Time and other painters

2009/07/01 Roa Zubia, Guillermo - Elhuyar Zientzia Iturria: Elhuyar aldizkaria

• Materialak

If you want to see the influence of time, wait. If you want to see the paper in yellow, for example, leave the paper in sight and be. Time will paint it. However, those who want to investigate the process do not. They do not wait.

Research should answer several questions, and fortunately, it is not necessary to wait to answer many of them. The first question is why. Why paper is degraded. Asked otherwise, what causes paper aging.

Fortunately, the human being has a great experience with paper. Paper has great importance for the human being, it is a favorite material. For interpersonal agreements to have value they must be written on paper. What happened in history must be confirmed by a role for historians to accept. Looking ahead, to know if the projects are viable, we analyze them on paper. It is not strange to have experience with paper. Moreover, there is a culture of taking care of paper.

For example, to protect the paper we do not let you wet. And we know what to do to make the paper as bitter as possible. We store it in a dark and dry place. However, we know that in a set of leaves the leftover is yellow more than the ones below. The difference is great. Therefore, it is not a bad idea to put another page that does not matter about the lot.

This is what answers the first question of researchers: contact with yellow air paper. In addition, the paper is affected by moisture and ultraviolet light. On paper, yellow and aging are synonymous. And to be fit, paper should take care of these two things; besides being in dry places, it should flee from contact with air and light (especially the Sun).

Water and air produce oxidation in the paper components and light launches photochemical reactions. In the 1970s, research into the degradation of polymers found that papers also generate free radicals (as in our body). But the main problem of paper aging is the formation of molecules that give it color, known chromophores.

Contribution of trees

It is believed that the next question chemicals should answer would be how chromophores are generated. But there is another previous question: what chromophores they are.

There is no single chrome. However, they have identified a main source of chromofos: lignin. Many times it is said that paper is made of cellulose, as they do it with tree wood and that the main component of wood is cellulose. But there are others who are part of the composition of the paper, including lignin.

The molecule is very long and complex, a natural polymer. It can break from many places and can affect many chemical reactions, both lignin and its parts. The possibilities of creating chromophores are varied. Therefore, chemicals do not seek certain chromophores in lignin degradation, but chromium families. They talk about lignoles, styling and other families. There are many molecules, and to fight the yellow of the paper, in short, we must avoid the creation of entire families.

In practice, paper makers incorporate light-absorbing products, photostabilizers into paper. They act as Lignina's bodyguard: the rays instead of reaching lignin reach them. However, for a correct selection of these products it is necessary to know the usual chromophores.

Another question. With already yellowish paper, what? Restorers need a response. Ancient historical documents are yellow and therefore difficult to read. Or they are degrading. The goal of restorers is to whiten the paper without triggering the ink. To do this you have to undo the paper chromophores without destroying the area.

The tradition of Chinese paper has given solutions to this. It seems a paradox, but the Chinese have used hot water to whiten the paper. Water, that enemy of paper! News The method has been studied by chemists and is not bad, but it is not very good and most restaurateurs have rejected it. Another surprising method uses light, another enemy of paper. In alkaline medium it is treated with visible light --not ultraviolet -, but it is not very effective either.

Modern chemistry has better solutions. Selecting the products according to the type of paper destroy the chromophores. Oxygenated water was a candidate and chlorinated products have been common. Currently, the most common are paper 'deacidification' products, along with those that give hardness to paper.

One last question, how will these products change over time? Again, we are in the beginning. The restorer cannot wait to see it. Simulation of the aging process. You can put the paper to 100ºC for 3 days. It is an opportunity. But paper degradation does not occur at this temperature, but at room temperature. Therefore, there are many old papers to analyze. Experts do not know how these roles were in youth, but they can compare what was observed with what was observed in accelerated aging and try to understand aging.

Yes, while you answer the questions there is no time left. Keep painting paper.