Issue of the Missing Tissue

TAD and more

Tissue papers are produced on complex machines. A modern tissue machine will operate at speeds of 2000 meters per minute (about 120 km/hour.) The wet end section is where the paper fibers (pulp) are formed into a sheet. A pulp slurry consisting of less than 1% wood fines and over 99% water is jetted onto a fabric that forms the paper sheet and also permits a large quantity of water to be drawn through it while leaving the formed paper on the top surface. After forming and drainage, the dry content of the sheet may be around 20%. So, there is a lot of water remaining to be evaporated in the drying process that follows. Generally, tissue machines are of two types; through-air-drying (termed TAD) and dry crepe (termed DC.)

It is beyond question that TAD products possess physical and mechanical characteristics superior to those made through more traditional techniques and which results in unmatchable softness. On TAD machines, after the paper is formed, it is transferred to a fabric that takes it over several large rolls where heated air is blown through the tissue and fabric, thus evaporating water. After transport over these rolls the paper is transferred to a large steam heated roll that is called a Yankee dryer. On DC machines the paper is formed in the same way but there are no rolls where air is blown through the sheet of paper. Rather, the sheet is pressed directly onto the Yankee under high pressure that squeezes water out and into the felt that transported the sheet.

The Yankee dryer may be about 5.5 metres in diameter and over this dryer are two hot air hoods positioned about 2cm above the dryer surface. From these hoods air at temperatures of up to 650°C is jetted onto the paper at velocities around 7000 metres per minute. During the very brief time (a small fraction of one second) that the sheet is on the Yankee, it is dried to its final moisture content of about 95%. To remove the sheet from the surface of the Yankee a steel blade is positioned against the surface of the roll to release the paper. This operation is termed “doctoring” and plays a key roll in increasing the bulk of the paper as the paper is “creped” by the doctor blade. The creped sheet “flies” through the air to the reel where it is wound onto a hard paper core. These rolls are then sent to other machines where they are converted into the final products: rolls of toilet paper; rolls of kitchen towels; boxes of facial tissue and packages of napkins.

Through air machines produce products with the highest bulk (measured in cc/gm) and this is accomplished because the paper is never subjected to high levels of pressing for removal of water. The paper is softer and feels more pleasant when contacting our body. But this feature is costly. TAD products have higher manufacturing cost. Pulp (the wood fines) is by far the most expensive cost in producing tissue. The next most expensive cost is energy. World wide energy costs have been increasing for several years and it is one reason prompting tissue machine manufacturers to seek better ways to make tissue paper. Since pressing on dry crepe machines is the reason that the products do not have as much bulk as TAD products, tissue machine manufacturers have developed different ways of removing water before the Yankee dryer and that permits the sheet to be transferred with considerably less pressing. These machines produce products with bulk almost equal to TAD products but at a much lower energy costs. Likely, these new DC machines will become the standard for making high quality tissue.

The energy for drying comes from steam inside the Yankee dryer and this steam is usually made using fuel oil or another petroleum product. The very hot air in the hoods comes from direct flame heating of air using natural gas or propane gas. It is important that the drying air be clean and free of odor. Saving energy should be of major concern to all people world wide. It lowers the costs of the products and also reduces the greenhouse gases that have been shown to cause global warming.