Bast Fibers

fibers contained in the stalks of terrestrial seed plants; long prosenchyma cells lacking in live contents. The walls of bast fibers are markedly and uniformly thickened, often with pronounced stratification, simple pores, and a very narrow cell cavity. The average length of a bast fiber is 1-2 mm, but primary fibers originating from the procambium are generally longer (20-400 mm), while secondary ones (of cambial origin) are shorter. The walls of bast fibers in many plants are saturated with lignin; in some plants the membranes of the fibers consist almost solidly of cellulose and are elastic and very strong.

Industrial bast fibers obtained by primary treatment of bast plants are widely used in the textile industry to manufacture yarn. The following kinds of bast fibers are distinguished: thin, pliable, slightly lignified fibers (flax, ramie) and coarse, thick-walled, markedly lignified ones (hemp, kenaf, jute, abaca). Industrial bast fibers consist of bundles of elementary fibers glued together. Yarn from flax fibers is used to make cloth; that from sisal, hemp, and abaca fibers, to make rope, cord, twine, strands for steel cable, and so forth; that from jute and kenaf fibers, mostly to make sacks. Abutilon fibers are brittle and therefore have limited use in the textile industry. They are used to make sacks and cable in which they replace jute fibers.

Primary processing of bast crops involves biological (retting) and mechanical (breaking, scutching, separation of bast) processes and includes the obtaining of stock (from flax, hemp, and sometimes kenaf stems) or bast (from jute and kenaf stems), and of bast fibers from the stock or bast. The commonest method of preparing stock is dew retting or spreading: after threshing, the stems (straw) are spread on a meadow or field in even rows where they lie for 15-25 days. Owing to the action of mold fungi (Cladosporium herbarum and Alternaria tenuis), heat, moisture, and light, pectin and other substances in the stalks decompose and the bond of the bast fibers with the surrounding tissues is destroyed. With cold-water retting, the straw in sheaves, bales, and containers is submerged in a body of water (retting pond, artificial rettery) for 10-15 days. The bast fibers are separated from the tissues as a result of the activity chiefly of pectin-fermenting bacteria (Clostridium felsineum and C. pectinovorum). In flax and hemp mills, retting in water heated to 36°-37°C is used to make industrial stock. The stock is obtained in 70-80 hours or in 24-48 hours if accelerators (urea, ammonia water) are used. The process can be speeded up even more by steaming the straw in autoclaves under pressure of 2-3 atmospheres (for up to 75-90 minutes) and then soaking it in a weak solution of soda ash, acids, and a special emulsion (for up to 30 minutes). The stock is dried and processed in breakers and scutchers where the fiber is freed from woody tissue (boon) and epithelial and parenchyma tissues.

Bast is separated from the stalks in a bast-separating machine, then subjected to biological retting and treated in a scutching and washing machine. The bast fibers are dried, softened, and graded. The technology by which bast is first obtained from stalks is also promising for flax.