Main articles: kraft process, sulfite process, and soda pulping
To make pulp from wood, a chemical pulping process separates lignin from cellulose fibers. This is accomplished by dissolving lignin in a cooking liquor, so that it may be washed from the cellulose fibers. This preserves the length of the cellulose fibers. Paper made from chemical pulps are also known as wood-free papers–not to be confused with tree-free paper. This is because they do not contain lignin, which deteriorates over time. The pulp can also be bleached to produce white paper, but this consumes 5% of the fibers. Chemical pulping processes are not used to make paper made from cotton, which is already 90% cellulose.
The microscopic structure of paper: Micrograph of paper autofluorescing under ultraviolet illumination. The individual fibres in this sample are around 10 µm in diameter.
There are three main chemical pulping processes. The sulfite process dates back to the 1840s, and it was the dominant process before the second world war. The kraft process, invented in the 1870s and first used in the 1890s, is now the most commonly practiced strategy. One advantage is a chemical reaction with lignin produces heat, which can be used to run a generator. Most pulping operations using the kraft process are net contributors to the electricity grid or use the electricity to run an adjacent paper-mill. Another advantage is the process recovers and reuses all inorganic chemical reagents. Soda pulping is a specialty process used to pulp straws, bagasse, and hardwoods with high silicate content.
There are two major mechanical pulps, thermo mechanical pulp (TMP) and groundwood pulp (GW). In the TMP process, wood is chipped and then fed into large steam-heated refiners where the chips are squeezed and made into fibres between two steel discs. In the groundwood process, debarked logs are fed into grinders where they are pressed against rotating stones and made into fibres. Mechanical pulping does not remove the lignin, so the yield is very high, >95%, but also causes paper made from this pulp to yellow and become brittle over time. Mechanical pulps have rather short fibre lengths and produce weak paper. Although large amounts of electrical energy are required to produce mechanical pulp, it costs less than chemical pulp.
Main article: deinking
Paper recycling processes can use either chemical or mechanical pulp. By mixing with water and applying mechanical action the hydrogen bonds in the paper can be broken and fibres separated again. Most recycled paper contains a proportion of virgin fibre in the interests of quality. Generally deinked pulp is of the same quality or lower than the collected paper it was made from.
There are three main classifications of recycled fibre:.
Mill broke or internal mill waste – this incorporates any substandard or grade-change paper made within the paper mill which then goes back into the manufacturing