Preservation of Paper and Textiles of Historic and Artistic Value

4 Methylmagnesium Carbonate—An Improved Nonaqueous Deacidification Agent

Downloaded by FUDAN UNIV on February 15, 2017 | http://pubs.acs.org Publication Date: December 17, 1978 | doi: 10.1021/ba-1977-0164.ch004

G E O R G E B. K E L L Y , JR., L U C I A C . T A N G , and M A R T A K. K R A S N O W Preservation Research and Testing Office, Library of Congress, Washington, D . C . 20540 Methylmagnesium carbonate, prepared by carbonation of a solution of magnesium methoxide, is compared with the magnesium methoxide as a deacidification agent for paper. Although the two materials are equally effective in deacidification and in prolonging the life of the paper, the methylmagnesium carbonate is two to 10 times as stable to water, depending upon the solvent used. This stability avoids premature precipitation, minimizes surface deposits, and increases the interval between cleanings of equipment, making the solution much more convenient to use. The use is covered by U.S. Patent No. 3,939,091, February 17, 1976, assigned to the Library of Congress. A royalty-free, nonexclusive license will be available.

p h e detrimental effect of acid i n paper has been well documented (1, 2, 3, 4), and numerous treatments have been proposed to alleviate the effect. Schierholz ( 5 ) , Barrow ( 6 ) , and others have proposed methods of neutralizing the acids i n paper based on aqueous treatments. However, many papers are sensitive to aqueous treatments, either because of the fragility of the paper or the tendency of the inks or colors to r u n when exposed to water, and for these papers a nonaqueous deacidification treatment is required. One of the most widely used nonaqueous deacidification processes is that proposed b y Smith (7) based on a methanol or methanol-Freon solution of magnesium methoxide. This process is very effective and leaves a good alkaline reserve, but the magnesium methoxide is very sensitive to water, and this leads to inconvenience i n the handling and processing of paper as a result of premature precipitation. These incon veniences were recognized, and a better nonaqueous deacidification agent r

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


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KELLY ET AL.

Improved Nonaqueous Deacidification Agent

was being sought when a reference to methymagnesium cabonate was found in the literature (10). This compound was easily prepared, was soluble i n nonaqueous solvents, and had been extensively investigated as a reagent for organic syntheses. It promised a much more nearly neutral treatment than magnesium methoxide and the prospect of hydrolyzing directly to magnesium carbonate. It was therefore decided to investigate its use as a deacidification agent i n comparison with mag nesium methoxide. Magnesium compounds are particularly attractive as deacidification agents as they tend to be more soluble than those of calcium or other alkaline earths, which makes their use more convenient and makes the attainment of an adequate alkaline reserve somewhat easier. Magnesium compounds had also been reported to act as stabilizers in oxygen woodpulping operations by Robert i n 1965 ( 8). The stabilizing effect was investigated i n detail by Gilbert et al. in 1973, who reported that the effect was a result of inactivation of metallic oxidation catalysts such as iron (9). Magnesium compounds therefore appeared to offer the possi bility of protecting the cellulose from oxidation as well as degradation. The role of magnesium compounds i n paper oxidation was discussed further by Williams (11). Experimental Methylmagnesium carbonate was prepared by saturating an 8% solution of magnesium methoxide i n methanol with carbon dioxide at room temperature ( 2 5 ° C ) over 2 hr. The resultant solution was clear and colorless. Portions were diluted 10:1 with various solvents and observed for 24 hr for evidence of precipitation. The methanol solution was evaporated to dryness under vacuum on a steam bath, and a friable, glossy white solid was recovered; solubility i n a number of solvents was determined at the ratio of 1 g of solid in 50 m L solvent. For treatment of papers, an 8% solution of magnesium methoxide was diluted to about 1% (0.116Af by analysis) with trifluorotrichloroethane (Freon T F ) and divided into two portions. One of these was saturated with carbon dioxide over 3 hr; the other was reserved for comparison. The rate of conversion was followed by periodic p H meas urements on the solution and on 2-mL portions of the solution which were diluted to 100 m L with distilled water. Similar portions were also absorbed on filter paper, dried, and the p H of the paper was measured. The magnesium methoxide and methylmagnesium carbonate solu tions were used to treat a series of three papers. E a c h 8V2 X 11-in. sheet was covered with a separate 25-mL portion of one of the solutions in a shallow enameled pan, saturated, drained, and dried. After treat ment, the treated sheets and the untreated controls were conditioned ( T A P P I T-402) and tested for physical properties: brightness ( T A P P I T-452 modified to use a single sheet against a white, glazed clay backing of 66 brightness); M I T fold ( T A P P I T-511); p H and titration (2.5 g

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

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PRESERVATION OF PAPER AND TEXTILES

Figure 1. Comparative stability to water of magnesium methoxide and methylmagnesium carbonate in methanol. Left flask: magnesium methoxide plus 0.5 mL water. Right flask: methylmagnesium carbonate plus 5 mL water.

paper was pulped with 250 m L water i n a Waring Blendor for 45 sec. The p H was measured on resultant pulp with a p H meter while stirring with a magnetic stirrer. Titration was with standard 0.1N N a O H to p H 7.0 if the paper was acid. If the paper was alkaline, excess standard acid was added, the solution was boiled for 1 min with agitation to expel C 0 , cooled, and back-titrated with 0.1ZV N a O H to p H 7.0); thickness ( T M I M o d e l 549E micrometer, average of 10 samples); tensile strength ( T A P P I T-494 modified to 15-mm strip); and accelerated aging ( T A P P I T-453 modified to 100°C and extended to 36 days). The convenience and operating characteristics of the two solutions were judged further by applying the solutions to paper by brushing with a 7-in. wide paste brush, the paper backed with blotting paper to absorb edge run over. Sufficient solution was applied to wet the papers thor oughly and distributed evenly with the brush. The convenience of spray ing was judged by spraying the paper with a manual pump-type plastic sprayer (Figure 1), pumping the solution five to seven times twice a day until plugging occurred. 2

Results and Discussion Methylmagnesium carbonate is reported by Finkbeiner and Stiles to have the formula C H O M g O C 0 C H • X C 0 where X may vary with solvent and temperature (10). W h i l e this formula is empirically correct, the structure has not been conclusively established and may be more complex than indicated. Methylmagnesium carbonate is much more stable to water than magnesium methoxide, as illustrated i n Figure 1. The methylmagnesium 3

2

3

2



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KELLY ET AL.


carbonate solution i n methanol was able to absorb 10 times as much water as the magnesium methoxide solution without precipitation, but it does precipitate with larger amounts of water. Pure methanol is not suitable as a solvent for much of the deacidification work since i t frequently causes movement of inks and dyes on the paper. T o overcome this problem, the working solution is usually the commercial 8% solu tion of magnesium methoxide diluted to about 1% with a poor solvent such as Freon T F . Such a mixture is satisfactory for most deacidification work and is also recommended with methylmagnesium carbonate. If the Freons should be restricted i n the future because of their damage to the ozone layer, the dilution could be made with a variety of other solvents which may include toluene, ether, acetone, tetrahydrofurane, or chlori nated hydrocarbons, which should provide considerable flexibility i n choosing a suitable replacement. In the methanol-Freon solvent mixture the methylmagnesium car bonate is only about twice as tolerant to water (as measured by the volume of water necessary to cause precipitation) as magnesium meth oxide, but this appears to be sufficient to eliminate most, if not all, of the problems with water sensitivity. T h e difference i n stability is apparently a result of the lower concentration of carbon dioxide i n the methanol-Freon mixture since the Freon is a much poorer solvent for carbon dioxide than is methanol. The solubility of carbon dioxide i n the solvent also affects the rate of reaction i n the preparation of methylmagnesium carbonate as shown in Table I. I n pure methanol the p H became constant after carbonation for 40 m i n at room temperature, indicating complete reaction, whereas i n the methanol-Freon solvent, the reaction takes about 2 hr. Table I.

Preparation of Methylmagnesium Carbonate Effect of Carbon Dioxide and Solvent on pH of 0.116M Solutions

Carbonation Time (min) 0 10 20 40 80 100 150

Methanol Solvent

pH 11.9 8.5 8.3 8.2 8.2

Appearance clear clear clear clear clear

Water Dilution pH" 10.4 7.3 6.8 6.8 6.8

Methanol-Freon TF Solvent"

pH 11.5 11.6 8.7 8.7 8.7 8.7 8.7

Appearance

Water Dilution pH"

clear clear cloudy cloudy si. cloudy v. si. cloudy clear

• 2 ml in 200 mL water. * 1 methanol to 7 Freon TF.


10.7 10.7 10.3 8.8 7.7 7.5 7.4

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In spite of the longer time required, there is an advantage i n using the methanol-Freon solvent mixture for the carbonation reaction since it contains a built-in indicator. The mixture becomes cloudy w i t h precipitate and then clears as the carbonation is continued. The reaction is complete when the solution becomes clear again, and it is then ready for immediate use. Although it would appear that the methylmagnesium carbonate solution with a p H of 7.4 would be a much milder treatment for paper than magnesium methoxide at p H 10.4, this is true only while the paper is i n contact with the solution. W h e n the solutions were added to filter paper and dried, the p H was about 10.2 with either treatment. This was a little surprising at first since the methylmagnesium carbonate-treated paper showed effervescence on treatment with acid immediately after drying, indicating that the methylmagnesium carbonate had hydrolyzed to magnesium carbonate, whereas the paper treated with magnesium methoxide required two days exposure to air before it showed noticeable effervescence with acid treatment. However, magnesium tends to form basic carbonates of varying composition depending upon the method of preparation, so it is presumed that the carbonate formed from the methyl magnesium carbonate is of the basic variety. This would explain the equivalence i n the p H of the two treated papers as it would require very little magnesium hydroxide present with the carbonate to raise the p H to the indicated level. If desired, the methylmagnesium carbonate can be recovered as a white, brittle solid by evaporation of the solution under vacuum at temperatures up to 100°C. Unlike magnesium methoxide, which becomes almost completely insoluble on recovery as a solid, methylmagnesium carbonate solid is very soluble in methanol but tends to dissolve faster if the methanol is presaturated with carbon dioxide. The solid form provides a convenient method of storage or shipment of the material, much like instant coffee. The effect of treatment with methylmagnesium carbonate and with magnesium methoxide on the properties and aging characteristics of paper was investigated using three papers, a bookpaper, an offset printing paper, and a handmade paper, with varying fiber composition and p H as shown i n Table II. E a c h paper sheet was treated i n a shallow tray with a separate 25-mL portion of the appropriate solution of equivalent con centration, sufficient to saturate the sheet. The sheet was then drained and dried to complete the treatment. The p H and alkaline reserve of the treated papers from the two solutions are closely equivalent, as shown in Table III. The treating operation provided a dramatic illustration of the differ ence i n stability of the two solutions. The magnesium methoxide solution


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KELLY ETAL. Table II.


No.

Improved Nonaqueous Deacidification Agent Papers Used in Deacidification Treatments

Identification

Composition

pH

Additives

1.

Commercial book

30% bleached southern kraft 70% bleached hardwood kraft

6.5

13 parts clay

2.

Commercial offset

30% bleachedsouthern kraft 70% hardwood soda

6.1

15 parts clay rosin & starch

3.

Laboratory handmade

33.3% refined rag 33.3%bleached kraft 33.3% bleached sulfite

5.3

1% Neuphor" 3% alum

° Modified rosin. formed gelatinous precipitate immediately on contact w i t h the paper and left visible deposits on the surface upon drying. Some of the deposits could be brushed off readily, but they could not be ehminated entirely without damaging the surface of the paper. I n the case of handsheets, the deposits were very heavy and were sufficient to have made writing or printing on the sheet difficult to read. This may have been a result of the somewhat rougher texture of these sheets which caused the deposits to adhere more readily than on the smoother commercial papers. The papers treated with the methylmagnesium carbonate solution showed no precipitation even when allowed to soak i n the solution for several minutes and had no visible deposits when they were dried. The texture of the treated papers was also quite different. The treated papers had a pleasant, natural feel, while those treated with magnesium methoxide felt stiff and harsh. The physical properties of the treated papers, the untreated controls, and the solvent-only treated papers, along with the percent fold retention after accelerated aging for 36 days at 100°C, are shown i n Table III. Since the treated papers all ended w i t h essentially the same chemical impregnant, a basic magnesium carbonate, properties were essentially the same from either treatment. There were some minor differences. The surface deposits from the magnesium methoxide showed u p as increased paper thickness and i n the case of the handmade paper, as significantly higher brightness from the white surface deposits. T h e solvent alone shows some beneficial effect, presumably a result of washing out of some of the free acid and other impurities i n the paper, but the improvement by the solvent alone is very small. Although the differences i n physical properties for the papers treated by the two solutions tend to be minimal, investigation of the techniques


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PRESERVATION O F PAPER AND TEXTILES

Table III.


Paper No.

Treatment

pH

Properties

Alkaline Reserve (MgC0 %) 3

1

None, control M g methoxide M g methyl carbonate

6.5 9.1 8.9

none 3.67 3.17

2

None, control M g methoxide M g methyl carbonate Solvent only

6.1 10.2 9.8 6.5

none 3.42 3.47 none

3

None, control M g methoxide M g methyl carbonate

5.3 10.3 10.3

none 4.62 5.12

°36 days at 100°C. of applying the solutions to paper confirmed that important differences exist i n the operating characteristics. The increased stability towards water permits the impregnation of paper b y the various methods with greater ease and less tendency towards premature precipitation. The advantage i n dipping operations has been discussed previously. W i t h large documents such as posters or maps, dipping is often inconvenient, and for these documents the application may be made with a wide paste brush. The solution is applied and brushed evenly i n sufficient amount to thoroughly dampen the paper. The brushes tend to become clogged with gelatinous precipitate after a period of time and need to be cleaned. The period of time between cleanings is about doubled with the use of methylmagnesium carbonate i n place of magnesium methoxide, and surface deposits on the paper are also substantially reduced, based on our experience. The spraying of the solutions was also investigated since magnesium methoxide has been available commercially as a pressure spray package for some time. Considerable difficulty with nozzle plugging has been experienced from these sprayers. Unfortunately, no facilities for filling pressure bottles were available, so a plastic, manual pump-type sprayer, similar to those used to apply window cleaners, was selected as a reason able alternative (Figure 2 ) . F i r m pressure on the pump gave a fine mist with good distribution i n a cone-type spray pattern with this sprayer, and the size of the orifice approximated that of the pressure bottles. Either of the solutions could be sprayed intermittently from these sprayers for a f u l l day with no sign of plugging under the conditions existing i n our laboratory ( 2 4 ° C , 45% r.h.). However, on standing over-


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KELLY ET AL.


of Treated Papers


MIT Fold (Y2W

Tensile Strength (kg)

Thickness Brightness Fold Retention (mm) (%) after Aging* (%)

MD

CD

MD

CD

558 532 483

470 549 691

5.43 5.51 5.10

3.46 3.50 3.75

0.097 0.122 0.098

565 670 575 593

316 449 448 226

6.18 6.83 5.60 4.91

3.22 3.17 3.30 2.51

5499' 5501" 5237' 6

6.30' 5.93' 6.39'

MD

CD

74.5 74.6 74.0

16 46 56

23 64 52

0.086 0.107 0.091 0.092

75.4 75.4 73.0 76.6

15 46 50 23

22 38 40 31

0.188 0.200 0.194

78.0 80.9 78.0

1' 54' 48'

Handmade sheet, random fiber orientation.

Figure 2. Pump-type sprayer used in spraying tests


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night, the magnesium methoxide sprayer became partially plugged and would not give a good spray pattern. The methylmagnesium carbonate sprayer still worked fine with no sign of plugging. The magnesium methoxide spray nozzle was cleaned with a fine wire and hard pumping to restore the spray pattern, and the test was continued, spraying the two solutions with five to seven pumpings twice a day on an exactly equal basis. W i t h daily clean out of the nozzle, the magnesium methoxide spray lasted six days before plugging so solidly that the nozzle could not be cleaned. The methylmagnesium carbonate sprayer maintained a good spray pattern for 10 days with no difficulty beyond an occasional, slight interference with the spray pattern on the initial one or two pumps after standing overnight. However, the spray nozzle cleaned itself each time without requiring any probing with a wire. The test lasted 11 days. Here again, the improved performance of the methylmagnesium carbonate i n this test is quite clear. Methylmagnesium carbonate and magnesium methoxide have been extensively tested i n the Restoration Office of the Library of Congress. O f the two, methylmagnesium carbonate has been found to be more stable to changes i n enivronmental conditions and to have a wider tolerance to many of the different application techniques. W h i l e it is perhaps too early to draw too many conclusions about the future uses, it does appear to be most useful in treating materials which cannot, for one reason or another, be subjected to aqueous treatments. Of particular interest is its success in the treatment of fragile manuscript material. In summary, methylmagnesium carbonate offers significant advantages i n the treatment of papers because of its much greater stability to water so that premature precipitation is avoided and the time required for clean ing equipment is materially reduced without sacrificing protection of the papers as a result of the treatment. The use of methylmagnesium carbonate for deacidification has been covered by U . S. Patent N o . 3,939,091 issued February 17, 1976. Literature Cited 1. Jarrell, T. D . , Hankins, J. M . , Veitch, F. P., Technical Bulletin No. 334, U . S. Department of Agriculture, Washington, D . C . (September 1932). 2. Hanson, F. S., "Resistance of Paper to Natural Aging," Paper Ind. Paper World, Feb. 1939, pp. 1157-1164. 3. Hudson, F. L . , "Acidity of 17th and 18th Century Books in Two Libraries," Pap. Technol. (1967) 8, 189-190, 196. 4. Barrow, W . J., "Permanence/Durability of the Book II," p. 39, Dietz Press, Richmond, 1964. 5. Schierholtz, O. J., U . S. Patent 2,033,452 (March 10, 1936). 6. Barrow, W . J., "Permanence/Durability of the Book III," "Spray Deacidifi cation," Dietz Press, Richmond, 1964.


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KELLY ET AL.


7. Smith, R. D . , "The Non-Aqueous Deacidification of Paper and Books," Ph.D. thesis, University of Chicago, December 1970. 8. Robert, A., Traynard, P., Martin-Borret, O., French Patent No. 1,387,853, February 6, 1 9 6 5 (see also U . S. Patent 3,384,533 (May 21, 1968)). 9. Gilbert, A . F., Pavlovova, E . , Rapson, W . H., "Mechanism of Magnesium Retardation of Cellulose Degradation during Oxygen Bleaching," TAPPI ( 1 9 7 3 ) 56 ( 6 ) 9 5 - 9 9 .

10. Finkbeiner, H. L., Stiles, M . , J. A m . Chem. Soc. ( 1 9 6 3 ) 85, 6 1 6 . 11. Williams, J. C., Fowler, C. S., Lyon, M. S., Merrill, T. L., A D V .CHEM.S E R . ( 1 9 7 7 ) 164, 3 7 .


R E C E I V E D February 9, 1977.


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Preservation of Paper and Textiles of Historic and Artistic Value

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