Sheng Qiang,Shunxi Song,,,Peiyao Wang,Yujie Shi,Linhao Li,Haitang Liu

1.College of Bioresources Chemical and Materials Engineering,National Demonstration Center for Experimental Light Chemistry Engineering Education(Shaanxi University of Science&Technology),Shaanxi University of Science and Technology,Xi'an,Shaanxi Province,710021,China

2.Tianjin Key Laboratory of Pulp&Paper,Tianjin University of Science&Technology,Tianjin,300457,China

Abstract:Mineral fillers are important for conserving raw fiber materials and reducing production costs in the paper industry.However,the increase in filler content will inevitably result in strength reduction,which limits the adding amount of filler in paper production.In this study,we designed a cellulose fiber/filler co-refining approach to improve the strength and optical properties of paper;moreover,the synergistic interaction between fibers and precipitated calcium carbonate(PCC)fillers in the co-refining process was investigated.Results of fiber separation and PCC particle size analysis showed that,compared with conventional refining,the content of fines increased,whereas the PCC particle size decreased.More importantly,composites were formed between the PCC and fines,which promoted strength improvement of paper.Physical tests show that the tensile index of paper with 15%PCC content increased by 22%compared with that of the paper filled by conventional method,whereas the brightness and opacity of paper improved by fiber/filler co-refining for a specified filler content.These findings provide a basis for the further development of co-refining filling technology.

Keywords:cellulosic fiber;mineral filler;filler composite;paper strength;optical properties

1 lntroduction

Mineral fillers are the second most utilized raw material for papermaking.As an important component in paper materials,mineral fillers are typically much less expensive than cellulosic fibers[1-2].Therefore,the substitution of cellulosic fiber with mineral fillers and the sustainable development of forest resources have been emphasized in the paper industry[3].

Mineral fillers such as ground calcium carbonate(GCC) or precipitated calcium carbonate (PCC)typically have small particle size,high brightness,high refractive index,and low water absorption,which can effectively improve optical and surface properties of paper,increase the dryness of wet web,and decrease steam consumption in the drying process.Since the 1970s,the filler content of writing papers has increased from 15%to 30%[4].In general,paper strength is primarily derived from hydrogen bonds between cellulosic fibers[5].However,mineral fillers are inorganic materials,and no chemical bond exists between the filler and cellulosic fiber.The presence of filler particles hinders fiber bonding through the steric effect;consequently,paper strength and paper machine operation are deteriorated,thereby limiting the filler content in paper[6-7].

To maximize the effect of fillers on the optical properties and reduce its negative effect on paper strength,significant effort has been expended to increase the filler content in paper.Filler preflocculation is a technology that aggregates filler particles via ionic polymers.The formed flocs increase the tensile strength and filler retention of paper[8-11].However,the optical properties of paper can deteriorate owing to the large flocculation size for a specified filler content[12-13].

Fiber loading can be performed to improve the bonding between fibers and increase the filler content by adding fillers inside the fiber cells.Currently,two main fiber loading methods exist:lumen loading via mechanical deposition,and cell wall loading viain-situprecipitation[14].Green[15]investigated the lumen loading of softwood kraft pulp with titanium dioxide via sufficient mechanical diffusion.Middleton et al[16]loaded PCC fillers in softwood bleached kraft pulp fibers,and Zakaria et al[17]developed magnetite powder to load such particles into lumens of unbleached,never-dried kenaf pulp.Allan[18]reviewed several studies pertaining to cell wall loading based on thein-situprecipitation of filler particles.Compared with the conventional addition of filler,lumen loading results in a moderate reduction in the light-scattering coefficient and increases strength of laboratory handsheets.However,it is difficult to fill a significant amount of filler and large-sized fillers inside the fiber.

Silenlus[19]used anin-situprecipitation strategy by injecting carbon dioxide into a calcium hydroxide solution containing fines to form composite fillers.Subramanian et al[20]injected carbon dioxide gas into mixed solutions of different types of fibers and calcium hydroxide to form a 1:1 mixture of PCC and fibers;subsequently,they investigated the effects of composite fillers prepared using different types of fines on paper properties.The results showed that the strength,light scattering coefficient,and opacity of paper with composite fillers were higher than those of ordinary PCC-filled paper under the same ash content.However,the preparation of composite fillers via thein-situdeposition method requires a strict control of the synthesis conditions;furthermore,it is only suitable for inorganic fillers that can be synthesized and not for natural fillers with different shapes.

Mechanochemical methods can effectively alter the physical and chemical properties of substances[21].They introduce the accumulation of mechanical energy by grinding,compression,shearing,friction,and stretching to modify the surface properties of materials[22].The typically used PFI refiner in the paper industry can be considered as a mechanochemical equipment that can generate strong shearing and friction forces between the fiber and mineral filler.When materials are processed in the gaps between grinding discs,they accumulate excessive potential energy,resulting in bond dissociation,defects,and other changes on the surface[23].Ribeiro et al[24]proved that this process not only reduced the particle size of activated carbon and titanium dioxide,but also enhanced their reactivity.

Herein,we propose a facile and efficient method to improve the strength and optical properties of paper using a PFI refiner,i.e.,a method known as corefining.In contrast to the typically used filling method,co-refining refers to a filling method where fillers are input into the fiber slurry and refined with cellulosic fiber.During this process,the fiber can be further refined with the assistance of filler,whereas the filler can be disintegrated into small particles.In addition,the composite structure of fines and fillers can be formed,thereby affording dual improvement in the strength and optical properties of paper,as compared with the filled paper using traditional method.

2 Experimental

2.1 Materials

Hardwood bleached kraft pulp was used as the raw material for this study.The filler used in this study was PCC with an average particle size of 6.38μm.Cationic polyacrylamide(CPAM,Mw 6-8 million g/mol)was used as the retention agent.

2.2 Preparation of co-refining slurry

To prepare the material,30 g hardwood bleached kraft pulp board were soaked in water for 12 h at room temperature and disintegrated for 15000 r in a di si nt egr at er wi t h a concent r at i on of 1.5 wt%.Subsequently,the pulp was filtered through a funnel.Next,the pulp was mixed with 50%PCC(based on the fiber mass)and refined for 8000 r at a concentration of 10 wt%.By contrast,the blank sample was refined without PCC for 8000 r.The refining equipment used was a PFI refiner(DCS-041PT,KRK,Japan),and the refining gap was fixed at 0.15 mm.

2.3 Fiber component separation and analysis

A vibrating screen(200 mesh)was used to separate the fibrous components.5 g pulp(o.d.)was diluted to a concentration of 0.125 wt%and then added to the vibrating screen. The fines were obtained via sedimentation,whereas the long fiber was obtained via filtration.The mass and solid contents of the different components were measured and calculated.For the corefined pulp,the content of fines was calculated by subtracting the ash from the fines component.

A fiber quality analyzer was used to analyze the fiber morphological parameters of the long fiber and fines components.

2.4 Particle size analysis

The particle size of the filler was determined using a Malvern Hydro 2000S laser diffraction equipment(Malvern,UK).When the ash content of the pulp was measured at 525℃,it was discovered that the residual ash contained primarily fillers because the cellulosic fibers were completely decomposed.The sample was ultrasonicated for 3 min to disperse the filler particles prior to testing.

2.5 Surface morphology observation of fibers

A TESCAN VEGA-3-SBH microscope was used to assess the morphology of the long fibers and fines.All samples were coated with gold and observed at an accelerating voltage of 20 kV.

2.6 Handsheet preparation and physical property evaluation

Handsheets containing different amounts of PCC were prepared to investigate the effect of co-refining on the paper properties.The target basis weight of the handsheets was 65 g/m2.CPAM at a dosage of 0.03%(based on the dry weight of the furnish)was mixed as a retention aid with the furnish for 15 s prior to the formation of the handsheets.

The handsheets were stored at 23℃and 50%RH for 24 h prior to testing.The paper properties,including the strength,structure,and optical properties,were measured.

3 Results and discussion

3.1 Characteristics of fiber and filler

Vibration screening was performed on the co-refined pulp to investigate the effect of co-refining on fines content.As illustrated in Table 1,the co-refining process decreased the long fiber content from 87.01%to 85.04%and increased the fines content from 12.99%to 14.96%,which improved the paper strength significantly[8,25].In addition,the beating degrees of the long fiber components from the blank and co-refined pulp samples were similar.These results indicate that a significant amount of fines was produced in the corefining process,thereby facilitating the improvement in fiber bonding.The increased amount of fines may have been due to fragments generated during fiber fibrillation.This can be attributed to the grinding effect of filler particles.Generally,mineral fillers are harder than cellulosic fibers;therefore,they facilitate the production of microfibrils and filaments shed from the surface of long fibers during co-refining.

PCC among the fines of the co-refined pulp was separated via ignition at 525℃,which ensured that the PCC components were not decomposed(TAPPI 413 om-93).Therefore,the particle size of PCC in the corefined pulp was measured indirectly based on the change in the ash particle size.As shown in Table 1,the PCC particles disintegrated,as expected,and the average particle size of PCC decreased to 5.06μm.The change in the particle size of PCC increased its light scattering coefficient,thereby enhancing the optical properties of paper.

Table 1 Properties of fiber and filler after component separation

Fig.1 shows a comparison of the weighted-length distribution of the separated fiber components.As shown in Fig.1(a),the average lengths of the long fibers from the blank and co-refined pulp samples were 0.706 and 0.708 mm,respectively.This indicates that co-refining did not significantly affect the fiber length.In addition,the average length of fines in the co-refined pulp sample was shorter than that in the blank pulp sample(33μm vs.47μm),as shown in Fig.1(b).This might be because the fines content(measuring 10-30μm in length)from the co-refined pulp was higher than the fines from the blank pulp sample.This may be resulted from the composite formed by PCC and fines during co-refining.

Fig.1 Comparison of weighted-lengths of long fiber(a)and fines(b)for blank and co-refined pulp samples

3.2 Morphology of fiber and PCC in co-refining system

The surface morphologies of the long fibers screened from different pulps are presented in Fig.2.Refining typically results in fiber fibrillation,as shown in Fig.2(a).However,in the co-refining system,the long fiber was more fibrillated as compared with that from the blank pulp sample.This is due to the addition of small and hard PCC particles during the co-refining process.The frictional force generated by PCC rendered the surface of the fiber more fibrillated,which generated more fines.In addition,the composite structure of fines encapsulating PCC can be observed,as shown in Fig.2(c),because co-refining imposes a strong mechanical force effect on PCC and fibers.With the continuous accumulation of mechanical potential energy,PCC and cellulose exhibited a decrease in size and an increase in surface activity[23,26], which contributed to the formation of PCC/fiber composites.This composite structure may significantly improve the paper strength.

Fig.2 Morphologies of long fiber and fines:(a)long fiber from blank pulp;(b)long fiber from co-refined pulp;(c)fines from co-refined pulp

3.3 Physical properties of handsheets

The effect of co-refining on the bulk of paper is shown in Fig.3.Compared with the conventional filling method,the PCC/fiber co-refining method yielded handsheets with a lower bulk.In addition,as the PCC content increased,the improvement in the paper bulk via co-refining filling was less prominent as compared with the conventional filling paper.This can be attributed to the change in the PCC particle size.When the conventional filling method was used,the particle size of the PCC did not change.However,the PCC particles disintegrated during the co-refining process,and the particle size decreased.The decrease in particle size of PCC enhanced the filling capacity under the same void volume,which ultimately decreased the paper bulk.

Fig.3 Comparison of paper bulk between conventional filling and co-refining filling

The tensile strength of paper significantly affects the operation of paper machines and paper printability[27].The tensile strength of paper decreased as the PCC content increased (Fig.4).Compared with the conventional filling method,the tensile index of paper produced via co-refining filling increased by 22%at a PCC content of 15%.When the tensile index of the paper was 43.8 N·m/g,the PCC content of paper produced via co-refining filling increased by 6 percentage points,indicating that the co-refining approach improved fiber bonding.

Fig.4 Comparison of tensile index of paper between conventional filling and co-refining filling

The decrease in paper strength due to the addition of fillers to paper is attributable to two reasons.First,the filler,as an inorganic component,cannot generate hydrogen bonds with the fiber,thereby destroying the fiber bonds and reducing the strength of paper.Second,under the same paper basis weight,an increase in the filler content results in a decrease in the fiber consumption,which consequently decreases the paper strength.The improvement in the tensile index of paper produced via co-refining filling indicated that a composite structure was formed between PCC and fiber during the co-refining process.In the disc refiner,the high-speed hydraulic shear enhanced the interaction between PCC and water on the surface of the particles.Ca—OH, Ca—O—Ca, and other groups were generated,and they promoted hydroxylation on the surface of the filler[28],which consequently increased the combination of fibers and fillers.

Filler addition can effectively improve the optical properties of paper,thereby reducing the risk of printthroughs.The effects of different filling methods on paper brightness and opacity are shown in Fig.5 and Fig.6.The brightness of the handsheets was not significantly altered by co-refining filling.However,co-refining filling can effectively improve the opacity of paper,as shown in Fig.6.The increase in the opacity implies that the paper possessed enhanced light scattering ability,thereby reducing print-through risks.The improved light scattering ability can be explained by the decrease in PCC particle size.For a specified filler content,the decrease in the particle size resulted in an increase in the number of particles and more filler-fiber interfaces in the structure,thereby increasing the number of reflections and refractions on the light path[27].Therefore,it can be concluded that co-refining filling improved the paper strength and optical properties simultaneously for a specified PCC content.

Fig.5 Comparison of paper brightness between conventional filling and co-refining filling

Fig.6 Comparison of opacity of paper between conventional filling and co-refining filling

The surface morphology of the paper prepared using different filling methods is shown in Fig.7.PCC is a type of aggregated filler composed of multiple singlefiller particles.The aggregate structure of PCC was destroyed during co-refining;instead,it appeared in the form of single particles as the specific surface area increased, which improved the light scattering performance of paper.Meanwhile,PCC and fiber intersected each other to form a composite structure,thereby resulting in a more compact paper structure and hence,paper with increased strength.Therefore,the utilization of fiber and filler co-refining can improve both the strength and optical properties of paper.

Fig.7 Morphologies of papers filled via(a)conventional filling paper;(b)co-refining filling paper

4 Conclusions

In this study,the interaction between the precipitated calcium carbonate(PCC)filler and cellulosic fiber during co-refining and its effect on paper properties was investigated.It was discovered that although corefining did not significantly affect the length of long fibers,it increased the content of the fines from 12.99%to 14.96%.In addition,the PCC particle size decreased,which improved the optical properties of paper.Meanwhile,in the co-refining process,a composite encapsulated structure of fines with fillers was formed,which improved the paper strength and increased the paper tensile index by 22%at 15%PCC content compared with the conventional filling paper.The formation of the composite structure and the decrease in PCC particle size resulted in an increase in the strength and optical properties of paper simultaneously.These two effects provide a theoretical and technical basis for the development of new,robust filling technologies.

Acknowledgments

This work was financially supported by Innovation Capability Support Program of Shaanxi(2020KJXX-082)and the Foundation(202005)of Tianjin Key Laboratory of Pulp&Paper(Tianjin University of Science&Technology).