Why do newspapers turn yellow over time?
[ 2006-07-19 10:18 ]
家里的報紙放久了，總是會變成黃色。有些書放久了，也會變成黃色。是因為紙老化了嗎？下面這篇文章可以給你一點啟示。
 Paper is made from wood, which is made up mainly of white cellulose. Wood also has a lot of a dark substance
in it called lignin, 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 of oxidation. 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:
纖維素
lignin：木質(zhì)素
oxidation：氧化
（英語點津 Annabel 編輯）
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