence could be mostly attributed to the degeneration of individual fiber strength.The contribution of Scott bond improvement from lignin removal during bleaching processesto sheet tensile strength was relatively small due to the fact that the amplification itself causedby bleaching chemicals was limited. On the contrary, Scott bond rising caused by beating wasso big that the sheet tensile strength increased greatly. The ratio of tensile index to zero-spantensile index of Masson pine CTMP was increased with the decrease of lignin content afterchlorine dioxide treatments, which indicated that the contribution of internal bonding strengthto tensile strength was higher than fiber individual strength for CTMP. It was also supportedby the regression analysis of tensile index to zero-span index and Scott bond of CTMP.Whatever chemical pulps or chemi-thermal mechanical pulps made from softwood plantmaterials, the sheet tensile strength depended more on relative bond area and internal bond strength based on the regression models made from the beating process. Or rather, the tensilestrength depended on relative bond area and interfiber bonding strength. And the influence ofrelative bond area on tensile strength was much higher than that of interfiber bond strength.The regression equations were as follows: T=6.217+R*（1.020+0.043S）for softwoodchemical pulps; T=3.770+R*（0.508+0.094S）for softwood chmi-thermal mechanicalpulps, where T=tensile strength index, R=relative bond area, S=specific bond strength.The research reports about zero-span tensile strength: The fibers separated fromsoftwood log without lignin removal from fiber cell wall would not possess the maximalindividual fiber strength naturally. The fiber strength initially in a low level increased to amaximal value, and then got down along with removal of lignin by chemicals treaments. Theevidence presented in the results demonstrated conclusively that the interfiber bonding, curledfibers and kinked fibers have little effect on zero-span tensile strength. The enhancement ofzero-span tensile index after beating was suggested to be ascribed to the variation of fiberelongation. The internal and external fibrillation of fibers during beating treatment modifiedthe fiber cell wall structure and made the fiber more flexible, which enhanced the fiberelongation. The difference of dry to rewet zero-span tensile index can be used to characterizethe degree of fiber damage. Both the bleaching and beating processes could result in anincrease of fiber damage.