Measurement Of Fiber Charge And Its Effects On Interfiber Bonding Ability Of High Yield Pulp

High yield pulp(HYP)fiber becomes increasingly used in many papers and paperboards.However,the weak interfiber bonding strength of HYP is the main drawback limiting its application proportions and scopes.Fiber charge is one of the most significant factors affecting the bonding strength of HYP.Systematically investigating the method for fiber charge evaluation and the influence of fiber charge on interfiber bonding strength of HYP becomes of great importance for improving the mechanism of HYP fiber bonding enhancement.Firstly,the method for fiber charge(including fiber surface charge and inner charge)measurement was investigated and improved by using polyelectrolyte adsorption theory.Series of polydiallyldimethyl ammonium chloride(poly-DADMAC)with different molecular weight(Mw)were used to treat the cellulosic fiber for accessibility evaluation.The applicability of polyelectrolytes with different Mws for surface charge and total charge determination was investigated.The results showed that different Mws meant different accessibilities.The low-Mw polyelectrolyte(Mw=7.5-15 kDa)could permeate into pores of fiber cell wall and adsorb with fiber inner charge through electrostatic interaction,while the high-Mw polyelectrolyte(Mw>100 kDa)was only adsorbed on fiber surface.On a condition that the salts concentration was much low,the high-Mw polyelectrolyte(Mw>100 kDa)could be used for fiber surface charge measurement in a short processing time(2 h),and low-Mw polyelectrolyte(Mw=7.5-15 kDa)could be used for fiber total charge measurement in a relatively long time(more than 9 h).Secondly,the polyelectrolyte adsorption technique was used for analizing the HYP fiber charge properties of different treatments.The HYP fiber(P-RC APMP)was subjected to three different treatments(TEMPO-mediated oxidation,CMC attachment and PFI refining)to alter the charge density in different regions of fiber.The results determined by polyelectrolyte adsorption method showed that TEMPO-mediated oxidation led to the increase of fiber bulk charge(means surface charge and inner charge increase together);CMC attachment increased fiber surface charge,and PFI refining resulted in the increase of surface charge and inner charge,along with fiber fibrillation.Charge enhancement of HYP could increase water retention value(WRV)and fiber flexibility,while it decreased fiber freeness.Compared with only surface charge inhancement,the bulk charge increase also showed more notable increase in WRV and fiber flexibility.Finally,the impact mechanisms of HYP fiber charge(surface charge and inner charge)on its interfiber bonding strength were discussed.The HYP fiber charge density of different regions showed different effects on fiber bonding ability.The shear bond strength per unit area(b)increased linearly with the fiber surface charge,with a regression coefficient of 0.88.Inner charge enhancement could improve fiber flexibility and deformability effectively,which benefited to the increased relative bonded area(RBA).The bulk charge increase showed much higher in Page bonding strength index(B)and tensile index enhancement than the only surface charge increase.During enhancing the tensile strength of handsheets,the process of increased fiber charge was more favorable than the process of mechanical refining in maintaining bulk of handsheets.