Revised October, 2018.
Fibers are a class of materials that are continuous filaments or discrete elongated pieces. As they occur in nature, they are important constituents of living things, as well as non-living mineral materials. Natural and man-made fibers are, in turn, often used in the manufacture of other materials.
Fibers are classified in several ways. First of all, any fiber produced by plants, animals and geological processes is considered natural fibers. Those composed of cellulose and lignin are considered vegetable or plant fibers. Some examples include the dietary fiber of vegetable foods, cotton for clothing, or hemp or flax for cordage. The fibers within and produced by animals may be far more complex with respect to their protein structures and functions. Some examples include insect and arachnid silks, animal tendon sinews and insulatory hair coatings such as wool and mohair. Finally, there are some filamentous mineral fibers in nature; perhaps the most common are asbestos and some naturally occurring silica-based glass fibers.
Fiber uses are many and varied. Plant fibers may be used alone but more often are a constituent of a composite product. They may be woven into strands; compressed or “felted” into mats, papers or films; or incorporated into composite materials with organic fillers, metallic materials or plastics. Today, due to concerns regarding the “renewability” and availability of resources and waste management, there is renewed interest in plant-origin natural fibers. These cellulosic materials include cotton, jute, linen, hemp, flax, kenaf, cornstalk, ramie, soybean and hemp.
Additionally, there are specialized markets for isolated dietary fiber that can be used as a supplemental food product ingredient. For example, oat fiber is rich in beta-glutan that clinical studies have shown significantly reduces LDL and total cholesterol in the human body. More than 100 million Americans have elevated cholesterol levels. For the 65 million who are "borderline high," (200-239 mg/dL), lifestyle modifications such as diet can play a role in maintaining normal cholesterol levels, according to the National Institutes of Health (NIH) National Cholesterol Education Program. Partly due to these types of findings, soluble fiber was the first whole food for which the Food and Drug Administration allowed a health claim. Since 1997 product labels have stated that: "Diets low in saturated fat and cholesterol that include soluble fiber from oatmeal may reduce the risk of heart disease." Most ingredient and food product manufacturers acknowledge that North American markets are seen as open and ready for fiber-rich products. Currently, most consumers have supplemental dietary fiber from oat or wheat bran rather than as a more palatable supplement or a food ingredient.
Fibers can be derived from any agricultural material. This may be wood fiber, or “stovers,” the field waste material from commodity corn or production. Wood may be harvested for fiber production; pulp wood for paper production is a good example. Wood waste from the lumber industry is also utilized in composite wood products or as fillers in other materials. For example, fine-screened and dried wood dust often is sold as specialized filler for epoxy compounds. In this situation, the fiber is being used in a micro-crystal structural applications.
There are also “fiber plastics,” which to date, have depended upon inorganic synthetic glass fibers, but concerns over worker health, biodegradability and other issues are forcing the industry to consider plant-derived alternatives.
Agricultural fibers have been crucial to human development. Clothing from cotton and other plant fibers were in use prior to the Bronze Age. Hemp and flax fibers were important materials used on the sailing ships that explored the world. Wool, cotton and animal fur clothed and insulated the world’s population. The economics of these agricultural fibers affected the development of world culture, from advancing world trade to inciting warfare, and the socioeconomic impacts led to better lifestyles for some and slavery for others.
The development of the world’s petrochemical resources during World War II led to inexpensive post-war petroleum that could be used for a host of synthetic materials, including fibers that had better functional characteristics than those previously obtained from plant materials.
Today, there is considerable interest in the “bast” fiber plants: hemp, flax, kenaf, jute and ramie. Long, slender primary fibers on the outer portion of the stalk characterize bast fiber plants. These usually are annual plants but may be perennials that grow from seed. They tend to be drought tolerant, have minimal fertilization needs and can be grown on a range of soils. Most bast fiber plants produce a “primary fiber” from the outer stalk and a secondary “core fiber” from the interior pithy stem material. Most current research and development interest centers on the primary fiber, but attention is now also focusing on applications for the core fiber since it too is a harvested, potentially valuable co-product.
Primary fibers tend to be long and supple with good tensile properties; primary fiber applications include use in clothing, woven composites, matting materials and complex composites. Core fiber, also sometimes referred to as “hurd,” tends to be less filamentous with poorer tensile and processing properties. However it has its own applications. Some types are twice as absorbent as wood shavings, making it an excellent animal bedding and landscaping mulch product. Also, it has been experimentally blended with lime to create a strong yet lightweight concrete and plaster product. Both types of fiber are biodegradable, and fibers from some plants possess anti-mildew and antimicrobial properties. Both types of fiber have been demonstrated to have numerous applications in composite fiber plastic products and similar materials.
National Cholesterol Education Program, National Institutes of Health.