| The production and consumption of paper and paperboard in China is ranked the first around the world,but the shortage of fiber raw materials has become a major bottleneck restricting the sustainable development of paper industry.Addition of mineral filler in papermaking process can not only improve paper optical properties and printability,but also save fiber raw materials,reduce energy consumption and production cost.The increase in filler content of paper has become a research hotspot and development trend of paper industry around the world.However,the increase in filler content in paper can cause strength loss and low retention rate,which negatively affect the runability of paper machine,and thus limit the further increase of filler content.Therefore,the development of new filler technology to alleviate the negative impact of filler on paper properties is the key to realize high filler content in paper.Filler preflocculation as an effective and economical filling method has attracted extensive attention from domestic and foreign scholars and papermaking companies.In the process of preflocculation,filler floc size and its stability in approaching system can significantly affect paper physical properties.In this work,precipitated calcium carbonate(PCC)was adopted to investigate characterization and flocculation mechanisms of filler floc,floc size regulation and Z-direction distribution of filler,and the enhancement of filler flocs.The detail content is as follows:1.Characterization and flocculation mechanisms of filler flocsCationic starch(CS),cationic polyacrylamide(CPAM)and the blend of cationic polyacrylamide and anionic polyacrylamide(APAM)were adopted as flocculants to preflocculate PCC filler.The shear resistance and re-flocculation ability of PCC flocs were investigated under pump circulation and stirring modes.Results showed that the floc size could increase by 6~9 times after addition of flocculants.Stirring condition,flocculant type and dosage can affect the floc size and floc stability markedly.Bridging mechanism of PAM played an essential role in preflocculation of PCC filler while charge neutralization played a major role for cationic starch.Compared to cationic starch,CPAM can produce a larger PCC floc size.In addition,the use of CPAM combined with APAM produced a larger floc size when the dosage of PAM was 1mg/g.The filler floc size could be up to 63?m when the ratio of CPAM/APAM was 2:3.With the extension of shearing time,the floc size was decreased obviously.When ultrasonic treatment was applied for 1min,floc structure was destroyed and the floc size was changed to the initial size of filler particles.Overall,filler flocs induced by PAM exhibited better shearing resistance ability while the flocs induced by cationic starch showed better ability of re-flocculation.The fractal dimension,strength factor and recovery factor were used to characterize the morphological characteristics,shearing resistance ability and re-flocculation ability of filler flocs.Results showed that flocculants had a great impact on fractal dimension.The fractal dimension of filler flocs induced by PAM flocculants was increased with increasing the dosage of flocculants.In addition,the floc size was decreased as stirring time and the fractal dimension was increased due to the destruction of floc structure by shearing.Strength factor was also increased with increasing the dosage of flocculants.The structure of filler flocs became compacted with increasing the dosage of flocculants,and thus improve the stability of filler flocs.Among the flocculants,cationic starch showed better recovery factor.When the dosage of cationic starch was 40 mg/g,the recovery factor was up to 74%.In contrast,the recovery factor of filler flocs induced by CPAM was only around 20%.2.Effect of floc size and filler distribution in z-direction on paper physical propertiesIn this part,CPAM was used to preflocculate PCC filler and the floc size was regulated by stirring speed.Effect of PCC floc size on fractal dimension and strength factor was investigated.Results showed that the filler floc size was changed from 23.2?m to 9.6?m when the stirring speed increased from 200 rpm to 500 rpm.Meanwhile,the fractal dimension were also increased correspondingly.In addition,the effect of floc size on filler retention and paper properties was also investigated.Results showed that filler preflocculation can effectively improve filler retention.When filler addition was fixed at 55%,filler retention rate could be up to 83.14%compared to filler without preflocculation.When the floc size was 15~18?m,the tensile index,burst index and tear index of paper containing 30%PCC were increased by 18%,15%and 16%,respectively.Besides,the brightness and opacity of paper were improved with decreasing floc size.However,filler preflocculation led to the bulk of air permeability of paper decreased slightly.When the floc size was 9.6?m,the bulk of paper containing30%PCC decreased 2.9%.Therefore,paper strength properties,optical properties and structural properties should be comprehensively considered to meet the requirements of paper grades when filler preflocculation was applied.The effect of filler distribution in z-direction on paper properties was also discussed.Radian factor was employed to characterize the degree of filler concentrated in middle layer.The relationship between radian factor and paper properties was explored.Results showed that the radian factor of filler distribution affected paper porosity,and thus paper properties.Generally,paper porosity was decreased with increasing F_h.When filler particles were concentrated in middle layer(F_h=1),the tensile index of paper could increase by10.7%compared to the paper with even filler distribution(F_h=1).In addition,when filler concentrated in middle layer could decrease paper bulk and brightness.3.Co-flocculation of CPAM/cellulose nanofiber/PCC and its effect on paper physical propertiesCellulose nanofiber(CNF)can be used as strengthen aid for improve the stability and re-flocculation ability of filler floc.Results showed that PCC/CNF composite floc was formed by bridging mechanism of CPAM.The use of CNF help to increase the contacting area of CPAM and PCC,and thus enhancing the shearing resistance of filler flocs.For a given stirring speed,the floc size can be up to 41?m when addition of 7%CNF.After ultrasonic treatment for 1min,the re-flocculation ability with CNF addition was enhanced significantly.The fractal dimension,strength factor and recovery factor were all improved as the dosage of CNF was increased.The optimal properties of floc could be obtained by addition of 5%CNF.The corresponding strength factor and recovery factor can be up to 90.64%and 53.7%,respectively.Effect of CPAM/CNF/PCC flocs on filler retention and paper physical properties was also explored.Results showed that addition of CNF helped to improve the stability and the size of filler flocs,which improved filler retention.When filler content of paper was 43%,the retention rate of the flocs induced by CPAM and CNF could increase by four percentage point which could be up to93.6%,compared to filler flocs induced only by CPAM.The use of CNF not only improve the stability of filler flocs but also help increase the bonds between cellulosic fiber and filler flocs,and thus increasing paper strength.Fiber bondability was improved with increasing the dosage of CNF,which resulted in the decrease in paper bulk.In contrast,when the dosage of CNF was 5%,filler content is 27%,the tensile index and tear index of paper filled with PCC flocs was increased by 16.5%and 17%,respectively.Therefore,CNF played an essential role in enhancing stability of floc and fiber bonding,which showed a great potential in improvement of paper properties. |
PDF Full Text Request