Design of a Liquified Gas Mass Deacidification ... - ACS Publications

11 Design of a Liquified Gas Mass Deacidification System for Paper and Books RICHARD DANIEL SMITH Downloaded by FUDAN UNIV on February 16, 2017 | http://pubs.acs.org Publication Date: December 17, 1978 | doi: 10.1021/ba-1977-0164.ch011

Wei T'o Associates, Inc., P.O. Box 352, Park Forest, Ill. 60466

The Wei T'o Nonaqueous Book Deacidification System, an in-progress pilot project at the Public Archives of Canada, is presented in terms of institutional background, system selection, characteristics of liquified gas systems, and current system design. The manual aqueous (magnesium bicarbonate) and nonaqueous (magnesium methyl carbonate: magnesium methoxide mixture with carbon dioxide in methanol and trichlorotrifluoroethane) deacidification solutions used at the Public Archives extend potential record life but are limited to unique, valuable records because of treatment cost. The system was chosen as a pilot project test because its chemistry was accepted, a history of successful use existed, and mechanical problems of handling liquified gas solvents appeared solvable. Deacidification of paper using liquified gas solvents to dissolve, carry, and deposit deacidification agents function analogously to chlorofluorocarbon refrigeration cycles. Successful treatment requires vacuum drying books to 0.5% moisture or lower, solution impregnation at pressures up to 200 psig, and flash drying to deposit the deacidification agent throughout the materials treated.

' " p h i s report on the W e i T o Nonaqueous Book Deacidification System at the Public Archives of Canada is divided into: (1) institutional background, (2) system selection, (3) characteristics of liquified gas systems, and (4) current system design. The focus on deacidification (neutralization of paper and impregnation of an alkaline buffer) occurs because most deterioration is caused by acid-catalyzed hydrolysis of cellulose (1,2). 149 Williams; Preservation of Paper and Textiles of Historic and Artistic Value Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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Downloaded by FUDAN UNIV on February 16, 2017 | http://pubs.acs.org Publication Date: December 17, 1978 | doi: 10.1021/ba-1977-0164.ch011

Institutional Background The Public Archives of Canada, established i n 1872, is a federal department of the government of Canada with many responsibilities. Among the latter are acquiring from many sources all significant archival material of every kind, nature, and description relating to all aspects of Canadian life and to the development of the country and for providing suitable research services and facilities to make this material available to the public ( 3 ) . Its eight collecting divisions are the Manuscript Division, Public Records Division, Library, Machine Readable Records, National M a p Collection, Picture Division, National Photographic Collection, and National F i l m Archives (including historical sound recordings). The Records Conservation Section, staffed with 20 persons, of the Administration and the Technical Services Branch currently provide con servation and restoration services for paper materials to the Public Archives and National Library of Canada. Studies, both within and without the Public Archives, indicate the present collection is deteriorat ing more rapidly than the Records Conservation Section can restore it. Moreover, the collections are growing at increased rates, and the stability of materials to be acquired is projected as no better than materials already i n the collection. The obvious solution of expanding the Records Conservation Section was established as unrealistic i n terms of rate of collection deterioration, personnel availability, and treatment cost. The collections are estimated to lose 50% of their folding endurance (a measure of capability of paper to resist wear from patron use) every 7.5 years. The leaves of many books are too weak for rebinding 60 years after the date of publication and break if turned after 100 years (4). The Section has been unable to recruit qual ified conservators for existing positions, and the competition for available conservators is increasing as more institutions establish conservation pro grams. The manual aqueous (magnesium bicarbonate) and nonaqueous (magnesium methyl carbonate: mixture of magnesium methoxide and carbon dioxide i n methanol and trichlorotrifluoroethane) deacidification treatments used by the Section are known to alleviate this deterioration, but cost restricts their application to unique, exceptionally valuable materials. Thus, on examination, the Public Archives found itself meeting its acquisition and service missions but failing to maintain its general collec tion i n useable condition. The Public Archives of Canada investigated alternate approaches for extending the useful life of its collections be cause the techniques practiced by the Records Conservation Section gave no promise of bringing the rate of records deterioration under control.

Williams; Preservation of Paper and Textiles of Historic and Artistic Value Advances in Chemistry; American Chemical Society: Washington, DC, 1978.

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System Selection The W e i T o system was selected for pilot trial because it appeared to best meet the requirements of the Public Archives of Canada ( 5 - 9 ) . The deacidification of paper is not a preservation panacea but a partial treatment known to extend the potential life of paper. F o r example, deacidification does not strengthen deteriorated and embrittled paper, an increasingly important aspect of the conservation problem. The impreg nation of 3-5% by weight of resin strengthening agents throughout a book is considered a straightforward operation for a liquified gas process but a difficult task for a vapor phase process. The analysis suggests that if it is necessary to use liquified gases for strengthening paper, the same pro cedure should be used for deacidification. Storage of archival library, and museum holdings at reduced relative humidities and temperatures functions similarly to deacidification i n reducing the rate at which the chemical reaction, given i n Figure 1, proceeds. Clearly, the reaction demonstrates acid attack, producing a chemically equivalent but partially hydrolyzed cellulose, and cannot proceed if water or energy are removed. The availability of water and energy to enter into the reaction can be reduced by controlling the rela tive humidity and storage temperature. The choice between cold storage as a form of preservation and deacidification itself can be made on the basis of cost. C o l d storage and the mass nonaqueous deacidification process being reported on were evaluated as equal in cost 30 months after treatment for a 1,000,000-volume library i n a 1970 study (10). There after, the annual maintenance and operation costs made preservation through air conditioning increasingly more expensive at a projected rate of $0.10/volume/yr.

Cellulose

•

-j-

Water

A C I C I catalyst

Hoat

^

Partially hvdrolyzed cellulose

Figure 1. Acid-catalyzed hydrolysis of cellulose



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The fact that holdings of the Public Archives and National Library of Canada are working collections, available for regular patron use, which may be called upon for loan and exhibition indicated the system chosen should provide protection when materials are outside institutional control. These missions raised questions about cold storage because new facilities would be required, possibly distant from use points, and re strictions on patron use introduced. The Records Conservation Section has five years of satisfactory expe rience treating a variety of unique works with W e i T o solutions. A survey of conservators and scientists i n other institutions verified the chemistry was sound and the results were aesthetically acceptable. The problems to be resolved were mechanical, involving equipment choice rather than the chemistry of the treatment (11). The hazards inherent to the system could be isolated and controlled at the treatment site. The solvent, approximately 90% dichlorodifluoromethane and 10% methanol by volume, has a maximum allowable concentration of 982 ppm i n air (12), a level many times more than expected i n workroom air. Incorpo ration of solvent recovery equipment not only reduces unit treatment cost but avoids a potential detrimental effect on the ozone layer i n the upper atmosphere by dichlorodifluoromethane. Peripheral benefits of the choice include a reduction i n oxidative degradation as magnesium compounds, particularly magnesium hydrox ide, sequester the trace metals, iron, copper, and cobalt, which catalyze oxidative degradation (13,14, IS). The elimination of iron also reduces the probability of foxing (staining) caused by mold interacting with iron i n damp storage conditions or records wet by water from burst pipes (16). In summary, the system was chosen because (1) the chemistry of the process was accepted, (2) a history of successful use existed, and (3) the problems to be resolved appeared solveable. Characteristics of Liquified Gas Systems The concept of using liquified gases to dissolve, transport, and de posit deacidification agents i n paper and books was pioneered at the Graduate Library School, University of Chicago (9,10). Liquified gas systems are seen as combining the merits of traditional liquid and vapor phase deacidification methods. L i q u i d deacidification solutions are used to protect more acidic papers because they introduce larger quantities of deacidification agent than available vapor phase systems do. The solvents are introduced as liquids and removed as vapors to facilitate rapid drying and the deposition of deacidification agent throughout the treated aggre gates of paper, that is, books. Typically, magnesium methoxide, the preferred agent, is prepared i n methanol at concentrations of 8-10%



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and subsequently diluted with dichlorodifluoromethane to a 0.5-1.0% concentration. These concentrations introduce four to 10 times more deacidification agent than required to neutralize the 0.1% by weight sulfuric acid equivalent typically found i n deteriorated paper. Prior to introducing the solution, the books to be treated are vacuum dried to low levels of dryness, that is, under 0.5% moisture. The liquified gas solution, maintained separately in anhydrous con dition, is impregnated i n books at pressures ranging from 70-200 psig. The treatment does not require a dwell or soaking time, but the solution must contact intimately the materials undergoing treatment. After drain ing the solution surrounding the wetted materials, the solvents are flash dried (removed rapidly by pressure reduction) to deposit the deacidifi cation agent throughout the books. Control during this stage is critical because the solution tends to migrate towards the periphery of the books before evaporating since the temperature of the books drops as a result of the heat consumed i n vaporizing the solvents. Migration is minimized by supplying heat during drying or by reducing the pressure to maintain the pressure differentials. The alkaline buffer deposited is expected to vary from a minimum near the spine or bound margin to a maximum at the head, tail, and fore-edge margins of the leaves i n treated books. This variation is considered advantageous because the heavier concentration is deposited in the edges of the leaves where acidic gases are prefer entially adsorbed from polluted air. Another way to visualize the treatment process is to contrast it to the refrigeration cycle in home air conditioners, freezers, or refrigerators. Schematics of the system and a home refrigerator are presented as F i g ures 2 and 3. In Figure 2, the liquid refrigerant, typically dichlorodi fluoromethane (Genetron 12, Freon 12, or Halocarbon 12) flows as a self-propelled liquid from the receiver to the evaporator. The pressure drops, and the refrigerant evaporates, thereby taking up heat and cooling the refrigerator. The low pressure gas (roughly 20 psig) is drawn into the compressor and compressed to perhaps 100 psig. This high pressure gas gives up heat in the condenser and condenses into a liquid to com plete the cycle by flowing back into the receiver. These same functions are the fundamental treatment blocks of the W e i T o System as shown i n Figure 3. In addition, the continual recycling is broken at the receiver. This introduces a requirement for two receivers. One receiver, called the storage tank, holds the deacidification solution. The other receiver receives the condensed solvent gases precisely as in the refrigeration cycle. A turbine pump is incorporated to move the solution to the process tank, the analog of the evaporator i n the refrigera tor. However, the books are thoroughly wetted i n the process tank, and excess solution is drained back to the storage tank prior to the process



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Figure 2. Refrigeration cycle

Figure 3. Liquified gas deacidification system



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tank functioning as an evaporator. The liquid solution impregnated i n books is flashed (vaporized rapidly) to produce a drying effect through out the wetted books. The vapor is recovered through condensation by compression and cooling just as i n refrigeration. The condensed solvents in the present system w i l l be transferred to cylinders and transported elsewhere for recycling into fresh deacidification solution. Where possible, the equipment and operating procedures were selected for additional benefit. F o r example, the vacuum pump and exit port of the vacuum drier were enlarged for emergency use i n drying water-wet books. I n future, additional treatments such as impregnating selected resins to strengthen weakened paper are to be tested. Current System Design The system originated i n June 1974 when W e i T'o Associates was asked to prepare a design for consideration. Arrangements were made with an American engineering firm to undertake construction i n January 1975. This first system d i d demonstrate feasibility of the process, but the quality of treatment was not satisfactory. Certain components proved unreliable, and concern existed as to whether the system met safety requirements. These operational problems are believed to have caused the engineering firm to become unwilling to complete the system. This attitude plus funding considerations and the need for local maintenance and repair service led to a decision to complete the system i n Canada. The initial step of identifying experienced Canadian engineering firms who were available, qualified, and interested i n undertaking completion occurred i n autumn 1976. During winter and spring 1977, the entire system was reviewed i n terms of chemical processing, refrigeration fabri cation standards, and component selection. The modified system which resulted is divided into three sections: (1) warm air and vacuum drying; (2) deacidification (includes evacuation, solution impregnation, deposi tion and drying, and solvent recovery); and (3) auxilary operations for presentation i n this progress report. The water contained i n air-dry books, 4 - 1 0 % by weight, if not re moved before treatment, is swept out of the books by escaping vapor to contaminate the recovered solvents. Removal before treatment offers the advantages of introducing a more uniformly deposited, tightly held de acidification agent, reducing contamination of the solution returned to the storage tank and increasing the savings i n solvent recycling, and the capability of using recovered solvent vapor to equalize pressure between the process and storage tanks. Approximately 50% of the water i n air-dry books is removed by drying for 24 hr i n an electric, forced-air bench drier at 140°F. The water remaining is removed i n a heated-shelf ( 1 2 0 ° F )



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vacuum drier operated at 2 m m H g or less overnight. The vacuum is returned to ambient pressure with air at 1-2 p p m of water when books are removed from the vacuum drier. T w o wire-frame, perforated metal floor baskets measuring 10 i n . by 18 i n . are placed i n the process tank for each of the five to six deacidification cycles per day. The books, dried to at least 0.5% by weight of water, are handled i n baskets for convenience in controlling exposure during drying and deacidification and preventing damage i n handling. The fragility—i.e., embrittlement—caused by dry ing is a relative matter and is far less than the loss of strength that occurs from wetting paper with water. Deacidified books are brought back to f u l l strength through gradual moisture regain prior to their return to the collection. The major steps i n the deacidification process are presented sche matically i n Figure 4. Following the loading of two book baskets, the process tank door is secured and the preset cycle is started with air evacuation to 1-2 m m H g . The pressure is then equalized with the storage tank by introducing the recovered solvent vapor from the receiver. The fill pump is started, and the process tank liquid filled. A small quan tity of solution is allowed to flow through the process tank, but the bulk is returned to the storage tank through the bypass line. Impregnation pressure is controlled by adjusting the pressure regulating valve. The fill pump is turned off, and the excess solution is drained back into the storage tank when the books are thoroughly wetted.

^urgeairp^^q_

Figure 4. Current system design



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The vapor filling the process tank and vapor from the solution wet ting the treated books is recovered by an oil-free compressor to approxi mately 7 psig. The books are brought rapidly to a damp dry condition by removing additional vapor with a vacuum pump. This small quantity of vapor is exhausted to the atmosphere because the cost of its recovery is not justifiable at this time. The damp dry books in baskets are placed in closed corrugated boxes for 24-48 hr to complete drying, moisture regain, and return to room temperature. Thereafter, the deacidified books are inspected for appearance and quality of treatment prior to returning them to the colletcions. The auxiliary operations include (1) a supply of solution (prepared elsewhere) i n 140 lb-net refrigeration cylinders and its transfer to the storage tank by nitrogen gas pressure, (2) pumping of the recovered solvents from the receiver to refrigeration cylinders for transportation and beneficiation elsewhere, and (3) a self-contained hot water system for heating the shelf vacuum drier. The period required for a book to be treated and be returned to the collection is estimated at two weeks. This allows a one-week backlog for efficient batching of like books and one week to pass through the two-day drying, one-day deacidification, and two-day recovery and inspection cycles. Deacidification cycles of about 60 min are anticipated. Most of this time is required for air removal, solvent recovery, and evacuation. Solution impregnation and removal requires only five to 10 minutes. A daily production of 150 deacidified books measuring 6 i n . by 9 i n . by 1 i n . is expected from the five to six cycles anticipated. Experience to date indicates equipment selection and system design are important considerations i n liquified gas deacidification systems. Operations using magnesium alkoxides or similar materials sensitive to trace quantities of moisture should be conducted under anhydrous con ditions, and in-line design constructions leading to vapor locks should be avoided. Components must be selected for exposure to an anhydrous, methanol/dichlorodifluoromethane, alkaline alkoxide solution. Acknowledgments The author gratefully thanks the Public Archives of Canada for per mission to publish this article and the Dupont Co., Wilmington, D e l . , and Dupont of Canada, L t d . , Kingston, Ontario, for guidance in handling liquified gas refrigerants. Literature Cited 1. Clapp, V . W., "The Story of Permanent Durable Book Paper, 1115-1970," Restaurator Supplement No. 3, 1972.


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2 . Smith, R . D . , " A Comparison of Paper in Identical Copies of Books from the Lawrence University, the Newberry, and the New York Public Libraries," Restaurator Supplement No. 2 , 1972. 3. Public Archives of Canada, Publication Catalogue No. S A 2 - 5 0 7 0 , p. 1, Ottawa: Ministry of Supply and Services, 1976. 4. Smith, R . D., "Maps: Their Deterioration and Preservation," Special Libraries (February 1 9 7 2 ) 63: 59-68. 5. Smith, R . D., "Treatment of Cellulosic Materials," U.S. Patent No. 3,676,182, Washington, D . C . , July 1 1 , 1 9 7 2 . 6. Smith, R . D . , "Preserving Cellulosic Materials Through Treatment with Alkylene Oxides," U.S. Patent No. 3,676,055, Washington, D . C . , July 1 1 ,


1972.

7. Smith, R . D., "Treatment of Cellulosic Materials," Canadian Patent No. 911,110. Ottawa, October 3, 1972. 8. Kelly, G . B., Jr., "Composition for Use in Deacidification of Paper," U.S. Patent No. 3 , 9 3 9 , 0 9 1 , Washington, D . C . , February 17, 1976. 9. Smith, R . D . , "New Approaches to Preservation," "Deterioration and Pres ervation of Library Materials," Winger, H . W., Smith, R . D . , Eds., Uni versity of Chicago Press, Chicago, 1 9 7 0 . 10. Smith, R . D . , "The Nonaqueous Deacidification of Paper and Books," Ph.D. dissertation, pp. 1 9 4 - 1 9 5 , University of Chicago, Chicago, 1970.

1 1 . Kelly, G . G . , Jr., et al., A D V . CHEM. SER. ( 1 9 7 7 ) 164,

62.

13.

37.

12. American Conference of Governmental Industrial Hygienists. "General Exact Solution for Mixtures of N Components With Addictive Effects and Different Vapor Pressures," "Threshold Limit Values of Airborne Contaminants and Physical Agents," pp. 43-45, A C G I H , Cincinnati, Ohio, 1 9 7 1 . Williams, J. C., et al., A D V . CHEM. SER. ( 1 9 7 7 ) 1 6 4 ,

14. Ericsson, B., "Oxygen Oxidation in Aqueous Alkaline Medium," Ph.D. dis sertation, Kungl Tekniska Hoskolan, Stockholm, 1 9 7 4 . 15. Sinkey, J. D . , "The Function of Magnesium Compounds in an OxygenAlkali-Carbohydrate System," Ph.D. dissertation, Institute of Paper Chemistry, Appleton, Wisc., 1 9 7 3 . 16. Waters, P., Restoration Officer, Preservation Office, Library of Congress, Washington, D.C., personal communication.

R E C E I V E D June 2 4 , 1 9 7 7 .


Design of a Liquified Gas Mass Deacidification ... - ACS Publications

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