Bleached paper is made by chemically treating paper pulp to remove lignin, a natural polymer that gives wood its brown color, resulting in a brighter, whiter product.
The process of making bleached paper typically begins with raw materials like wood fibers, recycled paper, or other plant sources. These materials are processed into pulp through mechanical or chemical methods. After pulping, the fibers still contain lignin, which needs to be removed or altered to achieve whiteness. This is where the bleaching stage comes in.
Bleaching Paper Pulp
Bleaching is a critical step to enhance the brightness and purity of paper pulp. According to the provided reference, two methods are commonly used for bleaching pulp, with oxidative bleaching being the main method.
Oxidative Bleaching
Oxidative bleaching utilizes oxidative bleach to destroy the structure of lignin and dissolve lignin to improve the purity and whiteness of pulp. This method involves using chemicals that react with lignin, breaking it down into soluble compounds that can be washed away.
Common bleaching agents used in oxidative bleaching include:
- Chlorine (Cl2)
- Chlorine dioxide (Clo2)
- Hypochlorite
The use of these chemicals effectively removes the color-causing lignin from the pulp fibers, significantly increasing their brightness and making the paper appear white. While other bleaching methods exist, oxidative bleaching remains the main method of bleaching used in the industry due to its effectiveness in removing lignin.
The Bleaching Process Steps (Simplified)
- Pulp Preparation: Raw fibers are pulped to create a fibrous slurry.
- Bleaching Treatment: The pulp is mixed with bleaching agents (like those used in oxidative bleaching) in carefully controlled conditions (temperature, time, chemical concentration).
- Washing: After the chemical treatment, the pulp is thoroughly washed to remove the reacted lignin fragments and residual chemicals.
- Further Processing: The now-bleached pulp is ready for further processing into paper sheets.
By removing or modifying the lignin through processes like oxidative bleaching, the paper achieves the desired white appearance that is common for printing paper, writing paper, and many other paper products.
