This paper was given at the Jan. 1997 conference of the Technical Section, Canadian Pulp and Paper Association, under the title "Mill Experiences with Precipitated Calcium Carbonate (PCC) in Papers Containing Mechanical Pulp," and will appear in Pulp and Paper Canada later in 1997. This condensed version is printed with the permission of the publisher.
A revolution in wood-free papermaking has occurred over the past decade.1 The availability of low-cost precipitated calcium carbonate (PCC), manufactured on individual paper mill sites, fueled the rapid expansion of alkaline papermaking. PCC provides many benefits, including excellent brightness and opacity at significantly lower cost than the clay or fiber it replaces.
Several concerns have slowed the move to alkaline papermaking with mechanical pulps. Most are directly related to the fact that, in water, PCC forces the pH to 8 or higher, causing lignin to darken. In the presence of a strong acid PCC dissolves, increasing calcium hardness.
Recently a new, "acid tolerant" (AT) technology that stabilizes the PCC suspension to neutral pH was introduced.2 Hardness typically increases by only 50 to 100 ppm, and pulp brightness losses are limited to 1 or 2 points. However, final sheet brightness improves because the PCC itself is extremely bright.
Handsheet and other laboratory studies have been published to explain this technology and expected results in groundwood papermaking.2 More recently, a large number of production trials and commercial operations have been monitored, so there are many real-world examples to draw on. [Trials in two newsprint mills are summarized in the full length paper. Results from one of the mills are given in this condensed version.]
Several AT-PCC trials of varying duration were conducted on a newsprint machine over a one-year period. The objectives were to replace the calcined clay used for opacity in several basis weights, and to look at the potential of making hi-brite grades by increasing the PCC content. Retention was accomplished with the single-component cationic retention aid normally used with clay.
The following material is drawn from the trials, mostly from the two longest, lasting one and two weeks.
When AT-PCC was added, headbox pH increased from 5.8 to 7.1 and was stable throughout the trial(s). The filler and overall first pass retention remained about the same as with clay, and Schopper-Riegler drainage was slightly faster. There were no operating problems associated with running the PCC.
The optical properties of the sheet were maintained or slightly improved when a portion of the clay was replaced with PCC. This was achieved despite a lower total filler content. Strength properties were unchanged.
It is well known that acid papers degrade over time.3 The poor archival properties of acid, wood-containing papers diminish their value.
The effects of accelerated aging on sheet properties were determined on samples of neutral/PCC and acid/clay papers. The conditions were 28 days at 80�C and 65% relative humidity, estimated to be the equivalent of about 120 years at 23�C and 50% RH.
Losses in zero-span tensile, breaking length and fold endurance were much greater with the acid paper (Table 1). The presence of PCC also lessened the effects of thermal aging on brightness reversion.
|Extract pH4||ISO Brightness (%)||Zero-Span Tensile (km)||Breaking Length (km)||MIT Fold (#)|
a) 1.2% clay, 51.1 g/m2
b) 1.6% PCC, 49.4 g/m2
c) 10.2% PCC, 48.4 g/m2
4Papers b and c were made with AT-PCC at a neutral pH. The extract pH does not reflect the pH of the wet end on the paper machine.
The pH extracts explain the improvements in paper permanence. Low pH accelerates the acid hydrolysis of cellulose. Neutral pH is advantageous; the higher the PCC content the greater the protection. However, brightness reversion upon UV irradiation was not affected.
[The authors advise against use of alum, if it is used at all, as the stock nears the paper machine. They also emphasize that pitch deposition problems are not encountered with neutral PCC in groundwood.]
1. Gill, R.A., "PCC Fillers: High Opacity and a Whole Lot More," TAPPI 1990 Neutral/Alkaline Papermaking Short Course, October (1990)
2. Evans, D.B., Drummond, D.K., Koppelman, M.H., "PCC Fillers for Groundwood Papers," Proceedings of the 1991 TAPPI Papermakers Conference (1991)
3. Zou, X., Gurnagul, N., Bouchard, J., Uesaka, T., "Accelerated Aging of Papers of Pure Cellulose: Mechanism of Cellulose Degradation and Paper Embrittlement," Polymer Degradation and Stability 43(3); 393-402 (1994)
[Postscript: In February, Minerals Technologies announced that it had signed an agreement with Myllykoski Paper Oy, a Finnish paper company, to construct a PCC plant using acid-tolerant technology. The mill produces groundwood supercalendered papers for magazines and catalogs. By the end of 1997, it will be making the world's first PCC-based supercalendered paper. -Ed.]