Editing Paper machine (section)

== Pulp types and their preparations ==
{{Main|Pulp (paper)}}
The plant fibres used for pulp are composed mostly of cellulose and hemi-cellulose, which have a tendency to form molecular linkages between fibres in the presence of water.  After the water evaporates the fibres remain bonded.  It is not necessary to add additional binders for most paper grades, although both wet and dry strength additives may be added.

Rags of [[cotton]] and [[linen]] were the major source of pulp for paper before wood pulp.  Today almost all pulp is of wood fibre.  Cotton fibre is used in speciality grades, usually in printing paper for such things as resumes and currency.

Sources of rags often appear as waste from other manufacturing such as denim fragments or glove cuts.  Fibres from clothing come from the cotton boll.  The fibres can range from 3 to 7&nbsp;cm in length as they exist in the cotton field.  Bleach and other chemicals remove the colour from the fabric in a process of cooking, usually with steam.  The cloth fragments mechanically abrade into fibres, and the fibres get shortened to a length appropriate for manufacturing paper with a cutting process.  Rags and water dump into a trough forming a closed loop.  A cylinder with cutting edges, or knives, and a knife bed is part of the loop. The spinning cylinder pushes the contents of the trough around repeatedly.  As it lowers slowly over a period of hours, it breaks the rags up into fibres, and cuts the fibres to the desired length.  The cutting process terminates when the mix has passed the cylinder enough times at the programmed final clearance of the knives and bed.

Another source of cotton fibre comes from the [[cotton ginning]] process.  The seeds remain, surrounded by short fibres known as linters for their short length and resemblance to lint.  Linters are too short for successful use in fabric.  Linters removed from the cotton seeds are available as first and second cuts.  The first cuts are longer.

The two major classifications of pulp are [[chemical pulp|chemical]] and [[mechanical pulp|mechanical]].  Chemical pulps formerly used a [[sulfite process|sulphite process]], but the [[kraft process]] is now predominant.  Kraft pulp has superior strength to sulphite and mechanical pulps and kraft process spent pulping chemicals are easier to recover and regenerate. Both chemical pulps and mechanical pulps may be bleached to a high brightness.

Chemical pulping dissolves the [[lignin]] that bonds fibres to one another, and binds the outer [[fibril]]s that compose individual fibres to the fibre core.  Lignin, like most other substances that can separate fibres from one another, acts as a debonding agent, lowering strength.  Strength also depends on maintaining long cellulose molecule chains.  The kraft process, due to the alkali and sulphur compounds used, tends to minimize attack on the cellulose and the non-crystalline [[hemicellulose]], which promotes bonding, while dissolving the lignin. Acidic pulping processes shorten the cellulose chains.

Kraft pulp makes superior [[linerboard]] and excellent printing and writing papers.

Groundwood, the main ingredient used in [[newsprint]] and a principal component of magazine papers (coated publications), is literally ground wood produced by a grinder.  Therefore, it contains a lot of lignin, which lowers its strength.  The grinding produces very short fibres that drain slowly.

Thermomechanical pulp (TMP) is a variation of groundwood where fibres are separated mechanically while at high enough temperatures to soften the lignin.

Between chemical and mechanical pulps there are semi-chemical pulps that use a mild chemical treatment followed by refining.  Semi-chemical pulp is often used for corrugating medium.

Bales of [[recycled paper]] (normally old corrugated containers) for unbleached (brown) packaging grades may be simply pulped, screened and cleaned.  Recycling to make white papers is usually done in a deinking plant, which employs screening, cleaning, washing, bleaching and flotation. [[Deinked pulp]] is used in printing and writing papers and in [[tissue paper|tissue]], napkins and [[paper towel]]s. It is often blended with virgin pulp.

At integrated pulp and paper mills, pulp is usually stored in high density towers before being pumped to stock preparation.  Non integrated mills use either dry pulp or wet lap (pressed) pulp, usually received in bales.  The pulp bales are slushed in a [re]pulper.