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Synthesis and application of calcium sulphate pigment for paper coating: Potentialand prospects
Nitin Kumar a,⁎, Nishi Kant Bhardwaj a, Swapan Kumar Chakrabarti b, Sunil Kumar b
a Department of Paper Technology, Indian Institute of Technology, Saharanpur Campus – 247001, Uttar Pradesh, Indiab Thapar Centre for Industrial Research & Development, Yamunanagar, 135001, Haryana, India
In the present study, calcium sulphate pigment has been synthesized experimentally from commercial gradecalcium chloride. The effect of various factors on the particle size of calcium sulphate formed in a wet processand its potential application as pigment in coating recipe has been investigated. The operating factors inves-tigated include concentration of calcium chloride solution, temperature, addition of different rheology mod-ifiers and their optimization. The particle shape and size distribution of synthesized calcium sulphatepigment have been studied through image analyzer, scanning electron microscope (SEM) and particle sizeanalyzer. Significant improvement was observed in various coated paper properties like brightness, white-ness, gloss, stiffness, surface strength and water retention properties of coating color with respect to conven-tional ground calcium carbonate (GCC) coating pigment.
Coating provides an excellent means of improving the aesthetic andprinting properties of paper. Coated paper is more uniform and morereceptive to ink to get better results during the printing application.The basic pigment in a coating color is usually ground/precipitatedcalcium carbonate (GCC/PCC) and China clay of different grades. Particleshape, size and size distribution are the main properties of the pigmentwhich determines the final coating structure, its optical properties, andthe resulting coated paper performance. Pigment particle shape is re-sponsible for the improvement in coating structure through physicalhindrance while particle size and size distribution improve coatingstructure through controlled consolidation. Particle size distributionand shape of the pigments also determine the pore size and pore vol-ume of the coating due to the variation in packing characteristics ofthe pigment. These in turn influence the paper properties like smooth-ness, gloss and printability [1,2].
The demand of quality coated paper has been increased so far, andthe resources are limited to add value to the product. This creates at-tention towards the development of new raw materials for papercoating. The present study aims to synthesize calcium sulphate pig-ment for coating application. There are different ways to manufacturepaper coating grade calcium sulphate pigment. In the present study
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the calcium sulphate is synthesized from commercial grade calciumchloride as the calcium chloride can be procured from various chem-ical industries as a waste. Calcium sulphate is usually used as a mix-ture with clay or other pigments in top coatings. By controlling theconditions of crystallization, both particle size and pigment shapecan be influenced. Limited reports are available on synthesis of calci-um sulphate and its application as a pigment in paper coating [3–6].This paper presents the effect of various process parameters on theparticle size of calcium sulphate and to identify the most importantfactors responsible for controlling the particle size. Attempts havealso been carried out to find its potential as pigment in coating formu-lation to improve the coated paper properties.
2. Experimental
2.1. Materials
Paper with a basis weight of 78 g/m2, having hardwood (80%) andsoftwood (20%) pulp combination, was used as a base for coating appli-cation. Commercial grade, coarser and finer particle size GCCs wereused in precoat and topcoat coating formulation respectively. Tapiocabased modified coating starch was used as a natural binder in precoatformulation. Styrene butadiene (Tg: 17 °C) based synthetic binder,polyacrylate based dispersing agent and calcium stearate based lubri-cating agent were used as additives to provide specific functions. Am-monium zirconium carbonate based compound was used as aninsolubilizer. Carboxy methyl cellulose (CMC) and acrylic based thick-ening agents were used as a natural and synthetic rheology modifier
41N. Kumar et al. / Powder Technology 218 (2012) 40–45
in topcoat and precoat formulation respectively. Hexa sulpho stilbenebased compound was used as an optical brightening agent (OBA).Thecommercial grade calcium chloride (CaCl2) and sulphuric acid wereused for the synthesis of calcium sulphate pigment.
CaCl2 solution of desired concentration was prepared using wateras a solvent. The natural rheology modifier was then added into thesolution. Temperature of the solution was maintained as per the re-quirement. Sulphuric acid was added slowly at the vortex of CaCl2 so-lution under fixed agitation. Stoichiometric amount of sulphuric acidwas added for the precipitation of calcium sulphate particles. Theprecipitated slurry was kept under agitation for 15 min for propermixing. Precipitated slurry of calcium sulphate pigment was thenwashed with water with the help of Buchner funnel until the pH of fil-trate reached in the range of 8–8.5. Calcium sulphate pigment wasdried for 3 hour in an oven at 105 °C. The effects of various experi-mental conditions, such as reaction temperature, CaCl2 concentration,and rheology modifiers on the particle size of calcium sulphate pig-ment were investigated. Details of experiment for the synthesis ofcalcium sulphate pigment and the properties of synthesized calciumsulphate pigment are given in Tables 1 and 2 respectively.
2.3. Coating color preparation
Required weight of calcium carbonate slurry was taken in beakerand kept under agitation. Clay slurry and calculated amount ofwater were added to it to get targeted solid concentration. Starchpaste was added slowly in pigment slurry to avoid any viscosityshock or lump formation. Cooked CMC paste was then added at thevortex of pigment slurry. The slurry was agitated at high speed forcomplete dispersion of CMC. The speed of the agitator was loweredto avoid any foam formation during addition of synthetic binder. Cal-cium sulphate pigment was then added slowly into the slurry. Lubri-cant, insolubilizer and synthetic thickener were added at the vortexafter fixed interval of time. Finally OBA was added to the color. ThepH of the color was adjusted to 8.5–9.0. The total solids of coatingslip were kept around 60%. Calcium sulphate pigment was used intopcoat coating color formulation alone and in different combinationswith ground calcium carbonate. The details of coating color formula-tion for precoat and topcoat coating application are given in Tables 3and 4 respectively.
2.4. Paper coating application
Base sheets were preconditioned for 24 hour at 23 °C and 55%relative humidity. The coating color was applied on 21.0×29.7 cm2
base sheets using an automatic bar coater (RK-Print Coat InstrumentsLtd., U.K., model K 101). Coat weights for precoat and topcoat applica-tion were maintained 15 and 10 g/m2, respectively by using bars ofdifferent number. Coated sheets were immediately placed in anoven maintained at 105 °C for 60 s to dry. Coated paper was
Table 1Process conditions during synthesis of calcium sulphate pigment.
Parameters
Temperature, °C
CaCl2 concentration, % by weight
Rheology modifier, % on CaCl2
Average particle size, µ
5
14.70
10
11.40
I
35
20
Nil
8.80
30
6.0
* With CMC.#with starch.
supercalendered in plant scale supercalander by applying a linearnip pressure of 76 bars, at 50 °C. All sheets were passed throughtwo nips.
2.5. Methods
The particle shape of pigments was captured through Carl Zeissimage analyzer; model Axio Scope A1, at 1000 magnification. Scan-ning Electron Microscope (SEM); model JSM–6510 LV, JEOL, wasused for micro pictures of pigments at 10,000 magnifications. Particlesize distribution of pigments used in paper coating application wasmeasured on Horiba make laser scattering particle size analyzer;model no. LA-920. Remi make medium duty stirrer; model RQ–124A, was used for the synthesis of calcium sulphate pigment at3000 rpm. Viscosity of coating slip was measured on brookfield vis-cometer; model no. DV-II pro viscometer. Water loss value was mea-sured on AAGWR water retention meter; model no. 250. Opticalproperties of coated sheets were determined by using data colorbrightness tester; model Spectraflash 300. Gloss value of the coatedsheets was determined on L&W gloss meter; model no. SE-224.L&W air permeance tester, model no. SE-166 was used to measurethe air permeance or porosity of coated sheets. Printing propertiesi.e. IGT pick velocity, ink setoff and print gloss were measured onIGT printability tester; model no. AIC2-5.
3. Results and discussion
3.1. Synthesis of calcium sulphate pigment
3.1.1. Effect of CaCl2 concentrationThe concentration of CaCl2 solution was seen to have effects on
controlling the particle size of calcium sulphate pigment. In thisstudy, the concentration of CaCl2 solution was varied from 5% to40% by wt. at 35 °C (Table 1). It was found that the higher concentra-tion led to much smaller particles. The pigment particle size at 40%
40
4.62
5
3.82
II
10
30
Nil
4.40
70
20.85
III
5
30
1
1.53*, 2.00#
Table 4Coating color formulations for topcoat application.
42 N. Kumar et al. / Powder Technology 218 (2012) 40–45
concentration was significantly higher (14.70 μ) as compared to 5%concentration (4.62 μ) (Fig. 1a–e). This may be attributed to nucle-ation predominating over crystal growth at higher concentrationleading to a decrease in particle size. Thus the maximum achievableconcentration of CaCl2 solution is favorable to get finer particle sizeof pigment. It was observed that at the level of 40% concentration ofCaCl2, the slurry converts into thick paste which was difficult to
Fig. 1. a. Image analysis of calcium sulphate pigment formed at 5% CaCl2 concentration (AveCaCl2 concentration (Average particle size 11.40 μ). c. Image analysis of calcium sulphate pigof calcium sulphate pigment formed at 30% CaCl2 concentration (Average particle size 6.0(Average particle size 4.62 μ).
disperse. Therefore further optimization study was performed at30% concentration.
3.1.2. Effect of reaction temperatureReaction temperature was also found an effective parameter to
control the pigment particle size. The effect of temperature was stud-ied from 5 °C–70 °C at 30% concentration by wt of CaCl2 solution(Table 1). It was found that the pigment particle size decreasedwith decreasing the reaction temperature. The Figs. 2a–c and 1dclearly indicate that the particle size significantly increased (3.60 μto 20.40 μ) on increasing the reaction temperature from 5 °C to70 °C. Increase in particle size may be attributed to the fact thathigher reaction temperature reduces the viscosity of solution duringsynthesis of pigment which results reduction in shear intensity be-tween pigment particles. Hence, the least possible temperature is infavor for the reduction of pigment particle size.
3.1.3. Effect of rheology modifiersUsing rheology modifier during the reaction process can modify
certain properties of the final precipitate. CMC and starch were usedas a rheology modifier to control the particle size of pigment. This
rage particle size 14.70 μ). b. Image analysis of calcium sulphate pigment formed at 10%ment formed at 20% CaCl2 concentration (Average particle size 8.80 μ). d. Image analysisμ). e. Image analysis of calcium sulphate pigment formed at 40% CaCl2 concentration
Fig. 2. a. Image analysis of calcium sulphate pigment formed at 5 °C temperature(Average particle size 3.82 μ). b. Image analysis of calcium sulphate pigment formedat 10 °C temperature (Average particle size 4.40 μ). c. Image analysis of calcium sul-phate pigment formed at 70 °C temperature (Average particle size 20.85 μ).
Fig. 3. a. Image analysis of calcium sulphate pigment formed by using CMC as a rheol-ogy modifier (Average particle size 1.53 μ). b. Image analysis of calcium sulphate pig-ment formed by using starch as a rheology modifier (Average particle size 2.00 μ).
Fig. 4. SEM image of calcium sulphate pigment.
43N. Kumar et al. / Powder Technology 218 (2012) 40–45
study was carried out at 5 °C and 30% concentration by wt. of CaCl2solution (Table 1). The results indicate that rheology modifiers signif-icantly reduced the particle size of calcium sulphate pigment; 1.53 μwith CMC and 2.00 μ with starch (Fig. 3a and b). It is well knownthat CMC and starch both increase the viscosity of the solution asthey show good water holding capacity. This increases the shear in-tensity between pigment particles resulting in reduction of pigmentparticle size. On comparing the results, 60% reduction in particlesize was achieved with CMC as a rheology modifier (Figs. 2 and 3).
3.2. Application in paper coating
The optimized conditions (reaction temperature 5 °C, CaCl2 con-centration 30% & CMC as a rheology modifier 1%) were used for thesynthesis of calcium sulphate pigment for paper coating application.The particle size distribution of synthesized calcium sulphate andfiner grade conventional GCC pigment is given in Table 2.
3.2.1. Effect on coating color rheologyThe reduction in Brookfield viscosity was observed on introducing
calcium sulphate in coating formulation (Table 5). The micro pictures
clearly indicate a blocky or roundish shape of GCC pigment particles,whereas calcium sulphate possesses needle like structure (Figs. 4–5).The aspect ratio (particle length to width ratio) is near to unity forGCC whereas it is much higher for calcium sulphate pigment. Thehigh aspect ratio of calcium sulphate pigment helps to orientate calci-um sulphate particles easily along the longer axis as compared to GCCunder shear force (Figs. 4–5) [4]. Hence drop in viscosity was ob-served with calcium sulphate pigment.
Coated paper properties depend on dewatering and immobilizationof the coating color into the base paper during coating application.Morethe water penetration more will be material shift from the coatingcolor to the base paper [7]. It has been observed that the water loss ofcoating color decreases (higher the value of water loss, the poor willbe the retention) on increasing parts of calcium sulphate pigment in
44 N. Kumar et al. / Powder Technology 218 (2012) 40–45
coating color formulation (Table 5). Possibly, the bigger particle size andneedle like shape of the calcium sulphate pigment create tortuosity forthe liquids to flowwhich results improvement in water holding capac-ity of coating color.
3.2.2. Effect on coated paper propertiesAs the inherent brightness and whiteness level of synthesized cal-
cium sulphate pigment were higher than finer grade GCC pigment(Table 2), therefore improvement was observed in brightness andwhiteness of coated sheet with introduction of calcium sulphate pig-ment in coating color formulation (Table 6).
Paper gloss is also an essential property of coated paper thatmakes the object look shiny or lustrous and depends on specular re-flection from the surfaces. Improvement in gloss was observed on in-creasing parts of calcium sulphate in coating formulation (Table 6).
Fig. 6. Effect of aspect ratio of pigment
The particle orientation during coating consolidation affects gloss be-cause tilted surface facets reflect light to different directions [8–10].Fig. 6 clearly indicates that the high aspect ratio pigment particles ali-ened properly during consolidation thus produce better gloss to coat-ed paper [10].
Paper stiffness is required while during printing of coated paper forthe proper feeding of paper from tray to printing unit, otherwise it maycause jamming problem. The aspect ratio of the pigment is responsiblefor the change in stiffness of coated paper. The effect of changing thepigment aspect ratio has also been quantified. It was found by the re-searchers that a coating layer containing high aspect ratio fine clayhas an elastic modulus 7–8 times higher than one containing onlyground calcium carbonate pigment [8,11]. The improvement in stiffnessof coated paper with using high aspect ratio calcium sulphate pigmentclearly proves the above statement (Table 6).
Surface strength was measured in terms of IGT pick velocity. Thehigher the values of pick velocity, the higher will be the surfacestrength. Surface strength of coated paper increased on increasing theamount of calcium sulphate in coating color formulation (Table 6).Thus offering an opportunity for reduction in binder amount. Possiblythe bigger particle size of calcium sulphate pigment has a lower specificsurface area as compared to finer gradeGCCwhichhelps in reducing thecoating binder demand [4].
Print gloss is an important property of printed coated sheet. Printgloss depends not only on the unprinted initial sheet gloss but also onthe ink setting rate during printing operation. Higher initial sheetgloss provides higher print gloss [12]. It is obvious that increasingthe number of pores in the coating layer increases the ink settingrate. Pigments having high aspect ratio shows slower ink settingrate. It clearly indicates the alignment of pigment particles during cal-endering which creates closed coating structure and tortuous path toflow the liquid through the coating. The extent of penetration into thecoated paper surface can be reduced by replacing conventional lowshape factor pigment with high shape factor pigment. This will resultin improved print gloss of final coated paper [8].The results indicatethat ink setting rate becomes slow (higher value of ink setoff indi-cates slower ink setting rate) after addition of calcium sulphate incoating color formulation (Table 6). It may be correlated with thelower air permeance value obtained in case of high shape factor calci-um sulphate pigment (Table 6). The air permeance gives an indicationof pore volume, the lower the pore volume, lower will be the ink set-ting rate (Table 6). The ink setoff value is almost in same range up to50% replacement of finer grade GCC with calcium sulphate pigment.Hence, the calcium sulphate based coatings have a lower ink require-ment and the slower ink setting will give higher print gloss as ob-served during the study (Table 6).
45N. Kumar et al. / Powder Technology 218 (2012) 40–45
4. Conclusion
The calcium sulphate was successfully synthesized under controlledconditions. Best results were achieved with using CMC (1%) as a rheol-ogymodifier at 30% concentration of CaCl2 solution and at 5 °C reactiontemperature. Application of calcium sulphate as a pigment in papercoating improves the water retention, brightness, whiteness, stiffness,surface strength in terms of IGT pick velocity and print gloss of coatedpaper. Result of study revealed the feasibility to synthesize coatinggrade calcium sulphate pigment which can be used as an alternatepigment in coating color formulation to get better results. The use of cal-cium sulphate is also ecologically sensible as it can also be producedfrom the waste of chemical industries.
Acknowledgments
Authors wish to thank Director, Thapar Centre for Industrial Re-search and Development for extending the facilities for the researchwork. Authors also acknowledge Dr. Sunil Kumar, Sr. Research Scientist(TCIRD), Dr. Sanjeev gupta, Sr. Research Scientist (TCIRD) andMr. VipulChauhan, Sr. Research Scientist (TCIRD) for their critical suggestionsduring experimental work.
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