CHANGES TO PCC STRUCTURE IN

Thad C. Maloney*, José Ataide, Juha Kekkonen, Henrik Fordsmand, Henrik Hoeg-Petersen

*JM Huber, Telakkatie 5A, 49460 Hamina, Finland; [email protected]

Abstract For precipitated calcium carbonate (PCC), it is well known that the degree of structuring has a strong influence on the performance of the pigment as a filler. “Structuring” refers to the aggregation of small primary particles into a larger secondary structure. A high degree of pigment structure results in high bulk and good optical performance. A long-standing question of papermakers concerns the possible break-down of PCC from an aggregated structure into primary particles. This could conceivably arise either from high shear forces in the wet end or from high structural pressure during web consolidation. This study attempts to determine if such a break-down occurs for S-PCC in fine paper production and to find out the implications for paper quality.

In the study, we demonstrate with lab sheets that when PCC structure breaks down there is a high negative impact on paper properties. For example, two-sidedness increases and bulk decreases. In order to examine the PCC particle properties on a , a new analytical technique was developed. The technique is based on the oxidation of chemical fibers under controlled conditions and the subsequent analysis of the ash residues with particle size techniques. Our studies indicate that this is an acceptable method to characterize fillers in pulp suspensions and paper handsheets. We go on to study the filler properties from the PCC plant, through the approach flow system, to the couch of a modern fine paper machine. Our analysis indicates that the S-PCC studied here did not break down to a significant extent on full scale paper machines, though we did find evidence of breakdown on a pilot machine. It was also found that the new test can give an indication of the amount of PCC flocs formed in the wet end and in web consolidation.

Introduction For uncoated woodfree copy paper, bulk is one of the most important quality parameters. Generally speaking, the optical properties that can be reached with today’s pulp bleaching methods, optical brighteners and fillers are adequate. Likewise, strength of fine paper is less of an issue than in the 1980’s and 1990’s. There are several reasons why bulk (at a specified smoothness) is so important to fine paper manufacturers. One reason is related to the manufacturing economies because high bulk allows one to decrease or to increase filler contents while maintaining adequate stiffness.

Precipitated calcium carbonate (PCC) has emerged as the filler of choice for today’s fine paper. This is because PCC gives excellent optics and bulk compared to its leading competing filler, ground calcium carbonate (GCC). Also, the on-site solution allows one to synthesize several different morphologies with different characteristics that can be used to help optimize a mill’s grade mix. Two of the important morphologies for uncoated copy paper are rhombohedral PCC (R-PCC) and scalenohedral PCC (S- PCC). R-PCC is not highly aggregated and has a low surface area-to-particle size ratio and thus gives excellent drainage and high paper machine speed. However, even more important in fine paper manufacture is S-PCC which is useful for manufacturing paper with high bulk.

Paper bulk at a given smoothness depends on several factors including the paper machine design and pulp quality variables. The filler type and amount also has an important role. A large particle size and narrow particle size distribution (PSD) increases paper bulk by decreasing the particle packing. Today, both these variables have been optimized with PCC and there is very little room left for further improvement by either of these appraoches. Another important variable for bulk is the amount of structure or aggregation. The success that S-PCC enjoys is largely due to its high amount of structure.

The structure of S-PCC is shown in Figure 1. It it’s a useful view to consider the arrangement of PCC in paper in terms of structural hierarchy. The arrangement of the crystal lattice in the primary level of structural hierarchy determines certain material properties such as index or refraction and melting point. For PCC used in paper manufacture the important crystalline types are aragonite and calcite. S- PCC is the latter. The shape and size of the individual crystals influences the specific surface area, which has a direct consequence on the consumption of wet end additives in papermaking. However, it is the higher levels of structural hierarchy which are most important for paper bulk.

1 2 3 4 Crystal Structure Crystal Morphology Primary aggregation Secondary aggregation

Figure 1. The levels of structural hierarchy in S-PCC: 1) molecular organization, 2) crystal morphology, 3) primary aggregation and 4) secondary aggregation (PCC flocs).

In the synthesis of S-PCC, the cigar shaped crystals grow out of a calcium hydroxide core during the carbonization reaction. After carbonization is complete, the single crystals are arranged in a 3- dimensional star-shaped pattern attached at the original core through ionic bonding forces. In some places on the S-PCC aggregate secondary nucleation may lead to further growth of cigar shaped crystals. This third level of structural hierarchy is what is normally associate with the PCC particle size. The size and structure of this aggregate has an important consequence for paper bulk.

We can also consider a fourth level of structure. In the paper machine wet end, colloidal and hydrodynamic forces may cause the PCC aggregates to flocculate together. Thus, in paper, there is a certain distribution of filler in the x, y an z directions that may be more or less homogenous (Karrila, S. et al. 2002). The bonding of these larger filler aggregates is much weaker than the primary aggregates, consisting mostly of electrostatic forces. It has been established through filler flocculation experiments (Holm M. and Manner, H. 2003) that this highest level of filler structure can influence paper properties e.g. optics, strength and bulk.

From the point that it is manufactured in the on-site plant to the finished paper PCC experience some rather severe forces (Pelton, R. H. 1984) that may break down the aggregates into individual crystals. In the wet end, the high shear in the pumps, cleaning units and in the formation zone (the highest shear is normal at the fan pump and headbox screen) may be adequate to break apart the primary aggregates. In press dewatering, the paper is subjected to high pressure, in the range of 10 MPa for a roll press, which could “crush” the aggregated PCC structure causing a loss of bulk. Severe pressures in the stack could also break apart the PCC structure causing a loss of bulk.

The objective of this study was to determine how deaggregation of S-PCC affects paper properties and to determine if, in fact, S-PCC breaks down during fine paper manufacture.

Materials and Methods In the first part of this study, S-PCC with a deliberately broken down structure was compared to conventional in-tact S-PCC. The S-PCC used in the study is FS-240 and also a similar grade FS-270. These are commercial grades of S-PCC produced by JM Huber with an average particle size of 2.4 µm and 2.7 µm, respectively, as measured with a Sedigraph. A 70/30 hardwood/softwood mixture of industrially prepared refined kraft pulp was used in handsheet making. A sample of the FS-240 was broken down by exposing a 20% slurry to 12 hours in an Ultra-Turrax T50 mixer exposed to a shear of 1.83x10 4 s -1. Our experiments showed that this was adequate conditions to detach a significant fraction of primary crystals from the aggregates.

80 g/m 2 handsheets were prepared at different filler content in the range of 22-28% filler content on a Fibertech dynamic sheet former. A cationic polymer – micro particle retention system was used: 200 g/ton Kemira Fennopol K3400 R + 2500 g/ton bentonite. The pH was not adjusted and no other additives were used. Sheets were calendered in a Gradek calender with 2+2 nips at 11 kN/m pressure at room temperature. Sheets were tested with standard ISO methods.

Two different methods for particle size analysis were used. A Sedigraph 5100 measures particle size distributions by sedimentation rate. The particle size by this method is especially effected by the particle specific volume. Some measurements were also performed with a Horiba LA-920. This method is also adequate to quantify PCC particle size distributions. However, because it is based on laser diffraction the resulting PSD does not necessarily equal that from Sedigraph. The PCC or the ash residues was first treated in a sonic bath for 30 minutes, followed by addition of a cationic dispersant, Fennofix 40, before particle size measurements. We have found that for a conventional PCC particle size distribution these are adequate dispersion conditions to break up flocs, but not to break down the primary aggregates to individual PCC crystals.

The PSD of the PCC in either the pulp furnish or paper was determined as follows: A sample of the dried furnish or paper was treated in a Leco TGA-601 thermo gravimetric analyzer under conditions sufficient to oxidize the cellulosic fibers. An 8 gram sample of the dried furnish or paper was heated first to 300 oC and then held constant for 2 hours. The oven gas was air. These are sufficient conditions to oxidize most of the organic material but not to cause the pulp to burst into flames. The temperature was then increased to 500 oC and held for 1 hour to remove any remaining organic material. However, this is well below 900 oC, at which temperature calcite decomposes. The ash residue was dispersed and measured with Sedigraph as described above.

Results and Discussion Handsheet experiments The effect of the high sheer treatment on the PSD is shown in Figure 2. The node centered around 5 µm is associated with the PCC primary aggregates. The small node at around 0.5 µm is cause by the primary crystals. In the unsheared PCC there is a very small amount of the individual crystals detectable. After exposure to shear the amount of individual crystals increases significantly changing the distribution from nearly mono- to bimodal. Furthermore the average size of the agglomerates also decreases.

14 PCC aggregates

12 Individual PCC crystals 10

8 FS-240 Sheared FS-240 6

4 MassFrequency (%)

2

0 0.1 1 10 100 Particle Diameter (µm)

Figure 2. The effect of shear treatment on the particle size distribution of FS-240 as measured in a Horiba LA-920.

The deaggregation of the PCC has a distinct effect on most paper properties. Figure 3 shows that when the S-PCC structure breaks down the bulk substantially decreases. Loss in bulk can be caused by either a decrease in particle size or an increase in the width of the PSD. In this case, the break down of the PCC agglomerates, resulting in the decreases in average particle size, contributes to the lower bulk. It also seems reasonable, that an increase in the fraction of single PCC crystals has resulted in a higher degree of PCC packing. The deagglomeration of the PCC has a substantial negative effect on optical performance, shown in Figure 4. This is rather interesting, as the average particle size of FS-240 is somewhat above the maximum for optimum light scattering. If the size of the PCC would have been reduced by synthesizing a smaller S-PCC aggregate, then the light scattering would be expected to increase. However, the individual PCC crystals in the size range about 0.1 µm wide x 0.5-1.0 µm long (by SEM) are below the optimum size for light scattering. Therefore, when the PCC is broken down by shear into individual crystals the light scattering decreases. Figure 5 shows that when the structure of the PCC breaks down the tensile strength decreases. This is because a higher number of particles per unit mass of PCC allows higher fiber surface coverage resulting in higher disruption of fiber bonds at a given filler content. Figure 6 shows that break down of the PCC has had a fairly small effect on the smoothness. However, the change to a bimodal distribution has increased the smoothness two- sidedness. This is because the separated PCC crystals are preferentially pulled through the sheet by dewatering forces and enriched on the wire side, increasing the smoothness of this side. Even-sidedness is clearly favored by a homogenous and narrow distribution of particles.

1.25 60 Untreated S-PCC Untreated S-PCC Sheared S-PCC Sheared S-PCC 1.20 50

1.15 40 Bulk (cm3/g) Bulk Tensile Index (Nm/g) Index Tensile

1.10 30 20 22 24 26 28 30 20 22 24 26 28 30 Filler content (%) Filler Content (%) Figure 5. The effect of deagglomeration Figure 3. The effect of shear on PCC on the machine direction tensile index of used in handsheets. handsheets.

4.0

/kg) 80 2 Untreated S-PCC

75 Sheared S-PCC 3.0

70 2.0 Untreated S-PCC, TS Untreated S-PCC, WS 65 1.0 Sheared S-PCC, TS Sheared S-PCC, WS PPS Smoothness (µm) Smoothness PPS 60 0.0 Light Scattering Coefficient (m LightScattering 20 22 24 26 28 30 1.10 1.12 1.14 1.16 1.18 1.20 1.22 3 Filler Content (%) Bulk (cm /g) Figure 4. The effect of PCC Figure 6. The top (TS) and bottom (WS) deagglomeration on sheet light scattering. side smoothness vs. bulk for handsheets made from normal S-PCC and deagglomerated S-PCC.

Breakdown of PCC on the paper machine. From the above study, it is seen that the break down of S-PCC’s structure causes a highly negative effect on the paper properties. In that study, rather severe conditions were used to breakdown the PCC. Shear rates on a paper machine may be even higher than used in our lab experiment (Pelton, R. H. 84), though exposure times to high shear typically much lower. The chemical conditions on a paper machine may be more contusive to PCC deagglomeration than in our lab experiment. Partial dissolution of the PCC will weaken the aggregate so that it may break down at low shear levels. In fact, in the lab we have used pH shocks in the range of 3-4 to completely deaggregated S-PCC even in the absence of a shear field. On a fine paper machine, exposure of the PCC to areas of low concentration, especially in the long circulation, and pH shocks from e.g. alum used in connection with AKD , could weaken the aggregates and facilitate break down.

To our knowledge, a good method does not exist for the measurement of filler PSD after the filler has been mixed with pulp or formed into a sheet. We attempted, in the first place, to solve this problem by dissolving the pulp fibers with a cellulase enzyme. However, we were not able to find an enzyme that could completely dissolve the fibers at a suitably high pH within a reasonable time frame. Therefore, we developed the burning test described in the experimental section. The validation of the test is shown in Figure 7. In this experiment, the PSD of FS-270 measured with the Sedigraph was compared to FS-270 that was heated under the conditions described in the experimental section. The FS-270 was also mixed with pulp in the ratio 3:1 pulp:PCC, dewatered, , dried and then heated under the above-mentioned conditions to completely oxidize the pulp fibers. The results show that the heating of the PCC, or heating in the presence of pulp fib does not significantly affect the PSD. SEM images of the ash residue shown PCC crystals that are similar to unburned PCC, indicating that these have not physically changed during heating. Also, the pH of the ash residue was 9.5 indicating that only a very small amount of CaCO3 had decomposed and formed CaO in the oxidation stage. The small amount of non-PCC inorganic material in the pulp was assumed to have negligible influence on the PSD.

50 FS-270; Unburned FS-270, Burned 40 FS-270 Mixed with pulp, burned 30

20 MassFrequency(%) 10

0 0.1 1.0 10.0 Particle Diameter (µm)

Figure 7. PSD of S-PCC, S-PCC heated under conditions of the oxidation test, and S-PCC mixed with pulp then heated under the same conditions to oxidize and remove pulp. The oxidation has no effect on the PSD.

The new PSD test was used to evaluate the possible breakdown of FS-270 in a pilot paper machine trial. The pilot machine was running at 1600 m/min under typical European fine paper conditions; 80 g/m 2 , 20-22% filler content and no other fillers present than FS-270. The results in Figure 8 compare the PSD of the virgin filler (dosing tank sample) to the PSD of the PCC in the headbox. The same analysis procedure was used for both samples. In Figure 8, a broadening of the distribution in both directions is observed. There is a small increase in the amount of particle below 1 µm, which is associated with the existence of individual PCC crystals. This shows that under the conditions of the pilot machine the FS-270 has begun to break down. Surprisingly, we also see the formation of a new node at about 7 µm. This indicates that some PCC flocs have formed that are roughly about the size of 2 primary PCC agglomerates.

Since the implementation of the fiber oxidation PSD test we have observed on many occasions the existence of large PCC flocs. We believe that these are flocs that are present in the original sample, but that the strength of the flocs is increased in the oxidation through a sintering-like process. Sintering occurs when the temperature of packed particles is raised to a point near, but below, the bulk melting temperature. The very high radius of curvature around sharp edges cause melting and welding at crystal contact points. The melting point of calcite is 1339 oC, which is well above the decarbonization point around 900 oC, which is also well above the test temperature of 500 oC. Therefore, for this material and under these conditions, we do not expect the same type of sintering that takes place in e.g. metal powders. However, some sort of welding of the PCC particles takes place. This may be exacerbated by the presence of organic and inorganic impurities in the sample. In any case, it is clear that in order for the flocs to be bonded together the primary aggregates must be in intimate contact. Therefore the amount of large particles – 7 µm and above- which is measured in the oxidation test gives information of the state of PCC flocculation the original stock or paper sample.

60 Primary Aggregates

50 Virgin SPCC SPCC in Headbox 40 Primary 30 Crystals PCC Flocs 20 MassFrequency (%) 10

0 0.1 1.0 10.0 100.0 Partical Diameter (µm)

Figure 8. The PSD for PCC leaving the PCC plant and in the headbox of a fine paper machine. A broadening of the PSD is seen.

One of the topics we were interested in this study was the possible breakdown of PCC in calendering. It seems possible that a high calendering load could cause the PCC structure to collapse causing a loss of bulk. If this were an important mechanism in the sheet consolidation process, then we would expect the amount of primary PCC crystals to increase with calendering pressure. In Figure 9, the PSD for PCC in paper calendered in a single soft calender at 80 kN/m is compared to the PSD from paper calendered at 200 kN/m. The samples for this study were taken from the same pilot machine study mentioned above. Figure 9 shows that the calendering has no effect on the amount of primary PCC crystals which are present. The primary PCC crystals in the samples has already formed in the wet end. The high calendering pressure has resulted in the formation of a small peak or large particles centered around 50 µm. This may be due to tighter consolidation of filler flocs which are then “sintered” together in the fiber oxidation step.

22 Primary Aggregates Heavy line 20 pressure 18 Primary Light line 16 Crystals pressure 14 PCC Flocs 12 10 8 6 Mass Frequency (%) 4 2 0 0.1 1.0 10.0 100.0 Particle Diameter (µm)

Figure 9. The PSD of FS-270 in a pilot machine soft nip calendar at low and high calendaring pressures.

It seemed from the pilot study that some PCC deagglomeration and flocculation processes may take place in fine paper manufacture. It was decided to investigate this phenomenon further on industrial scale paper machines. We studied three different high speed modern fine paper machines by taking a number of samples for the PCC plant through the approach flow all the way to the finished paper. The results from all these surveys were consistent – there is no evidence of significant PCC breakdown in the approach flow or on the paper machine. This is illustrated in Figure 10 which shows results from one of the machines. The slight increase in fine particles in the machine chest may be considered negligible, especially since the primary particles are no longer visible in the headbox or on the paper machine.

More detailed studies of the pumps and machinery in two PCC plants were also made and these showed no evidence of significant PCC breakdown in either plant. We believe that the initial evidence of PCC breakdown we found at the pilot machine headbox was due to the rather aggressive conditions on this machine. On this machine, operating speeds – thus sheer- were high and there is a significant recirculation of broke. Thus the PCC is exposed to longer periods of high sheer than is normally seen on an industrial fine paper machine. In Figure 10, it is seen that the amount of PCC agglomerates increases from PCC plant, to wet end, to the pope. This is consistent with observations from other paper machines. We believe that the formation of secondary PCC aggregates is caused primarily in the wet end by homoflocculation of the PCC and is further enhanced in the consolidation process as the PCC particles are brought into closer proximity. 40

PCC storage tank Machine chest 30 Headbox Before calender 20 After calender

Mass FrequencyMass (%) 10

0 0.1 1.0 10.0 100.0 Particle Diameter (µm)

Figure 10. The PSD from a full scale fine paper machine using PCC. Samples are taken along the process from the PCC plant to the finished paper at the pope reel.

The test which was used in this study clearly gives useful information on the state of agglomeration of PCC in stock suspensions and in paper. However, it is also apparent that there is some work remaining to be done on the test development, especially in understanding the conditions that lead to the formation of flocs or aggregates. Also, one should exercise caution in generalizing the results of the study to other paper machines and grades of PCC. In fact, it if the authors’ experience that different grades of PCC are aggregated with different bonding strengths, and it is clear that different paper machines have widely different chemistries and shear profiles.

Conclussions It was found that the breakdown of scalenohedral PCC to primary crystals causes a degradation in handsheet properties including lower bulk, worse optics and more two-sidedness. A fiber oxidation test was developed which seems to give a good indication of the state of agglomeration of PCC in pulp suspensions or in finished paper. Under rather extreme conditions in a pilot paper machine, it was found that the S-PCC begins to break down into primary crystals. However, in three industrial paper machines was found that the PCC did not break down in the wet end or in consolidation. In both pilot and full scale experiments it was found that flocs or secondary aggregates of PCC are formed. It is believed that these are formed in the wet end by homoflocculation and in web consolidation.

References

Holm M. and Manner, H. (2003). APPITA anual conference: 337-342

Karrila, S., Susilo, A. and Champine, J. (2002). 2002 Progress in paper physics seminar: 114-117

Pelton, R. H. (84). TAPPI 1984 Papermakers Conference: 1-6