Pulp is made from breaking down the fibrous part of plants, primarily trees, and refers to the main ingredient in the papermaking process. Pulp is used in consumer products like paper, paper cups, tissue and cardboard – and in specialty products as diverse as LCD screens, home décor, shoes and handbags, food casings and filters.
What is Pulp?
Pulp made from trees (wood fiber) is the most common source of fiber for papermaking. During the papermaking process, pulp is made from breaking down either wood chips or paper recovered for recycling. There are many different methods of producing wood pulp depending on type of paper and the characteristics needed in the final paper product. In general, there are two main methods of separating fibers from wood to produce pulp; one using chemicals and one using mechanical grinding.
- Chemical Pulp: Pulp obtained by digestion or cooking of wood with solutions of various chemicals. The paper produced from this pulp results in strong products that are unlikely to discolor. The principal chemical processes used are the sulfate (kraft), sulfite, and soda processes. Chemical pulps are used to make shipping containers, paper bags, printing and writing papers, and other product categories that requiring strength. The kraft pulping process is the most common process used. The process creates a strong fiber where the active cooking agent is a mixture of sodium hydroxide and sodium sulfide. The term "kraft" is commonly used interchangeably with "sulfate" and is derived from a German word which means "strong." Kraft pulps are usually broken down into two types; hardwood kraft pulps (made from trees with leaves) and softwood kraft pulps (made from trees with needles).
- Dissolving Pulp: A special grade of chemical pulp usually made from wood for use in the manufacture of regenerated or cellulose derivatives such as acetate, which is used in film, satiny fabrics, some eyeglass frames and from 1949 to 1963, Lego bricks.
- Fluff Pulp: Pulp made during a chemical, mechanical or combination chemical/mechanical process, usually bleached, used as an absorbent medium in disposable diapers, bed pads and hygienic personal products. Also known as "fluffing" or "comminution" pulp.
- Mechanical Pulp: Any wood pulp manufactured wholly or in part by a mechanical process, including stone-ground wood and chip mechanical pulp. Mechanical pulp has a greater yield than chemical pulping due to a high residual lignin or glue content left in the pulp following pulping. The downside is paper created from this pulping process is weaker and discolors easily when exposed to light. Uses include newsprint printing papers, specialty papers, tissue, toweling and paperboard.
- Recovered Paper Pulp: Pulp created from paper that was recovered for recycling. Sources of recovered pulp include newspapers, magazines, catalogs, office papers, cardboard boxes, shoe boxes, cereal boxes. In its simplest form recovered pulp is made by combining the recovered paper with water in a pulper that acts as a blender to separate and mix the fibers into a slurry, much like the consistency of cottage cheese. Depending on the desired final product, the recovered paper may be further processed to remove inks, coatings, sizing adhesives or other impurities (a process called de-inking). Recovered paper pulp can be used to produce new products made entirely of recovered content or from a blend of recovered paper pulp and pulp from wood chips.
Different pulp-producing processes produce different types of paper products. However, in the paper production process, different pulps are often combined like a recipe to create products with specific properties. For instance, high quality printing and writing grade papers such as copy and magazine paper typically use a larger percentage of bleached Kraft pulp with some lesser percentage of mechanical and recovered paper pulp.
What Kinds of Products are Made from Pulp?
Pulp is the base to almost all paper products. The paper your grocery ad is printed on, the packaging your cereal or medicine comes in, and the paper towels you use in the kitchen all started as wood pulp.
Fluff pulp is used in absorbent and personal care items like diapers, feminine hygiene and adult incontinence products.Dissolving pulp can be dissolved in a solvent and spun into textile fibers like rayon or Lyocell; chemically reacted to be formed into fibers or films; or used as a thickener for other products.
How is Wood Pulp Made?
There several ways to process wood chips into pulp. In chemical pulping, wood chips (from trees) are cooked using a chemical process in essentially a pressure cooker known as a digester. The wood fiber is separated into cellulose fibers, lignin (the wood glue that holds the tree together) and other substances such as sugars. Cellulose is an essential building block in the cell walls of trees and plants, helping to make them strong. The pulp is then washed to clean it and separate it from other substances such as lignin. After the pulp is washed, it is screened for further cleaning. The end product is a slurry that will be washed and bleached to standard brightness depending on its final end-use.
Mechanical pulping differs from chemical pulping in that pulp is created through the grinding of wood chips instead of cooking them. Wood chips are heated and fed into a machine that consists of two metal discs that rotate. The chips are broken up into smaller chunks and then into individual fibers as they are ground by the discs. They are blended with water to make a slurry, similar to chemical pulp and then run through the same process to dry it out.
For mills using paper recovered for recycling as raw material, pulp is made by mixing the recycled materials with water in what resembles a large blender called a repulper. In the repulper, the pulp is separated to create individual fibers in a slurry. From there, the pulp is washed and screened for further cleaning.
The slurry is then run through a variety of presses and heaters to remove much of the water. The pulp will either be sold to different facility or processed into a product on site.
Wood pulp is naturally light brown in color. Depending on the type of paper the pulp with will be used to produce, it may go through a bleaching process. Pulps used to make white copy paper, for example will be bleached to achieve the desired brightness, while pulps used to make paper grocery bags are unbleached.