Pulp is a broad term that refers to the main ingredient in the papermaking process.
The largest category of pulp is wood pulp, which is derived by chemically or mechanically removing the fibrous materials from wood chips. The second largest category of pulp is that derived from recovered paper by chemical or mechanical treatment of bales of recovered office paper, old newspapers, old magazines, and packaging materials. Finally, pulp can be produced from nonwood sources such as hemp, bagasse, straw, bamboo, kenaf, flax, and cotton.
There are several different manufacturing methods within the wood pulp category. The two broadest classifications are chemical pulp and mechanical pulp. The term chemical pulp is derived from the use of “cooking” chemicals to separate the hemicellulose (fibers) from the lignin or “glue” that binds these fibers. Chemicals are added to a big vat called a digester where they are cooked under steam heat and pressure. The two main chemical pulp methods are sulfite and sulfate (also known as kraft).
The most popular cooking method is the kraft, or sulfate, cooking process. The main cooking chemical is sodium hydrochloride, caustic soda, or lye. Sodium hydrochloride and other chemicals are mixed in the digester under heat and pressure conditions to “cook” wood chips to a uniform consistency. The resulting pulp or “slush” is then washed and bleached to standard brightness depending on its final end-use.
There are a variety of mechanical pulping methods. Thermomechanical pulp (TMP) uses a combination of heated wood chips and mechanical processes. Stone Groundwood (SGW), as the name implies, grinds or macerates the wood chips. Finally, Chemithermomechanical pulp (CTMP) uses a variety of chemicals, heat, and grinding techniques to produce pulp.
Different types of pulp produce different types of paper, although many papers use a combination or “blend” of several different types of pulp and recovered paper. This “recipe” is known as the fiber furnish. For example, high quality printing and writing grade papers such as copy paper, envelopes, magazine paper, and catalog paper typically use a larger percentage of bleached kraft pulp, some mechanical pulp, and some recovered paper pulp. Lower quality printing and writing papers use a larger percentage of mechanical pulp and mix in some bleached kraft pulp and recovered paper pulp. Newsprint uses mostly recovered paper pulp and mechanical pulps. Finally, packaging grades use mostly recovered paper pulp and brown (unbleached) kraft pulp.