Preparation of Composite Powder and Properties by Surface Modification of Inorganic Pigments for Papermaking

Abstract

In this study, functional composite-pigment was prepared by a dry impact-blending method, using the inorganic materials used for coating of titanium dioxide, GCC, clay, and talc. After considering the particle size of inorganic pigment, the selected core powder and fine powder were mixed at a suitable mixing ratio. When powder was electrified, their fine particles could adhere by static electricity on the larger ones termed ‘core particles’, and then form an ordered mixture. During the surface modification of pigments, the energy caused by fine powder adhering to core particles was measured, and utilized in examining the efficiency of the change of particle size surface modification. Afterward, these were susceptible to being composited by impact force, resulting from very high-speed air circulation. After putting these surface modified pigments in water, their static electricity properties were measured by particle charge detector and Zeta potential tester. From these results, it was revealed that the elemental powder properties of papermaking inorganic pigment could be improved.

1. Introduction

Particle coating or surface modification technology has been used for the synthesis of composite materials with desired end-use properties in many industries, including pharmaceuticals, food, cosmetics, ceramics, electronics and special chemicals. At present, most commercial powder coatings are done by wet coating methods such as solgel processes, wet chemical deposition, spray coating, dip coating, spinning disc coaters and a variety of fluidized bed coaters. (Cho and Min, 2000; Cho et al., 2001b) Wet particle coating is used primarily to form a barrier or film between the large particle and its environment. The coating material usually consists of a solute dissolved in an organic solvent or an aqueous suspension of the solute. The organic solvent is usually highly volatile (VOC).

VOCs have been implicated as a major precursor in the production of photochemical smog, which causes atmospheric haze, eye irritation and respiratory problems and even some VOCs are carcinogenic. These environmental drawbacks of wet coating methods have forced researchers in the powder technology field to find alternative methods for coating of powders.

Dry particle coating is a relatively new and alternative approach to wet coating methods and it has drawn attention of many researchers (Pfeffer et al., 2001).

Mechanical forces (mechanical impact, shearing etc.) are used in order to attach submicron-sized small particles onto relatively larger micron-sized particles, without using any solvents, binders, or even water. Since the size of the small particles is minute, Van der Waals interactions are strong enough to keep them firmly attached to the large particles. Depending on the choice of equipment, operating conditions, and particle properties, either a discrete or continuous coating of small particles can be obtained.

However, the domestic technological level is a real condition that is receiving basic study at some university universities and research institutes at present. Recently, the trend that demands multi-functional powder of high added value is continuously increasing in our industrial society. So, to invent effective future industry, intensive research and development for vanguard multi-functional material development that can break through the limit of existing powder properties is being conducted (Kim and Lee, 2001).

This research seeks to apply inorganic pigment for paper coating, to confirm the possibility that has been raised in several studies about the field of paper application of a surface modification technology, and evaluate the efficiency of such a surface modification technology. Pigment that occupies most of the component parts of coating color causes great effect on the properties of the physical and optical properties of the coated paper, and the printability. This pigment influences the electronic properties of particles size and shape in the coating color, according to smectic formation and combination form.

Usually, titanium dioxide particle, whose refractive index is high at the surface of inorganic composite particles, has been used for the surface modification of the white pigment itself, and to increase the scattering degree. The particle sizes of inorganic pigments for paper applications of clay, talc, GCC, and titanium dioxide were measured, then selected core particles and fine particles were mixed at a desirable mixing ratio. When electrified, the fine particles could adhere to the larger ones, due to static electricity. These powders were composited by impact force, resulting from very high-speed air circulation. During the surface modifying of pigments, the energy caused by the core powder’s adherence to fine powder was measured, and the surface modifying efficiency according to the change of particle size was also investigated (Zhao and Yang, 2003).

Finally, design and basic data are presented for the invention of composition particles that can give various abilities, achieving the objective of this research.

2. Materials and Experiments

2.1 Background on dry particle coating

Dry particle coating to change the surface properties or functionality of powders is a very important process for many industries. Typical applications include the modification of flowability, wettability, solubility, dispersibility etc. for particle properties. In dry particle coating processes, materials with relatively large particle size (core particles, 1–500 um) are mechanically coated with small particles (fine particles, 0.1–50 um), in order to create new functionality, or to improve their initial characteristics (Honda et al., 1987, 1989, 1992; Honda, 1992). Since the size of the small particles is minute, Van der Waals interactions are strong enough to keep them firmly attached to the core particles. Thus, either a discrete or continuous coating of fine particles can be achieved, depending on a variety of operating conditions, including processing time, rotation speed, weight fraction of fine to large particles, and particle properties. Multiple layering is possible, when using different coating materials, and processing them one after another (Hersey, 1975, 1977). In a typical dry particle coating process, core particles are mechanically mixed with fine cohesive powder. At the beginning of the operation, the fine aggregates adhere to the core particles in their immediate vicinity. When a core particle carrying fines adheres, its surface collides with a non-coated particle, and it transfers part of its fine particles to the latter. By friction and collision between the particles, the agglomerates of fine particles are gradually dispersed onto the surface of the carriers, which results in an increase in the coated surface area (Tatami et al., 2010).

Afterward, the mechano-fusion system greatly modifies the surface texture, by giving a high level of mechanical energy to the particles. Local melting and a partial or total penetration of the fine particle component into the body of the larger particles take place (Cho et al., 2001b). The dispersion of fine particles actually occurs from the earlier stages of the process. The dispersion rate and the degree to which the agglomerates are broken up depend strongly on the mechanical energy input, and therefore, on the type of mixer used.

2.2 Order mixture

The subject of dry particle coating is closely related to the subject of dry mixing of powders. Theoretically, a binary mixture process should mix two different species of powders, so that any sample taken from the mixture would contain the same proportion of the two powders (Kim and Lee, 2001). This is very hard to achieve in practice, when the powders are either cohesive, or they are very different in particle size. When powders are cohesive, they naturally form agglomerates, and mixing of these powders requires preliminary breaking up of the agglomerates. When the powders are very different in particle size, there is an increased tendency for segregation as the particle size becomes larger. However, when the particles to be mixed are very different in particle size, the smaller particles tend to adhere on the surface of the core particles (Lee and Seul, 2002). The adhesion force between the smaller particle and the larger particle is greater than the weight of the smaller particle, so the detachment of small particles from the surface of larger particles is difficult (Honda et al., 1988, 1991).

The advantage of an ordered mixture is that it provides a much better degree of homogeneity than random mixing, as long as the particle size distribution of the larger particles is not too wide. So, in terms of subsequent segregation, ordered mixtures are more stable than random mixtures. It was also shown by early researchers that having a very wide size distribution of the larger size particles may lead to “ordered unit segregation”, because of the nature of the poly-disperse core particles (Yeung and Hersey, 1979). Fig. 1 shows the concept of ordered mixture.

Fig. 1

Model flow of surface modification process.

2.3 Surface modification system

The surface modification systems, which make it possible to combine fine particles in dry condition, use physical methods that are classified into powder mixer and surface modification system. The fine particle and the core particle are fed into the powder mixer, where the two particles are mixed and dispersed to form an “Ordered (interactive) Mixture”. (Ouabbas et al., 2009; Yip and Hersey, 1977a, 1977b, 1977c) The order mixture is weighed, before being fed into the surface modification system, the main unit of the system. The surface modification system comprises a high-speed rotor, stator, and cycle tube. The processed material fed to the surface modification system is dispersed, and is repeatedly subject to mechanical effects of mainly impact force, compression, friction and shearing force, as well as the particles mutual effect, to uniformly embed film or sphere in a short time (Fig. 2). (Yoshihara and Pieper, 1999)

Fig. 2

Mechanism of surface modification.

2.4 Surface modification of the materials

This experiment used 5 kinds of inorganic pigment that are usually used for making paper by particle: two GCC, No. 1 clay, No. 2 clay and talc, which are used for surface modification of inorganic pigment. Table 1 displays the use of titanium dioxide (Anatase 80 %, Rutile 20 %) by fine particle, and the basic properties of matter of these pigments. (Pertti, 1985)

Table 1 Properties of materials

Division	Powder	Species	Shape	Chemical components	Particle Size (um)	Specific Gravity (−)
Core particle	GCCa	No. 1 GCC (Omya)	Amorphous	CaCO3b	1	2.93
		No. 2 GCC (Omya)			2
	clay	No. 1 clay (Engel Hard)	Plate	Al2SiO3	2	2.60
		No. 2 clay (Engel Hard)			4
	talc	Coating talc (Hanseng)	Amorphous	MgSiO2	6	2.80
Fine particle	titanium dioxide	P-25 (Degusa)	Rounded	TiO2	0.025	3.80

a  Ground Calcium Carbonate,

b  Chemical components

2.5 Surface modification of pigments

The added quantity for surface modification was calculated by retrieving the ratio between the specific gravity, and the particles size of core particles and fine particles used in the experiments. In particular, the mixing ratio between the mass and volume ratio of repeated experiments measured the efficiency of surface modification (Table 2). The required additional quantity calculated through the equation, is spouted to the surface modification system (NHS-0 model), for surface modification by titanium dioxide for inorganic pigments. (Kangwantrakool et al., 2001)

Table 2 Core particle and TiO₂’s compound ratio

Mass ratio	Core Particle	Fine Particle
No. 1 GCC: titanium dioxide	92.4	7.6
No. 2 GCC: titanium dioxide	96.2	3.8
No. 1 clay: titanium dioxide	95.7	4.3
No. 2 clay: titanium dioxide	97.8	2.2
talc: titanium dioxide	99.1	0.9
Volume ratio	Core Particle	Fine Particle
No. 1 GCC: titanium dioxide	94.1	5.9
No. 2 GCC: titanium dioxide	97.0	3.0
No. 1 clay: titanium dioxide	97.0	3.0
No. 2 clay: titanium dioxide	98.5	1.5
talc: titanium dioxide	99.3	0.7

The surface modification operating conditions used in the experiments were pre-treatment by powder mixer (1000 rpm, 1 min), and then modification by surface modification system (9,000 rmp, 4 min).

2.6 Measurement of adsorption energy

10 g of core particle GCC, clay, talc and fine particle titanium dioxide dispersed powder were added to each device of the electrostatic charge measuring system (TR-8651, SANKYO POWTECH). Frictional force and rotational force were added to rotate the rotor for 1 minute at 1200 rpm. After moving to the Faraday well using the suction force of the vacuum pump, each pigment was then measured by charge amount electrometer and quantity charged side container. At this time, the impact energy adsorption when the fine particle was modified further to the core particle surface was measured by using the Coulomb force and the Van der Waals charge (Ohashi et al., 1995).

2.7 Measurement of Zeta potential

A 1 % mixed solution was prepared when the surface modified papermaking pigment was charged surface modified pigment in distilled water, to examine whether the change in the electrical properties of the pigment was surface modification. After mixing for 5 minutes with a stirrer, the Zeta potential change of each pH of the mixed solution was measured by Zeta potential meter (ZETASIZER 3000HS, MALVERN).

2.8 Measurement of charge density

The charge density was measured by charge density meter (PCD 03 pH-S, HERCULES) to make a 1 % solution mixed with distilled water to a surface modified pigment.

3. Results and Discussions

3.1 Microstructure

The variations in respect of the titanium dioxide on the surface of inorganic pigments used as core particle via surface modifications were measured by FE-SEM (S-4300, HITACHI). As Figs. 3–5 show, the surface modification of treated pigment was accomplished better than that of primary sample, and titanium dioxide was overall well distributed (Yeung and Hersey, 1979).

Fig. 3

FE-SEM images of GCC as shown in the (a) GCC pigment and (b) modified with titanium dioxide.

Fig. 4

FE-SEM images of clay as shown in the (a) clay pigment and (b) modified with titanium dioxide.

Fig. 5

FE-SEM images of talc as shown in the (a) talc pigment and (b) modified with titanium dioxide.

In particular, the plate-like particle shape of the clay and talc surface of the surface-modified titanium dioxide was evenly distributed, and the surface modification effect was confirmed to be excellent. The basic model of the surface modification in the surface modification system has a spherical shape particle, but it appears that the modification effect is excellent, even in the case of the clay and talc as a plate (Mujumdar et al., 2004). The surface modification was performed in terms of mass ratio and volume ratio separately. Equivalent surface modification effect on the particle inorganic pigment for the paper surface-modified by titanium dioxide was found under scanning electron microscopy (FE-SEM) in the comparison of the measurement such as mass ratio and volume ratio.

Both of them showed uniform surface modification. And therefore all experiments were carried out by using the mass ratio.

3.2 Brightness and scattering index

Figs. 6 and 7 shows the degree of brightness and scattering Index of GCC, clay, talc, and titanium dioxide mixed with the sample, and the mass ratio of the sample surface modification treatment. The surface was modified in all samples, even if the brightness of the scattering index sample was larger than that of the simple enhanced mixing.

Fig. 6

Brightness contrast by Chromameter CT-300 (Mixing & modification).

Fig. 7

Scattering index by Elrepho 3300 (Mixing & modification).

Separate modification of each sample showed the increased scattering degree compared to the modification of mixed one because the number of times being refracted increases and the refraction angle is different when light goes through the pigments whose fine particles on the core particles have been modified (Cho et al., 2001a; Willets et al., 1958).

3.3 Adsorption energy

Table 3 shows the charge amount of the pigment in each, in order to measure the energy at which the fine particle is adsorbed to the core particle. There is a tendency of the particle charge to increase, as the particle size of the core particle is increased. In the case of supplying frictional force and rotational force to the core particle surface, it appeared that the amount of charge value increases, as the particle diameter increases (Yang et al., 2005). Fig. 8 shows a graph of the measurements of the charges of the Van der Waals force and the law of Coulomb force calculations. The core particle and fine particle adsorption energy of the particles were found to increase, as the ratio between the particles diameters increased. This is because of receiving a larger impact force of Van der Waals forces than the Coulomb, when friction occurred by mixing the two particles. The adsorption energy is large, and is better than the surface modification, which was confirmed through previous research (Ishizaka et al., 1992).

Table 3 Charge of a particle calculated from mean particle size, density and specific charge

Division	Particle Size (um)	Density (−)	Specific charge (Cg−1)	Value of −Q or q (C)
GCC	4	1.38	−2.86 × 10−7	9.13 × 10−19
clay	2	1.73	−1.85 × 10−7	2.13 × 10−20
talc	8	1.14	−8.79 × 10−7	7.14 × 10−16
titanium dioxide	0.025	2.05	+2.75 × 10−7	8.33 × 10−21

Fig. 8

Calculation results of the total energy of the monolayer particle coated system. (a) Curve GCC modified with titanium dioxide (b) Curve clay modified with titanium dioxide (c) Curve talc modified with titanium dioxide.

3.4 Zeta potential

The zeta potential was measured, to confirm the change in electrical properties in the solution of the surface modified pigment. In Fig. 9, the graph shows the Zeta potential of inorganic pigments. The zeta potential of titanium dioxide measured higher than clay and talc at isoelectric point 7. It is known that the relative aggregation propensity is strong, cohesive and pulp fiber is improved in clay and talc, with isoelectric point lower than titanium dioxide. The surface-modified talc and clay increased the isoelectric point of the Zeta potential. These results indicate that talc and clay particles reformed with titanium dioxide on the surface showed the characteristics of titanium dioxide particles. (Lee and Jeong, 2000) It appears that when preparing the coating pigment, the viscosity and fluidity of the pigment can be controlled, and when used in the filler, the change in quietness of electrical properties modulates the aggregation propensity of paper dust.

Fig. 9

Measurement of Zeta potential.

• (a) Non-modified paper pigments
• (b) clay modified with titanium dioxide
• (c) talc modified with titanium dioxide

3.5 Charge density

Fig. 10 shows the particle charge density of the surface-modified pigment (PCD). It could be seen that pigments have a charge amount, by supplying a rotational force to the frictional force resisting the dry particles, in the form that was affected by the electrical properties in aqueous form.

Fig. 10

PCD of mixing & modification pigments.

The charge density of GCC is the highest in the pigments. It is assumed that higher charge density can be accomplished as the contact area and the number of times being fractionated between the same particle increase when fractional and rotational forces apply to the spherical shape particle.

4. Conclusions

The inorganic pigments for paper manufacturing were mixed with titanium dioxide at certain proportions, and the following conclusions were drawn through consideration of the results after modifying the surface of the mixture using the surface modification system

There were no differences between the mixing ratios on a mass basis or on a volume basis of the Core Particles to Fine Particles as confirmed through scanning electron microscopy (SEM) of the surface-modified organic pigment, and the same effect of surface modification was shown.

The adsorption energy needed between the particles in the pretreatment step of the surface modification of inorganic pigment for paper manufacturing increased as the ratio of the particle size of the Core particles to Fine particles became larger. In addition, when measuring the adsorption energy, the particle size was observed to have a great influence on the Van der Waals force and the Coulomb’s force acting between particles. When surface modification with titanium dioxide was performed on clay, the whiteness showed more excellent results than the sample which was simply mixed. These results show the effect of the surface modification with titanium dioxide for which the whiteness is relatively higher than clay; however, it was considered that during the process of surface modification, the whiteness of the sample with surface-modification was further improved because of higher uniformity of the particle distribution and reduction in particle size through the spheroidization treatment of the particles in the irregular hexagonal form. In addition, the whiteness of the GCC and talc which were used to modify the surface with titanium dioxide was also improved, as in the surface modification of the clay. The reason is because the optical properties of the modified pigment are shown through the fine particles, as they are modified on the surface of the core particles.

When the Zeta potential was measured in order to determine the change in electrostatic properties, the cohesion tendency according to pH was reduced, and a change in the isoelectric point was shown in all samples which underwent surface modification with titanium dioxide. In the case of PCD measurement, the measured values of the surface-modified samples were higher than the simply mixed samples, due to increase of the quantity of electric charge generated at the time of the collision and friction between the core particles and the fine particles during the surface modification.

When the surface of the inorganic pigment for paper manufacturing was modified with titanium dioxide, the effects of the surface modification could be confirmed as a change in optical and electrostatic properties. It is considered that these results will provide a basis to compensate for and improve the problems in the process which can occur at the time of manufacturing the coating color for the coating on the surface of paper, as well as allow the addition of other inorganic pigments on the basis of these results in the field.

Acknowledgements

This study is supported by Kangwon National University.

Author’s short biography

Jun Hyung Cho

Jun-Hyung Cho is a professor in the Department of Paper Science & Engineering at Kangwon National University in Republic of Korea. He received his Master Degree in Engineering and Ph.D. degree in Chemical Engineering from Nagoya University in Japan from 1984 to 1989.

In 1996, he went for a one year visiting scholar at Wisconsin University in United State and he took another year of journey as a visiting scholar at North East Forest University in China in 2012. His research and consulting interests are in the area of pigment for Papermaking and Environment-friendly Material.

Yong Won Lee

Yong-Won Lee obtained the B.Eng. and M.Eng., degrees in Paper science & Engineering from Kangwon National University in Republic of Korea. He is presently a Ph.D student and is working at Han-kuk Paper Company R&D team. He has studied to develop the new functional pigments with environmentally-friendliness for paper-making industries. Recently, functional printing paper of the development pigments using surface modification method for special papermaking.

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