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What Is the Kappa Number?

Mary McMahon
Updated May 17, 2024
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The Kappa number is an assessment of how much lignin is present in pulp, which determines the amount of bleach that needs to be added to it if the goal is a white processed paper product. High Kappa numbers require more bleach, while lower numbers have less lignin and need less bleach. This number is also sometimes known as the pulp brightness measurement, and must be regularly checked at a pulp mill while the facility processes and generates pulp.

A standardized test can be used to determine the Kappa number. Standardization helps to ensure that results are consistent and readily understandable across platforms; a Kappa number of 10%, for example, is an absolute number that provides meaningful information. These numbers can be reported in discussions of the plant's efficiency and systems, and are also important for the logs associated with batch paper production. Plants need consistent papers because consumers rely on a standard brightness in the products they purchase.

To measure the Kappa number, a pulp mill can perform a test with potassium permanganate solution. It determines how much of the solution, in milliliters, is absorbed by a gram of the pulp. This allows the company to calculate and report the Kappa number. A pulp mill may have numerous testing stations to check on pulp as it moves through the processing stages, and can keep continuous logs on its batches of pulp and paper.

Pulp mills can use a variety of techniques to lower the Kappa number and cut down on the amount of bleach they need, including things like washing the pulp to remove excess lignin. These processes may add to processing time, and require careful monitoring to make sure they work as efficiently and effectively as possible. Companies that build machinery and develop processes for the paper industry look at ways to lower Kappa numbers and periodically release new products with increased functionality in this area.

Kappa number logs are typically kept with other records that pertain to production. A pulp mill can use this to determine how different kinds of timbers perform during pulping, and to track quality on its projects. If a company starts to turn out inconsistent pulps and papers, the logs can be checked to find out where the problem is originating, and how to address it. Inadequately maintained machines can be a potential cause, as they may work less effectively and thus remove less lignin during the pulping and processing procedures.

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Mary McMahon
By Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a About Mechanics researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

Discussion Comments
By bythewell — On Dec 03, 2011

@umbra21 - Not all paper is used in printing though. And much of the paper which is used in printing doesn't have to be perfect. I actually wonder what would happen if they did offer more kinds of paper that didn't use so much bleach. At the moment it usually more expensive to buy paper which is certified as being environmentally friendly. And fair enough, because I'm sure to make it entirely environmentally friendly takes a lot of effort.

But couldn't they make a paper that's just less harmful? Shouldn't cutting down the bleach make it less expensive to make, if you weren't so concerned about the quality?

I guess they just don't know if people would buy such a product.

I would though and I think others would too, if they understood how it works.

By umbra21 — On Dec 02, 2011

Paper mills aren't as bad as they used to be, although this is mostly because of regulations, rather than because they have found cleaner ways of making paper.

Unfortunately, people like white paper more than ever, because they need an absolutely white surface for printing. The bleach doesn't just make the paper white, it makes it evenly colored. If you take the bleach out of the equation, or even lessen it, you will get an uneven product, not just a darker product.

And that would lead to people not having very good printing and so forth.

As it says in the article, it's better for them to try and lower the kappa number before using bleach.

By browncoat — On Dec 01, 2011

It's a shame that people are so attached to paper being absolutely bleached and white. If that wasn't considered the norm in paper, I imagine that much less bleach would be needed and so less bleach and by products of bleach would be going into the environment.

Paper making is notoriously bad for the environment. One of my friends recently wrote a book about a paper mill and had to do some research and she was pretty shocked at how bad it can be. Basically, any river which receives discharge from a paper mill can expect almost 100% die off of anything that ever lived there.

And a lot of the problem is the bleach.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a...

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