Basically paper is made from cellulose fibers derived from plants. Cellulose consists of glucose chains which form the building block for most plant materials. The fibre in both wood, grass and other fibrous plants such as hyacinth weed is made of cellulose. These fibers are bound together to form parenchyma (are thin-walled cells that make up the inside of many non-woody plant structures including stems, roots, and leaves) and sclerenchyma cells (in woody plants bound together by lignin). In function cellulose gives wood its strength, while lignin gives it rigidity (stiffness). Pure cellulose is white, while lignin is dark brown or black
To make paper a form paper, cellulose is separated from lignin and a mixture of sugars called hemicelluloses and other minor chemical components. This separation can be accomplished mechanically or chemically in a process called pulping.
In mechanical process, solid wood is steamed and ground into fine fibers which are laterpassed through chemical bleaching process to produce (Semi-mechanical chemical pulp) such as the one used to produce newsprint paper grade.
In chemical process, small pieces of chipped wood is cooked under high temperature and pressure in either acid (sulphite pulping) or alkali (sulphate pulping) to dissolve lignin which is later extracted through washing process. At this stage the pulp (raw paper) is brown in color and it is thereafter bleached to make it white. Packaging paper is almost never bleached, the reason cartons are brown in color(Kraft grade).
To make it white and depending on the whiteness level required, the resultant pulp mixture is passed through different chemical bleaching methods. While bleaching improves the quality of paper, it also weakens the paper. For instance, if you bleach strongly to remove as much lignin as possible, the fibresbecomes weak. If you leave some lignin, such as case of newsprint, the paper ages faster due to reverse oxidation. The acidic compounds retained on the material during pulping causes a catalytic reaction that activates the hydrolytic decomposition (degradation) of cellulose
However, it is impossible to remove all the lignin. Lignin slowly oxidises and turns paper brown or yellowish. For high quality wood paper, acid pulping is used, but this method is also more polluting.
Pulp from hardwoods (broadleaved species) consists of shorter fibres which are stiffer and is often used for low grade paper like cartons. Softwood (needle-leaved) species have longer fiber which makes it more flexible.
Stage 1: Chipping/breaking down. – Breaking down wood and other cellulose material into smaller pieces for easier processing.
Step 2: Beating/cooking– Mechanical process of individualizing cellulose fibers and remover of lignin (binding material)
Step 3: Bleaching – Whitening using bleaching chemicals
Step 4: Formation– spreading of pulp slurry of fibers in a fast moving mat to form a sheet of paper
Step 5: Drying – Pressing paper mat between rollers to remove water hence reducing moisture content from 80-5%
Step 6: Conversion– Reducing the size of ready paper to usable sizes.