Why Do Old Books Turn Yellow?

When you go into a big library, you’ll see many old books that have become yellow and brittle. Why did that happen? You may once wondered that, let’s find out the reason.

Make your own ancient treasure map
Draw a treasure map on a sheet of paper. Meanwhile, ask mom or dad to make you a cup of
black tea (without milk or sugar). Pour the tea onto a plate, place your map in it and let it soak overnight. In the morning, take out the map carefully and let it dry in the sun. Does it now have an ancient, yellow effect? Show it to your friends and tell them that an ancient pirate gave it to you!

Lignin and paper
As we know, paper is made from wood. Wood is in turn made of carbohydrates like cellulose
and lignin. Lignin is a very complicated molecule that adds hardness to wood. More the lignin, hardier is the wood. However, in paper it is a problem. Over time, lignin breaks down to form many phenolic acids, which are yellow in colour. These acids then react with cellulose. This causes the paper to become very brittle.

That’s what happened when you put the map in tea. There was tannic acid in the tea, which reacted with the acid in the paper.

How to make books last
William Barrow was a librarian in the 1930s, who was very interested in knowing how to
preserve old books (perhaps some of them had old treasure maps!) He was the one who discovered that it was the acid from lignin that caused it.

Since then, paper manufacturers remove lignin from the wood pulp before it is made into paper. These require additional chemical reactions. In addition, the paper is made alkaline by adding calcium bicarbonate. If any lignin is left in the paper, when it forms acid, the calcium bicarbonate will immediately react with it and ‘neutralise’ it. This kind of paper is called acid-free paper.

All this makes the paper expensive. Things like newspapers, tickets, notebooks etc are therefore not printed on it. But all books nowadays are printed on acid-free paper.

Battery team gets a charge out of lignin

Creating energy from wood waste has progressed from novel idea to renewable energy work in development. Researchers from Poland and Sweden are using a waste product from the paper making process to develop a battery. That material is lignin. Olle Inganas, professor of biomolecular and organic electronics at Linkoping University in Sweden and Grzegorz Milczarek, a researcher at Poznan University of Technology in Poland, have completed a study that shows how it is done. They maintain that the insulating qualities of lignin derivatives can be combined with the conductivity of the polymer polypyrrole to create a composite material that effectively holds an electric charge.

Lignin acts as the insulator and polypyrrole as a conductor, holding an electric charge. Lignin is the substance found in plants, and it is stripped out of wood as a waste product during the paper-making process. In the researchers’ paper, “Renewable Cathode Materials from Biopolymer/Conjugated Polymer Interpenetrating Networks” published in Science, the authors provide more details on lignin and their methods.

“Brown liquor, the waste product from paper processing, contains lignin derivatives. Polymer cathodes can be prepared by electrochemical oxidation of pyrrole to polypyrrole in solutions of lignin derivatives. The quinone group in lignin is used for electron and proton storage and exchange during redox cycling, thus combining charge storage in lignin and polypyrrole in an interpenetrating polypyrrole/lignin composite.”

A clear advantage of their discovery would be in the ready availability of a natural material such as lignin as opposed to dependence on metal oxides such as those used in lithium-ion batteries. The researchers themselves, however, emphasize that their work needs further and extensive study; they recognize this is not at a stage for industrial-style development.

These rechargeable batteries are still limited, according to the researchers, because they slowly lose their electric charge as they sit idly. Milczarek also found that various lignin derivatives perform differently in the cathode, depending on how they are processed. With continued investigations, it may be possible to optimize the batteries. Another implication to a “wood” battery may be in cost, versus existing batteries, as there would not be a reliance on precious metals.

“The advantage of using a renewable material for charge storage is the enormous amount of this material that is already being produced on Earth by growing plants, which contain about 20 to 30 percent lignin,” according to Inganas. “It is also a low-value material, currently being used for combustion. Lithium-ion batteries, on the other hand, require metal oxides and some of those materials, such as cobalt, are rather rare.”

According to the International Lignin Institute, after cellulose, it is the most abundant renewable carbon source on Earth. Between 40 and 50 million tons per annum are produced worldwide as a mostly non-commercialized waste product.