家里的报纸放久了，总是会变成黄色。有些书放久了，也会变成黄色。是因为纸老化了吗？下面这篇文章可以给你一点启示。

    Paper is made from wood, which is made up mainly of whitecellulose. Wood also has a lot of a dark substance in it calledlignin, which ends up in the paper, too, along with the cellulose. The exposure of lignin to air and sunlight is what turns paper yellow.

    Lignin makes wood stiff and trees stand upright. You could say it acts as a glue to bind the cellulose fibers together. It is a polymer, a substance that is formed by the joining of simpler molecules into giant molecules that act differently than the smaller molecules do. Without lignin, it is said that a tree could only grow to be about 6 feet tall. Lignin also helps protect the wood from pests and other damage.

    Newsprint, which must be produced as economically as possible, has more lignin in it than finer papers. At the mill, the wood that will be turned into newsprint is ground up.

    Paper manufacturers utilize the benefits of lignin in some types of paper. Brown Kraft paper, the dark brown paper used in grocery store bags and cardboard is stiff and sturdy because it has more lignin in it, and because those kinds of paper aren't treated with bleaching chemicals. It doesn't matter how dark they are because the printing on them is limited.

    To make a fine white paper, the mill puts the wood through a chemical solvent process, which separates and discards the lignin. Pure cellulose is white, and the paper made from it will be white and will resist yellowing.

    Lignin eventually turns paper yellow because ofoxidation. That is, the lignin molecules, when exposed to oxygen in the air, begin to change and become less stable. The lignin will absorb more light, giving off a darker color. If newsprint were kept completely out of sunlight and air, it would remain white. After only a few hours of sunlight and oxygen, however, it will start to change color.

    cellulose: 纤维素

    lignin：木质素

    oxidation：氧化