While increased filler content has long been sought after by the paper industry, technical limitations have precluded the practice. The critical limiting factors include the loss in sheet strength and physical durability with respect to dusting at elevated ash levels. Filler disrupts the fiber-fiber bonding network of the sheet by reducing the number of fibers. The small filler particles also prevent effective contact of the fibrils.
For every 1% increase in ash content, the tensile strength of the sheet is reduced by 1-2%. Unfortunately, dry-strength agents do not perform well in the presence of filler. The loss in strength is also detrimental to paper machine operation, as well as subsequent printing operations. Low internal bond strength can cause problems such as delamination in the printing press, especially for high basis
Another challenge in increasing the ash content is that fillers are harder to retain in the system. This often leads to elevated retention aid dosages and poor formation of the sheet. Operational instabilities due to dusting or center roll picking are other potentially negative consequences of incorporating higher ash levels in graphic papers.
Figure 1 - Differentiating aspects of fillerTEK, Nalco’s filler preflocculation technology
New filler treatment
Nalco has developed its patent-pending FillerTEK technology to address the concerns of paper producers. This technology delivers an increase in filler content up to five units, while preserving the critical strength and optical attributes of the final sheet. The technology is based on increasing the particle size of the filler through a unique method of preflocculation. The net outcome is reduced interference of the filler with fiber-fiber bonding. The concept behind the technology was designed to circumvent some of the limitations of previous approaches of filler preflocculation. The differentiating aspects of this technology are summarized in Fig. 1.
The method involves a chemical treatment of a filler slurry to produce filler flocs with a well-defined particle size distribution. The controlled floc size eliminates detrimental effects on sheet strength and formation. The treatment also was especially designed to produce flocs that exhibit improved shear stability in paper machines. These attributes of the technology are achieved by a combination of chemical and mechanical approaches.
The third differentiating aspect of FillerTEK technology is return on investment (ROI). The program is economical, even for paper producers targeting a relatively low increase in sheet ash of 3-5%.
FillerTEK technology is a fit for customers using precipitated or ground calcium carbonate (PCC or GCC), or a blend of the two carbonates as their filler source. The technology has been demonstrated with a variety of furnish types, and the program is best suited for the uncoated or coated freesheet market. The chemical treatment is carried out on-site with a mill's existing filler slurry. This can be accomplished in-line or the treated filler slurry can be stored in a run tank; no residence or aging time is required before use.
Figure 2 - Impact of fillerTEK technology on internal bond strength of handsheets as measured by z-directional tensile (ZDT). The fillerTEK technology line was extrapolated. The black lines indicate that the filler level can be increased by 8.5 units at equal ZDT strength with the filler treatment
The FillerTEK technology concept of increased ash content without a loss in sheet properties was initially demonstrated in laboratory handsheet experiments. For this purpose, a furnish was prepared with 63% hardwood kraft pulp, 27% softwood kraft pulp and 10% percent broke, along with starch at 9 kg/ton, of which 5 kg/ton was added with ASA (dosed at 2.5 kg/ton), POSITEK 3G 8692 microparticle at 2 kg/ton, and Core Shell® 61067 cationic flocculant at 0.9 kg/ton. Additionally, four filler levels were investigated: untreated PCC at 20% and 28%, and FillerTEK technology treated PCC at the same levels. Eight handsheets were prepared for each condition.
The results for internal bond strength are plotted in Fig. 2. At 18% ash content, FillerTEK technology results in a 10% gain in z-directional tensile (ZDT), which allows for an increase in filler content of 8.5 units without a loss in internal bond strength. Again at 18% ash, the tensile index strength improves by 7% with FillerTEK technology, such that the ash content can be increased by three units at equal tensile strength.
The opacity data is illustrated in Fig. 3, and indicates a loss in opacity for the treated PCC sheets. This loss of 0.2 units can be recovered by a 1% increase in filler content. Similar trends are also observed for sheet brightness. Overall, the FillerTEK technology program results in a smoother and slightly less porous sheet compared with the untreated PCC samples.
The ability to provide strength improvements without insurmountable optical penalties is also a challenge of increased filler content. The use of a larger-sized filler particle can provide strength, but the opacity loss is so severe that it often cannot be recovered even with a 10-point increase in ash content. This point was demonstrated in handsheet experiments with two samples of PCC whose median sizes were 3.6 and 4.5 microns, as measured by light scattering using a Malvern Mastersizer. The ability to deliver higher ash content at equal strength and opacity is a fundamental aspect of FillerTEK technology.
Figure 3 - Impact of fillerTEK technology on the opacity of handsheets. The black lines indicate that the opacity loss due to the filler treatment can be recovered after a one-point increase in ash content
Validation on gapformer pilot machine
FillerTEK technology has also been demonstrated on the EuroFEX gap former pilot machine at Innventia in Stockholm, Sweden. The high-speed and high-shear environment of this pilot machine was considered to be a rigorous test for the filler flocs. Precipitated calcium carbonate was treated with the FillerTEK technology program and compared with the untreated filler. Filler levels of 20% and 30% were evaluated in a fine paper furnish comprised of 85% eucalyptus and 15% pine. An 80-g/m2sheet was produced at a speed of 1,000 m/min, and the following additives were used: cationic starch at 7.5 kg/ton, OBA at 4.5 kg/ton, and POSITEK 3G 8694 microparticle at 5 kg/ton.
The dose of NALCO® 74508 cationic flocculant was varied to maintain a constant white water consistency of 2.3 g/L for all trial conditions. The sheets were not surface sized and were dried off-line with a multi-cylinder open dryer. Samples produced on the pilot machine indicated that the filler content could be increased by five points at equal tensile strength, internal bond strength, brightness, opacity, and formation of the sheet.
One disadvantage of higher filler content is a loss in sheet bulk. The magnitude of loss depends on the filler type and furnish composition. Preflocculation of the filler can also produce an additional loss in bulk depending on filler type. If this is a critical parameter for a particular grade, the papermaker may be able to employ one of the following strategies to recover the bulk loss at the higher ash levels.
One option is to take advantage of the improved sheet smoothness and reduce the calendering load.
The use of a mechanical fiber source such as BCTMP is another approach to increase bulk.
Figure 4 - Z-directional ash distribution for commercial sheets Produced with and without fillerTEK treatment from the case study. A similar profile is maintained at the higher ash level
FillerTEK technology has been utilized by a mill producing 400 tons/day of copy paper and offset grades in the basis weight range of 75-105 g/m2. The majority of the mill's production is 75-g/m2copy paper with 18% filler content using a blend of PCC and GCC as the filler source. This is a non-integrated mill faced with increasing pulp prices. Previous attempts to increase the ash level were unsuccessful due to limitations in sheet strength as well as operational issues on the machine, such as dusting, deposits and poor retention. The technology was implemented across all grades and has been utilized continuously for over a one-year period. As a result, the mill has achieved a 5% increase in the ash content of its sheet while maintaining runnability.
Nalco installed equipment at the mill to treat the filler slurry. The resulting filler was then stored in a run tank and used as needed. The FillerTEK technology equipment has been successfully operated in a continuous mode with little maintenance.
A snapshot of the technology's performance is summarized by the sheet properties shown in Table 1. These results represent the average of 10 sheets randomly selected from a set of 10,000 sheets produced with and without FillerTEK technology. The overall ash level has been increased from 17.7% to 22.2%, or 4.5 units. In this comparison, the basis weight for the treated sheets was less than the untreated, i.e., 74.9 vs. 76.3 g/m2, respectively. At the 22.2% ash level, the technology delivered equal internal bond strength, tensile index, bulk, opacity, brightness, porosity, and sizing response. The treated filler produced a smoother sheet, and a slight loss in stiffness or bending resistance.
The mill also incorporated BCTMP into its furnish for cost savings and bulk advantage at the higher ash level. The ASA sizing dosage has remained steady even as filler use has increased. Additionally, the sheets described in Table 1 were subjected to a converting study to monitor dust formation. It was found that the FillerTEK technology sheets, at 4.5% higher filler content, generated 47% less dust than the sheets containing untreated filler. Figure 4 illustrates that the z-directional ash distribution in the sheet is similar at the elevated filler level.
Table 1 - Commercial case study summary of sheets produced using a blend of PCC/GCC with and without the fillerTEK technology. Results represent the average of 10 samples randomly selected from 10,000 sheets
|Sheet property||Untreated filler||FillerTEK treatment||Impact of FillerTEK technology|
|Value||Std. Dev.||Value||Std. Dev.|
|Sheet ash, %||17.7||0.50||22.2||0.50||+4.5 pt ash increase|
|Basis weight (g/m2)||76.33||1.17||74.87||0.92||Lower|
|Internal bond (ZDT)||571.7||18.0||574.0||10.6||Equal|
|Tensile index (Nm/g)||54.32||2.97||52.73||2.73||Equal, within std dev|
|Bending resistance (mN)||103.5||9.3||91.1||8.6||Reduced|
|Porosity (ml/min)||1157||87||1198||66||Equal, within std dev|
|PPS Roughness (um)||6.43||0.32||6.01||0.11||Smoother sheet|
|Sizing, HST (sec)||69.15||25.3||61.43||25.4||Equal, within std dev|
|Opacity at 75 g/m2, %||94.43||0.76||94.63||0.52||Equal|
Print trial of commercial sheets
Enhanced propensity for linting or dusting in the printing press is a substantial concern for paper producers interested in higher filler content. In an effort to address this concern, a print trial was conducted at Rochester Institute of Technology (RIT) Print Industry Center on 75 g/m2text grade sheets produced commercially with the FillerTEK technology program. Sheets containing 25% untreated PCC were compared with sheets containing treated PCC at 25% and 29% levels.
A Goss Sunday 2000 four-color, heat-set offset press was used to make 28,000 impressions.
FillerTEK technology was found to have no negative effect on pressroom runnability, and the operators noticed no distinction between the treated samples at 25% and 29% ash levels. Tape pulls from the printing blanket were analyzed to determine the linting propensity of the sheets. Specifically, calcium levels on the tapes were measured by energy dispersive x-ray spectroscopy. These tests indicated that the extent of linting was reduced on the top side of the sheet and was equal on the bottom side for the FillerTEK technology sheet at 29% ash compared with the untreated sheet at 25% ash.
FillerTEK technology effectively delivers cost-efficiency to paper producers by allowing them to utilize less expensive raw materials. This patent-pending technology is based on preflocculation of filler in a controlled manner to produce flocs with a defined size distribution and improved shear stability. The net result is that the treated filler can be incorporated at higher levels in the sheet without a loss of strength or optical properties. The technology has been demonstrated on the EuroFEX gap-former pilot machine and successfully implemented by a UCFS mill for more than a one-year period.
Michael Anconais program manager, graphic grades, andKatherine Broadusis senior research chemist, Nalco Company