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TECHNICAL PAPER

CALENDARING RESPONSE OF CALCIUM CARBONATE COATED BASE PAPER CONTAINING AGRO RESIDUE PULP

Sanjay Tyagi *1, Dr. A. K. Ray 2

KEYWORDS
GCC, Coating, PCC, Calendaring, Agro-residue

ABSTRACT

Calendaring of a calcium carbonate containing coating of base paper, having mainly agro residue pulp as the major furnish, indicated that PCC containing coating developed higher gloss and opacity than GCC. Brightness and bulk of PCC coated sheets were higher as compared to GCC. A smoother sheet is obtained with GCC along with ultrafine clay, while the print gloss is also high for GCC-containing coating formulations. Increasing calendaring temperature from 90 to 120 °C caused an increase in sheet gloss and smoothness and a drop in bulk and opacity. The gloss improvement effect was relatively lower in the case of PCC coating, as compared to GCC. The effect on print gloss with increased calendaring temperature was minor.

_____________________________________________________________

INTRODUCTION

Fine ground calcium carbonate (GCC) is used as a major component in coating, especially in top coating, due to its high brightness generation and its cost effectiveness. A small proportion of clay is usually added to boost the properties of print gloss, which is lacking in the case of GCC. PCC has come up as a new option to replace GCC. Typically used PCC in coatings are Aragonite and Calcite.

One of the important operations in coating is calendaring of coated paper. The main aim of calendaring the coated paper is to make the paper structure more suitable for printing. Coated paper are intended mainly for high quality products like magazines, catalogs, books etc. - often printed in multicolour printing processes where low roughness and uniform surface pore size are essential requirements for good print results. Low surface smoothness can lead to poor ink transfer and low print gloss, resulting in unevenness in the printed image.

Paper calendaring as a general term means treatment of the paper in a nip formed by two or more revolving rolls, The effect of calendaring on the paper properties depends on the speed, temperature, the diameter and surface condition of the calendar rolls, linear load between the rolls and the coating component of paper.

_____________________________________________________________ * Corresponding author, e-mail address: sanjaykhatuli1@rediffmail.com ; styagi77@yahoo.co.in
1 Research Scholar, Department of Paper Technology, IIT Roorkee (Saharanpur Campus), Saharanpur and Scientist B, Central Pulp & Paper Research Institute, Saharanpur (INDIA)
2 Professor, Department of Paper Technology, IIT Roorkee (Saharanpur Campus), Saharanpur (INDIA)


It is a process in which the improvement of some paper surface properties is usually accompanied by the impairment of other properties, such as optical and strength properties. Successful calendaring therefore depends on a balance between achieving the best possible surface properties without causing too much impairment of other paper properties. This is the reason why the calendaring result is often estimated by comparing against paper properties like gloss, bulk, opacity, brightness etc.

The coating colour composition also affects calendaring response to coated papers. The type of pigment/pigment blends, pigment particle shape and size, type and amount of binder used in a coating formulation all affect the coated paper properties tremendously.

In spite of there being many literature references calendaring and coated paper properties, there is no study on the influence of calendaring on coated paper made from the blend of agriculture residue pulp and softwood pulp. In the present study, an attempt has been made to investigate the effect of calendaring on a coated paper made from agricultural residue pulp and softwood pulp in the ratio 75:25 with different coating formulations containing calcium carbonate as the major pigment. The response of changing the calendaring temperature was also investigated.

Coating Paper structure and Influence of Calendaring

Coating colour properties affect the immobilization of the coating layer. During coating, surface roughness and pores of the base paper are filled with coating colour. At the same time, the roughness and bulk of the base paper are increased by deformation of the paper structure. Coating colour properties affect water transfer into the base paper during coating, as well as the structural changes of the base paper occurring between coating application and drying of the coating layer (1). Coating colour, having a high solids content, leads to better coating coverage (2). High solid content normally means a faster immobilization of the coating layer, which reduces the structural changes of the paper matrix under the coating layer.

The effect of calendaring on the properties of coated paper has been reported by several studies (3, 4, 5), and show that the main effects of calendaring are an increase in gloss and smoothness and a decrease in porosity. On the other hand, calendaring leads to lower opacity and brightness. These effects are largely attributed to the compacting and rearrangement of the paper structure during calendaring. This compacting of the paper structure also leads to lower opacity and brightness and, as a result, there is bulk reduction and loss of paper stiffness.

Viscoelastic behavior of coated layer

Rodal (6) described in his model the viscoelastic behavior of paper during calendaring. The polymers in the base paper and in coating layer are either amorphous materials like lignin, hemicellulose and synthetic binders, or partly crystalline materials like cellulose and starch. Deformation in the structure of the polymers are time dependent, partly permanent (plastic) and partly recoverable (viscoelastic). The viscoelastic behavior of polymers depends on molecular weight and structure of the polymer as well as the level of external stress, temperature and moisture content. The response of viscoelastic material to the applied stress can be seen as a combination of fast, viscous deformation and slow elastoplastic deformation.

Effect of the variable calendaring temperature on properties of coated paper

Increasing the claendering temperature is known to enhance gloss and smoothness and to reduce opacity and stiffness (7). At constant paper gloss, a higher calendaring temperature reduces the roughness of coated paper. Using the higher calendaring temperature (and lower linear load), focuses the deformation of the structure on the upper surface of the coating layer. An increase in temperature also results in less variation in the porosity of the coating layer but at the same time an increase in the gloss variation of the coated layer.

The use of high temperature, low linear load and high running speed will maximize surface effects and minimize bulk effects of the paper structure. Increasing the moisture content improves paper surface properties, but reduces opacity and brightness. Increasing the linear load increases gloss and smoothness, but again there is usually a reduction in opacity and brightness.

MATERIALS AND METHODS

Base paper: The base paper used in the study was collected from a paper mill having the furnish composition of 75% agricultural residues (bagasse/ wheat straw) and 25% soft wood chemical pulp. The properties of base paper are given in Table 1.

Table1.
Properties of base paper with furnish composition of
agricultural residues & soft wood in ratio 75:25

Property
Values obtained
Basis weight (g/m2)
75.9
Thickness (µm)
97
Bulk (cm3/g)
1.28
Tensile Index (N.m/g)
CD
MD
25.5
52.0
Tear Index (mN.m2/g)
CD
MD
4.80
3.90
Brightness (%)
85.6
Opacity (%)
92.8
Whiteness (%)
120.95
Sp. Sc. Coefficient (m2/kg)
42.0
   
Tensile stiffness Index (mN.m/kg)
CD
MD
4.51
7.90
Show through Index
42.0
Ash (%)
9.2
Porosity (Bendtsen) (ml/min)
84.2

Coating formulation: Five different pigments used in the coating formulation were collected from commercial suppliers /mills. They were GCC-1 (fine grade), GCC-2 (ultra fine grade), Clay-1 (ultra fine clay), Clay-2 (fine clay) and aragonite PCC. The physical properties of pigment were analyzed using a Horiba laser scattering particle size distribution analyzer. The results are given in Table 2.

Table 2.
Pigment Properties
Pigment type Particle size distribution Mean particle size(µm) ISO brightness (%) Sp. Surface area (cm2/cm3) % content
> 2µm
GCC I (fine grade)
Broad
5.28
90.0
15642
87
GCC 2 (ultra fine grade)
Broad
1.43
91.9
51467
90
PCC
Steep
1.35
95.2
48110
91
Clay 2 (fine clay)
Broad
3.51
92.6
30953
89
Clay 1 (ultra fine clay)
Broad
1.32
93.4
49180
91

The coating formulation is based on 100 parts pigment, 12 parts SBR latex, 1 part CMC, 0.12 part OBA and 0.5 part dispersant for all colours. The coating colours were prepared in laboratory coating mixer under constant stirring. The coating colour solids were kept in the range of 60 – 65%. The base paper was coated single sided with the coat weight of 10-12 gsm.

The different coating pigments were blended in following ratios:

Coating colour formulation no.
Pigment blend
1
80/20 GCC-1/Clay-1
2
80/20 GCC-1/Clay 2
3
100 GCC-2
4
80/20 PCC/Clay-1
5
80/20 PCC/Clay-2
6
100 PCC

Calendaring conditions:
After coating the sheets were calendared in a laboratory calendar supplied by DT Paper Science, Finland. The calendaring was performed using hard nip rolls under constant calendaring conditions of a temperature of 60 ºC and nip load of 130 kN/m at the speed of 22 m/min. In another set of experiments, the condition of calendaring pressure and speed remains the same and only the calendar temperature varies from 90 ºC to 120 ºC.

Evaluation of coated papers:
Calendared coated/uncoated sheets were conditioned at 271% relative humidity of 65±2% prior to testing of following properties as per the relevant standard methods listed below:

Grammage (g/m2) – ISO 534
Brightness (%) – ISO 2470
Gloss (at 75º) – Tappi 480 om 99
PPS roughness (micron) – ISO 8791/4
Opacity (%) – ISO 2471
Print Gloss – The sheets were printed using IGT Print density ink and gloss was measured

RESULTS AND DISCUSSION

(i) Response of constant calendaring conditions on coated paper properties

The base paper containing agricultural residue pulp along with softwood pulp in the ratio of 75:25 was coated with six different coating colour formulations prepared with calcium carbonate as major pigment. In the first trial all coated sheets were subjected to the constant calendaring conditions of temperature, nip load, number of nips and calendar speed. Table 3 and 4 show the properties of uncalendared and calendared paper. The results are interpretated using bar charts which give an assessment of relative performance quickly and easily. The error bars make the typical standard deviation for the test results.

Table 3. Uncalendared (coated) Paper properties
Property
Coating colour formulations
 
1
2
3
4
5
6
Gloss (%)
44.0
36.0
53.0
51.0
47.0
52.0
Bulk (cm3/g)
1.21
1.18
1.20
1.19
1.19
1.19
Brightness (%)
85.4
85.7
87.1
86.9
86.9
86.7
Opacity (%)
94.7
94.7
94.9
95.5
95.7
95.9
Smoothness (PPS) H-10 (micron)
1.63
1.61
1.64
1.48
1.57
1.63
Print gloss (%)
70.0
67.0
73.0
71.0
67.0
67.0

Table 4.
Paper properties of coated paper under constant calendaring conditions
Properties
Coating colour formulations
 
1
2
3
4
5
6
Gloss (%)
71.0
65.0
72.0
75.0
75.0
74.0
Bulk (cm3/g)
0.90
0.91
0.91
0.93
0.92
0.92
Brightness (%)
82.3
83.4
83.5
83.2
83.1
84.3
Opacity (%)
92.2
93.4
93.6
94.1
94.8
94.3
Smoothness (PPS) H-10 (micron)
0.79
0.87
0.90
0.85
0.81
0.90
Print gloss (%)
96.0
95.0
96.0
93.0
93.0
91.0

Fig 1
Fig. 1 Uncalendared sheet gloss of coated papers

Figure 1 depicts the results of uncalendared sheet gloss of coated paper with different colour formulations. With 100% calcium carbonate, both PCC (colour 6) and GCC (colour 3) gave about the same gloss level. When CaCO3 was partly replaced with clay, the combination of PCC and clay gives higher uncalendared gloss values (colour 4 & 5) than colour blended with GCC and clay (colour 1 & 2). The reason for the high gloss with PCC is probably the small particle size and steep particle size distribution of PCC as compared to GCC, which gives more densification of surface material between the nips of calendar rolls (Table 2). It brings the material into the same plane from both the top and bottom side and flattens out this material laterally by fiber collapse, as a result of the pressure pulse in the calendaring nip. Fig 2
Fig. 2 Calendared sheet Gloss (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 2 shows the result of calendared sheet gloss. Compared to Figure 1, as expected, there is an increase in the sheet gloss of all coating formulations. The coating formulations containing PCC (colour 4 & 5) give higher gloss values than GCC coatings. This may be due to the more rigid structure of GCC coatings (GCC is coarse compared to PCC), which is comparative less responsive to calendaring. The formulation containing PCC and fine clay gives the best results.
Fig 3
Fig. 3 Calendared sheet opacity (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 3 indicates the results of opacity of calendared sheets. All the PCC containing formulations (colour 4, 5, & 6) give higher opacity than coating colours containing GCC. The maximum opacity is given by the formulation containing PCC and fine clay (colour 5). This may be due to maximum light scattering displayed by this coating formulation given an open coating structure.
Fig 4
Fig. 4 PPS smoothness of calendared sheets (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 4 gives the PPS smoothness values of calendared sheets. The smoothest sheet is produced using colour 1 containing GCC and ultra fine clay. This may due to the formation of well packed micro-smooth surface where fine clay particles may be embedded well between GCC particles. The colour with 100% calcium carbonate (colour 3 & 6) gives a less smooth sheet as compared to the colours containing clay. This may be due to the more platy characteristic of clay particles, which tend to flatten more in calendaring roll nips.
Fig 5
Fig. 5 Calendared sheet brightness (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 5 reveals the result of sheet brightness of calendared sheets. Colour formulation 6 with 100% PCC gives the brightest sheet compared to other coating colours. There is no significant difference in sheet brightness coated with other formulations.
Fig 6
Fig. 6 Calendared sheet bulk (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 6 indicates the effect of calendaring on bulk of coated sheets. The formulations containing PCC (colour 4, 5 & 6) give slightly bulkier sheets compared to other formulations containing GCC. This may be due to steeper particle size distribution of PCC particles which produce an open coating structure with large air void volumes.
Fig 7
Fig. 7 Calendared sheet print gloss (under constant calendaring conditions of 60 ºC, linear load 130 kN/m and speed of 22 m/min)

Figure 7 shows the results of print gloss. All formulations containing GCC give higher print gloss values as compared to PCC containing colours. The paper coated with 100% PCC colour gives the lowest print gloss value. The reason for this is the open coating structure which is due to the steep particle size distribution of the PCC particles resulting in fast ink setting rates during printing. This locks in any micro-roughness from filament formed during film splitting when ink is applied on paper. (8)



Response of variable calendaring temperature on properties of coated paper:

Keeping the other calendaring conditions constant (i.e. nip load, calendar speed, and number of calendar nips), the temperature of the calendaring rolls varied from 90 °C to 120 °C and sheets coated with different formulations of coating colour were calendared. The different properties of coated paper were measured and the results dissipated in Figs. from 8 to 13.
Fig 8
Fig. 8 Variation of sheet gloss with variation in calendaring temperature

Figure 8 depicts the variation of sheet gloss coated with different formulations at two temperature ranges. Sheet gloss increased for all the colour formulations as the calendaring temperature increased from 90 °C to 120° C. The variation in gloss is smaller for colours containing PCC as compared to colours containing GCC. This indicates that coating with PCC in formulation can generate high gloss under the lower calendaring pressure and temperature.
Fig 9
Fig. 9 Variation of sheet brightness with variation in calendaring temperature

Figure 9 reveals the variation of sheet brightness with variable calendaring temperature. As expected, sheet brightness decreased for all the colours as the calendaring temperature increased from 90 ºC to 120 ºC. The highest brightness was consistently produced by colour 6 containing 100% PCC. The lowest brightness was given by colours containing ultra fine clay (colour 1 & 4). This is due to the presence of very small air voids which are inefficient at light scattering.
Fig 10
Fig. 10 Variation of sheet smoothness with variation in calendaring temperature

Figure 10 indicates the variation of sheet PPS smoothness with calendaring. As the temperature increases from 90 °C to 120 °C, the smoothness of the sheet increases for all colours. The colour containing ultra fine clay (colour 1 & 4) gives a smoother sheet as compared to other coating colours.
Fig 11
Fig. 11 Variation of print gloss with variation in calendaring temperature

Figure 11 shows the variation of print gloss with a change in calendaring temperature. As indicated, there is only a small variation in print gloss as a result of temperature. The colour formulations containing GCC give higher print gloss values than colour formulations containing PCC. The colour with 100% PCC gives the lowest values of print gloss.
Fig 12
Fig. 12 Variation of sheet bulk with variation in calendaring temperature

Figure 12 indicates the variation of sheet bulk with calendaring, which decreases only slightly with an increase in temperature for all colours.
Fig 13
Fig. 13 Variation of sheet opacity with variation in calendaring temperature

Figure 13 shows the results of the variation of sheet opacity with a change in calendaring temperature. The sheet opacity decreased for all the colours as calendaring temperature increased from 90 °C to 120° C. The colour containing PCC gives higher values of opacity. This is because of the high light scattering effect produced from the open coating structure of PCC with the steep particle size distribution.

CONCLUSIONS

1. Upon calendaring PCC containing coatings, colours showed relatively higher improvement in gloss than GCC containing coatings.
2. PCC containing formulations gave higher opacity than coating colours containing GCC.
3. Smoother sheet is obtained with GCC along with ultra fine clay.
4. Brightness and bulk of a PCC coated sheet is higher than that of a GCC coated sheet.
5. Print gloss is higher in the case of coating formulations having GCC.
6. Increase in calendaring temperature from 90ºC to 120ºC caused
• Increase in sheet gloss. However the effect was relatively less in case of PCC coating as compared to GCC.
• Decrease in brightness. The effect was lowest in case of PCC.
• Improvement in smoothness. Coating colours containing ultra fine clay gave smooth sheet.
• The variation in print gloss was minor.
• Bulk and opacity of sheet got reduced.

REFERENCES

(1) Gane, P and Hooper, J. – An evaluation of interaction between coating colour and base paper by coating profile analysis, Fundamentals of papermaking, 9th fundamental research symposium, Cambridge, p. 871-893 (1989)
(2) Baumeister, M – Quality optimization by control of coating structure, Tappi J. 64 p. 85-89 (1981).
(3) Trefz, M. - Film coated offset grades- theoretical aspects and practical experience, Tappi Coating conference proceedings 1995, p. 435 (1995).
(4) Hiller, R. – A study of Paper Improvements through supercalendaring, Tappi J. 49 (8), p.348- 352 (1996).
(5) Hunger, G., - The influence of calendaring on sheet and coating densification and on final printability, Tappi J. 50(7) p. 372- 379 (1967).
(6) Rodal, J. – Soft nip calendaring of paper and paperboard, Tappi J. 72 (5) p. 177-186 (1989).
(7) Esa Lebtinen - Pigment coating and surface sizing of Paper, Papermaking Science and Technology, Book 11, PI & TAPPI Press, Finland 2000
(8) Hiorns, A – Calendaring response of calcium carbonates in double coated woodfree paper, TAPPI 2001 Coating and Graphic arts Conference and Trade fair, San Diego, CA

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