Development of Deformable Calcium Carbonate for High Filler Paper.

Controlling the size and rigidity of calcium carbonate became possible. HCCs were developed and manufactured by the in situ reaction of carbon dioxide and calcium oxide, which were already preflocculated together with GCC using ionic polymers before the reaction. HCC is deformable under pressure during the papermaking process, and its degree of rigidity can be controlled by adjusting the fraction of calcium oxide. The size of HCC can be further controlled by adjusting shearing force. The more the fraction of calcium oxide, the more rigid the HCC and the smaller the diameter of the HCC. When used in papermaking, HCC increased the tensile strength and bulk of paper simultaneously without lowering other essential paper properties, and its deformable nature under pressure improved paper smoothness. Saving chemical pulp by 10% by replacing it with HCC, which is 3-4 times less expensive than the chemical pulp, was demonstrated successfully without lowering the essential properties of paper. Implementation of HCC in the paper mill may result in saving chemical pulp, drying energy, and production cost. The paper mill may utilize the carbon dioxide from the mill stack after purification for HCC preparation.

Introduction

Replacing chemical pulp with inorganic materials such as calcium carbonate without loss of key paper properties in papermaking is a dream come true for the papermakers and beneficial for forest conservation, drying energy reduction, and production cost saving.[1] In reality, achieving 1–2% increase in inorganic filler content in printing paper without lowering essential properties should be a great achievement in paper mill.

To produce highly filled paper, lumen loading of fillers was tried by corefining the fillers and the wood fibers.[2,3] After the refining, excess fillers were washed out. High strength and high first-pass retention were reported because fillers were located inside the fiber lumen, and lumen-loaded fillers did not disturb the interfiber bonding. However, fillers could not enter into the lumen in large amounts and only small-size fillers such as TiO2 could be effective to go into the lumens.

High filler-loaded paper has been studied using the filler preflocculation method where precipitated calcium carbonate (PCC) or ground calcium carbonate (GCC) was flocculated to around 20–40 μm in diameter by applying ionic polymers and controlled turbulence to regulate the floc size.[4−9] In another method called the agglomeration method, positively and negatively charged fillers were prepared separately and combined later in the papermaking process.[10] These preflocculation and agglomeration methods decreased the surface area of the fillers by making flocs with fillers and allowed more hydrogen bonding area between wood fibers to increase strength properties of the paper. High bonding properties of the paper sheet by these methods allowed more addition of fillers as long as other essential properties are acceptable. However, these filler preflocculation and agglomeration methods do not improve the bulk and stiffness of the paper sheet, which are the most difficult properties to achieve in developing high filler-loaded paper.

A few other approaches allowed some benefits in strength improvement but implementation in the mill was not very successful because, we believe, there was either complexity in the processes or trivial economic benefits. Silenius developed “Superfill,” which was made by in situ CaCO3 formation on fractionated fines, and next, washed out unattached fillers.[11] Superfill improved paper strength and smoothness, but not the bulk. Coflocculation of fibrous fines and fillers improved strength properties, opacity, and printing characteristics.[12] Cellulose nanofibril, more fibrillated one than the fines mentioned above, also has gained attention as the filler-modifying materials for high-loading paper because of its high surface area and bonding ability.[13−15] High strength properties were obtained by cooking of starch in the presence of fillers. The cooked filler–starch mixture was dried and ground to produce papermaking fillers.[16,17] Recently, preflocs by adding polymers to filler–starch mixtures can increase the strength of the filler-loaded paper by enhanced filler bondability,[18]

Hybrid calcium carbonate (HCC) was developed by us, where a mixture of GCC and calcium oxide was preflocculated using ionic polymers in the first step and then, carbon dioxide was injected into the mixture to make semirigid agglomerates of the GCC and newly formed PCC from the calcium oxide.[19−22] The HCC allowed the handsheets to improve both high bulk and high tensile strength simultaneously, which were almost impossible to achieve together in conventional papermaking.

In this study, we investigated the effect of polymer preflocculation during the HCC preparation process and the mechanism of how HCC properties were developed. In addition, we tried to vary the deformability or rigidity of HCC by varying the fraction of the calcium oxide addition level. If we can control the rigidity, we can control the retention rate and white water system in the papermaking process and the bulk of the paper. Furthermore, we wanted to know whether and how much HCC has the potential to replace the expensive bleached chemical pulp. Finally, the centrifugal pressure method was investigated to predict the bulk of the paper before papermaking by comparing the filler compacting degree.

Experimental Section

Materials and Handsheet Preparation

Two different GCCs were donated by Omya Korea Inc. located in South Korea, and their mean sizes were reported to us as 2.0 and 10.0 μm. Calcium oxide was purchased from Korea Showa Chemicals Co. As a retention aid for papermaking, cationic PAM (C-PAM. MW 5–7 million g/mol +5 meq/g) from CIBA Specialty Chemical Korea was used at 0.1% based on the dry weight of the GCC-containing furnish.

To make the handsheets containing fillers, we used a mixture (20:80) of commercial softwood bleached Kraft pulp (SwBKP; a mixture of hemlock, Douglas fir, and cedar) and hardwood bleached Kraft pulp (HwBKP; a mixture of aspen and poplar) as the wood fiber furnish, both of which came from Canada. These wood pulps were refined together in a valley beater until their freeness reached 500 mL CSF (TAPPI T227 om-99). Then, after mixing with fillers, we made handsheets of 60 g/m2 basis weight (TAPPI T205 sp-95). The ash content (TAPPI 413 om-93), bulk (TAPPI T411 om-97), tensile strength (ISO 1924), Bekk smoothness (TAPPI T479 cm-99), and Gurley stiffness (TAPPI T543 om-00) of the handsheets were measured according to the standard methods.

Preparation of HCC and Preflocculated GCC

To prepare preflocculated GCC (pGCC), 0.02% cationic PAM, the same PAM used as the retention aid, and 0.02% anionic polymer (Perform SP7200, MW 0.5 million g/mol −5.0 to −3.0 meq/mol Hercules, USA) were added to 2.0 μm GCC by weight sequentially at 2000 rpm. As soon as the GCC flocs were made, their sizes were measured using FlowCAM[23,24] and we called it pGCC. To prepare HCC10, HCC30, HCC50, HCC70, HCC85, and HCC100, 5.62, 16.9, 28.1, 39.3, 47.8, and 56.2 g of calcium oxide were mixed together with 90, 70, 50, 30, 15, and 0 g of 2.0 μm GCC, respectively, to produce 100 g of HCC for each condition. To perform preflocculation of GCC and calcium oxide, 0.03% cationic PAM and 0.04% anionic polymer based on the dry weight of the solid were sequentially added while stirring. After the preflocculation, carbon dioxide was injected into the preflocculated mixture at 30 °C and 350 rpm until a neutral pH was reached. For comparison, calcium oxide and GCC were mixed together with the same ratios as those of HCCs but this time without ionic polymers. We called them no polymer HCC (NHCC).

The sizes and shapes of the GCC, HCC, NHCC, and pGCC were determined using FlowCAM and a scanning electron microscope (S-4800 model. Hitachi, Japan). The FlowCAM dynamic imaging particle analyzer (Benchtop B3 Series, Fluid Imaging Technologies, USA) measured the volume moment mean values (D[4,3]) of the diameter.[24] A few drops of the calcium carbonate fillers were dried on the sample holders for further micrographic study.

Filler Evaluation for Its Bulk Development

We tried to obtain a method for comparing the bulk developing capability of the fillers before making handsheets. In our previous publication,[22] we developed a cylinder method for comparing the bulk developing capability of the fillers by applying low pressure and that result was quite well-correlated with the handsheet bulk. However, we found from the literature[25] that a paper web experiences much higher wet pressing pressure on the paper machine (3–18 MPa or 30–180 atm). With the cylinder method, it was impossible to apply this high pressure. The TAPPI standard handsheet method designated one specific wet pressing pressure (414 kPa or 4.14 atm), and with only the handsheets that were prepared according to the standard method, we can compare their properties. We tried to use a centrifuge that can apply high pressure on the fillers in water and to apply the water retention value (WRV) concept (the TAPPI UM 256) for the application of pressure. We put 12.5 g of fillers (GCC, pGCC, and HCC. O.D. weight by calculation) to the test tube that belongs to the centrifuge (MF600, Hanil Science Industrial, Republic of Korea) and filled water up to the 50 mL line for each test tube. The cross-sectional area of each test tube was 5 cm2, and the weight of the sample for each tube was 54 g (water plus calcium carbonate). The same amount of filler and water was present for each test tube, and we think that the density of the fillers should be the same. After 15 min. of centrifugation at the designated acceleration of 500, 1500, and 2500 G, for which pressures can be calculated as 5, 15, and 25 atm, respectively, we checked the height differences. Centrifugation of fillers is quite different from applying wet pressing pressure on the paper sheet that contains fillers. However, as long as it gave high correlation with the handsheet bulk, we think it may be worth trying. It was not a complicated procedure and gave exaggerated differences between fillers at these high centrifugal pressures.

Results and Discussion

Morphologies of Fillers

Micrographs of the fillers are shown in Figure . In the pGCC (Figure b), GCC particles (Figure a) were attached together to make flocs in water and maintained the shape until dried on a specimen holder. In the HCC case, the GCC particles and the newly formed PCC particles were attached or “physically bonded” together to form rigid agglomerates. Choi et al. had shown that they were a little deformable under the compressive pressure, while pGCCs were totally collapsed as much as GCCs at the same level of pressure.[22] NHCC (Figure d) consisted of small-size agglomerates and particles, while HCC had distinctive individual agglomerates. The crystal form of HCC50 was characterized by FTIR spectra and turned out to be calcite (Figure S1 in the Supporting Information).

Figure 1

SEM micrographs of (a) GCC, (b) pGCC, (c) HCC50, and (d) NHCC (no polymer HCC50) and (e) diameters of HCC and NHCC particles in water as determined using the FlowCAM.

The size of GCC (or NHCC 0) and its preflocculated one (pGCC or HCC0) measured using FlowCAM is shown in Figure e. The volume moment mean value (D[4,3]) of the GCC was bigger than the size the supplier informed us, 2 μm, but we can use it for comparing the relative size. Although not presented here, the size of 10 μm GCC was measured as 15.4 μm in the FlowCAM, which is larger than that of NHCC10. The size of pGCC or HCC 0 in Figure e was the largest, and the size of HCC decreased as the fraction of calcium oxide increased. It seemed that as the proportion of the newly formed calcium carbonate increases, the HCC floc becomes smaller and more rigid. However, the size of HCC without the polymer increased in proportional to the fraction of the calcium oxide. It seemed that during the in situ calcium carbonate formation, GCC particles gather around the newly formed calcium carbonate. More calcium carbonate formation from calcium oxide resulted in the size increase of the NHCC. The size of HCC 100 is very large compared to that of 2 μm GCC. This is because we used very low intensity agitation for HCC formation in this experiment.

Schematics of the Bulk Development Mechanism

We made a model describing the behavior of HCC when different fractions of calcium oxide were used, as shown in Figure . In Figure a, in the presence of ionic polymers but in the absence of calcium oxide (top in Figure a), carbon dioxide does not undergo reaction, and there is only the preflocculation of GCC by the ionic polymers to result in pGCC. This pGCC will have a very large size but no resistance to wet pressing pressure and gives low bulk in the handsheet. It is well known that the preflocculated GCC (pGCC) gives high tensile strength because of the fact that most of GCCs are aggregated by ionic polymers and almost no free GCCs are available.[8,22] At a low fraction of calcium oxide (middle in Figure a), calcium oxide made a small number of the newly formed calcium carbonates that have binding capability with GCCs. Binding GCCs by newly formed calcium carbonate and the presence of ionic polymers make the HCC size smaller than that of pGCC and give some resistance against the wet pressing pressure. This resistance against wet pressing pressure may increase the bulk of the handsheet. At a high fraction of calcium oxide (bottom in Figure a), calcium oxide makes a large number of the newly formed calcium carbonate that binds GCCs together tightly and the HCC became much less deformable and its size, the smallest. The ionic polymer still works to attach the GCCs to the body of HCC and to make almost no free GCCs available. This less-deformable HCC with no free GCCs was expected to make the highest bulk and high tensile strength in the handsheet. In case of high shear force during the papermaking process, the rigid HCC (bottom in Figure a) might help to resist against breakage.

Figure 2

Schematics of HCC formation with and without ionic polymers. (a) With polymers. Top: no CaO, middle: small amount of CaO, and bottom: large amount of CaO and (b) without polymers. Top: no CaO, middle: small amount of CaO, and bottom: large amount of CaO.

In Figure b, in the absence of calcium oxide and ionic polymers (top in Figure b), GCC does not change its size. The tensile strength should be very low because all the GCC particles are free and they block the hydrogen bonding between wood fibers. At low fraction of calcium oxide (middle in Figure b), calcium oxide makes a small number of the newly formed calcium carbonate that has binding capability with GCCs. They make small flocs that are resistant against wet pressing pressure, but there are no ionic polymers to bind the flocs and GCCs. Therefore, there are still a large number of free GCCs that may interfere the interfiber bonding between wood fibers. Resistance to wet pressing pressure increases bulk in the handsheet. The average size of the NHCC (HCC without the polymer, middle in Figure b) will be larger than that of the GCC (top in Figure b). At a high fraction of calcium oxide (bottom in Figure b), calcium oxide makes a large number of the newly formed calcium carbonates that bind more GCCs together tightly. NHCC became larger in size because of combining more GCCs by the newly formed calcium carbonate and less deformable. This NHCC will make high bulk in the handsheet but still has free GCCs because of lack of ionic polymers. These small-size, free GCC particles will interfere the interfiber bonding. From the model analysis, we think that we can control the size, the size distribution, and the rigidity of the HCC as we need by controlling the fraction of calcium oxide and by controlling the addition level of the ionic polymers.

Application of Centrifugal Pressure on the Fillers

We may need to know how much bulk will be developed in the sheet with the fillers before making handsheets. As we described the procedure of using a centrifugal device in the Experimental Section, we put the same amount of fillers and water in the individual test tube and applied the centrifugal pressure of 5, 15, and 25 atm for 15 min. We selected these pressures because in wet pressing on the paper machine, the maximum pressure was very high[25] and we wanted to see the compressed shape of the filler at a similar higher pressure than in the TAPPI standard handsheet method (TAPPI T205 sp-95). The height of the fillers after application of centrifugal pressure is shown in Figure a. At 0 atm pressure, pGCC gave much higher height than that of 2 and 10 μm GCCs. However, as soon as centrifugal pressure was applied, pGCC, which consists of the GCC, 2 μm, was collapsed to the height of the GCC, 2 μm. This was because the ionic polymers used in pGCC kept the floc shape at 0 atm but as soon as pressure was applied, ionic polymers could not withstand the pressure. The curve patterns of the centrifugal heights at three different centrifugal forces (5, 15, and 25 atm) were very similar. We made the relationship between centrifugal heights at 15 atm pressure and bulk of the handsheets as shown in Figure b and obtained very high regression coefficients (R2 = 0.9791 for 30% ash and R2 = 0.9621 for 40% ash). We also used the heights at 5 and 25 atm for the regression with bulk and obtained similar results as the 15 atm case. We may use this relationship to predict the bulk of the paper when different fractions of calcium carbonate and different types of ionic polymers for preflocculation were used. We show the test tube used in the centrifugal pressure method in Figure S2 in the Supporting Information section.

Figure 3

Centrifugal height and paper bulk relationship at different pressures for the HCC fillers. (a) Centrifugal height of different fillers and (b) relationship between the centrifugal height and paper bulk.

Physical Properties of Paper Sheets Containing Different HCC Fillers

Paper sheets containing 30 and 40% ash were prepared by adjusting the filler input. The presence of the ionic polymers that aggregate GCC and calcium oxide during HCC preparation determined the tensile strength of the paper, as shown in Figure a. Here, the tensile strength was converted to the breaking length, which compensates for the effect of basis weight variation. We expected that the behavior in Figure a where the NHCC prepared without ionic polymers had free, unattached calcium carbonate particles. These free particles interfere the interfiber bonding between wood fibers. In Figure b, the 2 μm GCC (or HCC0 without the polymer) and its preflocculated one, pGCC, (or HCC0 with the polymer) gave almost the identical sheet bulk. This meant that initially large-size pGCC flocs were totally collapsed to a flat shape inside the sheets. Although not presented here, the 10 μm GCC gave higher bulk than 2 μm GCC but lower than HCC10. From HCC10 to HCC100, the bulk of the sheets increased. The sizes of HCC85 and HCC100 were very small among the HCCs (Figure e) but gave high bulk because of their rigidity or resistance against compressive pressure during the papermaking process (Figure b). One interesting behavior is that 40% HCC curves crossed the 30% HCC curves. At lower fraction of calcium oxide, the bulk from the 40% HCC was lower than that from the 30% HCC. At higher fraction of calcium oxide (>HCC50), it was reversed. This meant that at the high fraction of calcium oxide (>HCC50), the more the ash content, the higher the bulk of the HCC-containing paper.

Figure 4

Properties of the HCC-containing paper. “HCC0” with and without polymers means pGCC (preflocculated 2 μm GCC) and 2 μm GCC, respectively. (a) Breaking length, (b) bulk, (c) Gurley stiffness, and (d) Bekk smoothness.

Stiffness is one of the essential properties of white printing paper and known to be difficult to increase. Stiffness is usually proportional to the elastic modulus and the cube of the thickness. Therefore, pGCC (HCC0 with polymers) and GCC (HCC0 without polymers) showed the lowest stiffness, while HCC-containing papers increased their stiffness according to the increase of calcium oxide fraction (Figure c). The NHCC without ionic polymers gave lower stiffness because of their lower tensile strength but still much higher than GCC or pGCC.

Smoothness is another essential property of the printing paper. Figure d shows the Bekk smoothness of the sheets and a high value in Bekk smoothness meant highly smooth sheets. The 10 μm GCC made the sheet with 30% ash content very rough because of its large size and incompressible nature (◆ black diamond dot in the figure). pGCC consisted of 2 μm GCCs and made as high smoothness as GCC. pGCC (HCC0 with the polymer) was completely collapsed in the sheet in the papermaking process from its initially largest size among HCCs in Figure e to its lowest bulk in Figure b. The size of HCCs was much larger than that of 10 μm GCC (15.4 μm in FlowCAM) but gave much higher smoothness (Figure d). The only explanation of this behavior should be that the HCCs were deformed under the pressure exerted in the papermaking process to make the sheet surface smooth. We believe that the HCC100 gave very low smoothness because of its high rigidity and large size. HCC10 was the most deformable among the HCCs but still had higher bulk and stiffness over 2 μm GCC, while the other properties remained the same. Paper-folding endurance is important in respect of cracking at the folded line. We presented in Figure S3 in the Supporting Information section that the HCCs did not lower the folding properties.

Model Analysis of High Bulk by HCC

Figure shows how the small-size HCC with a high amount of calcium oxide gave higher bulk than the large-size HCC with a low amount of calcium oxide. The low fraction of calcium oxide resulted in large size but more deformable HCC (Figure a). This was because less amount of the newly formed calcium carbonate made connection between GCCs, and ionic polymers played a major role to make flocs for HCC. The high fraction of calcium oxide made small size but less-deformable HCC (Figure b). This was because more amount of the newly formed calcium carbonate made connection between GCCs tightly and ionic polymers played a minor role to make flocs for HCC. During the calcium carbonate formation, it seemed that the polymers kept on agglomerating GCC particles continuously, while the newly formed calcium carbonate was covering most surface of the flocs. This is because the carbon dioxide should have difficulty to get inside the flocs. As more reaction occurs, thicker covering should be formed to result in rigid HCCs. Most of the GCC particles should reside inside the HCC because of the binding effect of ionic polymers and the effect of connecting GCCs by the newly formed calcium carbonate. This made the HCC without free GCC wandering and endowed the HCC-containing paper high tensile strength. The tight small-size HCC will have high resistance against the pressure encountered during the papermaking process and results in high-bulk paper. The high-strength and high-bulk paper should come out.

Figure 5

Schematics of the bulk development model for HCC. (a) Low fraction of calcium oxide case and (b) high fraction of calcium oxide case.

High Filler Trial Sheet

From the experimental results we discussed, we may summarize that the HCC-containing sheets had higher bulk, higher tensile strength, and higher stiffness than the ones containing 2 μm GCC. We may apply these results to printing paper by replacing more wood fiber with HCC. The sheet containing 40% HCCs may have equivalent physical properties to the one containing 30% 2 μm GCC. We selected four important physical properties and compared them in Figure . In Figure a, the breaking lengths of the paper containing 40% HCC30 and 40% HCC50 were equivalent to those of the paper containing 30% GCC. Ten percent more fillers did not lower the bulk of the handsheets for HCC (Figure b). Stiffness of all the HCC-containing sheets was much higher than that of GCC-containing sheets in spite of 10% higher filler level for the HCCs (Figure c). In printing paper, smoothness and stiffness are usually much more important than the breaking length in the routine quality checklist. In Bekk smoothness, the HCC10 was the best among HCCs (Figure d), and we believe that it was from its high deformable nature. When four important printing paper properties such as bulk, breaking length, Bekk smoothness, and stiffness were compared among the sheets containing HCCs and 2 μm GCC, HCC30 was the most favorable candidate to replace 10% wood fibers.

Figure 6

Property comparison between papers containing 30% GCC (2 μm) and 40% HCC. (a) Breaking length, (b) bulk, (c) Gurley stiffness, and (d) Bekk smoothness.

Ten percent more HCC fillers in the printing paper means 10% chemical pulp saving, which is 3–5 times more expensive than HCC. In other words, this leads to protection of forest by less wood cutting, energy saving in pulping and papermaking, and production cost reduction. The US EPA (Environmental Protection Agency) reported that the forest carbon sequestration effect of chemical pulp saving is 3.05 tCO/short ton in the manufacture of office paper.[26]

Conclusions

HCC increased the bulk, stiffness, and tensile strength of paper sheets simultaneously while providing surface smoothness as much as GCC at the equivalent ash content. The rigidity of HCC could be controlled by adjusting the fraction of calcium oxide in the preparation of HCC. A high fraction of calcium oxide made the HCC more rigid and less deformable. The size and rigidity of HCC helped raise the paper bulk. The rigidity of the HCC may help withstand the high shearing force encountered in the process of commercial paper production. The presence of ionic polymers in the formation of HCC played an important role in holding the small size particles that may interfere the hydrogen bonding between wood fibers. By controlling the rigidity of HCC properly, we could control the surface smoothness of the paper too. Ten percent saving of chemical pulp without losing essential properties of printing paper was demonstrated successfully using HCC technology. This means less use of wood fibers, significant energy savings in the pulping and papermaking process, and production cost reduction. The HCC technology should be considered as one of the greenhouse gas reduction technologies with respect to wood savings and process energy reduction in paper industry. The centrifugal height method was introduced to predict the paper bulk before papermaking.