Volume 15, Number 8
Dec 1991

Lignin, CaCO₃ and the Revision of the ANSI Paper Permanence Standard

The most widely used permanent paper standard in the world is ANSI Z39.48. The international standard now under development by ISO is based on it, as are the national standards in many countries, despite the fact that it covers only uncoated paper for printing.

ANSI Z39.48 is being revised, as all formal standards must be from time to time, in order to improve it and relate it more closely to modern manufacturing and test methods. Its coverage is also being expanded to include coated paper. The revision is being carried out by a committee of the National Information Standards Organization (NISO), referred to as Standards Committee II or SCII. ("II" is not a Roman numeral, but the double letter "I.") SCII has commissioned research and testing to guide and inform its efforts, as such committees often do. In early December, the results of some of that research were made public by Betsy Humphreys, Chair of SCII, in a letter to NISO members and other interested parties reprinted below. Only one selection from the enclosures she refers to in her letter is reprinted here, the part about lignin. In the last draft, a maximum of 7.5% lignin was permitted; this has now been lowered to 1%. If this draft is approved by members in the next vote, the 1% lignin level will be part of the new version of this standard.

When this revision is finally approved, though, the story will not be over, because further research needs to be done, as Ms. Humphreys says, to lay the basis for the next revision. Putting together a standard is like having a child: your work only begins with the act of creation.

Betsy Humphreys' letter, dated December 5, 1991, follows.

The version of ANSI Z39.48-199X which was circulated for balloting and comments from December 10, 1990-March 11, 1991 received 2 negative votes and a number of critical comments. NISO Standards Committee II (SCII) has reviewed and carefully considered all comments received and has also commissioned and reviewed the results of additional testing on the effects of lignin in alkaline paper. Based on these deliberations, the Committee has amended the proposed standard as follows:

1. The scope of the standard has been expanded to address retention of the original color as well as retention of the physical strength of paper. Comments received indicate that paper which retains its physical strength, but undergoes significant color change over time, is not considered suitable for materials to be permanently retained by libraries and archives. Although ANSI Z39.48-1984 does not address the issue of color change, its paper stock provisions (i.e., "no groundwood") effectively excluded lignin (given the book paper-making technology of the early 1980s), and therefore excluded the primary known cause of color change. At that time, it was generally believed that lignin also contributed to the deterioration of the physical strength of paper. The results of tests commissioned by the Committee indicate that papers with up to 7.5% lignin and alkaline reserves ranging from 2% to 20% can exhibit excellent retention of physical strength after 24 days of artificial aging at 90°F [i.e., °C] and 50% RH. Such papers may also undergo significant color change when exposed to sunlight however. There is concern that such color change can affect the long term legibility and reproduceability of information recorded on the paper.
2. The maximum allowable percentage of lignin has been reduced to 1% due to the role of lignin in darkening of paper and the lack of data on the effect of lignin on the absorption of environmental pollutants by alkaline paper.
3. The wording of the paper stock requirement for coated paper has been amended to correct an inadvertent discrepancy in the allowable lignin levels for coated and uncoated papers.

Testing commissioned by SCII has resulted in new information about the effect of lignin on alkaline paper. Further research is needed. The Committee encourages paper manufacturers and paper scientists to conduct this research expeditiously so that it can contribute to a thorough re-examination of the lignin issue during the next revision cycle for ANSI Z39.48.

As with the previous draft of the revision, a summary of all major issues examined by the Committee is included in the accompanying paper, "Issues Related to the Revision of ANSI Z39.48." The section entitled "Mechanical Pulps, Lignin and Color Change" contains new information. Other sections are identical to those circulated with the previous draft. Also attached is a summary of the differences between this proposed revision and the current text of ANSI Z39.48-1984.

Again, SCII thanks all those who provided thoughtful and helpful comments on the previous drafts of the revision of ANSI Z39.48. We believe that the version currently being circulated for ballot represents a reasonable and responsible interpretation of the available information on the many complex issues related to paper permanence.

Betsy L. Humphreys

Mechanical Pulps, Lignin, and Color Change

Since the current standard was developed, the paper industry has begun to use chemi-thermo-mechanical and other new types of mechanical pulps. These pulps do not contain groundwood and therefore meet the paper stock requirement in the current standard. Such pulps do contain significant amounts of residual lignin. Paper tests commissioned by the Committee and conducted by other institutions indicate that alkaline papers with up to 7.5% residual lignin can exhibit retention of durability equivalent to alkaline papers with less than 1% lignin. Papers with 7.5% lignin and alkaline reserves ranging from 2% to 20% were tested and all showed excellent retention of durability after 24 days of artificial aging at 90°F [i.e., °C] and 50% RH. Although it now appears that lignin levels of 7.5% or even greater do not by themselves promote deterioration of physical properties [in alkaline paper with an alkaline reserve--ed.], the presence of lignin definitely causes the darkening of paper that is exposed to light. There are as yet insufficient data to determine whether lignin may also contribute to an alkaline paper's absorption of harmful environmental pollutants. Two voting members of NISO and several other interested parties objected to the 7.5% lignin level included in the draft revision previously circulated for voting because: (1) the darkening of paper can affect the long term legibility and reproduceability of the text and images on it and (2) the combined effect of lignin in alkaline paper and common environmental pollutants is not understood. The darkening of lignin on exposure to light is worrisome in part because many important documents are housed for years in uncontrolled conditions in houses, offices, and laboratories before they are transferred to libraries and archives for permanent retention.

The 1984 Standard did not directly address the issue of color change. Given the state of paper-making technology in the early 1980s, however, the "no groundwood" specification in the 1984 standard effectively excluded lignin from permanent paper and therefore excluded the major known cause of darkening of paper. Now that research commissioned by SCII has revealed that lignin apparently does not contribute to deterioration of physical strength, the issue of color change must be considered separately. Based on the negative votes and comments received on the draft revision previously circulated for ballot, SCII had decided to lower the allowable lignin level to 1% or less to ensure that paper which meets the standards for permanence exhibits excellent retention of color as well as retention of physical strength. Additional research is definitely needed on the effects of larger percentages of lignin in alkaline paper, and this issue will deserve careful re-examination when ANSI Z39.48 goes through its next revision cycle.

The Kappa number, specified in the proposed revision as the method for determining the percentage of lignin was developed as a test for pulp, not finished paper. To confirm that the test was suitable for finished paper the Committee conducted Kappa tests on samples of finished paper for which the Kappa values of component pulps and the amount of lignin content were known. The paper Kappa numbers obtained were consistent with the pulp values and the known amounts of lignin. The Committee also received comments that the Kappa number test does not provide reliable results for very high yield pulps. An examination of available test data indicates that the Kappa number is reliable for pulps and papers with less than 20% lignin, which is well above the 1% to 7.5% levels considered by SCII.

[For further information on this topic, contact Patricia Harris at the National Information Standards Organization (NISO), PO Box 1056, Bethesda, MD 20817 (30V975-2814).