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Progress in Organic Coatings 79 (2015) 31–36

Contents lists available at ScienceDirect

Progress in Organic Coatings

j o ur na l ho me pa ge: www.elsev ier .com/ locate /porgcoat

alcium sulphate as pigment for improved functional properties ofoated paper

mit K. Singhal, Sunil Kumar ∗, Sanjeev Gupta, Nishi K. Bhardwaj, Raghavan Varadhanvantha Centre for Industrial Research & Development, Yamuna Nagar 135001, Haryana, India

r t i c l e i n f o

rticle history:eceived 7 July 2014eceived in revised form 18 October 2014ccepted 2 November 2014

eywords:igmentalcium sulphateoated paperrightnesshiteness

a b s t r a c t

In the last few years, interest and demand of high bright paper have forced paper manufacturers tothink new ways to improve brightness and whiteness of coated paper. Pigment coating is widely used toenhance the optical properties such as brightness, whiteness, and gloss of paper and paperboard. Theseoptical properties are the most important for end user and also determine the final cost of coated paper.Calcium sulphate has inherent better optical properties compared to other conventional pigments forexample ground calcium carbonate, precipitated calcium carbonate and kaolin clay. The present studywas carried out with an aim to synthesize calcium sulphate using waste procured from phosphoric acidindustry and to study its impact on the rheology of coating color as well as optical properties of coatedpaper. Addition of calcium sulphate improved the water retention property of coating color which canbe helpful for improving the machine runnability. The results also revealed that calcium sulphate can

be used as a pigment to produce coated paper of high brightness and whiteness. The brightness andwhiteness of the coated paper were improved 4 and 15 points, respectively by using 50 parts of calciumsulphate as a pigment replacing clay from the coating formulation. The surface strength in terms of IGTpick value of coated sheets was found significantly comparable using calcium sulphate as pigment. Theprint gloss results were observed analogous with matt grade coated paper.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The principal need for coating is to improve certain properties ofaper to make the printed image sharp, clear and more appealing tohe eye [1,2]. The same carried out by filling the non-uniform sur-ace of paper by applying pigmented coating and supercalenderingo get desired properties of coated paper.

Pigments represent the major part of any coating color whichonstitute about 80–85% by weight. Pigment characteristics, suchs particle size, size distribution, and morphology, play fundamen-al role in determining the coated paper quality like brightness,hiteness, gloss, print gloss etc. Different type of pigments e.g.

round calcium carbonate (GCC), precipitated calcium carbonatePCC) and clay are used for paper coating [3]. GCC and clay are the

ost widely used pigments in paper industry and exploited in aarge range of coating applications [4–6]. The major part of the cal-

ium carbonate is GCC processed from chalk, limestone and marble.alcium carbonates have high brightness, whiteness and oil absorp-ion, but they form matt coatings. In addition, calcium carbonate

∗ Corresponding author. Tel.: +91 1732 292720; fax: +91 1732 292748.E-mail address: sunilic2001@gmail.com (S. Kumar).

ttp://dx.doi.org/10.1016/j.porgcoat.2014.11.002300-9440/© 2014 Elsevier B.V. All rights reserved.

coatings produce good printability and require low binder demandin water-based coatings. Fine clay is used for better gloss, opacityand printability of coated paper because of its smaller particle sizeand platy shape i.e. a layered structure [6].

In the last few years, demand of high bright paper has forcedpaper manufacturers to think new ways to improve bright-ness/whiteness of paper [5]. Production of high bright bleachedpulp, addition of high bright fillers and optical brightening agent(OBA) in the paper are selectively utilized to increase the bright-ness and whiteness of paper [7,8] while the coated paper with highbrightness/whiteness level can be produced by using high brightpigments like GCC/PCC, addition of OBA/OBA carrier and reducingthe load of supercalendering. But by reducing the supercalender-ing load, brightness and whiteness increased but the desired glosscould not be achieved.

Calcium sulphate (CS) has inherent better optical propertiescompared to other conventional pigments like GCC, PCC and kaolinclay. So it can be used to improve brightness/whiteness of paper.Calcium sulphate abundantly exists in nature in three different

forms according to the degree of hydration: gypsum (calciumsulphate dihydrate, CaSO4·2H2O), bassanite (calcium sulphatehemihydrate, CaSO4·0.5H2O) and anhydrite (CaSO4). Calciumsulphate also find its application in different industries as food

3 Organic Coatings 79 (2015) 31–36

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Table 1Characteristics of different pigments used in coating color formulation.

Properties CS GCC Clay

Brightness, % ISO 98.1 89.3 82.1CIE whiteness 97.8 85.1 72.2Yellowness, % 0.24 2.62 6.51L* 99.3 96.6 94.7a* 0.03 0.16 0.36b* 0.11 1.30 3.23

retention value was measured on water retention meter (AA-GWR)as per Tappi Test Method T 701 pm-01. Basis weight of coated sheetswas measured as per Tappi Test Method T 410 om-98. Thickness ofcoated sheets was measured on L&W thickness tester as per Tappi

Table 2Coating color formulation.

Ingredients Control A B C D E F

2 A.K. Singhal et al. / Progress in

dditive, timbering, medical material, inorganic filler or intensifiern composite [9].

Phosphoric acid is produced either by acidulation of phos-hate rock by a mineral acid (sulfuric or hydrochloric acid) in

wet process or by burning of phosphorus produced throughlectro-thermal process. Hydrochloric acid method generates largeuantities of waste slurry containing calcium chloride and createnvironmental problem on discharge. For manufacturing 1 ton ofechnical grade phosphoric acid, generally about 1.2 ton of wastealcium chloride is generated [10].

This reported work is the extended work carried out earlier asentioned elsewhere [11]. The current research work deals with

he synthesis of calcium sulphate from effluent rich in calciumhloride collected from a phosphoric acid industry with an aim toevelop different grades like matt or high gloss coated paper usingalcium sulphate as pigment through single application of coat-ng color. The synthesized CS was characterized by using variousechniques like chemical analysis, X-ray diffraction, and scanninglectron microscopy. The synthesized CS was also analyzed for itsptical properties, particle size distribution, use in coating colorormulation, effect on coating color properties and finally effect onoated paper quality.

. Experimental

.1. Materials

Coating base paper of 60 g/m2 basis weight, having hardwoodnd softwood pulp combination, was used for coating applica-ion. Commercially available GCC (white powder, brightness ISO:9.3%, 75% particle ≤2 �m), coating clay (brightness ISO: 82.1%, 48%article ≤2 �m), synthesized CS as pigments along with styreneutadiene based synthetic binder (solid: 50%, Tg: 17 ◦C) and poly-crylate based dispersing agent (solid: 42%, nature: anionic) weresed in coating formulation. Starch (brightness ISO: 85%) andolymeric acrylic copolymer (solid: 25%, nature: anionic) weresed as a natural and synthetic rheology modifier, respectively.hemical containing six-sulphonic group of triazinyl stilbene moi-ty and ammonium zirconium carbonate (AZC) based compoundsere used as OBA (solid: 20%, nature: anionic) and insolubilizer

solid: 25%, nature: anionic), respectively. Calcium stearate basedompound was used as lubricant (solids: 50%). These differenthemicals were added to the color to have desired impact on coat-ng rheology and final coated paper properties.

.2. Synthesis of CS

Industrial waste rich in calcium chloride was procured from anndian chemical company producing technical grade phosphoriccid. The CS was synthesized at ambient temperature under opti-ized conditions without addition of any rheology modifier and

urfactant as reported in our previous work [11]. The synthesizedS was characterized for its optical and other properties as given inable 1 [12].

.3. Characterization of CS

Optical properties were measured by brightness tester (Elrepho,&W). Horiba Laser Scattering Particle Size Analyzer (LA-950V2)as used to determine the median particle size and particle sizeistribution of all the pigments. The crystalline phases of theS products were investigated by wide-angle X-ray diffraction

PANanalytical X’Pert PRO). The wavelength of the monochromatic-ray beam was 1.54 A and the range of the 2� scan was 8–80◦. Theiffraction peaks of CS phases were compared to standard patternJCPDS, Card No. 041-0225).

Median particle size (D50), �m 0.21 0.94 1.71% Particles ≤2 �m 92 75 48Span, {(D90-D10)/D50} 2.5 3.4 6.5

Surface morphology of product was characterized by scanningelectron microscopy (SEM, JEOL JSM 6510 LV) after coating with anapproximately 50 A thick gold layer.

2.4. Preparation of coating color

Procedure for making coating color was followed as given inreported work [11]. Required weight of calcium carbonate slurrywas taken in beaker and kept under agitation. Clay slurry and cal-culated amount of water were added to it to get targeted solidconcentration. Starch paste was added slowly in pigment slurryto avoid any viscosity shock or lump formation. The speed of theagitator was lowered to avoid any foam formation during additionof synthetic binder. CS 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 50%.

The coating color containing various parts of CS with otheringredients was prepared as given in Table 2. Coating color for-mulation marked “control” was prepared without addition of CS.The parts of GCC and other additives were fixed throughout theexperiments in all batches (marked as A to F).

2.5. Paper coating application

The coating color was applied to 21.0 cm × 29.7 cm size basepaper sheet using an automatic bar coater (K Control Coater, Model101). The sheets were preconditioned for 24 h at 27 ◦C and 65% rela-tive humidity. The coat weight was maintained around 13–14 g/m2

with a thickness of 12–14 �m with bars used for applying the coat-ing color on base paper. The coated sheets were immediately placedin an oven maintained at 105 ◦C for 60 s to dry. Coated paper wassupercalendered in plant scale supercalender by applying a linearnip pressure of 76 bars at 50 ◦C. All the sheets were passed throughsingle nip.

2.6. Characterization of coating and coated paper properties

Viscosity of coating color was measured on Brookfield viscome-ter (RVDV-II + Pro) as per Tappi Test Method T 648 om-97. Water

GCC 15 15 15 15 15 15 15Clay 85 80 75 65 55 45 35CS 0 5 10 20 30 40 50Other additives Constant

A.K. Singhal et al. / Progress in Organ

TdouT

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Fig. 1. X-ray diffraction graph of CS.

est Method T 411 om-94. Gloss value of the coated sheets wasetermined on L&W Gloss Tester as per Tappi Test Method T 480m-92. Optical properties of coated sheets were determined bysing brightness tester (Elrepho, L&T) as per Tappi Test Method

452 om-92.

. Result and discussion

.1. Characterization of CS

The optical properties, shape, mean particle size and particle size

istribution of pigments used in paper coating have great impactn coating color properties and end paper quality [4,5,8]. Dried CSas characterized for its optical properties, particle size, and par-

icle size distribution and was found to be the finest and has better

Fig. 2. SEM micrographs showing ne

ic Coatings 79 (2015) 31–36 33

optical properties among all pigments (Table 1). The median parti-cle size for CS was 0.21 �m as compared to 0.94 �m and 1.71 �mof GCC and clay, respectively. The characteristic peaks of CS wereconfirmed by X-ray diffraction technique as shown in Fig. 1 [13].As reported, the shape of GCC was cubic prismatic while that ofclay was platy. The shape of synthesized CS was needle like as con-firmed by SEM micrographs (Fig. 2) and the similar observationswere reported elsewhere [13,14]. CS has better optical propertiesas compared to other pigments used in coating formulation andthus would help in improving the similar properties in final coatedpaper.

3.2. Effect of addition of CS on coating color properties

Rheological properties and water retention of coating colorare crucial parameters in the pigment coating of paper. Theseproperties of coating color have been the subject of numerousinvestigations over the years [15]. The results of these investiga-tions revealed that water retention property is a function of particleto particle interactions of the pigment, shape of pigment, polymericemulsion and natural/synthetic rheology modifier used in coatingcolor formulation including the base paper [16–19]. The viscosityof the coating color is linked with machine runnability. In presentstudy, the attempts were made to adjust the viscosity of coatingcolor to the desired level (500 ± 100 cP at 100 rpm) while maintain-ing the solid level of coating color at 50 ± 0.5%, and also to impartthe necessary degree of water retention.

Results revealed that addition of CS in coating color had a directrelationship with the low shear viscosity, which increased with

the increase of CS parts. Water holding capacity of the coatingcolor was improved (lower the value, better is the water holdingcapacity) with increase in parts of CS in coating color and will havebetter machine runnability (Fig. 3). The increase in viscosity and

edle shape morphology of CS.

34 A.K. Singhal et al. / Progress in Organic Coatings 79 (2015) 31–36

Fig. 3. Effect of addition of CS on water retention value of coating color.

Fig. 4. Effect of addition of CS on brightness of coated paper.

Fig. 5. Effect of addition of CS on whiteness of coated paper.

A.K. Singhal et al. / Progress in Organic Coatings 79 (2015) 31–36 35

of CS o

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Fig. 6. Effect of addition

mprovement in water retention value might be due to particleize and particle size distribution of CS, as CS is finer than bothCC and coating clay and finer particles behave similarly.

.3. Effect of addition of CS on optical properties of coated paper

The brightness of coated paper was increased from 86.1 to 90.5%hich is quite substantial gain in brightness for paper industry

Fig. 4). The other alternate of increasing the brightness and white-ess of coated paper could be addition of higher parts of OBA alongith OBA carrier [20]. But both of these ingredients are very expen-

ive per unit weight of any coating color and shall increase theost of coating color to a very high level, which is not acceptableo the industry. In similar way, whiteness of coated paper showedemarkably noticeable improvement by 15 points through additionf 50 parts of CS in coating color (Fig. 5). These improvements inptical properties of coated paper were due to the inherent opticalroperties of CS as mentioned earlier. However, the gloss of coatedaper was reduced from 55% to about 30% by increasing the partsf CS in coating formulation (Fig. 6). The CS had positive impact onrightness, whiteness, shade and fluorescence but negative impactn gloss of coated paper which might be due the fact that parts oflay were reduced in all other batches as compared to control andts particle are not like clay i.e. platy in shape. The platy shape oflay imparts gloss to the paper and because of this fact, higher partsf clay are used for high gloss paper and vice versa. CS could be useds pigment with other finer clay to have higher gloss in the coatedaper or for manufacturing matt finish coated paper with high levelf brightness and whiteness to meet the customer demand. The sur-ace strength in terms of IGT pick value was greater than 100 cm/shich indicate a good value without any printing defects with all

oating formulation. The print gloss of coated sheets was foundnversely proportional to the amount of CS in coating formulation

hich might be due to the reduced parts of clay and needle shapetructure of CS.

. Conclusion

The coating grade CS with narrow particle size distributionD50: 0.21 �m), higher brightness (98.1%) and whiteness (97.8) asompared to other conventional pigments was successfully synthe-

ized from industrial waste under optimized process parameters.S could be used as pigment to produce premium quality coatedaper used in magazines and publications where the paper musteproduce photos to meet the highest quality standards. Addition of

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n gloss of coated paper.

CS in coating color formulation provided better optical propertiesto coated paper which could not be achieved by any other pig-ment under study. Improvement was observed in brightness andwhiteness of the coated paper with 4 and 15 points, respectively byusing 50 parts of CS as a pigment replacing clay from the coatingformulation.

The surface strength in terms of IGT pick value was found com-parable for all coated paper under study. The print gloss of coatedsheets decreased with increase in CS parts in coating formulationwhich might be due to the reduced parts of clay and morphology ofCS. However, even with 50 parts of CS, the print gloss was as goodas required for matt grade variety of coated paper. On the otherhand, the synthesis of CS from industrial waste and its use in paperindustry will reduce the environmental pollution as well as protectthe natural resources.

Acknowledgement

Authors wish to thank the Department of Science and Technol-ogy (DST), New Delhi, for providing the financial support to carryout this research work (Project No. DST/TSG/NTS/2011/151).

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