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MASS DEACIDIFICATION: AN INITIATIVE TO REFINE THE DIETHYL ZINC PROCESS

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Kenneth E. Harris and Chandru J. Shahani
Preservation Directorate
Library of Congress
Washington, D.C.
October 1994

SECTION 1: Summary - Introduction - Background - DEZ Refinement

Summary

In the summer of 1992, the U.S. Congress approved a Library of Congress (LC) Action Plan aimed at refining the diethyl zinc (DEZ) mass deacidification process and assisting in the development of other deacidification processes. This report deals with the DEZ phase of the two year initiative. It describes the progress achieved in this effort and recommends changes that should be considered in any future application of the DEZ process.

Background:

Objective of this project:

The Developmental Effort:

Operational difficulties:

Optimal performance of Akzo pilot plant:

Conclusions:

DEZ Pros and Cons:

Other assessments of DEZ technology:

Future possibilities:

Introduction

Deacidification processes are used to extend the life of books, manuscripts and other paper-based materials by neutralizing acids in paper and impregnating it with an alkaline reserve that will continue to protect it from acid-induced degradation in the future. Over the past two decades, the Library of Congress has been at the forefront of the development of deacidification processes that can be applied en masse to large collections. In its search for an ideal mass deacidification process, the Library invented and further developed the diethyl zinc (DEZ) gaseous process and also brought about key improvements in solvent-based, liquid phase processes. In the conservation science community, gaseous processes have traditionally been deemed to have a significant edge over liquid phase, solvent-based processes, mainly because they preclude any possibility of adverse effects on inks, dyes and colored pigments in manuscripts, color plates and book covers due to bleeding or softening of inks. The following quotation from the inventor of the Wei T'o liquid phase process (or a "liquified gas process") sums up the strong bias that existed for a gaseous mass deacidification process:

" Many researchers dream of finding a fountain of youth. In search for deacidification methods, the fountain of youth seems to me to be a gaseous method of deacidification. A gaseous method would be ideal, absolutely ideal from every point of view, if it could be made to work."

-- Dr. Richard D. Smith in Canadian Library Journal, Vol. 36, 1979, p. 326.

It can be said with confidence now that the DEZ gas phase process works. Since the Akzo Chemicals (formerly Texas Alkyls) pilot plant began operation in December 1987, over 200 scientific and commercial batch runs to deacidify library books, maps manuscripts and other paper-based materials from domestic and international collections have been completed without any safety problems. Problems which persisted with this process until recently have been resolved as a result of the present initiative.

Background

Earlier gas phase deacidification processes employed alkaline chemicals that could be vaporized and deposited in paper, such as ammonia, morpholine, and cyclohexylamine carbonate (CHC). All of these chemicals suffered from the fact that they deposited an alkaline reserve that was not permanent, since whatever could be vaporized once would vaporize again over time. In addition, these chemicals constantly exude an odor which is not pleasant and can also present potential health hazards.

In the 1970's, the Library of Congress, with the encouragement of the U. S. Congress, launched a research program to identify methods to neutralize the acid in books that led to their embrittlement. This research by Dr. John Williams, Research Officer, and senior scientist George Kelly, laid the foundation for most of the current mass deacidification processes. They filed four patents for new processes. One of these patents, filed in 1975, was for the invention of a gas phase deacidification process using the chemical diethyl zinc. They described the process by which DEZ vapor rapidly penetrates the fibers in book papers, neutralizes the acids present, and deposits a uniform, stable alkaline reserve of zinc oxide to protect the paper from future acid damage. Laboratory scale experiments were followed by a few successful small-scale experiments at a GE facility.

Library scientists continued development of the DEZ process through the 1980's with the design, construction, and operation of scaled-up test facilities. Like most research projects that dare to chart unfamiliar ground, the DEZ research and development effort had its ups and downs. Twelve test runs carried out by Northrop Services, Inc. in a retrofitted space simulation chamber had established the promise of this process. A pilot plant was then constructed at NASA's Goddard Space Flight Center, in which the process was to be perfected. The worst setback came when a fire broke out as the new pilot plant equipment was being subjected to an operational test without any books in the chamber. Subsequent analysis of this incident showed it to be the result of pathetically poor engineering practice and design. Water reacts vigorously with DEZ, giving off zinc oxide, ethane (a flammable gas), and heat. The heat can cause ethane to catch fire, unless this reaction is carefully controlled by limiting the amounts of water and DEZ that are allowed to mix. In the incident at the Northrop Services facility at Goddard, water was pumped into a chamber in the belief that all DEZ had been pumped out. In fact, a few hundred pounds of DEZ had been pumped into the chamber. Material balance, a fundamental ingredient of chemical process operation, was entirely lacking. Not surprisingly, this incident raised great alarm, as well as opposition to the DEZ process and set its development back by several years.

The Library then turned to a manufacturer of diethyl zinc to design and safely operate a DEZ small scale test plant. The Commerce Department granted a license to Texas Alkyls (now Akzo Chemicals, Inc.) to promote commercial application of the DEZ process. In 1989, a small scale test facility was constructed at the Texas Alkyls plant in Deer Park, Texas, with a capacity for deacidifying up to 500 books in each treatment cycle. The engineering of the DEZ technology was successfully demonstrated in a series of 23 tests without any safety problems. However, the process appeared to have some persistent problems with the quality of the treatment. Nevertheless, the process had reached a state of maturity at which the Library felt that an acceptable product could be produced with proper operational procedures in place.

In 1989, Congress directed the Library to consider for its deacidification services all technologies that could safely and effectively meet its preservation needs, rather than confining its attention to the DEZ process alone. The Library issued in September 1990 a request for proposals from all firms interested in providing deacidification services to preserve the Library's collections. Based upon the findings of a board of technical experts and test results obtained from an independent testing lab, LC canceled the procurement in the fall of 1991. Though each of the three processes that were submitted for consideration slowed the rate of loss of strength in acid paper by at least 300% percent, two of the processes failed to meet one or more of LC's preservation requirements and each resulted in a variety of aesthetic damages or other problems in treated books.

Of the three processes evaluated by an expert panel, only Akzo's DEZ process met all of the preservation requirements specified in the procurement effort (neutralization of acids in paper, provision of a specified level of evenly distributed alkaline reserve for future protection, and extension of the life of paper). For this reason, and also because the advisory board determined that the DEZ process had the greatest potential at that time for being improved in the short term to meet the Library's requirements, LC proposed that Congress approve an initiative to minimize problems associated with DEZ treatment, such as distinctive odors in treated books and iridescent rings on book covers and coated paper stock.

The Library placed a notice in the Commerce Business Daily (CBD) in the summer of 1992 to inform interested parties of LC's intention to enter into a sole source contract with Akzo Chemicals for a research and development effort to refine the DEZ deacidification process. A contract was signed in December 1992 for 12 test runs at Akzo's Book Preservation Facility in Texas.

The DEZ Refinement Effort

Objective: The primary emphasis of the recent research and development initiative was on the elimination of objectionable odors in treated books and on minimizing visual damage, such as iridescent rings and chemical attack on some pyroxylin-coated book covers as well as rings on coated paper. An additional factor was the effect on adhesives used in bookbindings and in labels. The scope of this study was to extend to all types of papers, inks, book covers, and other materials. Process parameters during the drying, permeation, rehydration and post-treatment steps needed to be optimized so that library books could be deacidified in a consistent and reproducible manner, with an adequate alkaline reserve and without process-related damage.

The Technical Team: To obtain broad-based professional assistance with its DEZ refinement initiative, the Library assembled a multi-disciplinary team of consultants. They included scientists from academia, technical experts in chemical engineering and paper technology, and professionals from the library and conservation fields, who helped LC design this research and development program. Consultants and LC staff members who participated in this effort are listed in the Appendix.

Methodology:: Consistent with the Library's earlier policy of not experimenting with its collections, all of the library books used in this project were expendable books from the Library's Exchange and Gift Division. As an aid in monitoring the effects of process variables from one experimental run to another, two types of test books were routinely included in all runs in the pilot plant. One of the books was a soft-covered book made by sewing together a 1-inch thickness of unprinted, alum-rosin-sized paper. This book was made to exclude any adhesives or other extraneous materials other than the paper itself and cotton thread. This is the "white test book" to which reference may be found occasionally in this report. The variation of alkaline reserve deposit among different experiments was routinely tracked by measurements on treated papers from this test book. The other Library test book, frequently referred to as the "blue test book" is constructed of a small variety of different papers that include newsprint, acidic alum-rosin-sized paper, an alkaline paper, a coated paper, supercalendered paper, and Whatman paper. The composition of this book is presented in the Appendix.

Potential Causes of Odor Formation: None of the known chemical reactions of DEZ leads to the formation of an odorous chemical. Therefore, any odor(s) that form must result from uncharacterized side-reaction(s). A logical path to solving this problem would be to analyze the volatile products from DEZ-treated paper. If the odor-causing chemical could be detected, then the chemical reaction that produces this chemical could be recognized, and hopefully, eliminated. This was exactly the approach employed in earlier work carried out by the Library well before the 1991 procurement process. The presence of several compounds in trace concentrations was discovered. However, none of the compounds accounted for the characteristic DEZ treatment odor, which persists for several months after treatment. Immediately after treatment, there was also a sweet alcoholic odor, much of which dissipated after the post-treatment step in which treated books were flushed with air for three days. Since none of the chemical compounds identified by GC-MS (gas chromatography coupled with mass spectrometry) has a smell resembling the "DEZ odor," the chemical that caused this odor must have been present in too small a concentration to be detected by the sensitive analytical means employed. Without a substantive clue, the process of deciphering the main cause of odor was reduced to sheer speculation.

In the present effort, two studies were commissioned to dig a little deeper, one at Colorado State University and the other at NIST. The latter suggested that the odor had to be an intrinsic part of the deacidification process with DEZ. The report was very logical. However, by the time this work was completed, the laboratory-scale deacidification at the Universities of Delaware and Houston were finding it impossible to develop an odor in DEZ-treated books. They were able to generate an odor later at higher temperatures.

The only other recourse that seemed to make sense was to compile all the different possibilities that could lead to odor formation and then eliminate them one by one. The following potential causes of odor were outlined as a result of a discussion among LC staff, LC consultants, and Akzo staff:

A systematic experimental effort was undertaken to examine each of these hypotheses. The effort at the pilot plant level was supplemented with laboratory experiments carried out by consultants at the Universities of Houston, Delaware, and Colorado State.

The chronology of events in the next section of this report describes the progressive elimination of most of these possibilities. Finally, it was the laboratory experiments that established that it was only at higher operational temperatures that the deacidification treatment was accompanied by an odor. In fact, most of the laboratory-scale experiments, which were carried out at room temperature, were unsuccessful at reproducing the odor that had generally come to be associated with the DEZ deacidification treatment. The challenge from then on was to get Akzo to lower two key temperature levels, one for DEZ evaporation and the other for book temperature in the permeation step when DEZ reacts with moisture in paper to produce zinc oxide. At first, LC's request was turned down on the grounds that these changes would undermine the safety of the plant. To their credit, Akzo personnel reconsidered this issue and became more receptive to LC's proposal for lower temperature levels.

It was then discovered that the plant design would not allow the lowering of the DEZ vaporization temperature to desired levels. Attempts to lower the DEZ vapor temperature from the normal 120°F to 70°F in the treatment chamber required reducing the chamber pressure. The normal pressure of 35 Torr was lowered to about 15 Torr to prevent condensation of DEZ on the books during permeation. However, after several test runs at the lower pressure and temperature, the permeation cycle experienced DEZ flow upsets. An engineering analysis of the problem indicated that the lower pressures in the chamber and DEZ vaporizer piping caused higher vapor velocities. A resulting pressure drop in the piping increased the pressure in the upstream DEZ vaporizer, which in turn prevented vaporization of the DEZ at the lower target temperature. This caused flooding of the vaporizer and process upsets.

To stabilize the process at lower DEZ temperatures, the DEZ circulation rates had to be cut back until uneven book permeation treatment occurred. This problem could have been overcome by increasing the throughput of DEZ gas into the reaction chamber by adding a parallel supply pipe. This modification would have delayed operations by perhaps 2 to 3 months and would have cost $10,000 to $15,000. However, this option could not be entertained as Akzo had already decided to shut down the plant.

Several test runs conducted for the Library in 1993 demonstrated significant progress in addressing the issues of odor and occasional physical damage in some treated books. Then, on December 13, 1993, Akzo announced it had decided to shut down its Book Preservation Facility for business reasons effective at the end of the first quarter of 1994. This action signaled Akzo's assessment "of limited prospects for the adoption of DEZ in the near future." In letters to institutions with which it had contracts, Akzo reported this action was "despite our firm belief that [DEZ] represents the best available technology to address a need that is real and truly worldwide." (See the Appendix)

Following Akzo's announcement, the chemical company continued to cooperate with the Library to complete its proposed cycle of 12 DEZ test runs. After much trial and error, the last two runs (numbered 11 and 12) were accomplished under conditions which were not what LC had specified, though these runs provided the lowest temperatures that Akzo managers and engineers were confident of being able to repeat. Those two runs demonstrated that odors as well as the chemical attack observed on some covers and adhesives could be greatly reduced or even eliminated in the vast majority of books treated at lower book and DEZ temperatures, while still achieving a higher alkaline reserve.

Assessment of odor in treated books: Odor is one of those elusive perceptual qualities that means different things to different people, and possibly even to the same person at different times. Therefore, it was necessary to establish a dependable and reproducible methodology for the assessment of odor in library books treated in different test runs over several months. An odor panel comprised of three conservators was set up to evaluate odor in treated books by comparison against two DEZ-treated books from earlier Test Run 3, which established the baseline parameters. This procedure was modeled after TAPPI Test Method T483. It was challenging, and perhaps impractical, to try to establish objective odor protocols that applied consistently over time to books treated from one run to another. Objectivity may have been compromised by the fact that DEZ odor, even in control books, is fugitive by nature and, therefore, dissipates over time. The books against which the constant comparison was being made smelled less and less as they continued to be riffled repeatedly, with the possible result that the evaluation of books from each succeeding run grew progressively more demanding. In any future studies, an instrumental method that quantifies odor emission would be most desirable.

Assessment of acceptability of treated books: Another panel composed essentially of upper management was set up to determine the acceptability of treated books from one run to the next, by a purely subjective evaluation of any perceptible changes in the appearance of the covers and the textblock, as well as the formation of any odors. An average of ten managers examined and scored 30 to 50 books from Test Runs 4 through 12. The average scores are presented as a bar chart in the Appendix.

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