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As paperboard manufacturers enter a new millennium, they are finding new ways to achieve the quality and productivity demands of today’s marketplace. Producers of white top linerboard and bleached paperboard are currently reducing operating costs and improving quality by converting from acid to alkaline Papermaking technology combined with calcium carbonate filler and coating pigments.
During the past 20 years, the North American free-sheet printing and writing papers market has seen a steady transition from acid to alkaline Papermaking technology. Improved brightness, opacity, and cost savings through filler for fiber replacement drove this transition. White top linerboard and bleached paperboard manufacturers are beginning to recognize these same benefits as they convert from acid or neutral pH chemistries to alkaline Papermaking chemistry. Although the cost savings per ton is not as great as it was for free-sheet markets, these typically conservative paperboard segments are willing to explore viable cost savings approaches that provide a competitive advantage.
BLEACHED PAPERBOARD TRENDS. The bleached paperboard market is moving toward higher brightness and a bluer shade in North America. This shift is most noticeable in lightweight bleached board grades or bristols. The caliper range for these grades is typically 7 to 14 point (thousandths of an inch). This shift is also occurring in mills that produce heavier weight grades as well. The increased brightness and bluer shade is allowing mills to be more competitive in the higher brightness European coated paperboard markets.1
Some manufacturers are also offering increased sheet gloss and print gloss for differentiation. The improved gloss grades are being targeted for high-end applications, such as cosmetic, pharmaceutical and tobacco cartons.
Forecast bleached board production for 2000 is 5.6 million tons in North America. The breakdown of grades is as follows:
• Folding carton: 2.1 million tpy
• Milk carton and food service: 1.2 million tpy
• Export: 1.8 million tpy
• Other grades: 500,000 tpy
Total production for domestic markets (U.S. and Canada) is expected to decline from 4.3 million to 4 million tpy through 2004 due to efforts to reduce packaging and increased use of alternate packaging materials, such as plastics. This domestic decline may be offset by increased demand in the export market resulting in a modest growth rate of 1.1% through 2004. Southern mills should be able to cost-effectively fill most of the growth in overseas demand.2 This will place increasing demands on domestic producers to meet the higher brightness standards of the European market.
Bleached board brightness targets (TAPPI brightness) are transitioning from a range of 82% to 84% up to 86% to 90%. Some paper mills are also simultaneously improving gloss with target ranges (Hunter) moving from a range of 55% to 65% up to 65% to 70%. The application of hot-soft nip calendering will help mills meet higher gloss applications while maintaining bulk and stiffness that is critical to performance.
North American producers of bleached paperboard have traditionally run their machines under acid wet-end conditions (pH of 4.5 to 5.5). The continuing need to achieve higher brightness specifications has led to calcium carbonate use in paperboard coatings. Machine runability issues azise from the acid/base reaction between the wet-end environment and the calcium carbonate in coated broke. The major problems from this reaction are:
• Foam/entrained air caused by the liberation of carbon dioxide (CO2) from the dissolution of carbonate pigments
• Deposit formation from particulate or dissolved calcium related to carbonate pigment use
• System upsets that cause sizing, retention, and drainage problems as a result of calcium carbonate dissolution
• Machine corrosion from acid digestion intended to reduce the above noted problems.
Board mills are evaluating and commercializing alkaline/precipitated calcium carbonate (PCC) filler technologies to eliminate these problems while reducing manufacturing costs.
The use of UV over-print varnishes to impart gloss and rub resistance in printed board has grown steadily during the past 20 years. Environmental concern with these varnishes has also been growing and has already prompted German companies to move away from this technology. Observation to determine if these environmental concerns will carry over to other European and North American printers is warranted. The impact of this could place more emphasis on sheet gloss for coated board production.3
Recently a producer of fluorescent whitening agents (FWAs) received FDA approval to use, as an indirect additive, low levels of FWA in board grades that contact food products.4 This will provide paperboard producers with a new method of achieving brightness targets.
WHITE TOP LINER TRENDS. The white top linerboard market is also going through a quality transition. Early white top grades were produced with only bleached fiber constituting the top ply. This resulted in a mottled surface appearance called “mottled white” linerboard. In recent years the quality of white top grades has improved and many mills now produce “full” white top linerboard that contains mineral filler for increased brightness and hiding power.
In 2000, an estimated 2.1 million tpy of white top linerboard will be produced. This represents a 10-year compound growth rate of approx. 4.1%. The split between full white top and mottled white top is expected to be 1.1 million tpy and 1 million tpy, respectively. The 10-year compound growth rate for white top has been more than 18%, while mottled growth rate has declined by nearly 1%/yr.5
White top linerboard has shown a steady growth rate as a percent of the outside ply of corrugated containers (Table 1).5 Mini-flute applications are relatively small, but they are growing rapidly and tend to be used in applications with higher graphic quality re-quirements. As mini-flute technology gains acceptance, it is expected to use significant levels of white top.
| TABLE 1.North American evolution of white top/mottled linerboard as a percentage of total outside linerboard.5 |
| |
1980 |
1990 |
1998 (est.) |
| Outside liner (million tpy) |
6.648 |
8.915 |
10.300 |
| White top (milliom tpy) |
.704 |
1.367 |
2.000 |
| Percentage (%) |
11 |
15 |
19 |
Higher quality graphics for white top grades are evolving. Print jobs that were once one or two-color flexography are giving way to four-color flexography and offset printing. This places new demands on the surface characteristics (e.g., smoothness and porosity) of white top linerboard. Improved post-print flexography quality is taking market share away from pre-print litho as well as pre-print flexo.
The Metsa Botnia mill in Kemi, Finland, found a niche market for its family of high graphic quality white top linerboard sheets that utilize pigmented surface treatments. The improved sheet brightness and smoothness provide a superior quality surface for multi-color flexo and litho print applications.
Improving the appearance or hiding power of the bleached ply over its natural kraft base is the key to developing a full white top sheet versus the mottled sheet. To obtain this appearance, paper producers have historically added the following fillers:
• Hydrous and calcined kaolin
• Titanium dioxide
• Ground calcium carbonate (GCC)
• Silica and silicates.
All of these fillers provide sufficient brightness for mills to achieve a typical 72% GE TAPPI brightness standard, but their hiding power differs greatly. The low hiding power characteristic of hydrous kaolin and GCC causes mills to use more bleached fiber than they would prefer. Silica, calcined kaolin, and titanium dioxide are costly alternatives that exhibit better hiding power.
White top linerboard producers in North America are currently looking for improved brightness and appearance. New brightness standards of approximately 76% GE TAPPI and higher are developing in North America to close the gap with European producers. FWAs may also play a role in developing higher brightness grades of white top linerboard.
FIT FOR FILLERS. In white top linerboard, PCC and narrow particle size distribution GCC fillers are finding opportunities to replace traditional less efficient and/or more costly fillers such as clay, titanium dioxide, silica, and traditional GCC products. The shift to alkaline Papermaking technology, combined with carbonate filler technologies, allows board manufacturers to improve quality parameters such as brightness (76+ GE brightness), appearance (hiding power), and flexographic printability which were too costly to achieve under acid conditions.
Bleached paperboard manufacturers have historically not used traditional mineral fillers due to concern with reduced stiffness from sheet densification and bond strength loss. Bleached board producers are now looking at the bulking benefits of scalenohedral PCC’s (described below) much the same way that bond paper producers looked at the products in the free-sheet market. Paperboard products receive constant competitive cost pressures from alternative plastic products and electronic media.6 Increasing quality while reducing cost is therefore a major objective for these mills.
As with most precipitation processes, precipitation of calcium carbonate produces products with a very narrow particle size distribution. This offers benefits that are difficult to match with other mined and milled minerals, such as GCC and hydrous or calcined kaolins. The new narrower particle size distribution GCC has improved optical performance but still falls short of the opacifying efficiency of a 1.3 mm average particle size scalenohedral PCC.
PCC is commonly produced in two mineral forms, known as calcite and aragonite. The calcite mineral form will typically be produced in two crystal shapes, either scalenohedral or prismatic morphologies. The aragonite mineral form will typically be produced with an orthorhombic (needle-like) morphology. The scanning electron micrographs show these morphologies.
The scalenohedral—calcitic form of PCC is the most widely used for paper filling. These are high brightness families of products (96% to 98% TAPPI, 94% to 96% ISO) that range from 0.8 µm to 2.7 µm average particle size (APS). Maximum light scattering for opacity is found at 1.3 µm APS. This form of PCC also offers sheet bulk that milled minerals cannot match economically. When produced with a 2.1 µm to 2.7 µm APS the bulking characteristic is maximized, while bond strength is optimized, and size demand is minimized. This helps paperboard mills maintain the critical stiffness and sizing at maximum filler levels.
BLEACHED BOARD APPLICATION. Bleached board mills that have made the conversion to alkaline typically add from 3% to 9% scalenohedral PCC with an average particle size ranging from 2.1 µm to 2.7 µm. At these filler levels, they are able to maintain or increase board bulk while balancing other critical bleached board properties. The replacement of expensive bleached fiber with PCC filler results in a significant manufacturing cost savings ranging from US$3 to US$8/ton of finished paperboard.1
Board brightness also improves by two points or more at these filler levels. In coated board applications, a brighter baseboard makes it easier for mills to meet their new higher brightness targets.
Figure 1 shows the properties achieved by one alkaline bleached board mill. Almost all of the mill’s major board properties either improved or remained unchanged since switching from an acid to an alkaline wet-end system along with the use of 5% PCC filler. Even the number of days between boil-outs has dramatically improved. Other paper mills and researchers have documented similar benefits.8,9
Figure 1. Comparison of PCC filled alkaline versus acid 12-point board from commercial paper machine.7
PCC acts as a pH buffer in the bleached board wet-end and provides alkalinity, eliminating the need for sodium bicarbonate. Alkaline mills have noted that the wet-end is much more stable with the alkaline chemistry in place. Runability has also improved due to fewer deposits and less foam (entrained air) compared with acid systems.
The natural cationic surface charge on the PCC particle gives it the ability to adsorb or tie-up pitch, stickies, and other anionic trash much like talc. This property helps promote improved paper machine runability.
The mill has realized benefits to date including:
• Increased base board brightness
• Ability to increase carbonate content in their coating for brightness
• Less shade reversion; inventory fading is reduced
• More stable wet end runability
• Fiber savings through lower cost filler use
• The replaced fiber is used on another machine to increase its productivity.
The mill continues to optimize its board and is cautiously moving to higher PCC filler levels.
WHITE TOP LINER APPLICATION. In 1999, six North American white top linerboard producers switched to PCC or narrow distribution GCC fillers with five of the six converting from acid to alkaline technology. Four of the six use PCC.
In white top linerboard grades, the addition of a 1.2 µm to 1.3 µm average particle size filler PCC offers maximum light scattering (hiding power) as well as brightness, bulk, and improved flexographic printability as benefits. The appearance of the sheet can be maintained or improved, depending on whether the goal is to maximize quality or maximize cost savings. The new narrow distribution GCC offers properties approaching this PCC product. These highly efficient PCC fillers tend to be more expensive on a pound per pound basis, but the total furnish savings realized by white top producers when utilizing PCC more than offsets any filler price differential.
The especially high light-scattering and brightness characteristic of this PCC enables mills to increase white top brightness while reducing bleached fiber coverage. A 42-lb (205-gsm) white top linerboard sheet can reduce its requirement of bleached fiber coverage by as much as 5% (e.g., 36% to 31% top ply coverage).
The redistribution of furnish with more natural and less bleached fiber can save mills several million dollars per year in bleached fiber cost. Total sheet strength is maintained due to a higher level of stronger natural furnish usage. These properties are achieved through the application of 8% to 15% PCC filler in virgin bleached top ply and 5% to 10% PCC in a recycled (mixed office waste) bleached top ply. When bleached recycled fiber is used, a combination of filler carryover and lower fiber strength limits PCC addition. Meeting the increasing industry brightness standards will become more difficult for mills using recycled fiber in their top ply. PCC filler will help these mills achieve the new higher brightness demands.
Changing from kaolin or traditional GCC filled white top grades to PCC filled grades opens up (increases) the sheet’s porosity. This allows uniform controlled flexographic ink receptivity and results in clear, sharp image quality. The open sheet does however result in slightly higher ink consumption. The predominant feeling is that the print quality improvement outweighs the slight increase in ink use.
By utilizing the principles of light absorption and reflectance explained by Kubelka and Munk,10 mills can take advantage of excess brightness by dying back. This provides mills with a tool to improve opacity and/or appearance.
Brightness increases of one to seven points can be achieved while reducing bleached ply coverage. The level of brightness improvement depends on mills’ ob-jectives for fiber reduction and starting filler combination. Figure 2 shows how brightness is improved by the addition of a 1.3 µm PCC filler at a mill while still achieving equal sheet strength. This mill saw a significant reduction in cost from the replacement of TiO2.
Figure 2. White top linerboard and brightness improvement using PCC filler for Mill No. 1.
Abrasion issues should always be considered when it comes to machine wire life and converting operations wear. The abrasion characteristics of 1.2 µm to 1.3 µm PCC are significantly better than GCC and calcined kaolin. The SEM photo in Figure 3 shows a Einlehner Abrasion comparison between a calcined clay and PCC.
Figure 3. Abrasion of PCC versus calcined kaolin for white top linerboard.7
ROBERT A. GILL is senior scientist paper technology, paperboard filling and WILLIAM J. HASKINS is business development manager, Specialty Minerals Inc., Bethlehem, Pa.
REFERENCES
1. W.J. Haskins and R.A. Gill, “Paperboard Filling Experiences with Precipitated Calcium Carbonate (PCC),” Scientific and Technical Advances in Boardmaking Conference, PIRA International, London, U.K., October 25-26, 1999.
2. RISI, North American Pulp and Paper Review, March 2000.
3. D. Bailey, “A Comparison of the Coated Board Industries in Europe and North America,” TAPPI Coated Paperboard Conference, Kalamazoo, Mich., November 1999.
4. CIBA Technical Bulletin, 2000—”CIBA Tinopal for FDA Applications.”
5. W. Baerlocker, “Graphic Linerboard,” Eighth International Containerboard Conference, Miller Freeman Inc., New Orleans, La., September 17-18, 1998.
6. I.I. Pikulik, N.A. Poirier, and F. Leger, “Papermaking in the Third Millennium,” Pulp and Paper Canada, Vol. 100, No. 10, p. 23-27, 1999.
7. R.A. Gill and W.J. Haskins, “Paperboard Filling Experiences with Precipitated Calcium Carbonate (PCC) for the New Millennium,” Proceedings of the 2000 TAPPI Papermakers Conference, Vol. 2, p. 811-821.
8. P.L. Whiting, “Contaminant Control on a High Speed Paper Machine,” Proceedings of the 1997 TAPPI Engineering and Papermakers Conference, Book No. 2, p. 661-668.
9. T. Miyanishi, Y. Kamijo, and H. Ono, “Adsorption of Anionic Dissolved and Colloidal Substances on the Calcium Carbonate Fillers,” Proceedings of the 1999 TAPPI Papermakers Conference, Vol. 2, p. 429-436.
10. E. Bohmer, “Filling and Loading,” Chapter 15, Pulp and Paper, 3rd Ed., Volume 3, Edited by J.P. Casey, p. 1521-1527, 1981.
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