The Alkaline Paper Advocate

Volume 3, Number 6
Dec 1990

NISO Draft Standard Allows Significantly More Lignin

The ANSI/NISO standard for permanent paper for library and archival use, Z39.48, is being revised to cover both coated and uncoated paper and to incorporate the results of recent research. A revised draft was made available in November for review by all interested persons. The review period ends February 28, 1991. There is a hitch: First you have to buy the draft revision, and it costs $30. (Order from National Information Standards Organization, PO Box 1056, Bethesda, MD 20827.)

This version of the standard has some surprises in it, relative to a) method of pH determination, b) allowable lignin content, and c) replacement of tear resistance by the tear index. (The tear index, unlike tear resistance, does not vary with the weight of the paper. It is calculated by dividing the tear resistance in millinewtons by the grammage of the paper.)

The draft standard comes with a summary of issues related to this revision. That summary is excerpted below, with the permission of Betsy Humphreys, chair of Standards Committee II (SCII), which is preparing the revision. For a similar discussion of the previous draft revision, see the November 1989 issue of this Newsletter, p. 52: "Comments Invited for Z39.48 Revision." Issues covered in that discussion and not revised in the new comments are not repeated here.

Issues Related To The Revision Of ANSI Z39.48

Artificial Aging

... The strongest argument against the incorporation of accelerated aging in the standard is the absence of any evidence suggesting that some papers which can meet the criteria in this proposed revision do in fact exhibit poor retention of durability after accelerated aging. All data known to the Comittee indicate that adherence to the criteria described in the proposed revision is a sure predictor of acceptable retention of durability after accelerated aging-

Given that fact and the number of questions about accelerated aging that unanswered, SCII has concluded that there is no need to add a requirement for artificial aging to this revision of Z39.48.


...The first draft of the proposed ISO standard for permanence of paper included a maximum pH of 9.5. Tests done by the Library of Congress indicate, however, that commercial papers with pH levels as high as 10.3 are as permanent as papers in the 7.5 to 9.5 pH range. Based on these data, SCII has incorporated a maximum pH of 10 in the revised standard.

Mechanical Pulps

Since the current standard was developed, the paper industry has begun to use chemi-thermomechanical 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 confirmed that alkaline papers with up to 7.5% residual lignin exhibit retention of durability equivalent to alkaline papers with less than 1% lignin. For this reason, SCII has set a lignin percentage of 7.5% in the proposed revision. The specific prohibition against groundwood has been removed. Papers with significant groundwood cannot meet the tear resistance criterion in the standard.

Recycled Pulps

... Alkaline papers containing recycled pulp can be suitable for long term retention in libraries and archives.

Some alkaline recycled papers meet the tear resistance requirement and some don't. The proposed revision will also screen out recycled paper with very high amounts of residual lignin.

Manufacturing Materials and Methods

SCII received a few comments on the desirability of adding specific prohibitions against the use of certain sizing materials and of specifying that the paper must be made by certain processes. The Comittee believes that, as much as possible, the standard mast specify measurable attributes of finished paper and leave the selection of production methods and materials to paper manufacturers. To do otherwise could penalize the development of innovative ways to make permanent paper and have a negative effect on beneficial advances in paper technology.

Coated Paper

The issues previously discussed apply to both coated and uncoated paper. Another set of questions applies to coated paper only. Coated paper was first introduced for high volume applications in the 1930s, but its technology has undergone tremendous change in the last 50 years. Early coated papers differ so substantially from those produced today that testing of naturally aged specimens is largely irrelevant to the task of establishing specifications for permanence of current coated paper. As the name implies, coated paper contains a base or core paper to which a coating is applied. Because of their coatings and adhesives, coated papers contain more different substances than uncoated papers. As slight differences in the makeup of coatings can affect the quality and cost of finished coated papers, paper manufacturers are naturally reluctant to provide information on the exact formulation of these papers.

The core of a coated paper may be alkaline, neutral, or acidic. Most coatings are alkaline at least at the time they are applied, although some owe their alkalinity to substances which quickly evaporate after the paper is made. If the coating is very alkaline, the extraction pH of the coated paper as a whole is likely to be alkaline even if the core paper is acidic. An initial question addressed by the Committee was whether an alkaline coating could, in effect, neutralize acid in the core paper, i.e., was a paper with an alkaline coating "permanent" even if its core paper was acidic? Both the opinions of paper experts and the results of private studies done by paper manufacturers indicated that acid in the core of a coated paper would have a negative effect on permanence. Paper testing commissioned for SCII has corroborated the view that the core of a coated paper must be acid-free to ensure reasonable permanence. Standard tests for determining the pH of the core paper exclusive of its coating are yet to be developed. For this reason, the proposed revision specifies reliance on manufacturer's certification of the pH of the core paper as well as qualitative methods for determining the core pH of coated papers-

Comments received in response to the discussion draft of the revised standard pointed out that paper which is technically "uncoated" may have a very lightweight, but highly alkaline surface coating sufficient to yield an line cold extraction pH for a paper that is actually acidic. SCII has therefore specified manufacturer's certification of the pH of the paper and qualitative methods for determining the underlying pH of the paper for uncoated papers as well.

Another question was whether a coated paper containing a neutral core and an alkaline coating could be as permanent as a coated paper with an alkaline core. The paper testing done for the Committee indicates that this may indeed be the case. Experts consulted agree that it is logical that an alkaline reserve in a coating could protect a neutral core from environmental pollutants. Additional testing related to this question would certainly be desirable, but data currently available support a minimum required pH of 7 for the core of coated paper and an alkaline reserve for the paper as a whole of 2% or more.

Because a significant part of its weight is coating rather than fiber, the results of physical tests on coated paper differ substantially from the results of the same tests performed on uncoated paper of equal basis weight. In comparing the initial folding endurance and tear resistance of alkaline coated papers with the current standard, The Committee found that almost none of the coated papers met the requirements as stated for uncoated paper. For example, a 60 lb. coated paper did not meet the standard for 60 lb. uncoated paper. Approximately 33% of the gross weight of the 60 lb. uncoated [i.e., coated] papers tested was coatings however, so the core papers were actually 40 lb. stock. The alkaline 60 lb. coated papers generally did meet the standards for folding endurance and tear resistance for 40 lb. uncoated paper. Since the printing qualities of coated paper are essential for some kinds of publications, it would serve no useful purpose to establish a standard for permanence of coated paper that excludes virtually all available alkaline coated paper. For this reason, SCII is proposing that requirements for initial durability of coated paper be based on the weight of the core of the paper.

Two other issues specifically related to the permanence of coated paper were brought to the Comittee's attention. These were: 1) the potential for unacceptable color changes in the coating over time, and 2) the possibility that age will weaken the bond between the coating and the core paper so that the coating will flake off. Professor Joseph Brown, a umber of the Committee, directed color change and strength of bond tests on the Committee's coated paper samples and found no evidence of a need to address either of these properties in the standard.

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