Calcium Chloride Modified Filler for Papermaking

Mar 27, 2014 - ABSTRACT: Starch, sodium oleate, and calcium chloride were used for the modification of calcium carbonate filler, so that the negative ...
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Starch/Sodium Oleate/Calcium Chloride Modified Filler for Papermaking: Impact of Filler Modification Process Conditions and Retention Systems As Evaluated by Filler Bondability Factor in Combination with Other Parameters Xiujie Huang,† Zaisong Sun,‡ Xueren Qian,† Jinsong Li,§ and Jing Shen*,† †

Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Material Science and Engineering College, Northeast Forestry University, Harbin 150040, China ‡ Rizhao Polytechnic, Rizhao 276826, China § Mudanjiang Hengfeng Paper Co., Ltd, Mudanjiang 157013, China ABSTRACT: Starch, sodium oleate, and calcium chloride were used for the modification of calcium carbonate filler, so that the negative impact of filler addition on paper strength can be reduced by surface encapsulation with cellulose-bondable starch/oleic acid complexes. The impact of filler modification process conditions and retention systems was evaluated based on the filler bondability factor (in terms of tensile strength), filler retention, and tensile strength/brightness/opacity of filled paper. Noticeably, at the filler addition level of 20%, filler modification resulted in an increase in filler bondability factor of about 130%. The use of cationic polyacrylamide or polyacrylamide/bentonite resulted in an increase in retention of modified filler in papersheets from 57.3% to 72.3% or 76.4%. The filler bondability factor was also improved by using these two retention systems. fibers is high, filler modification with starch for improved filler/ paper properties is promising. An interesting aspect in this area is related to the attachment of starch/fatty acid inclusion complexes to filler surface, wherein the water-solubility of these complexes is much lower than that of unmodified starch, which translates to their stable anchoring to the filler particles (i.e., without being easily dissolved in the water phase).15 In our previous work, the concept of filler modification for papermaking by encapsulation with starch/fatty acid inclusion complexes with the aid of calcium ions was proposed and demonstrated.26 A feature associated with the concept is that it can be applied to the modification of both acid-stable fillers (e.g., Kaolin clay) and acid-unstable fillers (i.e., unmodified calcium carbonate fillers). In the current study, the concept of filler bondability factor was proposed, and the impact of filler modification process conditions and retention systems was evaluated by filler bondability factor in combination with other parameters.

1. INTRODUCTION In the paper industry, cellulosic fibers are the essential component for the production of various paper products. Meanwhile, mineral fillers are widely used as substitutes for cellulosic fibers, particularly in the case of printing and writing paper grades.1−5 There are distinct advantages associated with the use of mineral fillers in papermaking.6 As fillers are usually much less costly than cellulosic fibers, their use can reduce the production cost. Fillers are also indispensible for improving the critical properties of paper products including brightness, opacity, smoothness, and printability/writability. Other advantages include reduction of energy consumption in the dyer section of the papermaking process and possibilities of adding unique functions to cellulosic paper.7,8 However, due to the interference of filler particles with fiberto-fiber bonding in a cellulosic fiber network, the addition of fillers can have a negative impact on paper strength, which is most pronounced at high filler addition levels.9,10 This is basically the key factor limiting the amount of filler particles that can be incorporated into cellulosic paper. To increase the competitiveness of traditional cellulosic paper products, there is a strong interest in the industry to increase the filler content of cellulosic paper while paper properties and papermaking processes are not negatively influenced. This can essentially be targeted by improving the interaction of filler particles with cellulosic fibers with the aid of chemical additives.11,12 In this regard, engineering of fillers with synthetic or biobased polymers is one of the interesting research topics.13−25 Filler modification by deposition/precipitation of starch on filler particles has been reported in the literature.13,15,22−25 Since the cost of starch is low and its bondability with cellulosic © 2014 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. Unmodified corn starch with a dryness of 90.65% was obtained from Shandong Runyin Biochemical Engineering Co., Ltd., China. Calcium carbonate filler (precipitated calcium carbonate powder) with an ISO brightness of 92.4% was purchased from Guangxi Guilin Wuhuan Co., Ltd., China. Sodium oleate and calcium chloride were analytical grade reagents. The fully bleached kraft pulp (derived from softwood) imported from Canada was supplied by Received: Revised: Accepted: Published: 6426

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Figure 1. Schematic illustration of bonding characteristics of unmodified filler and bonding-material-encapsulated filler.

bentonite was used, the dosages of cationic polyacrylamide and bentonite were 0.05% and 0.5%, respectively, based on the total dry weight of pulp and filler. Paper-sheets with target grammage of 60 g/m2 were prepared by using a ZQJ1-B200 mm sheet former. After sheet formation, the wet paper-sheets were pressed at 0.4 MPa for 5 min prior to drying. The amount of filler particles was 20% based on the target dry weight of the paper-sheets. After conditioning in a glass desiccator, the tensile strength of the paper-sheets was tested by using a ZL-300A tester (China). Paper brightness and opacity was tested by using a YQ-Z-48A tester (China). For the determination of the properties of varying types of paper-sheets, the conditions were kept consistent so as to make fair comparisons. 2.4. Determination of Filler Retention and Filler Bondability Factor. Filler retention was determined by incinerating the paper-sheets at 525 °C followed by calculation based on the weights of ash and the added filler material, and the detailed information was published elsewhere.21 Filler bondability factor (FBF) as a parameter for filler modification process optimization was defined as:

Mudanjiang Hengfeng Paper Co. Ltd., China. Prior to papersheet preparation, the pulp was processed in a Valley refiner, and the final beating degree of the refined pulp was 31 oSR. 2.2. Filler Modification. The filler modification process was largely based on the previously published procedures.26 Calcium carbonate filler with a dry weight of 15 g and a given amount of starch powder (starch dosage varies between 15% and 40%, based on the dry weight of filler) was added to a 500mL four-necked flask. Distilled water was added to the flask to adjust the total weight of the aqueous mixture to 100 g. Under stirring at 200 rpm, the mixture was heated to 95 °C, and was cooked at this temperature for 1 h to induce the substantial swelling/solubilization of starch in the system (a reflux condenser was used so as to avoid water loss throughout the cooking process). Subsequently, sodium oleate solution was added, and the mixture was again heated to 95 °C. At 95 °C, the mixture was mixed/cooked for 30 min to allow the entry of sodium oleate into the helical structure of the starch molecules in the aqueous system. Upon cooling to less than 30 °C, under stirring the well-dispersed mixture was diluted to 500 mL, 200 mL of which was poured into a 0.2 mol/L calcium chloride solution (200 mL), and starch/oleic acid complexes could be precipitated/anchored onto calcium carbonate particles in the presence of calcium ions (according to our previous work,26 the presence of calcium ions is essential in inducing the deposition/ precipitation of starch/oleic acid complexes onto filler particles). Finally, the excess calcium ions in the modified filler slurry were sufficiently washed out by centrifugation. For comparison purposes, in one set of experiments, starch, calcium carbonate filler, and sodium oleate were added to the flask followed by mixing and cooking, which can be referred to as “all-at-once” addition in contrast to the above-mentioned “sequential” addition. It is noted that the dosage of starch mentioned in this study was based on the dry weight of calcium carbonate, whereas the dosage of sodium oleate was based on the dry weight of starch. 2.3. Preparation of Paper-Sheets and Determination of Tensile Strength, Brightness, and Opacity. The aqueous slurry containing pulp fibers and filler particles was sufficiently mixed. When the single retention system involving cationic polyacrylamide was used, its dosage was 0.05%, based on the total dry weight of pulp and filler. When the dual retention system involving cationic polyacrylamide and

filler bondability factor strength of filled paper = × filler retention × 100 strength of unfilled paper

In the current study, the strength of filled or unfilled paper was determined in terms of tensile strength, and the filler bondability factor was calculated.

3. RESULTS AND DISCUSSION 3.1. Concept of Filler Bondability As a Parameter for Filler Modification Process Optimization. The negative impact of filler addition on paper strength is essentially due to the poor bondability/compatibility of filler particles with cellulosic fibers.15,18,22,26−28 Thus, the presence of filler particles can impair the interfiber bonding characteristics,29 which is an important source of paper strength. However, the encapsulation of filler particles with a bonding material (i.e., the material can be bondable to fibers) can significantly enhance filler bondability, and hence improved strength properties of the filled paper (see Figure 1). In the case of filler modification with starch or other cellulose-bondable materials,30 the process 6427

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optimization can be based on the critical properties of the filled paper. Noticeably, for a given filler addition level, the strength of the filled paper may be indicative of filler bondability. However, this is also dependent upon filler retention. Thus, for comparison of the bondabilities of different fillers with fibers, filler retention (or ash retention) and paper strength must be considered simultaneously. In this context, the concept of filler bondability factor (see the Materials and Methods section) involving the calculation based on filler retention and paper strength was proposed in this study. This parameter is expected to be useful in evaluating the bondabilities of various fillers at a given filler addition level (e.g., 20%). It is noted that at a very low ash content of the paper-sheets (for example, much less than 1% based on the dry weight of paper-sheets), paper strength might be improved by the presence of fillers due to enhanced sheet formation; thus, this particular case is excluded from the scope pertaining to the concept of filler bondability factor. Due to the fact that the filler addition level for many printing and writing paper grades is high enough to have a negative impact on paper strength, this proposed factor is expected to be useful in many practical applications in the paper industry. In the current study, the filler bondability factor was evaluated in terms of tensile strength and filler retention, and the impact of the filler modification process conditions and retention aids was investigated based on the filler bondability factor in combination with other parameters. 3.2. Effect of Filler Modification Process Conditions on the Use of Modified Filler in Papermaking. Our previous study showed that starch/fatty acid complexes were effectively precipitated/deposited on filler particles with the aid of added calcium ions; however, the deposition/precipitation of starch on filler was rather poor when no calcium ions were introduced.26 Here, the role of calcium ions is presumably related to their reaction with the soluble fatty acid salts (e.g., sodium oleate) located in the helical structure of starch. It is worth noting that the reaction product, i.e., the calcium salt of fatty acid, is insoluble in aqueous medium, thus facilitating the precipitation of dissolved starch molecules in the presence of filler particles. The addition of calcium ions instead of strong or weak acids in the filler modification process enables the applicability of this concept to various types of fillers including acid-unstable calcium carbonate. For this filler process concept, the evaluation of impact of filler modification process conditions is still needed. The effect of sodium oleate (a soluble fatty acid salt) dosage on filler bondability, filler retention, and paper properties is shown in Figure 2. The increase in sodium oleate dosage in the range of 0−3% resulted in significantly increased filler retention and filler bondability factor; however, both filler retention and filler bondability factor did not show a noticeable increase upon further increase of sodium oleate dosage. As the source of guest molecules for entering into the helical structure of host molecules (i.e., starch molecules),31 the dosage of sodium oleate expectantly played an important role in the filler modification process. Basically, the sodium oleate dosage should be sufficient to induce the formation of water-insoluble starch/oleic acid complexes on filler particles.26 On the other hand, the addition of excess sodium oleate could not contribute to filler encapsulation with precipitated starch; rather, the formation of calcium oleate uncomplexable with starch was found to have no favorable impact on filler bondability improvement. Sodium oleate dosage had an observable effect on the tensile strength and opacity of filled paper. The negative

Figure 2. Effect of sodium oleate dosage on filler bondability, filler retention, and paper properties (tensile strength, brightness, and opacity) (addition of starch and sodium oleate: “sequential” addition; stirring speed during adding starch/sodium oleate/calcium carbonate composition to calcium chloride solution: 200 rpm; starch dosage: 20%).

effect associated with sodium oleate addition on paper brightness can be attributed to its color, which is in agreement with published work by Huang et al. (2013).26 On balance, sodium oleate dosage of 3% was preferred. In the typical step of our filler modification process, the aqueous mixture containing starch and calcium carbonate was cooked followed by the addition of sodium oleate. As an alternative to this approach, starch, calcium carbonate, and sodium oleate were mixed simultaneously followed by cooking in aqueous medium, which can be referred to as “all-at-once” addition in contrast to the typical approach of “sequential” addition. In terms of large-scale production of the engineered filler products, the “all-at-once” approach would be more easily implemented. In view of this, these two approaches were compared, and the results are shown in Figure 3. Evidently, the “sequential” approach gave better results in comparison to the “all-at-once” approach. Particularly, the “sequential” approach resulted in a significantly higher filler bonadability factor. It might be considered that the “sequential” approach was more favorable for effective starch deposition/precipitation on filler particles. In the filler modification process, upon the completion of sequential steps of cooking and sodium oleate treatment, the 6428

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Figure 3. Comparison of two different approaches associated with the addition of starch and sodium oleate (i.e., “all-at-once” addition and “sequential” addition) in terms of filler bondability, filler retention, and paper properties (tensile strength, brightness, and opacity) (sodium oleate dosage: 3%; stirring speed during adding starch/sodium oleate/ calcium carbonate composition to calcium chloride solution: 200 rpm; starch dosage: 20%).

Figure 4. Effect of stirring speed (during adding starch/sodium oleate/ precipitated calcium carbonate composition to calcium chloride solution) on filler bondability, filler retention, and paper properties (tensile strength, brightness, and opacity) (sodium oleate dosage: 3%; addition of starch and sodium oleate: “sequential” addition; starch dosage: 20%).

aqueous mixture containing starch/sodium oleate/calcium carbonate was poured into calcium chloride solution to induce the precipitation/deposition of starch inclusion complexes on filler particles. In this step, the mixing speed can be critical, as it may affect the mixing sufficiency and interactions of various components in the system. In fact, mixing is a rather critical issue in the papermaking process, for example, in terms of the use of retention additives.12,32 The impact of stirring speed during adding starch/sodium oleate/calcium carbonate composition to calcium chloride solution is shown in Figure 4. The stirring speed exhibited a distinct impact on filler retention and filler bondability factor; however, it only gave somewhat minor or negligible effect on the tensile strength, brightness, and opacity of filled paper. On balance, the stirring speed of 500 rpm gave the optimum results. It is likely that a very low stirring speed was not favorable for sufficient mixing, whereas a very high stirring speed might lead to starch detachment from filler particles in the starch deposition/precipitation step. As a cellulose-bondable material, the use of starch as a filler surface coating agent is essential for enhancing filler bondability. In this sense, it is desirable to investigate the impact of starch dosage during the filler modification process. As shown in Figure 5, with the increase in starch dosage from 15% to 20%, filler bondability factor, filler retention, and paper tensile strength significantly increased; however, the increase in starch dosage in the range of 20−40% did not have a

pronounced positive impact. This indicates that the starch dosage of 20% was sufficient to ensure the effective coverage of starch on the filler particles. On the other hand, the effect of starch dosage on paper brightness and opacity was not quite pronounced. Thus, in terms of the use of modified filler material in papermaking, the starch dosage of 20% gave the optimum results. Based on the above discussion, the optimum filler modification process conditions were (1) sodium oleate dosage of 3%, (2) “sequential” addition of starch and sodium oleate, (3) stirring speed (during adding starch/sodium oleate composition to calcium chloride solution) of 500 rpm, and (4) starch dosage of 20%. Under these conditions, filler modification resulted in a significant improvement in filler bondability factor from 5.3 to 12.5 (Table 1). This translates to about 130% increase in filler bondability factor. Also, in the absence of retention aids, filler modification improved filler retention by about 100% (Table 1). This significant filler retention improvement can be attributed to the aggregation of filler particles during the process of filler modification with starch/fatty acid complexes.13,15,22,26,33 On the other hand, surface encapsulation of filler particles with starch/fatty acid complexes is likely to contribute to improved hydrophobic sizing of the paper,33 presumably due to aggregation of filler particles and the hydrophobic nature of fatty acids. Based on the results shown in Table 1, it can be concluded that filler 6429

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fines) effectively in the paper. Thus, the effect of retention aids on the use of modified filler (prepared under the abovementioned optimum conditions) was investigated. Cationic polyacrylamide and cationic polyacrylamide/bentonite were used as two retention systems, i.e., a single polymer system and a microparticle retention system. As shown in Figure 6, for unmodified filler and modified filler, the use of two retention systems led to improved filler

Figure 5. Effect of starch dosage on filler bondability, filler retention, and paper properties (tensile strength, brightness, and opacity) (sodium oleate dosage: 3%; addition of starch and sodium oleate: “sequential” addition; stirring speed during adding starch/sodium oleate/calcium carbonate composition to calcium chloride solution: 500 rpm).

Table 1. Comparison of Unmodified Filler with Modified Filler in Terms of Their Use in Papermaking in the Absence of Retention Additives

paper samples unfilled paper unmodified calcium carbonate filled paper modified calcium carbonate filled paper

filler bondability

filler retention (%)

tensile index (Nm/ g)

brightness (% IS0)

opacity (%)

5.3

27.0

51.3 42.8

71.9 72.6

77.8 79.8

12.5

57.3

48.7

72.3

83.5

Figure 6. Effect of retention aids on use of modified fillers in papermaking. Note that F, Fa, Fb, MF, MFa, and MFb refer to (1) unmodified filler without the use of retention aids, (2) unmodified filler used in combination with cationic polyacrylamide, (3) unmodified filler used in combination with cationic polyacrylamide/ bentonite, (4) modified filler without the use of retention aids, (5) modified filler used in combination with cationic polyacrylamide, and (6) modified filler used in combination with cationic polyacrylamide/ bentonite, respectively.

retention and filler bondability factor. The microparticle retention system showed a higher efficiency in comparison with the single polymer system, which may be explained by the synergy between the polymer and the microparticle.34,35 For modified filler, the use of cationic polyacrylamide or cationic polyacrylamide/bentonite resulted in an increase in filler retention from 57.3% to 72.3% or 76.4%; in the case of unmodified filler, the corresponding filler retention increased from 27% to 58.4% or 62.8%. The positive effect of retention systems on filler bondability factor was also quite pronounced. For example, the filler bondability factor of the modified filler increased from 12.5 to 14.4 upon the use of the microparticle retention system. This positive effect can be related to the parttime role of cationic polyacrylamide as a bonding material during paper-sheet formation.

modification substantially mitigated the negative impact of filler addition on paper strength, which may allow the improvement in paper opacity and brightness by increasing the amount of filler particles in the paper matrix. 3.3. Effect of Retention Systems on the Use of Modified Filler in Papermaking. For modern papermaking processes associated with the use of mineral fillers, the use of retention aids is a usual practice. Essentially, in the area of papermaking wet-end chemistry, the retention aids are mainly targeted for retaining the fine components (fillers and fiber 6430

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It is evident that the modified filler was superior to unmodified filler in terms of filler bondability, filler retention, and paper tensile strength. In the case of paper opacity, no distinct differences associated with these two fillers were found. However, upon filler modification, the positive impact of filler addition pertaining to the improvement of paper brightness was reduced, which might be explained by the color (brownness) of oleic acid and lower brightness of starch in comparison to calcium carbonate.26

(8) Wang, J.; Liu, W.; Li, H.; Wang, H.; Wang, Z.; Zhou, W.; Liu, H. Preparation of cellulose fiber-TiO2nanobelt-silver nanoparticle hierarchically structured hybrid paper and its photocatalytic and antibacterial properties. Chem. Eng. J. 2013, 228, 272−280. (9) Hubbe, M. A.; Gill, R. A. Filler particle shape vs. paper properties − A review. TAPPI Paper Summit − Spring Technical and International Environmental Conference, 2004. (10) Hubbe, M. A. Prospects for maintaining strength of paper and paperboard products while using less forest resources: A review. BioResources 2014, 9, 1634−1763. (11) Ankerfors, M.; Lindstrom̈ , T.; Sod̈erberg, D. The use of microfibrillated cellulose in high filler fine papers. TAPPI PaperCon Conference, 2013. (12) Hubbe, M. A.; Nanko, H.; McNeal, M. R. Retention aid polymer interactions with cellulosic surfaces and suspensions: A review. BioResources 2009, 4, 850−906. (13) Cao, S.; Song, D.; Deng, Y.; Ragauskas, A. J. Preparation of starch-fatty acid modified clay and its application in packaging papers. Ind. Eng. Chem. Res. 2011, 50, 5628−5633. (14) Chauhan, V. S.; Bhardwaj, N. K. Enhancing paper properties by preflocculation of talc using amphoteric starch. Nord. Pulp Pap. Res. J. 2013, 28, 248−256. (15) Deng, Y.; Jones, P.; McLain, L.; Ragauskas, A. J. Starch modified kaolin fillers for linerboard and paper grades: A perspective review. Tappi J. 2010, 9, 31−36. (16) Ding, M.; Liu, W. Improvement of paper properties with PCC modified by sodium alginate. China Pulp Paper 2012, 31, 15−18. (17) Fatehi, P.; Hamdan, F. C.; Ni, Y. Adsorption of lignocelluloses of pre-hydrolysis liquor on calcium carbonate to induce functional filler. Carbohydr. Polym. 2013, 94, 531−538. (18) Nelson, K.; Deng, Y. Enhanced bondability between inorganic particles and a polysaccharide substrate by encapsulation with regenerated cellulose. J. Appl. Polym. Sci. 2008, 107, 2830−2836. (19) Sang, Y.; McQuaid, M.; Englezos, P. Pre-flocculation of precipitated calcium carbonate filler by cationic starch for highly filled mechanical grade paper. BioResources 2012, 7, 354−373. (20) Shen, J.; Song, Z.; Qian, X.; Song, C. Chitosan-coated papermaking grade PCC filler prepared by alkali precipitation: properties and application. 2nd International Papermaking and Environment Conference, 2008. (21) Shen, J.; Song, Z.; Qian, X.; Yang, F. Carboxymethyl cellulose/ alum modified precipitated calcium carbonate fillers: Preparation and their use in papermaking. Carbohydr. Polym. 2010, 81, 545−553. (22) Song, D.; Dong, C.; Ragauskas, A. J.; Deng, Y. Filler modification with polysaccharides or their derivatives for improved paper properties. J. Biobased Mater. Bioenergy 2009, 3, 321−334. (23) Yan, Z.; Liu, Q.; Deng, Y. Improvement of paper strength with starch modified clay. J. Appl. Polym. Sci. 2005, 97, 44−50. (24) Yoon, S. Y.; Deng, Y. Experimental and modeling study of the strength properties of clay-starch composite filled papers. Ind. Eng. Chem. Res. 2007, 46, 4883−4890. (25) Zhao, Y.; Hu, Z.; Ragauskas, A. J.; Deng, Y. Improvement of paper properties using starch-modified precipitated calcium carbonate filler. Tappi J. 2005, 4, 3−7. (26) Huang, X.; Shen, J.; Qian, X. Filler modification for papermaking with starch/oleic acid complexes with the aid of calcium ions. Carbohydr. Polym. 2013, 98, 931−935. (27) Li, L.; Collis, A.; Pelton, R. A new analysis of filler effects on paper strength. J. Pulp Pap. Sci. 2002, 28, 267−273. (28) Yang, H.; Qiu, L.; Qian, X.; Shen, J. Filler modification for papermaking with cationic starch and carboxymethyl cellulose: A comparative study. BioResources 2013, 8, 5449−5460. (29) Yan, D.; Li, K. Evaluation of inter-fiber bonding in wood pulp fibers by chemical force microscopy. J. Mater. Sci. Res. 2013, 2, 23−33. (30) Lourenço, A. F.; Gamelas, J. A. F.; Zscherneck, C.; Ferreira, P. J. Evaluation of silica-coated PCC as new modified filler for papermaking. Ind. Eng. Chem. Res. 2013, 52, 5095−5099.

4. CONCLUSIONS Calcium carbonate filler was modified by surface encapsulation with starch/oleic acid complexes with the aid of excess calcium ions. Filler bondability factor in combination with other parameters were employed for evaluation of the use of modified filler in papermaking. The impact of filler modification process conditions and retention systems was investigated. At the filler addition level of 20%, filler modification resulted in a significant increase of filler bondability factor from 5.3 to 12.5. For unmodified filler and modified filler, the use of cationic polyacrylamide or cationic polyacrylamide/bentonite simultaneously enhanced filler bondability factor and filler retention. Particularly, the microparticle retention system was most effective in filler bondability/retention improvement. These results would add to the useful basics related to improved use of mineral fillers in the paper industry by filler engineering.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-130-89981727 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge support from the Fundamental Research Funds (DL12CB08) for the Central Universities of China, the National Natural Science Foundation of China (Grant No. 31100439), the Program for New Century Excellent Talents in University (NCET-12-0811), and the Program for Returned Overseas Chinese Scholars from the Ministry of Human Resources and Social Security of China.

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REFERENCES

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