| Fillers are used in papermaking industry mainly for two simple reasons: they improve certain properties,such as brightness,opacity,smoothness and printing properties and if they are cheap,they reduce production costs.However,increasing the filler content in paper causes the deterioration of paper strength and affects the runnability of paper machines.In the paper forming process,the adsorption of chemical additives and the mechanical retention produced by fiber network cause the accumulation of filler particles.Finally,it exist in the form of filler aggregates in paper,and the filler aggregates affect every macroscopic mechanical and optical behavior of paper.Therefore,it is greatly significant to reveal the mechanism of the influence of fillers on the paper strength properties from the perspective of the morphology and distribution characteristics of fillers aggregates in order to improve the strength properties of paper and increase the papers filler content.The research work of this dissertation consists of three parts.Firstly,the image characterization method of paper filler aggregates was established;Secondly,the effects of filler average particle size,particle size distribution and morphology on the morphology and distribution characteristics of the filler aggregates were studied.The paper properties were correlated with the morphology and distribution characteristics of the filler aggregates and the regression models were verified.Finally,on this basis,the methods of filler pre-flocculation and chemical modification were used to adjust the characteristics of filler aggregates,thereby achieving the purpose of improving the paper properties.(1)Establishment of characterization method of filler aggregatesTaking the paper filled with PCC(precipited calcium carbonate)as an example,and the paper was layered by tape bonding method.Firstly,the grayscale features of filler aggregate images obtained by secondary electron mode and backscattering mode of SEM(scanning electron microscope)were studied.It was found that the gray-scale difference between the filler and fiber was larger in the images which obtained by backscattering mode,and that was beneficial to extract the feature information of the filler aggregates.Secondly,the image quality was improved by median filtering noise reduction and image gray enhancement by MATLAB software.Finally,Image J and Excel software were used to extract and calculate the filler aggregates in handsheets.The results showed that the morphological characteristics of the filler aggregates can be more accurately characterized by dividing the paper into 10 layers,and the medium particle size of filler aggregates was characterized by a “mountain-type” distribution in Zdirection of paper.(2)Correlation between the morphology and distribution characteristics of filler aggregates and paper propertiesBased on the commonly used fillers in papermaking industry,such as GCC(ground calcium carbonate),PCC and clay,seven kinds of fillers with different morphological characteristics were prepared by means of filler classification and filler blending.The effects of median particle size,particle size distribution and morphology of filler on the morphology and distribution characteristics of filler aggregates were investigated.The results showed that the medium particle size of filler aggregates in the seven kinds of paper all had a "mountain-type" distribution in the Z-direction of the paper.And the medium particle sizes of the filler aggregates in the papers were larger than those of original fillers.When the medium particle size of filler aggregate was larger,the standard deviation of the Z-direction distribution of filler aggregate was also larger.In addition,the fractal dimension of area-perimeter method can be used to quantitatively characterize the morphology of paper filler aggregates.The larger the fractal dimension,the more complex the surface morphology of filler aggregates.The regression analysis of characteristics of filler aggregates(i.e.,medium particle size,particle size distribution,standard deviation of particle size distribution in Z-direction,fractal dimension)and the paper tensile index,paper light scattering coefficient and bulk contributed by filler aggregates were carried out.The results showed that there was a significant linear correlation between the paper tensile indexes of and the Zdirection particle size distribution standard deviation,medium particle size and particle size distribution of the filler aggregates,and the correlation coefficient r was 0.9931.The Z-direction particle size distribution standard deviation of the filler aggregates had the most significant influence on the paper tensile index.In addition,there was a significant positive linear correlation between the light scattering coefficient of the paper and the particle size distribution and fractal dimension of the filler aggregate,and the correlation coefficient r was 0.9656.The particle size distribution and fractal dimension had similar effects on the paper light scattering coefficient.There was a significant correlation between the filler aggregates bulk and the fractal dimension,medium particle size and particle size distribution of the filler aggregate.The fractal dimension of the filler aggregate had a positive correlation and the greatest influence on the filler aggregates bulk.Conventional filling and pre-flocculation filling paper were prepared by using PCC-I(4.2 μm)and PCC-II(7.7 μm)fillers,and the fitted regression model was verified.The results showed that the regression model of the distribution of filler aggregates and the tensile index of paper had a certain theoretical guiding significance.Increasing the Z-direction particle size distribution standard deviation of filler aggregates was helpful to increase the paper tensile index.But the higher medium particle size and higher particle size distribution of filler aggregates were not conducive to the improvement of paper tensile index.In addition,the regression model of the characteristics of filler aggregates and paper light scattering coefficient was more suitable for the analysis of conventional paper-filling method,and the regression model of filler aggregates characteristics of and the bulk contributed by filler aggregates should be analyzed under the same filling mode.In addition,X-ray microtomography method was compared with the method established in this paper.The results showed that the X-ray microtomography method,as a nondestructive testing method,was more accurate in characterization of large size aggregates formed by pre-flocculation method,but it has some shortcomings,such as long detection time and high price.The image characterization method established in this dissertation was more suitable for the characterization of the morphology and distribution of conventional paper filling aggregates,and has the advantages of short time and low cost of analysis and testing.(3)Regulation of the morphology and distribution of filler aggregates and improvement of paper propertiesPCC-Ⅰ and PCC-II fillers were pre-flocculated by CPAM(cationic polyacrylamide).16 μm and 24 μm flocs were obtained by controlling shear strength and shear time.The effects of different flocs sizes on the paper properties were investigated.The results showed that the 16 μm flocs had better effect on improving the paper tensile indexes and tear indexes than the 24 μm floc.When PCC-I which has smaller particle size was pre-flocculated to 24 μm,there were many micro pores inside the aggregate,which improved the light absorption coefficient of the paper and slowed down the decrease of light scattering coefficient and opacity of the paper.Two different particle size PCC fillers(PCC-(1): 4.9 μm;PCC-(2): 8.0 μm)and talc(14.1 μm)were pre-flocculated in order to improve the properties of paper by regulating the structure of filler aggregates.The results showed that when the paper filler content was 28%,PCC-(1)filler can form a complex "sandwich" filler aggregate structure with talc,and the paper tensile index and tear index increased by 29.1% and 28.1%,respectively.This structure was beneficial to reduce the negative effect of pre-flocculation on paper opacity.Based on the influence of the particle size distribution in Z direction of the filler aggregate on the paper strength,the Z-direction particle size distribution curve of the paper filler aggregate was controlled by the layered method.The small particle size filler PCC-(1)was distributed on both sides,and the large particle size filler PCC-(2)was distributed in the middle layer of the paper sheet to increase the standard deviation of the aggregate distribution in the Z dimension of the paper.The results showed that when the paper filler content was 30%,compared with the conventional filling method,the tensile indexes of paper with designed structure was increased by 35.7%,and the air permeability was increased by 24.5%.Meanwhile,the paper bulk and brightness were not damaged.In addition,the chemical modification of fillers can also control the morphology of filler aggregates by affecting the characteristics of filler particles.The modification of starch-sodium stearate and hydrochloric acid were used to regulate the surface chemical properties of the filler,which not only could cause the filler to aggregate and increase in size,but also made the surface of the filler with hydroxyl groups,which was beneficial to improve the paper properties.The results showed that the size of the fillers increased after modified by different routes of starch-sodium stearate,and the surface of the particles was coated with starch-sodium stearate complex in varying degrees.Starch gelatinization →addition of filler→addition of sodium stearate,this modified route could increase the tensile index of the filled paper by 22%.The modification of calcium silicate with hydrochloric acid can realize the regulation of the micro-nano scale of the filler aggregate.In the process of modification,calcium silicate reacts with hydrochloric acid to form orthosilicate which was a polyhydroxy small molecule.It can not only promotes the aggregation between the filler particles,but also helps to increase the bonding between the filler and the fiber.When the paper filler content was 30%,the tensile index and tear index of the modified filler-filled paper were increased by 33.0% and 18.1%,respectively. |
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