One of Over 700 Sites
	with Guide Alan Bruzel, Ph.D. Bio | Contact

Subjects

Acids Bases pH Analytical Chem Ask an Expert Astrochemistry Atomic Structure Biochemistry Chemical Databases Chemical Warfare Chemistry Clip Art Chemistry History Chromatography Computational Chem Convert/Calculate Crystallography Electrochemistry Environmental Chem Fun Sites General Chemistry Graphics Software How Things Work Inorganic Chem K-12 Grade Lessons Organic Chemistry Periodic Tables Physical Chemistry Polymer Chemistry Spectroscopy Stoichiometry Terminology Toxic Chemicals Subject Library  All articles on this topic

More Sites On This Topic

Apply to become a partner for this site.

Alkaline Paper

Dateline: 05/04/98

By Alan Bruzel

Mechanical shredding of wood chips into pulp produces newsprint for newspapers and comic books. An advantage of newsprint, exploited by high-speed printing, is its efficient ink absorbency. Unfortunately, over time the paper yellows due to oxidation of lignin, a complex polymer present in wood to an extent of 16 to 34 percent. For esthetic reasons, the paper for stationery and hard cover texts must be mostly cellulose, free from its association with lignin. The high quality wood pulp required for these applications is obtained by chemically digesting wood chips in the presence of heat and pressure. The two major chemical digestion procedures are the sulfite process using calcium bisulfite and sulfurous acid, and the sulfate process using sodium sulfate. The solubilized lignin is washed away, and the remaining wood pulp is bleached. Chlorine has been used for bleaching, but is an environmental concern, and is being replaced by chlorine dioxide, which, in turn, may yield to other chemicals (hydrogen peroxide, oxygen, or ozone), or to enzymes. The enzyme laccase not only bleaches, but removes lignin and may replace the traditional chemicals used in the pulping process.

After bleaching, hydrophobic compounds (sizing chemicals) are added to prevent ink from smearing during printing. Sizing is a mixture of hydrated aluminum sulfate and rosin. (The aluminum sulfate is referred to as alum in the papermaking trade, although true alum is a hydrated double sulfate of a monovalent and trivalent metal.) The alum coagulates the rosin allowing it to precipitate into the paper fibers making them more water (and ink) resistant.

Chemical treatment of wood pulp produces an attractive, high-quality, white paper that takes a crisp imprint. It survives only 30 to 40 years. The pulping, bleaching, and sizing processes generate hydrochloric and sulfuric acids. The atmospheric pollutant sulfur dioxide, derived from the burning of sulfur-containing fuels, also produces sulfuric acid. Strong acids such as these hydrolyze paper's primary constituent, the long polymer cellulose, to shorter polymers, resulting in brittleness and deterioration.

Paper manufactured before the mid-1800's was not autolytic. Then, the cellulose from which paper was made came from rags and textile waste; cotton is almost pure cellulose. However, the industrial revolution made the mills more efficient resulting in less waste, and the rag sellers could not satisfy the demands of the paper manufacturers. A new source of cellulose for papermaking was required. The 1844 invention by Keller and Voeter of the mechanical pulping of wood met this need. Alum/rosin sizing and chemical wood pulping led to more acid-generating residues. The result is that paper manufactured from cotton rags more than a century ago, if protected from atmospheric pollutants, will outlast the newer paper produced from wood pulp. The remedy is to deacidify the "newer" paper, and to institute manufacturing processes that will generate alkaline paper for future use.

Conservators deacidify valuable books and documents by treating the paper with a mildly alkaline solution – cellulose degrades in highly alkaline solutions of pH 10 and above – that leaves a carbonate salt in the fibers of the paper. Deposition of the water-insoluble calcium carbonate uses a process called double decomposition. The paper is treated with an aqueous calcium chloride solution, and is allowed to dry. An aqueous solution of sodium carbonate is then applied. The reaction of calcium chloride and sodium carbonate precipitates calcium carbonate. Careful washing removes the water soluble sodium chloride.

CaCl₂ + Na₂CO₃CaCO₃ + 2NaCl

Deacidification of pages printed with water soluble ink employs salts soluble in non-aqueous solvents. Here, a methanolic solution of one percent barium hydroxide or magnesium methoxide, Mg(OCH₃)₂, is applied to the document. Atmospheric carbon dioxide neutralizes barium hydroxide to barium carbonate. The water already in the paper (at 50% relative humidity, paper can hold about 8% moisture) hydrolyzes magnesium methoxide to magnesium hydroxide. Atmospheric carbon dioxide similarly converts this hydroxide to the carbonate.

Prevention of future damage from acids requires the treated paper to contain an alkaline reserve of carbonate of three percent by weight. Deacidification treatment may extend the life of valuable books and documents by 500 years; costs are estimated at $300 million for the preservation of US collections.

The best method of preserving printed works is not to use acidic paper. The manufacturing of alkaline paper for hard cover and paperback books forced a change in the papermaking process. The chemically digested wood pulp is now neutralized with sodium carbonate. Newer hydrophobic sizing agents such as alkyl ketene dimer and alkenyl succinic anhydride are replacing the acid-generating alum/rosin sizing. Calcium carbonate fillers, introduced in the 1980's, developed into today's precipitated calcium carbonate technology. This allows production of paper with up to 30% filler which not only neutralizes present and future acidic reactions, but adds to the paper's opacity.

Neutrality is pH 7, but paper of pH 6 and above is designated acid-free. A paper with a pH of 8 to 8.5 is referred to as permanent paper. Newsprint, by comparison, is pH 4.5 when new, and pH 3.5 when degraded.

Care must be exercised in selecting appropriate alkaline paper. Photographic albums need acid-free, neutral paper, rather than materials with an alkaline reserve, such as the permanent papers. Printers using sheetfed presses must make adjustments for alkaline paper's lower stiffness. Alkaline paper manufacturing is not without drawbacks. The water systems of papermaking facilities provide a warm, pH neutral, nutrient rich growth medium for bacteria and other organisms, such as nematodes. These conditions originate from environmental regulations encouraging recycling of wastewater, and temperatures in some areas of the facility reaching 110^o F (43^o C). In these circumstances, the biocidal property of chlorine dioxide will be needed even when its bleaching activity has been supplanted by other chemicals.

Production of alkaline paper has dramatically increased since 1976. At that time, less than one percent of the books received by the US Library of Congress were made of alkaline paper. Twenty-five percent of American books and 50% of European books used alkaline paper in 1981. The disparity is explained by the European papermaking industry's greater access to calcium carbonate. By 1991, 85% of American books were printed on alkaline paper. Books made on alkaline paper are to be so marked, to prevent unnecessary preservation efforts.

Is this expenditure of labor and material justified? Is any writer's work worthy of this chemically induced longevity? Or would the muse be better served if our writer had
"Bent o'er the desk, or, born to useful toils,
Been taught to make the paper which he soils ..."
Lord Byron, "English Bards and Scotch Reviewers," lines 407-408.

Recommended Web resource for additional information:

Drying Doctor Corporation
Designer of software programs for the papermaking and chemical processing industries.

Previous Features

Subscribe to The Chemistry Newsletter
Name	Email