Efficacy of dispersion of magnesium silicate (talc) in papermaking
S1061
Table 1
Physico-chemical and optical properties of talc fillers of different particle size distribution.
Particular
Talc-1
Talc-2
Talc-3
Talc-4
Talc-5
ISO brightness, %
92.5
92.7
93.7
94.1
93.2
Ionic nature
Anionic
Anionic
Anionic
Anionic
Anionic
Colloidal charge demand, l eq/g
+1.1 501 9.0
+1.6 373 9.2
+2.0 455 9.3
+2.1 410 9.1
+2.1 492 9.1
Zeta potential, mV
pH
Particle shape
Lamellar
Lamellar
Lamellar
Lamellar
Lamellar
2.4. Handsheet preparation and testing
2.2. Characteristics of talc fillers
The fillers were dispersed in water to 10% (w/v) slurry prior to the addition in the refined pulp stock of 1% consistency (w/v). The paper handsheets of 60 g/m 2 with target ash content of around 15% were prepared as per the Tappi test method T 205 sp-02. The ash content in paper was determined at 525 C as per the Tappi test method T 211 om-93. The ash con- tent and first pass ash retention (FPAR) were calculated with the following formula: Ash content in paper ; % ¼
The fillers were characterized for optical and physico-chemical characteristics; brightness, colloidal and surface charge, parti- cle size distribution (PSD), and shape. The moisture free fillers were compacted in the dice with the help of arbor press. The filler dice was then used to measure the optical properties in brightness tester (Datacolor Spectraflash 300). Filler suspen- sion (10% w/v) was filtered through a 300 l m screen and the pH of the filtrate was measured with the help of a pH meter. The colloidal charge or ionic behavior of 10% (w/v) slurry of fillers was examined on Mutek particle charge detector (PCD 03 pH). The surface charge on fillers was determined in the form of zeta potential on Mutek system zeta potential meter (SZP 06). About 500 ml filler sample (10% w/v) was ta- ken and mixed thoroughly before measurement. The PSD of the fillers was measured using Laser scattering particle size dis- tribution analyzer (Horiba LA950S2). The talc fillers were wet- ted with ethanol and then dispersed in deionized water to make 10% (w/v) slurry. In the case of Talc-3 filler, the particle size of filler was also measured after dispersing it in different manners i.e. in water only, with wetting agent, with dispersing agent, and with both wetting and dispersing agent. The measurement conditions in the analyzer were kept constant for all talc fillers. The particle shape of the fillers was determined by X-ray dif- fraction (Bruker AXS, D8 Advance, Switzerland) using Cu K a radiation. The micrographs of fillers were taken on Field emission scanning electron microscope (Quanta, FEI, Czech Republic).
o : d : weight of ash in paper ð g Þ o : d : weight of handsheet ð g Þ 100
ð 1 Þ
Ash in paper ð % Þ Filler added based on pulp and filler ð % Þ 100
FPAR ; % ¼
ð 2 Þ The light scattering coefficient of paper was measured on brightness tester (Datacolor Spectraflash 300) as per the Tappi test method T 519 om-02. The scattering coefficient of filler was calculated from the following formula: S sheet ¼ S unfilled sheet ð 1 L Þþ LS filler ð 3 Þ where S is the light scattering coefficient and L is the filler loading amount.
3. Results and discussion
2.3. Wetting and dispersion of talc filler
3.1. Physico-chemical and optical properties of talc fillers
Initially, the effect of agitation time on dispersion of talc fil- ler was studied. The talc filler was dispersed in deionized water (no wetting agent and dispersant) for different time periods of 30, 60, 90 and 120 min in an emulsifier at 2000 rpm speed. Secondly, the dosage of wetting agent and dispersant were optimized based upon the viscosity behavior of talc slurry. The talc filler was first diluted with deionized water at 50% solids (w/v). The slurry was agitated in high speed emulsifier for around 30 min. The wetting agent was added to the diluted filler slurry, agitated for around 5 min and the rheology of talc slurry was measured in terms of Brookfield viscosity. A graph between dosage of wetting agent/dispersant and viscosity of talc slurry was plotted to get the minima of viscosity. The dosage was selected based upon the lowest viscosity of talc slurry. The optimized dos- age of wetting agent and dispersant were then used together to completely disperse the talc filler.
The filler characteristics are important for the optical and structural developments in the paper matrix. All talc fillers were having almost comparable optical properties. They were anionic in nature which was indicated by their cationic colloi- dal charge demand and anionic zeta potential. The cationic charge demand was indirectly proportional to the particle size of talc filler. It was increased on decreasing the particle size, due to exposure of more oxide surfaces. All mineral fillers were alkaline in nature with a pH of around 9.0–9.2 (Table 1).
3.2. Particle size distribution and shape of talc fillers
The particle size distribution (PSD) and shape of fillers are the important factors responsible for the retention of filler and light scattering in paper. The PSD of Talc-1 was the broadest among all talc fillers followed by Talc-2, Talc-3, Talc-4 and
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