Of the many methods that have been devised for neutralizing acidic paper and buffering it against future acid attack, three show the most promise for mass deacidification in the next few years. One, the Wei T'o method, has been used at the Canadian Archives for years. The active agent in the Wei T'o method is methyl magnesium carbonate, which precipitates as magnesium carbonate. A second method, diethyl zinc, is planned for use by the Library of Congress. Its program has been set back about a year by safety problems. In this process, zinc carbonate is deposited within the paper from the gaseous phase of the DEZ. A third method, the Koppers process, was quietly tried out at Berkeley while the patent was pending (AN, Dec. 1984, p. 100), found to work fine, and then quietly and mysteriously dropped (AN, Feb. 1985, p. 4). Deacidification of books may have been a bit too far removed from the Koppers Company's main business, which is commercial roofing, to justify the investment for development.
Now that the patent f or the Koppers process has been granted, the process is a matter of public record. The company manager has kindly supplied a copy of the patent for publication in the Newsletter. Excerpts are reproduced below.
UNITED STATES PATENT - No. 4,522,843
Date of Patent: Jun. 11, 1985
DEACIDIFICATION OF LIBRARY MATERIALS
Inventor: Robert A. Kundrot, 3801 D Logans Ferry Rd., Pittsburgh, Pa. 15239
Appl. No.: 573,739
Filed: Jan. 25, 1984
References Cited: U.S. Patent Documents [here are listed eight patents from 1936 to 1977, by Schierholz, Fluck et al., Langwell, Smith (2) and Williams et al. (3)]
A method is provided of deacidifying books, imaged paper and other imaged material having a cellulose base comprising treating said material with suitable alkaline particles of basic metal oxide, hydroxide or salt dispersed in a gas or liquid in an amount and for a time sufficient to increase the pH of the material and provide an alkaline buffer without impairing the image thereon, said liquid consisting essentially of an inert halogenated hydrocarbon and a surfactant.
[The problem of acidity of most book paper of the last 100 years is described, and earlier deacidification methods reviewed.]
Unfortunately, all of these processes suffer from one or more of a nunber of drawbacks that have prevented their wide-spread acceptance. These drawbacks include high cost, toxicity, complexity of treatment, residual odor, deleterious effects on certain types of paper and inks, lack of an alkaline reserve, and the necessity of drying the book or paper to very low moisture contents before treatment.
It has now been discovered that acidic cellulosic materials can be treated with nontoxic inexpensive materials in a manner which obviates or minimizes many of the problems of the prior art including the necessity for drying the book or paper prior to treatment. This method can be used on cellulosics (paper) even when such paper is imprinted and or bound. More particularly, it has been discovered that books, paper and other material having a cellulose base can be preserved by treatment with alkaline material particles of basic metal oxides, hydroxides or salts (hereinafter alternatively ref erred to as alkaline or basic material) in an amount and for a time sufficient to increase the acidic pH of the material and provide an alkaline buffer. Quite surprisingly, it is not necessary to neutralize the acids present within the confines of the treatment period. Rather, a basic metal oxide, hydroxide, or salt of suitable particle size is distributed through the cellulosic or paper web wherein these particles slowly stop and neutralize the acidic compounds present or produced during aging. These basic materials are also present in sufficient amounts to buffer against deacidification by other acidic influences to which the paper may later be subjected in storage.
The alkaline materials are regularly available materials and are preferably chosen from those which are relatively non-toxic. These particles are of such a size that they do not substantially interfere with any image, are colorless, and provide good coverage. Submicron or near sub-micron particles are suitable as these can be suspended in a gas or inert liquid which obviates the need for solutions or solvents which contribute to many of the drawbacks of current methods. Particles of these dimensions are also tightly held within the paper matrix and do not loosen under normal use. Typical BET surface areas range from 50 to 200 m2/g which provides high probability of acid contact and interdiction.
The invention will be further described in the following detailed description of the invention.
The cellulosic materials can be treated with any suitable basic metal oxide, hydroxide or salt. Suitable materials are the oxides, hydroxides, carbonates and bicarbonates of the Group 1 and 2 metals of the Periodic table and zinc. Preferred are the materials in which the cation is magnesium, zinc, sodium, potassium, or calcium. Particularly preferred are the relatively non-toxic oxides, carbonates and bicarbonates of magnesium and zinc and the hydroxides of sodium, potassium and calcium. Representative examples include magnesium oxide, magnesium carbonate, magnesium bicarbonate, zinc carbonate, zinc bicarbonate, zinc oxide, sodium hydroxide, potassium hydroxide and calcium hydroxide. Magnesium oxide is most preferred. the predominant particle size (95-99%) is preferably between 0.01 and 0.9 micron, the average particle size is preferably between 0.2 and 0.6 micron and most preferably is about 0.4 micron. Typical surface areas are between 50 and 200 m2/g BET preferably about 170 m2/g.
The particles can be formed by burning the elemental metal and collecting the smoke, attrition of the preformed oxides or calcination of the elemental salts. For example, basic magnesium carbonate can be calcined at 450°C-550°C to produce a polydisperse high activity magnesium oxide with an average particle size of 0.4 microns and a predominant particle size between 0.1 and 0.9 micron.
The particles can be applied in the paper making process or to the finished paper by electrostatic transfer such as in a xerographic process, by a dispersion in a gas, or by a suspension in an inert liquid. In the case of a liquid suspension of the particles, the liquid chosen is preferably inert and possessing a high enough vapor pressure to allow its removal from the book or paper after exposure. Liquids which are well suited for this purpose are halogenated hydrocarbons. Typical materials include Dupont Freon Fluorocarbons such as Freon 11 (trichloromonofluoromethane), Freon 113 (l,l,2-trichloro-1, 2, 2-trifluoroethane), and Freon 114 (l,2-dichloro-1,1,2,2-tetrafluoroethane), and Allied Chemical Genetron 11 and 113 and mixtures. The suspension is less prone to settling and/or agglomeration if a surfactant is employed to overcome surface tension and charge attraction effects. Typical materials include surfactants such as ICI Solsperse 6000 and 3000 and 3M Fluorad FC 740 and 721. Mixtures of these surfactants can be employed. A preferred surfactant is a fluorinated alkyl ester known as Fluorad FC 740.
The amount of surfactant and alkaline material will depend in part on the length of treatment and the amount of deposition desired. Generally, however, the concentration of alkaline material will be between about 0.01% and about 0.3% and the surfactant between about 0.005% and about 1.0%. A most preferred range for the basic material particles is between about 0.01% and about 0.2%, and a most preferred range for the surfactant is between about 0.005% and about 0.5%.
In the case of unbound or single sheets of paper, deposition may take place using a gas or air supported dispersion. Active methods of deposition enhancement such as aerosol impingement, filtering through the paper and electrostatic attraction have proved promising for increasing the rate of deposition. Impingement of the gas supported dispersion on the paper combined with electrostatic attraction is particularly effective. In this method, paper is placed against a charged plate and the field so created is used to attract the particles to the paper.
The preferred method for bound sheet materials such as books or manuscripts is the use of a suspension in a liquid. The liquid is used not only to disperse the particles, but also to open the bound material to provide uniform treatment. By the use of spray nozzles or motion imparted to the bound material while submerged, pages can be easily separated and exposed to the particles. In a liquid suspension, one is able to obtain a higher concentration of particles in the treating media and deposit the necessary amount of alkaline material in a shorter time. By the use of halogenated hydrocarbon/surfactant combinations, the stable concentration of submicron particles can be increased from 20-30 milligrams/cubic foot in a gas to 1-100 grams/cubic foot in a liquid. At the higher concentrations, one immersion into the treating medium for a few seconds will usually suffice to deposit the required amount of basic material. At the lower concentrations, two or more immersions or a longer immersion time is required to achieve the same effect. After immersion, the inert liquid is evaporated, recovered and recycled.
The following examples will serve to illustrate the invention. All parts and percentages in the specification and claims are by weight, unless otherwise indicated.
Sample imaged acidic sheets were treated with an air supported dispersion of MgO powder (average 0.4 micron particle size with a predominant particle size between 0.01 and 0.9 micron and a concentration of 25 mg/cubic foot). The sheets were hung in a glove box adapted to control the humidity at 32% and the initial temperature was 22°C at standard atmosphere. The particles were transported to the glove box through lines connected to the exhaust of a Tost Air Mill. After a three-hour exposure to the static air dispersion, the pH of the paper increased to 6.6 from an initial pH of 4.4 and the image was not impaired.
Three sample imaged acidic sheets with a pH of 4.3 were dried in an oven at 50°C for one hour end then placed in a glove box at conditions described in Example 1. Then a magnesium oxide dispersion in air as described in Example 1 was pumped into the box, and a container of warn water (40°C) was uncovered. The relative humidity went form 34 to 94% in ten minutes and the water container was closed after ten minutes and the dispersion treatment was discontinued in 1½ hours. The humidification treatment of the paper increased the deposition rate by more than twofold over that in Example 1. The sample sheets facing the dispersion had a pH from 7.2 to 8.7.
A sheet of imaged acidic book paper (pH 4.0) was placed in contact with the charged sphere of an electrostatic generator (WINSCO Model N 100-v). A stream of dispersed particles described in Example 1 was directed against the paper for approximately 5 seconds. The pH of the paper after exposure was 8.5, and the image was not impaired.
A liquid treating suspension was prepared by adding 3.2 G. [sic] (0.20%) of MgO (prepared by calcining basic hydromagnesite at 500°C for 3 hours) to 1000 ml of Allied Chemical Genetron 113 containing 0.78 g (0.05% of 3M Floored FC 740 surfactant. This suspension was used to treat single sheets of imaged acidic book paper by submerging each sheet into the suspension for 20 seconds. the sheets were then air-dried. These sheets (40) along with an equal number of untreated sheets were subjected to accelerated aging according to TAPPI standard T 453 m-48 for up to 28 days. After samples were removed from the oven, the folding endurance test values of the paper were determined by using an MIT folding endurance tester (TAPPI standard T 511 su-69). The pH values were determined with a flat probe electrode according to TAPPI standard T 529 pm-74. The results were as follows:
ACCELERATED AGING TESTS AT 105°C. EFFECT ON M.I.T. FOLD ENDURANCE AND pH
Time in Days Untreated Treated
at 105°C Fold pH Fold pH
0 108 6.4 105 9.4
14 43 6.0 97 8.1
28 7 5.4 35 8.2
Note: Fold endurance is the number of double folds under a 0.5 Kg tension before failure. Paper is considered brittle and unusable at 5 or less folds.
[There are three more examples and a list of 24 claims, which are omitted due to lack of space here. Copies can be sent to anyone who sends a self-addressed envelope with their request.]