Paper Machine - Pulp Types and Their Preparations

Pulp Types and Their Preparations

Rags of cotton and linen were the major source of pulp for paper before wood pulp. Today almost all pulp is of wood fiber. Cotton fiber is used in specialty 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. Fibers from clothing come from the cotton boll. The fibers can range from 3 to 7 cm in length as they exist in the cotton field. Bleach and other chemicals remove the color from the fabric in a process of cooking, usually with steam. The cloth fragments mechanically abrade into fibers, and the fibers 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 fibers, and cuts the fibers 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 fiber comes from the cotton ginning process. The seeds remain, surrounded by short fibers 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 and mechanical. Chemical pulps formerly used a sulfite process, but the kraft process is now predominant. Kraft pulp has superior strength to sulfite and mechanical pulps. Both chemical pulps and mechanical pulps may be bleached to a high brightness.

Chemical pulping dissolves the lignin that bonds fibers to one another, and binds the outer fibrils that compose individual fibers to the fiber core. Lignin, like most other substances that can separate fibers 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 sulfur 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 fibers that drain slowly.

Thermomechanical pulp(TMP) is a variation of groundwood where fibers 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, napkins and paper towels. 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 pulper.