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Standard: |
TAPPI |
Method: |
TAPPI T 233 |
Title: |
Fiber length of pulp by classification |
TAPPI T 233 Bauer McNett is designed to measure the weighted average fiber length of a pulp. If a fiber is 1 mm in length and weighs w mg, then for a given pulp, the weighted average length (L) is Σ(wl)/Σw, or the sum of the products of the weight times the length of each fiber divided by the total weight of the fibers in the specimen. Use of either Clark type or Bauer-McNett type classifier will produce identical results within the stated precision.
A Bauer McNett type classifier for TAPPI T 233 is used for this test. The classifier has four tanks set up in a cascade arrangement from the largest openings to the smallest. The weight percent on the four screens and the percent that passed through the smallest screen (the fines portion) is reported.
Fiber length is a fundamental property of pulp. There have been indications that, provided the coarseness of the fibers remains constant, the tensile strength of a sheet made from relatively unbeaten pulp (other factors being the same) will vary as L1/2, the burst strength will vary as L, the fold endurance as L5, and the tear resistance as L3/2. These relationships will be true independent of the uniformity of lengths of the fibers, whether they are a mixture of various lengths or all of one length, but the numerical values of the exponents will decrease as the pulp is beaten. The density of the sheet will be appreciably affected by L if the fibers are the same coarseness.
The numerical average fiber length is of much less significance. If the fibers are of mixed lengths, it is an arbitrary measurement, largely influenced by the lower limit adopted, below which a particle is no longer regarded as a fiber. The weighted average fiber length of a pulp may exceed the numerical average length several times, if particles down to a length of 0.01 mm are included in an estimate of the numerical averages.
Ideally, it would be desirable to classify a pulp sample into a very large number of fractions and weigh and measure the length of each in order to determine the weighted average length. In practice, however, satisfactory results may be obtained from the weights and lengths of five graduated fractions which are provided by a four-screen classifier.
The Bauer-McNett type classifier requires 10 oven dry grams for each test replicate. To perform adequate testing, the equivalent of 24 oven dry grams is desired. This ensures proper disintegration and that a duplicate can be tested on each sample.
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