Natural Ground and Precipitated Calciumcarbonate in Woodfree Papermaking Maximilian Laufmann
Abstract Globally CaCO3 has become the dominant filler in the production of woodfree uncoated paper and coating base stock. CaCO3 fillers can be produced by specified grinding of a natural CaCO3 deposit based on chalk, limestone or marble. Ground limestone- or marble- based pigments are commonly named natural Ground CaCO3 or GCC. Another possibility of producing CaCO3 fillers represents the precipitation of CaCO3 from CaO via Ca (OH)2 and CO2. Such a chemical Precipitation based CaCO3 product is termed PCC. For the production of PCC, the step of burning natural CaCO3 to CaO remains current industrial practice. This means that both the GCC as well as the PCC manufacturing requires the availability of a suitable natural CaCO3 deposit. For PCC the choice is usually limited to limestone. GCC fillers are by far predominantly produced in a wet grinding process and delivered to the papermills as ready ready to use slurries with high solids contents of 65 to 72 %. PCC fillers are more commonly produced on-site at the papermill but there are also off site solutions.
Key words: Fillers, Natural Ground CaCO3 GCC, Precipitated CaCO3 PCC, GCC/PCC blends, uncoated paper, coating base paper
Maximilian Maximilian Laufmann, Laufmann, Technical Service Manager Rene Schneider, Schneider, Technical Service Plüss-Staufer 4665 Oftringen Switzerland
Phone ...41 62 789 2429 Fax ...41 62 789 2358
In this paper, GCC with its natural rhombohedral particle shape is compared in a papermaking papermaking environment with a typical filler PCC of scalenohedral morphology (S-PCC). Although other PCC morphologies are potentially possible, this comparison is based on the globally most common PCC particle shape for use as a paper filler, i.e. the scalenohedral morphology. It is primarily this type of S-PCC, which can add significantly to paper thickness. In order to demonstrate the basic differences of both types of pigment, pi gment, used either separately or in combination in a paper filling application, uncoated woodfree paper with 80 g/m2 and 22 % filler loading was produced on a pilot paper machine. Only the type of filler was changed or the ratio of the fillers in the blend situation. All other parameters where kept constant. The overall basic results showed that natural ground CaCO3 produced easier drainage and drying of the paper web, showed a lower size demand, a more closed sheet structure and higher paper strength. Higher paper strength potentially allows for higher filler loading, always an interesting tool to reduce paper cost. The GCC with higher fineness, while exhibiting a relatively low specific surface, produced higher opacity vs. the standard GCC filler. The precipitated CaCO3 applied in this comparison provided higher light scattering. In comparison to GCC, the scalenohedral shaped PCC filler produced higher paper bulk. As a consequence a distinctly more open sheet structure was noticed. After calendering, the SPCC filled sheet showed a slightly lower PPS roughness. Importantly, despite the disparity in paper bulk between the GCC and S-PCC loaded papers, there was was no difference difference in paper stiffness. stiffness. Commercial experience confirms the trends observed in the pilot study. The specific individual situation and the choice of pigment configuration strongly affects these tendencies. Useful compromises offer the combination of
GCC and scalenohedral-PCC either as a ready- touse premixed product or each type of pigment added individually via a dosage point into the paper machine system. Particularly less modern paper production units can exhibit drying capacity limitations in the after drying section. In such situations a combination of both pigments, GCC and S-PCC, is especially of advantage to the papermaker. In coating base stock, GCC as a primary filler also carries the potential of a faster draining and drying sheet structure, compared to an S-PCC application. This is particularly pronounced at higher basis weights and higher primary filler loading levels. The GCC filled sheet usually exhibits significantly higher internal bond values vs. the S-PCC loaded paper. This holds the potential of higher primary filler loadings at equal internal paper bond. S-PCC produces higher base paper bulk compared to GCC. However, this higher bulk diminishes with calendering of the coated sheet. After coating and printing, no difference in terms of surface roughness, paper and print gloss and print quality, initiated by the choice of primary filler, has been noticed.
1. Manufacturing Processes for GCC and PCC The chosen selective micronisation process of natural CaCO3 from a deposit of chalk, limestone or marble is dependent on the final end-product application. World-wide a variety of mineralogical deposits can be used as GCC filler pigments for the paper industry. In addition to their mineralogical composition and, hence, resulting pigment brightness, the logistical aspects in relation to the papermill are important and crucial factors for the choice of a deposit. Based on the current high pigment brightness requirements, mainly marble or limestone-based raw materials are used for GCC filler pigments. The pigment brightness can reach or exceed 95 % Elrepho (R-457) depending on the raw material quality and the individually adjusted production processing. Micronisation is usually performed using the so-called wet ground process. CaCO3 deposit crushing
CaCO3 deposit
coal, gas, oil
CO2 CO2
crushing burning
Introduction
hydration
grinding
CO2
Over decades now, the desire for higher brightness and its strategic implementation in the market place has been clearly observed. Despite sporadic critical evaluation of the sense or realistic need for this brightness trend, there has been no halting influence on the market behaviour. In comparison to kaolin qualities, calcium carbonate fillers with their much higher pigment brightness are important and have proven themselves as the main cost-saving option to improve the basic paper brightness. As a result of this brightness asset and their world-wide availability, calcium carbonate fillers have become one of the most important pigments in the field of wood free paper grades. Calcium carbonate fillers are today divided into two main categories; the natural ground CaCO3 GCC and the precipitated grades of CaCO3 PCC. The following presentation evaluates closely both pigment types regarding their manufacturing process and specific properties as well their application comparison in woodfree uncoated paper as both single pigment and in blends. Also highlighted are their properties as primary paper filler when used in coated paper qualities.
precipitation screening
screening
CaCO3 GCC
CaCO3 PCC
Figure 1 GCC vs. PCC “Production Principles”
If necessary, impurities are mechanically or physico-chemically removed. Highly automated production processing and large production units are especially economic. The end-products are usually in the form of rheologically stable filler slurries with a solids content from 65 up to 72 %. GCC slurries dedicated to the paper coating industries are supplied today with solids contents up to 78 %. A sophisticated logistics chain is a vital component for the overall raw material and paper product profitability. The slurry supplied by ship, railway or road transport is loaded into the storage tank of the paper mill and used today without any further treatments in the paper making process. The manufacturing process of PCC fillers for the paper industry also begins from a natural CaCO3 (figure 1). Due to some special technical aspects, the manufacturing process is confined to the use mainly of limestone deposits. Additionally, the 2
selected limestone has to fulfil some basic mineralogical requirements. High manganese and iron contents exemplify a very negative influence on the brightness of the end-product. As a result of the original limestone quality, interactions can occur which also negatively influence the optical and crystalline properties of the precipitated product. The above criteria mean that not every readily available limestone deposit is an acceptable source for the manufacture of the essential intermediary raw material, burned lime CaO, needed in the PCC manufacturing process. A stringent selection has to be made. As an example of these severe restrictions, one on-site PCC operation (on-site meaning PCC being manufactured within the confines of the paper mill itself) in South Africa is today forced, due to limited availability of suitable local raw material, to cover a part of their demand by sourcing from Europe. The calcination of localised limestone to CaO usually requires fossil energy. In the PCC manufacturing process, CaO is then further hydrated to calcium hydroxide Ca(OH)2. Burnt lime reacts strongly exothermically in contact with water and Ca(OH)2, is highly alkaline. In one or more reactors a re-carbonation with CO 2 is made to produce chemically precipitated CaCO3, PCC. The necessary CO2 gas can be gained from the on-site sulphate pulp line operation (limekiln) or from the flue gases of the fossil-based paper mill's power station. It is also vital to consider not only the availability of CO2, but also qualitative parameters such as gas density, temperature, purity of CO2 etc. Therefore, gas-cooling and cleaning systems for the CO2 have to be installed. An additionally installed integrated storage capacity for liquid CO2 is often needed to ensure against temporary shortage of CO2. The control of the particle size and fineness of the PCC quality is made via monitoring of the precise reaction parameters and the addition of specific reaction additives. Such a PCC has after screening a typical solids content of approximately 20 % and is stored on-site until usage. By so-called off-site PCC operations, the PCC filler is shipped to the paper mill with a solids content between 35 and 52 % and if necessary, rheologically stabilised. PCC qualities in dry powder form are also available in the market and are mainly used in special paper grades.
2. Applications - Natural Ground CaCO3 and Precipitated CaCO3 Morphological differences
Natural ground CaCO3 filler in its simplest form inherits from nature a rhombohedral particle shape (figure 2). The morphology of precipitated CaCO3, however, can be adjusted within certain technical and chemical limits. These differences can lead to significant variation in the production costs of a PCC.
GCC
S-PCC
Figure 2 GCC and S-PCC Fillers
World-wide, the most common PCC particle shape used in the paper industry is scalenohedral (figure 2). Primarily, this structure is coupled with the property of giving the paper a higher bulk. If desired, particle shapes other than the scalenohedron can be manufactured but are coupled with higher technological requirements and additional costs. To simplify presentation of the following results, scalenohedral PCC is termed S-PCC (figure 2).
3. GCC and S-PCC in the Production of Uncoated Woodfree Paper 3.1. Comparisons made on a pilot paper machine Trial conditions
With the objective to compare both materials as single pigment or in blends, trials have been performed on a pilot paper machine. The emphasis of these trials were on the pigments themselves and their direct and indirect effects in the application including process as well as the achievable paper quality. The working width of the pilot paper machine was 70 cm, the speed 80 m/min. The furnish: 25 % pine sulphate (30 °SR) and 75 % birch sulphate pulp (30 °SR). As sizing agent, AKD including a 3
promoter was used, at a dosage of 0.12 % (dry/dry). A good retention of the filler and the sizing agent was achieved using a micro-particle retention system, supported by additional 0.5 % cationic starch. Following normal production conditions, a market-available anionic OBA (0.2 %) was also added (1/3 in the wet-end, 2/3 in the size press). The grammage was 80 g/m 2 and the adjusted filler content in paper set at 22 %. The specific pigment data corresponding to the filler grades used are listed in figure 3. The GCC used, 60/7.0, i.e. 60 % < 2 µm with a specific surface area of 7.0 m2/g, is a European standard quality from a marble resource. The GCC grade, 85/7.5, (85 % < 2 µm, 7.5 m2/g specific surface) also from a marble-based source, is designed to develop a higher paper opacity in comparison to a standard GCC.
A reduction of the S-PCC portion of the filler by inclusion of GCC in combination shows improved drying results. Higher dry content leads to a positive development of higher initial sheet web wet strength. This trend, in respect of the dewatering after the wire suction roll as a function of pigment choice, is confirmed by the values seen after the third press (figure 5).
51
7.0
7.5
6.5
Sedigraph fineness < 2 µm % < 1 µm %
60 35
85 60
45 15
aps
µm
1.5
0.8
2.1
%
94.5
95.0
95.5
Brightness R-457
m²/g
Figure 3 Pigment Data
The quality of the S-PCC used in these trials represents a typical S-PCC filler commercially available from an on-site production unit. All 3 filler pigments show a high pigment brightness of approximately 95 % Elrepho R-457.
GCC (% < 2 µm) 60 85
50 49 48 47 46
100
75/25
GCC
GCC GCC S-PCC 60 / 7.0 85 / 7.5 Spec. surface BET
% Dryness after 3rd press
50/50
25/75
100 0
S-PCC
Blends
Figure 5 Drainage
The effect of this slow-down of dewatering with S-PCC as filler can be at least partly explained by a more voluminous structure of the pigment and the paper formed out of it. A higher bulk mass requiring subsequently higher drainage power. If a PCC with a prismatic structure had been used, the dewatering behaviour would therefore be positively influenced as seen with a GCC, but, as a result, such a product, compared to GCC, would not give a higher paper bulk. With a higher content of S-PCC in the blend, or with 100 % SPCC, a drastic increase in size press pick up is observed at the conventional size press.
3.2. Papermaking process
As shown in figure 4, the usage of 100 % S-PCC displays a clearly lower dry content after the wire suction roll compared to 100 % GCC.
3.0
l/min Starch / Polymer GCC (% < 2 µm)
2.8
60
85
2.6 2.4
30
2.2
% Dryness at couch roll GCC (% < 2 µm)
29
60
85
2.0
100
75/25
GCC
28
50/50
25/75
Blends
100 0
S-PCC
27 26 25
100
GCC
75/25
50/50
Blends
Figure 4 Drainage
25/75
100 0
S-PCC
Figure 6 Size Press - Pick Up
The origin of this phenomenon is surely due to the higher paper bulk and open, more porous, paper structure coupled with a higher paper humidity leading to a lower sizing development before the 4
size press. On the occasion of a different investigation on the same pilot machine regarding an organic filler to achieve extra high paper bulk. A significantly higher pick up of starch in the size press similar to S-PCC was obserbed. Due to the technical equipment available on the pilot machine the steam demand in the pre-drying section unfortunately could not be recorded, but only in the post-drying section. Trials including an increased S-PCC portion of 50 % or more in the sheet web showed much higher steam demand.
GCC/S-PCC blends up to 100 parts S-PCC, are both nearly linear. S-PCC, in the form of single particles of filler, gives, in comparison to GCC, a higher bulk.
1.5
2.5
GCC 85 % < 2 µm
1.4 1.3
15 kN/m
1.2 30 kN/m
1.1 1.0
kp/cm²
cm³/g
GCC (% < 2 µm) 60 85
100
75/25
GCC
50/50
25/75
100 0
S-PCC
Blends
2.0 1.5
Figures 9 Bulk
1.0 0.5 0.0
100
75/25
GCC
50/50
25/75
100 0
S-PCC
Blends
Figure 7 Steam Demand after Dryers
The simple explanation for this follows the comments already made regarding higher starch demand in the size press with high amounts of SPCC in respect of increased pore bulk. 3.3. Paper quality
The production of the paper in the pilot machine was finished by passing the sheet through a machine calender. The calender pressure was adjusted to 15 and 30 kN/m, respectively. When using S-PCC as single filler or in a blend with GCC, higher paper thickness together with higher bulk are noticeable.
This creates in the paper, through further inter particle flocculation, a relatively high voluminous structure. The resulting separation between the paper fibres increases and thereby paper thickness and bulk are increased. It could be possible that chemical and physical parameters of PCC are influencing additionally the mechanistic development of paper bulk. Importantly, however, the higher paper bulk generated by an S-PCC pigment is not automatically leading to a measurable increase of paper stiffness (figure 10). A research of market paper confirms this latter observation.
0.5
mNm Resonance
L x Q² GCC 85 % < 2 µm
0.4 0.3 0.2
15 kN/m 30 kN/m
0.1 110
µm
GCC (% < 2 µm) 60 85
105
0.0
100
GCC
75/25
50/50
Blends
25/75
100
0
S-PCC
100 95
Figure 10 Stiffness
90 85
100
GCC
75/25
50/50
Blends
25/75
100 0
S-PCC
Figures 8 Thickness
The increase of paper thickness and bulk, in these trials starting from 100 parts GCC through
The expectation that it might have been possible when using an S-PCC, arising from the resulting higher paper bulk, to reduce the grammage from 80 down to 75 g/m2 and maintain the paper thickness has not been fulfilled. A reduction of the grammage especially in this specific range leads to a drastic loss of paper stiffness and, naturally, lowers the opacity significantly. 5
0.45
the papermaking process are more densely consolidated and therefore disturb less the fibrefibre bond.
mNm Stiffness (Resonance) without Filler with GCC 60/7.5
0.40 0.35 0.30 0.25 0.20 65
70
75
80
85
g/m² Basis Weight
Based on results of a lab study the tensile strength decreases when an average S-PCC particle size of approx. 2 µm is exceeded. Noticeably, S-PCC grades of lower fineness, used to create highest paper bulk, are exhibiting high dusting tendencies. A well-known problem in the copy machine. The larger fibre-fibre separation generated by an S-PCC results, in comparison to GCC in higher paper porosity.
Figure 11 Stiffness vs. Basis Weight
It is also clear, that it is apparently necessary to set a standard minimum of 80 g/m2 to provide a trouble-free handling in European copy machines (given the higher filling rates in comparison to North America). This can also be connected with these findings. The resulting S-PCC induced increase in separation between the single inter-fibre and fibrefloc connectivity leads consequently to a reduction of the occurrence of H-bonding and, therefore, to a reduced paper strength.
1400
ml/min. Bendtsen
GCC (% < 2 µm) 60 85
1200 1000 800 600 400
100
75/25
GCC
50/50
25/75
Blends
100 0
S-PCC
Figure 13 Porosity Bendtsen 85
N MD
GCC (% < 2 µm) 60 85
80 75 70 65 60
100
GCC
75/25
50/50
Blends
25/75
100 0
S-PCC
Figure 12 Tensile
The resulting difference in tensile strength between 100 % S-PCC and 100 % GCC 60/7.0 is highly significant; GCC 85/7.5 also shows in comparison to GCC 60/7.0, a lower tensile. This is explained by the greater fineness of GCC 85/7.5 compared to GCC 60/7.0. Though not demonstrated specifically here, but known from former lab and practical tests, there is a significant difference between GCC and S-PCC regarding the influence on the paper internal bond (Scott bond). The usage of S-PCC as filler leads, in comparison to a standard GCC product, to a significantly lower internal bond. The hi gher Scott bond with GCC can be explained as a result of GCC's broader particle size distribution curve. The flocculated GCC aggregates formed during
Through combinations/blends of S-PCC and GCC the porosity can be positively influenced, i.e. the paper surface can be made more closed. Normally in practice, the porosity of the paper structure is effectively adjusted via further fibre refining which of course is coupled with additional energy costs. Associated with the additional energy demand of stronger refining is also an increased hygroscopic sensitivity of the fibres and so also of the paper. Recently, in Asia, a new large production line of wood free base sheet went into production. Today, the furnish is a mix of tropical hardwood but it is planned in the near future is to use a fast-growing acacia. Tropical Hardwood vs. European Birch Bulk Brightness Opacity
+ 20 % + 4 % points + 1 % point
Light scattering Tensile Internal bond
+ 15 % - 30 % - 50 %
Porosity
+ 30 %
Figure 14 Tropical Hardwood vs. European Birch 6
These pulps are showing, in comparison to a standard European fibre mix, a naturally higher bulk and significantly higher light scattering. Indeed the tensile strength and Scott bond are comparatively much lower and the porosity higher (open paper sheet). Based on these technical aspects and on the wish of highest possible filling ratio this paper maker chose GCC as filler so as to retain advantage of the natural bulk of the fibres without destroying the paper strength.
darker fibre is better hidden. Negatively influenced by an increased S-PCC amount in these trials is the CIE brightness.
145
%
GCC (% < 2 µm) 60 85
140 135 130 125
The values in figure 15 relate the influence of pigment on roughness of the paper surface. With an increased amount of S-PCC a trend to lower roughness can be observed. Comparing 100 parts S-PCC to 100 parts GCC, the difference ranges from between 0.5 to 0.8 µm, measured with the Parker Print Surf. No significant influence of these measured roughness differences on a print sample could really be detected.
6.5
µm PPS GCC 85 % < 2 µm
6.0
120
100
75/25
GCC
25/75
100 0
S-PCC
Blends
Figure 17 CIE Whiteness
One explanation is to be found in the different type of charge of the pigments and their interaction with the anionic optical brightener. Both GCC qualities were anionic wet ground produced, while PCC showed a slight naturally developed cationicity.
15 kN/m
5.5 5.0
30 kN/m
4.5 4.0
100
75/25
GCC
50/50
25/75
100 0
S-PCC
Blends
Figure 15 Roughness PPS
As an effect of the particle structure, and a steeper particle size distribution curve, S-PCC shows the expectedly higher light scattering effect. This higher light scattering effect is the basis for influencing positively the paper opacity.
70
m²/kg
GCC (% < 2 µm) 60 85
65
As already mentioned, all three fillers showed a similar pigment brightness of 95 % with Elrepho R-457. The brightness differences measured in the manufactured base sheet are therefore minimal.
60 55 50 45
100
75/25
GCC 90
50/50
%
50/50
25/75
100 0
S-PCC
Blends
GCC (% < 2 µm)
60
85
Figure 18 Light Scattering
89 88 87 86 85
93
100
75/25
GCC
50/50
25/75
Blends
100 0
S-PCC
%
GCC (% < 2 µm) 60 85
92 91 90
Figure 16 Brightness R-457 - UV
The very slightly higher brightness with S-PCC in comparison to GCC is mainly due to its comparatively higher light scattering coefficient as a function of filler-fibre voids. The slightly
89 88
100
GCC
75/25
50/50
25/75
Blends
100 0
S-PCC
Figure 19 Opacity at 80 g/m² 7
Also with GCC the light scattering effect can be influenced via increased particle fineness and steeper particle size distribution curve, to improve paper opacity. This is demonstrated by GCC 85/7.5. Also presented are the results with the blend of GCC/S-PCC. As extensively described in the relevant literature, sizing with AKD is often coupled with difficulties when using S-PCC. This phenomenon could also be noticed in these trials. As presented in figure 20, at a given dosage of 0.12 % AKD (dry/dry) with even just 50 % S-PCC, there results a totally insufficient sizing of the paper.
60 50
GCC
z i n g i s n o
30 20
100
75/25
GCC
50/50
25/75
100
S-PCC
Blends
Also, in association with this evolution of alkalinity, a sizing loss (size reversion) over a certain time period must be recognised. To minimise this risk, generally in paper production right from the beginning, higher amounts of AKD are added. Analytical analysis of PCC filled papers confirm higher quantities of hydrolysed AKD (ketone). 3.4. Summary of the trials on the pilot paper machine
g/m² Cobb (60) 0.12 % AKD
40
these remaining traces of Ca(OH)2. Under specified shear stress conditions and nitrogen saturation this source of alkalinity (out of the crystal core) can be made evident.
S-PCC
Brightness
Brightness
Drainage
Caliper
Dryness
Bulk
Strength
Light scattering
Density
Roughness
Sizing
Figure 20 Sizing
In a following trial made under the same conditions, a 50 % higher AKD demand was necessary to reach the desired sizing. The primary reason for such a high AKD demand is to be found in both the higher paper humidity of the sheet web when entering the pre-drying section as well as the remaining quantity of Ca(OH)2 when using PCC. At high temperature and high moisture levels the reaction between AKD and water (hydrolysis) is very strong. This hydrolysis of AKD is additionally supported by the remaining traces of Ca(OH)2 (alkaline). 13
pH
S-PCC slurry
12
S-PCC powder 11
S-PCC slurry
10
GCC
9 8
0
30 60 Shearing min.
90
Figure 21 GCC / PCC and pH
The higher pH values typically seen when using PCC in comparison with GCC are generated from
Figure 22 GCC vs. PCC “Criteria’s”
Both filler types GCC and S-PCC are suitable to provide a high base sheet brightness. The natural rhombohedral GCC leads to comparably easier dewatering and drying of the sheet web. On the other hand S-PCC provides a significantly higher paper bulk. This higher paper bulk, however, does not automatically leading to additional paper stiffness. GCC as filler reduces the paper strength less than S-PCC. This gives an advantage with GCC for higher filling levels and the associated savings. The specific structure of S-PCC and the steep particle size distribution curve provide this pigment with a high light scattering coefficient for good opacity. GCC 85/7.5 also with its increased fineness and steeper particle size distribution curve shows, in comparison to GCC 60/7.0 a higher light scattering coefficient and provides higher paper opacity. A paper with GCC as filler compared to S-PCC shows a significantly lower paper porosity. The paper produced with S-PCC has, after calendering, a lower surface roughness. It has to be mentioned that no noticeable differences could be found on printed samples related to this lower surface roughness. Due to the rhombohedral particle structure and the resulting better dewatering/drainage properties, as well as improved pH stability, the demand of 8
sizing agent is lower with GCC compared to SPCC. The combination of both pigment types in different ratios allows a fast and flexible adjustment for the required paper quality. The specific optimal pigment ratios to achieve defined quality parameters can be easily fine-tuned. 3.5 Notes from the field
The tendencies noted on the pilot paper machine have been confirmed on several occasions on existing production lines. The basic differences generated by the pigment are mainly dependent on the respective technical conditions. Considering that the aim is to produce marketable paper quality, direct pigment comparison is heavily influenced by several necessary correcting measures in practice. As a matter of fact, the new huge production machines are mostly having sufficient drainage and drying capacities built-in. A potential bottle-neck in the post-drying section is mainly to be found in older production units. A combination of S-PCC (higher paper thickness) with GCC is, when facing insufficient drainage and drying capacities, of great advantage. With such an alternative, the papermaker obtains a high paper bulk and high strength. The demand of AKD is kept on normal levels and the production speed corresponds nearly to an operation using 100 % GCC.
(
+ = improved )
Refining Demand Drainage PM Speed Bulk CIE Whiteness Opacity Stiffness Roughness Burst
S-PCC GCC/S-PCC O
(+)
O O O
++ ++ + -
O
O
O
+
O O
O
Figure 23 GCC /. S-PCC Blends (com. experience)
A typical experience relating to this (figure 23: a European paper manufacturer made several trials to produce a high quality 80 g/m2 copy paper using 100 % S-PCC of different origins on a Fourdrinier machine. The customer saw that dewatering became distinctly worse and consequently had to reduce PM speed by about 10 %. Additionally, the open paper structure could only be closed by higher fibre refining which
required a 20 % increase of energy demand. With a filler combination 50 % S-PCC / 50 % PCC the customer went back to the original machine speed and could reduce also the required fibre refining (figure 23). At identical smoothness levels the paper thickness decreased only by 2 µm and the opacity by 0.5 %. With this pigment formulation an increase of paper strength of nearly 10 % was obsered. In Europe, huge amounts of copy paper with a filler load up to 28 % are produced and absorbed by the market. An analysis of this paper, shows a successful implementation of 100 % GCC.
4. GCC and S-PCC as Primary Filler in Woodfree Coating Base Paper Major properties required for a woodfree coating base paper are sufficient tensile strength, good internal bond (Scott bond) as well as a stable water absorption and wetting properties at high dimensional stability. High brightness and acceptable opacity are simply a must with a closed surface being considered of great advantage. Pigment ratio including primary and secondary filler is, depending on the grammage, at a level of about 12 to 20 %. In a specific trial on the pilot paper machine an 80 g/m2, coating base paper was produced. As primary filler, 8 and 12 % GCC and S-PCC, respectively, were compared. As a result of the addition of coated broke, the amount of secondary pigment in the coating base paper was 8 %. Using a compromise between double coating and single coating practice, a single coat of 15 g/m2 per side of coating colour was applied with bent blade. The coated paper was afterwards calendered and offset printed. The results of this trial can be summarised as follows.
Pilot und Commercial : GCC
Drainage, Strength
S-PCC
Bulk prior to calendering
After coating and calendering GCC vs. PCC Roughness, Paper- / Print gloss, Print quality
s c e n r e f e f i d n o
Figure 24 GCC and S-PCC in Coating Base Paper
Again, the GCC filled paper showed higher favourable drainage properties of the sheet web. 9
Also the internal bond (Scott bond) was, with a standard GCC, significantly higher. As expected, the coating base paper produced with S-PCC as primary filler showed higher bulk, a property which after calendering of the coated paper was no more visible. Basically, it can be stated that after coating and calendering no significant differences regarding roughness, paper and print gloss as well as printability could be detected. What was influenced, however, were the optical properties of the coated paper by those of the coating base paper (being generated by the filler). In practice, using GCC as primary filler is of particular advantage in papers of higher grammages. The reason behind this is mainly the better dewatering and drainage properties and, as already mentioned, the higher Scott bond with this filler. Higher and better drainage gives the possibility to increase production speed. The higher Scott bond with standard GCC will allow basically a higher filler loading. High filling levels positively influence opacity and brightness of the paper. At the same filler content, a paper with S-PCC as primary filler will, due to the higher light scattering coefficient of the pigment, have a higher opacity in comparison to one with a standard GCC. The difference is mainly dependent on the added amount of primary filler. A GCC 85/7.5 filler will lead to similar paper opacity as when using less finer S-PCC grades. The combination of standard GCC and S-PCC as primary filler in woodfree coating base paper is in the US not unknown. A close look at t he pigments from the coated broke together with S-PCC primary filler are already showing the combination GCC/clay/S-PCC.
Conclusion Both, the pilot and commercial examples illustrate the important attributes of GCC and S-PCC. They are very different in performance and therefore provide a wide range of options for producing papers with various end-use demands. GCC provides strength and higher loading potential, easier drainage and drying, low size demand, and higher sheet density. PCC provides higher opacity at lower filler loadings and lower density (higher bulk). In this way, papermakers can choose the attributes necessary for a particular grade. Both type of pigments provide high brightness at a low cost. 10
