Bleaching of wood pulp

Bleaching of wood pulp is the chemical processing carried out on various types of wood pulp to decrease the color of the pulp, so that it becomes whiter. The main use of wood pulp is to make paper where whiteness (similar to but not exactly the same as "brightness") is an important characteristic. The processes and chemistry described in this article are also applicable to the bleaching of non-wood pulps, such as those made from bamboo or kenaf.

Brightness is a measure of how much light is reflected by paper under specified conditions and is usually reported as a percentage of how much light is reflected, so a higher number represents a brighter or whiter paper. In the US, the TAPPI T 452 or T 525 standards are used. The international community uses ISO standards. The following table shows how the two systems rate high brightness papers, but there is no simple way to convert between the two systems because the test methods are so different. Note that the ISO rating is higher and can go above 100. This is because today’s white paper manufacturing uses fluorescent whitening agents (FWA). Because the ISO standard only measures a narrow range of blue light, it is not an adequate measure for the actual whiteness or brightness.

Newsprint ranges from 55-75 ISO brightness. Writing and printer paper would typically be as bright as 104 ISO.

While the results are the same, the processes and fundamental chemistry involved in bleaching chemical pulps (like kraft or sulfite) are very different from those involved in bleaching mechanical pulps (like stoneground, thermomechanical or chemithermomechanical). Chemical pulps contain very little lignin while mechanical pulps contain most of the lignin that was present in the wood used to make the pulp. Lignin is the main source of color in pulp due to the presence of a variety of chromophores naturally present in the wood or created in the pulp mill.

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