Sheet Holes in the Tissue Industry
When does a hole in tissue paper become critical in tissue making process? What size hole is still acceptable? How many holes per fabric length are allowed? The answer to these questions are related to the tissue product being produced.
For example, a producer of diapers may not accept a single hole. A toilet paper manufacturer, on the other hand, may allow some holes with an opportunity to downgrade to a middle layer of the toilet paper. However, it is always a question of the hole size and number of holes.
The target is to reduce the number of holes or at least to reduce the size of the hole. Holes smaller than 2 mm are normally accepted (Figure 1). Tissue with holes bigger than 10 mm (Figure 4) will be scrapped or called internal broke which can be recycled. The only way to reduce the size of the hole is to reduce the size of the deposit creating the hole. If the deposit cannot be removed by the first path, try to at least reduce the size of the deposit.
What are stickies and pitches?
Stickies is another name for a deposit on the fabric which belong to the group of gluing particles. Gluing particles are differentiated into stickies, pitches, and white pitches. Stickies come from hotmelts and wax. White pitches come from dispersion glue and artificial binders (from coated broke). Pitches come from rosin agglomeration from mechanical pulp. Papers are coated or laminated to get a certain property of the paper end product. If the tissue mill uses recycled paper in its furnish, these components will be a part of the raw material.
Virgin fiber tissue mills which recycle their own broke can have issues with deposits too. In this case, the deposit comes from tape or glue between the layers. The stock preparation of a virgin fiber mill to reduce gluing particles is usually not very good. The possibility to reduce the amount of deposits is limited and there will always be some remaining gluing particles in the stock. Therefore, these mills reduce the amount of internal broke which they use for production and throw the rest away.
Recycled tissue mills have good stock preparation, but more gluing particles in their raw material. These mills know many ways to reduce, mask, or disperse gluing particles so they are less harmful. But, pump energy and other chemicals often destroy the masking, or the small particles start to agglomerate over time. There are still some gluing particles that reach the wire section (Figure 2). One way to minimize gluing particles is by using finer screening. However, it may impact the fiber losses. Many mills prefer to deal with consequences on the forming fabric and process rather than lose fiber.
How do gluing particles create holes in tissue paper?
The forming fabrics carry numerous fibers which cannot be removed with a full-width shower. The stock goes through the head box carrying gluing particles. The tendency for a gluing particle to stick on a fiber, which is embedded on the fabric, is much higher compared to a gluing particle sticking on the forming fabric. So, the first small deposit sticks on the fiber and is embedded into the fabric. Next the fibers start to stick on the deposit, again with some gluing particles. This cycle repeats. If it’s not removed, the deposit grows and grows.
This process, fibers -> gluing -> particle -> fibers, starts very small. You can imagine that stickies bigger than 10 mm will never pass any screens or cleaner. At the beginning of this process, the deposits are on the surface and are relatively easy to clean. Over time, the deposit is pressed into the fabric. Then much more energy is needed to clean the fabric. For this reason, forming fabrics must be cleaned continuously to keep stickies small and avoid the deposit embedding into the fabric structure.
Deposits can be located on the fabric, felt, and even in the tissue paper (Figure 5). But the biggest problem, holes, is caused by deposits on the outer fabric. The stock is delivered out of the head box between the two forming fabrics (the outer fabric, also called top fabric, and the inner fabric, also called bottom fabric which embraces the plain breast roll). Drainage of the stock goes in one direction, through the outer fabric. Pulp dewatering travels in one direction, through the outer fabric. If the dewatering channels are blocked due to a sticky, a hole on the sheet will form. The deposit steals the fibers and keeps them on the surface or the fibers are washed away. Or, there is a reduced amount of fibers where the deposit was located.
A future post will discuss avoiding or stopping this process and ways to effectively clean the tissue fabric.