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Enhancing Egyptian kaolinite via calcination and dealumination for application in paper coating.

Abstract The aim of this study was to optimize the modification of Egyptian kaolinite for application in paper coating. The study focused on four modification methods; sedimentation process, chemical bleaching, calcination, and dealumination. The effect of these methods on the structure of kaolinite was studied using X-ray fluorescence (XRF), X-ray diffractometer (XRD), Fourier transform infrared spectrometer analysis, and field emission SEM. The original and modified kaolinites were applied in paper coating mixtures. The results of XRF analysis showed that the [Fe.sub.2][O.sub.3] and Ti[O.sub.2] in Egyptian kaolinite were reduced chemically via sodium dithionite from 0.41% to 0.25% and 2.20% to 2.00%, respectively. Calcination at 900[degrees]C, followed by acid activation and bleaching, showed a further decrease in Fe2[O.sub.3] and Ti[O.sub.2] impurities to 0.012 and 1.45 (wt%), respectively. XRD results revealed that all characteristics reflection of kaolinite disappeared upon calcination. SEM investigation showed a significant reduction in kaolinite particle size. Calcination and dealumination of kaolinite did not improve coated paper roughness, while air permeance and optical properties significantly increased in comparison with commercial kaolinite. In addition, a significant improvement was observed in coaled paper mechanical properties including burst, tensile strength, stretch, and tensile energy absorption with respect to original and commercial kaolinite. In contrast, the kaolinite fraction <2 [micro]m highly improved paper gloss, print density, and print gloss, more than calcined kaolinite and its modified pigments. In con elusion, dealumination of calcined kaolinite did not show any further change in all coated paper properties compared to the calcined ones.

Keywords Kaolinite, Calcination, Acid activation. Paper coating, Optical properties, Print quality

Introduction

The main reason for applying pigment coatings to paper and paperboard is to improve its printability, optical properties, and smoothness. In its simplest form, a pigment coating suspension consists of pigment such as kaolinite, calcium carbonate, titanium dioxide, water, binder, and other additives. (1) Pigment is the abundant component in the coating and is naturally the most important factor affecting the properties of the coating materials. Kaolinite is one of the most widely used pigments in the paper industry and is utilized in a wide range of coating applications. (2) It consists of a dioctahedral 1:1 layer structure with a general composition of [Al.sub.2] [Si.sub.2][O.sub.5][(OH).sub.4]. Structurally, it is made up of alternate octahedral gibbsite and tetrahedral silica layers giving a platy structure. (3) It has many desirable physical and chemical properties including white color, fine particle size, platelet particle structure, low abrasiveness, and chemical inertness. (4) The inherent impurities in kaolinite, such as titanoferrous minerals, iron oxides, mica, etc., vary its color from yellow to dark brown. (5) It has to be processed for industrial applications especially in paper coatings, which demand very high brightness, low yellowness, and other physical and optical properties. (6)

Kaolinite brightness can be modified by several methods such as fractionation by centrifuge or sedimentation process, (7) chemical bleaching, (8) intercalation (9,10) and calcination or thermal treatment. (6) Kaolinite, when thermally treated, produces a product of significant industrial applications called "calcined Kaolinite" or "metakaolinite" with characteristic properties. Calcination increases the brightness and whiteness and decreases the color values, giving an overall improvement in optical properties. (11) Another one of the most common chemical modifications of kaolinite used for both industrial and scientific purposes is acid activation (acid leaching) which involves the treatment of kaolinite with a mineral acid solution, usually HC1 or [H.sub.2]S[O.sub.4], causing disaggregation of kaolin particles, elimination of mineral impurities, and dissolution of the external layers. The acid treatment is beneficial in terms of increased surface area, porosity, and number of acid centers with respect to the original kaolin. (11,12)

Although kaolinite is present in large amounts in many localities in Egypt, it is characterized as a hard and massive pigment and has various amounts and kinds of impurities, which means that it does not satisfy the required specification of paper production. (13,14) The major impurities are quartz, feldspars, anatase, and iron expressed as separated phases or substituted in the kaolinite lattice. Unfortunately, these phases commercially influence important characteristics such as brightness, hydraulic conductivity, rheology, and other chemical properties. Consequently, Egyptian kaolinite needs careful selection of processing techniques to get kaolinite of an almost ideal composition with minimum ancillary mineral impurities.

The aim of this work is to optimize several modification methods for removing the impurities associated with Egyptian kaolinite to make it suitable as paper coating pigment. The study will facilitate using locally produced coated paper having excellent brightness, light scattering, and mechanical properties.

Experimental

Materials

The original kaolinite (K) sample was supplied from the El Tieh plateau, Sinai, Egypt and was purified using standard sedimentation techniques to produce the <2 pm kaolinite fraction (KF). Sodium dithionite ([Na.sub.2][S.sub.2][O.sub.4]> Sigma--Aldrich), hydrochloric acid (HCl, Sic Company, assay 30%), and sodium hexametaphosphate (Fine Chem, [Na.sub.6][P.sub.6][O.sub.18]) were used as received without any further treatment. A commercial kaolinite (CK) sample supplied by Rakta Company was used as a control sample for comparing the results. A bleached kraft paper of 80 g/[m.sup.2] was used as base paper. Commercial offset ink for printing was supplied by Paints and Chemical Industries Company (Packin), Egypt.

Modification of Egyptian kaolinite

Preparation of <2 [micro]m kaolinite fraction (KF)

The original kaolinite sample was dispersed in water at a high solid percentage (60%), and sodium hexametaphosphate (1% of dried kaolin) was added for stabilizing the kaolinite as a colloidal suspension. Small amounts of Ti[O.sub.2] and of [Fe.sub.2][O.sub.3] impurities could be flocculated and separated by a sedimentation process after 24 h. The colloid <2 [micro]m kaolinite fraction was centrifuged and then was dried in an oven at 60[degrees]C.

Calcination of Egyptian kaolinite

The <2 [micro]m kaolinite fraction was calcined in a programmable muffle furnace at 900[degrees]C for 2 h, and the obtained calcined kaolinite (metakaolinite, Cal K) was further subjected to acid treatment.

Chemical bleaching by sodium dithionite

4 g of calcined kaolinite was stirred thoroughly in 50 mL of distilled water and 2 g of [Na.sub.2][S.sub.2][O.sub.4] was added; then the mixture was digested for 30-50 min in a water bath at 40[degrees]C. The sample was centrifuged and treated twice with 0.02 N HCl for 10-15 min. Afterward, the sample was washed with distilled water and then dried. The product is referred to Cal K/d (Table 1).

Acid leaching (dealumination of calcined kaolinite)

The calcined kaolinite was treated with 2 N and 4 N HCl at 100[degrees]C for 4 h at ratio of 1:2.6 (w:v). The activated samples were washed with water thoroughly to ensure that pH was at 7, and then they were dried overnight at 60[degrees]C. The products are referred as Cal K/ 2 N HCl and Cal K/4 N HCl. The acid-leached samples were also bleached using the same procedure as mentioned before. The samples are referred to (Cal K/2 N HCl/d and Cal K/4 N HCl/d) as shown in Table 1.

Application of modified Egyptian kaolinite in paper coating

Preparation of coating mixtures

The paper coating mixtures used in this study consisted of 100 parts each of kaolinite pigment (commercial, original, and the modified kaolinite pigments), 15 pph binder, and 0.3 pph of dispersant (pph = part per 100 parts of dry pigment). Each pigment was dispersed in distilled water in the presence of sodium hexametaphosphate at a solids content of 50%. The pH was adjusted to 8.5 using a 1 M NaOH solution.

Preparation of coated paper samples

A wire-wound coating bar, for applying the coating mixtures, was chosen to give a 6 pm wet film using a K-bar semiautomatic coater (model NOS K101, R&K Print Coat Instruments Ltd., UK). All coated paper sheets were calendered at 80[degrees]C and a pressure of 90 bars for 20 min.

Printing of coated paper samples

Coated paper samples were prepared in strips of 6 x 30 [cm.sup.2]. Offset printing was carried out at 125 N/cm printing pressure, 0.3 m/s printing speed, and 2.0 micron ink film thickness using an 1GT printability tester, model AIC 2-5. The temperature was kept at 24[degrees]C throughout the printing process.

Characterization of the modified pigments and coated paper

Mineral identification by X-ray diffraction analysis (XRD) was carried out using a Philips powder diffractometer (type PW1730) with Ni-filter, Co radiation ([delta] = 1.79) at 30 kV and 20 mA. The samples were finely ground, mounted randomly on an aluminum holder, and analyzed by X-ray diffraction using a scanning speed of 4[degrees] 2[theta] per minute. The scans were limited with the range 2[theta] = 2[degrees]-65[degrees].

Quantitative determination of the oxides presented in the kaolinite samples and loss of ignition (LOI), that was obtained by heating sample powder to 1000[degrees]C for 2 h, were carried out using X-ray fluorescence spectroscopy (XRF) (Philips PW 1410). Tube voltage and current for a tungsten (W) target were 40 kV and 60 mA, respectively. Fourier transform infrared spectrometer (FTIR-460 plus, JASCO model 6100, Japan) was used for the characterization of the original kaolinite and its modified forms. The spectra were recorded on a single beam spectrometer with a resolution of 4 [cm.sup.-1] at room temperature in the range 400-4000 [cm.sup.-1]. The samples were ground with KBr (1:100 ratio) and mounted as a tablet to the sample holder in the cavity of the spectrometer.

The morphology of the samples was investigated using field emission scanning electron microscopy (FESEM, Quanta FEG 250, Holland). The brightness and the color values (L, a, and b) of the modified pigments were determined by light reflection at 457 nm considering BaS[O.sub.4] as the reference to 100% whiteness (ASTM, 1990). The properties of coated papers including modified kaolinite pigments were evaluated using standard tests for physical, optical, and mechanical properties. Coated paper roughness (according to ISO 8791-2) and air permeance (according to ISO 5636) were measured, in mL/min and [micro]m/Pa.s, respectively, using a roughness tester (Bendtsen; model K531, Messmer Bunchel). ISO brightness and opacity measurements were conducted on a brightness and color meter instrument (model 68-59-00-002, Buchel-B.V, Netherlands) according to standard methods of ISO 2470-1 (2009) and ISO 2471 (2008), respectively. A micro gloss meter was used at an angle of 75[degrees] to measure the paper and print gloss. The burst strength was determined using a burst tester; model BT-10 T1S Techlab systems, reporting ISO 2758-3 (2009). The tensile strength, stretching, and tensile energy absorption were measured in kN/m according to ISO 1924-2 standard using a Tensile Test machine, T-series; model H5KT, Tinius Olsen Ltd., at 1 kN. The print density was determined by an X-rite densitometer (Model Denis EYE 700), according to ISO 5.3 standard, with an accuracy of [+ or -] 0.02.

Results and discussion

X-ray diffraction study

Kaolinite and kaolinite fraction <2 [micro]m

The X-ray diffraction patterns of the El Tieh kaolinite (K) and its <2 [micro]m fraction (KF) are illustrated in Fig. 1. The El Tieh sample is disordered, poorly crystalline kaolinite (Hinckley index = 0.55), and contains traces of quartz and sparse mica, calcite, and feldspar. Kaolinite (K) is indicated by its characteristic reflection at 7.14 [Angstrom] (001) and 3.56 [Angstrom] (002), quartz (Si[O.sub.2]) at 3.33 [Angstrom], feldspar at 3.23 [Angstrom], and calcite at 3.03 [Angstrom]. Mica appears at 10 [Angstrom]. Anatase (Ti[O.sub.2]) appears at 3.51 [Angstrom]. The XRD pattern of <2 [micro]m fraction (KF) shows the characteristic reflections of kaolinite at 7.14 [Angstrom] (001) and 3.56 [Angstrom] (002). Meanwhile, all the characteristic reflections of non-clay minerals disappeared. The failure to detect the iron oxide in the kaolinite sample may be due to the poor sensitivity of XRD to iron oxide present at a low level and in a small particle size.

Calcined kaolinite (metakaolinite)

All the characteristic reflection of kaolinite disappeared upon calcination up to 900[degrees]C due to the dehydroxylation of kaolinite and formation of metakaolinite as shown in Fig. 2. Ti[O.sub.2] (anatase form) was recognized in the XRD pattern of metakaolinite by its characteristic reflection at 3.51 [Angstrom]. The presence of a low broad 20 (20[degrees]-28[degrees]) peak band with a high intensity indicates the presence of metakaolinite.

The broad band observed over the 2[theta] range 20[degrees]-28[degrees] in the XRD pattern of the metakaolinite is due to its unsymmetrical nature and the retention of the two-dimensional reflections of the original kaolinite. (15)

X-ray fluorescence analysis

The original kaolinite was received as finely divided, pale-yellow powder, indicating the presence of colored impurities in the sample in terms of titanium dioxide and iron oxide as determined by chemical analysis using the X-ray fluorescence, as shown in Table 2. Theoretically, kaolinite contains 46.51% Si[O.sub.2], 39.54% [Al.sub.2][O.sub.3], and 13.95% loss in ignition (LOI). (16) The silica content in the El Tieh kaolinite is slightly higher and LOI is lower, which indicates the presence of "free" silica and other impurities. The results revealed a slight decrease in Ti[O.sub.2] and [Fe.sub.2] [O.sub.3] contents in the kaolinite fraction <2 [micro]m (KF) after the sedimentation process. Further reduction in both Ti[O.sub.2] and [Fe.sub.2][O.sub.3] was obtained after the bleaching of the KF sample to reach to 2.00 and 0.25 (wt%), respectively. The presence of some residual iron content in the kaolinite after bleaching may be attributed to the fact that most iron oxides are in the structure of aluminum silicates which cannot be easily removed by the bleaching technique as previously proved by Hassan and Salem. (17) Also, it has been shown that anatase is concentrated in the fine end of the kaolinite particle size distribution. (18) Therefore, during dispersion of the original kaolinite using sodium hexametaphosphate, only small amounts of Ti[O.sub.2] were removed (Table 2). Boulis and Attia found that the anatase presents as very fine Ti-bearing grains disseminated throughout the kaolinite groundmass. (19) Calcination of KF pigment up to 900[degrees]C reduced LOI from 13.92 to 0.78 due to dehydration and dehydroxylation of kaolinite to produce metakaolinite, which is the most active form of kaolinite (Table 2.) The bleaching of metakaolinite (Cal K/d) reduced the iron oxide content from 0.24 to 0.12 (wt%). Chemical analysis of dealuminated metakaolinite revealed a reduction in [Al.sub.2][O.sub.3], Ti[O.sub.2], and [Fe.sub.2][O.sub.3] in the order Cal K/4 N HCl < Cal K/2 N HCl, and Si[O.sub.2] increased in the opposite order. It is clear that acid activation mainly affects the chemical composition of the octahedral layer, while the tetrahedral layer is less prone to acid effects because of strong bonds among silicon and oxygen ions. (20) The bleaching of dealuminated metakaolinite (Cal K/2 N HCl and Cal K/4 N HCl) reduced the [Fe.sub.2][O.sub.3] contents to 0.023% and 0.012%, respectively. Further reduction was also obtained in Ti[O.sub.2] reaching to 1.78% and 1.45%, respectively.

FTIR analysis

The Fourier transform infrared spectra of original kaolinite (K), metakaolinite (Cal K), and dealuminated metakaolinite (Cal K/2 N HC1) are shown in Fig. 3. The original kaolinite sample (k) (Fig. 3a) exhibited all characteristic hydroxyl stretching bands attributed to the inner-surface hydroxyls oriented toward the interlayer at 3692 [cm.sup.-1]. The bands at 3623 and 917 [cm.sup.-1] are assigned to those hydroxyl groups oriented toward the vacant sites in the external layers of the kaolinite structure. Additional broad stretching bands of kaolinite at 3473 and 1637 [cm.sup.-1] arc attributed to associated water adsorbed on the external surface. (21) Important changes in the spectra were observed after thermal and acid treatment of the (k) sample. The bands at 3692 and 3623 [cm.sup.-1] were considerably diminished and they disappeared altogether after calcination and acid leaching (Figs. 3b and 3c). The Si-O-Si parallel stretching band at 1031 [cm.sup.-1] disappeared when the sample was subjected to thermal and acid treatment, while the band at higher frequency (1098 [cm.sup.-1]) corresponding to the Si-O-Si perpendicular stretching band appeared. The band at 1098 [cm.sup.-1] is assigned to amorphous Si[O.sub.2]. (22) Loss in crystalline order through chemical bond breakage and bending, produced a final solid with a high silica gel-like content. (23,24) The presence of a band at 907 [cm.sup.-1] is generally considered as evidence for a three-dimensional amorphous silica phase.

Morphology

SEM was used to probe the changes in the morphological features of original kaolinite (K) after calcinations and dealumination (Fig. 4).

Figure 4a clearly shows kaolinite books of varying sizes with low aspect (crystal width to thickness) ratios. Individual platelets are noticeable. Some pseudo-hexagonal edges on kaolinite platelets are observed and some have rough edges. The surfaces of the kaolinite flakes show that they are a composite of smaller particles <600 nm across. Some of the stacks show remnants of euhedral developed edges but most are somehow ragged. The basal surfaces of these stacks are rough and give the appearance of degradation from an interlocking mosaic of kaolin "crystallites." Small individual platelets (below 700 nm in diameter) are the cause of this decay and they also differ in morphology. Some of them display the pseudohexagonal shape but others are irregular.

The change in the morphological features of kaolinite (K) after calcinations and dealumination is shown in Fig. 4b. A significant reduction in the sizes of the kaolinite plates upon treatment was observed. However, the plate-like particle shapes present in the original kaolinite sample were preserved after acid leaching, as previously reported by Madhusoodana et al. (25) According to Mackenzie et al., (26) this plate-like particle shape is one of the unique features of mesoporous silica derived from kaolinite, and differs from the equiaxial particle shapes of materials from other silica sources. The white rims of Si[O.sub.2], which appear both around and inside the crystals, suggest that the amorphous Si[O.sub.2] is retained, despite the dissolution of metakaolinite in hydrochloric acid. (27)

Optical properties of the modified kaolinite pigments

In order to quantify the effectiveness of treatment methods in this study, the brightness and the color values (L, a, and b) were measured. Brightness is the percentage reflection of light at a wavelength 457 nm. The Lab system based on the idea of color opposite has been developed to give a better representation. The L is a measure of lightness/darkness and varies from 100 for perfect white to 0 for absolute black. The coordinates a and b define the chromaticity plane whose center is neutral or gray. Coordinate a indicates red/green color (the more positive the value of a, the greater the reddishness, and the more negative, the more greenishness). The yellow/blue shade is represented by b (the positive the value gives yellowness and the negative corresponds to blueishness. (11) The optical properties of metakaolinite (Cal K) and dealuminated metakaolinite pigments in comparison with K, KF, and KF/d pigments are illustrated in Table 3. The low brightness of the K sample is attributed to mineral impurities associated with kaolinite in terms of titanium dioxide and iron oxide. The brightness significantly improved during these processes where the [Fe.sub.2][O.sub.3] content was reduced from 0.38% to 0.25%, indicating that part of the iron in the sample is "free" and leachable. The brightness of the bleached kaolinite fraction <2 [micro]m (KF/d) rose up to 89.42% compared with 83.63% and 87.0% for the kaolinite (K) and the kaolinite fraction <2 [micro]m (KF), respectively. The iron removal showed that -35% of the total iron in the clay is "free" in nature, and the rest is present in the structure of either kaolinite or the ancillary mineral (mica or titania). In addition, the lightness (L) also improved and reached to 91.33%. The a and b values had brought down in the order KF/d < KF < K after the sedimentation and chemical bleaching processes which reduced the titanium dioxide and iron oxide contents as shown previously in Table 2. Metakaolinite (Cal K) has higher brightness and lightness than KF. The calcination process increased the brightness and decreased the color values, giving an overall improvement in optical properties. The percentage increase in brightness and lightness reached 11.3% and 5.3%, respectively. The bleaching of metakaolinite (Cal K/d) further reduced the iron content. The values of a and b have brought down in the order Cal K/d < Cal K < KF. The analysis of dealuminated metakaolinite samples indicated that [Al.sub.2][O.sub.3], [Fe.sub.2][O.sub.3], and Ti[O.sub.2] decreased (Table 2) upon increasing the concentration of acid. The brightness of Cal K/2 N HCl and Cal K/4 N HC1 increased to 93.20% and 93.57%, respectively (Table 3). However, the bleaching of dealuminated metakaolinite reduced the iron percentage, causing an increase in brightness up to 93.95% and 94.05% with a percentage increase of around 12.4% for Cal K/2 N HCl/d and Cal K/4 N HCl/d, respectively. The lightness also increased and reached 94.93 and 94.97 for Cal K/2 N HCl/d and Cal K/4 N HCl/d, respectively. The a and b values of these samples also decreased and a showed a negative value. This significant decrease in reddishness can be attributed to the removal of some free iron oxide impurities along with the titanoferrous minerals.

Coated paper specifications

Roughness

Roughness is an indirect measurement of paper smoothness. It is an important parameter for the writing and printing process. It concerns the surface contour of paper and it should be low to attain good printing properties. Figure 5 represents the effect of calcination and dealunrination of the El Tieh kaolinite on coated paper roughness before and after calendaring in comparison with the kaolinite fraction <2 [micro]m (KF) and the commercial one. The results showed that calcination led to a substantial increase in coated paper roughness. The calcination process irreversibly converts the kaolinite into a material called "metakaolinite" and causes the kaolin particles to stick or fuse together into porous aggregates and results in a rough surface. (28) The figure also shows that KF has the lowest roughness compared with metakaolinite and commercial pigment. Further dealumination of metakaolinite and bleaching caused a slight increase in coated paper roughness. Calendaring reduced the roughness of all coated paper samples. This can be attributed to the compression effect of calendaring which orients and levels off the disordered kaolinite plates; as a result, the micro roughness of the coated surface is reduced. (29)

Air permeance

Air permeance (porosity) is a measure of the extent to which a paper surface will allow the penetration of a gas or liquid such as air or ink through its surface. It is a function of various stages of the paper-making process. Surface sizing and coating work to seal surface fiber, reducing the paper porosity. The end use applications of paper require different porosity levels. Paper used in packaging should have high porosity for faster filling, less breakage, and high performance. Printing processes require papers of different porosities, e.g., high-speed web offset printing for newspaper printing requires highly porous paper for rapid ink absorption, while sheetfeed offset printing often requires low porous paper to promote proper ink drying and to increase ink gloss. In our work, calcined kaolinite and its modified pigments have higher air permeance than KF and the commercial sample (Fig. 6). This may be attributed to the porous structure created by calcined kaolinite. (28) The calendaring process has little effect on the air permeance of the prepared coated papers.

Brightness

Coated paper brightness is a useful parameter for controlling the appearance of manufactured paper. It is the most important selling feature. Figure 7 reveals that calcination followed by dealumination and bleaching significantly enhanced the brightness of coated paper compared with original and commercial kaolinite. The increase reached about 12.3% for the (Cal K) sample. Calcination and acid leaching significantly reduced the [Fe.sub.2][O.sub.3] and Ti[O.sub.2] contents as previously mentioned in Table 2. In addition, the calcination process creates a porous structure, i.e., increases air permeance (Fig. 6), leading to improved the brightness and light scattering. (28) The dealumination of the calcined kaolinite has no further effect on the coated paper brightness.

Opacity

Opacity is important to prevent the appearance of the printed text on the reverse side of the paper sheet. It is directly linked to light scattering within the coating structure. Metakaolinite and its modified pigments have higher opacity than the original kaolinite and the commercial sample, as shown in Fig. 8. On continued heating of kaolinite up to 900[degrees]C, the particles begin to fuse together, forming aggregates; the final result is a large number of kaolin-air interfaces and relatively high internal pore volume, leading to increased light scattering and opacifying properties. (28,30) There is no significant difference in coated paper opacity after dealumination of calcined kaolinite.

Gloss

Gloss is the specularly and diffusely reflected light component measurement against a known standard. Gloss is important for magazine advertisement printing. The level of gloss desired is very dependent on the end use of the paper. Figure 9 shows that calcined kaolinite and its modified pigments have higher gloss than original kaolinite K, but it is lower than that of CK and KF. Decreasing gloss of calcined kaolinite and its modified pigments is due to the high value of roughness, as shown in Fig. 5. A slight decrease in gloss was obtained upon using the dealuminated calcined kaolinite. Calendaring slightly enhanced the gloss of all samples.

Mechanical properties of coated paper

The stability of the strength and mechanical properties of paper has become the most useful indicator for its permanence or chemical stability. The mechanical properties of the coated paper having the modified kaolinite pigments in terms of burst strength, tensile strength, stretch, and tensile energy absorption are represented in Table 4. Metakaolinite and dealuminated metakaolinite pigments exhibited higher burst strength than the original kaolinite and kaolinite fraction--the increase reached 8%. Further bleaching of this kaolinite has no effect. Calcination and dealumination of Egyptian kaolinite significantly increased the tensile strength, stretch, and tensile energy absorption of coated paper having these pigments compared with the CK, K, and KF samples. The increase reached 17.5%, 10.5%, and 19.7%, respectively.

Dealumination of the calcined kaolinite did not show any significant effect on the mechanical properties compared to the calcined ones.

Print quality

Table 4 shows the effect of different calcined kaolinite pigments on print density. Calcined kaolinite and its modified pigments decreased the print density of coated paper compared with CK and KF, but it is still higher than that of the original kaolinite. The lower values of print density may be due to the impact of the high paper surface roughness of these pigments, as shown in Fig. 5. The calcined kaolinite and its modified pigments have higher print gloss than the original kaolinite, but it is lower than KF and CK, as shown in Table 4. According to Preston et al., (31) rougher substrates will have both lower paper gloss and generally lower print gloss. Low print gloss is an indication of a rough print surface, and this often means that the print density is also low. (32)

Conclusion

Egyptian kaolinite was successfully modified using sedimentation, chemical bleaching, calcination, and dealumination methods. The iron and titanium were reduced chemically via sodium dithionite from 0.41% to 0.25% and 2.2% to 2.00%, respectively. The calcination process increased the brightness and decreased the color values giving an overall improvement in optical properties--the increase in brightness reached [approximately equal to]12.4% of leached and bleached calcined kaolinite. The results revealed that calcination, acid leaching, and bleaching of calcined kaolinite did not improve coated paper roughness, while air permeability was improved compared with a commercial pigment. The optical properties of coated paper, as brightness and opacity, were highly increased, while coated paper gloss decreased. Decrease in gloss is correlated to a high value of coated paper roughness. The modified Egyptian kaolinite (calcined, leached, and bleached) significantly increased the tensile strength, stretch, and tensile energy absorption of coated paper having these pigments compared with the CK, K, and KF samples. The increase reached 17.5%, 10.5%, and 19.7%, respectively. In addition, the kaolinite fraction <2 [micro]m highly improved the printability of coated paper (print density and print gloss) more than calcined kaolinite and its modified pigments. Dealumination of calcined kaolinite did not show any further change in all coated paper properties compared to the calcined ones.

DOI 10.1007/s11998-015-9672-5

S. El-Sherbiny ([mail]), F. A. Morsy

Chemistry Department, Faculty of Science, Helwan University, Cairo, Egypt

e-mail: samyadd@yahoo.com; samya.elsherbiny@gmail.com

M. S. Hassan, H. F. Mohamed

Central Metallurgical R&D Institute CMRDI, Helwan, Cairo 11421, Egypt

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Table 1: Original and modified Egyptian kaolinite pigments

Methods                 Pigment samples                Code

                        Original kaolinite             K
Sedimentation process   Kaolinite fraction <2 micro]   KF
Thermal treatment       Calcined kaolinite             Cal K
                          (metakaolinite)
Acid leaching           Leached calcined kaolinite/2   Cal K/2 N HCl
                          N HCl
                        Leached calcined kaolinite/4   Cal K/4 N HCl
                          N HCl
Chemical bleaching      Bleached kaolinite fraction    KF/d
using sod. dithionite     <2 [micro]m
                        Bleached calcined kaolinite    Cal K/d
                        Bleached leached calcined      Cal K/2 N HCl/d
                          kaolinite/2 N HCl
                        Bleached leached calcined      Cal K/4 N HCl/d
                          kaolinite/4 N HCl

Table 2: XRF measurements of Egyptian modified pigments

Oxides                  K        KF      KF/d    Cal K    Cal K/d

Si[O.sub.2]           47.87    46.25    46.25    56.48     56.48
[Al.sub.2][O.sub.3]   36.78    39.96    39.96    37.10     37.10
Ti[O.sub.2]            2.20     2.16     2.00     2.00      2.10
[Fe.sub.2][O.sub.3]    0.41     0.38     0.25     0.24      0.12
L.O.I                 13.35    13.92    13.91     0.78      0.71

Oxides                Cal K/2   Cal K/4   Cal K/2   Cal K/4
                       N HCl     N HCl    N HCl/d   N HCl/d

Si[O.sub.2]            66.48     76.48     66.48     76.48
[Al.sub.2][O.sub.3]    26.10     17.20     26.87     17.10
Ti[O.sub.2]             1.81      1.76     1.78      1.45
[Fe.sub.2][O.sub.3]     0.09      0.05     0.023     0.012
L.O.I                   0.68      0.58     0.46      0.23

Table 3: Optical properties of modified pigments

Pigment samples   ISO brightness     L       a       b

K                     83.63        89.79    0.40    5.88
KF                    87.00        90.55    0.18    5.00
KF/d                  89.42        91.33    0.11    4.68
Cal K                 92.30        93.08    0.09    2.49
Cal K/d               93.05        94.17    0.08    2.11
Cal K/2 N HCl         93.20        94.56    0.06    2.02
Cal K/4 N HCl         93.57        94.63    0.21    1.86
Cal K/2 N HCl/d       93.95        94.93   -0.39    1.53
Cal K/4 N HCl/d       94.05        94.97   -0.24    1.48

Table 4: Effect of calcined kaolinite and its modified pigments
on mechanical properties and print quality of coated paper

Samples            Burst     Tensile    Stretching
                  strength   strength      (%)
                   (kPa)     (kN/mm)

CK                 208         5.04        1.90
K                  179.5       5.08        1.82
KF                 189.6       5.16        1.86
CaL K              193.5       5.97        2.01
CaL K/d            193.5       5.95        2.01
Cal K/2 N HCl      193.4       5.94        2.01
Cal K/4 N HCl      193.3       5.92        2.01
Cal K/2 N HCl/d    193.4       5.93        2.01
Cal K/4 N HCl/d    193.4       5.92        2.01

Samples            Tensile     Print    Print
                    energy     gloss   density
                  absorption
                    (J/m)

CK                  66.00       16      0.88
K                   58.50        3      0.35
KF                  61.80       15      1.29
CaL K               70.03       10      0.71
CaL K/d             70.57       10      0.73
Cal K/2 N HCl       70.43        8      0.44
Cal K/4 N HCl       70.56        8      0.56
Cal K/2 N HCl/d     70.56        9      0.66
Cal K/4 N HCl/d     70.73        9.4    0.69


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Author:Sherbiny, Samya El-; Morsy, Fatma A.; Hassan, Mervat S.; Mohamed, Heba F.
Publication:Journal of Coatings Technology and Research
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Geographic Code:1USA
Date:Jul 1, 2015
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