Paper Manufacturing Process

Manufacturing of Paper is not as simple as we think. In a nutshell Paper making is obtaining cellulose fibres, cleaning and washing them thoroughly, suspending them in water to form a mat, take out water and dry it in to a sheet. But technically the process is a long one.

The basic raw material required to make Paper is Fibre. Cellulose is the principal component of all vegetable tissues and fibres and is the most important organic material on the earth.

The cellulose required for manufacturing Paper can be obtained from Wood, Cotton, Straw, Sugarcane, Bamboo and esparto grass. Cereal straws such as Wheat, Rice, Barley and Oat can also be used to get cellulose. The cellulose required should have sufficient fibres with good strength and should be economically viable for making Paper.

The primary source of obtaining cellulose is Wood. Wood is available in abundant, easy to grow and has desirable length and strength of the fibres. In a tree there are two main components, cellulose and lignin. Lignin is the substance which binds the cellulose fibres of the tree. The Lignin content can be about 23 to 30% in a normal tree. Lignin is Yellow in colour which turns in to dark Colour when exposed to Light.

For making Paper we need only Fibres and NO LIGNIN. So we need a process so as to get only Fibres and separate Lignin from the Wood.

After removal of the bark (Skin) wood is forced against abrasive grinding stones. One more method is to cut wood in to small pieces called Chips. The Chips are than placed between grooved surface of two refiner discs operating in opposite direction. Water is added along with the Ships. The material thus obtained contains both Lignin as well as Cellulose. This is called Mechanical Process.

The Pulp obtained is called Mechanical Pulp. When Paper is made from this pulp it has good opacity and high bulk. But it has less strength due to short fibres and the paper has a tendency to discolour due to presence of Lignin.

Another process is called Thermo Mechanical Pulp wherein the wood chips are passes through steam before going to the refiners. Since the wood chips become loose they tend to get crushed easily leading to longer Fibre. The Lignin content also reduces because of steam.

There is one more process called Chemi – thermo - Mechanical process (CTMP) where Chemical is added along with Steam.

This is a common process used for getting fibres. Logs of Wood are cut in to chips and cooked under heat and pressure. Chemicals are added to separate Fibre and Lignin. This is done in a Cooker called Digester. This process gives longer and stronger fibres as they float freely in the process.

Chemicals such as Sodium Hydroxide and Sodium Sulphate are used and converted to Sodium Sulphite. This Sodium Sulphite is the main agent for cooking.

This process is more common in all manufacturing units. This process also allows other waste chemicals get extracted from the main flow which can be used to make other products.

The cellulose after cooking turns Brownish in Colour due to the presence of Lignin. Here for making Paper such as Kraft paper this Pulp is washed and is ready to make paper.

In case of requirement of White Paper Bleaching is another process through which the Colour of the Pup is changed and the finished product also gets stability.

Washing removes inorganic and organic Chemicals. Bleaching removes and alters Lignin without damaging the cellulose. Series of Bleaching process are carried out to get a high bright Pulp and thereby Paper.

The stock preparation process involves addition of non fibrous material with the pulp to get the final product.

The most common additives are Starch, Calcium carbonate, Clay, Titanium Dioxide, talc, Calcium Sulphate, Colour dyes and pigments and Optical Brighteners.

The cellulose should now be processed mechanically so as to get a bonding of fibres and there by surface to surface contact. This process is called Refining or beating. A beater or Refiner is used for this process wherein the pressure is adjusted according to the requirement of the finished sheet.

This step is now the make a sheet of paper. The fibres are suspended in the water and moved to the unit at the start of the paper machine called Head Box. The Head Box is set so that the solution of 99% water along with fibres will drop on a moving screen and form a sheet like structure.

The water will fall on the sides as well as by suction to form a sheet. Since the screen is moving at a high speed the fibres tend to align themselves with the direction of the screen. The formation of fibres in this direction is called GRAIN direction. As the fibres leave the screen they are than passed through rollers and pressed to take out more water. This is called press box.

The paper is than passed through large steam dryers which evaporate the remaining moisture so as to get a sheet of paper.

The final step of making paper is Calendaring. Here paper is passed through chilled rolls to get smoothness.

The paper is than wound on large rolls at the end of the machine.