亚洲成aⅴ人在线观看_亚洲欧美日韩综合一区在线观看_国产无码电影一区二区三区_国语精品91自产拍在线观看二区

食品伙伴網(wǎng)服務號
 
 
當前位置: 首頁 » 專業(yè)英語 » 資源技巧 » 正文

為什么報紙會變黃

放大字體  縮小字體 發(fā)布日期:2008-03-04
核心提示: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 whitecellulose. Wood also has a lot of a dark substance in it calledlignin, which ends


    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 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: 纖維素

    lignin:木質(zhì)素

    oxidation:氧化    

 

更多翻譯詳細信息請點擊:http://www.trans1.cn
 
關鍵詞: 報紙 變黃
分享:

 

 
推薦圖文
推薦專業(yè)英語
點擊排行
 
 
Processed in 0.866 second(s), 171 queries, Memory 1.36 M