There are two fundamental steps in papermaking. First, the fibrous raw material, or cellulose, from pulpwood, nonwood fibers, or recycled papers is converted into pulp, a mass of fibers suitable for papermaking. Second, the pulp is interwoven and bonded into a structure known as paper. Pulping permits papermaking fibers to be separated from wood.
The three primary types of pulp are mechanical pulp,thermo-mechanical pulp (TMP), and chemical pulp. Each type of pulp results in varying degrees of paper quality.
Wood itself is made up of both cellulose fiber and lignin, a glue-like substance that holds the fibers together. Generally speaking, the more lignin that is removed from the wood, the better the quality of the paper. Lignin tends to discolor paper over time. Chemical pulp removes lignin from pulp, while mechanical and thermo-mechanical pulping do not.
Mechanical pulp. The principal types of mechanical pulp are groundwood, produced by forcing pulpwood against a revolving, abrasive grinding stone; and refiner mechanical pulp (RMP), produced by passing wood chips through a disk refiner instead of pressing the wood against the grinding stone.
Thermo-mechanical pulp. Thermo-mechanical pulp (TMP) is made by cooking wood chips or sawdust under pressure with steam, softening the lignin. The softened chips are then ground into pulp by passing through a disk refiner and bleached for whiteness. The heat generated by disk refining and preheating the wood chips softens the lignin that holds the fibers together, permitting fiber separation with little fiber damage. These papers tend to be whiter and brighter than mechanical pulp papers and have good opacity for two-sided printing. Many high-quality newspapers are printed on TMP.
Chemical pulp. Chemical pulp is the most expensive pulping method, resulting in the highest quality papers. In this process wood chips are cooked under pressure in a cooking liquor designed to break down and remove the lignin from the pulp. The pulp is cleaned and bleached, resulting in a bright white paper that has great permanence. Chemical pulping liberates the cellulose fibers from the lignin that bonds them together.
During chemical pulping, chemicals dissolve the lignin and hemicellulose in wood to free cellulose fibers from the wood. The bleaching of chemical pulp removes residual lignin and hemicellulose. Chemical pulping has a yield of 45%~55%, compared to the yield of 90% or more common with mechanical pulping methods. Chemical pulping processes use acids, such as sulfurous acid, and alkalis, such as lye (caustic soda).
Semichemical pulps are produced by first mildly cooking wood chips with chemicals, most commonly sodium sulfite or a small amount of an alkaline salt, such as sodium carbonate, bicarbonate, or hydroxide. The cooking partially removes and softens the lignin, weakening the bond between fibers. After cooking, the chips are passed through one or two disk refining steps for fiberizing. A wash removes the chemicals. Semichemical pulping retains a considerable amount of lignin, and consequently, yields are typically 60%~80%. Bleaching reduces the yield, as it does with any pulping method.
Bleaching of pulp. Unbleached wood pulp ranges in color from cream to dark brown; a brown paper bag is made of unbleached pulp. Bleaching removes residual lignin in the pulp without causing severe chemical damage to the cellulose fibers. Bleaching a pulp makes the resulting paper whiter, which improves printing contrast. Whiter pulps produce colored papers that are more brilliant.
Stock preparation and refining. The fibers obtained by pulping must receive further treatment before they can used to make high-quality paper. This treatment is called stock preparation, and it entails fiber refining and the mixing of fibers with nonfibrous materials to produce a paper of the desired properties. Paper is formed on the papermaking machine, but many of the characteristics of paper are determined during stock prepartion.
During refining, beaters, conical-type refiners (jordan), or disk refiners are used to subject the fibers to varying degrees of brushing, cutting, fraying, and shortening actions depending on the properties desired in the paper. As a result, the fibers swell and soften and their surfaces develop fine hairs, all of which promote bonding to make a stronger paper.
During stock preparation, the cellulose fibers are mixed with fillers, internal sizing, dyes and colored pigments, and other additives that impart properties to paper.
Paper forming. Most printing papers are made on fourdrinier papermaking machines, Figure 5-3, named after the Fourdrinier brother, Henry and Sealy. The pulp is mixed with large amounts of water before forming stage, resulting in a 99% water ratio. The mixture of pulp and water, called a furnish, flows into a continuously cycling fine screen through which water drains-leaving the fibers in a thin sheet or web on its surface. The fibers align with the moving wire, which forms the paper's grain direction. The web transfers from the wire to a continuous felt blanket that carries it to steam-heated dryer cylinders.
In the wet or forming section of the paper machine, the diluted is formed into a mat of fibers, from which most of the water is removed by gravity or vacuum.
Paper machines also have a press section, where as much water as possible is removed from the web by pressing and suction, and a drying section, where the remaining water is removed, this time by evaporation caused by hot drying cylinders.
Paper leaving the dryer has a moisture level from 2% to 8%, depending on its desired end use.
Surface-sized paper is made by passing the partially dried web between a roller-based coating system that applies the sizing material. The size pres is located in the dryer section of the machine. After being dried, the paper web is calendered between polished steel rollers to give it the desired smoothness, making it machine-finish (MF) paper.
Supercalendering. Supercalendered paper is made by running machine-finish paper through the supercalender, Figure 5-4, a machine consisting of alternate polished steel compressed paper or cotton rollers running together under high pressure. It compresses the paper and increases surface smoothness the paper and increases surface smoothness and gloss.
Coated papers. The press operator often uses the printing press to put a varnish coating on the paper after it has been printed, usually for protective or aesthetic purposes. However, the papermaker also may apply a coating to the surface of paper to make the paper more suitable to its end use. Coated papers are stocks that have received a mineral coating, usually clay, to improve their printability and appearance. They are available in a variety of finishes. Coatings improve paper whiteness, pick resistance, water resistance, ink holdout, paper opacity, gloss, and overall print quality. Usually, paper is coated on two sides (C2S). Labels and some specialty products are only coated on one side (C1S). Please see the scheme of a huge papermaking machine in Figure 5-5.