Volume 4, Number 2
May 1991

TAPPI Papermakers, April 7-1 Seattle

The annual Papermakers Conference is put on by the Paper and Board Manufacturing Division. This time there were 66 papers, 11 posters and three panels or round tables --not bad for a division with only 340 s. I think attendance was something over 800, dawn a bit because of the recession but not as low as feared. Novel developments and findings in a number of fields were reported on the use of calcium carbonate in groundwood papers, the effect of alum and lignin on permanence, and slipperiness, among other topics. In addition, there was a panel on alkaline conversion at three mills, and a session on alkaline sizing. committees, including the Paper Permanence Committee(Ad Hoc), net the day before the conference.

Calcium Carbonate in Groundwood

Max Laufmann of Pluess-Staufer AG, a Swiss supplier of calcium carbonate, gave a paper on the manufacture of neutral groundwood-containing papers. (His paper, like all the others here, was co-authored but for simplicity only the presenter is mentioned.) He said 70% of all European freesheet papers were alkaline in 1989, and predicted that all of them would be alkaline by 1995. Furthermore, about 38% of all coated groundwood paper there is produced under neutral or pseudo-neutral conditions. Groundwood paper production at a neutral pH was not adopted in the U.S. until late last year, but interest is high and the paper is well received on the market. It was not attempted earlier, as Bruce Evans of Pfizer said in his paper, because of pitch problem, darkening of the paper, and a "mind barrier" about any pH over 6. Two secrets of success in making neutral groundwood papers: 1) control of anionic trash, the main focus of the paper, and 2) choice of point of addition for the calcium carbonate (and for alum too, if it is used) so as to minimize the opportunity for the carbonate to interact with the low-pH alum and groundwood.

Vijay Mathur of G.K. Carbonate presented a paper on acid-resistant precipitated calcium carbonate (PCC). PCC, he said, is valued for the brightness it gives to paper, but operates to reduce brightness in some wood-containing sheets, by some mechanism quite independent of pH. A proprietary means of treating PCC has been developed that will lower the pH of PCC by two points (from 9.8 to 7.8) and make it less ready to react with alum. Brightness and opacity are improved, and less alum is required. (Some but not all PCC has a pre-treatment pH of 9.8. Most sources put it closer to 7.5 or 8.0. Bruce Evans of Pfizer, in his paper on PCC fillers for groundwood papers, gives a range of 7.5 to 8.2 for untreated PCC, and 5.0 to 7.0 for PCC treated by his own company's process.)

Alum, Lignin and Permanence

Gerard Rose of Nalco presented a paper entitled "The Effect of Aluminum on the Permanence of Papers" in the session sponsored by the Paper Permanence Committee. It described trends in the use of alum, reviewed the scant literature on aluminum's effect on permanence, and found a preponderance of evidence suggesting that aluminum in an alkaline sheet will not adversely affect its permanence. He summarized some recent German work comparing the effect of alum and polyaluminum chloride (PAC) on permanence: alum, with its lower pH, had a worse effect than the PAC, even though PAC contains twice the aluminum.

In the discussion afterwards, Chandru Shahani of the Library of Congress pointed out that deacidified papers containing high levels of alum aged very well in the oven.

Betsy Humphreys of the National Library of Medicine described the latest revision of the American National Standard for the Permanence of Paper (Z39.48, the standard developed by the National Information Standards Organization or NISO), including its controversial specification for a of 7.5% lignin. This specification was based on testing of several sets of paper done for the committee. If all the papers were carbonate-filled, they probably performed very well, because alkaline buffered lignin-containing papers have been tested at the Library of Congress and are known to performed well under accelerated aging. However -and this is a major difference between accelerated and natural aging-the oven does not duplicate the effect of air pollutants over a long period of time. A new set of papers is being tested in the presence of gaseous pollutants to see the effect of carbonate loadings of 2%, 6% and 20% on permanence of CTMP papers.

There have been verbal reports of buffered paper and board (i.e., deacidified or containing calcium carbonate) that have exhibited large drops in pH within a period of three to ten years under normal storage conditions. This may be because the carbonate was used up in reaction with pollutant gases. Two recent studies, one French and one Swedish, suggest that accelerated aging with pollutants uses up the alkaline reserve in paper rather quickly if it is exposed to the moving gas. This is a new line of investigation, and the study designs have taken some criticism, but they show effects similar to those observed in natural aging recently.

Robert Johnson of DuPont gave two papers on permanence and color reversion of CTMP in fine papers. Only the one on permanence will be described here. CTMP, or chemithermomechanical pulp, is a new type of very high-yield pulp that can be blended with chemical pulps at high levels, 30-50% of the total fiber, to make commercially acceptable printing and writing papers. Brightness reversion is still a problem because of the high level of lignin, but it can be reduced substantially by the use of calcium carbonate fillers.

Papers containing up to 45% CTMP and up to 25% calcium carbonate filler -were compared with unbuffered groundwood and buffered and unbuffered freesheet. No unbuffered CTMP paper was included in the study. Both dry and mist aging were done for up to 50 days, and changes in fold, tear and brightness were measured. Both brightness and strength were retained well by the 30% MT, which lost brightness no faster than the unbuffered freesheet, and retained its strength even better than the buffered freesheet. Most of the tables and figures in the paper relate to the rate of loss of properties during accelerated aging. Apparently loss rates for both tear and fold were strongly correlated only to pH (average during aging), and not to fiber type, or even to the presence of carbonate filler.

Johnson's results confirm previous reports in the literature that color-causing reactions are not the same as those causing loss of strength: the rate of yellowing, but not the rate of loss of fold, is directly related to the percent of groundwood pulp. In his literature review he cites a study by LeThi reporting that rapid aging of acidic groundwood is largely due to the fines (fiber fragments, hemicelluloses and ray cells) produced during the pulping process. State-of-the-art CTMP pulps, Johnson says, have much lower levels of such fiber fragments.

(The effects of fines on aging is well documented in the literature, by the way, as shown by a 79-page literature review published in 1985 as a bound-in supplement to Art & Archaeology Technical Abstracts: "Three Fundamental Aspects of Cellulose Deterioration," three annotated bibliographies prepared by Robert L. Feller, Sang B. Lee, and Mary Curran. The stapled binding it almost unreadable close to the binding edge; rebinding is advised. But the paper, editing and printing are excellent. The second bibliography of the three is "Hemicelluloses: Their influence on Paper Permanence.")

Slipperiness

Neutral sizing can make paper slippery if measures are not taken to control this effect, but slipperiness has other causes too. Mike Withiam of Haber gave a paper on the effect of fillers on coefficient of friction (COF). He compared talc, hydrous and calcined kaolins, and several precipitated silica and silicate fillers, but not calcium carbonate. Synthetic silicate, and to a lesser t, calcined kaolin, increased COF significantly. Other causes of slipperiness: presence of even small amounts of fatty acids or soaps in the fiber furnish after pulping and washing; wood species (pine is worse than hemlock, for instance); and the size and porosity of filler particles.

Joe Marton, in the session on alkaline sizing, noted that hydrolyzed AKD makes paper slippery. The paper needs to be heated in the dryer section to cure the AKD, but too much heat will hydrolyze it.

Alkaline Sizing and Alkaline Conversions.

Betty Moyers of Hercules gave a well-received paper on diagnosis of sizing loss problems. (This is more than retention: the size has to be evenly distributed, and it has to anchor itself in the sheet; sizing has to be developed, and then it must not disappear over time.)

Many factors, she said, could upset the delicate chemical balance of the system and throw sizing off: additives (glycols are the worst, often found in defoamers), contaminants (e.g., oils or high ionic content in the water), precipitated calcium carbonate (which ties up the size, if it can, before the size can react with the fiber), and the amylose fraction of starch (which appears to coil around the hydrophobic tails of the AKD molecules and put them out of business).

Charles Farley of American Cyanamid (which has been making alkaline size since the fifties) spoke on the use of alum to improve ASA sizing efficiency. Alum, aluminum chloride, polyaluminum chloride and polyaluminum silicate-sulfate all boost the sizing efficiency of ASA and reduce anionicity. ASA must be used very soon after being emulsified, or it loses all its sizing effect; but if you add a very small amount of alum, it regains it, at least for a short while. Alum is best added early in the system (e.g., to the machine chest, where it will unavoidably react with the calcium carbonate filler and lower the pH temporarily), and be sell dispersed before the ASA is added. The alum-ASA-stock mixture must be used soon too--within 20 or 30 minutes-for best sizing effect. Mike Williams (Boise Cascade/Wallula), Scott Fruhwirth (E.B. Eddy/Port Huron) and Rick Glisson (Simpson Pasadena/San Jacinto mill) gave accounts of their mills' alkaline conversions, with the obstacles they met and overcame or learned to live with. The first two accounts are in the proceedings.

The San Jacinto mill , Glisson said, completed its conversion to alkaline in May 1990. They did not decide on AKD until after several three-day trials. One surprise was the importance of addition points. Another was the emergence of problems (e.g., slip and occasional size fugitivity) that had not been apparent in the three-day trials, sometimes three or four months later. Most problem were solved or reduced.