EFFECT OF KAOLIN ADDITION TO CALCIUM CARBONATE PRECOATS
Tony Hiorns, Tara Nesbitt
Imerys Paper Europe
Par Moor Centre
St. Austell
Cornwall
UK
PL24 2SQ
ABSTRACT
The use of engineered ground calcium carbonate (GCC) in precoating of woodfree paper is one way of achieving higher optical performance.  However, relative to a standard GCC, lower coating solids must be used.  The addition of an appropriate kaolin to a standard GCC can give similar optical performance
to engineered GCC alone but with benefits of higher runnable solids and a smoother, less absorbent coating layer.
INTRODUCTION
The use of high levels of calcium carbonate in precoating is well established, especially in woodfree paper grades.  In Europe, the easy availability and high brightness of ground calcium carbonate (GCC) means that some paper producers now use it as the sole precoat pigment.  The arrival of engineered GCC’s means that it is possible to get improvements in precoat smoothness and optical performance, whilst still using 100% GCC. However, the steeper particle size distributions of engineered GCC’s means that they have greater high shear viscosity levels and also a tendency to dewater quickly.  This can lead to coater runnability problems and results in the use of lower coating colour solids levels.
Kaolin has a relatively high particle aspect ratio.  This can result in good fibre coverage and also help to reduce coating colour dewatering.  When high and low aspect ratio pigments are blended together, synergistic effects can occur leading to a relatively open, disordered coating structure with higher than expected light scattering.  Therefore, it was proposed that a kaolin with the right particle
size and shape distribution could be used with a standard GCC and light scattering levels similar to an engineered GCC might be obtainable [1,2].  Also, the runnability of the standard GCC/Kaolin blend could be significantly better than the engineered GCC alone. EXPERIMENTAL DETAILS
Pigment Characterisation
Two ground calcium carbonates (GCC’s) were used as the base pigments.  One of the GCC’s has a standard or broader particle size distribution (psd) and the other was engineered to have a steeper psd (denoted s-GCC and e-GCC respectively).  A range of six kaolins was used, with a range of mean particle sizes, shapes and psd steepness.  The particle shape factor is an estimate of the mean aspect ratio (particle diameter/particle thickness) of the kaolin [3,4].  The two GCC’s have a very blocky shape and thus have no significant particle anisotropy.  The steepness of the psd is defined as d30/d70 x 100.
Table 1. Pigment properties
Pigment Powder Sedigraph Particle Size Data Particle B’ness < 2 µm < 1 µm < ½ µm < ¼ µm Mean Size Steepness Shape13460日元等于多少人民币
(ISO, %) (%) (%) (%) (%) (µm) Factor Standard GCC 93.3 75 48 27 15 0.99 29 x Engineered GCC 93.5 88 59 28 12 0.80 41 x Kaolin 1 89.2 79 56 34 15 0.82 30 21 Kaolin 2 88.0 83 67 46 21 0.60 29 19 Kaolin 3 86.0 85 62 39 18 0.69 32 72 Kaolin 4 79.4 54 35 21 10    1.74 20 69 Kaolin 5 83.0 65 46 30 14    1.14 22 63 Kaolin 6 82.4 72 50 30 17 0.97 25 61
Base Paper Characterisation
Three woodfree base papers were used.  Base 1 and 2 had similar surface roughness levels, but Base 1 was sized
and Base 2 was unsized.  Base 3 had been calendered on one side and hence the two sides had similar water absorbency levels but had different roughness levels.
Table 2. Base paper properties
Base Side Substance R o R o R∞R∞ B’ness Opacity Y’ness PPS Cobb “basis weight” 457 nm 550 nm 457 nm 550 nm ISO DIN DIN 1000 kPa  (g/m2) (%) (%) (%) (%) (%) (%) (%) (µm) (g/m2)
1    1 80 82.8 80.5 91.6 89.7 91.6 89.7 -0.9    6.8 20
2 91.5  -0.5 7.3
2    1 8
3 75.0 74.8 80.5 81.8 80.5 91.5    2.2    6.7 85
2 80.6  2.2 6.9
3    1 52 67.8 66.1 80.5 84.9 80.5 77.8 7.1 7.7 18
2 70.
3 68.8 81.8 86.0 81.8 79.9 7.0    3.6
(Note: All optical measurements made with no UV component by using a 400 nm cut-off filter.)
Coating Details
The formulations were selected to study the effect of adding 20% kaolin to a GCC precoat.  It was thought that adding significantly more kaolin than this amount would lead to an unacceptable loss in paper brightness and
that adding less than this would lead to only small effects.  A simple starch (Nylgum A45 starch)/latex
(Dow 945
SBR latex) binder combination was used throughout.  The pH of each formulation was adjusted to ~8.5 using a老山狙击手
4% NaOH solution.
Table 3.  Coating Formulations
Formulation    1 2 3 4 5 6 7 8 9 10 11 12 13 14 GCC 100
80 80 80 80 80 80
Standard
80 80 80 80 80 80 Engineered
GCC 100
1 20 20
Kaolin
2 20 20
Kaolin
3 20 20
Kaolin
Kaolin
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5 20 20 Kaolin
Kaolin
6 20 20
窦骁身高
Latex 8 8 8 8 8 8 8 8 8 8 8 8 8 8 SBR
Starch    4 4 4 4 4 4 4 4 4 4 4 4 4 4
A Helicoater TM was used at 800 m/min with a pond head and a 0.451mm thick blade at a blade holder angle of 45°.  One side of the three base papers was coated with a coat weight range of 6 to 14 g/m2.  Each formulation
was coated at the highest possible colour solids level that could be used without any runnability defects occurring.  To achieve the full coat weight range, it was sometime necessary to dilute the coating colour in order
to obtain the lower coat weights.  The low shear (Brookfield 100 rpm) and medium shear (Bohlin at 10,000 s-1) viscosities were measured.
Paper Testing
The coated papers were allowed to condition for 24 hours at 23°C and 50 % humidity before paper testing.  An Elrepho 3300 was used to measure the optical characteristics of the coated papers with a UV filter employed to remove any effects of optical brightening agents in the base paper.  Paper brightness (ISO), opacity (DIN) and yellowness (DIN) were measured.  The single sheet (R o) and stack of paper (R∞) brightness measurements were
used to calculate the Kubelka-Munk light scattering (S) and absorption (K) coefficients.  A Parker Print Surf was
used to measure surface roughness at a pressure of 1000 kPa with a hard backing.  An ink receptivity test (K&N)
was performed to give an estimate of the coating absorbency.  In this test, an excess of an oil-based ink containing a soluble dye is applied to a paper and then the excess is removed after 2 minutes.  A brightness meter
is adjusted to read 100 with the original sample and the inked sample is then measured.  The higher this percentage difference value, the greater the absorption of the paper.  The results for each formulation were then interpolated to a coat weight of 10 g/m2.
RESULTS
Coating Colour Rheology
Figure 1 shows how the low and medium shear rate viscosities of the coating colours vary with runnable coating solids.  The coating colours containing the e-GCC were at lower coating solids leve
ls compared to those containing the s-GCC.  At the lower shear rate, the points for both the s- and e-GCC lie on the same general trend of higher viscosities at higher solids.  The coating colours containing the e-GCC are at lower viscosity levels (500 to 1200 mPa.s) relative to those containing the s-GCC (850 to 2400 mPa.s).  At the medium shear rate, there is more scatter in the viscosity results, with the viscosity ranges overlapping considerably (80 to 210 mPa.s for e-GCC and 100 to 245 mPa.s for s-GCC).
Figure 1.  Low and medium shear viscosity of Kaolin/GCC coating colours
If the medium and low shear viscosities are plotted against each other, then an indication of the rheological behaviour of the coating colours can be seen, as shown in Figure 2.  The steeper trend for the e-GCC colours
indicates that these colours are less shear thinning than those based on s-GCC.
The scatter on the viscosity plots at both low and medium shear rates is due to the wide range of kaolin types used in this study.  The kaolins with the highest shape factors tended to give the highest viscosity levels.
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2
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Blade deflection was used to control the coat weight – the higher the deflection, the higher the blade
pressure.  On the very smooth base paper (Base 3 – side 2), it was not possible to obtain high coat weights with all of the formulations without runnability problems, such as skip coating or spits and streaks.  On all of the base papers, the formulations containing the s-GCC ran at similar blade deflection values, but at significantly higher coating solids levels, ranging between 1 and 6%.
There were no consistent or obvious differences in blade deflection and coating solids between the high and low absorbency base papers, as shown in Figure 3a.  A significantly higher blade deflection was required on the rough base compared to the smooth base, as shown in Figure 3b.  This indicates that the base paper property determining the coat weight in this study is the roughness.
特洛伊希文Coated Paper Roughness
The roughness levels of the coated papers are shown in Figure 4.  The addition of kaolin results in a smoother coated paper, although the magnitude of the decrease in sometimes quite small.  The base paper with the lower absorbency (Base 1) gave slightly rougher coated papers compared to the base with the high absorbency (Base 2).  The smooth base paper (Base 3 –side 2) maintained a large advantage over the rougher base paper (Base 3 – side 1).
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Coated Paper Absorbency
The absorbency of the coating layer was estimated using the K&N test, as shown in Figure 5, where a high number indicates a more absorbent surface.  The more absorbent base paper (Base 2) gave lower K&N values compared to the less absorbent base (Base 1).  The smoother base paper (Base 3 - side 2) also gave lower K&N values compared to the rougher base (Base 3 – side 1).  As expected, when e-GCC was used, the coatings were
more open and absorbent.  The addition of kaolin resulted in only very minor decreases in absorbency.
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Coated Paper Optical Performance
The brightness and opacity levels of the three base papers were significantly different (~80 vs. ~92 and ~78 to ~92 respectively).  This makes comparing any differences in brightness and opacity levels due to the coating very difficult.  Therefore, the Kubelka-Munk light scattering (S) and absorption (K) coefficients were determined for the base papers and all of the coated samples.  This allows the optical performance of the coating layer to be calculated and separated from that of the base paper.  It also allows the differences in pigment colour (e.g. the low and high brightness kaolins used) to be separated from changes in actual light scattering.  However, the S and K coefficients are always subject to large errors, as the calculations are very sensitive to changes in brightness levels
and coat weight.  Also, there can be problems with the calculations when the paper has a very low K value, as is the case with woodfree base papers and high brightness pigments.  This means that it is
difficult to compare the S and K values for the different base papers.