KAPPA NUMBER OF PULP

Kappa number is a key test method for determining the level of lignin remaining in a sample of finished or in-process pulp. It is thus a measure of the completeness of the pulping process for many kinds of chemical and semi-chemical pulps, both bleached and semi-bleached. Kappa number is based upon the reaction of a strong oxidizing chemical, potassium permanganate, with lignin as well as small levels of certain other organic impurities remaining in the pulp at various stages of its processing. Kappa number gives the maker of the pulp, as well as the papermaking user of the pulp, valuable information about the properties of the pulp as well as the paper made from it, particularly with regard to the level of residual lignin present.

This kappa number standard applies to many kinds of chemical, semi-chemical, unbleached and semi-bleached pulps within the kappa number range 1 to 100. Above a kappa number of 100, precision of the test may decrease, and the relationship between kappa number and lignin content may decrease, depending mainly upon the wood species from which the pulp is made. There is no general and unambiguous relationship between the kappa number and the content of lignin of other organic impurities in a particular pulp. The relationship varies according to the wood species and the pulping, delignification, and bleaching procedures used to produce a specific chemical pulp. Kappa number is essentially a straight line relationship with both Klason lignin and chlorine number for pulps below 70% total pulp yields (1). The percentage of Klason lignin in a pulp sample whose kappa number is determined by the procedure in this standard test method may be approximated using the following equation

Lignin level (%) = Kappa number × 0.15

If the kappa number is to be used to determine a precise numerical value regarding the amount of lignin present in a specific pulp of interest, a more precise relationship can be established by testing the specific pulp of interest. There are claims that the precision can be improved if hexuronic acid in the unbeached pulp or brown stock are measured (TAPPI T 282) and their contributions are accounted for in the measured Kappa number.

As written, this standard is intended for use in the laboratory testing of pulps. It is recognized, however, that kappa number is widely used as an in-process test in the pulp and paper mill, in some cases with modifications. Section 16 of this standard includes informative information regarding the unintended or unexpected impact that certain deviations from the standard can have on data accuracy, precision, or both.

Likewise, this standard does describe the use of automated instruments for measuring kappa number. The user of such equipment shall verify the applicability of such equipment for any intended use, and do such testing as may be required to determine the agreement of results from any automated testing equipment used with results obtained using the procedure in this test method.