TAPPI T 261 Fine Fraction by Weight of Paper Stock by Wet Screening

The standard “TAPPI T 261 Fine fraction by weight of paper stock by wet sieving”, developed by the Technical Association of the Pulp, Paper and Converting Industries (TAPPI), describes a procedure for determining the weight percent fine content of paper stock or pulp samples by means of a single-screen classifier. A modified procedure allows the apparatus to be used to measure the tendency of the fine fraction to be retained by the fiber fraction under conditions of gradual turbulence. The separation of suspended particles of different shapes and sizes by wet sieving can be accomplished using a variety of devices.

Paper stock samples, particularly those taken from the paper machine circulation system, are characterised by having different proportions of particles, usually classified as fibres and fines. The proportions of these two components can vary considerably depending on the material, stock preparation and whether mineral fillers have been added to the stock. The fine fraction has a greater tendency to pass through the wire and be recycled during sheet formation, leading to fines accumulating in the headbox. The extent of this accumulation is an indicator of the holding performance of the machine and affects, for example, drainage, felt filling and saveall loading.

A suspension, such as paper stock, contains a range of solid particles varying in size by small increments. Any dividing line between what is defined as fiber and what is fine is an arbitrary decision. Based on published literature and microscopic examination, it is recommended that the dividing line be particles that will pass through a 76 μm diameter round hole or a nominal 200 mesh sieve. Other hole sizes may also be selected.

This technique can be used to measure the tendency of the fine fraction to remain with the fibre rather than following water into the permeate under gradient turbulence in simulating paper machine conditions. The determination of fine retention is particularly important in determining the effect of gradient turbulence in the evaluation of additives such as retention aids.

To perform adequate testing, the sample must be representative of the system from which it was drawn. A minimum of five grams of oven-dried pulp is needed for retesting each sample.

The refining process results in the formation of small, broken fiber fragments, commonly known as fines. Fines are defined as particles that pass through a round hole of 74–76 μm diameter or a nominal 200-mesh sieve. Fines include broken fiber segments, fibrils, lamellar fragments, ray cells, and so on.

Fine grains are divided into two groups according to their shape:

• The first group includes flake-like or coarse particles known as flour matter, flakes and pieces. These fine particles are described as being rich in lignin, with a small length-width ratio and low bonding potential. They are known to have a negative effect on the mechanical properties of paper, as they inhibit the formation of fiber bonds.
• The second group of fine particles includes the finer, flexible and swellable cellulose-rich particles known as slime, fibrils or ribbons. This group of fine particles has a large length-to-width ratio and good bonding properties. They are known to contribute to the increased strength of the paper.

In general, the wet screening method is used to screen clayey, sticky and very small-sized materials that are difficult to screen dry.

Our organization, which has been trying to support businesses from every sector with its testing, measurement, analysis and evaluation studies carried out in a wide range for years, has a strong staff that closely follows the developments in the world in the field of science and technology and constantly improves itself. In this context, testing services in accordance with the "TAPPI T 261 Wet sieving and fine fraction according to the weight of the paper stock" standard are also provided to businesses.