Mass Deacidification: An Initiative to Refine the Diethyl Zinc Process

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

Acknowledgments

The authors would like to acknowledge the cooperation and assistance of Tom Albro, John Bertonaschi, Martha-Lucia Sierra, and Terry Wallis, conservators who served on a panel that was responsible primarily for conducting an odor evaluation of selected books from each DEZ test run. Paul Gray assisted Ms. Sierra with the selection and transportation of test books to the Akzo facility. Ms. Sierra participated actively in many aspects of the DEZ initiative and did much to facilitate the project, including the preparation of books for evaluation by assessment panels. We are grateful to the Library managers who agreed to serve on a Management Mass Deacidification Assessment Panel that met several times to evaluate the extent of acceptability of treated books.

The Library is particularly indebted to Sylvia Subt, John Koloski, and James R. Bond of the Government Printing Office for undertaking scanning electron microscopy of DEZ-treated coated papers, and to Professor David Hoffman, University of Houston, and Professor Klaus Theopold, University of Delaware, for undertaking important laboratory-scale experiments that provided new insights.

Identified in the Appendix are consultants and LC staff members who participated actively on a technical team that provided excellent guidance for this R&D effort. A few of the consultants, particularly George (Larry) Eitel, a chemical engineer, played an active role in assisting with the planning of individual test runs, along with the Akzo engineer, Mildred Jeffery, whose cooperation and knowledge of plant operations were key elements in the success of this project.

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:

At the end of the procurement effort to obtain mass deacidification services in 1991, the expert advisory panel concluded that the DEZ process showed the greatest promise for meeting the Library's technical requirements.

However, this process also had some unacceptable flaws:

Objective of this project:

Minimize these remaining problems in no more than 12 runs in Akzo's pilot plant in Houston, Texas.

The Developmental Effort:

Cause of odor a mystery since known chemistry cannot explain it. Several possible sources of odor formation investigated:

Impurities in diethyl zinc and the nitrogen gas used in the process
Degradation products present in paper
Air leaks in chamber
Tight or loose packing of books
Amount of alkaline reserve
Temperature in DEZ vaporization chamber
Temperature in reaction chamber

Series of experiments in the Akzo plant and in the laboratory undertaken systematically ruled out all other possibilities for odor formation except the two temperature parameters.

Lowering of these temperatures significantly lowered the extent of odor formation.

About 4500 treated books stored in a room with no air circulation did not give off a noticeable odor. By contrast, 500 books treated for the procurement effort exuded a distinct odor.

Chemical interaction between book covers and DEZ was also reduced at lower operational temperatures. Chemical attack on adhesives was also decreased at lower temperatures.

Operational difficulties:

Inefficient plant design does not allow DEZ vaporization temperatures under 110°F.

Minor modification to plant needed but impossible because of Akzo's decision to shut down the plant.

Optimal performance of Akzo pilot plant:

Last two runs (numbered 11 and 12) employed optimum conditions to produce the best treatment that the Akzo plant, with its inherent limitations, was capable of producing in a consistent and reproducible manner.

Achieved alkaline reserve of 2 to 3% calcium carbonate equivalents (LC's minimum requirement is 1.5%).

There were no iridescent rings on glossy papers or chemical attack on book covers.

Odor level was minimal.

A management panel that examined books deacidified in these culminating runs found 98% of the covers and textblocks acceptable and 94% of the treated books to be free of unacceptable odors.

Conclusions:

Odors and chemical attack on some book covers are caused by higher temperatures.

Iridescent rings on coated papers and covers are eliminated by maintaining a high enough DEZ flow rate.

Near-perfect treatment is achievable.

Problems are resolvable with minor engineering changes.

DEZ Pros and Cons:

Main advantages:

Inks do not run or bleed since no solvent employed.

This would be an essential requirement for treatment of archival manuscripts, but printed inks in books are less vulnerable.

Can deposit high concentrations of alkaline reserve, extremely uniformly, regardless of the thickness or size of a book.

All types of papers, including coated and dense, supercalendered papers, are thoroughly treated.

Zinc oxide deposit in paper least likely to produce color shifts in inks and pigments since treated paper is almost at neutral point (pH barely above 7). Calcium and magnesium-based processes provide a much more basic (higher pH) environment and increase the possibility of color changes in images.

A mature process which has been studied extensively for effectiveness; also extensive toxicology data available.

Disadvantages:

Pyrophoric nature of DEZ needs good engineering, good safety practice and constant maintenance.

A complex technology, for which engineers and other technical staff are needed 24 hours a day.

Plant must be situated away from the library. Therefore, books need to be transported to and from the plant.

Substantial capital outlay (5 to 10 millions) needed for a new plant.

Economies of scale can only be achieved by building a plant with a substantial production capacity, necessitating a large capital outlay.

Other assessments of DEZ technology:

Harry Ransom Humanities Research Center of the University of Texas recently concluded that "... it is now clear that the process works well for archives and manuscript materials."

The General State Archives of the Netherlands determined this year that, of three processes studied for them by the highly regarded TNO Centre for Paper and Board Research in the Netherlands, DEZ is best suited for the treatment of archival materials and manuscripts.

A. Liénhardy in Restaurator in 1994 (Vol. 15, No. 1, p.1) found DEZ and Bookkeeper to be the two most promising technologies among seven mass deacidification processes compared.

Future possibilities:

Library needs to decide whether to invest heavily in a new DEZ plant at this time or to keep its options open for the future.

Mass deacidification is a technology that has not yet matured to its fullest extent. Therefore, continue to encourage the development of such technologies.


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.

Project Chronology

The following table provides a chronologically arranged descriptive summary of the DEZ R&D initiative, describing briefly the objectives and results of the test runs and of the complementary laboratory research efforts.

Date

Activity and objective

Comments

1/12/93

Test Run 1: Establish whether odor will be generated in absence of inks, adhesives, covering materials and degradation products from aged paper.

Odor was generated in test books made of new paper (no degradation products) without any adhesives, book-covering materials, etc. Therefore, odor must be due to interaction of DEZ with book paper. Baseline operational parameters were established for future runs. Books treated uniformly, 1% alkaline reserve.

2/93

Initiate consultation contracts with Professors Klaus Theopold, University of Delaware, and David Hoffman, University of Houston, to carry out laboratory studies that would complement the pilot plant effort.

Parallel laboratory scale experimentation would attempt to comprehend the chemistry underlying the problem of odor formation.

2/20/93

Test Run 2: Establish whether degradation products in aged paper play any role in odor formation.

The same test books as used in Run 1 (no adhesives, inks, etc.) were used to fill the chamber, but these books were aged artificially for different aging periods. Established that degradation products in aged paper do not generate odor.

3/93 thru 5/93

Establish laboratory setups at Universities of Delaware and Houston to simulate the DEZ process in the Akzo pilot plant.

This laboratory scale work would complement the knowledge gained from test runs in the pilot plant and aid in determining the course of future test runs.

6/93 thru 9/93

Laboratory Experiments at Universities of Delaware and Houston: The objective of these experiments was to reproduce the odor effect encountered in the Akzo pilot plant, and then to attempt a solution.

Fifteen experiments were performed under a variety of conditions in an effort to reproduce the odor generated in the plant. These experiments failed to produce any odor, except for a mild odor after permeation at 140F (the highest temperature achieved in the pilot plant). Even this odor was so fleeting that very little of it was left by the time the treated books arrived at the Library inside sealed plastic bags. These experiments definitively established that it was possible to deacidify books with DEZ without producing an odor.

Evidently, some different chemical reactions occur in the pilot plant than those that we were able to recreate in the laboratory. A significant possibility is that the higher DEZ evaporation temperature employed in the plant induces a small fraction of DEZ to decompose before it has a chance to react with paper. These unknown decomposition products exist only briefly to react through a different, uncharacterized chemical route to produce the odorous compounds. Since high odor levels are generally associated with noticeable chemical attack on book covers, these degradation products of DEZ must also react with other materials present in book bindings.

6/14/93

Test Run 3: Study effect of reduced permeation time. Longer permeation time increases the probability of undesirable reactions, besides extending treatment cost.

No adverse effects due to reduction in permeation time from 8 hours to 4 hours. Treatment was extremely uniform, but odor developed and chemical damage persisted on some covers, especially on blue test book covers. Alkaline reserve was still at the 1% level and would be kept there until all other issues had been resolved.

9/29/93

Test Run 4: This run went to great lengths to remove all volatile chemicals, including those present originally in the books, and others generated in different steps in the process. The book temperature during permeation was also reduced to 110F, as compared to 130F in Run 3 and 140F in Run 2. This was the lowest temperature Akzo would allow at this time from a safety standpoint.

Treated books had a noticeably reduced odor in comparison with previous test runs.

An internal LC-management deacidification assessment panel evaluated a limited sample of these books and found the treatment to be acceptable.

7/93 thru 10/93

NIST study to investigate odor formation.

Employed available data to suggest plausible causes of odor formation. Concluded that odor may be intrinsic to the reaction of DEZ with a variety of chemical compounds in books and the treatment facility.

9/93 thru 10/93

Study by Prof. L. S Hegedus at Colorado State University to analyze the odor causing chemicals.

Prof. Hegedus ascribed odor formation to reactions caused after decomposition of diethyl zinc, rather than by diethyl zinc itself. The chances of such a decomposition process would decrease as the DEZ and book temperatures in the permeation phase of the process are lowered.

11/10/93

Test Run 5: Attempted to reduce DEZ temperature in the evaporator to 75F. Too high a pressure in the evaporator condensed DEZ to a liquid, which stopped the flow of DEZ vapor to the chamber. DEZ temperature had to be stepped up to continue the permeation process. The rest of the run was conducted at conditions that were about the same as in Run 4.

The attempt to lower DEZ temperature in the evaporator was a good effort to push the system to the limit. Unfortunately this attempt did not succeed.

Odor level was about the same as in Run 4. It became evident with this run that chemical damage on covers is also much reduced along with the odor as DEZ and book temperatures during the permeation process is decreased.

12/14/93

Test Run 6: This renewed attempt at reducing book temperature during the permeation step did not succeed, as the written instructions were misinterpreted by plant operators and permeation was completed at a higher temperature.

In spite of the higher DEZ temperature in the evaporator and high book temperature (140F) during DEZ permeation, the treated books have been evaluated relatively favorably by the Conservation Office odor panel.

1/5/94

Test Run 7: This treatment was completed at a reduced temperature as desired. The book temperature ranged between 83 and 105F.

Books were uniformly treated at the lowest DEZ temperature attained thus far and had the lowest odor level yet experienced. Also, chemical damage to pyroxylin covers was noticeably absent.

1/17/94

Test Run 8: Attempted to lower DEZ temperature below the level attained in Run 7. In addition, a higher alkaline reserve of 1.5 to 2.5% calcium carbonate equivalents was attempted. Runs 1 thru 7 incorporated alkaline reserves of 0.8 to 1.0% calcium carbonate equivalents.

Due to limitations in plant design, DEZ flow shut off at a temperature in the low eighties before permeation was completed. Nevertheless, an alkaline reserve of 1.5 to 3.0% calcium carbonate equivalents was obtained at a low odor level. The goal of achieving an acceptable product was viewed as a distinct possibility if the DEZ temperature could be kept around 90F.

2/28/94

Test Run 9: Objective was to obtain a high level of alkaline reserve while maintaining low enough temperature conditions to minimize odor formation.

One and a half hours into permeation, the DEZ temperature in the vaporizer dropped to 70°F and the DEZ flow was curtailed. The temperature was then raised by an Akzo technician to 135°F, as compared with LC's objective of 90°F. As anticipated, odor level was high after the treatment. However, these books were lost in transit from Akzo to LC for several months. By the time they were received and evaluated, much of the odor had dissipated. The management panel found 91% of these books acceptable from an odor tolerance perspective, and 99% of the covers and textblocks were acceptable.

3/5/94

Test Run 10: Repeat process plan as proposed for Run 9, with relaxation to allow Akzo a maximum DEZ temperature of 100°F -- an allowance of 10°F, since they contend that they are working at the edge of the capability of the plant.

Akzo started permeation with DEZ temperature at 130°F--30° above the compromise upper limit, 50° above the desired upper limit. The temperature was progressively lowered to 100°F a few hours after the start. Most of the books in this run were made up of super-calendered paper, which has a much higher moisture content. Normal drying left too much moisture in the books, generating a lot of heat and giving much higher alkaline reserves (above 3 percent). Permeation time of 10 hours was not enough to react with all of the moisture. Several books remained partially untreated. Higher book temperatures led to higher cover damage. Lower DEZ flow rate led to stagnant areas in the chamber and creation of iridescent patterns on book covers and coated paper.

3/30/94

Test Run 11: Repeat the Run 10 process plan.

Akzo operators conducted the run under conditions that defined an operational comfort zone with the DEZ temperature just above 100 degrees for most of the process. All books were uniformly treated. Alkaline reserves easily exceeded the Library's minimum requirement of 1.5 percent calcium carbonate equivalents. The management assessment panel rated 92% of the books to have acceptable odors; the ratings of acceptable covers and textblocks were 98.9% and 98.1% respectively.

4/2/94

Test Run 12: Repeat the Run 10 process plan.

Operational conditions were about the same as those in the previous run. The objective was to establish good repeatability under standardized conditions. The process was again completed smoothly at temperatures that were 20 to 30 degrees higher than desired, but the books were treated uniformly at an alkaline reserve level that exceeded the Library's minimum requirement. The management assessment panel rated 95.2% of the books to have acceptable odors, which is the best rating of all 12 runs. They rated 97.4% of the covers and 98.9% of the textblocks acceptable.

7/94 td>

Final receipt of treated books.

8/94

Conservator and management panels conclude assessments.

Miscellaneous observations

Uniformity of Deposition of Zinc Oxide within Paper: Recent work reported by MacInnes and Barron (Journal of Materials Chemistry, v. 2, pp. 1049-1056, 1992) indicated that scanning electron microscopy (SEM) data showed that the zinc oxide deposited by the DEZ process in coated paper did not penetrate under the coating. This observation was contrary to SEM data that had been obtained by the Library during the earlier developmental work on the DEZ process. Further investigation was undertaken to resolve these conflicting sets of data. Ten different coated paper samples selected at random from more than 300 books treated in Run 4 were analyzed by SEM and X-ray microanalysis in LC's Research and Testing laboratories; further confirmation was obtained from John Koloski and James Bond at the Government Printing Office, who also mapped the distribution of zinc oxide across the edge of each of the ten papers. All of the papers subjected to these analyses, without exception, showed the zinc oxide to be deposited most uniformly throughout the paper matrix under the coating. This is, in fact, one of the best advantages that the DEZ process has to offer. DEZ can penetrate through all kinds of paper, and through the thickest books, to deposit finely divided zinc oxide most uniformly. It can do so, not only because it is a gas, but because it is attracted towards the bound water molecules dispersed evenly throughout the book mass and reacts with them to form zinc oxide, which constitutes the alkaline reserve.

Effect of DEZ Treatment on Photographic Materials: It is not surprising now and then to find microfiche attached inside a book cover. Also, one of the most significant problems in photographic preservation is the acid hydrolytic degradation of cellulose nitrate and acetate-based photographic film. The possibility of using a mass deacidification process for neutralizing the acids in large quantities of film appeared attractive.

A few samples of microfiche, microfilm and motion picture film were included in one of the test runs with DEZ. The results were disastrous. The films curled and shrank badly. The effect was much worse than what one would expect from only the drying of film. It is clear that any photographic film material would be lost if exposed to DEZ.

Optimization of the DEZ process: The resolution of the problems of odor and visual damage did not come about easily and even entirely satisfactorily, as is evident from the preceding account. At least four of the twelve runs were compromised due to stoppages induced by limitations in the plant design or its operation. As a result, it was not possible to extend this effort to optimization of the process to make it run more economically. In any case, since Akzo decided to close down this plant, its optimization would have been of limited value. It needs to be pointed out that there is plenty of room for cutting back on several steps in the process, especially in the nitrogen flushing before the permeation step. Nitrogen consumption, which was a significant part of the total operational cost, as well as process time could have been cut down significantly. The drying step also needed to be made more efficient by using a higher capacity pump and a wider bore tubing connection to the chamber -- improvements suggested by LC personnel soon after the plant had been built. However, Akzo management was not persuaded that such a plant modification would make a significant difference in the rate of drying of books. Another possibility was to introduce a pre-drying step in a separate chamber to save chamber operation time.

The drying step needed to be further standardized. Process conditions for this step needed to be correlated firmly with the weight of books to be treated. The drying conditions, used successfully in most of the runs, caused Test Run 10 to fail, because most of the books in this run were composed of supercalendered paper, which is much heavier and, therefore, has a significantly higher moisture content. The average drying conditions were inadequate for this run, with the result that too high an alkaline reserve was deposited on the outer margins (about 4 percent calcium carbonate equivalents), while the insides of the books remained untreated when permeation was stopped.

Advantages of the DEZ Technology

It is a mature process with extensive processing and toxicology data developed and most process-related issues resolved at the Library's expense. Even with occasionally languid attention from Akzo management, the pilot plant worked in a problem-free fashion for 6 years without any significant problems.

Even totally soluble inks will not run or bleed when papers containing them are treated with the gaseous diethyl zinc process. This feature makes this process invaluable for the treatment of archival materials and manuscripts, which are often written or annotated in soluble inks. However, the Library's mass deacidification program is focused primarily on books, where the danger of solubilization of printed inks by solvents is not as great. The ink solubilization problem in printed books is much more readily contained than with manuscript inks. A recent study, funded by the National Endowment for the Humanities and conducted by the Harry Ransom Humanities Research Center of the University of Texas, concluded that "...it is now clear that the process works well for archives and manuscript materials." The General State Archives of the Netherlands also determined this year that, of the three processes studied for them by a Dutch agency, DEZ is the best process for archival materials and manuscripts.

The DEZ process can deposit high concentrations of alkaline reserve, extremely uniformly, throughout all kinds of paper, including coated and supercalendered paper, without any change in the feel or texture of paper.

The final pH range of 7.5 - 7.8 is least likely to change the colors of inks and pigments in treated materials as compared with calcium and magnesium-based treatments, which give a final pH in the range of 9 to 10.5 pH units.

Disadvantages of the DEZ Technology

Because of the pyrophoric nature of the chemical, a DEZ plant cannot be situated at or near library or archival buildings. Therefore, books and documents must be transported to and from the plant, thereby increasing the chance of damage to books and manuscripts and adding to treatment costs.

An essential requirement for this process is a well-engineered and well-maintained plant that can manage the risks inherent in the process without any compromises. This requirement translates into relatively higher capital costs for plant start-up, as well as for operation and maintenance.

Economies of scale can only be achieved by building a plant with a substantial treatment capacity, again necessitating a large capital investment.

Conclusions

Odors are caused by higher temperatures. It has been clearly established that there is a definite and proportional relationship between temperature levels to which DEZ is exposed in the vaporization chamber and in the main reaction chamber during the permeation step and the resulting odor level in treated books and the chemical attack on book covers, adhesives, and labels. The higher the temperature to which DEZ is exposed, the stronger are the odors and the chemical attack on book materials. These adverse effects can be minimized, and even eliminated, by promoting process conditions and plant design that minimize the heat to which DEZ is exposed. Books treated at ambient DEZ temperatures do not develop any odor. A logical possibility is that a small fraction of DEZ decomposes before it can react with the moisture in paper. The unknown products of this decomposition, which may well be transient in nature, lead to the formation of the odorous compounds, as well as the chemical attack on some book covers, labels, and coated papers.

It has also been established that tight or loose packing of books; the amount of alkaline reserve; reactions of DEZ with degradation products, unknown paper chemicals and adhesives; phases of the moon and the positions of various planets and constellations do not have any influence on the observed adverse effects of DEZ treatment.

Iridescent rings on coated paper and covers have been eliminated. Iridescent rings on reflective surfaces, such as smooth coated paper and book covers appear when the reaction between DEZ and moisture in these materials is a relatively static, diffusion-controlled process. This is a special case of well-established phenomena in gas-solid interactions, which lead to formation of products in concentric spheres when the penetration of the gas into the solid matrix is controlled mainly by diffusion. In the case of DEZ reaction with the moisture in paper, these concentric deposit patterns can be eliminated by maintaining a constant turbulence in the DEZ flow within the reaction chamber. The key parameter is the DEZ flow-rate. For the Akzo chamber, the minimum acceptable flow-rate was established at 150 pounds per hour. Higher flowrates are better, but DEZ flow into the chamber must be sacrificed to obtain lower vaporization temperatures. In spite of a satisfactory flow rate, if the permeation process is upset and comes to a standstill, even briefly, the formation of the iridescent ring pattern is unavoidable.

Problems are resolvable with minor engineering changes. It is believed that minor modifications in the plant could have been performed under normal circumstances to totally eliminate odor formation. However, Akzo's decision midway through this project to shut down the plant early in 1994 precluded any such initiative.

Near-perfect deacidification of books is achievable. Given the less-than-satisfactory circumstances under which the Library had to conduct a significant portion of this research and development initiative, it is heartening that the final two redundant test runs demonstrated that near-perfect results can be achieved by deacidifying books with the DEZ process, which is the only gas phase process known to leave a permanent alkaline reserve within paper.

Future of the DEZ Technology

The technology behind the DEZ process is now a tried and proven reality. It has been demonstrated to work well enough to meet all of the Library's requirements. It would be hard for any liquid phase, solvent-based process to match (1) the uniformity with which this process can deposit the alkaline reserve within even bulky, oversized books and within dense, supercalendered and coated paper, and (2) lack of any adverse effects on colored media. However, the utilization of this technology comes at a significant cost. These costs can be contained only with a significant capital investment in a large enough plant. If cheaper processes become available that are almost as good, safer, and probably more convenient, such costs may be hard to justify.

Since Akzo Chemicals' untimely decision to shut down the existing plant forecloses the Library's near-term ability to experience the efficiency of the DEZ technology at a large production capacity, and because of the high capitalization cost for a new DEZ plant, the Library will continue to assess the benefits offered by other promising, simpler, and cheaper deacidification technologies.

The Library's patents on the DEZ process expired in 1993 and 1994, and Akzo Chemicals terminated its DEZ license with the U.S. Commerce Department effective September 1994 (see Akzo's June 1994 letter in the Appendix). If any other institution or company in the U.S. or elsewhere decides to apply the DEZ technology to deacidification of library collections and archival materials, the Library will be pleased share the extensive technical expertise it has acquired in the development of this process.

Conservators' Report on the Effects of DEZ on Bound Materials
By Martha-Lucia Sierra and Terry Wallis

Technical Summary of the Library of Congress DEZ Book Deacidification Process Research and Development Tests 1 to 12, 1993-1994
By George L. Eitel

*** Last Update 1/20/95 (efm) ***


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